Faruduy Discuss. Chem. SOC., 1989, 87, 121-132 Chemistry and Catalysis at the Metal/ Metal Oxide Interface Jas Pal S. Badyal, Roger M. Nix,? Trevor Rayment and Richard M. Lambert* Department of Chemistry, University of Cambridge, Cambridge CB2 1 EP Under appropriate conditions, single-crystal observations are capable of yielding information about reaction mechanisms and transport phenomena which can assist in understanding the behaviour of practical, high metal area, supported catalysts. In the case of chemical catalysis by metals, the literature contains a number of well documented examples of which illus- trate the success of this approach. Corresponding model studies of the metal/oxide interface are less common, although in principle they should be capable of improving our understanding of those systems in which metal/support interactions play an important role in determining the catalytic chemistry.This paper deals with the application of such methods to two areas of synthesis gas chemistry; additionally, correlated measurements have been made on the structure and reactivity of the corresponding high-area catalysts. The usefulness of such a combined approach will be illustrated with reference to methanol synthesis over copper/rare-earth oxide systems and the behaviour of Ru/TiO, catalysts. It has long been recognised that interactions taking place at the metal/metal oxide interface can play an important part in determining the properties of heterogeneous catalysts.’ In particular, the strong metal-support interaction (SMSI) has been the subject of extensive research, although the origin of this effect is still a controversial subject.’h Indeed, it has been argued that the SMSI phenomenon is a particular manifestation of a more general type of metal/metal oxide interaction’ and the present paper deals with two different metal/metal oxide systems in which the observed behaviour is strongly dependent on an intimate interaction between the two phases.In both cases, we have examined the detailed structural and chemical properties of the systems by using single-crystal specimens to model certain relevant aspects of behaviour. These observations have been correlated with measurements carried out on the appropri- ate dispersed or high-surface-area materials. Oxide-supported metal catalysts exhibiting uniquely high activities for a variety of reactions may be prepared by oxidation of bimetallic rare-earth/ transition-metal com- pounds.’,’ Such catalysts appear to exhibit much higher activities than ostensibly similar materials prepared by more conventional procedures,’~~ that the identity of the active site in catalysts derived from alloy precursors is a particularly interesting question. Since Tauster’s original observations,” an increasing number of metal oxide systems have been reported as exhibiting SMSI-like behaviour.Correspondingly there have been a number of attempts to address aspects of this problem by the application of single-crystal model systems.“.” We report here on the activation and performance of highly efficient methanol synthesis catalysts prepared from Cu/ Nd alloys using correlated XRD and reactivity measurements.Complementary information about basic aspects of the surface chemistry of this system has also been obtained by measurements on the oxidation of Nd/Cu ultrathin single-crystal alloy films. Similarly, we have investigated the structure, mobility + Present address: Department of Chemistry, Queen Mary College, Mile End Road, London El 4NS. 121122 Metal/ Metal Oxide Interface and adsorption behaviour of well characterized TiO, films on Ru(0001) over a range of temperature which is pertinent to the conditions required for inducing SMSI behaviour in practical Ru/TiO, catalysts; these results are also complemented by measurements on dispersed Ru/TiO, materials. Experimental Preparation of the Cu( loo), Cu( 11 1) and Ru(0001) orientated single-crystal specimens followed standard techniques; the experimental arrangement, methods for dosing with Nd and TiO, and procedures for heating and in situ cleaning of the specimens in an ultra-high-vacuum environment have been described elsewhere.13.14 Surface analysis was by means of LEED-Auger spectroscopy and thermal desorption data were obtained using a multiplexed mass spectrometer with a collimated ion source sampling aperture. “ , 1 4 Reactivity measurements on the Nd/Cu alloy-derived