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Russian Chemical Reviews 70 (2) 91 ± 104 (2001) Modern chemistry of nitrous oxide A V Leont'ev, O A Fomicheva, MV Proskurnina, N S Zefirov Contents I. Introduction II. Atmospheric nitrous oxide III. Structure and properties IV. Decomposition of nitrous oxide V. Chemistry of nitrous oxide VI. Applied aspects of the chemistry of nitrous oxide VII. Conclusion Abstract. are oxide nitrous of chemistry the of trends Modern Modern trends of the chemistry of nitrous oxide are discussed. and properties physical structure, its on Data discussed. Data on its structure, physical properties and reactivity reactivity are generalised. The effect of N the and environment the n o are generalised. The effect of N2O on the environment and the possibility of its utilisation are considered.Attention is focused on possibility of its utilisation are considered. Attention is focused on the processes in which the oxidising potential of nitrous oxide can the processes in which the oxidising potential of nitrous oxide can be employed. The bibliography includes 329 references be employed. The bibliography includes 329 references. I. Introduction Nitrous oxide was discovered by J Priestley in 1772. After experi- ments performed by H Davy in 1799, N2O received the name `laughing gas' from its action. In the mid-nineteenth century,N2O assumed importance and found wide application as a soft anaes- thetic. Presently, nitrous oxide is used in medicine primarily in mixtures with other more powerful inhalation anaesthetics.1 Since nitrous oxide is non-toxic and readily soluble in lipophilic media, it finds use in food industry as a foaming agent.Nitrous oxide is one of three compounds containing N7O bonds which were discovered in interstellar space.2 In the last two decades, the chemistry of nitrous oxide has been substantially developed. In one aspect or another, N2O was mentioned in a large body of research. In the present review, the abundant data on nitrous oxide are surveyed. Particular attention is given to studies in which N2O was described as the reagent. The role of N2O in the destruction of the ozone layer giving rise to the so-called `greenhouse' effect is also considered. A V Leont'ev, O A Fomicheva, MV Proskurnina Department of Chemistry, M V Lomonosov Moscow State University, Leninskie Gory, 119899 Moscow, Russian Federation. Fax (7-095) 939 02 90.Tel. (7-095) 939 50 59 (A V Leont'ev, O A Fomicheva), (7-095) 939 16 20 (M V Proskurnina). E-mail: alexandr@org.chem.msu.ru (A V Leont'ev), marina@org.chem.msu.ru (M V Proskurnina) N S Zefirov Department of Chemistry, M V Lomonosov Moscow State University, Leninskie Gory, 119899 Moscow, Russian Federation, Institute of Physiologically Active Compounds, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-095) 913 21 13. Tel. (7-095) 939 16 20. E-mail: zefirov@org.chem.msu.ru Received 23 October 2000 Uspekhi Khimii 70 (2) 107 ± 122 (2001); translated by T N Safonova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n02ABEH000631 91 91 92 93 94 99 101 II.Atmospheric nitrous oxide 1. Sources of atmospheric N2O and its environmental effects The environmental effects of nitrous oxide have been the subject of much attention of ecologists over the last several decades. This interest is primarily associated with the fact that the role of `laughing gas' (previously, it had been thought that this gas was quite harmless) in the atmospheric processes is beginning to emerge. It was realised that nitrous oxide plays a role in destruc- tion of the ozone layer and gives rise to the greenhouse effect. It was demonstrated that the average residence time of nitrous oxide in the lower layers of the atmosphere is about 120 years due to the lack of processes necessary for chemical conversions of N2O.3 Nitrous oxide gradually diffuses from the troposphere to the stratosphere, where it decomposes partly through photolysis and partly in reactions with reactive O and .OH species formed upon photolysis.4 hn N2+HO N2O N2+O(1D), N2O+O(1D) 2 NO, N2O+O(1D) N2+O2 , N2O+.OH 2.Nitric oxide (NO), in turn, reacts with ozone to formO2 andNO2 . The latter undergoes disproportionation to give NO and O under the action of solar radiation, this process occurring repeatedly.5 Nitrous oxide is the third most abundant minor component of the atmosphere, exceeded in amount only by carbon dioxide and methane. It is believed that these gases along with ozone and Freons are of importance in processes providing thermal balance of the Earth.Their molecules adsorb heat given up by the Earth as infrared radiation resulting finally in a noticeable increase in the temperature of the troposphere and the greenhouse effect.6, 7 Although the contribution of N2O to the total greenhouse effect is evaluated at 6%, the `greenhouse-forming' power of nitrous oxide is approximately 260 times larger than that of carbon dioxide.4, 5 Taking into account the residence time of nitrous oxide in the atmosphere and the fact that its discharge to the atmosphere from anthropogenic sources cannot be terminated immediately, ecologists have expressed serious concern over the contribution of N2O to the greenhouse effect.Analysis of air samples taken from cavities in land ice in Greenland demonstrated that the nitrous oxide content of the92 atmosphere remained rather constant (*280 ppb) over the last thousand years.8 Beginning in the mid-eighteenth century, the growth in industry was accompanied by a progressive increase in the N2O content in the atmosphere. Presently, the concentration of N2O in the Northern Hemisphere reaches *314 ppb and continues to increase at a rate of *0.5 ± 0.9 ppb per annum. Thus, the nitrous oxide content of the atmosphere increases by 0.2% ± 0.3% annually. The same trends are true for the Southern Hemisphere. Identification of all sources of discharge of nitrous oxide to the atmosphere and quantitative estimation of the contributions of these sources are stubborn problems.For various reasons, the scatter in the data obtained by different researchers is very wide.4, 9 However, the available models are steadily improved with the aim of gaining reliable information on the processes influencing the total balance of emission of N2O to the atmosphere and its decomposition.10 In 1990, emission of nitrous oxide to the atmosphere was *12.7 million tons 11 of which about 35 percent were of anthro- pogenic origin. Processes of nitrification and denitrification in soils of tropical and temperate regions and in oceanic waters are the major natural contributors ofN2O. It is assumed with a rather low accuracy that these processes are responsible for 45%, 25% and 30% of all natural emission ofN2O, respectively.Heating and power plants,12, 13 motor transport (18%),14, 15 some industrial chemical processes (12%) 16 and agriculture (70%) 17 are recog- nised to be the major sources of N2O of anthropogenic origin. One cannot say that no efforts are being undertaken to decrease the discharge of nitrous oxide to the atmosphere. Thus, the largest world's manufacturers of adipic acid (*5%± 8% of the total anthropogenic emission of N2O) concluded an agree- ment 18 and committed themselves to supply plants with special technological cycles and to reduce the amount of N2O exhausted into the atmosphere by more than 90% by 2000. New catalysts and catalytic processes for decomposition of N2O are being developed.19, 20 However, agriculture remains the major contrib- utor of nitrous oxide.The amount of nitrous oxide can be reduced by decreasing the use of nitrogen-containing fertilisers and by reducing flooded areas under rice crops where anaerobic processes ofN2Oformation occur. However, it is difficult to accomplish this goal because the growth of population generates a need for the solution of food problems, while modern highly productive agricultural crops require the intensive use of nitrogen fertilisers. Therefore, it was predicted that the N2O content in the atmosphere will be increased by 0.2% ± 0.3% annually in the coming years, as has been observed in recent years.11, 21 2. Catalytic purification of waste gases from N2O Presently, a number of catalytic processes for the neutralisation of pollutants are available and successfully used.22 Purification of waste gases from N2O is one of the steps aimed at reducing its discharge to the atmosphere.Apparently, the optimum procedure involves catalytic decomposition of nitrous oxide. For a long time, investigations in this field were carried out from the standpoint of fundamental science.19 The available systems often did not satisfy strict requirements imposed upon industrial catalysts for gas purification. More recently, many available catalytic systems have been reinvestigated under `pseu- doindustrial' conditions, viz., in the presence of complex gas mixtures, at temperature modes typical of heating furnaces and at high flow rates.23, 24 The world's large manufacturers of adipic acid, for example, DuPont (the catalyst is CoO ± NiO/ZrO2), BASF and Asahi (the catalyst is Cu/Al2O3), developed new technologies for the utilisation of N2O.25, 26 However, the avail- able catalytic compositions are far short of optimum.In addition, a wide variety of industrial sources of N2O emission as well as operating conditions varying over a wide range call for a search for new more perfect catalysts. In addition, it was established that installations used for the neutralisation of NO and NO2 can serve as sources of nitrous oxide.25 Denitrification of waste gases in A V Leont'ev, O A Fomicheva, MV Proskurnina, N S Zefirov industrial scrubbers is generally performed using selective high- temperature reduction of nitrogen oxides NOx with ammonia or hydrocarbons in the presence of a catalyst.27 The conditions and parameters of the reactions of N2O with reducing agents [RH,28 ± 33 NH3 (see Refs 34 and 35) or CO36 ± 38] have also been studied extensively.Since the maximum conversions of NO and N2OintoN2 are achieved at different temperatures, the possibility of the simultaneous removal of these gases is an important criterion for the estimation of the efficiency of catalysts used for the purification of waste gases from nitrogen oxides.39 ± 41 It should be mentioned that catalytic procedures for the purification of waste gases from nitrogen oxides are generally more favourable both from the economical standpoint and from the viewpoint of the efficiency of a reduction in the amount of NOx to the maximum permissible concentrations.The adsorption methods can also be employed for the solution of this problem. Thus Centi et al. 42 demonstrated that Ba ± ZSM-5 zeolites can be used for the selective removal of nitrous oxide from gas flows containing 0.1% ± 0.2% of N2O, and mixtures enriched with nitrous oxide (5%) can be either utilised as an oxidant or directed to refineries for subsequent decomposition. III. Structure and properties 1. Structure of the nitrous oxide molecule Generally, the structure of the nitrous oxide molecule (the symmetry group C?u) is described by the resonance forms A and B. + + 7 7N N O B N N O A It is believed that the N-oxide form A makes the major contribu- tion to the geometric and electronic structure of the molecule.The total effective charges on the atoms in the form A are given below: Ref. qO qN(O) qN Calculation procedure 0.47 0.62 0.44 CNDO/2 4-31G MP3 43 44 45 70.33 70.55 70.33 70.14 70.07 70.11 The opposite charge distribution in the resonance forms A and B accounts for the relatively low dipole moment of the molecule (0.161 D).46 The N7N bond (1.128A) in nitrous oxide is sub- stantially shorter than the standard N=N bond (1.24A, MeN=NMe) and is only insignificantly longer than the N:N bond (1.10A,N2). TheN7O bond (1.184A,N2O) is quite similar to the standard N=O double bond (1.21A, MeN=O).47 There- fore, the N7N and N7O bond orders in nitrous oxide are estimated at 2.73 and 1.61, respectively.48 It is easy to see that the representation of theN2O molecule by the resonance forms A and B formally assumes the presence of ambidentate properties, which can be manifested in the reactions of nitrous oxide both with electrophilic 49 and nucleophilic 50 agents.7 + NH2 H2N N N O7 N N O7 A + + 7 7 PR3 N N O PR3 N N O B + 7 The zwitterionic resonance forms A and B can be supple- mented with the form C (1,3-dipole) , N N O C which can be involved in cycloaddition.Modern chemistry of nitrous oxide Data on the formation of stable five-membered rings in the reactions of N2O with dipolarophiles are lacking. The theoretical possibility of this cycloaddition was discussed.51, 52 Bridson-Jones et al.53 suggested that dihydro-1,2,3-oxadiazoles can be formed as intermediates in high-temperature oxidation of alkenes with nitrous oxide. It should be noted that the non-classical representation of the nitrous oxide molecule as a biradical D47, 54 also allows an adequate description of the behaviour of N2O in chemical reactions. + 7 N N O N N O D A Within the framework of the molecular orbital method, this description implies that one-electron delocalisation of the 2ppx and 2ppy orbitals of O7 results in the fact that electrons occupy the px and py bonding molecular orbitals which form one-electron px(NO) and py(NO) bonds and weak partial px(NN) and py(NN) bonds. 2. Br��nsted basicity Based on the data on the equilibrium constants of several tens gas- phase reactions B2H++B1 B1H++B2 the absolute gas-phase basicity scale for a wide range of com- pounds from N2 to tert-butylamine was proposed.55 According to this scale, the proton affinity of nitrous oxide (137.8 kcal mol71) is comparable with those of methane (130.2 kcal mol71) and carbon monoxide (141.9 kcal mol71).Olah et al.44 prepared (and characterised) MeON�¢2 by the reaction N2O +MeF MeON�¢2 SbF¡¦6 , SO2F2, SbF5 780 8C However, the authors did not detect the hydroxydiazonium ion NNOH+ in the reaction of N2O with superacids. The protonated form of nitrous oxide in the gas phase was detected by IR spectroscopy.56 The spectral characteristics were not unambiguously assigned to particular isomers, viz., to NNOH+ or +HNNO.Calculations 57, 58 demonstrated that pro- tonation of the oxygen atom is more favourable than protonation of the nitrogen atom. However, the results of these calculations depend substantially on the electron correlation methods applied.59 3. Physical properties Nitrous oxide is a colourless diamagnetic gas with a weak pleasant odour and sweetish flavour. It is incombustible but can maintain burning. The nitrous oxide molecule is very stable. In the absence of a catalyst at 400 ¡À 530 8C, thermal decomposition of N2O is insignificant (0.22% ¡À 0.23%) 60 2N2O 2N2 +O2 . The activation energy Ea of this non-adiabatic spin-forbidden process is*59 kcal mol71.61 Nitrous oxide possesses a high critical temperature.At room temperature under high pressure (50.1 atm, 21 8C), it can exist as a liquid and can be stored in steel cylinders. In laboratory conditions, N2O is generally prepared either by thermal decom- position of ammonium nitrate or by the reaction of potassium nitrosyl sulfate with sulfuric acid.61 The physical properties of nitrous oxide are given below.62 ¡À 64 790.86 788.48 71.7 36.5 Melting point /8C Boiling point /8C Critical pressure /atm Critical temperature /8C 93 Critical density /g cm73 Density of N2O (liq) at788.48 8C/g cm73 Density of N2O (g) at 0 8C/mg cm73 Enthalpy of formation (DfH0298) /kcal mol71 Free energy of formation (DfG0298) /kcal mol71 Entropy (S 0) /cal K71 mol71 Thermal capacity (C0 0.452 1.226 1.997 19.6 24.6 52.52 9.19 p) /cal K71 mol71 IV. Decomposition of nitrous oxide 1.Thermal decomposition Homogeneous thermal decomposition of nitrous oxide is one of the most extensively studied chemical reactions.65, 66 This process is often used as a model for testing new theories and experimental systems 67 and it is also widely employed for the preparation of atomic oxygen O(3P).68 The following most important elementary stages resulting in decomposition of N2O are distinguished: N2+O+M, N2+O2 , NO+NO, N2+.OH, (1) (2) (3) (4) (5) N2+HO+M N2O+O N2O+O N2O+H N2O+.OH From the above-listed equations it can be seen that decom- position of nitrous oxide can occur either as a result of collisions with the third body M [reaction (1)] or through secondary reactions with the oxygen atom or with radical species [reactions (2) ¡À (5)].The nature of the species M and the character and concentrations of the radical species have an essential effect on this process. In most of the early investigations, the kinetics of decom- position of nitrous oxide was examined in the presence of argon or nitrogen at 850 ¡À 1300 8C. However, the formation of nitrous oxide in industrial apparatuses used for denitrification of waste gases and in heating and power plants, which are important anthropogenic contributors of N2O,9 occurs at moderately high temperatures (600 ¡À 900 8C) in complex gas mixtures.The neces- sity of gaining an insight into the kinetics of these processes gave impetus to investigations of thermal decomposition of N2O at 600 ¡À 900 8C in the presence of carbon dioxide, water vapour, hydrogen, methane, oxygen and other gases.69 ¡À 71 2. Heterogeneous catalytic decomposition A search for new alternative oxidants, on the one hand, and the necessity of the environmental protection aimed at reducing industrial discharge of nitrous oxide to the atmosphere, on the other hand, gave impetus to the development of efficient proce- dures for catalytic decomposition of nitrous oxide. These proce- dures were devised primarily by large manufacturers of adipic acid. Thus the BASF, Bayer, DuPont and Asahi companies use catalytic systems for decomposition of N2O in waste gases,72 whereas the Solutia company employs recirculation of N2O to the technological cycle and utilises it as an oxidant for the direct preparation of phenol from benzene.73 Problems of a search for efficient catalysts of decomposition of nitrous oxide were covered in an excellent review 25 in which the data on decomposition of N2O with the use of different heteroge- neous catalysts published up to 1996 inclusive have been surveyed and analysed.In the cited review, the mechanism and kinetics of decomposition of N2O were considered, the role of different oxygen forms occurring on the surface of catalysts was discussed and the problem of catalyst poisoning was touched on.However, a body of the published data in this field increases very rapidly and we believe that it is necessary to survey the data published in 1997 ¡À 2000 (Table 1).74 ¡À 106 The discovery of isothermal oscillation of concentrations of N2O, N2 and O2 upon decomposition of nitrous oxide on the94 Table 1. Catalytic systems for decomposition of nitrous oxide. Catalyst Catalysts fixed on inert supports Cu, Co, Ni, K, Mg, Mn, Ba Rh, Ru CuO, Ru, RuO2 Cu Rh CuO, CoO, Fe2O3 Zeolite catalysts [Fe], [Al] ± ZSM-22 Co, Cu, Fe ± ZSM-5 Oxide catalysts CaO, MgO Ex-hydrotalcites Mg, Co, Ti, Al, La, Rh Spinels (MgxCo17x)Co2O4 Cu ± ZSM-5 zeolite catalyst 107 was mentioned in a review.25 Later, a mechanism was proposed according to which the concen- trations of the components change in a wave-like fashion due to cyclic oxidation ± reduction of active copper(I) centres responsible for decomposition of N2O.108 ± 110 Recently, oscillation of nitrous oxide on the Rh/ZrO2 (Ref.111) and Fe ± ZSM-5 (Ref. 112) catalysts was observed. 3. Photochemical decomposition Photochemical decomposition of nitrous oxide affords N2, O2 , NO and NO2 .113 Spin-forbidden primary processes (6) and (7) occurring under exposure to light (l&250 and 200 nm, respec- tively) are convenient procedures for the preparation of atomic oxygen in investigations of the kinetics of different reactions in the gas phase.114, 115 hn N2O N2+O(3P) hn N2O N2+O(1D) In recent years, considerable attention has been paid to electronic, vibrational and rotational states of both photofrag- ments, viz., N2 and O,116, 117 with the aim of giving a comprehen- sive description of N2O photolysis.Great interest was also expressed in photochemical reactions of nitrous oxide in the atmosphere 4 because photolytic decom- position of N2O is the key process in the conversion of nitrous oxide in the stratosphere. 4. Photocatalytic decomposition Presently, photocatalysis is the most vigorously evolving line of photochemistry,118, 119 and this method can be successfully used for decomposition of nitrous oxide. Thus photodecomposition of N2O under irradiation with UV light (l<300 nm) at room temperature readily proceeds on the surfaces of ZnO,120 TiO2,25 Pt/TiO2 121 or Cu ± ZSM-5.122 Copper(I) compounds applied to inert supports,123 Ag, Cu/TiO2 124 or ions of different transition metals in zeolite matrices 125 exhibit high catalytic activities.Possible mechanisms of decomposition, in particular, on zeolites ZSM-5, were discussed.126, 127 Based on the data from IR and UV spectroscopy, it was concluded 127 that the reaction of N2O with Ref. Support 74 ± 79 C 81, 82 88 ± 91 92 ZrO2 Al2O3 C, Al2O3 , SiO2 , ZSM-5 USY, NaY, Al2O3, 96±98 ZrO2 , FSM-16, CeO2 , La2O3 SiO2 , MgO, CaO 105 80 83 ± 87 93 ± 95 99 ± 104 106 (6) (7) A V Leont'ev, O A Fomicheva, MV Proskurnina, N S Zefirov the Ag+± ZSM-5 system afforded the Ag+±N2O complex whose photoactivation resulted in decomposition of nitrous oxide into N2 and O2 .Photodissociation of nitrous oxide on heterogeneous catalysts may be very useful not only in denitrification but also in oxidation or destruction of organic compounds. Photocatalytic degradation of salicylic acid 128 in saturated aqueous solutions of N2O in the presence of ZnO or ZnO/TiO2 is an example. V. Chemistry of nitrous oxide 1. Thermodynamical analysis of the use ofN2Oas an oxidant A change in the Gibbs free energy DrG0 can be used as a criterion for the possibility of a particular reaction occurring. Thus the reactions can take place and proceed spontaneously if DrG0 (in kcal mol71)<0, whereas in the range of 0 ± 10 kcal mol71, the reactions are unlikely. This situation calls for further investiga- tion. If DrG0>10 kcal mol71, the reactions cannot proceed; however, special conditions can help in performing these proc- esses.The DrG0 value at which the reaction under consideration cannot be carried out depends on many factors and the above- mentioned values can be considered only as a very rough estimate. The thermochemical data for hypothetical and actual reac- tions of N2O with the reagents X or HX, which were calculated as differences between the total changes of the enthalpies (DfH0298) or the Gibbs free energies (DfG0298) of formation of the final and starting compounds, are given in Tables 2 ± 5. XO+N2 (g), X+N2O (g) 2XO+3N2 (g)+H2O (g). 2XH+3N2O (g) The required thermodynamical properties of individual com- pounds were taken from handbooks.129 ± 133 It should be emphas- ised once again that the data in Tables 2 ± 5 serve only as a qualitative estimate of the possibility for the reaction to occur.In addition, the above-mentioned data should be compared with each other with caution because the compounds exist in different Table 2. Changes in the Gibbs free energy of the reactions of nitrous oxide with inorganic compounds. DrG DrH 298 298 Reaction product (XO) Substrate (X or HX) POCl3 (g) CO2 (g) SO3 (g) NH2OH (sol) N2 (g) H2O2 (g) H2O (g) CuO (sol) PCl3 (g) CO (g) SO2 (g) NH3 (g) NH3 (g) H2O (g) H2 (g) Cu (sol) 787 786 742 7 72305 780 755 788 789 743 734 72106 777 757 Note.Hereinafter, the values are given in kcal mol71. Table 3. Changes in the Gibbs free energy of the reactions of nitrous oxide with some classes of organic compounds. Substrate (X) DrG DrH 298 298 Reaction product (XO) PhSSPh (liq) Me2SO (liq) Me2SO2 (sol) MeNCO (liq) PhN(O)=NPh (sol) PhSH (liq) Me2S (g) Me2SO (liq) MeNC (g) PhN=NPh (sol) 7 750 773 77 757 758 780 777 742 O HO O (sol) OH (sol) 747 735Modern chemistry of nitrous oxide Table 4. Changes in the Gibbs free energy of the reactions of nitrous oxide with aliphatic and aromatic hydrocarbons. Substrate (X) CH4 (g) MeOH (g) HCOH (g) n-C3H8 (g) MeCH(OH)Me (g) Me3CH (g) Me3COH (liq) H2C=CH2 (g) CH2=CHMe (g) CH2=CHCH2OH (g) CH2=CHCHO (g) PhH (g) PhMe (liq) PhCH2OH (liq) PhCHO (liq) PhCO2H (sol) PhEt (liq) (liq) OH (liq) Table 5. Changes in the Gibbs free energy of some reactions involving nitrous oxide. Reaction type 4Na (sol)+3N2O (g)+NH3 (g) ? ? NaN3 (sol)+3NaOH (sol)+2N2 (g) 2NaNH2 (sol)+N2O (g) ? ? NaN3 (sol)+NaOH (sol)+NH3 (g) C6H11NH2 (g)+N2O (g) ? C6H11N3 (liq)+H2O (liq) 727 2NH3 (sol)+N2O (g) ? NH4N3 (sol)+H2O (liq) 738 2Na2O (sol)+N2O (g)+NH3 (g) ? 7112 ? NaN3 (sol)+3NaOH (sol) physical states and the determined Gibbs energies and enthalpies of the initial and final compounds may be inexact.Therefore, N2O is potentially a powerful oxidant. However, only the data on high-temperature oxidation of organic substrates on heterogeneous catalysts and several examples of oxidation of some substrates in homogeneous reactions catalysed by transi- tion-metal complexes are available in the literature.Evidently, this inertness is associated with the high activation energy of decom- position of nitrous oxide. Apparently, this problem can be solved by searching for suitable catalysts. 2. The use of N2O in oxidative catalysis Considerable progress has been achieved in oxidative catalysis over the latter part of the twentieth century;134 however, the advances in oxidative hydroxylation of paraffins and aromatic compounds are still poor. Generally, the reactions using atmos- pheric or molecular oxygen as an oxidant proceed with low selectivity (sometimes these reactions lead to complete oxidation of the substrate).Presently, several major approaches to the solution of this problem are available, viz., the employment DrG DrH 298 298 Reaction product (XO) 750 757 782 761 763 773 74 MeOH (g) HCOH (g) HCO2H (g) MeCH(OH)Me (g) Me2C(O) (g) Me3COH (liq) Me3COOH (liq) 751 768 781 761 774 764 7 (g) 754 747 O 757 773 782 764 759 777 7137 CH2=CHCH2OH (g) 756 CH2=CHCHO (g) CH2=CHCO2H (g) PhOH (g) PhCH2OH (liq) PhCHO (liq) PhCO2H (sol) PhCO3H (sol) PhCOMe (liq) 764 782 762 761 776 794 715 7128 OH (sol) 7 782 O (liq) 7 759 DrG DrH 298 298 7322 7349 770 771 7 78 7 95 of membrane-type reactors,135, 136 the separate performance of reduction and reoxidation of catalyst's surface (two-step process) 137 or the use of alternative oxidants.In the latter case, major hopes are placed on the use of hydrogen peroxide on titanium silicate catalysts 138 and nitrous oxide on ZSM-5 zeolite catalysts.139 Two main lines of investigation, viz., selective oxidation of aliphatic (aromatic) hydrocarbons and oxidative condensation of lower alkanes, were mentioned in reviews 139, 140 devoted to the employment of N2O as an oxidant under conditions of heteroge- neous catalysis. However, in spite of a large number of stud- ies,141 ± 151 the latter reaction is, in Krylov's opinion,152 substantially less useful than other oxidative conversions of hydrocarbons.Actually, direct oxidation reactions of benzene to phenol or of methane to methanol are of importance. Phenol is still prepared by the oxidation of cumene, and methanol is prepared by the conversion of the synthesis gas. A multitude of catalytic systems, including those involving N2O as an oxidant, viz., various metal oxides (mixed solid solutions doped with ions of other metals fixed on inert supports), heteropolyacids and their salts, zeolites, etc., were examined in direct oxidation reactions of benzene and methane.139, 140 Some catalysts of hydroxylation of methane and benzene with nitrous oxide are listed in Table 6. However, attempts to perform the selective conversion of methane to methanol failed. Table 6. Catalysts of oxidative hydroxylation with the use of nitrous oxide.Hydrocarbon Ref. Catalytic system Benzene 153, 154 156 ± 159 Fe ± Si, FePO4 , H±Ga ±FER H, Na, H± [Al], Fe ± ZSM-5 Methane 155 160 ± 162 163, 164 165 166 Fe ± Si, FePO4 , H±Ga ±FER H, Na, H± [Al], Fe ± ZSM-5 Sr- or Ca-hydroxyapatites Mo/SiO2 12-phosphomolybdic acid The major progress has been achieved in performing the oxidation of benzene to phenol.167 A new procedure for the synthesis of phenol, which is based on the use of Fe-containing ZSM-5 zeolites and is characterised by high selectivity with respect to both phenol (97% ± 98%) and nitrous oxide (85%), was developed by researchers from the G K Boreskov Institute of Catalysis of the Siberian Branch of the Russian Academy of Sciences in cooperation with the Solutia company (USA).Suc- cessful tests of a pilot plant were performed and the construction of the industrial plant (Pensacola, Florida) rated at 140 thousand tons of phenol per annum is being brought to completion.168 It should be noted that the results of many investigations in this field were covered by patents.169 ± 176 The nature of high catalytic activity of zeolites in oxidation of benzene is the matter of vivid debates. Presently, two major hypotheses are available. One of them is based on a conventional view of zeolites as acid catalysts whose activity is associated with Lewis 156, 159, 177 and (or) Brùnsted 178, 179 acidic centres. However, a special investigation 180 did not confirm the hypothesis based on the concept of Brùnsted centres.The second hypothesis was proposed by Panov et al.139, 181 These authors believed that the interaction of N2O with the surface of Fe-ZSM-5 zeolites afforded a special form of surface oxygen arbitrarily called a-oxygen. In its properties, a-oxygen differs substantially from the known surface forms of oxy- gen.182, 183 a-Oxygen cannot be formed upon adsorption of molecular O2 . It was assumed that a-oxygen resembles somewhat active oxygen of monooxygenases for which hydroxylation of C7H bonds is a typical reaction. Quantum-chemical calculations96 of the thermodynamical and geometric parameters of the forma- tion of a-oxygen and its successive reactions with benzene or methane were performed.184, 185 It was demonstrated 162 that CH4 reacted virtually quantita- tively with the a-form of oxygen, which arose as a result of decomposition of N2O on Fe-ZSM-5 zeolite, at room temper- ature.However, the resulting methanol was strongly bound to the zeolite surface and its desorption into the gas phase did not occur. An increase in the temperature to 250 ± 300 8C led to vigorous CO evolution due to secondary conversion of the product. A search for possible catalysts of selective oxidation of hydro- carbons is not restricted to zeolites. Different research groups made efforts to search for a means of activation ofC7Hbonds.186 Wang et al.187 succeeded in performing oxidative carbonylation of methane on Rh/FePO4 at 300 ± 400 8C in the presence of CO and nitrous oxide to form MeOH, MeCO2H and MeCO2Me.Partial oxidation of methane with nitrous oxide under conditions of plasma discharge 188 gave rise to HCHO and MeOH in a total yield of 10%, which is comparable with the results obtained with the use of the best catalytic systems. On the whole, the problem of direct oxidation of methane to form methanol, in spite of efforts made, is still far from solution. 3. Oxidation reactions involving N2O As mentioned above, nitrous oxide is a potentially strong oxidant, but it is very inert for kinetic reasons. Under mild conditions,N2O reacts only with highly reactive compounds, such as organo- boranes,189 silaethenes 190 or disilenes.191 N2+(Et2B)2O+C2H4+H2O BEt3+N2O C6H14 715 8C O N SiMe2 N Me2Si C(SiMe3)2+N2O C SiMe3 Me3Si (Me2SiO)n+(Me3Si)2CN2 O O O2 R2Si SiR2+N2O SiR2 R2Si R2Si SiR2 PhH 20 8C O Me Pri.Me, R= Me In the absence of a catalyst, N2O does not react with alkanes, arenes, amines, esters and aliphatic alcohols even at 200 ± 300 8C and 100 ± 500 atm.53 Under these conditions, alkenes and alkynes form predominantly carbonyl compounds.192, 193 O +N2O 300 8C, 500 atm 7N2 The reactions of nitrous oxide with a solution of triethyl phosphite or cyclohexyl isocyanide in benzene (20 atm, 130 8C) afforded triethyl phosphate (80%) or cyclohexyl isocyanate (20%), respectively.194 A series of oxides of tertiary phosphines were prepared in quantitative yields 195 with the use of nitrous oxide in the supercritical state (100 ± 140 atm, 60 ± 100 8C).The reaction of N2O with nitrogen trifluoride in the presence of an excess of SbF5 gave rise to NF2O+Sb2F¡11 whose vacuum pyrolysis afforded NF3O.196 Until recently, procedures for the preparation of NF3O by direct chemical oxo transfer from an O-donor to NF3 were lacking. F F F F 150 8C F N Sb NF3+N2O+SbF5 FF F O N N NF2O+SbF¡6 +N2 A V Leont'ev, O A Fomicheva, MV Proskurnina, N S Zefirov Oxidation of aqueous solutions of alcohols, amines and ethers with nitrous oxide in the presence of Pt or Pd black proceeds according to a radical mechanism.197 It is assumed that reductive decomposition of N2O adsorbed on platinum gives rise to N2 and the Pt(.OH) radicals, which are strong electrophilic oxidants.Me2C .OH+H2O+Pt, Me2CHOH+Pt(.OH) N2+Me2CO+Pt(.OH). Pt(N2O)+Me2C .OH On the whole, activation of N2O is generally a necessary condition for its involvement in oxidation as the oxo donor. 4. Reactions with nucleophilic reagents The reactions of N2O with melts of amides of alkali metals and alkaline-earth metals giving rise to the corresponding azides are among the most important reactions of nitrous oxide with nucleophilic reagents.50 Since sodium azide is rather stable, it serves as the starting compound for the preparation of other azides, and this reaction provides the basis for the industrial production of NaN3.198 NaN3+NaOH+NH3 . 2NaNH2+N2O Sodium azide is also formed in the reaction ofN2O either with Na in liquid ammonia NaN3+3NaOH+2N2 , 4Na+3N2O+NH3 or with sodium oxide 61 110 ± 190 8C NaN3+3NaOH.2Na2O+N2O+NH3 The reaction of N2O with ammonia over the Ni ±Al2O3 catalyst afforded ammonium azide 63 Ni ±Al2O3 NH4N3+H2O. 2NH3+N2O Diazo transfer from nitrous oxide to anions of different anilines was carried out (2 ± 5 atm, 20 8C) 199 and the correspond- ing aryl azides were isolated (10% ± 35%). The reaction of N2O with the monolithium derivative of 1,1- dibenzylhydrazine (1) proceeded successively through the forma- tion of N-azidoamine 2 and N-nitrene 3.200 Bn Bn N NOLi 7LiOH N NLi+N2O H N NH 1 Bn Bn Bn Bn 7N2 N N 3 N N3 2 Bn Bn 1,2-Diphenylethane (12%) and N,N-dibenzylbenzalhydra- zone (11%) were identified as the major products of the above reaction. The reactions of N2O with phosphorus ylide Ph3P+CH¡2 (see Ref.201) or with MeLi 202 gave rise to diazomethane (in 25% or 70% yields, respectively). It should be noted that, with rare exceptions, the reactions of nitrous oxide with anions of organo- lithium compounds afforded mixtures of N- and O-products due to the ambidentate character of the former. In addition, the possibility of direct oxidation of organometallic compounds with nitrous oxide must not be ruled out. Based on the results of investigations into the reactions of N2O with different organo- lithium compounds, two pathways were proposed. Pathway a (see Ref. 203) R2CHOLi +N2 R2CHOLi +N2 R2CHLi +N2O R2CHN NOLi R2CN2+LiOH H+ R2CN2+R2CHLi R2C N NCHR2 H R2C N NCHR2 LiModern chemistry of nitrous oxide Pathway b (see Refs 204, 205) PhOLi +N2 PhN NOLi PhLi +N2O PhLi PhN NPh+Li2O 2 PhLi N2O Ph Ph+PhN NPh Li Li PhN NPh N2O PhLi Ph2NNPh Ph2NNPh Li H.Ph2NNPh H Ph2NNPh NPh+Ph2NH Ph2NN To our knowledge, the procedure for the preparation of diazoferrocene (4) (the yield was 25%) developed by Nesmeyanov et al.206 is the only example of the successful application (from the synthetic standpoint) of diazo transfer involving nitrous oxide. Fe N N Fe BuLi, THF±Et2O N2O,715 8C Fe 4 Recently, this procedure was used for the preparation of N=N-bonded ferrocene oligomers.207, 208 Grignard reagents, unlike organolithium compounds, do not react with nitrous oxide.203 However, it was demonstrated 209 that the reaction of PhMgBr with N2O in the presence of CuCl (10 mol.%) afforded small amounts of PhN=NPh (2%) and Ph2NH (1%).Previously, it has been found 210 that organocal- cium analogues of Grignard reagents, for example, PhCaI, reacted with N2O to yield azobenzene (8%), biphenyl (10%) and traces of benzidine, phenol and hydrazobenzene. The potential of this reaction as a new procedure for the formation of C7N bonds was examined. 211 It appeared that the yields and the ratio of the reaction products depend substantially on the solvent and the type of substitution in the benzene ring. The maximum yield of azobenzene (60%) was achieved in dimethoxy- ethane. However, substantial amounts of biphenyl obtained as a by-product impair the synthetic significance of this reaction.The reaction of N2O with Ph2Ca afforded azobenzene as the major product (10%).211 This provides evidence that PhN=NPh can be formed by the direct insertion of N2O at the R7M7R bond, and not just by the reaction of diazotate R7N=N7OM with the second R7Mmolecule, as has been suggested previously.210 5. Reactions with metal-complex compounds Nitrous oxide can act as the oxo donor in reactions with metal- complex compounds. The pioneering studies in this field were performed by Bottomley et al.212 ± 218 The authors used N2Oas an oxidant for the synthesis of a series of oxometallocenes with terminal and(or) bridging oxygen atoms, such as oxocyclopenta- dienyl derivatives of titanium,213, 214 chromium 215, 216 and vana- dium.217, 218 Generally, the reactions of low-valence cyclopentadienyl derivatives of transition metals with N2O afford oxometallocenes containing the central metal atom in an intermediate oxidation state, whereas the reactions with O2 give rise to oxo clusters containing the metal atoms in the highest oxidation state.212 Thus oxidation of dicyclopentadienylchromium with oxygen 212 or nitrous oxide 216 afforded two different derivatives, viz., 5 and 6, respectively.[(Z-C5Me5)Cr(O)(m-O)]2+(C5Me5)2 , 2(Z-C5Me5)2Cr+2O2 5 97 4(Z-C5Me5)2Cr+4N2O [(Z-C5Me5)Cr(m3-O)]4+4N2+2(C5Me5)2 . 6 298) From Bottomley's standpoint 212 this fact cannot be explained only by thermodynamical reasons [the energy of the N7O (D0 bond in N2O is *40 kcal mol71 and the energy of the O7O bond in O2 is 119 kcal mol71].133 The difference in the structures of oxometallocenes should be determined by the reaction mecha- nisms, which are poorly understood so far because of their complexity.The reactions of titanocene and vanadocene 219 with nitrous oxide proceeded through the formation of the monomers [(Z-C5Me5)2]M(O) (M=Ti or V) whose decomposition afforded the oxo clusters [(Z-C5Me5)M4(m-O)6]. More recently, many researchers used nitrous oxide along with conventional oxo donors (PyO, OPPh3, Me2SO or O2) for the preparation of different terminal (bridging) oxo metal com- plexes.Thus the reactions of N2O with the tetraaza[14]annulene ± Sn(II) complex,220 U(OC6H3But2-2,6)3 (see Ref. 221) and V[(Me3SiNCH2CH2)3N] 222 were reported. Cyclopentadienyl derivatives of uranium,223 samarium,224 zirconium 225, 226 and hafnium 227 also reacted with nitrous oxide to give oxo com- pounds. The heterobinuclear zirconium ± iridium oxo complex 7 was prepared.228 Bu N t But N IrCp* Cp2Zr N2O 7N2 IrCp* Cp2Zr O7 Cp=C5H5; Cp*=Me5C5. The reactions of N2O with metal complexes can be accom- panied by insertion of the former at the (Ln)M7R bond. Generally, these reactions afford compounds of the (Ln)MOR type as the final products. However, intermediate complexes 8 containing the nitrous oxide molecule were isolated and charac- terised in the case of titano- 229 and zirconocene 230, 231 systems.Ph Ph Ph Ph Ph Cp2 Zr Cp2 Zr Cp2 Zr 20 8C 7N2 N2O 0 8C O +N N Ph 7 8 O Nickel compounds reacted with N2O to form stable alkyl- (aryl)oxy complexes 9 and 10.232 ± 234 OEt N2O (bipy)NiEt2 7N2 Et (bipy)Ni 9 bipy is 2,20-bipyridyl. PMe3 Ni N2O (Me3P)2Ni O O 7N2,7Me3PO Ni 11 Me3P 10 It was proposed 232 that the reaction with nickel complex 11 proceeded via intermediates analogous to the zirconium com- plexes 8. The insertion of N2O at the (Ln)M7R bond giving rise to the (Ln)MOR compounds was described for the complexes of Hf 235 and Ru.236, 237 The reactions of nitrous oxide with some metal complexes proceed according to unique mechanisms.Thus the molybdenum complex 12 is the only known metal complex, which can cleave the N7N bond in the N2O molecule.23898 N(Ar)R N2O Et2O, 25 8C 2 R(Ar)N Mo 12 N(Ar)R ON N N(Ar)R N(Ar)R Mo Mo + R(Ar)N R(Ar)N N(Ar)R N(Ar)R R=C(CD3)2Me; Ar=C6H3Me2-3,5. Based on the results of calculations, it was suggested 239 that the reactions of this complex with N2O and N2 proceed by different mechanisms, viz., by bimolecular and monomolecular mechanisms in the cases of nitrous oxide and N2 , respectively. Two different complexes were obtained in the reactions of iridabenzenes 13a,b, which differ by the substituents in the phosphine ligands.240 PR3PR3 PR3PR3 N2O Ir Ir 7N2 PR3 PR3 13a,b O R=Et PR3CO Ir 7PR3 H PR3PR3 R=Me HIr PR3 PR3 O R=Et (a), Me (b).It was found 241 that the reactions of the [Cr(OCP)Li2(THF)4] complex (OCP is octaalkylcalix[4]pyrrole) with N2O gave rise to two different oxygen-containing products depending on the solvent used. 6. Metal-complex catalysis of oxo transfer Apparently, the high activation energy of decomposition of N2O (Ea=59 kcal mol71) 61 N2O (g) N2 (g)+O (g) (1D), DH8=85 kcal mol71, N2O (g) N2 (g)+O (g) (3P) , DH8=40 kcal mol71, is the only factor that prevents the wide use of nitrous oxide as an oxo donor. Powerful inexpensive nitrous oxide, which can be readily removed from the reaction medium and does not introduce impurities, could be a valuable oxidant. A search for suitable catalysts of N2O decomposition seems to be the most promising way of solving this problem.Oxo transfer from the reagent to the substrate under the action of metal-complex catalysts is used in various reactions.242 Acyl(alkyl) peroxides, molecular oxygen, amine N-oxides, hydrogen peroxide and aryliodoso compounds are most popular oxo donors. Attempts to use nitrous oxide have also been undertaken, but no noticeable success has been achieved. Generally, elimination of the O atom from N2O either gives rise to oxo derivatives of complexes which cannot participate in the catalytic cycle for various reasons, or leads to the oxidation of ligands involved in the coordination sphere about the metal atom. Thus the use ofN2O in the catalytic cycle of oxidation of PPh3 to form Ph3PO in the presence of HCo(N2)(PPh3)3 was reported.243 The following reaction scheme was proposed: A V Leont'ev, O A Fomicheva, MV Proskurnina, N S Zefirov N2 N2O H3Co(PPh3)3 HCo(N2O)(PPh3)3 HCo(N2)(PPh3)3 N2O H2 PPh3 OPPh3 Oxidation of carbon monoxide is catalysed by carbonyl complexes of Rh,244 Ru, Os or Fe.245 N2O+CO cat N2+CO2 .It was suggested 245 that the reactions in the [Rh] ±CO±N2O and [Fe] ±CO±N2O systems proceeded through different mecha- nisms. In the case of rhodium complexes, the catalytic cycle involves one-electron transfer from [Rh(CO)]7 to N2O yielding N2O7., its capture by the [Rh4(CO)11]27 anion and subsequent elimination ofN2 and CO2 (the fragment A of the resulting cluster 14 is shown).37 CO Rh Rh O7 +N OC N7 Rh Rh A [Rh(CO)4] .+N2O7. [Rh(CO)4] +N2O [Rh4(CO)11]27+N2O7. [Rh4(CO)11(N2O)]37. 14 [Rh4(CO)10]37.+N2+CO2 In the presence of iron carbonyls, the nucleophilic attack of N2O on the reactive intermediates occurs giving rise to an intermediate adduct 15. 7N + N2O N [Fe2(CO)8]27 Fe2(CO)¡87O 15 [Fe2(CO)7]27+CO2+N2 It should be mentioned that molecules which are incorporated into complexes and can regenerate these complexes in the catalytic cycle serve as substrates in the above-considered oxidation reac- tions involving N2O. These reactions are not versatile, which impairs their practical significance. Oxidation of CO246, 247 and PR3,248 which are incorporated into metal complexes, with nitrous oxide beyond the catalytic cycle is also documented. The reactions of nitrous oxide with cyclopentadienyl complexes of cobalt at 130 8C resulted in their decomposition to form the corresponding furans and but-2-ene-1,4-dials.249 Oxo transfer from N2O to triphenylphosphine in the presence of [(CN)5Mo(O)](PPh4)3 proceeded through the formation of [(CN)4Mo(O)2]27 (16), the latter serving as an oxidant.250 27 37 O O VI IV +N2 +N2O O (CN)4Mo (CN)5Mo 16 27 O 2 +Ph3P O (CN)4Mo 16 47 O O +Ph3PO (CN)4Mo O Mo(CN)4 17Modern chemistry of nitrous oxide However, the reaction of the dioxo complex 16 with the substrate afforded compound 17, which cannot be involved in subsequent reactions, and these reaction cannot be performed in the catalytic mode.Triphenylphosphine was oxidised with nitrous oxide in an autoclave at 80 8C in the presence of phthalocyanine complexes of Al and Cu.251 In the studies cited, the possibility of the use ofN2O as the oxo donor in reactions catalysed by metal complexes was merely mentioned. However, a series of investigations in which N2O was used in the catalytic cycle were published in recent years. Thus it was demonstrated 252 that Ru(II)(TMP)(THF)2 (TMP is tetrame- sitylporphyrin) reacted with nitrous oxide under mild conditions. The reaction of the resulting complex, viz., of trans- Ru(VI)(O)2(TMP), with styrene afforded finally the starting Ru(II) complex and oxirane, thus closing the catalytic cycle.Yamada et al.253 examined the possibility of activation of nitrous oxide with some low-valent transition-metal complexes. Oxida- tion of PPh3 was used as the model reaction in the presence of acetylacetonates of the corresponding metals as precatalysts. It was found that Co- and Ni-containing compounds exhibit high activities. The reactions were carried out in the presence of 20 mol.% of metal acetylacetonate and 40 mol.% of diisobutyl- aluminium hydride under atmospheric pressure of N2O. M(acac)n, Bui2AlH Ph3PO Ph3P N2O, 20 8C, PhMe Fe(acac)3 18 Ru(acac)3 43 Co(acac)2 98 Pd(acac)2 Ni(acac)2 0 89 M(acac)n Yield Ph3PO (%) VI. Applied aspects of the chemistry of nitrous oxide 1. Reduction in protic media Reduction of nitrous oxide with a copper-containing enzyme, viz., N2O reductase, is one of the stages of microbiological denitrifica- tion 254 NO NO¡ NO¡ N2O N2 , 2 3 which, in turn, together with nitrogen fixation and nitrification comprise the global biogeochemical cycle of nitrogen in nature.At the same time, nitrous oxide can serve as a substrate both for natural nitrogenases 255, 256 and for synthetic protic nitrogen- fixing systems.257 N2+H2O. N2O+2e7+2H+ It was demonstrated 258 that N2O reacts also with aprotic synthetic nitrogen-fixing compositions. Li7Me3SiCl7MCln+N2O THF, 20 8C, 24 h 7LiCl [M N(SiMe3)n]+(Me3Si)3N+Me3SiOSiMe3 M=Cr, Fe, Co, Ti. A number of other metal-containing enzymes (CO dehydro- genase 259 and methionine synthetase 260) also react with nitrous oxide.Aqueous solutions of strong reductants, such as CrCl2 , can also reduce nitrous oxide 261 although this reduction proceeds very slowly. The rate constant of this reaction increases more than 108 times if nitrous oxide is present as a ligand in the [Ru(NH3)5(N2O)]2+ complex.262 This complex, which was iso- lated in its individual state as salts with PF ¡6 or BF¡4 ,263 is presently the only known stable metal complex containing N2O as a ligand. The mode of coordination of nitrous oxide in this complex is not finally established;264 however, according to calculations,265 the Ru7NNO mode of bonding is preferential. 99 The reactions of N2O with solutions of some transition-metal compounds were investigated.266 In particular, it was demon- strated that the reduction of N2O in aqueous solutions with potassium borohydride was catalysed by cobalt complexes Co(bipy)3(ClO4)3 and Co(DMG)2PyCl (DMG is dimethylgly- oxime) and by vitamin B12 .Co(II)+N2 . Co(I)+N2O In the absence of a reductant, vitamin B12 is oxidised by N2O according to the following scheme Cob(III)alamin+N2+2OH7, Cob(I)alamin+N2O+H2O 2Cob(II)alamin. Cob(I)alamin+Cob(III)alamin The discovery of this reaction, which proceeds both in vitro and in vivo, was of importance for anaesthetics and hematol- ogy.267 Tetraaza[14]annulene compounds of Ni(I) and Cu(I) also proved to be capable of reducing N2O in the two-electron mode.268 The Ni(II)Ann complex (Ann is tetraaza[14]annulene) was subsequently used as a catalyst of electrochemical reduction of nitrous oxide and showed high efficiency and chemical stabil- ity.269 Reduction of N2O in aqueous solutions is a thermodynami- cally favourable process.However, in most cases the reactions in the absence of a catalyst proceed very slowly for kinetic reasons. 