Faraduy Discuss. Chem. SOC., 1989, 87, 1-12 Some Aspects of Catalyst Characterisation and Activity Peter B. Wells School of Chemistry, The University, Hull, HU6 7RX Knowledge of the connection between catalyst structure and catalytic activity is reviewed, examples being selected from reactions involving monophasic catalysts (oxides and sulphides, metal single crystals, zeolites) and multi- phasic catalysts (especially supported metals). Contributions to our knowl- edge from surface science, conventional structural and characterisation studies, EXAFS spectroscopy, and in situ methods of catalyst evaluation are considered. Evidence that reversible displacements of atoms in the active phase may occur during catalytic conversions is noted, as is the importance of permanently retained hydrocarbonaceous species in the establishment of reproducible catalytic activity in metals.Throughout, the complementary nature and roles of structural information and mechanistic studies is empha- sized. Faraday Discussions are special events in the scientific life of the United Kingdom and of the wider community at which we address major topics in a format of discussion that is peculiarly our own. Catalysis has been the subject of several Discussions. The first, in 1921, was entitled ‘Catalysis with reference to Newer Theories of Chemical Action’. At that meeting Langmuir proposed that reaction occurred at surfaces between adjacently adsorbed molecules, these adsorbed states being restricted to monolayers, and Lindemann discussed the kinetics of surface reactions.In 1932, at a Discussion of ‘The Adsorption of Gases on Solids’, Lennard-Jones identified the separate contributions of physical and chemical adsorption to the energy profile associated with the approach of a molecule towards a surface, and thereby located the origin of the activation energy for adsorption. In 1950 (Discussion no. 8), with Lennard-Jones now in the chair, the scientific community gathered here in Liverpool to debate the so-called geometrical and electronic factors in catalysis; the former was advanced by Beeck, who had prepared and examined oriented nickel films, the latter by Schwab and Eley who had applied electronic theories of the metallic state to catalysis by alloys. In 1966 (Discussion no. 41) and again in 1981 (Discussion no.72) we were particularly concerned with the nature of adsorbed intermediates and with the manner in which catalysts directed reaction paths in order to achieve selectivity. Today, against a background of advances in surface science, and of Discussions in 1959 and 1971 on the chemical reactivity on non-metallic solids, we are gathered to discuss ‘Catalysis by well Characterised Materials; the emphasis is clearly on studies of reactions occurring at surfaces of which we have a substantial knowledge as to their structure. In 1836 Berzelius drew the existing word ‘catalysis’ from common usage into the scientific vocabulary.’ It is variously reported of Leibig and of Bunsen that they retorted ‘If he gives the phenomenon a name, he will think he understands it’. Perhaps it is as well, at the outset of this Discussion, to remind ourselves that catalysis is a process of molecular interconversion, and whereas it behoves us to characterise our catalysts to the best of our ability, we must not be tempted into believing that, because we can describe the catalyst in great physical detail, we are thereby adding to our knowledge of catalysis.It is the connection between the characteristics of the well documented surface and the observed molecular interconversions that so intrigues us at the present 12 Catalyst Characterisation and Activity time; this is the topic that we are here to address and to which I have been asked to provide an introduction. Some catalysts consist of single chemical phases, whereas others are multiphasic.Single-phase catalysts may be subdivided into those that exhibit catalysis as a property of the external surface, and those which possess intracrystalline space so that catalysis can occur throughout the bulk of the material. These materials have been termed ‘accurate solids’ by Weisz’ or, more recently, as ‘uniform solids’ by Thomas.’ Single-phase Catalysts First, let us consider the extent to which a monphasic material which catalyses a reaction at its external surface presents to adsorbing molecules a configuration of atoms represen- ted by a simple truncation of the bulk structure. VzOs catalyses hydrocarbon oxidation in a manner that suggests that the active sites are V=O groups at the surface. Thus, activity for butane oxidation at 725 K varies with oxygen pressure in a manner that correlates with surface V=O concentration as determined by infrared ~pectroscopy.~ Also, the activity for ortho-xylene oxidation to phthalic anhydride at 673 K correlates with the extent to which the (010) face of V 2 0 s is presented to the gas phase, as judged by electron microscopy and X-ray diffraction, and this face is rich in V=O groups.”