catalysts were made in two separate systems. In the first, the reactor was incorporated into the sample stage of an X-ray powder diffractometer, thereby permitting concurrent measurement (and hence direct correlation) of catalytic performance and bulk structural transformations.The second reactor system was additionally equipped with facilities for in situ measure- ment of Cu surface areas (using the N 2 0 frontal chromatographic method) and a variety of transient/ T-programmed techniques. In both cases, alloy activation was carried out in the pure synthesis gas feed (1 : 2 CO/Hz, <20 ppm COz) under similar conditions to those employed for methanol production (8-20 bar, 423-473 K). Results Methanol-synthesis Catalysts derived from Nd/Cu Intermetallic Compounds Upon exposure to synthesis gas at high pressures, rare-earth-Cu alloy precursors undergo visible degradation; this reflects substantial bulk transformation of the alloy at the microscopic level. This ‘activation’ stage is essential to the initiation of methanol synthesis, the alloys themselves exhibiting no measurable activity, and the process is exemplified by the behaviour of NdCuz as revealed by the sequence of X-ray diffraction patterns shown in fig.1A. Under the relatively mild conditions employed in this experiment (8 bar/448 K) several distinct stages in the conversion of the alloy are clearly evident. The first involves bulk absorption of hydrogen to yield an intermetallic hydride of closely related structure. The chemical reactivity of this intermetallic hydride is, however, substantially greater than that of the parent alloy and the conversion is followed by rapid decomposition and the onset of oxidation to yield an intimate mixture of rare-earth oxide, binary rare-earth hydride and elemental copper.’ Oxidation of the rare-earth component proceeds by dissociative chemisorption of CO: gasification of the deposited carbon that is inherent to the activation process leads to a high transient level of methane (fig.IB). Most importantly, however, there is a very stong correlation between the appearance of the rare-earth oxide and elemental copper phase and the initiation of methanol-synthesis activity. The ultimate activity of the catalysts is very dependent upon the efficiency of the activation process; this in turn is very sensitive to the conditions employed and the nature of the starting alloy. A number of general conclusions can be drawn from studies on an extensive range of alloys of varying composition and stoichiometry.( i ) AIIoy activation can be achieved using a variety of oxidizing media (e.g. 02, steam, N,O); however, none of these yield catalysts as active as those obtained using synthesis gas under optimum conditions. ( i i ) The nature and extent of the interaction of the alloy precursor with hydrogen is crucial to the subsequent oxidation. Indeed, pretreatment of the alloy charge in pure hydrogen at low temperatures can significantly enhance thef. P. S. Badyal, R. M. Nix, T. Rayment and R. M. Lambert 123 0 35 45 55 B 6 12 t / h Fig. 1. (A) synthesis-gas activation of NdCu, : sequential diffraction patterns obtained during treatment at 8 bar/448 K. ( a ) Virtually untransformed starting alloy, ( b ) strongest peak from intermetallic hydride marked with an asterisk.(Partially masked peaks at ca. 38 and 44.5 originate from sample holder.) ( B ) Qualitative g.c. activity data for NdCu, activation in CO/H2 at 15 bar/423 K. Peak of methane yield correlates with oxidative decomposition of intermetallic hydride. (-) CH,OH, (- - - ) CH,. efficiency of the syngas activation. (iii) At higher pressures, activation can proceed at lower temperatures, yielding catalysts that show extraordinary initial synthesis activities. XRD and electron-microscopic characterization of fully activated catalysts reveals only the presence of rare-earth oxide and copper particles with average particle sizes of ca. 30 and ca. 200 A, respectively. In situ measurement of the specific copper surface areas, however, yield values that are typically (0.5 m2 g - ' , substantially less than expected on the basis of the Cu particle size.This suggests that a substantial amount124 Metall Metal Oxide Interface /- 573 - 4 7 3 4 -co*- v time Fig. 2. Variation of product yield (arb. units) upon (A) increasing the reactor temperature to 573 K, ( B ) introducing 2% C 0 2 into the gas feed. Initial conditions: 15 bar CO/H2, 473 K. 