Furaday Discuss. Chem. SOC., 1989, 87, 91-105 GENERAL DISCUSSIO N Prof. B. E. Conway (University of Ottawa, Ontario, Canada) addressed Prof. G. C. Bond: In the paper of Bond et al. it is shown that catalytically active oxides may be dispersed in a ‘monolayer’ state. It would be interesting to know if these materials are in a truly two-dimensional state or whether they exist in microscopic clusters of metal plus oxide ions, equivalent in amount to a monolayer or less (ca. 1015 particles per real cm’). This presumably may be the case for the species Prof. Bond refers to as being in a ‘supramonolayer’ state of paracrystalline V205, as characterized by Raman spectros- copy. The question, therefore, that I would like to ask is if it is known whether, in some quasi-three-dimensional phase, the catalytic activity of such species on a support is substantially different from that of the true, two-dimensional monolayer phase? Also the stoichiometries of the true two-dimensional and the paracrystalline phases may be different.The reason I mention this is that, in electrocatalytic oxidations at noble-metal surfaces, e.g. those of Pt, Pd, Rh, Ru, Au, a clear distinction can be made by means of cyclic-voltammetric experiments’ between the formation and reactivity of the initial sub-monolayer of OH/O species that can be electrochemically deposited and the more extended oxide film in which OH or 0 species have undergone place-exchange with underlying metal atoms; i.e. the interphase has become reconstructed, giving a quasi- three-dimensional phase oxide, but still in a film 2-3 atomic diameters in thickness.The latter, arising from the two-dimensional chemisorbed species, is distinguished by a characteristic irreversibility between its formation and reduction (plate l ) , whereas the truly two-dimensional chemisorbed species can be formed and reduced in an entirely reversible manner, as indicated in plate 1. The reversibility is judged by the identity of the Gibbs energy (as an electrode potential) for formation of a given state of the oxide with that for its reduction. Only in the two-dimensional chemisorbed state are such ‘oxide’ films on the noble metals active as catalysts or mediators in small-molecule electro-oxidations, e.g. of CH,OH, HCOOH, HCHO; reconstructive conversion to the phase-oxide species, causes inhibition of such reactions.’ In the case of baser metals, such as Ni, under cyclic voltammetry, multilayers of NiO or Ni’O’OH (depending on potential) are immediately formed in an irreuersible way.By performing cyclic voltammetry at Ni at 180 K in a H20(100/~)-CH,0H(900/o) solution of NaOH, we have shown that an equivalent monolayer of OH on Ni can be generated, but not reversibly as at Pt at low temperature.’ Therefore, this ‘monolayer’ is probably not a true two-dimensional phase but rather consists of islets of nucleated three-dimensional NiO, having quite different behaviour from that of the Pt ‘monolayer’ oxide. 1 H. A. Kozlowska, B. E. Conway and W. B. A. Sharp, J. Electroanal. Chem., 1973, 43, 9. 2 M. W. Breiter, Elecrrochim. Acra, 1963, 8, 447; 1963, 8, 457. Prof.G . C. Bond replied: With regard to Prof. Conway’s question, I firmly believe that the catalytic ability of the true monolayer species in the case of the V205/Ti02 system is indeed superior to that shown by the particulate ‘paracrystalline’ form which is formed above the monolayer when the V205 loading exceeds the monolayer capacity. That this is so is clearly demonstrated in fig. 8 of our paper, where the rate of propan-2-01 decomposition expressed per g V205 decreases monotonically as the Vz05 loading is increased. Similar plots have been recorded for o-xylene oxidation [see for example ref. ( 5 ) of our paper]. 91Faraday Discuss. Chern. SOC., 1989, Vol. 87 I c) C 2 7 u Plate 1. Progression of cyclic voltammetric current us. potential profiles showing resolution of reversibly formed and reduced two-dimensional oxide films (OH on Pt) at Pt from irreversibly formed and reduced reconstructed quasi-three-dimensional oxide film.Conditions: 6 mol dm-3 aq. H2S04 at 233 K; progression of cyclic voltammograms taken to successively increasing anodic potentials. Maximum OH coverage corresponds to 1.2 equivalent monolayers. (Integrals of current us. potential profile, up to various potentials, corresponding to coverages by OH formed or reduced). General Discussion (Facing p. 91)92 General Discussion I I I 1 f' Fig. 1. F.t.i.r. spectra of the surface species arising from adsorption of toluene ( a ) and benzene ( b ) on vanadia-titania (full lines) and pure titania (broken lines) at room temperature.The absorbance scales are not the same for the different spectra. One must, however, take care not to generalise, because the MoO3/TiO2 system behaves differently (see fig.9 of our paper). Here the conversion of propan-2-01 decomposition is initially proportional to the MOO,, and there is no indication that the supramonolayer species are less active than those in the monolayer. The conversion does, however, decrease when the MOO, concentration exceeds 3%. Prof. G . Busca ( University of Genoa, Italy) said: When vanadia-titania 'monolayer' samples activated in vacuum are put into contact with toluene vapour at room tem- perature, a surface reaction occurs producing an adsorbed species whose F.t. i.r. spectrum is shown in fig. 1 ( a ) , full line. Both the i.r.spectrum and the chemical behaviour suggest that this is a benzyl species.' Also, benzene vapour reacts at room temperature in the same conditions. The spectrum of the adsorbed species arising from benzene reactive adsorption [fig. l(b), full line] is composed by the superposition of the spectra of more species. The two quartets in the regions 1620-1520 cm-* and 1500-1440 cm-' indicate that different mono- and di-substituted benzenes are present. In particular all bands observed after phenol adsorption (producing mostly phenate species) are also present after benzene adsorption. Broader bands in the region 1900-1630 cm-' are probably due to adsorbed quinones. In the same conditions both toluene and benzene adsorb reversibly without any reaction on pure TiOz [broken lines in fig.1 ( a ) and (b)]. Vanadia-titania 'monolayer' catalysts after treatments that cause desorption of water show both in F.t.i.r. and in laser Raman spectra a typical sharp band at ca. 1035 cm-'.