2. Electrocatalytic reduction Presently, electrochemical processes are successfully used both in the synthesis of a great variety of compounds and in environ- mentally safe technological processes.270, 271 It is known that N2O along with Freons plays a crucial role in the destruction of stratospheric ozone and hence maximum effort is expended to decrease its discharge to the atmosphere. As mentioned above, high-temperature catalytic processes are most often used for the neutralisation of nitrogen oxides.However, electrochemical reduction of nitrous oxide has advantages over thermal destruc- tion because the former process is not accompanied by the formation of by-products, viz., NOx . Reduction of N2O to molecular nitrogen is thermodynamically favourable. N2O+2H++2e7 N2+H2O, E8=+1.77 V. The kinetics of electrochemical reduction on platinum electro- des in alkaline 272 and acidic 273, 274 solutions was examined by cyclic voltammetry. A model was proposed 273, 274 according to which adsorbed hydrogen plays the key role in reduction of N2O. N2 (g)+OH (ads). N2O (ads)+H (ads) On the other hand, Ahmadi et al.275 believed that decom- position of adsorbed nitrous oxide is the limiting stage of this process.N2 (g)+Pt7O. Pt7N2O (ads) The stage of subsequent reduction on the electrode, on the contrary, proceeds very rapidly. Pt+H2O. Pt7O+2H++2e7 Electrochemical reduction of N2O on other metal 276, 277 and semiconducting oxide 278, 279 electrodes was investigated. With Au, Pd, Cu, Ag, ZnO or In2O3 , high current efficiency was achieved. Tetraaza[14]annulene-nickel complexes,269, 280 cobalt por- phyrins,281, 282 myoglobin 283 and iron ± molybdenum com- plexes 284 fixed on graphite electrodes can be used as catalysts of electrochemical reduction of nitrous oxide. A possibility of performing the destruction of NOx on a cathode under conditions of a gas diffusion electrode seems to be promising.Both platinum 285 (or modified platinum 286, 287) elec- trodes and electrodes prepared from other transition metals 288 are employed as cathodes. In all cases, reduction of N2O to N2100 proceeds smoothly without the formation of by-products. In addition, it was demonstrated 285 that N2O7H2 (1 M KOH) fuel cells can be used as chemical sources of electrical current. Catalytic reduction of N2O with atomic hydrogen was suc- cessfully carried out in a special electrochemical membrane-type reactor.289 Reduction of nitrous oxide with carbon monoxide was also performed in an electrolytic cell supplied with a solid oxide electrolyte.290 Considerable attention is given to the development of electro- chemical methods for the analysis of nitrous oxide in liquids.291, 292 These methods, unlike chromatographic methods, allow one to monitor changes in the N2O content on-line.Undoubtedly, these gas analysers will find wide use in medicine, food industry and systems of environmental monitoring. 3. Radiation-induced reactions in aqueous media Ionising radiation in water induces processes giving rise to radicals, ions and stable species:293 .OH H2O2 H3O+ .H e H2 ¡aq 0.06 0.29 0.04 0.08 0.29 7 Product of water radiolysis Radiochemical yield G/mmol J71 e¡ Jones 308 believed that the N4O2 molecule is planar (the symmetry group D2h). However, Manaa and Chabalowski 309 In aqueous solutions saturated with N2O, solvated electrons reasoned that this conformation is only intermediate one between aq react with nitrous oxide to give the .OH radicals, the total two equivalent boat conformations (the symmetry C2u).Accord- radiochemical yield [G(.OH)] of the latter being increased to *0.58 mmol J71. .OH+N2+OH7. N2O+e¡aq This procedure for the generation of hydroxyl radicals, which are powerful electrophilic oxidants, is rather widely used in the laboratory practice.294, 295 In addition, the ability of N2O to react with solvated electrons attracts attention of researchers engaged in the application of ionising radiation to purification of sewage. It is well known that radiation treatment (cathode-ray or gamma radiation) in combination with other chemical or biological methods for the purification of liquid waste often proves to be convenient for the decomposition of harmful impurities.296 The efficiency of this purification can be improved by introducing compounds which transform water radiolysis products to radicals acting as reductants (oxidants).297, 298 Bubbling of nitrous oxide through sewage may improve the efficiency of the purification process.299 However, the presence ofN2Osometimes substantially impairs the efficiency of this process.298 Not only gamma radiators or cathode-ray accelerators, but also ultrasonic probes and UV lamps can be employed as sources of ionising radiation. Thus photoinduced oxidation of aqueous solutions of Am(III),300 Np(IV), Pu(IV),301 aliphatic alcohols 302 and amino acids 303 saturated with nitrous oxide was reported.Shilov et al.304 demonstrated that *35% of Np(V) existing as a suspension of its hydroxide in 1 M LiOH solution saturated with N2O was converted into Np(VII) upon sonication (44 KHz) for 40 min. Oxidation of organic substrates with nitrous oxide under the action of ionising radiation can occur both in the gas phase 305 and in non-aqueous media.306 4. Nitrous oxide oligomers as potential high-energy materials Semiempirical calculation procedures are finding increasing appli- cation in the search for new high-energy fuels and explosives. Before proceeding to the development of procedures for the synthesis of this type of compound, calculations are performed to elucidate whether conditions necessary for the preparation of the desired high-energy compound are fulfilled.In a series of studies, calculations for hypothetical nitrous oxide oligomers were performed. Thus, Vol'pin et al.307 examined the possibility of the existence of the cyclic trimer (N2O)3 (18). The heats of its formation (DH0) calculated by different methods are given in Table 7. A V Leont'ev, O A Fomicheva, MV Proskurnina, N S Zefirov O7 +N N N+ +N N N O7 7O 18 The potential barrier to the (N2O)3?3N2O decomposition reaction of the benzene-like molecule 18 (the DEa value varies from 33 to 85 kcal mol71) is rather high, and it could exist due to kinetic stability. Selected geometric and thermodynamical parameters (see Table 7) of the hypothetical cyclic structure N4O2 (19) were discussed.308, 309 O N N N N O19 ing to the calculations performed by these authors, the potential barrier to this transformation is approximately 7 ± 10 kcal mol71.Table 7. Heat of formation DH0 of nitrous oxide cyclic oligomers. Ref. Oligo- DH0 /kcal mol71 mer QCSISD MINDO/3 MNDO AM1 DZPSCF MP2 748 7155 73.2 782.0 717 748.5 18 19 307 308 309 7100.5 778.5 786.3 The assumed structures and properties of open-chain,310 crown-like 311 and (NO)3-grafted 312 nitrous oxide oligomers were considered with the use of ab initio calculations. Jones and Csizmadia 310 ± 312 believed that the occurrence of such metastable compounds is quite possible. The authors reasoned that high pressures, which have become accessible in recent years, can provide a decrease in the activation energy necessary for polymer- isation of nitrous oxide and favour the formation of these unusual structures.It should be noted that the (N2O)n polymers in the form of ionic clusters 313, 314 or weakly bound complexes 315, 316 have long been known. However, these systems are of no interest as high- energy compounds. Thus the trimer of nitrous oxide, which has been observed in a molecular beam by IR spectroscopy,315 exists as a non-planar acyclic weakly bound molecular complex charac- terised by the energy of dissociation into monomers equal to *3.5 kcal mol71. 5. Nitrous oxide in the supercritical state In the late 1980's, liquids in the supercritical state rekindled interest owing to their unique properties and unusual sensitivity with respect to small changes in the state parameters. The major fields of their application are supercritical liquid extraction,317 chromatography 318 and synthetic chemistry.319, 320 Nitrous oxide in the supercritical state [N2O(sc)] is more polar than CO2(sc), which allows one to use the former with greater success both for extraction of different organic compounds 321, 322 and as a mobile phase in chromatography.323, 324 In the synthetic chemistry, N2O(sc) can be employed either as a reagent 195 or as a solvent.325 Oxidation of tertiary phosphines with N2O(sc) or withModern chemistry of nitrous oxide the N2O(sc)/CO2(sc) mixture was performed.195 However, attempts to oxidise other classes of organic compounds, including compounds sensitive to oxidation by atmospheric oxygen, such as aldehydes, failed.195 Poh et al.195 reported that secondary alcohols can be oxidised under analogous conditions in the presence of palladium black.In addition, in some cases 326, 327 the use of N2O(sc) in the presence of rather low concentrations of organic compounds led to explosions. This is a serious obstacle to the wide application of N2O(sc) as a reagent. However, it is believed that the use of co-solvents, two-phase systems or carefully selected conditions can make these reactions less dangerous. VII. Conclusion Analysis of the data published over the last fifteen years demon- strated that the majority of studies concerned in one way or another with nitrous oxide have been carried out within the framework of either the environmental chemistry or heteroge- neous catalysis.It should be noted that these lines of investigation are closely related. On the one hand, there is a need to diminish the emission of nitrous oxide by 70%± 80% to stop an increase of its content in the atmosphere. This aim should be aided by employing new selective catalysts of decomposition of nitrous oxide in motor vehicles, heating and power plants and industrial processes. On the other hand, there are grounds to believe that N2O will be used as one of the major oxidants in industry. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (2) 105 ± 129 (2001) Organic hydrotrioxides V V Shereshovets (deceased), S L Khursan, V D Komissarov, G A Tolstikov Contents I. Introduction II. Synthesis of hydrotrioxides III. The structure, thermochemistry and spectroscopy IV. The mechanism of formation of organic hydrotrioxides V. Decomposition of hydrotrioxides VI. The mechanism of thermal decomposition of hydrotrioxides VII. Organic hydrotrioxides as oxidants Abstract. structure, synthesis, the on studies of results The The results of studies on the synthesis, structure, thermochemistry organic of capacity oxidising and thermochemistry and oxidising capacity of organic hydrotrioxides hydrotrioxides are generalised. Particular emphasis is placed on the analysis of are generalised. Particular emphasis is placed on the analysis of thermal on and ROOOH of decomposition catalytic and thermal and catalytic decomposition of ROOOH and on gener- gener- ation oxygen.molecular singlet and radicals free of ation of free radicals and singlet molecular oxygen. Problems Problems concerning decomposition and formation of mechanisms the concerning the mechanisms of formation and decomposition of of organic The considered. also are hydrotrioxides organic hydrotrioxides are also considered. The bibliography bibliography includes references 154 includes 154 references. I. Introduction The chemistry of trioxides, viz., compounds with a7O7O7O7 fragment, has been a matter of considerable interest during recent decades.1± 5 The family of trioxides comprises hydrogen trioxide HOOOH, organic hydrotrioxides ROOOH, dialkyl- and diaryl- trioxides ROOOR.Primary ozonides (1,2,3-trioxolanes) and transannular ozonides can be considered as cyclic analogues of dialkyl- and diaryltrioxides. Phosphite ozonides [e.g., (RO)3PO3], ozone ± arene complexes ArX .O3 as well as alkali metal and ammonium ozonides also belong to trioxides. The nomenclature of compounds with the general formula ROnR0 has not been settled yet. For n=2, this formula corre- sponds to the class of peroxides, the simplest representative of which is hydrogen peroxide (or hydrogen dioxide), HOOH. Extension of the oxygen chain by an O atom gives compound HOOOH, which by analogy should be named `hydrogen trioxide'. Replacement of H atoms by organic radicals leads to `hydro- trioxides' (HTO), ROOOH, and `trioxides', ROOOR.`Tetraox- V V Shereshovets Institute of Organic Chemistry, Ufa Scientific Centre of the Russian Academy of Sciences, prosp. Oktyabrya 71, 450054 Ufa, Russian Federation S L Khursan, V D Komissarov Bashkir State University, ul. Frunze 32, 450074 Ufa, Russian Federation. Fax (7-347) 235 60 66. Tel. (7-347) 223 67 27. E-mail: khursanSL@bsu.bashedu.ru (S L Khursan) G A Tolstikov N N Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, prosp. Akad. Lavrent'eva 9, 630090 Novosibirsk, Russian Federation. Fax (7-383) 335 47 52. Tel. (7-383) 234 38 50. E-mail: gtolstik@nioch.nsc.ru Received 28 September 2000 Uspekhi Khimii 70 (2) 123 ± 148 (2001); translated by AMRaevsky #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n02ABEH000622 105 107 107 112 114 121 123 ides' RO4R are formed in the radical-chain oxidation of organic compounds, in the stage of quadratic termination of the oxidation chain induced by peroxyl radicals ROO..Hydrotrioxides of different classes of organic compounds have the general formula ROOOH and should probably be named by analogy with hydro- peroxides ROOH. For instance, the term `acylhydrotrioxides' refers to aldehyde hydrotrioxides. In this review, we analyse the publications devoted to the synthesis, structure and decomposition of HTO and their use as oxidants of various organic compounds. The latest review con- cerning this topic was published by PlesnicÆ ar in 1992.4 Since then, numerous papers dealing with various aspects of the chemistry of HTO have appeared and a considerable body of new information on their structures and reactivity has been obtained.Progress in the studies of HTO necessitates generalisation of these data, refinement or even revision of some statements in PlesnicÆ ar's review 4 concerning, e.g., the efficiency of generation of 1O2 in the decomposition of HTO. In addition, the kinetics of the synthesis of ROOOH, the synthesis of ROOOH on surfaces, thermochem- istry of HTO and related compounds, radical and catalytic decomposition of HTO, the use of HTO as oxidants of unsatu- rated, organophosphorus, nitrogen- and sulfur-containing organic compounds are considered in detail in the present review. We also present comprehensive information on the kinetics of thermal decomposition of HTO and report a critical analysis of the data on the efficiency of generation of singlet molecular oxygen in the thermal decomposition of ROOOH.In his review,4 PlesnicÆ ar did not dwell on these problems or only briefly outlined them. Finally, it should be noted that studies carried out by the researchers from the Russian Federation have been incompletely covered in that review.4 Therefore, we deemed it our duty to consider in the present review not only the results of studies carried out during the last eight years but also those reported before 1992. For a rather long time, HTO were thought of only as hypo- thetic labile intermediates in the studies of the mechanisms of ozonisation of saturated organic compounds.However, once the fact that hydrotrioxides are stable at low temperatures had been revealed, the chemistry of HTO has progressed rapidly. Methods for the synthesis of HTO have been developed which allow the preparation of HTO at concentrations as high as*1 mol litre71. Organic HTO appeared to be both efficient oxidants in reactions with various organic substrates and convenient chemical sources of singlet molecular oxygen in organic media.106 A hypothesis that hydrotrioxides are formed as intermediates in the ozonisation of certain organic compounds RH was first put forward independently by Price and Tumolo 6 and by White and Bailey 7 to rationalise the results obtained in studies of the reactions of ozone with ethers 6 and aromatic aldehydes.7 Rapidly, this hypothesis has gained acceptance and was nearly simulta- Table 1.Hydrotrioxides derived from aromatic aldehydes, ethers, linear and cyclic acetals, alcohols and ketones. Formula Compo- R1 und O R1 HO O O 1a 1b 1c 1d 1e 1f 2 Me O OO O HR1 Me O R2 OO O H 3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m O R2 R1 O R2 OO O H 4a 4b 4c 4d 4e 4f 4g 4h 4i O R1 O OO O H 5a 5b 5c 5d 5e 5f R2 R1 O H OO O H 6a 6b 6c 6d 6e 6f 6g 6h 6i 6j 6k O R1 Me 7a 7b OOOH neously used by different authors in their studies on ozonisation of saturated hydrocarbons,8± 10 alcohols,9 acetals 11 and silanes.12 Murray et al.13 ± 15 were the first who have provided strong experimental evidence for the existence of HTO. They have shown that unstable peroxide compounds 1a, 2 and 3a (Table 1) accu- mulated in the reaction medium during the low-temperature ozonisation of benzaldehyde, 2-methyltetrahydrofuran and iso- propyl methyl ether, respectively.A signal at d 13.1 was present in the 1H NMR spectra of compounds 1a, 2 and 3a, which was tentatively assigned to the OOOH proton. Ref. R2 In further studies, 1H NMR spectroscopy has become the basic method for identification and quantification of HTO in solution. Table 1 lists hydrotrioxides derived from aromatic aldehydes, ethers, linear and cyclic acetals, alcohols and ketones and Table 2 presents hydrotrioxides derived from hydrocarbons known to date. Organic compounds of other carbon subgroup elements also form hydrotrioxides upon oxidation by ozone. Hydrotrioxides of organosilicon and organogermanium com- 15, 16 16 16 16 16 16 H2-Cl 3-Cl 4-Cl 2-F 4-OMe Table 2. Hydrotrioxides derived from hydrocarbons.15 Formula Ph Me Me Me Me Me Me Me Me Ph 4-MeC6H4 4-MeOC6H4 4-FC6H4 4-ClC6H4 4-BrC6H4 OOOH OOOH 13 17 17 17 17 17 17 H Et 1817 17 17 17 16 Et Et Et Et Pri Ph 4-MeC6H4 4-ClC6H4 4-BrC6H4 Me 19, 20 19, 20 19, 20 19, 20 19, 20 19, 20 19, 20 19 ± 21 19, 20 Me Me Me Me Me Me Me Et Et Me Ph 3-MeC6H4 4-MeOC6H4 4-FC6H4 4-ClC6H4 4-BrC6H4 Me Ph 22, 23 23 18 19, 20, 22 19, 20 19, 20 HMe Prn Ph 4-MeC6H4 3-ClC6H4 18, 24, 25 26 26 Me Me Me Me Me Me H H 18 HHHH 2618, 26 26 26 26 26 17 Me CH2Cl Et Prn Me CH2OH Et MeCHOH CH2CH2OH Me Me Ph Ph3COOOH EtMe2COOOH 27 27 Me Prn V V Shereshovets, S L Khursan, V D Komissarov, G A Tolstikov Me OOOH MeOOOH OOOH OOOH OOOH OOOH OOOH MeOOOH MeOOOH Compound 8910a 10b 11 12 13 14 15 16 17 18 19 Ref.28, 29 30 30 30 18, 31 30 30 30 18, 31, 32 18, 31 18, 31 18, 31 18, 31Organic hydrotrioxides pounds have been detected (Table 3). Presumably,HTOrepresent labile intermediates in the ozonisation of stannanes,36 though their formation from organotin compounds has not been reported so far. Table 3. Hydrotrioxides derived from organoelement compounds. Ref. R Compound Formula Me R Si OOOH Me 20a 20b 20c 20d 33, 34 33 33 33 H4-Me 4-MeO 4-Cl R3SiOOOH 21a 21b Et Bun 35, 36 37 22 36 Et3GeOOOH II. Synthesis of hydrotrioxides It was only recently that the methods for the synthesis of the simplest trioxide, HOOOH (23), have been proposed. Hydrogen trioxide 23 is formed upon oxidation of 9,10-dihydroxyanthra- cene, 2-ethyl-9,10-dihydroxyanthracene or hydrazobenzene by ozone (778 8C, acetone-d6 or methyl acetate).34 O OH R R O3 +HOOOH 23 O OH R=H, Et.O3 PhN NPh+HOOOH. PhNH NHPh Hydrogen trioxide 23 is also formed in the decomposition of silane hydrotrioxides.33, 34 All HTO listed in Tables 1 ± 3 can be synthesised using a rather simple and virtually identical procedure consisting of bubbling a cooled O3±O2 or O3±N2 gas mixture through a liquid substrate RH or its solution at reduced temperature (720 to 778 8C). After completion of the reaction, the unconsumed ozone is removed by blowing air or (if necessary) an inert gas and then the concentration of HTO in the solution is determined by NMR spectroscopy. The yield of ROOOH with respect to the consumed RH depends on the substrate structure and varies from 1%± 2% for cumene hydrotrioxide 8 to 70%± 90% for acetal, alcohol and silane hydrotrioxides.A modified procedure for the synthesis of HTO involves ozonisation of organic compounds on the surface of silica gel (the so-called `dry' ozonisation).18, 31, 32, 38, 39 It was used to obtain the hydrotrioxides 11 and 15 ± 19, which could not be prepared in the liquid-phase ozonisation.18, 31, 32 Highly polar silica gel surface stabilises the ionic intermediates of the ozonisation reaction, thus favouring the formation of ROOOH even from non-activated hydrocarbons. It can be hypothesised that `dry' ozonisation will also be convenient for the synthesis of otherHTOwhich cannot be obtained by liquid-phase ozonisation. The kinetics of formation of compounds 3m,16 4h,21 6d,25 6f 16 and 7b 27 have been studied.At sufficiently low temperatures where thermal decomposition of HTO can be ignored the kinetic curves of accumulation of HTO have the shape of saturation curves, which means that the yield of ROOOH (a) calculated with respect to the ozone consumed decreases with an increase in conversion. The extent of the reaction is related to the amount of the ozone absorbed D(O3) and the yield of ROOOH is calculated by the formula a=âROOOHä . DÖO3Ü The yield calculated with respect to the ozone consumed (a0) is obtained by extrapolating the value of a to the zero time [D(O3)=0]. The a0 values for a number of HTO are listed in Table 4. The decrease in the yield with the increase in the extent of the reaction is explained by consumption of the HTO in the reactions with ozone.O3 O3 products. ROOOH RH The rate constants for the first (k1) and second (k2) stages of ozonisation of 1,1-diethoxyethane at 760 and 780 8C are k1=6.861074 and 2.561075 litre mol71 s71 and k2= 3.061073 and 1.261074 litre mol71 s71, respectively.21 Table 4. Yields of hydrotrioxides (a0) calculated with respect to the consumed ozone. Compound Temperature /8C O 760 Prn (7b) Me OOOH Me Me 778 O Pri (3m) OO O H OEt 778 760 OEt (4h) Me OO O HHO H (6d) 798 778 750 Et OO O HMe Me 778 740 O H O (6f) O O H III. The structure, thermochemistry and spectroscopy 1. The structure a. Equilibrium molecular geometry of hydrotrioxides Experimental data on the spatial structure ofHTOare lacking and this was established only from the results of semiempirical (EHT, CNDO/2, MINDO/3, AM1) 40 ± 43 and ab initio quantum-chem- ical calculations.33, 44 ± 54 Ab initio calculations of the equilibrium molecular geometry of HOOOH have been carried out at different levels of theory, from simple calculations in the one-determinant approximation with the STO-2G basis set to sophisticated calculations with different TZP basis sets augmented with diffuse functions and intensive treatment of electron correlation.Both partial 44 ± 48, 50 and full optimisation of the molecular geometry 33, 49, 51, 52, 54 were employed. Tables 5 and 6 present the results of ab initio calcu- 107 Ref. a0 /mol mol71 27 0.5 16 0.45 21 21 0.51 0.45 25 25 25 0.90 0.80 0.20 16 16 0.98 0.80108 Table 5. Results of ab initio calculations of the equilibrium geometries and total energies of HOOOH.r(OO) /AÊ r(HO) /AÊ Basis set y(HOO) /deg 102.5 103.6 104.1 103.2 103.5 99.6 100.3 100.8 100.2 1.392 1.437 1.432 1.373 1.373 1.539 1.442 1.445 1.441 1.002 0.959 0.958 0.953 0.949 0.992 0.980 0.982 0.972 RHF/STO-3G RHF/4-31G RHF/6-31G RHF/6-31G* RHF/6-31G** MP2/4-31G MP2/6-31G* MP2/6-31++G* MP2/6-31G** Note. Hereafter, the Etotal are given in hartree. Table 6. Results of ab initio calculations 54 of the equilibrium geometries and total energies of hydrotrioxides H3COOOH and H3SiOOOH. Basis set r(OOH) /AÊ r(XOO) /AÊ r(XO) /AÊ y(OOO) /deg r(OH) y(OOH) /deg /AÊ Compound H3COOOH 105.9 106.1 106.5 102.4 103.4 103.9 1.003 0.958 0.957 1.392 1.440 1.433 1.396 1.436 1.430 RHF/STO-3G 1.453 RHF/4-31G 1.438 RHF/6-31G 1.439 Compound H3SiOOOH 106.2 103.9 0.958 1.429 1.441 RHF/6-31G 1.771 lations 53 of the molecular structures ofHOOOH,H3COOOHand H3SiOOOH with full geometry optimisation.The data listed in Tables 5 and 6 reveal salient features of the molecular geometry of hydrotrioxides. 1. The r(O7O) distances in trioxides are close to those in peroxides. Blint and Newton 46 found that the O7O bond length can be calculated with a reasonable accuracy using the empirical Pauling equation (1) re=rse7aln(n), where re is the equilibrium bond length (A); rse is the equilibrium length of a bond with the order of unity; a=0.351 A is the empirical constant; and n is the bond order. According to calculations with the 4-31G basis set, r(O7O)&1.41 ± 1.44 A in polyoxides.The O7O bond in HOOOH is 0.03 to 0.04 A shorter than in HOOH. A similar tendency of changes in the O7O bond length is observed for the pairs H3COOOH/H3COOH and H3SiOOOH/H3SiOOH,54 which seems to be a general feature. 2. The torsion (dihedral) angle j in the HTO molecules characterises the positions of the substituent atoms with respect to the plane passing through threeOatoms. According to the most sophisticated calculations,49, 54 j&75 ± 80 8. The calculations revealed two stable conformations, anti (A) and syn (B), for the hydrogen trioxide molecule, which differ in positions of the H atoms with respect to the trioxide plane. H H HO O O O O O H B A The assumption of the existence of stable anti and syn conformations of hydrogen trioxide molecule (23) is indirectly confirmed by the results of IR and microwave spectroscopic studies of hydrogen trisulfide, a close analogue of HOOOH.55 According to the results of quantum-chemical calculations carried out at the MP2/TZ+P and QCISD/TZ+P levels of theory, the anti conformation of HSSSH molecule is 0.25 kcal mol71 (87 cm71) more stable than the syn conformation.V V Shereshovets, S L Khursan, V D Komissarov, G A Tolstikov Ref. Etotal j /deg y(OOO) /deg 49 49 33 49 51 49 49 52 51 84.2 83.5 83.9 80.1 80.8 78.3 78.5 79.6 78.7 105.9 106.0 106.5 107.2 107.4 104.4 106.1 106.4 106.2 7222.58049 7225.21862 7225.44304 7225.53362 7225.54584 7225.62077 7226.09075 7226.10273 7226.11275 Etotal j(XOOO) j(OOOH) /deg y(XOO) j(HXOO) /deg /deg /deg 83.5 82.4 82.6 87.8 85.8 86.0 179.1 179.0 179.0 106.5 107.7 108.1 7261.16718 7264.18963 7264.45417 79.3 84.1 180.2 109.6 7515.49701 For the HOOOH molecule, conformation A is more energe- tically favourable (by 3.51.0 kcal mol71) than conformation B (see Refs 43, 44, 46, 49).This gives a rationale of the low stability of 1,2,3-trioxolanes, which are intermediates of the ozonolysis of alkenes, compared to linear trioxides. The molecule of primary ozonide (1,2,3-trioxolane 24) adopts an `envelope' conforma- tion 56, 57 in which the positions of methylene groups formally correspond to the syn conformation of the trioxide. CH2 H2C O3 CH2 CH2 O O O 24 Hence, the energy of conformational transition and strain energy in the five-membered ring [*6 kcal mol71 (see Ref. 58)] contribute to the destabilisation of the primary ozonide.As a consequence, the activation energy of further transformations of compound 24 is *10 kcal mol71 lower than those of thermal decomposition of dialkyl trioxides.5 Hydrotrioxides derived from alcohols, ethers and acetals can form intramolecularly hydrogen-bonded structures only if these molecules adopt the syn conformation.13, 14, 16, 17, 19, 20, 24 HO H CH2OH CH2OH H CH2 O O O O O O O O O HHence, the enthalpy of formation of a hydrogen bond, DH8, includes the anti ± syn interconversion energy.18 Radical decom- position of the cyclic intermediate removes the conformational strain, thus decreasing the activation energy for this process. 3. TheH7O7OandC7O7Obond angles (see Tables 5 and 6, respectively) in hydrotrioxide molecules are somewhat larger than in the corresponding hydroperoxides.The O7O7O bond angles in different HTO (104 ± 107 8) are in good agreement with the IR spectroscopic estimate.59 4. Conformational analysis of theHOOOH molecule has been carried out.49 The potential energy surface for simultaneousOrganic hydrotrioxides O¡O on the trioxide molecule is more pronounced than rotation of the OH groups about the O7O axes in the molecule was calculated. Analogous semiempirical AM1 calculations were carried out for hydrogen trioxide and tetraoxide.43 The nature of the barriers to rotation was interpreted based on the results of the Fourier analysis of the conformational potential of the polyox- ides. The conformational behaviour of polyoxides is dominated by the repulsion between the lone electron pairs of the neighbour- ing oxygen atoms and by stabilisation of the hydropolyoxide owing to delocalisation of the lone electron pair (nO) over the s*-antibonding orbital of the neighbouring O7O bond (or O7C and O7H bonds).18, 54 The latter effect provides an explanation for the shortening of the r(O7O) distance in trioxides compared to peroxides, since the stabilising effect of hyperconjugation nO ± s that of hyperconjugation nO ± sO¡OH on the peroxide molecule, thus favouring a decrease in the O7O bond length.54 Analysis of the potential surface for internal rotation of hydrogen trioxide suggests that the most favourable conversional process between two global energy minima (A1 and A2) for the anti conformation of HOOOH involves a two-step flip-flop rotation rather than simultaneous rotation of the OH groups.43, 49 H O H H O O trans O H H O O H O A1 O O cis H O H H H O O trans O O O H H B O O O cis b.Association of hydrotrioxides The binding energies of the intermolecularly hydrogen-bonded dimers were calculated taking dimerisations of HOOOH, H3COOOH and H3SiOOOH as examples.33, 53 O O R H O O H R O O R=H, CH3, SiH3 . Relatively high binding energies (E=6 ± 8 kcal mol71 per O7H bond) are close to the hydrogen bond energy in cyclic dimers of carboxylic acids.60, 61 This confirms the assumption that self-association is a salient feature of HTO molecules. It should be noted, however, that the computational method (RHF/6-31G) employed by PlesnicA ar et al.33, 53 overestimates the energies of hydrogen bonds. Indeed, augmentation of the basis set with p-polarisation functions for hydrogen and d-polarisation func- tions for other atoms leads to more realistic results, viz., the binding energy of H3COOOH in the eight-membered dimer is 4.5 kcal mol71 per hydrogen bond.18 The effect of water on the decomposition ofHOOOHhas been studied.52, 54 The water molecule acts as a bifunctional catalyst, thus facilitating the decomposition of hydrogen trioxide intoH2O and 1O2.This reaction is preceded by the formation of a six- centred hydrogen-bonded complex. The binding energy in this complex is 1.5 kcal mol71 lower than in the cyclic dimer of HOOOH. The calculated height of the potential barrier to H HO O O B HO O OH A2 109 decomposition of hydrogen trioxide (15.1 kcal mol71 relative to noninteracting reagents) is in good agreement with the experi- mentally found effective activation energies for the decomposition of HOOOH.34, 52, 62 Ab initio calculations of intramolecularly hydrogen-bonded structuresROOOHhave been carried out taking methanol hydro- trioxide (6a) as an example.18 The most stable conformers of the molecule are the anti (C) and syn (D) conformers with nearly equal energies. H H C C H H H O O OH OH O O O OD C H The anti conformer C is stabilised by the hydrogen bond between the H atom of the alcoholic hydroxy group and the most distant (from the carbon atom) O atom of the trioxide fragment.The syn conformation D favours the formation of a hydrogen bond between the OOOH hydrogen and alcoholic O atom. The binding energies of structures C and D calculated at the MP2//RHF/6-31G** level of theory are 2.2 and 5.4 kcal mol71, respectively. In conclusion, mention should be made that HTO can form associates with carbonyl compounds.18 In addition to the signal at d=13, the 1H NMR spectrum of 1,4-dimethylcyclohexyl hydro- trioxide (16) in acetone exhibits two peaks at d=4.15 and 4.35 which disappear irreversibly upon heating.It is well known that hydrogen peroxide 63 and hydroperoxides 64, 65 readily react with aliphatic and alicyclic ketones to give a-hydroxy hydroperoxides and a-hydroxy peroxides. Presumably, a similar reversible reac- tion occurs in the system HTO16 ± acetone resulting in a deriva- tive of 2-R-trioxy-2-hydroxypropane 25.18 Compound 25 exists in two forms, which accounts for the presence of two OH proton resonance peaks in its 1H NMR spectrum.Me Me Me Me H O O O ROOOH +MeCOMe OH O O O O R 25 R Me. R=Me 2. Thermochemistry In this Section, we outline briefly the possibilities of estimation of the enthalpies of formation and bond dissociation energies in hydrotrioxides. For more detailed data, see a recent review.66 The experimentally determined thermodynamic characteris- tics of HTO (enthalpies of formation and of phase transitions, entropies, etc.) are lacking. Therefore, thermochemical calcula- tions of the thermal effects of the reactions involving ROOOH were carried out using approximate computational schemes, in particular, the approach of additivity of thermochemical incre- ments developed by Benson et al.58, 67, 68 In the framework of Benson's scheme, the enthalpies of formation of hydropolyoxides are related by simple relationships (2) DfH8(ROOOH)=DfH8(ROOH)+DfH8[O7(O)2], DfH8(ROOOOH)=DfH8(ROOOH)+DfH8[O7(O)2], (3) where the second term in the right side of Eqns (2) and (3) is the polyoxide group increment, which characterises the enthalpy contribution of the central O atom of the trioxide fragment.As will be shown below, the assumption that the enthalpy of formation changes linearly in the order ROOH7ROOOH7110 ROOOOH is rough and leads to substantial discrepancies between the results of thermochemical studies carried out by different authors. Practically, the most convenient way of calculating the DfH8(ROOOH) values is to use Eqn (2). A number of experimen- tally determined enthalpies of formation of hydroperoxides has been reported.69 Good agreement with experimental DfH8(ROOH) values is also observed when using the method of isodesmic reactions as well as ab initio calculations with extended basis sets and inclusion of electron correlation (CI or perturbation theory).70, 71 Unfortunately, high computational cost of real-time calculations of DfH8(ROOH) allows them to be carried out only for the simplest substituents R.Thermochemistry of peroxides has been analysed using a large body of experimental data on the enthalpies of formation (see Ref. 72 and references cited theirein), increments for calculating DfH8(ROOH) in both the gas phase and condensed phase were proposed. The polyoxide group increment, DfH8[O7(O)2], which plays the key role in calculations of the enthalpies of formation of hydrotrioxides, cannot be calculated from the available experi- mental data on DfH8 and bond dissociation energies in related compounds.Since the accuracy of indirect estimates is insuffi- cient, the numerical value of DfH8[O7(O)2] has been the subject of systematic revisions.70, 73 ± 77 a. Comparison of the enthalpies of formation The first estimates of DfH8[O7(O)2] (256 and 194 kcal - mol71, see Refs 73 and 74, respectively) were made based on the assumption that thermodynamic properties change linearly in the order ROR07ROOR07ROOOR0 and from comparison of the enthalpies of formation for pairs of compounds H2O and H2O2,73, 74 MeOH and MeOOH and MeOMe and MeOOMe.74 This approach was found to be very rough. In particular, the results obtained suggested that tetaroxides can be synthesised at temperatures below 80 ± 100 K.74 Later, it has been shown 78 ± 80 that tetraoxides ROOOOR (R=tert-Alk, ArAlk) are rather stable at T4170 K, which indicated overestimation of the DfH8[O7(O)2] value reported in earlier studies.73, 74 b.Comparison of thermal effects of reactions Another procedure for the calculation of the polyoxide group increment was used by Nangia and Benson.75 Based on the reaction equations (4) 2ButOO, ButOOOOBut (5) ButO+ButOO, ButOOOBut (6) 2ButO ButOOBut with the known thermal effects DH8(4)=9.01.5 78, 79 and DH8(6)=38.01.0 kcal mol71 (see Ref. 81) and using the addi- tivity rule, according to which DH8(5)=0.5[DH8(4)+DH8(6)], they DH8(5)=23.50.8 kcal mol71 found that and DH8(6)7DH8(5)=DH8(5)7DH8(4)=14.50.8 kcal mol71.By representing the enthalpies of formation of the molecules under study as the sums of the enthalpies of formation of the thermochemical groups, one can readily obtain the expression for the calculation of the polyoxide group increment DH8(6)7DH8(5)=DfH8(RO)7DfH8(ROO)+DfH8[O7(O)2]. (7) Using the known values of the enthalpies of formation of the MeO. and MeOO. radicals, the new polyoxide group increment DfH8[O7(O)2] was found to be 16.82.2 kcal mol71 (see Ref. 75). A year later, Nangia and Benson corrected this estimate and reported that DfH8[O7(O)2]=14.8 kcal mol71.76 This approach has a drawback consisting of the assumption of linear changes in the DH8 of the reactions (4) ± (6). Recent experiments showed that DH8(5)=20 ± 21 kcal mol71 (see Refs 18, 82, 83).Mention has been made of the nonlinear character of changes in V V Shereshovets, S L Khursan, V D Komissarov, G A Tolstikov the enthalpies of formation in the order `peroxide ± trioxide ± te- traoxide'.84 c. Quantum-chemical calculations of the enthalpies of formation The use of quantum-chemical methods has opened new possibil- ities for the thermochemical analysis of the reactions of HTO and higher polyoxides.44, 46, 50, 70, 71, 77, 82, 84 ± 87 However, direct calcu- lations of the enthalpies of formation of polyoxides can lead to large errors owing to incomplete inclusion of electron correlation in the ab initio computational schemes and inadequate parametrisation of semiempirical methods. This can be illustrated as follows.The enthalpy of formation of MeOOOH obtained from MP4SDTQ/6-31G*//MP2/6-31G* calculations is 722.2 kcal mol71 and must correspond to the energy of dissoci- ation of the weakest O7O bond, D(MeO7OOH); however, D(MeO7OOH)=35.8 kcal mol71 (see Ref. 70). The value can- not be regarded as realistic despite the high level of quantum- chemical calculations. It is only 2 to 3 kcal mol71 lower than the corresponding bond dissociation energies in peroxides. However, the temperature intervals of stability of these compounds are not comparable: peroxides are rather stable at room temperature, while the lifetimes of HTO under these conditions are within several minutes. Therefore, the numerical estimates of the poly- oxide group increment obtained from both semiempirical MNDO77 and ab initio 70 calculations of DfH8 of polyoxides {DfH8 [O7(O)2]=13.11.5 and 9.60.1, respectively} should be used with caution.d. Quantum-chemical calculations of bond dissociation energies The best results can be obtained from calculations of bond dissociation energies (strengths). This is due to mutual cancella- tion of the errors of calculations of DfH8 for polyoxide and its decomposition products. Systematic studies of the O7O and O7H bond dissociation energies in organic polyoxides (see Table 7) have been carried out.18, 82, 84 ± 86 This allowed calcula- tions of the enthalpies of formation for a number of peroxides, trioxides and tetraoxides. It was found that the difference DfH8(ROOOR0)7DfH8(ROOR0) is 5 to 8 kcal mol71 larger than DfH8(ROOOOR0)7DfH8(ROOOR0). The reason for this discrepancy was explained based on the results of analysis of the conformational potentials of polyoxide compounds H2Ox (x=2, Table 7.Bond strengths D (kcal mol71) in organic hydropolyoxides. D Ref. Bond type R7OOOHa RO7OOHb ROO7OHb HO7OOH HOOO7H ROOO7Hd R7OOOOHa RO7OOOHa ROO7OOHa HOO7OOH 63.61.2 18 20.41.0 (MNDO) 86 20.81.1 (AM1) 86 23.4 c 88 (25.50.6)+(1.970.49) .s*(R) (MNDO) 86 (23.80.6)+(1.380.49) .s*(R) (AM1) 86 30.1 c 88 24.8 (CI(SDQ)) 50 25.0 (MNDO) 86 25.2 (AM1) 86 82.6 70 80.80.8 18 63.61.2 18 20.81.4 18 8.71.2 84 7.1 (MNDO) 82 12.1 (MNDO) 77 25.10.5 18 80.80.8 18 ROOO7OHa ROOOO7Ha aR=Alk. bR is unsubstituted, halo-, hydroxy- or alkoxy-substituted alkyl. cR=Me. dR is unsubstituted, halo-, hydroxy- or alkoxy-substitu- ted alkyl, H, HCO or MeCO.Organic hydrotrioxides 3, 4).43 It was shown that the enthalpies of formation in the series `peroxide ± trioxide ± tetraoxide' do not change linearly as it would follow from the additivity method owing to additional stabilisa- tion of peroxides by the dipole ± dipole interaction between the alkoxy groups.Therefore, the DfH8[O7(O)2] group increment can be calculated as the difference between the enthalpies of formation of a trioxide and tetraoxide with the same substituents. Calculations of the enthalpies of formation of trioxides from the corresponding peroxides should be carried out with inclusion of the energy of conformational rearrangement of the molecule (*5 kcal mol71). For practical calculations of DfH8 values in polyoxides we recommend the following relationships (8) DfH8(ROOOR0)= =DfH8(ROOR0)+(17.61.6) kcal mol71, (9) DfH8(ROOOOR0)= =DfH8(ROOOR0)+(11.11.0) kcal mol71.Calculations using Eqns (8) and (9) allow correct reproduc- tion of the intervals of thermal stability of trioxides and tetra- oxides and give the bond dissociation energies that are close to the activation energies for decomposition of polyoxides (provided that Ea is corrected for the normal value of the pre-exponential factor). 3. Spectroscopic studies a. UV, IR and Raman spectra Scarce experimental data indicate that the electronic spectra of HTO are similar to those of related peroxide compounds. The optical spectrum of HOOOH obtained in the pulse radiolysis experiments is similar to that of hydrogen peroxide and exhibits a somewhat larger molar extinction, e&100 litre mol71 cm71 (l=240 nm).62 Weak absorption at l>320 nm in the spectrum of benzaldehyde hydrotrioxide (1a) can be assigned to the S1 (pO¡O?sO¡O) transition.The position and spectral features of this signal are similar to those of aromatic endoperoxides.89 Thus, theUVspectra ofHTOexhibit no characteristic bands suitable for identification and determination of the concentration of ROOOH. Since almost any reaction of the synthesis or decom- position of HTO proceeds to give peroxides as side or major products, one can conclude that investigations into the chemical nature of organic HTO based on the UV spectra is hardly probable. Specific features of the IR spectra of hydrogen and deuterium trioxides and tetraoxides allowed Giguere and Herman 59 to relate the observed frequencies to vibrations of the zigzag chain of the O atoms linked by single bond.They estimated the O7O bond length at*1.5 Aand theO7O7Obond angle at 100 to 120 8 and concluded that internal rotation of the OH groups in free molecules is only weakly hindered, whereas in the solid state their orientation will be strongly dependent on the possibility of hydrogen bonding, as in hydrogen peroxide. More recently,90 normal coordinate analyses for HOOOH and HOOOOH mole- cules were performed. Nonplanar hydrogen polyoxide structures exhibited fairly good correspondence between the calculated frequencies and experimental IR spectrum and the existence of several spatial isomers was suggested.Further quantum-chemical studies have confirmed in general the above-mentioned conclu- sions 59, 90 (see Table 3). Raman spectra of hydrogen polyoxides have been studied.91, 92 The fundamental skeletal vibrations of the HOOOH and HOOOOH molecules and their deuterated ana- logues were identified and interpreted (Table 8). The frequencies observed are consistent with the C2 symmetry of the molecular structure in the form of a skew chain of O atoms linked by single bonds. Vibrational spectra of organic hydroperoxides have not been studied as yet. Table 8. Vibrational spectra of hydrogen polyoxides.92 Compound Frequencies /cm71 IR Raman HOOOH *855 7755 DOOOD HO(1)O(2)O(3)O(4)H *857 7760 7777 *855 500 755 *857 497 760 *855 *764 450 *98 *823 7 DO(1)O(2)O(3)O(4)D *857 *768 *440 *823 435 7777 *828 *98 *8287 *430 b.NMR studies Organic HTO can be conveniently identified by NMR spectro- scopy owing to the presence of characteristic signals of the hydrotrioxide fragment. Resonance absorptions of other groups are less characteristic. The chemical shifts of the 1H, 13C and 29Si nuclei in the NMR spectra of a number of HTO and related compounds 17, 29, 33 are listed in Tables 9 and 10. Typical of HTO is the signal of the OOOH proton at d*13 in the 1H NMR spectra, the position of the signal being virtually independent of the nature of ROOOH.13 ± 17, 19 ± 21, 23 ± 30, 33, 34, 36, 37 In the series ROH±ROOH±ROOOH, the chemical shift of H increases from d 0.5 ± 5.5 forROH to d 7.6 ± 9.2 forROOH93 and d 12 ± 14 for ROOOH.Unlike alcohols and hydroperoxides,93 dilution of HTO solutions has little effect on the chemical shift of the trioxide proton. Based on these observations, it was concluded that hydrotrioxides form strong intramolecularly hydrogen- bonded complexes.13, 14 A peculiarity of the 1H NMR spectra of hydrotrioxides is the appearance, in some cases, of the splitting of the OOOH proton signal into two signals.14, 16, 17, 19, 20, 24 The chemical shifts of these signals also show little change with dilution. Heating of the samples results in broadening and coalescence of the lines. The coalescence temperature depends on the chemical nature of both ROOOHand the solvent. For instance, two peaks in the 1H NMR spectrum of 2-methyltetrahydrofuran hydrotrioxide (2) coalesce at727 8C.14 For hydrotrioxide 3b obtained from methyl a-meth- ylbenzyl ether in Freon 11/12, two peaks appear in its 1H NMR spectrum only at 750 8C and lower temperatures, whereas no peak coalescence is observed in diethyl ether, acetone-d6 and ethyl acetate even at710 8C.17 The origin of two resonance signals for the OOOH proton is still to be clarified.This can be due to either the `chair ± boat' conformational transition of the six-membered ring resulting from the formation of intramolecular hydrogen bond or the presence of two conformations with theRgroup at the O atom in either axial or equatorial position. Hydrotrioxides derived from cyclic acetals 19, 20 can form various complexes owing to nonequivalence of the two acetal O atoms with respect to the OOOH group. Studies of the 1H NMR spectra of hydrotrioxides in different solvents 17 showed that though the chemical shifts of two peaks vary insignificantly, their relative intensities can vary over a rather wide range (see Table 9).In diethyl ether, the more intense low- field peak of the hydrotrioxide 3b disappears upon heating with a somewhat higher rate compared with the high-field signal. The 111 Assignment ns(O7O) d(O7O7O) nas(O7O) ns(O7O) d(O7O7O) nas(O7O) ns(O7O) n[O(2)7O(3)] ds(O7O7O) dtors[O(2)7O(3)] nas(O7O) das(O7O7O) ns(O7O) n[O(2)7O(3)] ds(O7O7O) dtors[O(2)7O(3)] nas(O7O) das(O7O7O)112 Table 9. Chemical shifts and multiplicities of signals in the 1H and 13C NMR spectra of organic hydrotrioxides and related compounds. X Compound d 1H Me 1C X 2 1.17 d (Me) 1.50 (Me), 5.18 (OH) 1.52 (Me), 10.95 (OOH) 1.64 (Me), 13.67 (OOOH) HOH OOH OOOH (8) Me (see a) Me C X 1.33 d (Me), 4.65 q (H), 5.3 (OH) 13.20, 13.23 (1.2 : 1) (OH, OOOH) 12.95, 13.12 (1 : 1) (OH, OOOH) HOOOH (6k) OOOHb (6k) OH Me C X OMe HHc OOH OOHc OOOH (3b) OOOHb (3b) OOOHd (3b) 71.35 d (Me), 3.05 (OMe), 4.12 q (H) 10.6 (OOH) 1.65 (Me), 3.35 (OMe), 8.85 (OOH) 1.60 (Me), 3.25 (OMe), 13.19, 13.54 (1 : 1.2) 1.80 (Me), 12.81, 13.04 [1 : (7 ± 10)] (OOOH) 1.57 (Me), 12.90, 13.15 (1 : 1.2) (OOOH) Note.The spectra were measured in (CD3)2CO at760 8C, the chemical shifts are given in ppm relative to tetramethylsilane; the signals in the 1H NMR spectra are singlets unless otherwise specified; the ratios of the signal intensities are given in parentheses. a At773 8C. b In Et2O.c In CDCl3. d In MeCO2Et. Table 10. Chemical shifts of signals in the 1H, 13C and 29Si NMR spectra of hydrotrioxides derived from organosilicon compounds and related compounds. Compound X d 1H Me 1Si X 1 Me 0.30 (Me), 4.40 (H) 0.32 (Me), 4.5 ± 5.5 (OH) 0.43 (Me), 11.2 (OOH) 0.55 (Me), 13.96 (OOOH) 13.64 (OOOH) 13.40 (OOOH) 0.33 (Me) 0.40 (Me) HOH OOH OOOH (20a) OOOH (20a) a OOOH (20a) b OSiMe2Ph OOSiMe2Ph Note. The spectra were measured in (CD3)2CO at778 8C; the chemical shifts are given in ppm relative to tetramethylsilane. a In MeCO2Et. b In Me2O. possibility of assigning the weaker signal to the ROOOH formed from the solvent was rejected.17 Based on the results of the concentration dependence and temperature variation studies, PlesnicÆ ar et al. 17 concluded that HTO can form either intramolecularly hydrogen-bonded struc- tures or rather strong eight-membered intermolecular associates.In conclusion mention may be made of the first identification of isopropyl alcohol hydrotrioxide (6f) and hydrogen trioxide (23) by 17O NMR spectroscopy.94, 95 The experimental results are in reasonable agreement with those of the ab initio GIAO/MP2/ 6-311++G** calculations of the 17O chemical shifts. For HO(1)O(2)O(3)H (23), the experimental and calculated chemical shifts for the O(2) atom are 421 and 433 ppm, respectively, while d[O(1,3)]=305 (cf. the calculated value d=306). Hydrotrioxide Me2C(OH)[O(1)O(2)O(3)H] (6f) is characterised by d[O(1)]= 368, d[O(2)]=445 and d[O(3)]=305 (relative to H217O). Prob- ably, 17O NMRspectroscopy of hydrotrioxides would serve as the basis for a breakthrough in understanding of the structure of organic HTO in the condensed phase.V V Shereshovets, S L Khursan, V D Komissarov, G A Tolstikov d 13C C(Me) C(2) C(1) other C atoms 777 24.3 32.3 26.67 7 148.8 150.7 145.5 134.5 34.6 72.1 84.0 104.3 147.3 7 25.1 7 7 7 7 7 7 69.9 105.26 105.15 24.7 24.0 25.4 24.6 26.0 144.4 143.9 142.5 140.5 134.3 56.2 56.1 49.7 49.9 7 7 7 7 7 7 7 79.7 79.7 106.4 106.7 108.1 103.2, 104.1 108.1 d 29Si(1) d 13C C(Me) C(1) 716.95 3.30 14.31 17.31 71.1 3.1 2.8 2.7 140.6 143.5 139.6 138.57 7 7 7 7 7 143.1 139.5 3.9 2.8 70.96 16.6 IV. The mechanism of formation of organic hydrotrioxides 1. Experimental results The reactions of ozone with C7H bonds of organic compounds leading to hydrotrioxides have been proposed to follow three main mechanisms, viz., a concerted 1,3-dipolar insertion of ozone (route a),6, 8, 11, 96, 97 homolytic abstraction of an H atom by ozone followed by cage recombination of the radical pair (route b) 9, 10, 96, 98 and ionic (molecular) mechanism (rou- te c).10, 11, 76, 98 R O O a R C O H R R b R C H+O3 R +HOOO R c R++HOOO7 Obviously, the variety of transformations of organic com- pounds in the reactions with ozone to give HTO cannot be described by a common mechanism.In this connection, theOrganic hydrotrioxides study by Giamalva, Church and Pryor 99 is remarkable. They proposed that the reactions of O3 with a number of cage hydro- carbons proceed by the insertion mechanism (route a) with the feasible transition states considered to be a set of resonance structures including both the radical (route b) and ionic (route c) mechanisms as the limiting cases.The dominating reaction mechanism depends on the nature of the substrate and oxidation conditions. Let us consider the experimental grounds that serve as the basis for inferences about the most plausible mechanism of the ozone reaction with the C7H bond. a. Reaction kinetics The pre-exponential factors for the reaction RH+O3 (Table 11) are 2 to 3 orders of magnitude lower than those of typical radical abstraction reactions of the H atom.101 The estimate logA&6.5 obtained for the 1,3-addition reaction (route a) (see Ref. 76) is close to the activation parameters listed in Table 11; however, the experimental activation energies are incompatible with the ener- gies of formation of pentacoordinated carbon.Taking into account the strain energy in the five-membered ring in the transition state of the 1,3-addition reaction, the expected Ea value must lie between 20 and 26 kcal mol71 (see Ref. 76). Table 11. Activation parameters of the reactions of organic compounds with ozone.100 Solvent Substrate logA Ea /litre mol71 s71 /kcal mol71 13.7 cyclo-C6H12 PhPri EtOH 8.7 7.3 9.1 10.0 7.9 18.5 7.71 6.4 4.88 6.15 6.81 6.26 10.7 cyclo-C6H11OH MeCOEt cyclo-C6H12 PhPri EtOH MeCOOH CCl4 CCl4 CCl4 b. Kinetic isotope effect Relatively high kH/kD values (see Table 12) indicate substantial weakening of the C7Hbond in the transition state of the reaction RH+O3. A large kinetic isotope effect is evidence for the linear transfer of a proton and its symmetric localisation in the transition state.102 This conclusion contradicts the mechanism of 1,3- cycloaddition and precludes linear arrangement of the C, H and O atoms. Table 12.Kinetic isotope effect in the oxidation of organic compounds by ozone.10, 96, 100 T /K Solvent Substrate kH/kD 4.1 ± 4.5 3.8 ± 4.1 2.4 ± 4.5 5.9 295 273 273 293 cyclo-C6H12 ± cyclo-C5H10 Freon 11 Me2CO CCl4 cyclo-C6D12 PhCD2OCMe3 MeCD2OCMe3 MeCD2OH c. Relative reactivity Ozone exhibits a much higher selectivity toward primary, secon- dary and tertiary C7H bonds (0.003 : 1 : 87) as compared to alkoxyl (0.08 : 1 : 33.7) and peroxyl (0.024 : 1 : 10) radicals.4 d. Structure of a substrate It is remarkable that compounds used as substrates in the liquid- phase ozonisation to give HTO, viz., oxygen-containing com- 113 pounds (acetals, ethers, alcohols), hydrocarbons (cumene, cage compounds, including those containing the cyclopropane frag- ment) and silanes exhibit a common feature consisting of the possibility of effective stabilisation of the derived carbocation.Ozone was found to react with the benzylic C7H bond of fluorene at a much lower rate than with the analogous bond in diphenylmethane and 9,10-dihydroanthracene derivatives.103 The low reactivity of fluorene is due to the fact that the formation of a carbocation from fluorene is less energetically favourable than from the other two compounds.103 Studies within the same class of compounds showed that electron-donor substituents accelerate the reaction of the RH+O3 type and that the apparent rate constant for the reaction series for substituted toluenes (r=72.07) substantially differs from the r values for peroxyl (70.76) and alkoxyl (70.34) radicals.4 e.Solvent effect The yields of HTO in the oxidation of aliphatic and aromatic acetals by ozone in polar solvents (acetone, diethyl ether) are much higher than in dichloromethane and pentane.4, 19, 20 f. Formation of charge-transfer complexes Taking into account the high electron affinity of ozone, one can assume that the reaction with the appropriate substrate will proceed with partial transfer of the electron density to the O3 molecule. Indeed, the reaction of ozone with alkylaromatic com- pounds (e.g., cumene) results in a charge-transfer complex, which was proved by spectroscopic methods.104 ± 107 Further nonradical transformation of the complex results in the hydrotrioxide.This is confirmed by a substantial increase in the yield of the hydro- trioxide 8 upon irradiation of the reaction mixture by light in the spectral region corresponding to absorption of the charge-transfer complex;28, 29 the absence of kinetic isotope effect in the reaction of ozone complex with toluene-d8 108 and by acceleration of the decomposition of ozone ± arene complexes in polar solvents.109 2. Thermochemical analysis of the mechanisms of formation of ROOOH Thermochemistry of the reactions of ozone with the C7H bonds was investigated taking the reaction with ethanol as an example. Detailed kinetic studies 24, 100 of the reaction revealed its first order with respect to both reagents, the rate constants were determined over a wide temperature range (see Table 11) and the dependence of the rate constant on the chemical nature of solvent was investigated.At T=293 K, the rate constant for the gas-phase reaction EtOH+O3 is 0.720.06 litre mol71 s71. Let us consider a feasible mechanism for this reaction. a-Hydroxyethyl hydrotrioxide 6b was reliably established 24, 100 to be the major product. As mentioned above, the mechanism of 1,3-dipolar insertion (route a), for which logA&6.5 and Ea&20 ± 26 kcal mol71, is inconsistent with the observed Arrhe- nius parameters (see Table 11). The enthalpies of the reactions proceeding by routes b and c can be calculated using the following values (in kcal mol71): DfH8(EtOH)=756.1,110 DfH8(O3)=34.0,110 DfH8(MeC.HOH)= 714.53,111 DfH8(MeC+HOH) = 1393 111 and DfH8(HOOO.)