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. This has been confirmed by STEM microanalysis, which shows the presence of copper intimately associated with the rare-earth oxide phase.8 There are two other features of these alloy-derived catalysts that warrant special comment, both of which relate to the deactivation to which these materials are very susceptible. Fig.2 shows the effect of (A) increasing the reactor temperature to 573 K, and (B) introducing 2% C 0 2 to the gas feed, for NdCu-derived catalysts that were initially operating in C02-free synthesis gas at 473 K. Clearly, the catalysts are strongly deactivated by these treatments. Furthermore, the deactivation is essentially irreversible in both cases. Surface Structural Chemistry and Chemisorption Properties of Nd/Cu Ultrathin Films and Properties of the Neodymium Oxide/Cu System The Nd/Cu Bimetallic System: Nd/Cu( 100) and Nd/Cu( 11 1 j The morphology and growth mode of neodymium on both the (100) and (1 1 1 ) faces of copper was studied over a range of conditions. A direct comparison of these two systemsJ.P. S. Badyal, R. M. Nix, T. Rayment and R. M. Lambert 125 reveals strong similarities in behaviour that usefully serve to summarize the most important features. In both cases, the Auger uptake curves at 300 K exhibited a number of ‘breaks’ in the variation of the intensity of the Nd signal that can be attributed to the formation of distinct Nd monolayers. This continued until at least 2 monolayers had formed: the subsequent behaviour was less well defined, however, and some crystallite nucleation or simultaneous mutlilayer growth probably occurs. Photoemission results are consistent with this interpretation, valence- and core-level copper emission exhibits a monotonic attenuation. The formation of the first monolayer was accompanied by a substantial (ca.1.7-1.8 eV) fall in the work function and its completion marked by LEED patterns of rather poor quality that are indicative of a relatively low-density monolayer in both systems [(4.4-2.8) x l O I 4 atom cm-’I. No well ordered submonolayer structures were observed. As the coverage was increased above the monolayer a further small decrease (ca. 0.1-0.2 eV) in work function was registered before saturation was achieved, but no further LEED patterns were observed, indicative of the formation of disordered layers of neodymium exhibiting a relatively open-packed structure. If films deposited at 300 K were annealed then substantial changes in the film morphology and composition are evident. The situation is best defined, however, if the films are annealed during Nd uptake.The limiting work function under these conditions is significantly higher than that observed at 300 K. Furthermore, both AES and XPS show removal of Nd from the immediate surface region. In the case of Nd uptake at 900 K on Cu( 11 1) there is, indeed, complete saturation of the Cu and Nd AES signals for Nd doses above ca. 2 monolayer equivalent and the establishment of a characteristic, well defined valence emission spectrum that is invariant with further Nd deposition. The saturation of the AES signals correlates with the appearance of a well defined (8 x 8) LEED pattern; this structure was invariably observed for nominal Nd coverages from 2 monolayers up to at least 5 monolayers after deposition at 900 K and was stable (for the higher coverages) upon further annealing to temperatures in excess of 1000 K.At lower doses, deposition at elevated temperatures (ca. 500-900 K) yielded a (2 x 2) LEED pattern. This then transformed smoothly into the (8 x 8) pattern at ca. monolayer Nd coverage. The situation on the Cu(100) substrate is more complex. Uptake at 800 K yielded a pseudo-hexagonal c( 10 x 2) structure for Nd doses exceeding 1.5 monolayer, that was stable to much higher coverages (>4 monolayer). Subsequent annealing of this structure (>2 monclayer Nd) at higher (>800 K) temperatures first gave a complex hexagonal ( a = 20.0 A) pattern, then a rotated ( J 3 7 x J 3 7 ) pattern, and finally any of a number of structures as the temperature was progressively increased. The important point is that there is a direct correspondence between the (2x2) LEED pattern observed on the (1 11) face and the c( 10 x 2) pattern on the (100) face of copper.Furthermore, the ( 8 x 8) pattern is equivalent to the more complex hexagonal LEED pattern seen on the Cu( 100) surface. The