摘要:

J . Chem. SOC., Faraday Trans. I, 1982, 78, 3499-3508 Electron Spin Resonance Studies of the Radicals Formed from C-Nitroso Compounds and Olefins Part 2 .-Reactions of Fluoro-ole fins with 2,4,6-Trichloroni troso benzene and with Nitrosobenzene BY LESLIE H. SUTCLIFFE* AND ANNA ZILNYK The Department of Inorganic, Physical and Industrial Chemistry, The University of Liverpool, Donnan Laboratories, Grove Street, P.O. Box 147, Liverpool L69 3BX Received 22nd January, 1982 The dark reactions of a number of fluoro-olefins with 2,4,6-trichloronitrosobenzene and with nitroso- benzene have been investigated. The fluoro-olefins CF,CFCl, CF,CFBr, CF,CFI and CF,CCl, gave free radicals, but none were detected with CF,CH,, CF,CFH, CF,CFCF, or CFClCFCl. The temperature at which the reactions are allowed to occur is a critical factor in determining the type of radical produced: at least two radicals are formed for each olefin studied.A temperature-dependent e.s.r. spectrum was found for the reaction between CF,CFBr and nitrosobenzene in iso-octane : activation parameters for the interconversion process of the radical PhN(O)CF,CFBr- about the CF,-N(O) bond were found to be 21.8 1 .O kJ mol-I, 23.8 0.6 J mol-I K-I for the free energy, enthalpy and entropy, respectively, at 298 K. Similarly, a temperature-dependent spectrum was detected from the reaction of CF,CFBr with 2,4,6-trichloronitrosobenzene in CFC1,: the slow limit could not be reached but the enthalpy of activation for interconversion of the radical ArN(0)CF2CFBr- was estimated to be 21 1.1 kJ mol-' and 6.6 5 kJ mol-'.There is considerable interest in the reactions between closed-shell compounds under mild conditions that give free radicals. Some of these reactions involve spin traps, so it is essential to be able to distinguish such a reaction from one in which a spin trap reacts with a free radical. In the first paper of this series1 nitrosodurene2 (2,3,5,6-tetramethylnitrosobenzene) was shown to react thermally with fluorinated olefins, and in many cases long-lived free radicals were observed by e.s.r. spectroscopy. The predominant radical structure was deduced to be ArN(O)--CF,-CXY- (I) where X,Y = F,C1; F,Br; C1,Cl; F,I; Br,Br. We now present the results of an investigation into the reactions of 2,4,6-trichloronitrosobenzene and of nitrosobenzene with some fluoro-olefins.Mulvey and Waters3 have made a detailed e.s.r. study of the reactions between non-fluorinated olefins and 2,4,6-trichloronitrosobenzene in benzene. They concluded that aromatic nitroso compounds add to olefins by a one-electron transfer from C=C to N=O bonds to give a biradical: ArNO +CH,CHR + ArN(O)-CH,-CHR. Further reaction by radical combination, hydrogen abstraction or spin trapping by the nitroso compound give the observed nitroxides. Two types of e.s.r. spectra were 34993500 E.S.R. STUDIES OF C-NITROSO RADICALS discerned, one having two beta-hydrogen splittings and the other with one beta- hydrogen splitting. However, it has been shown4 recently that brief exposure to visible light results in nitroxide-radical formation from 2,4,6-trichloronitrosobenzene dissolved in benzene in the absence of olefin: this is thought to be a spin adduct of a cyclohexadienyl radical derived from the solvent benzene.v i s i b l e light A r N O - A h 0 JA The spin adduct (11) has aN = 1.255 mT and a? = 0.530 mT. It was also shown4 that, in the dark, neither trans-stilbene nor triphenylethene react with trichloronitrosoben- zene in benzene, but brief exposure to visible light results in the formation of nitroxide (11). Mulvey and Waters3 were unaware of the photochemical effect and reported the following coupling constants: (i) for the trans-stilbene system.: aN = 1.26 mT, aF = 0.59 mT (1H); (ii) for the triphenylethene system: aN = 1.27 mT, af = 0.525 mT ( I H). When CFCl, was used as solvent trans-stilbene and trichloronitrosoben- zene4 gave radical signals attributed to ArCH(C,H,)--CH(C,H,)N(o)Ar after irradiation with visible light.A similar experiment with triphenylethene produced a nitroxide having only a nitrogen triplet in its e.s.r. spectrum. We have found that trichloronitrosobenzene does not react in the dark with neat trichloroethene or 1,2-dichloroethene, but Mulvey and Waters3 reported coupling constants: aN = 1.27 mT, aH = 0.56 mT (1H) and aN = 1.31 mT, aH = 0.52 mT (lH), respectively. We irradiated a solution of 1,2-dichloroe+$ene and trichloronitrosoben- zene with ultraviolet light and obtained an e.s.r. spectrum having coupling constants of aN = 1.23 mT, aH = 0.775 mT (1H) and a* = 0.10 mT (1H) which we assigned to ArCHCl-CHCl-N(0)Ar.A different situation obtains for non-1-ene and oct-1-ene. In the dark, trichloronitrosobenzene in neat non-1-ene gives an e.s.r. spectrum having the following splittings: aN = 1.24 mT, a r = 0.93 mT (2H), ag = 0.06 mT (2H), a,” = 0.028 mT (IH). Similarly, oct-1-ene gives aN = 1.24 mT, aF = 0.91 mT (2H), a: = 0.06 mT (2H), a,” = 0.028 mT. Mulvey and Waters3 obtained similar values but they did not manage to resolve the gamma couplings. These long-chain 0: :fins have a structure suitable for reacting with trichloronitrosobenzene via an ‘ ene ’ mechanis~,~ followed by oxidation of the hydroxylamine formed6-8 to give the nitroxide radicals observed. A r ’ A 2 0 I / - c-c, I . ‘C\ NO / AT Storage of the non-1-ene and oct-1-ene samples for half an hour at room temperature gives rise to different e.s.r.spectra: for non-1-ene ax = 1.21 mT and uH = 0.46 mT (IH), and for oct-1-ene aN = 1.23 mT and aH = 0.55 mT (1H). The concentration of these radicals was greatly increased by irradiation of the samples with ultraviolet light: we assign the spectra to the radicals ArCH,CHArN(6)CH2[CH,I,CH,, (111), where n = 4 or 5. The radicals may be produced by the addition of Ar to the CH,= part of the olefin followed by spin-trapping of the resultant radical. In this paper we present the e.s.r results obtained from the dark reactions of a number of fluoro-olefins. The temperature at which the reactions are allowed to occur is a critical factor in determining the type of spectrum observed. The fluoro-olefinsL. H. SUTCLIFFE AND A. ZILNYK 3501 CF,CFCI, CF,CFBr, CF,CFI and CF,CCl, gave free radicals, but none was detected from CF,CH,, CF,CFH, CF,CFCF, or CFClCFCl. Nitrosobenzene has been used in place of 2,4,6-trichloronitrosobenzene in some experiments.EXPERIMENTAL MATERIALS The 2,4,6-trichloronitrosobenzene was given to us by Prof. W. A. Waters who prepared it by the method described by Holmes and B a ~ e r . ~ Nitrosobenzene was bought from Aldrich Ltd and used without further purification. CF,CCl, and CFCI, were supplied by I.C.I. while all the other fluoro-olefins were obtained from Bristol Organics. SAMPLE PREPARATION This was carried out in a manner similar to that described in the first paper of this series.I Solutions of the nitroso compound were left at room temperature in order to allow the dimer to dissociatelo to monomer before adding the olefin.E.S.R. MEASUREMENTS A Varian E-4 X-band spectrometer was used. Complex spectra were digitised so that they could be converted to the second-derivative form thus allowing line positions to be measured automatically and with precision. The digitiser comprised a 1 MHz crystal clock which connected the E-4 signal output to a Data Dynamics 1183 paper-tape punch. The spectrum was digitised into ca. 5000 points and then the paper tape was processed by the University of Liverpool 1906s computer. Sample temperatures were maintained to & 0.5 OC using a Varian V-4540 temperature controller. Temperatures were measured before and after recording spectra with the aid of a Comark type 521 5 digital thermometer, having a precision of & 0.1 O C .The spectrometer field scan was calibrated by using a saturated aqueous potassium carbonate solution of potassium peroxylamine sulphonate'l (low-field uN = 1.3064 mT. high-field aN = 1.3 1 18 mT). E.s.r. spectra were simulated using either acomputer program written by Dr W. R. Mcilwaine or one including exchange written by Dr M. F. Chiu of the University of York. Irradiations were carried out using a Varian UVlOO mercury lamp fitted with a Chance OX1 filter . RESULTS 2,4,6-TRICHLORONITROSOBENZENE Normally, after the reactants had been mixed, an e.s.r. spectrum began to develop on warming the sample to ca. - 30 OC; the sample was then kept below - 15 "C for 1-1; h to allow the radical concentration to build up. With CF,CFBr a signal coming mainly from one radical (radical A) was observed (see fig.1). The spectrum is reversibly temperature dependent owing to modulation of the hyperfine splitting constants of the two non-equivalent beta fluorines by internal r0tation.l The fast limit is observed at - 30 O C but the slow limit was not reached on cooling. Since spectral parameters were not available from the slow limit it was not possible to obtain all the energetics for the interconversion process, but we estimate the enthalpy of activation to be 21 5 kJ mol-I: our previous work' showed this value t o be solvent-dependent. Other splittings arise in the spectrum from two meta hydrogens, one nitrogen and one gamma fluorine. The fluorine coupling constants show an intrinsic temperature dependence (see table 1).The radical is stable for several hours if stored below - 15 O C , but if the temperature is allowed to rise above this even for a few minutes then a new intense e.s.r. spectrum develops (radical B). The broadline spectrum (line-width 3 0.2 mT) has been interpreted as arising from one3502 E.S.R. STUDIES OF C-NITROSO RADICALS FIG. 1 .-First-derivative e.s.r. spectrum of the radical from 2,4,6-trichloronitrosobenzene and CF,CFBr in CFCI, at - 18.5 OC: (A) experimental, (B) computer simulation using a line-width of 0.04 mT. nitrogen (aN = 0.975 mT) and one beta fluorine (aF = 2.66 mT). The signal lasts for ca. 1 h at room temperature but disappears quickly if heated above 40 OC. A sample prepared and examined immediately at room temperature gives the e.s.r.spectrum of radical B (fig. 2) that reaches its maximum concentration after a few minutes. Lowering the temperature increases the complexity of the spectrum owing to the presence of at least one other radical. CF,CFCl and CF,CFI gave results (see tables 2 and 3) like those of CF,CFBr, and again a radical similar to radical A predominates at low temperature and has aL. H. SUTCLIFFE A N D A. ZILNYK 3503 TABLE 1 .-HYPERFINE COUPLING CONSTANTS (mT) OF THE PREDOMINATING LOW-TEMPERATURE RADICAL FORMED FROM 2,4,6-TRICHLORONITROSOBENZENE AND CF,CFBr IN CFC1, 255.2 1.015 3.13 0.110 0.055 237.5 1.015 3.16 0.095 0.055 208.9 1.010 3.24 0.050 0.055 186.2 1.015 3.28 0.040 0.055 171.6 1.015 3.32 0.037 0.055 163.2 1.015 3.36 not 0.055 resolved Temperature coefficients : d(agF1 +af2)/dT = -2.5 x mT K-l; d(a,F)/dT = 9.5 x mT K-l.1.0 ml - FIG. 2.-First-derivative e.s.r. spectrum of the radical formed from 2,4,6-trichloronitrosobenzene and CF,CFBr in CFCl, at 21 OC. temperature-dependent spectrum. Again the spectrum is complicated by the presence of other radicals, possibly like radical B. Second derivatives of the spectra were used to effect simplification. However, ENDOR measurements would probably give more satisfactory results. With CF,CCI, the predominant spectrum at low temperature comprises splittings from one nitrogen, two beta fluorines and two meta hydrogens (see table 4): the radical is stable below - 15 OC. Increasing the temperature leads to the formation of a radical having a 1 : 1 : 1 triplet from one nitrogen atom, the coupling constant (aN = 1.01 mT) being temperature-independent.3504 E.S.R.STUDIES OF C-NITROSO RADICALS TABLE 2.-HYPERFINE COUPLING CONSTANTS (mT) OF THE PREDOMINATING LOW-TEMPERATURE RADICAL FORMED FROM 2,4,6-TRICHLORONITROSOBENZENE AND CF,CFCl IN CFCI, aN $1 + aF2 265.6 0.995 3.10 0.120 0.058 244.8 1 .oo 3.14 0.105 0.058 228.1 1 .oo 3.18 0.095 0.058 1 .oo 3.23 0.070 0.060 209.4 192.4 1 .oo 3.26 0.060 0.058 175.6 1.01 3.30 0.052 0.058 Temperature coefficients : d(afl+af2)/dT = -2.2 x mT K-l; d(a,F)/dT = 7.6 x 10-4 mT K-l. TABLE 3 .-HYPERFINE COUPLING CONSTANTS (mT) OF THE PREDOMINATING LOW-TEMPERATURE RADICAL FORMED FROM 2,4,6-TRICHLORONITROSOBENZENE AND CF2CFI IN CFC1, aN 3.04 0.135 0.060 297.2 1.01 261.2 1 .oo 3.10 0.120 0.060 250.6 1.01 3.1 1 0.113 0.058 233.7 1.01 3.13 0.105 0.058 207.9 1.01 3.20 0.080 0.058 d(aBF,+aF2)/dT = - 1.8 x lo-, mT K-l; d(a,F)/dT= 6.2 x lop4 mT K-l.Temperature coefficients : TABLE 4.-HYPERFINE COUPLING CONSTANTS (mT) OF THE PREDOMINATING LOW-TEMPERATURE RADICAL FORMED FROM 2,4,6-TRICHLORONITROSOBENZENE AND CF,CCl, IN CFC1, 259.5 1.045 2.990 0.055 245.8 1.040 3.020 0.055 229.3 1.040 3.060 0.055 210.2 1.045 3.080 0.055 191.2 I .045 3.120 0.055 172.2 1.050 3.160 0.055 Temperature coefficient: d(afl + a,F,)/dT = - 1.9 x lo-, mT K-* NITROSOBENZENE Nitrosobenzene reacts readily with fluoro-olefins to give e.s.r. spectra: warming a sample from - 100 to - 60 OC in the e.s.r. cavity produced strong signals. In all cases, the spectra are complex showing that several radicals are formed simultaneously, thus presenting an ideal case for ENDOR measurements.When CF,CFBr was used the spectrum from a single radical could be observed occasionally if the temperature was kept below -30 “C during radical formation. The spectrum was assigned to PhN(o)CF2CFBr- in a manner similar to the spectra of fluoroalkylphenyl nitroxides of known structure.12 The temperature dependence of the spectrum can be explainedL. H. SUTCLIFFE A N D A. ZILNYK 1.0 mT I 3505 FIG. 3.-First-derivative e.s.r. spectrum of the radical formed from nitrosobenzene and CF,CFBr in iso-octane at -67.6 O C : (A) experimental, (B) computer simulation using a line-width of 0.03 mT. as modulation by internal rotation of the hyperfine coupling constants of the two non-equivalent beta fluorines.The slow limit is observed at ca. - 100 OC, and 30 *C was the highest temperature at which measurements could be made before the radical decomposed. Spectra were simulated for a range of temperatures (see fig. 3 and 4 for examples), and from the rate constants obtained activation energetics of the interconversion process were evaluated by a least-squares-fitting computer program : the values found are 21.8 k 1 .O kJ mol-l, 23.8 1.1 kJ m o t 1 and 6.6k 0.6 J mol-l K-l at 298 K for the free energy, enthalpy and entropy, respectively. The fluorine and nitrogen coupling constants have an intrinsic temperature dependence (see table 5). With all the fluoro-olefins, marked colour changes were observed when reaction occurred with nitrosobenzene. The blue colour of nitrosobenzene turns to orange and then finally to dark red: similar colour changes have been noted13 during reactions of arylnitroso compounds with styrene and with methyl methacrylate.3 506 (A) E.S.R.STUDIES OF C-NITROSO RADICALS 1.0 mT I FIG. 4.-First-derivative e.s.r. spectrum of the radical formed from nitrosobenzene and CF,CFBr in iso-octane at 29.0 OC: (A) experimental, (B) computer simulation using a line-width of 0.03 mT. Product analysis using a combined gas-liquid chromatographic and mass- spectroscopic analysis of reaction mixtures of nitrosobenzene and CF,CFBr has established that a number of low-molecular-weight compounds are formed together with a solid which is probably a polymer. Mono-, di- and tri-bromo derivatives have been found that also contain phenyl groups: these may be ~xazetidines.~~ DISCUSSION From the e.s.r.spectroscopy of reactions of trichloronitrosobenzene we have assigned the predominant radical (radical A) at low temperatures to ArN(O)CF,CXY-L. H. SUTCLIFFE AND A. ZILNYK 3507 TABLE 5.-HYPERFINE COUPLING CONSTANTS (mT) AND INTERCONVERSION RATE CONSTANTS (s-l) OF THE RADICAL FORMED FROM NITROSOBENZENE AND CF,CFBr IN ISO-OCTANE rate T/K constant aN $1 4 2 4 a: a: P 302.3 295.3 265.8 241.3 225.7 205.7 187.6 174.3 9.5 x 108 7.0 x lo8 2.3 x lo8 7.2 x 107 6 . o ~ 107 3.5 x 105 8.0 x lo6 2.0 x 106 0.968 0.968 0.960 0.970 0.960 0.950 0.947 0.947 1.71a 2.180 1.71a 2.180 1.71a 2.240 1.71" 2.263 1.718 2.290 1.72 2.300 1.72 2.305 1.72 2.320 0.045 0.049 0.054 0.056 0.065 0.080 0.080 0.080 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.225 0.225 0.230 0.233 0.233 0.237 0.243 0.243 Temperature coefficients: d(aN)/dt = +2 x mT K-l; d(uf2)/dT = - 1.1 x mT K-l; d(a,F)/dT= - 3 x mT K-l; d(azp)/dT= - 1 .4 ~ mT K-1. a The coupling constant is assumed. where XY = C1,Cl; F,CI; F,Br; F,I. A likely structure for the main radical (radical B) at higher temperatures when CF,CFBr or CF,Cl, are used is ArN(6)CXY CF,-. (V> However no gamma fluorine structure was resolved. Both CF,CFCl and CF,CFI may give radicals having structure (V), but the e.s.r. spectra are difficult to assign owing to the presence of other radicals having wide lines. It has been shown15 that free-radical addition to this type of fluoro-olefin occurs mainly at the =CF, end. Thus for radical A the addition of At, or any other radical, followed by spin trapping of the resultant radical can be discounted.Irradiation of the samples with ultraviolet light does not result in formation of radical B at low temperatures, so we exclude structures of the type ArN(o)CXYCF,Ar. Ginsburg and co-workers16* l7 have reported a number of reactions between trifluoronitrosomethane and fluoro-olefins, and from e.s.r. spectroscopic evidence they concluded that radical ions are formed. They suggest that at low temperatures either (VI) or (VII) may be formed: CF,N(6)CF2CXY ( W CF,N(O)CF,CXY[N(CF,)OCF,CXY],N(CF,)OCF,CXY. (VW At higher temperatures it is believed that another radical ion predominates, namely CF3N(6)CXYCF,. (VIII) Although the radical-forming reactions studied by Ginsburg and coworkers bear some similarity to those reported here we do not believe that radical anions could be observed in a non-stabilising solvent and in the absence of a suitable counter-ion.We have carried out preliminary experimentsLs with trifluoronitrosomethane, and are now making a more detailed study.3508 E.S.R. STUDIES OF C-NITROSO RADICALS TEMPERATURE DEPENDENCE OF HYPERFINE COUPLING CONSTANTS Radicals formed from trichloronitrosobenzene show hardly any change in aN with temperature. The radical from nitrosobenzene and CF,CFBr has an aN value having a positive temperature coefficient (see table 5). Temperature-dependent aN values of nitroxides of known structure have been reported12+19 and interpreted in terms of out-of-plane torsional motions of the nitroxide group : a positive temperature coefficient probably means that the radical has a planar minimum-energy conformation or is non-planar and has a low barrier to inversion.2o Both the beta and the gamma fluorinecouplingconstants are temperature-dependent.Positive and negative coefficients have been reportedl2I 21* 22 for other fluoroalkyl nitroxides: the temperature dependence of beta fluorine coupling constants has been explained in terms of preferred conformations, restricted rotations or non-planar radical centres.lg Note (table 5 ) that the beta fluorine temperature dependence we observe comes entirely from one of the fluorine nuclei: but note that a& was given a fixed value of 1.71 mT for spectra simulated for temperatures above 225.7 K, since only the sum of a& and a& can be observed.Although gamma fluorine coupling constants can be appreciably temperature-dependentl29 21* 22, no attempt has been made to explain these long-range effects. We thank Dr W. R. McIlwaine for constructing the data-acquisition system and for helping with computer programs. We are also indebted to Mr A. Mills for his skilful g.1.c. and mass-spectrometric analysis. A. Z. thanks the University of Liverpool for the award of a scholarship. S. A. Fairhurst and L. H. Sutcliffe, J. Chem. SOC., Faraday Trans. I , 1979, 75, 1521. S. Terabe, K. Kuruma and R. Konaka, J. Chem. SOC., Perkin Trans. 2, 1973, 1252. D. Mulvey and W. A. Waters, J . Chem. SOC., Perkin Trans 2, 1978, 1059. C. Chatgilialoglu and K. U. Ingold, J . Am. Chem. Soc., 1981, 103, 4833. H. M. R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1969, 8, 556. ti A. B. Sullivan, J . Org. Chem., 1966, 31, 281 1. G . T. Knight, Chem. Commun., 1970, 1016. R. E. Banks, M. G. Barlow and R. N. Haszeldine, J . Chem. SOC., 1965, 4717. R. R. Holmes and R. P. Bayer, J. Am. Chem. SOC., 1960, 82, 3454. lo R. R. Holmes, J . Org. Chem. 1964, 3076. l1 R. J. Faber and G. K. Fraenkel, J. Chem. Phys., 1967, 47, 2462. l2 S. Terabe, R. Konaka, Bull. Chem. SOC. Jpn, 1973, 46, 825. l 3 L. Sumegi, F. Tudos and I. Kende, Acta Chim. Acad. Sci. Hung., 1975, 68, 75. l4 D. A. Barr and R. N. Haszeldine, J. Chem. SOC., 1955, 1881. l5 D. C. Nonhebel and J. C. Walton, Free Radical Chemistry (Cambridge University Press, Cambridge, l6 V. A. Ginsburg, A. N. Medvedev, S. S. Dubov, M. F. Lebedeva, M. N. Vasil'eva and L. L. l7 V. A. Ginsburg, A. N. Medvedev, L. L. Martynova, M. N. Vasil'eva, M. F. Lebedeva, S. S. Dubov 1974), chap. 8. Martynova, Zh. Obshch. Khim., 1967, 37, 611. and A. Ya. Yakubovich, Zh. Obshch. Khim., 1965, 35, 1924. N. G . Hargreaves, Ph.D. Thesis (Liverpool University, 1976). l9 P. D. Sullivan and E. M. Menger, Adu. Magn. Reson. 1977, 9, 1. 2o A. T. Bullock and C. B. Howard, J . Chem. SOC., Faraday Trans. I , 1980, 76, 1296. 21 K. J. Klabunde, J . Am. Chem. SOC., 1970, 92, 2427. 22 E. G. Janzen, B. R. Knauer, J. L. Gerlock and K. J. Klabunde, J. Phys. Chem., 1970, 74, 2037. (PAPER 2/ 126)

