摘要:

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. Wells 1 1 to achieve apparently enormous turnover frequencies in alkene isomerisation;’2 the metal-phthalocyanine-in-faujasite catalysts mimic enzymes in their terminal-methyl oxidation ~electivity;~’,’~ modified metals capable of optical selectivity8’-86’s8 have, in a formal sense, enzyme-like properties. As our scientific ability to characterise materials improves further, who knows where we shall be able to take our subject of catalysis? References 1 J. J. Berzelius, Edinburgh New Philosoph. J., 1836, 21, 223. 2 P. B. Weisz, Pure Appl. Chem., 1980, 52, 2091. 3 J. M.Thomas, Angew. Chem. (international edn.), 1988, 27, 1673. 4 K. Mori, A. Mijamoto and Y . Murakami, J. Phys. Chem., 1985, 89, 4265. 5 M. Gasior and T. Manchej, J. Catal., 1983, 83, 472. 6 A. Legrouri, T. Baird and J. R. Fryer, React. Solids, 1988, 5, 53. 7 K. Tanaka and T. Okuhara, Proc. Climax 3rd Int. Con,$ Chem. Uses of Molybdenum, ed. H. F. Barny and P. C. H. Mitchell (Climax, Ann Arbor, Michigan 1979), p. 170. 8 M. 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Nguyen, J. Synth. Org. Chem. Jpn, 1979, 37, 173. 82 Y. Orito, S. lmai and S. Niwa, Nippon Kagaku Kaishi, 1979, 11 18. 83 Y. Orito, S. lmai and S. Niwa, Nippon Kagaku Kaishi, 1980, 670. 84 Y. Orito, S. lmai and S. Niwa, Nippon Kagaku Kaishi, 1982, 137. 966, 62, 433. 85 H. U. Blaser, H. P. Jallet, D. M. Monti, J. F. Reber and J. T. Wehrli, paper presented at the 1st Int. 86 P. Meheux, R. B. Moyes, I . M. Sutherland and P. B. Wells, unpublished work. 87 C . M. Mate and G. A. Somorjai, Surf Sci., 1985, 160, 542. 88 Y. Izumi, Adti. Caral., 1983, 32, 215. Symp. o n Heterogeneous Catalysis and Fine Chemicals, Poitiers, 1988. Paper 9/01711 I; Received 11 th April, 1989

ISSN:0301-7249    

DOI:10.1039/DC9898700001

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Furuduy Discuss. Chem. SOC., 1989, 87, 13-21 Oxidative Dimerization of Methane over well defined Lithium-promoted Magnesium Oxide Catalysts Jack H. Lunsford," Mark D. Cisneros, Paul G. Hinson, Youdong Tong and Hongsheng Zhang Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A. Pure MgO samples having greatly different morphologies and lithium- promoted catalysts derived from these materials have been examined by electron microscopy. In addition, their ability to generate and react with gas-phase methyl radicals has been determined using e.s.r. in combination with a matrix-isolation technique. Lithium caused the MgO to sinter severely at 700 "C, but the resulting material showed improved activity for the gener- ation of methyl radicals. These factors gave rise to catalysts which were moderately active and selective for the oxidative dimerization of CH4 at 700 "C (up to 20% conversion, 66% C2+ selectivity).The results are par- ticularly significant in that catalysts which had been prepared such that the lithium was initially present as LizO on the surface exhibited almost constant C2+ production as the LizO was converted to Li2C0,. Thus, neither Li20 nor Li2C0, is the active phase for the selective conversion of CH,, . A catalyst prepared from LilC03 and MgO sintered much less extensively, produced more CH; radicals and gave higher yields of C2+ hydrocarbons. A com- parison of these catalytic properties suggests that surface area and related morphological factors do not have an adverse effect on the oxidative dimeriz- ation reaction, provided the surface can be rendered inert to secondary reactions with methyl radicals.Among the growing number of catalysts which have been reported to be active and selective for the oxidative dimerization of methane, lithium-promoted MgO ( Li'/ MgO) is the most thoroughly studied.'-'' Although Li+/MgO is not particularly active, it ranks as one of the better catalysts with respect to combined CH, conversion and selectivities for higher hydrocarbons, with C2+ yields of ca. 20%. (Here C2 refers to ethane and ethylene and C2+ refers to all hydrocarbons except CH,.) In the initial work on this system Lunsford and co-workers' proposed a mechanism that involves the formation of CH; radicals at specific 0- centres. These centres result from the substitution of Lit for Mg*+ ions in the metal oxide.The CH; radicals may either enter the gas phase, where they couple to form C2H6:4 CH;+CH;+M --+ C2H,+M ( 1 ) CH;+O, % CH3OS + CO,. (2) or react with 0, to form CH30S radicals: CH30; is an intermediate in the formation of C02. Moreover, in a typical catalyst bed the CH; radicals also would collide many times with the surface before they couple, and these surface reactions may result in the formation of CO,, probably via the formation of CH30- ions: (3) CH;+OI- -+ CH,O,+e +-+ CO,. The C2+ yield by secondary is limited both by these undesirable reactions involving CH; radicals and reactions of the hydrocarbon products. 1314 Oxidative Dimerization of Methane Mehandru et all6 showed in a theoretical study that an 0- ion in a Mg,,O:of cluster model is indeed capable of abstracting an H' atom from CH4, and reaction of the resulting CH; radical with the surface to form CH,O, is favoured when corner Mg*+ sites are available.These results suggest that roughened surfaces or small crystallites would promote the non-selective oxidation of methane; by contrast extended (100) surfaces would favour the oxidative coupling reaction. An attempt has been made in the present study to test the relationship between surface morphology and reactivity. First, the reaction rates of CH; radicals with several well characterized, pure MgO and Li-promoted MgO surfaces were determined and secondly, the catalytic properties of Li+/MgO catalysts prepared from different starting materials were compared. Experimental The magnesium oxides used in this study were Johnson Matthey Puratronic Grade (99.998%), MgO (JM), and Aldrich Gold Label (99.99%), MgO (A). The former was chosen because it exists in the form of small cubes (see below) similar to those which have been observed when Mg is burned in air.In order to prevent the conversion of the oxide to the hydroxide, which would occur using the conventional aqueous impregna- tion method, the lithium was added as methyl-lithium in diethyl ether (Aldrich, 1.4 mol dmP3). A conventional catalyst was prepared using Aldrich Gold Label lithium carbonate (99.997% ). A catalyst designated as Li+/MgO (JM) was prepared from the Johnson Matthey MgO by first heating the pure oxide under vacuum to 300 "C for 12 h and to 320 "C for 1 h.In a glovebox (under argon) the LiCH, solution was added to the MgO, and the ether was removed by evacuation. The catalyst powder was pressed into wafers and sieved to 20-40 mesh chips without exposure to air. While still under an inert atmosphere the catalyst (4g) was loaded into a reactor. Above the catalyst was a layer of 20-40 mesh fused-quartz chips which served to reduce the free volume and to preheat the reactant gases. The sample was subsequently heated for 6-8 h in flowing O2 at the reaction temperature prior to introducing the feed mixture. A second catalyst was prepared via Mg(OH)2, which was obtained by boiling Aldrich Gold Label MgO in water and evaporating the slurry to dryness. The material was converted back to the oxide by heating it for 2 h at 500 "C under flowing 02.The catalyst, Li+/MgO (A), was then prepared in the same manner as the Li+/MgO (JM) catalyst. A more conventional catalyst, Li+/ MgO( Li2C03), was prepared by heating an Li2C03 solution containing Aldrich MgO to 100 "C for several hours and then evaporating the slurry to dryness. The sample was kept at 140 "C for 3 days in a drying oven, sieved to 20-40 mesh and heated in the reactor to 700°C under flowing oxygen. Reactions were carried out in a single-pass fused-quartz reactor similar to that described previously." The reactor was 22 mm i.d., and the gases exited the catalyst bed into 2 mm i.d. tubing. The total pressure was 760 Torr?; this included a feed of 315 Torr CH, and 62 Torr 02, with the remainder being He and a small amount of N,, which served as an internal standard.The flow rate was 50 cm3 min-'. The samples were analysed by ICP after the catalytic reaction and the Li contents, in wt O/O, were the following: Li'/ MgO (JM), 4.8; Lif/ MgO (A), 4.5; Li+/ MgO ( Li2C03), 5.5. The matrix-isolation electron spin resonance (m.i.e.s.r.) system used to study the formation and reactions of methyl radicals has been described in detail elsewhere.'* The system includes a heated catalyst that is upstream from a sapphire rod maintained at 15 K. The reactant gases pass over the catalyst and through a leak into a differentially t l Torr = 101 325/760 Pa.Faraday Discuss. Chem. SOC., 1989. Vol. 87 Plate 1. ( a ) MgO (JM) after thermal treatment at 320 "C; ( b ) MgO (A) after thermal treatment at 500 "C; ( c ) Li+/MgO (JM) used catalyst; ( d ) Li+/MgO (A) used catalyst.Catalytic reactions were carried out at 700 "C. J. H. Lunsford et al. (Facing p. 14)Faraday Discuss. Chem. SOC., 1989, Vol. 87 Plate 2. ( a ) MgO (JM) after thermal treatment at 700 "C; ( b ) MgO (A) after thermal treatment at 700 "C; ( c ) Li+/MgO (Li2C03) used catalyst. J. H. Lunsford et al.J. H. Lunsford et al. 15 pumped region. Radicals that emanate from the catalyst bed are frozen in an argon matrix on the sapphire rod and are analysed by e.s.r. spectroscopy. The system has been modified so that a second catalyst may be placed between the radical source and the leak. In this manner it is possible to determine not only the ability of catalysts to generate radicals but also the rate at which radicals react with metal oxides.Both the used catalyst samples and the pure MgO samples were heated in flowing O2 for 1.5-2.0 h at 500°C. The oxides were first operated as radical generators at 700°C and then as scavengers at 470 "C. After use as a radical generator the Li+/MgO (JM) was given an intermittent treatment in 0, at 500 "C to remove carbon from the surface. When reactions of CH; radicals with the oxides were being studied, Sm203 was the radical source. Electron micrographs of the starting materials and the used catalysts were obtained using a Philips model EM 400T transmission electron microscope at an accelerating potential of 120 kV. Carbon-coated 3 mm diameter copper grids were dusted with the samples which had been ground into a powder.Magic-angle spinning solid-state n.m.r. (m.a.s. n.m.r.) spectra of 'Li in the used catalyst Li+/MgO (JM) were obtained using a Bruker MSL-300 spectrometer. Data were collected with the samples spinning at 3-4 kHz. Chemical shifts are reported relative to Li+ in an aqueous LiCl solution. Downfield chemical shifts are taken to be positive. Results Electron Micrographs Electron micrographs for the pure samples and the used catalysts are shown in plates 1 and 2. The state of the MgO (JM) prior to the addition of LiCH3 is described in plate 1( a ) . The MgO used to obtain this micrograph had been heated under vacuum at 300 "C for 16 h and then at 320 "C for 1 h. Although the morphology is generally cubic, a closer inspection of the micrographs reveals terraces on the surface.The MgO (A) was prepared by the decomposition of Mg(OH), at 500 "C under flowing 02, and the general platelike morphology of the sample depicted in plate 1( 6) reflects the relic structure of Mg(OH), . I 9 During the catalytic reaction Li+ promotes extensive sintering of the Li+/MgO, which is reflected in a growth of the particles from ca. 30 nm in the MgO (JM) to ca. 300 nm in Li+/MgO (JM) [plate l(c)]. This sintering has been studied previously by Mirodatos et aL6 Although only relatively few particles are shown in plate l ( d ) , the shape and size distribution are representative of the catalyst sample. By comparison, the particles of the used Li+/MgO (Li,CO,) sample [plate 2 ( c ) ] have a much broader size distribution and, on average, are considerably smaller than either of the samples prepared using LiCH3 as the source of lithium.Lithium ions, rather than thermal treatment of the sample to 700 "C under reaction conditions, are responsible for the extensive sintering reported here. Plater0 et aZ.,,' for example, found that thermal treatment of MgO at 800°C in H20 resulted in particles which were only ca. 80nm on a side. The MgO samples prepared for the m.i.e.s.r. experiments were heated to 700 "C and the electron micrographs are shown in plates 2( a ) and ( 6 ) . Clearly, the MgO (A) sample formed regular cubes which remained in the relic structure of hexagonal Mg(OH)2. The size of the cubes (ca. 7 nm) was considerably smaller than the corresponding size of the MgO (JM) particles. M.A.S.N.M.R. of 'Li in MgO The substitution of Li+ for Mg2+ is a prerequisite for the formation of [Li+O-] centres in Mg0,2'*22 yet the extent of this ionic substitution has been determined only approxi- mately. Since 'Li is a quadrupolar nucleus one might expect that the local symmetry16 Oxidative Dimerization of Methane 30 20 10 0 -10 -20 -30 -40 shift (ppm) Fig. 1. Lithium-7 n.m.r. spectra of an Li'/MgO (JM) used catalyst: ( a ) before washing with cold water to remove excess Li,C03, ( b ) after wishing. of the Li' ion would be reflected in its m.a.s. n.m.r. spectrum. The high cubic symmetry of the MgO crystal would give a single narrow line, whereas the low symmetry of Li2C03 would yield a multiplet of lines. The results of the n.m.r.experiments, shown in fig. 1, conform to these expectations. Spectrum ( a ) is that of a used Li+/MgO (JM) catalyst sample which contained an appreciable amount of lithium as Li,CO, , as determined by X-ray diffraction. The broad resonance, centred at 1 .O ppm, presumably results from the Li2C03. This resonance is broad because of magnetic dipole interactions between 7Li and other 7Li nuclei or perhaps between 7Li and 'H from adsorbed water. A weak, narrow resonance at 2.3 ppm is attributed to 7Li in a high-symmetry crystal field; i.e. Li+ ions which have substituted for Mg2+ ions in MgO. The relative areas of the narrow resonance and the broad resonance indicate that ca. 0.08% of the lithium is inside the crystal. This value corresponds to 0.004 wt % Li in MgO, which falls within the range 0.004-0.006 wt O h , as determined by Abraham and co-workers." When the sample was washed briefly in cold water to remove the surface Li2C03, the broad line was greatly diminished relative to the narrow resonance at 2.3 ppm, as shown in spectrum ( b ) , and a narrow resonance was observed at 1.0 ppm, with the accompanying quadrupolar splitting.The latter resonances are attributed to highly dispersed Li2C03. Methyl-radical Generation and Reaction The ability of the several samples to generate gas-phase CH; radicals and to react with these radicals is summarized in table 1 . The differences in the specific rates ( i e . rates per unit surface area) of radical formation over MgO (JM) and MgO (A) are within experimental error. When compared on the basis of unit surface area, it is evident that the addition of lithium has a marked positive effect on the rate of radical formation.The difference in specific activities of the