lattice parameter of the ( 2 ~ 2 ) - t y p e structure is in very good agreement with that observed for NdCu2 layers present in the bulk intermetallic compound, NdCu,. It is the (8 x 8) structure, however, whose formation most closely correlates with the signal saturation observed in AES and UPS. I t is therefore proposed that the (2x2) LEED pattern corresponds to a thin alloy film containing either one or two NdCu’-type layers, whilst the (8x8) LEED pattern represents a thick alloy film of NdCu, stoichiometry with a structure based upon that of the bulk intermetallic compound, but showing some form of longer-range periodicity.In summary, however, the uptake of neodymium on single-crystal copper surfaces yields predominantly disordered overlayers at low temperatures, but at elevated tem- peratures Nd/Cu intermixing at the atomic level occurs to give well ordered alloy thinMetall Metal Oxide Interface 0.5 z! 0 0 .- x 0 I B 1 1 1 I 300 500 7 0 0 900 T l n d X l K Fig. 3. (A) Nd 3d5,, XP spectra of the oxidation of a 5 monolayer unannealed Nd film at 300 K (exposure in L). (B) Effect of heating to progressively higher temperatures during a sequence of low-temperature CO adsorption/desorption cycles. Nd coverage 1 monolayer (unannealed); T,,,, , maximum temperature achieved during TPD sweep.films; these alloy films provide well characterised metal surfaces suitable for studying the surface chemistry of rare earth/Cu catalyst precursors and the Cu/rare-earth oxide catalyst systems derived from them by in situ oxidation. Oxidation Chemistry of Nd-containing Films on Cu(100) and Properties of the NdO,/Cu( 100) Interface Whilst the prior incorporation of hydrogen is essential to achieve substantial oxidation and activation of the bulk alloys, this is not the case for the ultrathin pure Nd and Nd-Cu alloy films grown in u.h.v. Indeed, the bulk intermetallic hydrides are not stable in u.h.v. and uptake of hydrogen into the Nd overlayers grown on Cu( 100) is surprisingly slow. A direct consequence of the absence of both absorbed and gas-phase hydrogen, however, is that there is no facile method for removal of carbon deposited during oxidation by CO.In this section, therefore, we include work carried out using both O2 and CO as the oxidizing media, recognizing that whilst O2 is not the preferred activating agent for the bulk alloys, model systems obtained in u.h.v. using O2 are directly relevant to the intimate mixture of rare-earth oxide and copper present in the active catalysts. The rate of uptake of O2 on thick (>3 monolayer) pure Nd films at 300 K is initially fast but falls away rapidly as the kinetics become controlled by the diffusion of oxygen into the bulk of the film. The latter process is substantially facilitated by heating. The limit of the initial uptake appears to correspond to a stoichiometry of NdO, (x = l ) , and this stage of oxidation was accompanied by a substantial shift in the Nd 3dS,, peak to higher binding energy (fig.3A). At higher O2 exposures there is a shift in the peakJ. P. S. Badyal, R. M. Nix, T. Rayment and R. M. Lambert 127 maximum back towards lower binding energies and the development of a well defined low-binding-energy shoulder, ultimately yielding a peak similar to that observed for At low initial coverages (<2 monolayer) there is some evidence for aggregation of the oxidized neodymium at high O2 exposures (ca. 100 Lt) even at 300 K; this was more clearly evident, however, upon heating to moderate temperatures (550-800 K). The reactivity of the Nd-Cu alloy t i n films towards O2 at 300 K was not found to be significantly different from that of the pure Nd overlayers.Rapid O2 uptake was accompanied by destruction of the corresponding alloy LEED pattern and some apparent segregation of Nd to the surface, but in other respects the behaviour of the systems was very similar to that described above for Nd overlayers. All of the oxidized films obtained at 300 K were disordered to LEED. Oxygen dosing at elevated temperatures or subsequent annealing of oxidized films did, however, yie!d patterns corresponding to a hexagonal structure with lattice parameter of ca. 3.8 A, which is very similar to that expected for the unreconstructed basal plane of A-type Nd,O, or the (1 11) face of the C-type sesquioxide. The rotational orientation and degree of ordering of the oxide depended upon both the Nd coverage and the extent of annealing.In