‘ In these examples, the surface behaves as would be expected if it were a simple truncation of the bulk structure.For an anisotropic solid the situation is more complex. MoS2 has a well known layer structure and important differences have been demonstrated between the catalytic activities of the basal and edge planes. Tanaka and co-workers mounted similar quantities of cut and uncut single crystal MoSz in parallel reactors and examined reactions which depended either on the presence of low co-ordination number molybdenum sites (present in the edge planes) or on the presence of Bronsted acidity (located on the basal planes).’ Thus, the rate of hydrogen isotope exchange in ethene (requiring Mo sites) occurred rapidly on cut but not on uncut crystals, whereas isomerisa- tion of 2-methyl-but- 1-ene to 2-methyl-but-2-ene (requiring acid sites) occurred at comparable rates over each catalyst bed. Such experiments further encourage the belief that the surface of a solid has properties that are predictable based on a knowledge of the bulk structure.This postulate can be tested more rigorously by examination 01 low surface area metal catalysts. In ammonia synthesis at 20 bar? and 870 K over (1 121), ( 1 120), ( l O i O ) , and (0001) faces of single crystal rhenium the rates were about 2 x lo”, 8 x lo“, 5 x lo”, and 5 x 1013 NH, molecules formed per square centimetre per second;*.’ this is thus a highly surface-sensitive reaction.Activity has been attributed to the presence in the surface of atoms having coordination numbers of ca. 7, and to the face being very open, enabling dinitrogen to diffuse freely along the troughs ‘which might be important for the dissociation of nitrogen on these planes.” On this basis, turnover frequencies per site were judged to be 314, 98, 6, and 0.03 respectively.’ LEED experiments conducted at 870 K immediately after evacuation of reactants and products from the high pressure cell showed the surface to have the original structure, i.e.it had not undergone permanent reconstruction, and it may reasonably be supposed that the observed structure was indeed responsible for the catalysis. It is remarkable how similar are the surface geometries of the ( 1 121) face of Re and the (1 1 1 ) face of Fey (the most active iron face); this again supports the view that in some systems the surface is represented by a truncation of the bulk. However, the situation in general is well known to be more complex than this. Certain faces of a number of metals important in catalysis undergo reconstruction when clean, and for others reconstruction accompanies adsorption. The delicacy of such ? 1 bar= 10’ Pa.P. B. Wells 3 processes is shown in Debe and King's study of the (100) face of tungsten;''''' below 200 K an ordered phase is formed'* [describable as W{ 100}(J2 x J2)R45"] in which sympathetic lateral displacements of surface atoms of 0.014-0.020 nm occur so that kinked rows and channels are formed in the topmost atomic layer." At higher tem- peratures, a surface phase-transition occurs to give a disordered structure; the lateral displacements are unaltered by the direction of displacement that has become ran- d ~ m ." ? ~ ~ The concept of the surface being a truncation of the bulk is thus bruised. An early example of an adsorbate-induced displacive reconstruction was reported by King and Thomas in 1980 for hydrogen adsorbed on W{lOO} at, say, 300 K and moderate coverage ( O H = 0.12).14 Hydrogen adsorption has the effect of displacing pairs of surface tungsten atoms so as to create surface dimers.A study by electron energy-loss spectroscopy by Willis has provided vibrational spectroscopic evidence for this hydrogen bridging mode and a detailed account of the movement of substrate tungsten atoms during this surface reconstruction. l 5 Subsequently, row pairing has been observed during hydrogen adsorption on the (1 10) planes of Ni and Pd. If we envisage dynamic hydrogen adsorption and desorption at these surfaces, or if we imagine the progress of hydrogen- deuterium exchange, we realise that the surface metal atoms must be in a state of intermittant motion. This is an aspect of catalysis that has for too long been ignored; many investigators have considered motions of adsorbed molecules with respect to a supposedly static catalyst surface.In future we should give greater consideration to the dynamic aspects of the catalytically active surface. Time does not permit the discussion of surface reconstructions initiated by carbon, sulphur, and other adsorbates. Suffice it to say that, where adsorbate-metal bonds are of a strength comparable with the metal-metal bonds of the active phase, the perturbation of the surface structure of a metal catalyst may be profound. Woodruff and co-workers' observation of the rotative surface displacement of Ni atoms by C is a good example.'' Such systems have been considered from a theoretical standpoint by Ibach and others" and are clearly of great practical importance in the catalysis of hydrocarbon reactions by metals.These particular surface reconstructions occur without change in the density of atoms in the surface layer. In other systems, surface atom density is not conserved; for example, Moritz and Wolf" have demonstrated by LEED intensity analysis that rows of atoms are missing in the clean surface of Au( 110) and that there are considerable displacements of atoms extending at least three layers deep. I mention this, in