two used Li+/MgO (JM) and Li'/MgO (A) catalysts may not be as great as indicated, since there is considerable error in the determination of surface areas in the range 0.2-0.4 m2 g-I. The electron micrographsJ. H. Lunsford et al. 17 Table 1. Generation of gas-phase methyl radicals and their reaction with catalytic surfaces" rel. rate of formation rel. reaction efficiency - 7 --I metal surface oxide area/m2 g-' g-' m ' g-- ' m - MgO (JM) 38 3 0.3 26 0.7 MgO (A) 92 9 0.5 100 1 Li+/MgO (JM) 0.18 18 500 8 Li'/MgO (A) 0.45 20 220 5 Li+/ MgO ( Li2C03) 2.7 100 180 3 " T = 700 "C, P = 1 Torr for CH; radical formation; T = 470 "C, P = 1.7-1.9 Torr for CHI, radicals reacting with a catalyst surface. Flow rates: Ar = 3.7 cm3 min-'; CH4 = 1.1 cm' min-'; O2 = 0.025 cm3 min-'. of plates l ( c ) and ( d ) , in fact, suggest that the surface areas are comparable, which indicates that the specific rate of CH; radical formation is essentially equivalent.The specific activity of the three lithium-promoted catalysts may be the same within experi- mental error. On a per gram basis, however, the Lif/MgO (Li,C03) catalyst was considerably more active in generating CH; radicals than either of the two catalysts prepared using LiCH3. Reaction efficiencies of the oxides are reported relative to quartz chips by using the equation where [CH;], and [CH;lS are the radical amounts determined by e.s.r. with quartz chips and the oxide of interest, respectively, as the scavenger.Quartz chips are almost inert for both generating and scavenging methyl radicals. The amounts of the two MgO samples and the two used Li+/MgO catalysts were adjusted so that the total surface area within each set was approximately equivalent. In addition, a small correction was made to account for the fact that the Li+/MgO samples were weak radical generators under the reaction conditions. That is, the rate of radical formation was determined with no Sm203 in the system, but with Li'/MgO present in the location of the radical scavenger. Without this correction the values of ER for the Li+/MgO (JM and A) samples were actually negative. The errors introduced by this effect and the small surface areas make a comparison of the specific reaction efficiencies for the Li+/MgO catalysts meaningless.Nevertheless, it is evident that on a per gram basis the reaction efficiency is small. In view of the very different morphologies it was surprising that the two pure MgO samples exhibited specific activities which were similar. Catalytic Results The results of the catalytic experiments are summarized in fig. 2 and table 2. The Li'/MgO (JM) samples, which had been oxidized but not exposed to CO?, nearly reached steady-state activity for the formation of C2+ products within the first 5 min of operation at 700°C, and that same state was maintained for 27 h. By contrast, the formation of gas-phase C02 was negligible for the first 2 h, consistent with the conversion of Li20 to Li2C03 on the surface.Moreover, the CH, conversion decreased from 18 to 15%, and the reaction was oxygen limited over this same period. Based on the gas-phase composition alone one might conclude that the reaction was 100% selective for the18 Oxidative Dimerization of Methane i '-v-v-v-v+.- v-v-v O M = 1 2 3 4 5 6 Fig. 2. stream 62 Torr Change in CH, and O2 consumption and product distribution as a function of time on over 4 g Li+/MgO (JM) operating at 700 "C. The feed gas consisted of 315 Torr CH, and ' O-, at a flow rate of 50 cm3 min-'. Methane converted: 0, to C,H,; A, to C2HJ; 7 , to C,s; +, to CO-,. 0, Total pressure of CH, converted; 0, total pressure of O2 converted. Table 2. Oxidative dimerization of methane over Lis/ MgO catalysts" selectivity CH4 yield catalyst T/"C conv.(%) (%) C2H, C-,H, C, CO-,h Li+/MgO (JM) 650' 14.4 6.1 25.0 17.7 3.5 53.8 700 14.6 8.4 28.4 29.0 4.6 38.0 Li+/MgO (A) 700 13.0 7.7 29.6 29.3 4.2 36.9 Li+/ MgO ( Li2C03) 7 00 20.3 12.1 26.5 33.1 6.3 34.2 '' Data are reported for catalysts that had been on stream for 10 h.( Surface area of the catalyst used was 2.3 m2 g I . Selectivity for CO was < 1 '/o. formation of C2+ hydrocarbons, but the absence of CO? in the gas phase is only a result of surface reactions between COz and LizO. Almost identical results were obtained with the Li+/MgO (A) catalyst, but the Li'/MgO (Li,CO,), which of course had a full complement of surface carbonate at the beginning of the reaction, reached a steady state with respect to CH, conversion and the formation of all products within the first 10 min of operation.That steady state was maintained for 26 h. As shown in table 2, the steady-state methane conversions, C2 and C3 selectivities, and C2+ yields were very similar for the Li'/MgO (JM) and the Li'/MgO (A) catalysts. The C2+ yields of 7.7-8.4'/0 were unexpectedly poor for these catalysts. The results were even worse when the Li'/MgO (JM) catalyst was operated at 650°C. The Li'/MgO ( Li,CO,) catalyst, however, exhibited considerably better performance, both with respect to CH, conversion and C2+ selectivity. Based on the greater surface area of this material it is not surprising that the activity was greater, but the improved selectivity must reflectJ. H.Lunsford et al. 19 an increase in the concentration of CH; radicals. A C2H4:C2H6 ratio of nearly unity was observed for both the Li'/MgO (JM) and Li+/MgO (A) catalysts; however, this value was 1.2 for the Li'/ MgO ( Li,C03) catalyst. Discussion The electron micrographs of plates 1 and 2 demonstrate that the morphology of the steady-state Li+/MgO catalysts is not influenced by the form of the starting MgO, but the type of lithium compound which is employed has a more significant effect on the state of the final material. Since the resulting catalysts prepared from LiCH3 and MgO (JM) or MgO (A) were essentially the same, it was not possible to test the theoretical prediction of Mehandu et af.I6 that Mg2+ in low-coordination (e.g. corner sites) would limit the desorption of CH; from the surface and thereby modify C2+ selectivity.The catalytic results obtained wih Li+/ MgO ( Li2C03) suggest just the opposite; i.e. that the C2+ selectivity was greater for this material, which had a higher surface area and a greater number of small crystallites. The pure MgO samples also exhibited quite different morphologies or crystallite size, but the radical scavenging experiments demonstrated that the MgO (A) sample, with many more corner sites, did not manifest a significantly greater specific activity for reaction with methyl radicals than did the MgO (JM) sample. Thus, the importance of corner sites, defect sites, kinks etc. which would enhance the concentration of Mg2+ in low coordination is not evident from these experiments.In fact, a much more important factor appears to be the surface area available for reaction. The presence of Li2C03 obviously inhibits the surface reactions of CH; radicals. This in part may be a result of particle growth, and hence a decrease in the number of low-coordination sites, but it may also result from a partial coverage of the surface by an inactive phase. Previously X-ray photoelectron spectroscopic studies showed that at 7 wt o/o Li up to 50% of the surface was covered by Li2C03.' The image contrast in the exterior region of the catalyst particles of plate 1 also suggests extensive coverage of the surface by Li2C03. The results of table 1 confirm that lithium also promotes the generation of gas-phase CH; radicals. A correlation between the presence of [Li+O-] centres and the rate of formation of CH; radicals has been shown previously,2 and the catalytic results (see below) further demonstrate that specific sites on the surface, in contrast to a Li2C03 or LizO phase, are responsible for the formation of C2+ compounds.There is good agreement between the ability of the three catalysts to generate CH; radicals, per gram of sample, and the steady-state catalytic results. The fivefold greater activity of the Li'/MgO (Li,CO,) in producing radicals is consistent with the higher CH4 conversion and particularly the C2+ selectivity. It is important to note that the rate of formation of ethane uia radical coupling is proportional to the square of the CH; radical concentration. These results also indicate that surface area per se does not adversely affect the C, yield, as was suggested by Aika and co-worker~.'~ However, for a catalyst to be active and selective the available surface ( a ) must be effective in generating methyl radicals and ( b ) must not function as a sink for methyl radicals by secondary reactions.The potential role of Li2C03 or Li20 as active centres for the oxidative dimerization of CH, is indirectly addressed in the non-steady-state catalytic experiments. Initially the Li+/MgO (JM) catalyst used to obtain the data of fig. 2 contained Li20 on its surface, but during the first 2.5 h of reaction this oxide was converted to the carbonate. In this same period, however, the rate of formation of C, and C3 hydrocarbons remained essentially constant, indicating that the formation of C2H6, as well as subsequent reactions to C2H4 and C3 products, did not occur on the LizO or Li2C03 phase.Recent experiments in our laboratory show that pure Li,O is somewhat active for the partial oxidation of CH, at 700°C, but at the lithium levels used in the present study the contribution of Li,O would be small. Moreover, Li,C0320 Oxidative Dimerization of Methane is essentially inactive, therefore as the carbonate is formed the small contribution of LizO to the activity would decrease. It is important to contrast the constant rate for the formation of C2+ hydrocarbons observed over the Li+/MgO (JM) catalyst at 700 "C with the transient behaviour reported by Korf et al.9 for an Li+/MgO catalyst operating at 800 "C. Mirodatos et a1.6 have shown that under reaction conditions at 750°C there is a substantial loss of lithium from the surface, which, in part, explains the results of Korf et al.As pointed out in the Introduction, the specific centres which form CH; radicals, and thus give rise to C2+ hydrocarbons, have been identified as surface 0.- ions in equilibrium with bulk [ Li+O-] centres. The 'Li n.m.r. spectra of fig. 1 provide additional evidence for the substitution of Li' ions for Mg'+ in the MgO crystal. Recent measurement of the kinetic isotope effect (KIE) for CH, and CD4, reported by Cant et al.,I4 show that the slow step in the oxidative dimerization of methane over Li+/MgO involves the breaking of a C-H or C-D bond. A similar conclusion has been reached in our laboratory based on the KIE for the formation of CH; radicals.The observation of a KIE is surprising since 0- ions in their purely ionic form are capable of abstracting a hydrogen atom from CH, at temperatures well below 25 "C.,, The reaction occurs with almost no activation barrier. If 0- ions are indeed the catalytic centres at 700 "C, their activity must be modified by the delocalization of charge, so that they should more properly be described as 0'- ions. As an alternative, the active centres may actually be 0, ions on the surface which result from the reaction Ozonide ions, formed by this reaction, have been identified on the surface of MgO" and on Li+/ MgO catalysts which have been quenched from elevated temperatures.', This type of ion also reacts with simple alkanes, but at considerably higher temperatures than the 0- centres.'6 0-+o, ---* 0,.( 5 ) Conclusions Although lithium promotes sintering of MgO, the alkali-metal ion enhances the flux of CH; radicals entering the gas phase when compared with that observed over pure MgO, both on the basis of sample mass and unit surface area. The greater flux of CH; radicals from the Li'/MgO samples is, in part, a result of the presence of additional active centres on the surface. No positive evidence was found in support of the theory that morphology per se influences the reactions of CH; radicals with a surface, but subtle effects could have gone undetected. Sintering obviously limits these secondary CH; radical reactions. Catalysts derived from Li2C03 are less subject to sintering than those derived from LiCH3, and the larger surface area gives rise to both a greater CH; radical flux and a greater C2+ yield.Neither Li,O nor Li2C03 is active for the formation of C, hydrocarbons, but rather specific centres which are derived from the substitution of Li' ions for Mg2+ ions in the oxide appear to be responsible for the activation of CH,. We thank Dr P. Chu for assistance with the solid-state n.m.r. studies. This work was supported by the National Science Foundation under grant CHE-8617436. References 1 T. Ito and J. H. Lunsford, Nature (London), 1985, 314, 721; T. Ito, J-X. Wang, C-H. Lin and J . H. 2 D. J. Driscoll, W. Martir, J-X. Wang and J. H. Lunsford, J. Am. Chem. SOC., 1985, 107, 58. 3 C-H. Lin, T. Ito, J-X. Wang and J . H . Lunsford, J.Am. Chem. SOC., 1987, 109, 4808. 4 K. D. Campbell, E. Morales and J . H . Lunsford, J. Am. Chem. SOC., 1987, 109, 7900. 5 J . H. Lunsford, in Oxygen Complexes and Oxygen Activation by Transition Metals, ed. A. E. Martell Lunsford, J. Am. Chem. SOC., 1985, 107, 5062. and D. T. Sawyer (Plenum Press, New York, 1988), pp. 265-272.J. H. Lunsford et al. 21 6 C. Miradatos, V. Perrichon, M. C. Durupty and P. Moral, in Catalyst Deactivation, ed. B. Delmon and 7 G. A. Martin and C. Mirodatos, J. Chem. SOC., Chem. Commun., 1987, 1393. 8 J. B. Kimble and J. H. Kolts, Energy h o g . , 1986, 6 , 226. 9 S. J. Korf, J. A. Roos, N. A. de Bruijan, J. G. van Ommen and J. R. H. Ross, J. Chem. SOC., Chem. 10 J. A. Roos, A. G. Bakker, H. Bosch, J. G. van Ommen and J. R. H. Ross, Cataf. Today, 1987, 1, 133. 11 G. J. Hutchings, M. S. Scurrell and J. R. Woodhouse, J. Chem. SOC., Chem. Commun., 1987, 1862. 12 G. J. Hutchings, M. S. Scurrell and J. R. Woodhouse, J. Chem. SOC., Chem. Commun., 1987, 1388. 13 G. J. Hutchings and M. S. Scurrell, Appf. Cataf., 1988, 38, 157. 14 N. W. Cant, C. A. Lukey, P. F. Nelson and R. J. Tyler, J. Chem. SOC., Chem. Commun., 1988, 766. 15 V. T. Amorebieta and A. J. Colussi, J. Phys. Chem., 1988, 92, 4576. 16 S. P. Mehandru, A. B. Anderson and J. F. Brazdil, J. Am. Chem. SOC., 1988, 110, 1715. 17 H-F. Liu, R-S. Liu, K. Y. Liew, R. E. Johnson and J. H. Lunsford, J. Am. Chem. SOC., 1984, 106,4117. 18 W. Martir and J. H. Lunsford, J. Am. Chem. SOC., 1981, 103, 3728. 19 A. G. Shastri, H. B. Chae, M. Bretz and J. Schwank, J. Phys. Chem., 1985, 89, 3761. 20 E. E. Platero, D. Scarano, G. Spoto and A. Zecchina, Faraday Discuss. Chem. SOC., 1985, 80, 183. 21 Y. Chen, H. T. Tohver, J. Narayan and M. M. Abraham, Phys. Rev. B, 1977, 16, 5535. 22 J-X. Wang and J. H. Lunsford, J. Phys. Chem., 1986, 90, 5883. 23 E. Iwamatsu, T. Moriyama, N. Takasaki and K. Aika, J. Chem. SOC., Chem. Commun., 1987, 19. 24 K. Aika and J. H. Lunsford, J. Phys. Chem., 1977, 81, 1393. 25 N. B. Wong and J. H. Lunsford, J. Chem. Phys., 1972, 56, 2664. 26 Y. Takita and J. H. Lunsford, J. Phys. Chem., 1979, 83, 683. G. F. Froment (Elsevier, Amsterdam, 1987), pp. 183-195. Commun., 1987, 1433. Paper 8/05049J; Received 19th December, 1988