ISSN:0300-9599    

DOI:10.1039/F19827803499

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faradqy Trans. 1, 1982, 78, 3509-3518 Evidence for the Participation of an Isomerization Pathway in Diazirine Photolysis Study of Primary Processes and Energy Partitioning BY JUAN M. PEREZ Department of Physical Chemistry, Facultad de Ciencias, Universidad de Alicante, Apartado 99, Alicante, Spain Receiued 22nd February, 1982 CNDO/S calculations have allowed the band centred at 322.9 nm in the photolysis of diazirine to be considered as an A” c A’ transition. Correlation diagrams constructed using 3H-diazirine as a model and with the conservation of individual electronic angular momenta are used to explain some of the unusual features in the decomposition reactions of diazirines. Calculated energy-partitioning data indicate that there are two photodecomposition pathways with differing efficiencies in converting available energy into internal energy.Evidence shows that diazo-compounds are intermediates in diazirine photolysis and that the efficiency of this pathway accounts for 25”/, of the total reaction. Substituted diazirines are known to cleave, whether thermally or photochemically, into a carbene fragment and a nitrogen molecu1e.l However, the detailed mechanism of this fragmentation remains obscure. It has been shown experimentally that an excited 3H-diazirine molecule can isomerize into diazomethane or break up to give methylene and a nitrogen molecule. Two different reaction paths have been postulated. In the first, an excited 3H-diazirine molecule forms excited diazomethane directly2 which then loses its excitation energy via radiationless processes giving ground-state diazomethane, or breaks up into CH, and N,.In the second, the primary process is again the fragmentation of excited 3H-diazirine again into CH, and N,. In a subsequent step these species recombine and ground-state diazomethane is f ~ r m e d . ~ From ab initio SCF-CI calculations4 these sequencies both seem realistic. Recently, Frey and Penny5 have reinterpreted the photolysis of 3-chloro-3- methyldiazirine. In their opinion, both the fission and isomerization pathways of excited diazirine play a cruci,al role in the reaction mechanism. Moreover, Figuera et aL6 point out that in the same system at least two pathways to the formation of vinyl chloride are required to explain the experimental results, but they do not provide information on the nature of both pathways.Correlation diagrams constructed using 3H-diazirine as a model can be used to explain some of the unusual features displayed by these decomposition reactions. However, increased knowledge of the primary processes occurring in diazirines could be obtained by studying how energy is partitioned among the photolysis products along a series of compounds. This paper supplies theoretical calculations which in conjunction with experimental data available should provide a better insight into the decomposition of diazirine. 35093510 DIAZIRINE PHOTOLYSIS CALCULATIONS MOLECULAR-ORBITAL CALCULATIONS OF EXCITED STATES OF DIAZIRINES The CNDO/S method was used, and a configuration interaction over 30 excited states was carried out.The molecular geometry used was taken from the literat~re.~ The molecules were placed on the YZ plane and the Z axis was chosen as the C, axis. In this way, the molecular orbital perpendicular to the molecular plane become a” and b, in the point groups C, and C,,, respectively. The electronic transitions of low energy are shown in table 1. Table 2 includes the electronic populations of the atoms directly involved in the dissociation (C and N) with 3H-diazirine as a representative case, and table 3 gives the electron configuration of the low lying states of 3H-diazirine. TABLE 1 .-ELECTRONIC TRANSITIONS BETWEEN SINGLET STATES OF ALIPHATIC DIAZIRINES ~~ ~~~~ oscillator band maximuma strength orbital state molecule transitions transitions calc.expt. calc.b expt. 3H-diazirine a, + b, B, +- A , 326 322.9“ 0 - a, + a, A , t A , 231 - a2 t b, B,4- A , 226 320.6 0.04 - 3,3’-dimethyldiazirine a,+ b, B, t A , 327 358.1d 0 - a, 4- a, A , +- A , 212 - B , t A l 169 359.8 0.03 - a, + b, - 0 - 0 a In nm; evaluated from the integrated molar extinction coefficient of the band, see J. G. Calvert and J. N. Pitts, Photochemistry (John Wiley, New York, 1967), p. 172; see ref. 12(a); see ref. 12(b). TABLE 2.-ELECTRONIC POPULATIONS OF THE CARBON AND NITROGEN ATOMIC ORBITALS OF 3H-DIAZIRINE excited state ground atom/orbital state 1 st 2nd 3rd C S 1.011 1.01 1 P, 0.851 0.854 PY 1.130 0.671 pz 0.995 0.994 Na s 1.642 1.623 pz 1.172 0.944 Py 1.235 1.218 pz 0.995 1.488 1.010 0.825 1.129 0.994 1.574 1.159 0.835 1.494 1.010 0.850 1.131 0.918 1.642 1.174 1.235 1.100 a Both nitrogen atoms have the same populations because of symmetry.J. M.PEREZ 351 1 TABLE 3.-ELECTRON CONFIGURATIONS IN c2, SYMMETRY OF THE LOW-LYING STATES OF THE MOLECULES INVOLVED IN THE 3H-DIAZIRINE PHOTODECOMPOSITIONa 3 H-diazirineb methenec nitrogend a 1s electrons have been omitted; obtained from the CNDO/S results described above; S. Yan Chu, A. K. Q. Sin and E. F. Hayes, J. Am. Chem. SOC., 1972,94,2969; M. Orchin and H. H. Jaffe, The Importance of Antibonding Orbitals (Houghton Mifflin Co., Boston, 1967), p. 31. PHOTODECOMPOSITION CORRELATION DIAGRAMS Correlation diagrams were constructed for 3H-diazirine photodecomposition. First we correlated the molecular orbitals of 3H-diazirine with those of the same symmetry in methylene and nitrogen. Then we constructed the electron configurations of the low-lying states of the species involved and the related states of reactants and products whose molecular orbitals were intercorrelated.The ordering and symmetry of the 3H-diazirine molecular orbitals and the electronic composition of its excited states were obtained from CNDO/S calculations. Those of nitrogen and methylene were taken from the literature (see table 3). We assumed, for energetic reasons, that nitrogen is formed in its ground electronic state. It can be shown that this assumption does not exclude any significant correlations. Correlation diagrams were constructed assuming conservation of the C,, symmetry along the reaction path and also for paths of lower symmetry (see fig.1). The basic principle of individual electronic-orbital angular momenta conservation was described.8 PHOTOISOMERIZATION CORRELATION DIAGRAMS A correlation diagram was also constructed for the photoisomerization of 3H- diazirine and diazomethane, assuming conservation of CS(osy plane) symmetry along the reaction path, this plane being the only symmetry element (see fig. 2). The ordering and symmetry of diazomethane molecular and the electronic composition of its excited states were obtained from CNDO/S calculations. Table 4 shows the electronic configuration in C, (oZy plane) symmetry of the low lying states of diazomethane.3512 DIAZIRINE PHOTOLYSIS 7 2 --. x 2 6 'u 0 7 2 --- x P C e, 0 lene (3) FIG. 1 .-Correlation diagrams for 3H-diazirine decomposition constructed under the assumption that nitrogen is formed in its ground electronic state, maintaining: (a) C,, symmetry, (b) the plane out and (c) the plane oZu.cS&z plane) 3H -diazirine, d i azomethane I / 4 1 FIG. 2.-Correlation diagram for the 3H-diazirine isomerization to diazomethane, assuming conservation of the cue plane. TABLE 4.-ELECTRON CONFIGURATIONS IN c,, SYMMETRY OF THE LOW-LYING STATES OF DIAZOMETHANE~ a 1s electrons have been omitted; the molecular geometry was taken from M. Martin, V. Menendez and J. M. Figuera, Rev. Roum. Chim., 1976, 21, 31.J. M. PEREZ 3513 ENERGY-P ARTITIONING CALCULATIONS The study of the partition of energy amongst photolysis products along a series of compounds can be used to elucidate the connection between structure and energy partitioning.In order to calculate the percentage of energy removed by the organic species in photodissociation we must know the total energy available for distribution among the fragments and the part that is carried off by the organic fragment. The total energy available for partitioning will be the sum of the light absorbed by the reactant, its thermal energy and the exothermicity of the reaction at 0 K. The contribution of thermal energy was neglected. The light intensity absorbed was calculated following procedures already de~cribed.~ The exothermicity of the reaction was summarised as the difference between the standard heats of formation of reactants and products. These data were calculated according to the MIND0/3 method with the Rinaldi optimization procedure.Heats of formation, together with the most important characteristics of the respective geometries, are shown in table 5. TABLE 5.-GEOMETRIES AND STANDARD HEATS OF FORMATION IN SUBSTITUTES DIAZIRINES AND RELATED CARBENES AH? bond lengths/nm angles/rad /kJ mol-l C1-N C1-C2 Cl-HI ClNN HlClN C2ClN me th yldiazirine 0.145 0.150 0.110 t-butyldiazirine 0.145 0.157 0.110 3,3’-dimethyldia~irine~ 0.147 0.152 - Cl-C2 C1-HI 1.148 2.039 2.202 186.4 1.151 - 2.056 161.8 1.149 2.007 2.251 248.9 HlClC2 C2ClC2 methylcarbene 0.145 0.1 12 3,3’-dime thylcar benea 0.145 - t- bu t ylcarbene 0.150 0.124 1.915 - 322.8 - 2.190 249.8 1.954 - 360.7 “C,, symmetry in 3,3’-dimethyldiazirine and 3,3’-dimethylcarbene; C, symmetry in the others. The method of determining the distribution of energy is based on the dependence of the rate of unimolecular decomposition of the species formed upon their energy content.The calculation of the energy distribution functions of the excited olefins was carried out by fitting the experimental ratio of the amount decomposed to the amount stabilised (DIS), using the relationship between this quantity and the microscopic unimolecular rate constants, k(E), where ( k i ) is the average rate constant for the ith process. The rate constants k(E) were calculated using R.R.K.M. theory. All the internal vibrations of the molecule were taken to be active degrees of freedom. Activated complexes were chosen under the constraints that they should reproduce available Arrhenius pre-exponential factors, since R.R.K.M.results depend mostly on the A factor and not on the exact model of the activated complex.1o Vibrational assignments of molecules and activated complexes and collision parameters used are given in the Appendix.3514 DIAZIRINE PHOTOLYSIS The weighting function employed in this work is one which corresponds to the two-channel mechanism of olefin formation P(E) = PF(E) + (1 -P) F’(E’) (2) where p is the weighting factor, and F(E) and F ( e ’ ) are assumed to have normal gaussian functions. Calculations were carried out in the following systems: 3-methyldiazirine, 3,3’- dimethyldiazirine and t-butyldiazirine, for which suitable data are available in the 1iterature.ll The values ofweighting factors and gaussian widths employed in the energy distribution function, P(E), are similar for all cases studied.They are determined by a trial-and-error method, which consists of modifying the location of two gaussians, Emp and Ekp, until a reasonable reproduction of the experimental results is obtained. The calculated energy-partitioning data are shown in table 6 . The theoretical curves that best fit the experimental values of the rate constants are plotted in fig. 3. TABLE 6.-ENERGY PARTITIONING DATA (kJ m0l-l) E:,, pathb E& oc efficiency (Eolelin)d ~~~~ methyldiazirine 513 (a) 463 21 0.90 430 3,3’-dimethyldiazirine 523 (a) 417 21 0.80 42 1 (b) 350 50 0 . 6 8 (b) 334 50 0.64 (b) 409 50 0.64 t-but yldiazirine 636 (a) 500 21 0.79 537 a Energy available for partitioning between olefin and nitrogen; path (a), weight factor equal to 0.25; path ‘b) weight factor equal to 0.75; parameters of energy distribution function of excited olefins; olefin average energy relative to path (a) and calculated by eqn (3), see text.6 2 pressure/Torr FIG. 3.-Plot of the rate constants for ethylene (O), propylene (a) and 1,l’-dimethylcyclopropane (*) decomposition produced by photolysis of 3-methyldiazirine, 3,3’-dimethyldiazirine and t-butyldiazirine,” respectively. Experimental data from Frey et al.” Smooth lines, calculated values using distribution function made up of two gaussians with weights 1 :3.J. M. PEREZ 3515 DISCUSSION The low-resoIution electronic spectra of 3-methyldiazirine, 3,3’-dimethyldiazirine and t-butyldiazirine have been recorded by Frey et a1.l1 All spectra exhibit a similar shape.Robertson et ~ 1 . ~ have studied the high-resolution electronic spectra of 3H-diazirine and 3,3’-dimethyldiazirine ; they are very diffuse and rotational structure is not observed. The lowest n*-n+ transition allowed appears to be B, +- A, and is observed to occur at 322.9 nm. The agreement of CNDO/S calculations in band intensity and position with experimental and calculated values is about that expected from the method used. The lowest-energy transition increases the electronic population of the two p z nitrogen orbitals, while that corresponding to the py carbon orbital decreases. In addition to this band, Robertson et al. observed another transition 2.5 nm away from the former. They consider the second band to have arisen from the other non-bonding n- orbital.The above conclusion is based on the iissumption that both transitions originate from non-bonding orbitals and that the separation of these levels is small, owing to a symmetric and antisymmetric combination. However, Vasudevan et ~ 1 . ’ ~ found from SCF-CI calculations that there exists a large n+-n- separation of 288 kJ mol-l. A separation of 410 kJ mol-1 is also obtained from the results of Robin et al.,14 thus confirming that the second band observed in the neighbourhood of the n*-n+ transition has a different origin. Finally, Lombardi et ~ 1 . ’ ~ have pointed out the possibility that this band arises from a triplet-singlet transition, n*-n. The following two transitions calculated according to the CNDO/S method (see table 1) appear far apart from the second band, and it is impossible to attribute such a variation to the method of calculation employed.It seems reasonable to ascribe these transitions to the electronic system that appears below 200 nm. Starting with the electronic populations of the atoms directly involved in the dissociation (C and N) with 3H-diazirine as a representative case (see table 2), the interpretation of primary photochemical processes in terms of the bonding and antibonding character of the molecular orbitals involved is not without its difficulties. To investigate why these compounds photodecompose so effectively we constructed the correlation diagrams shown in fig. 1. These show photodecomposition from the first excited state to be forbidden. If photodecomposition from this state takes place retaining C, symmetry (the y z plane) throughout the reaction path, the process is endothermic and a potential-energy barrier arises between 193 and 338 kJ 11101-l.On the other hand, some diazirine substitutes are shown to fluoresce6*16 during the B, -+ A , transition. Thus it is evident that a fraction of the excited molecules undergo a process by which they decompose. However, Frey5 has reported that in 3-chloro- 3-methyldiazirine photolysis the quantum yield, aD, is close to unity (at 325 nm), and Over a considerable range of pressures CD, did not vary with pressure. To investigate the possibility of another photodecomposition pathway, we constructed the photoisomerization diagram shown in fig. 2. This diagram shows that photoiso- merization from the first excited state of 3H-diazirine is allowed when C, symmetry ( y z plane) is retained throughout the reaction path and that it yields diazomethane in the excited state A”(1).A similar conclusion is propounded by D e ~ a q u e t , ~ who indicated that if the excited system does not undergo intersystem crossing and remains on the ‘n-n* curve no dissociation will occur and the n,n* singlet state of diazomethane will then be populated. Frey et ~ 1 . ~ presented experimental evidence that in 3-chloro-3-methyldiazirine photolysis, dichloroethane results from the reaction of chloromethyldiazomethane3516 DIAZIRINE PHOTOLYSIS with HCI produced in the system. Also, direct formation of the diazocompound has been observed in the thermolysis of 3-butyl-3-pheny1dia~irine.l~ In order to gain information about the distribution between the two fragments of the excess energy due to the overall exothermicity of the process, we studied the energy partitioning between the photolysis products in a series of compounds, as this can be used to determine the connection between structure and energy partitioning. Thus we calculated the energy-distribution functions for those systems for which suitable data are available.The calculated energy-partitioning data are shown in table 6. Two main observations emerge from an inspection of this table. The first concerns the rather different efficiencies of the two pathways in converting available energy into internal energy. Path (a) is of greater efficiency than path (b).The second is connected with the relative importance of both pathways, and shows that path (a) is responsible for almost 25% of the total reaction. In the above discussion we have shown (by arguments based on the symmetry of the electronic states of species involved) that a continuous increase in the NNC angle on going from 3H-diazirine to diazomethane opens an allowed path for decomposition from the first singlet electronic state of diazirine compounds. This state is reached when the diazirine compound is irradiated in the long-wavelength band whose photon energy is close to 380 kJ mol-l. Thus, the average energy of the olefin formed by means of this pathway can be calculated by eqn (3) ( Eolefin) = [hv + AH*(diazirine - carbene)] x F+ AH*(carbene - olefin) (3) where F stands for the energy percentage carried out by the carbene group.For the photodecomposition of diazocompounds from the first singlet electronic state F is found to be ca. 65%.971a In addition, these systems show a very narrow energy distribution within the photolysis fragments and depend only slightly on the charac- teristics of the alkyl chain. If adequate data in table 5 are substituted in eqn (3) the values shown in the last column of table 6 are obtained. The agreement between calculated values and those obtained by application of the R.R.K.M. theory can be considered as satisfactory, bearing in mind the approximate nature of methods used. Thus there is a striking similarity between efficiencies for energy conversion in diazoalkane photodecomposition and those corresponding to path (a).This fact supports the above conclusion that the diazo compound is an intermediate product in diazirine photolysis. In addition, Frey et al. report that in 3-chloro-3-methyldiazirine photolysis the low-pressure results show the photoisomerization pathway to be over 27% of the primary process (this figure becomes 23% at very low pressures). The value found in this work is 25%, which can satisfactorily be compared with the experimental value reported by Frey. Path (b) represents a second means of photodecomposition of excited substituted diazirines. The results shown in table 6 show that there is a fairly wide spread of energies among the excited olefins. These data indicate that the primary process of photodissociation is not as simple as in the case in which the fragments fly apart with little interaction.The existence of intersystem crossing between the non-dissociative singlet state ln - z* and the dissociative triplet 3n - W* has been pr~pounded,~ but the participation of triplet states seems to be ~n1ikely.l~ In any case, this path must involve the intermediate formation of a carbene, followed by rapid intramolecular hydrogen transfer, yielding an olefin with sufficient internal energy to allow further decomposition.J. M. PEREZ 3517 This work was sponsored by Comision Asesora de Investigacion Cientifica y Tecnica, Spain. APPENDIX ETHENE The high-pressure Arrhenius parameters given by Benson and Hangerz0 were used [log(A/s-l) = 13.5 and E, = 354.5 kJ mol-'1.The vibrational frequencies (in cm-l) of the moleculez1 and activated complex are given below. ( S / D ) ratios are calculated using Z = 2.7 x lo7 Torr-l s-l. molecule complex 825 1443.5 780 730 943.2 2989.5 800 2989 949.2 3019.3 840 3019 995 3105.5 900 r.c. 1050 3272.3 620 3272 1342.4 1623.3 620 1623 PROPENE The high-pressure Arrhenius parameters given by Chappell and Shawz2 were used [log (Als-l) = 16.1 and E, = 357.8 kJ mol-'1. The frequencies used both for the molecule and activated complex are given below. ( S / D ) ratios are calculated using 2 = 3 x lo7 Torr-l s-l. complex (C-C rupture) 3090 1378 920 3272 1444 c.r. 3013 1443 912 3184 61 1 943 2954 1419 578 3019 1419 825 2954 1652 963 3002 1623 250 2992 1298 99 1 3002 1050 250 2933 1172 1045 2989 995 200 1474 428 225 1420 145 200 The activated complex frequencies for C-H scission were taken from ref.(23). 1, D DIME THY LCY c LO PRO PA NE The high-pressure Arrhenius parameters given by Flowers and FreyZ4 were used [log ( A / s - l ) = 14.8 and E, = 261 kJ mol-'1. The vibrational frequencies (in cm-l) of the mo1eculeZ5 and activated complex are given below. ( S / D ) ratios were calculated using 2 = 3.2 x lo7 Torr-' s-l. The two internal rotors corresponding to the rotations of methyl groups have been neglected because their reduced inertia moments cancel out in the expression for the rate constant. 1143518 DIAZIRINE PHOTOLYSIS molecule complex 3099 3080 3018 3018 2977 2976 2976 2975 2899 2899 1502 1470 1648 1466 1464 1418 1381 1378 1358 1281 1102 1070 1040 995 987 983 970 964 930 866 844 73 1 675 269 33 1 314 290 - - r.c.301 8 3018 3018 2970 2976 2976 2975 2899 2899 1502 1470 1468 1466 1464 1418 1381 1378 700 700 900 700 1040 990 980 983 970 964 930 866 400 73 1 350 369 33 1 314 290 - - 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 H. Durr, Top. Curr. Chem., 1975, 55, 87. H. M. Frey, Adv. Photochem., 1966, 4, 225. C. B. Moore and G. C. Pimentel, J. Chem. Phys., 1964,41, 3503. Bigot, R. Ponec, A. Sevin and A. Devaquet, J. Am. Chem. SOC., 1978, 100, 6575. H. M. Frey and D. E. Penny, J. Chem. SOC., Faraday Trans. I , 1977, 73, 2010. J. M. Figuera, J. M. Perez and A. Tobar, J. Chem. SOC., Faraday Trans I , 1978, 74, 809. L. Pierce and Sr. V. Dobyns. J. Am. Chem. SOC., 1962, 84, 2651. D. M. Silver, J . Am. Chem. SOC., 1974, 96, 5959. J. M. Figuera, J. M. Perez and A. P. Wolf, J . Chem. SOC., Faraday Trans. I, 1975, 71, 1905. T. B. Alfasi, S. W. Benson and D. M. Golden, J . Am. Chem. SOC., 1973, 95, 4784. H. M. Frey and I. R. D. Stevens, J. Chem. SOC., 1965, 1700; 1963, 3514; 1965, 3101. L. C. Robertson and J. M. Merritt, (a) J. Mol. Spectrosc., 1966, 19, 372; (6) J. Chem. Phys., 1972, 56, 2919. K. Vasudevan and W. E. Kammer, Chem. Phys., 1976, 15, 103. M. B. Robin, H. Basch, N. A. Kuebler, K. B. Wiberg and G. B. Ellison, J. Chem. Phys., 1969, 51, 45. J. R. Lombardi, W. Klemperer, M. B. Robin, H. Basch and N. A. Kuebler, J. Chem. Phys., 1969, 51, 33. P. H. Hepburn, J. M. Hollas and S. N. Thakur, J . Mol. Spectrosc., 1975, 54, 483. B. M. Jennings and M. T. H. Liu, J . Am. Chem. SOC., 1976, 98, 6416. J. M. Figuera, E. Fernandez and J. M. Avila, J. Phys. Chem., 1974, 78, 1348. J. M. Figuera and A. Tobar, J. Photochem., 1979, 10, 473. S. W. Benson and G. R. Hanger, J. Phys. Chem., 1967, 71, 1735. G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, New York, 1946). G. A. Chappell and H. Shaw, J. Phys. Chem., 1968, 72,4672. J. W. Simons, B. S. Rabinovitch and F. H. Dorer, J. Phys. Chem., 1966, 70, 1076. M. C. Flowers and H. M. Frey, J . Chem. SOC., 1959, 3953. L. M. Sverdlov and E. P. Krainov, Opt. Spektrosk., 1959, 7 , 460. (PAPER 2/326)

ISSN:0300-9599    

DOI:10.1039/F19827803509

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 3519-3527 Water Solubility in Molten-salt Mixtures A Theory for Selective Ionic Hydration BY GIUSEPPE A. SACCHETTO* Institute of Analytical Chemistry of the University, Via Marzolo 1, 35100 Padova, Italy AND ZDENfiK KODEJS Institute of Inorganic Chemistry of the Czechoslovak Academy of Sciences, Majakovskiho 24, 160 00 Prague 6, Czechoslovakia Received 1st March, 1982 A statistical thermodynamic treatment based on the quasi-lattice model of molten salts is developed to explain the non-linear dependence of the enthalpy of transfer of water from liquid water to an infinitely dilute solution of water in a binary molten-salt mixture on the molar composition of the mixture. The derived equation is used to fit previously determined data for the enthalpy of transfer of water from liquid water to (undercooled) liquid LiNO, + NH,NO, mixtures in the whole composition range; a satisfactory fit is obtained for a reasonable choice of the values for the quasi-lattice coordination numbers of LiNO, and NH,NO,.Literature data for the enthalpy of dissolution of water in molten LiNO,+NaNO, and LiNO, + KNO, mixtures are in qualitative agreement with the predictions of the present theory. Investigations of the dissolution of simple gases (e.g. inert gases, hydrogen and oxygen) and of some more complex molecular compounds (e.g. water and dimethyl- sulphoxide) in molten salts are currently receiving increased attention,l? as they can throw light on the different types of interactions between neutral molecules and ions in highly ionic liquid environments and can also have technological applications in such fields as molten-salts-based systems in nuclear reactors, electrowinning of metals from ionic melts and the utilization of geothermal energy sources.Two main methods have been adopted for the investigation of the dissolution of gases in molten salts. One method is based on direct measurements of the solubility of gases in molten ~ a l t s ~ - ~ and the other is based on vapour-pressure measurements for highly concentrated solutions of salts in molecular liquids (such as water or dimethylsulphoxide) when a suitable equation relating the activity and composition of a molecular compound in highly concentrated solutions (e.g. the activity isotherm of Stokes and Robinson6) is used in this case to calculate the activity coefficient of the molecular compound at infinite dilution in the liquid (or undercooled liquid) salt.2* This method is particularly suitable for those molecular compounds which are not thermally stable and/or chemically inert with respect to molten salts.It can be used to determine solubility data in undercooled molten salts and the data can then be extrapolated for temperatures above the melting point of the salt. The dissolution of water in molten nitrates has been extensively investigated by both method^.^-^ Some of these studies were performed using binary mixtures of metal nitrates, whose cationic composition was varied over a wide range (e.g. LiNO, + NaNO,, LiNO, + KN0,4 and LiNO, + NH,NO, mixtures2).The enthalpy of dissolution, AHsoln, of gaseous water in molten nitrates and the 3519 114-23 520 WATER SOLUBILITY IN MOLTEN SALTS enthalpy of transfer, AH,,, of water from liquid water to infinite dilution in molten nitrates were calculated from the temperature dependences of solubility data1* * and of vapour pressure data,2* respectively; their dependences on the composition of some binary nitrates mixtures have also been given.lT To our knowledge, there exists no interpretation of the non-linear dependences of the dissolution enthalpyl and of the transfer enthalpy2 on the molar composition of the binary nitrate mixtures. Indeed, the only suggestion is that of 'a non-random distribution of the water molecules between the hydration spheres of the two cations', given in ref.(2). In the present paper a statistical thermodynamic treatment is developed to explain the main features of the transfer (and solution) properties, assuming as the starting points the basic concepts of the quasi-lattice model of molten salts, as previously developed 6y BlanderlO and by Bombi and Sacchetto.ll THEORETICAL We must first make a reasonable assumption about the type of quasi-lattice sites occupied by neutral molecules entering the molten salt; this has previously been discussed by Sacchetto et For water-containing systems, extensive investigations were carried out by Braun- stein and coworkers7 on hydrous nitrate melts, where they adopted the anionic site occupancy hypothesis for the water molecules. In the following treatment we shall assume the same kind of occupancy, by postulating that water molecules compete with nitrate ions for occupancy of the sites of the anionic sub-lattice, which is equivalent to assuming that only cations are hydrated.We make also the simplifying assumption that in the dilute solutions considered here only simple cation-water pairs (M+-H,O), and not more complex groups such as H,O-M+-H,O, are formed. According to a formalism similar to those previously adopted in treatments based on the quasi-lattice model, the transfer of water from liquid water to an infinitely dilute solution of water in a single molten salt of the type AY can be expressed by the following simplified scheme A+-Y- + H,O A+-H,O + Y- (1) which represents the replacement of one Y- anion in the anionic sub-lattice of the AY molten salt with one water molecule from liquid water.The displaced anion can, for instance, be associated with a vacant cation site [see e.g. ref (7), p. 2441.t Assuming that the interaction energies for any given pair of particles (ions, neutral molecules and formally also cationic vacancies) involved in the process, i.e. the so-called pairwise ' bond' energies E , are mutually independent, the energy change per mole, AE&, associated with the process considered above can be expressed as follows where EA-H2P [ = N(cAPHpO + E ~ - , . ~ . - corresponds to the energy involved in the formation of one mole of A+-H,O pairs in the particular framework expressed by reaction (I), and EHzO [= J1TEHz0-H20] is the energy for breaking one mole of water-water bonds (electrical charges are omitted for simplicity).Similarly we can write for the salt BY B+-Y-+H,OSB+-H,O+Y- (3) t As a consequence of the choice of only anionic-sites occupancy by water molecules the possible anionic vacancies are not taken into consideration.G . A. SACCHETTO AND z. KODEJS 352 1 where the energy change per mole, AEE, can be expressed as in eqn (2) The energy parameters AE& and AEE are to be considered as temperature- independent, because they are defined as true internal energy changes.” In a similar way the transfer of water from liquid water to infinite dilution in a binary molten-salts mixture of the type [xAY + (1 - x) BYI can be expressed by the following scheme (A+, B+)-Y-+H,O=(A+, B+)--H,O+Y-. Both reactions (1) and (3) are superimposed to yield the overall process reaction (9, the contributions from each of them depending on the different relative probability of either reaction pathway.The energy change per mole associated with reaction ( 5 ) , AEtFjx, can thus be calculated according to the following combination of AE& and AE,s, A e i X = WA AE& + (1 - w,) AE,B, (6) where wA is a probability factor which is evaluated by means of a statistical mechanical calculation. The relative probabilities of the two possible reaction pathways (1) and (3) are related to the equilibrium distribution of the water molecules between the two kinds of available anionic sites, those near the A+ and B+ cations, respectively. Thus wA and (1 - w,) are equal to the fractions of A+-H,O and B+-H,O pairs, respectively, over the total amount of cation-water pairs formed at equilibrium.We now calculate, using the assumptions introduced above, the equilibrium distribution of the water molecules in the system AY+BY+H,O. Let n A and nB denote the numbers of A+ and B+ ions, respectively, and nHZO be the number of water molecules interacting with the cations from anionic sites. If we denote the quasi-lattice coordination numbers of molten AY and BY by the symbols 2, and Zz, respectively, and define Zmix as a formal quasi-lattice coordination number of the molten mixture AY+BY, then Z,n, and Z2nB are the total number of sites around A+ and B+ cations, respectively, available to accommodate water molecules, and Zmix nH20 is the total number of cationic sites around the water molecules dissolved in the melt.If a’ and b’? are used to denote the fractions of the positions adjacent to A+ and B+, respectively, occupied by the water molecules in any arbitrary distribution of the species, then 2, nA a’ and Z , nB b’ are the numbers of A+-H,O and B+-H,O pairs, respectively, and their sum gives the total number of cation-water pairs in the melt, The energy associated with the arbitrary distribution is given by the following (8) expression E = 2, nA Q’EA-H,~ + 2, nB b’EBdH,-,. The number of configurations, a’, of the ternary system AY + BY + H,O is defined (9) as a’ = R+‘a-‘ = R+’Q-’LR-’ where a+’ refers to the distribution of the cations on the cationic sites and f&’ and R,’ refer to the distributions of anions and water molecules on the anionic sites around A+ and B+ cations, respectively. A B t Primed symbols will be used to indicate an arbitrary distribution of the ions and unprimed symbols will denote the equilibrium distribution.3522 WATER SOLUBILITY IN MOLTEN SALTS Random mixing of the cations in the cationic sub-lattice gives The numbers of configurations arising from the distribution of the anions (Y- and H,O molecules) are as follows ZInA! Z,n,a’![Z,n,(l -a‘)]! and The most probable (equilibrium) distribution is found by maximizing the probability of an arbitrary distribution, P‘{ = In [a’ exp ( - E‘/AT)]), with respect to a’, at constant composition, i.e.by means of the following operation d In [a’ exp (- E‘/kT)] da‘ = 0. (13) By replacing a’ And E’ from eqn (8)-(12) and by using Stirling’s approximation, In n! = n In n - n, we obtain for the equilibrium distribution a 1-a - = -exp [- (EA-H,O - ~R--H,O)/RTl. b I - b Since we consider here only very dilute solutions of water in molten salts, we can take a 6 I and b 6 1.Eqn (14) then reduces to (15) a - = exp [ - (AE& - AE,B,)/RT] b where AES - AE,B, is written in place of EAeHZ0 - EBdHPO, according to eqn (2) and (4). The statistical factors W , and (1 - w,) in eqn (6) are the relative probabilities of the two possible reaction pathways expressed by reactions (1) and (3) and thus are equal to the fractions of A+-H,O and B+-H,O pairs, respectively, over the total number of cation-water pairs (see above); for the equilibrium condition we can write and z, nB b 1-W - A - ~ , n , a + ~ , n , b ‘ Dividing eqn (1 6) by eqn (1 7), and substituting for the a / b ratio from eqn (1 5), we obtainG .A. SACCHETTO AND z. KODEJS 3523 in which nA and nR are expressed as functions of xAY, according to the well known relationship: xAY = nAY/(nAy + n g y ) = nA/(nA + nB). Putting for simplicity (19) p = exp[-(AE,A,-AEtB,)/RT] (20) 21 2 2 @ = - and and solving for wA, we obtain Substituting eqn (21) into eqn (6) we finally have DISCUSSION From eqn (22) it is seen that AQix is not a simple mole-fraction-based linear combination of the two AE,, contributions arising from the two single salts AY and BY. Each contribution is in fact 'weighted' by a factor which depends on the energy difference AE& - AEtB,, the temperature, the spatial (or configurational) Z J Z , factor and the molar composition of the salt mixture.For instance, at constant temperature A m i x is a non-linear function of the molar composition, except for the trivial case AEk = AEtB,. It can also be easily demonstrated that the curvature of A Q i x as a function of xAY is such that the values of A m i x are nearer thc more negative of the two AE,, values for single salts than expected on the basis of a simple linear interpolation between the two AE,, values. At constant xAY, on the other hand, A G i x increases with increasing temperature and its limiting value for T + co is given by A e i x = XAY AE& + (1 - X A ~ ) AEE. (23) Thus, the linear dependence of A Q i x on molar composition is only a limiting behaviour for very high temperatures and/or small energy differences AE& - AEtB,.The above features of the quantity A G i X are shown in a graphical simulation in fig. 1. The choice of the values for AE$ and AEtB, used for the simulation is quite arbitrary and is essentially conditioned by the need to have AE& different from AEZ in order to enhance the curvature and sensitivity of the plots with respect to the varied quantities, such as the temperature and the ratio of quasi-lattice coordination numbers. The curves (a), (b) and ( c ) refer to three different values of the temperature, for 8 = 1 , i.e. 2, = 2,. Note that the shapes of the curves are influenced only by the difference AE,A,-AE,B, and not by the individual AEtr values. As regards the influence of the ZJZ, ratio, we must first recall that similar salts cannot have their quasi-lattice coordination numbers too different from one another [see, e.g.the average coordination numbers for liquid salts in ref. (1 3)]. 2 values in the range 4-6 were assumed in many applications of the quasi-lattice model of molten salts.' The influence of changing the Z J Z , ratio in the range from 6/4 to 4/6 can be seen in fig. 1, where curves (b), ( d ) and ( e ) are drawn for the same temperature (340 K). The effect is rather significant; the importance of the choice of proper values for 2, and 2, will be reconsidered below.3 524 0.0 -1.0 - I - 0 E - .Y . 5 -2.0 -3.0 WATER SOLUBILITY IN MOLTEN SALTS 0.0 0.5 XAY 1 .o FIG. I.-Graphical simulation of the dependence of the energy change A@'" on the molar composition of the AY +BY salt mixture (x,,), as derived by eqn (22).AE& = -3.0 kJ mol-I, AEE = 0.0 kJ mol-l. (a) T = 310K, 8 = l;(b) T = 340K, 8 = l;(c) T = 370K, 8 = 1; ( d ) T = 340K,8= 6/4;(e) T = 340K, B = 416. A relationship between the energy change involved in the process represented by reaction (l), AEP,, and the experimentally measurable enthalpy of transfer of water from liquid water to the liquid salt AY, AH&, can be suggested on the basis of the following considerations. When a molecule of water is transferred from liquid water to an anionic site in a liquid salt AY, it must form Z , water-cation 'bonds' to the A+ cations. If the energy change involved in the formation of the 2, successive water-cation bonds were not affected by saturation effects,14 we could write the following simple relationship7 AH&.= Z , AE&. (24) However, as saturation effects could not be excluded in every real system, we should write: AH& d Z , AE& (25) as Z , AE& represent an upper limiting value for the experimental AH& quantity. In verifying the ability of eqn (22) to represent the dependence of the experimental AHEix values on salt mixture composition, we shall use, as a first approximation, eqn (24) to calculate AEP,. As only condensed phases are involved in the transfer process, internal energy changes can be considered equal to enthalpy changes.G. A. SACCHETTO AND z. KODEJS 3525 In the same approximation we assume and AH,R, = 2,A.E: AHgix = ZmixAKix where Zmix, the formal quasi-lattice coordination number of the molten mixture AY + BY, can be calculated as a function of Z, and 2, according to the following reasoning.Eqn (7) applied to the equilibrium condition gives The factors n A a/nHzO and nR b/nHZ, represent the fractions of the total number of cation-water 'bonds' in the AY+BY+H,O system which are A+-H,O and B+-H,O bonds, respectively. As shown above these two factors are equal to wA and 1 - wA, respectively, so we can write where wA is given by eqn (21), together with the definitions of 8 and j? [eqn (19) and (20), respectively]. In the definition of p [eqn (20)] the energy terms AE& and AE,B, can be expressed as a function of AH& and AHE, respectively, through the use of eqn (24) and (26); we then obtain By combining eqn (22), (24), (26) and (27) and solving for Amix, the equation is obtained where Zmix, p and 0 have the meanings defined above [eqn (29), (30) respectively].(30) following (31) and (19), A test of the validity of the equation for AHgix can be performed by fitting it to the data for the enthalpy of transfer of water from liquid water to infinite dilution in the (undercooled) liquid LiNO, + NH,NO, mixtures, which were determined from vapour pressure measurements by Sacchetto et all5 (for the LiNO, + H,O and for the NH,NO,+H,O solutions) and by Bombi et a/., (for the LiNO,+NH,NO,+H,O mixed-salt systems). All the data were obtained using the same dew-point apparatus and can be considered as a consistent set in the temperature range 320-380 K. The values of the enthalpy of transfer [as plotted against x ~ , ~ ~ ~ ~ in fig.2 of ref. (2)] are given in table 1 . In view of the procedure adopted fGr their derivation,, these values must be regarded as average transfer enthalpies in the temperature range and may be considered to be subject to an uncertainty of kO.2 kJ mol-l. A first trial to fit the reported AHt,. data by means of eqn (3 1) was made by assuming that the quasi-lattice coordination numbers of LiNO, (Z,) and of NH,NO, (2,) have TABLE VA VALUES OF THE ENTHALPY OF TRANSFER XLiN03 0.00 0.25 0.50 0.75 1 .00 AH,,./kJ mol-l 4.1 - 1.2 - 4.4 -7.1 - 7.33 526 4.0 0.0 - I - z ;z --- - 4 . 0 -8.0 WATER SOLUBILITY IN MOLTEN SALTS 0.0 0.5 LiNO, 1.0 FIG. 2.-Comparison between experimental and calculated values of the enthalpy of transfer of water from liquid water to LiNO, +NH,NO, (undercooled) liquid mixtures, AHtYix, at 350 K.(0) Experimental data from ref. (2) (vertical bars indicate the estimated uncertainty). Calculated curves for (a) 2, = Z , = 3; (6) 2, = Z, = 4; (c) Z , = Z , = 5 ; ( d ) 2, = 2, = 6. the same value. Fig. 2 shows a plot of the experimental AH,, data together with the least-squares optimized curves, obtained by fitting the data at the average temperature of 350 K with the following sets of 2 values: 2, = 2, = 3,Z, = Zz = 4, 2, = 2, = 5 and 2, = 2, = 6.t The reported curves agree with the general trend of the experimental data, although the point at x L ~ N O , = 0.75 appears to be too low to be fitted by eqn (3 1) with the above assumption about the coordination numbers.The best fit is, however, obtained with the set 2, = 2, = 4, a value which appears to be quite reasonable in respect of the usual 4-6 range (see above). On the other hand, note that the 2 values derived by this procedure are presumably lower than the real ones, as a consequence of the fact that the above-mentioned saturation effect could not be taken into account [see eqn (25)]. A further trial was made to refine the fit by using sets of different 2, and 2, values; in view of previous considerations, however, the difference between 2, and 2, was not allowed to exceed f 2 . Under this assumption the best fit was obtained with the set 2, = 4 and 2, = 6. The corresponding fitting curve (not shown) has a distinct, although small, asymmetry, so that its curvature is stronger in the high xL~NO, range.This corresponds to the trend shown by the experimental points. Note, however, that in view of the many approximations of the model proposed and especially owing to the approximate character of eqn (24), (26) and (27), not too t It is well known that the average coordination numbers of molten salts are real, but integers are generally assumed in the applications of the quasi-lattice model.G. A. SACCHETTO AND z. KODEJS 3527 much importance can be given to the resulting values of Z , except for the fact that they fall into a range which compares well with that proposed in previous papers on this subject. Data for the enthalpy of dissolution of water in molten LiNO,+KNO, and LiNO, + NaNO, mixtures were calculated by Field1 mainly from the solubility data of Bertozzi4 and Peleg.5 However, as can be seen in fig.8 of ref. (I), although the dependence of AHsoln on xLiNO3 is non-linear and the curvature is as expected on the basis of eqn (31) applied to dissolution enthalpies, these data are not very useful for a quantitative test owing to their uncertainty and dispersion. We conclude that the most important and general features of the dependence of the enthalpy of transfer (or dissolution) of water on the composition of the salt mixtures are correctly predicted by the present theory, although for a more fruitful and detailed test further experimental data are required. This work was carried out within the framework of the agreement between the National Research Council of Italy (C.N.R.) and the Czechoslovak Academy of Sciences (c.S. A.V.). P. E. Field, in Advances in Molten Salt Chemistry, ed. J. Braunstein, G. Mamantov and G. P. Smith (Plenum Press, New York, 1973), vol. 3. G. G. Bombi, G. A. Sacchetto and C . Macca, Extended Abstracts, 31st Meeting of the international Society of Electrochemistry, Venice, 1980, vol. 2, p. 455. B. Cleaver and D. E. Mather, Trans. Faraday Soc., 1970, 66, 2469; P. E. Field and W. J. Green, J. Phys. Chem., 1971,75,821; F. Paniccia and P. G. Zambonin, J. Chem. Soc., Faraday Trans. I , 1972, 68,2083; E. Desimoni, F. Paniccia and P. G . Zambonin, J . Chem. Soc., Faraday Trans. 1, 1973, 69, 2014; F. Paniccia and P. G. Zambonin, J. Chem. Soc., Faraday Trans. 1, 1973, 69, 2019; S. Allulli, J . Phys. Chem., 1969, 73, 1084. G. Bertozzi, 2. Nuturforsch., Teil A, 1967, 22, 1748. M. Peleg, J. Phys. Chem., 1967, 71, 4553. R. H. Stokes and R. A. Robinson, J. Am. Chem. Soc., 1948,70, 1870. M-C. Abraham, M. Abraham and J. Sangster, J . Chim. Phys., 1979, 76, 125. M-C. Abraham, M. Abraham and J. Sangster, Can. J. Chem., 1980, 58, 1480. ’ J. Braunstein, in ionic interactions, ed. S. Petrucci (Academic Press, New York, 1971), vol. I. lo M. Blander, J . Phys. Chem., 1959, 63, 1262. l 1 G. G. Bombi and G. A. Sacchetto, J. Electroanal. Chem. Interfacial Electrochem., 1972, 34, 319. G. A. Sacchetto, G. G. Bombi and C. Macca, J . Chem. Soc., Faraday Trans. 1, 1976, 72, 1972. l3 H. A. Levy and M. D. Danford, in Molten Salt Chemistry, ed. M. Blander (Wiley, New York, 1964), l4 M. Blander, J . Chem. Phys., 1961, 34, 432. l5 G. A. Sacchetto, G. G. Bombi and C. Macca, J . Chem. Thermodyn., 1981, 13, 31. pp. 109-125. (PAPER 2/372)

