Furuduy Discuss. Chern. Soc., 1989, 87, 161-172 GENERAL DISCUSSION Dr R. Burch (University of Reading) began the discussion of the paper by Waller et al.: In the very last sentence of your paper you comment that the activity (of Cu catalysts) for reverse shift is a better guide to methanol synthesis activity than the N,O-determined copper area. I should like to add that in our experience the N,O-determined copper area is also a poor guide to the activity of supported copper catalysts for methanol synthesis from CO-C02-H2 mixtures. We have recently reported’ a strong support effect when Cu/ZnO catalysts are compared with Cu/Si02 catalysts. Furthermore, we have observed2 that there is a synergy between Cu and ZnO even when they are physically separated. We have concluded that ZnO may play a much more active role in the synthesis of methanol than has generally been realised.Do you consider that a similar active participation of ZnO in the reverse shift reaction could account for your observa- tions? 1 R. Burch and R. J. Chappell, Appl. Catal., 1988, 45, 131. 2 R. Burch, R. J. Chappell and S. E. Golunski, Catal. Lett., 1988 1, 439. Prof. F. S. Stone (University of Bath) replied: Our results certainly do not suggest that N,O-determined copper area is a poor guide to methanol synthesis activity. It is more a question of relatively small uncertainties among the various ways of carrying out the N 2 0 experiment. We have no evidence to suggest that active participation of ZnO can account for our observations of the beneficial effects of ageing or the slight decline in activity for the catalyst derived from the 205 min aged precursor indicated in fig.7-9. Dr K. C. Waugh ( I . C. I. C&P Group, Billingham, Cleveland) (communicated): I should like to make some observations on Dr Burch’s comment of Prof. Stone’s elegant paper. Dr Burch reported in his comments that the methanol synthesis activity of a physical mixture of silica-supported copper and silica-supported zinc oxide was greater than that of the individual components when tested alone. The result is as intriguing as it is difficult to explain. However, I believe that Dr Burch’s rationale that it betokens some unique synergy, albeit at a distance, between copper and zinc oxide, is both premature and fallacious. The main reason for declaring it to be premature is that Dr Burch has looked at Cu/Si02, ZnO/Si02 mixtures only.In the absence of his having tested Cu/Si02 physically mixed with MnO/ S O z or A1203 or MgO/ SO, etc. he cannot logically conclude as to a ‘unique’ synergy. He has countered the suggestion that there might be some migration of the zinc oxide to the copper during the reaction by separating the Cu/Si02 after the reaction and showing its post- and pre-reaction activities to be the same within experimental error. This is exactly as I would have thought since the suggestion pre-supposes some form of synergy between the copper and the zinc oxide which we and other groups have shown not to exist.’-3 It was the culminating conclusion of a vast body of research in I.C.I. that the methanol synthesis activity was a linear function of the copper metal area and that this conclusion held regardless of the nature of the support or lack of it,’ fig.1. We should not and cannot abandon this work when faced with Dr Burch’s interesting result. Rather this information should be used as a basis of explanation. Taking it as read that the methanol synthesis activity is a function of the copper metal area the only explanation of Dr Burch’s work is in a re-distribution of the copper 161162 General Discussion 12 .o 1 1 .o 10.0 9.0 8 .O 7.0 6 .O 5 .o 4 .o 3 .o 2 .o 1 .o 10 20 30 40 Cu-metal area/m2 g-’ Fig. 1. Methanol synthesis activity as a function of copper-metal area. a, CuO/ZnO/A1,0, (60: 30: 10); +, CuO/ZnO/AI20, (45: 37: 18); 0, CuO/SiO,; A, CuO/A1203; A, CuO/MgO; 0, CuO/MnO; H, CuO/ZnO.over the surface of the ZnO/Si02 during the course of the reaction; post-reaction examination of this material was not done. However, even if it had been done the result might well have been misleading, since in order to separate the Cu/SiO, from the ZnO/ Si02, it would have been necessary first to passivate both materials by controlled oxidation, a process which could induce sintering in the newly formed Cu/ZnO/SiO,. Necessarily, therefore, the experiment which should have been carried out, is the in situ measurement of the post-reaction copper-metal area by whatever means. The absence of such a measurement allows us endless speculation. 1 K. C. Waugh, Appl. Catal., 1988, 43, 315. 2 M. Bowker, R. A. Hadden, H. Houghton, J.N. K. Hyland and K. C . Waugh, J. Catal., 1988, 109, 263. 