particular, no oxide LEED patterns were observed for annealing tem- peratures below 550 K. Insufficient annealing resulted in rotational disorder, especially at high Nd coverages. The oxide structures have high thermal stability; they were still observed (albeit with reduced intensity) after annealing at temperatures greater than 1100 K. In the first case, the c(2 x 2) and (42 x 2d2)R45" structures characteristic of the 0-Cu( 100) system were often additionally present at low Nd precoverages. Despite the obvious high-temperature stability of these structures, AES and UPS results after very high-temperature annealing are consistent with some dissolution of NdO, entities into the bulk of the copper substrate (note, however, that both Nd and 0 are also independently capable of bulk dissolution into copper at high temperatures).The oxidation behaviour of the Nd/Cu substrates is significantly different in certain important respects when CO, rather than 02, is used as the oxidant. Dissociative chemisorption of CO was induced by the presence of surface Nd at all temperatures. At low Nd coverages, low temperature (<200 K) molecular chemisorption on exposed copper was also evident from TPD results. The alloy films exhibited a significantly lower irreversible CO uptake at 300 K than the pure Nd overlayers; this kinetic inhibition could be overcome by raising the substrate temperature during CO exposure. In other ways, however, the metal-oxide interface generated from the alloy films did not differ significantly from that obtained by oxidizing the Nd overlayers.In addition to the obvious presence of carbon, the major differences exhibited by oxidized films obtained using CO as the oxidant were: (i) CO exposure (300-600 K) did not yield neodymium in its maximum oxidation state: CO-oxidized films gave Nd 3d spectra similar to those obtained at low O2 exposure ( i e . a high apparent binding energy and no distinctive low-binding-energy shoulder); (ii) annealing at high tem- peratures did not yield any of the ordered oxidized layers obtained after O1 exposure. There were, however, some similarities in the characteristics of the CO- and 02-oxidized films. In particular, annealing at temperatures >550 K again gave rise to aggregation of the oxidized layers at low Nd coverages ((2 monolayer) to expose underlying Cu( 100) surface.This was evident from the increase in the low-temperature, molecular CO chemisorption capacity (fig. 3B) and the observation of the characteristic c(2 x 2)CO- Cu( 100) LEED pattern at low temperatures following such treatment. In addition, there was again evidence for some NdO, dissolution at very high temperatures. bulk Nd203. t 1 L (langmuir) = loh Torrs.128 Metal/ Metal Oxide Interface 1 . 0 - 0.6 m h m 0 . 5 0 0 2 0 . 4 v 2 D % 0.3 0 0 0 . 2 0.1 ( b ) O ~ l j I I I I I 0 0 . 2 0.4 0 . 6 0.0 1.0 1 . 2 1 . 4 Ti coverage/monolayer Q 0.0 0 0 1 d 0.6 h - v 2 n 2 21 0 . 4 0 . 2 O ! , I I I I I ~ 0 0.2 0 . 4 0 .6 0 . 0 1.0 1 . 2 1.4 Ti coverage/monolayer Fig. 4. ( a ) Effect of Ti predosing on uptake of P-CO by ruthenium: desorption yield as a function of Ti coverage. ( b ) Effect of Ti on P-H, yields as a function of Ti coverage. Ti/ Ru( 000 1 ) ModeZ Studies Initial studies examined the interaction between metallic titanium and a well character- ized Ru(0001) surface; subsequently, the behaviour of this bimetallic system towards H2 and CO was also investigated. Titanium deposited on Ru(0001) at 300 K exhibits a layer-by-layer growth mode. A weak LEED pattern appears a t loadings close to monolayer completion, corresponding to a ( J 9 l x J9l)R5.2' coincident titanium over- layer, in which the Ti-Ti separation is ca. 8% greater than the corresponding distance in pure 11.c.p.titanium.13 "The fingerprint TPD spectrum (p-CO) for saturation doses of CO on clean ruthenium was utilized to investigate the influence of adsorbed titanium species on neighbouring ruthenium sites. Uptake of p-CO is strongly suppressed by titanium dosing; this effect is markedly non-linear, indicating that islands of titanium exert a significant long-range influence on the chemisorption of CO by bare ruthenium sites [fig. 4 ( a ) ] . In addition to the clean surface p-CO peaks, two new features appear in the desorption spectra: experiments using isotopically labelled CO show that the low-temperature feature is due to an associative CO species bound to titanium atoms, while