ISSN:0300-9599    

DOI:10.1039/F19827803519

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 3529-3535 Theory of Volumetric Behaviour of Hydrous Melts The Systems LiN0,-H,O and NH,N03-H,O BY ZDENEK KODEJS Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Majakovskeho 24, 160 00 Prague 6, Czechoslovakia AND GIUSEPPE A. SACCHETTO* Institute of Analytical Chemistry of the University, Via Marzolo 1, 35100 Padova, Italy Received 1st March, 1982 An equation for the dependence of the molar volume of a hydrous melt on its molar composition is derived on the basis of the concept of the apparent molar volume of water and from a previous calculation of the distribution of water molecules between sites near ions and sites near water molecules in the melt. The densities of LiN0,-H,O and NH,NO,-H,O liquid mixtures have been measured at 333 K over the composition ranges 0.05 < x(LiN0,) c 0.3 and 0.1 < x(NH,NO,) < 0.5, and have been used, together with literature data for the systems NH,NO,-H,O, (Lin,5 K,,,)NO,-H,O, Ca(NO,),-H,O and Cd(NO,),- H,O, to test the validity of the derived equation and to calculate the values of the physical parameters involved.Many investigations of the volumetric behaviour of electrolyte solutions have confirmed the suitability of the concept of the apparent molar volume (a.m.v.) of a sa1t.l All deviations from the additivity of V z (the molar volume of the pure solvent) and V: (the molar volume of the pure salt) are included in the value of the a.m.v. of the salt, which then depends on the salt concentration. At the other end of the composition range, i.e.for the so-called ‘hydrous melts’, and for their non-aqueous analogue^,^ the concept of the a.m.v. of the salt cannot be applied. For this type of system a different approach has been proposed by Kodeji and Slams,* who formulated the alternative concept of the ‘apparent molar volume of water’, based on the assumption that the following equation holds for the total volume of the solution: V = n, Vw + n, V,O where Vw is the a.m.v. of water. In highly concentrated solutions the fraction of water molecules that have the typical bulk water structure is rather small, as most of the water is strongly influenced by the ions. The concept of the a.m.v. of water has been used to derive a relationship between the molar volume of the melt, V,, and its composition [see eqn (2) of ref.(4)], which for a binary salt-water system at fixed temperature reduces to the following expression : where x, is the mole fraction of the salt, R , is the mole ratio of water (i.e. R , = nw/ns), and A , C and D are empirical parameters. The main scope of the present paper is to provide a theoretical explanation of the physical meaning of the parameters in eqn (2) and to show how the general features 35293530 VOLUMETRIC BEHAVIOUR OF HYDROUS MELTS of the dependence of V, on x, can be predicted on this basis. The proposed treatment starts from a model for solutions of water in molten-salt mixtures recently developed by Sacchetto and KodejL5 Density measurements for the LiN0,-H,O and NH,NO,- H,O systems have also been performed and the density data are used to verify the ability of the derived equation to fit experimental molar-volume data with its parameters taking physically reasonable values.Further tests on the following systems are performed using literature data : NH,N0,-H,0,6 (Lio.5 K0,5)N03-H20,7 Ca(NO,),- H,Os and Cd(NO,),-H,O.g THEORETICAL The quantity within square brackets in eqn (2) represents the concentration- dependent molar volume of water, V,, in a salt-water liquid mixture: By introducing expressions for A and C which are derived at the concentration limits of the binary salt-water system, and by replacing R , with (1 -x,)/x,, the following equation is obtained for V, : 1 -x, 1 + x,(D - 1) vw" vw = Dxs v,"+ 1 +x,(D- 1) (4) where V$ is the molar volume of water at infinite dilution in the liquid salt.Eqn (4) is formally very similar to an equation recently derived by Sacchetto and KodejS5 by means of a statistical-mechanical calculation based on the quasi-lattice model used to explain the composition dependence of enthalpies of transfer of water from liquid water to infinitely dilute solutions of water in molten-salt mixtures. They assumed two types of sites to be available for a water molecule entering the liquid salt mixture AY-BY, and calculated the equilibrium distribution of the water molecules between these two types; the following equation for the enthalpy of transfer of water from liquid water to a binary molten-salt mixture, AHgix, was obtained: where the quantities Z are quasi-lattice coordination numbers, and AH,, are enthalpies of transfer of water from liquid water to the pure molten salts.The quantity/? is defined as and 0 = ZAy/ZBy. The statistical weighting factors for AH&y/ZAy and AH,B,y/Z,, in eqn ( 5 ) are formally similar to those for V$ and V z in eqn (4). There remains only to correlate the product PO with the parameter D. We can suppose that in a liquid salt-water mixture two types of sites having different energy levels are available to accommodate water molecules. One type of site is in the immediate neighbourhood of the ions of the salt; the effective molar volume of water at this type of site may be regarded as the molar volume of water at infinite dilution in the molten salt, V,". The second type of site is in the bulk water structure; the effective molar volume of water at this site is the same as the molar volume of pure water, V:.The different effective molar volumes of water at the two types of site may essentially be ascribed to the differentz. KODEJS AND G. A. SACCHETTO 353 1 modes and densities of packing of the water molecules in the hydration spheres of the ions and in bulk water. The distribution of water molecules between these two types of site is governed by statistical factors which are essentially the same as those calculated in our previous paper5 [see eqn (5)]. Thus the effective molar volume of water in a salt-water mixture, V,, results from a non-linear combination of V z and V;, as given by eqn (4). The parameter D is given by PO; in the present case AHSa't = exp( -2) Zsalt RT (7) as the enthalpy of transfer of water from liquid water to bulk water is equal to zero [cf.eqn (6)], and 0 is defined as the ratio Zsalt/Zwater; the quantity Zwater can be interpreted as an average coordination number of water in liquid water and an approximate value of 4 can be taken from models of liquid-water structure.1° Values of 2 of ca. 4-5 were found to be acceptable for various nitrates in ref. (5). The value of 0 is then taken approximately equal to 1. By combining the assumption r/s = V,O [see eqn (l)] with eqn (4), where D is replaced by PO, we have the following equation for the dependence of the molar volume of a liquid salt-water mixture on the mole fraction of the salt: 1 -x, 1 +x,(ae- 1) v:) (1 - x s ) v, = v,ox,+ ( vz+ 1 +x,(a@- 1) where P is given by eqn (7) and 0 is assumed equal to 1 (see above). EXPERIMENTAL Reagent-grade LiNO, and NH,NO, (Merck) were vacuum desiccated and stored over P,O,.The densities of the aqueous solutions were determined by means of a DMA digital densimeter (A. Paar K.G., Austria) ; the experimental procedure and precision of the measurements have been discussed previ~usly.~ The densities of the LiN0,-H,O and NH,NO,-H,O solutions were measured at 333 K in concentration intervals ranging from xs = 0.05 and 0.1, respectively, up to the saturation limits. The experimental density data are summarized in table 1, together with the corresponding molar volumes of the mixtures. TABLE EXPERIMENTAL VALUES OF DENSITY AND MOLAR VOLUME OF THE LiN0,-H,O AND NH4N03-H20 SYSTEMS AT 333 K LiN0,-H,O NH,NO,-H,O d vm d vm X S /g cm-, /cm3 mol-l XS /g cm-, /cm3 mo1-I 0.05 1.0819 19.00 0.10 0.10 1.1721 19.72 0.15 0.15 1.2547 20.45 0.20 0.20 1.3303 21.20 0.25 0.25 1.3990 21.98 0.30 0.30 1.4613 22.78 0.35 0.40 - 0.45 0.50 - - - - - - - - 1.1177 1.1695 1.2129 1.2497 1.2825 1.3 105 1.3383 1.3627 1.3870 21.67 23.36 25.08 26.83 28.56 30.31 32.00 33.71 35.353532 VOLUMETRIC BEHAVIOUR OF HYDROUS MELTS I 0 0.5 1 .o x, FIG.1 .-Graphical simulation of the dependence of the molar volume of salt-water mixtures on the mole fraction of salt, as derived from eqn (8) (molar volume is expressed in cmS mol-l, enthalpy in kJ mol - I ) . V," = 25, AHtr = 0; (d), Vg = 15, AHtr = - 16; (e), V," = 15, AHtr = 16. T = 333 K, 2 = 4, B = 1 , V: = 40, V," = 20. (a), VF = 15, AHtr = 0; (b), VF = 20, AHtr = 0; (c), RESULTS AND DISCUSSICN SIMULATION It is of interest to show graphically how the main features of the dependence of Vm on x,, as given by eqn (8), vary with the parameters.Let us first discuss the simple case where AH,, = 0. There are three different possibilities according to the relative magnitudes of V: and V z : if V z = V:, we get a linear function for Vm as a function of x, [i.e. an additivity relationship between V: and V,O, see line (b) in fig. I] ; if V z < V,", negative deviations from simple add'tivity are obtained [line ( a ) ] ; if V z > V,", positive deviations are obtained [line (c)]. Only few water transfer-enthalpy data are available (see below). The values of AH,, found are usually negative, but positive values can be also found.On the other hand, it is reasonable to consider that the molar volumes of water at the two above-mentioned types of site are generally different. The most frequent case for real systems is certainly Let us now suppose that V: < V:. A negative value for AH,, not only enhances the negative deviations from additivity [cJ curve ( d ) with curve (a)] but also introduces a significant asymmetry effect, so that the deviations are more pronounced in the low-x, region than in the high-x, region. In the opposite case, when a positive value for AH,, is chosen the negative deviations from additivity are reduced [cf. curve (e) with curve (a)] and an asymmetry effect is also evident in this case. A positive value of AHtr is known only for the NH,NO,-H,O system (see below); only very small devia- tions from additivity are expected in this case.For the less frequent case where V$ > V: the deviations from additivity are expected to be positive in any case. Different asymmetric shifts may also be found in this case, depending on the sign of AHt,. We can assert that the only condition for perfect additivity of the molar volumes V$ < V:.'oz. K O D E J S A N D G. A. SACCHETTO 3533 of water and salt is V$ = V;, whatever the value of AHtr. However, a good apparent additivity can be obtained, in spite of the inequality of V," and V:, if Alltr has a positive and sufficiently large value. This situation is rather infrequent in practice (see below for the NH,NO,-H,O system). We can thus say that the additivity of molar volumes of liquid salt-water mixtures should be the exception rather than the rule.COMPARISON WITH EXPERIMENTAL AND LITERATURE DATA To test the applicability of eqn (8), we first use the experimental density data for LiN0,-H,O and for NH ,NO,-H,O solutions as obtained from our measurements (table 1). Further density-data sets were collected from the literature for the systems NH,N0,--H,0,6 (Li, K, 5)N0,-H,0,5 Ca(NO,),-H,OH and Cd(NO,),-H,Og (see table 2). We first decide which of the parameters of eqn (8) can be considered as known a priori and which can be evaluated from the fit. Some of them are in fact known with a good degree of accuracy: e.g. the temperature and the molar volume of liquid water.'l The value of 2 is taken as 4 (see above); however, it does not play a decisive role as its choice influences only the term AHt,./Z, whose uncertainty comes mainly from the value of AH,, (see below).Difficulties arise in the evaluation of AHtr for several systems. In fact, only in the case of LiN0,-H,O and NH,NO,-H,O have transfer-enthalpy values been correctly evaluated1, by applying the Stokes-Robinson ' adsorption-hydration' mode113 to vapour-pressure data and by taking into account the 'internal' entrcpy contribution to the water-transfer process., For the other systems reported in table 2 approximate A l l t r values can be estimated from E, - El data (the 'net' adsorption energy13), which are available in the literature. The contribution of the 'internal' entropy change is estimated to be ca. 40-500/, of the total E,- El effect in the case of the LiN0,-H,O system.l2?l4 On applying the same correction to the Ea-E, values for the (Lio.5 KO S)NO,-H,O and the Ca(NO,),-H,O systems1, we obtained the AHtr values given in table 2.column 3. For the Cd(NO,),-H,O system, for which directly measured E.', - E, values are not available, we calculated an approximate AHtr value from that for the Ca(NO,),-H,O system by using the quotient 0.9, which is obtained by dividing the extrapolated value of Ea-El for the calcium-containing system by that of the cadmium-containing system as reported by Sangster et ~ 1 . ~ ~ 7 16* The remaining parameters V$ and V': were instead obtained from the fitting treatment and are reported in table 2, columns 4 and 5, respectively. For the Ca(NO,),-H,O and Cd(NO,),-H,O systems the best fit of eqn (8) and more reliable values of the parameters V," and V,P were obtained by using equivalent volumes and equivalent fractions instead of the corresponding molar quantities.This approach has been adopted previously by other authors in treating density data for ultraconcentrated solutions of bi-univalent electrolytes [see e.g. ref. (1 7)]. We have tested the effect of the uncertainties inherent in the transfer-enthalpy values on the calculated values of the parameters V'$ and V:. We found that changing the AH,,/Z values by 30p/,, a change large enough to cover the most unfavourable case, results in small changes in V:, not exceeding 2:{; the changes in V'Z are larger, but they never exceed 10%. The V: values resulting from the fitting treatment can be compared with the molar * The E,- E, values for the Ca(NO,),-H,O and Cd(NO,),-H,O systems given by these authors are the result of extrapolations over very wide concentration ranges pzrformed on data for aqueous solutions of mixtures of (Ago ,Tl,,,)NO, with Ca(NO,), and with Cd(NO,),, respectively; for this reason they are not reliable.Moreover, the datum for the calcium-containing system does not agree with that given in ref. (14). The quotient 0.9 adopted above is perhaps more reliable.3534 VOLUMETRIC BEHAVIOUR OF HYDROUS MELTS TABLE 2.-vALUES OF THE PARAMETERS OF EQN (8) AT 333 K salt A& v: V; Vs"d x, range /kJ mol-l /cm3 mol-1 /cm3 mol-l /cm3 mol-1 NH,NO, 0.1-0.5 4.0 17.1 53.0 - LiNO,-KNO,b 0.5-0.8 - 4.2 15.4 43.5 43.6e 0.05-0.27 - 8.4 13.3 37.lC 36.3f NH,NO,a 0.25-0.45 4.0 15.9 53.4 - LiNO, 0.05-0.3 - 7.3 16.5 35.1 33.4e CdfN03)2 0.05-0.35 - 7.5 12.4 37.4c - V: is 18.32 cm3 mol-l and 19.09 cm3 m o t 1 at 333 K and 392 K, respectively;13 a data from ref. (6); (L& K,,,)N03 at 392 K; equivalent volume; values extrapolated from density data reported in the indicated references; ref, (18); f ref. (19). 0 0.2 0.4 0.6 0.8 1.0 xs FIG. 2.-Comparison between experimental and calculated deviations of molar volumes of mixtures from purely additive behaviour. 0, NH,N03-H,O (this work); a, NH,NO,-H,O [ref (6)]; 0, LiN0,-H,O; +, (Li,,,K,.,)N03-H,0; A, Ca(NO,),-H,O; x , Cd(NO,),-H,O. volumes of the undercooled liquid salts calculated from their densities as extrapolated on the basis of density-temperature equations for the molten salts when available in the literature (table 2, column 6).No such data are available for NH,NO, and Cd(NO,),. The agreement between the two sets of V,O values may be regarded as satisfactory, in view of the large uncertainty which affects the above procedure. The larger discrepancy between the V . values for LiNO, can be ascribed to the rather narrow concentration range over which the density could be measured for the LiN0,-H,O solutions. The equivalent volume obtained for Cd(NO,), is almost equal to that for Ca(NO,),. Since the two cations are similar in size, this finding provides a reasonable justification for the calculation presented. From table 2 the V$ values are seen to decrease as the ionic potential of the cation increases.The (L& Ko.,)NO,-H,O system does not fit into the observed pattern; however, the value of V," is rather uncertain, as the measurements were restricted toz. KODEJS AND G. A. SACCHETTO 3535 the salt-rich concentration range. The observed pattern also provides support for the value of V z for the NH,NO,-H,O system calculated from our data, in contrast to that calculated from the data of Sharma and Gaur? The scatter of the experimental V, data about the calculated V, against x, curves can be seen in fig. 2, where deviations, AVm, from the linear additive behaviour of V: and V,O are shown by way of illustration. The fit is good for all systems except NH,NO,-H,O, for which systematic trends are apparent in both sets of data. The small, but not negligible, systematic discrepancy between our data and those of Sharma and Gaur6 is reflected in the above-mentioned discrepancy in Vg values.This work was carried out within the framework of an agreement between the National Research Council of Italy (C.N.R.) and the Czechoslovak Academy of Sciences (c. S .A .V.). F. J. Millero, in Water and Aqueous Solutions, ed. R. A. Horne (Wiley-Interscience, New York, 1972), chap. 13. * J. Braunstein, in Ionic Interactions, ed. S. Petrucci (Academic Press, New York, 1971), vol. I, chap. IV. G. A. Sacchetto, G. G. Bombi and C. Macca, J . Electroanal. Chem. Interfacial Electrochem., 1974, 50,300; Z. KodejS, G. A. Sacchetto, C. Macca and G. G. Bombi, Ann. Chim. (Rome), 1978,68, 151. Z . KodejS and I. Slama, Collect. Czech. Chem. Commun., 1980, 45, 17. G. A. Sacchetto and Z . KodejS, J . Chem. Soc., Faraday Trans. I , 1982, 78, 3519. R. C. Sharma and H. C. Gaur, J , Chem. Eng. Data, 1977, 22, 41. W. W. Ewing and R. J. Mikovsky. J . Am. Chem. Soc., 1950,72, 1390. W. W. Ewing, J. Phys. Chem., 1953, 57, 245. New York, 1972), chap. 10. G. E. Boyd, J . Chem. Eng. Data, 1977. 22, 413. l 2 G. G. Bombi, G. A. Sacchetto and C. Macci, Extended Abstracts, 31st Meeting of ISE, Venice, September 1980, ed. E. Vecchi (Institute of Polarography and Preparative Electrochemistry of the National Research Council, Padua, 1980), p. 455 and paper to be published. ' J. Braunstein, L. Orr and W. MacDonald, J. Chem. Eng. Data, 1967, 12, 415. lo C. M. Davis and J. Jarzynski, in Water and Aqueous Solutions, ed. R. A. Horne (Wiley-Interscience, l 3 R. H. Stokes and R. A. Robinson, J . Am. Chem. SOC., 1948, 70, 1870. l4 H. Braunstein and J. Braunstein, J. Chem. Thermodyn., 1971, 3, 419. l 5 J. Sangster, M.-C. Abraham and M. Abraham, J. Chem. Thermodyn., 1979, 11, 619. J. M. Sangster, M.-C. Abraham and M. Abraham, Can. J. Chem., 1978, 56, 348. S. K. Jain, J . Chem. Eng. Data, 1977, 22, 383. G. P. Smith, and G. F. Petersen, J . Chem. Eng. Data, 1961, 6, 493. l 9 W. J. McAuley, E. Rhodes and A. R. Ubbelohde, Proc. R. Soc. London, Ser. A, 1966, 289, 151. (PAPER 2/373)