3 W. X . Pan, R. Cao, D. L. Robberts and G. L. Griffin, J. Catal., 1988, 114, 440. Dr R. Burch (University of Reading) responded: In reply to Dr Waugh’s comments we should like to make the following remarks. ( a ) We have not used the expression ‘unique’ synergy. Indeed it would be foolish to do so since we have data showing that, for example, Ga,O, also promotes Cu catalysts for methanol synthesis. ( b ) The question raised concerning the transfer of Cu to ZnO/Si02 catalysts can be answered easily. Post-reaction chemical analysis of the ZnO/ Si02 particles failed to detect any Cu on these particles. Therefore, it is not appropriate to suggest, as Dr Waugh has done, that ‘the only explanation’ of our results is ‘a re-distribution of the Cu over the surface of the ZnO/SiO,’.Indeed, our analytical measurements indicate that this is the least likely explanation of our results. ( c ) Dr Waugh comments that they and other groups have shown that synergy between Cu and ZnO does not exist. Currently, this appears to be a minority view since several groups’-5 have independently reported support effects with Cu catalysts for methanol synthesis. Furthermore, the work on Cu/rare earth intermetallics”’ clearly shows no correlation between Cu surface area and methanol activity. 1 B. Denise, R. P. A. Sneeden, B. Beguin and 0. Cherifi, Appl. Catal., 1989, 30, 353. 2 W. R. A. M. Robinson and J. C. Mol, Appl. Catal., 1988, 44, 165.General Discussion 163 3 J. C . Frost, Nature (London), 1988, 577.4 M. S. W. Vong, M. A. Yates, P. Reyes, A. Perryman and P. A. Sermon, 9th Int. Congr, Catal., Calgary, 5 J. Barbier, Th. Fortin, Ph. Courty and P. Chaumette, Bull. Chim. SOC. France, 1987, 925. 6 G. Owen, C. M. Hawkes, D. Lloyd, J. R. Jennings, R. M. Lambert and R. M. Nix, Appl. Catal., 1987, 7 R. M. Nix, T. Rayment, R. M. Lambert, J. R. Jennings and G. Owen, J. Catal., 1987, 106, 216. 1988, p. 545. 33, 405. Dr R. B. Moyes (University of Hull) said: According to fig. 7 the activity for the test reaction reaches a maximum with ageing time in the mother liquor of the precursor to the catalyst. I should like Prof. Stone to speculate on the structural possibilities which might explain this result. Prof. Stone answered: The ageing process in the mother liquor will not involve a change in overall Cu/Zn ratio of the malachite once all the aurichalcite has disappeared.However, it will lead to a more uniform distribution of the Cu” and Zn2+ ions in individual crystallites, and I see that as advantageous for achieving high Cu dispersion in Cu/ZnO after decomposition and reduction. Extended ageing will presumably lead to ripening in the zincian malachite suspension. The latter is likely to be deleterious for surface area in the final catalyst, and there is evidence for this in the measured areas reported in the paper. I ascribe the maximum in the activity of the catalyst with precursor ageing time to these effects. Prof. J. B. Moffat ( University of Waterloo, Ontario, Canada) then said: I believe that you have put your finger on an aspect of catalyst preparation which is too infrequently studied.In your work on the effects of precursor ageing, you observed, at least during the first 30 min of the process, the production of more finely divided solid. My belief is that d G / d A cannot be negative. Would you care to comment on this from a surface chemistry point of view? In other words, what is the driving force for a reduction in particle size? Prof. Stone replied: Prof. Moffat asked about the paradox of a decreased particle size on ageing. The driving force for this is the change in chemical composition of the malachite phase on ageing, the increased zinc content leading to a more stable structure, whose small crystallites form rapidly. Prof. M. Ichikawa (Hokkaido University, Sapporo, Japan) commented: It is important to choose the catalyst precursors for the unique metal centres of CuO-ZnO in methanol synthesis as reported by Prof.Stone et al. However, I would like to concentrate much more on the further structural changes of the catalysts upon admission of the reactants such as CO + H2 and how to reach the static states of the catalysts in the working conditions, resulting in the production of the new metal centres involving the methanol synthesis. According to the previous work by Ueno and co-workers’ by EXAFS evaluation for Cu/ZnO catalysts, the size and morphology of Cu clusters on ZnO are essentially affected (changeable) by the presence or absence of CO and/or H2. Thus, I wonder whether the well characterised metal centres on your ZnO-CuO catalysts prepared from some particular precursors will retain their characteristic structures and chemical compositions under the working conditions of syngas.1 K. Tohji, Y. Udagawa, T. Mizushima and A. Ueno, J. Phjx Chem., 1985, 89, 5671. Prof. Stone added: Prof. Ichikawa is quite right to stress that changes in the size and morphology of the copper crystallites in the Cu/ZnO catalyst under