a high-temperature feature is due to the autocatalytic recombination of dissociatively chemi- sorbed CO from on and around the titanium islands, as titanium diffuses away into the underlying ruthenium to form a Ti/Ru alloy phase.Hydrogen chemisorption results in a surface hydride species of limiting stoichiometry, TiH3 (at a monolayer of titanium precoverage); this species decomposes at ca. 600 K with concomitant formation of a Ti/Ru surface alloy. Hydrogen spillover from the TiH3 to the adjacent bare ruthenium sites has been shown to occur in the submonolayer region [fig. 4(6)]. For sufficiently thick titanium films, the stoichiometry of the hydrideJ. P. S. Badyal, R. M. Nix, T. Rayment and R. M. Lambert 129 0 . 7 0 . 6 0 h - 0 . 5 0 0 1 v cr: 0.4 2 0 % 0 . 3 0 u 0 . 2 0 . 1 0 0 0 . 2 0 . 4 0 . 6 0.8 1.0 1 . 2 1 . 4 TiO, coverage/monolayer 2 0 .4 4 \ 0 0.2 0 . 4 0 . 6 0.8 1.0 TiO, coverage/monolayer Fig. 5. ( a ) CO desorption yield per surface ruthenium atom (O), as a function of TiO, (x = 2) coverage. ( b ) Hydrogen desorption yield ( O ) , as a function of TiO, (x = 2) coverage. films begins to approach that of the bulk hydride, TiH2. The TiH3 surface hydride is itself very active in the dissociative chemisorption of CO, this being accompanied by a very pronounced destabilization of the hydrogen atoms associated with the original hydride phase. Alloy formation at low titanium precoverages on Ru(0001) leads to an ordered (2 x 2) surface compound which is two layers thick; this structure corresponds to maximal formation of strong Ru-Ti bonds. For larger amounts of titanium predeposition transfor- mation to a disordered phase occurs; it is not clear whether this is due to the limitations of surface + bulk diffusion. TiO,/ Ru( 000 1 ) Model Studies We have attempted to simulate the decoration model of SMSI by investigating the growth morphology, structure, kinetic behaviour and chemical properties of TiO, films on Ru(0001) as a function of oxide loading and temperature.The uptake of TiO, at room temperature was determined by AES to follow a monolayer-by-monolayer growth mode; this is consistent with TiO, moieties wetting the metal surface in a similar way to the decoration model proposed for SMSI. At monolayer coverage, the system exhibited a weak (1 x 1 ) LEED pattern, assigned to the formation of a TiO, overlayer in registry with the Ru(0001) plane.Selective removal of oxygen atoms chemisorbed on the bare ruthenium sites (in the submonolayer region of TiO, coverage) was performed by using a hot cathode-ion gun operating in a background pressure of 2 x Torrt -t 1 Torr = 101 325/760 Pa.130 Metall Metal Oxide Interface 0.7 0.6 rn h rn 0.5 0 0 2 0.4 v 2 0 % 0.3 0 u 0.2 0.1 0 0 0.2 0.4 0.6 0 . 0 1.0 1.2 1.4 TiO, coverage/monolayer 0 0.2 0.4 0.6 0.8 1.0 TiO, coverage/ monolayer Fig. 6. ( a ) CO desorption yield per surface ruthenium atom ( O ) , as a function of TiO, ( x = 1) coverage. ( b ) Hydrogen desorption yield ( O ) , as a function of TiO, ( x = 1 ) coverage. H2 , and in line of sight of the specimen, the latter being held at 575 K (no alloying occurs under these conditions). The stoichiometry of this TiO, phase was found to correspond to x = 2 using quantitative AES and XPS measurements.In the submonolayer regime, such TiO, (x = 2 ) , moieties deposited on to Ru(0001) at room temperature lead to simple site-blocking behaviour for the subsequent chemisorption of p-CO on bare ruthenium sites [fig. 5 ( a ) ] . However, the loss of ensembles of surface ruthenium atoms hinders hydrogen chemisorption much more severely [fig. 5( b ) ] . H 2 + C 0 coadsorption measurements indicate that the TiO, species are very highly dispersed, possibly because the method of deposition involves preadsorbed oxygen atoms acting as anchors for the incident titanium atoms. LEED, Auger and XP spectroscopy measurements show that with high loadings of TiO, at temperatures characteristic of the preparation conditions required for SMSI, excess TiO, diffuses into the bulk metal, leaving an expanded, reduced titanium oxide film ( x = 1) which completely coats the surface.At submonolayer coverages, this reduced TiO, species leads to an increase in the amount and strength of adsorption of H2 on adjacent bare ruthenium sites with respect to the unreduced TiO, (x = 2 ) species [fig. 5 ( b ) ] . p-CO chemisorption shows an increase in binding energy, and the TiO, species have more than