ISSN:0300-9599    

DOI:10.1039/F19827803529

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 3537-3544 Thermal Analysis of the Decomposition Mechanism of Platinum and Palladium Tetrammine Faujasite X BY DIETER EXNER, NILS JAEGER, KARIN MOLLER AND GUNTER SCHULZ-EKLOFF* Applied Catalysis Research Group, Department of Chemistry, University of Bremen, D-2800 Bremen 33, Federal Republic of Germany Received 4th March, 1982 The decomposition of platinum and palladium tetrammine faujasite X has been studied by temperature- programmed desorption (t .p.d.) spectroscopy, differential thermal analysis and derivative thermogravimetry . Well resolved t.p.d. spectra allowed the deduction of a detailed mechanism with respect to autoreduction, ammonia evolution and the reaction of ammonia with acid sites. Characteristic differences observed between the palladium and platinum samples are related to the lower stability of the palladium tetrammine complexes within the faujasite matrix.Noblz-metal-loaded faujasite catalysts find wide applications in petrochemical processes.’? Since the activity depends on the metal dispersion, the factors influencing the dispersion process have been investigated in several studies of noble-metal-loaded fa~jasites.~-ll In a recent study Reagan et a1.12 discussed the decomposition of platinum and palladium ammine complexes exchanged into zeolite Y, and the catalytic properties of the resulting reduced platinum and palladium zeolites. The present study applies temperat ure-programmed desorp tion (t . p. d .) spectroscopy, differential thermal analysis and derivative thermogravimetry, and focusses on the evaluation of the decomposition mechanism in a vacuum, or in argon or oxygen.The use of zeolite X was of interest because of previous observations that metal agglomerates far exceeding supercage dimensions can be grown and stabilised within the matrix under special experimental conditions. EXPERIMENTAL SAMPLES Zeolite NaX (Na~6~(A~02)~6(slo2)l~6} . nH20) was prepared by hydrothermal cry~tallisation.~~~ l4 The composition (Si/Al ratio) was determined by chemical analysis and X- ray fluorescence spectroscopy. The preparation of iron-free samples could be checked by flame atomic absorption spectroscopy and by electron spin resonance spectroscopy. Iron is claimed to influence the platinum dispersion in faujasites.15*16 Ion exchange was carried out with solutions of 0.01-0.05 mol dm-3 Pt (NH,),Cl, and Pd(NHJ4C12 at room temperature.The loaded zeolites were washed until chloride-free. The crystallinity of the PtNaX and PdNaX zeolites was largely maintained, as has been confirmed by X-ray diffraction, electron microscopy and a dynamic method” involving the nitrogen physisorption capacity being normalised to the standard NaX. The samples PtX 13, PtX 25, PtX 42, PtX 52, PdX 7, PdX 13, PdX 25, PdX 43, where the numbers denote the degree of exchange, have been used in the following investigations. 35373538 DECOMPOSITION OF FAUJASITE x METHODS TEMPERATURE-PROGRAMMED DESORPTION The decomposition of platinum and palladium ammine complexes in faujasite X was studied in a vacuum (pumping time constant < min; residual gas pressure < lo-' mbar) on the one hand, and in the presence of additional media (argon or oxygen, 1 bar) on the other.A linear heating rate of 5OCmin-l was used in all cases. The ion currents of the evolved compounds were recorded by a computer (PDP 11) coupled to a quadrupole mass spectrometer (Q 200, Leybold-Heraeus, Koln). The relevant mass-spectrum region could be recorded in steps of 5 O C . The desorption rates of ammonia and nitrogen are given in the figures below. For studies in additional media (argon or oxygen) a thin layer of the sample (50 mg cm-2) was deposited onto a glass frit in an external reactor, and the dried-gas medium was streamed with a flow rate of 70 cm3 min-' during both the pretreatment (room temperature, 12 h) and the temperature programmes.To record the compounds evolved during the ammine complex decompositiy process, the exit of the reactor was connected to the quadrupole mass spectrometer recipient uia a leak valve. DIFFERENTIAL THERMAL ANALYSIS AND DERIVATIVE THERMOGRAVIMETRY The measurements were carried out with a thermobalance (L 81, Linseis, Selb), using tempered (1 100 "C) alumina (Merck, Darmstadt) as a reference in flowing dried-gas media during the pretreatments (room temperature, 12 h) and the decomposition studies. Flow rates of 70 cm3 min-' and heating rates of 5 O C min-' were used. Sample charges of 40 mg cm-2 in Pt crucibles were applied. Additional studies of the decomposition of platinum and palladium tetrammine chlorides confirmed the results reported by Wendlandt et al.l*, l9 RESULTS DECOMPOSITION IN A VACUUM Fig.1 shows rates of desorption of ammonia and nitrogen evolved in the temperature-programmed decomposition of platinum and palladium ammine com- plexes in faujasite X which have been degassed and dehydrated in a vacuum at room temperature overnight. The decomposition of platinum tetrammine faujasite X starts at ca. 50 "C; it reaches the first significant maximum in the rate of ammonia desorption at ca. 200 "C and the last maximum between 300 and 350 "C. The decomposition of the corresponding palladium compound, measured by ammonia desorption, starts immediately above room temperature and shows its first maximum at ca. 75 O C . The ammonia desorption is largely complete by ca. 300 O C .The following qualitative features can be drawn from the t.p.d. spectra. (1) The pattern of the spectrum depends on the degree of exchange. (2) Ammonia and nitrogen are liberated in several steps. (3) The ammine complexes of palladium are less stable than those of platinum. (4) In the case of palladium complexes ammonia desorption is never accompanied by nitrogen desorption. The following values for the ratio NH,/N, have been estimated for the totally evolved amounts of the two gases: 5 (PtX 13, vacuum), 6 (PtX 25, vacuum), 8 (PtX 42, vacuum), 9 (PtX 52, vacuum) and 6 (PtX 52, Ar). For all palladium samples ratios of ca. 6 were found. Overall H,/N, ratios of ca. 1 could be determined for all samples. The platinum samples PtX 42 and PtX 52 exhibit maxima of hydrogen evolution shifted to higher temperatures in comparison with those of nitrogen evolution, indicating higher adsorption energies for H, than for N, at the reduced platinum.Activation energies for ammonia evolution have been estimated using procedures given by McCarty and Madix20 and Chan et aL21 and were 144 kJ mol-1 (PtX 13, 240 "C; PtX 25,240 "C), 108 kJ mol-l (PtX 42,310 "C), 88 kJ mol-1 (PtX 42,210 "C), 80 kJ mol-1 (PtX 52, Ar, 310 "C) and 52 kJ mol-1 (PdX 43, 100 "C).3539 D. EXNER, N. JAEGER, K. MOLLER AND G. SCHULZ-EKLOFF d (a 1 PtX 13 PtX 25 I I 1 I I I b o 100 200 300 400 500 r 1 I 1 I I b 0 100 200 300 400 500 T/"C 0 x Y .- E +.I C .d A ( a ) I PtX 42 \ PdX 4 3 0 r 1bO 200 I 300 1 400 I 500 I # T/"C FIG. 1 .-Rates of evolution (arbitrary mass-spectral intensities) of ammonia (-) and nitrogen (---) in the temperature-programmed decomposition ( 5 K min-I), of (a) platinum and (6) palladium tetrammine faujasite X in a vacuum.3 540 DECOMPOSITION OF FAUJASITE x The rates of desorption of water are more than two orders of magnitude lower than those of ammonia up to 450 "C.Above 500 "C a marked evolution of water from the partial destruction of the protonated zeolite lattice is observed. The maximum rate of water desorption is shifted to lower temperatures with increasing degree of exchange. DECOMPOSITION IN ARGON Desorption spectra for temperature-programmed decomposition in streaming argon are given in fig. 2. Ammonia is desorbed at higher temperatures than in a >r E c 4- .- Y .- I \ x +- 0 c W c .I Y .- I I I I I I b r 1 I I I I + 0 100 200 300 400 500 0 100 200 300 400 500 FIG.2.-Rates of evolution (arbitrary mass-spectral intensities) of ammonia (-1 and nitrogen (----), d.t.a. curve, and rate of weight loss in the temperature-programmed decomposition ( 5 K min-') of (u) platinum tetrammine faujasite X (PtX 52) and (h) palladium tetrammine faujasite X (PdX 43) in argon. TPC T/"C vacuum, with a maximum rate at ca. 300 O C , corresponding to the results of Reagan et a1.I2 The maximum rate of ammonia desorption from the decomposing palladium complex is accompanied by the maximum rate of nitrogen desorption, unlike the vacuum case. The maximum rate of ammonia desorption from the platinum complex at ca. 300 "C corresponds to an endothermic peak in the differential thermal analysis [fig.2(a)] and to a maximum rate of weight loss in the derivative thermogravimetry [fig. 2(a)]. A broad endothermic peak at ca. 250 "C in the differential thermal analysis has no corresponding maximum rate of weight loss in the derivative thermogravimetry. No marked corresponding effects in differential thermal analysis and derivative thermogravimetry are observed in the decomposition of the palladium complex [fig. 2(b)], because of the broader temperature region of ammonia evolution. Water is only partially removed during the pretreatment at room temperature with the dried medium prior to thermal analysis. This was indicated by rates of desorptionD. EXNER, N. JAEGER, K. MOLLER AND G. SCHULZ-EKLOFF 3541 of water ranging in the same order of magnitude as the desorption rates of ammonia.Large amounts of water could be removed if the pretreatment temperature was raised to 150 O C . However, the spectral patterns were not changed by this thorough dehydration, indicating that water has no additional effect in the presence of a medium. However, the d.t.a. and d.t.g. spectra suffer from the fact that the ammonia desorption is overlapped by the water desorption. They are nevertheless helpful for supporting conclusions which will be mainly drawn from the t.p.d. spectra. T I \ I \ ' \ ! \ r I 1 I I I b 0 100 200 300 400 500 FIG. 3.-Rates of evolution (arbitrary mass-spectral intensities) of ammonia (-) and nitrc,:en (---), d.t.a. curve, and rate of weight loss in the temperature-programmed decomposition ( 5 K min - I ) of platinum tetrammine faujasite X (PtX 52) in oxygen.Tf"C DECOMPOSITION IN OXYGEN The use of oxygen instead of argon as a medium in the decomposition of the platinum ammine complex strongly increases the N,/NH, ratio evolved at ca. 300 "C with a maximum rate (fig. 3). A simultaneous, strong desorption of water is observed. Derivative thermogravimetry gives a corresponding maximum rate of weight loss (fig. 3), and differential thermal analysis exhibits a strong exothermic peak at 300 O C (fig. 3). DISCUSSION DECOMPOSITION IN A VACUUM The resolution of the desorption spectra obtained for the temperature-programmed decomposition of dehydrated platinum and palladium tetrammine faujasite X samples in a vacuum can be interpreted by the following reactions.3 542 DECOMPOSITION OF FAUJASITE x Pt SAMPLES Below 2OOOC the evolution of ammonia is due to the decomposition of the tetrammine complex [Pt (NH3),l2+ -+ [Pt(NH,),]2i + (4 - x)NH,.(1) Above 200 O C further decomposition of the platinum ammine complex includes the autoreduction of Pt2+ ions [Pt (NH3),]2i + Pto - (NH) + 2Hi + (X - l)NH, 2PtoNH + 2Pt0 + N, + H,. (2 a) (2 b) Reactions (2 a) and (2 b) occur simultaneously. reduction process (2 a). They react with additionally liberated ammonia At this point Brsnsted-acid sites ZOH (Z:Zeolite) are formed during the auto- Z-OH + NH, + Z-O-NH;. (3) Since overall ratios H,/N, z 1 could be estimated, and ratios NHJN, z 6 were found, the proposed autoreduction mechanism [reaction (2 a)] can be supported, in the course of which a complete reduction of the metal ions should have taken place.Ratios NHJN, > 6 are indicative of incomplete reduction. This is observed for the samples PtX 42 and PtX 52, which contain more than 4 platinum tetrammine complexes per supercage. It might be postulated that up to 4 platinum tetrammine complex ions can be located in the tetrahedral supercages by forming partial bonds with the zeolite lattice and which can be decomposed more easily under autoreduction. In these cases ammonia evolution should always be accompanied by nitrogen evolution, as is observed for PtX 13 and PtX 25. If the supercage is loaded with more than 4 platinum tetrammine complex ions, then some of the complex ions might be able to liberate ammonia ligands without simultaneous autoreduction, leading to an incomplete overall reduction and to ammonia evolution not accompanied by the simultaneous evolution of nitrogen (PtX 42 and PtX 52).The reason for the easier autoreduction of those platinum complexes which are partially coordinated to the zeolite lattice might be found in the relatively low bond symmetry of this coordination, which will be more favourable to the splitting of N-H bonds which is necessary in the autoreduction process. In the temperature range between 300 and 400 O C the liberation of ammonia ions from the acid sites can be observed: Z-O-NH,+ -+ ZOH + NH,. (4) Above 450 O C nitrogen and hydrogen are evolved from the decomposition of NH, desorbed from sites of high acidity. Pd SAMPLES Reactions (1) and (2a) can also be postulated for the decomposition of the palladium tetrammine faujasite X.Reaction (2 b) is shifted to temperatures above 400 O C . This hypothesis includes the assumption of palladium imide species being more stable than platinum imide species. Such species are found to be relatively stable surface compounds in the catalytic decomposition of ammonia on 23 and palladium.24 An insufficient amount of hydrogen is evolved to support an alternative assumption of NH, decomposition evolved from acid sites. In the case of palladium no significant desorption of NH, from Brsnsted-acid sites is observed. This could be due to the fact that most of the ammonia has left the zeoliteD. EXNER, N. JAEGER, K. MOLLER AND G. SCHULZ-EKLOFF 3 543 structure at rather low temperatures and is not available to neutralize acid sites formed in the autoreduction step.A comparison between platinum and palladium tetrammine faujasite X on the one hand (fig. l), and platinum and palladium chloridel8V l9 on the other, with respect to the temperature at which ammonia evolution due to the decomposition process begins, leads to the conclusion that the cation complexes are less stable in the faujasite matrix than in the chloride compound. The more pronounced destabilisation of the palladium cation complex could be due to the higher tendency of palladium, as compared with platinum, to form covalent bonds with oxygen, which can be deduced from the relatively weak shift of the Pd(3d5,,)X.p.s. signal in Pd025 as compared with Pd, and the relative strong shift of the Pt(4f,,,)X.p.s.signal in Pt026 as compared with Pt. Thus the expected stronger covalent bond between palladium and the faujasite lattice oxygen should facilitate the liberation of the ammonia ligands. The estimated activation energies for the evolution of ammonia from the decom- posing complexes (52-144 kJ mol-l) are within the range of values found for the desorption of ammonia from p l a t i n ~ m ~ ~ ~ 27 on one hand, and from Brlansted-acid sites28 on the other. DECOMPOSITION I N ARGON Less resolved temperature-programmed desorption spectra are obtained if the decomposition is carried out in argon (fig. 2). In the competitive reaction paths of ammonia evolution and reaction with acid sites, the former will be favoured in a vacuum and the latter should be favoured in the presence of a medium, owing to the slower diffusion of the ammonia molecules out of the zeolite framework and, consequently, the increased probability of their reaction with an acid site.The endothermic peak aroung 250 O C in d.t.a. indicates the liberation of ammonia from the platinum at the same temperature as in a vacuum, while no increased rate of weight loss can be observed in d.t.g. For palladium samples the simultaneous desorption of NH, and N, can now be observed due to the fact that NH, is available at higher temperatures as compared with the vacuum process and can now be catalytically decomposed. DECOMPOSITION I N OXYGEN A strong exothermic peak is found in the differential thermal analysis of the platinum tetrammine decomposition (fig.3) at a temperature corresponding to the maximum rate of ammonia desorption on one hand, and the maximum rate of catalytic ammonia decomposition on the other; the latter can be deduced from the maximum rate of nitrogen evolution. Since the catalytic decomposition of ammonia produces hydrogen, the exothermic peak should be due to the formation of water from this hydrogen and the medium oxygen. This is supported by a simultaneous maximum for the desorption of water observed in the t.p.d. spectrum. The heat produced in the water-formation reaction should be responsible for the enhanced ammonia decom- position, which can be seen from the large N2/NH, ratio. There was no indication in the mass spectra of the formation of nitrogen oxides. CONCLUSIONS The study of the well resolved t.p.d.spectra of the decomposition of platinum and palladium tetrammine complexes within a faujasite X matrix allows the deduction of a detailed mechanism with respect to autoreduction, ammonia evolution and reaction of ammonia with acid sites.3 544 DECOMPOSITION OF FAUJASITE x Characteristic differences observed between the palladium and platinum samples may be related to the lower stability of [Pd(NH3)J2+ complexes within the faujasite matrix. Financial support by the Senator fur Wissenschaft und Kunst der Freien Hansestadt Bremen is acknowledged. A. P. Bolton, Am. Chem. SOC. Monogr., 1976, 714. E. Gallei, Chem. Ing. Tech., 1980, 52, 99. P. Gallezot, in Catalysis by Zeolites, ed. B. Imelik et al. (Elsevier, Amsterdam, 1980), p.227. J. A. Rabo, V. Schomaker and P. E. Pickert, Proc. 3rdInt. Congr. Catal., (North Holland, Amsterdam, 1965), vol. 2, p. 1264. J. A. Rabo, P. E. Pickert and R. L. Mays, Znd. Eng. Chem., 1961, 53, 733. vol. 2, p. 1329. E. Czaran, K-H. Schnabel and M. Selenina, 2. Anorg. Allg. Chem., 1974, 410, 225. J. C. Vedrine, M. Dufaux, C. Naccache and B. Imelik, J. Chem. Soc., Faraday Trans. I , 1978,74,440. C . Naccache, N. Kaufherr, M. Dufaux, J. Bandiera and B. Imelik, Am. Chem. SOC. Symp. Ser., 1977, 40, 538. lo P. Gallezot, A. Alarcon-Diaz, J. A. Dalmon, A. J. Renouprez and B. Imelik, J. Catal., 1975,39, 334. l1 (a) D. Exner, N. Jaeger and G. Schulz-Ekloff, Chem. Zng. Techn., 1980, 52, 734; (b) D. Exner, N. Jaeger, R. Nowak, H. Schriibbers and G. Schulz-Ekloff, J. Catal., 1982, 74, 188; (c) G. Schulz- Ekloff, D. Wright and M. Grunze, Zeolites, 1982, 2, 70. ti R. A. Dalla Betta and M. Boudart, Proc. 5th Int. Congr. Catal. (North Holland, Amsterdam 1973), l2 W. J. Reagan, A. W. Chester and G. T. Kerr, J. Catal., 1981, 69, 89. l3 D. W. Breck and E. M. Flanigan, Molecular Sieves (Society for Chemical Industry, London, 1968), l4 H. Kacirek and H. Lechert, J. Phys. Chem., 1975, 79, 1589. l5 D. Geschke, H. Winkler and D. Wendt, Z. Phys. Chem. (Leipzig), 1973, 252, 235. l6 J. B. Uytterhoeven, Acta Phys. Chem., Szeged, 1978, 24, 53. l7 S. Briese-Gulban, H. Kompa, H. Schrubbers and G. Schulz-Ekloff, React. Kinet. Catal. Lett., in press. W. W. Wendlandt and J. P. Smith, The Thermal Properties of Transition-metal Ammine Complexes (Elsevier, Amsterdam, 1967), p. 179. W. W. Wendlandt and L. A. Funes, J. Inorg. Nucl. Chem., 1964, 26, 1879. p. 47. 2o J. G. McCarty and R. J. Madix, Surf. Sci., 1976, 54, 121. 21 C. N. Chan, R. Aris and W. H. Weinberg, Appl. Surf. Sci., 1978, 1, 360. 22 C. E. Melton and P. H. Emmett, J. Phys. Chem., 1964, 68, 3318. 23 D. G. Loffler and L. D. Schmidt, J. Catal., 1976, 41, 440. 24 K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Surf. Sci., 1976, 59, 302. 25 K. S. Kim, A. F. Gossmann and N. Winograd, Anal. Chem., 1974, 46, 197. 26 K. S. Kim, N. Winograd and R. E. Davis, J. Am. Chem. Soc., 1971,93, 6296. 27 J. L. Gland, Surf. Sci., 1978, 71, 327. N. Y. Topsse, K. Peddersen and G. Derouane, J. Catal., 1981, 70, 41. (PAPER 2/384)