=15.3.18 The DfH8(HOOO7) value can be found using the gas-phase acidity of HOOOH (352 kcal mol71 obtained from the ab initio MP4//MP2/6-31++G* calculations 52) and DfH8(HOOOH)=712.2 kcal mol71 (see Ref.18). Thus we get DfH8(HOOO7)=725.9 kcal mol71. The activation energy of the gas-phase reaction of ethanol with ozone was estimated using the experimental rate constant k=0.72 litre mol71 s71 provided that A=16108 litre mol71 s71 (see Ref. 112); this gives Ea=10.9 kcal mol71.114 The enthalpy of a reaction proceeding by the radical route b is DH8(b)=DfH8(MeC. HOH)+DfH8(HOOO. )7 7DfH8(EtOH)7DfH8(O3)= =15.3714.5734.0+56.1=22.9 kcal mol71. Comparison of the DH8(b) and Ea values suggests that the homolytic mechanism for the formation of the hydrotrioxide 6b is energetically unfavourable.In addition, both thermochemical estimates 18, 66 and quantum-chemical calculations 70, 71, 113 pre- dict that the radical HOOO. is less stable compared to its decomposition products (HO. and O2) and decomposes almost barrierlessly. This reduces the role of the radical HOOO. in the formation of HTO from saturated organic compounds to a minimum. Thus, the homolytic mechanism (route b) is inconsis- tent with thermochemical estimates. The enthalpy of the gas-phase reaction of the hydride ion transfer (route c) can be calculated using the following equation DH8(c)=DfH8(MeC+HOH)+DfH8(HOOO7)7 7DfH8(EtOH)7DfH8(O3)+Eq . Here, Eq is the energy of the Coulomb interaction within the ion pair (7e2/r), where r is the inter-charge separation. Assuming that r=2.650.05 A,76 we get DH8(c)=139725.9734.0+56.17125=10.2 kcal mol71.As can be seen, the calculated Ea value is in good agreement with DH8(c). Calculations of DH8(c) values for liquid-phase reactions face considerable difficulties owing to the lack of reliable methods for calculating the solvation energies of reactants, DH8(solv). A qualitative estimate, DH8(solv)<0, can be obtained by simply reasoning that the ionic character of the transition state will favour its solvation compared to the electrically neutral EtOH and O3. It is also clear that DS0(solv)<0. Therefore, the logA and Ea values for the liquid phase will be smaller than those obtained for the gas phase, which is consistent with the experimental results (see Table 11).Nangia and Benson 76 used the Kirkwood formula for calculating the change in the Gibbs free energy of solvation. However, it was noted 100 that this approach a priori implies too strong a dependence of the rate constant for the reaction on the dielectric constant of the medium. We believe that a consistent explanation is that the actual transition state is not purely ionic but includes to a great extent the contribution of charge transfer from MeC+HOH to HOOO7. 3. Formation of hydrotrioxides in the ozonisation of hydrocarbons Experiments show that the most plausible mechanism for the reaction of ozone with the C7H bonds of organic compounds involves a hydride transfer of hydrogen to ozone followed by cage recombination of the ion pair (route c).The reaction is favoured by some factors responsible for the stabilisation of the carbocation formed: the structure of the substrate, polar substituents in the substrate, the presence of electron-donor substituents at the reaction centre. In the absence of these factors the most plausible mechanism for the reaction RH+O3 involves a homolytic abstraction of a H atom by the ozone molecule. The energy profiles of the reactions of ozone with the C7H bonds of methane, ethane, propane and isobutane have been studied.113 In the transition state, theHatom is nearly orthogonal to the plane passing through three oxygen atoms, which indicates the involvement of the p-MO of ozone in the formation of a new bond.The C7H7O angle is rather large (167 to 174 8) and the C7H and H7O distances are close (1.28 and 1.25 A for methane and 1.27 and 1.34 A for isobutane, respectively). This is consistent with the results obtained in studies of the kinetic isotope effect (see Section IV.1). According to quantum-chemical calculations, all the localised transition states V V Shereshovets, S L Khursan, V D Komissarov, G A Tolstikov have large contributions of biradical structures. The radical HOOO. formed in the reaction is extremely unstable and decom- poses into the oxygen molecule and the hydroxyl radical R.+HOOO. R.+O2+.OH. RH+O3 Thus, the probability of the homolytic mechanism of the ozonisation of hydrocarbons containing non-activated C7H bonds in the gas phase (or in non-polar solvents) is very low.On the other hand, the contribution of radical structures to the transition state decreases as the number of alkyl substituents in the a-position relative to the C7H bond involved in the reaction increases.113 Hence under specific conditions (e.g., with appropri- ate substituents and medium) the ionic mechanism can become even more energetically favourable. This can occur in the ozoni- sation of alkanes in superacidic solutions at 778 8C.114, 115 The reaction proceeds via a transition state with the 3c-2e bond between the protonated ozone, HOOO+, and the C7H bond. The pentacoordinated carbonium ion either undergoes further decomposition or loses a proton to give the corresponding hydro- trioxide: + OOOH R R R C H++OOOH R C R3COOOH.7H+ H R RYet another specific procedure for the preparation of hydro- trioxides of non-activated organic compounds is `dry' ozonisa- tion, i.e., the reaction of ozone with hydrocarbons adsorbed on a polar surface. The synthesis of HTO 11 and 15 ± 19 by `dry' ozonisation of aliphatic, alicyclic and alkylaromatic hydrocar- bons on silica gel has been reported.18, 31, 32 O3, SiO2 RH ±70 8C ROOOH. 11, 15 ± 19 It was also noted in these studies that hydrotrioxides 11 and 15 ± 19 cannot be obtained by conventional liquid-phase oxida- tion.Triphenylmethylhydrotrioxide 18 can be prepared not only by `dry' ozonisation but also in solution. However, low solubility of triphenylmethane at temperatures near 770 8C precludes the synthesis of Ph3COOOH in solution. In addition, this synthesis requires thermal activation or photoactivation of the intermediate complex ArH .O3, as in the case of cumyl hydrotrioxide.28, 29 In `dry' ozonisation, the polar surface acts as an activator.We can distinguish several factors responsible for the differ- ences between the low-temperature ozonisation of hydrocarbons on the surface and the ozonisation in solution. First, the concen- trations of reagents on the surface of the adsorbent can be very high, which cannot be achieved in solution at low temperature. As a result, ozonisation on the surface proceeds much faster.116 Second, low mobility of the adsorbed reagents, intermediates and products reduces the probability of side reactions and, hence, increases selectivity of the process. Finally, the surface of silica gel is a highly polar anisotropic medium, which stabilises the ionic intermediates.Thus, the formation of hydrotrioxides ROOOH by `dry' ozonisation is yet another argument in favour of the ionic mechanism of the reactions of ozone with the C7H bonds of saturated organic compounds. V. Decomposition of hydrotrioxides 1. Reaction products. Chemiluminescence The thermolysis products of hydrotrioxides 1a, 2, 3d, 3m, 4h, 5a,b, 8 and 20a are presented in Table 13. It should be noted that with a few exceptions (see notes to Table 13) the decomposition products were analysed together with the products formed in the synthesis of ROOOH; because of this, the yields of each of the products were calculated with respect to ozone consumed to give theOrganic hydrotrioxides Table 13.Thermal decomposition products of ROOOH. ROOOH O HO (1a) O O Me O O (2) O O HMe Me Me O C (3a) OO O HPh Me Me O C (3b) OO O H Me Me Pri O C (3m) RH OO O HOEt Me C OEt (4h) OO O H H OO C O (5a) O O H Me OO C (5b) OO O H Me HO C H (6b) OO O H Me Me O C O H (6f) O O H Ph Me O C O H (6k) O O H PhMe2COOOH (8) CD3 COCD3 PhMe2SiOOOH (20a) CD3 COCD3 Note. Hydrotrioxides ROOOH were obtained by oxidation of the corresponding substrates RH. a The yields are given with respect to ozone consumed in the reactions, D(O3). b The product undergoes further cyclisation and dehydration to give 2-methyl-4,5-dihydrofuran. c The yields are given with respect to HTO. d The yield is given with respect to D[ROOOH]; decomposition of the hydrotrioxide 8 in the presence of ionol results in PhMe2COH as the only reaction product.Solvent RH RH CD3COCD3 Me2CO MeCO2Et RH RH RH RH ButCOMe Me2CO MeCO2Et Product PhCO2H MeCOCH2CH2CH2OH Me2CO MeOH HOOH Me2C(OMe)OOH PhCOMe MeOH PhCO2Me PhCO2H Me2 CO:Me2CHOH:MeCO2Pri=1 : 1.3 : 2.7 MeCO2Et EtOH HCOOCH2CH2OH MeCO2CH2CH2OH MeCHO MeCO2H HCOOH HOOH Me2CO MeCO2H HOOH PhCOMe PhCO2H PhMe2COH PhCOMe PhMe2SiOH PhMe2SiOSiMe2Ph H2O2 PhMe2SiOOH 115 Ref. Yield a /mol mol71 14 0.96 14 0.70 b 95 0.32 0.29 0.05 0.03 17 0.50 0.14 0.20 0.05 96 20 0.80 ± 0.85 0.75 ± 0.80 23 1.0 23 1.0 25, 100 0.31 0.56 0.12 0.58 95 0.37 0.39 0.11 17 0.65 0.05 29 33 0.8 ± 0.9 c 0.1 ± 0.2 d 0.9 c 0.08 0.09 0.05116 particular product, D[P]/D(O3). All the reactions are accompanied by evolution of molecular oxygen including 1O2.According to the data listed in Table 13 thermal decomposi- tion reactions of the hydrotrioxides derived from benzaldehyde (1a), acetals (4h, 5a,b), cumene (8) and dimethylphenylsilane (20a) in which the reaction products are formed in nearly 100% yields exhibit the highest selectivities. As was mentioned above, silane- derived hydrotrioxides decompose to give, in particular, HOOOH.33, 34 Taking hydrotrioxide 6b as an example, it was shown that the yields of the major decomposition products, viz., D[P]/D(O3)=0.300.04 for MeCHO, 0.560.03 for MeCOOH, 0.110.03 for HCOOH and 0.570.03 for H2O2, are virtually independent of temperature in the range between778 and 0 8C.25 Decomposition of HTO is accompanied by chemilumines- cence in the visible spectral region.16, 21, 27, 117 ¡¾ 119 The chemilumi- nescence spectra of compounds 8, 6d, 6f, 6e, 7a, 3m and 1 exhibit maxima (l) at 425, 425, 405, 405, 405, 405 and 515 nm, respec- tively.No chemiluminescence was observed in the thermolysis of the hydrotrioxide 6b. Based on comparison of both the fluores- cence and phosphorescence spectra of the carbonyl compounds which resulted from decomposition of ROOOH and the chemi- luminescence spectra in the visible spectral region it was concluded that the emitters of chemiluminescence are excited carbonyl groups ( C=O*). One of the most interesting and important properties of hydrotrioxides is their ability to generate 1O2 during thermal decomposition.13, 14, 16, 17, 19 ¡¾ 21, 27, 31 ¡¾ 33, 35, 52, 89, 117, 118, 120 ¡¾ 122 The formation of singlet molecular oxygen was proved by carry- ing out specific reactions of singlet oxygenation of unsaturated organic compounds (see Section VII.1).Yet another convincing proof of the formation of 1O2 is intense chemiluminescence in the IR spectral region with a maximum at*1270 nm, which is typical of O2 (1Dg).123 Chemiluminescence in the visible and IR spectral regions has also been observed in the decomposition of adamantyl hydro- trioxide 15 on the surfaces of silica gel and alumina.32, 39 Spectral characteristics of chemiluminescence were found to be identical with those observed in the liquid phase. The yield of singlet molecular oxygen was determined by three methods. The first method involved measurements of the amount of the 1O2 acceptor consumed (1,3-diphenylisobenzofuran, 1,2-dimethylcyclohexene or tetraphenylcyclopentadien- one).13 ¡¾ 15, 17, 19, 20, 28, 29, 35 The yield of singlet molecular oxygen (F0=D[1O2]/D[ROOOH] /mol mol71) was calculated based on the assumption that the amount of 1O2 that evolved in the reaction (D[1O2]) is equal to the amount of acceptor consumed (D[A]).However, this approach provides only semiquantitative data on the yield of 1O2,124 so it is not surprising that the results obtained in the experiments with different 1O2 acceptors differ substan- tially. For instance, the D[1O2]/D[ROOOH] values for 1,1-dieth- oxyethane-derived hydrotrioxide using (4h) found tetraphenylcyclopentadienone and 1,3-diphenylisobenzofuran were 205% and 545%, respectively.20 Quantitative determi- nation of the yield of 1O2 requires a kinetic approach based on rigorous analysis of all possible consumption channels for the 1O2 acceptor, hydrotrioxide and singlet molecular oxygen.The draw- back of this method is that the acceptor of singlet molecular oxygen can be oxidised by the hydrotrioxide, which leads to overestimation of the results of measurements of the F0 values. Generation of 1O2 in the decomposition of ROOOH, its deactivation and quenching by acceptors A was found 16 to follow a mechanism similar to that of the thermolysis of organic phosphite ozonides 124 k0 P0+O2, 1O2 , ROOOH kd 1O2 O2, hn, kr 1O2+A AO2 , V V Shereshovets, S L Khursan, V D Komissarov, G A Tolstikov kq 1O2+A A+O2 , ln (10) a k0t , where k0, kd, kr and kq are the rate constants for thermal decomposition of the hydrotrioxide, radiative deactivation of singlet molecular oxygen and chemical and physical quenching of 1O2 by the acceptor A, respectively; and P0 is the decomposition product of the hydrotrioxide.Based on this mechanism, for the steady state we get I0 It 0 (11) I I a 1 a ktdaAa, A . (12) aAa0 ¢§ aAa? a F0Fr ¢§ OktdU¢§1

ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (2) 131 ± 146 (2001) Hydrogen-containing carbon nanostructures: synthesis and properties B P Tarasov, N F Goldshleger, A P Moravsky Contents I. Introduction II. Synthesis of hydrofullerenes III. Properties of hydrofullerenes IV. Sorption of hydrogen by carbon nanotubes and graphite nanofibres V. Conclusion Abstract. (full- nanomaterials carbon of interaction the on Data Data on the interaction of carbon nanomaterials (full- erenes, their and nanofibres graphite nanotubes, carbon erenes, carbon nanotubes, graphite nanofibres and their metal- metal- doped results New surveyed. are hydrogen with modifications) doped modifications) with hydrogen are surveyed. New results on on the are hydrofullerenes of properties and preparation the preparation and properties of hydrofullerenes are presented.presented. The as nanomaterials carbon of use the for prospects The prospects for the use of carbon nanomaterials as reversible reversible hydrogen sorbents are discussed. The bibliography includes 183 hydrogen sorbents are discussed. The bibliography includes 183 references. I. Introduction Hydrogen has indisputable advantages over other energy carriers as it is a versatile, highly efficient and environmentally safe energy carrier. This provides good prospects for its wide application in power engineering, especially as a fuel for vehicles transport.1 However, there is a very serious problem hampering the use of hydrogen as a fuel, namely, hydrogen storage and transportation.None of the currently available methods for the storage of hydro- gen (in the gas state under high pressure, in the liquid state, as metal hydrides and hydrides of intermetallic compounds, or in the adsorbed state at low temperatures, see Table 1) meets the requirements imposed on hydrogen storage systems, e.g., by the US Department of Energy (DOE) Hydrogen Plan (content of hydrogen 56.5 mass% or 563 kg m73) 2 and by the Interna- tional Energy Agency (content of hydrogen 55 mass %, release of hydrogen at temperatures below 373 K).3 Therefore, the development of new, more efficient methods for the storage and transportation of hydrogen is still an important task the success of which will determine, in many respects, further progress in the development of the `hydrogen' technology and power engineering.Hydrogen accumulation based on the reversible sorption of hydrogen is one of the most promising and popular methods for the storage of hydrogen. The most popular sorbents include hydride-forming metals and intermetallic compounds 4, 5 as well as active carbons prepared by various methods.6±9 Fullerenes and their derivatives, carbon nanofibres and nanotubes have been suggested repeatedly over recent years as hydrogen-accumulating B P Tarasov, N F Goldshleger, A P Moravsky Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-096) 576 40 09. Tel. (7-095) 913 21 09. E-mail: btarasov@icp.ac.ru (B P Tarasov), gold@cat.icp.ac.ru (N F Goldshleger), moralex@icp.ac.ru (A P Moravsky) Received 18 August 2000 Uspekhi Khimii 70 (2) 149 ± 166 (2001); translated by S S Veselyi #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n02000621 Table 1.Traditional hydrogen storage methods.3 Method and condi- Hydrogen tions for hydrogen storage Hydrogen content in the volume content /kg m73 sorbent (mass %) 100 7.7 Gaseous H2 (300 K, 10 MPa) 100 71 150 120 4.0 7.6 1.4 1.9 3.7 85 95 80 0.05 ± 2 *1.± 2 Liquid H2 (20 K) Metal-hydride TiH2 MgH2 LaNi5H6.7 TiFeH2 Mg2NiH4 Cryoadsorption activated carbon (155 K, 6.9 MPa) substrates.2, 6, 10, 11 In the present review, we consider the data on the interaction of molecular hydrogen with these carbon nano- materials (NM) and the prospects for their application as rever- sible sorbents for the storage of hydrogen. II.Synthesis of hydrofullerenes The presently known methods for the preparation of hydrofuller- enes,{ such as the Birch ±HuÈ ckel reduction of fullerenes, hydro- boration, catalytic hydrogenation of fullerenes, reduction of fullerenes based on non-catalytic transfer of hydrogen from a donor to an acceptor, etc., have been surveyed in detail in several reviews.12 ± 14 However, the rapid development of studies in this field requires a continuous analysis of the newly appearing methods for the synthesis of hydrofullerenes, especially because no ideal procedure for the preparation of hydrofullerenes with reliably controlled composition is yet available.{ Hydrofullerenes are also called fullerene hydrides or hydrogen-contain- ing fullerene compounds; sometimes they are also called hydrofullerites in order to emphasise the solid state of the product. 131 131 135 138 143 Drawbacks of the method heavy containers, small volume capacity high losses, high cost insufficient capacity; necessity of heating; sensitivity to admixtures necessity of cooling and compression132 1. Hydrogenation of fullerites and metal fullerides with molecular hydrogen Chemical transformations occurring in mechanical mixtures of powders of a fullerite (C60, C60/C70) and hydrogen-sorbing metals and intermetallic compounds (Pd, V, LaNi5, LaNi4.65Mn0.35, CeCo3) under the action of molecular hydrogen or deuterium have been studied in a wide pressure and temperature range.15 ± 19 It has been found that hydrogenation at 1.0 ± 5.0 MPa and 573 ± 673 K results in mixtures of metal hydrides MHy and hydrofullerenes C60Hx: {C60Hx+MHy}. {C60+M}+H2 Heating of the resulting mixture {C60Hx+MHy} in an inert atmosphere results first in dehydrogenation of the metal hydride and then (at 800 K) dehydrogenation of the hydrofullerene to give hydrogen and fullerene ± metal compositions.{ <600 K {C60+M} .{C60Hx+M} {C60Hx+MHy} *800 K 7H2 7H2 Hence, we are dealing with a system which can accumulate hydrogen under certain conditions and release it under other conditions.This reaction cycle can be repeated many times, but the side reactions, such as polymerisation and carbide formation, restrict the prospects of application of these systems for hydrogen accumulation. Depending on the temperature, the reaction of the pre- synthesised fullerides C60Pt and C60Pd4.9 with hydrogen (the H2 pressure being 1 ± 3 MPa) results either in hydrogen compounds of metallofullerides <C60MHx> or in mixtures of hydrofuller- enes C60Hx (x=2 ± 26) with Pt or PdHy, respectively. 400 ± 550 K <C60PtHx> C60Pt +H2 600 ± 700 K C60Hx+Pt 400 ± 550 K <C60Pd4.9Hx> C60Pd4.9+H2 600 ± 700 K C60Hx+PdHy Upon heating to 800 K, almost all the hydrogen is evolved from the reaction products, and the resulting mixture of the fullerite with the metal (Pd, Pt) can be hydrogenated again in the temperature range from 400 to 700 K to give mixtures of the hydrofullerene with PdHy or Pt.20 ± 22 The source of gaseous hydrogen and its purity affect consid- erably the hydrogenation of solid fullerenes and the state of the reaction products. For example, the hydrogenation of crystalline fullerene (fullerite) with hydrogen evolved upon the decomposi- tion of hydrides of intermetallic compounds results in crystalline hydrofullerenes containing from 10 to 30 hydrogen atoms per fullerene molecule.23, 24 The content of hydrogen in the product increases with the number of the `heating to 673 K± cooling to 300 K' cycles. The thermal cyclic hydrogenation mode not only provides a deeper process but also gives samples with a more homogeneous distribution of the components over the volume.The IR spectra of the resulting hydro- or deuteriofullerenes show intense absorption bands in the regions of C7H (2800 ± 3000 cm71) and C7D (2050 ± 2200 cm71) stretching vibrations. The relative intensities of the absorption bands of the starting C60 decrease abruptly (Fig. 1). X-Ray diffraction analysis of the reduction product shows that hydrofullereneC60Hx (or deuteriofullereneC60Dx) retains the crystal structure of the parent fullerite; the a0 parameter of the face-centred cubic (FCC) lattice increases monotonically with the { In certain cases, heating above 950 K results in irreversible formation of metal carbides.B P Tarasov, N F Goldshleger, A P Moravsky 12 2000 3000 1000 n /cm71 Figure 1. IR spectra of hydro- (1) and deuteriofullerenes (2) C60H(D)18 ± 24.24 amount of hydrogen (or deuterium) in the hydrofullerene (Fig. 2).} Heating of the hydrides synthesised by this method to 800 K results in intense evolution of hydrogen. The IR spectrum of the solid product of the thermal decomposition of hydrofullerene manifests bands corresponding to C60 and a series of bands the positions of which are close to those of the photo- or thermobari- cally polymerised C60 with the cyclobutane rings between the fullerene molecules,26 which indicates that thermal dehydrogen- ation is accompanied by polymerisation of the fullerene cages.a0 /A C60D24 C60H18 C60H10 C60 14.5 14.4 14.3 14.2 14.1 x 5 20 15 10 0 Figure 2. Dependence of the FCC lattice parameter a0 of hydro- and deuteriofullerenes C60H(D)x on the number of hydrogen and deuterium atoms.25 The samples dehydrogenated at 800 K can be hydrogenated again, but repeated `hydrogenation ± dehydrogenation' cycles result in the accumulation of polymerised fullerene.25 The addi- tion of NH3, HI or C2H5I (5 mass%± 10 mass %) to the hydro- gen evolved from a metal-hydride accumulator accelerates considerably the hydrogenation of the fullerite but also increases the degree of polymerisation of the fullerene molecules formed upon the dehydrogenation of the hydrofullerene.3 Studies of the magnetic properties of hydrofullerenes prepared by hydrogenation of fullerene with hydrogen liberated from an accumulator based on hydrides of intermetallic compounds of nickel and rare-earth metals 23 showed that the samples contained ferromagnetic particles; this was explained by the gas-phase transfer into the reactor of traces of metal or metal hydride particles by hydrogen which has evolved from the accumulator.One could assume that fine particles of nickel and/or nickel ± rare- earth metal intermetallides act as catalysts of the solid-phase hydrogenation of fullerene. The use of a nickel catalyst for hydrogenation of solid fullerite, e.g., by controlled addition of volatile nickel compounds to the gas fed into the reactor, could } According to the reported data,25 the lattice parameters for C60Hx and C60Dx are similar for equal x values.TransmissionHydrogen-containing carbon nanostructures: synthesis and properties provide more efficient reversible systems for hydrogen accumu- lation. The solid-phase hydrogenation of fullerenes continues to attract the attention of scientists from the viewpoint of both practical application (hydrogen storage) and synthesis of individ- ual hydrofullerenes. For example, solid-phase deuteration of C60 at 373 ± 773 K and pD2=15 MPa gave deuterides with the com- positions C60D8, C60D18 and C60D36 (FCC structure, a0=1.423, 1.448 and 1.500 nm, respectively).27 Controlled doping of full- erene with palladium decreases (down to 373 K) the temperature of hydrogen or deuterium sorption relative to that for the undoped C60 sample at a gas (H2 or D2) pressure of 100 MPa.28 A neutron diffraction study of palladium-doped fullerene showed no characteristic peaks of metallic Pd.Similarly, these peaks were not observed in the samples treated with hydrogen. Treatment of fullerene-containing carbon black by hydrogen plasma gives hydrofullerenes with low hydrogen content (x=1 ± 6).29 Pro- longed treatment does not increase the amount of sorbed hydro- gen.Progress is also observed in the field of liquid-phase hydro- genation of fullerenes. Cheaper heterogeneous catalysts have been developed and methods for more selective and extensive hydro- genation have been found. For example, the hydrogenation of C60 on the Ni/Al2O3 catalyst 30 occurs under milder conditions (tol- uene, 423 ± 523 K, 2.5 ± 7.5 MPa) than the hydrogenation on the Ru/C catalyst.31 The reaction occurs selectively to give C60H36.The reaction selectivity can primarily be accounted for by the high dispersion of nickel deposited onto Al2O3.} A decrease in the reaction temperature decreases the selectivity (similarly to the reduction of C60 with dihydroanthracene in the presence of an organic catalyst 32). Liquid-phase hydrogenation of C60 with hydrogen (at atmos- pheric pressure, in the presence of 4% Pd/SiO2, 5% Ru/Al2O3 or 5% Rh/Al2O3) or with hydrogen donors (cyclohexane, decalin, etc.) at 323 ± 343 and 348 ± 433 K, respectively, results in hydro- fullerenes C60Hx.33 In some experiments, Pd/SiO2 was used as the catalyst for the hydrogenation of fullerene by hydrogen transfer from a donor (cyclohexane, cyclohexene) to the acceptor (C60).Titration of the hydrogen peroxide formed due to the oxidation of C60Hx with 2-ethylanthraquinone in the presence of O2, chemical determination of hydrogen in C60Hx, as well as IR and NMR spectra indicate that the C60 reduction products have the compo- sition C60H42 ± 46.33 Unfortunately, the results of mass spectro- metric determination of the hydrofullerenes formed have not been reported; this might provide direct information on the composi- tion of C60Hx.33 In a search for systems for the hydrogenation of fullerenes and dehydrogenation of hydrofullerenes which would comply with the requirements for the practically applicable hydrogen accumula- tors, Loutfy et al.34 have examined whether soluble catalysts could be employed for hydrogenation of the C=C bonds and if salt melts could be used as the reaction media for hydrogenation.They also studied the effect of modification of the fullerene electronic structure on the hydrogenation process. This approach was based on the achievements in studies of the activation of the C±Hbonds by metal complexes,35 on the one hand, and on the data on the efficient catalytic hydrogenation of the C=C bonds with hydro- gen in the presence of Cs6C70,36 on the other hand. Alkoxy compounds of the Group IV metals were found to be efficient as the soluble catalysts for fullerite hydrogenation and hydrofullerite dehydrogenation.This study 34 has shown that the use of certain soluble catalysts made it possible to decrease considerably the activation energy of hydrogenation and the temperature of the process. For example, the dehydrogenation of (C60/C70)H36 occurred at 573 K (6 h) to give (C60/C70)H22 (the loss of } The results obtained 30 agree with the conclusion on the possible involve- ment of highly-dispersed nickel particles in the solid-phase hydrogenation of C60.23, 24 133 1.74 mass% H2), while the repeated hydrogenation of (C60/C70)H22 occurred at 423 K (12 h, 3.6 MPa) to give (C60/C70)H*45. It has also been shown that doping of fullerene by an alkali metal facilitated markedly the fullerene hydrogena- tion.The dehydrogenation in this system occurs at considerably lower temperatures than the non-catalytic dehydrogenation of solid hydrofullerites. The use of operating temperatures below 400 K is important for hydrogen-accumulating systems, hence these results 34 dem- onstrate that an almost acceptable temperature range can be achieved. 2. Reduction of fullerenes in the `metal ± hydrogen donor' system The reduction of fullerenes is carried out using many traditional methods of organometallic chemistry (see, e.g., the review 37). For example, lithium in liquid ammonia in the presence of ButOH was used for the reduction of C60.38 Li, NH3 (liq), ButOH C60H2n . C60 The process stopped in the step of the formation of C60H32 ± 36 (see Refs 12 and 13).The products with a lower hydrogen content, e.g., C60H32, were probably formed during the analysis or storage of the samples. The composition C60H36 of the reduction product of C60 fullerene with lithium in liquid ammonia has also been confirmed by other researchers.39 Polyfullerenes obtained by photo-induced and radical poly- merisation of C60 cages (the radical polymerisation is preferable for the synthesis of polyfullerenes) are also reduced according to the Birch ±HuÈ ckel method to give hydropolyfullerenes with a mean hydrogen content of *3.5 mass% (for C60H36, 4.76 mass% H2).40 Hydropolyfullerenes, unlike monomeric hydrofullerenes with an equal hydrogen content, are almost insoluble in CS2. This implies that the reduction at low temper- atures does not involve cleavage of the C7C bonds between the fullerene cages in the polymers; this has been confirmed by gel- permeation chromatography.The IR spectra of hydropolyfuller- enes contain intense bands in the region of C7H stretching vibrations. The 1H and 13C NMR spectra indicate structural similarity between polymeric and monomeric hydrofullerenes (the positions of the hydrogen atoms in the fullerene cage). The use of polyfullerenes does not increase the degree of hydrogena- tion of the fullerene cage, and hence, the amount of stored hydrogen. Thus, polyfullerenes are not superior to C60 molecules in hydrogenation and dehydrogenation reactions. The reaction of C60 with EtAlCl2 and activated magnesium [C60 :Al :Mg=1 : (50 ± 150) : (30 ± 50); 293 ± 295 K] in a THF± toluene mixture in the presence of Cp2TiCl2 (1 mol.% ± 3 mol.%) results in fullerene-containing aluminacyclopropane (yield 75%± 90%), hydrolysis of which in D2O results in a partially deuterated fullerene derivative.41 Metallation of C60 with an excess of Et3Al in the presence of Cp2ZrCl2 gives 2,3-fullerenoaluminacyclopen- tanes, hydrolysis of which results in mixtures of hydrogen- containing ethylfullerenes of the type HmC60Etm (m=1 ± 12).42 The reduction ofC60 andC70 in the `metal (M) ± proton donor' system, where M=Zn (E=70.76 V), Ti (E=71.63 V), Al (E=71.66 V) orMg (E=72.37 V), was also studied.43 ± 46 It was shown that the reaction of C60 with the Zn/Cu redox couple and a proton donor in toluene , unlike the reaction of C60 with Zn dust,43 results in a more efficient fullerene reduction which occurs via the consecutive formation ofC60H2, C60H4, and finally,C60H6; the target compounds can be isolated in each step.44 ± 47 Prolonged heating of a C60 ± Zn/Cu ±H2O mixture in toluene results in two C60H6 isomers in the ratio 6 : 1, these amount to 30%± 40% with respect to the starting C60.The 1H and 13C NMR data show 1,2,33,41,42,50-C60H6 to be the main reaction product.44, 45134 H H H H H H The hydrogen atoms in 1,2,33,41,42,50-C60H6 are not exchanged with deuterium atoms from D2O on the NMR time scale, i.e., the pKa of C60H6 is much higher than that of ButC60H, which rapidly exchanges protons with the medium.48 An attempt to isomerise 1,2,33,41,42,50-C60H6 in the presence of a Pd/C or Pt/C catalyst (as has been done for 7,8-C70H2, cf. Ref 49) resulted in rapid dehydrogenation of hydrofullerene instead of the expected reaction.44 The reaction in the C70 ± Zn/Cu ±H2O system under certain conditions results in preferential formation of C70H10, 7,8,19,26,33,37,45,49,53,63-C70H10 being the major isomer.50 For various reasons (the presence of an oxide film, difficult separation of the products from the reaction mixture, etc.), efficient and selective reduction of fullerenes in the presence of other metals (Ti, Al, Mg) 44, 46 was found to be impossible.Water is the most suitable proton donor for the preparation of reduced fullerene derivatives in the Cn ± Zn/Cu ±H2O systems (n=60, 70).Hydrofullerenes with low hydrogen content, viz., C60H2, C60H4, C60H6 and C70H10, were obtained in good yields.44, 50 The isomer distribution is controlled by the reduction kinetics. The use of strong acids as proton donors for the reduction of fullerenes gives hydrofullerenes with higher hydrogen content.51 In toluene, the reduction of the fullerene cage in the C61H2 fulleroid in the presence of Zn/Cu occurs more slowly than that of fullerene,52 but the reaction in o-dichlorobenzene occurs quite efficiently to give three isomeric compounds C61H4 which are light- and air-sensitive even at low temperatures. The reaction of sodium naphthalenide with C60 in THF gives 1,10,2,20-tetrahydro- bi[60]fulleren-1-yl C120H2 together with other products (including 1,2-dihydrofullerene).53 Compound C120H2 is also formed upon hydrolysis of the salt (Me2NC6H4)3C+[C60]¡ obtained by the reduction of C60 with the crystal violet radical.54 The reaction of C60 with dilithiumacetylenide results in bis(hydrofullerenyl)acetylene C122H2 (yield 6.6%).53 It should be noted that deprotonation of C120H2 is accompanied by fast dissociation of the transient C27 120 dianion to the C760 monoanion, whereas deprotonation of C122H2 results in the C27 122 dianion.53 3.Reduction of fullerenes by organic compounds The reduction of fullerenes based on non-catalytic hydrogen transfer from a donor (e.g., dihydroanthracene) to an acceptor (e.g., C60 fullerene) results in selective formation of C60H36 { or C60H18, depending on the reaction conditions.55, 56 H H 623 K C60+ C60Hn+ H H The hydrogenation of C70 with dihydroanthracene results in a series of hydrofullerenes C70Hn (n=36 ± 46), C70H36 being the major product.Anhydrous hydrazine in benzene reduces C60 and the endohe- dral compound 3He@C60 into the lower hydrides C60H2±C60H8 and 3He@C60H2 ± 3He@C60H8; their compositions and struc- { Lobach et al.56 paid much attention to standardisation of the conditions for the preparation of C60H36 because the synthetic procedure affects considerably the properties and structures of hydrofullerenes. B P Tarasov, N F Goldshleger, A P Moravsky tures were determined using 1H and 3He NMR spectroscopy (in the case of 3He@C60).At high hydrazine concentrations, the reaction mixture also contained higher hydrofullerenes, viz., C60H18 and C60H36.57 Photoinduced electron transfer (PET) plays an important role in the reduction and oxidation of fullerenes. Since the one-electron oxidation potential of the triplet-excited state of C60 is 1.14 V (relative to the saturated calomel electrode),58 electron donors having less positive oxidation potentials can be used for the photoreduction of C60. For example, the photoreduction of C60 with allyltributyltin in the presence of CF3COOH results in the selective formation of 1-allyl-1,2-dihydrofullerene, C60-1,2-(H)C3H5,59 while the reaction of C60 with tributyltin hydride in benzene occurs (under irradiation 59 or at elevated temperatures 60) to give C60-1,2-(H)SnBu3.If 10-methyl-9,10- dihydroacridine is used as a hydrogen donor in the presence of PhCN ±CF3COOH in photoreduction of C60 (l>540 nm), full- erene is reduced selectively to give 1,2-C60H2 (70%).59 H H hn C60+ PhCN ±CF3COOH NMe 1,2-C60H2+ N + Me This photoreduction reaction of C60 is superior to, e.g., the hydroboration of C60 (see Ref. 61) because of its selectivity and because mild reducing agents are used. However, in the absence of irradiation this reaction does not occur even at 373 K. The bimolecular rate constant for quenching of the triplet- excited C60 (3C60) by 10-methyl-9,10-dihydroacridine (kq= 4.36109 litre mol71 s71) and the mechanism of the photoreduc- tion of C60 have been determined.62 Similarly to the two-electron reduction of C60 in the presence of 10-methyl-9,10-dihydroacridine, the one-electron reduction of C60 involving 1-benzyl-1,4-dihydronicotinamide and its dimer occurs through photoinduced electron transfer from the electron donor to 3C60.63, 64 The difference between the redox and acid- base properties of the radical cations formed upon PET can define the reaction pathway.Heating (100 8C, toluene) or pulse photolysis (Nd3+ :YAG laser, l=532 nm, benzonitrile) of a mixture of C60 with the Hantzsch ester (diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5- dicarboxylate) under anaerobic conditions result in fullerene hydrogenation products and diethyl 2,6-dimethylpyridine-3,5- dicarboxylate (the dehydrogenation product of the Hantzsch ester).H H COOEt COOEt EtOOC C60Hx+ EtOOC C60+ Me Me Me Me N NH The progress of the dehydrogenation of the Hantzsch ester with the fullerene was monitored by 1H NMRspectroscopy and a signal at d 5.93 characteristic of 1,2-C60H2 could be detected if the reaction was carried out at room temperature. Heating of the reaction mixture in the presence of excess Hantzsch ester results in hydrofullerenes with the composition C60Hx (x=2, 4, 6) identi- fied by mass spectrometry.65, 66 Illumination of the same mixture by visible light under aerobic conditions (room temperature), in agreement with the known data 67 on the sensitising effect of C60, completely converted the Hantzsch ester in the presence of fullerenes to give the correspond-Hydrogen-containing carbon nanostructures: synthesis and properties ing pyridine derivative; the concentration of C60 remained unchanged.65 Tertiary amines are widely used in the photoreduction of fullerenes.Prolonged irradiation (l=350 nm,N2) of a mixture of C60 with tertiary amines RNMe2 (R=Ph, Me) gives 1,2-C60H2 along with a pyrrolidine derivative of C60 (fullereno[C60]pyrroli- dine).68 The ESR method was used to study many photoreduction reactions of C60. For example, it was found that the C¡60 radical anion is formed in the presence of photo-activated TiO269 or triethylamine 70 in a mixture of organic solvents 69, 70 and in an aqueous solution.71 The photochemical reduction of C60 in the presence of TiO2 or ascorbic acid was studied.72 It was found that the C60 fullerene is reduced photochemically by triethylamine to give C¡60 .Prolonged irradiation of the reaction mixture results in C60H7 (the C2¡ 60 protonation product) which is decomposed to C¡60 in a dark reaction.73 Irradiation of C60 in the presence of certain acceptor-type photosensitisers (9,10-dicyanoanthracene, 1,4-dicyanonaphtha- lene, N-methylacridinium and 2,4,6-triphenylpyrylium hexafluo- rophosphates) results in 1-substituted 1,2-dihydro[60]fullerenes following introduction of proton donors such as N,N-dimethyl- formamide, 1,3-dioxolane, phenylacetaldehyde, methyl formate, tert-butyl alcohol, propionic acid, etc.74, 75 2 H PET 1 C60+R±H R Co-sensitisers, e.g., biphenyl, are used in order to increase the 60 radical cation formed was shown to Cá C60 60 , yield of the products.The Cá possess electrophilic character. hn PET H7Cá60 +R , Cá60 +RH H7C607R . H7Cá60 +R + e7 4. Other methods for the preparation of hydrogen-containing fullerene derivatives Dihydrofullerene C60H2 was obtained in low yield upon sonica- tion of C60 solutions in decahydronaphthalene.76 The formation of dihydrofullereneC60H2 upon electrochemical hydrogenation of C60 in 30% aqueous KOH was also reported.77 The electrolytic hydrogenation of C60 and dehydrogenation of C60H2 are rever- sible.77 C60Hx+xOH7 . C60+xH2O+x e7 Mechanochemical reactions involving fullerenes and graphite have been studied.78 ± 81 Mechanochemical treatment of graphite in an atmosphere of hydrogen (1 MPa) at room temperature results in nano-structured graphite which adsorbs hydrogen both physically and chemically.81 The method of `high-speed vibration milling' (under these conditions, crystalline C60 is pulverised into particles with sizes smaller than 1 mm) was found to be highly efficient for the supramolecular complexation of fullerenes and their derivatives with g-cyclodextrin and sulfocalixarene.79 The yields of [4+2]-cycloaddition products of fused aromatic com- pounds (anthracene, tetracene, pentacene and naphtho[2,3-a]pyr- ene) to C60 obtained according to this procedure are higher than the yields of the same products obtained by heating the compo- nents in solution.80 The solid-phase reaction of C60 with KCN, unlike the homogeneous reaction, gives the C120 dimer.82 This reaction also occurs in the presence of other salts (K2CO3, AcOK), metals (Li, Na, K, Mg, Al and Zn) and organic bases (4-di- methylamino- and 4-aminopyridine).82 It is probable that the mechanochemical method can also be applied for the synthesis of 135 hydrogenated fullerene derivatives { (similarly to the hydrogena- tion of organic compounds with hydrogen 84). III.Properties of hydrofullerenes 1. The state of hydrogen in solid hydrofullerenes In the majority of experiments on the reduction of C60 and C70, mixtures of fullerene hydrides are obtained. Solid samples of hydrofullerenes with the same molecular formula but prepared by different methods differ strongly in their solubilities, light and air resistances, abilities to withstand prolonged storage and in other properties (see, e.g., Refs 13, 51, 85). Of hydrofullerenes, C60H36 has been studied most thoroughly.Electron-impact and field-desorption mass spectrometric analysis showed that C60H18 is the first stable thermal decomposition product of C60H36 prepared under the conditions of non-catalytic transfer of hydrogen from a donor to an acceptor.56 The inter- mediate hydrides C60Hn (n=33 ± 35) are much less stable than C60H36. Intense peaks of C60H18 and C60 appearing in the mass spectrum of a sample of C60H36 after its prolonged storage indicate that C60H36 undergoes decomposition; the presence of the C60 peak is evidence of possible reversible addition of hydro- gen to fullerene.The formation of C60H36 as the only reduction product of C60 under non-catalytic hydrogen transfer conditions is also confirmed by the spectra obtained using matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS): the most abundant peak is that of C60Há35, the fragmentation is insignificant, the intensities of the satellite peaks corresponding to the oxidation products C60HxO+ are much lower than in the case of the C60H36 obtained by the Birch reduction.39 As noted above, solid hydrofullerenes decompose at temper- atures above 800 Kin an inert or in a hydrogen atmosphere to give hydrogen and fullerenes.23, 86 The thermal decomposition product of crystalline C60D24 at 823 Kmainly consists of fullerene (Fig.3) a I (arb. units) 40 200 b 50 c 100 0.3 0.2 0.1 1/d /A¡1 Figure 3. Diffractograms of the parent fullerene (a), fullerene deuteride C60D24 (b) and the product of thermodecomposition of the deuteride (c).25 with the distance between the C60 molecules considerably exceed- ing that in the parent fullerene (the a0 constants of the FCC lattice of the decomposition product and C60 are 1.452 and 1.417 nm, respectively). Presumably, the increase in the fullerene crystal lattice constant resulting from hydrofullerene decomposition can be useful in the synthesis of alkali metal fullerides with super- conducting properties.86 The position of the C1s peak maximum in the X-ray photoelectron spectrum of the C60D24 deuteride is shifted towards lower bond energies in comparison with that of C60, which makes it possible to conclude that a small positive charge exists on the deuterium atoms.The peak becomes narrower { Apparently, the microwave irradiation method 83 can also be used for the preparation of hydrogenated fullerene derivatives.136 as the deuteride undergoes thermal decomposition, and its `centre of gravity' shifts to higher bond energies.86 Particular interest in the fundamental and applied aspects is felt in the synthesis of hydrofullerenes in which the hydrogen atoms are located both outside and inside the fullerene cage. This is a way to increase the hydrogen capacity of fullerenes. Therefore, studies of the properties of individual hydrogen molecules located in a closed space are also of interest.For example, it was found that compounds C60H52 and C60H48 fluoresce in the long-wave region. This phenomenon was interpreted taking account of the possibility of penetration of hydrogen molecules into a closed hydrofullerene cavity.87 Hydrofullerites obtained under ultrahigh hydrogen pressures, pH2=0.6 and 3.0 GPa, were studied by inelastic neutron scattering (INS) spectroscopy.88 The spectra showed that the synthesis of C60Hx (pH2=0.6 and 3.0 GPa, 620 K, 24 h) followed by cooling the sample to 77 K under a high pressure of H2 is accompanied by the formation of a solid solution of hydrogen in the C60Hx lattice (x&24 and 32 ± 46 for pH2=0.6 and 3.0 GPa, respectively).88, 89 The main features of the INS spectrum of C60H32 obtained at pH2=3.0 GPa 90 are similar to those for C60H*23 obtained at pH2=0.6 GPa.91 Presumably, a considerable fraction of hydrogen (*3 H2 molecules per C60 molecule) are located as H2 molecules in the interstices of the C60Hx lattices.Upon heating to 293 K, theH2 molecules leave the sample with heat evolution. Dissolved molecular hydrogen is detected by the rotational transition peaks in the H2 molecule (*15 and 30 meV) in the difference INS spectra between the `quenched' (H2-containing) and heated (H2-free) products. The possibility of polymeric binding of the C60H32 hydrofullerite at high H2 pressure (3.0 GPa) is suggested by the presence of a peak near 13 meV in its INS spectrum.The low-energy part of the INS spectrum (<9 meV) for C60H32 is believed to indicate 88, 89 the presence of an endohedral hydrogen atom unbound with carbon. Hydrofullerites obtained at pH2=0.6 and 3.0 GPa and T=520 ± 620 K and then quenched at T<120 K exhibited ferromagnetic properties up to T&300 K. Their magnetisation was as high as M=1.2 emu g71 at H=1 T.92 According to analytical data, the concentration of ferromagnetic admixtures in hydrofullerenes is insignificant. Magnetisation which disappears upon heating of the samples above 340 K or upon prolonged storage at room temperature is a property inherent in hydro- fullerites obtained by various methods.92, 93 Probably, their mag- netisation is caused by the presence of unstable transient radical- type electronic states.The penetration of hydrogen inside the cage, polymerisation of the fullerene cages upon dehydrogenation and other high-energy transformations of the C7C and C7H bonds in sterically hindered fullerene lattices could be the reasons for the generation of quasistable carbon atoms with unsaturated valence or delocalised unpaired electrons. 