particular, to draw attention to the fact that, since calculations of densities of states rely on the accurate specification of the location of atoms, such reconstructions imply that electronic densities of states at the surface will differ from that of the bulk to a greater degree than might otherwise have been expected. To summarise, the characterisation of the surface region of a metal single crystal in terms of its geometrical and electronic description may in certain instances be a complex task.The corresponding complexities for oxide and sulphide single crystals are only now being addressed. The problem is compounded for high-area catalysts having supported active phases consisting of many small metallic particles. Zeolites We now turn to consider a class of materials which are uniform catalysts in a three- dimensional sense. This class includes zeolites and derivative materials and, without the same rigour, pillared clays. The 1950 Discussion contains one paper which refers to the use of zeolite as a cracking catalyst,'" a very early report. The field expanded enormously in the 1960s so that large monographs became available in the following decade.20 Zeolites are crystalline aluminosilicates having intracrystalline space to which4 Catalyst Characterisation and Activity organic and inorganic molecules and ions may gain access and where they may react.The structures of these materials may be determined by X-ray diffraction, may be imaged by high-resolution electron microscopy, may be probed by Al- and Si-n.m.r.," and the behaviour of molecules in the intracrystalline space may be modelled theoretically and presented visually by use of molecular graphics. To attempt to review this field would be absurd; to summarise the main scientific and industrial thrusts is the least that should be done to pay tribute to an area in which catalyst characterisation and its relationship to activity has been demonstrated in a particularly spectacular manner.Zeolite composition may be expressed as M,,;Al,Si,-,O,.n H 2 0 where M represents non-framework cations and Al,Si2- .04 represents the framework components; sorbed water may also be present. These materials depend, for their catalytic activity, on the presence of Brmsted acid sites created at the point of aluminium substitution in the silica lattice. Acidity may be modified by: (i) variation of the Al/Si ratio, stronger acidity being linked with isolated aluminium sites in a silica-rich environment2? and with the local environment of the pore structure; (ii) substitution of non-framework cations by NH,' and subsequent thermal conversion to H+; (iii) the alternative substitu- tion of non-framework cations by hydrated rare earth ions and subsequent processes, e.g. La( H20)3-t + zeolite -+ La( OH)2' + H' zeolite; (iv) substitution of framework ele- ments by others from Groups 111, IV, and V (B, GA, Ge, P), which give new families of materials (AlPOs, SAPOs etc.).Acidic sites are contained within the pores and cavities. The internal surface area provided by the pore system is frequently in the range 600-800m2g-', whereas the external surface area is less than 1% of the total. Zeolite structures may be classified according to pore size, and in particular according to the number of 0 atoms contained in the ring or channel that defines the pore size. Thus faujasite (Linde X and Y), mordenite, and zeolite-L contain 12T windows and are representative of large pore zeolites; ZSM-5, ferrierite, and EU-1 with 10T windows are typical of medium pore zeolites, whereas Linde A and erionite with 8T pores are small pore zeolites.Atlases of zeolite structures23 provide details concerning pore size and geometry together with information concerning the nature of the channel systems, e.g. whether or not they are interconnected. The structures of zeolites are such that they (a) limit the sizes of molecules entering the pores, (b) regulate the approach of reactants to the sites, (c) limit the size of achievable transition states, and (d) control the passage of products away from the site [see S. M. Csicsery, in ref. (19)]. The materials are shape selective by virtue of (a), and the rates of diffusion of reactants to the sites and diffusion of products away from the sites [(b) and (d)] largely determine the products formed.The diffusivities of hydrocar- bons differ enormously even when they are of the same carbon number. For example, the diffusivity of trans-but-2-ene in H-zeolite T is more than six orders of magnitude greater than that of n-butane.24 Or again, the diffusivity of p-xylene in medium-pore ZSM-5 is four orders of magnitude greater than that of the ortho- and meta-isomers.2s Limitation of transition state geometries is particularly valuable in hindering coke formation within medium-pore and small-pore zeolites. A useful summary of these factors is contained in ref. (2). 