ISSN:0300-9599    

DOI:10.1039/F19827803537

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1 , 1982, 78, 3545-3560 Electron Spectroscopic Studies of Galena and its Oxidation by Microwave-generated Oxygen Species and by Air BY STEPHEN EVANS* AND ELIZABETH RAFTERY Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, Dyfed SY23 1NE Received 5th March, 1982 The reaction of (100) surfaces of PbS crystals (natural galena) with reactive oxygen species generated by microwave discharge in NO has been compared with aerial oxidation in studies using angle-resolved X-ray and ultraviolet photoelectron spectroscopies. Reaction with the discharge products was much faster, facilitating the study of this process in a controlled environment. The final products of the two processes were similar. Initial attack by atomic oxygen at Pb sites led to the formation of oxide species with the liberation of free sulphur, most of which was lost from the surface.Some, however, remained trapped under an oxidized layer. Sulphate species were also formed, probably by direct attack on PbS. Surfaces of good crystallinity (assessed via their photoelectron diffraction patterns) and chemical purity could be regenerated from oxidized surfaces by 800 eV argon-ion bombardment followed by annealing at ca. 350 O C . Core-level binding energies (&) for bulk PbS, PbSO,, PbO, PbCO,, PbS,O, and 5Pb(OH), * Pb(NO,), were obtained via gold decoration and confirmed by reference to adventitious carbon. The Eb of the surface sulphate and oxide differed from those of the individual bulk lead salts, and they were inferred to coexist in an intimate mixture which probably also contained hydroxide. After the oxidized surfaces were heated to ca.350 O C only the sulphate and some of the oxide remained. The uptake of oxygen at surfaces of lead@) sulphide, a compound which occurs naturally in large single crystals as the mineral galena, has long been of interest. Galena is the principal ore of lead, and processes used in its extraction involve complex oxidation processes at PbS crystal surfaces.l This has led to a considerable body of research [briefly reviewed in ref. (I)] directed towards identification of the chemical species present in oxidized galena surfaces, from which, however, a general consensus has not become apparent. PbSO,, PbSO,, PbS,O,, PbO, elemental sulphur, PbS,O, - PbO, PbSO, - PbO and PbS,O, have all been reported as oxidation products under differing conditions [see ref.(l)], the first five resulting from aerial oxidation although no one group has reported more than three of these species. Two reports of thiosulphate formation (detected by infrared spectroscopy)2v were not supported by more recent X.P.S. data,' from which it was concluded that PbS,O, could not comprise > 5% of the total products. On the other hand, fundamental studies of PbS surfaces themselves, undertaken for their intrinsic interest and because of their technological importance in infrared emission and detection, have yielded a comprehensive picture of their electronic s t r u c t ~ r e . ~ - ~ In addition, these studies have shown that sulphur can desorb from the surface in vacuu, and subsequently be replaced reversibly by oxygen,, and that the adsorption of oxygen at room temperature is molecular, rather than dissociative.However, no discrete oxidized phase could be detected, even after 10l2 Lf expos~re.~ 1 L (Langmuir) = Torr s; 1 Torr = 133.3 Pa. 35453546 OXIDATION OF PbS STUDIED BY ELECTRON SPECTROSCOPY There have thus been no reports of the oxidation of galena through to ‘bulk’ products in an ultra-high vacuum system (where oxidation conditions can be more closely controlled than in external oxidation) : the reaction of pure dry ground-state molecular oxygen with PbS is negligibly slow. To bypass this problem, while retaining control of the oxidation environment, we have adopted a more active oxidant, microwave-excited nitric oxide, which contains excited molecular (l A) and atomic oxygen species.* Using this oxidant we have been able to form a variety of chemical species on galena surfaces, including several of those reported for aerial oxidation. Previous studies have used PbS surfaces prepared either by epitaxial deposition5 or by the cleavage in vacuum of natural galena.4q6 The latter is probably the better method, but it is difficult to prepare surfaces by this means which have sufficient area (ca.1 cm2) for X.P.S. studies. We have therefore investigated the effects of ion bombardment and annealing on air-cleaved crystals to ascertain whether chemically and structurally acceptable (100) surfaces can thereby be regenerated in vacuo as required.The effect of prior ion bombardment alone on the oxidation process has also been investigated. The techniques employed include angle-resolved X-ray and ultraviolet photoelectron spectroscopies (X.P.S., u.P.s.) and X-ray photoelectron diffraction (X.p.d.). EXPERIMENTAL Measurements were made using an AEI/Kratos ES200A electron spectrometer with Mg Kct and He I/He I1 photon sources, a PHI 04-131 ion gun, a rotatable probe and a preparation chamber (base pressure, low 1 0-lo Torr region), in which exposure to microwave-excited NO (2450 MHz; ca 100 W; pressure near sample ca 5 x Torr) was carried out as previously described.s During the course of this work (cu. 3 years) three excitation arrangements, of differing activity, have been used: most of the data were collected using the least efficient source (denoted 11) of ‘active oxygen’.Sources I and I1 were both nominally identical to the previous arrangement,8 but differed in their accuracy of alignment. The most recent source (111) had a shorter distance between plasma and sample, and was much more active than the others. Some results from source I1 and all the data from source I11 were collected digitally using a microcomputer-based data ~ y s t e m . ~ All the electron binding energies (Eb) reported here conform with the standards recom- mended by Bird and Swift.lo However, as a result of incidental instrument repairs and recalibrations, the kinetic energy (Ek) scale offset (the apparent spectrometer work function) varied considerably during the course of the study.Absolute Ek values from spectra relating to oxygen sources I, I1 and I11 are thus not directly equivalent. Six galena crystals were studied, each cleaved in air to ca. 12 x 7 x 2 mm from specimens supplied by Hilary Corke Minerals (from Ladywash mine, Eyam, Derbyshire; cleaves A-D) and by Gregory Bottley and Co. (cleaves E-F). Cleave E failed to yield a flat cleavage and was mechanically polished before examination. However, this very flat surface, ideal for angle- resolved X.P.S. work, cannot have been accurately (100) over its whole area. Heating a galena crystal attached directly to a copper probe tip resulted in a vigorous reaction, with rapid destruction of the crystal, migration of copper to the sample surface, dissolution of the probe tip and the formation of globules of metallic lead at the interface.A gold-foil separator was therefore inserted between tip and crystal in subsequent experiments. All samples initially suffered from carbonaceous contamination. Oxidation of one as-inserted sample was achieved with source I (cleave A; Pb 4flC 1s height ratio 36: 1). However, the presence of contamination often inhibited attack on the underlying sulphide when using source I1 because of its lower oxidizing activity. The most active oxidation source (111) was able rapidly to remove this contamination [cf. ref. (1 l)]. However, most specimens were oxidized only after ion bombardment (700 eV, 2-5 PA) and usually after subsequently annealing in uucuo for 0.5-2 h at the highest available temperature. This was estimated by infrared pyrometry as cu.350-400 OC at the sample surface. Cleave DS. EVANS AND E. RAFTERY 3 547 was first oxidized after annealingwithout prior bombardment. The initial Pb 4f: C is peak-height ratios for these surfaces lay in the range 189-390. Ion-bombarded surfaces and surfaces annealed following ion bombardment were examined before oxidation by both U.P.S. and X.P.S. (Pb 4f, S 23, Ar 2p, C Is, 0 Is), and the angular dependences of the Pb 4fand S 2s peaks were monitored to provide, uia X.p.d. effects, evidence relating to the extent of ion-induced disorder. The angular dependences of the Pb 4f signals were re-examined after prolonged oxidation. Oxidations were conducted stepwise, recording at least Pb 4f, 0 1s and S 2s X.P.S. after each oxidation period.Valence u.P.s., C 1s X.P.S., and/or S 2p X.P.S. were sometimes also recorded. However, few S 2p spectra were collected, because they coincided with energy-loss structure from the Pb 4f peaks (making background subtraction difficult) and their doublet character led to undue complexity in curve resolution (see fig. 3). After extensive oxidation some surfaces were heated and then re-examined by X.P.S. Six bulk compounds of lead were studied to provide Eb for comparison with those of the oxidized surfaces. Commercial samples of PbSO, and PbO were used, while PbSO,, PbS,O, and PbCO, were prepared from AnalaR lead(I1) acetate and the appropriate AnalaR sodium salt in stoichiometric quantities. As the existence of pure Pb(OH), is questionable,', a preparati~n'~ yielding SPb(OH), * Pb(NO,), (a suitable model for surface hydroxide) was adopted. The compounds were dried in a vacuum desiccator, and abraded surfaces of pressed pellet samples14 were examined by X.P.S.The PbO initially exhibited a pronounced doublet 0 1s signal : the high Eb component (OH) was removed by heating in UCLCUO to ca. 400 OC. The Pb 4f, S 2s and 2p, 0 Is, C 1s and N 1s peak areas were obtained to confirm the stoichiometries of the surfaces.14 Each sample was gold-decorated in the preparation chamber and the Eb calculated using Au 4fas reference.15 For the most important model compounds (PbSO, and PbO) and for PbS itself gold was evaporated stepwise to ensure that the reported Eb values were not coverage- dependent, while for the other compounds Au 4f: Pb 4fratios of 1-3 were achieved in a single evaporation.The peak profiles were often distorted, suggesting that decomposition might have accompanied the deposition of gold, although differential sample charging was thought more likely to have been the cause. Eb values for comparison were therefore also calculated from the original spectra using adventitious C 1s as a reference. Gold evaporation was continued on PbS until a visible layer had been deposited. The Au 4f peak intensity was then compared with the average Pb4f intensity before evaporation to estimateI6 the inelastic mean free path (i.m.f.p.) for ca. 11 13 eV electrons in PbS. RESULTS AND DISCUSSION GALENA SURFACES (I) CLEAVED I N AIR On cleave A, inserted into the vacuum within 15 min of cleavage ( i e .after the equivalent of > loll L oxygen exposure), the symmetrical Pb 4f and S 2s peaks had full widths at half maximum (f.w.h.m.) of 1.1 and 2.1 eV, respectively. The 0 1s peak area was < 0.3% that of Pb 4J reflecting well under a monolayer coverage. The absence of any evidence for oxidation confirms previous (11) &+-BOMBARDED SURFACES The Pb 4fX.p.s. peaks were skewed to higher Ek (f.w.h.m. now 1.35 eV), indicating partial reduction to elemental Pb: the broader S 2s peaks were unaffected. When the electron take-off angle was increased from 10 to 6 7 O , the high-E, Pb 4f component was roughly doubled in relative intensity, suggesting that the concentration of elemental Pb increased towards the surface. The angular dependence of the Pb 4fand S 2s peak intensities (fig. 1) showed marked diffraction structure, indicating that the crystal largely retained its structural integrity.3548 OXIDATION OF PbS STUDIED B Y ELECTRON SPECTROSCOPY annealed n 1.0 5 2 1.0 5 U - a 11 N .d - 0.5 - 0 30 60 * ''6' " ' ' ' ' L " 0 I" ion-bombarde d - I .. . l . I . I . . 0 30 60 0 I" FIG. 1 .-Normalized X.P.S. peak intensities from PbS (100) as a function of electron take-off angle, 8, for Pb 4f (0) and S 2s (a) signals from an ion-bombarded surface and a surface annealed after ion bombardment. All take-off angles are given with respect to the normal to the surface. However, recent work on GaAs" suggests that increasing the ion-beam power beyond a critical region would have resulted in more serious disorder. The angle- integrated Pb 4ffS 2s area ratios confirmed that the proportion of lead within the mean sampling depth was much higher than in stoichiometric PbS (see table 1 below).Some argon was retained by the crystal (Ar 2p w 0.35% of Pb 4farea). The Ar 2p E b (relative to the Fermi level) was 242.1 eV, cf. 248.6 eV in the gas phase.18 Approximating the difference between the reference levels in the two measurements as the work function, 3.7 eV,19 yielded the extra-atomic relaxation energy for the implanted Ar as 2.8 eV, close to those for monatomic Ar implanted in other solids.20* 21 No evidence for the formation of Ar clusters22 was found. The He I U.P.S. [fig. 2(a)] showed less fine structure than that of epitaxially grown (100) PbS surface^,^ suggesting that the extreme surface was appreciably disordered. No contribution from the free Pb metal to the He I valence band could be identified, because of the relatively featureless nature of the Pb valence band.ll However, the He II/? Pb 5d signals did show low E b shoulders at ca.0.9 eV separation [fig. 2(a)], confirming the presence of elemental Pb near the surface. Nevertheless, U.P.S. (from a smaller sampling depth) did not reflect a higher concentration of elemental Pb than the X.P.S. data. The elemental Pb was thus not localized at the vacuum interface, despite the increasing concentration of Pb in the near-surface region revealed by the FIG. 2.-He I and He I1 spectra from PbS. (a) Cleave F after ion bombardment (45 min, 800 eV, ca. 2 PA), showing the Pb 5d (metal) contribution at ca.0.9 eV to lower binding energy of Pb 5d (PbS), an identical shift to that seen on Pb 4f[136.9 eV1* cf: 137.8 eV (table l)]. (b) The same surface after annealing. Note the Pb 5d shift to lower binding energy, and the marked angular dependence of the He I spectrum. (c) Polished surface (E) after ion bombardment, annealing and oxidation by source I11 for 15 min. The He I spectra were 5-point quadratically smoothed 15 times, and the He I1 spectra in (a) and (6) 50 times [see ref. (23)]. The small residual undulations in these He I1 spectra are not significant. The He I1 spectrum in (c) was smoothed 200 times, and raw data are also shown for comparison. The Pb (metal) shoulders in (a) were still detectable after 200 smoothing cycles.S. EVANS A N D E. RAFTERY 3 549 .0.3 eV - . . 1 - . Pb 5d(Ht . ?. He I1 \* - *. . . ' . - . . . . . . . . *..J . - - . ". FIG. 2.-For legend see facing page. F A R 13550 OXIDATION OF PbS STUDIED BY ELECTRON SPECTROSCOPY X.P.S. angular dependence. The presence of excess Pb distributed below the surface would be expected to make the crystal strongly n-type throughout the X.P.S. sampling depth:4 this expectation was confirmed by the shift of 0.2kO.I eV to lower Ek found on ion bombardment for the Pb 45 S 2s and Pb Sd(He IID) peaks [Fig. 2(a) and (b); table 1 below]. The Au 4f-Pb 4f energy separation was smaller in the first stages of the stepwise gold decoration. This may have resulted from alloy formation with the free Pb released by the ion bombardment, or from changes in the Au 4f Eb with crystallite size.15 An appreciable Pb (metal) peak (ca.14% of the Au 4f area) remained even after deposition of a visible layer, due apparently to surface segregation of the free Pb. A correction for this was made when estimating the i.m.f.p. for I 1 13 eV electrons in PbS. A value of 21 A was obtained, less than the formula of Seah and D e n ~ h ~ ~ for compounds suggested, but close to that predicted for an elemental solid. (111) SURFACES ANNEALED AT 350-400 O C AFTER ION-BOMBARDMENT No metallic Pb was detected either by Pb 4fX.p.s. or He I1 u.P.s., which showed symmetrical Pb 5dpeaks of f.w.h.m. 0.8 eV, close to the value reported5 for epitaxially grown material. The Ar 2p peak had been eliminated, and the Pb 4fand S 2s X.P.S. peak widths and Eb values were the same as for cleaved PbS.The angle-integrated Pb 4f/S 2s intensity ratio (cf. table 1) was much lower than for the ion-bombarded surfaces, confirming that the excess Pb had been eliminated without serious loss of sulphur. The X.p.d. patterns (fig. I) were slightly enhanced relative to those from ion- bombarded surfaces, reflecting improved order. The patterns were also slightly superior to those from an air-oxidized cleaved surface (F). The Pb and S X.p.d. patterns were similar, but not identical: although the Pb and S sites have the same geometry and coordination they differ in the identity of the neighbouring atoms. As expected25 the patterns resembled closely those from (100) surfaces of the isostructural salts NaCl and KBr.26 The He I U.P.S. valence band [Fig.2(b)] resembled that from epitaxially grown PbS.5 Marked angular variations in the relative intensities of its several components were found, as expected for a well ordered single crystal. Ion bombardment followed by annealing thus yielded (100) PbS surfaces generally comparable with surfaces prepared by cleavage or epitaxial growth. BULK COMPOUNDS Stoichiometries derived from the X.P.S. peak areas are shown in table 1. All are in satisfactory agreement with the expected formulae, confirming the chemical integrity of the sample surfaces. The separations of the Au 4fand Pb 4f peaks were smaller for low Au coverages on both PbS (see above) and PbSO, although not on PbO. However, they became constant over the range of coverage preferred for energy calibration, 0.3 < (Au 4f/Pb 4f) < 3.Au 4f-derived E b values from this coverage range are compared in table 1 with E b values obtained using the adventitious C 1s peak as reference. The two methods gave results within 0.4eV for all but one compound (the hydroxide, with a discrepancy of 0.7eV). These E b values could therefore be applied within such error limits in comparisons with surface species. OXIDATION OF GALENA SURFACES (I) RESULTS The major features of the oxidation process were similar with all active oxygen sources and all methods of sample preparation. Chemical compositions estimated from the spectra are given in table 2.S . EVANS AND E. RAFTERY 3551 TABLE 1 .-STOICHIOMETRIES AND BINDING ENERGIES FOR LEAD COMPOUNDS FROM X.P.S. core-electron binding energiesa stoichiometry Au reference from peak C reference ‘fitted’ compound areas (k 0 1s S 2s Pb 4f,7/2 Pb 4-7,~ Pb 4f712 PbSC PbS(Ar+) PbSO, PbS203 PbSO, PbO 5Pb(OH), Pb(NO3), PbCO, Pbl.O0SLOld - 225.2 Pb1.22S1.ood - 225.3 Pbo~Q2Sl~o04~l 531.8 232.6 226.1 Pbl~03Sl~o03~l 531.2 231.0 N 1s c 1s Pbl.oCo.Q703.1 531.2 138.8 pb1.08s2.002.9 531’2 23 1.7 Pbl .Oo0.Q2 529.1 - pb5.8016.0N2.3 532’1e 405.9 137.6 137.6 - 137.8 139.5 139.3 138.7 138.4 138.8 139.2 139.1 139.0 - 137.8 138.0 138.3 138.0 138.7 138.3 - - 289.3 138.9 137.7 a Au 4f7,2 = 83.98 eV; C 1s = 284.7 eV.l0 ‘Fitted’ binding energies are the apparent values required to construct the best approximation, judged by eye, to the spectra of oxidized galena surfaces on the assumption that these phases might be present and possibly charged.They have no significance in any other context. Using Pb 4f, 0 Is, C Is, N Is, S 2s and S 2p peaks (as appropriate) except for PbS20, (S by S 2p only), and the method of ref. (14). Heated to ca. 380 O C , with or without prior ion bombardment: stoichiometry given is average of three surfaces, showing k 10% variation. The Au-referenced binding energies were derived from those for the ion-bombarded surfaces by the subtraction of the mean kinetic-energy shifts observed on annealing after bombardment. Peak intensities integrated over all accessible take-off angles (cf. fig. 1). Major peak; minor peak at ca. 529.2 eV. Only cleave A (unannealed) was oxidized with source I. Reaction had almost ceased after 60-120min oxidation, yielding the spectra shown in fig.3(a). No angular- dependence data were taken on this sample. In the earlier stages, the high-E, 0 1s feature was more prominent, as in the spectra reported below. The effect of heat on the final oxidized surface is shown in fig. 3(b). Several crystals were oxidized with source 11. Data from one typical cleaved surface, oxidized after (a) annealing, ( b ) argon-ion bombardment alone and ( c ) bombardment followed by annealing, and obtained in each case at two electron take-off angles, are shown in fig. 4. Reaction became too slow to monitor after ca. 120 min exposure. No major differences were observed in the rates of reaction following different surface preparation and the kinetic data were therefore plotted together (fig. 5 ) . No core-level photoelectron diffraction effects were detected for any of the oxidation products.Two crystals were oxidized with source 111, one cleaved (examined both before and after argon-ion bombardment) and one polished (studied after bombardment). As the reaction was much more rapid using this source, the earlier stages of oxidation could not be studied. Typical X.P.S. and U.P.S. are shown in fig. 6 and 2(c), respectively. Exposure of a crystal to air for several days before insertion into the vacuum system resulted in substantial oxygen uptake, with the development of chemically shifted Pb and S peaks similar to those produced by the excited oxygen sources. The oxidized layer was, however, very thin, and these shifted components were only obvious at high 115-2t b ) 0 Is c Is s 2s Pb &f A AA l .. . . 1 . . . . 1 . . -. 1 . . ' . ' . . L 715 720 725 960 965 970 FIG. 3.- indicate c P b 4f energy loss+ El m U W 4 rn r m 0 c c3 ? S2- kinetic energy/eV I . . . . l . . . . l . . . . l . . . . l . . . . l . . . . l . . . . I . . . . l . v 1080 1085 1090 1095 1100 1105 1110 1115 1120 z v1 cd kinetic energy/eV m -0 Is, C Is, S 2s, S 2p and Pb 4fX.p.s. (0 = 15O) from cleave A: (a) after 126 min oxidation using source I; (b) the same surface after heating to 350-400 O C . Assignments 0 the identities of species only: the correlation lines do not here accurately preserve either the Eb or Ek separations of the bulk compounds. In (b) there appears (uniquely) 2 0 0 cd .e to be an appreciable charging shift for the oxidized layer.2S. EVANS A N D E. RAFTERY 3553 1: PbO PbS kinetic energy/eV PbS,O, ‘ Pb(OH 1; PbCO PbSO, 1 I ......... ..PbCO, ........... ..PbS04 x 1003554 OXIDATION OF PbS STUDIED BY ELECTRON SPECTROSCOPY n c CI ." a 5 ." c ' 0 % a 0 60 120 I 1 0 0 0 I I200 I 1 --T-7--- 0 60 120 time/min time/min FIG. 5.-Plots of peak height against oxidation time (source 11) for the lowest (a) and highest (b) Ek S 2s components [assigned to sulphate and sulphide species, respectively: see section ($1, and the low-& (c) (largely sulphate and hydroxide) and high-E, (d) (oxide) 0 1s components. Bars indicate ordinate scales in counts per second. 0, Annealed after cleavage; 0 , ion-bombarded; 0, ., a, 0, annealed after ion bombardment . electron take-off angles (cleave F; see table 2 and fig.7). The substrate Pb 4fpeaks still gave strong X.p.d. patterns, comparable with those from other surfaces. (11) OUTLINE ASSIGNMENT OF MAJOR X.P.S. COMPONENTS The two new components of the S 2s signal seen in fig. 3(a), with roughly equal intensities at the one take-off angle used, might suggest the presence of thiosulphate, but such an assignment is not compatible with the later data. Similar spectra obtained with source 111 had an angular dependence (fig. 6 ) which demonstrated that the two components had different depth distributions: the species with the higher Eb was nearer the surface than the other. The lower Eb component was less obvious when using source 11, but attempts to fit the spectra of fig. 4 using a Du Pont curve-resolver confirmed its presence at low take-off angles.The S 2s spectra recorded at high take-off angles could, however, be fitted with only two components. Comparison of the spectra with those of the bulk compounds then indicated that higheSt-Eb S 2s component must be assigned to sulphate, and the other new component to elemental sulphur. None of the compounds other than thiosulphate had a S 2s component near the relevant energy, and the peak was close in energy to that expected from data for sulphur itse1f.l. 27* 28 The low-& 0 1s component was similarly assigned to oxide species, the sulphate 0 1s ionizing to higher Eb. However, the bulk Eb values were not an acceptable match to those of the oxidizeds 2s . ...: . .. ... .. . .. ~ 8 . i . .. . 1. c .% . * I . * * . . .. . . . . . . , J . . . ' l . ' " 1 " 15 720 725 . . . I , r , , l , , , l l . , . . l , . l . l r r . . l 1010 1015 1020 1025 1030 1035 .. . . .. . . . . \. , -. . . . . . . :*. . . . . .: . , . . *.: . . * p.::.,: .. ... :;*:-.*::- .. . . . I *;. '. . .:. . . . . . .* . .. .. . . * .: .* .*. :: .,*. . :. * . . ..:. - . :* .a- so:- . .- . z. - . . . . I t .. e =loo *----------..- I " ~ ~ ~ ~ ~ ~ ~ ' ~ ~ 1104 1109 1114 1119 1124 1129 . . . . ' . 9 = 67' FIG. 6.-0 Is, S 2s and Pb 4fX.p.s. recorded at two take-off angles from the polished PbS surface (E), after 3 min oxidation using source 111. Estimated curve resolutions are shown for the S 2s spectra, which still had rather poor signal-to-noise ratios even after data collection times of cu. 60 min.3556 OXIDATION OF PbS STUDIED BY ELECTRON SPECTROSCOPY .. . . . . ' . . '2: ,..#' . . . * -. . . 'I i , , p FS. EVANS A N D E. RAFTERY 3557 surfaces. The correlation lines on fig. 4 were drawn preserving the energy separations between peaks, but adjusting their absolute positions to give the best mean fit (estimated visually) to the spectra. The optimum shifts required varied considerably in both magnitude and direction (see table l), making it clear that charging of the oxidized species was not responsible. Although alignments for most of the compounds studied are shown, it must be stressed that conclusive evidence was available at this point only for elemental sulphur, sulphate and oxide species. (111) QUANTIFICATION OF X.P.S. INTENSITY DATA Accurate quantification was not possible, because the species produced had different depth distributions within a sampling depth which increased between the 0 1s and the higher-Ek S 2s and Pb 4fpeaks.Nevertheless, a useful indication of surface stoichiometry was achieved, using a model established for homogeneous solids14 but omitting the factor EE.5 allowing for the variation of sampling depth with Ek. This is accurate only for infinitely thin overlayers, and we may thereby have underestimated the oxygen content by up to 25%. Attempts were made to rationalize the resultant stoichiometries for a number of oxidized surfaces. The results are shown in table 2. Resolution of the S 2s spectra into three components yielded the PbSO,, elemental sulphur and PbS mole fractions, and estimation of the proportion of the 0 1s signal in the low-E, (oxide) shoulder provided the PbO mole fraction.If any detected Pb remained unassigned, the presence of Pb(OH),, formed from traces of water in the oxidant, was inferred. The formation of PbCO, from carbonaceous contamination cannot, however, be ruled out as an alternative explanation. Excess oxygen was frequently observed, and some may have been associated with the carbonaceous material itself. The discrepancies between the rationalized and observed oxygen contents must arise partly from the inadequacies of the quantification procedure, but resolution of selected 0 1s spectra using the Du Pont instrument confirmed that the high-Eb peak always consisted of at least two components. Unfortunately, the variety of reasonable fits precluded any quantitative use of the resolved spectra.(IV) INTERPRETATION A N D DISCUSSION The good general agreement between the Eb values determined independently by reference to C 1s and Au 4fstandards (table 1) suggested that a chemical explanation be sought for the poor correlation between the Eb values on the oxidized surfaces and those of the bulk compounds [see section (II)]. As the Eb of the end-products of aerial oxidation (fig. 7) were closely similar to those generated by excited oxygen, the formation of metastable oxidation products by excited-oxygen attack seemed unlikely. Experimental error was also discounted, especially as a similar discrepancy had already been reported' between bulk PbSO, and the surface sulphate species produced by aerial oxidation.The disappearance of the X.p.d. patterns on oxidation demonstrated that the oxidized layers were not epitaxial. Indeed, the basic sulphate formed was clearly not even a well characterized polycrystalline compound : the sulphate/oxide ratio (see table 2) varied with oxidation time, take-off angle (i.e. depth) and, especially, from one source to another. The only chemical shift which could be accurately determined (k0.2 eV) for the layer was S 2s (SO:-)-S 2s (PbS), and this too varied, from ca. 7.2 eV (source I) through ca. 6.8 eV (source 11) to ca. 6.6 eV (source 111), loosely paralleling changes in the sulphate/oxide ratio. The oxidized layers evidently possessed neither regular structure nor uniform composition. Their existence as intimate mixtures of species, rather than discrete phases, seems the best explanation of the3558 OXIDATION OF PbS STUDIED B Y ELECTRON SPECTROSCOPY TABLE 2.-sTOICHIOMETRIES OF OXIDIZED PbS SURFACES FROM X.P.S.mol xb (1 00) fig. no. surface treatment cleave A as inserted cleave D argon-ion bom barded cleave F argon-ion bombarded cleave E polished, argon-ion bombarded cleave D argon-ion bombarded, annealed cleave D annealed only cleave E polished, argon-ion bombarded cleave F argon-ion bombarded cleave F 126 rnin ox. source I Above heated 35 min ox. source I1 30 rnin ox. source I1 30 rnin ox. source I1 97 min ox. source I1 127 rnin ox. source I1 3 min ox. source 111 1 min ox. source I11 air for several days 15 15 15 67 10 67 10 67 15 67 15 67 10 67 10 67 67 in surface layer substrate 0 / O a PbSO, S PbO Pb(OH), PbS Oc(%) - 17 65 16 23 23 19 14 13 15 17 25 36 12 11 18 19 14 15 - 7 - 14 4 12 4 14 - 16 - 10 - 6 - 21 31 35 47 62 41 47 56 48 38 57 26 37 68 89 57 57 30 38 19 273 - 30 23 15 15 23 27 29 10 18 23 25 6 33 42 25 15 33 47 27 23 9 12 - - 19 55 25 23 35 96 105 103 98 74 134 153 113 159 69 71 63 61 100 127 128 146 86 a At a takeoff angle of 10-1 5 O 63 % of the signal comes from within a depth of 16 8, (0 1s)-21 8, (Pb 4f) : at 67 O these depths reduce to 6-8 A.From X.P.S. peak area ratios as described in the text. The figures for the substrate reflect the proportion of PbS detected, the sum of all the overlayer species being set to 100%. Percentage of detected oxygen included in formulation: if < 100, the 0 1s peak was larger in area than required for the rationalization given, and vice versa.The large deviations often found emphasize the limited accuracy of the procedures used. Resolution of the 0 1s (oxide) peak was particularly subjective, small changes in position and width permitting large changes in its peak area. Whenever excess oxygen was detected, the size of this component was minimized, to optimize the overall match to the total 0 1s intensity (i.e. maximizing the proportion of hydroxide). It is possible that in many entries oxide has been substantially underestimated, and hydroxide correspondingly overestimated.S. EVANS A N D E. RAFTERY 3559 differences in Eb values between the oxidized surfaces and the individual bulk compounds. Core-electron Eb values depend not only on the charge on an atom but also on the potential due to the surrounding ions, and the magnitude of their electronic relaxation as the electron leaves.The angular-dependence experiments [see section (11) above, fig. 4 and 6 and table 21 all showed (by the disappearance of its S 2s components at high 9) that there was a negligible proportion of elemental sulphur at the vacuum interface, while the 0 1s spectra (fig. 6) revealed a tendency for the proportion of oxide to increase towards that interface. The elemental sulphur was thus trapped at the galena surface by an overlayer of sulphate and oxide. The observation (e.g. fig. 4) of high-9 spectra showing a galena S 2s component but no elemental sulphur was probably a consequence of protection of part of the original surface by carbonaceous contamination [cf.ref. (1 l)]. With oxidation source 111, capable of eroding contamination rapidly, both components were lost at high angles (fig. 6). The large increase in the sulphate/oxide ratio on heating (table 2 and fig. 3) suggested that much of the oxide diffused into the bulk of the crystal at temperatures of 350-400 OC, while the elemental sulphur evaporated. The U.P.S. data confimed that the extreme surface was largely oxide, and that coverage of initially clean galena by oxidation products was complete. The lowest-& part of the He I valence band (largely S 3p) was totally lost early in the oxidation, and the higher-& structure (largely 0 2p), which developed concurrently, resembled that obtained from oxidized Pb.ll Moreover, the Pb 5d Eb value for an oxidized surface [fig.2(c)], close to that of annealed PbS, was much nearer to that expected for PbO (0.2-0.7 eV higher, assuming chemical shifts as in table 1) than to that for PbSO, (1.1 - 1.9 eV higher). The presence of oxide in all the oxidized layers, and its rapid development in the earliest stages of oxidation (fig. 9, suggested that oxide formation was a primary step, attack at Pb releasing elemental sulphur. The rapid reduction in the S/Pb ratio as the reaction proceeded implied that most of this sulphur was lost from the surface. Since elemental sulphur was never detected in the absence of sulphate, the sulphate was probably formed by direct attack on the galena rather than by oxidation of sulphur. The elemental sulphur peaks were much less prominent in the data obtained using the least active source (11), implying that more than one oxygen species was active in the oxidation, and that their concentration ratio varied from source to source.If recombination of 0 atoms were responsible for the reduction in activity in the sources with a long path from discharge to sample, source I1 would have been relatively rich in lA molecular oxygen. It could then be inferred that lA oxygen can oxidize elemental sulphur (with desorption) but not galena. Conversely, the oxide peaks were largest using source I11 (expected to yield the highest concentration of atomic oxygen species), and least obvious following aerial oxidation. These observations again suggested that oxygen atoms initiated disruption of the surface much more readily than did excited molecules.No marked increase in reactivity was observed for the ion-bombarded surfaces, which must have had a much higher concentration of surface defects, indicating that the initial attack of 0 atoms is not defect-dependent. CONCLUSIONS Large-area (100) galena surfaces for surface chemical studies can be prepared by low-energy argon-ion bombardment followed by annealing at 350-400 OC. Such surfaces exhibit good order and do not deviate by more than a few percent from the ideal s t oichiome try. Oxygen atoms attack (100) galena surfaces at Pb and probably also at S sites.3560 OXIDATION OF PbS STUDIED BY ELECTRON SPECTROSCOPY Reaction is not confined to defects. Pb(I1) oxide species form, and while substantial quantities of S are lost from the surface (probably as SO,), some elemental sulphur remains trapped at the interface between the semiconductor and its oxidation products.Sulphate species are also formed, and the sulphate and oxide coexist in an unstructured layer (which may also contain other species, principally hydroxide). Aerial oxidation yields similar products but at greatly reduced rates and in different proportions. Thiosulphate is not a significant oxidation product under any of the conditions studied. When an oxidized surface is heated strongly in uacuo, much of the oxide, the hydroxide and the elemental sulphur are lost (the former by diffusion into the bulk), and only an intimate mixture of lead(rI), sulphate and oxide groups remains.We thank the SERC for support, including an Advanced Fellowship (to S.E.). We also thank D. J. Keast and K. Downing of Gillette Industries Ltd for curve-resolving facilities and assistance, and Prof. J. S. Anderson, F.R.S. and Dr J. M. Adams for useful discussions. A. S. Manocha and R. L. Park, Appl. Surf. Sci., 1977, 1, 129. R. G. Greenler, J. Phys. Chem., 1962, 66, 879. G. W. Poling and J. Leja, J. Phys. Chem., 1963, 67, 2121. T. Grandke and M. Cardona, Surf. Sci., 1980, 92, 385. A. L. Hagstrom and A. Fahlman, Appl. Surf. Sci., 1978, 1, 455. T. Grandke, L. Ley and M. Cardona, Phys. Rev. B, 1978, 18,3847; Solid. State Commun., 1979,32, 353. F. R. McFeely, S. Kowalczyk, L. Ley, R. A. Pollak and D. A. Shirley, Phys. Reu. B, 1973, 7 , 5228. S. Evans, Proc. R. SOC. London, Ser. A, 1978, 360, 427. S. Evans and D. A. Elliott, Surf. Interface Anal., 1982, in press. lo R. J. Bird and P. Swift, J. Electron Spectrosc. Relat. Phenom., 1980, 21, 227. S. Evans and J. M. Thomas, J. Chem. SOC., Faraday Trans. 2, 1975, 71, 313. l2 D. Greninger, V. Kollonitsch, C. H. Kline, L. C. Willemsens and J. F. Cole, Lead Chemicals (International Lead Zinc Research Organization Inc., New York, 1975), p. 61. l3 J. L. Pauley and M. K. Testerman, J. Am. Chem. SOC., 1954, 76, 4220. l4 S. Evans, R. G. Pritchard and J. M. Thomas, J. Electron Spectrosc. Relat. Phenom. 1978, 14, 341. l5 S. Evans, in Handbook of X-ray and U. V. Photoelectron Spectroscopy, ed. D. Briggs (Heyden and Son, l6 S. Evans, R. G. Pritchard and J. M. Thomas J. Phys. C, 1977, 10, 2483. l7 I. L. Singer, J. S. Murday and L. R. Cooper, Surf. Sci., 1981, 108, 7. London, 1978), p. 121. G. Johansson, J. Hedman, A. Berndtsson, M. Klasson and R. Nilsson, J. Electron Spectrosc. Relat. Phenom., 1973, 2, 295. lS J. C. Riviere, Solid State Surface Science, ed. M. Green (Dekker, New York, 1969). 2o P. H. Citrin and D. R. Hamann, Chem. Phys. Lett, 1973, 22, 301. 21 C. D. Wagner, Faraday Discuss. Chem. SOC., 1975, 60, 291. 22 S. Evans, Proc. R. SOC. London, Ser. A, 1980, 370, 107. 23 A. Proctor and P. M. A. Sherwood, Anal. Chem., 1980, 52, 2315. 24 M. P. Seah and W. A. Dench, Surf. Interface And., 1979, 1, 1 . 25 S. M. Goldberg, R. J. Baird, S. Kono, N. F. T. Hall and C. S. Fadley, J. Electron Spectrosc. Relat. 26 S. Evans and E. Raftery, unpublished results. 27 R. K. Clifford, K. L. Purdy and J. D. Miller, AIChE. Symp. Ser., 1974, 71, 138. 2a D. M. Hercules, Anal. Chem., 1970, 42, 20A. Phenom. 1980, 21, 1 . (PAPER 2/392)