* This band broadens and shifts down (to 980cm-') if water is adsorbed. From the coincidence of i.r. and Raman bands we have concluded that this is due to V=O stretching of non-coupled single vanadyl species, while from the perturbation of this band2 and of its first overtone3 we have concluded that these species are coordinativelyGeneral Discussion 93 unsaturated on the water-free samples. From comparison with the spectra of vanadium inorganic compounds it seems likely that these species are nearly octahedral. According to the results of several other authors, e.s.r.data show the presence of coordinatively unsaturated V02+ vanadyl species having a nearly octahedral environment on activated samp~es.~ We have proposed that the active sites for hydrocarbon activation on vanadia-titania are similar to those as expected on the surface of vanadyl pyrophosphate catalysts, i.e. vanadyl species in a nearly octahedral incomplete c~ordination.~ Therefore, my questions are: (i) What do you think about the role of coordinative unsaturation on vanadium (ii) Have you characterized spectroscopically your catalysts in water-free environ- centres for the activation of hydrocarbons? ments? 1 G. Busca, F. Cavani and F. Trifir6, J. Catal., 1987, 106, 471. 2 C. Cristiani, P. Fozatti and G. Busca, J. Catal., 1989, 116, 586.3 G. Busca, Langmuir, 1986, 2, 577. 4 G. Busca, G. Centi, L. Marchetti and F. Trifir6, Langmuir, 1986, 2, 568. 5 G. Busca, G. Centi and F. Trifiro, J. Am. Chem. Soc., 1985, 107, 7757. Prof. G. Centi (Department of Industrial Chemistry and Materials, Bologna, Italy) continued: In your discussion about the species of vanadium on the Ti02 surface and on their role in the mechanism of o-xylene oxidation you don't mention the existence of different valence states of vanadium. We have a series of evidencele6 regarding the presence of VIV on TiOz (anatase) and on its role in the activation ( H-abstraction)2 of o-xylene. For example, it is possible to determine by chemical analysis6 that VlV forms by simple calcination (500°C) from a mechanical mixture of V205 and Ti02 (anatase).The amount of Vlv that it is possible to determine after2 extraction of Vv species corresponds to the formation of a complete layer on Ti02. Spontaneous reduction of Vv occurs at high temperatures in air and in the absence of reducing agents, due to the specific reaction of vanadium with the Ti02 surface. The VlV which forms is very stable and does not reduce2 or reoxidize. The same effects occur at lower temperatures in the presence of reducing agents, such as during catalytic tests and is a more general feature of the V-Ti02 catalysts. Furthermore, our evidence indicates that the formation of this stable Vlv is the key for the stabilization of the selective upper layer of Vv. After the catalytic tests the V205 that has not reacted during calcination spreads on the surface of Ti02 forming a layer, which greatly enhances catalytic behaviour, and a supramolecular region, which also increases the catalytic behaviour, by a factor of nearly 4-5 times that of the theoretical monolayer. By chemical analysis one can demonstrate that both these species have a V": VlV ratio of 2 : 1 and that the catalytic behaviour may be correlated to the amount of this specific reduced V-oxide phase.Both phases are characterized by an i.r. band at 995 cm-'. Do you have indications about the presence of different valence states of V on TiOz and on the nature of the modifications that occur during the catalytic reaction? 1 F. Cavani, G. Centi, F. Parvanello and F. Trifiro, in Preparation of Catalysts ZV, ed. B. Delon and P. Grange (Elsevier, Amsterdam, 1987), p.227. 2 G . Busca, L. Marchetti, G. Centi and F. Trifir6, J. Chem. Soc. Faraday Trans. 1 , 1985, 81, 1003. 3 F. Cavani, G. Centi, J. Lopez Mieto, D. Pinelli and F. Trifiro, in Hererogeneous Caralysis and Fine 4 G. Busca, L. Marchetti, G. Centi and G. Trifir6, Langmuir, 1986, 2, 568. 5 F. Cavani, G. Centi, E. Foresti, F. Trifiro and G. Busca, J. Chem. Soc., Faraday Trans. 1, 1988, 84, 237. 6 G. Centi, D. Pinelli and F. Trifiro, J. Mof. Cataf., submitted. Chemicals, ed. H. Guismet and J. Baroult (Elsevier, Amsterdam, 1988), p. 353. Prof. Bond said: I would like to reply to the related comments and questions posed by Prof. Busca and Prof. Centi. Their work has added significantly to our understanding94 General Discussion of the structure and activity of V205/Ti02 catalysts, and I am confident that the application of F.t.i.r. spectroscopy in particular will in the course of time resolve outstanding questions concerning the structures of adsorbed intermediates.In our experience the predominant oxidation state of vanadium in calcined materials prepared either by impregnation or grafting is Vv; this conclusion is based on X.P.S. evidence, admittedly on samples that had been exposed to the atmosphere, and although e.s.r. shows only small amounts of VIv ( < 5 % ) its concentration in the working catalyst may well be greater. I am, however, frankly surprised by Dr Centi's observation that the monolayer species are entirely VIv after calcination, but note that his material was prepared from a mechanical mixture of V205 and Ti02.This may account for the difference between his finding and those of