reaction164 General Discussion 20 LO 60 80 100 0 t/min Fig. 2. Activity of 8% Cu/ZnO in CH30H decomposition at 429 K (a) and CO/ H2 at 523 K (b) [with X-ray diffraction patterns before ( c ) and after (d) use] showing catalysis-induced appearance of Cuo(lll) at 42.5" 28 (shaded). conditions need to be borne in mind. On long-term usage there is a decrease in methanol synthesis activity.However, the precursor ageing effect still shows through, even after those considerations have been applied. Both factors are therefore important. Dr P. A. Sermon (Brunel University) (communicated): (i) We have observed with Cu/ZnO that activity for methanol synthesis (and decomposition) changes with time on stream (fig. 2), producing additional Cuo (fig. 3). (ii) Is there a clear relationship between activities in C02/H2 at 1 bar, and CO/C02/H2 at 50 bar, confirming the pivotal role of C02? (iii) In your fig. 1 what percentage of your catalysts is sufficiently crystalline to be X-ray detectable ? Prof. Stone replied: With regard to the first point raised by Dr Sermon, whilst this increase in Cuo may occur in the absence of C 0 2 , it is very unlikely that with C 0 2 present in the feed there is an increase in the amount of Cuo with time.This is not in any sense to deny that CO may not be a better reductant for copper oxide than H2. On Dr Sermon's second point, I would argue that there is indeed a clear relationship in that there is a good correlation between the results in fig. 6 and 7 for the catalysts derived from precursors aged up to 140min.General Discussion 165 01 I I I I I I I 30 60 90 120 150 180 time/min Fig. 3. Reduction of CuO at 423 K with 6 kPa H2 (0) and CO (0). Clearly CO is a faster and more effective reductant for CuO to Cu (22.5% CuO remaining) than H2 (32.5% CuO remaining). As to the proportion of our catalysts which are X-ray detectable, the EM results [our plate l ( b ) ] imply that all the material in the unaged precursor is sufficiently crystalline to be seen by X-rays.There is evidence, however, from the intensities of the malachite pattern that some of the material in the samples aged for 30min may be undetectable by X.r.d. There is also some indication for this from the particularly high SA obtained after calcination of this precursor (our table 1). Prof. V. Ponec (Gorlaeus Laboratory, Leiden University, The Netherlands) made the first comment on the paper by Lambert et al: My comment concerns fig. 1B. It looks here as if Cu were already present but not yet active and needed some induction period to be activated to start to produce CH30H. CH, is produced first and this means H 2 0 is (probably) produced too.H 2 0 can convert Cu to CU"+ which is then stabilised by Nd20+,, the oxide. Moreover, one can imagine, also, other ways by which Gun+ can be formed to become the active centre of CH30H synthesis. My question is, what were the reasons (not mentioned in your paper) which led you to ignore the last possibility completely? Drs R. M. Nix and R. M. Lambert ( University of Cambridge) replied: The 'induction period' prior to onset of methanol synthesis is the period during which extensive solid-state transformations are occurring. Specifically, the conversion of the intermetallic precursor into an intimate intergrowth of elemental copper and rare-earth oxide crystal- lites. This activation process proceeds via the incorporation of hydrogen into the lattice of the alloy to yield both ternary alloy hydrides and, by a hydrogenolysis process, also the binary rare-earth hydride (the exact mechanisms and proportions of hydride forma- tion being very dependent upon the intermetallic precursor concerned).This behaviour is well illustrated by the representative in situ X.r.d. spectra shown in fig. 1A. Further- more, it has been shown that there is a direct correlation between the rise in synthesis activity and the concurrent growth of the (X.r.d.-visible) copper and rare-earth oxide phases.'166 General Discussion Methane is a by-product of the activation process and not of a catalytic reaction. It is generated during the oxidation of the various metal/alloy hydrides by reactions such as 2NdCu - H, + 3CO -+ 3CH,+ 2Cu + Nd203.Generation of water is not therefore a significant factor and, indeed, the use of water vapour as the oxidizing media for alloy activation is detrimental to the ultimate catalytic activity; the favoured activation process is an in situ activation in a syngas feed following low-temperature pretreatment in pure hydrogen. A wide range of results points to the activity of these materials being associated with very highly dispersed copper in the poorly crystalline rare earth oxide matrix (such copper is evident at levels up to 25 at.'