ISSN:0300-9599    

DOI:10.1039/F19827803545

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982,78, 3561-3571 Characteristics of Asbestos Minerals Structural Aspects and Infrared Spectra BY MARIE-JOSE LUYS, GILBERT DE ROY, ETIENNE F. VANSANT* Department of Chemistry, University of Antwerp (U.I.A.), Universiteitsplein 1, B-26 10 Wilrijk, Belgium AND FRED ADAMS Received 8th March, 1982 The infrared spectra of U.I.C.C. standard asbestos have been investigated in relation to their mineral structure. A quantitative account is given of the surface hydroxy species. The effect on the spectra of pretreatments in dilute acid and basic solutions have been studied with reference to the carcinogenic activity of asbestos minerals. Asbestos is the name given to a group of naturally occurring silicate minerals in the serpentine and amphibole series, possessing a fibrous habit.They have been identified as biologically active agents capable of fatal lung scarring, asbestosis, pleural and peritonial mesothelioma, lung cancer and gastrointestinal cancer. The activity of the fibres is attributed to both morphological and chemical fact0rs.l The surface characteristics of asbestos minerals may be important in determining their adsorption of environmental carcinogenic species. The use of infrared spectroscopy in this field offers many opportunities for identification2 and structure elucidation3 of these minerals. The internal fibrous structure is reflected by the spectral shape, and the influence of pretreatment conditions can be evaluated therefrom. The spectra also contain information relating to the structural and surface hydroxy groups.In this work the infrared spectra of asbestos samples have been studied and the influence of structural aspects on biological activity is discussed. EXPERIMENTAL Amosite, anthophyllite, crocidolite, and Rhodesian and Canadian chrysotile samples were kindly supplied by the U.I.C.C. Pneumoconiosis Research Unit, Johannesburg. Their prepar- ation has been described at length* and their physical properties are well known.5 The samples were analysed using energy-dispersive X-ray emission spectrometry, according to a procedure published elsewhere. The chemical stability of the asbestos minerals was investigated by treatments at 75 OC in 0.1 mol dm-3 NaOH or 0.1 mol dm-3 HC1 solution. The contact time was 30 min in each case. Thereafter the samples were thoroughly washed and dried at 50 OC.Infrared spectra were obtained using a Beckmann 4240 double-beam grating spectrometer, equipped with a variable reference-beam attenuator. The scan range extended from 4000 to 250 cm-l, using a conventional slit programme with a setting of 3 mm at 3000 cm-l. The scan speed was selected at 150 cm-' min-', as a compromise between accuracy and hydration time. The specimens were obtained using 150 mg of a 0.5 % mixture of asbestos fibre (freshly dried) in KBr, which was pressed (9 ton cm-2)f into 1.3 cm2 dies. t 1 ton = lo3 kg. 356 13562 CHARACTERISTICS OF ASBESTOS MINERALS The positions of the absorption bands were determined by visual inspection of the spectra. The error on these readings was estimated to f 2 cm-l, except for shoulders, where the accuracy was worse.The relative intensities of the hydroxy bands were determined from the absorbances;' the background correction was performed manually. RESULTS AND DISCUSSION The chemical analysis data of the U.I.C.C. standard asbestos samples are collected in table 1. The compositions of these samples are very similar to those determined by other worker~.~~ 8-10 Approximate formula unit compositions were determined from TABLE I.--CHEMICAL COMPOSITION (Wt %) OF ASBESTOS MINERALS sample amosi te anthop hylli te crocidoli te RCa CCb Na A1 Si S c1 K Ca Ti V Cr Mn Fe c o Ni c u Zn As Rb Sr Pb Mg - 1.7 0.5 19.9 0.1 0.03 0.36 0.46 0.04 0.02 0.02 1.71 0.02 0.01 0.05 0.02 0.005 0.002 34.5 - - - 16.6 0.5 28.3 0.3 0.01 0.50 0.30 0.02 0.12 0.18 6.5 0.0 1 0.12 0.02 0.03 0.004 0.002 0.003 - - 3.3 I .o 20.8 0.1 0.01 0.15 1.09 0.04 0.01 0.01 0.1 1 0.02 0.01 34.7 - - 0.001 0.030 0.001 - 27.4 0.4 22.4 0.2 0.05 0.35 0.03 0.29 0.07 3 .O 0.01 0.28 0.02 0.003 0.00 1 0.002 0.008 - - - - 28.1 0.3 21.5 0.1 0.18 0.04 0.24 0.03 0.08 0.10 4.1 0.0 1 0.13 0.05 0.01 0.001 0.002 0.004 - - a Rhodesian chrysotile ; * Canadian chrysotile.these data. The calculations were performed on the basis of O,,(OH), for the amphiboles and 05(OH), for the sepiolites. All contributions < 0.1 atom per formula unit were neglected, and the results are collected in table 2, which also contains the idealized formulae as a basis for comparison. A fair agreement is observed, although the cation content of the sepiolites tends to be overestimated.THE AMPHIBOLE MINERALS The amphiboles belong to the inosilicate group. They occur as double chains of linked silica tetrahedra [fig. l(a)J which form the asbestos axis. These chains are cross-linked with bridging cations, which within the structure alter the interplanar spacings and the angle at which the adjacent units are stacked. The ions appear in four distinct sites [fig. 1 (b)]. The M, and M, sites are coordinated to four oxygen atoms (attached to one silicon) and two hydroxy groups, whereas the M, sites are surrounded by six oxygen atoms.l1? l2 Mossbauer spectroscopic data suggest that these sites haveM-J. LUYS, G . DE ROY, E. F. VANSANT AND F. ADAMS 3563 nearly perfect octahedral symrnetryl3 with mean bond distances M,-0 = 2.12 A, M,-0 = 2.1 I A and M,-0 = 2.1 1 A.14 The individual bond lengths deviate by no more than 0.02, 0.05 and 0.01 A, respectively.According to the Mossbauer parameters13 the M, site has an eight-coordination with lower symmetry. These ions are coordinated to four oxygen atoms (attached to one silicon atom) at a mean distance of 2.33 A, and to four oxygens (attached to two silicon atoms) at 2.7 A.14 TABLE 2.-FORMULA UNIT COMPOSITIONS sample approximate formula unit composition idealized formula unit Si, Al, ... . 0 0 8 OH 0 Mg, Fe, . . . FIG. 1 .-Amphibole structure, showing the silica double chain (a) and the sandwich structure (b). The cations occupying the M, positions determine the way in which the individual sandwiches agglomerate. In the monoclinic amphiboles (amosite and crocidolite) all chains are oriented in parallel and the oxygen atoms of two neighbouring sandwiches are nearly eclipsed.The packing density is determined by the mean M, cation radius, the larger cations yielding the closest packing.15 In this group a very wide range of isomorphous substitution can be achieved. Anthophyllite, however, displays orthorhombic structure. This phenomenon only occurs within a very limited substitution range. In the orthorhombic amphibole all chains lie parallel to the c-axis, but they are staggered in the other directions, yielding shorter bond distances. The densest packing is obtained when the M, cations are ~ma11.l~3564 CHARACTERISTICS OF ASBESTOS MINERALS The infrared spectra contain two regions of specific interest.In the 1300-250 cm-l range the lattice vibrations reflect the structural differences between the individual samples. Fig. 2 contains the lattice vibration spectrum of the amphibole samples, and the vibrational frequences are collected in table 3. The hydroxy stretching region (fig. 3) contains information concerning the cation distribution. I I I I I I I I O 3 1000 700 400 G1crn-l FIG. 2.-Infrared spectra of amphiboles in the lattice vibration region: A, amosite; B, anthophyllite; C, crocidolite. LATTICE VIBRATIONS The lattice vibrations consist of stretching and bending distortions in the lattice structure. In the 1200-900 cm-l region several major absorption peaks occur. The three spectra exhibit an intense and complex band near 1000 cm-l which is attributed to asymmetric stretching vibrations by species of the type X-0-Y (X,Y = Si, Al, Mg, Fe etc.).ls* l7 This absorption is considerably broadened in the anthophyllite spectrum as compared with the monoclinic samples. This observation is consistent with the closer stacking in the orthorhombic structure and the consequently larger number of vibrational contributions to this band.15 The bands at 1140 and 1085 cm-lM-J.LUYS, G . D E ROY, E. F. VANSANT A N D F. ADAMS 3565 are attributed to the symmetric stretching modes of tetrahedral and octahedral XOX species, respectively.l59 l7 The latter argument can be correlated with the chemical composition of the samples. In crocidolite a larger contribution by FelI1 ions is expected relative to amosite; this can be traced from the spectra as a larger line width and a 20 cm-l shift towards higher frequencies.In the three amphibole samples a shoulder is detected at 900 cm-l which may be attributed to Si-0-A1 or A1 0-H vibrations.18 The 800-600 cm-l region contains the stretching vibrations of X-0 species (X = Si, Al, Mg, Fe etc.) It contains several sharp peaks, but a detailed assignment could not be obtained. The considerable difference in this region between the anthophyllite spectrum and the other amphiboles should be related to differences in structure. The pattern in this region originates from five atomic species within an oxygen-hydroxy environment which can reside in various structurally inequivalent sites. TABLE 3 .-VIBRATIONAL FREQUENCIES OF AMPHIBOLES (IN Cm-') sample X-0-Y stretch X-0 stretch amosite 1134 1084 1000 895 776 -- 730 704 660 640 crocidoli te 1150 1105 930 900,880 780 - 730 695 660 640 anthophyllite - 1101 1010" 918,900 775 755 730 694 670 - sample bending amosite 530 500 480 - 427 -I 338 304 250 anthophyllite 535 500 472 450 425 350 330 307 264 crocidolite 545 505 - 445 410 350 315 305 255 a Very broad.Italicised entries indicate a shoulder. The bending vibrations in the region 600-250 cm-l form a complex absorption band with a mineral-dependent shape. In the anthophyllite spectrum a broader range of frequencies is observed which confirms the structural characteristics of the monoclinic mineral. The difference we observe in the amosite and crocidolite spectra can only be attributed to compositional effects.The presence of a considerable amount of Na and FelI1 ions in the crocidolite mineral may be responsible for the different and more complicated pattern of the X-0-Y bending vibrations. The infrared spectra therefore reflect the compositional and structural differences between the amphibole minerals. Their overall shape, however, is characteristic of the mineral type and consists of well defined regions. CATION DISTRIBUTION Another region of interest is located in the hydroxy stretching-vibration range. In general four peaks are detected, with positions depending on the M, and M, occupancies which are bonded to the vibrating hydroxy g r o ~ p . l ~ - ~ ~ A tighter M-OH bond will result in a OH vibration absorbing at a lower frequency. Thus the presence of FelI1 and FeII cations should produce slightly different frequencies ( 5 cm-l apart) which are not resolvable in our spectra.We have therefore determined the overall iron content, irrespective of its oxidation state. The (M, M3M1) occupancies yield the frequencies collected in table 4.19-24 Fig. 3 contains the hydroxy stretching pattern of the amphibole minerals. From the absorbances the relative contributions of the peaks were determined, and the ion3566 CHARACTERISTICS OF ASBESTOS MINERALS TABLE 4.-HYDROXY STRETCHING FREQUENCIES IN AMPHIBOLES code (M, M, M,) populations frequency/cm-l A (MgMgMg) 3669 €3 3655 C 3639 D (FeFeFe) 3619 (Fe Mg Mg) (Mg Fe Mg) (Mg Mg Fe) (Fe Fe Mg) (Fe Mg Fe) (Mg Fe Fe) A B C IV 366" I I 3640 I 3625 3670 3660 I 3625 3640 3620 FIG. 3.-Hydroxy stretching patterns of amosite (A), anthophyllite (B) and crocidolite (C).TABLE 5 .-RELATIVE ABSORBANCES AND CATION DISTRIBUTION intensity cation number M,, cation number M,, sample A B C D Fe Mg Fe Mg amosite 0 6 19 1 1 2.1 0.9 3.8 0 anthophyllite 10 0 0 0 0 3.0 1 .o 2.7 crocidolite 0 0 10 15 2.6 0.4 3.6 0 distributions in the MI and M, positions were calculated according to table 4. The results are collected in table 5. The M,, populations were determined from the formula unit compositions by subtraction. In the amosite and crocidolite samples virtually all the Mg ions are located in the M, and M, positions between the paired chains, and the M, and M, positions are mainly occupied by Fe. This distribution fits the packing conditions for these minerals, and is in agreement with data published el~ewhere.~, In anthophyllite all the iron seems to occur in M, and M, sites, although the packing conditions should favour Mg.The distribution obtained in this work can, however, be accepted because of the low iron content of the sample. Moreover, it has been confirmed by X-ray and Mossbauer methods that Fe has a preference for the M, site in all the amphibole minerals. If the amount of Fe exceeds the number of M, positions (i.e. 2.0 atoms per formula unit)M-I. LUYS, G. D E ROY, E. F. VANSANT A N D F. ADAMS 3567 a symmetry transition occurs and the orthorhombic amphibole is converted to its monoclinic 25 THE SERPENTINE MINERALS The structure of chrysotile is the magnesium analogue of kaolin. Planar-linked silica tetrahedra face an adjoining brucite sheet composed of magnesium ions which are coordinated octahedrally with oxygens and hydroxy groups (fig.4). Two of the three apical oxygens in the silica sheet replace hydroxy groups in the brucite sheet to FIG. 4.-Chrysotile structure. complete the chrysotile structure.26* 27 The distance between these adjacent sheets is ca. 7.3 A, with symmetry repetitions every 14.6 A. The physical shape of chrysotile is determined by the bond lengths in the tetrahedral and octahedral sheets. Their ratio is expressed by the morphological index,2s yielding a value of 76-77 in the two samples we investigated. The calculation was performed assuming all iron to be present in the octahedral layer and an FeI1/Fe1I1 ratio of 1.0. The high index value is indicative of intense intracrystalline torsions, which are released by sheet curvature.In contrast to halloy~ite,~~ chrysotile is curved with the brucite sheet to the outside, forming tubular crystals with an internal free diameter of ca. 76 A. This morphology has been confirmed by electron micrograph^^^ and diffraction 31 The infrared spectra of the chrysotile samples contain a number of significant features in the lattice-vibration region as well as the hydroxy stretching frequencies. Because of the similarity of the spectra, fig. 5 and 6 only show the Rhodesian chrysotile patterns. LATTICE VIBRATIONS The lattice-vibration pattern (fig. 5) contains a number of major peaks. Their frequencies are listed in table 6 and agree with those obtained by Yariv et aL3 The3568 CHARACTERISTICS OF ASBESTOS MINERALS 1075 cm-l absorption can be attributed to symmetric stretching modes of SiO and (Mg, Fe) 0 species.According to polarization data3 these vibrations are identified as out-of-plane stretchings, and they appear at nearly identical frequencies because of the strong coupling between the tetrahedral and octahedral sheets. This coupling also induces a small desymmetrization and a consequent infrared activity of these vibrations. The 1025 cm-l absorption is also attributed to symmetric stretching modes, but these should occur in the curved plane. The 954 cm-l peak results from I I 1 1 - 1 1 I I I 1400 1100 800 500 200 F/cm-' FIG. 5.-Lattice-vibration spectra of Rhodesian chrysotile (A), Rhodesian chrysotile treated in dilute acid (B) and Canadian chrysotile (C).asymmetric stretching modes of both silica and brucite, with a strong coupling between the two. It probably also contains part of the in-plane stretching modes, whose degeneracy could be lifted by the curvature of the layers3 An attempt has been made to assign the vibrational bands in the 600-250 cm-l region. The bands are primarily caused by the more complicated vibrations of the tetrahedral (v, v,) and octahedral (v, v3 v, v5 v6)16 entities. It should be borne in mind that all vibrations influence each other in a very complicated way. The 630 and 600 cm-l vibrations are attributed to the vlv3 stretching modes of the magnesia octahedra occurring in the plane of the curved sheets. The splitting of this absorption is again determined by the lifting of degeneracy of the modes in the linear and curved directions.The 560 cm-l peak is attributed to v, out-of-plane bending vibrations of silica tetrahedra. The v, bendings of the Mg octahedra probably cause the 480 cm-l (out-of-plane) and the 435 and 400 cm-l (in-plane) absorptions. The 380 and 360 cm-l peaks cannot be identified clearly but it is suggested that in-plane v2 Si-0 and v5 Mg-0 bending vibrations might contribute to them. The 285 cm-l peak is probably caused by the v6 Mg-0 bending mode.M-J. LUYS, G. D E ROY, E. F. VANSANT A N D F. ADAMS 3569 TABLE 6.-LATTICE VIBRATIONS OF CHRYSOTILE ASBESTOS sample vibrational frequency/cm-l Rhodesian 1075 1034 964 630 605 565 490 435 392 380 360 285 Canadian 1084 1025 954 635 600 560 480 432 400 384 355 274 HYDROXY GROUPS In the hydroxy stretching region we observe two fundamentally different species.A very broad band was detected near 3440 cm-l and two sharp hydroxy peaks were obtained at 3645 and 3685 cm-l (fig. 6). The very broad 3440 cm-l band must contain mostly hydroxy species, since the H,O bending vibration region (near 1650 cm-l) contains no appreciable absorption. I c v I --r----- 0 3LOO 2800 1700 tL( 5lcrn-l FIG. 6.-Hydroxy patterns of chrysotile samples : (A) Rhodesian chrysotile, (B) Rhodesian chrysotile treated in dilute acid and (C) Canadian chrysotile. The assignment of the three bands has been performed on the basis of the chrysotile structural data. Three crystallographically inequivalent hydroxy species are present in the crystal, as indicated in fig.4. They appear in two planes: (OH), in the plane which is common to the tetrahedral silica and the octahedral brucite sheet, and (OH), and (OH), in the outer plane of the chrysotile tubes.26 The position of the magnesium ions, however, is not mid-way between these two planes, but is shifted to the 0-(OH), plane. Therefore the (OH), band strength is expected to be lower as compared with those of the other species. It may be assigned to the 3440 cm-l vibration, although the width indicates extensive hydrogen bonding. This is a situation comparable to the3570 CHARACTERISTICS OF ASBESTOS MINERALS behaviour of the inner hydroxy groups of silica.,, It is therefore suggested that the (OH), species form hydrogen bonds with their oxygen neighbours.The two absorption bands at 3645 and 3685 cm-l are assigned to the (OH,) and (OH), stretching vibrations. The ratio (OH), : (OH), = 2 is confirmed by the infrared spectrum, where a value of 2.2 is observed. It is, however, remarkable that the (OH), and (OH), surface hydroxy groups are not involved in hydrogen-bonding interactions. The curvature of the sheets can explain the lack of mutual interaction, because of the longer in-plane distances and the unfavourable geometry involved. However, we would expect some interaction between adjacent tubes, or between these species and the water of hydration. Experiment clearly demonstrates that these phenomena did not occur in our samples. CHEMICAL STABILITY OF ASBESTOS MINERALS Spectra were recorded of asbestos samples which had been treated with dilute NaOH solution.In each case the characteristic lattice pattern was identical to that of the original samples. This observation clearly demonstrates the stability of these minerals in dilute basic solutions. Moreover, the hydroxy stretching region was not modified by the pretreatment. The base-treated minerals therefore have identical bulk and surface structures. The spectra of the amphibole minerals treated in dilute HCl solution did not change with respect to the original patterns. The pretreatment conditions used are insufficient to change the amphibole structure. Literature data1 indicate that after very long exposure times a slight degradation can occur. However, chrysotile samples treated in acid media yield spectra with characteristics which differ from those of the original samples.Fig. 5 and 6 contain the spectra of the acid-treated Rhodesian chrysotile sample (the spectrum of Canadian chrysotile is similar). In the lattice vibration region several absorptions are enhanced and new ones appear. The out-of-plane vibrations at 1075, 560 and 285 cm-l increase in intensity, while the in-plane 1025 cm-l stretching vibration intensity is weakened. Moreover, the occurrence of free silica stretching vibrations at 1200 cm-l and out-of-plane SOH and MgOH deformations at 800 cm-l confirm the overall degra- dation of symmetry and the structural breakdown of the chrysolite lattice. This trend is sustained by the increasing amount of water of hydration which is detected at 3410 and 1640 cm-l.We therefore suggest that as a consequence of the acid treatment the chrysolite lattice has been partially degraded probably even into amorphous, hydrated silica and magnesium (hydr)oxide. However, under the conditions of this test a considerable amount of chrysotile remains intact, and the general shape of the spectra is conserved. CONCLUSIONS The biological activity of asbestos minerals is determined by their surface structure and their reactions in uiuo. It has been suggested that the activity of chrysotile is much higher than that of the amphiboles, at least in the initial stages following exposure.lY 33 From the structural characteristics of the minerals it is known that the chrysotile surface acquires a positive net charge on contact with a solution of near-neutral pH.,, This type of charge centre is very effective in causing membrane functions to deteriorate, and its activity is not very susceptible to neutralization by surfactants.Moreover, the outer surface, composed of hydroxy species, offers considerable possibilities for interactions with tissue. This is in accord with our findings that the hydroxide species at the surface of the chrysotile tubes are free under normal conditions, and that the surface is not hydrated.3571 M-J. LUYS, G. D E ROY, E. F. V A N S A N T A N D F . ADAMS The chrysotile structure is decomposed in a low-pH medium, as opposed to the amphibole minerals which are stable in dilute acid, at least for the short interval of time used in our experiments. The breakdown of chrysotile fibres is a process which ultimately might decrease the mineral's interactive possibilities and consequently lower its biological activity.During the breakdown period, however, major alterations may occur in the vicinity of the fibres, including oxidation-reduction reactions of organic compounds contained in cells. The surface of the fibres in all cases could act as a catalyst for the denaturation of proteins and the alteration of macromolecules. Amphibole fibres are therefore thought to be the more active species in chronic diseases. A. M. Langer and M. S. Wolf€, Inorganic and Nutritional Aspects of Cancer (Plenum Press, London, 1977), vol. 91, chap. 3. R. W. Luce, U S . , Geol. Suru., Prof. Pap., 1971, 750B, 199. S. Yariv and L. Heller-Kallai, Clays Clay Miner., 1975, 23, 145.V. Timbrell, J. G. Gilson and I. Webster, Int. J. Cancer, 1968, 3, 406. V. Timbrell, in Pneumoconiosis, Proc. Int. Conf. Johannesburg (Oxford University Press, Cape Town, 1969), p. 28. P. Van Espen, Ph.D. Thesis (University of Antwerp, 1978). N. L. Alpert, W. E. Keiser and H. A. Szymanski, Theory and Practice of Infrared Spectroscopy (Plenum Press, New York, 2nd edn, 1970). J. C. Rabbit, Am. Mineral., 1948. 33, 268. T. F. Bates, Am. Mineral., 1959, 44, 78. vol. 30, p. 87. lo R. E. G. Rendall, Biological Eflects of Mineral Fibres (I.A.R.C. Scientific Publications, Lyon, 1980), l1 E. J. Whittaker, Acta Crystallogr., 1956, 9, 855. l2 B. E. Warren and D. I. Modell, 2. Kiistallogr., 1930, 75, 11. l3 G. M. Bancroft, A. G. Maddock and R. G. Burns, Geochim. Cosmochim. Ada, 1967, 31, 2219. l4 J. Zussman, Acta Crystallogr., 1955, 8, 301. l5 E. J. W. Whittaker, Acta Crystallogr., 1960, 13, 291. l6 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds (Wiley- Interscience, New York, 1978). E. Flanigen, in Catalysis by Zeolites ed. J. A. Rabo (Am, Chem. S o c . Monogr., Washington D.C., 1976), vol. 171. l8 V. Stubican and R. Roy, Am. Mineral., 1961, 46, 32. lo R. G. J. Strens, Chem. Commun., 1966, 15, 519. 2o G. M. Bancroft, A. G. Maddock, R. G. Burns and R. G. J. Strens, Nature (London), 1966, 5065, 21 R. G. Burns and R. G. J. Strens, Science, 1966, 153, 890. 22 R. W. T. Wilkins, Am. Mineral., 1970, 551, 1993. 23 R. G. Bums and C. Greaves, Am. Mineral., 1971, 56, 2010. 24 R. G. Bums and F. J. Prentice, Am. Mineral., 1968, 53, 770. 25 S. Ghose, Acta Crystallogr., 1961, 14, 622. 26 E. J. W. Whittaker, Acta Crystallogr., 1956, 9, 855. 27 F. L. Pundsack, J. Phys. Chem., 1956, 60, 361. 28 T. F. Bates, Am. Mineral., 1959, 44, 73. *' G. De Roy, I. Verhaert and E. F. Vansant, Reel. Trau. Chim. Pays-Bas, 1981, 100, 162. 30 T. F. Bates, L. B. Sand and J. F. Mink, Science, 1950, 111, 512. 31 J. Zussman, G. W. Brindley and J. J. Comer, Am. Mineral., 1957, 42, 133. 32 A. V. Kiselev and V. I. Lygin, Infrared Spectra of Surface Compounds (J. Wiley, New York, 1975). 33 W. G. Light and E. T. Wei, Environ. Res., 1977, 13, 135. 913. (PAPER 2/406)