2. Thermodynamic analysis of the dehydrogenation of hydrofullerenes The thermodynamic properties of the C60H36 hydrofullerene obtained by non-catalytic hydrogen transfer 56 have been studied in the temperature range from 4.8 to 340 K.94, 95 The following values of thermal capacity, enthalpy, entropy and Gibbs free energy were found for the hydrofullerene obtained at T=298.15 K and p=101.325 kPa: C p (298.15)=690 J K71 mol71; H8(298.15)7H8(0)=84.94 kJ mol71; S 8(298.15)=506 J K71 mol71; 7[G8(298.15)7H8(0)]=66.17 kJ mol71.High temperature is required for the dehydrogenation of hydrogen-containing fullerene derivatives. Quantum-chemical calculations of the C7H bond energy in fullerene hydrides performed by the semi-empirical AM1 method gave the values of 306.27, 298.32, 295.80 and 293.72 kJ mol71 for C60H12 (Th), B P Tarasov, N F Goldshleger, A P Moravsky C60H18 (C3) and C60H36 (T and Th), respectively.96 Heating of C60H36 under isothermal conditions (594 and 610 K, 20 h) results in dehydrogenation of the sample and evolution of gaseous H2.In this case, the composition of the solid residue is `C60H6': C60H36?C60H18?`C60H6'. The dehydrogenation occurs step- wise 97 and is accompanied by sublimation of C60H36 and C60H18. The estimated saturated vapour pressures of C60H36 and C60H18 at 594 K are 9.861079 and 5.461079 MPa, respectively. 3. Specific features of hydrofullerene structures The C60D24 deuteride obtained by deuteration of crystalline fullerene with gaseous D2 is a crystalline powder with an FCC lattice (a0=1.455 nm) 23 and a crystallite size of 51 ± 56 nm.25 The synthesis of C60D36 with an FCC lattice (a0=1.500 nm) has been reported.27 On the other hand, C60H36 samples obtained by hydrogenation under high pressure 98 or in the presence of iodine 99 have body-centred cubic (BCC) lattices.100 Theoretical and experimental studies of the structures of the most stable (T, Th, S6 and two D3d) isomers of the C60H36 hydrofullerene have been carried out.99 ± 103 Comparison of the experimental 1H and 13C NMR spectra of the solid C60H36 (see Ref.99) with the calculated ones 104 made it possible to assume a structure with the T symmetry 105 for C60H36 obtained by non- catalytic hydrogen transfer.56 According to X-ray emission and photoelectron spectral data, quantum-chemical calculations and electrochemical behaviour, the most probable structure for the C60H36 hydrofullerene is that with the T symmetry and with four isolated benzenoid (non-hydrogenated) rings located on the sur- face of the C60H36 hydrofullerene in the vertices of an imaginary tetrahedron (Fig.4).103, 106, 107 Comparison of the CKa spectra of C60, C60H36 and C6H6 prompts the conclusion that the p-electron systems of hydro- fullerenes and benzene are similar; the localisation of the HOMO electron density of the C60H36 hydrofullerene on the benzenoid rings allows one to assume that Z6 metal complexes based on C60H36 can be synthesised.106, 108 Studies of the electrochemical behaviour of the C60H36 hydrofullerene indicates that C60H36 is a rather `hard' molecule with low reactivity in redox reactions. The electrochemical electronegativity decreases in the series C60>C60H2>C60H36&C6H6. The approximate equivalence of the electronegativities of the two latter compounds can also be evidence in favour of the T-symmetrical structure of C60H36.107 The presence of a large number of reaction centres in the fullerene molecules results in a large number of possible isomers of hydro- and fluorofullerenes.Based on a combination of the MO LCAO method with molecular mechanics, Breslavskaya and D'yachkov 109 predicted the structures of the most stable CnXk T Th D3d S6 Figure 4. The most stable C60H36 isomers (one of the two D3d is shown).103 The thick lines denote the non-hydrogenated C7C bonds.Hydrogen-containing carbon nanostructures: synthesis and properties isomers with high hydrogen and fluorine contents (X=H, F; n=76, 78, 84) and compared the theoretical data with the available experimental results.According to the neutron diffrac- tion structural data 110 for C60D36 prepared according to a reported method 99 by deuteration of C60 at 623 K and a D2 pressure of 3.4 MPa in the presence of iodine as a promoter, the structure with the T symmetry is most stable for this deuterioful- lerene as well [a structure with the T symmetry has been suggested previously 99 as the most stable one for C60H36 (IR and 13C NMR spectral data)]. Billups et al.111 studied the 3He spectra of 3He@C60H36 samples prepared by reduction of 3He@C60 with lithium in liquid ammonia 38 and by non-catalytic hydrogen transfer.55 The 3He NMR spectra of both 3He@C60H36 samples contain peaks at d 77.7 and 77.8; the third peak at d 716.45 was assigned to the chemical shift of the 3He@C60H18 hydrofullerene.Compar- ison of the experimental spectra 111 with the calculated chemical shifts for the most stable 3He@C60H36 isomers [d 710.8 (T ), 77.7 (D3d 0), 76.1 (S6), 73.4 (Th) and 75.6 (D3d)] 104 makes it possible to assume that the D3d 0 isomer is formed in both cases. In addition, based on the 3He NMR spectra of the sample prepared by reduction of 3He@C60 with lithium in liquid ammonia, the conclusion was made as to the low probability of the formation of the isomer with the Th symmetry which had previously been suggested 38 for a 3He@C60H36 sample. According to the reported data,112 the isomer with the S6 symmetry (see Fig. 4) is the most stable C60H36 hydride, though the difference between the energies of the ten most stable isomers is less than 62.76 kJ mol71.In the previous studies by the same authors, isomers with the T and D3d symmetries were noted as the most stable ones of the large number of possible structures suggested for the C60H36 fullerene hydride.101 Thus, it has been shown for C60H36 that not only different chemical methods can result in isomers with different symmetries, but also products with different isomer compositions can be formed within a single procedure (see, e.g., Refs 85 and 106). In the case of the C60H36 molecule, the problem of the most stable structure remains open and requires more accurate theoretical calculations. It is quite possible that certain methods for the synthesis of C60H36 can produce thermodynamically unstable isomers (e.g., due to kinetic factors).In order to verify the validity of this assumption, one has to perform in-depth experimental and theoretical studies, including the isomerisation of C60H36. A reliable determination of the C60H36 structure is also interesting from the practical respect. For instance, the knowledge of its exact structure is very useful when one plans the syntheses based on this hydrofullerene. 4. The reactivities of hydrofullerenes According to the reported data,113 a hydride with the net composition C60H18.7 with a0=1.476 nm is stable up to 703 K in an Ar atmosphere. Further increase in the temperature results in hydrogen evolution and is accompanied by decomposition of part of the fullerene molecules and formation of graphite platelets and methane.} The use of catalytic amounts of ruthenium or platinum did not decrease the temperature of hydrofullerene dehydrogenation. The dehydrogenation of C60H36 on treatment with a strong hydrogen acceptor, viz., 2,3-dichloro-5,6-dicyano-1,4-benzoqui- none, which is widely used in the analysis of hydrofullerenes, was found to be non-quantitative, since in addition to C60, its derivatives were also formed.56 Thermogravimetric (TG) and differential thermogravimetric (DTG) studies of C60H36 in the presence of oxygen were carried out.30 It was found that C60H36 is partially oxidised at 550 Kto give C60H36Ox (x&4.8) and that its combustion occurs at 745 K.As it might have been expected, these } Presumably, the character of thermal decomposition and the crystal lattice parameters of hydrofullerenes depend considerably on the con- ditions of fullerene hydrogenation. 137 temperatures are much lower than the temperatures of partial oxidation and combustion of C60 (705 and 888 K, respectively).30 Complexes of polyaromatic compounds and fullerene with plat- inum immobilised on silica exist in the temperature range 323 ± 373 K and can be repeatedly hydrogenated and dehydro- genated.114 It is these properties that are required for practically usable systems for the storage of hydrogen: good reproducibility of hydrogenation ± dehydrogenation cycles in a temperature range as low as possible. Deprotonation of 1,2-C60H2 in the presence of Bu4NOH (benzonitrile, anaerobic conditions) is accompanied by the for- mation of the C2¡ 60 dianion which quickly reacts with a number of alkyl halides to give mono- and dialkylfullerenes.115 H H 1) R7X base 2) H+ R H 60 Ph2CHBr, PhCH2Br, MeI, propargyl bromide and allyl bromide are used as the alkylating reagents. However, attempts at alkylation of the C27 formed with octyl bromide failed. Analysis of the absorption spectra for mono- and dialkylated C60 derivatives shows that the sp3 carbon atoms in the monoalkyl derivative are in the 1,2-position, while in dialkyl-C60 derivatives, they are in the 1,4-position. Fukuzumi et al.116 studied the mechanisms of formation of R2C60 and R(R0)C60 dialkylfuller- enes from chemically obtained C2¡ 60 and various alkyl halides (RX and R0X) differing both in the nature of the R and R0 radicals and of the halogen atom.In the first step, transfer of one electron from the C2¡ 60 dianion to an alkyl halide occurs to give RC¡60, which is transformed to R2C60 or R(R0)C60 according to the SN2 mechanism. It was shown that protonation of ButC¡60 with trifluoroacetic acid initially gives 1,4-ButC60H; the latter is then quickly trans- formed to 1,2-ButC60H which is more stable thermodynamically. Photoinduced transfer of an electron from a soft reducing agent, 1-benzyl-4-tert-butyl-1,4-dihydronicotinamide, to triplet-excited C60 results eventually in selective two-electron reduction of C60 to give ButC¡60, which reacts with PhCH2Br or ButI in the next step to give 1,4-But(PhCH2)C60 (see Ref.63) or But2C60 (see Ref. 117), respectively. Photoalkylation of C70 occurs similarly to give ButC¡70 (and ButC70H after protonation); the formation of five regioisomers is possible for ButC¡70, unlike AlkC¡60 which gives only one product.118 60 radical anion or the C260¡ dianion The reactivities of the C¡ manifest themselves not only in reactions with alkyl halides. The higher C7H bond strength in lower hydrofullerenes in compar- ison with the higher ones results in the possibility of the transfer of a hydrogen atom onto these anions from higher hydrofullerenes. It was shown by cyclic voltammetry that the reaction between C60 and C60H36 in a propylene carbonate ± toluene mixture is induced by electron transfer and eventually involves transfer of a hydrogen atom from C60H36 to the fullerene.119 The pathway according to which the C¡60 radical anion and the C260¡ dianion electrogenerated from C60 react with C60H36 was considered as the most probable variant of the transformation; this reaction involves the transfer of a hydrogen atom from C60H36 and results in C60H2 (proton transfer is impossible since the calculated 120 pK1 for C60H36 is 31.35 and C60H2 is a rather strong acid 121, 122 with pK1=4.7 and pK2=16).In order to estimate the possibility of formation of138 C60H2 from C60 and C60H36, Tkachenko et al.119 calculated the mean C7H bond energies in hydrofullerenes C60H2 and C60H36, which were found to be 295 and 279 kJ mol71, respectively.Thus, cleavage of a C7H bond in C60H36 with subsequent formation of a C7H bond in C60H2 is energetically favourable. The relatively week C7H bond strength in higher hydro- fullerenes enables its homolytic cleavage by other relatively weak acceptors as well. Elimination of a hydrogen atom from C60H36 under mild conditions can also be carried out by treatment with the 2-(p-fluorophenyl)hexafluoroisopropyl radical.123 Heating (353 K) of hydrofullerenes C60Hx (x=6 ± 18) with Hg[N(SiMe3)2]2, Ti[N(SiMe3)2](NEt2)3 and other metal com- plexes also results in elimination of a hydrogen atom from C60Hx and formation of fullerene-containing oligomers, e.g., 7[(C60Hn)Hg2]m7.124 mC60Hx+2mHg[N(SiMe3)2]2 7[(C60Hn)Hg2]m7+2mHN(SiMe3)2.IV. Sorption of hydrogen by carbon nanotubes and graphite nanofibres Much attention has been given recently to the sorption of hydro- gen by such carbon nanostructures as carbon nanotubes, graphite nanofibres and their metal-doped modifications. Carbon nanotubes (NT) with low-defect structures were found in 1991 in the products of electric-arc evaporation of graphite.125 However, long before that, similar NT} with a high content of structural defects have repeatedly been detected in solid products of hydrocarbon pyrolysis on metal catalysts.126 Owing to its high efficiency, the catalytic pyrolysis of hydrocarbons is still widely used for the practical production of long (up to millimetres) carbon nanostructures, including NT and the so-called vapour- grown carbon fibres (VGCF).A graphene sheet rolled in a seamless cylinder is the main element of carbon NT. Multi-walled nanotubes (MWNT) con- sisting of graphene sheets rolled as a cylindrical spiral also exist. Vapour-grown carbon nanofibres can consist of parallel-sided graphene platelets perpendicular to the fibre axis (platelet-type VGCF), of nested graphene cones (herring-bone VGCF), of truncated cones (conical layer nanotubes, CLNT), etc. Single-walled nanotubes (SWNT) are closed networks built of quasi-sp2-hybridised carbon atoms. The network consists of hexagonal cells on the side surface of the cylinder. Open and closed SWNT exist; in the latter, the cylinder ends are closed by hemispheres (`fullerene halves') consisting of hexagonal and pentagonal cells.The SWNT diameter ranges within 0.8 ± 5 nm (most often, 1 ± 2 nm) and its length is 1 ± 500 mm (most often, 5 ± 50 mm). Carbon SWNT have a rather narrow diameter distri- bution, in contrast to activated carbons in which the sizes of macro-, meso- and micropores differ by a factor of several hundred.127 A typical feature of SWNT is the formation of rather strong molecular aggregates (named as bundles, strands, ropes, braids, etc.), in which the axes of separate SWNT are parallel to each other. The shortest distance between the carbon skeletons of the adjacent SWNT in the bundles is 0.32 nm. This value is close to the size of the van der Waals distance between graphene layers in graphite and can be changed by introduction of inorganic or organic intercalants, surfactants or by chemical modification of the cylindrical surface of the NT.Ideal (defect-free) multi-walled nanotubes (MWNT) are built of SWNT inserted one into another. IdealMWNTare formed in a carbon arc if no metal catalyst is added to the graphite to be vaporised. The distances between the layers inMWNTare close to the interlayer distance in graphite. The outer diameters of electric- arcMWNTare 5 ± 40 nm(most often 15 nm), the inner diameters are 0.8 ± 5.0 nm (mostly 1 ± 2 nm), and the lengths are 0.5 ± 70 mm. } These nanotubes were called tubular graphite fibres. B P Tarasov, N F Goldshleger, A P Moravsky The synthesis of SWNT and MWNT is accompanied by the formation of other carbon modifications: fullerenes, multilayer polyhedral nanoparticles, graphite microcrystals and amorphous carbon.Furthermore, mixtures of products obtained by the catalytic method contain metal particles. The content of carbon NT in the synthesis products does not exceed 20%± 70% (for different methods), therefore in all cases purification from admix- tures is the most laborious stage in the preparation of pure carbon NT.Many publications (see, e.g., the corresponding re- views 128 ± 133) report the state-of-the-art of the methods for the preparation and study of the properties of carbon nanostructures and their use. 1. Sorption of hydrogen by single-walled carbon nanotubes The carbon material obtained by the electric-arc method and containing single-walled carbon NT (in the form of bundles containing 7 ± 14 NT), amorphous carbon and particles of a catalyst (for example, Co) adsorbs 5 mass%± 10 mass% H2 (with respect to the pure SWNT) at 273 K and 300 Torr followed by cooling to 90 K.134 According to the temperature-programmed desorption (TPD) data,134 hydrogen is desorbed from nanotubes and activated carbon at 133 K.However, if a sample containing SWNT is preliminarily subjected to a treatment which opens the `plugs' at the tube ends, a second peak almost at room temperature (290 K) appears on the TPD curve. This peak is explained by the evolution of hydrogen adsorbed inside the SWNT due to the penetration ofH2 molecules through the open tube ends.No high- temperature peak of this type is observed in the desorption of hydrogen from activated carbon samples or from fullerene soot obtained in the absence of a catalyst. It is believed 134 that in the carbon material obtained by the electric-arc method, the `high- temperature' hydrogen is mainly located in the SWNT channels, whereas the `low-temperature' hydrogen is adsorbed on amor- phous carbon, on the outer surface of the SWNT and/or in the intertube space of the SWNT bundles. The amount of adsorbed hydrogen (99.9999% purity) for relatively pure carbon SWNT obtained by the laser method (see Refs 135 and 136) exceeds 8 mass% H2 at 10 ± 12 MPa and 80 K.137 In this case, the mean diameter and the specific surface of a separate SWNT are*1.3 nm and 1300 m2 g71, respectively.The diameters of the SWNT bundles are *6 ± 12 nm. The outer specific surface of the bundles measured by the BET method is 2855 m2 g71. The amount of adsorbed hydrogen depends on the number of `sorption ± desorption' cycles. For freshly prepared samples of pure SWNT, the C:H ratio of &1 (*8.25 mass% H2) is achieved at 80 K and 7 MPa of H2 (Fig. 5, curve 1). H/C 1.0 2 0.8 1 3 0.6 0.4 0.2 6 8 10p /MPa 2 4 0 Figure 5. Dependence of the amount of hydrogen adsorbed by an SWNT at 80 K on pressure in the first (1) and subsequent cycles (2 and 3).137Hydrogen-containing carbon nanostructures: synthesis and properties Hydrogen is initially adsorbed on the outer surface of an SWNT bundle.An increase in pressure to 12 MPa during subsequent cycles increases the amount of absorbed H2, which is attributed to the penetration of H2 molecules into the intertube space of the bundles. As a result, the intertube space increases and the bundle breaks into separate SWNT; the outer SWNT surfaces are completely covered and the inner surfaces are partially covered by H2 molecules (Fig. 5). The increase in the intertube distance inside the SWNT bundle can be detected by X-ray diffraction and NMR spectroscopy.138 It has been shown that an SWNT bundle expands after immersing a carbon SWNT sample in a nitric acid solution and shrinks to the original configuration after deinterca- lation of HNO3, i.e., this change is reversible.Liu et al.139 synthesised carbon SWNT by the electric arc method (the production rate of SWNT was 2 g per hour) in the presence of hydrogen [a nickel ± cobalt ± iron powder (Ni :Co : Fe=3 : 0.75 : 0.75) was used as the catalyst, and FeS was used as theSWNTgrowth accelerator]. The content ofSWNT in the carbon material obtained by this method was *60%, the meanSWNTsize was 1.85 nm, the diameter of theSWNTbundles was about 20 nm. Upon treatment of the SWNT samples, which included, in particular, heating at 770 K, the amount of adsorbed hydrogen (room temperature, 10 MPa) reached 4.2 mass %, which corresponded to the ratio H:C=0.52. Up to 78% of the adsorbed hydrogen desorbed under standard conditions. The remaining hydrogen (from 0.52 mass% to 0.95 mass% H2 for different SWNT specimens) desorbed on heating the sample to 423 K.Studies of the hydrogen-sorbing properties of a carbon material obtained by the electric-arc method 140 and containing *70 mass%SWNTshowed that it adsorbs about 3.5 mass%H2 at 10 MPa and with multiple repetition of the cycles `cooling to 77 K± heating to 300 K'.3, 6 Interesting results were reported 141 that up to 6.5 mass%± 7.0 mass%of hydrogen was absorbed by an SWNT at room temperature and 0.1 MPa. The specific surface and the gas-sorbing capacity of carbon materials depend considerably on the preliminary thermal treat- ment. It was shown 142 that heating of an SWNT sample, which has preliminarily been acid-etched, above 600 Kin vacuo results in the liberation of different gases (CH4, CO, CO2 and H2).After thermal pretreatment, SWNT adsorb a considerably greater amount of xenon at 95 K: the SWNT samples annealed at 1073 K absorbed 20 times more Xe than the samples heated to 623 K. The increase in the adsorption capacity of SWNT was explained by the effect of high-temperature annealing, which removes the carboxyl groups blocking the ends and defects of the NT walls (Fig. 6). Apparently, such a thermal treatment should also affect the hydrogen adsorption capacity of NT. Hydrogen can also be stored in SWNT using an electro- chemical process which is reversible. The relatively low but quite reliably measured hydrogen capacity in the process charge (HT+x H)+xOH7 HT+x H2O+x e7 is about 0.4 mass% for a number of SWNT specimens.In this case, SWNT/Au is the negative electrode, Ni is the positive electrode, Hg/HgO/OH7 is the reference electrode, and 6 M KOH is the electrolyte. The capacity decreases by only 30% after 100 charge ± discharge cycles.143 The maximum content of hydrogen in carbon NT specimens reduced with dihydroanthracene at 623 K or with lithium under modified Birch conditions corresponds to the `C9H±C10H' and `C8H' compositions, respectively;144, 145 this is several times less than in the systems using physical hydrogen adsorption at high pressures. The desorption of hydrogen from such hydrogen- containing nanotube derivatives occurs only at 773 K, which implies that hydrogen is chemically bound.145 2. Sorption of hydrogen by graphite nanofibres Inconsistent data on the hydrogen-sorbing properties have been reported for vapour-grown carbon fibres (VGCF). The highest 139 HO O O C O C C OH HO O C O OH HO C T 7CO2,7CO,7CH4,7H2 Figure 6.Schematic drawing of chemically treated single-walled NT before and after heating above 600 K.142 hydrogen-sorbing capacity of VGCF (11 mass%± 67 mass% H2) has been reported by Chambers et al.146 The graphite nano- fibres were obtained by catalytic pyrolysis of carbon-containing gases on the nanoparticles of a metal catalyst containing iron, nickel and copper, and then purified from the catalyst particles. The VGCF with a length of 10 ± 100 mm and a diameter of 3 ± 50 nm obtained by this method can be assigned to the platelet-type or conical layered structures.146 In a favourable conformation, the distance between the graphene sheets is 0.337 nm, while the gas-kinetic molecular diameter of hydrogen is 0.289 nm.The sorption of hydrogen at 12 MPa and 298 K was studied on a Sieverts-type apparatus. In a series of studies,147, 148 attention was drawn to the necessity of careful preparation of the samples intended for the H2 sorption capacity measurements. It was noted that VGCF having developed systems of slit-shaped nanopores with a great number of open edges have a perfect configuration for hydrogen adsorption. Owing to this unique VGCF structure, the hydrogen molecules interact not only with the graphene sheet surface but also with the adjacent H2 molecules; it is believed that this may result in the capillary condensation of hydrogen at anomalously high temperatures.146 Moreover, the sorption of H2 is accompa- nied by the expansion of pores (crevices), which can also result in polymolecular sorption of hydrogen.Comparison of X-ray dif- fraction spectra of VGCF samples before and after hydrogen adsorption indicates that the interlayer distance d between sepa- rate graphene sheets increases to 0.347 nm after H2 adsorption (d=0.340 nm for the initial VGCF). As the adsorbed H2 is removed from the sample, the parameter d returns to the original value (0.340 nm).147, 148 The results on the hydrogen sorption by VGCF reported by Chambers et al.146 have evoked both much interest and many critical comments,{ as all attempts to reproduce this experiment failed.6, 149, 150 Moreover, the reported high values of H2 absorp- tion considerably exceed even the highest theoretical estimates.2, 151 { It may well be that an error was made in the evaluation of the amount of hydrogen adsorbed by the VGCF.For example, the same study 146 reported data on the sorption of hydrogen by palladium and intermetallic compounds which were much higher than the generally accepted values.140 StoÈ bel et al.150 studied the adsorption of hydrogen on two types of carbon materials: on various specimens of amorphous high-porosity activated carbon and on VGCF synthesised in the presence of a catalyst.The adsorption of H2 by activated carbon measured at 296 K and an H2 pressure of 12 MPa was 1.6 mass %, which is in good conformity with calculations on the physical adsorption of hydrogen. Under these conditions, the adsorption of H2 by VGCF did not exceed 1.2 mass %. The authors 150 did not obtain high H2 absorption on carbon nano- fibres with parallel-laminated structure prepared by them; how- ever, they noted an unusual behaviour of the material in a hydrogen atmosphere, which may be due to the fibre nanostructure. The hydrogen-sorbing properties of a lithium- and potassium- doped carbon material prepared by catalytic decomposition of methane have been studied. After removal of the catalyst, the material contained up to 90% VGCF{ with an outer diameter of 25 ± 35 nm, with mainly platelet or conical-layered structures.152 Doping with lithium or potassium [Li(K) :C&1 : 15] occurred under conditions of the solid-phase reaction between VGCF and Li(K)-containing compounds such as carbonates and nitrates.Hydrogen uptake reaches 20 mass%for the Li-containing VGCF at 653 K and 14 mass% for the K-containing VGCF at room temperature, which amounts to *160 and 112 kg (H2) m73, respectively.152 Multiple repetition of the H2 absorption ± desorp- tion process results in an insignificant (*10%) loss of the sorption capacity of the material. The specimens doped with lithium had higher chemical stabilities under ordinary conditions than those doped with potassium. The high sorption capacity of Li(K)-doped VGCF is attributed to the layered VGCF structure with large interlayer clearance (0.347 and 0.335 nm for VGCF and graphite, respectively) between the small-size (50 nm and less) graphene sheets in the VGCF, as well as to the presence of an alkali metal.Without alkali doping, the absorption of H2 by the VGCF decreases abruptly. Chen et al.152 have also obtained anomalously high values of H2 sorption by graphite doped with Li andK[up to 35%± 70% of the sorption capacity of Li(K)-VGCF at 473 ± 673 K and pH2=0.1 MPa]. The high hydrogen sorption by VGCF doped with alkali metals was explained by possible dissociative hydro- genation of the carbon sorbent, in which the alkali metal is a catalytic active centre.This hypothesis is supported by the fact that a band characteristic of the Li7H bond vibrations (*1420 cm71) appears in the IR spectrum of a Li-doped VGCF specimen upon treatment with H2. In the temperature range from 473 to 673 K, the intensity of the Li7Hvibrations correlates with the hydrogen absorption. Subsequently, the hydrogen atoms are transferred to the carbon network and add to carbon atoms. The appearance of an absorption band in the region of the C7H stretching vibrations confirms the dissociative mechanism of VGCF interaction with hydrogen. An increase in temperature causes the desorption of hydrogen and, correspondingly, disap- pearance of the bands assigned to vibrations of the C7H bond. Thus, systems based on Li(K)-VGCF can store hydrogen at elevated temperatures.While recognising the validity of the qualitative dependences of hydrogen sorption by carbon nanomaterials doped with alkali metals,152 Yang 153 assumed that these results might have been affected by the presence of moisture in the hydrogen used. Comparative experiments on the absorption of extremely pure (99.999%) absolutely dry and moist hydrogen by the lithium- doped VGCF obtained using the above technique 152 have shown that the adsorption was 2 mass% and 12 mass% H2, respec- tively. It was assumed 153 that the anomalously high hydrogen { Chen et al.152 relate this material, which contains layered truncated graphene cones, to carbon nanotubes. In accordance with the terminology accepted by Dresselhaus 2 and by us, it should be attributed to graphite nanofibres.B P Tarasov, N F Goldshleger, A P Moravsky sorption (up to 20 mass %) by the VGCF doped by Li and K152 can be due to the formation of alkali metal hydroxides and hydrates in a humid atmosphere. The graphite nanofibres obtained by pyrolysis of CO in the presence of Co/La2O3 also absorb H2;154 preheating of the samples at 1173 K is necessary in order to provide a high sorption efficiency (see also Ref. 148). The samples preheated at 673 K absorbed much less H2, whereas graphite was found to be completely inert under identical conditions. The effect of thermal treatment of a carbon material surface on the adsorption of hydrogen is shown by the data listed in Table 2. It was found that etching followed by treatment of carbon nanostructures increased their specific surfaces and the hydrogen-sorbing capacities, which resulted in the stable absorp- tion of 5 mass% H2 by the samples treated with a NaOH solution.155 The VGCF-containing carbon soot obtained by catalytic pyrolysis of ethylene on nickel at 900 K adsorbs about 2.5 mass% of hydrogen at 5 ± 7 MPa and with repetition of the cycles `cooling to 77 K± heating to 300 K'.3, 6 Table 2.Effect of pretreatment of carbon nanotubes on hydrogen adsorp- tion at pH2=10 MPa and T=293 K.155 Conditions of NT pretreatment H2 (mass %) 2.67 1.16 Mechanochemical treatment (30 min), etching (65% HNO3, 72 h) Mechanochemical treatment (30 min), etching (65% HNO3, boiling) Heating with NaOH (3 mol litre71, 1 h), washing 5.15 with H2O, drying at 373 K (2 h), calcination at 823 K (1 h), treatment with H2SO4 (15%, 1.5 h), heating at 373 K (10 min), washing with H2O, drying at 373 K Catalytic decomposition of acetylene at 1173 K in the pres- ence of Co/SiO2 results in the formation of carbon nano- fibres.156, 157 The sorption capacities of such metal-free carbon fibres at 10 MPa hydrogen pressure is*1 mass%H2.Although the structure of the carbon material obtained by Klyamkin et al.156, 157 is similar to that of the VGCF obtained by Chambers et al.,146 their sorption capacities differ strongly (1 mass% H2 156 and 67 mass%H2 146), despite the similar behaviour of the speci- mens in the stage of hydrogen desorption.According to the reported data,158 multi-walled carbon NT absorb up to 1 mass% H2 at 10 MPa and 298 K. The amount of adsorbed H2 increases with an increase in pressure in the range of 10 ± 200 MPa. The CO:H2 ratio affects the specific surface of the nano- carbon fibre obtained upon pyrolysis of CO in the presence of a Fe7Ni7Cr alloy and its sorption capacity with respect toH2: the hydrogen uptake by the VGCF decreases with an increase in the H2 content in the starting mixture.159 High-precision calorimetric methods 160 were used to study the adsorption capacity of a number of carbon materials, includ- ing VGCF obtained according to the reported technique,161 and commercial SWNT. The materials studied were found to be poor H2 adsorbents under the experimental conditions (H2 adsorption at 303 K,H2 desorption at 373 K), and their adsorption capacities did not exceed that of activated carbon, which disagrees with other reported data.146 Ozaki et al.162 compared the adsorption of hydrogen on platinum-coated carbon fibres prepared by two methods.The first method involved the preparation of a homogeneous mixture of a polymer with a metal complex followed by high-temperature carbonisation of the specimen. High-temperature activation of this specimen in water vapour or in CO2 resulted in the migration of the metal particles to the outer surface of the carbon carrier. The second method employed a mixture of polymers, viz., oneHydrogen-containing carbon nanostructures: synthesis and properties which underwent pyrolysis and one which underwent carbon- isation.The addition of a metal (Pt) compound to the pyrolysable polymer made it possible to deposit the metal on the walls of the pores formed upon pyrolysis. The H2 adsorption for the material obtained according to the second method depends on the amount of platinum added: a small amount of platinum favours H2 spillover and higher H: Pt ratios. A decrease in the H: Pt ratio with increase in the platinum concentration is due to an increase in the metal particle sizes.162 It was shown that thermal treatment of carbon nanofibres prepared by pyrolysis of ethylene in the presence of a Fe7Ni7Cu alloy (Fe :Ni : Cu=85 : 10 : 5) in argon at 1273 K resulted in a noticeable increase in the amount of the stored hydrogen (up to 6.5 mass%H2).163 The importance of pretreatment of a specimen before hydrogen sorption has also been noted in other stud- ies 148, 154 where the necessity of a special thermal treatment of VGCF in order to remove the chemisorbed gases has been shown.By analogy with NT (see Refs 142, 155), it may be surmised that chemical treatment of VGCF and their subsequent high-temper- ature annealing increase the specific surface of a VGCF and its activity with respect to H2 adsorption. After an appropriate treatment, VGCF and NT obtained under catalytic pyrolysis conditions (carbon source: benzene or methane; precursor of the catalytically active species: ferrocene) can adsorb up to 4.0 mass%± 5.7 mass%H2.164 The absorption of pure hydrogen (99.999%) by graphite fibre with a parallel-laminated structure (98% of VGCF in the mixture) starts immediately without an induction period to reach 10 mass%± 13 mass% H2 in the first cycle (Table 3).The H2 adsorption and desorption rates are high.166 Upon repeated adsorption ± desorption cycles, the sorp- tion capacity of the material decreases to approximately 70% of Table 3. Effect of pretreatment of a graphite nanofibre on hydrogen adsorption at pH2=11 MPa and T=373 K.165 H2 (mass %) Conditions of VGCF pretreatment Mean VGCF diameter /nm 12.38 12.82 80 90 10.03 100 10.1 125 evacuation, 5 h, 373 K treatment with 37% HCl, washing with H2O, evacuation, 5 h, 373 K dispersion in ethanol, treatment with 37% HCl, washing with H2O, evacuation, 5 h, 373 K dispersion in ethanol, treatment with 37% HCl, washing with H2O, evacuation, 5 h, 373 K Note.The graphite nanofibre was obtained by pyrolysis of benzene at 1373 ± 1473 K in the presence of Fe and S. p /MPa 12 108642 6 4 2 10 H(mass %) 8 Figure 7. Hydrogen desorption isotherm at 300 K.166 141 the original value, which is thought to be due to the destruction of the graphite fibre structure. The graphite nanofibre obtained by pyrolysis of acetylene and ethylene at 873 K in the presence of Ni adsorbs up to 10 mass% H2 following keeping for 16 h at room temperature at 12 MPa. After a decrease in pressure, almost all hydrogen is released (Fig.7).166 Despite the observed discrepancies in the available experi- mental data (cf., e.g., Refs 148, 150, 158, 165), it is obvious that NT and VGCF are actually characterised by high capacities with respect to hydrogen. 3. Sorption of hydrogen by nanostructured graphite The sorption of hydrogen on activated carbons at 133 K and pH2=2 MPa is 0.5 g H2 per 1 kg of C (Ref. 8) and 2 mass% at 155 K and pH2=6.9 MPa.150 A uniquely high, though hard to explain, hydrogen-sorbing capacity was observed for carbon of AX-21 grade with 3000 m2 g71 overall material surface and 700 ± 1800 m2 g71 micropore surface; the mass and volume densities of H2 at 77 K and 5.0 MPa were 100 g H2 per 1 kg of the adsorbent and 32 kg H2 per 1 m3, respectively.9 The relationship between the sorption capacity and the volume of micropores has been analysed for a series of activated carbons.It has been found that a pore size optimal for the adsorption of hydrogen exists.167 Thermal desorption spectroscopy was used to study the sorption of hydrogen by nanoporous carbon (NPC).168 It was shown that the maximum amount of H2 that can be adsorbed by NPC is 15.6 at.% at 266 MPa and 773 K. Mechanochemical treatment of graphite in an extra-pure hydrogen atmosphere (99.9999%) at room temperature and starting hydrogen pressure of 1.0 MPa results in nanostructured graphite which adsorbs H2 well.81 After milling for 80 h, the maximum amount of adsorbed H2 for such a sample is 7.4 mass% (CH0.95). This value is comparable to the values of H2 absorption reached for nanotubes and superactive carbon at temperatures below 100 K and H2 pressures of about 5 MPa.Enoki et al.169 studied the properties of nanostructured graph- ite and its intercalated compounds. The composites prepared by mechanical grinding ofMgand graphite in the presence of organic additives (THF, cyclohexane, benzene) were found to be efficient hydrogen-adsorbing materials.170, 171 The introduction of an organic additive is necessary for the composite structure to form and its hydrogen-adsorbing properties to appear. 4. Theoretical calculations of hydrogen adsorption by carbon nanomaterials The mechanism of the uniquely high hydrogen sorption by carbon NM has not yet been clarified.Different variants have been considered: physical adsorption and chemisorption of H2 mole- cules on the graphene sheet surface; the arrangement of more than one layer of H2 molecules between the graphene planes; capillary condensation of hydrogen inside the nanotubes and in the space between the graphene sheets at anomalously high temperatures; the charge state of hydrogen in carbon nanomaterials. Therefore, the calculated maximum hydrogen capacities of NM differ strongly as well. The Monte-Carlo method was used 172 to calculate the adsorption capacities of carbon materials with crevice-shaped and cylindrical pores with respect to gaseous H2 at various temperatures and pressures. The calculated values of H2 sorption were compared to the experimental results obtained for the specially prepared microporous activated carbon (AC).A linear relationship between the adsorption capacity of hydrogen and the specific surface of AC was found. The best AC samples had a specific surface of 2290 m2 g71; the corresponding maximum amount of adsorbed H2 was 0.6 mass %. This value is in good agreement with the calculated value of 0.7 mass% H2 for ideal slit-shaped pores with S=2600 m2 g71 at 300 K and pH2=6 MPa.142 According to the reported data,172 carbon materials with slit- shaped pores (d=0.7 nm) are the optimum H2 adsorbents at room temperature and pH2=10 MPa. The highest calculated H2 volume density is equal to 14 kg m73 (mass content equal to 1.3 mass% H2). According to the data of Rzepka et al.,172 the sorption capacities of carbon materials with slit-shaped pores are higher than those of materials with cylindrical pores over practi- cally the entire temperature and pressure range studied.It was noted that polymolecular (two-layer) sorption of hydrogen can be observed at 77 K in pores with d=1.0 nm even at low H2 pressures; this corresponds to the gravimetric capacity of the carbon material equal to *2.0 mass% and 5.5 mass% H2 for cylindrical and slit-shaped pores, respectively. Therefore, the observed high values of H2 uptake by single-walled nanotubes (5 mass%± 10 mass% H2 at room temperature) 134 cannot be explained by pure physical adsorption occurring in SWNT due to strong capillary forces.Calculations 172 of physical H2 adsorption at room temper- ature and pH2=11 MPa on carbon material with slit-shaped pores with d=0.34 nm corresponding to the interlayer distance in VGCF146 imply that hydrogen cannot be adsorbed by this material. Since under these conditions the gravimetric capacity of a microporous carbon material with d=0.7 nm is as low as 1.3%, the increase in d observed for VGCF in the course of hydrogen adsorption cannot result in adsorption equal to 11% ±67% reported in Ref. 146. Using the model of VGCF structure with sorbed hydrogen shown in Fig. 8 for the calculation of H2 adsorption, it has been shown 173 that the distance of 0.34 nm between the graphene layers is insufficient for the incorporation of hydrogen molecules.Moreover, a graphite nanofibre with d=0.9 nm can adsorb as low as 0.46 mass%± 1.6 mass% H2 at 298 K and pH2=5 ± 11.2 MPa. Analysis of H2 adsorption on a graphite surface taking into account the real sorption conditions and the geometric parameters of the hydrogen molecule and the graphite layer gives the value of *2.8 mass% H2 for one H2 layer adsorbed on a separate graphene sheet.2 In real systems, the intermolecular forces, the interaction of the H2 molecules with the material surface and the presence of surface defects can result in the formation of addi- tional sites for H2 adsorption. Considering the possible formation of the second hydrogen layer, the value of H2 sorption on the carbon surface was assessed to be*4.1 mass %.Doping of graphite with K, Rb and Cs increases slightly its sorption capacity with respect to H2; the H2 content due to Figure 8. Cell model for the calculation of hydrogen adsorption by a platelet-type graphite nanofibre (pore diameter 0.9 nm). The dark and light circles indicate the carbon atoms and hydrogen molecules, respec- tively.173 B P Tarasov, N F Goldshleger, A P Moravsky chemisorption corresponds to the C8KH2/3 composition, and the H2 content due to physical adsorption corresponds to the C24KH2 composition. However, the hydrogen content is small in both cases (see Ref. 2 and the references therein). On the other hand, for VGCF doped by a metal, high H2 absorption was obtained even at a low metal : carbon ratio (1 : 15), apparently because of partial reduction of carbon in the graphite layer due to the catalytic effect of alkali metals.152 However, other researchers are skeptical about the high values of H2 adsorption obtained 152 (see, for example, Refs 153 and 174), despite the elegant hypoth- esis put forward by Chen et al.152 for the explanation of the absorption of hydrogen by nanomaterials doped by alkali metals.Two geometrical models have been suggested for hydrogen sorption by carbon NT bundles.2 In the first one, hydrogen is considered as a liquid capable of filling the free space unoccupied by the carbon atoms of the nanotube. The use of density values of 0.071 and 2.26 g cm73 for liquid hydrogen and carbon atoms, respectively, results in the H2 adsorption equal to 2.3 mass %.According to the second model which accounts for the adsorption of hydrogen molecules with a gas kinetics diameter of 0.289 nm both inside the SWNT and in the inter-tube space of SWNT bundles (Fig. 9), the overall H2 adsorption for a (10,10)NT bundle } (the NT diameter being 1.38 nm) was found to be 4.0 mass% H2. Of these, 3.3 mass% H2 corresponds to the adsorption inside the tubes and 0.7 mass% H2, to adsorption between the NT in the bundle. Under high-pressure conditions, the compressibility of hydrogen and intermolecular interaction might result in a denser packing of the hydrogen molecules and hence in an increase in its adsorption on carbon NT. In order to confirm this assumption, a detailed calculation has been carried out 175 which has shown that the density of hydrogen in this case can be, in fact, higher than follows from purely geometrical considerations. Computer simulation of hydrogen adsorption by SWNT and a carbon material with idealised slit-shaped pores has also been carried out.151 The hydrogen ± hydrogen and hydrogen ± carbon interactions were simulated using the Silvera ± Goldman and Crowell ± Brown potentials, respectively; quantum effects were also taken into account.Adsorption of hydrogen inside the NT, between the tubes in the bundles and on the outer SWNT surface was assumed in the calculations. The computer simulation of hydrogen absorption by SWNT and activated coal is in favour of SWNT but does not confirm the high values of hydrogen adsorption for SWNT and VGCF found experimentally.134, 146 Figure 9.A typical arrangement of H2 molecules adsorbed on triangu- larily packed carbon nanotubes.2 } The numbers in parentheses indicate the indices used for the description of the nanotube structures; they are related directly to the NT diameter and the chiral angle.Hydrogen-containing carbon nanostructures: synthesis and properties Calculations ofH2 adsorption inSWNT bundles show 151 that the dense packing of SWNT results in lower gravimetric and volume densities of hydrogen even at sufficiently high adsorption potentials. Low adsorption can result from the surface inaccessi- bility in dense SWNT bundles. The results of calculations using the Monte-Carlo method carried out for bundles of parallel SWNT demonstrate a strong dependence of the gravimetric adsorption of hydrogen on the bundle diameter, i.e.on its specific surface.176 At 77 K and pH2=10 MPa, the adsorption of H2 for isolated (10,10)NT and for bundles consisting of three and seven (10,10)NT were 9.6 mass %, 7.0 mass%and 5.5 mass %, respectively. A depend- ence of the gravimetric capacity of NT on the inter-tube distance in a bundle was also noted. The increase 138 in the inter-tube distance inside the SWNT bundles observed upon their treatment with nitric acid allows one to assume that the parameter g=a ± d (where g is the van der Waals gap, a is the lattice spacing, d is theNTdiameter) used in the calculations of H2 adsorption can be optimised.177 Optimisation of the packing geometry (the g parameter) for SWNT has been carried out in order to obtain the maximum hydrogen adsorption on SWNT at 298 and 77 K.With this purpose, a series of NT have been studied, viz., (9,9)NT with d=1.22 nm, (12,12)NT with d=1.63 nm and (18,18)NT with d=2.44 nm. It has been found that the optimum g value is a function of temperature and equals 0.6 and 0.9 nm at 298 and 77 K, respectively. It has been shown that none of the SWNT packing geometries studied (the triangular and square ones) can provide high hydro- gen adsorption at room temperature. Hydrogen adsorption capacity approximating that recommended by the International Power Agency can be obtained on carbon NT with d=1.22 nm and g=0.9 nm only at 77 K and pH2>5 MPa.Calculations for charged carbon NT by the Monte-Carlo method have confirmed their somewhat higher ability to adsorb H2 in comparison with uncharged NT: the presence of a charge results in an increase in H2 adsorption by 10%± 20% and 15%± 30% for 298 and 77 K, respectively.178 However, the hydrogen mass and volume densities at 298 K are still too small even for charged tubes. 