% by EDAX)., This copper could, in principle, be present as individual atoms or ions (i.e. CU"+) but at the levels concerned it is more likely to be present as small ( < l o A) clusters; this proposition is supported by EXAFS data on Cu/Th02 catalysts.' 1 R.M. Nix, T. Rayment, R. M. Lambert, J. R. Jennings and G. Owen, J. Catal., 1987, 106, 216. 2 G. Owen, C. M. Hawkes, D. Lloyd, J. R. Jennings, R. M. Lambert and R. M. Nix, Appl. Catal., 1987, 33, 405. 3 J. C. Frost, Nature (London), 1988, 334, 577. Prof. A. Baiker (ETH Zurich, Switzerland) said: I have two questions, the first concerns the deactivation of the Nd/Cu derived catalyst in the presence of CO, shown in fig. 2B). I am wondering whether the authors have an explanation for this behaviour. It may be interesting to mention that we have not observed a similar deactivation due to the presence of CO, with catalysts prepared by in-situ activation from amorphous Cu7Zr3 alloys.' Do you have any suggestion for the different behaviour of these catalysts? The second question concerns the occurrence of segregation of the constituents during the transformation of the alloy to the final catalyst, e.g.during the oxidative decomposition of the alloy. Segregational phenomena have been reported for a number of amorphous alloys (see paper 15 in this series). Do you observe segregation, and if so, what are its consequences on the preparation of the alloy-derived catalyst? 1 D. Gasser and A. Baiker, Appl. Catal., 1989, 48, 279. Drs R. M. Nix and R. M. Lambert responded: The deactivation of the NdCu-derived catalyst upon exposure to process gas containing CO, is a feature that is common to all the RE/Cu alloy-derived catalysts'.' and also to Cu/ThO, catalysts3 The extent of deactivation and also the degree of recovery after removal of CO, from the feed varies significantly: from complete deactivation with zero recovery (e.g.Ce/Cu catalysts) to only partial deactivation and complete recovery (e.g. Cu/Th02 catalysts). In the case of the highly basic rare-earth oxide containing catalysts, isotopic labelling experiments indicate the formation of a surface carbonate and the extent of deactivation/recovery of NdCu and CeCu catalysts has been associated with the decomposition temperatures of this ~ a r b o n a t e . ~ If this reasoning can be extrapolated to catalysts obtained from amorphous Zr/Cu alloys, then the different behaviour may arise from the significantly weaker interaction of CO, with ZrO,. In this context it is interesting to note that recent work of colleagues of ours at I.C.I.has shown that catalysts obtained from ternary RE/Cu/Zr alloys are significantly less susceptible to C 0 2 poisoning than the binary RE/ Cu catalysts but can exhibit comparable activities.''5 The concept of surface segregation is well defined when looking at near-surface transformations (such as the oxidation of amorphous alloy ribbons) and, indeed, we have observed apparent surface segregation of NdO, during oxidation of ultra-thin alloy films on copper single-crystal substrates. In the case of the bulk intermetallic compounds, however, the solid is largely transformed into a microporous, intergrowth of crystallites and it is not clear to us exactly what meaning should be attached to 'surface segregation' in such systems.General Discussion 167 1 J.R. Jennings, R. M. Lambert, R. M. Nix, G. Owen and D. G. Parker, Appl. Caral., 1989, 50, 157. 2 G. Owen, C. M. Hawkes, D. Lloyd, J. R. Jennings, R. M. Lambert and R. M. Nix, Appl. Card. 1987, 3 J. C. Frost, Nature (London), 1988, 334, 577. 4 R. M. Nix, R. W. Judd, R. M. Lambert, J. R. Jennings and G. Owen, J. Catal., 1989, 118, 175. 5 G. Owen, C. M. Hawkes, D. Lloyd, J. R. Jennings, R. M. Lambert and R. M. Nix, Appl. Card., in press. 33, 405. Prof. J. Cunningham (University College Cork, Republic of Ireland) addressed the authors: My question to Drs Badyal and Nix seeks clarification concerning the suggestion on pp. 123-124 that 'a substantial amount of copper is present in another form, specifically a form that is undetected by both HREM and XRD, and also inert or inaccessible to N 2 0 titration'.Not only is that suggestion reminiscent of references to 'missing-copper' made by Herman et al. in their studies of Cu/ZnO catalysts,' but also it echoes reservations expressed in the paper by Waller et al. concerning the adequacy of N20- determined copper area. Against this background it would be helpful to have clarification from Drs Badyal and Nix as to (i) whether they consider the missing copper in the system to take the form of small particles dispersed within the oxide phase and (ii) whether they associate the N,O-measured copper surface area wholly or in part with that fraction of the particles whose surfaces partially obtrude through the oxide surface. 