ourselves and others. We did of course establish some years ago' that VIv ions dissolve into the anatase lattice quickly at ca. 700 "C, so what Dr Centi sees may be an early stage of this process, i.e. ViV ions occupying regular lattice sites at the anatase surface. What is, however, certain is that V205/Ti02 catalysts containing no more V205 than equates to a single monolayer can show high selectivity in o-xylene oxidation, and I do not therefore readily accept that an intervening layer of V'" species is invariably necessary to stablise a reactive layer of Vv species. As to Prof. Busca's questions, I am afraid we have not undertaken spectroscopic characterisation of our catalysts under water-free conditions.I do, however, accept that co-ordinating unsaturated vanadium species are probably present under reaction condi- tions and that they may play a role in hydrocarbon activation. We have, however, chosen to represent the activation of the methyl group as involving oxidative addition to a V=O group [see fig. 10( 6) of our paper], rather than as addition to a coordinatively unsaturated site. Our approach to the mechanism of o-xylene oxidation is described in ref. (46) of our paper. 1 G. C . Bond, A. J. SBrkany and G. D. Parfitt, J. Card., 1979, 57, 476. Prof. H. Knozinger ( Univerpity of Munich, Federal Republic of Germany) commented: The authors mention formation of oxide monolayers by spreading of an active oxide over a supporting oxide [(ref.(7) and (12)]. In fact, MOO, spreads over A1203 as shown by ion-scattering spectroscopy. This is the case in the presence and absence of water vapour. The atmosphere, however, has a significant effect on the structure of the spread material. Raman spectroscopy detected MOO, exclusively under dry conditions. This material cannot be detected by X-ray diffraction and must therefore be well dispersed. When water vapour was present during spreading, a surface polymolybdate was formed, this chemical transformation being independent of the spreading process [see ref. (12)]. The oxyhydroxide is an intermediate for the molybdate formation but not required for the spreading. Interestingly, on SiOz as supporting oxide neither spreading nor molyb- date formation did occur.Prof. Bond responded: I am grateful to Prof. Knozinger for his comments. It is of course well established that it is much more difficult to obtain stable monolayers on Si02 than on many other oxides. Dr J. A. H. MacBride ( University of Durham) (communicated): The mechanisms of oxidation of propan-2-01 to acetone, and of o-xylene to 2-methylbenzaldehyde7 by oxide-supported vanadium pentoxide proposed by Bond et al. do not involve reduction of vanadium. The former predicts H2 as a product (or intermediate), a reaction well known on metallic catalysts (e.g. Cu) but surprising on a transition-metal compound of high oxidation state. An alternative possibility, proposed by analogy with chromic acid oxidation of alcohols via chromate esters,' taking Bond's fig.4 type C vanadium speciesGeneral Discussion 95 (but see below) is: V V CH / \ path A lpath B / 1, The surface vanadate ester (3) could give the ketone and a V"' species [perhaps re-oxidized to V"] by base capture of a - H (path A, shifts-) or the (also observed) alkene (5) by capture of /3 - H (path B, shifts#"*) without redox change. A related mechanism for oxidation of methyl (6) to formyl (9) could be: Subsequent oxidation of formyl to carboxyl could be written in analogy with sequences starting from (1) (C=O replacing H-0) or from (6) (C=O replacing CH2) but the proposed mechanism for chromic acid oxidation of aldehydes in solution2 suggests the sequence:96 General Discussion While these polar reactions are convenient to write, the possibility of radical reactions, perhaps involving reduction of two vanadium (v) atoms to V'", should not be ignored, particularly in the oxidation of side-chains of aromatic compounds.Concerning the mechanism of methanol oxidation on supported phosphomolybdate catalysts discussed by Serwicka et al. (pp. 173-187) the question arises whether the species methylated on bridging oxygen (stated in discussion to have been detected spectroscopically) lies on the primary pathway from methanol to methanal. Given that it does, the mechanism for this reaction may be rewritten more simply (and more conventionally) as: 0 0 0 (131 (1C) t The shifts shown at (16), (Serwicka's 111 fig. 9, p. 185) avoid the confusing suggestion that carbon is electron-rich at any stage; repair of the reduced Keggin structure (17) by an oxygen atom is represented by (15).A simpler mechanism for alcohol oxidation on a high-oxidation-state metal-oxygen array such as the Keggin structure or supported (or bulk) vanadium pentoxide, say of surface structure types D and F considered by Bond (fig. 4), whereby hydroxyl oxygen is retained in the carbonyl product (e.g. CH3180H --* CH2 = l 8 0 ? ) may be written:General Discussion 97 Correspondingly the methyl-to-formyl oxidation considered above would be: t Oxidations by surface species of Bond's types A and E could be similar, e.g. R7C =O H H 0 0 I I \ o w / / \ M(x- 2 ) 1 J . March, Advanced Organic Chemistry (McGraw-Hill, 1977), p. 1083 and references therein. 