2 Calculations using the density functional theory performed in order to search the optimum conditions of hydrogen sorption by SWNT showed that there are two types of H2 chemisorption sites on the outer and inner NT surfaces.179 Hydrogen is sorbed in the atomic state on the outer and inner SWNT surfaces, and also in the molecular form } inside the SWNT channel.181 The emergence of a new absorption band at 4226 cm71 in the Raman spectrum of NT samples after their electrochemical charging (for gaseous H2, 4161 cm71) assigned to the absorption of molecular hydrogen was considered as proof of the presence of adsorbed hydrogen.The H2 capacity of cylindrical channels of carbon SWNT increases linearly with the NT diameter in accordance with the expression: cH (mass %)514.3(d/d0) [where 14.3 is the amount of H2 (in mass%) adsorbed by (10,10)NT, d0 is the diameter of (10,10)NT]. The maximum H2 storage capacity of carbon NT is limited by the energies of intermolecular repulsion between hydrogen molecules located in the inner SWNT cavity and those between H2 molecules and SWNT walls.Lee et al.181 calculated the sorption of hydrogen by multilayer diameters.181 carbon nanotubes. Hydrogen atoms were found to be preferen- tially adsorbed on the outer NT surface and the sorption capaci- ties ofMWNTwith respect toH2 were found to be independent of their The maximum capacities of } The inelastic neutron scattering spectra of SWNT after hydrogen adsorption on them display a peak at ca. 14.5 meV corresponding to rotational transitions in the H2 molecule. The presence of this peak indicates that physical adsorption of H2 occurs under the experimental conditions; the peak intensity decreases with a temperature increase in the range of 25 ± 40 K.180 143 (5,5)@(10,10)MWNT and (10,10)SWNT are 7.7 mass% and 14.3 mass%H2, respectively.179, 181 V.Conclusion Carbon nanomaterials attract interest as promising materials for the development of hydrogen accumulators. Theoretically, fullerene can be hydrogenated to C60H60 with- out destruction of the fullerene skeleton; this corresponds to 7.7 mass% H2 (0.92 m3 of H2 per 1 kg of C60H60). However, attempts at synthesis and characterisation of the hydrofullerene with this composition have failed so far. Compound C60H60 should be extremely unstable; the excessive strain of this molecule stems from the appreciable deviation of the configuration of bonds between the carbon atoms from the most stable tetrahedral configuration. According to molecular mechanics calculations, the strain in the C60H60 molecule can be decreased if at least ten hydrogen atoms in the C60H60 molecule are bound endohedrally (this is the most stable conformation for the C60H60 composi- tion).182 The methods used in practice for the reduction of fullerenes give C60Hx compounds with 4.5 mass%± 6.0 mass% H2.Their preparation requires high temperatures and H2 pressures or reactive reducing agents (see, e.g., Refs 12 and 13). Analysis of numerous studies on the synthesis, structures and reactivities of hydrofullerenes shows that the practical applications of fullerenes and their metal derivatives as hydrogen sorbents require that the actual sorption capacities are increased, the hydrogenation is accelerated, the dehydrogenation temperature is decreased and the hydrogenation ± dehydrogenation reversibility is ensured.In addition, side reactions should be eliminated.3 Nevertheless, C60Hx and C70Hx hydrofullerenes have already been suggested, for example, as modifying additives for the carbon anodes of reversible Li-batteries.183 Other carbon nanostructures, such as single-walled carbon nanotubes and vapour-grown carbon fibres, as well as their metal- doped modifications, are quite promising as hydrogen-storage media. This is evident from Table 4 which shows generalised data on the hydrogen-sorption capacities of SWNT and VGCF, which in certain cases exceed considerably the values required for mobile hydrogen-storage systems. However, the data on the amount of hydrogen accumulated by carbon NM obtained by different authors differ considerably.The reasons for the discrepancies lie in the absence of reliable methods for the preparation of pure SWNT and VGCF and the absence of generally accepted techni- ques for their certification, for example, with respect to purity, degree of their `openness', nanotube diameter and interlayer distances, and the amount of metal catalysts. In addition, the Table 4. Sorption characteristics of various carbon nanomaterials. T /K Material pH /MPa Ref. 2 Maximum capacity (mass %) 80 90 300 300 293 77$300 a 300 7.18 0.04 10.± 12 0.1 105.± 10 110.1 11 298 ± 773 373 77$300 a 473 ± 673 137 1342, 139 141 1553 146 152 1663 152 152 167 SWNT SWNT SWNT SWNT SWNT SWNT VGCF VGCF VGCF VGCF Li-VGCF K-VGCF VGCF 5.± 10 0.1 0.1 12 473 ± 673 300 8.25 5.± 10 4.2 6.5 ± 7 1.1 ± 5.2 3.5 11.± 67 0.4 10.± 12 2.5 20 14 10 a `Cooling to 77 K$heating to 300 K' process.144 hydrogen-sorbing capacity depends crucially on the pretreatment of carbon nanomaterials and the purity of hydrogen used. There- fore, the results obtained in the studies on hydrogen sorption characterise only a particular material and cannot as yet be used for comparison of the efficiencies of different types of carbon NM.The mechanism of the uniquely high hydrogen sorption by carbon nanomaterials is not clear, either. Nevertheless, the existence of high hydrogen capacities in nanocarbon materials is obvious.This allows one to hope that nanocarbon materials (provided that they are accessible) can be used in actual hydrogen-storage systems. The experimental and theoretical materials presented in this review testify to a significant increase in the scope of research on the synthesis and hydrogen-sorbing properties of carbon nano- structures aimed both at the solution of fundamental scientific problems and at the development of mobile hydrogen-storage systems. This is also evident from presentations submitted at various conferences on carbon nanomaterials, hydrides and hydrogen power engineering. This review was financially supported by the Russian Foun- dation for Basic Research (Project Nos 99-03-32647 and 00-03- 32018) and the Russian Scientific and Technical Programme (Project No.99005). References 1. D Yu Gamburg, V P Semenov, N F Dubovkin, L N Smirnova Vodorod. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (2) 147 ± 159 (2001) Metallic nanosystems in catalysis V I Bukhtiyarov,MG Slin'ko Contents I. Introduction II. Physicochemical properties of nanostructures III. Reactivities and catalytic properties of nanostructures IV. The prospects for the application of nanostructures in catalytic processes V. Conclusion Abstract. catalytic in nanosystems metallic of reactivities The The reactivities of metallic nanosystems in catalytic processes are considered. The activities of nanoparticles in catal- processes are considered. The activities of nanoparticles in catal- ysis are due to their unique microstructures, electronic properties ysis are due to their unique microstructures, electronic properties and high specific surfaces of the active centres.The problems of and high specific surfaces of the active centres. The problems of increasing the selectivities of catalytic processes are discussed increasing the selectivities of catalytic processes are discussed using several nanosystems as examples. The mutual effects of using several nanosystems as examples. The mutual effects of components of bimetallic nanoparticles are discussed. The pros- components of bimetallic nanoparticles are discussed. The pros- pects for theoretical and experimental investigations into catalytic pects for theoretical and experimental investigations into catalytic nanosystems and the construction of industrial catalysts based on nanosystems and the construction of industrial catalysts based on them are evaluated. The bibliography includes 207 references them are evaluated.The bibliography includes 207 references. I. Introduction Metallic catalysts supported on various carriers (e.g., SiO2, a- and g-Al2O3, zeolites, magnesium and chromium oxides, carbona- ceous materials, etc.) are widely employed in chemical, petro- chemical and oil-refining industries as well as for purification of gaseous industrial wastes and motor transport exhausts from toxic components. The production of efficient industrial catalysts includes selection of inert carriers of definite porous structures, their coating with particles of active metals of predetermined compositions, sizes and structures and the most efficient unimodal distribution of these particles over the carrier surface.Catalytic reactions occurring on the catalyst's surface consist in adsorption of molecules of the starting compounds, their migrations, chemical conversions of adsorbed substances and subsequent desorption of the products formed. The gas-phase components are dissolved in the near-surface layers of the catalyst thus changing its reactivity. In some cases where a certain degree of coating of the catalyst's surface with the adsorbed molecules is achieved, its spontaneous and adsorption-induced reconstruction takes place. The energies of lateral interactions of the adsorbed species, their mobilities and the energies of binding of the interacting molecules to the catalyst are the crucial factors which V I Bukhtiyarov G K Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, prosp.Akad. Lavrent'eva 5, 630090 Novosibirsk, Russian Federation. Fax (7-383) 234 37 66. Tel. (7-383) 234 17 71. E-mail: vib@catalysis.nsk.su MG Slin'ko State Scientific Centre of the Russian Federation `L Ya Karpov Institute of Physical Chemistry', ul. Vorontsovo Pole 10, 103004 Moscow, Russian Federation. Fax (7-095) 975 24 50. Tel. (7-095) 917 78 70. E-mail: slinko@polymer.chph.ras.ru Received 20 November 2000 Uspekhi Khimii 70 (2) 167 ± 181 (2001); translated by R L Birnova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n02ABEH000637 147 147 150 154 157 determine the rates of the elementary catalytic steps mentioned above.The efficiency of a catalyst depends on how fast and concertedly do the active sites on the catalyst's surface adsorb the reacting species, retain them during the chemical conversions and liberate the reaction products to be recovered for the next catalytic cycle. The maximum reaction rate is reached when the adsorption, reaction and desorption possess the optimum energies of binding of molecules to the catalyst (the Sabatier ± Balandin principle). The rates of individual elementary stages depend differently on the size of the active metal particles; hence, the total rate of the reaction varies with a change in the particle size. The knowledge of the structure and properties of the active sites and their distribu- tion over the catalyst's surface is of primary importance for establishing a correlation between the structure and properties of the catalyst's surface and the particle size.It is also desirable to establish the number of adsorbed reacting species and metal atoms involved in an elementary catalytic act. II. Physicochemical properties of nanostructures The intermediate position occupied by nanoparticles between the metals in bulk and individual atoms predetermines the deviation of their physicochemical properties from those of bulk metals, on the one hand, and those of isolated atoms, on the other hand. Such characteristics of nanoparticles with the sizes ranging from several tens to units of nanometres are affected as the electronic, mag- netic, optical properties and many others.The changes in the surface structures and electronic properties of metallic nano- particles present the greatest interest for specialists in catalysis, because these characteristics determine primarily specific inter- actions of the reactants with the surface of the active component, the nature and reactivities of the adsorbed species and eventually the activities and selectivities of the nanosystems in heterogeneous catalysis. 1. The structure and geometry of the surface The atoms inside bulk crystals have optimum chemical bonds with all adjacent atoms. If the number of metal atoms in the particle (N) is less than 13, it has a close packing in which each atom represents a surface atom. For N513, two kinds of atoms exist, viz., the inner and surface ones.The proportion of surface atoms in a cluster { depends on the total number of atoms. { Many investigators believe that in contrast to particles, clusters comprise a strictly definite number of atoms and thus represent nearly monodisperse structures with sizes less than 10 nm.148 55 76 13 92 Number of atoms in a cluster Proportion of surface atoms (%) 561 45 349 52 147 63 These data refer to clusters with `shell' structures which are characterised by close packing of their metal atoms (see below). For N=13, such clusters have only one inner layer (consisting of only one atom, whereas for N=55 and N=147, these have two and three inner layers of atoms, respectively. Then, the particle size increases from 0.8 to 2 nm.In particles with sizes of 2 ± 10 nm, the numbers of surface and inner atoms are comparable, whereas in particles with sizes>30 nm the fraction of surface atoms is less than 2%. Hence, the proportion of surface atoms decreases with an increase in the particle size, their coordination properties with respect to the reactants change too. Small particles form many different faces; their number and parameters depend on the method used for their preparation. The properties of the particles approach those of bulk crystals with an increase in the particle size.The surface structures of metallic particles are studied by theoretical and experimental methods.1± 4 The theoretical approach is based on the analysis of the equilibrium form of crystals 1 determined by the minimum value of Gibbs' free energy.The total energy depends on the thermodynamic properties of the bulk phases, the contribution of the planar surface and the plane intersection energy, which corresponds to the contribution of curved or rough surfaces. Obviously, the surface structures of nanoparticles are determined by the ratio of contributions of planar and rough surfaces, which in turn depends on the particle size. For large particles, the contribution of rough surfaces is insignificant and the equilibrium form of a crystal is determined by the minimum surface energy. The contribution of the curved surface increases with a decrease in the particle size. Hence, the surface of nanoparticles contains predominantly planar, planar and rough or predominantly rough areas depending on their sizes.Experimental studies of surface structures are complicated, since the methods employed should ensure resolution at the atomic level. Transmission electron microscopy (TEM) is one such method. Studies of nanoparticles using TEM corroborated the presence of planar and rough surface areas and made it possible to follow their alterations with a change in the nano- particle size.3, 4 Thus a combination of TEM with computer simulation of surface structures showed that the surface of platinum particles (*8 nm) consists of (111) faces (*37%), (100) faces (28%) and rough areas (35%).4 It is noteworthy that the rough areas contain faces with higher Miller's indices, e.g., (001), (113), (012) and (133) faces.The results of computer simulation studies of rough areas and of platinum particles as a whole are presented in Fig. 1. In recent years, scanning tunnelling microscopy (STM), which ensures resolution at the atomic level, has been widely used for the study of surface structures of metallic nanoparticles.5±9 It was found that surface structures of small (1 ± 2 nm) metallic particles are determined by their shapes which in turn strongly depend on the nature of the support. At the same time, such low coordinated sites as edges and kinks strongly affect the interaction of a surface with adsorbed species. The use of STM for the analysis of surface structures of metallic nanoparticles seems promising, since in contrast to TEM this method allows in situ analysis of surface structures, i.e., in the course of a catalytic reaction.An alternative procedure is based on the adsorption of molecules the spectral characteristics of which are sensitive to the structure of the adsorption centre. Successful application of this method entails the choice of a suitable molecular probe and measurements of its spectral characteristics in the adsorption on single crystals with different surface structures. Like STM, this method permits one to obtain information in situ, which is extremely important in studies of catalytic reactions, especially those where the molecular probe is one of reactants.NMRand IR spectroscopic studies of CO adsorption on platinum and palla- dium clusters supported on SiO2 is an example.10 ± 12 It was shown V I Bukhtiyarov, MG Slin'ko (001± ) (111± ) (111± ) 110 (010) (100) 320 (111) (11±1) (001) Figure 1.The types of faces on the surface of a platinum particle (8 nm) and a structural model of a site marked by a dashed line.4 The light and dark circles designate the atoms localised at different levels. that part of the 13CO is adsorbed on platinum catalysts in a linear rather than a bridge-like form, the proportion of the linear fraction being increased with an increase in the degree of platinum dispersion from 16%± 19% to 56%.10 This effect was attributed to the fact that the surfaces of smaller platinum particles contain more low-coordinated sites.Asimilar conclusion was made by Xu et al.11 who showed that the major part of the CO is adsorbed on small palladium particles in a linear rather than in a bridge-like form. This points to a higher concentration of defect/edge sites in small nanoparticles in comparison with large ones. An analysis of spectra of theO1s core level recorded after low- temperature adsorption of O2 was proposed by us for investiga- tion of surface structures of supported silver. This approach is based on the previously established regularity, viz., the greater the number of defects on the silver surface the lower the minimum temperature needed for dissociation of oxygen molecules.13 ± 15 In other words, the amount of atomic oxygen after low-temperature adsorption correlates with surface defects of the supported silver particles.Studies of low-temperature (T=140 K) adsorption of O2 on silver particles deposited onto a graphite surface revealed that the form (atomic or molecular) of the chemisorbed oxygen depends on the sizes of the silver particles. The molecular form is absent in the case of small (< 10 nm) particles which suggests a high degree of defectiveness of their surface structures, while this is present with an increase in the particle sizes to 20 ± 40 nm. The latter circumstance points to the formation of (111) and (110) low- index faces. It is worth noting that the dimension-dependent effect is manifested in the epoxidation of ethylene on silver catalysts in the range of particle size from 40 to 50 nm.These data altogether suggest that the surfaces of small clusters do not exclusively contain low-index faces, such as (111), (110) or (100) faces. The greater part of the surface, which increases with a decrease in the nanoparticle size, is occupied by curved areas. This circumstance is of crucial importance for the catalysis by nanoparticles, since the adsorption properties of rough surface defects (e.g., angles, steps, edges, etc.) differ from those of flat surfaces. Therefore, simple extrapolation of exper- imental data obtained for single crystals to real catalytic systems is impossible. In this connection, studies of reactivities of metallic nanoparticles become especially urgent.2. Electronic properties As the number of atoms in nanoparticles is limited, their electronic structures occupy an intermediate position between the discrete energy levels of free atoms and continuous energy bands of the bulk metal. The dependence of electronic structures of metallic particles on their sizes was studied in particular by Kubo,16 who suggested that the spectrum of the energy levels of nanometre particles should resemble that of a large molecule rather than that of a bulky solid. The electronic levels of atoms in a large metallic particle are close to one another and eventually overlap to form energy bands. The expression:Metallic nanosystems in catalysis (1) h à EF N , where EF is the Fermi level energy andNis the number of atoms in a metallic particle, was proposed to evaluate the distance between the neighbouring energy levels (h).Estimation of the maximum size of a particle where changes in its electronic properties can occur as calculated by this simple formula gives the value of *2 nm. Presumably, this is the threshold size where significant changes in the electronic properties of supported metallic nano- particles are observed. This hypothesis was confirmed by exper- imental data. It was found that the properties of nanoparticles change so drastically in the size range of 2 ± 5 nm that a `metal ± non-metal' transition becomes very likely. This conclusion is based on the increase in the binding energy of electrons (Eb) at core levels 16 [the binding energy was measured by X-ray photo- electron spectroscopy (XPS)] with a decrease in the sizes of metallic nanoparticles.In the general case, the changes in DEb are due to the contribution of the following terms: (2) DEb=DE+DER+DEF , where DE is the `true chemical shift' equal to the difference between the energies of different initial states of the element under study,{ DER is the change in the relaxation energy and DEF is the difference between the Fermi level energies. The last two terms in equation (2) are determined by the effects of the final state as a result of photoelectron emission, viz., the DER value reflects the changes in the relaxation energy upon the screening of photoelectron vacancies caused by redistribution of electrons in the solid, whereas DEF reflects possible changes in the Fermi level of a sample relative to that of the spectrometer.} If the conductivity of the sample is high enough, an electrochemical equilibrium is established between the sample and the spectrom- eter. In this case, the positive charge induced by photoelectron emission is neutralised due to the transition of electrons from the `earth' to the sample and the contribution of DEF to the total change in DEb will be insignificant. If the electronic conductivity of the sample is not high enough to compensate for the charge due to photoemission, the positive charge is accumulated on the surface of the sample.The electrons passing through this pos- itively charged layer lose their kinetic energies, which results in the overestimation of the apparent binding energy.This suggests that the Fermi levels of the sample and the spectrometer are no longer at equilibrium, which results in the charging effect due to accumulation of unknown surface potential. The subdivision into terms proved to be useful for establishing the reasons for the change in DEb with a decrease in the metallic nanoparticle sizes. It was suggested (see, e.g., Refs 17 and 18) that it is the relaxation effect that underlies the positive shift of the binding energy in small metallic nanoparticles (2 ± 3 nm) by 0.5 ± 1.0 eV in comparison with the binding energy of the bulk metal. However, studies of alloys 19, 20 and those employing isochromatic spectroscopy 21, 22 revealed that the observed shift in the binding energy cannot be attributed to the relaxation effect alone.It was concluded that the changes in the initial state of atoms in metal clusters with particle sizes of less than 2 ± 3 nm are due to the `metal ± non-metal' transition. This conclusion is of fundamental importance, since the changes in the initial states of atoms in small clusters should influence (and they do influence, as will be shown below) their reactivities with respect to the gases to { The XPS method owes its second name which is widely used by researchers viz., `electron spectroscopy for chemical analysis' (ESCA), to the term DE. } `The Fermi level of spectrometer' is a popular term used to define the Fermi level of the material (mostly, gold) of which the energy analyser of the spectrometer is made.149 be adsorbed. It is evident that the catalytic properties of clusters cannot remain unchanged under these conditions. In contrast to the initial-state and relaxation effects, the contribution of the charging effect to total changes in DEb was not taken into consideration by the majority of authors who dealt with studies of electronic properties of metal clusters. Such an approach is justified with respect to homogeneous insulator specimens where the magnitude of the charging potential is identical for all inner levels and can be easily taken into consid- eration in the analysis of XPS spectra by the internal standard method.The situation is more complicated in heterogeneous systems where different phases are characterised by different conductivities. In this case, the surface charges are different in individual phases of the specimen, which generates the so-called differential charging of the more conducting phase in comparison with the less conducting one. Apparently, this situation is quite realistic in the case of heterogeneous catalysts which contain metallic nanoparticles supported onto the surface of dielectric substrates (Al2O3, SiO2, etc.). Indeed, it was demonstrated 23, 24 that metallic crystallites with the sizes ranging from five to several tens of nanometres manifest the properties of conducting metals by providing more efficient compensation of the surface charge in comparison with the surface of insulating substrates.As a result, the charge potential on metallic particles is lower than that of the dielectric substrate, and the corresponding lines of the substrate in the XPS spectra manifest a greater shift in the binding energy (DEb) than those of the supported metal. Then, the shift of the spectra by a value equal to the charge of the substrate, the lines of which are generally used as internal standards in calibration of XPS spectra, will produce an apparent negative shift in the lines of the corresponding spectra of the metals. The latter effect is manifested exclusively upon photoemis- sion; however, it can be of tremendous importance for character- isation of reactivities of various particles including silver particles.The hypothetical role of the conductivity electrons in the for- mation of chemical bonds between the metal surface and chem- isorbed molecules which was put forward as early as 1937,25 was corroborated later by quantum-chemical calculations.26 The role of the conductivity electrons in the formation of chemical bonds was confirmed experimentally by the presence of the Knight shift in the 13C NMR spectra of carbon oxide adsorbed on supported metallic particles.27, 28 The effects of differential charging can be taken into account with the help of those parameters of XPS spectra which do not depend on the surface charge. One of them is the modified Auger parameter a, which represents the sum of the binding energies of the most intense line in the XPS spectrum of the element under study and the kinetic energy of the corresponding Auger peak.29 Since the charge potential increases the binding energy of the core level and decreases the corresponding kinetic energy of the Auger peak by the same value, their sum is not changed with changes in the charge.With the use of the Auger parameter we have demonstrated that it is the differential charging of supported nickel and silver particles that is responsible for the changes in the binding energies of their core levels (2p3/2 and 3d5/2, respectively) in catalysts (both fresh and treated with the reaction mixture) for butadiene hydrogenation (Ni/C) and ethylene epoxidation (Ag/ Al2O3).30 ± 32 Treatment of nickel particles supported onto carbon filaments with the reaction mixture causes a shift in the overall Ni spectrum (2p3/2) by 1.1 eV towards higher values of Eb with a simultaneous decrease in the Ni :C ratio.30 The constancy of the parameter a and the absence of a shake-up satellite in the Ni 2p3/2 spectrum indicate that both fresh and treated samples contain metallic nickel, whereas the shift in the Ni spectrum 2p3/2 is due to the differential charging.Studies of Ag/Al2O3 catalysts where the sizes of silver particles vary from 10 to 100 nm revealed a negative shift in the Ag 3d5/2 spectra by 0.3 ± 0.4 eV for particles with the sizes above 50 nm.31, 32 In this case, the parameter a was constant for all150 specimens and equal to 726.2 eV.Comparison of this parameter with the known values of a for metallic silver (726.0 ± 726.3 eV) and silver oxides (724.0 ± 724.5 eV) suggests that the samples under study contain particles of metallic silver. This conclusion was confirmed by the results of an X-ray phase analysis and scanning tunnelling microscopy, which have demonstrated the presence of the a-Al2O3 phase and metallic silver in the speci- mens.33 The observed change in Eb(Ag 3d5/2) is determined by the effect of a differential charging of the supported silver, which in turn is due to the inner conductivity of large silver particles and the lack of conductivity of the surface of the non-conducting sub- strate.31 This conclusion is in good agreement with the results of NMR studies of silver catalysts 34, 35 according to which the Knight shift of the 147Ag signal disappears if the size of silver particles is less than 50 nm.Since the Knight shift is determined by hyperfine interactions of silver nuclei with the conductivity electrons, it is clear that the size of several tens of nanometres is critical with respect to the conductivities of supported silver particles. The great importance of such changes in the properties of silver conductivity electrons for catalysis is illustrated in Fig. 2. As can be seen, the rate of ethylene epoxidation decreases more than 10-fold with a decrease in the sizes of silver particles below 50 nm. It will be shown below that this correlation depends on the form of oxygen adsorbed on silver.Eb(Ag 3d5/2) /eV 367.8 6 1 368.0 4 2 2 368.2 0 d /nm 40 80 60 20 Figure 2. The dependence of the rate of ethylene epoxidation (1) and the binding energy Eb(Ag 3d5/2) (2) on the average size of silver particles in non-promoted Ag/Al2O3 catalysts. A correlation was also found 36 ± 38 between the deviation of electronic properties of supported nanoparticles from those of bulk metals and the catalytic activities of nanoparticles. III. Reactivities and catalytic properties of nanostructures The existence of size effects in electronic and structural properties of nanostructures implies that their reactivities with respect to gaseous molecules also differ from those of bulk metals, this difference being more pronounced in smaller particles.The first evidence for such dependence was obtained in catalytic studies which demonstrated that the rates of numerous reactions cata- lysed by metallic nanoparticles vary in the same range 39 as their electronic and structural properties.10 ± 12, 19 ± 21, 40 ± 42 Catalytic reactions usually occur on the catalyst's surface. Therefore, the activities of catalysts are conventionally referred to unit surface area. This quantity, which is termed `specific catalytic activity' (SCA), is determined by the chemical composition, sizes of crystallites, structures of surface faces, structural irregularities and miscellaneous defects. Therefore, the knowledge of changes in the catalytic properties of the catalyst's surface caused by changes in both the sizes of crystallites and surface structures of their faces 10717 r /(C2H4 molecule) m72 s71 V I Bukhtiyarov, MG Slin'ko is of fundamental importance for the theory and practice of catalysis.Although the structures of nanocrystals can differ from those of macroscopic crystals due to exposure of other types of faces on their surface,43 we thought it appropriate to analyse, at least briefly, the results of investigations of applied single crystals with different surface orientations as the subjects. This is espe- cially important, because the results of such studies are very often used as references for illustrating the dependence of reactivities of metallic nanoparticles on their sizes, as will be shown below.1. The reactivities and catalytic properties of single crystal faces The properties of atoms in the crystal lattices of metals are determined by their specific atomic environment, i.e., the number of adjacent and more remote neighbours, and the sizes of the corresponding coordination zones. Therefore, the reactivity of a surface depends essentially on the localisation of atoms on one or another crystallographic face and the parameters of their atomic environment. A correlation between the orientation of the crystal faces and their catalytic activities was established by A A Balan- din as early as 1929.44 In the past years, the number of inves- tigations of this kind has increased manifold. The rates of carbon monoxide oxidation on (111), (100), (410) and (210) faces of the single crystal Pt0.25Rh0.75 at different temperatures, total pressure of 261077 mbar and the CO:O2 ratio of 2 : 1 differ markedly.45 On heating, the reaction rates on different planes increase rapidly but differently in the range of 400 ± 500 K and then decrease slowly after reaching a certain maximum.The higher the strength of lateral interactions of the CO molecules, the higher the temperature at which the maximum rate is achieved. The oxidation of CO on the faces of Pt, Pd, Ir, Rh and Ru crystals proceeds in a similar way. The catalytic activity of the (111) face of iron in the synthesis of ammonia from nitrogen and hydrogen is about 430 times higher than that of the closely packed (110) face and 13 times higher than that of the (100) face.46 It is also known that aromatisation and cyclisation of hydrocarbons occur on the (111) face of platinum, whereas the (100) face is responsible for their isomerisation and hydrogenolysis. The presence of curved areas, e.g., (10 8 7) favours the formation of undesired products, such as propane and ethane.The atoms in the surface layer can be at equilibrium with the atoms in the second layer. Since the bonds of the surface atoms are not compensated, their equilibrium positions can differ from their positions in the bulk. Such `surface reconstruction' was observed in the CO oxidation catalysed by platinum and some other metals. For example, the reaction occurring on the (110) face of platinum single crystals is accompanied by spontaneous reconstruction of the surface from the (161) state into the (162) state.Surface rearrangements are due to interactions between the platinum atoms, CO molecules and oxygen atoms.47 ± 49 The higher reac- tivity of the (110) face of platinum in comparison with other faces can be the reason for autooscillations of the reaction rate.48 ± 54 On this face, spatial-temporal self-organisation in the form of pro- gressive spiral and stationary reaction waves and chaotic regimes were observed in the nano- and micrometre ranges. On the other hand, the oxidation of carbon monoxide by oxygen on the (111) face of platinum at low partial pressures of CO and O2 and at 300 ± 700 K was unaccompanied by changes in the surface struc- ture and, correspondingly, by autooscillations of the reaction rate. The activation energy of a catalytic reaction depends on the coverage of the catalytic surface with oxygen and carbon mon- oxide.47 The adsorbed oxygen atoms increase the reaction rate, while adjacent CO molecules decrease it. The distribution of atoms and molecules adsorbed on the surface of a catalytic species influences substantially the dependence of the rates of elementary stages of the catalytic reaction on the degree of coverage of the catalytic surface.Spatial-temporal self-organisation was also observed in a nanometre range in the interaction of NO and H2 on the (110) face of rhodium single crystals at partial pressures of nitrogenMetallic nanosystems in catalysis oxide and hydrogen of 1076 ±1074 mbar and at temperatures on the catalyst's surface of 300 ± 800 K.55 ± 58 Phase transitions of the `order ± disorder' type and various superstructures of adsorbed hydrogen atoms were found. The nitrogen atoms initiated the rearrangement of the original surface resulting in the appearance of the (361) ±N superstructure recorded by low energy electron diffraction.This was transformed into the (261) ±N superstruc- ture with an increase in the degree of coverage of the catalyst's surface with the adsorbed nitrogen.59 Lateral interactions of adsorbed particles and their mobilities exert a crucial effect on the formation of structures on a nanometre range. The NO+H2 system on Rh(110) was also characterised by anisotropy of the near-surface layer and the presence of unusual reaction autowaves of different shapes.Non-linear phenomena, e.g., spatial-temporal self-organisation in nanosized particles, significantly influence both the spatial structure of the reaction autowaves at the meso level and the rate of the catalytic reaction. The effects of the structural factor on SCA can also be explained by assuming that the surface of small metallic clusters contains many more low-coordinated atoms than that of bulk metals.60 Indeed, the proportion of surface atoms, particularly those localised in the apices as well as on the boundaries and faces and characterised by lower coordination numbers than atoms localised inside the crystals, increases essentially in crystallites the sizes of which exceed 5 nm.If this is true, SCA will depend on the changes in the crystallite sizes in the range of not more than 1 ± 5 nm, whereas the catalytic activities of larger crystallites will be determined by the structures of the faces exposed on the surface of the nanoparticles. 2. The reactivities and catalytic activities of nanostructures The first attempts to establish a correlation between the rate of a catalytic reaction and the particle size of the active component were undertaken by Kobozev in the late 1930's.61 These inves- tigations were continued in the early 1950's by Boreskov et. al.62, 63 who have formulated a rule of relative stability of SCA of catalysts having identical chemical compositions.A similar conclusion was made later by Boudart et al.64, 65 who have named this kind of reaction `structure-insensitive reactions'. However, the rates of many reactions do depend on the degree of dispersion of the active component, and these reactions have been named `structure- sensitive reactions'. Further studies of the dependence of catalytic activities of metals on the particle size have revealed 66 ± 76 that all metal-catalysed reactions can be classified into four main groups, viz.,(a) SCA depends only weakly on the size of the supported metal particles (structure-insensitive reactions); (b) SCA drops with a decrease in the particle size (the so-called negative size effect); (c) SCA increases with a decrease in the particle size (the positive size effect); (d) maximum SCA is characteristic of particles of intermediate sizes. Seemingly, in the case of structural insensitivity or a positive dimensional effect the sizes of nanoparticles of the active compo- nent in certain systems can be minimised, thus affecting favour- ably the economical parameters of the industrial process.Unfortunately, this assumption was refuted by some exam- ples.77, 78 It was shown 77 that the rate of even such structure- insensitive reactions as the hydrogenation of hydrocarbons decreases by more than two orders of magnitude in those cases where iridium nanoclusters (d&2 ± 3 nm) supported on MgO were used instead of conventional Ir/MgO catalysts, whereas the tenfold increase in the activity of the Irn/g-Al2O3 catalyst in the hydrogenation of toluene is explained 78 by the formation of iridium agglomerates during the annealing of fresh samples in hydrogen at 573 K.78 Obviously, such drastic differences in the behaviour of SCA as a function of the particle size are due to changes in the reactivities of the particles with respect to the participants of the catalytic 151 reaction. This accounts for a great number of publications devoted to the analysis of reactivities of nanoparticles.However, real specimens used in catalytic studies appeared to be unsuitable subjects for establishing an exact correlation between the changes in the catalytic properties and reactivities. Progress in this field could be achieved, if model systems were used.These were prepared by covering flat samples of a substrate with metal particles using physical (e.g., deposition of bulk metals in high vacuum) or chemical methods (e.g., introduction of chemical compounds of a metal followed by reduction and/or calcination). For the first time, the reactivities of metallic clusters were studied in detail using the oxidation of CO on platinum metals as an example.79 ± 82 This choice has been made owing to the large body of experimental material obtained in precision studies of this reaction on bulk metals.83 ± 86 These results could be used for the analysis of reactivities of nanostructures supported on metals. Thus the earlier established mechanism of this reaction carried out on the (111) face of palladium 83 was used to explain the peculiarities of CO oxidation on palladium clusters supported on a-Al2O3.79, 80 The threefold increase in the rate of CO2 formation at temperatures>500 Kupon a decrease in the sizes of palladium clusters from 4.9 to 1.5 nm was attributed to the increased coefficient of CO adhesion to edge and corner palladium atoms the number of which increases with a decrease in the particle size.79 This explanation was corroborated by experiments 87 in which the reactivities of palladium clusters with respect to CO were studied by temperature-programmed desorption (TPD).It was shown that the TPD spectra of CO molecules adsorbed on palladium with the particle size of 27 nm and on the (111) face of palladium single crystals coincide, whereas the corresponding spectra of desorption of CO with a particle size of 2.5 nm display an additional peak at lower temperatures.Similar TPD spectra were recorded by following the CO desorption from palladium with a particle size of<5 nm supported on mica. This suggests that the formation of weakly bound forms of carbon monoxide is charac- teristic of small palladium particles and does not depend on the nature of the substrate.88 Presumably, it is the appearance of a weakly bound adsorbed form of CO that is responsible for the increased rate of CO2 formation with a decrease in the sizes of palladium nanoparticles.79, 80 Quite opposite regularities were observed by following the temperature-programmed desorption of CO molecules from plat- inum particles supported on Al2O3 (Ref.89) and mica (Ref. 90), viz., the intensity of the low-temperature peak of the CO desorp- tion decreased with a decrease in the size of the platinum particles (>4 nm). It was thus concluded that CO molecules are more strongly adsorbed on small particles, which can account for the decrease in the rate of CO oxidation on small (< 2.5 nm) platinum particles.81 Comparison of CO adsorption on supported platinum and rhodium particles revealed that the ratio of low- and high-temper- ature components in the TPD spectra of rhodium particles (in contrast to the corresponding parameters in the TPD spectra of platinum particles), only weakly depends on the size of the metal particles.91 Presumably, this peculiarity of CO adsorption on rhodium is the reason for the structure-insensitivity of the CO oxidation on Rh-containing catalysts.82 These data suggest that a correlation exists between catalytic activity in the CO oxidation on various platinum metals and the strength of the CO± metal bond despite significant differences in the manifestations of the size effect.The fact that the structure- insensitivity of the CO oxidation on rhodium, its negative sensi- tivity on palladium and its positive sensitivity on platinum could be explained on the basis of `single crystal data' gave impetus to the studies of the behaviour of nanoparticles in other catalytic systems.82, 92, 93 For example, the results of TPD studies of NO molecules 92 provided an explanation for the 45-fold decrease in the rate of the CO+NO reaction with a decrease in the size of rhodium particles on Rh/a-Al2O3 from *70 to 1 nm (see Ref.82). According to these data, the major part of the nitrogen152 is desorbed from large particles at 463 K and only a small part is desorbed at 553 K. The amount of nitrogen adsorbed at high temperatures increases with decrease in the particle size. Stronger adsorption of nitrogen on the surface of small rhodium particles can be responsible for the structure-sensitivity of the CO+NO reaction on rhodium.82 It is of note that the nature of the support (a-Al2O3, y-Al2O3, SiO2) has practically no effect on the activity of rhodium particles in this reaction.Yet another example is the CH4+O2 reaction and O2 adsorption on Pt/Al2O3 catalysts with average sizes of platinum particles of<2 and 20 nm.94 It was found that the catalysts with larger particle sizes are ninefold more active in the methane oxidation, these are characterised by lower heats of oxygen adsorption (250 kJ mol71 instead of 280 kJ mol71 for smaller particles) and the oxygen adsorbed on them is more reactive with respect to hydrogen. The latter assumption is based on the fact that the oxygen adsorbed on a platinum catalyst with a particle size of 20 nm is totally desorbed by hydrogen at 195 K, whereas in the case of smaller platinum particles it does not react with hydrogen at temperatures up to 276 K.These findings point to a weakening of the bond between the platinum surface and the adsorbed oxygen with an increase in the particle size and a concomitant increase in the reaction rate.94 Studies aimed at establishing a relationship between the changes in the reactivities of nanoparticles and their electronic and structural properties are of fundamental importance. The discovery of a dependence of the heat of desorption ofCO on the crystallographic orientation of platinum faces 95 ± 97 aroused debates concerning the effect of the structural factor on the heat of CO adsorption