ISSN:0301-7249    

DOI:10.1039/DC9898700013

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Furaduy Discuss. Chem. SOC., 1989, 87, 23-32 Mechanistic Relationships in the Activation of Methane and the Conversion of Methanol on Heteropoly Oxometallates Shamsuddin Ahmed, Slavik Kasztelant and John B. Moffat" Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl Heteropoly oxometallates have been employed in the heterogeneous catalysis of the conversion of methanol to hydrocarbons and the partial oxidation of methane. In the former process methanol is first protonated at Bransted-acid sites, but at higher temperatures the heteropoly anions are partially methy- lated subsequent to scission of the C-0 bonds in protonated methanol. With silica-supported 12-molybdophosphoric acid the exchange of the pro- tons by cations effectively poisons the catalyst for the conversion of methane.Introduction of a chloro-additive to the feedstream in the latter process produces changes in the conversion and selectivity which are markedly different for the molybdenum- and tungsten-containing catalysts. With the former the conversion of methane is increased while the selectivity to partial oxidation products is decreased. With the latter the former comments are also applicable but selectivities to methyl chloride reach as high as 90 mol '30. The results from cation exchange and the addition of a chloro-additive are interpreted and a mechanism with common features is proposed for the methanol and methane conversion processes. Heterogeneous catalysts are many and varied, from monocationic oxides to multiele- mental complexes and from supported monometallic to the complex multicomponent solids such as those employed in ammonia synthesis.The functionality of a given catalyst is usually related in a complex way to its chemical composition, crystallographic structure and surface and bulk properties. Two catalysts with differing functionalities are invariably found to possess contrasting features of the aforementioned properties. Heteropoly oxometalates with anions of Keggin structure are interesting examples of isostructural catalysts, that is, those in which the elemental composition may be changed to produce an alteration of the catalytic properties while the structural features are retained. These heteropoly oxometallates are ionic solids with discrete anions and cations.' The latter may be simple one-atom inorganic ions or multi-atom organic species.The anions (fig. 1 ) are large cage-like structures with a central atom such as phosphorus which is surrounded by four oxygen atoms arranged tetrahedrally. Twelve octahedra with oxygen atoms at their vertices and a peripheral metal atom such as tungsten or molybdenum at their centres envelope the central tetrahedron and share oxygen atoms with each other and the central atom. The structures of the heteropoly anions are semiquantitatively similar for either molybdenum or tungsten as peripheral metal atoms, although the bond lengths are slightly different with, for example, the peripheral metal-terminal oxygen atom bond lengths being 1.66 and 1.70 8, for Mo and W, respectively.' There is considerable interest in the methanol-to-gasoline recess,* and in particular the mechanism by which the carbon-carbon bond is produced.'In addition, considerable t Permanent address: Direction de recherche CinCtique et catalyse, lnstitut Francais du PCtrole, 1 & 4, av.de Bois-PrCau BP 311, 92506 Rueil Malmaison Cedex, France. 2324 Mechanistic Relationships Fig. 1. Heteropoly anion of Keggin structure (KU, Keggin unit). Large circles, central atom and peripheral metal; small circles, oxygen atoms. attention has recently been directed to processes for the activation of methane and its conversion to a more suitable chemical feedsto~k.~-~ Earlier work in this laboratory has shown that methanol can be converted to hydrocarbons (>C,) on 12-tungstophosphoric acid (HPW), while methane is oxidized on 12-molybdophosphoric acid (HPMo); the anions of the two catalysts are structurally identical, but they contain tungsten and molybdenum, respectively, in the peripheral position.'-I4 Photoacoustic FTIR studies in this laboratory have shown that methanol is protonated at the Brflnsted-acid sites in HPW and at high temperatures the scission of the C-0 bond in CH30Hl results in the methylation of the oxygen atoms of the heteropoly anion.13914 The observation that methanol and methane can be converted on structurally similar but chemically different catalysts provides an interesting opportunity to explore the mechanistic relationships between such processes.In the present work the effects of additives to the solid and gas phase in the conversion of methane are reported and the rationalization of the observations is compared and contrasted with the mechanism for the methanol-to-hydrocarbons process on isostructural heteropoly oxometallates.Experimental The supported heteropoly oxometallates were prepared by impregnation of the silica (Grace-Davison grade 407, 740 m2 g-', 8-40 mesh) with aqueous solutions of the heteropoly acid, which was recrystallized before use. The solutions were evaporated to dryness at 80 "C and the solids calcined at 350 "C for 2 h. Particle sizes of 8-15 mesh were employed for the studies reported here. In the exchange studies, loadings of 16 wt O h HPMo on silica were employed, corresponding to 0.068 KU nm-* of support surface.In some cases 23.9 wt % HPMo was employed. The supported salts of the heteropoly acids were prepared by the impregnation of the calcined supported heteropoly acids with an excess of an aqueous solution of the cation in the form of the carbonate, acetate or nitrate, depending on the solubility. After evaporation to dryness at 80 "C the solids were calcined at 200 "C forS. Ahmed, S. Kasztelan and J. B. Mofat 25 cation (Cs)/KU ( x (Li)/KU (0) Fig. 2. Relative N 2 0 turnover rate of the CH4/N20 reaction after addition of caesium carbonate ( x ) or lithium carbonate (0) to silica-supported 12-molybdophosphoric acid (HPMo). X, 0 16 wt '/o HPMo; +, W 23.9 wt '/o HPMo. Reaction temperature 843 K, W = 0.5 g, F = 30 cm' min-', CH4 (67 mol %), N 2 0 (33 mol %).1 h. The reaction temperature was 570 "C, the catalyst weight was 0.5 g, the flow rate was 30 cm3 min-' and the feed composition was 67 mol '/o CH4, 33 mol YO NzO. Pretreat- ment was performed for 1 h in helium at the reaction temperature. A fixed-bed continuous-flow reactor was employed for studies of the catalysed process. The catalysts were preheated in helium for 1 h at the reaction temperature. An on-stream Hewlet-Packard 5890 gas chromatograph was used for all analyses. The chloro-additive was added to the feed stream by passing helium through a saturator containing the liquid held at 0 "C. The flow of helium was adjusted so that after dilution following introduction into the main flow of reactants ( CH4-N20) the desired concentra- tion of the additive in the feed was obtained.Experiments in the absence of CH, showed that the chloro-additive may also be oxidized, although methyl chloride is found in the effluent only when methane is present in the feed stream. To correct for the oxidation of the chloro-additive experiments were performed in duplicate, one in the absence, another in the presence of CH,, with care being taken to ensure that residence times were equivalent in each case. Since the experimental procedure followed provides results effectively obtained with a chloromethane-pretreated catalyst, the duration of the reaction performed in the absence of CH, was kept constant in each series of experiments. Results The exchange of the protons contained in the silica-supported heteropoly acids by various cations decreases the conversion and modifies the selectivity.Typical results are shown in fig. 2-4. It is evident that as the number of cations introduced into the catalysts increases, the turnover number, expressed in terms of the oxidant, decreases. In all cases the turnover number appears to approach that expected for the supported cation itself as the number of cations contained in the supported heteropoly oxornetallate is increased. Interestingly, while the selectivity to CO is larger than that to COz in the26 Mechanistic Relationships 0 2 4 6 8 cation (Cs)/KU Fig. 3. Selectivity of the CH,+ N 2 0 reaction after addition of caesium carbonate to silica-supported 12-molybdophosphoric acid (HPMo). Open symbols (16 wt YO HPMo). Closed symbols (23.9 wt YO HPMo).Open symbols not joined by line (Cs/Si02). Reaction conditions as in fig. 1. A, CO; 0, C02; 0, CH20. 50 A 0- 0 A lo O<O 0 4 -- - - I I 1 I I I I n I 0 2 4 6 8 cation (Li)/ KU Fig. 4. Selectivity of the CH4 + N 2 0 reaction after addition of lithium carbonate to silica-supported 12-molybdophosphoric acid (HPMo). Open symbols (16 wt '/O HPMo). Closed symbols (23.9 wt O/O HPMo). Open symbols not joined by line (Li/Si02 and SO2, respectively). Reaction conditions and symbols as in fig. 1.100 h $ e o - - E" 6 0 - x > 4 0 - 0 v c) .- .- CI - g 20- 0- S. Ahmed, S. Kasztelan and J. B. Moffat - :o co I h 3 - b 8 2 - ? v E: 0 .- E s 0 0- 1 I - A P 27 Fig. 5. Effect of addition of dichloromethane (DCM) or tetrachloromethane (TCM) to CH, feedstream over HPMo/Si02.(A, P refer to absence and presence, respectively, of chloro- additive). ( a ) Additive DCM, ( b ) - ( d ) additive TCM, ( a ) - ( c ) 20% loading of HPMo, ( d ) 10% loading. Reaction temperature 450 "C. F = 60 cm3 min-'. ( a ) w = 1.05 g, CH4/ N 2 0 = 1, DC M 0.30 mol YO ; ( b ) w = 1.0 g, C H,/ N20 = 4, TC M 0.12 rnol % ; ( c) w = 3 .O g, C H,/ N 2 0 = 4, TCM 0.17 mol YO; ( d ) w = l.Og, CH4/N20=4, TCM 0.17 rnol O/O. absence of the cations, the introduction of either lithium or caesium produces a reversal of the selectivities and again an approach to that expected with the cations themselves. In contrast, with barium and magnesium as the added cations the selectivities to CO and to COz are altered but the relative order is preserved.Both from the figures and other data (not shown) it is evident that poisoning of the catalysts resulting from the introduction of the particular cation approaches completeness as the number of cations inserted reaches 3-4 per heteropoly anion. This strongly suggests that the cations replace the exchangeable protons, and further that the protons play a vital role in the catalytic process. The addition of small quantities of a chlorine-containing additive to the feed stream in the conversion of methane also results in significant changes of the conversion and selectivities (fig. 5-7). With silica-supported 12-molybdophosphoric acid the addition of small quantities (<0.3 rnol "/o) of either dichloromethane (DCM) or tetra- chloromethane (TCM) increases the conversion of methane but decreases the selectivity to formaldehyde, while the selectivity to carbon monoxide increases.However, a substantial increase in the overall yield of H2C0 occurs. The production of carbon dioxide remains low while small quantities of methyl chloride are produced. As observed in the absence of any chloro-additive, the conversion in the presence of TCM is at a maximum for ca. 20 wt "/o loading of HPMo on the silica support and decreases with either decrease or increase of the loading from that at which the maximum is observed. In contrast the selectivity to formaldehyde is highest at low loadings. The effect of increasing residence time with the chloro-additive is semi-quantitatively similar to that observed where the additive is not present, with the conversion increasing and the selectivities to formaldehyde and carbon monoxide decreasing and increasing, respec- tively.With times-on-stream up to 8 h and a reaction temperature of 375 "C the selec- tivities to H2C0 and CO decrease and increase, respectively, within the first hour (but28 100 h 8 80- - 0 60- x c) .- .? 4 0 - * 0 0 - 20- 0 - h =" 7 0 8 v C 0 v) Q) > 0 .- 0 - Mechanistic Rela tionships - - I - A P A P I-I MC F MC Fig. 6. Effect of addition of dichloromethane (DCM) or tetrachloromethane (TCM) to CH, feedstream over HPW/Si02. (A, P refer to absence and presence, respectively, of chloro-additive). ( a ) Additive DCM, ( b ) - ( d ) additive TCM. ( a ) , ( b ) , ( c ) Reaction temperature 450 "C, w = 2.0 g ; ( a ) , ( b ) , ( d ) F=60cm'min-'; ( a ) , ( b ) , ( d ) CH,/NzO=4; ( d ) W=3.Og, T=525"C; ( c ) F = 11 cm3min-', CH,/N20= 1; ( a ) DCM=0.30mol YO; ( b ) TCM=0.21 rnol %; ( c ) TCM= 0.38 rnol YO; ( d ) TCM = 0.17 rnol %.to a relatively minor extent) and subsequently remain virtually constant. Concomitantly the conversion increases and, for times greater than 1 h, changes very little. The rates of product formation are also enhanced when a chloro-additive is present. The selectivity to and the rate of formation of H,CO are maximum for a CH4/N20 ratio between 4 : 1 and 8 : 1, while the conversion decreases continuously as this ratio is increased from 1 : 8 to 54: 1. Not surprisingly, the conversion of methane increases with increase in reaction temperature while the selectivities to H2C0 and CO decrease and increase, respectively.With 12-tungstophosphoric acid as catalyst in the conversion of methane the introduc- tion of either DCM or TCM to the feed stream produces changes which are distinctly different from those observed with 12-molybdophosphoric acid (fig. 6 and 7). For brevity comments will be restricted to observations obtained with TCM. With increasing amounts of TCM the conversion increases and appears to be approaching a constant value. Concomitantly the selectivity to H2C0 suffers a precipitous drop, while the production of CH&l increases and both attain virtually constant values with 0.1 mol '/O TCM. Note that selectivities to CH3Cl as high as 90 mol '/O are attainable with 0.55 mol YO TCM in the feedstream. A maximum in selectivity to methyl chloride is reached at a 5 wt '/O loading of HPW on the silica support with a decrease in this selectivity for further increases in the loading.However, the conversion of methane increases from the small value obtained on the support itself to a constant value at a loading of 20-30 wt %. Increases in the residence times produce increases in the conversion of methane and selectivity to CO, while that to CH3Cl decreases. With increase in the CH4/N20 ratio the selectivities to CO and CH3Cl decrease and increase, respectively, each passing through a point of inflection at a CH,/N20 ratio of ca. 1 : 1. Experiments in which the catalysts were pretreated as per usual with an He-N,O-CCl, mixture followed by exposure of the catalysts to a flow of CH4-N20, but with CCl, absent, provided evidence for the retention of chlorine on the catalyst (fig.8). WithS. Ahmed, S. Kasztelan and J. B. Moflat 29 0.4 h 2 0.3 0 8 v C .- 0.2 5 > s 0. I 0.1 0.2 0.3 0.4 0 . 5 TCM in feed (mol Yo) Fig. 7. Selectivity and conversion for various concentrations of TCM in the conversion of methane on HPW/Si02 ( w = 2 . O g , F=60cm3min-', T=45O0C, CH4/NzO=4). A, CO; 0, CO,; 0, H2CO; D, CH,CI; +, CH, conversion. both catalysts the conversions are initially relatively high but decrease to the values expected in the absence of the chloro-additive. The selectivities are initially similar to those expected when the chloro-additive is present in the feedstream but rapidly shift to those found when the chloro-additive is absent. It is interesting to note that the production of methyl chloride on HPW continues for ca.3 h. Although not shown in the figure, the rate of production of methyl chloride on HPW in the absence of CCl, from the feed is similar to that found in its presence. Discussion As noted in the introduction, earlier PAS FTIR studies from this laboratory have shown that with for example HPW, methanol interacts with the Brflnsted-acid sites to form protonated methanol (fig. 9), whose characteristic bands are identified in the infrared Since methanol is capable of diffusing into the bulk structure of HPW, all protons will, in principle, be capable of interaction with methan01.'~ Since the protons are coulombically bound to the terminal oxygen atoms of the heteropoly anion, the protonated methanol is presumably localized in this environment.While the protonated methanol exists at room temperatures and slightly above, a temperature of 150°C, for example, is sufficient to produce a C-0 bond scission to form water and methyl cations, which are evidently bound to the oxygen atoms of the heteropoly anions14 (fig. 10). Further heating produces oligomerization products which are believed to result through a carbene process.30 70 Mechanistic Relationships - - A Fig. 8. Selectivity to products and conversion of methane in the absence of chloro-additive in the feedstream after pretreatment of the catalyst with tetrachloromethane (TCM). Open symbols: 20 wt O/, HPMo/Si02. Pretreatment in helium/N20: 1 : 1 with 0.12% TCM for 1 h. Reaction temperature 450 "C, catalyst 1.05 g, flow 60 cm3 min-', CH,/N20 = 1 : 1.Filled symbols: 20 wt YO HPW/Si02. Pretreatment in helium/N20: 1 : 1 with 0.35 mol YO TCM for 2 h. Reaction temperature 450 "C, Catalyst 2.0 g, flow 1 1 cm3 min-', CH,/N20 = 1 : 1. A, A, CO; 0, 0, H2CO; V, CH,CI; 0, +, conversion. \ / \ / H' \ / \ 'H + Fig. 9. Protonated methanol. Exchange of the protons by larger cations in the silica-supported HPMo has the principal effect of reducing the activity of the catalyst in the methane-conversion process. Temperature-programmed desorption, exchange and reduction experiment^'^'^^ have shown that, at temperatures between 400 and 600 "C the protons in the heteropoly acids are desorbed as water apparently through a process which strips oxygen atoms from the heteropoly anion: KUOH + KUOH ---* KUO+ KUD + H20.In so doing a relatively small portion of the total oxygen atoms in the heteropoly anionS. Ahmed, S. Kasztelan and J. B. Moflat 8-0 31 0 Fig. 10. A fragment of the methylated heteropoly anion. are replaced by vacancies. The substitution of the larger cations for the protons effectively eliminates the vacancy production process. The oxidant N20 is believed to be capable of supplying oxygen to the vacant sites by a process of dissociative chemisorption and methyl cations or radicals will interact with these oxygen sites to produce oxygenated products. It is evident from the data that while the vacancy elimination process is undoubtedly the principal result of the cation exchange, other subsidiary processes are also operative.Since the results show a dependence on the nature as well as the number of cations, an electronic factor which may increase the electron density on the heteropoly anion is apparently also operative. This may favour the formation of active charged oxygen species (0") through the dissociation of N20: KUO + N20 -+ KUO" + N2 where O* may be 0- l7 or 02-,18 as suggested by various workers. Further, it is expected that the cations themselves may possess an activity in the conversion process. The effects of addition of carbon tetrachloride to the feedstream in the conversion of methane are strikingly different with the two silica-supported catalysts, HPMo and HPW. In the former case only a small quantity of methyl chloride is formed, while the conversion of methane is increased and the selectivities to CO and H2C0 are increased and decreased, respectively.In sharp contrast with HPW the conversion of methane and the selectivity to CH3C1 are both increased, while those to CO and H2C0 are decreased. It should be recalled that earlier work from this laboratory has shown that HPW is active and selective in the conversion of methanol to hydrocarbons (C > l), whereas with HPMo methanol is largely converted to deep oxidation products. Furthermore, the conversion of methane on HPW is a factor of 200 smaller than that on HPMo, although the selectivities to H2C0 are similar. Extended Huckel calculations have shown that the protons in HPW are more mobile and hence more acidic than those in HPMo, while the anionic oxygen atoms in HPW are more tightly bound than those in HPMo.19 The observations that hydrocarbons are formed from methanol on HPW while oxidation products are obtained on HPh!fo appears to be consistent with the results and interpretations of these calculations. It has been pointed out2' that the activation of the C-H bonds in methane becomes more difficult as the surface oxygen species become more stable.The lower conversion of methane on HPW, where the oxygen is more tightly bound, than observed with HPMo, where the oxygen atoms are more labile, is consistent with the aforementioned generalization. The effects of introduction of carbon tetrachloride provide some interesting insights into the mechanism for the conversion of methane. It is well known2' that alkanes react32 Mechanistic Relationships with carbon tetrachloride in the gas phase by a free-radical chain mechanism: R‘+CCl, ---* RCl+’CCl, RH + ‘CC1, + R’ + HCC1,.While the possibility of this purely gas-phase process cannot be entirely dismissed, other mechanisms appear to be more plausible in the present system. The observations that the products in the presence of the two catalysts HPMo and HPW are significantly different provides evidence for the direct involvement of the solid phase. The observation that the process takes place for a finite time in the absence of the chloroadditive subsequent to the pretreatment of the catalyst in the presence of tetrachloromethane provides further evidence for the direct participation of the catalyst. The near absence of chlorinated products other than CH3Cl (and HCl) rules out the dominance of a purely gas-phase process.Jt is of relevance to note that in the conversion of methanol on HPW at 350°C the dominant products are C2 and higher hydrocarbons, while at a temperature of 400°C methane is the major p r ~ d u c t . ~ As noted earlier, spectroscopic studies have shown that the oxygen atoms of the heteropoly anion are methylated in the conversion of methan01.’~”~ In the conversion of methane with HPMo the more labile oxygen atoms occupying the vacancies created by the loss of water can be removed to form oxygenated products, in contrast with HPW where the oxygen atoms are more tightly bound. In the presence of a chloro-additive, chlorine is evidently incorporated on the catalyst, although the nature of the binding is at this time unknown.Methyl species, possibly in radical form, may interact with the incorporated chlorine to produce methyl chloride. With HPW the chlorine is apparently less tightly bound and can be relatively easily removed from the surface by interacting methyl species. The financial support of the Canadian Natural Sciences and Engineering Research Council is gratefully acknowledged. References 1 M. T. Pope, Hereropoly and Zsopoly Oxomeralures (Springer-Verlag, Berlin, 1983). 2 C. D. Chang, Catal. Rev. Sci. Eng., 1985, 26, 323. 3 See e.g., G. J. Hutchings, L. J. van Rensburg, W. Pick1 and R. Hunter, J. Chem. SOC., Faraday Trans. 4 R. Pitchai and K. Klier, Card Rev. Sci. Eng., 1986, 28, 13. 5 N. R. Foster, Appl. Card., 1985, 19, 1. 6 J. S. Lee and S. T. Oyama, Card. Rev. Sci. Eng., 1988, 30, 249. 7 H. Hayashi and J. B. Moffat, J. Card., 1982,77,473; 1983,81,61; 1983,83, 192; in Catalytic Conversion of Synthesis Gas and Afcohols ro Chemicals, ed. R. G. Herman (Plenum Press, New York, 1984), p. 395. 8 S. Kasztelan and J. B. Moffat, J. Caral., 1987, 106, 512; 1988, 109, 206; 1988, 112, 54. 9 S. Kasztelan and J. B. Moffat, J. Chem. Soc., Chem. Commun., 1987, 1663. I , 1988, 84, 1311. 10 S. Kasztelan, E. Payen and J. B. Moffat, J. Card., 1988, 112, 320. 11 J. B. Moffat, in Methane Conversion, A Symposium on the Production of Fuels and Chemicals from 12 S. Ahmed and J. B. Moffat, Appl. Card., 1988, 40, 101. 13 J. G. Highfield and J. B. Moffat, J. C a r d , 1985, 95, 108. 14 J. G. Highfield and J. B. Moffat, J. Card., 1986, 98, 245. 15 B. K. Hodnett and J. B. Moffat, J. Card., 1984, 88, 253. 16 B. K. Hodnett and J. B. Moffat, J. Caral., 1985, 91, 93. 17 L. Mendelovici and J. H. Lunsford, J. Card., 1985, 94, 37. 18 R. EIAmrany, Y. Barbaux and J. P. Bonnelle, C a r d Today, 1987, 1, 147. 19 J. B. Moffat, J. Mol. Caral., 1984, 26, 385. 20 H. H. Kung, Ind. Eng. Chem. Prod. Res. Dev., 1986, 25, 171. 21 J. A. Dawari, S. Davis, P. S. Engel, B. C. Gilbert and D. Griller, J. Am. Chem. Soc., 1985, 107, 4721. Natural Gas (Elsevier, Amsterdam, 1988). Paper 8/04492K; Received 14rh December, 1988