98% of zeolites used in industrial processes are of the large pore faujasite type and are used to crack petroleum to gasoline (300Gg per year worldwide in 1987)." Being large-pore materials, they accept a wide range of reactants, coking is rapid, and the catalyst has to be regenerated continuously.Y-zeolite is commonly used at a loading of ca. 10-40% with a clay binder as an amorphous matrix; rare earths may be present to improve H-transfer so as to produce alkanes and aromatics in preference to alkenes and naphthenes, and increased Si/ A1 ratios reduce acid site concentrations and hence The industrial applications of zeolites as catalysts have recently beenP. B. Wells 5 the extent of secondary cracking. Chen and Degnan summarise the situation27 by saying ‘Hybrid rare-earth-exchanged high-silica zeolites have become available, allowing the refiner to select from a wide variety of cracking catalyst compositions’.Catalyst design tailored to specific needs has become a reality, as a direct result of our synthetic and characterisation skills. Other specialised cracking processes are of importance. Medium-pore zeolites such as ZSM-5 admit normal and slightly branched alkanes and so are valuable as dewaxing catalysts, used to improve the physical properties of diesel Finally, the small pore zeolite erionite is used to remove Cs- and C,-alkanes (which have a low octane number) from gasoline as LPG.23203*7 Thus, shape selectivity is evident.*, A comprehen- sive account of the chemistry of catalytic cracking is given in ref. (28). Reactions of aromatic molecules within medium-pore zeolites have achieved con- siderable importance.For example, ZSM-5 is active for the alkylation of toluene by methanol to give para-~ylene.*~-*~ The selectivity of this reaction has been attributed to transition-state limitation, the proposal being that bulkier transition states required to produce meta- and ortho-xylene are not achievable, even at the intersections of the pore systems in ZSM-5. However, it should be noted that xylene isomerisation, in which mixtures of ortho- and meta-isomers of xylene are fed to ZSM-5, results in high yields of para-xylene. Thus, a more prudent interpretation of selective toluene alkylation would seem to be that all isomers are formed in equilibrium and that para-xylene diffuses away from the reaction site preferentially. Indeed, as mentioned earlier, the diffusivity of para-xylene in this zeolite exceeds that of the other isomers by several orders of magnitude.Two further processes merit mention. The Mobil methanol to gasoline (MTG) process using ZSM-5 type catalysts is now capable of full-scale production in New Zealand (the chemistry has been reviewed3’), and the BP/UOP Cyclar process for the conversion of LPG to aromatics and by-product hydrogen over a gallium-containing medium-pore zeolite is to be demonstrated this year at a plant currently being com- missioned at Grangerno~th.~’ The role of gallium in this catalyst is not clear, but a patent indicates3* that gallium impregnation into the zeolite rather than gallium substitu- tion of framework elements gives the preferred catalyst. A further dimension is introduced when a zeolite is used as a matrix within which to mount to a further active phase such as a transition metal.Much distinguished work has been done involving the incorporation of catalytically active Group 8 metals in zeolites using the combined properties of shape selectivity provided by the zeolite and hydrogenation/dehydrogenation activity or oxidation activity provided by the metal.33 Far-infrared spectroscopy has made a particular contribution to characterisation in this area.34 The restriction of molecular motion in a reactant molecule at such a metal site is demonstrated by the reaction of cyclopentane with de~terium.~’ At an unrestricted platinum or palladium surface at about 315 K (e.g. Pt/silica, Pd film) hydrogen atoms on both sides of the ring may undergo exchange during one residence of the hydrocarbon molecule at the metal ~ i t e .~ ’ , ~ , This gives C5HsD5 and CSDIo as products, the latter being formed from CSHsD5 molecules that achieve ring turnover before desorption. When the platinum active phase is located in the supercage of a Y-zeolite ring turnover cannot be achieved3’ because of spatial restrictions and exchange is restricted to one side of the ring and to the formation of C5H5D5. Finally, in this section, I draw your attention to the work of Mitchell and Drew3’ and of Herron and T01man~*?~~ who have reported the catalytic properties of iron and cobalt phthalocyanines synthesised in the faujasite supercage. Such materials are active for the selective oxidation of the terminal methyl groups of unactivated alkanes with iodosobenzene as the oxygen source.For example, Mitchell reports that the cobalt centre in his catalyst carries adsorbed oxygen and acts as a site for the oxidation of6 Catalyst Characterisation and Activity methylcyclohexane to cyclohexylmethanol. This ability to use zeolite cages as ‘reaction vessels of molecular d i m e n s i o n ~ ’ ~ ~ is capable of much further explotation. Multi-phase Catalysts We now turn to a consideration of catalysts which are non-uniform according to the Thomas definition3 and which consist of an active phase on a support. I shall consider supported metal catalysts in particular, but supported sulphides and supported oxides also fall into this category. Over the last two decades, great efforts have been made to demonstrate appropriate procedures for the characterisation of supported metals, and