with changes in the particle size.94 It is known that the spectra of the temperature-programmed desorp- tion of carbon monoxide from the closely packed (111) face of platinum have the main peak at 400 K,95 whereas its desorption from the more open (110) face of platinum or from platinum foil is accompanied by the appearance of an additional peak at 510 K, the relative intensity of the high-temperature peak being increased with an increase in the densities of the monoatomic steps.96, 97 Comparison of spectra of temperature-programmed desorp- tion of CO molecules from platinum single crystals and Pt/Al2O3 particles revealed that the weakening of the CO7Pt bond on large particles is due to predominant localisation on their surface of terraces the structure of which is identical with that of the (111) face, whereas small particles contain a large number of coordina- tively unsaturated sites, which favours strengthening of this bond.89 The presence of such low-coordinated sites was related to possible dissociation of the CO molecules on small platinum particles.87 Their structure was simulated as a (100) step separat- ing the (111) terraces.It was suggested that the repulsion of carbon atoms adsorbed on such sites from the CO molecules adsorbed on the (111) face of platinum weakens the CO7metal bond.89 The expediency of the structural approach is also confirmed by the fact that the shape of TPD spectra of the CO molecules did not depend on the orientation of the faces of Rh single crys- tals,98, 99 which is in complete agreement with the structure- insensitivity of the CO+O2 reaction on rhodium-containing catalysts.The decrease in the heat of adsorption of oxygen and the corresponding increase in its reactivity with an increase in the size of platinum nanoparticles supported on alumina was attrib- uted to the increased number of smooth areas on the catalyst's surface. The electronic approach to rationalise unusual reactivities of metallic clusters was largely developed in studies with the use of methods suitable for the analysis of the nature of adsorbed particles together with the electronic properties of metal atoms.10, 100 ± 103 Studies of CO adsorption on palladium, copper and nickel clusters supported on graphite revealed that a `metal ± dielectric' transition and strengthening of the CO7Mbond with a decrease in the cluster size are observed within the same range of particle sizes.103 It was found that in small particles the decrease in the intensity of the C1s signal which is associated with the V I Bukhtiyarov, MG Slin'ko adsorbed CO (Eb=285.9 eV) occurs at higher temperatures.Moreover, the strengthening of the CO7M bond on nickel particles results in the dissociation of CO as can be evidenced from the appearance, in the TPD spectrum, of a C1s signal with Eb=284.2 eV characteristic of elemental carbon, after heating of the sample to 300 K. It was assumed that the observed increase in the reactivities of small nickel particles with respect to the CO oxidation is accounted for by closeness of the 3d-levels of Ni and the 2p*-orbital of CO. If the sizes of palladium and nickel nanoparticles are less than 3 nm, their metallic properties are weakened, which correlates with an increase in their reactivities towards oxygen 101 and hydrogen sulfide,102 respectively.The conclusion about the strengthening of the O7Pd bond was based on data from micro- calorimetry according to which the heat of adsorption of O2 remains unchanged (Q&210 kJ mol71) for particle sizes ranging from 1000 to 3 nm, but increases drastically (up to 335 kJ mol71) for smaller particles.101 The nature of the support does not influence the heat of adsorption of O2 on palladium particles sprayed over SiO2, Z-Al2O3 and SiO2±Al2O3. The conclusion about enhanced dissociation of H2S on smaller nickel clusters is based on the increase in the relative intensity of the S 2p line (Eb=162.0 eV) characteristic of S27 ions in comparison with the line (Eb=164.5 eV) related to hydrogen sulfide adsorbed as a molecule with a decrease in the particle size.102 One should not rule out the possibility that the proportion of rough surfaces in particles with sizes varying from 3 to 5 nm is greater, since they contain a large number of low-coordinated atoms on the steps, in the edges, etc.Apparently, the electronic and structural factors are closely interrelated and their effects on the reactivities of metallic clusters cannot be differentiated. The correctness of this statement can be illustrated by the results of experiments carried out by Zilm et al.10 Comparison of 13C NMR spectra of adsorbed CO molecules recorded with and without magic angle spinning allowed the authors to suggest that the significant decrease in the diffusion rates of carbon monoxide on the surface of small palladium particles is due to stronger binding of CO to these particles.It was concluded that strongly bound CO with a lowered ability for diffusion is linearly adsorbed on edge and corner palladium atoms the proportion of which increases with a decrease in the particle size. Moreover, comparison of the Knight shift for the linear and bridge-like forms of CO demon- strated that these low-coordinated sites are characterised by electron deficiency resulting from changes in their Fermi levels relative to those of the bulk metal. It is worth noting that the differences in the properties of the conductivity electrons of supported nickel 30 and silver 31, 32 par- ticles, on the one hand, and those of the corresponding bulk metals, on the other hand, also correlate with changes in the surface structures of the particles, which become more defective after their treatment with the reaction medium.In the case of Ni7C catalysts for butadiene hydrogenation, this conclusion was based on TEM data, whereas for silver particles active in ethylene epoxidation it was based on the disappearance, from the XPS spectra, of the O1s line characteristic of the adsorbed oxygen. The variations in the routes of NO decomposition and the CO+NO reaction with a decrease in the size of palladium particles supported on silica was explained by the effect of low- coordinated sites, with lower electron density on the NO adsorp- tion.103 The strengthening of the NO7Pd bond in such low- coordinated sites the existence of which was confirmed by comparison of infrared absorption spectra of CO molecules adsorbed on palladium particles and on (111) and (110) faces of palladium single crystals,104, 105 favours the dissociation of NO into nitrogen and oxygen atoms.In the absence ofNOadsorbed as molecules on the surface of small palladium particles,N2O cannot be formed as was demonstrated using the temperature-pro- grammed reaction (TPR). As mentioned above, the number of papers which attribute the changes in selectivity caused by a decrease in the size of metallicMetallic nanosystems in catalysis nanoparticles to changes in the chemical nature of their surface adsorption in comparison with adsorption on bulk metals will unquestionably be increasing. The above-cited NMR studies of CO adsorption on palladium supported on SiO2 represent a first step in this direction.10 It was shown that two types of CO molecules on the surfaces of small palladium particles differ in the degree of mixing of their electronic levels with the electronic zones of the metal.The correlation between the nature of the adsorbed molecules and the catalytic properties (in the first place, selectivities) of supported nickel and silver particles has been demon- strated.30, 31, 106 The TPD spectra of hydrogen following its adsorption for 2 min on two catalysts of selective hydrogenation of butadiene differing in their Ni :C ratios are depicted in Fig.3. The selectivity of butadiene hydrogenation to butene on a catalyst with a low carbon content (Ni :C ratio of 0.4) was 45%, whereas that on a catalyst with a higher carbon content (Ni :C ratio of 0.1) reached 95%.30 As can be seen from Fig. 3, the more selective catalyst gives one TPD peak at 320 K, whereas the TPD spectrum of the less selective specimen contains a weakly expressed addi- tional peak corresponding to more strongly bound hydrogen (at 360 K). These peaks were assigned based on the TPD data and work function measurements of hydrogen adsorption on speci- mens of bulk nickel (single crystals and foil) obtained in earlier studies.107, 108 In experiments where hydrogen exposure varied, it was established that hydrogen in a high-temperature form is initially adsorbed on the nickel surface and the work function increases markedly.The TPD peak at 360 K was ascribed to the negatively charged hydrogen ions, H7. The intensity of the high- temperature peak becomes constant with an increase in the hydrogen exposure, whereas the 320 K peak continues to increase gradually. The work function did not change in the studied range of coverage of the nickel surface with hydrogen, which points to the absence of a charge on the low-temperature form of hydro- gen.107 High reactivity of hydride ions is a possible reason for non- selective hydrogenation of butadiene on specimens with high Ni :C ratios (*0.4). The lack of this form in specimens with the Ni :C ratio of &0.1 (see Fig. 3) is the reason for increased selectivity.Unfortunately, these conclusions cannot be corrobo- rated by XPS, since hydrogen does not generate a photoelectron spectrum. This approach is quite useful in studies of different forms of oxygen adsorption on silver. The screening of O1s signals from the adsorbed oxygen by the oxygen of the substrate (a-Al2O3) used in industrial catalysts of ethylene epoxidation was eliminated by replacing the latter with a model substrate (graphite).106 The oxygen adsorbed on the surface of silver particles with the sizes of 10 nm produces one peak with Eb=530.5 eV, while larger particles produce an additional peak with Eb=528.5 eV.Com- parison of these values with the Eb values for oxygen adsorbed on bulk silver specimens prompted a conclusion about the formation [H2] (arb. units) 4 2 3 1 21 T/K 600 500 400 300 Figure 3. The TPD spectra of hydrogen after its 2-min adsorption at P=10 Pa and T=270 K on Ni7C catalysts with the Ni :C ratio of 0.1 (1) and 0.4 (2). 153 of two forms of oxygen adsorbed as atoms, viz., the peak with a lower binding energy is related to nucleophilic oxygen (O27), while the second peak is related to the electrophilic form (Od7). It was shown 109, 110 that the formation of ethylene oxide requires the co-existence of these two forms of the adsorbed oxygen on the silver surface: the electrophilic oxygen participates in the forma- tion of ethylene oxide directly by reacting with ethylene, while the nucleophilic oxygen serves as a source of surface Ag+ ions (surface silver oxide), which are the sites of adsorption of ethylene molecules.The key step of the ethylene epoxidation can be presented as follows: Od¡ ads +C2H4 ads (Ag+) C2H4O. With account taken of the sharp drop in the concentration of nucleophilic oxygen with a decrease in the size of the silver particles in the range of 50 ± 20 nm, the mechanism proposed provides an explanation for the sharp decrease in the rate of epoxidation on silver catalysts with a particle size of less than 50 nm.76 The reaction rate decreases as a result of less effective adsorption of ethylene on small particles which contain no Ag+ sites.Since the nucleophilic oxygen is active exclusively in com- plete oxidation of ethylene, its disappearance due to a decrease in the size of the catalyst's nanoparticles will increase the selectivity of ethylene oxide formation. This suggestion has been confirmed experimentally by Verykios et al.111 who have observed a contin- uous increase in the selectivity with a decrease in the size of silver particles from 50 to 20 nm. These data and the results of other investigations 109, 110 suggest that this size range is an optimum for particles of highly reactive and highly selective silver catalysts for ethylene epoxidation. Studies of the dependence of selectivities of chemical reactions on the average sizes of coated nanoparticles of the active compo- nents revealed both positive and negative effects and, in some cases, their independence of the particle size (see, e.g., Refs 112 ± 114).We shall consider only the results of several experiments which are promising for use in practical catalysis. The dependence of selectivities of the synthesis of higher hydrocarbons from CO and H2 on Co7MgO and Co7ZnO catalysts on the sizes of coated cobalt particles was established.115 The smaller the average sizes of cobalt nanoparticles, the higher the selectivity expressed by the Anderson ± Schulz ± Flory distri- bution coefficient (a). It was suggested that the amount of dissolved hydrogen responsible for complete hydrogenation of hydrocarbon intermediates decreases with a decrease in the particle size, eventually resulting in the formation of heavier hydrocarbons.The selectivity of hydrodesulfurisation of thio- phene increased with a decrease in the sizes of ruthenium nano- particles supported on alumina.116 Comparison of Co/Al2O3 catalysts accelerating steam conversion of ethanol and prepared from different Co-containing precursors revealed that the selec- tivity of this reaction decreases in the following order: Co (carbo- nyl)>Co (chloride)>Co (acetate)>Co (nitrate).117 Since the sizes of metal particles increase in the reverse order, the observed decrease in selectivity was attributed to the enlargement of cobalt nanoparticles. And, finally, it was found 118 that the selectivity of n-pentane hydrogenolysis on Pt/Y-zeolite catalysts increases linearly with decrease in the size of the metallic nanoparticles.Even these few examples illustrate the remarkable effects of the average sizes of active metal nanoparticles on the relative rates of individual steps of multiroute catalytic reactions which influ- ences the selectivities of formation of the reaction products. The catalytic activities manifested by nanosystems in reactions in which bulk specimens or even supported large crystallites are virtually inactive corroborate this statement. Gold nanoparticles which, in contrast with the bulk metal, manifest catalytic activity in certain reactions including those promising for industrial use, provide an illustrative example.In the 1980's, Japanese investi- gators succeeded in demonstrating that Au nanoparticles sup- ported on Fe2O3 or Co3O4 represent active catalysts for low- temperature oxidation of CO.119 This finding has stimulated the154 appearance of numerous publications devoted to the analysis of the catalytic properties of these systems. The dependence of their catalytic properties on the nature of the substrate, size of gold particles and further treatment of freshly prepared specimens was studied.120 ± 125 It was shown that the size of gold nanoparticles is the key factor which determines the activity of a catalyst in CO oxidation; this activity changes 2 ± 3-fold with different substrates (TiO2, Al2O3, SiO2), whereas the reaction rate increased by one and even two orders of magnitude with a decrease in the gold particle sizes below 4 nm.120 ± 122 The maximum catalytic activity was observed in particles with a size of 3.2 nm.123 It is of note that gold-containing catalysts manifest high activity even at low temperatures (250 ± 270 K), whereas such a catalytically active metal as platinum is inactive at these temperatures due to the blocking of its surface by adsorbed CO molecules.126 The range of reactions catalysed by gold nanoparticles appeared to be rather broad.It was demonstrated that impreg- nated Au/SiO2 and Au/Al2O3 catalysts manifest high selectivities in the hydrogenation of buta-1,3-diene to butene.127 The selectiv- ity of hydrogenation of diene mixtures into alkenes on Au/ZrO2 catalysts reached 100%.128 Studies into hydrogenation of CO2 on Cu-, Ag- and Au-containing catalysts supported on ZrO2 revealed that these metals are catalytically active in methanol synthesis.129 Gold nanoparticles used as catalysts for low-temperature reduc- tion of NO with propene have certain advantages.130 Unlike Rh-,131 Pt- 132 and Pd-containing 133 catalysts with which N2 is formed instead of N2O at temperatures above 570 K, the for- mation of nitrogen on Au/Al2O3 proceeds efficiently even at 370 K.The use of gold nanoparticles as catalysts of propene epox- idation provides a more interesting example. The gas-phase oxidation of propene by an oxygen ± hydrogen mixture on Au/TiO2 catalysts ensures nearly 100% selectivity of propene oxide formation at a 1% conversion of C3H6.134 A detailed study of various factors revealed that the selectivity of this reaction depends critically both on the nature of the substrate and the size of gold nanoparticles.If the particle size is less than 2 nm, the reaction performed on gold catalysts changes its route in such a way that propane is formed with nearly 100% selectivity.135 This finding is in good agreement with the high activity of the low- percentage Au/SiO2 catalyst in the hydrogenation of alkenes.136 It may thus be concluded that gold nanoparticles with sizes ranging from 2 to 4 nm catalyse propene epoxidation, whereas smaller particles catalyse its hydrogenation. It was found that the selec- tivity of formation of some industrially important products increases considerably with a decrease in the particle size.137, 138 For example, Ptn clusters within the structure of the zeolite LTL provided a nearly 100% selectivity of dehydrogenation of n-hexane to benzene.137 A vast variety of physical instrumental methods, such as molecular beam technique, low energy electron diffraction, elec- tronic Auger spectroscopy, photoelectron X-ray spectroscopy, vacuum tunnelling microscopy, nuclear magnetic resonance, electron spin resonance, Raman spectroscopy, calorimetry, TPR, TPD, infrared spectroscopy, EXAFS, MoÈ ssbauer spectroscopy, etc., came into use in the last decades of the XXth century for studying surfaces of heterogeneous catalysts at the molecular level.However, despite the high precision achieved in the techni- ques for measuring and the control of the state of the catalytic surfaces, the `cognitive' potential of the experimental results is rather limited because these measurements are usually indirect and the information derived from them demands quantitative processing and interpretation. This can be done only with the use of mathematical models. It is clear that successful interpretation of the results obtained requires that the mathematical models and the methods employed for their analysis provide an accurate description of the whole body of experimental results.139 ± 146 The lattice gas model is an example. The main difference between ordinary and lattice gases is that in the former case a particle can have any coordinates, whereas molecules of lattice V I Bukhtiyarov, MG Slin'ko gases occupy definite positions in space, viz., those in the knots of a certain regular lattice where elementary processes can occur.These processes are described either by computational methods conventionally used in statistical physics (e.g., the Monte-Carlo and cellular automaton methods) or by using cluster approxima- tions, which allow simple estimation of thermodynamic parame- ters of a system and the rates of elementary surface acts based on the classical transition state theory developed byM I Temkin for heterogeneous catalysis. Sequential implementation of a series of elementary steps results in changes in the state of the lattice cells and the evolution of the system.The Monte-Carlo method considers evolution as a random Markovian chain of elementary events occurring in the lattice cells. The cellular automaton approach also represents an effective tool for simulating the dynamics of elaborate spatially distributed systems based on the analysis of their local behaviour. To obvious disadvantages of stochastic methods, one can relate the large body of calculations, whereas their main advantage is the possibility of precise control over the position of a particle on the surface of a catalyst. The deterministic approach entails the solution of systems of nonlinear differential equations derived from the main kinetic equation using certain approximations.The energy of lateral interactions of the adsorbed species involved in reactions and their mobilities are the main parameters which characterise their surface layer. The latter represents an open, non-ideal, non-linear system which exchanges mass and energy with the environment. The use of the non-linear dynamics method made it possible to gain considerable experience in mathematical simulation of catalytic systems starting at the molecular level.143 ± 146 Non-linear events, such as self-organisa- tion, the appearance of chemical turbulence, etc., have also been studied. However, the attempts to simulate the effects of the particle size on such events have not been undertaken yet. IV. The prospects for the application of nanostructures in catalytic processes 1.Ligand-stabilised transition metal clusters The catalytic properties of supported particles can differ essen- tially from those of bulk metals; therefore, new routes of catalytic processes should not be ruled out. It can thus be assumed that the advantages of nanostructures prepared by conventional methods as highly selective catalysts cannot always be realised due to the broad size distribution of particles and their sintering in the course of the catalyst's operation. In supported metallic catalysts obtained by industrial synthesis, which includes impregnation of substrate granules with solutions of active metal compounds followed by calcination and/or reduction, strong interactions between metallic nanoparticles and the substrate surface are usually absent, which results in the statistical distribution of particles according to their sizes and also facilitates their sintering.There are several ways to solve this problem, namely, mod- ification of the support aimed at generating a sufficient number of centres for nucleation of metallic particles,147 the choice of materials (a substrate and an active component) providing the epitaxial growth of nanoparticles owing to the structural match- ing of parameters of crystal lattices,148 the use of novel substrates with an original geometry of the pore volume,149 etc. The size homogeneity of nanostructures can be achieved through the use of ligand-stabilised metallic clusters.150, 151 Isolated transition metal nanoclusters which are the most commonly used as active compo- nents of supported metal catalysts, are prepared by the following techniques: (1) reduction of transition metal salts; (2) reduction and removal of ligands from organometallic compounds; (3) elec- trochemical synthesis.151 Studies into the kinetics of growth of nanoclusters revealed that clusters containing the so-called magic number of atoms are characterised by regular geometry of the outer surface and manifest enhanced stabilities.The other name of these clusters, viz., `full-shell clusters', reflects the presence of fully packed layers surrounding the central metal atom. The number ofMetallic nanosystems in catalysis the metal atoms (N) in the nth-layer is determined by the equation:151 (3) N=10 n2+2 (n>0).Thus, full-shell clusters contain 13 (1+12); 55 (13+42); 147 (55+92); 309 (147+162), etc., atoms (see Section II.I). The enhanced stabilities of these clusters stem from their closely packed structures which ensure the formation of the maximum number of metal ± metal bonds and, as a consequence, the mini- mum surface energy. An excellent survey of growth mechanisms of nanoclusters has been given in reviews.152 ± 154 It was shown that the formation of iridium nanoclusters stabilised by the P2W15Nb3O97 62 polyoxoan- ion follows an autocatalytic surface growth mechanism, which provides an explanation for the formation of isolated full-shell clusters in a kinetically controlled regime of synthesis.Exper- imental evidence for these conclusions can be derived from the formation of nearly monodisperse rhodium nanoclusters (d=40.6 nm) in the reduction of polyoxoanionic Rh(I) com- plexes with hydrogen in acetone.154 The nature of nanoclusters stabilised by ligands entails the possibility of their additional stabilisation by electrostatic and steric interactions. Electrostatic or inorganic stabilisation is achieved by adsorption of ions on the electrophilic surface of a metal.155 Such adsorption results in the appearance, on the surface of nanoclusters, of a double electrical layer which provides Coulombic repulsion of like-charged individual species. Steric or organic stabilisation is provided by the surrounding of the metal centre of the nanocluster with a layer of bulky structures,155 such as polymeric units 156 or surfactants.157 The geometric sizes of these adsorbates favour the formation of a steric barrier which prevents the contacts between the metal centres of nanoclusters.It is evident that some of the stabilising agents, e.g., polyoxoanions, combine electrostatic and steric functions.151, 158 Stabilisation of nanoclusters with ligands presents substantial interest as regards the synthesis of supported catalysts characterised by enhanced stabilities and homogeneous size distribution of metal particles. In addition, this makes the use of metallic nanoclusters promising for homogeneous catalysis, as will be shown below. The catalytic properties of metallic nanoclusters have been studied by several groups of investigators (for a review see Ref.151). In 1981, Schmid et al. 159 succeeded in synthesising a cluster with the composition Au55(PPh3)12Cl6 by passing B2H6 through a solution of PPh3AuCl in benzene. Later, Rh55(PPh3)12Cl6, Ru55(PBut3)12Cl20, Pt55(AsBut3)12Cl20 clusters were synthesised using diborane as the reductant.160, 161 Substitu- tion of hydrogen for diborane made possible the synthesis of and Pt*561phen*36O*190 ± 200 Pt*309phen*36O*3010 clus- ters.162, 163 As can be seen, all these clusters are `magic' ones, which provides additional evidence in favour of enhanced stabil- ities of the full-shell nanostructures cited above. First attempts at using Ru clusters as catalysts in homogeneous hydrogenation reactions were unsuccessful because of their rapid decomposi- tion.163 Therefore, further efforts of this research group were directed to the use of nanoclusters for the synthesis of supported heterogeneous catalysts.It was shown that palladium clusters with sizes of 3 ± 4 nm stabilised by ligands manifest high activities and selectivities in the hydrogenation of hex-2-yne into cis-hex-2-ene. It was shown that the activity of a catalytic system depends on the nature of the stabilising ligands, while the selectivity is always equal to 100%. Rhodium and palladium nanoclusters supported on TiO2 and Al2O3 appeared to be highly efficient catalysts of hydroformylation 164 and hydrogenation 165 reactions.Unlike the authors of the above-cited papers, in the early 1980's Vargaftik, Moiseev et al.166 carried out a successful syn- thesis of ligand-stabilised clusters which are active and stable in homogeneous catalysis. The Pd*561phen*60(OAc)*180 and Pd*561phen*60O*60(PF6)*60 clusters catalysed oxidative acetox- ylation of a series of light organic compounds, e.g., of ethylene into vinyl acetate, of propene into allyl acetate and of toluene into 155 benzyl acetate, with 95%± 98% selectivity. Moreover, these catalysts effected the oxidation of primary alcohols into ethers and aldehydes and of secondary alcohols into ketones.166 Yet another example of a successful application of metallic nano- clusters in homogeneous catalysis is the use of rhodium clusters stabilised by polyoxoanions, which manifested high selectivities in the hydrogenation of cyclohexene.154 This was ascribed to the double stabilisation (both electrostatic and steric) of the clusters by polyoxoanions.167 Notwithstanding, the majority of successful applications of metallic nanoclusters as catalysts is connected with the use of systems fixed or immobilised on the surface of porous and non- porous supports, e.g., SiO2, Al2 O3 or TiO2.It was shown, in particular, that 5% colloidal rhodium supported on activated charcoal with a narrow size distribution of clusters (1.2 ± 2.2 nm) was nearly 3 times more active in the hydrogenation of butyroni- trile than the corresponding industrial catalyst which also con- tains 5% Rh on activated charcoal with a greater scatter of particle sizes (1 ± 5 nm).168 In catalysts stabilised by ligands, the size of rhodium particles varies from 1.2 to 2.0 nm, whereas standard catalysts contain predominantly large crystallites, while the fraction of small particles (1 ± 5 nm) is small.A representative list of reactions which are efficiently catalysed by clusters stabi- lised by ligands is given in a review.151 This list includes various reactions of enantioselective hydrogenation,169 hydrosilyla- tion,170 hydropyrolysis and hydrogenolysis,171 CO and CO+H2 oxidation,172 cycloaddition reactions,173 etc. This can be supplemented with examples of enhanced stabil- ities of catalysts based on clusters stabilised by ligands under conditions of a catalytic reaction.Thus the `lifetime' of a catalyst containing palladium nanoclusters in the reaction of cyclooctene hydrogenation was 96 000 turnovers, whereas that of an industrial catalyst (e.g., palladium on activated charcoal) was 38 000 turn- overs.168 The Ir*300 nanoclusters did not change their activities after 32001000 hydrogenation turnovers, which is comparable with the corresponding value (39501000 turnovers) for the industrial catalyst (7.9% Ir/g-Al2O3) and markedly exceeds the `lifetime' of Ir/Z-Al2O3 (`Exxon', USA) equal to 1740250 turn- overs. Ligand-stabilised metallic clusters are extremely interesting subjects for the development of fundamental concepts of catalysis by nanostructures, since it allows one to control precisely the size distribution of nanoparticles, to establish relationships between their sizes and catalytic properties and even to govern the activities and selectivities of catalysts based on them.However, the high cost of such catalysts in comparison with ordinary specimens is an obstacle to their wide industrial application. Only electrochemical synthesis makes it possible to prepare relatively large (e.g., hundreds of milligrams) amounts of catalysts based on metal clusters stabilised by ligands. Therefore, industrial applications of such nanoclusters in large-scale catalytic processes is hardly probable. However, their use in fine organic syntheses where high selectivity of formation of the target product is the main requirement imposed on a catalyst can be of considerable practical interest. It is anticipated that the attainment of 100% selectivity will become one of the main challenges to catalytic science in the next few decades.174 This can be exemplified by successful recent applications of clusters stabilised by ligands in various hydro- genation reactions including enantioselective hydrogena- tion.175, 176 2.Bimetallic clusters: molecular design of active centres Studies into bimetallic systems have the same long history as studies of individual metals. This is primarily due to the search for the ways of increasing the activities, selectivities and stabilities of monometallic catalysts. The first patents which suggested the use of Pd ±Au catalysts for hydrocarbon conversions instead of palladium ones appeared in the 1960's.177 In the past years, the number of investigations (including those having industrial appli- cations) which demonstrate the advantages of bimetallic catalysts156 over monometallic ones has increased manifold.178 ± 184 Most of these studies dealt with the dependences of catalytic activities related to the surface metal atoms and their selectivities on the ratios of metals.For example, the maximum activity of Pt ± Re/ SiO2 catalysts containing 75% rhenium exceeds those of individ- ual platinum and rhenium supported on SiO2 by more than two orders of magnitude.180 Rubidium, palladium or platinum addi- tives increased the activities of nickel-containing zeolites in butyronitrile hydrogenation in both gas and liquid phases.181 The steady-state activity of Pd ± Re/Al2O3 catalysts in the hydro- genolysis of 2,2-dimethylpropane passes through an obvious maximum at a rhenium content of 50 at.%.182 Examples of enhanced selectivity in the formation of various products through multiroute catalytic reactions are even more impressive.The above-cited patent 177 claims that the selectivity of n-heptane isomerisation increases from 66.9% to 81.3% with a change in the Pd :Au ratio of Pd ± Au/Al2O3 catalysts from 6 : 1 to 1 : 2 (mass %). The selectivity of monometallic palladium and gold catalysts is equal to 50% and*25%, respectively. The selectivity of 1% Ru/SiO2, which is a catalyst of cyclohexane dehydrogen- ation to benzene, increases, following introduction of inactive copper and silver (1 mass %), from 76% to 94% and 87.9%, respectively.183 The enhanced selectivity of nitrogen formation and decreased yields of ammonia were observed in the reduction of NO with hydrogen with an increase in the rhenium content in bimetallic Pt ± Re/SiO2 catalysts.184 Modification of platinum nanoparticles on silica with tungsten made it possible to reach 99.2% selectivity of methanol formation in the reaction of CO2 with hydrogen, the conversion being 2.6%.185 It is of note that platinum alone catalyses the formation of CO with nearly 100% selectivity, whereas tungsten is inactive in this reaction.It was shown 181 that all M± Cu/NaY catalysts (M=Ru, Rh, Pd, Pt) manifest high selectivities in the formation of tertiary amines from acetonitrile.The unusual properties of bimetallic catalysts gave impetus to the studies of their surface using model systems and sensitive physical methods. The experimental results have been generalised in the reviews.186, 187 It was found that the introduction of a second metal influences the catalytic properties of the first metal through the following mechanisms: (1) by changing the electronic properties of the first metal atoms (as has been shown previously, the electronic properties of metals influence the nature and reactivities of adsorbed particles); (2) by changing the number of identical atoms within the structure of the adsorption centre (as is known, adsorption of molecules from the gas phase often requires a combination of specifically arranged atoms rather than a single `landing' site).In the first case, we deal with an electronic (ligand-associated) effect, whereas in the second case the multiplet ensemble effect is manifested.178, 186, 187 With regard to real catalysts, this is, most probably, artificial, therefore, the existence of such effects has been demonstrated using only model systems. Direct evidence for the electronic effect was obtained in experiments with adsorption of simple gases on the surface of a metal single crystal covered with a monolayer of atoms of another metal. Concrete systems for such experiments were selected on the basis of structural matching of this monolayer and the plane of a metal substrate.188 ± 191 For example, a palladium monolayer was deposited onto the (110) face of tungsten the hexagonal structure of which ensured a practically complete coincidence of Pd7Pd distances in the monolayer (0.2740 nm) and onto the (111) face of the bulk Pd single crystal (0.2751 nm).188 The activation energy of desorption of CO molecules from the surface of Pd/W(110) appeared to be by 85 kJ mol71 smaller than that from the surface of the Pd(111) single crystal.In addition, the preexponential factor in the equation of CO desorption rate increased by five orders of magnitude. Combined use of both factors caused a decrease in the desorption temperature from 510 K on Pd(111) to 330 K on the monolayer of Pd/W(110) in the case of a low degree of coverage by CO.A similar effect was observed when palladium was deposited V I Bukhtiyarov, MG Slin'ko onto the surface of the Re(0001) hexagonal face.189 An analysis of UV photoelectron spectra of the palladium monolayer on the (110) faces of tantalum 190 or niobium 191 revealed that the densities of filled electronic states of palladium near the Fermi levels in these systems are significantly decreased in comparison with that on Pd(111). As a result, the electronic structure of the surface layer of palladium becomes similar to that of the Group IA metals characterised by weak chemosorption bonds with CO. This can be explained by the formation of covalent bonds between palladium and niobium due to rehybridisation of d- and sp- orbitals, although a small charge transfer can also take place.Alloys with high negative enthalpies of formation are related to the systems in which the electronic effect plays a crucial role.187 Depending on the strength of the interaction between the main and incorporated metals, the ensemble effects are conventionally divided into simple and mixed ones.186, 187 Simple effects are largely manifested in systems where a reactive metal is mixed with an inert one. For example, the multibound state of CO on platinum and nickel is not realised following their fusion with copper.192, 193 The triply bound form of NO which predominates on the (111) face of platinum is not detected on the surface of the (363)-SnPt2/Pt(111) alloy where triatomic platinum multiplets are completely absent.194 Decisive evidence for the existence of a mixed ensemble effect was obtained in the study of a copper monolayer deposited onto the (111) face of nickel single crys- tals.195, 196 Despite the higher strength of binding of the bridge-like form of CO to the nickel surface in comparison with the linear form, the latter was exclusively formed at low coverage of carbon monoxide.This was inferred from the results of high-resolution electron energy loss spectroscopy (HREELS) which point to the absence of Ni7Ni couples on the surface of this bimetallic system. However, a bridge-like form was also detected at higher degrees of CO coating, which is due to the presence of Ni7Cu couples.196 The mutual effects of individual components are characteristic of many bimetallic systems.Therefore, several attempts have been undertaken to use these regularities for the directed synthesis of bimetallic catalysts with predetermined properties. Thus Besen- bacher et al.197 proposed to introduce atoms of gold into a nickel- containing catalyst of the steam conversion of methane in order to reduce the rate of its coking. The choice of gold was based on the results of fundamental investigations of single crystals and porous specimens of the catalyst using physical methods and quantum- chemical calculations of activation energies of the abstraction of the first hydrogen atom from the methane molecule and carbon atom adsorption on pure and modified nickel surfaces.The conditions for gold introduction were also established. This resulted in the synthesis of an Au7Ni bimetallic catalyst, which, in contrast to the nickel catalyst, preserved its activity in n-butane conversion due to the absence of graphite deposits on its surface. The most remarkable achievements in the design of bimetallic catalysts can be expected from the use of nanoclusters which allows molecular design of active centres. There exist several synthetic routes which ensure control over the composition and properties of the nanostructures formed,198 viz., combined or sequential reduction of metal salts, electrochemical synthesis and reduction of double complexes. Studies of synthetic nanoclusters by physical methods aimed at elucidating their structures acquires special importance, since in addition to the crystal structure of the bulk alloy, bimetallic nanoclusters contain unusual structures which have no analogues among bulk materials.These include two types of structures, viz., `core ± shell' and `cluster-in-cluster' types. Core ± shell structures were detected in Pd7Pt clusters stabi- lised by poly(N-vinyl-2-pyrrolidone) (PVP) with a Pd : Pt ratio of 4 : 1; they were prepared by the reduction of a H2PtCl67PdCl2 mixture.199 ± 201 Elucidation of exact structures of these nano- clusters required the use of a combination of methods, e.g., X-ray phase analysis, TEM, XPS, UV-visible absorption spectroscopy (UV-Vis) and EXAFS.Analysis of the UV-Vis spectra and X-ray diffractograms confirmed the formation of bimetallic nanoclus-Metallic nanosystems in catalysis ters. Using the TEM approach, it was shown that the Pd7Pt clusters formed are homogenous according to their sizes (the mean size was 1.5 nm) and belong to `magic' clusters with N=55.169 A conclusion about the saturation of the nanocluster surface with palladium was inferred from the quantitative XPS data.202 And, finally, the coordination numbers for palladium and platinum were determined using EXAFS. Comparison of experimental results with the values calculated for two models, viz., a model with inner platinum nuclei and a statistical distribution model, revealed that the former best fits the experimental results.199 The experimental data altogether allowed one to propose a structural model of a Pd7Pt nanocluster in which a monolayer of 42 palladium atoms covers a nucleus consisting of 13 platinum atoms.198 Later, bimetallic nanoclusters containing core ± shell structures could successfully be obtained by stabilisation of Pd7Au, Pt7Au and Rh7Pt systems by polyvinylpyrroli- done.203, 204 In bimetallic nanosystems of the `cluster-in-cluster' type, the atoms of one component form a cluster, whereas the atoms of the other component cover only some of its planes (not necessarily as a monolayer) to form an intertwining structure. The fact that such structures are formed in Pd7Au clusters with a change in the Pd :Au ratio from 4 : 1 to 1 : 1,203 implies that it can represent a modified structure with an inner nucleus.Additional evidence in favour of this suggestion can be derived from the transformation of Pd7Pt nanoclusters with inner nuclei into `cluster-in-cluster' structures in their application onto inorganic materials.205 With- out going into details about the similarities and differences of these two structures, it should be noted that the surface of nanoclusters of the `cluster-in-cluster' type contains some centres which consist of two kinds of atoms, i.e., the ensemble effect is possible, whereas only the electronic (ligand) effect is a priori possible in structures with inner nuclei. It is the ligand effect that is responsible for the enhanced catalytic activities of PVP-stabilised Pd7Pt clusters in the hydro- genation of cycloocta-1,3-diene into cyclooctene.Indeed, the maximum increase in activity (more than twofold in comparison with pure palladium) was observed for the Pd : Pt ratio of 4 : 1,200 i.e., under conditions where palladium atoms completely covered the inner platinum nucleus. A similar dependence was observed in the case of Pd7Au nanoclusters.203 The fourfold increase in the catalyst's activity in comparison with those of individual compo- nents was observed in the hydrogenation of cyclopentadiene into cyclopentene on Pd7Rh nanoclusters.198 Yet another example of the ensemble effect is the reaction of selective hydration of acrylonitrile to acrylamide: CH2=CH7CN+H2O CH2 =CH7CONH2 on bimetallic Cu7Pd nanoclusters stabilised by PVP.206 It was found that this reaction occurs on two adjacent palladium and copper atoms: the coordination of the C=C bond takes place on the palladium atom, which brings the C:N bond close to the adjacent copper atom which effects hydration.This results in the enhancement of activity and nearly 100% selectivity of the amide synthesis. It was shown also that the supported Pd7Mo/SiO2 catalyst based on a bimetallic precursor manifests higher activity in the CO+NO+O2 reaction than analogous specimens syn- thesised from individual organometallic complexes.207 V. Conclusion The fundamental knowledge of catalytic properties of nanosys- tems at the molecular level and practical application of their peculiarities largely determine progress in industrial catalysis and, correspondingly, the economic potential of certain branches of the chemical industry and the environmental situation.The unique microstructures of nanosystems in comparison with ordinary metals and chemical compounds confer novel properties on them. The characteristic features of nano- and microgeometry of nanosystems and their high specific surfaces 157 open up new opportunities for the design of highly selective and highly reactive perfect catalysts. Some of these features can be synergistic, e.g., in bimetallic clusters where one component influences the properties of the adjacent component. Detailed investigations into such interactions will allow the synthesis of novel selective catalysts with predetermined properties.It is the selectivity that is the key problem in the theory and practice of catalysis. And if the problems of sizes and compositions of nanoclusters which ensure their optimum selectivity, detailed electronic structures of cluster complexes, etc., are solved, surface reactivity will be put under control. Nanosystems are a `natural bridge' between homogeneous and heterogeneous catalysis. Many metals, especially Group VIII metals, which manifest the highest activity as heterogeneous catalysts form numerous cluster compounds endowed with unique catalytic properties. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (2) 161 ± 176 (2001) Electrochemically active species and multielectron processes in ionic melts V I Shapoval (deceased), V V Solov'ev, V V Malyshev Contents I. Introduction II. Model concepts on the structures of ionic melts III. Structures, chemical and electrochemical properties of titanium-containing fluoride melts IV. Structures, chemical and electrochemical properties of boron-containing fluoride melts V. Effect of cations on the mechanisms of chemical and electrochemical reactions in ionic melts VI. Theoretical and experimental studies of cation ± anion interactions in nitrate- and carbonate-containing melts VII. Multielectron electrochemical reduction of oxoanions of high-melting metals and non-metals in ionic melts Abstract.of formation of mechanisms the for concepts model The The model concepts for the mechanisms of formation of electrochemically processes multielectron and species active electrochemically active species and multielectron processes in in ionic titanium-containing and boron- carbonate-, nitrate-, ionic nitrate-, carbonate-, boron- and titanium-containing fluo- fluo- ride of importance fundamental The generalised. are melts ride melts are generalised. The fundamental importance of the the acid-base formation of mechanism the in melt a of properties acid-base properties of a melt in the mechanism of formation of of electrochemically and nitrate- for shown is species active electrochemically active species is shown for nitrate- and carbo- carbo- nate-containing by confirmed is fact This melts.nate-containing melts. This fact is confirmed by electrochemical electrochemical measurements for constants force of calculations by and measurements and by calculations of force constants for oxy- oxy- anions. species active electrochemically of form optimum The anions. The optimum form of electrochemically active species has has been the on depend abilities reduction their established; been established; their reduction abilities depend on the cationic cationic composition the of properties adsorption the melt, a of composition of a melt, the adsorption properties of the electrode electrode surface bibliography The strength. field electric the and surface and the electric field strength. The bibliography includes includes 218 references 218 references.I. Introduction Complex multicomponent systems containing ions of polyvalent metals are used as reaction media in the preparation of inorganic and organic compounds by high-temperature electrochemical synthesis, for the implementation of processes in high-temper- ature current sources, in electrometallurgy, in electroplating and other areas where a specific approach to the control of chemical processes in melts is required. Molten salts are widely used in practice, that is why the study on their structures, chemical and electrochemical properties is an issue of current interest.1±3 This review covers the state-of-the-art of a fundamental problem of electrochemistry of ionic melts, viz., determination of the nature of electrochemically active species (EAS) involved in electrochemical reactions and the conditions required for multi- electron transformations at electrode ± electrolyte interfaces.The electrochemically active species are groups of ions formed in a melt due to acid-base interactions and possessing the highest abilities to accept electrons in the double electric layer. To date, V I Shapoval, V V Malyshev V I Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, prosp. Palladina 32/34, 252680 Kiev, Ukraine. Fax (38-044) 444 30 70. Tel. (38-044) 444 14 62. E-mail: synthesis@ionc.kar.net (V V Malyshev) V V Solov'ev Yu Kondratyuk Poltava State Technical University, Pervo- maiskii prosp.24, 36601 Poltava, Ukraine. Fax (38-053) 256 18 96. Tel. (38-053) 227 46 48 Received 15 August 2000 Uspekhi Khimii 70 (2) 182 ± 199 (2001); translated by S S Veselyi #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n02ABEH000619 161 162 164 165 166 167 169 this problem got both the theoretical background and was confirmed comprehensively by experiment for not-too-compli- cated electrochemical systems with simple one-element ions. If the components of a system arise from complexes, the electrode reactions are supposed to involve multiple-charged ions with complex compositions. Such systems can exist both in aqueous electrolytes and in ionic melts. The problem of theoretical description of multielectron electrochemical reactions in ionic melts has not been solved unambiguously so far.In order to describe experimental results on multielectron transformations at interfaces of electrochemical systems, one mainly uses the con- cepts on acid-base equilibria and the concepts on electron density redistribution mechanisms during the formation of new chemical bonds or under the influence of the outer-sphere cations and anions on the intracomplex bonds. The concepts of the mechanism of formation of electrochemi- cally active species in ionic melts based on the analysis of thermodynamic and kinetic parameters are phenomenological. The lack of systematic studies on the relationship between chemical reactions of EAS formation in the bulk melt and their electrochemical reduction and on the effect of the adsorption properties of electrode surfaces and the external electric field on the acid-base interactions in ionic melts not only predetermines the existence of controversial results in the interpretation of the relationships obtained experimentally but also prevents the for- mulation of basic principles of EAS formation and control of electrochemical processes.The most abundant information on the interaction between the species in a melt and on the processes of their electrochemical reduction can be obtained from data on the changes in the electronic state of the species.4±7 Therefore, it seems expedient to use the results of both theoretical and modern experimental research methods for the substantiation of EAS formation mechanisms in ionic melts.Once this mechanism has been established, it will be possible to develop a theoretical basis for creating the general principles for control over multielectron processes. In ionic liquids (ionic melts), the acid-base interactions manifest themselves explicitly,8, 9 but it is difficult to conceive their mechanisms because of their complexity. A cation ± anion interaction model is used for the description of this mechanism. In accordance with this model, the acid-base properties of a medium with respect to a particular anion are changed by addition of different types of cations to a dilute solution. Let us consider how this model `operates' for such anions as NO¡3 , CO23 ¡, TiF26 ¡, BF¡4 , WO2¡ and MoO2¡ (hard bases) and such cations as Cs+, Rb+, 4 4162 K+, Na+, Li+, Pb2+, Ba2+, Sr2+, Ca2+, Mg2+, Be2+ (hard acids) in nitrate-, carbonate-, boron- and titanium-containing fluoride melts that are widely used in practice.2±4 II. Model concepts on the structures of ionic melts The physicochemical properties of ionic melts containing poly- coordinated anions, viz., NO¡3 and CO23 ¡, are determined by the electronic structures of the oxygen-containing anions and the character of their interactions with the melt cations.There is only the near-range order in ionic liquids, which creates problems both for the interpretation of the experimental relationships and for the theoretical descriptions of the melt structures.This can explain the existence of various model concepts as to the structures of ionic melts. Studies on the changes in the intra-anion bonds depending on the cationic environment (changes in the force constants of the bonds and in bond angles were analysed) in melts of monovalent metal nitrates have led to the conclusion that contact cation ± anion pairs exist in these melts.10, 11 Based on this conclusion, a model of an ion pair was built. This model has been used, e.g., for the explanation of the features of IR and Raman spectra of LiNO3 and alkaline-earth nitrate melts, namely, the low-frequency com- ponent of the spectrum resulting from the M7O vibrations and splitting of one of the frequencies in the NO¡3 spectrum.The quasi-lattice model of the melt structure accounts for the concerted vibrational movement of the ions. Considering the mean statistical distribution of the inter-ion potential, the exis- tence of two sublattices, namely, cationic and anionic, was assumed.12, 13 This model denies the existence of ion pairs due to the comparability of lifetimes of the vibrational levels and the diffusional migration of species. Based on this quasi-lattice model, the existence of [M(NO3)4]37 complexes was assumed;14 this was confirmed by spectroscopic data.15 The Kucharski ± Flengas quasi-chemical model 16 and the three-sphere Ferland model 17 account for the statistical exchange of cations in sublattices of mixed nitrate melts. Complex ions existing in the melt are regarded as fragments of the ionic structure rather than isolated species of an anionic sublattice.16 The asymmetry of the cation-anionic environment and the existence of independent kinetic species are explained 18 by effects of various cations, in agreement with the assumptions made elsewhere.10, 11 For objective reasons, the above models cannot claim to provide full description of the structures of ionic melts, but each of them states unambiguously that the structure depends signifi- cantly on the cationic compositions.This dependence has been confirmed by spectroscopic data for systems with polycoordinate anions, viz., complexation products. In fact, the geometries and types of complex species in melts containing insignificant amounts of transition metal salts as well as in melts with oxygen-containing anions have been established based on spectroscopic data.19 Analysis of the spectra of sulfate melts revealed a considerable contribution of p-interactions to the chemical bonds of the metal complexes studied.15 The characteristic vibration frequencies were found to depend on the cationic composition for individual and mixed melts containing molecular ions as the starting structural elements.In many cases, C7O and S7O vibration frequency shifts for outer-sphere cations Cs+>K+>Na+>Li+, accom- panied by band broadening due to the polarising effect of the cations, were observed in carbonate- and sulfate-containing solutions.15 However, the reasons for the frequency shifts in the spectra of melts were explained either qualitatively 20, 21 or on the basis of various model assumptions on the melt structures (the quasi- lattice model,13 the ion-pair model,10, 11 etc.).In addition, mutu- ally exclusive interpretations of the same data can be found as well.The use of spectroscopic methods for the study on dilute oxygen-containing melts encounters considerable difficulties in the interpretation of the bands because of their low intensities. On V I Shapoval, V V Solov'ev, V V Malyshev the other hand, information on the acid-base interactions in ionic liquids can be obtained by studying electrode processes. Let us consider certain specific features of electrode processes in melts, in particular those containing nitrate and carbonate ions.Neglecting of the role of acid-base properties of the medium has traditionally led researchers to mutually exclusive conclusions in the explanation of the same features of electrode processes. For example, Delimarskii et al.22, 23 showed that a cathodic process with a 100% yield of carbon was possible in the electrolysis of a molten equimolar mixture of lithium and potassium carbonates. They noted a specific feature of the voltammetric curves, viz., the existence of a region with a distinct limiting current at potentials 0.4 ± 0.5 V, and considered that this region corresponded to the evolution of carbon preceding that of the alkali metal.23 The existence of limiting currents in voltammograms has also been reported in other studies on cathodic processes in molten carbo- nates (see, e.g., Refs 24 ± 27), but these features have been inter- preted differently by different authors.Studies on the possibility of carbon evolution preceding the discharge of alkali metal cations 28, 29 showed that cathodic carbon evolution depends on the electrolyte cationic composition and can occur not only from carbonates but also from chlorides, either supplemented with thermally unstable carbonates or saturated with gaseousCO2. The dependence of the amount of carbon evolved on the cationic composition of the carbonate electrolyte showed the dominating role of the reducing ability of CO2 formed in the redox reaction(1) (2/m)Mm++CO2¡ M2/mO+CO2 .3 As in the case of carbonate-containing melts, the development of concepts on the mechanism of formation of electrochemically active species in nitrate-containing melts was facilitated by the existence of many different interpretations of the features of cathodic processes. In fact, the nature of the electrode process preceding the electrochemical reduction of alkali metals, which has first been reported by Lyalikov and Karamzin,30 was inter- preted in different manners. Rust and Duke,31 who studied acid- base equilibria, assumed that it is the dissociation product of the anion, viz., the nitronium ion NOá2 , that is reduced in electro- chemical and chemical reactions. A similar conclusion was reached by Delimarskii et al.32 However, no direct measurements could prove the existence of NOá2 in nitrate melts.It was also found 33 that noticeable amounts of NOá2 cannot be formed in the presence of strong acids; instead, a compound is formed the properties of which coincide with those ofNO2. This subsequently undergoes one-electron reduction into nitrite. Usanovich 34 believes that [H2NO3]+ ions rather than NOá2 exist in aqueous melts. A different viewpoint regarding the nature of the electrode process preceding the alkali metal evolution was given by Deli- marskii et al.:35 this process results from direct NO¡3 reduction, irrespective of the electrolyte cationic composition. A similar interpretation has been given in other studies,36 ± 38 but the reasons for the direct electrochemical nitrate ion reduction have not been explained.The currently available experimental results on electrode processes in molten salts, particularly in nitrate melts, are used as a base for the simulation of various schemes of formation of electrochemically active species and the corresponding electrode processes. Let us consider some of the models. Tkalenko et al. applied the regularities of the electrochemical 3 reduction of NO¡3 in aqueous solutions 39 to melts;40 ± 43 this allowed them to propose a model scheme of the cathodic process in concentrated nitrate melts. According to this scheme, the NO¡ ion in the bulk melt is an ellipsoid with the negative charge distributed over the periphery. Near the electrode surface, the ion is subjected to a combined effect of the cation and electrode fields, therefore its state differs from that in the bulk.It is believed that at negative potentials (cathodic polarisation) the NO¡3 ions are polarised additionally by the electrode force field in such a way that the positively charged nitrogen atom is displaced towards theElectrochemically active species and multielectron processes in ionic melts electrode relative to the oxygen atoms. The nitrate ion polarised by the electrode field is regarded as an activated complex. Species that form activated complexes under the action of both the electrode field and cation field in the double electric layer are considered electrochemically active.43 It is the NO¡3 ions that undergo discharge upon cathodic polarisation, hence they can be regarded as electrochemically active species.We feel that the above scheme of the formation of electro- chemically active species has a number of drawbacks. First, though the authors do not deny the effect of the electrolyte cationic composition, they believe that the cation ± anion inter- action is limited to indirect anion polarisation (through variation of the field potential). They rule out the formation of metal complexes as electrochemically active species; this disagrees with the concept on the chemical interaction of species in melts. In addition, this assumption contradicts the results of studies on the electrochemical reduction of oxygen-containing anions 4, 44, 45 and spectroscopic data.15, 19, 46 Second, the unreasonable overestima- tion of the electrode field effect requires a thorough consideration. And, finally, while noting the possible effect of both the electrode field and cations, the authors do not estimate the contributions of each of the effects to the formation of the activated complex.A different model of the formation of electrochemically active species in melts has been proposed by Chernukhin et al.47, 48 The model is based on the concept on inter-species interactions in condensed media. It was suggested 48 that inter-species interac- tions result in concerted groups of ions consisting of one anion and surrounding cations. The anion can be transformed to the activated state and undergo electrochemical transformation.In their study on electrode processes in ternary mixtures, Chernu- khin et al. assumed that the inter-species interactions in the double electric layer and in the melt bulk have similar natures. Within the framework of the concept that melts are dynamic systems { and based on the results reported by Belen'kaya et al.50 (see footnote {), it was assumed 48 that the state of an anion depends on the concerted action of the ionic environment. The activation of anion discharge was explained by the polarising action of the electrode field resulting in a definite anion orienta- tion and, subsequently, localisation of its pairwise inter-species interactions, preferably with cations having the strongest polar- ising effect.This enhances the non-equivalence of theN7Obonds and prolongs considerably the life-time of the group consisting of the anion and the first-sphere cations. Such a group has been called the activated complex.48 For example, the formation of an activated complex is given by the following scheme for a binary melt containing potassium and calcium: 7E Kán¡xCa2xá¡1{Ca2+[ONO2]7} Kán¡xCa2xá¡1{Ca2+...O...[(NO2]+}, (2) 3 where n is the coordination number (c.n.) of the anion during discharge, which is constant irrespective of the cationic composi- tion; Kán¡xCa2xá¡1{Ca2+[ONO2]7} is the binary melt formula which emphasises the polarising effect of the cation on the NO¡ anion by designating the anion as [ONO2]7. Within the scope of Scheme (2), the electrode reaction has the form: 2 e7 7E (3) CaO+NO¡ Ca2++NO¡ {Ca2+...O...[NO2]+} 2 .3 For similar lithium-containing melts, an intermediate compound LiO7 can be formed: 2 e7 7E Li++NO¡ {Li+...O...[NO2]+} 3 LiO7+NO¡2 . (4) Based on the above model for the formation of electrochemi- cally active species, the appearance of additional cathodic waves { These views have later been generalised by Delimarskii et al.49 { Belen'kaya et al.50 used Raman spectroscopy to study the changes in the geometry and the potential function of nitrate ion in molten alkali nitrates. It was shown that the bond lengths and strengths in molecular ions can be changed with strengthening of inter-species interactions in the melt.Therefore, the reactivities of species in molten media can be controlled directly by varying their compositions. 163 in the voltammograms upon addition of strongly polarising cations, e.g., in melts of sodium and potassium nitrates, can be explained by an increase in anion asymmetry. The shift of the reduction wave upon addition of strongly polarising cations to the melt occurs due to the gradual replacement of the background cations in the nitrate ion environment by cations with a higher polarising strength. This scheme, unlike that proposed by Tka- lenko,43 is generally consistent with the concept on the chemical interactions between species in melts and with the data of numerous spectroscopic studies on concentrated nitrate melts. However, certain aspects of the mechanism discussed above are debatable.Shapoval et al.4, 44 generalised their own results and other data and explained some features of the electrochemical reduction of oxygen-containing anions. It was assumed that electrode proc- esses in molten salts with oxygen-containing anions are limited by a preceding chemical reaction. For example, the formation of electrochemically active species in the reduction of the carbonate ion in lithium-containing melts was represented by the scheme:(5) n Li++CO2¡ LinOn72+CO2 . [LinCO3]n¡2 3 3 A similar scheme of the preceding acid-base reaction has been proposed for nitrate-containing melts based on data for NO¡ reduction in a KCl ± LiCl eutectic as the background:45 (6) 2Li++NO¡ Li2O+NOá [Li2NO3]+ 3 2 .As follows from Eqns (5) and (6), it is not anions but their reaction products with cations of the molten media that are electrochemically active species. 3 The proposed anion ± cation interaction scheme agrees most closely with the concepts on the chemical reactions of species in ionic melts, but this has a phenomenological character.51 The anion ± cation interaction mechanism can be comprehended more precisely by studying it at an electronic level with the use of quantum-chemical calculations and by comparing them with experimental data. The CO2¡ and NO¡3 anions which are most sensitive to variations of the acid-base properties of a melt are most convenient for the comparison of the electrochemical reduction characteristics. The scheme for the formation of electrochemically active species in oxygen-containing melts proposed by Shapoval et al.4, 44 does not account fully for the function of cations as acids which, like protons in aqueous media, can add to anions and thus accelerate electrochemical reduction.The complex nature of interaction between species in ionic media creates difficulties for their theoretical description. Therefore, it is expedient to consider a model scheme of cation ± anion interaction which makes it possible to change the acid-base properties of the medium with respect to a selected anion by gradual accumulation of cations in the anion's first solvation shell (cationised anion).52 The cation ± anion interaction for carbonate- and nitrate-containing melts can be represented as follows: {Mmá [CO3]27}(mn72)+ n (7) nMm++CO2¡ 3 M2/mO+CO2 + (n72/m)Mm+ , {Mmá n [NO3]7}(mn71)+ (8) nMm++NO¡3M2/mO+NOá2 + (n72/m)Mm+ , where m is the cation charge, n is the reaction order with respect to the cation (coordination number).Thus, cation ± anion interac- tion can result either in cationised anions (metal complexes) or in anion dissociation (both under direct influence of cations and via intermediate formation of `short-lived' metal complexes). Earlier quantum-chemical calculations 53 and electrochemical data 51, 54 showed that cation ± anion interactions in nitrate-con- taining melts result mostly in nitrate metal complexes, whereas in carbonate-containing melts, two mechanisms of the EAS forma- tion in accordance with Scheme (7) are equally probable.On the164 other hand, the model proposed, which is valid in dilute melts as the background, does not account for the effects of the electrode surface adsorption properties and the electric field on the charac- ter of acid-base interactions and on the charge transfer elementary act. 3 A calculation of the total energies, charges and populations for carbonate ions showed 55 that the presence of uncharged electrode surface atoms (this surface was simulated by a linear cluster of three carbon atoms) favours the electron density redistribution in an isolated CO2¡ anion according to the s,p- mechanism (where the s-component predominates), which, in turn, weakens the intra-anion bonds. Estimates of contributions from the electrode surface and from the cation field to the changes in the C7O bond energy showed that the latter contribution predominates, i.e., the electrode surface does not affect in princi- ple the mechanism of the EAS formation within this model.This mechanism is primarily determined by the melt cationic composi- tion. 3 The cation ± anion interactions are affected considerably not only by the adsorption effects but also by electric fields.56 Comparison of the energy characteristics and atomic charges and orbital populations in an isolated CO2¡ anion and in an anion located near the electrode surface under the effect of the cation field and an external electric field (E=261010 V m71) showed that cleavage of C7O bonds occurs with formation of CO2.It is believed 57 that the effect of the electrode surface lowers the activation barriers for two- and four-electron CO2¡ 3 reduction in comparison with the similar magnitudes for an isolated anion. The additional effect of the electric field increases the effect discovered (by a factor of *2) and favours the direct discharge of the CO2 molecule (9) C+2O27 . CO2+4e7 The geometric sizes of metal complexes resulting from cat- ion ± anion interactions in nitrate-containing melts are much larger than that of the linear cluster of three carbon atoms mentioned above. This fact prompted a search for new clusters simulating the electrode surface.58 The optimum four-cluster variant selected is an aromatic system of 12 carbon atoms (48 basis orbitals) which ensures an adequate representation of the donor-acceptor properties of an uncharged electrode surface. Interaction of a surface cluster with an NO¡3 anion initiates the electron density redistribution in it according to the s,p-mecha- nism (with the s-component predominating) which weakens the intra-anion bonds.An additional influence of the field from the melt cations, e.g., Li+ and Be2+, on the NO¡3 anion results in a considerable (three- to fourfold) increase in the N7O bond energies. In this case, the p-component predominates in the s,p- mechanism. Both of these factors increase theN7O bond energy. The trend towards strengthening the N7O bonds is preserved as the c.n.of the cation is increased. Solov'ev et al.59 emphasised that the overall effect of the electrode surface and melt cations on the EAS formation mechanism is determined by competition of two opposite trends in electron density redistribution in NO¡3 and concluded that the electrode surface effect on the mechanism of formation of EAS, such as metal complexes {Mmá n [NO3]7}(mn71)+, is insignificant within the framework of Scheme (8). Calculations carried out by Solov'ev et al.60 showed that the external electric field (261010 V m71) imposed on a cationised nitrate ion located near the electrode surface virtually does not change the energy of N7O intra-anion bonds against the back- ground of the rather strong interaction of NO¡3 with Li+ or Be2+.It was found that the contributions of both electric field and electrode surface do not greatly affect the cation ± anion inter- action. It has been found 58 (from activation barrier calculations) that the effect of the electrode surface increases considerably the reactivity of metal complexes with respect to two-electron reduc- tion. This effect increases with increase in the cation specific V I Shapoval, V V Solov'ev, V V Malyshev charge and coordination number. The trend towards lowering of activation barriers is preserved under an external electric field. An estimate of the electrode surface effects, the cation field and the external electric field on the elementary charge transfer act showed that the adsorption properties of the electrode surface play the most significant role in these processes.The optimum coordina- tion numbers were found to be 2 for NO¡3 surrounded by Li+ cations and 3 for the same anion surrounded by Be2+ cations. Taking electrochemical measurement data into account,61 electro- chemical reduction can be represented as follows (10) {Mmá n [NO3]7}(mn71)++2e7 NO¡2 +nM2/mO + (n72/m)Mm+ . Experimental studies 62 not only confirm the formation of EAS according to Scheme (8) but also suggest unambiguously the catalytic nature (caused by the depolariser regeneration) of nitrate reduction waves in the presence of strongly polarising cations. An excess of the latter is believed to result in regeneration of the original EAS Mm+,7E (11) 3NO¡2 NO¡3 +2NO+O27 .n 2 The possible realisation of Scheme (11) follows from quan- tum-chemical calculations of interactions between Mmá . . .NO¡ (M=Li+, Be2+) in an electric field with a strength of E=0; 0.2; 261010 V m71 (see Ref. 55). Similar calculations showed the priority of cation ± anion interactions in the EAS formation in ionic melts. This fact was confirmed in IR and Raman spectro- scopic studies on the effect of the melt cationic composition on the force constants of the N7O, C7O, W7O and Mo7O bonds in NO¡3 , CO23 ¡, WO24 ¡ and MoO24 ¡ anions. The force constants calculated for the [MNO3]0 and [M2CO3]0 systems (M=Cs+, Rb+, K+, Na+, Li+) suggest 53, 62 a trend towards an increase in the stretching force constants of the N7O and C7O bonds compared with the same bonds in isolated NO¡3 and CO23 ¡ anions as the cation specific charge increases in the series from Cs+ to Li+.The observed effect of the increase in the N7O bond strength due to anion cationisation was confirmed by quantum- chemical calculations and by analysis of electronic absorption spectra.15, 19 In carbonate melts containing the Cs+, Rb+ andK+ cations, the C7O force constants tend to increase, which also agrees with quantum-chemical calculations. 4 4 Calculations of force constants for [MEO4]0 systems (M=Naá2 , Liá2 , Pb2+, Ba2+, Sr2+, Ca2+; E=W, Mo) showed 61 that the E7O bond stretching force constants and the deforma- tion force constants in EO2¡ increase with the increase in the cation specific charge.The increase in the force constants of these interactions is believed to result from an increase in the degree of delocalisation of the valence electrons of the EO2¡ anion on the cation orbital. In turn, this stabilises the electronic energy levels of the inner shells in EO2¡ 4 . III. Structures, chemical and electrochemical properties of titanium-containing fluoride melts To date, data on the structure of dilute titanium-containing melts in fluoride ± chloride systems are very scarce, therefore even the simplest models of the structures of these melts are unavailable. However, there are a number of publications devoted to the studies of these melts. Analysis of the electronic absorption spectra of the fluoride- containing system KCl ± NaCl ± NaF with the use of the ligand field theory 63 revealed transient coordination forms of the Ti3+ ion resulting from mixed coordination with Cl7 and F7 anions.In the presence of a 100-fold excess of F7 with respect to TiF3, the formation of a trigonally distorted complex was detected. Com- plexation of lower titanium compounds with fluoride ions in KCl ± NaCl melts was reported 64 ± 66 and it was noted that the c.n. with respect to the F7 ligand in the complexes formed is 3 ± 4.Electrochemically active species and multielectron processes in ionic melts The existence of [TiF4Cl2]27 complexes, presumably with dis- torted octahedral symmetry, in K2TiF6 ± NaF melts was stated.67 The possible formation of titanium tetrafluoride in these melts was also reported by Rolin.68 The formation of poorly soluble compounds MTiF4 in titanium-containing melts cannot be ruled out.69, 70 Thus, based on the results of the few studies on the structure of titanium-containing fluoride ± chloride melts, one can assume that they contain the mixed [TiF4Cl2]27 complex and that the inter- action mechanism of the melt species, which results in the compounds MTiF4, depends considerably on the cationic compo- sition.The electrochemical reduction of titanium-containing fluo- rides in the presence of chlorides was studied by Smirnov et al.70 The steady-state polarisation curve method was used to study the reduction of Ti4+ in the 0.14 K2TiF6 ± 0.86 NaCl melt at 1033 K.The titanium deposition mechanism was studied by recording the potentiometric voltammetric characteristics of electrochemical reduction on steel, graphite and titanium electrodes (tetravalent titanium ions were assumed to be the EAS). A polarographic study on the electrochemical reduction of K2TiF6 with a KCl ± NaCl melt as the background was first carried out by Vitlaci and Nechel 71 who have established the two-electron nature of the charge transfer to the TiF2¡ 4 anion. Analysis of results reported in these studies 70, 71 suggests unambiguously the possibility of formation of new titanium- containing species upon dissociation of the original complex, viz., chloride ± fluoride titanium complexes with titanium in various valent states and cationised titanium-containing anions.These species can react with the melt components. In such cases, a multistep electrode process is observed, and the transfer act can involve several species. 6 A higher level of experimental data interpretation (with calculation of diagnostic criteria of electrochemical reduction) distinguishes a series of studies 65, 66, 72 which served as a basis for the assumption on the formation of TiF4 molecules from the original species TiF2¡ and the cationised anion {Mmá n [TiF6]27}(mn72)+. However, the studies discussed above and those by Juman and White 73 do not report any quantitative data on the electrochemical reduction kinetics and merely suggest the electrode reaction mechanisms.6 The most comprehensive data on the electrochemical proper- ties of titanium-containing fluoride melts have been obtained in the studies on the electrochemical reduction of the TiF2¡ anion with a KCl ± NaCl melt as the background.74 ± 76 The possible formation of new EAS, viz., dissociation products of the original complex, was also reported in these studies. Three reduction waves were found in the cathodic polarisation curves. The first wave was interpreted as a result of charge transfer to the dissociation product, i.e., the TiF4 molecule, and the second one, as a result of four-electron charge transfer directly to the complex anion. The third wave could not be interpreted on the basis of the experimental data.The authors of the majority of studies discussed above did not relate the results obtained to the effect of the melt cationic composition on the EAS formation mechanism. It seems most reasonable to study this mechanism with the use of quantum- chemical methods which enable the simulation of EAS formation at the electronic level by relating it to electron transfer during electrochemical reduction. The successful application of the cation ± anion interaction model to the studies on carbonate- and nitrate-containing melts stimulated the use of this model for the study on melts containing complex anions. A scheme of the cation ± anion interaction in titanium-containing fluoride melts was represented as follows:77 {Mmá n [TiF6]27}(mn72)+ (12) nMm++TiF2¡ 6 TiF4+2F7+nMm+ .165 According to this scheme, the cation ± anion interaction can involve the formation of cationised anions (metal complexes) {Mmá n [TiF6]27}(mn72)+ or dissociation of the TiF26 ¡ anion to give titanium tetrafluoride. Calculations showed 78 that dissociation of an isolated TiF2¡ 6 6 6 anion is impossible for energetic reasons: comparative analysis of atomic orbital populations in an isolated TiF2¡ 6 anion and that in cationic environment showed that electron density redistribution cannot result in cleavage of the Ti7F bond in this anion. Reactions of the Na+ or Li+ cations with the TiF2¡ anion yield metal complexes with coordination numbers equal to 2, 4 and 6. The Ti7F bond energies decrease with the increase in the c.n.under the influence of cations, whereas the energies of the cation ± anion bonds increase. The interaction between Mg2+ cations and TiF2¡ at c.n.=2 results in cleavage of two Ti7F bonds. Further increase in the number of Mg2+ cations in the first solvate shell results in further dissociation of the TiF2¡ 6 6 anion. Weakening of Ti7F bonds due to the cation ± anion inter- action results from electron density redistribution in the TiF2¡ anion according to the s,p-mechanism. The electron density redistribution increases with the increase in both c.n. and the cation specific charge.6 It was noted 79 that the cation ± anion interaction lowers the activation barriers for two- and four-electron reduction of metal complexes of the TiF2¡ anions and titanium tetrafluoride molec- ular associates in comparison with the corresponding values for isolated TiF2¡ 6 and TiF4.Predominance of four-electron reduction was established for metal complexes; on the contrary, two- electron charge transfer predominates for isolated TiF4. Volkov 80 does not rule out that the EAS formation mecha- nism is affected by chloride anions of the background electrolyte. The existence of stable ion-molecular associates {Cl¡2 [TiF4]0}27 in the melt bulk is probable. This follows from an estimate (based on quantum-chemical calculations) of the interaction of Cl7 ions with the TiF4 molecule. The activation barriers for two- and four- electron reduction of TiF4 increase considerably in the presence of chloride anions.Hence, anionic environments (unlike cationic ones) do not increase the reducing ability of EAS. 6 6 It has been shown 81 that the electrode surface effect results in electron density redistribution in cationised TiF2¡ anions and in TiF4 molecular associates, as well as in isolated TiF4, according to the s,p-mechanism with predominance of the s-component. Hence, the electrode surface does not affect considerably the EAS formation mechanism but only enhances the cation ± anion interaction effect. Comparative analysis of atomic charges and atomic orbital populations 82 showed that the electric field ini- tiates electron density redistribution in the TiF2¡ anion with predominance of the s-component and in TiF4 with predom- inance of the p-component.The combined effect of the electrode surface and the electric field results in considerable lowering of the activation barrier for EAS four-electron reduction.ATiF2¡ 6 anion surrounded by six cations and a TiF4 molecule surrounded by four alkali metal cations have the optimum reducing ability. In our opinion, these results are in full compliance with the idea of outer- sphere cationisation in melts containing anions with complex compositions 3, 44, 45 confirmed by electrochemical studies on tungstate- and molybdate-containing systems.83 IV. Structures, chemical and electrochemical properties of boron-containing fluoride melts Much less data are currently available on the structures and properties of boron-containing fluoride melts than on titanium- containing fluoride melts; these data have almost not been interpreted.Raman spectroscopic studies on the NaBF4 and NaF± NaBF4 melts 84 showed that the structure of the BF¡4 anion remains unchanged at temperatures up to 880 K. A similar conclusion was made by Prostakov et al.85 who also noted that various complex associates can exist inKBF4 ±KF± CsF melts. Pirina and166 Prostakov 86 used data on the specific densities and electric conductivities of the KBF4 ±KF ± CsF system to calculate the stabilisation energies of these complex associates. They also stated that complex anions [M(BF4)4]37 with tetrahedral configurations were formed predominantly and noted that mixing of salts having the same cation resulted in gradual substitution of anions to give mixed complexes of the types [MF4]37, [M4F]3+, [M(BF4)4]37, [M(BF4)6]37, [M(BF4)3F]37 and [M(BF4)2F2]37. A study on solubility of BF3 in KCl ±KF melts at 870 ± 970 K showed 87 that after dilution of the melt with respect to fluoride or after addition of strongly polarising cations (Li+, Mg2+, Ba2+, Ca2+), the system contains (in addition to the BF¡4 species) mixed fluoride ± chloride boron complexes and BF3 molecules in equi- librium with halide ions.Based on data on boron equilibrium potentials in KCl ±KF±KBF4 melts,88, 89 it was concluded that the majority of boron ions exist as higher fluoride and fluoride ± choride complexes, viz., [BF4]7 and [BF3Cl]7, respectively.It was assumed 90 taking account of viscosity and specific density data for NaCl ± NaBF4 and NaCl ± RbBF4 melts that the structures of tetrafluoroborate melts comply with the model of `solids' (with retention of the short order arrangement of species). The ambiguous interpretation and scantiness of data on the 4 structures and physicochemical properties of boron-containing melts often result in conflicting concepts on the electrochemical reduction processes in these melts. Studies on the cathodic process in the LiF ±KBF4 ±KF system showed that reduction of the BF¡ anion involves simultaneous transfer of three electrons.91 It was assumed that diffusion of the BF¡4 anions from the melt bulk is not a limiting step of electrochemical reduction, and this is an indication of the process complexity.Myshalov 92 who studied the electrochemical reduction in boron-containing melts con- cluded that boron reduction follows a two-step mechanism. Vasil'eva et al.93 studied the possibility of electrochemical prepa- ration of boron from borax-containing melts; they detected several waves on the polarisation curves originating from the stepwise reduction of the oxygen-containing anion. 4, BF3 Cl7 and BF3, is following EAS forms can be assumed: {Mm4 á[BF4]7}(4m71)+, {Mmá tion mechanism was complemented as follows: The reduction kinetics of boron compounds in fluoride- containing melts have been studied most thoroughly by Tsiklauri et al.94 ± 96 Analysis of reduction chronovoltammograms and interpretation of the spectral characteristics of the ternary molten system KCl ± NaCl ±KBF4 suggest that the chloride ± fluoride melt contains several boron-containing equilibrium forms.Reduction of three of them, viz., BF¡ believed to account for two waves in the voltammetric curves. This conclusion does not contradict the data by Zarutskii et al.97 On the other hand, the EAS formation mechanism was not related to the melt cationic composition 98 which is important in EAS formation in the melt bulk. The EAS formation scheme in fluoride boron-containing melts is represented as follows:99 {Mmá n [BF4]7}(mn71)+ (13) nMm++BF¡4BF3+F7+nMm+ . Preliminary calculations showed that dissociation of an iso- lated BF¡4 anion is impossible for energetic reasons.It was found that the interaction of this anion with Na+ and Mg2+ cations demonstrates a trend towards a decrease in the intra-anion bond energies, while the interaction with the Li+ cations, the trend is towards their increase. Distribution of atomic charges and pop- ulations of the atomic orbitals of these systems indicates strength- ening of the B7F bonds in metal complexes of the anion with Li+ cations due to predominance of the p-component in the s,p- mechanism of electron density redistribution in BF¡4 . In metal complexes with Na+ and Mg2+ cations, the s-component was found to predominate. The possible dissociation of the BF¡4 anion upon cation ± anion interaction cannot be ruled out.The interaction of one of the products of reaction (13), viz., the BF3 molecule, with the melt cations has been studied.100 Analysis of the interaction energies of V I Shapoval, V V Solov'ev, V V Malyshev BF3 with Na+ or Li+ for the cases where the coordination numbers of these cations are equal to 1, 2 and 3 showed that cleavage of the B7F bonds does not occur, though they are weakened considerably. On the other hand, interaction of two Mg2+ cations with BF3 results in decomposition of the BF3 molecule; in this case, the formation of {Mgmá 2 [BF3]0}2m+ asso- ciates in the melt bulk is possible. It was noted 101 that the cationic environment lowers the activation barriers for two- and four-electron reduction of the products of reaction (13), i.e., metal complexes of the BF¡4 anion and associates of the BF3 molecule, in comparison with the corresponding values for the isolated BF¡4 and BF3 species.Predominance of two-electron reduction over four-electron reduction is observed both for cationised anions and for BF3. Spectroscopic studies on the molten ternary system KCl ± NaCl ±KBF4 suggest that mixed chloride ± fluoride com- plex ions [BF4-xClx]7 exist in the melt.102 Analysis of the effect of the background electrolyte chloride anions on the interaction of the BF¡4 species with one or two lithium cations (carried out by quantum-chemical calculations) showed the possible formation of both [BF4-xClx]7 complexes and their associates in the cationic environment.The {[BF3]0Cl7}7 species is the most reduction- stable. The activation barrier for two-electron reduction of these associates is lower than that of the BF¡4 anion without the cationic environment. The maximum decrease is achieved for the {Li+[BF3Cl]7}0 system. However, though the effect of the activation barrier decrease for BF¡4 and BF3 in a cationic environ- ment is greater than that for the {Li+[BF3Cl]7}0 associate, one cannot rule out a possible decrease in the latter. Solov'ev 103 assumed that the effect of the electrode surface in boron-containing fluoride melts causes electron density migration in EAS according to the s-mechanism, resulting in an insignif- icant weakening of the B7F bonds. It was therefore concluded that, within the model scheme selected, the electrode surface does not affect greatly the EAS formation mechanism.Imposed electric field (E=261010 V m71) enhances the trend towards weakening of the B7F bonds in {Mmá n [BF4]7}(mn71)+ and strengthens slightly these bonds in other EAS.104 On the other hand, the decrease in the activation barriers for reduction due to the surface and electric field effects results in a considerable increase in the reducing ability of cation ± anion interaction products. Taking this into consideration, the 2 [BF3]0}2m+, BF3, {M+[BF3Cl]7}0. Thus, the EAS forma- (14) nMm++BF3 {Mmá n [BF3]0}mn+ , (15) {Mmá nMm++[BF47xClx]7 n [BF47xClx]7}(mn71)+ . Analysis of electrochemical measurements of BF¡4 and BF3 reduction with the KCl ± NaCl melt as the background has been carried out by Tsiklauri et al.95, 96 without interpretation of the EAS reduction waves.Based on the calculations, the existence of an additional EAS (besides BF3 and associates of chloride ± fluoride complexes) was assumed,104 namely, {Mmá n [BF3]0}mn+; the probability of its reduction is higher than that of mixed chloride ± fluoride associates. V. Effect of cations on the mechanisms of chemical and electrochemical reactions in ionic melts The effect of the solvent nature on the mechanism and rate of chemical reactions in ionic melts was first discovered by Rust and Duke 31 in a study on acid-base equilibria in nitrate-containing melts. It was shown that replacement of a solvent with weakly polarising cations by a solvent with strongly polarising cations accelerates the reactions.This effect was explained by activation of the reacting molecules by the cationic environment. In other studies, the role of the melt cationic composition was ignored. For example, in studies on the electrochemical reduction of oxygen-Electrochemically active species and multielectron processes in ionic melts containing anions 26, 35, 36 it was assumed that, irrespective of the electrolyte cationic composition, the anions themselves partici- pate in the cathodic reaction. The specific features of the cathodic reduction of anions in the presence of cations were explained by various factors excluding the acid-base interaction. Certain model concepts used in studies on aqueous solutions were applied to ionic melts.In an analysis of the possible direct discharge of the nitrate ion in aqueous solutions, Lopatin105 used the `cationic bridges' concept which makes it possible to explain the effect of the solution cations on the cathodic process. Having studied the anion discharge kinetics near the zero charge potential of the electrode, he explained the effect of cations on the discharge rate by a decrease in the negative reduction potential of the nitrate ion with increase in the concentration and the cation charge. The possibility of direct discharge of the nitrate ion has also been noted.106 Kvaratskheliya 39 studied the effect of the cation nature on the shift of NO¡3 cathodic reduction waves in aqueous electro- lyte solutions. In his opinion, one of the reasons for the effect of salts with polyvalent cations on the positive shift of the cathodic wave is the formation of cation ± anion pairs in which one of the N7O bonds is weakened under the effect of the cationic environ- ment.In studies on electrode processes in melts of individual nitrates with various cationic compositions,40 ± 42 it was concluded that the activity of the nitrate ion is determined by its environment and hence depends on the acid-base properties of the melt. Differences in the positions of the NO¡3 electrochemical reduction wave were explained by the different charge densities of the cations whose force field affects both the NO¡3 anions and the O27 ions formed in the cathodic process.Attempts were made to find an empirical relationship between the composition of a nitrate-containing melt and the reactivities of its components. For example, Fletcher et al.107 reported an inverse dependence of the isobaric potential of activation ofNO¡3 electrochemical reduction on the solvent cation radius. In a study on the self-diffusion and charge transfer in carbonate- and nitrate-containing melts, the existence of metal complexes such as MCO¡3 , MNO¡3 and M2NOá3 (the latter being most probable for the lithium cation) in the melt was assumed and the possibility of the following elementary chemical reactions in a melt was noted:108 (16) acid. Mn++base Of interest are the studies by Shams El Din et al.109, 110 reporting a comparative estimate of the acidities of alkali and alkaline-earth metal cations.According to these data, the acidity changes in the series of cations: Ca2+>Li+>Sr2+>Ba2+> Na+, K+. One of the first theoretical studies on the effect of the medium on the elementary act of chemical reactions in the condensed phase can be found in a series of works.98, 111 ± 113 They considered certain classes of chemical and electrochemical processes and calculated the kinetic parameters of the elementary act of charge transfer, the mechanism of which being dynamically affected by the macroscopic medium. In their opinion, the dynamic effect is determined by fluctuations or changes in the ionic environment surrounding the reacting species; chemical reactions are assumed to occur in a polar liquid and to be accompanied by a considerable charge density redistribution in the reacting species.110 It was noted 111 that short-range effects predominate in reactions in ionic melts.Proof that the cationic composition of the medium affects the mechanisms of electrochemical reactions in nitrate-containing melts was obtained in a series of studies 47 ± 49 where a different scheme for the EAS formation mechanism in such melts was suggested (see Section II). However, we feel many statements of this scheme to be debatable because of the doubtful assumption that the nature of inter-species interactions in the electric double layer is the same as in the melt bulk.The electrochemical reaction mechanisms have also been found to be affected by cations for 167 titanium- 66, 69, 70 and boron-containing 84 ± 86, 97 halide melts. Unfortunately, the results obtained were not accompanied by conclusions or generalisations. The results of experimental studies 28 ± 30, 44, 45 on the kinetics of electrode processes with conjugate acid-base reactions in oxy- chloride molten salts demonstrate that the cationic composition of the melt affects the rate and mechanism of the reactions occurring in the melt. These studies are based on the idea that the electro- chemical activities of oxygen-containing anions originate from the cation ± anion mechanism of the EAS formation specific for molten salts (see Section II).This mechanism can be regarded as acid-base interaction where the decisive role belongs to the acid properties of the cations. The formation of EAS and their reactions in the near-electrode layer occur at a limited rate since these processes involve complete restructuring of the reacting species. 