J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2803-2807 Characterization of Carbon-supported Ruthenium-Tin Catalysts by High-resolution Electron Microscopy G. Neri and R. Pietropaolo Faculty of Engineering, University of Reggio Calabria, 89100 Reggio Calabria, Italy S. Galvagno and C. Milone Department of Industrial Chemistry, University of Messina, 98166 Messina, Italy J. Schwank Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA Ruthenium and bimetallic ruthenium-tin catalysts supported on high-surface-area carbon are characterized by high-resolution electron microscopy (HREM) and CO chemisorption. The characterization results give important insights into the reasons for the remarkable catalytic properties of Ru-Sn bimetallic catalysts in selective hydro- genation reactions of a,/.?-unsaturated aldehydes.In the monometallic Ru/C sample, ruthenium is present as small particles, showing no variation in the crystalline structure and lattice parameters with respect to the bulk state. At low Sn : Ru ratios, the average metal particle size is smaller than for monometallic Ru/C, whereas at higher Sn : Ru ratios Ru particles agglomerate on the surface of the support. According to HREM observations, most metal particles in the bimetallic catalysts possess a structure consistent with that of elemental ruthenium. CO chemisorption results indicate that the surface of the ruthenium particles is partially, covered by tin species. Only in the samples with high tin loading are a few Ru-Sn alloy particles found, in addition to a majority of particles having the structure of pure ruthenium.The high selectivity of Ru-Sn/C catalysts for selective hydro- genation of the C-0 group in a,/.?-unsaturated aldehydes can thus be attributed to the presence of small ruthe-nium particles whose surface is covered with tin species which are most likely in ionic form. RuSn alloy particles do not appear to make a significant contribution to the catalytic activity. Bimetallic catalysts are commonly used in many important processes in the petroleum industry. They also have potential applications in the synthesis of fine chemicals for the per- fumery and food industries. In our laboratory bimetallic systems containing platinum and one element of Group 14, such as Ge and Sn, have been investigated.These catalysts have remarkably high selectivity in the hydrogenation of a,/.?-unsaturated aldehydes to the corresponding unsaturated alcohols.’** Ruthenium is an attractive catalyst for the hydrogenation of carbonyl compounds. Moreover, the addition of tin to monometallic catalyst has been found to affect favourably the catalytic activity and/or selectivity towards the desired prod- ucts. Basset et aL3 reported that Ru-Sn supported on SiO, is a very active and selective system for the hydrogenation of ethyl acetate to ethanol under mild conditions. Ru-Sn cata-lysts, promoted with boron, have been shown to catalyse the selective hydrogenation of esters and unsaturated aldehydes to alcoh01.~ The effect of tin has been attributed to modifi- cations of the structural and/or electronic properties of ruth- enium either by dilution of the active surface atom ensembles or by formation of an alloy with the second metal com- ponent.Similar results obtained in our laboratory on Ru-Sn sup-ported on high-surface-area carbon confirm that addition of tin to ruthenium catalyst strongly modifies the performance of the noble meta1.5*6 In the selective hydrogenation of a,/.?-unsaturated aldehydes addition of tin to ruthenium changes the product distribution leading to a selectivity higher than 90% towards the formation of unsaturated alcohol.6 It also causes, at low tin loading, an increase of the catalytic activity. Higher tin loadings poison the active centres and the cata- lytic activity decreases.The chemical state and location of tin relative to ruthenium in bimetallic Ru-Sn catalysts is still an open question. Some authors have suggested that Sn is present as ionic tin; however, alloy formation has been detected by XRD in Ru-Sn catalysts promoted with b~ron.~,~,’ The objective of this work is to use HREM to obtain microstructural information on the Ru-Sn/carbon catalysts previously used for selective hydrogenationSv6 and to find a relationship between the structure of these bimetallic cata- lysts and the catalytic properties. Direct imaging of lattice planes by HREM has already proved to be a versatile tech- nique to obtain information on the crystallographic structure of small metal particles8 It should be possible to determine where the tin is located in these catalysts and to what extent Ru-Sn alloy particles are present.Experimental Catalyst samples were prepared by using the incipient wetness technique. Carbon (Chemviron SCXII, 80-100 mesh, surface area 900-1100 m2 g-’) was co-impregnated with aqueous solutions of RuCl, and Sndl, having the appropri- ate metal concentration. The catalysts were dried at 393 K in air for 1 h followed by reduction under flowing H, at 673 K for 2 h. They were then stored in air and reduced in situ under very mild conditions, 343 K and 0.1 MPa H, for 1 h, before catalytic tesk5v6 The ruthenium loading was held con- stant at 2 wt.