2 As ref. ( l ) , p. 641. Prof. Bond responded to Dr MacBride: I have to say I believe that dehydrogenation of propan-2-01 and the oxidation of o-xylene to o-tolualdehyde are quite different processes, although the initial steps in each may show similarities [see fig.lO(6) of our paper]. The former reaction gives H2 as a product and is catalytic in the sense that no significant reduction of Vv to V"' occurs. It takes place at temperatures at least 200 K lower than that at which reduction of V205 by H2 starts. No O2 is present and there is therefore no source of 0 atoms, as implied by Dr MacBride's first reaction scheme. On the other hand, oxidation of o-xylene to o-tolualdehyde [of which only the opening gambit is shown in fig. 10(6)] does involve abstraction of an 0 atom from the surface, with subsequent re-oxidation of V'" to Vv.H2 is not a product of this reaction. Indeed, one recalls that the Mars-van Krevelen mechanism, which has been widely invoked to explain selective oxidations, was first formulated to describe this reaction. Prof. A. Baiker (ETH Zurich, Switzerland) said: I would like to comment on some points raised by the authors. The first concerns the statement made that calcination i Dioxygen is represented as the singlet for convenience only.98 General Discussion does not produce significant change in the structure of the precursor when grafting methods are used. In our studies we found that calcination has a significant influence on the structure of the immobilized species and on the concentration of potential immobilization centres (acidic OH groups) on the support.Another point which requires some comment is the suggestion that the structure of monolayers is homogeneous. High-resolution electron microscopy, electron spin reson- ance and X.P.S. studies performed on the vanadia-titania and vanadia-silica systems have clearly shown us that monolayers are neither chemically nor structurally homogeneous. Thus we cannot regard them as structurally uniform. As regards your statement that the grafting method is capable of producing monolayers only I may add that we have recently shown that the grafting method is of great potential to prepare not only monolayers, but also multilayers. This procedure is possible, since the vanadia species immobilized in the first layer possess acidic hydroxyl groups which can be utilized to anchor further vanadyl-alkoxide species [see ref.(14) in your paper]. This procedure has been used successively to tailor the structure of vanadia on titania catalysts for the selective catalytic reduction (SCR) of NO with ammonia.’ Finally, when discussing the potential of supported monolayers of oxides, it seems noteworthy to mention that the support interaction in such well dispersed systems provides another very interesting possibility to tailor the catalytic properties of such systems. An illustrating example for this behaviour has recently been presented.’ The intrinsic activity for SCR of vanadia monolayer species supported on titania could be improved by almost 500 times by using a mixed oxide support of 20% titania-silica. 1 A. Baiker, P.Dollenmeier, M. Glinski and A. Reller, Appl. Catal., 1987, 35, 351. 2 A. Baiker, P. Dollenmeier, M. Glinski and A. Reller, Appl. Catal., 1987, 35, 365. Prof. Bond said: In reply to Prof. Baiker, I should say that the structural changes produced by calcination are nothing like so great when grafting methods are used as when impregnation by the oxalato-vanadyl complex is employed: ’ thus for example the first product of the reaction of V0Cl3 with Ti-OH is thought to be 0 C1 \ / V / \ 0 0 Ti Ti so that ‘calcination’ simply results in the hydrolysis of V-Cl to V-OH. My colleagues and I demonstrated several years ago [ref. (6) of our paper] that the resulting V-OH groups are indeed capable of reacting with further V0Cl3, in agreement with Prof. Baiker’s statement [see also ref.(14) of our paper]. As regards the homogeneity, or otherwise, of the monolayer species, we may possibly have overstated the case in our paper. We simply wished to contrast the supported monolayer situation with that of the face-specificity shown by the unsupported oxide, as demonstrated by Prof. Baiker’s own work with V205, as well as that of others with MOO,. Prof. Baiker’s work on Si02-Ti02 as support is clearly very significant, and the phenomenon merits closer study. 1 G. C. Bond and S. Flamerz, Appl. Caral., 1989, 46, 89. Prof. K. I. Zamaraev (Siberian Branch of the USSR Academy of Sciences, Novosibirsk, USSR) commented: I should like to draw your attention to the fact that fig. 2B (p. 136) and 5 (p. 139) of our paper seem to provide a support for the suggestion made by Prof.General Discussion 99 G.C. Bond that, when finely spread over the surface of the supports, transition-metal oxides can form quite novel species with structure and reactivity different from those for the species present in the transition-oxide itself. From "Vn.m.r. spectra of fig. 2B one can see how a novel vanadium(v) site is formed when V205 is finely spread over the surface of SO2. From kinetic data of fig. 5 one can see how vanadium(v) sites attached to the surface of S O 2 contribute to the activity of supported vanadium catalyst for SO2 oxidation to SO,. Prof. Bond added: I am of course familiar with Prof. Zamaraev's pioneering use of V n.m.r. spectroscopy in the study of supported vanadium catalysts. It is good to know that this technique confirms the unique structures exhibited by these materials, and in conjunction with other methods it is likely to prove a powerful procedure for structure determination. Dr J.C . Vedrine (Institut de Recherches sur la Catalyse, CNRS, Villeurbanne, France) said: I agree with your interpretation of X.P.S. data (fig. 5) when I,/ ITi reaches a plateau with V205 'towers' (needles) covering a small fraction of the support. I was just wondering if it was V205 particles 'independent' of the support rather than as towers which could be evidenced by TEM and EDX-STEM. I do not understand the fig. 3 data. I understand the 995 cm-' band (V205) variations but not those of the 640cm-' band assigned to anatase which goes to zero intensity. This should be true for surface groups but not for bulk species.Can you be more specific on the interpretation of the 640 cm-' band and explain this zero intensity? Isopropyl alcohol decomposition is known to characterise: Brgnsted acid site (via propene formation), basic site (via acetone formation) and redox site (via acetone if air is used in the feed). Could you summarize the experimental catalytic conditions (temperature, air in the flow gas)? We have studied this reaction in air on MoO3/SiO2 with Mo content: at low Mo content propene was formed first, then propanal (electrophilic attack) before reaching MOO, type reaction products (acetone and propene).' Did you observe any propanal in the products? 