ISSN:0301-7249    

DOI:10.1039/DC9898700023

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1989, 87, 33-45 New Families of Catalysts for the Selective Oxidation of Methane John M. Thomas,* Wataru Ueda,"? John Williams and Kenneth D. M. Harris Davy Faraday Research Laboratory, The Royal Institution, 21, Albemarle Street, London W1X 4BS The catalytic performance of 11 chemically distinct crystalline solids, all of which have layered structures of either the Sillen, perite or bipox type in which infinite sheets of [ Bi2O2I2+ or [ PbBi02]+ are interleaved with other layers such as chloride ions, metal chlorides or tungsten oxide, is reported. Several kinds of well characterized Sillen structures, designated X I , X2, X3, X , , as well as regular intergrowths thereof, were tested. Some of these monophasic materials show good C2 selectivity with ratios of C2H4/C2H, ranging from ca.unity to in excess of 30 at temperatures close to 1000 K for the oxidative coupling of methane. Scope exists for further structural modification and the design of superior catalysts of other bismuth-containing solids based on fluorites and pyrochlores. 1. Introduction With the increasing availability of a range of microporous catalysts based on silicon aluminium phosphate (SAPO), which are efficient in converting methanol to alkenes, it has been argued that a commercially attractive means of utilizing the vast quantity of available methane is to convert it first to a mixture of hydrogen and carbon monoxide ('syngas') by oxidation, then to utilize a methanol-synthesis catalyst, based for example on Cu/ZnO, and then to fine-tune the subsequent conversion of methanol to short-chain alkenes.The scope for fine-tuning is indeed considerable, now that many metal SAPO (e.g. Co- or Mn-substituted SAPOs) catalysts are available, but the challenge, both commercially (especially in the development of geographically remote sources of petrochemicals and power) and scientifically is to develop a catalyst for the direct, oxidative coupling of methane using oxygen. Quite apart from the oxides described by other contributors to this Discussion, there are already numerous candidate catalysts from which one may choose. Thus, many different categories of solids have been shown to exhibit greater or lesser degrees of catalytic activity and selectivity in the selective oxidation, by oxygen, of methane to ethene, ethane and larger hydrocarbons.Structures based on rock-salt, but rich in vacancies, as well as those based on perovskites, spinels, the Suzuki phases and variants of the rare-silicate mineral kentrolite (Pb,Mn,Si,O,) have also been shown'-, to possess good catalytic performance for the selective production of ethene and ethane by oxidative coupling of methane. we have shown that there is a potentially very large family of oxidative coupling catalysts for methane, all containing bismuth and all derived, in one structural form or another, from bismuth oxyhalides. Many of these catalysts have been shown to be monophasic solids, and it is the relationship between structure and catalytic performance that is the principal focus of our paper. In earlier ?Present address: Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan.3334 Selective Oxidation of Methane The kind of question we seek ultimately to resolve centres upon whether one particular structural variant or set of variants within the large family of bismuth-containing, layered solids10-16 now established to exist (see below) possesses exceptional catalytic perform- ance and, if so, why. This question is of profound fundamental interest in understanding the key attributes and mode of operation of the so-called uniform heterogeneous catalysts. In uniform heterogeneous catalysts, by d e f i n i t i ~ n ” , ’ ~ all, or nearly all, atoms in the bulk of the solid either participate directly or are implicated in the crucial catalytic act.Prominent examples are the SAPOs mentioned above, and, par excellence, zeolitic catalysts, clays and pillared clays. Zeolites, ALPOs and SAPOs are not considered further here, except in as much as to emphasize that they constitute some of the best-known examples of well characterized materials. We focus instead on the bismuth oxyhalides. Ternary oxides such as bismuth vanadium molybdenum oxides (for example BiMo,-,V,04)” or the various types of bismuth molybdates (Bi,MoO,, Bi2M0209 and Bi2M030,2),20 all of which are” efficient selective oxidation catalysts for hydrocarbons such as propene, are in this class and they are known to entail sacrificial loss of oxygen. Structural oxygen is released from the catalyst,?’ thereby converting it to a non- stoichiometric solid.The anion deficiency is made good when reactant gaseous oxygen is taken up by the catalyst. This is the notion associated with the work of Mars and van Krevelen.?, To date, no definitive study has been made to ascertain whether the various types of bismuth oxyhalide catalysts described below do indeed have significant standing concentrations of vacancies that are, as outlined above for the bismuth molybdates and vanado-molybdates, implicated in the crucial acts of catalysis. Such questions are best resolved by kinetic measurements using isotopically labelled reactants (see Discussion). What we concentrate upon here, however, is an investigation of new catalysts, character- ized by X-ray diffraction, almost all of which seem to function best in their crystalline, monophasic states.2. Structural Background Recognizing that the bismuth ion has a marked tendency”’ to abstract hydrogen atoms from hydrocarbons, we concentrate in this section on the structural aspects of hitherto unexplored (in the context of their use as selective oxidation catalysts) oxyhalides of bismuth. We first recall that bismuth oxychloride itself can give rise to so-called X I , X2, X3 and X , structural types, as well as intergrowths thereof. The X I type, exemplified by LiBi3O4CI2, is shown in fig. 1 . I t consists essentially of layers of Bi,O:’ between which are regularly intercalated layers of halide ions. The X, structure is that of BiOCl (fig. 2) and the salient features of X3 are shown in elevation in fig.3(a). Recurrent inter- at the sub-unit-cell level are possible in the family of bismuth oxyhalides, as witnessed by phases designated XlX3 or XlX,X3 in fig. 3( b ) and (c), respectively. The occurrence of such intergrowths is identified through the magnitude of the Fbserved unit cell (z-direction) dimension, which is 6.065 8, for X I (c = 12.13 A), 7.37 A for X2, 10.845 8, for X, and 13.435, 16.91 and 18.215 A for X1X2, XIX, and X2X3 structures, respectively. Structures designated by X , X I X 2 , X ,X2X2 and X ,X2X3 have c = 19.50, 20.805 and 24.28 A, respectively; and X2X2X,, X,X,X, and X,X,X, have c = 25.585, 27.755 and 29.06 A. Until very recently the nature of the X , set of structures was unknown. Harris et al.“ have, however, elucidated the structure of at least one member of this series. It is the structure taken up by the compound Cs,Bi,,Ca,CI,,O,, in which, as shown in fig.4, there is a layered portion of the well known CsCl structure (containing only partial occupancy of the Cs sites) flanked on either side by modified bismuth oxychloride layers.I. M. Thomas et al. 35 Bi 0 x 1 - a0 Fig. 1. Schematic illustration of typical Sillen phases derived from BiOCI. The so-called X , phase (typified by ABi,O,CI,; A = Li, Na, K) is shown in ( a ) where there is a single layer of halogen (represented by large circles). Another view of this structure is shown in ( b ) . Key for ( a ) : Bi (closed circles), 0 (small open circles), C1 (large open circles). of the X, type, within which the Bi3+ and Ca” ions occupying the same type of site.Doubtless there are other variants of X, yet to be structurally clarified. Isostructural with the [Bi2O2I2+ layer is the [PbBi02]+ layer, which, in view of the comparable X-ray scattering amplitudes of Bi and Pb, is very difficult to distinguish from it by X-ray diffraction. Sillen recognized that a whole family of structures may be formed by recurrent intergrowths at the sub-unit-cell level of halogen or alkali-metal- halogen layers of formula [HI and [AH] (H = C1, Br, I; A = Li, Na, K, Rb). Indeed it is now realized that the mineral perite, PbBiOzCl (fig. 5), and BiOCl (fig. 2) are the paradigms of the so-called Sillen phases, namely: [ PbBiO,][CI] and [ Bi20z][CI][CI]. [Bi20J2+ layers also occur in a recurrent fashion in the Aurivillius phasesz6 where the interleaving components are perovskitic and of general formula [A,_, Bn03,,+ ,I2- where n represents the thickness of the perovskitic layer.(Perovskite itself has the formula AB03 and consists of a cube of BO, octahedra at each vertex with the A ion at the cube centre.) A compound which belongs to the simplest member of the large family of the Aurivillius phases is [BizO,][WO,], shown schematically in fig. 6. It is theoretically, and in practice, possible14 for structures to be formed containing elements of both the Sillen and Aurivillius phases. Some of the simplest examples (with n = 1 in the notation used above for the perovskitic component), shown in fig. 7, have layer sequences given by: [ PbBi02]2’[C1]-[ Bi2O2J2+[ WO,]’+ and [ Bi20,]”[CI]-.[ Bi202]*+[ Nb0413-. These compounds, PbBi3W08CI and Bi,NbO,CI, are the first mem- bers of the ~ o - c a l l e d ’ ~ bipox (bismuth-perovskite-oxyhalide) family of structures. Mixed Aurivillius-Sillen structures consisting of recurrent oxychloride and multiply condensed36 Selective Oxidation of Methane C 7.4 A x2 a Fig. 2. The XI structure, typified by BiOCl in which there is a double layer of halogen atoms ( a ) . Elevation view of the structure ( 6 ) showing unit cell repeats. (Key as for fig. 1). perovskitic layers ( n = 2 , 3 , . . . in the component [An-1Bn03n+l]), as well as those in which components of double halogen layers (as in BiOCl itself) are recurrently inter- leaved with pervoskitic layers, may also be envisaged, and have indeed been synthesized.The compound PbBi,ReO,Cl,, illustrated in fig. 8, and reported by A~kerman,'~ is one such compound. There are very many other layered compounds of bismuth containing the [Bi2O2I2+ sheets. Some of these are devoid of halide, like the Bi202C03 and Bi203(C03)2 which have the X , and X2 structures, respectively; others contain both the [Bi2O2I2+ sheets and the halides, like Bi240,,Xl, ( X = C1 or Br), the structure of which is not yet unambiguously established. The family of warm superconductors of general formula29 Bi2Sr3- yCa,Cu,08+ ,. also has the [ Bi202]'+ layer. Yet others, formedzo*21,30-38 by high-temperature treatment of Bi203 with one or other of Nb205, V205, Ta205, W 0 3 , MOO, and TiOz and combinations thereof, display a rich diversity of both familiar and hitherto unknown structural types encompassing Aurivillius phases, bipox phases, perov- skites, pyrochlores, half pyrochlore-half perovskites, fluorite-like and otherI7 structures.We have not systematically examined the catalytic properties of these numerous variants in the context of methane-coupling oxidations. Preliminary experiments sugged4 that many of these, depending upon their precise structure and composition, show promise as effective catalysts for the production of C2 hydrocarbons.J. M. Thomas et a]. ( a ) x3 a C c / 2 C x3 a x1 x1 x3 a Fig. 3. Elevation views of the X, ( a ) , X,X3 ( b ) , and X,X,X, ( c ) structures typified by the compositions LiCa2Bi304Clh, LiCaBi,04C14 and Li,Ca,Bi,O, 2Cl,o, respectively. [Key as for fig.W l . 3. Experimental The desired catalysts were prepared in the main by heating a stoichiometric mixture of the relevant halide (for example LiCl, NaCl or CsCl and CaCl,), Bi,03 and BiOCl at temperatures generally in excess of 1000 K in sealed platinum tubes or in alumina crucibles. Preparations effected with open alumina crucibles tended to yield products that were less phase-pure. These conditions were satisfactory for materials possessing X I , X2 and X3 structures. Details have been given elsewhere for the preparation of Cs2Bi,oCa6C112016,'2 PbBi3W08C1,'4 and the synthetic perites PbBi02C1 and CdBi02C1.1S All materials were sintered and well crystallized and in some cases (as with Cs,Bi ,,Ca,CI 120!6) good enough to permit single-crystal X-ray analysis.X-Ray powder38 A Selective Oxidation of Methane B CI C s l cs2 0 M( BilCa) 0 Fig. 4. Two representations of the stacking of the constituent ions in the structure of the X, catalyst, Cs2Ca,BiloOl,Cl12. In (B) conventional ionic radii are used. The structure is based on modified bismuth oxychloride layers of the XI type interleaved with portions of modified CsCl structure (note that the partial occupancies of the sites Csl and Cs2 differ12). diffractometry was used to ascertain phase purity. In general, only monophasic materials were subjected to catalytic testing. Catalytic performance was assessed using a fixed-bed reactor, made of quartz, with a conventional gas-flow system at atmospheric pressure. 2.0 g of catalyst, mixed with quartz powder as diluent was held between plugs of quartz wool.The internal diameter of the tube reactor used in London was 30 mm, that in Tokyo 25 mm. (Our results encompass measurements on both systems.) Gas adsorption measurements (krypton B.E.T.) indicated that most catalysts had surface areas of ca. 1 m2g-'. Before methane was introduced, the catalyst bed was pretreated at reaction tem- perature with a stream of oxygen and nitrogen flowing for 1 h. Reaction temperatures were in the range 970-1030 K; partial pressures of methane and oxygen were 20 and 10 kPa, respectively, the total flow rate being 50 crn' m-', nitrogen being used as gaseous diluent. Products were analysed gas-chromatographically, as described previously.' ' Recovered catalysts were analysed by X-ray powder diffractometry, especially with a view to testing whether any decomposition of the original catalyst had occurred during use.Greatest attention in this work is paid to those (the majority reported here) which remained monophasic throughout the catalytic runs (see fig. 9 for a typical example). No attempt was made to explore the surface structure of the catalysts with a view to detecting the presence of an amorphous phase, or surface reconstruction or impurity. Any crystalline impurity present must be of particle size smaller than ca. 100 A diameter and/or at bulk levels of less than 1%.J. M. Thomas et al. 39 Fig. 5. Representation of the perite structure exhibited by the mineral PbBiOzCl [after ref. (15)J. 4. Results Table 1 summarizes typical data obtained using the range of bismuth oxychlorides, the structures of which were discussed in section 2.Also included for comparison are the results for an oxyfluoride and an oxybromide (which are isostructural with the corre- sponding alkali-metal bismuth oxychloride) each of the XI structural type. We have reported earlier' that the bismuth oxychlorides are superior in stability and in general catalytic performance to their analogous oxyfluorides and oxybromides, and that liber- ation of chlorine during use is significantly more pronounced for BiOCl (X2 type) than for XI oxychlorides such as ABi,O4CI2 (A = Li, Na, K). With each of the experimental facilities we often observed time-dependent catalytic behaviour. Thus, the amount of CH, converted over LiX, (catalyst 2, table 1) dropped, over a period of some 5 h of continuous use, to almost half of its initial value, there being, over the same period, more than a doubling in the Cz selectivity.It was also noted that poorly crystalline preparations invariably exhibited inferior catalytic perform- ance, emphasizing the importance of phase purity and crystallinity. We found that some40 Selective Oxidation of Methane Fig. 6. The Aurivillius phase with n = 1 (see text) typified by yBi2W0, where there are infinite layers of [BizO-,]’+ and [WO,]’- [after ref. (14)]. Fig. 7. Idealized structure of PbBi,WO,CI showing the mixing, at unit-cell-level, of the Sillen and Aurivillius structures. of our preparations (for example, those crystallizing with X3 structures) tended to undergo spontaneous phase changes after exposure to moist air.The practical con- sequences of this effect have not yet been explored. Evidently, in situ X-ray studies are called for.J. M. Thomas et al. 41 Fig. 8. An example of a mixed Aurivillius-Sillen structure, typified by PbBi,ReO,CI,. 5. Discussion In analysing the various trends in performance for these structurally well characterized families of bismuth oxychloride catalysts, it is useful to bear in mind that one of the principal roles of the individual catalyst is to facilitate generation of methyl radicals. Early indications, recently e ~ t e n d e d , ~ " ' ~ leave little doubt that methyl radical production is favoured at the solid catalyst, and possibly in the gas phase. M i m ~ , ~ * for example, in an elegant series of experiments using CH,/CD4 and '2CH4/13CH4 (plus oxygen) mixtures, over a wide variety of oxide-based catalysts, has shown that, from the observed dominant production of CD3CH3, methyl (and not methylene) radicals are crucial intermediates in the catalytic coupling of CH, to higher hydrocarbons.There is also no doubt that C2H4 is a secondary product generated from the C2H6 by further hydrogen abstraction, most likely in the gas phase. ( Mims38 has shown that CD3CH3 and CD2CH2 coexist in a typical product stream.) This proof of there being both a surface (heterogeneous) and parallel gas-phase (predominantly homogeneous) set of reactions involved in the catalytic coupling of methane to ethane, makes it more difficult (in the absence of detailed kinetic and analytical studies) to pinpoint the key attributes of the mode of operation of the bismuth oxychlorides, even though they are structurally very well characterized.The scatter in results (compare, for example, catalysts 6 and 7, table 1 ) is to some degree a consequence of this fact. The measure of irreproducibility we have found from experiment to experiment, and between ostensibly the same experiment on the two different sets of apparatus, is a further manifestation of the composite hetero-homo nature of the overall reaction. Atoms of bismuth and chlorine are well known to facilitate abstraction of hydrogen from hydrocarbons, so that, in general, we expect and do indeed find that the bismuth oxychlorides are among the premier families of catalysts for methane coupling.It is not clear, however, whether chlorine has to be completely liberated from the solid42 Selective Oxidation of Methane I 1 3.0 10.7 18.4 26.1 33.8 41.5 2 6 / O 49.2 56.9 64.6 72.3 80.0 Fig. 9. X-Ray diffraction of the perite catalyst prior to and after use. ( c ) is taken from the JCPDS file. ( a ) PbBi02CI recovered after CH, oxidation tests at 700 "C; ( h ) PbBi02CI prepared in a Pt tube; ( c ) lead-bismuth oxide chloride/perite. Table 1. Typical catalytic performance of phase-pure bismuth oxychlorides CH4 0 2 structural T converted converted C2 no. formula type'' / K ('%) 1 ( "/o ) selectivity C,H4/C2H, 1 2 3 4 5 6 7 8 9 10 I1 12 13 N aC a B i 0,C 1 (, Li C a B i 0,C 1 <, L i C a B i 0, C I (, LiCaBi,O,Cl, Li,CazBi,Ol2C1,,, Li Bi30,CI LiBi3O4Cl2 Na Bi 304 F2 N a Bi 304 Br, BiOCl Cs2Ca,Bi,,,OI6C1,2 Pb Bi02Cl PbBi3W0,CI 993 33.8 993 41.7 972 33.1 993 21.9 993 17.7 993 15.6 1023 22.1 1003 8.0 1003 32.2 1023 23.0 994 21.3 1023 13.7 1023 9.2 68.6 99.7 89.7 47 .0 37.2 35.5 18.5 91.6 46.3 - - 43.2 46.5 18.0 67.0 66.8 62.9 77.1 51.3 25.8 71.1 51.1 72.9 71.4 34.7 25.1 4.1 2.8 2.1 3.5 I .3 16.2 4.3 3.4 1.8 1.4 - I' Catalysts 1-11 inclusive are all structurally of the Sillen type (see section 2).Table 2.Variations in catalytic performance in a sub-set of closely related Sillen-type solids (at 993 K ) S C, yield percentage of CH4 conversion 0 2 c2 2 Li3Ca2Bi,,0, ,C1 ,,, 14/mmm 45.56 0.22 66.7 17.7 37.2 66.8 11.8 e LiCaBi,O,CI, I4/mmm 16.64 0.33 50.0 21.9 47 .O 67.0 14.7 z LiCa Bi , 0,C I , I4/mmm 21.29 0.67 0.0 41.7 99.7 46.5 19.4 5 formula spacegroup c/A Ca/Bi single CI layer ( ) conversion ('/" ) selectivity ("/o ) (Yo ) Li Bi 30,C12 14/mmm 12.03 0.0 100 15.6 35.5 62.9 9.8 s P w44 Selective Oxidation of Methane catalyst in order to function effectively in this regard.So far as the lifetime of the catalyst is concerned, the less the loss of constitutional chlorine from the solid the better. Our experimental results (on catalysts 2,4, 5 and 6) offer some clues as to the relative importance of various structural features. It is to be noted (table 2) that we have systematically varied the proportions of the same al kali-metal and al kaline-earth-metal cations within this sub-family of sillenite-based catalysts. Individual members of this sub-family all possess the same crystallographic space group; they all have similar layered structures; and as the percentage of single chloride layers varies from zero to 100 the ratio of the Ca/Bi ions changes from two-thirds to zero.It is noteworthy that the best catalytic performance, judged by the methane conversion and C , yield, is found for the X3 structure in which there is no single layer of CI ions. We know that BiOCl, which has the X2 structure, tends to lose its chlorine under reaction conditions rather freely. Could it be that the fewer the number of discrete halogen layers, the better the performance because the chlorine, while still available, is not too labile? The key to the effectiveness of the catalysts reported here may be linked to the coexistence of both bismuth and chlorine in structures which are quite stable (some, as shown above, being more stable than others).It is not yet known whether, as with all other methane selective oxidation catalysts (containing constitutional oxygen) that have so far been t e ~ t e d , ~ ' - ~ ' the oxygen in the solid is used sacrificially, as it is in other examples*' involving uniform heterogeneous catalysts. Such confirmatory tests are in hand. We also plan to assess the separate influence of bismuth and chlorine by synthesizing new solids, such as the perites based on (PbBi02)+ (where the proportion of bismuth in each corrugated (Bi,O,)?+ sheet is halved) and other oxychlorides in which the (M2O2),+ sheets are totally free of bismuth. As mentioned earlier, there are many other complex oxides containing bismuth, which form structures such as the fluorites and pyrochlores that tolerate high concentra- tions of defects and display rapid ionic and electronic transport.Bearing in mind the known catalytic, electrocatalytic and advantageous electrical properties of these monophasic, crystalline materials,43.44 and especially their tendency for smooth uptake of gaseous oxygen to be facilitated when bismuth is present in their bulk,4' it is prudent systematically to investigate along the lines outlined here, their role as catalysts and el ect ro ca t a I y s t s 40*45.46 for the oxidative coupling of methane. This work was supported by the S.E.R.C., the Ramsay Memorial Trust (whose trustees awarded a Japanese Ramsay Fellowship to W.U.) and by BP Research International (through a studentship to K.D.M.H., maintenance grant to J.W.and an equipment grant to J.M.T.). References 1 W. Hinsen, W. Bytyn a n d M . Baerns, Proc. 8th Int. Congr. Catal. (Verlag Chemie, Berlin, 1984), vol. 3, p. 581. 2 R. Pitchai and K. Klier, Catal. Rec. Sci. Eng., 1986, 28, 13. 3 K. Otsuka and M. Hatano, Shokuhai, 1987, 29, 46. 4 J . M. Thomas, Xiankuan Zhang and J. Stachurski, J. Chem. Soc., C'hem. Commun., 1988, 162. 5 R. Kurt Ungar, Xiankuan Zhang a n d R. M. Lambert, Appl. Catal., 1988, 42, 41. 6 T. Ito a n d J. H. Lunsford, Nature (London), 1985, 314, 721; see also J. M. Thomas, Nature (London), 1985, 314, 669; J. A. Labinger, K. C. Ott, S. Mahta, H. K. Rockstad and S. Zaumalan, J. Chem. Soc., Chem. Commun., 1987, 543.7 J. P. Bartek, J. M. Hupp, J. F. Brazdil and R. K. Grasselli, Symposium o n Hydrocarbon Oxidation, New Orleans (ACS, Washington D.C., 1987). vol. 32, 774. 8 J. A. Sofranko, J. J. Leonard, C. A. Jones, A. M. Gaffney and H. P. Withers, Symposium o n Hydrocarbon Oxidation, New Orleans (ACS, Washington D.C., 19871, vol. 32, 763. 9 E. Iwamatsu, T. Moriyama, N. Takasaki a n d K. Aika. J. ('hem. Soc., C'hem. Commun., 1987, 19. 10 W. Ueda and J. M. Thomas, J. Chem. Soc., Chem. Commun., 1988, 1148. 11 W. Ueda and J. M. Thomas, Proc. 9th Inr. Congr. Caral., Calgary, 1988, 2 , 960.J. M. Thomas et al. 45 12 K. D. M. Harris, W. Ueda, J. M. Thomas and G. W. Smith, Angew. Chem. Int. Ed., 1988, 27, 1364. 13 J. F. Ackerman, Muter. Res. Bull., 1982, 17, 883. 14 J. F.Ackerman, J. Solid State Chem., 1986, 62, 92. 15 J. Ketterer and V. Kramer, Muter. Res. Bull., 1985, 20, 1031. 16 T. Isozaki and W. Ueda, unpublished work. 17 J. M. Thomas, Angew. Chem., Int. Ed., 1988, 27, 1673. 18 J. M. Thomas, Solid State lonics, 1988, in press. 19 W. Ueda, C-L. Chen, K. Asakawa, Y. Moro-oka and T. Ikawa, J. Catal., 1986, 101, 969. 20 D. Buttrey, D. A. Jefferson and J. M. Thomas, Philos. Mag. A, 1986, 53, 897. 21 W. Zhou, J. M. Thomas, D. A. Jefferson and M. Alario Franco, J. Phys. Chem., 1987, 91, 512. 22 C. R. Adams and T. Jennings, J. Catal., 1963, 2, 63. 23 P. Mars and D. W. van Krevelen, Chem. Eng. Sci. Suppl., 1954, 3, 41. 24 C. N. R. Rao and J. M. Thomas, Acc. Chem. Rex, 1985, 18, 113. 25 L. G. Sillen, 2. Anorg. Allg. Chem., 1941, 246, 115.26 B. Aurivillius, Arkiu. Kemi., 1950, 463, 499. 27 G. Blasse, G. J. Dirken, G. Greaves and S. K. Blower, Muter. Res. Bull., 1988, 23, 1591. 28 A. Lagercrantz and L. G. Sillen, Arkiv. Kemi. Mineral. Geol., 1948, 25A, 1. 29 A. K. Cheetham, A. M. Chippendale and S. J. Hibble, Nature (London), 1988, 333, 21. 30 W. Zhou, J. M. Thomas and D. A. Jefferson, Proc. R. Soc. London, Ser. A , 1986, 406, 173. 31 W. Zhou, D. A. Jefferson and J. M. Thomas, J. Solid State Chem., 1987, 70, 129. 32 J. M. Thomas, D. A. Jefferson and G. R. Millward, JEOL News, 1985, 23E, 1. 33 W. Zhou, Ph. D. Thesis (University of Cambridge, 1987 1. 34 W. Ueda, W. Zhou, D. A. Jefferson and J. M. Thomas, unpublished work. 35 J. M. Thomas and W. J. Thomas, Introduction to the Principles of Heterogeneous Catalj.sis (Academic 36 K. D. Campbell, H. Zhang and J. H. Lunsford, J. Phys. Chem., 1988, 92, 750. 37 G. J. Hutchings, M. S. Scurrell and J. R. Woodhouse, J. Chem. Soc., Chem. Commun., 1988, 253. 38 C. A. Mims, to be published and personal communication. 39 P. F. Nelson and N. W. Cant, J. Phys. Chem., 1988, 92, 6176. 40 K. Otsuka, M. Hatano and T. Komatsu, Cad. Todaj: in press; see also K. Otsuka, K. Suga and I. Yamanaka, Caral. Lett., in press. 41 A. Ekstrom and J . A. Lapszewicz, Symposium on Direct Conversion of Methane to Higher Homolayers, Division of Petroleum Chemistry, Los Angeles (ACS, Washington D.C., 1988), p. 430. 42 0. V. Buevskaya, A. I . Suleimanov, S. M. Aliev, and V. D. Sakolovskii, React. Kiner. Catal. Lett., 1987, 33, 223. 43 M. P. van Dijk, A. J. Burggraf, A. N. Cormack and C . R. A. Catlow, Solid State Ionics, 1985, 17, 159. 44 M. P. van Dijk, J. H. H. terMaat, G. Roelofs, H. Bosch, G. M. H. van de Velde, P. J. Gellings and A. J. Burggraf, Reel. Trav. Chim. Pavs-Bas, 1984, 103, 38. 45 S. J. Korf, H. J. A. Koopmans, B. C. Lippens Jr and A. J. Burggraf, J. Chem. Soc., Faradaj, Trans. 1, 1987, 83, 1485. 46 M. Stoukides and C. G. Vayenas, J. Catal., 1981, 70, 137. Press, London, 1967) p. 383. Paper 81050441; Received 19th December, 1988