% in all catalysts, whereas the tin loading was varied from 0 to 1.56 wt.%.The numerical value in the cata- lyst code, reported in Table 1, indicates the nominal atom% of Ru on the basis of the nominal metal loading; e.g. catalyst code Ru95/C refers to a sample with 95 atom% Ru and 5 atom% Sn. Chemisorption of CO was measured in a conventional pulse system operating at room temperature. The catalyst sample (0.1-0.5g) was reduced in flowing hydrogen (at 673 K for 2 h) followed by flushing for 3 h in a helium stream at 673 K with subsequent cooling in flowing helium to room tem- perature. Calibrated CO pulses were injected by means of a sample loop into the helium carrier gas and detected by a thermal conductivity detector. A mixture of 10 vol.% CO in He was used to inject a small amount of CO.By comparing the amount of CO reaching the detector and the amount of CO injected into the system, the quantity of CO adsorbed on Table 1 Composition and characterization of Ru/C and Ru-Sn/C samples catalyst code Ru (wt.%) Sn (wt."/,) CO :Ru" d/nmb Ru100/C 2.0 -0.261 5.9 (5.3)' Ru95/C 2.0 0.12 0.220 3.5 Ru90/C 2.0 0.26 0.115 3.5 Ru80/C 2.0 0.58 0.096 3.4 -dRu70/C 2.0 1.01 0.003 eRu60/C 2.0 1.56 --d aRatios given relative to one Ru atom. By TEM. 'By CO chemi- sorption. Not evaluated. Below the detection limits. the catalyst could be determined. Additional pulses were injected until no further CO uptake was noted. Blank experi- ments on the carbon support proved that there was no measurable uptake of CO on the support itself. Table 1 lists the various catalysts, their nominal loading, CO : Ru ratio, and average particle sizes obtained from transmission elec- tron microscopy. Under the experimental conditions used, Sn by itself does not adsorb measurable quantities of CO.Electron microscopy studies of the catalysts were per-formed on a JEOL 2000 FX instrument operating at 200 kV and a JEOL 4000 EX microscope operating at 400 kV. The latter instrument was equipped with a top entry state goni- ometer for high-resolution work and directly interfaced with a Gatan camera for real-time image processing. The catalyst specimens for electron microscopy were pre- pared by gently grinding the powder samples in an agate mortar, suspending and sonicating them in isopropyl alcohol, and placing a drop of the suspension on a holey carbon copper grid.After evaporation of the solvent, the specimen was introduced into the microscope column. During speci- men preparation, the samples were exposed to air. The top entry stage goniometer in the JEOL instrument is not well suited for interfacing with sample manipulators, and there is, at present, no commercial high-resolution electron' micro- scope available with in situ reduction capability. The lattice spacings of the metal particles were obtained by Fourier transformation of the lattice fringe image, utilizing a 40 (41 h S 30-W 0>.g 20-I-U 10. m0-. r d/nm 40 I 1 J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 Table 2 Crystallographic data for Ru and Ru-Sn alloys lattice constant/A component crystal system a C ~~~~ ~ Ru Ru,Sn, hexagonalcubic 2.706 9.351 4.282 - Ru,Sn, RuSn, tetragonal tetragonal 6.172 6.389 9.915 5.693 solid-state camera (COHU) interfaced with a Macintosh computer for image calculation. The distances on the Fourier transform were calibrated using an Si( 111) specimen, with a (220) lattice spacing of 0.192 nm. Table 2 shows a listing of the crystal structure data for the metallic components which can exist in the Ru-Sn system. Results and Discussion Plate 1 shows a typical low-magnification transmission elec- tron micrograph of the monometallic Ru100/C catalyst. The ruthenium particles are clearly visible on the surface of the carbon support.The particle size distribution obtained by measuring the diameter of several hundred particles in micro- graphs of this sample is reported in Fig. 1. The surface mean diameter, d,, calculated from the equation: d, = 1,n,d:/1,n,d? was 5.9 nm, where n,represents the number of particles of a given diameter, d,. Chemisorption of CO has been used to