5 1 1 T. C. Liu, M. Forissier, G . Coudurier and J . C. Vedrine, J. Chem.Soc., Faraday Trans. 1, 1989,85, 1607. Prof. Bond replied: As I recall the TEM work on the V205/Ti02 system, the needle-like growth of V205 crystals seems to be nucleated by the TiOz surface, and we have taken this as supporting evidence for our structural model. We do not fully understand why the Raman bands due to the support decrease in intensity so quickly as the coverage by the second component increases. We have, however, observed this behaviour with a number of systems, for example with Mo0,/Ti02 [ref. (27) of our paper]. We take it to be a real phenomenon, but one which requires further study. Isopropyl alcohol decomposition was investigated by passing N2 through the liquid at a controlled temperature: no air was present. Reaction temperatures are given in the legends to fig.8 and 9: further details are to be found in ref. (42) of our paper. We did not observe formation of propanol at any time. Prof. J. M. Thomas (Royal Institution, London) said: Your paper is an interesting one. One can now confidently expect the arrival of many new high-performance catalysts based on the idea of having a monolayer or so of one oxide or hydroxy-oxide grafted chemically onto the surface of another. My point is to emphasize that scope now exists for choosing (i) different polymorphs of a given (substratum) oxide, and (ii) different crystal faces of a given polymorph. It is well known that monolayers of vanadium oxide on TiOz (for the catalytic oxidation of o-xylene to phthalic anhydride) function efficiently only on anatase, not100 General Discussion on rutile or brookite.Likewise, some of the faces of anisotropic crystalline oxides such as MOO, are catalytically more active and selective than others. As well as the structure of the surface of the substratum, ease of wetting of the active component added to it, is also likely to be important. Prof. J. Haber (Polish Academy of Sciences, Krakow, Poland) added: The authors mention spreading which takes place on heating mechanical mixtures of oxides as a technique for preparation of oxide monolayers. It should, however, be emphasized that spreading occurs because of the phenomenon which can be interpreted in terms of solid-solid wetting in oxide systems’-2 and which plays an extremely important role not only in preparation but also, and mainly, in determining the behaviour of oxide monolayers in the course of catalytic reactions.Consideration of the equilibrium conditions at the interface between two solids and the gas phase show that spreading of one solid over another solid to form a thin film would occur only if the mobile phase adheres to the immobile phase (the support) more strongly than it coheres to itself. In the opposite case, even if a monolayer were obtained by an appropriate technique, e.g. grafting, the coalescence of the monolayer would take place resulting in the formation of heterogeneous mixture of crystallites because of the tendency of the system to attain a lower free-energy level. Indeed, experiments showed4 that when a reactor was filled with a mechanical mixture of vanadia with anatase, which is wetted by the former, and o-xylene-oxygen mixture was then passed through the reactor, a continuous improvement of the performance was observed with the time-on- stream until high activity and selectivity were attained, characteristic for a monolayer VO,/anatase catalyst.Conversely, when a monolayer VO,/rutile catalyst was prepared by grafting and placed in the reactor, very good performance was initially observed, but the activity and selectivity decreased rapidly with the time-on-stream to become similar to those of a mechanical mixture of vanadia and rutile. X.r.d. examination confirmed that segregation to form a heterogeneous system took place. It should be noted that the surface free energy of oxides depends on the habit of crystallites and is strongly influenced by adsorption, particularly of polar substances, and by incorporation of foreign ions into the surface layer of the lattice. Therefore, all these factors may be expected to have a profound effect on the stability and properties of monolayer catalysts.On discussing the reactivity of oxide monolayers the authors make reference to the t.p.r. technique. It should be borne in mind that such a general parameter as the position of the t.p.r. peak is of rather little value and may be misleading in characterization of the monolayers. In order to describe the reactivity of a monolayer three types of information are important: 5,6 kinetics of reduction; stoichiometry of reduction, i.e. the final degree of reduction which in the broad range of temperatures is a parameter characteristic for the system; ability of the monolayer to activate the molecules of the reducing gas.Indeed, the data quoted by the authors in table 1 show that VO, supported on A1203 and on SiO, exhibit similar t.p.r. peak positions, whereas we have shown’ that VO, on alumina is reduced by CO at 400°C, whereas VO, on silica cannot be reduced by CO at all. In a description of experiments on the decomposition of isopropyl alcohol the authors state that V0,/Ti02 and Mo0,/Ti02 give propene as the major product, the rate of decomposition being highest for low loadings. This is surprising because well established earlier data exist7’* indicating that, on VO,/anatase samples of low loadings, acetone is the major product of isopropyl alcohol decomposition. Moreover, on VO,/anatase samples obtained by grafting’ no propene whatsoever could be detected i.e.Brfinsted acidity was apparently completely eliminated. Propene clearly appeared in the products in the case of samples, in which the monolayer coverage was surpassed. These observa- tions permitted this method to be proposed for the determination of the monolayer capacity.’General Discussion 101 1 J. Haber, Pure Appl. Chem., 1984, 56, 1663. 