there has been concern to show that methodology is transferable from one laboratory to another, in order that literature data can be treated with confidence.Thus, standard catalyst projects have been undertaken in many parts of the world, including the U.S., the U.K., Japan, and Europe; I shall describe briefly the fourth of these. In 1976 a group of about 20 chemists from European universities with a common interest in catalysts by metals formed a study group under the auspices of the Committee of Science and Technology of the Council of Europe and embarked on a programme of catalyst characterisation. The first study concerned a Pt/silica code-named EUROPT- 1.The group wished to determine whether their apparatus, techniques, and procedures were comparable, and whether they could together characterise EUROPT-1 with sufficient confidence to be able to offer the catalyst to the scientific world as a reference material. They chose to study a highly loaded Pt/silica for which the platinum active phase would be visible in electron microscopy, so that the dispersion of platinum could be determined both from a particle-size distribution and from measurements of selective gas adsorption (H2, CO, 0’). The program also envisaged the measurement of surface area, pore-size distribution, chemical composition and trace element analysis. 6 kg of 6.3% Pt/silica was manufactured by Johnson Matthey Chemicals plc, of which half was distributed among the participants, and half was set aside to provide marketable standard samples. The programme commenced in 1977.Having identified the quantities to be determined, measurements were made using apparatus that was to hand; no effort was made to lay down specific procedures because resources were not available to construct new apparatus. Participants made those measurements that they felt most confident to perform, and results were compared. The project was completed in 1983, and the published five-part report appeared in 1985.40 EUROPT-1 contains 6.3% Pt and has a surface area of 185 f 5 m’ g-I. The material shows no signs of ageing as yet. The platinum particle size distribution extends from 1.0 to 3.5 nm and is centred at 1.8 nm; 75% of the platinum particles are s2.0nm in diameter and this corresponds to a dispersion close to 60%.The platinum in the as-received catalyst is almost completely oxidised, but may be reduced without change in platinum particle size distribution at temperatures up to 623 K. The extent of sintering above this temperature has been examined. Adsorption of hydrogen, carbon monoxide, and oxygen was examined over a wide range of temperature and pressure; the adsorption stoichiometry was found to be quite complex. The extents of adsorption when correctly interpreted are consistent with a platinum dispersion of 60% ; however, the study demonstrated that determinations of dispersion from selective gas adsorptions alone would, for this catalyst, have been unsafe. The success of this project lies in the fact that it demonstrated that concordant information on the characterisation of a supported metal catalyst can be obtained from a range of laboratories.A close reading of the report also shows that careful characterisation is a painstaking process involving, for example, selective adsorption over a wide range of temperatures and pressures, which cannot be replaced (except for comparative purposes) by simplified procedures or single-point measurements. A further section of the report, dealing with catalytic properties of EUROPT- 1 appeared in 1988.4’ The generalP. B. Wells 7 methodology of catalyst characterisation in this area has been reviewed recently by Scholten et ~ 1 . : ~ and engineering aspects of design, preparation, and performance by Lee and and by K ~ m i y a m a .~ ~ Characterisation of the detailed structure and morphology of metal particles in supported metal catalysts presents a considerable additional challenge. Substantial progress in this area is being made using the techniques of extended X-ray absorption fine structure (EXAFS) spectroscopy and the corresponding near-edge spectroscopy (NEXAFS or XANES). The application of these spectroscopies to the characterisation of supported metals has recently been EXAFS spectroscopy provides information (bond distances, neighbour types, coordination numbers) concerning the short-range environment surrounding an interrogated atom averaged over all environ- ments present in the sample. Results can therefore be used to devise models for metal particle structure and morphology.Theoretical considerations suggest that small isolated clusters of metal atoms should adopt special geometries, particularly icosahedra, in preference to the close-packed structures of the bulk metals.47 EXAFS spectroscopy should provide a clear test as to whether these structures exist in small metal particles present in supported catalysts. As Joyner has pointed a 55-atom icosahedral particle would provide a wide variety of interatomic distances and coordination numbers, whereas an f.c.c. particle of comparable size would provide only the expected a, J2a, and J3a distances. The platinum particles in EUROPT-1, for example, provide EXAFS spectra which conform to the