obtain complementary average particle size data. The average Ru particle size, d,, was determined from the CO uptake by the expression d, = 6V/S, where V is the ruthenium metal volume and S the surface area of Ru. The metal surface area was measured assuming a ruthenium surface density of 1.63 x1019 atoms m-2 and a stoichiom- etry CO : Ru = 1 : 1.' The agreement between the average Ru particle size calculated from chemisorption and the Ru particle size determined by transmission electron microscopy (Table 1) is satisfactory within the accuracy of both methods. The same Ru100/C sample was also examined in a high- resolution electron microscope operating at 400 keV where the lattice structure of the metal can be resolved.An HREM image of a ruthenium particle of about 3-4 nm diameter is 50> 1 40-s v 5 30-C -12345678910 d/nm "1234567910 d/nrn d/nrn Fig. 1 Particle size distributions of Ru 1OO/C and Ru-Sn/C samples:(a)Ru100/C; (b)Ru95/C; (c) Ru90/C and (d)Ru80/C J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 1 Micrograph of the monometallic Ru100/C Plate 2 HREM image of Ru100/C showing the [1213] orientation of a ruthenium particle of about 3-4 nm diameter 1 nm Plate 3 HREM image of Ru100/C showing a 2 nm ruthenium particle having a hexagonal shape oriented with the [OOOl] plane parallel to the electron beam G.Neri et al. (Facing p. 2804) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 nm 10 nm Plate 4 TEMs from Ru-Sn bimetallic samples: (a)Ru90/C; (b)Ru70/C 20 rim Plate 5 Higher-magnification TEM of the Ru70/C sample. The agglomerate shown is composed of Ru particles of about 3 nm in diameter. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 nm Plate 6 Thin section of Ru60/C showing small particles -2nm Plate 7 High-resolution micrograph of a metal particle of the Ru60/C sample.The Fourier-transform patterns indicate an fcc structure with d spacings of 0.221 and 0.211 nm, respectively, corresponding to an Ru,Sn, alloy. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 shown in Plate 2. The Ru(10i0) and Ru(li01) planes with spacings of 0.234 and 0.205 nm, respectively, were imaged consistent with the lattice fringes of the hcp Ru structure. Very small metal particles in close contact with a support can show deviations of the lattice parameter from the bulk values for the metal. For example, small palladium particles supported on titania have been reported to show a lattice expansion in the (111) planes of palladium.” For small platinum particles supported on alumina the single-crystal structure was found to be preserved for particles as small as 1 nm,” while other authors” reported a decrease in the lattice parameter for platinum proportional to the reciprocal of the platinum particle size.The latter result was attributed to surface stress effects caused by the high surface : volume ratio of small particles.” On Pt/C and Pt/SiO, we have recently observed no deviation from the bulk fcc value in the lattice spacing of small particle^.'^*'^ On Pt/SiO, only a few par- ticles of about 1 nm diameter or less showed a change in the lattice parameter. Literature data on lattice parameters of supported ruth- enium are very scarce. Datye et a/.’’ have shown that Ru supported on MgO preferred low-index planes with the {OOOl}, { lOi0) and { 101l} planes of Ru being exposed.Their results show no significant variation from the hexagonal structure characteristic of the bulk ruthenium. Our observa- tions here are in agreement, as the analysis by Fourier trans- formation of lattice images of particles in Ru100/C showed no deviation from the lattice parameter of the bulk structure within the experimental error associated with the measure- ment of the d spacings,16 even for metal particles as small as about 1 nm diameter. This indicates that most of these par- ticles were single crystals. Numerous well faceted metal par- ticles of hexagonal shape are present on the external surface of the carbon support. We observed such a clear particle faceting only in the monometallic Ru sample.Plate 3 showed one Ru particle of about 2 nm oriented with the electron beam parallel to the [OOOl] direction. In the bimetallic cata- lysts containing Sn, the small metal particles were generally spherical, rather than faceted. Anno and Hoshino have pre- dicted that the equilibrium shape of small supported particles should be dependent on size.17 Below 2 nm the metal par- ticles would be spherical, whereas above this size they would be flat and faceted. In agreement with the above findings, Buglass et d.18 have reported faceted particles in ruthenium crystallites larger than 2 nm on their Ru/C