2 J. Haber, T. Machej and T. Czeppe, 1985, 151, 301. 3 J. Haber, T. Machej and R. Grabowski, Solid State lonics, 1989, 32/33, 887. 4 M. Gasior, J. Haber, T. Machej and T. Czeppe, J. Mol. Catal., 1988, 43, 359. 5 J. Haber, A. Kozlowska and R. Kozlowski, J. Catal. 1986, 102, 52. 6 J. Haber, A. Kozlowska and R.Kozlowski, Proc. 9th Int. Cong. Catal, Calgary 1988, (The Chemical 7 M. Gasior, 1. Gasior and B. Grzybowska, Appl. Catal., 1984, 10, 87. 8 B. Grzybowska-Swierkosz, Mat. Chem. Phys. 1987, 17, 121. 9 B. Grzybowska-Swierkosz, in Catalysis by Acids and Bases, ed. K. Tanabe (Elsevier, Amsterdam, 1985) Society of Canada, 1988), p. 1481. p. 45. Prof. Bond replied: Both Prof. Thomas and Prof, Haber have drawn attention to the importance of solid-solid wetting in the formation and stability of oxide monolayers. Undoubtedly, when two solid phases are mixed and heated, the formation of a monolayer of one upon the other is conditional on there being a decrease in the Gibbs free energy of the system; and when a monolayer is prepared, by whatever means, its stability vis-a-vis the aggregated state likewise depends on thermodynamic factors.This approach, however, overlooks certain important considerations and concepts. First, monolayer species prepared by impregnation or grafting are not (as we were at pains to point out in our paper) simply a two-dimensional lamella of the corresponding bulk oxide: the competing processes of spreading and aggregation, therefore, constitute chemical changes and not simply physical changes. Secondly, thermodynamic arguments do not explain anything; as chemists we are interested in the chemical principles underlying the interaction of one oxide phase with another, and in the type of bonds that are formed. Thirdly, there have to be mechanisms whereby spreading and aggrega- tion occur, and this too is a legitimate field of enquiry. There is evidence [see ref.(7) of our paper] that the migrating species are volatile oxyhydroxides, which may form the same monolayer species as those formed by other methods. Thus the chemical changes mentioned above may well be hydration and dehydration. The sensitivity of the monolayer to the geometry of the underlying surface, mentioned by both Prof. Haber and Thomas, has not yet been fully explained, and in view of the comparatively small differences between the anatase, rutile and brookite structures in the case of Ti02, is somewhat surprising. However, as Prof. Thomas points out, the way to the systematic study of the structural effects is now open. Prof. Haber has drawn attention to limitations of the t.p.r. technique in the characteri- sation of oxide monolayers. I am, however, only partly in sympathy with his comments: the stoichiometry of the reduction is readily evaluated by t.p.r.(see fig. 2 of our paper for an example), and where multi-step reduction occurs (as with MOO,) the oxidation state of the ion after each step is easily deduced. There is of course additional information to be obtained from isothermal measurements, although activation energies can be derived from t.p.r. results with some assumptions. One of its chief advantages is the speed with which measurements can be made, and I know of no case where they have been positively misleading. I regret that we were not able to refer to all the relevant literature concerning isopropyl decomposition on V205/Ti02 catalysts.Our own observations show (see fig. 8 of our paper) that acetone constituted about 70% of the products at 220°C on all the V20s loadings used. The extent to which BrGnsted sites exist seems to depend critically on the pretreatment applied; some very discordant results have been reported for TiOz itself [see ref. (42) of our paper]. A precise comparison of our results with those of Professor Haber must take account of the effect of reaction temperature on selectivity as well as the thermal history of the catalysts employed. Prof. J. B. Moffat (University of Waterloo, Ontario, Canada) said: Professor Bond has provided us with some interesting information on the interactions of transition-metal102 General Discussion 8 20 I 965 8 500 1000 wavenumber/cm-' Fig.2. oxides, including MOO, on various supports, including Si02 , and has noted the evidence for a two-dimensional monolayer of 0x0 or hydroxyoxo species with structures and properties different from those for the unsupported oxides. It seems relevant, to me at least, to show you some of our recently published results' for MOO, on Si02, particularly since they provide evidence for the formation of a heteropoly oxometalate, namely 12-molybdosilicic acid, which continues to be of catalytic interest in our laboratory for methane conversion.2 The laser Raman spectra (LRS) of bulk 12-molybdosilicic acid ( H4SiMo,2040, abbreviated to HSiMo) shows characteristic bands at 995, 969, 911, 636 and 252 cm-' [fig. 2(a)]. A silica-supported HSiMo catalyst has a spectrum [fig.2(b)] which shows the same bands and confirms the presence of HSiMo after calcination at 500°C. The spectra of MOO, deposited on silica at a pH of 2,7,11 [fig. 2( c ) , ( d ) , and (e), respectively] display the bands characteristic of HSiMo in addition to that at 495 cm-' attributed to the silica support. A band at 960cm-' provides evidence for the presence of the he~tamolybdate.