1 R. G. Herman, K. Klier, G. W. Simmons, B. P. Finn and H. B.Bulko, J. Card., 1978, 56, 407. Drs R. M. Lambert and R. M. Nix (University of Cambridge) replied: As indicated in our response to Prof. Ponec, we believe the excess copper ( i e . that not present in the form of the larger Cu crystallites visible by X.r.d.) to exist in the form of small copper clusters entrained in the rare-earth metal oxide matrix. Indeed, some of the highest levels of activity were exhibited by catalysts derived from NdCu alloys (exten- sively pretreated in pure hydrogen at < 100 "C) which contained no X.r.d.-visible copper particles. The results of N20 titrations on a range of alloy-derived catalysts have been described elsewhere:' the values obtained are consistent with the X.r.d.-visible copper representing only a fraction of the total amount present but also require that the excess copper is non-titratable under standard conditions (60 "C).This either could be due to almost complete encapsulation of the small clusters under reaction conditions or might arise from electronic modification of the redox properties of the small clusters as a result of coordination to the oxide matrix. Certainly, the extent to which the synthesis mechanism is associated with the rare-earth oxide surface as opposed to the small copper clusters has still to be resolved. 1 R. M. Nix, R. W. Judd, R. M. Lambert, J. R. Jennings and G. Owen, J. Card., 1989, 118, 175. Prof. M. W. Roberts ( University of Wales College of Cardin (communicated ): Could you comment on the origin of the Nd 3d5,, peak that develops at low binding energy (977 eV) on exposure of the overlayers to oxygen even at low (1.5 L) exposure.What was the O(1s) binding energy and how did this vary with oxygen coverage? Does the stoichiometry, NdO, where x = 1 .O, imply that the fast initial oxygen interaction occurs throughout the bulk of a Nd overlayer even when this is many layers thick? How was x estimated? Was it related to the calculated concentration of the higher oxidation state of neodymium, Nd2+, and the latter estimated from the Nd 3d5,, spectrum (your fig. 3)? Drs R. M. Lambert and R. M. Nix replied: The low-energy satellite peak in the Nd 3d5,, spectra at the higher O2 exposures has an energy shift and relative intensity characteristic of Nd3': such satellites are observed in many of the 3d spectra of the light rare-earth metal elements and arise from final-state configuration interaction.In neodymium metal itself the main peak corresponds to a poorly screened 4f" final state and there is negligible intensity in the well screened f"" satellite.168 General Discussion For the five-monolayer Nd film the 01s peak initially grew at constant binding energy (ca. 529.25 eV), but a small shift was observed concurrent with the appearance of the Nd satellite, ultimately to give a peak at 528.7 eV (similar behaviour was observed for Nd films of different thickness). The oxygen stoichiometry at various stages in the oxidation process can be estimated by comparison with the 0 1 s signal intensity at saturation exposure. Using this method an overall stoichiometry of NdO,-l is arrived at after 2.5 L 02.Some degree of oxygen concentration gradient through the film might be expected but all the experimental evidence suggests that this is small during the fast initial oxygen interaction with this and all other ultra-thin films ( < 5 monolayers of Nd). Prof. Roberts made the further comment in response to the authors’ reply: The Nd(3d5,,) spectra for the interaction of a 5 ML Nd film with oxygen (fig. 3A) show evidence for the presence of possibly three different states of Nd in the early stages of oxidation (after 0.6 L and 1.5 L exposure): Nd’, Nd3+ (as evidenced by intensity at 978 eV, the low energy satellite from Nd203) and a species characterised by a binding energy of 983 eV, possibly Nd2+. Under such conditions of inhomogeneity, estimating the stoichiometry of the oxidised film by comparing the intensity of the O( 1s) peak with that at saturation (presumably taken to correspond to Nd203) is not appropriate. My question was therefore to ascertain what quantitative procedure of Nd( 3d5,,) spectra analysis was followed to establish that the stoichiometry of the metal oxide overlayer corresponds to NdO, with x = 1.0.Prof. A. Zecchina (Turin University, Italy) said: The presence of TiO, islands is supposed to lead electronic charge transfer to neighbouring ruthenium atoms, which therefore bond to CO with stronger energy. Could you be more detailed on this point which is crucial for understanding the SMSI effect? In particular, why does electronic charge flow from TiO, to Ru and not vice versa? Is it a problem of semiconductor-metal junction? Is this the only way to explain the strengthening of the Me-CO bond? Drs R.M. Lambert and J. P. S. Badyal replied: Titanium deposited in an ambient atmosphere of oxygen at a pressure of 1 x lop6 Torr results in the laying down of an ultra-thin TiO, film where ‘as deposited’ stoichiometry corresponds to x = 2 at monolayer completion. X.P.S. measurements yield a value of 459.2eV for the T i ( 2 ~ , , ~ ) binding energy: this corresponds to Ti02. Annealing of TiOz films to temperatures characteristic of SMSI behaviour results in a 3.2 eV decrease in binding energy, which is consistent with the transformation TiOz --* TiO. This is further supported by LEED and Auger measurements.’ In the case of TiO? submonolayer quantities, we observe simple site-blocking on CO chemisorption; in the case of TiO, the presence of reduced titanium ions may be expected to lead to charge transfer to the neighbouring ruthenium metal with a comcomitant increase in CO binding energy, consistent with our observations.1 J . P. S. Badyal, A. J. Gellman, R. W. Judd and R. M. Lambert, Cural. Leu. 1988, 1, 41. Prof. J. B. Moffat then remarked: Miyazaki’ has carried out BEBO calculations of the interaction of a number of diatomic molecules including CO on various one- component metals and has predicted that a molecular state should exist with CO and all metal surfaces although the depth of the energy well, not surprisingly, varies with the metal. In all cases, an activation energy was predicted to separate the molecular state from the totally dissociated state.You have shown that on neodymium/coqper intermetallic compounds oxidation of the rare-earth component proceeds by dissociative chemisorption of CO and that at low Nd coverages and low temperature (<200 K) molecular chemisorption on exposed copper was also evident.Genera 1 Discussion 169 Do you inevitably see the molecular state and is this a necessary precursor to the dissociated state? How, if at all, is the observation of the molecular state related to the catalytic process? In connection with the titanium/ruthenium studies and your comment regarding hydrogen spillover do you have information on how the Ru-H energy compares with that of Ti-H? 1 E. Miyazaki, J. Caral., 1980, 65, 84.Dr Lambert, Dr Badyal and Dr Nix replied: The molecular CO state observed at low temperatures and low Nd coverages in the TPD studies on the model systems is associated with chemisorption on exposed copper surface; the binding characteristics of this species are not significantly perturbed by the presence of pre-adsorbed neodymium (which is itself oxidized by dissociative adsorption during the initial stages of exposure). At initial Nd coverages greater than a monolayer, however, (i.e. when there is no exposed copper present) CO is still rapidly and dissociatively adsorbed by the neodymium at 300 K and there is no evidence for a molecular precursor state. Further molecular adsorption on the oxidized neodymium was not observed under high vacuum conditions at either 77 or 300 K but will certainly be an important feature of the high-pressure catalytic chemistry.We have observed two TPD features on exposing H2 to Ti/Ru(0001).' The low- temperature feature due to atomic hydrogen associated with the bare Ru(0001) patches showed an initial increase [our fig. 4(6)] due to hydrogen spillover from the TiH, islands; these TiH, species give rise to the appearance of a sharp hydrogen desorption feature at high temperature which increases in intensity with titanium precoverage. 1 J. P. S. Badyal, A. J. Gellman and R. M. Lambert, J. Catal., 1988, 111, 383. Dr A. R. Gonzalez-Elipe (Instituto de Ciencias de Materiales de Sevilla, Seville, Spain) said: In relation to the formation of hydride species in your Ti/Ru and TiO,/Ru systems I would like to mention that in previous work using i.r., H'-n.m.r., e.p.r., TPR and X.P.S." on M/Ti02 catalysts (where M: Rh, Pt or Ni) we have postulated the formation of such species in 'real catalysts' through 'spillover' of hydrogen atoms from the metal to the reduced TiOz support according to the reaction: TiVi'+ Rhs-H -+ (Ti-H)3++ Rh,.In addition we have presented evidence that such species could produce (i) an additional suppression of the H2 and CO adsorption in the SMSI ~ t a t e , ~ - ~ (ii) an enhanced mobility of the reduced TiO, support leading to 'decoration' of the metallic particles6 and (iii) the reduction of CO to give CH,OH.' Owing to the rather different experimental conditions used in your work with 'model systems' I would like to ask you a few questions. First, I would like to know what are the conditions to generate hydride species in your TiO, islands deposited on Ru(0001).Its formation is reported in your paper but you do not give details on this point. My second point is do you observe an additional suppression of CO adsorption in your TiO,/Ru system when hydride species are present? Finally, I would like to know your opinion on the role of such hydride species in hydrogenation or hydrogenolysis reactions occurring on these M/Ti02 systems. 