ISSN:0300-9599    

DOI:10.1039/F19827803561

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982,78, 3573-3586 Hydrogenolysis of Alkanes with Quaternary Carbon Atoms over Pt and Ni Black Catalysts BY HELGA ZIMMER, PAL TETENYI AND ZOLTAN PALL* Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 77, Hungary Received 9th March, 1982 Hydrogenolysis of hydrocarbons with quarternary C atoms (neopentane, neohexane, 2,2,3-trimethyl- butane, 2,2- and 3,3-dimethylpentanes and 2,2,3,3-tetramethylbutane) has been studied over Pt and Ni black catalysts. The reactivities of different types of C-C bond have been determined. The probability of C-C bond rupture where one of the carbon atoms is quaternary is inversely proportional to the bond dissociation energy. On Pt, two essential types of hydrogenolysis can be distinguished.One reaction is responsible for the breaking of internal C-C bonds attached to the quaternary carbon atom and the other for demethylation. With larger molecules, the former reaction is preferred and the surface intermediate should be 1 ,Cdiadsorbed, while that for the latter reaction is 1,3-diadsorbed. Nickel, as previously suggested, causes terminal C-C rupture, although with branched hydrocarbon reactants internal C-C bond rupture is also possible, presumably uia 1,4-adsorption. General features of hydrogenolytic reactions depending on the nature of the metal catalyst have been observed and summarized by various Three formal characteristics, the activity of the metal and the depth and pattern of hydrogenolysis, have been proposed as being sufficient to describe this reaction.* An optimum adsorption strength is needed for maximum catalytic activity; neither too strong nor too weak metal-hydrocarbon interaction favours the formation of the intermediates involved in hydrogen~lysis.~ As far as the depth of hydrogenolysis is concerned, two different sorts of behaviour have been observed:3 single bond rupture over Pt, Pd, Rh and Ir and deep fragmentation (multiple rupture) over other Group VIII metals.Deep fragmentation over Ni has been attributed to demethylation.6 Ponec and coworkers’ defined a factor enabling them to distinguish between terminal C-C rupture and ‘ true’ multiple fragmentation. The demethylating character of nickel has been confirmed by studies with n-butane which, in the presence of an excess of hydrogen, undergoes almost random single bond rupture on platinum.8 Leclercq et aZ.@? lo studied extensively the hydrogenolysis of various hydrocarbons over a Pt-AI,O, catalyst.They defined a ‘reactivity factor’ w to characterize the pattern of hYdrogenolYsis actual rate of rupture statistical rate of rupture ’ cr)= The w factors for all hexane isomers were determined for single hydrogenolysing metals (Pt, Pd, Rh and Ir). Internal rupture was more characteristic of Pt and Ir; thus, taking into account the terminal C-C rupturing character of Ni, the position of preferred rupture was shifted towards internal C-C bonds with metal catalysts in the lower rows of the Periodic Table.ll Montarnal and Martino12 attributed this behaviour to the increasing ‘softness’ of the metal in terms of Pearson’s concept of acidity .35733574 HYDROGENOLYSIS OF ALKANES OVER Pt A N D Ni The pattern of hydrogenolysis depends on the structure of the hydrocarbon rea~tant.~-ll Leclercq et aL9 arrived at the conclusion that the relatively small but consistent variations of hydrogenolysis patterns on Pt as a function of the structure of hydrocarbons can be explained by assuming various possible intermediates for hydrogenolysis, such as 1,2-, 1,3-, 1,4- and I ,5-diadsorbed species. The structure of the molecule has less influence on the activity of hydrogenolysis. Tetknyi13 found that propane underwent hydrogenolysis with a higher rate and a lower energy of activation than ethane over several metals. This effect was attributed to the possibility of 1,2,3-interactions between the propane molecule and the catalyst.The fact that hydrogenolytic properties of neopentane are closer to those of ethane than to those of propane indicates that 1,3-interaction must be less favourable for the rupture of this molecule. Whereas the rate of C-C bond rupture in ethane showed differences as large as 5-6 orders of magnitude over several Group VIII metals, the differences in activities of the same metals for 3-methylpentane hydrogenolysis was much less: ca. 2 orders of magnitude.13 All these observations point to the importance of three carbon atoms interacting with the catalyst surface (most likely via multiple C-M bonds') whereas the rest of the molecule ex5ibits compensating, directing and other effects of secondary importance.Attention has to be paid to the influence of hydrogen on the catalytic reaction. The rate of skeletal rearrangement reactions (cyclization and isomerization) has been found to have a maximum as a function of the hydrogen pressure14 as has also been observed for hydrogenolysis.*? l5 This was attributed to the hydrogen regulating the concentration of surface intermediates of different degrees of dissociation for each reaction.16 Working with fixed hydrogen and hydrocarbon partial pressures has the disadvantage that one cannot be sure on which side of the opkimum the reaction takes place. Therefore comparison of the reaction rates (or even of the activation energies") includes an element of uncertainty. This may be why we find such discrepancies in the literature regarding the order with respect to hydrogen: negative for the hydrogenolysis of ethanel8? l9 and pentane,20 positive for n-heptane,21 isoheptanes,229 23 butane and isopentane20 and a change from positive to negative with maxima for propane, isobutane and neopentane.20 The study of compounds containing quaternary carbon atoms has the advantage that some mechanisms can be excluded because certain types of adsorption (for instance 1,2- and 1,3-diadsorbed species) are sterically hindered or impossible.Leclercq et aL99 lo concluded, on the basis of the reactivity of 2,2,3,3-tetramethylbutane, that in these cases 1,4-diadsorption could become predominant. In the present work we report a study carried out using hydrocarbons with quaternary carbon atoms (neopentane, neohexane, 2,2,3-trimethylbutane, 2,2- and 3,3-dimethylpentanes and 2,2,3,3-tetramethylbutane) in order to gain a deeper understanding of the nature of hydrogenolytic processes.A comparison of Pt and Ni blacks seemed to be reasonable in order to elucidate the question of whether nickel, which selectively cleaves terminal C-C bonds, is also able to cleave internal C-C bonds attached to the quaternary carbon atom. In the case of such hydrocarbons very pronounced structural effects can be expected, whereas with other hexanell. l3 and heptane isomers the above-mentioned compensating effect of the rest of the large molecule (not interacting with the surface) may mask primary structural effects. EXPERIMENTAL The experiments were carried out in a static circulating apparatus described earlier,24 connected to a gas chromatograph with a Squalane capillary column.Reaction mixturesH. ZIMMER, P. TETENYI AND z. PALL 3575 contained 1.23 kPa hydrocarbon and 2-60 kPa hydrogen. Platinum black catalyst was precipitated from H,PtCl, at 275 K with HCHO in the presence of KOH25 and pretreated with hydrogen at 630 K.2s Its surface area was ca. 1.8 m2 g-l (measured by H, adsorption using the B.E.T. method) and its mean crystallite diameter was ca. 30 nm (measured by X-ray diffraction). Nickel black catalyst was prepared from Ni(OH), and reduced with hydrogen in s i t ~ . ~ ’ Its specific surface area was CQ. 4 m2 g-l and crystallite size ca. 15 nm. The catalyst samples (0.10 g Pt black and 0.34 g Ni black) were regenerated before each experiment in air and hydrogen in the case of Pt and in hydrogen in the case of Ni at the reaction temperature, which was 600 K for Pt and 500 K for Ni.Hydrocarbons were Merck, Fluka or EGA products, with a purity > 99.5%. Oxygen was removed from them before use by subsequent cooling and outgassing at liquid-nitrogen temperature. Initial reaction rates were determined from analyses of samples taken after 5 min contact time. Plots of conversion against time justified this approach. 25 20 - I N I m f - 15 a0 I 2 3? \ 10 5 1 FIG. 1.-Initial rate of rupture of the bond between the quaternary carbon atom and its most substituted neighbouring carbon atom as a function of hydrogen pressure. 0.10 g Pt black catalyst, T = 603 K, p(HC) = 1.23 kPa.3576 HYDROGENOLYSIS OF ALKANES OVER Pt A N D Ni RESULTS The initial reaction rates (w,) for hydrogenolysis of the C-C bonds between the quaternary carbon atom and its most substituted neighbouring carbon atom are shown in fig. 1 as a function of the partial hydrogen pressure over platinum black catalyst. The rates were calculated from the rate of appearance of the larger fragment of the molecule in the gas phase.Fig. 2 illustrates the hydrogenolysis rates for the bonds between quaternary and primary C atoms per C-C bond (CI-CIv) calculated in a similar manner. In the case of 2,2,3,3-tetramethylbutane the gas phase contained 2,2,3-trimethylbutane in trace amounts only. Reactivity factors9 are given in table 1, together with the literature data. The gem-disubstituted pentanes have numerous other reaction possibilities : we observed dehydrogenation and isomerization as well as C,- and C,-cyclization.An example of the composition of the products is given in table 2. Fragmentation was the only reaction observed with neopen tane and 2,2,3,3- tetramet hylbutane under similar conditions (conversions 2.72 and 17.7 %, respectively) whereas 2,2,3- trimethylbutane gave 4.93 % fragments and 0.1 % toluene. I I I I I I 1 2 3 4 5 6 FIG. 2.-Initial rate of rupture of the CI-C,, bonds. Pt black catalyst, conditions as in fig. 1. m,)1104 paTABLE ~.-HYDROGENOLYSIS REACTIVITY FACTORS (w) OVER Pt BLACK CATALYST, T = 603 K, p(HC) = 1.23 kPa, p(H2) = 2-60 kPa p(H2)lkPa 2.6 6.0 9.7 15.2 28.5 41.3 a 1 w1 0.08 0.35 0.22 0.3 1 0.91 1.43 0.8 4.61 4.21 3.56 3.51 2.27 0.71 2.2 0.14 0.15 0.57 0.57 0.61 - 0.35 w2 0 3 % p(H2)lkPa 3.3 5.9 14.9 24.0 32.1 38.8 a 0 1 0.09 0.14 0.51 0.57 0.49 0.46 0.25 - - 0.09 0 2 0 3 5.22 5.10 4.13 3.90 3.72 3.67 3.7 - - - - 1% 0 4 0.50 0.47 0.33 0.39 0.82 0.95 1.45 ~~ p(H,)/kPa 6.3 14.5 24.2 31.3 39.5 - 0.16 0.35 0.35 0.30 3 2.09 1.90 2.25 2.70 .- 0.75 0.75 0.40 - p(H2)lkPa 6.4 14.1 31.1 39.1 61.3 a 0 1 0.01 0.02 0.03 0.08 0.20 0.7 0 2 5.95 5.92 5.88 5.72 5.17 2.7 0 3 0.01 0.01 0.01 0.02 0.12 0.6 < 0.01 > 6.94 a 0.45 4.3 F a Ref.(9): T = 573 K, p(HC) = 10 kPa, p(H2) = 90 kPa.3578 HYDROGENOLYSIS OF ALKANES OVER Pt A N D Ni TABLE 2.-PRODUCT COMPOSITION (mOl%) FROM 2,2- AND 3,3-DIMETHYLPENTANES. PT BLACK CATALYST, T = 603 K, p(CH) = 1.23 kPa, p(H,) = 15 kPa, AFTER 5 min CONTACT TIME.initial hydrocarbon product 2,2-DMP 3,3-DMP 2 < c, G XA - 2,2-DMP 3,3-DMP 2-MHx 1,l-DMCP toluene and/or DMCP’ benzene 2 conversion (%) 1.49 2.63 0.02 - 0.08 - 0.69 - 0.70 0.34 - 0.1 1 6.82 2.82 0.39 0.33 0.05 9.25 7.22 - - - TABLE 3.-PRODUCT COMPOSITION (Ill01 %) FROM ISOBUTANE, NEOPENTANE, 2,2,3-TRIMETHYL- BUTANE AND 2,2,3,3-TETRAMETHYLBUTANE. 0.34 g Ni BLACK CATALYST, T = 505 K, p(HC) = 1.23 kPa, p(H,) = 4.3 kPa, AFTER 5 min CONTACT TIME. initial hydrocarbon products A 2 conversiona 1.31 0.180 3.62 0.335 c, c2 c, i-C, i-C, neopentane neohexane 2,3-DMB 2.58 0.700 7.66 0.934 0.535 0.524 0.045 0.520 0.206 - 0.01 8 0.461 0.282 - - 0.017 - - - 1.60 - - - 1 .oo - - - 0.03 1 - - - - a Mol % of reactant consumed. 2,2,3,3-Tetramethylbutane was also reacted over nickel black catalyst.Similar bell-shaped curves (table 3) were observed for the formation of isobutane and propane. Some comparative experiments were also carried out with neopentane, 2,2,3-trimethylbutane and isobutane at constant hydrocarbon and hydrogen pressures. We observed maximum isobutane formation from 2,2,3,3-tetramethylbutane at this partial hydrogen pressure.H. ZIMMER, P. TETENYI AND z. P A A L 3579 1,3 - Ib - 1,s - x --+----t U M M M M + M M SCHEME 1. 1,3 - l,L - -+-+ M M t-t- M M SCHEME 2. DISCUSSION PLATINUM BLACK CATALYST We will first summarize our approach and expectations. Various diadsorbed species leading to C-C bond splitting can be presumed, e.g. terminal adsorption, scheme 1, and internal adsorption, scheme 2. 1,2-diadsorption is neglected owing to the higher activation barrier in ethane hydrogenolysis involving such adsorption.l3 Single or multiple metal-carbon bonds may be present. If hydrogenolysis involved mainly or exclusively a particular type of adsorption, it would be possible to predict the sequence of reactivities of various hydrocarbons, on the basis of geometric and/or energetic considerations: prevailing type of adsorption expected reactivity order internal 1,3- x, >>(/-xA-x > 2 t t t t terminal 1,3- X>>c>><V-->c,P>>\< internal 1,4- +- - > terminal 1,4- terminal 1,4- W > % > % - F ? - terminal 13- ></\-'>c We have found that (1) the bond between the quaternary carbon atom and its most substituted neighbour carbon atom is broken with the highest selectivity in all cases (table l), (2) the order of reactivity for the rupture of this type of bond is (fig.1): 2,2,3,3-TMB > 2,2,3-TMB > 2,2-DMB > 2,2-DMP - 3,3-DMP - neo- pentane, and (3) for the rupture of the CI-CIV bonds, a nearly opposite sequence was3580 HYDROGENOLYSIS OF ALKANES OVER Pt A N D Ni observed (fig. 2): neopentane > 2,2-DMB > 3,3-DMP > 2,2-DMP > 2,2,3-TMB % Thus, the rate sequence for alkylsubstituted butanes supports the assumption of terminal 1,4-adsorption. The production of C,-cyclic products from gem-dimethyl- pentanes (table 2) gives evidence also for the 1,5-interaction of the molecule with the catalyst. The slower demethylation can be explained by terminal 1,3-adsorption.The hydrogenolysis rates measured at the maxima of the curves (fig. l), where the hydrogen order becomes zero, increase as the C-C bond dissociation energy decreases in the different compounds. Such great differences were observed only between ethane and higher alkanes and were explained by the more favoured 1,3- and/or 1,2,3-adsorption of the latter species being responsible for the higher reactivity and lower energy of activation observed with larger molecules.1* 13, 29 However, in hydrocarbons without quaternary carbon atoms at least one of the carbon atoms in the C-C bond to be broken may be attached to the surface and dissociated. The nearly random reactivity factors observed with platinum indicate that small differences like the different electric charges of the individual carbon atoms may be important in determining which carbon atom will be preferentially adsorbed.ll7 307 31 Namely, terminal carbon atoms carry higher partial negative charges than secondary and especially tertiary carbon On fresh catalysts, the initial adsorption of the latter types will be preferred, thus giving an enhanced probability of bond rupture to the internal positions [table 1 and ref.(9) and (1 l)]. According to Balandin’~~~ concept of the energetics of catalytic reactions, their driving force must be hidden in the difference between the bonds to be broken and to be formed, including the bonds formed between the reactant and the catalyst. Assuming that the introductory phase of hydrogenolysis is C-H bond dissociation,’ Balandin’s equation should be for the breaking of a C-H bond over Pt (and forming H-Pt and alkyl-Pt species) 2,2,3,3-TM B.QCH, Pt = Qc-H - &-pt - Qc--Pt- (1) Taking literature values for individual bond energies, with respect to primary, secondary and tertiary carbon atoms, the results are given in table 4, and show that energetics as expressed by eqn (1) must be an important factor in determining the direction of hydrogenolysis. This is true even if we do not consider the H-Pt and C-Pt bonds in eqn (1) to be chemically stable. The same conclusion can be drawn from spectroscopic data on mercury-alkyls where the experimentally determined force constants for the C-Hg bond were found to be 2.52 for ethyl, 2.37 for isopropyl and TABLE 4.-ENERGIES INVOLVED IN THE INTERACTIONS OF VARIOUS CARBON ATOMS WITH Pt CATALYST type of kJ mol-l carbon atom QH-ptb Qc-ptc QcH,P~’ Qc--Ptc QCH,P? primary 410 239 95 76 98 73 tertiary 38 1 239 89 53 86 57 secondary 396 239 92 65 92 65 a Ref.(28); ref. (33); the energy of the C,,-Pt bond has been taken from ref. (34), others were calculated assuming that the following ratio is valid : QR-cr : Qpt-cII : Qpt-cIII - QHWcI: QH--CII: QH-cIIr; as in note (c) but using the force constant ratios of Hg-alkyls from - ref. (35): QPt-CI Q p t - c I I : Q P ~ - c I I I = Q H ~ - c I : QHg-cII QHg-CrII-TABLE 5.-C-C BOND DISSOCIATION ENERGY AND RELATIVE REACTIVITIES OF SOME HYDROCARBONS c-c energya corresponding substituents in scheme 3 /kJ mol-l alkyl groups RZ R; R3 R; alkane relative reactivity of the CZ-C, bond ruptureb not measured 343 ethyl-ethyl H H H H n-butane 335 eth yl-isopropyl CH, H H H 2-methylbutane not measured 306 isopropyl-t-butyl CH, CH, CH, H 2,2,3- trimethyl butane 2.82 283 t-butyl-t-butyl CH, CH, CH, CH, 2,2,3,3-tetramethylbutane 4.06 322 ethyl-t-butyl CH, CH, H H 2,2-dimethylbutane 1 .oo a Ref.(28). Measured at the maxima of the reaction, rate against p(H,) curves, conditions given in fig. 1.3582 HYDROGENOLYSIS OF ALKANES OVER Pt A N D Ni 2.22 for t-butyl substituents (in N ~ m - ’ ) . ~ ~ QcH, Pt values for different types of carbon atoms calculated from Hg-alkyls are in fair agreement with data extrapolated from C-H bond energy values (table 4). Another factor influencing the energetics of bond rupture is the possibility of n-allylic interaction whenever 1,2,3-adsorption is possible.This means a considerable lowering of the energy of activation calculated on the basis of Balandin’s equations.36 The situation is totally different with 1,4-adsorption. Here the C-M bonds are not formed with the participation of the carbon atom of the C-C bond to be broken and, owing to the quaternary character of at least one of the carbon atoms, n-allylic interactions are not possible. Here the energy of the C-C bond to be cleaved will influence the reaction more directly. The situation is shown in scheme 3 and the corresponding data are shown in table 5 . The relative reactivities of substituted SCHEME 3. butanes with quaternary carbon atoms are inversely proportional to the C,-C, bond energy. No measurements are shown for n-butane and methylbutane, where 1,2,3,- adsorption may overcompensate the effect of changing bond energy.As distinct from this Anderson-type reaction involving at least two metal atoms, another possibility would be the formation of metalacyclopentane with participation of one metal atom (scheme 4). Analogous metalacyclobutanes were proposed by O’Donohoe et al.37 Such organometallic complexes are known to form surface n-complexes, as shown in scheme 4.t c c SCHEME 4. The position of the maxima in the hydrogenolysis curves is shifted towards higher partial hydrogen pressure with increasing carbon number. This is in agreement with the results of Gault and coworkerszz who found the same tendency for hydrocarbons containing no quaternary C atom. We agree with Leclercq et al.lS who believe that it is due to the adsorption properties of the hydrocarbons.With increasing carbon number, the ‘coefficient’ for the reactive adsorption of the hydrocarbons increases and the molecule can more easily displace hydrogen on the reactive sites. It would be desirable to check this hypothesis by more direct measurements of hydrocarbon and hydrogen coadsorption. t We thank one of the referees for drawing our attention to this point.H. ZIMMER, P. TETENYI AND z. PALL 3583 The ‘ lazy ’ C-C splitting of the gem-dimethylpentanes indicates that their internal 1,4-type adsorption either is not predominant or does not occur at all. The behaviour of 3,3-dimethylpentane gives us a further information: though the structure of the molecule would permit a terminal 1,4-interaction with the metal surface, it does not play an important role compared with 1,5-type adsorption (cf.table 2). This strong tendency for terminal adsorption can be interpreted either by assuming a dicarbyne-type surface species in hydrogenoly~is,~~ or by simple steric effects; that is, the end methyl groups of a long carbon chain are the most accessible for the catalyst surface atoms. Formation of metalacyclohexanes (analogously to scheme 4) should be sterically preferential. This species should, however, lead to another type of C-C bond splitting analogous to the ‘ring opening’ of cyclopentane~.~ NICKEL BLACK CATALYST Nickel selectively cleaves terminal C-C bonds3? 6~ l3 Due to the strong interaction between nickel and hydrocarbon, hydrogenolysis is confined to the place of primary adsorption and presumably involves 1,2-intera~tion.~~ Such adsorption is impossible with neopentane and 2,2,3,3-tetramethylbutane and these molecules undergo frag- mentation at a rate which is almost an order of magnitude slower (table 3).Tetra- methylbutane also gives isobutane and propane as well as methane. Their appearance (and the lack of detectable amounts of larger demethylated fragments, i.e. 2,2,3- trimethylbutane, neohexane and neopentane) serves as evidence of direct internal C-C bond rupture over nickel. The example of 2,2,3-trimethylbutane shows that the C-C bond rupture commences where 1,2-interactions are possible, giving considerable amounts of neohexane and neopentane. Isobutane and propane formation can also be explained by the same consecutive mechanism (scheme 5).There is also another possibility for the cleavage SCHEME 5. ’ I M M SCHEME 6. of the central bond: direct rupture caused by a 1,4-type interaction with the metal (scheme 6). With tetramethylbutane only scheme 6 may be operative. The comparison cf C, and C , fragment yields from trimethyl- and tetramethyl-butanes indicates that their formation via direct and stepwise fragmentation (schemes 5 and 6, respectively) should occur with comparable rates. To select between possible pathways, we calculated the ratio of fragments of two consecutive demeth ylating steps 116-23584 HYDROGENOLYSIS OF ALKANES OVER Pt A N D Ni 10 20 30 tlmin FIG. 3.-Ratios of concentration of the ( 2 - 1)th fragment to the concentration of the ith fragment from 2,2,3-trimethylbutane as a function of reaction time.Ni black catalyst, conditions as in table 3. x , neopentane/neohexane; 0, propane/isobutane; A, isobutane/neopentane. as a function of the reaction time (fig. 3). In the case of an exclusive consecutive mechanism, we would expect all these values to increase with increasing contact times. Instead we found decreasing isobutane/neopentane ratios (step 3) while the neo- pentane/neohexane (step 2) and propane/isobutane (step 4) conversion ratios increased. This clearly indicates that for isobutane formation there is another path- way. The relatively low isobutane/neopentane ratio means that this other reaction route is not favoured. The propane/isobutane ratio is, at the beginning, near to unity, as expected from scheme 6, and the difference in favour of isobutane indicates the occurrence of scheme 5 .The methane balance also gives evidence for the direct rupture of internal C-C bonds. If all the smaller hydrocarbon products were formed by consecutive demethyl- ation according to scheme 5 , the methane balance should be written as [C,] = [neohexane] + 2[neopentane] + 3[isobutane] + 4Epropane]+ 5[ethane] (2) for 2,2,3-trimethylbutane. These calculated [C,] values are compared with the experimental data in table 6. In the case of isobutane and neopentane, methane formation is always greater than the calculated values, which means that there is also extensive breakdown of ethane (scheme 5). On the contrary, 2,2,3,3-tetramethylbutane gives much less methane than the calculated amount, which indicates direct C,,-CIv bond cleavage (scheme 6).2,2,3-Trimethylbutane represents an intermediate type: the calculated methane yield is greater than the experimental yield at the beginning of the reaction but with increasing contact time extensive demethylation should prevail. The above points indicate that several types of adsorption can occur simultaneously: mainly 1,2-adsorption leading to demethylation and 1,4-adsorption resulting in rupture of the central bond. Demethylation can also occur through 1,3-adsorption,H. ZIMMER, P. TETENYI AND z. PALL 3585 TABLE 6.-METHANE BALANCE FOR SOME HYDROCARBONS AS A FUNCTION OF REACTION TIME. Ni BLACK CATALYST, CONDITIONS GIVEN IN TABLE 3. hydrocarbon calc.measured /min (molx) (molx) time [GI [CJ isobutane neopen tane 5 12 21 31 5 15 25 35 2,2,3- trimethylbutane 5 2,2,3,3-tetramethylbutane 2 17 27 4.5 7 9.5 18 23.5 1.6 6.6 12 20 0.1 1 0.24 0.36 0.41 6.6 25.0 41 .O 0.77 1.94 3.1 4.0 8.5 11.4 2.6 15 32 54 0.70 1.94 2.62 3.2 6.1 37.0 70.0 0.27 0.84 1.5 2.3 5.8 8.7 M M SCHEME 7. M M SCHEME 8. as in the case of neopentane, but this reaction is much slower (scheme 7). This reaction may have given 2,3-dimethylbutane and isopentane from 2,2,3- trime thy1 bu tane (table 3). We cannot exclude an internal 1,3-type interaction of 2,2,3-trimethyl- butane (scheme 8), although the contact of the tertiary carbon atom with the surface is sterically hindered. Regarding the relative inactivity of 1,3-type interactions (see table 3), we think that internal C-C bond rupture uia 1,4-interactions may be more favoured.We thank Dr A. Sarkany for helpful discussions.3586 HYDROGENOLYSIS OF ALKANES OVER Pt AND Ni J. R. Anderson, Adv. Catal., 1973, 23, 1. H. Matsumoto, Y. Saito and Y. Yoneda, J. Catal., 1971, 22, 182. Z. Pail and P. Tetenyi, Nature (London), 1977, 267, 234. Z. Paal and P. TttCnyi, in Catalysis, ed. G. Bond and G. Webb (Specialist Periodical Report, The Chemical Society, London, in press), vol. 5, p. 80. L. Guczi, K. Matusek, A. Sarkany and P. Tetenyi, Bull. SOC. Chim. Belg., 1979, 88, 497. H. Matsumoto, Y. Saito and Y. Yoneda, J. Catal., 1970, 19, 101. L. Guczi, A. Sarkany and P. Tetenyi, J. Chem. SOC., Faraday Trans. I , 1974, 70, 1971. G. Leclercq, L. Leclercq and L.Maurel, J. Catal., 1977, 50, 87. lo G. Leclercq, L. Leclercq and L. Maurel, Bull. SOC. Chim. Belg., 1979, 88, 599. l1 Z. Paal and P. TetCnyi, React. Kinet. Catal. Lett., 1979, 12, 131. R. Montarnal and G. Martino, Rev. Inst. Fr. Pet., 1977, 32, 367. l3 P. Tetenyi, Acta Chim. Acad. Sci. Hung., 1981, 107, 237. l4 Z. Padl and P. TetCnyi, Dokl. Akad. Nauk SSSR, 1971, 201, 11 19. l5 Z. Paal and P. Tktinyi, Kern. .Kozl., 1975, 43, 165. l6 Z. Paal, Adv. Catal., 1980, 29, 273. l7 G. A. Martin, J. Catal., 1979, 60, 345. J. H. Sinfelt, J. Catal., 1972, 27, 468. lB G. Leclercq, L. Leclercq and L. Maurel, J, Catal., 1976, 44, 68. 2o F. Garin and F. G. Gault, J. Am. Chem. SOC., 1975, 97, 4466. 22 P. Parayre, V. Amir-Ebrahimi, F. G. Gault and A. Frennet, J. Chem. SOC., Faraday Trans. I , 1980, 23 P. Parayre, V. Amir-Ebrahimi and F. G. Gault, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1723. 24 L. Guczi and P. Tetenyi, Acta Chim. Acad. Sci. Hung., 1967, 51, 275. 25 P. Tetenyi and L. Babernics, Acta Chim. Acad. Sci. Hung., 1963, 35, 419. 26 T. Baird, Z. Paal and S. J. Thomson, J. Chem. SOC., Faraday Trans. I , 1976, 69, 50. 27 P. Tttknyi, L. Babernics, L. Guczi and K. Schachter, Acta Chim. Acad. Sci. Hung., 1964, 40, 387. 28 S. W. Benson, Thermochemical Kinetics (Wiley, New York, 1968), p. 215. 2B P. Tetenyi, L. Guczi, Z. Paal and A. Sarkany, Kem. Kozl., 1977, 47, 363. 30 J. K. A. Clarke, J. J. Rooney, Adv. Catal., 1976, 25, 125. 31 I. I. Levitsky, A. M. Gyul’maliev and E. A. Udal’tzova, J. Catal., 1979, 58, 144. 32 A. Balandin, Adv. Catal., 1969, 19, 1. 33 K. Christmann, Bull. SOC. Chim. Belg., 1979, 88, 519. 34 P. Tetenyi, Acta Chim. Acad. Sci. Hung., 1967, 54, 267. 35 P. L. Goggin, G. Kemeny and J. Mink, J. Chem. SOC., Faraday Trans. 1,1976,72, 1025; J. Mink and P. L. Goggin, J. Organomet. Chem., 1980, 185, 129; J. Mink, DSc. Thesis; J. Mink, personal communication. ’ J. R. H. Van Schaik, R. P. Dessing and V. Ponec. J. Catal., 1975, 38, 273. V. P. Sokolov, V. P. Shport and N. M. Zaidman, React. Kinet. Catal. Lett., 1976, 4, 179. 76, 1704. 36 P. TetCnyi, L. Guczi and A. Sarkany, Acta Chim. Acad. Sci. Hung., 1978, 97, 221. 37 C. O’Donohoe, J. K. A. Clarke and J. J. Rooney, J. Chem. SOC., Faraday Trans. I , 1980, 76, 345. 38 V. Amir-Ebrahimi and F. G. Gault, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1735. (PAPER 2/416)