4 The assumption on the cation ± anion interaction in molten electrolytes was confirmed in studies on the electrochemical reduction kinetics of oxygen-containing anions in chloride and oxide melts. Detailed studies on the electrochemical reduction of WO2¡ with an equimolar KCl ± NaCl melt as the background 4 and of a sodium tungstate melt 114 showed that the tungstate ion itself is not an electrochemically active species but it can generate such species.Owing to the cation ± anion interaction, a sufficient excess of strongly polarising cations results in outer-sphere cationisation of the WO2¡ 4 anion to give symmetrically cationised species. Thus, the electrochemical reduction of oxygen-containing anions in the presence of molten chlorides as the background is preceded by a slow acid-base reaction which is the rate-determin- ing step of the electrode process. The EAS are formed due to the interaction of the oxygen-containing anion with the molten electrolyte according to the cation ± anion interaction mechanism determined by the acid-base properties of the medium. VI. Theoretical and experimental studies of cation ± anion interactions in nitrate- and carbonate-containing melts 3 3 The results of experimental studies discussed above 28 ± 30, 39 ± 42, 44 ± 49, 108 ± 110 suggest that the reduction of oxy- gen-containing anions depends on the cationic composition of the melt.Hence, it is necessary, first, to estimate the possibility of the formation of cationised anions (metal complexes) in principle and second, to study the effect of cations on the reducing abilities of anions. In application to nitrate- and carbonate-containing melts, the cation ± anion interaction was represented by Schemes (7) and (8). The CO2¡ anion, the CO2 molecule and their complexes with the lithium cations (up to five-coordinate complexes) were selected as model compounds for preliminary quantum-chemical calculations.A preliminary energy-based estimate of cation ± anion interaction pathways for carbonate-containing melts based on a semiempirical calculation showed 115, 116 the possibility of formation of a broad spectrum of metal complexes of the CO2¡ anion with lithium cations and the effect of the cation nature on the increase in the reducing ability of cation ± anion interaction products. Analysis of the energy profiles of potential surfaces of the interaction of NO¡3 and CO23 ¡ with Li+ or Be2+ showed that the interaction of cations preferentially occurs along the bisecting line of any bond angle of the anion; there are local energy minima in the monodentate position of the cation with respect to the anion.117, 118 If interaction between cations and anions is suffi- ciently strong (it is more efficient in the case of Be2+), the weakening of N7O intra-anion bonds under the effect of the cation was found to differ noticeably from that ofC7Obonds.As calculations showed, bidentate interaction of an anion with a cation results in electron density migration from the 2s, 2px and 2py orbitals of the O(1) and O(2) atoms to the corresponding168 cation orbitals with formation of s-bonds. As a result, an effective positive charge s+ is transferred to the oxygen atoms. s+ O(1) s7 M (17) O(3) E O(2)s+ In turn, this favours the electron-donating effect of the 2p2 orbitals of O(3) s7 O(1) s+ s7 M (18) O(3) E O(2) with formation of p-bonds. In complexes formed by the nitrate ion, electron density 3 3 redistribution along the p-bonds under the effect of cations occurs within the NO¡3 anion and causes strengthening of the N7O bond.On the contrary, the s-mechanism of electron density redistribution predominates in the carbonate ion and in the corresponding complexes, resulting in a considerable weakening of the C7O bonds. Thus, the role of metal cations is to effect the polarisation of an anion as a whole according to the s,p- interaction mechanism with predominance of one of the inter- action types in the isoelectronic ions NO¡ and CO2¡ (see Refs 119, 120). The results of semiempirical and ab initio calcu- lations coincided qualitatively, which afterwards permitted the use of semiempirical calculations which are less laborious.The gradual increase in the number of the first solvate shell cations, results in the weakening of C7O bonds in CO2¡ 3 with an increase in the coordination number. For the coordination number equal to 2, cleavage of one C7O bond occurs. Con- versely, a trend towards strengthening of the N7O bonds is observed for NO¡3 . This conclusion was confirmed by data on charge transfer and electron density migration within anions for the entire coordination number range and by spectroscopic data.121 Analysis of the energies of the frontier molecular orbitals showed that the ability of NO¡3 to undergo reduction is higher than that ofCO2¡ 3 . Cationisation results in a decrease in the energy gap between the highest occupied and lowest unoccupied molec- ular orbitals in these anions, which suggests that the cation ± anion interaction has a favourable effect on the ability of anions to undergo reduction.122, 123 Comparison of the reduction activation barriers for NO¡3 and CO23 ¡ and their metal complexes also indicates an increase in the ability of anions to undergo reduction due to the cation ± anion interaction.This interaction decreases the reduction activation barriers for the entire series of com- plexes.123 Electric field is one of the most important factors affecting the reduction in melts. The results of quantum-chemical calculations were used to estimate the effect of the electric field on the acid-base interaction and on the elementary charge transfer act.A uniform electric field was simulated by a system of opposite charges located symmetrically at a long distance from the molecule. The calcu- lations were carried out using a programme developed for this purpose.124 It was shown 125 that a field which enhances the specificity of cation ± anion interaction (predominantly, s-type in carbonate ion and p-type in nitrate ion) affects considerably the reducing ability of the interaction products; specifically, it lowers the activation barrier. Shapoval et al.126 reported the results of an experimental examination of the acid-base interaction mechanism by determin- ing a number of electrochemical parameters of the electrode process affected most strongly by the acidity of the melt.The model, Scheme (8), determined the choice of the starting system for the study on cation ± anion interaction in melts. Studies on NO¡3 electrochemical reduction in a K, Na, Cs/Cl eutectics as the background (T=793 K) showed that, like the K, Na, Rb/Cl V I Shapoval, V V Solov'ev, V V Malyshev 3 eutectics, it is weakly acidic with respect to the NO¡3 anion and is suitable for studies on NO¡3 anions with strongly polarising cations. Introduction of Mg2+, Ca2+ or Li+ cations to a nitrate- containing chloride melt at concentrations no higher than a two-, four- and hundred-fold excess, respectively, with respect to NO¡ shifted the electrochemical reduction wave towards negative potentials.127 ± 129 Further increase in the concentration of the added anions shifted the discharge potential to more positive region.Analysis of the chronovoltammograms for theNO¡3 anion in the presence of, e.g., a strongly polarising Ca2+ cation suggested unambiguously the catalytic character of nitrate reduc- tion waves in the presence of excess Ca2+. It was shown 127, 129 that the excess of these cations is not only consumed in the formation of qualitatively new electrochemically active species, viz., metal complexes, but also results in regeneration of the depolariser in agreement with the experimental Scheme (11). The coordination number of the Ca2+ cations determined in the presence of a considerable excess of these cations equalled 2, while that in the case of excess magnesium cations equalled 3.Calculations of heterogeneous rate constants of charge transfer and regeneration of the original EAS indicate that introduction of cations to the melt catalyses the overall process,127, 130 while the EAS formation during the electrochemical reduction of NO¡3 in the presence of Li+, Ca2+,Mg2+ occurs according to the cation ± anion interaction mechanism in accordance with Scheme (8). 3 33 3 Experimental studies on the cation ± anion interaction of CO2¡ with strongly polarising cations 131 ± 133 indicate that this can be described by Scheme (7). Experimental studies confirmed certain regularities obtained from quantum-chemical calculations.134 For example, an increase in the rate constants of charge transfer and regeneration of the original EAS with an increase in the specific charges and concen- trations of cations in a melt corresponded to a decrease in the activation barrier for EAS reduction.The reduction potential shift by more than 0.6 Vupon replacement of Na+byMg2+ correlated with the calculated changes in the N7O bond energies in metal complexes formed by the nitrate ion. The absence of electro- chemical activity in CO2¡ in the presence of weakly polarising cations in a melt correlates with high activation barriers of two- and four-electron reduction. The experimentally found reduction ofCO2¡ 3 in the same potential range in the presence of cations with various polarising strengths correlates with the calculated data, according to which CO2¡ is characterised by a trend towards dissociation.Similar results were also obtained in other studies 51, 135 on the electrochemical reduction of CO2¡ in tung- state melts with various cationic compositions. 3 The results of a theoretical and experimental study on the mechanism of cation ± anion interaction in carbonate and nitrate- containing melts were reviewed by Solov'ev.136 It is generally acknowledged that this mechanism includes the formation of cationised anions (metal complexes) [MnNO3](mn71)+ and disso- ciation of CO2¡ under the effect of cationic environment. How- ever, this mechanism being applied to dilute melts did not account for the effect of the electrode surface and the electrical field on both the character of cation ± anion interaction and the elemen- tary charge transfer act.The electrochemical reduction characteristics measured depend on a combination of various factors. The adsorption properties of the electrode surface (together with the temperature mode and the electrical field) affect strongly the character of electrochemical reduction in melts. Considering the surface of a solid electrode as possessing a discrete structure and the differ- ences in the structures of the melt ± electrode interfaces (at zero charge potential) caused by individual features of the electrode and melt surfaces, one should take into account the interaction of the surface atoms with adsorbed species of the melt. In accordance with the general physical concepts, this results in changes in the electronic properties of the interface.A preliminary estimate (quantum-chemical calculations) of the electrical field effect on the reactivity of cation ± anion interaction products 125 agrees withElectrochemically active species and multielectron processes in ionic melts real processes of electrochemical reduction in melts. Such proc- esses occur on electrode ± melt interfaces where a certain potential gradient exists. Electrochemical reduction and cation ± anion interaction are influenced not only by adsorption effects but also by the electrical field (external field and double layer field). Therefore, more detailed studies of the field effect on electro- chemical reduction (on the energy parameters of cation ± anion interaction products and on their reactivities) are necessary.Thus, three basic factors should be taken into account in the analysis of the EAS formation mechanism and electrochemical reduction in nitrate- and carbonate-containing melts, viz., the acid-base properties of a melt, the electrode surface properties and the effect of the electric field. VII. Multielectron electrochemical reduction of oxoanions of high-melting metals and non-metals in ionic melts Multielectron electrochemical discharge ± ionisation reactions can occur with simultaneous charge transfer or through a number of consecutive steps, with transfer of one or several electrons involved in each of the steps. It was noted 137 ± 139 that the mechanisms of these reactions can be described by determining experimentally the effective transfer factors or the stoichiometric factors of the elementary steps.However, due to the diversity of ionic states in electrolytes and structures of cathodic deposits, it is unlikely that a general (common) mechanism for the electro- chemical deposition of polyvalent metals and non-metals will ever be established.140, 141 One can govern the reactivities, solubilities and electrochemical properties of the starting and transient forms by changing, e.g., the temperature and electrolytes. There are several viewpoints on the mechanisms of electro- chemical reduction of high-melting polyvalent metals from ionic melts. According to one of them (cf. Ref. 141), electrochemical reactions result in cathodic deposition of high-melting metals as solid oxygen-containing compounds (oxides and bronzes).A different viewpoint was expressed by Spitsyn et al.142 who believe that the deposits in question consisted of products of electro- chemically initiated polymerisation of high-melting metal ions. At the moment, it is hardly possible to prefer the involvement of one particular species over another in reduction. However, there is no doubt that after multicharged species obtain the first electrons, they become reactive and initiation of polymerisation of the species becomes possible. In this case, the situation resembles that observed in aqueous solutions.141 It is believed that the alkali metal acts as the reduced state stabiliser in melts and in non- aqueous solutions; in aqueous solutions, this role is played by hydrogen.However, the mechanism suggested is questionable, especially in the case where solid oxygen-containing compounds are not formed on the cathode. It was assumed in certain publications (see, e.g., Refs 143 and 144) that the discharge and ionisation of polyvalent metals in aqueous solutions occur via transient ions with intermediate valence, the kinetic and thermodynamic stabilities of which depend on the nature of the metal and the electrolyte. Many of the intermediates formed in the bulk of the near-electrode layer possess high reactivities; they react with each other (dispropor- tionation) and with the solution components (redox reactions). As a consequence, the life-times of these ions are small.It is a matter of discussion whether these concepts can be applied to ionic melts. Polyakov 145 analysed the studies on the electrochemical behaviour of niobium in chloride, oxochloride, chloride ± fluor- ide, oxochloride ± fluoride, chloride and oxofluoride melts. It was concluded that both multistep discharge (according to virtually every possible scheme for pentavalent metals, including the transient formation of clusters and intermediates) and single-step discharge of niobium-containing ions are possible. It is generally acknowledged that niobium is reduced in two or three steps in 169 chloride ± fluoride electrolytes. The number of steps of niobium reduction in oxochloride ± fluoride melts depends on the O:Nb ratio (for O:Nb41, a single-step process is observed).The diversity of niobium oxofluoride and fluoride complexes existing in melts based on NaCl ± KCl and the variety of mecha- nisms of their electrochemical reduction have been noted in a number of studies.146 ± 151 To date, much remains unclear in the chemistry and electrochemistry of niobium in salt melts.145 A general problem, as in studies on other metals, is the establishment of correlations between the data from physicochemical and spectroscopic studies and the results of electrochemical measure- ments obtained by linear voltammetry, chronopotentiometry and other related methods. In addition, discharge of different-ligand complexes and release of their anionic shells in the near-cathode layer can involve the formation of species with compositions other than those of species in the melt bulk.6 6 The laws of electrochemical behaviour of tantalum resemble those of niobium in similar melts.147 In their surveys Kuznetsov et al.152, 153 noted diverse views on the electrochemical reduction mechanism of zirconium in fluo- ride, chloride, fluoride ± chloride and oxofluoride melts. Some researchers consider that discharge of ZrF2¡ to give the metal is preceded by various intermediate steps (formation of complex fluorides nMF. ZrF2, poorly soluble complex salts MZrF4, etc.). Others believe that the reduction of ZrF2¡ is a multistep (involv- ing two or even three steps) or a one-step process.The existing disagreement can be explained by differences in the experimental conditions and in techniques for the interpretation of results.154, 155 It is presently believed 155, 156 that the electrochem- ical reduction of zirconium in chloride ± fluoride melts occurs either according to the auto-inhibition scheme or with four- electron reversible single-step charge transfer, depending on the concentration of the F7 ion. Of particular interest are theoretical studies on the stepwise character of charge transfer. For non-adiabatic reactions, the discharge of polyvalent ions is usually represented by a sequence of single-electron charge transfer steps with concomitant inter- valent interactions, which are also considered to be one-electron processes.157, 158 However, a series of studies on the features of electrochemical reduction of the elements in question did not reveal any steps in the discharge of the corresponding oxo, oxohalide and halide complexes.Studies on adiabatic electro- chemical electron transfer reactions showed 157, 158 that at a certain ratio between the energy of environment reorganisation accompanying the electron transfer and the Coulombic interac- tion energy of valence electrons in the reagent, the consecutive single-electron discharge can become multi-electron discharge. In this case, the elementary charge transfer act can be compared to a continuous stream of electrons which cannot be separated into separate steps. To date, the stepwise character of multielectron processes could not be detected by methods of electrochemical analysis.The use of quantum-chemical methods was suggested for this pur- pose.57, 79, 101, 104 A specially developed technique was used for comparison of the activation barriers of multi- and single-step electrochemical EAS reduction with optimal compositions and forms in carbonate-, nitrate-, boron- and titanium-containing melts.57, 104 For boron-containing melts it was found 57, 101 that the activation barriers for two-step reduction of BF¡4 anions and BF3 molecules are lower than those for single-step reduction. This law is no longer valid in the case of boron-containing metal complexes. It was concluded for titanium-containing fluoride melts 57, 79 that single-step reduction is more favourable than multi-step reduction.Thus, no common mechanism of electrochemical reduction of multivalent metals and non-metals has been established to date. Different states of the ions of these metals in electrolytes, structures of cathodic deposits and conditions of the process (temperature and the nature of the electrolyte) can favour one or another electrochemical reduction mechanism. The ambiguity170 and diversity of electrochemical reduction mechanisms of EAS can also be due to differences in the melt compositions (different anionic and cationic compositions, admixtures) and in features of experimental techniques (for example, electrode polarisation rates, purity of reagents, materials of electrodes and atmosphere in voltammetry). The possible `masking' of the apparent stepwise mechanism by autoinhibition should also be noted.Therefore, in each particular case of single-step electrochemical reduction, one should speak of a narrow potential range, which practically cannot be resolved by the existing methods. The processes of electrochemical evolution of high-melting metals and non-metals from ionic melts underlie a new promising method for the preparation of carbides, borides and silicides of Group IV ± VIA metals, i.e., high-temperature electrochemical synthesis.3, 159 It should be noted that there is as yet no generally accepted opinion on the mechanism of EAS formation involving these compounds. In this Section, we survey the results of experimental studies on the mechanisms of formation and struc- tures of the EAS of high-melting metals.1. MoO2¡ 4 and WO24 ¡ anions with chloride melts as the background in the presence of strongly polarising cations 4 7 4 4 Systematic studies on the electrochemical behaviour of molybdate and tungstate ions in oxohalide electrolytes were carried out by many research groups. Electrochemists from the Sverdlovsk scientific school studied the electrochemical deposition from melts based on alkaline-earth metal chlorides. It was found 160 that anodic dissolution of molybdenum in CaCl2 ± CaO melts produces MoO2¡ ions; if an admixture of MoO3 is present, the formation of Mo2O2¡ ions is more probable. Depending on the melt temperature on the cathode, either molybdenum oxide Mo2O3 or metallic molybdenum are deposited.161 Deposition of molybdenum occurs from electrolytes with acidic properties caused by the presence of calcium cations.160 ± 162 It has been shown by electrochemists from the Kiev scientific school 163, 164 that EO2¡ anions (E=Mo, W) in an equimolar KCl ± NaCl melt as the background (973 ± 1073 K) do not man- ifest electrochemical activity up to decomposition potentials of the background electrolyte.The equilibrium potential of molybde- num (tungsten) is not reached in these systems, as noticeable metallic corrosion occurs. The acidic character of alkali and alkaline-earth metal cations (Li+, Ba2+, Ca2+, Mg2+, Al3+) was established.165, 166 Introduction of cations with high polar- ising strength (Li+, Ba2+, Ca2+) to melts containing EO2¡ 4 anions gives rise to a reduction wave at more positive potentials than the decomposition potential of the background electrolyte.167, 168 Acidification of the melt results in a considerable increase in the reduction wave of molybdate (tugstate) ions and its displacement to the positive range of potentials.If the melt is acidified with a small concentration of Ca2+ cations, one wave is observed; two waves are observed at [Ca2+]&(4 ± 5) [WO2¡ 4 ]: the first wave reaches a maximum, and a second wave appears at more negative potentials. Upon acidification with Li+, Ba2+ or Ca2+ cations, the WO2¡ reduction waves maintain their kinetic nature. This follows from the absence of a linear dependence of the limiting current on the cation concentration and from the activation energy of the limiting current (88.2 kJ mol71 for [Ca2+]?0).As the melt is acidified, the surface kinetic constant increases by an order of magnitude and reaches the values comparable to the diffusion constant, while the activation energy of the limiting current decreases to 12.4 kJ mol71 (see Ref. 169). 4 If a 3.5 ± 4.0-fold excess of Mg2+ cations with respect to EO2¡ is introduced, an almost proportional relationship between the limiting current of the reduction waves and the concentration of these cations is observed.170, 171 The maximum limiting current is reached at [Mg2+]&4 [EO2¡ 4 ]. Under these conditions, the mass transfer rate constant [(1.5 ± 2.0)61073 cm s71] is comparable to the diffusion constant; this suggests the diffusion nature of the steady-state waves.V I Shapoval, V V Solov'ev, V V Malyshev Introduction of Al3+ cation also resulted in a proportional dependence of the limiting current on the concentration of this cation. At [AlCl3]>0.5 [EO2¡ 4 ], strong evolution of gases (chlor- ine, chlorine oxides) and a decrease in the limiting current were observed. 4 4 4 4 In a chloride melt as the background, the electrochemical reduction of MoO2¡ and WO2¡ anions to the metals in the presence of various cations (Li+, Ba2+, Ca2+, Mg2+, Al3+) occurs at various potentials. On going from Li+ to Al3+, the reduction potentials are shifted in the positive direction by more than 1 V.The limiting current of MoO2¡ and WO2¡ electro- chemical reduction is reached at a non-equivalent concentration of cations. The shift of the reduction half-wave potentials (E1/2) upon a ten-fold increase in cation concentration is 0.2 ± 0.4 V. Such a significant E1/2 shift upon changing the concentration and the cation polarising strength results from the different composi- tions of the electrochemically active species due to changes in the acid-base properties of the melt. As Shapoval et al.170, 171 report, this shows that, first, variations of the acid strength and the cation concentration change the acid-base reaction stoichiometry, which cannot be described by dissociation of the anion; second, it cannot be described by a reaction resulting in the complete loss of oxygen ions by the metal.The EAS formation mechanism in melts with various cationic compositions can be presented by the following scheme: {Mzá x [EO24 ¡]}zx72 (19) xMz++EO2¡ 4 EO2Öx¡1Ü +xMz+O27 , 4¡x where E=Mo, W;Mz+=Li+, Ba2+, Ca2+, Mg2+, Al3+. The direction of shift of equilibrium (19) is determined by the polarising strength and concentration of the cation and by the anion strength. In any case, the cation ± anion interaction occurs through a step of the formation of metal complexes. 4 4 Acidification of a melt containing MoO2¡ or WO2¡ by Li+, Ba2+, Ca2+ or Mg2+ cations results in outer-sphere cationisation of anions giving metal complexes {Mzá x [EO24 ¡]}(xz72)+. Acid- ification of the same melts by the Al3+ cation results in decom- position of the anions to give EO2Öx¡1Ü¡ species.4¡x 4 The outer-sphere cationisation of WO2¡ anion by the Ca2+ cation was studied by IR emission spectroscopy.172 It was shown that at [Ca2+]>[WO2¡ 4 ] , symmetric cationisation occurs to give the {Ca2á 4 [WO24 ¡]}6+ species. Shapoval et al.170, 171 also observed an increase in the electron- accepting properties of molybdate and tungstate ions with increase in their degree of cationisation and increase in the acid strength of the cation. This indicates that multielectron processes might have occurred. Apparently, the specific features of multi- electron processes are caused by the particular structure of the cationised EAS.For example, in the case of Ca2+ and Li+cations, two electrons are transferred in the electrode process (20) WO2+x CaO+(27x)O27 , {Cax[WO2¡ 4 ]}2(x71)+2e7 where 14x42. For x=4 in the presence of Mg2+, Al3+ in the melt, transfer of six electrons occurs in a practically unresolvable range of potentials. The multielectron electrochemical processes are described by the following overall reactions: (21) E+2Li2O+2O27 , {Liá4 [EO24 ¡]}2++6e7 E+4CaO , (22) {Ca2á 4 [EO24 ¡]}6++6e7 E+4MgO . (23) {Mg2á 4 [EO24 ¡]}6++6e7 In the case of acidification by the Al3+ cations (24) Al+2O27 + (2+x)Cl7 . AlO2Clx2áx +6e7Electrochemically active species and multielectron processes in ionic melts 2. CO2¡ 3 anions with chloride and carbonate melts as the background The cathodic evolution of carbon upon electrolysis of molten carbonates has first been reported by Haber and Tolloczko 173 who obtained carbon deposits from a BaCl2 ± BaCO3 melt at 853 K.Systematic studies on electrolysis and electrode processes involving carbonates have been carried out by electrochemists from the Kiev and Sverdlovsk scientific schools. Delimarskii et al.22 have found that graphite powder is the only cathodic product in the electrolysis of an equimolar Li2CO3±K2CO3 mixture at 853 ± 873 K (current efficiency *100%); carbon dioxide and oxygen (in 2 : 1 ratio) are the anodic products. The cathodic process is believed 174, 175 to include the reaction (25) CO2¡ CO2+O27 3 and reaction (9), while the anodic process includes the reactions (26) CO2¡ CO2+(1/2)O2+2e7 , 3 (27) O27 (1/2)O2+2e7 .The ratio of the parallel reactions (26) and (27) is determined by the current density. It has been noted that carbon is not formed in the electrolysis of pureK2CO3. If this melt is saturated withCO2 (under pressure), carbon is precipitated on the cathode. Shapoval et al.176, 177 concluded that a certain CO2 concentration is required for carbon deposition from molten chlorides; it can be achieved by addition of thermally unstable carbonates or by saturation of the melt with gaseous carbon dioxide. Trunov and Stepanov 178, 179 showed that a platinum electrode which contacts with a mixed stream of carbon dioxide and oxygen is reversible with respect to a carbonate melt.They believe that an equilibrium (between carbonate ions and molecules of CO2 and O2) is established on the electrode. Smirnov et al.180 studied the polarisation of platinum and nickel electrodes in a melt of alkali metal carbonates. It was shown that, depending on the current density, various electrode reactions occur resulting in formation of carbides, evolution of free carbon and carbon(II) oxide, as well as discharge of alkali metal cations. The polarisation depends on the concentration of carbonate ions and is due to their diffusion from the bulk melt to the surface. The mechanisms of anodic processes in chloride ± carbonate melts in various atmospheres (He, CO, CO2 or 2CO2+O2 mixture) have been studied in detail.181 Bartlett and Johnson 27, 182 calculated the decomposition potential of alkali and alkaline-earth metal carbonates and considered all possible variants in the reduction of carbonate ions.Based on the calculations, it was concluded that the precipitation of the alkali metal is energetically more favourable at all temperatures for K2CO3; for Na2CO3, the evolution of carbon is possible up to 1143 K, while that of sodium is possible above 1143 K. Reduction of lithium, calcium, magnesium and barium carbonates gives carbon. 3 4 6 4 In a series of studies,183 ± 187 the formation of new species upon dissolution of carbon dioxide in molten Li2CO3 ± Na2CO3±K2CO3 eutectics was reported. The formation of dicar- bonate ions was confirmed by potentiometric titration, by struc- tural studies on a melt and by determination of the free energy (DH=7142 kJ mol71) of the reaction between CO2 and CO2¡ to giveC2O2¡ 5 (see Ref. 183).The possible existence of such species as CO2¡ (see Refs 184, 185), C2O2¡ (see Ref. 184), C2O2¡ (see Ref. 186) and CO2¡ 3 2 (see Ref. 187) was noted. In K, Na, Rb/Cl melts as the background, the carbonate ion does not show electrochemical activity.188 An increase in temper- ature to 700 8C does not favour the electrochemical reduction of CO2¡ 3 either. The addition of a 500-fold excess of LiCl with respect to CO2¡ 3 does not produce a wave on the voltammograms. As the Li+ concentration is increased, a small wave of CO2¡ electro- chemical reduction stretched along the potential axis is observed in the potential range from 0.7 to 0.9 V.The Ca2+ cation affects the electrochemical reduction of CO2¡ more strongly than Li+ 3 171 3 does. A noticeable wave is observed even with a 200-fold excess of Ca2+. The waves in the presence of Ca2+ are also extended along the potential axis. The wave heights were not found to depend explicitly on the cation concentration. The effect of the Mg2+ cation was found to be most pronounced. A distinct wave appears even with a four-fold excess of Mg2+. Further increase in the Mg2+ concentration increases the wave height, and it reaches a maximum with a 100-fold excess of Mg2+. Evacuation of the system after reaching the maximum wave height decreases the latter.The addition of Mg2+ after evacuation increases the wave again. It should be noted that the electrochemical reduction of CO2¡ in molten chlorides as the background occurs in the same potential range, irrespective of the polarising force of the cation. Hence, the electrode processes in melts with different cationic compositions involve the same species, namely, CO2. The electrochemical reduction of CO2¡ 3 under the action of strongly polarising cations occurs at much lower temperatures than the thermal decomposi- tion temperatures of the corresponding carbonates. Hence, the formation ofCO2 does not result from thermal decomposition but is a consequence of the polarising effect of cations. 3. MoO2¡ 4 , WO24 ¡ and CO23 ¡ anions in oxide melts as the background in the presence of strongly polarising cations 44 7 It is known 189 ± 192 that polytungstate and polymolybdate melts Na2EO4±EO3 (E=Mo, W) can be considered as melts of eutectic mixtures Na2EO4±Na2E2O7 containing the Na+, EO2¡ and E2O2¡ ions. The ratio between the concentrations of EO2¡ and E2O2¡ 7 ions is determined by the equilibrium (28) 2EO2¡ 4 E2O2¡ 7 +O27 .Presumably, the introduction of Li2WO4 or ZnWO4 as well as of WO3 in a sodium tungstate melt complicates the structure of tungsten ± oxygen groups, for example, owing to the reaction (29) LixO(27x)7+W2O2¡ 7 . 2WO2¡ 4 +x Li+ 4 However, if this concept is applied to moltenMIEO4±MIIEO4 mixtures (MI=Li, Na, K; MII=Ca, Zn, Ba, Sr, Mg, Al), some contradictions arise.First, if the equilibrium (28) took place upon introduction of tungstates of strongly polarising cations, e.g., CaWO4, in a sodium tungstate melt, this would have occurred in melts containing an excess of the Ca2+ cation with formation of the tungstate ion dimer. However, studies on the electrochemical behaviour of the CaCl2 ±CaWO4 ±CaO system at high temper- atures 152 and the IR spectra of a KCl ± NaCl ±Na2WO4 ± CaCl2 melt (for [Ca2+]>[WO2¡ 4 ]) 14 indicate that the tungstate ion retains its tetrahedral structure. Second, the solubility of tung- states (molybdates) with strongly polarising cations in sodium or potassium tungstate (molybdate) decreases with the increase in the cation specific charge.193 These facts can imply that the strength of Mz+±EO2¡ interaction increases with increase in the specific charge and that the concentration of oxygen ions in the molten mixtures decreases.In our opinion, the cationic composition of tungstate and molybdate mixtures, like that of oxochloride melts, should affect considerably the EAS composition and the parameters of the electrode process. Kushkhov et al.194, 195 studied the melts of Na2WO4 ±MEO4 mixtures (M=Li, Ca, Zn, Ba, Sr, Mg, Al; E=Mo, W). The addition of tungstates and molybdates with strongly polarising cations shifts the oxygen electrode potential towards more positive values, i.e., decreases the activity of the O27 ion. An increase in the cation specific charge in the series Ba2+, Sr2+, Li+, Mg2+, Al3+ leads to a more significant shift.In the cases of addition of aluminium tungstate (molybdate) and tungsten (molybdenum) trioxide, the concentration dependences of the oxygen electrode potential practically coincide. It can thus be assumed that if a strongly polarising cation is present in a melt, an acid-base reaction of the molybdate (tungstate) ion with the Al3+ cation is possible172 (30) 2Al3++3EO2¡ 3EO3+Al2O3 . 4 According to reactions (28) and (29), the parameters (poten- tials, rates) of electrochemical evolution of tungsten and molyb- denum should be virtually identical in melts with identical acidities, irrespective of the cationic composition. However, an increase in the cation specific charge shifts the electrochemical reduction wave towards more positive potentials.For example, the shift upon transition from Na+ to Al3+ is*1.0 V. This fact cannot be explained using the viewpoint that EAS are formed in melts of sodium tungstate containing molybdates and tungstates of strongly polarising cations according to reaction (28). In melts with equal acidities, the specific charge on cations affects considerably the electrochemical reduction potential. In fact, the electrochemical reduction potentials of tungstate ion in the presence of barium, magnesium and aluminium cations are within 1.8 ± 1.9, 1.5 ± 1.7 and 1.0 ± 1.2 V, respectively. A similar situation is observed for molybdate ion. The acid-base (cation ± anion) interactions occurring in a melt of sodium tungstate upon addition of the carbonate ion are judged by changes in the e.m.f. of a cell with oxygen electrodes.135 Upon addition of carbonates, the oxygen electrode potential is shifted towards negative values, indicating an increase in the oxygen ion activity in the melt.The addition of lithium carbonate shifts the oxygen electrode potential much more strongly than that of sodium carbonate. This is due to the fact that the stability constant of sodium carbonate is higher than that of lithium carbonate.196 For the same reasons, a melt of sodium tungstate containing lithium carbonate has a lower pCO2. The electrochemical reduction of carbonate ion in sodium tungstate as the background can be detected only upon addition of lithium carbonate.The absence of a linear dependence of the reduction wave current on the lithium carbonate concentration and the decrease in the diagnostic criterion jp/V1/2 (where jp is the peak current density, V is the potential sweep rate) with an increase in the polarisation rate implies the kinetic nature of electrochemical reduction waves of carbonate ion. Electrochemi- cally active CO2 species are formed as a result of cation-anionic interaction of the strongly polarising Li+ cation with the carbo- nate ion. The formation of CO2 can be described by the above scheme [see Eqn (5)]. The addition of sodium carbonate increases pCO2 considerably, therefore the CO2 reduction wave is not observed in the voltammetric curves. 4. Effect of PO¡3 anions on the formation of electrochemically active species in the presence of MoO2¡ 4 and WO2¡ 4 anions 3 7 Introduction of PO¡3 in KCl ± NaCl ±Na2WO4 (Na2MoO4) melts gives two waves on the voltammograms.197, 198 An increase in PO¡ concentration within 0.01<[PO¡3 ]/[EO23 ¡]<0.2 increases the current of the second wave, whereas the potentials and currents of the first wave remain unchanged. The current of the second wave is proportional to the PO¡3 concentration. Potentiostatic electrolysis of molybdate (tungstate)-containing melts with the specified concentration ratio of molybdate (tungstate) and meta- phosphate anions gives a deposit of the corresponding high- melting metal on the cathode.Our assumption on the participation of the dimeric ionic forms of molybdenum and tungsten in the electrode processes in question is based on the following data.It has been shown 199 that introduction of sodium metaphosphate in a purely tungstate melt shifts the potential of the tungsten electrode from 71.8 to 71.0 V. It is at 71.0 V that discharge of W2O2¡ ions occurs; the existence of these ions has been proven both electrochemi- cally 200 and by high-temperature radiography.201 4 4 Based of voltammetric data,197, 198 the effect of PO¡3 on the electrochemical reduction of MoO2¡ and WO2¡ can be inter- preted as follows. The role of PO¡3 involves the formation of two EAS forms according to the scheme V I Shapoval, V V Solov'ev, V V Malyshev E2O2¡ 7 +PO34 ¡ (31) PO¡3 +2EO24 ¡ 2EO3+PO3¡ 4 +O27 .The formation of ditungstate ions in a sodiumtungstate melt in the presence of PO¡3 anions has also been reported byMalyshev.202 5. Effect of neutral MoO3, WO3 and B2O3 molecules on the formation of electrochemically active species in the presence of MoO2¡ 4 and WO24 ¡ anions The effect of tungsten trioxide on the electrode processes in tungstate melts has been studied in detail by a research group directed by Academician A N Baraboshkin.189, 191, 200 Contra- dictory opinions on the ionic composition of tungstate melts exist at present. It has been noted above that the melts of the Na2WO4±WO3 system can be regarded as melts of the Na2WO4±Na2W2O7 eutectic mixture and that they mostly consist of a mixture of Na+, WO2¡7 4 andW2O27 ¡ ions.The discharge of W2O2¡ occurs more easily than that of the tungstate ion. This is evident from a thermodynamic calculation of the decomposition potentials of the corresponding com- pounds 203, 204 and from experimental data:190, 191 introduction of small amounts of WO3 increases abruptly the potential of tung- sten evolution and decreases the decomposition potential of polytungstate melts. 4 The effect of MoO3 on electrode processes in the sodium tungstate melt and the EAS formation mechanism were studied by potentiometric and voltammetric measurements (a cell with oxy- gen electrodes was used).205, 206 Measurements of the equilibrium potentials of the oxygen electrode showed that in the presence of up to 2 mol.% ± 3 mol.% MoO3 in the sodium tungstate melt, the pre-logarithmic factor of the dependence of the oxygen electrode potential on MoO3 concentration is 0.120 V.This means that in this concentration range molybdenum exists mainly as MoO2¡ ions. The pre-logarithmic factor of this dependence at concen- trations 7 mol.% ± 20 mol.% is 0.112 V. Hence, molybdenum is present mainly as dimolybdate ions Mo2O2¡ 7 in this concentration range. The experimental dependences match the calculated ones, which confirms the validity of the chosen model of the ionic composition of the melt. 7 Measurements of the potential of a molybdenum electrode versus MoO3 concentration showed the existence of two equili- bria.206, 207 At MoO3 concentration up to 4 mol.% ± 5 mol.%, metallic molybdenum is in equilibrium with a melt containing Mo2O2¡ ions.The concentration dependence of the equilibrium potential is well described by the Nernst equation. The number of electrons per one electroactive species calculated from the slope of the plot of the molybdenum electrode potential against MoO3 concentration, (dE/dlog [MoO3]), equals 1.5. This value is con- sistent with the equilibrium (32) Mo+7MoO2¡ 4 . 4Mo2O2¡ 7 +6e7 7 At high MoO3 concentrations (>5 mol.%), the activity of Mo2O2¡ increases, and a tungstate ± molybdate melt is in equili- brium with the dioxide MoO2 rather than with metallic molybde- num (33) MoO2+3MoO2¡ 4 . 2Mo2O2¡ 7 +2e7 The number of electrons per one electroactive species, also calculated from the slope dE/dlog [MoO3], equals 1.This value describes the equilibrium (33). The formation of dimolybdate and ditungstate ions in tung- state ± molybdate melts in the presence of B2O3 was noted.208, 209 6. Effect of fluoride and fluoroaluminate ions on the formation of electrochemically active species in the presence of CrO3, MoO3, WO3, CrO2¡ 4 , MoO24 ¡ and WO24 ¡ in chloride ± fluoride melts Tungsten and molybdenum trioxides can be stabilised in chloride melts by binding them in oxofluoride complexes which are moreElectrochemically active species and multielectron processes in ionic melts stable. Such complexes have been described in the literature; they are obtained by treatment of tungstates and molybdates with hydrofluoric acid.We were able to find data on the existence of these compounds in melts in only one publication.210 The composition range of oxohalide melts where tungsten and molybdenum exist as stable oxofluoride complexes was deter- mined by studying KCl ± NaCl ± NaF ±EO3 melts (E = Mo W) with different [NaF]/[EO3] concentration ratios.211, 212 Based on the results of these studies, the overall electrode process of electrochemical reduction of the oxofluorotungstate complex can be presented as follows: (34) W+2O27+4F7 , [WO2F4]27+6e7 (35) 2O27+[WO2F¡4 ]27 WO2¡ 4 +4F7 , (36) 2[WO2F4]27 +6e7 W+WO2¡ 4 +8F7 . 4 4 As noted above, MoO2¡ and WO2¡ in melts of chlorides of weakly polarising cations (Na+, K+) as the background manifest no electrochemical activity up to decomposition potentials of the background electrolyte.The addition of fluoride ion to a sodium chloride melt containing molybdate and tungstate ions does not change the shapes of the voltammetric curves. The molybdate and tungstate ions are stable in NaCl ± NaF melts; the O27 is not replaced by F7 with the formation of oxofluoride complexes, and they do not manifest electrochemical activities up to decomposi- tion potentials of the background electrolyte.213, 214 4 4 6 x It is known that aluminium fluoride complexes possess acceptor properties with respect to O27 ions and hence can be used for controlling the acid-base properties. On introduction of AlF3¡ in a sodium chloride melt containing MoO2¡ and WO2¡ anions, waves at 0.7 ± 1.0 V appear on the voltammetric curves.163, 164 Acid-base interaction of EO2¡ 4 with AlF36 ¡ is accom- panied by replacement of F7 by O27 in the aluminium environ- ment to give AlOF1¡x complexes (x=2 ± 5), while EO2¡ 4 is bound in oxofluoride complexes. For example, for x=2 (37) EO2F2¡ 4 +2AlOF¡2 .EO2¡ 4 +2AlF36 ¡ 4 4 The proof that tungsten and molybdenum are evolved from oxofluoride complexes in the NaCl ±Na3AlF6±Na2EO4 systems is as follows. First, the deposition potentials of tungsten and molybdenum from these systems correlate well with the deposi- tion potentials of these elements from WO2F2, MoO2F2 relative the oxygen electrode calculated from thermodynamic data.203, 204 Second, the deposition potential of tungsten (molybdenum) from the system in question coincides with the deposition potential of tungsten (molybdenum) from the NaCl ± KCl ± NaF ± EO3 sys- tem, in which the high-melting metal exists as EO2F2¡ (see Refs 215, 216).The formation of EO2F2¡ and its subsequent electrochemical reduction to the metal in a KF±MoO3±B2O3 melt was assumed by Japanese researchers.213 The electrochemical reduction waves recorded for a NaCl ±Na3AlF6 melt containing EO3 and Na2EO4 both separately and simultaneously are virtually identical. Hence, the reaction between tungstate (molybdate) and fluoroaluminate ions results in EO2F2¡ 4 and AlOF1x¡x. 4 Taking into account the linear dependences of the reduction wave peak current on the EO3 (EO2¡ 4 ) concentration and on the polarisation rate and using the mass transfer constant of (5 ± 6)61073 cm s71, Malyshev et al.214 reached a conclusion on the diffusion nature of the limiting current.The dependence of the potential of molybdenum or tungsten electrode in a NaCl ±Na3AlF6 melt on the EO3 or EO2¡ concentration vs. the Pt, O2/NaCl ± 0.05 Na2MO4 half-cell is well described by the Nernst equation. The number of electrons per one electroactive species calculated from the slope of plots of the metal electrode potential against theEO3 (orEO2¡ 4 ) concentration equals 5.80.2. For these melts, the polarisation curves of the molybdenum and tungsten electrodes are characterised by low overvoltage; the cathodic and anodic segments of polarisation curves are symmet- ric up to the overvoltage of 0.35 V.Based on the slopes of the polarisation curves in the current ± overvoltage coordinates, the 173 number of electrons participating in the electrode process was found to be 5.5 ± 6.1. These data suggest the reversibility of the charge transfer step. The overall electrochemical reaction can be represented as follows: (38) E+2AlOF3¡ 4 +8F7 . EO2F2¡ 4 +2AlF36 ¡ +6e7 6 44 Several studies 214, 217, 218 deal with the effect of fluoride complexes AlF3¡ and SiF2¡ 6 , manifesting acid properties with respect to O27 ions on the electrochemical reduction of CrO2¡ 4 in K, Na/Cl, F melts as the background. The chromate ion in halide melts as the background with weakly polarising cations, such as Na+ andK+, does not manifest electrochemical activity up to the decomposition potentials of the support electrolyte.The forma- tion of EAS can be represented by an equation analogous to Eqn (38). The Cr2O3 formation potential coincides with its evolution potential from CrO2F2¡ vs. the oxygen electrode calculated from thermodynamic data.203, 204 The dependence of the chromium electrode potential in a NaCl ±Na3AlF6 melt (50 mass%: 50 mass %) on CrO2¡ concentration at 1173 K obeys well the Nernst equation with the number of electrons per EAS equal to 3.00.1. The electrochemical deposition of Cr2O3 occurs without a noticeable overvoltage; analysis of polarisation curves gives 5.7 ± 6.1 electrons.These data suggest the reversibility of the charge transfer step. The overall electrochemical equili- brium can be expressed as follows: Cr2O3+AlOF3¡ 4 +10F7 . (39) 2CrO2F2¡ 4 +AlF36 ¡ +6e7 Studies on the electrochemical reduction of oxoanions of high- melting metals in the presence of chloride, chloride ± fluoride and oxide melts with various acid-base properties made it possible to reveal three mechanisms of EAS formation:3, 4 anion cationisation with formation of {Mzá x [EOn/2+1]27}(xz72)+ species, oxoanion decomposition to give {EO[(n+2(17x)]/2Hal2(x-1)+y}y7 oxohalide complexes, formation of [E(1+x)On(1+x)/2+1]27 dimer complexes where E=Cr, Mo, W, C; M=Li+, Ba2+, Sr2+, Mg2+, Zn2+, Al3+. The direction of EAS formation is determined by the acid properties and the concentration of Mz+, EOn, PO¡3 , F7 and the stability of the EO2¡ n anion.One can create conditions enabling various multielectron electrochemical processes to favour a particular reduction mech- anism in ionic melts, by constructing electrochemically active species with various structures and compositions (in other words, by controlling the cation ± anion interactions). 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