samples. Transmission electron micrographs (TEM) of some bimetallic samples are presented in Plate 4. Catalysts with low tin content (< 20 atom%) exhibit metal particles smaller than those observed on the monometallic sample (Fig.1). When the tin loading increased (samples Ru70/C and Ru60/C) a massive agglomeration of smaller particles was observed. These agglomerates consisted of many small crys- tallites clustered together to form larger aggregates distrib- uted randomly on the carbon surface (Plates 4 and 5). These large agglomerates are predominantly located at the external surface of the carbon. In addition, there are numerous small crystallites clearly visible through thin sections of the carbon support (Plate 6). These smaller crystallites are probably located inside the small pores of the carbon support, and therefore, their growth appears to be limited by the diameter particles in the bimetallic catalysts with low tin content (<20 atom%) revealed on all particles examined a hexagonal struc- ture characteristic of pure, elemental ruthenium.This raises the question of where the tin is located. One possibility is that tin species, either in zero-valent or oxidized form, deco- rate the surface of these ruthenium particles. Such sub- monolayer or monolayer quantities of tin would not be visible in HREM images. Furthermore, it is very likely that the exposure of the specimen to air prior to examination in the microscope would have caused surface oxidation of the tin species, if present. An oxidized tin surface layer would not give rise to sufficient diffraction contrast to become vi$ble in the micrograph.Therefore, HREMs have to be interpreted in conjunction with surface-sensitive probes, such as chemisorp- tion and catalytic probe reactions. To test the hypothesis of Sn species covering the surface of Ru particles, the CO :Ru ratio was used to measure the Ru dispersion (Ru atoms on the surface :total Ru atoms). From the metal particle size dis- tribution histogram shown in Fig. 1 it is apparent that the average particle sizes of the bimetallic catalysts are smaller than that of the monometallic RulOO catalyst. If the metal particles in the bimetallic Ru-Sn/C catalysts consisted of pure ruthenium without any tin on the surface, there would be a larger number of ruthenium surface sites available for CO chemisorption. Therefore, one would expect an increased uptake of CO, compared with the Ru100/C sample.Experi- mentally, the opposite trend is observed, and the CO :Ru ratio on the Ru-Sn/C samples decreased with increasing tin content (Table 1). This suggests that tin, which does not adsorb CO under our experimental conditions, blocks some of the active ruthenium surface sites. Fig. 2 shows the value of Ru dispersion in the bimetallic catalysts normalized to the Ru dispersion of the Ru100/C sample (relative dispersion), and plotted as a function of the tin loading. Relative disper- sion values were calculated by dividing the CO :Ru uptake ratio determined from chemisorption experiments on the bimetallic samples by the CO : Ru uptake value obtained for the RulOO sample.The experimental points fit well the nor- malized Ru dispersion which would correspond to a mono- layer growth of tin on the surface of Ru, as indicated by the dashed line in this figure. This is in agreement with an inter- action between Ru and Sn possibly occurring through the formation of Ru-Sn bimetallic surface aggregates or surface alloys not extending beyond the first few atomic layers of the particles,” or it could be due to the presence of ionic tin species, most likely in the form of tin oxides. Even if the particle surface should be reconstructed as a consequence of air exposure of the reduced sample, HREM can give valuable information on the structure of the particle core. The finding that the metal particles in the Ru-Sn samples with low tin loading had a structure consistent with \ g 0.8 Q \ .-2 \ \0.6 .-\ \U p 0.4 o\ .