1 J. C. Conesa, P. Malet, G. Munuera, J. Sanz and J. Soria, J. Phys. Chem., 1984, 88, 2986. 2 J. Sam, J. M. Rojo, P. Malet, G. Munuera, M. T. Blasco, J. C. Conesa and J. Soria, J. Phys. Chem., 3 A. R. GonzLlez-Elipe, G. Munuera, J. P. Espinos, J. Soria, J.C. Conesa and J. Sanz, Proc. 9th Inr. Cotig. 4 J. Sanz and J. M. Rojo, J. Phys. Chem., 1985, 89, 4974. 5 A. Munoz, A. R. GonzLlez-Elipe, G. Munuera, J. P. Espinos and V. Rives Arnau, Specrrochim. Acra, 1985, 89, 5427. Catal., Canada 1988, ed. M. J. Phillips and M. Terman, vol. 3, p. 1392 Part A, 1987,43, 1599.170 General Discussion 6 G. Munuera, A. R. Gonzalez-Elipe, J. P. Espinos, J. C. Conesa, J . Soria and J. Sanz, J. Phys. Chem., 1987, 91, 6625. Drs Badyal and Lambert replied: We do not claim to have generated a hydride species on TiO, phase on Ru(0001); however, we were able to generate a Ti-H species following hydrogen exposure to Ti/ Ru(0001).' This species is extremely efficient in CO dissoci- ation and we suggest that such strongly bound hydrogen species may be responsible for the enhanced catalytic behaviour of Ru/Ti02 catalyst which have been reduced at high temperature.1 J. P. S. Badyal, A. J. Gellman and R. M. Lambert, J. Catal., 1988, 111, 383. Prof. A. K. Datye (University of New Mexico, Albuquerque, U.S.A.) had the following question and comment: The influence of TiO, on adsorbed CO in fig. 6 ( a ) is shown to be more pronounced than a simple site-blocking effect. However, reduced TiO, species are known to wet metal surfaces and spread on them. Hence the question is: How was the T i 0 surface coverage measured and was it measured before or after the CO adsorption. Does adsorption and dissociation of CO lead to any restructuring of these oxide overlayers? Your results imply that the TiO,H,. species may be responsible for the altered metal behaviour in the SMSI state.However, the increase in CO hydrogenation activity is not more than a factor of 3 after high-temperature reduction. The major difference in activity say between Pt/Ti02 and Pt/ S O z is seen even before high-temperature reduction on the fresh catalyst. Drs Badyal and Lambert replied: As explained in our paper, the T i 0 surface coverage was measured by Auger spectroscopy, calibration being provided by the known growth morphology of this phase on Ru(0001).' No restructuring of the oxide phase appears to be induced by CO desorption or dissociation. Regarding the differences between Pt/Ti02 and Pt/Si02, we too have observed similar differences in behaviour for equivalent metal loadings in the case of Ru/Si02 and Ru/Ti02 before high-temperature reduction; our results indicate clearly that this initial difference is due to differences in metal dispersion.2 1 J.P. S. Badyal, A. J. Gellman, R. W. Judd and R. M. Lambert, Catal. Left., 1988, 1, 41. 2 J. P. S. Badyal, R. M. Lambert, J. C. Frost, C. Riley and K. Harrison, J. Catal., submitted. Prof. A. K. Cheetham (University of Oxford) had a question for Prof. K. I. Zamaraev (Institute of Catalysis, Novosibirsk, U.S.S.R.): The last slide of your talk, and the abstract of your paper, shows a three-step catalytic cycle that achieves the oxidation of SOz to SO3 without the involvement of VIv species. What is the evidence that step 3, which involves the loss of bridging SO, and the incorporation of 02, proceeds via a single reaction of Vv species rather than a two-stage process, for example - S O , 0 2 v v ___, VlV ___, vv Prof.Zamaraev replied: Note that the first stage of the two-stage process proposed by Prof. Cheetham actually corresponds to the upward direction of reaction 4 of scheme 1 from our paper. As indicated in our paper, the rate of oxidation with O2 of the VIv complexes formed in reaction 4 is much less than the rate of the overall catalytic reaction under steady-state conditions. This means that the catalytic reaction does not proceed via the stepwise mechanism consisting of alternating steps of Vv reduction with SO:- anion to V" and subsequent oxidation of V" back to Vv with Oz. Moreover, the rateGeneral Discussion 171 CO poisoning expt 40 0 1 2 3 CO adsorbed/ 10' mol Fig.4. Ethene polymerization rate over chromocene/silica catalyst as a function of amount of CO adsorbed. (Initial C2H, pressure = 100 Torr; temperature = 40 "C; (Cr = 4.5 wt%, CO/Cr = 0.33% .) of the catalytic reaction under both steady-state and non-steady-state conditions is proportional to the concentration of Vv and does not correlate with the amount of V" in the catalysts. This suggests that only the Vv species are involved in the catalytic cycle. Under these circumstances it seems more likely that step 3 of our three-step catalytic cycle proceeds uia a transfer of two electrons from SO:- ligand to O2 ligand in the coordination sphere of the binuclear V v complex r SO:- s0:- (see p. 13 of our paper) rather than via the two stage