ISSN:0300-9599    

DOI:10.1039/F19827803573

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 3581-3593 Influence of Macroscopic Structure on the Gas- and Surface-phase Flow of Dilute Gases in Porous Media BY DAVID NICHOLSON* Department of Chemistry, Imperial College, London SW7 2AY AND JOHN H. PETROPOULOS Department of Chemistry, Democritos Nuclear Research Centre, Aghia Paraskevi, Athens, Greece Received 16th March, 1982 Calculations using model systems are presented which illustrate the effect of macroscopic heterogeneity of the porosity on the gaseous- and surface-phase flow of gas through porous media. It is argued that macroscopic heterogeneity is likely to be present in many systems studied in the laboratory and that its effect on flow properties could be significant. Analogies with previous work on pore structure are drawn.INTRODUCTION In previous paperP4 we considered the effect of pore structure on gaseous Knudsen flow in porous solids both with and without accompanying surface flow. For this purpose, structure factors K , and K , were defined and their behaviour studied in detail theoretically. If D, and D, are the gas-phase and surface diffusion coefficients of the porous medium and D; and Di are the corresponding coefficients for an idealized ‘ standard medium ’, then K , = D,/D; and K , = D,/Di. (1) The standard medium consists of a bundle of identical long uniform cylindrical capillaries oriented in the direction of flow of radius re = 2&/A, where E is the pore volume and A is the specific surface area of the pore walls per unit volume of the real porous medium.If the mean speed of the gas molecules is u and diffuse reflection of these molecules from the solid surfaces is assumed, then (2) In our studies2y4 the real porous medium was simulated by a network consisting of a regular square or cubic array of junctions interconnected by cylindrical capillaries, the radii of which were assigned at random from a distributionflr). D, and D,, and hence K~ and K , , were then calculated as a function off(r) (defined between limits rl, r2), the connectivity (i.e. the number, nT, of capillaries meeting at a typical junction) and the surface to gas-phase flux ratio in a capillary of radius rm = +(rl + rz), namely DZ = &ire = Br,. where k , is the Henry’s law adsorption coefficient. In practice, only K~ can be determined, because there is no expression for DZ comparable to eqn (2) for Di.Thus, model studies, like the aforementioned one 35873588 FLOW OF GASES IN POROUS MEDIA undertaken by US,^^^ are very useful for the purpose of providing guidance for (i) understanding the observed behaviour of K~ and (ii) making informed guesses about K , . The network model suggests2 that I C ~ and K , can be written as I C ~ = K E E ~ and K , = K:R, = K ~ Q (4) where K: = K: is the 'orientation' or 'anisotropy' factor, which has the ideal value of 1 / 3 in an isotropic All other pore structure effects (which in our network model are essentially the radius distribution, the connectivity and the randomness of radius assignment) are therefore included in E~ and E,, which have ideal values of unity, corresponding to a network of uniform radius.It is now becoming increasingly evident that the porous media studied in practice are often non-homogeneous on the macroscopic ~ c a l e . ~ - ~ This is not surprising because the most common objects of study are pellets formed by uniaxial compaction of powders in die^,^-^ which is known to lead to uneven local density of packing (i.e. non-uniform porosity).1° Other methods of pelletization also do not necessarily guarantee macroscopically homogeneous products. It is therefore necessary to examine the effect of macroscopically non-homogeneous structure on both gaseous and surface flow before a realistic theoretical analysis of the flow behaviour of porous media described in the literature can be made. Here we examine the effect of macroscopic heterogeneity, described by simple functions, on the behaviour of icg and K , .DEFINITION AND EVALUATION OF MACROSCOPIC STRUCTURE FACTORS Consider a porous medium in the form of a slab of dimensions I, = I, lg, It or a cylinder of length 1 and radius Zu. A gas concentration difference ACg is applied in the axial direction X , i.e. across the flat surfaces at X = 0 and X = I (the remaining surfaces being blocked). The resulting steady-state permeation flux at any X is J = UPdC,/dX ( 5 ) where dCg/dXis the local gas-phase concentration gradient, Pis the local permeability coefficient and U is the cross-sectional area normal to the direction of flow, given by either Z u l , or npY in the case of the slab or cylinder, respectively. The permeability coefficient is made up of gas-phase and surface components, namely (6) where A,, is the specific surface area per unit volume of the solid material making up the porous medium and e, K~ and K , are local value^.^ Experimentally an integral permeability coefficient is determined from the integrated P = Pg+P, = EDg+AksD, = ~ K ~ B E ~ / A , , ( ~ - E ) ~ A , ~ , ( ~ - E ) K , D ~ form of eqn ( 5 ) F = JI/UACg (7) where p is identical with P only when the porous medium is macroscopically homogeneous.In macroscopically non-homogeneous media, E and/or 7cg and K , , and hence P, vary locally and P is some average value, which can also be analysed into gas-phase and surface components, namely where b, and b , a r e integral diffusion coefficients and kg and Z, are analogous toD.NICHOLSON AND J. H. PETROPOULOS 3589 I C ~ and K , but also include the departure from ideal behaviour caused by the inhomogeneous macroscopic structure, E" is the observed overall porosity and A , 5 is considered not to vary appreciably with E, a usually reasonable appr~ximation.~ Pg is conventionally determined from the corresponding helium permeability, Pg, He, as p g = pg, He(MHe/Wi where A4 is the molecular weight of the gas. B g ( ~ g ) and 8, then follow from eqn (8). PROPERTIES OF 2,AND Es We now proceed to consider the properties of the macroscopic structure factors when P is a function of position in the axial or lateral directions, i.e. when P = P(X) or P = P(Y). CASE OF P = P ( x ) Integration of eqn (5) between X = 0 and X = I and comparison with eqn (7) shows that P = [ S,'dx/P(x)]-' where x = X / I .Similarly, Also E" = J:Edx. Hence, the expressions for the observed structure factors are (9) where P and Pg are given by eqn (6). It can be shown quite generally (see Appendix) that, if the macroscopic non- homogeneity of the porous medium is due principally to the local variation of E , whilst K, 1: constant, then R, < K , . If the local variation of icg is also appreciable, the corresponding result is f l tg -= J I C ~ ~ X . 0 The above results may be illustrated by numerical calculations using linear and parabolic E(X) functions E ( X ) = ~ , ( l +ax) E(X) = E,( 1 + ax - ax2) where E , = E (x = 0) and a = constant. These functions should be representative of porosity distributions resulting from one-ended or symmetrical two-ended powder compaction, respectively [cf- fig.96 of ref. (1 O)]. In each case I C ~ = K , = K , = constant was assumed and E , and a were chosen so as to have E" = 0.5 and E,,,/E, = 1.3, 1.6, 2.0 and 2.4 [where is the maximum value of E ( X ) in 0 < x < 11. The results3590 FLOW OF GASES IN POROUS MEDIA obtained under these conditions are practically the same for both functions and indicate a smooth decrease of R,/Ic, below unity as the degree of non-homogeneity, measured by E ~ ~ ~ / E ~ , increases (see fig. 1). The behaviour of CS was studied by analogous computations, using the same e(x) functions, as a function of the surface to gas-phase permeability ratio of the standard medium, namely A: k, DE (1 --E‘)”)22BE2 = 0.01,O.1, 1 and 10. The results obtained are given in fig. 2. Note that I~,/IIc, tends to increase with E ~ ~ ~ / E ~ substantially, especially at low surface to gas-phase permeability ratios. 1.4 1.2 c 1.0 Kg K O - 0.8 0.E 1 I I 0 1.5 2.0 2.5 EITlax/fO FIG. 1 .-Gas-phase structure factors for P = P( Y), (C/M,, > 1) and P = P(X), ( I ~ / M , < 1). Full l k s , linear variation of E . Broken lines, parabolic variation of E . The behaviour of R, and R, for the case when the macroscopic porosity varies along the direction of flow is thus very similar to that of IC, and IC, for the ‘ serial capillary model’,’* as would be expected intuitively. CASE OF P = P(Y) For a porous medium in the form of a slab with P varying along one of the lateral directions Y, eqn (5) is modified to and comparison with eqn (7) shows that = f P(y)dy 0D.NICHOLSON AND J. H. PETROPOULOS where y = Y/lu. Similarly, B ugc2 Pg(y)dy =2 -[--dy. A , 0 1-& Also E"= [:&dy. 3591 1 I I I 1.5 2 .o 2.5 ~m,x/EO FIG. 2.-Surface flow structure factors for P = P( Y). Full lines and broken lines are for linear and parabolic porosity functions, respectively, as in fig. 1. Hence the observed structure factors are given by u g = - - ~ f i g - FgAo(l-&) = - f L d y 1-E" K c 2 DE 2BE2 2 0 1 - & Again, in the case of tg it can be proved generally (see Appendix) that, if Eg N constant and the local variation of E is primarily responsible for the micro- scopic non-homogeneity of the porous medium, & > K ~ . If there is also appreciable local variation of icg, the corresponding result is dy -l 'g ' lo a1 * For a cylindrical medium, eqn (1 3) must be rewritten as J = -J:'2nYdypdCg = U B 2yPdy.dX3592 FLOW OF GASES I N POROUS MEDIA Introducing o = y 2 and comparing with eqn (7) we find which is exactly analogous to eqn (14). The expressions for p,, E, Rg and Rs are similarly analogous to eqn (15)-(18) and the result of eqn (19) is again obtained in the form The above results are illustrated by numerical examples, given in fig. 1, in which the same functions as before, namely e(w) = E,( 1 + am) e(o) = &,( 1 + ao - ad) were employed with the same values of ern ax/^, and E and with o = y or o = y 2 representing a slab or cylinder, respectively. As expected, f g / ~ , increases smoothly above unity with E,,,/E,.The behaviour of Rs is quite simple in this case, as shown by eqn (IS), and should not normally deviate materially from the mean value of K,. Thus, as might be expected, the behaviour of R, and Rs in the case of POI) closely resembles that of K~ and K , for the ‘parallel capillary model’.1y3 CONCLUSION There now exists convincing evidence that many porous media studied in practice are macroscopically non-uniform and, as indicated in the Introduction, some general trends in the pattern of the non-uniformity can be given, although precise quantitative description may be much more difficult to achieve. In this work we have shown, by choosing physically reasonable models for the spatial variation in porosity, how measured gas and surface diffusion coefficients would be affected by macroscopic non-uniformity .Similarly macroscopic heterogeneities may also occur in polymer films, usually cast in the form of a rectangular slab, and although the equations have been derived here in terms of the properties of a porous medium, the trends indicated would also apply in such cases. Note that, according to the curves shown in fig. 1, the effects of radial and axial variations in porosity act in opposite directions, and it is conceivable that in some systems both types of macroscopic heterogeneity could coexist and only very small modifications to the structure factor E~ would be anticipated. Although interpretation of actual data is not feasible until precise information about this property is available, it clearly emerges from this work that macroscopic heterogeneity is likely to be important in practice. APPENDIX The following results are obtained by repeated application of the Schwarz inequality Ibfi(~)~ a dx f f 2 ( x ) . a dx. . . > ( ff,(x)f2(x) a . . . d x y in conjunction with (a) eqn (9) and (10) or (b) eqn (1 6) and (1 7).D. NICHOLSON AND J. H. PETROPOULOS 3593 CASE OF P = P(x) D. Nicholson and J. H. Petropoulos, J . Phys. D, 1968, 1, 1379. D. Nicholson and J. H. Petropoulos, J. Phys. D, 1971, 4, 181. D. Nicholson and J. H. Petropoulos, J. Phys. D, 1973, 6, 1737. D. Nicholson and J. H. Petropoulos, J. Phys. D, 1975, 8, 1430. M. F. L. Johnson and W. E. Stewart, J . Catal., 1965, 4, 248. C. N. Satterfield and S. K. Saraf, Znd. Eng. Chem., Fundam., 1965, 4, 451. ’ J. H. Petropoulos and P. P. Roussis, J . Chem. Phys., 1968,48,4619. * P. P. Roussis and J. H. Petropoulos, J. Chem. Soc., Faraday Trans. 2, 1977, 73, 1025. K. Tsimillis and J. H. Petropoulos, J. Phys. Chem., 1977, 81, 2185. lo e.g. C. G . Goetzel, Treatise on Powder Metallurgy (Interscience, New York, 1949), vol. 1, chap. 8 and 9. (PAPER 2/454)

ISSN:0300-9599    

DOI:10.1039/F19827803587

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1 , 1982, 78, 3595-3603 Electron Transfer in Solids Temperature Dependence of Dielectric Relaxation and Conductivity in Mixed-valence Potassium Manganate-Permanganate BY DAVID R. ROSSEINSKY* AND JAMES S. TONGE Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD Received 18th March, 1982 The site-transfer conductivity expression o = ne2a2 v/6 kT has been further tested for K,(MnO,), by observations of the d.c. conductivity, o, and of the electron-transfer frequency, v, from dielectric relaxometry, over a temperature range, providing good agreement both individually and in activation energies E: v / v o = exp [-(6080 K)/T] and o/oo = exp [-(5908 K)/T] with v, = 4.8 x 10l2 Hz and oo = 84.7 l2-l cm-l. The Marcus-Hush semi-classical formulation for E as a sum of Ein = 3 k, k,Ar2/(kl + k,) and Eout = fe2(+r;’ +&l- r;;) (ny2 -&;I) together with summary adoption of v, = kT/h in the site-transfer expression, gwes a complete theoretical formulation for the conductivity as o = (ne2a2/6h) exp [ - (Ein + Eout)/ka or numerically (204 l2-l cm-l) x exp [ -(5300 K)/T], acceptably according with experiment. KMnO, and K,MnO, have lower conductivities; dielectric relaxometry on KMnO, does not show the Cole-Cole behaviour which we associate with simple electron transfer in such materials, and K,MnO, is not amenable to dielectric relaxometry. The combination of two approaches has recently thrown further light on the site-transfer (hopping) mechanism of d.c.conductivity in mixed-valence solids and in donor-acceptor adducts.The first involves the application of a phenomenological relation’ between the natural frequency v of electron transfer between the transfer sites and the value of the d.c. conductivity = ne2a2 v/6 kT where n is the number density of charge carriers, equated to the donor concentration, and a is the distance between the transfer ~ i t e s . l - ~ The second is the recognition that in such systems electron transfer represents a polarisation mechanism, the dielectric relaxation time of which will be the inverse of the electron-transfer frequency v.2-4 An array of relaxation frequencies about the most probable value gives Cole-Cole beha~iour,~ and in either case the frequency v is inferred from the value of the applied frequency at the maximum of a semicircle or circular-arc plot of the imaginary permittivity E” against the real, E’, where E” is calculated from the frequency-dependent conductivity less the d.c.value, 0, when appreciable. An alternative is a linear plot (arising from straightforward modification5 of the functions generating Cole-Cole arcs) as used in our original study2 of K,(MnO,),. K,(MnO,), is an alternating MnO, - MnOi- mixed-valence solid stoichiometrically comprising equal parts of KMnO, and K2Mn04. We report here extensions of our measurements, with minor modifications, to temperatures other than ambient and to 35953596 ELECTRON TRANSFER IN SOLIDS the parent compounds. In more detail we compare the K,(MnO,), frequency values with the electron-transfer rates for aqueous reaction, MnO; + MnOi-, prima facie a closely similar process to site transfer in the solid.EXPERIMENTAL The reagents and methods were much as in ref. (2), but in addition to bridge measurements (10 Hz-100 kHz) a phase-sensitive detector (Brookdeal 9505) and, once, a Solartron 1172 were used for confirmation. An Oxford CF4 cryostat was used down to -70 OC for cooling, and a helium-filled thermally insulated vessel with heater and thermocouple, for heating of compacted discs, in o measurements. The dielectric measurements were performed at or above ambient temperature because the circuitry associated with the cryostat introduced irresolvable interference. A variety of contacts for conductimetry were tried for the three solids, including a four-probemercury-contact cell for K3(Mn04),.6 Platinum or gold platings vacuum-deposited on the compacted discs served best, but even these decayed because of oxidation by all three highly reactive solids.Pressed-on platinum discs were the most constant, but not very reproducible. Special treatment of the data was thus devised (as below) to avoid errors arising from these problems. Values now reported replace ref. (2). A one-off relaxation measurement on K3(Mn04), was made on a Solartron 1172 frequency- response analyser, to which we had momentary access. Controlled by an Apple computer, the frequency range lou4 Hz-1 MHz is scanned automatically, and real and imaginary impedances outputted ; for a simple resistive-capacitive circuit this gives Cole-Cole-like response and an identical means of estimating v (see fig.3 later). RESULTS AND DISCUSSION Gold-plated electrodes and guard rings were used in measurements treated by the linear method in ref. (2). However, as o in the present measurements was measured after dielectric studies, to avoid polarisation, the values obtained were a lower limit because of electrode deterioration. This uncertainty made the low-frequency part of the Cole-Cole plot uncertain (hence the use2 of the linear plot which emphasizes high frequencies). Heavy Pt electrodes gave good Cole-Cole plots but erroneous E, and E, values (as shown by the non-retrieval of vacuum values with a void replacing the sample.) In order to make use of low-frequency data, it was decided to treat o in the relaxation analysis as an adjustable constant giving the best circularity of (minor) arc.This resulted in increases in o of up to 30% from the finally observed value, but these were more usually ca. lo%, which led to 10% to three-fold increases in the derived values of v. (With other materials and conditions, no such dependence on fitted o was found.) The assumption of arc circularity in fitting o proved a convenient procedure, and OFIT was always shown to be within a small or moderate (30%) interval of oOBS. Further confirmation of the benefit of this procedure is the generally improved fit of v derived via eqn ( 1 ) to oOBS or gFIT (table 1). The Cole-Cole plots are shown in fig. 1. $2-l cm-l. The four-probe mercury contact gave a value 1.4 x R-l cm-l, which was not sustained because of rapid Hg oxidation, as shown by a grey-white colouration.While possibly an upper limit for o, it could comprise the result of numerous surface effects, and we have preferred to take the through-disc values as being representative of the bulk conductivity . The direct comparison of oDIEL calculated from vet and eqn (1) with oOBS or oFIT, when d.c. and dielectric-relaxation data were measured within short intervals of time (< 24 h) on the same sample, gave entirely satisfactory agreement (table 1). For 296-298 K, deviations from average in one (oOBS) closely follow deviations in the other The final 298 K value of oOBS found over 7 samples was (2.1 0.5) xD. R. ROSSEINSKY AND J. S. TONGE 3597 75 r E 0 50 100 150 200 E l FIG. 1.-Typical Cole-Cole plots from K,(MnO,), at various temperatures: (a) 296, (b) 319, (c) 325, ( d ) 337 K.The numbers on the curves are the applied frequencies in kHz. (oDI EL), each being an intrinsic property of the sample [according to eqn (l), the same property]. The agreement now found between oOBS and (TDIEL provides strong support for eqn (1). This agreement greatly exceeds that originally found2 for a variety of materials, for which sample-by-sample comparisons were mostly not available ; thus a single perylene+hloranil sample3 yields oOBS = 3.2 x and oDIEL = 2.7 x Temperature dependences of both vet and oOBS in fig. 2 give activation energies presented in table 2. To these results are added the relevant data from electron-transfer studies of the reaction MnO, + MnOi- -+ MnOi-+ MnO, R-l cm-l, showing better agreement than do the averages.23598 ELECTRON TRANSFER I N SOLIDS TABLE 1 .-D.C.CONDUCTIVITY, DIRECTLY OBSERVED, FITTED TO COLE-COLE ARCS AND CALCULATED FROM DIELECTRIC RELAXATION FREQUENCY lo7 oll2-l cm-l ~ ~~~ sample TIK OBS FIT DIEL 1 2 3 4 5 6 337 325 318 296 282 330 292 298 298 307 298 296 27.5 10 5.3 1.75 0.78 1.75 1.60 2.15 5.4 2.3 3.5 19.0 29 13.2 6.1 2.1 0.83 22.4 2.0 1.8 2.45 6.1 2.5 3.85 21.4 12.9 6.5 2.07 (1.8 * 1)" 20.5 1.77 1.62 2.26 5.77 2.33 3.81 a Poorly defined arc. 2.5 3.0 3.5 4.0 4.5 5.0 1 0 3 IT FIG. 2.-Log ooBS and log v plotted against 1/T for K,(MnO,),. Sample A: a, T decreasing; 4, from start; 0, T increasing; giving o/SJ-l cm-I = 546 exp [ -(5592 K)/A. For sample B (+ and 0, respect- ively); o/SJ-' cm-' = 84.7 exp [-(5908 K ) / q and v/Hz = 4.8 x lo*, exp [-(6080 K)/q.D. R.ROSSEINSKY AND J . S. TONGE 3 599 TABLE 2.-vALUES OF 298 K ELECTRON-TRANSFER FREQUENCY (V/HZ) FROM AQUEOUS-SOLUTION RATE CONSTANT, FROM d.c. CONDUCTIVITY AND FROM DIELECTRIC RELAXOMETRY, AND THE CORRESPONDING ACTIVATION ENERGIES (E/kJ mol-I) FOR KMn0,-K,MnO, a Same sample. in aqueous s~lution.~ We quote here the intramolecular transfer frequency for 298 K in water (based on a juxtaposition constant2 of 0.1 dm3 mol-l), which is within a factor of five of those observed for d.c. conductivity [from eqn (l)] and for dielectric relaxometry, so confirming the original comparison.2 Furthermore, the activation energies are almost identical within experimental error ; that for the solution reaction has had an estimated value of -2.2 kJ mo1-1 for the simple cou.lomb interaction subtracted from it.This appears to be the first establishment of an identity of electron-transfer rate in both solid and liquid phases. The observation furthermore emphasizes the discrete nature of the transfer acts, which can be viewed in the solid as occurring within a narrowly defined, just half-filled density of states in the band gap, which overlap so weakly as to be virtually localised. If a common mechanism, discrete site-to-site transfer, underlies all three phenomena, then a similar analysis of the activation energy applies. This is quite well understood for the aqueous p r o ~ e s s , ~ - ~ ~ the activation energy containing a coulombic approach term not applicable in the solid and two terms involving 'reorganisation'.The inner reorganisation energy Ein is due to prior bond-length adjustments in the reactant coordination sphere, here in Mn-0 bonds, to values intermediate between the equilibrium values (a consequence of the Franck-Condon principle). The activation energy Ein attributable to a bond-length adjustment, Ar, is1, where k , and k , are the breathing-mode force constants. For the present system, E., has been estimatedlO as 12 kJ mol-l for Ar = 0.05 A and 6.8 kJ mo1-1 for Ar = 0.02 A, which an EXAFS study shows to be accessible at room temperature.12 The higher value is adopted here, partly on the grounds that in the solid (rather than in solution, for which the estimates were made) greater adjustment will be called for in achieving the electronic overlap required, in view of the immobility of the MnO, ions on lattice sites relative to the solution state.The 'outer' activation energy term Eout arises in essence from the polaron trap, the Franck-Condon principle disallowing electrostatic equilibration of ambient dielectric with the transferring charge, apart from the electronic polarisability part represented by nf, the square of the refractive index. The semiclassical theory of Marcus8* 11* l3 and Hushs gives for Eout14 (3) where rl and r2 are the radii of MnOy and MnOi-, r12 is the distance between the anion centres, and E , is the static permittivity ( E , and e are in e.s.u.). The use of the observed E , as the bulk permittivity about spherical anions in a lattice has proved quite realistic for alkali halides,15 such a model yielding calculated ion sizes in good accord3600 ELECTRON TRANSFER IN SOLIDS with experiment.With Y, x r2 x 2.8 A, r12 x 8.4 A and E, = 175 from our Cole-Cole plots, only n: is required. For KMnO, n, = 1.59, and from comparisons of n, for MHSO, and M2S0, (M = Na, K, Rb) n, for K2Mn0, should be ca. 1.62 and for K,(MnO,), 1.6. These values give Eout = 31.7 kJ mol-l and with Ein = 12 kJ mol-1 a total of 44 kJ mol-l, which is in accord with observed values (table 2). The pre-exponential of the transfer frequency (fig. 2) is found to be v, = 5 x 10l2 Hz, a magnitude often deemed typical of phonon frequencies16117 or an electron delo- calisation frequency in the pretransfer stage. A naive classical interpretation would predicate the slower of these alternatives, but v, clearly has a more complex origin involving processes akin to spin-lattice n.m.r.relaxation,ls requiring a more detailed mode119 of electron-phonon interaction than we have available. For the present pis a h we summarily insert the value kT/h = 6 x 10l2 Hz, taken from transition-state statistical mechanics. The conductivity can thus be expressed as kT h 0 = (ne2a2/6 kT) - exp [ - (Ein + Eou,)/RT] (4) where Ei, and Eout are given in eqn (2) and (3). A firm theoretical basis for v, is thus all that is required for a comprehensive understanding of the site-transfer d.c. conductivity. It is worthwhile to note that complex impedance measurement, fig. 3, on the independent Solartron frequency-response analyser, and involving a different but consistent numerical analysis, yielded v = 6.3 kHz and oDIEL = 2.2 x lo-' R-l cm-l, in total agreement with the point-by-point bridge and phase-sensitive detector permittivity measurements.It was of further interest that a second relaxation with a characteristic frequency of 6 Hz was also observed, conceivably due to ionic motion (K+). Clearly the corresponding conductivity contribution would be very small, but FIG. 3.-Complex impedance plot for K,(MnO,), at 296 K, from Solartron frequency-response analyser, indicating two relaxation processes. Applied (oscillation) frequencies shown. with an activationenergy sufficiently different from theelectronic, the ionic conductivity could predominate in favourable temperature ranges, verifiable by temperature- dependence studies on the low-frequency relaxation.K2Mn0, AND KMnO, Attempts at studying K,MnO, were particularly bedevilled by oxidation of contacts. Many were tried, the better o results being summarised in table 3 for a variety of contacts, most being vacuum-deposited. Gold contacts usually failed more rapidly than Pt, although that quoted survived untarnished for several hours. The higher values never persisted, but ca. 2 x L2-l cm-l may be accepted as the value for the material, probably free of electrode-deterioration effects. However, since grinding ofD . R. ROSSEINSKY A N D J. S. TONGE 360 1 -7.5 -- - 8 . 0 - s d I C -? - 8 . 5 - ---. v 0 -9.0 TABLE 3.-cONDUCTIVITY COBS OF K,MnO,: EFFECT OF CONTACTS AND OF TIME - - vac. dep." Pt +pressured leads < 1 2.0x 10-8 vac.dep. Pt + Ag-paint leads vac. dep. Au+vac. dep. Cu < 1 5 x 10-8 vac. dep. Pt + Ag-paint leads < 1 3.0 x lo-* 15 (evacuated) 1.2 x 10-10 Pt disc, pressure < I 4.8 x all the above > 12 8 x a Vacuum-deposited. s 2.9 3.1 3.3 3.5 lo3 KIT FIG. 4.-Temperature dependence of c7 for KMnO,. K,MnO, results in some disproportionation, traces of consequent K,(MnO,), in the compaction could be responsible for some enhancement of 0. It was fruitless to attempt dielectric relaxometry, because of the pronounced instability (although rapid scanning, as with the Solartron apparatus, within minutes of electrode application might work). KMnO, was much more amenable to study, oOBS values being steady for several weeks with Pt or Au/Cu contacts for any one sample at between 7 x and 0-1 cm-l, depending on the sample.Regardless of whether typical or highest values are considered, o varies as K,(MnO,), > KMnO, > K,MnO, (the latter inequality requiring revision of an earlier footnoted conclusion based on limited dataso). The activation energy for a KMnO, sample having oOBS (298 K) = 1.2 x Dielectric-relaxation studies on KMnO, are obscured by a critical dependence on the value of o employed, and it is not unequivocal that Cole-Cole arcs apply; at 298 K 0-l cm-l was found to be 56 f: 3 kJ mol-1 (fig. 4).3602 ELECTRON TRANSFER I N SOLIDS 0 50 100 150 200 250 f ’ FIG. 5.-Cole-Cole plots for KMnO, at 337 K, for CT = (3.4k0.4) x lop8 Q-l cm-l . (%BS = 3.4 x lop8 l2-l cm-l gives the middle curve; the other two curves for f 10% values emphasize the doubt in inferring conformity with an arc.) only continued increase of E” with E’ is observed, although arc dependence can be induced at higher temperatures by slight (ca.10%) adjustments of a; only at 337 K is a definite arc found (fig. 5). There is no simple evidence of a site-transfer mechanism, although it is not precluded. CONCLUSIONS The agreement between d.c. conductivity of K,(MnO,), and the calculated value from dielectric relaxometry has now been greatly enhanced. These both agree with the MnOL-MnOi- electron-transfer rate in solution, and all three activation energies (d.c. conductivity, relaxometry and aqueous reaction) also agree closely. The details of the aqueous reaction being already largely understood, a quite complete account of the conductivity process is now available.As expected, the conductivities of K,MnO, and KMnO, are lower, and of unresolved mechanism. We are indebted to the S.E.R.C. for a scholarship and equipment, to Mr T. E. Booty for technical expertise, and to Mr N. M. Rosseinsky for programming these and ancillary computations. S. J. England, P. Kathirgamanathan and D. R. Rosseinsky, J. Chem. SOC., Chem. Commun., 1980, 840. D. R. Rosseinsky, J. A. Stephan and J. S. Tonge, J. Chem. SOC., Faraday Trans. 1, 1981, 77, 1719. S. Bone, J. Eden, P. R. C. Gascoyne and R. Pethig, J. Chem. SOC., Faraday Trans. I , 1981,77, 1729. B. C. Bunker, M. K. Kroeger, R. M. Richman and R. S. Drago, J. Am. Chem. SOC., 1981,103,4254. C. F. J. Bottcher and P. Bordewijk, Theory of Electric Polarisation (Elsevier, Amsterdam, 1978). D. R. Rosseinsky, R. E. Malpas and T. E. Booty, J. Phys. E, 1977, 10, 1236. R. A. Marcus, J. Chem. Phys., 1956, 24, 966. N. S. Hush, Trans. Faraday SOC., 1961, 57, 557. 1971), p. 247. ’ J. C. Sheppard and A. C. Wahl, J. Am. Chem. Soc., 1957, 79, 1020. lo J. M. Hale, in Reactions of Molecules at Electrodes, ed. N. S. Hush (Wiley-Interscience, London, l1 R. A. Marcus, Discuss. Faraday Soc., 1960, 29, 21. l 2 T. K. Sham and B. S. Brunschwig, J. Am. Chem. SOC., 1981, 103, 1590. l3 R. A. Marcus, in Tunnelling in Biological Systems, ed. B. Chance, D. C. DeVault, H. Frauenfelder, l4 N. S. Sutin, in Tunnelling in Biological Systems, ed. B. Chance, D. C. DeVault, H. Frauenfelder, R. A. Marcus, J. R. Schrieffer and N. Sutin (Academic Press, New York, 1979), p. 109. R. A. Marcus, J. R. Schrieffer and N. Sutin (Academic Press, New York, 1979), p. 201.D. R. ROSSEINSKY AND J . S. TONGE 3603 l5 D. R. Rosseinsky, J. Chem. SOC. A , 1971, 608. l7 I. G. Austin and N. F. Mott, Science, 1970, 168, 71. M. B. Robin and P. Day, Adv. Znorg. Chem. Radiochem., 1967, 10, 263. H. L. Friedman, personal communication. R. R. Dogonadze, A. M. Kutznetsov, M. G. Zakaraya and J. Ulstrup, in Tunnelling in Biological Systems, ed. B. Chance, D. C. DeVault, H. Frauenfelder, R. A. Marcus, J. R. Schrieffer and N. Sutin (Academic Press, New York, 1979), p. 145. 2o D. R. Rosseinsky and R. E. Malpas, J. Chem'. SOC., Dalton Trans., 1979, 749. (PAPER 2/466)

ISSN:0300-9599    

DOI:10.1039/F19827803595

出版商:RSC    

年代:1982

数据来源: RSC