f.c.c model. Renouprez et al. claim that 1.2 nm platinum particles in zeolite-Y at 80 K conform best to the icosahedral although, in the presence of hydrogen at higher temperatures, these same particles exhibit structural properties that appear to approach that of the face-centred cubic model more closely.Here we have an example of particle morphology changing when adsorption occurs, and again your attention is drawn to the possibility that particle morphology is mobile during adsorption/ desorption and during catalysis. Configurational changes of metal clusters have been observed by EXAFS spectros- copy. For example, the bi-capped tetrahedral structure which the cluster O S ~ ( C O ) , ~ exhibits in the free state is only partially retained on absorption of the cluster carbonyl onto silica; a proportion of the molecules 'unfold' on the support surface, as indicated by the appearance of new long non-bonded distances between osmium atoms.so The process is reversible in that the osmium clusters can be washed off the silica support as the original material.EXAFS spectroscopy sometimes reveals the presence of very short metal-metal bonds. An EXAFS study of small rhodium clusters in a 0.57 wt% Rh/alumina has shown the presence of the Rho-Rho distance at 0.265 nm and of a Rh"-Rh"+ distance at 0.195 nm, the former with a coordination number of 5.5 and the latter 0.6.46 The rhodium ions are interpreted as providing the anchoring of the rhodium metal cluster to the oxygen ions of the alumina support. Use of EXAFS spectroscopy to probe catalytically active sites presents difficulties and challenges.We are well aware that some reactions (such as hydrogenolysis) are structure sensitive and appear to require a group of metal atoms to form an active site, whereas others (such as hydrogenation) are less sensitive to catalyst structure and may, if necessary, proceed at isolated metal atom sites. My research group has recently demonstrated by EXAFS spectroscopy that the sudden disappearance of ethane hydro- genolysis activity in a series of osmium catalysts derived from metal cluster carbonyls was due to the nuclearity of the cluster having dropped below a critical level." As expected, the hydrogenation activities of all the cluster-derived catalysts, including that inactive for hydrogenolysis, were normal. The minimum critical level that defines the smallest active site for hydrogenolysis has yet to be identified.Particularly useful information by way of characterisation can be obtained when a catalyst sample can be interrogated in EXAFS at two metal edges. For example, at an8 Catalyst Characterisation and Activity early stage in the development of the technique, Sinfelt and co-workers drew conclusions about the structure of bimetallic particles.52 They studied, inter alia, supported osmium- copper catalysts for which the average metal particle diameter was 0.16nm and the 0s:Cu atomic ratio was 1 : l . Spectra at the 0 s L,-edge and the Cu K-edge showed that the coordination number of 0 s and 0 s was 9, and of Cu about 0 s was 2, whereas that of 0 s about Cu was 6 and of Cu about Cu was 5 (all values kca.1). The model proposed was of a central core of osmium atoms covered with small patches or multiplets of copper atoms located on the surface. Other models, such as layers of Cu atoms on rafts of osmium, could be ruled out, and the copper concentration was insufficient to provide a complete surface layer around the osmium cores. We may be reasonably satisfied that procedures and expertise are now available to characterise supported metal catalysts ex situ in the manner described for EUROPT-1 accompanied, where appropriate, by EXAFS spectroscopy. However, characterisation should be taken one stage further; the most valid information is that which relates to the catalyst in situ in its working condition. Again, I draw on examples involving supported metals.One assumption that we tend to make is that the structure of the active phase in situ is the same as that detected ex situ after reaction, or is that expected on thermodynamic considerations. In the majority of instances this may well be the case, and few laboratories are routinely equipped with X-ray diffraction facilities for in situ structural studies. However, Palczewska reported in 1976 that palladium particles of high disper- sion are unable to attain the P-hydride phase,’, and this influences selectivity in ethyne hydrogenation. In a poster at this meeting HutchingsS4 reports that a Fischer-Tropsch catalyst, containing cobalt in the expected f.c.c. phase as examined ex situ, is transformed in situ so that the cobalt acquires the hitherto unknown b.c.c.structure under hydrocarbon synthesis conditions at 493 K. Examination of the catalyst after reaction shows that the structural change is reversible. Hopefully, such changes are exceptional. These two reports merely provide a flavour of the surprises that can be expected of in situ studies of the structure of working catalysts. Characterisation of Non-transient Hydrocarbon Components at Catalyst Surfaces When catalysis occurs, reaction may involve not only the reactants in their adsorbed states, but also other adsorbed species derived from the reactants