ISSN:0301-7249    

DOI:10.1039/DC9898700033

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1989, 87, 47-64 GENERAL DISCUSSION Prof. M. S. Spencer (University of Wales, Cardifl) (communicated). Much of my comment applies generally to work on methane coupling rather than specifically to Prof. Lunsford’s paper. For more than half a century gas-phase oxidation has been intensively studied, including the selective oxidation of methane,’ but little of this work is considered in the interpretation of methane-coupling results. Heterogeneous reaction steps are commonplace in nominally ‘homogeneous’ systems. Diffusion paths in a methane- coupling catalyst are typically a factor of lo6 shorter than in a reactor used for gas kinetics, so heterogeneous steps must be expected in some, if not all, of chain initiation, propagation, branching and termination.It is folly to attempt an arbitrary separation of the process into heterogeneous and homogeneous reactions. Professor Lunsford’s experiments to measure directly the efficiency of catalyst sur- faces in methyl radical generation and reaction are most welcome. It would be interesting to see these results, and others, used in an analysis of methane coupling as a conventional ‘homogeneous’ methane-oxygen chain reaction, with added heterogeneous steps for, e.g. chain initiation ( CH4 dissociation), branching [CH,02(a) ---* ?] and termination [2CH,(a) -+ C2H61. 1 D. M. Newitt and P. Szego, Proc. R. SOC. London, Ser. A, 1934, 147, 555. Prof. J. H. Lunsford (Texas A&M University, Texas, U S A . ) replied: The modelling of coupled heterogeneous-homogeneous reactions which occur during CH4 oxidation is, of course, an important objective for researchers in the field.Such models are presently limited by the almost complete absence of rate data on the reactions that occur between the radicals and the metal oxides that are commonly employed. For example, the initial reaction between a CH; radical and a metal oxide is a matter of conjecture, and the absolute rates of such reactions are not known. Clearly, additional research is needed in this area of kinetics and surface dynamics. Prof. M. W. Roberts (University of Wales, Cardifl) said: You mention the possibility of one of three different oxygen species being active in CH4 activation: 0-, 0, and 06-. We have drawn attention elsehwere to experimental evidence for the high reactivity of transient surface oxygen species in H-abstraction reactions.”2 The stable chemisorbed oxygen species are unreactive under the same experimental conditions. Do you think that the active species may be surface oxygen transients, possibly vibrationally excited? 1 A.F. Carley, M. W. Roberts and S. Yan, J. Chem. SOC., Chem. Commun., 1988, 267. 2 C. T. Au and M. W. Roberts, J. Chem. SOC., Faraday Trans. 1, 1987, 83, 2047. Prof. J. Cunningham (University College Cork) followed on from this: I would like to address Prof. Robert’s question as to the possible similarity between Lif/NiO and Li’/MgO, and develop an ensuing clarification that the positive hole in Li+/MgO is located on oxygen, whereas that in Li’/NiO is located on nickel. This ‘positive-hole’ character of 0- in MgO is represented within the Kroger-Vink notation by assigning a positive charge to that region of an MgO lattice which contains an 0- at a site normally occupied by 02-.As a consequence of this overall positive charge, such a ‘lattice’ 0- species can indeed be expected to exhibit properties strongly modified from those reported for 0- species in the gas phase. On this basis I do not have any difficulty with Prof. Lunsford’s adoption of a different notation namely 0’- in his paper, to distinguish it from 0-(g). Carried through to its logical conclusion, it suggests that a species such 4748 General Discussion as 0; formed from reaction of a lattice 0- with gaseous O2 would have significantly different properties from gas-phase 0,. Prof.Lunsford responded: In a sense we view the ozonide ion, O,, as a transient oxygen species which is vibrationally excited. Previous e.s.r. results have shown that 0; ions on MgO are unstable over a period of several hours at room temperature, and under reaction conditions the surface reactions 0-+o, s 0; 3 o;+o may 0ccur.l With our quenched Li+/MgO samples 0, has also been detected.* The availability of electrons for reduction of oxygen, however, is a major difference between Li+/MgO and the Zn (0001) surface which you have examined. In the former case the number of such electrons is very limited, whereas in the latter case they are available to form transient species such as surface O,, which is ultimately reduced to surface and bulk 02- ions. 1 N. B. Wong and J.H. Lunsford, J. Chem. Phys., 1972, 56, 2664. 2 J-X. Wang and J. H. Lunsford, J. Phys. Chem., 1986, 90, 5883. Dr R. Burch (Reading University) continued: The question I should like to raise concerns the actual coupling step in the formation of ethane from methane. It is clear from your earlier work that methyl radicals are released into the gas phase when methane is passed over Li/MgO catalysts in the presence of oxygen. However, Hatano and Otsuka' have recently claimed on the basis of kinetic measurements that methyl coupling occurs on the surface of an Li/NiO catalyst. Similarly, Wada et ~ 1 . ~ have argued on the basis of flow-rate studies that no ethane is formed in the gas phase over La/B203 catalysts. Do you have any information on the rate of release of methyl radicals from oxides, such as Li/NiO, in which one of the cations has clear redox characteristics? Is there any reason to exclude the possibility of surface coupling over catalysts of this type? 1 H.Hatano and K. Otsuka, J. Chem. SOC., Faraday Trans. I . , 1989, 85, 199. 2 S. Wada, T. Tagawa and H. Imai, Appl. Catal., 1989, 47, 277. Prof. Lunsford replied: Preliminary experiments on an Li+/ NiO catalyst using our MIESR system revealed that the rate of formation of gas-phase CH; radicals was unexpectedly low relative to the overall catalytic properties. It may well be that there is a class of catalysts which binds CH; radicals to the surface in such a way that coupling can occur prior to more extensive oxidation of the reactive intermediate. Prof. Cunningham said: A simple calculation indicates that, for an MgO powder of surface area ca.40 m2 g-I, the amount of Li+ sufficient to yield one monolayer coverage on MgO is only ca. 0.2 wt%. Consequently, in our studies at University College Cork upon Li'/MgO and other surface-doped MgO materials we have focused our main efforts upon materials with wt% loadings equivalent to monolayer coverage amounts, in the belief that effects of the dopants upon oxidative dimerization and other processes would thereby originate mainly from surface and near-surface regions. This belief has been strengthened by the results of depth-profiling studies upon Li+/ MgO materials, obtained in collaboration with Prof. Hirschwald at the Free University, Berlin, and now shown in fig. 1. This figure contrasts the relatively shallow depth-profile obtained by dynamic SIMS for lithium close to the surface of a monolayer-doped 0.24 wt% Li+/ MgO material, with the presence of lithium at greater concentration and to much greater depth in a Li+/MgO material prepared in identical fashion but with a lithium loading equivalent to ca.10 monolayers.General Discussion 49 1 . 5 0.5 I I I I 1 15 45 75 105 135 165 sputtering time/min Fig. 1. Depth profiles of Li/MgO at a fixed current density of 200 nA ern-:, but at different Li loadings: ( a ) 0.15 wt% of Li/MgO, and ( 6 ) 2.4wt% Li/MgO. Prof. Lunsford added: Your depth-profiling results raise some interesting questions concerning the influence of the near-surface region in the oxidative dimerization of methane.Our earlier study' showed that upon increasing the amount of Li+ from 0.2 to 1 wt%: ( a ) the surface area decreased from 37 to 8 m g-', ( b ) the specific activity for CH, conversion increased by a factor of 7 and ( c ) the C2 selectivity increased from 18 to 55%. Thus, the specific productivity of C2s greatly exceeded the fivefold increase in Li+ loading. The profiles of fig. 1 clearly show that the region near the surface both is richer in Li+ and extends further into the MgO with the sample containing 2.4 wt% Li'/ MgO than with the other sample. The more intensive and extensive Li+-rich near-surface region may be responsible for the greatly improved catalytic results that were observed when the Li+ content was increased. Obviously, CH, is activated only on the surface, but the concentration of active centres at the surface (e.g. 0'- or 0,) may be strongly influenced by the presence of [ Li+O-] centres in the near-surface region via hole transport, i.e.[L~+o-]+ 0: I-* [L~+o'-] + 0,. 1 T. Ito, J-X. Wang, C-H. Lin and J. H. Lunsford, J. Am. Chem. SOC., 1985, 107, 5062. Prof. Cunningham made a third comment: On the basis of the lithium depth-profile data just illustrated for Li+/ MgO material doped at the monolayer-equivalent level, we considered it probable that modifications of the defect structure of such material, such as the proposed creation of (Li-0-) pair defects, would be concentrated close to the surface. A temperature-programmed oxygen isotope exchange (TPOIEX) procedure was therefore developed in order to establish whether the 0- ccjmponent of such defects would exhibit earlier onset temperature for isotope exchange with gas-phase than the lattice 0,- anions at the surface of undoped MgO.Our results from application of this TPOIEX procedure to Li'/MgO are summarised in fig. 2, which represents them in terms of the atom fraction of oxygen-18 remaining in the gas phase (having initial composition of 0.5 mole fraction of "02 plus 0.5 mole fraction '"0,) over Li+/MgO or over MgO whilst temperature was ramped from 620 to 760 "C. It is clear from this figure that an isotope exchange process involving the exchange of l80 from the gas phase with 0 from the surface of the solid oxide onsets at a much earlier temperature and/or is 1650 General Discussion 0.6 0.5 0.4 00 C w .- - 2 0.: ct Y 5 (d 0.; 0.1 c I I 1 I 1 I I I 610 630 650 670 690 710 730 750 770 temperature/"C Fig.2. Comparison of atom fraction 18 plots for ( x ) MgO and (0) Li/MgO: both pretreated in oxygen at ca. 480°C for 1 h. more efficient over Li+/MgO than over MgO. In our poster at this meeting we show that this process must be of a form usually termed R , , viz. ~ 1 8 0 n - + 160180 160;,, + 1802(g, ( 1 ) ( g ) - Since there have been many proposals that R,-type oxygen exchange proceeds via a triatomic oxygen intermediate,' our results are consistent with Lunsford's proposal in his paper that 0, intermediates may be important in the initial step for activation of CH, on Li+/MgO surfaces. 1 K. Tanaka, .I. Phys. Chem., 1974, 78, 555. Prof. 0. V. Krylov (Academy of Sciences, Moscow) (communicated): In connection with the paper presented by Prof.Lunsford I should like to present our data on the same topic. We studied the processes of reduction and re-oxidation of some catalysts for the oxidative coupling of methane-Li20/Mg0, PbO/A1203, K20/A1203. It was shown that the reduction of the samples with H2 and CH, proceeds in the temperature range 400-700 "C. Among the gaseuos products, C2H6, C2H4, C02, H20 were found. At temperatures 400-500°C the rate of H20 formation becomes much lower than that for H2 (CH,) consumption. Under these conditions the heats of hydrogen adsorption QH2 can be directly measured from calorimetric data. The energies of 0-H bonds, Eo-H,~ were calculated by ( 1 ) H2 + W N S --* 2(OH)s using the values of QH2, in table 1 .Surface 0 - H groups can react with the consequent formation of oxygen vacancies and following re-oxidation via the traditional scheme of re-oxidation including the dehydroxylation stage: 2(OH), 3 2(0),+0,+H,O (2) o+;o, - (O)',. (3)General Discussion 51 Table 1. The parameters of interaction of H7 and CH4 with samples: QH2, E , - , , activation energies of H2 ( EL2) interaction and C 2 hydrocarbons formation ( E & ) ; rates of C 2 hydrocarbons ( Wcz) and COz ( Wco,) formation and rate of Hz consumption ( W,?) Li20/Mg0 210 95 90 320 7.5" 0" 31" K 2 0 / Alz03 1 10 52 120 270 l S h 4.3 71h PbO/A1203 55 80 180 250 1.7h 1.9h 154h "650 "C; h700 "C. But the interaction of oxygen with Li20/Mg0 pretreated in CH, or H2 leads to the formation of H20, i.e.the re-oxidation process proceeds as some sort of oxidative dehydrogenation of surface OH groups: 4(OH),+O2 ---* 4(0),+2H,O. (4) The absence of surface oxygen vacancies in the case of Li,O/MgO treated in H2 at 600°C was proved by re-oxidation experiments with N20 as the oxidant. Under these conditions the amount of N2 is equal to that formed on oxidized Li,O/MgO and corresponds to catalytic decomposition of N20: After additional treatment (at T = 700 "C in He) of the reduced sample the amount of N2 formed in the first pulses was much higher than that for the oxidized or the unheated in He samples of Li2/Mg0. In this case the treatment of the reduced sample causes the dehydroxylation of the sample surface and interaction of N20 with vacancies formed by the treatment takes place The activation energy of the re-oxidation of pretreated in He Li20/Mg0 (100 kJ mol-') and the heat of the interaction of oxygen atoms with the anion vacancies (420 kJ mol-') were determined.Thus, the re-oxidation process going via the formation of vacancies must be more suppressed in the presence of both methane and N 2 0 because of H20 formation which shifts the equilibrium state of reaction (2). This conclusion is in good agreement with the results reported by Prof. Lunsford and co-workers.' They observed a decrease of activity of Li,O/MgO catalyst with respect to CHI, radicals formation in the presence of a CH4-N20 mixture, while the formation of CHI, radicals in the presence of a CH4-02 mixture proceeds with a constant rate. Methane oxidation in the presence of oxygen and nitrogen oxide over a silica gel supported molybdenum catalyst has been rather extensively studied.Formaldehyde, CO, C02, and, in the presence of water, methanol were the basic reaction products. However, the methane conversion and the selectivity of formaldehyde generation were not high. The catalyst structure is known to have a marked effect on catalytic activity in certain oxidation reactions. My colleagues at the Institute of Chemical Physics essentially modified the catalyst structure by mechano-chemical activation.' The systems: SO2-supported Moo3 (I), Moo,-SiO, mixture (11), and a mixture of low-dispersity Mo and Si02 (111), as well as MOO and Si02 oxides were milled in a quartz vibrating mill for times of various duration (table 2).The catalytic activity was studied in a differential reactor in the presence of nitrogen oxide or air at 600 "C and contact time 0.5 s, both before and after activation. The milling of catalysts representing a specific mixture of meta molybendum or of MOO, with silica gel decreases the catalytic activity. For the metal and its oxides mixed with Si02, methane conversion becomes several times lower, but the formaldehyde 2N20 + 2N,+0,. ( 5 ) Li+( - ) + N 2 0 -+ Li'O- + N2. (6)52 General Discussion Table 2. Effect of mechano-chemical activation on the activity of Mo-containing catalysts in methane oxidation (20% CH4, 60% N20, 20% Ar) catalyst ~ selectivity (Yo) activation time CH, conversion (.r)/min (Yo 1 CH2O co co2 2% MoO3/SiOz (supported) 2% MOO,-SiO, (mixture) 10% MoO,-SiO, (mixture) 10% Mo-SiO, (mixture) M003 SiOz 0 180 0 180 0 180 0 180 0 180 0 180 1.3 1 .1 1.2 0.6 2.0 8.1 2.1 1.1 1.2 46 26 28 57 10 33 14 16 70 - - 100 low-active 100 100 - - 9 73 18 - - inactive 26 30 44 26 28 48 " Wt% of Mo in MOO,. generation selectivity attains up to 100°/~. The catalytic activity of silica-gel-supported MOO, does not change upon mechano-chemical activation. As shown by the X-ray method, mechano-chemical activation of samples I1 and I11 results in dispersion of the Mo and MNoO, crystals from 1000 to 300 A, whereas only amorphization of S O 2 occurs for sample I. It can be seen from the TEM patterns that besides being reduced in size, the molybdenum crystals become enveloped by S O , [plates l ( a ) and ( b ) ] .Along with the electron micrograms, the formation of capsules representing Si0,-enveloped Mo particles is confirmed by the results for catalyst 111 interaction with oxygen at 600 "C. With no mechano-chemical activation Mo stoichiometrically converts to MOO,. After 5 h treatment in the quartz mill the uptake of oxygen by the catalyst decreases by a factor of 1.5. As seen from X-ray data, after methane oxidation as well as after interaction with oxygen, the initial catalyst I11 completely oxidizes to MOO, while the mechano-chemically activated samples left in the reaction medium or subjected to oxygen represent different valent molybdenum oxides and metal Mo. It seems that mechano-chemical activation results in specific interaction of silica gel not only with Mo, but also with Moo3 particles and is accompanied by enveloping of these particles with SiOz.As a result of this the active catalyst oxygen contributing to carbon dioxide formation becomes fixed. As found by a specific experiment, CO forms essentially by formaldehyde decomposition. After activation of the catalyst C H 2 0 decomposition markedly diminishes and no CO appears in the reaction products. The absence of such an effect for catalyst I seems to be caused by the fact that, even in the course of the catalyst preparation, there occurs chemical bonding between the two phases responsible for catalytic activity and unaffected by mechano-chemical activation. 1 J. Lunsford et af., J. Am. Chem. SOC., 1985, 58. 2 G. A. Vorob'eva, A. A. Firsova, A. A. Bobyshev and D.P. Shashkin, Mechanical activation of oxide molybdenum-containing catalysts for methane oxidation to formaldehyde. Collected papers of the All-Union Conference on Chemical Syntheses Using Single-carbon Molecules, Nauka, Moscow, p. 78. Prof. J. B. Moffat (Uniuersity of Waterloo, Ontario) said: As all of us know, the earliest work on structure-sensitive reactions was largely concerned with bimetallic surfaces. We recall the impressive work by Sinfelt and coworkers on the Cu-Ni and Cu-Ru surfaces with the hydrogenolysis of ethane and the dehydrogenation of cyclo-Faraday Discuss. Chem. SOC., 1989, Vol. 87 Plate 1. ( a ) Initial mixture of Mo crystals with SiO,; ( b ) after 5 h milling. General Discussion (Facing p. 52)General Discussion 53 Table 3.Catalyst preparation route product precipitation selectivity (YO ) ignition selectivity (% ) ethane 32 9 ethene 31 9 carbon dioxide 23 53 carbon monoxide 14 18 hexane. Although there have been admittedly fewer examples of structure sensitivity in oxidation reactions there is evidence that structure sensitivity may occur in such processes. I recall the work of Tatibouet and Germain on the oxidation of methanol on Moo3 catalysts, in my opinion a convincing demonstration of structure sensitivity. We may also recall the work of one of our colleagues present here today, namely that of Volta and Vedrine, on propene oxidation on MOO,, another unambiguous example of structure sensitivity in oxidation processes. Recent work by Lambert and coworkers on the oxidation of ethene also suggests that structure sensitivity may exist in this process.Furthermore, I believe that Prof. Ponec will show us another example in the selective oxidation of n-butane. It is my understanding, Prof. Lunsford, that you interpret your results as suggesting the absence of structure sensitivity, although you have been careful to note that subtle effects may not have been observed. In view of the aforemen- tioned rather positive examples of structure sensitivity in oxidation processes I am wondering if you would care to speculate for us on the probability that structure sensitivity exists in the oxidative dimerization process. Prof. Lunsford replied: For monophasic oxides structure sensitivity may very well exist in the oxidative dimerization process; however, where the active sites consist of isolated defects or impurity centres it seems that structure sensitivity, in the usual sense, would not be a factor.Dr G. J. Hutchings (Liverpool University) added: In the presented paper it is clear that the addition of Li to the two samples of MgO produces materials that are very similar, and hence it is not possible to make any comparisons with the theoretical predictions of Mehandru et al.' However, in the absence of Li the situation may be less confused since the two MgO samples exhibit very different crystal sizes [MgO(A) 7 nm cubes, MgO (JM) 30 nm cubes]. Table 1 [ref. (l)] gives data for the relative CH; scavenging efficiency for these two MgO samples and it can be concluded that the MgO(A) is superior to the MgO(JM) sample in this respect. Is it possible to comment on the relative reactivity of these two MgO samples for methane coupling since it may then be possible to make some comparisons with the predictions of Mehandru et al.? 1 S.P. Mehandru, A. B. Anderson and J . F. Bradzil, J. Am. Chem. Soc., 1988, 110, 1715. Prof. Lunsford answered: We did not obtain catalytic data on the pure MgO samples, although the comparison you suggest would be interesting. Prof. R. W. Joyner (Liverpool University) made further comments: Prof. Lunsford has presented limited support for structure sensitivity in methane oxidation over mag- nesium oxide catalysts. We have performed experiments on magnesia catalysts prepared in different ways, therefore having quite different morphologies. Catalysts have been prepared by precipitation, which is known to give MgO with a very small crystallite size, (typically <5 nm cubes), and also by ignition of magnesium ribbon in air, followed by calcination to decompose any nitride phase.This latter approach generates much larger, regular cubes, with side 50-100 nm. The selectivity of the two catalysts prepared by these routes is quite different, and is shown in table 3.54 Genera 1 Discussion Electron microscopy of the used catalysts shows that the observed changes in the morphology of the catalysts are only those which are to be expected from the thermal treatment,' and that the reaction gases appear to have little influence on catalyst structure. Magnesium oxide catalysts show high selectivities to molecular hydrogen, (>N% ).Separate studies of the interaction of CO-H20 and C2H4-H20 mixtures with the catalysts have indicated that there are two main sources of hydrogen, the water gas-shift reaction: CO+H20 - CO,+H,, and the steam cracking of ethane to ethene: C2H6 ---* C2H4+H2. Magnesium oxide has no significant activity for the steam reforming of methane or ethane under the conditions of interest, [ T == lo5 K, P(CH,) = 47 kPa, CH4/02 = 5.75, O2 conversion ca. 98%]. These results will be reported in detail elsewhere.2 1 A. F. Moodie and C. E. Warble, J. Crystal Growth, 1971, 10, 26. 