~-' 1.r. and 1.r. spectra of solutions resulting from washing the Mo03/Si02 catalysts with acetonitrile confirm the presence of HSiMo species on the surface of the s u ~ p o r t . ~ 1 S. Kasztelan, E. Payen and J . B. Moffat, J. Catal., 1988, 112, 320. 2 S. Kasztelan and J. B. Moffat, J. Catal., 1989, 116, 82, and references contained therein. 3 E. Payen, S. Kasztelan, J. Grimblot and J.P. Bonnelle, J. Raman Spectrosc., 1986, 17, 233. 4 H. Jeziorowsky and H. Knozinger, J. Phys. Chem., 1979, 83, 1166. 5 L. Wang and W. K. Hall, J. Catal., 1982, 77, 232. 6 J. Leyrer, B. Vielhaber, M. I . Zaki, Z . Shuxian, J . Weitkamp and H. Knozinger, Mater. Chem. Phys., 7 E. Payen, S. Kasztelan, J. Grimblot and J . P. Bonnelle, Polyhedron, 1986, 5, 157. 1985, 13, 301. Prof. Bond responded: Dr Moffat's suggestion that species resembling, if not identical to, 12-molybdosilicic acid are formed in the MoO3/SiO2 system is a most interestingGeneral Discussion 103 one, and opens up the possibility of the formation of analogous species with other systems such as V205/Si02. As we stated in our paper, it is difficult to form oxide monolayers on SiO,; a contributing factor may well be the greater stability of a heteropoly oxometallate phase. Prof.T. J. Pinnavaia (Michigan State University, East Lansuing, U.S.A.) began the discussion of the paper by Prof. Cheetham as follows: Lateral and transverse layer distortion can play important roles in the pillaring of 2 : 1 larger lattice silicate clays. For instance, in-plane rotation of the SiO, tetrahedra will occur in order to optimize keying of the pillar into the ditrigonal cavities of the gallery surfaces. Also, transverse distortions can result in layer sagging and a reduction in gallery height as one moves laterally away from the pillar. What tetrahedral twist angle was found for the end-on orientation of anilinium vermiculite? How does the energy change with twist angle? Does your modelling allow for transverse distortion? Were the latter distortions indicated for anilinium vermiculite? Prof.A. K. Cheetham (University ofoxford) replied: We wish to stress that the clay calculations allow each atom in the layer to respond to the proximity of the anilinium ions (see section 4.2); both lateral and transverse distortions are automatically taken into account. We have not calculated the energy as a function of the twist angle because our objective was to converge to the minimum-energy structure. Prof. J. B.Nagy (Facultks Universitaires Notre Dame de la Paix, Namur, Belgium) asked: The polarizability plays an important role in intermolecular interaction; how are you going to introduce this into your calculations knowing that the polarizability for zeolites and, for example, SAPOs are different? It is also well known that the adsorbed molecules do modify the zeolitic structure.On the other hand, the layered compounds dimension is very much dependent on the amount and size of the adsorbed molecules. Did you introduce this possible variation or are these systems also considered as rigid? Prof. A. D. Buckingham ( Cambridge University) added: Prof. B.Nagy mentioned the possible importance of molecular polarizability in influencing the interaction of molecules with a zeolite crystal. The induction energy may be significant when electric fields are strong, as they are near ions. However, induction energy is non-additive so it cannot be represented adequately by a pairwise-additive potential. This can easily be appreciated by considering the interaction of a proton, H+, with an atom, A.The field, F, of the proton induces a dipole in A and causes an induction energy -+CUE'*, where cy is the polarizability of A. But in the symmetric trimer H'-A-HH' there is no field at A and no dipolar induction energy, although the two pairwise interactions of the protons with A remain unchanged at --+cyF*. Induction energy therefore requires incorporation of many-body effects into a simulation, and this is rarely done (polarization has been included in a few cases, e.g. by Barnes et al.'). A year ago in Durham, there was a Faraday Discussion on solvation, and in his introductory paper' Prof. H. C. Friedman drew attention to computations of Pettitt and Rossky3 showing an apparent attractive force between two CI- ions in aqueous solution, and suggested that there may be hydrogen-bond bridges of Prof.P. Suppan, Prof. B. E. Conway and I independently the type communicated comments"104 Genera 1 Discuss ion drawing attention to the possible importance of the induction energy in providing an attractive force between the two anions or two cations. What has this to do with catalysis? We have read in the press, though not yet in the scientific literature, of the possibility of fusion of deuterons at room temperature in a palladium cathode. The Coulomb repulsion of two D+ ions, or of D+ and H+ would need to be substantially reduced by an attractive force bringing them closer together, thereby enhancing the tunnelling to nuclear fusion.Induction energy may provide a catalytic influence, the polarizable material being the metallic environment of the pair of deuterons. 1 P. Barnes, J. L. Finney, J. D. Nicholas and J. E. Quinn, Nature (London) 1979, 282, 459. 2 H. L. Friedman, Faraday Discuss. Chem. SOC., 1988, 85, 1. 3 B. M. Pettitt and P. J. Rossky, J. Chem. Phys., 1986, 84, 5836. 