which are permanent or long-term temporary residents on the surface. Some such species may simply block the surface, others may participate in the progress of reaction by facilitating hydrogen- atom transfer.Characterisation must extend to an examination of such species, in order that the origins of activity can be properly understood. In 1966, Cormack, Thomson and Webb at Glasgow reported experiments in which C-labelled ethene was adsorbed on several metal/alumina catalysts at 293 K and the fraction of adsorbed hydrocarbon that could not be removed by hydrogenation or evacuation was determined from the measured radioactive count rate of the catalyst surface.” The fractions were: Pd, 65%; Ni, 24%; Rh, 23%; Ir, 16%; Pt, 7%. The corresponding fractions for “C-labelled ethyne adsorption were higher. Isotherms for the adsorption of I4C-labelled ethyne, ethene, and propene showed two distinct regions, the so-called primary and secondary regions, the primary region corresponding to permanent retention of hydrocarbonaceous species on the metal surface to the extent of ca.a monolayer and the secondary region corresponding to adsorption on top of this first adsorbed layer.” The stoichiometry of the permanently retained material was determined; for example, values for ethyne and ethene adsorption on 5% Rh/silica5’ were C2Hl.x and C2H,.I, and for propene adsorption on 5% Rh/alumina,sx was C,HS.2. This led to the statement by Thomson and Webb of a new view of metal-catalysed 14P. B. Wells 9 hydrocarbon reactions, which is well summarised in a Chemical Communication of 197659: '. . . We propose, as a general mechanism for hydrogenation, a model.. . (in which). . . hydrogenation should be interpreted as hydrogen transfer between an adsor- bed hydrocarbon species, M-C2H,, and adsorbed olefin.It should not be regarded as hydrogen addition direct to adsorbed olefin'. This later received support from studies of supported low-nuclearity osmium clusters derived from Os6( CO, 8); ethene hydrogena- tion over fresh catalysts of this type was accompanied by alkyl-ligand formation detected by infrared spectroscopy, and initial activity increased as hydrogen atom transfer from such hydrocarbon ligands came to predominate over transfer from simple metal atom sites6' Finally, in a Chemical Communication of 1977'' the Glasgow group stated ". . . under reaction conditions the catalytic hydrogenation is associated with the secon- dary adsorption region.. . the secondary adsorption.. . involves the formation of over- layers'.Thus, the concept that it is molecules adsorbed in the secondary adsorption region, ie., in a second adsorbed layer, that contribute to molecular turnover was securely established (although it appeared as an anti-Langmuirian heresy at the time). This work was soon to receive substantial support and firm structural definition from surface-science studies when it was reported by I b a ~ h , " , ~ ~ S o m ~ r j a i , ' ~ - ~ ~ and K e s m ~ d e I ~ ~ that the normal adsorbed state of ethene at modest temperatures on the (111) faces of Rh, Pd, and Pt is not adsorbed --C2H4 but adsorbed -C2H3 in the form of ethylidyne. The structure, packing, and reactivity of ethylidyne on (1 11) faces of the noble Group 8 metals has provided a detailed characterisation of the surface state of the ~ystem,".'~ and other related work has provided information on the presence at higher temperatures of other hydrocarbonaceous residues such as adsorbed -CH, -CH2, --CH,, -C2H, and --C2H2.This has led to new formulations by Somorjai of the mechanism of ethene hydrogena- tion at modest temperatures' (below 350 K for Rh) where the ethylidyne species are long-lived. These mechanisms appear not to involve the participation of the conventional diadsorbed state of ethene and the half-hydrogenated state adsorbed-thyl. This presents problems to those of us who have used Kemball t h e ~ r y ~ ' , ~ ' to interpret in some detail the deuterium distributions in the products of ethene hydrogenation obtained over a range of metals and under a variety of condition^.^'-^^ The Somorjai mechanism does not easily explain the experimental observations that all possible deuteriated ethenes and ethanes are formed as initial products, and that (in the C, system) hydrogenation is accompanied by double-bond isomerisation.Moreover, each Group 8 metal provides a deuterium distribution in the products which, after the application of Kemball theory, provides a characteristic fingerprint for the metal. In particular, the fingerprint for the h.c.p. metals Ru, Re, and 0 s differs greatly from that for the f.c.c. metals Rh, Pd, Ir, and Pt. Clearly, the metal has a profound influence on the course of reaction, however Thomson- Webb and Somorjai data are interpreted. Considerable work needs to be done to draw together this body of varied information, paying due regard to the various different forms of catalyst employed (evaporated films, supported metals, metal crystals). It is sobering that in 1989 we are re-assessing the mechanism of the hydrogenation of the simplest alkene! However, we must accept that, for metal-catalysed hydrogenation at ca.room temperature, a well characterised catalyst in its working state