2 J. S. J. Hargreaves, G. J. Hutchings and R. W. Joyner, Proc. 2nd Eur. Workshop Methane Activation, to appear in Catal. Today. Prof. M. Ichikawa (Hokaido University, Sapporo, Japan) said: I would like to address your proposed mechanism for the oxidative CH, coupling catalysed on Li+/MgO.In your paper you suggest that the CH; radical, most probably the intermediate for C, hydrocarbons, is directly derived from a homolytic C-H splitting of CH, by 0- sites on the Li+-promoted MgO surface. My questions are as follows: ( 1 ) From a thermodynamic point of view, which is energetically most favourable for the production of CH' radicals: a heterolytic C-H bond activation causing CH, species, which may be converted by electron abstraction (via the redox process), or a direct homolytic H-abstraction from CH, catalysed on an 0- site? Additionally, is there any evidence to find a possible role of CH, species incorporated in the oxidative coupling of CH4? CH, H' CH,+[O-M'] [M ' + -01 ' li-.'I' CH3-CH3 + CH;+[M+-O-] (2) Have you observed any kinetic isotopic effect by using d,-methane or "O-MgO/ Li for the oxidative methane coupling reaction on this particular catalyst or other oxide catalysts such as Y2O3 and Ce2O3? If so, how do you rationalize the data for your proposed mechanism of CH, activation? Prof. Lunsford replied to each of these questions: (1) The heterolytic breaking of the C-H bond by 0- is favoured both by the gas-phase reaction studies of Bohme and Fehsenfeld' and by the theoretical calculations of Mehandru et al. [ref. (16) in the paper]. I have no information on the energetics of the homolytic process which you propose. (2) Although we have not observed a kinetic isotope effect using CH, and CD,, Cant et al.[ref. (14) in the paper] have observed such an effect and the implications are briefly discussed in terms of 0'- or 0, as the actual active site. Another possibility is that a substantial fraction of the CH4 could react via gas-phase chain-branching reactions that are initiated by surface-generated CH; radicals. 1 D. K. Bohme and F. C. Fehsenfeld, Can. J. Chem., 1979, 47, 2717.General Discussion 55 Table 4. yield (YO) CH4 conv (YO) H2C0 CO CH3CI 20% HPMo/Si02 w = 1 g (paper, fig. 5) no TCM 0.35 0.21 0.13 0 0.17 mol% TCM 1.2 0.44 0.54 0.14 no TCM 0.1 0.06 0.032 0 0.17 mol% TCM 0.3 0.03 0.033 0.24 20% HPW/Si02 w = 2 g (fig. 7) Prof. A. K. Datye (University of New Mexico, Albuquerque, U.S.A.) in response to informal remarks made by Prof.P. B. Wells: Prof. Wells suggested that the presence of water may be causing a rounding off of the MgO cubes. However, the phenomenon of thermal roughening alone may account for the rounded corners seen in the used catalysts. In the case of ionic crystals, such as NaCl, Heyraud and Metois' have shown that while the equilibrium shape at low temperatures is a cube, at higher temperatures all the edges and corners become rounded. Comparison of the used catalyst with MgO cubes heated in inert atmospheres may allow a better estimate of the morphological changes caused by the reactive environment. 1 J. C. Heyraud, and J. J. Metois, J. Cryst. Growth, 1987, 84, 503. Dr G. J. Hutchings addressed Prof. Moffat: As noted in your paper the addition of tetrachloromethane during methane activation gives different effects on selectivity for the two structurally related heteropoly oxometallates, which may be indicative of the underlying oxidation reaction mechanism.It is the differences observed for the activity and selectivity under comparable conditions that allows one to rule out the operation of a purely homogeneous gas-phase reaction mechanism on the addition of the halo- compounds. Perhaps the difference in the two systems, i.e. Mo and W, is in the ability to activate the oxygen rather than the methane. Consider the data at comparable conditions, i e . 450 "C, CH4/N20 = 4, F = 60 cm3 min-' (table 4). It is clear that the yield of CH,Cl for the two catalysts is comparable, whereas the overall methane conversions are very different with the Mo catalyst giving much higher conversions.This could suggest that the CH3Cl is predominantly formed via a gas-phase reaction, but that the Mo system activates oxygen more effectively. Do you have any comments on the relative mechanisms of oxygen activation with these two heteropoly oxometallates ? Prof. Moffat replied: Work in our laboratory' with heteropoly oxometallates pre- treated with tetrachloromethane (TCM) prior to conversion of methane in the absence of TCM has shown that, at least initially, the observations are similar to those found where a small quantity of TCM is added continuously to the feed stream. This suggests that the catalyst is participating in the process involving chlorine-containing species and that chlorine in as yet an unknown form is incorporated on and/or in the solid.Earlier EXH calculations from this laboratory on heteropoly oxometallates of Keggin structureL have predicted tht the terminal oxygen peripheral metal bond of the heteropoly anion is weaker in the molybdenum-containing anions than in those containing tungsten. These results are consistent with those from earlier temperature-programmed desorption, exchange and reduction experiments.3 The evidence for the importance of the proton as shown in the present work has been interpreted as related to the formation of oxygen vacancies resulting from the extraction of oxygen atoms from the anion by the protons.56 General Discussion In view of the aforementioned, the formation of vacancies may be predicted to be more facile with molybdenum-containing than with tungsten-containing anions.Furthermore, it is suggested that the presence of vacancies, at least initially, is necessary for the oxidation process. Thus it would appear that the molybdenum heteropoly oxometallate should be more active in the processes involving the transfer of oxygen. 1 S. Ahmed and J . B. Moffat, J. Catal., in press. 2 J. B. Moffat, J. Mol. Catal., 1984, 26, 385. 3 B. K. Hodnett and J . B. Moffat, J. Catal., 1985, 91, 93. Prof. J. Haber (Polish Academy of Sciences, Krukow, Poland) then said: In your model of the interaction of methanol with the Keggin unit you are assuming that the methoxy species are formed with the participation of the terminal an-bonded oxygen atoms. Intuitively such an assumption may seem plausible; however, more detailed discussion leads to the conclusion that it is rather the bridging oxygen atom which is the most probable site of the reaction.The following arguments may be raised in favour of such an alternative: (i) the HOMO of the Keggin unit is mainly composed of lone-pair orbitals of the bridging oxygen atoms.' (ii) due to the distortion of the octahedron resulting from the shift of the metal centre towards the terminal oxygen atom the latter becomes more acidic, whereas the bridging oxygen atom becomes more basic;' (iii) 13C n.m.r. spectra recorded after interaction of methanol with the Keggin unit point directly to the bridging oxygen atom as the site where CH3 group is linked;' (iv) "0 n.m.r. spectra of 12-molybdophosphate solutions indicate that electron- density charge on bridging oxygens is higher than on terminal ones.4 1 Taketa, S. Katsuki, K.Eguchi, T. Seiyama and N. Yamazoe, J. Phjas. Chem. 1986, 90, 2959. 2 J. B. Goodenough, Proc. 4th Int. Con$, Chemistry and Uses of Molybdenum, Golden Colorado 1982, ed. 3 W. E. Farneth, R. H. Staley, P. J . Domaille and R. D. Farlee, J. Am. Chem. SOC., 1987, 109, 4018. 4 R. I. Maksimovskaya, M. A. Fedotov, V. A. Mastkhin, L. I . Kuznetsova and K. 1. Matveev, Dokl. Akad. H . F. Barry and P. G. H. Mitchell (Climax Molybdenum Co., Ann Arbor 1982), p. 1. Nauk SSSR, 1978, 240, 117. Prof. Moffat, in reply to this, then said: Photoacoustic FTIR spectra obtained in this laboratory have shown that methanol is protonated at room temperature by such heteropoly oxometallates as 12-tungstophosphoric acid.On stepwise heating to 150 "C the bands associated with protonated methanol diminish in intensity and a sharp band develops at 1453 cm-' and progressively increases in intensity. This band is attributed to the CH3 symmetric deformation in the CH30 group. Subtraction of the room- temperature spectrum from that of the heat-treated sample (fig. 3) revealed the progress- ive development of a band at 1022cm-] (inset, fig. 3 ) which appears to relate to the progressive formation of a metal alkoxide-type structure. For example, the C -0 stretch in W(OCH3)6 appears at 1070cm-'.' These observations suggest that the methyl group [CH:] is attached to a terminal oxygen atom of the heteropoly anion. Earlier EXH calculations from this laboratory predict that the magnitude of the charge on the terminal oxygen atom should be higher than that on the bridging oxygen atom of the anion.? It should be noted, however, that attack of a methyl group on a bridging oxygen atom of the anion may lead to a scission of one of the W-0 bonds of the bridge, leaving the methyl group apparently bound to a terminal oxygen atom.However, it seems more reasonable to assume that the methyl groups may be bound to both terminal and bridging oxygen atoms of the anion and a steady state or equilibrium may develop between the methyl groups associated with these two centres.General Discussion 57 (el (dl + CH30H at 25 "C 0 I 1 1 I I I 2000 1600 1200 800 wavenumber/cm-' Fig. 3. Effect of stepwise heating in U ~ C U O on spectrum of 'irreversibly sorbed' CH,OH on 12-tungstophosphoric acid.( h ) 50, ( b ) 70, ( c ) 110, ( d ) 150 "C (inset peak obtained by subtraction of spectrum of pre-evacuated acid, normalized at 1080 cm-'; ( e ) effect of dosing ( d ) with excess CH,OH at 25 "C and evacuation at 25 "C. 1 D. C. Bradley, M. H. Chisholm, M. W. Extine and M. E. Stager, Inorg. Chem., 1977, 16, 1794. 2 See, for example, J. B. Moffat, in Prepararion of'Caralysts IV. Srud. S u r - Sci. Catal., ed. B. Delmont, P. Grange, P. A. Jacobs and G. Poncelet (Elsevier, Amsterdam, 19871, vol. 31. Dr E . M. Senvicka (Polish Academy of Sciences, Krakow, Poland) said: Regarding Prof. Moffat's paper, I would like to comment on the nature of surface species responsible for catalytic activity in methane oxidation.Optimum temperature reported here for this reaction seems to exceed considerably the stability range known for heteropoly acids. It is therefore conceivable that Keggin anions act as a precursor of an active phase rather than survive in situ under reaction conditions. Results obtained in our laboratory seem to confirm such a hypothesis. Fig. 4 shows the i.r. spectra recorded for 0.1 monolayer H,PV2Mo,,,O,,,/SiO2(400 m' g-I) catalyst subjected to various treatments, after subtrac- tion of the silica background. The fresh sample (solid line) displays all the bands expected for the Keggin anion uiz. 785,865,960 and 1065 cm-'. The spectrum recorded immediately after 4 h calcination in air at 673 K (dashed line) shows that the Keggin structure has collapsed, to be replaced by decomposition products, possibly a mixed molybdenum-vanadium oxide system, with characteristic bands at 830 and 1015 cm- I .Simultaneously, the originally yellow sample turns white. However, on exposure to air (ca. 70% natural humidity), at room temperature, the sample recovers most of its original colour within half an hour, and corresponding changes in the i.r. spectra follow (dotted58 * . J I I I General Discussion 700 800 900 1000 1joo wavenumber/ cm-’ Fig. 4. 1.r. spectra of 0.1 monolayer H5PVzMo,0040/Si02 (400 mz g-’). (-) As prepared, yellow; (- - -) immediately after calcination in air at 673 K for 4 h, white; (. * - - .) after calcination in air at 673 K for 4 h stored for 0.5 h open to air at room temperature, yellow.line). It is obvious that a swift reconstruction of Keggin units takes place at room temperature under ‘moist’ conditions. This result indicates that one has to be very cautious about concluding on the surface composition of a working catalyst on the basis of investigation of a spent catalyst only. Examination in situ seems necessary for an unequivocal identification of the catalytically active phase. Prof. Moffat responded: Results from our laboratory have provided information on the nature and properties of catalysts prepared by supporting heteropoly oxometallates on a support such as silica.’.’ The observation that the rates of formation of products in the conversion of methane at 843 K increase linearly with loading of 12-molybdophos- phoric acid [H3PMo,2040, abbreviated to HPMo) on Si02 and at low values of loading extrapolate to those results found for the support itself clearly indicates that the active species are associated directly with the supported materials (fig.5). The effect of the temperature and duration of pretreatment on the activity, selectivity and remaining molybdenum loading of a 23 wt% HPMo/SiO, catalyst provides evidence for the effect of the support in the enhancement of the thermal stability of the heteropoly oxometallate (fig. 6). The conversion and selectivities in the oxidation of methane are approximately constant for pretreatment temperatures up to 773 K and for higherGenera 1 Discussion 59 H3PMo12040 loading (wto/o 1 501 I 0 0.9 - I v) -w 0.7 E - l 0, ;2- 0.5 2 c) c 0 .- Y < 0.3 E 0 .I 0 0 100 20 0 300 H3PMo,,0,0 loading/ mol KU g-' Fig.5. Effect of the HPMo loading of the support on the production rate of the different products of the CH4-N20 reaction at 843 K. Reaction conditions: CH4 (67%), N 2 0 (33'/0), W=OS g, F = 30 cm3 min-'. ( x ) N2, (+) total carbon detected, (a) HzO, (V) CH30H, (A) CO, (0) CO:, (0) CH20.60 General Discussion I I I * 1 I I I 1 J O $ , 60 0 80 0 1000 0 20 LO 64 80 100 - 20 - - 10 - calcination temperature/ K time of calcination/h Fig. 6. Effect of the temperatures of calcination over 16 h (left) and of the time of calcination at 823 K under air (right) on the CH4 conversion, selectivity, and Mo loading of the 23-HPMo catalyst. Reaction conditions: CH, (67%), N 2 0 (33%), TR = 843 K, W = 0.5 g, F = 30 cm3 min-'.( A ) CO, (0) COz, (0) CH20, ( x ) CH, conversion, (0) Mo loading. temperature the conversion decreases sharply, while the production of CO and CO, remains constant up to 900 K. Above 900 K the production of H2C0 and CO decreases, while that of C02 increases, all three apparently approaching the values expected for the silica support alone. It is evident that the activity of the HPMo catalysts can be related to the presence of a thermally sensitive species whose degradation products have a substantially reduced activity in the oxidation of methane and the thermal stability of the HPMo is enhanced by the presence of the support. The existence of PMo, ,O& anions with Keggin structure on various silica-supported samples was confirmed from the infrared spectra of solutions resulting from the washing of the samples with acetonitrile (fig.7). The characteristic bands of the Keggin structure at 1080 and 969-960 cm-' are still present and intense even after heating at temperatures up to 923 K for 16 h in air, although some diminution of the intensity of the bands can be observed. These observations clearly demonstrate that the Keggin structure still exists even after such vigorous pretreatment as heating in air at 913 K for 16 h. Raman spectra of silica-supported 12-molybdosilic acid ( H,SIMoI2O,,,) show the presence of the Keggin structure even after heating to 773 K for 2 h in air (not shown, see fig. 2 in comment on Bond paper p. 102). 1 J. B. Moffat and S. Kasztelan, J. Curd., 1988, 109, 206. 2 S. Kaszteian, E. Payen and J .B. Moffat, J. Card., 1988, 112, 320. Dr R. Burch (University of Reading) addressed Prof. J . M. Thomas: In your paper on layered oxychlorides for methane activation you show that for certain catalysts there is a very high ethene/ethane ratio. It is now fairly well known that even a smallGeneral Discussion 61 1063 969 883 1200 1000 800 wavenumber/ cm-' Fig. 7. Infrared spectra of acetonitrile solution after washing of the following supported HPMo samples calcined under different conditions. ( a ) Bulk H3PMo,204; ( b ) 1.16 HPMo, 350 "C, 2 h; (c) 11.1 HPMo, 350 "C, 2 h, then 20.1 HPMo sample; ( d ) 350 "C, 16 h; (e) 450 "C, 16 h; (f) 550 "C, 16 h; (g) 640 "C, 16 h; ( h ) 640 "C, 16 h, followed by a rest at 570 "C, 10 h; (i) 730 "C, 16 h. concentration of chlorine radicals in the gas phase can catalyse the rapid dehydrogenation of ethane to ethene, particularly in the presence of residual oxygen.Since in your experiments you are using quite large reactors and quite slow volumetric flow rates the residence time of the primary product (ethane) will be long. It seems quite possible, therefore, that much of the ethene observed could be formed by a purely gas-phase radical-catalysed dehydrogenation of ethane. Do you have any evidence that ethene is produced on the surface of your catalysts? Furthermore, do you have any information on the amount of chlorine being released from the various catalysts used in your work and is there any correlation between the rate of loss of chlorine and the ethene/ethane ratio?62 35 30 25 5 20- ..$ 15- 10 Genera 1 Discussion - - - - 5 - 0 080 I L 0 0 Fig. 8. Correlation between C2H4/C2H, ratio and CH4 conversion, data taken from table 1 . Dr G. J. Hutchings made the next comment regarding Prof. Thomas' paper: The catalytic performance of LiCa2Bi3O4CI6 for the methane-coupling reaction is indeed most promising. This compound contains the X3 structural unit and possesses the highest activity for the production of C2 hydrocarbons of all the oxyhalide structures investigated. It is interesting to compare the activity of this catalyst with that of catalysts previously researched. Based on a surface area of 1 m' g-' and using the data in table 1 of the paper it is possible to calculate a specific activity for LiCa2Bi304C16 of 7.8 x mol C2 m-2 h-' at 700 "C and 26 x mol C2 m-2 h-' at 720 "C, for a CH4 feed-rate of 132 x mol m-2 h-'.This is comparable to the specific activity of 15% Na/CaO' which is 10.4 x mol m' h-'. On this basis it is possible to place LiCa2Bi304C16 into the activity series that has been calculated for methane-coupling catalysts' Li/Sm203 > LiCa2Bi304C16 = Na/CaO > K/CaO > Sm2O3 = Li/CaO > Li/MgO > MgO. It is therefore apparent that this compound is a particularly active catalyst, and further modifications could improve its specific activity. The extremely high ethene/ethane ratio observed with the oxyhalide catalysts is also most interesting. A number of reactions for ethene formation from ethane are possible: (i) non oxidative dehydrogenation, (ii) oxidative dehydrogenation, and (iii) steam cracking.On inspection of the data in table 1 of the paper it would appear that there exists a correlation between the C2H4/C2H6 ratio, observed for the oxyhalide catalysts, and either the methane or oxygen conversion (fig. 8). Since the concentration of water increases with increasing conversion, then it is possible that the high ethene/ethane ratios could be due to homogeneous gas-phase ethane steam cracking to ethene. Van Kasteren et al.3 have commented that the post-reactor heated volume is a critical parameter controlling the ethene/ethane ratio. It is possible to comment on the possibil- ity of such secondary reactions occurring. 1 C. H. Lin, T. Ho, J-X. Wang and J . H. Lunsford, J. Am. Cbem. Soc., 1987, 109, 4808. 2 G. J.Hutchings, M. S. Scurrell and J. R. Woodhouse, Chern. Soc. Rev., 1989, 18, 251. 3 H. M. N. van Kasteren, J. W. M . Geerts and K. van der Wiele, Proc. 9th Int. Congr. Catal., ed. M. J . Phillips and M . Ternan (Chem. Inst. Canada, 1988), 2, 930.General Discussion 63 Prof. J. B. Moffat had three questions for Prof. Thomas: Prof. Benson has shown that the process in which a chlorine atom extracts a hydrogen atom from methane Cl+CH4 + HCl+CH, is a fast reaction and produces the expected HCl. You have carefully noted that it is not clear whether or not chlorine is removed from the catalyst during the process. Do you have any additional information in this regard concerning the deactivation of the catalyst? You have also commented on the possibility that less labile chlorine atoms are preferred.Do you have any further information on this topic? Have you tested for chlorine and/ or chlorine-containing species in the product? Prof. J. H. Lunsford had similar questions: From a technological standpoint the production of ethene, rather than ethane, is an important consideration in the oxidative dimerization of methane. The two catalysts, NaCa2Bi3O4Cl6 and LiCa2Bi,0,C1, are therefore most interesting because of the very large C,H,/C,H, ratios (table 1 in paper) that were obtained. Otsuka et al.' have previously observed large C2H4/C2H6 ratios over LiCl/ Ni-oxide and LiCl/ Mn-oxide catalysts; however, the C2H4 selectivity decreased sharply after 2 h on stream. These authors suggested that chlorine atoms in the catalysts diminished with time by evaporation, decomposition and reaction with the reactants. Do you have any evidence that chlorine atoms play a role in your catalytic system to give the unusually large C2H4/C2H6 ratios? For example, to what extent is chlorine lost from these catalysts and is there a concomitant formation of chlorinated hydrocarbons? As the activity of catalyst 2, table 1, decreased and the selectivity increased, did the C2H4/C2H6 ratio change significantly? 1 K.Otsuka, Q. Liu, M. Hatano and A. Morikawa, Chem. Lert. 1986, 903. Prof. Thomas replied to all these questions: Drs Burch, Hutchings, Moffat and Lunsford all raise important issues concerning our bismuth oxyhalide catalysts. The key points are worth repeating, namely that the activity of these monophasic solids is about as high as that of the best catalyst so far reported for the oxidative coupling of methane. And the ethene to ethane ratio is very high. We have recognized from the outset' that the role of chlorine atoms could be critical in the good performance of our catalysts. Indeed it was a deliberate act on our part to insert both C1 and Bi (each being well known hydrogen extractors) into the designed solid catalyst with the explicit intention of engendering conditions conducive to the production of CH, and other radicals, which would then produce desirable products either homogeneously in the gas phase or heterogenously through the agency of the surface of the catalyst. The possibility that homogeneous, gas-phase steam cracking of ethane to ethene also occurs must certainly not be discounted. Dr Hutchings is right to draw attention to the critical role of the post-reactor heated volume, the point first highlighted by van Kasteren et al. We do know, and this has been emphasized already,"2 that halogen tends to be lost from catalysts such as LiCa,Bi304C1, and BiOCl under the operating conditions described in our paper. The degree of loss varies considerably depending upon the structure of the catalyst. Whilst we have not made a systematic study of the relationship between chlorine loss and catalyst performance, we do know that some oxhyalides lose their halogen much less readily than others during use. In work carried out since our paper was written, we have prepared, and studied the catalytic performance, of monophasic solid solutions of BiOCl and LaOCl as well as BiOCl and SmOCl. The results, which will be described el~ewhere,~ are encouraging so far as the stability of these solid solution64 General Discussion catalysts is concerned. But the ethlene/ethane ratios fall as a result of replacement of Bi by either La or Sm. More work using other techniques is clearly called for. 1 W. Ueda and J. M. Thomas, Proc. 9th Congr. Card., Calgary, 1988, 2, 960. 2 W. Ueda and J. M. Thomas, J. Chem. SOC., Chem. Commun., 1987, 19. 3 J. Williams, J. M. Thomas, J. Kent and R. H. Jones, in preparation.