4 Faraday Discuss. Chem SOC., 1988, 85, 78, 79 and 83. Prof. Cheetham replied to Prof. B.Nagy’s question: The only significant polarisation term that is not taken into account in the zeolite calculations is the charge-polarisation interaction between the exchangeable cations and the guest molecules. This term is particularly difficult to incorporate because it cannot be estimated correctly with a two-body atom-atom potential (comment by Prof.Buckinghan). On the other hand, the excellent agreement that we obtain between calculated and experimental quantities suggests that the semi-empirical parameterisation of eqn ( 1 ) is largely compensating for the neglect of this term. We have not yet carried out any calculations on ALPOs or SAPOs, but we would certainly need to determine a new set of semi-empirical parameters in order to allow for the different polarisabilties. With respect to the clay calculations, the structures are not held rigid but are permitted to relax in response to the presence of the guest species (see section 4.2). Dr G. J. Hutchings (Liverpool University) (communicated): The possibility of using the computational approach to aid the elucidation of reaction mechanisms would appear to be exciting.In the paper the work of Vitrivel et al.”’ is discussed with respect to the mechanism of methanol conversion to hydrocarbons. The mechanism, arrived at using a quantum-mechanical approach, involves the donation of H- from gas-phase methanol yielding a gas-phase CH20H reactive intermediate. From an experimental viewpoint, most evidence”‘ now indicates that interaction of methanol with the Br@nsted acid sites of the zeolite yields CH: OHz which then methylates the surface of the zeolite to form a surface methyloxonium intermediate. CH3 I -0 0 0- \ -/+\ / -0-A1 Si-0- \ 0- / -0 A number of pathways have been cited3-‘ for the subsequent reaction of this surface-bonded reaction intermediate. It is interesting to note that one of these3 involves the donation of H- from gas-phase methanol to the methyloxonium intermediate to form methane H-CH2-OH CH,+ [CH’OH] J CH3 I - CO SAl- / + \si’ - SAl--O- S i gGeneral Discussion 105 This would appear to be very similar to the proposal of Vitrivel et uZ.'~' Is it possible to comment on how these computational methods may be extended to incorporate surface-bound reaction intermediates as well as gas-phase reactant molecules? 1 R.Vitrivel, C. R. A. Catlow and E. A. Colbourn, Proc. R. Soc. London, Ser. A, 1988, 417, 81. 2 R. Vitrivel, C. R. A. Catlow and E. A. Colbourn, J. Phys. Chem., submitted. 3 G . J . Hutchings, M. V. M. Hall, F. Gottschalk and R. Hunter, J. Chem. SOC, Faraday Trans. 1, 1987, 4 G. J. Hutchings, L. Jansen van Rensburg, W.Pick1 and R. Hunter, J. Chem. Soc., Faraday Trans. I , 5 T. R. Forrester and R. F. Howe, J. Am. Chem. SOC., 1987, 109, 5076. 6 C. D. Chang, Stud. Surf: Scz. Catal., 1988, 36, 127. 83, 571. 1988, 84, 1311. Prof. Cheetham replied: The calculations described by Vitrivel et al. represent one of the first quantum-mechanical treatments of reactivity within a zeolite cavity. The results are at variance with the generally accepted mechanism for this reaction, but it is interesting that at least one step in the reaction may involve H-atom donation. Future strategies will be more complex and will probably include a more sophisticated treatment of the host structure together with a quantum-dynamical approach to the interaction between the reacting molecule and the active site, as described in section 5.Prof. Moffat then asked: As is well known, in the application of theoretical techniques to fluid-solid systems, where the surface is simulated by a cluster, the number of atoms necessary to represent the surface is usually unknown; would you comment on the evidence which you may have for the adequacy of your representation? To which Prof. Cheetham replied: The evidence that the size of the cluster in our zeolite calculations is adequate is very strong. First, we have carried out calculations as a function of the cluster size and confirmed that our model, which typically has a cut-off radius of 12 A, has converged. Secondly, the excellent agreement between calculated and experimental quantities corroborates the validity of our approach. The clay calculations involve long-range electrostatic terms and have been carried out in reciprocal space by the Ewald method. Dr J. C. Vedrine said: In your calculations you have mainly presented a van der Waals type approach for zeolitic matrices and adsorbates. This is interesting. However, if you want to extend, as you said, to chemistry you have to introduce A1 and then acidic sites which play an important role in adsorption of such molecules as aromatics, olefins, pyridine and even alkanes. How do you introduce chemistry into your model and how many atoms (or unit cells) do you consider in your calculations? Prof. van Santen ( University of Technology, Eindhoven, The Netherlands) supplied the final question on this paper. Calculational results concerning the selective stability of the hypothetical DF and the ECR-1A silica zeolites are mentioned in this paper. The DF structure is found to be stabilized by 16 eV. The computations used are based on the use of full formal charges on lattice cations. This implies that the electrostatic Coulomb potential contributes significantly to the total cohesive energies. It would be of interest to know the relative densities of the two structures studied, since dominance of the electrostatic contributions implies the more dense structure to be the most stable. Do the authors believe that the predictions of relative stabilities remain the same, if non-full formal charges on the lattice cations are used? Prof. Cheetham aqswered: The DF structure (57.54 A3/Si02) is indeed more dense than ECR-1A (58.15 A3/Si02). However, the use of full, formal charges should not be taken literally because the electrostatic force-field has been adjusted in order that it will reproduce the observed properties of known materials such as SiOz and A1203. The difference in energy between the DF and ECR-1A structures is certainly small and we should not forget that any entropic differences are not taken into account.