just a simple site-blocking influence [fig. 6 ( a ) ] . The presence of reduced titanium ions at the edges of the TiO, islands will lead to electronic charge transfer to neighbouring ruthenium atoms, these Ti0,-Ru interface regions could then act more strongly as centres for CO chemisorption and H2 dissociation.We have recently identified highly dispersed ruthenium particles under SMSI conditions; these are in contact with a non-bulk-like TiO,H, species (which is responsible for dispersing the ruthenium particles). This TiO,H, species is generated at the Ti0,-Ru interface (x = 1 ) via the spillover of hydrogen atoms from the metal to the reduced upp port,'^ and would appearJ. I? S. Badyal, R. M. Nix, T. Rayment and R. M. Lambert 131 to be related to the TiO, (x = 1) reduced species which can be generated on the model system. Discussion On the basis of the results reported here, there would appear to be a number of features common to the chemistry of catalysts prepared from rare-earth/copper alloy precursors and more conventional oxide-supported metal catalysts in the SMSI condition.In both cases the model studies demonstrate the reduced dissociation activity of ordered (unhydrided) alloy surfaces towards CO, relative to the corresponding Nd or Ti overlayers. This is particularly evident in the Ti/Ru data, where an abrupt change in chemisorption properties correlates with the destruction of the ordered alloy phase. Likewise, oxidation of both bimetallic systems leads to a separation into metal and oxide phases; at sufficiently elevated temperatures, however, there is some dissolution of Ti (or Nd) oxide in Ru (or Cu). In the case of Ti/Ru this high temperature treatment leaves a highly reduced form (TiO,=l) of the metal oxide on the surface, whilst in the case of the Nd/Cu system the oxides obtained at temperatures pertinent to methanol synthesis are highly disordered and in certain instances also substoichiometric relative to NdzO3.Because of the preparative methods employed in each case the oxide phases are formed in intimate contact with the transition-metal component. It seems possible that these special defect (Nd/Cu) or reduced (Ti/Ru) oxide/metal junctions may be effective in the activation of CO towards methanol synthesis or methanation, respectively. Another interesting aspect concerns the role of hydrogen and hydrides in these systems. The Ti/Ru observations show that TiH, thin films on Ru are extremely active in the dissociation of CO, and it is the facility with which CO-induced oxidation of hydride phases at low temperatures occurs which is likely to play an important part in the production of ultra-dispersed active metal species from alloy precursors.Thus bulk Nd/Cu intermetallic compounds can initially from bulk hydrides under activation conditions, and these materials react rapidly with CO to form highly active Cu/rare-earth oxide catalysts. The high performance of both these catalyst types appears to be associated with the presence of ultra-dispersed transition-metal species embedded or entrained in an oxide phase whose structure differs from that of ordinary bulk oxides. In the Ru/TiO, system it appears that high-temperature reduction leads to the formation of a hydrogen- containing oxide phase'' which entrains very small metal particles and spreads to expose a large active area.Hydrogen atoms trapped at oxygen vacancies in the reduced oxide lattice might be expected to activate chemisorbed CO in a similar (though less pro- nounced) manner to that observed for the CO-induced oxidation of bulk rare-earth/Cu hydrides and TiH thin films. The rare-earth/ transition-metal precursors generate these very small oxide-embedded metal centres by a more direct but energetically less favour- able route. The intimate metal-oxide system is generated in a rather more direct route from the RE-containing intermetallic precursors by the predominantly kinetically con- trolled oxidative decomposition of the pseudo-atomic dispersion of metals present in the starting alloy. References 1 G. C. Bond and R. 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Cutal., 1988, 111, 383. 14 R. M. Nix and R. M. Lambert, Surj Sci., 1987, 186, 163. 15 J. P. S. Badyal, K. Harrison, C. Riley, J. Frost and R. M. Lambert, in preparation. 16 J. P. S. Badyal, CertiJicate of Postgraduate Studies (University of Cambridge, 1986). Paper 8/05065A; Received 20th December, 1988