ISSN:0301-7249    

DOI:10.1039/DC9898700047

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1989, 87, 65-77 Structure and Reactivity of Transition-metal Oxide Monolayers Geoffrey C. Bond,* Saad Flamerz and Rashid Shukri Department of Chemistry, Brunel University, Uxbridge UB8 3PH Transition-metal oxides supported on and chemically interacting with the surface of Si02, A1203, TiOz etc. form discrete monolayers of species unlike those found at the surface of the unsupported oxides, being in effect two- dimensional compounds. Areas of their application include alkene meta- thesis and polymerisation, selective reduction of NO by NH3, and selective oxidation of aromatics. They are readily detected by Raman spectroscopy, FTIR and EXAFS/XANES, and thermal methods and XPS assist their characterisation. They can exhibit strong acidic character, mainly of the Brfinsted type; whereas isopropyl alcohol decomposition on Mo03/Ti02 gives chiefly dehydration, the main product over V205/Ti02 is acetone. The unique ability of the latter system to catalyse the selective oxidation of o-xylene to phthalic anhydride is attributed to the ability of the surface species to undergo oxidative addition at V=O groups and to effect the elimination of H atoms in adsorbed radicals as water.When the amount of supported oxide exceeds the monolayer capacity, two further forms of the oxide are detectable, both differing in their reactivity from that of the bulk oxide. The Concept of Oxide Monolayers From the earliest days of the use of metals as catalysts, advantage was taken of their ability to be prepared in a highly dispersed form.The realisation that very small metal particles are, however, thermally unstable unless they are distributed over the surface of a support led to the development of a variety of procedures designed to achieve this end. Interest then centred on whether progressive reduction in particle size might ultimately lead to a diminution of specific catalytic activity, either through loss of metallic character or excessive interaction with the support. It is, however, now felt that although there are well established cases of particle-size effects and of reactions that are deemed structure-sensitive, these in the main are second-order effects which cannot disguise the inherent differences in the catalytic properties shown by various metals. By contrast the notion that there might be some benefit in using oxides in a dispersed form has been slower t o develop.This is because it is easily possible to prepare many single or more complex oxides in high-surface-area forms, either by providing them with microporous structures or by making them as small non-porous particles, both of which may show the desired thermal stability. Moreover the economic motivation to maximise the fraction of catalytically active species at the surface, so important with the noble metals, is usually absent in the context of oxides. Indeed high surface areas, especially if arising from microporosity, are positively inimical to obtaining high selec- tivities in hydrocarbon oxidations. However, over the years a number of observations have been made, suggesting that there are indeed great advantages to be gained by supporting a catalytically active oxide on the surface of another, and some of these combinations are now employed in industrial processes on a very large scale.Whatever the motivation or rationale behind the original work on these systems, it has now become clear that in certain circumstances a supported oxide may show properties entirely different in kind from those of the unsupported oxide; these variations cannot be attributed simply to an increase in the available surface area. Over the past 6566 Oxide Monolayers decade in particular there has been a growing appreciation of the fact that supported oxides frequently exhibit structures quite unlike those found at the surface of the unsupported oxide, and that these are due to the creation of a single monolayer of the active oxide on the support.’ Such monolayers are readily formed by oxo-compounds of transition elements in Groups IV to VII on supports such as SiOz, A1203 and TiO, etc.Most of this paper will be concerned with such systems. Before starting to enquire further into methods for creating oxide monolayers and the structures they contain, it is of interest to note that the various systems which can be classified as supported oxides have been developed independently, and in some cases simultaneously, for the purpose of achieving quite different practical objectives. These include (i) SO, oxidation (e.g. K20-Vz0,/Si02), (ii) selective oxidation and ammoxida- tion of aromatics (e.g. VS05/Ti0,), (iii) selective reduction of NO by NH3 (e.g.V20,/Ti02), (iv) alkene metathesis and isomerisation (e.g. Re/AI,O,, Mo/A1,03 etc.), (v) alkene polymerisation (e.g. the Phillips Cr/Si02 catalyst) and (vi) hydrodesulphurisa- tion and related processes (e.g. promoted Mo03/A1203 and W03/A1203), where the oxide form is the precursor to the sulphides formed in situ. In the fourth and fifth cases, the active species are most probably coordinatively unsaturated ions in a lower oxidation state, or even zero-valent metal atoms, the support serving to stabilise these in a way that the unsupported oxide cannot. In the last case, although the form of the supported oxide precursor, and the manner of its interaction with the promoter, are still subjects for debate, the support appears to play a crucial role in facilitating the formation of the active structures.It is a curious feature of catalysis research that concepts and techniques developed for one application are not always easily or willingly transferred to another. This follows because most industrial and academic workers confine their attentions to quite narrow fields and fail to realise the relevance of what is being done elsewhere. It is not, however, the purpose of this paper to attempt a comprehensive review of supported oxides, instructive as this might be. We will, however, attempt a few comparisons between different systems, in an attempt to identify the chemical factors at work. There are other areas in which the idea of a supported oxide is relevant. In the preparation of supported-metal catalysts, the metal salt (nitrate, chloride etc.) is frequently calcined before the reduction step is performed.Calcination may perform a number of useful functions, but its success depends ultimately on the formation of a well dispersed oxide phase, capable on reduction of giving small metal crystallites. Relatively little curiosity has been shown concerning the structure of such oxide phases, except where they interact excessively with the support (e.g. NiO and other base-metal oxides with SO,, AIz03 and MgO). Intervention of an oxide stage in the preparation may sometimes assist the stability of the final metal particles through allowing a ‘chemical glue’ of incompletely reduced oxide to be formed at the metal-support interface. Much attention has also been paid to the role of oxide promoters for supported metals; insofar as these oxides are not fully reduced under working conditions, the concepts of supported oxides and oxide monolayers may help to understand how they perform their function.Preparation and Characterisation of Oxide Monolayers In the various examples of supported oxides mentioned above, it is probable that in no instance does the active oxide, or do the ions of the active element, form a complete and coherent layer, and indeed it may not be necessary or even desirable that it should. It is difficult to conceive of the possibility that the entire surface of a microporous support could be coated with another oxide, without at least leaving some of the finest pores untouched or without some pore-blocking.There is one very clear instance of where good catalytic performance seems to depend critically on the ability to create a full monolayer of the active phase. It has been knownG. C. Bond, S. Flamerz and R. Shukri 67 for some time' that V,O, supported on TiO, is a much more effective catalyst for oxidation of o-xylene to phthalic anhydride than is unsupported V205 or say V205/Si02. This unique property of V2O5/TiO2 has resulted in much speculation, but it now seems clear that the active component comprises a single monolayer of an 0x0-vanadium designated as VO,/TiO, to stress the fact that the monolayer species do not in the least resemble those found at the surface of V z 0 5 . Moreover any uncovered patches of TiOz surface are probably responsible for non-selective ~ x i d a t i o n .~ The weight concentration of VO, (expressed as V20s) necessary to form a monolayer6 is only ca. 0.09% mP2; industrial catalysts typically contain ca. six times this loading, but the excess probably serves only as a source of VO, to maintain the monolayer in as complete a state as possible. Part of the success of TiOz (anatase) as a support is therefore probably due to the fact that it can be obtained with an acceptable surface area (ca. 10 m' g-I) as non-microporous particles of only a few tens of nanometers in size. Higher surface areas are detrimental to obtaining high selectivities in selective oxidations. Much of our own work has therefore been performed with pigmentary anatase of this kind, or alternatively with Degussa P-25 TiO:, (ca.50 m2 g-I). The traditional method of fabricating V,05/Ti02 catalysts (and indeed many other supported oxides) is a simple impregnation of the support with an aqueous solution of a suitable omp pound,^ or its adsorption onto the support from solution.8 Because NH4V03 is only poorly soluble in water, oxalic acid is often added, this giving on heating a solution of the vanadyl oxalato-complex ( NHJ, [V0(C2O4),]. After drying and calcination at ca. 450 "C, and providing the amount of complex used is controlled, a single monolayer can result; but equally it is possible to produce any desired loading of V205, either more or less than the monolayer equivalent. More reliable methods of forming monolayers are however based on 'grafting' techniques, in which a vanadium compound reacts either in the gas p h a ~ e ~ .~ or from ~olution"'~ with surface hydroxyl groups in a stoichiometric reaction to form the monolayer species, depicted as, for example VOX3+2Ti-OH - VOX(OTi),+2HX. No more than a monolayer is laid down even if an excess of the reagent is employed, for reaction ceases when all the hydroxyl groups have r e a ~ t e d . ~ Suitable compounds include VOCl,, VO(OR)3 where R is iso-C,H,, and VO(acac), .6*10 Whereas, when using the oxalato-complex, calcination is necessary to decompose it and to form the monolayer, its essential structure is present from the start when grafting methods are used: calcination has been shown not to produce any significant change." One other general technique for making oxide monolayers deserves mention.A simple mechanical mixture of the oxides when heated leads to the spreading of the active oxide over the supporting oxide.','' This method has been used successfully with Moo3, W03 and V205 on a number of oxides; in the case of Moo3, The presence of water assists the process through the formation of a volatile oxyhydroxide. A wide range of characterisation methods has been used to identify the monolayer structure and the additional phases resulting when more than a monolayer of the active species is present. Among the most informative are temperature-programmed reduction (t.p.r.),6~13-" infrared and laser Raman spectroscopy ( i.r., FTI R, LRS),6*13.1Z-20 X-ray photo e 1 e c t r o n spectroscopy ( X P S ) , 6*2 ,' ' E X A F S , () i on - s ca t t e r i n g s p e c t ro s c o p y ( I S S ) , ' X-ray diffraction (XRD) 17.19323 and u.v.-visible spectroscopy in the diff use-reflectance mode. While it might be imagined that identification of species present in a single monolayer would tax the capacity of some of these techniques, they do in fact yield much useful information of a generally concordant nature.Although many of the finer points of structural characterisation remain to be resolved, there is quite sufficient known for oxide monolayers to be regarded as 'well characterised surfaces'. The use of oxide monolayers in fundamental studies of catalysed reactions has several attractive features. One is using in effect a two-dimensional compound that68 Oxide Monolayers should be homogeneous in structure; one therefore avoids complications due to different activities of the various crystal planes exposed in an unsupported oxide.Structures may change under reaction conditions through redox or hydration/dehydration processes, but these changes are confined to the monolayer, and migration of oxide ions to and from the sub-surface region does not occur. Oxide monolayers also appear to be quite thermally stable, although in the VOy/Ti02 system the dissolution of V4+ ions into the support is detected above ca. 460°C.24*75 Ease of formation and thermal stability of monolayers has been related to the ratio of the charge on the support cation to the sum of the cation and oxide ion radii.26 For Si02 this ratio is large and so surface compmnd formation is not encouraged; for A1203 it is much lower, thus accounting for the ready formation of mixed oxides [e.g.AI(Mo0,)J at high temperature. Ti02 and ZrO, have intermediate values, and these would therefore be the supports most likely to give stable oxide monolayers. Structures of Supported Oxides Discussion of the structures found in supported oxides is subdivided into ( i ) structures observed when the loading of the active phase does not exceed the monolayer capacity, and (ii) structures seen at higher loadings. As noted above, with pigmentary anatase of ca. 10 m2 g-' surface area, the monolayer limit corresponds to ca. 0.9 wt '/o V20s; with Degussa P-25 this figure rises to ca. 4'/0.~ These values are somewhat less than the 'theoretical' monolayer capacities based on the density of V atoms in a single lamella of V205.With supports of higher surface area, or with active oxides of greater molecular mass, the theoretical monolayer limits corespond to eve! higher loadings. Thus, for example, a theoretical monolayer of SnO, on Si02 of 200 m' g-' corresponds to a loading of ca. 47 wt '/o SnO:,. We have always found it helpful with the systems we have studied to prepare by aqueous impregnation a series of samples having a range of loadings of the active phase, with a view to inferring the formation of the monolayer, and the point at which other structures intervene, by some change in the properties of the materials. Thus with V205/Ti02 the t.p.r. results6 show very clearly that up to the monolayer loading there is only a single peak, corresponding to Vv-* V'II, whose T,,, is at ca.450°C, while the use of loadings above 4% gives rise to an additional peak at higher temperature (fig. 1). The former attains a limiting size, but the latter continues to increase as the loading is raised (fig. 2); it is therefore straightforward to attribute the former to the monolayer species and the latter to some other phase. Chemical analysis of samples made by grafting give confirmatory evidence of the monolayer capacity, and their t.p.r. indicates the validity of assigning the low-temperature peak to the monolayer species Laser Raman and FTIR spectroscopy also serve to distinguish clearly between monolayer species and other forms. The characteristic sharp Raman peak at 995 cm-' due to the V=O...V vibration appears only when more than a monolayer is present (fig.3), and the appearance of bands at 820 and 996 cm-' characteristic of the MOO, lattice also indicate the end of the monolayer phase." Bands observed within the monolayer region suggest the presence of tetrahedral vanadate species on A1203 and TiO2;I3 these are isolated at low loading, and on the basis of EXAFS/XANES analysis'0*28 have been formulated as structure A (fig. 4). However, the monolayer species are capable of reacting further with a source of vanadyl and also show marked acidic properties, strongly suggesting that -OH groups are present (see below). There is considerable evidence to show that monolayer species react readily and reversibly with water,'9*'0329.'0 so that under hydrous conditions the VO, species may be properly formulated as B or C (fig.4), whereas on heating there may be dehydration to A or to an octahedral polyvanadate species3' such as D. With MOO, and WO., there is evidence with both SO2 and Ti02 supports for both tetrahedral (fig. l ) . 6G. C. Bond, S. Flamerz and R. Shukri 69 300 600 900 T /'C Fig. 1. Reduction of V0,/Ti02 (low-area washed anatase); ( a ) 0.9% V20s and ( c ) 8.8% V205 prepared by aqueous impregnation with ammonium vanadyl oxalate solution; ( b ) 0.8% V205 , prepared by grafting method using VOCI, .6 M h a b rn v 20 10 0 2 4 Fig. 2. Dependence of Hz uptake on VzOs content: circles first t.p.r. peak; squares, second t.p.r. peak; triangles, total.6 The full line corresponds to the theoretical uptake to reduction of Vv to V"'.species (e.g. E) and octahedral polymeric species (e.g. F),'* although spectroscopic appear to argue against occurrence of an O=Mo=O grouping, and to favour species such as G (fig. 4). The ratio of Mo:Ti in the monolayer is only ca. one-half, whereas with V:Ti it is ca. unity; a model has been suggested to account for this differen~e.~' Many authors are curiously reluctant to try to depict the structures of which they speak, and even more reluctant to show how they might fit onto the support70 Oxide Monolayers 0 v,os (wt O/O) Fig. 3. Variation in intensities of 995 cm-’ band (V=O...V) and of 640 cm-’ band (anatase) with V,O, content for catalysts prepared by aqueous impregnation of washed low-area anatase with ammonium vanadyl oxalate solutions.6 A 8 C D 0 0 OH I ,OH-.I1 OH-. 0 0 o’b‘o E F G Fig. 4. Representation of possible structures in monolayers of VO, and MOO,. lattice. One notable exception to this stricture is represented with the work of Courtine and c o - w ~ r k e r s , ~ ~ but unfortunately his model of coherent V20s lamellae in register with the anatase surface lacks a physical basis. We turn now to consider the supramonolayer region. In the V2OS/TiO2 system, as the V20s content is raised, the appearance of the 996 cm-’ Raman band precedes that of the second peak in the t.p.r. spectrum.‘ This has been interpreted as evidence, in the 1-5% V20s range for a support of ca. 10m2g-’, for the formation of a ‘disordered’ VzOs phase in which V=O---V links exist but which has the same reducibility as the monolayer species.Above 4% V,Os, ‘paracrystalline’ V2OS occurs, this phase being a needle-like growth of V2OS, growing away from the surface, but distinctly more easily reduced than normal VIOs. Examination of Mo0,/Ti02 prepared by aqueous impregna- tion provides some evidence for a ‘disordered’ or amorphous MOO, phase succeedingG. C. Bond, S. Flarnerz and R. Shukri 71 0. 