-\nc \-of the pores.For these catalysts showing large agglomer- ?!0.2 ations, it is not possible to derive a reliable particle size dis- tribution. On Pt-Sn catalysts, dispersed on the same carbon support, it has recently been shown that addition of tin causes increases of the average particle size, and at the highest tin loading large particles with raft-like structures were 0bser~ed.l~ Fourier transformation of the image structure of individual Ob 10 20 \%I To 20 100 Sn/( Ru + Sn) Fig. 2 Normalized Ru dispersion as a function of the Sn : (Sn + Ru) ratio: (---) calculated Ru dispersion which would correspond to a monolayer growth of tin on the surface of ruthenium that of pure Ru is very meaningful.It rules out the formation of bulk alloys in the catalysts with low tin loading. A different picture emerged for the catalysts with high tin loading, Ru70/C and Ru60/C. Most metal particles in these catalysts still possessed the structure of pure ruthenium. However, a few particles were found with the structure of ruthenium-tin alloy. There was no evidence for the presence of separate, pure tin particles or tin oxide particles on the carbon support. Plate 7 shows one particle of about 20 nm on sample Ru60/C which does not show the hexagonal struc- ture of pure ruthenium. The d spacings, evaluated from the Fourier transfomation of the structure image, are 0.221 and 0.211 nm, respectively, very close to the values of the (330) and (420) planes of the fcc Ru3Sn7 alloy.20 The bulk phase diagram of the Ru-Sn system shows the existence of three different alloy phases.Crystallographic data of these alloys are given in Table 2. RuSn, and Ru,Sn3 have a tetragonal structure whereas Ru3Sn7 is fee.,' The presence of RuSn, tetragonal alloy cannot be ruled out because the similarity in the lattice spacing with Ru3Sn7 makes it very difficult to dif- ferentiate the structure image of these two alloys.22 Despite air exposure during specimen preparation for microscopy, it was possible to preserve Ru-Sn alloy structures with tin in its zero-valent form. This is in agreement with our prior HREM observation of tin alloys in Pt-Sn/C13 and Pt-Sn/SiO, cata-lyst~.~~Of course, this does not preclude that the surface layer of these alloy particles may be oxidized and/or recon- structed.Our findings are in agreement with those of Desh- pande et aL7 who identified by X-ray diffraction (XRD) the Ru3Sn7 alloy in Ru-Sn-B unsupported catalysts. On alumina-supported catalysts they were unable to identify any alloy formation, probably owing to the very low amount of alloy, and to limitations of the XRD technique. It is unlikely that the few Ru3Sn7 bulk alloy particles observed here are responsible for the change in activity and selectivity. The majority of the particles in the samples with high tin loading had a core of pure ruthenium, and CO chemisorption gave again evidence that the ruthenium surface is partially blocked by tin species.The catalytic behaviour of the Ru/C and Ru-Sn/C cata-lysts towards the hydrogenation of C-C and C-0 groups has previously been tested in the reduction of 8-methyls tyrene, hydrocinnamaldehyde and cinnam-aldeh~de.~.~It has been observed that on addition of tin, the activity (expressed per g of Ru) towards the hydrogenation of the C=C double bond decreased, whereas the rate of hydro- genation of the C=O group went through a maximum at intermediate Sn :Ru ratios. Addition of tin in the hydro- genation of cinnamaldehyde increased the selectivity towards the formation of the unsaturated alcohol. On the basis of the characterization results reported in this paper, the catalytic behaviour of the Ru-Sn catalysts for hydrogenation of unsaturated aldehydes can be explained.Hydrogenation of the C-C double bond depends on the availability of ruthenium surface sites which are responsible for H, activation. Addition of tin blocks the ruthenium surface, thereby decreasing the number of ruthenium active sites and lowering the C-C double-bond hydrogenation activity. However, tin species on the ruthenium surface, when present in ionic form, would enhance the reactivity of the C=O group. Ionic tin can increase the polarization of the C=O bond, making it more reactive towards the attack of a weak nucleophilic agent such as the hydrogen chemisorbed on the nearby Ru atoms.This model of ionic tin decorating the surface of ruthenium particles is consistent with the observed activity and selectivity trend^.^,^ However, we cannot entirely rule out that, after reduction, zero-valent tin species are present on the surface of ruthenium particles, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 perhaps forming surface alloy or bimetallic Ru-Sn surface aggregates. Such submonolayer tin concentrations would go unnoticed in HREM images, and it would probably be impossible to maintain these tin species in a reduced state during specimen preparation for microscopy. The presence of tin surface species can alter the adsorption characteristics of the ruthenium surface either through an electronic effect (ligand effect) and/or through a dilution effect, which results in a modification of the geometry of the active sites needed for the reaction (ensemble effect).Conclusions The microstructural HREM characterization data of carbon- supported Ru and Ru-Sn catalysts, and the particle morph- ologies and size distributions measured by transmission electron microscopy, greatly facilitate the interpretation of activity and selectivity trends reported for selective hydro- genation of a,/?