process -so, 0, vv ___* V'V L vv proposed by Prof.Cheetham. Prof. J. H. Lunsford (Texas A&M Uniuersily, U.S.A.) began the discussion of Prof. Zecchina's paper: Attempts to identify active surface species by spectroscopic techniques are often frustrated by the fact that only a small percentage of a potentially active phase is involved in the catalytic cycle. We have found that a chromocene-on-silica catalyst, prepared by the sublimation technique, is active for ethene polymerization, but the sites responsible for this activity are extensively poisoned by the addition of carbon monoxide as shown in fig. 4. These results suggest that <0.33'/0 of the Cr is active for the polymerization reaction. The addition of 0.023 Torr of CO to a Cp,Cr/Si02 wafer initially yielded weak infrared bands at 2004, 1970 and 1831 cm-'.The integrated area of the band at 2004 cm-' was only 0.36% of the total area observed for the carbonyl bands with excess CO in the cell. Thus, it appears that the infrared bands reported by172 General Discussion Zecchina et al. in their fig. 3 do not reflect the active chromium species, but by observing the sample following the addition of very small amounts of CO it may be possible to probe the active site. Prof. Zecchina replied: Your results are very interesting because they confirm that under low ethene pressure the active sites are very scarce. In our model the small number of active sites simply derive from the fact that they are located at the narrow boundary between the free and the supersaturated (self-poisoned) regions.In this respect it is most interesting to consider that 0-CrCp(CO), complexes derive not only from the SSi-0-Cr-Cp 'active centres', but also from the 2%-0-Cr-Cp . . . CpCrCp inactive centres (reaction 2). On this basis, the bands of fig. 3 are more useful for elucidating the basic chemistry of the surface species than for probing the active sites. However, they are also relevant for establishing the structure of the active centres because they show that the Cr(Cp), poisoning the sites can be displaced by incoming ligands (actually CO). There is no reason for not extending this concept to ethene, especially when it is used under high pressure (as in the industrial process). Dr A. F. Masters (University of Sydney, New South Wales, Australia) said: It is probably unnecessary to invoke an qs=q3 equilibrium of the cyclopentadienyl ring of the metallocycle in the mechanism of scheme 8, as discussed in the text.The q5 e 37 cyclopentadienyl equilibrium relieves electronic saturation. Your metallocycle is formally electronically unsaturated (14 e) and is easily able to interact with ethene from an electron-counting point of view. My first question relates to the identity of the intermediate (6) of scheme 5 , and hence to the related intermediates, ( a ) of scheme 5 , and those of scheme 3. It seems that the only evidence for intermediate (6) is the decrease in the intensity of the 0-H infrared absorption at 3748 cm-', and the assumption that this reduction in intensity is caused by the reaction of a surface OH group with CrCp,, generating dicyclopentadiene. Is there any other evidence for ( b ) ? For example, have silanols been reacted with chromocene in homogeneous solution? With regard to the infrared spectra of your fig. 3, were these and your other spectra obtained via transmission or reflectance? If your assignments are correct, the group of six bands between 1920 and 1579 cm-' would presumably arise from the interaction of CO with supported chromocene [cf, eqn (2)]. Do you have any suggestions as to the identity(ies) of the product(s)? What CO pressure was used for these reactions? Finally, what percentage of chromium-containing species in your silica matrix would you estimate as representing the active catalyst? Prof. Zecchina replied to each of these comments: The reaction of Cr(Cp), with silanols in solution with formation cyclopentadiene is well known from the papers of Karol et al. (quoted in the references). The i.r. spectra have been performed in transmission by using thin silica wafers. With regard to the two triplets in the 1900-1579 cm-' range, formed under 40 Torr of CO at room temperature, we can only mention that they have carbonyl frequencies very unusual for Cr"-Cr" complexes. Negatively charged multicarbonyl species have to be invoked. Their detailed structure is, however, unknown: further research is needed. The active sites for ethene polymerization under low pressure are present only in the narrow boundary layer between the free and the supersaturated regions, so their total number is small. However, following our model, the number of active sites depends upon the ethene pressure. Under real industrial conditions (200 psi) ethene can displace the extra Cr( Cp), , poisoning the potentially active sites.