is one for which we must take into account the nature of permanent or long-lived hydrocar- bonaceous residues at the surface. The direct detection of adsorbed ethylidyne and ethylidene is of great importance in that it extends our horizons in terms of what is acceptable in discussions of mechanism, and new horizons always generate excitement. Twenty years ago my group at Hull studied by deuterium-tracer methods the hydrogenation of buta-1,2-diene at 293 K over and demonstrated that the reproducible and sudden onset of butane formation (selectivity breakdown) was due to the intervention of hydrogen addition to adsorbed butylidene.By extrapolation, we attributed the premature breakdown of10 Catal-vst Characterisation and Activity selectivity in ethyne hydrogenation to ethylidene hydr~genation.~~ That proposal, which was not warmly received at the time, now appears more acceptable. The adsorption of ethene as adsorbed ethylidyne shows that this adsorbate achieves maximum ligancy with the metal surface via hydrogen-atom transfer. To what extent shall we now find this to be a general phenomenon in hydrocarbon reactivity at metal surfaces? The widespread occurrence of such processes will cause us to consider many novel reaction mechanisms.To take one example, Joyner has recently discussed the mechanism of the Fischer-Tropsch hydrocarbon synthesis in the light of current surface science and proposes: ( i ) an initiation step involving reaction between two adsorbed methylene species to give adsorbed ethylidene; (ii) further reaction with adsorbed methylene gives adsorbed n- and iso-propylidene (via a metallocyle and a carbonium ion).*' In this way, rapid homologation and chain-branching characteristics are simultaneously interpreted. If the maximum ligancy of adsorbed intermediates is indeed important, then we shall see many new mechanistic proposals of this type. Finally, I wish to mention a class of metal-catalysed reactions that throw down new challenges to the characterisation of the working catalyst.I refer to attempts being made in my laboratory and elsewhere to create heterogeneous catalysts capable of producing optically active products, for example by asymmetric hydrogenation. We are studying the little reported Orito in which platinum modified by the presence of a cinchona alkaloid at its surface becomes active for the hydrogenation of an a-ketoester (e.g. methyl pyruvate) to an optically active product [ e . g . R-( +)- methyllactate]. This reaction occurs over cinchonidine-modified EUROPT- 1 at room temperature under 10 bar hydrogen pressure giving an optical yield close to 90% .86 Deuterium tracer studies show that the reaction is a simple addition of two deuterium atoms across the a-keto group.One approach to an interpretation is to suppose that the large L-shaped cinchonidine molecules undergo ordered adsorption on the platinum particles so as to leave exposed shaped ensembles of platinum atoms appropriate for the formation of one optical form of the product. Such an ordered adsorption has been simulated by molecular graphics. This model, if correct, prompts the question as to what causes cinchonidine to exhibit such an ordered adsorption. Curiously, good optical yields are obtained only if the cinchonidine solution and the platinum catalyst are equilibrated in the presence of oxygen. Anaerobic modification provides inferior optical yields.x6 One postulate is that ordered co-adsorption of oxvgen and cinchonidine is occurring (similar, in some respects to the ordered co-adsorption on Pt( 11 1 ) of carbon monoxide and and that adsorbed oxygen is removed by hydrogen in the initial stages of reaction leaving the shaped ensembles in place for optically selective a-ketoester hydrogenation to proceed.If we are to progress other than empirically towards the highly desirable goal of designing optically selective catalysis we need basic knowledge from surface science concerning the behaviour of large organic molecules at metal surfaces and possibly, as I have hinted, of ordered co-adsorption behaviour. That stretches the concept of catalyst characterisation somewhat, and indicates one path among many that surface science and catalysis must tread together.I must close. As my title indicated, I have chosen to discuss some aspects of this very extensive subject; my choice has been personal, and I apologise to those of you who would wish that other topics had been mentioned. In particular, I regret that 1 have not been able to address in situ characterisation in any detail. However, I hope that I have demonstrated that our understanding of catalytic activity is now inextricably bound up with our knowledge of catalyst structure and of the fine detail and atomic make-up of the catalytically active surface. We who work in heterogeneous catalysis tend to regard somewhat wistfully the natural catalysts, the enzymes, and envy Nature the selectivities and turnover frequencies that it is able to achieve at ambient temperatures with catalysts that have developed on the evolution timescale.Our science is now beginning to emulate, albeit clumsily, that of the natural world. Some zeolites are ableP. B. 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