-unsaturated aldehyde^.'.^ HREM proves that in the monometallic Ru/C sample nanometre-scale ruthenium crystallites are present showing no deviation in crystal struc- ture and lattice parameters with respect to bulk ruthenium. In the bimetallic Ru-Sn/C catalysts, at low tin loading (<20 atom%), the majority of the metal particles have a structure consistent with that of pure ruthenium, without any evidence for bulk alloy formation.In these samples the metallic par- ticles have an average particle size smaller than that observed in the monometallic Ru/C sample. While electron microscopy gives information about the structure of the metal particles, it is not surface sensitive and cannot differentiate whether or not tin species decorate the surface of these ruthenium particles. CO chemisorption data complement the microscopy data and provide information about the number of Ru surface sites. From the decrease in CO uptake despite a decrease in particle size, we can con- clude that tin must be decorating the surface of these Ru par- ticles, partially blocking the Ru sites needed for CO chemisorption.This conclusion is consistent with the observed decrease in C=C bond hydrogenation activity with increased tin loading, as it is known that Ru sites are required for this reaction. From the enhanced selectivity on the Ru-Sn catalysts for hydrogenation of the C=O group of unsaturated aldehydes, we can infer that under reaction con- ditions a significant fraction of the tin on the surface must be in ionic form. In the catalysts with high tin loading (>20 atom%), HREM reveals that only a few metal particles are present in the form of ruthenium-tin alloys, while the structure of most of the metal particles still corresponds to that of pure ruth- enium.Similar to the case of the low-loading catalysts, CO chemisorption indicates that tin species partially block the ruthenium surface. These catalysts have very low activity, indicating that the presence of alloy particles is not a domi- nant feature controlling the catalytic behaviour. The high selectivity for C-0 bond hydrogenation supports the hypothesis that the excess of tin (i.e.that not involved in alloy particles) is present in ionic form. Support by CNR for G.N. is gratefully acknowledged. We thank Dr. John Mansfield and Mr. Bryan Demczyk of the University of Michigan Electron Microbeam Analysis Labor- atory for valuable assistance in the microscopy work. References 1 Z. Poltarzewski, S. Galvagno, R. Pietroapolo and P. Staiti, J.Catal., 1986, 102, 190. 2 S. Galvagno, Z. Poltarzewski, A. Donato, G. Neri and R. Pietropaolo,J. Chem. SOC.,Chem. Commun., 1986, 1729. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3 P. Louessard, J. P. Candy, J. P.Bourneville and J. M.Basset, in Structure and Reactivity of Surfaces, ed. C. Morterra, A. Zecchina and G. Costa, Elsevier, Amsterdam, 1989,p. 591. 4 V. M. Deshapande, K.Ramnrayan and C. S. Narasimhan, J. Catal., 1990,121, 174. 5 S. Galvagno, A. Donato, G. Neri and R. Pietropaolo, Cutal. Lett., 1991,8,9. S. Galvagno, A. Donato, G. Neri, G. Capannelli and R. Pietropaolo, J. Mol. Catal., 1993,78,237.V. M. Deshpande, W. R. Patterson and C. S. Narasimhan, J. Catal., 1991, 121, 165. D.J. Smith, D. White, T. Baird and J.R. Fryer, J. Catal., 1983, 81,107. 9 J. R. Anderson, in Structure of Metallic Catalysts, Academic Press, New York, 1975. 10 J. W. M. Jacobs and D. J. Schryvers,J. Catal., 1987,103,436. 11 D. White, T. Baird, J. R. Fryer, L. A. Freeman, D. J. Smith and M. Day, J. Catal., 1983,81, 119. 12 C. Solliand and M. Flueli, Surf: Sci., 1985, 156,487. 13 G. Neri, S.Galvagno and J. Schwank, J. Chem. Sw., Faruduy Trans., submitted. 14 A. Sachdev and J. Schwank, J. Catd., 1989,lu), 353. 15 A. K. Datye, A. D. Logan and N. J. Long, J. Catal., 1988,109, 76. 16 R. Sinclair and G.Thomas, Metall. Trans. A, 1978,9, 373. 17 E.Anno and R. Hoshino, Sur$ Sci., 1984,144,567. 18 J. G. Buglass, S. R. Tennison and G. M.Parkinson, Catal. Today, 1990,7,209. 19 X. Wu, B. C.Gerstein and T. S. King, J. Catul., 1990,123,43. 20 ASTM, X-Ray Powder Data File 26-504. 21 M.Hansen, in Constitution of Binary Alloy, McGraw-Hill, New York, 1958,p. 3257. 22 ASTM, X-Ray Powder Data File 18-1 143. 23 J. Schwank, K. Balakrishnan and A. Sachdev, in New Frontiers in Catalysis, Proc. ZOth Znt. Congress on Catalysis, ed. L. Guczi, F. Solymosi and P. Tetenyi, Elsevier, Amsterdam, p. 905, 1993. Paper 3/07648B;Receioed 31st December, 1993