8 0.6 0 - 4 0.2 0 0 5 15 20 v,05 (wt O/O) Fig. 5. Dependence of I " / & , ratio on V205 content for catalysts prepared by grafting V0Cl3 (@) and VO(Bu'O), (0) onto Degussa P-25 Ti02.?' The curve is calculated for a 12.5% coverage of the monolayer by 'towers', and the arrow indicates the one-monolayer point. the monolayer species;17 the intensity of a Raman band at 982cm-I passes through a maximum as the MOO, content is raised, and has been attributed to a metastable phase.27 MoOJTiO, is reduced in two distinct steps (MeV'--+ Mo'"- Moo, see fig.6), the paracrystalline form being the less easily reduced; a structure for Mo1"0,/Ti02 has been ~uggested.'~ The appearance of four peaks in t.p.r. of low-loading samples suggests that tetrahedral and octahedral species are not equally reactive. Reduction of Ti0,- supported WO, occurs in one stage to Wo (fig. 5), but here the paracrystalline form is more easily reduced than the monolayer phase.27 The manner in which the ratio of the intensities of XPS peaks due to the supported oxide ions and those of the support (e.g. Iv/ITi etc.) varies with the loading of the active oxide gives a clear indication that second and subsequent coherent monolayers3' are not formed,16 as the ratio typically attains a limiting value when the loading exceeds ca. two monolayer equivalents (fig.6 ) . This has been interpreted quantitatively" as being caused by the occurrence of 'towers' of the 'disordered' and 'paracrystalline' V 2 0 5 , formed on the monolayer but covering only a comparatively small fraction of it (fig. 7). These 'towers' ultimately become the needle-like growths of V20s mentioned above; they are observed by electron microscopy. Reactivity of Oxide Monolayers Two particular facets of the properties of oxide monolayers are relevant to their catalytic behaviour: (i) their ease of reduction and (ii) their acidic character. Reference has already been made to the usefulness of t.p.r.in distinguishing between monolayer species and other phases formed at higher loading. The ease of reduction of the monolayer species should reflect their activity in oxidation reactions, and is conveniently measured by the temperature at which the reduction rate is a maximum, T,,,. Its value depends somewhat on the experimental conditions used" ( H2 concentration, flow rate, sample72 Oxide Monolayers l " 1 " I " 300 600 900 lz00 T / "C Fig. 6. ( a ) Reduction of 1.3% MoO,/TiO-, (unwashed low-area anatase) prepared by aqueous impregnation using ammonium heptamolybdate solution. ( b ) Reduction of 1.2% WO, /Ti02 (washed low-area anatase) prepared by aqueous impregnation using ammonium metatungstate solution. Ti% Fig.7. Representation of structures formed in V,OS/TiO, catalysts: ( a ) VO, monolayer; ( b ) 'disordered' V,05 ; ( c ) 'paracrystalline' V20,. size, heating rate etc.); factors concerned with the sample itself that may affect the value of T',, include (i) the chemical identity of the monolayer species, (ii) the precursor from which it is formed, (iii) its concentration on the surface and (iv) the kind of support used. In table 1 we have brought together values of T,,, for a number of systems, based on our own work or on published sources. Since T';,, seems to vary with concentration of the monolayer species,6 as well as with experimental conditions, too much stress should not be laid on a precise comparison of the values cited in this table, but certain trends do stand out clearly: (i) monolayer species are most easily reduced on TiOz andG.C. Bond, S. Flamerz and R. Shukri 73 Table 1. Reducibility of supported transition-metal oxides supported oxide support wt '/o T,.,,/"C ref. A1203 A1203 Si02 SiOz TiO, ' I TiO, TiO, " 3-0, A1203 SiO, Ti02 " TiO, " Zr02 A1203 6.1 3.1 5.8 0.5 3.4 4.0 1 .o 4.1 18.1 2.9 16.1 1 .o 2.0 4.6 2.75 2.3 3.0 4.9 507 47 0 532 517 442 43 6 480 450 450,873 460,840 606,736 520,780 490,760 490,750 ca. 400 282,317 247 202 14 this work 14 this work 14 6 6 this work this work 15 this work 27 this work this work 37 37 37 this work Values of T,,,%,, reported above were obtained using ca. 5% H2/N2 or H,/Ar, and heating rates of ca. 5 K min-'. Degussa P-25 TiO?. Low- area anatase. 25-0,; (ii) for VO, species, reducibility decreases as TiO, = ZrO, > A1203 > Si02 > MgO but for ReO, and MOO, ths sequence of A1203 and S O 2 is reversed; (iii) the first stage of the reduction of MOO, takes place at a slightly higher temperature than does the reduction of VO,; (iv) ReO, is much more easily reduced than VO, or MOO,, provided it does not interact too strongly with the support." Whatever the chemical factors at work, the choice of Ti02 as a support for VO, for oxidation reactions must be at least partly based on the greater ease of reduction shown by this system.A number of studies have been reported recently, bearing on the acidic character of supported transition-metal oxides. 1831')~293'0375~38339 Many such systems show good activity for hydr~cracking,~" but the well established techniques of NH, desorption and spectral observation of adsorbed pyridine provide further detail.1929*70 Lewis acidity associated with coordinatively unsaturated sites is sometimes d e t e ~ t e d , ~ ' but Bronsted-type acidity is more frequently seen.'x33s First-principles quantum-mechanical calculations" show that the latter depends on the number of doubly bound 0 atoms and is enhanced by delocalisation of charge into the support. We have examined isopropyl alcohol decomposition on V0,/Ti024' and MoO,/TiO, systems, the latter being prepared by aqueous impregnation using solutions of either ( NH4)6M07024.4H20 or H2[ MoO7(C20,)].H20, followed by drying and calcination. Both low-area anatase and Degussa P-25 TiO, have been used as supports; these have only low activity at the temperatures used (ca.220 "C for VO,/TiO,; ca. 180 "C for MoO,/TiO,) and tend to give propene as the chief product. For VO,/TiO,, rates expressed per g V205 are much higher for low loadings of V,O, than for the unsupported oxide, and decrease with increasing loading (fig. 8), suggesting that the activity of the monolayer species is superior to that of the condensed phases formed when the monolayer capacity is exceeded. The selectivity to propene is much lower (30-40'/0 ) than for Ti0274 Oxide Monolayers u '0 2 4 6 8 1 0 0 v,o, (wt % ) Fig. 8. Isopropyl alcohol decomposition on V205/Ti02 catalysts: dependence of rate per g V 2 0 5 at 220°C, activation energy and selectivity to propene on V,Oi content.'' 20 16 h 8 v 12 C 0 m 0 .- $ 8 s 4 I00 80 60 g 40 20 v A > 4.4 .- .- c) % / I 1 I .1 I+ 0 0 I 2 3 4 100 MOO, (wt % ) Fig. 9. Isopropyl alcohol decomposition on MoO,/TiO, catalysts. Dependence of rate and selectivity to propene at 180 "C on MOO, content.G. C. Bond, S. Flamerz and R. Shukri 75 I I fi ,ci H-0 H k- 0 Fig. 10. ( a ) Mechanism for dehydrogenation of isopropyl alcohol. ( b ) Mechanism for the activation of the -CH, group in o-xylene. and is almost independent of V205 content, and activation energies are also notably lower.42 The situation with the MoO,/TiO, system is somewhat less straightforward, in that rates expressed per g of catalyst increase steadily with MOO, (at least up to 3 wt '10, see fig. 9), suggesting that the monolayer species are no more active than the phases that succeed them.This system is, however, significantly more active than is VO,/TiO?. Propene selectivities remain high (80-95% ) except in the sub-monolayer region, and activation energies decrease somewhat irregularly, as MOO, contents increase. Reaction Mechanisms While MoO,/TiO, catalysts are considerably more active than V0,/Ti02 catalysts for dehydration of isopropyl alcohol, the latter are better at dehydrogenation than dehydra- tion (fig. 8).@ While dehydration can be readily explained if the catalyst surface has Brgnsted-acid sites ( i.e. acidic hydroxyl groups), dehydrogenation is likely to involve the VO, species acting as an electron acceptor, with the consequent formation of an adsorbed isopropoxyl radical (fig. 10). The ability of V0,/Ti02 to catalyse both types of reaction is believed to offer the basis of an explanation for its high selectivity in o-xylene oxidation.The general outline of this reaction is well understood;3*33 o- tolualdehyde is a primary product, and this is successively oxidised to phthalide and phthalic anhydride, but the latter is also formed from the earliest stages of the reaction. A rake-like mechanism therefore accounts for these features. Recent in situ spectroscopic have provided direct evidence for the initial activation of a -CH, group, and a detailed but admittedly speculative account has been given" of the subsequent steps. This is based on structure C in fig. 4 as the operative form in the VO,/TiOz monolayer, and the opening step is represented as the oxidative addition of a -CH, group across a V=O bond, followed by loss of water (fig. 10).Insertion of an 0 atom into the V-C bond gives an adsorbed methoxy species, which then by dehydrogenation forms an adsorbed carboxy species (i.e. V-CO-C,H,CH,). The whole reaction can be explained in terms of addition steps involving V=OH groups and H-elimination steps in which the V-OH groups participate. I t thus appears that the unique ability of V0,/Ti02 to catalyse o-xylene oxidation to phthalic anhydride with high selectivity, a property not shared so far as is known by other supported oxides, is due to ( i ) the76 Oxide Monolayers comparative weakness of the V=O bond and ( i i ) a structure that permits the simultaneous existence and use of V=O and V-OH groups. Conclusion Reaction of oxo-salts of elements in Groups IV to VII with a supporting oxide (SiOz, A1203, TiOz etc.), followed where necessary by calcination, leads to the formation of a two-dimensional monolayer of 0x0 or hydroxyoxo species having structures and proper- ties distinctly different from those found at the surfaces of the unsupported oxides.They are responsible for selective reduction and oxidation catalysis, they show strong Brensted acidity and they are in effect new chemical compounds. They are in general more easily reducible than the unsupported oxide, the reducibility depending upon the support: ReO, is, for example, easily reduced to Re' on TiOz, but at low loading on A1203 is hard to r e d ~ c e . ~ ' Ease of formation and thermal stability also varies from one support to another; monolayer species are not for example readily formed on SiO?, 13318 although precursors of V, Mo and Ti in oxidation state +3 have been successfully References 1 F.Roozeboom, T. Fransen, P. Mars and P. J. Gellings, Z. Anorg. Allg. Chem., 1979, 449, 25. 2 M. S. Wainwright a n d N. R. Foster, Catal. Rev. Sci. Eng., 1979, 19, 211. 3 G. C. Bond a n d P. Konig, J. Catal., 1982, 77, 309. 4 G. C. Bond a n d K. Bruckman, Furudajs Discuss. Chem. Soc., 1981, 72, 235. 5 I . E. Wachs, R. Y. Saleh, S. S. Chan and C. C. Cherisch, Appl. Catal., 1985, 15, 339. 6 G. C. Bond, J. P. Zurita, S. Flamerz, P. J. Gellings, H. Bosch, J . G. van Ommen a n d B. J . Kip, Appl. 7 H. Knozinger, Mater. Sci. Forum, 1988, 25-26, 223. 8 K. Segawa, D. S. Kim, Y.Kurusu and I . E. Wachs, in Proc. 9th Int. Congr. Catal., ed. M. J . Phillips a n d M. Ternan (Chem. Inst. Canada, Ottawa, 19881, vol. 4, p. 1960. 9 T. Hattori, M. Matsuda, K. Suzuki, A. Miyamoto a n d Y. Murakami, in Proc. 9rh Inr. Congr. Catal., ed. M. J. Phillips and M. Ternan (Chem. Inst. Canada, Ottawa, 1988), vol. 4, p. 1640. 10 T. Tanaka, H. Yamashita, R. Tsuchitani, T. Funabiki a n d S. Yoshida, J. Chem. Soc., Furado?. Trans. I , 1988, 84, 2987. 11 G . C. Bond a n d S. Flamerz, Appl. Cutal., 1989, 46, 89. 12 J. Leyrer, R. Margraf, E. Taglauer and H. Knozinger, Sur$ Sci., 1988, 201, 603; J . Leyrer, M. 1. Zaki a n d H. Knozinger, J. Phys. Chem., 1986, 90, 4775. 13 F. Roozeboom, M. C. Mittelmeijer-Hazeleger, J. A. Moulijn, J . Medema, V. H. J . d e Beer and P.J. Gellings, J. Phys. Chem., 1980, 84, 2783. 14 J . Kijenski, A. Baiker, M. Glinski, P. Dollenmeier a n d A. Wokaun, J. Catal., 1986, 101, I . 15 H. C. Yao, J. Cutal., 1981, 70, 440. 16 R. B. Quincy, M. Houalla a n d D. M. Hercules, J. C'atal., 1987, 106, 85. 17 T. Machej, B. Doumain, B. Yasse a n d B. Delmon, J . Chem. Soc., Faraday Trans. I , 1988, 84, 3905. 18 G . Busca, Mater. Chem. Phys., 1988, 19, 157. 19 H. Miyata, T. Mukai, T. O n o a n d Y. Kubokawa, J. Chem. Soc., Farudaj, Trans. I , 1988, 84, 4137. 20 S. S. Chan, I . E. Wachs, L. L. Murrell, L. Wang a n d W. K. Hall, J. Phj9.s. Chem., 1984, 88, 5831. 21 Z-X. Lu, Z-D. Ling, H-J. Fan, F-H. Li, Q. Bao a n d S. Zhang, Appl. Phj1.s. A, 1988, 45, 159. 22 G. C. Bond, J . P. Zurita a n d S. Flamerz, Appl.Catal., 1986, 27, 353. 23 B. M. Reddy, Ind. J. Chem., Sect. A , 1988, 27, 101. 24 M. Galantowicz, M. Gqsior, B. Grzybowska and J. Sloczynski, Przem. Chem., 1983, 63, 87. 25 V. A. Nikolov, D. G. Klissurski a n d K. I . Hadjiivanov, in Catulj'st Deactiuation 1987, ed. B. Delmon 26 F. Roozeboom, T. Fransen, P. Mars and P. J. Gellings, Z. Anorg. Allg. Chem., 1979, 449, 25. 27 G. C. Bond, S. Flamerz a n d L. J. van Wijk, Catal. Today, 1987, I , 229. 28 R. Kozlowski, R. F. Pettifer a n d J . M. Thomas, J. Phys. Chem., 1983, 87, 5176. 29 H. Miyata, K. Fujii a n d T. Ono, J. Chem. Soc., Farads!, Trans. I , 1988, 84, 3121. 30 T. Kataoka and J. A. Dumesic, J. Catal., 1988, 112, 66. 31 G. Hausinger, H. Schmelz and H. Knozinger, Appl. Catal., 1988, 39, 267. 32 J. Bernholc, J . A. Horsley, L. L. Murrell, L. G. Sherman a n d S. Soled, J. Ph!vs. Chem., 1987, 81, 1526. 33 M. Cornac, A. Janin a n d J . C . Lavalley, Polyhedron, 1986, 5, 183. 34 A. Vejux a n d P. Courtine, J . Solid State Chem., 1978, 23, 93. Catul., 1986, 22, 361. a n d G. F. Froment (Elsevier, Amsterdam, 19871, p. 173.G. C. Bond, S. Flamerz and R. Shukri 77 35 M. Inomata, K. Mori, A. Miyamoto, T. Ui and Y. Murakami, J. Phys. Chem., 1983, 87, 754. 36 H. Bosch, B. J. Kip, J. G. van Ommen and P. J. Gellings, J. Chem. Soc., Faraday Trans. 1 , 1984,80,2479. 37 Do Trong On, Thesis (University of Paris 6, 1988). 38 K. Arata and H. Mino, in Proc. 9th Inr. Congr. Card., ed. M. J. Phillips and M. Ternan (Chem. Inst. 39 F. A. Ivanovskaya and D. Kh. Sembaev, Russ. J. Phys. Chdm., 1987, 61, 253. 40 L. L. Murrell, D. C. Grenoble, C. J. Kim and N. C. Dispenziere, J. C a r d , 1987, 107, 434. 41 H. M. Ismail, C. R. Theocharis, D. N. Waters, M. I. Zaki and R. B. Fahim, J. Chem. Soc., Faraday Trans. I , 1987, 83, 1601. 42 G. C. Bond and S. Flamerz, Appl. Card., 1987, 33, 219. 43 R. Y. Saleh and 1. E. Wachs, Appl. Card., 1987, 31, 87. 44 H. Miyata, T. Mukai, T. Ono, T. Ohno and F. Hatayama, J. Chem. Soc., Farads). Trans. 1, 1988,84,2465. 45 A. J. van Hengstum, J. Pranger, S. M. van Hengstum-Nijhuis, J. G. van Ommen and P. J . Gellings, 46 G. C. Bond, J. Cutaf., 1989, 116, 531. 47 E. Vogt, J. van Dillen, J. Geus, J. Biermann and F. Janssen, in Proc. 9th Inr. Congr. Caral., ed. M. J. Canada, Ottawa, 1988), vol. 4, p. 1727. J. Caral., 1987, 101, 323. Phillips and M. Ternan (Chem. Inst. Canada, Ottawa, 1988), vol. 4, p. 1976. Paper 8/04991B; Received 14rh December, 1988

ISSN:0301-7249    

DOI:10.1039/DC9898700065

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1989, 87, 79-90 Computational-chemical Assessments of well characterised Uniform Catalysts Anthony K. Cheetham,*T Julian D. Gale,? Andreas K. Nowak, Brian K. Peterson,? Stephen D. Pickett and John M. Thomas Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21, Albemarle Street, London W1 X 4BS The use of computer-simulation procedures to model the behaviour of zeolites and other well characterised, uniform catalysts is discussed. We describe the prediction of the location of solute molecules and the estimation of heats of adsorption by molecular mechanics (MM) procedures. I n addi- tion, the use of Monte Carlo (MC) and molecular dynamics (MD) techniques to study sorbates at high loadings is considered, together with the calculation of diffusion coefficients by MD.The assessment of the stability of known and hypothetical zeolites by lattice-simulation procedures is described and we explore the extension of the method to pillared clays. The possibility of modelling a catalytic reaction mechanism by quantum-mechanical pro- cedures is also examined. 1. Introduction There is a very large class of heterogeneous catalysts in which all, or almost all, of the atoms in the bulk of the solid participate directly, or are implicated indirectly, in the key catalytic processes of the overall reaction. Well known examples are zeolites, silico-aluminophosphates (SAPOs), as well as certain clays and pillared clays. In all these cases, the active sites are uniformly distributed in the bulk of the solid. Because of the microporosity of these structures, however, the active sites are only accessible to gaseous reactant species of the requisite size and shape. Uniform heterogeneous catalysts, primarily because their structures may be elucidated by the traditional tech- niques applicable to the study of bulk solids,'-5 are therefore very well characterised in comparison with heterogeneous catalysts consisting of supported or multiphasic solids6 In addition to being able to pinpoint the nature of the active sites in uniform heterogeneous catalysts, a wealth of experimental methods is also available for the determination of other key features of their catalytic performance.Thus the siting of reactants, products and poisons in actual or model zeolitic catalysts, as well as their binding energies and mobilities, emerge from the application of a wide variety of spectroscopic, diffraction-based and thermodynamic methods.Neutron and X-ray diffraction studies on powdered uniform catalysts are especially revealing, as are high- resolution, multinuclear solid-state n.m.r. methods, high-resolution electron microscopy, and 'inelastic' spectroscopies, be they Raman- or neutron-based. The class of uniform heterogeneous catalysts is so large that experimental techniques alone are unlikely to be able to cope with the almost endless number of variants (brought about, for example, by changes (subtle or marked) in Si/Al ratios or distributions, in framework compositions and structures, in the character and distribution of exchange- able cations etc.) of individual catalysts that are theoretically, or in practice, available for use. We turn under these circumstances to computational assessments, both as a t Also at Chemical Crystallography Laboratory, University of Oxford, 9 Parks Road, Oxford OX1 3PD.7980 Compu ta t ional-chemica 1 Assessment of Catalysts means of rationalising information already gained and of guiding our quest to retrieve that which is yet to be won. With the appropriate combination of simulations and quantum-mechanical calculations, insights into the mechanisms of heterogeneous cataly- sis that are elusive by direct, experimental methods may also be obtained. In this paper we focus on the following topics, all of which are amenable, to a greater or lesser degree, to computational assessments: (i) the preferred sites at which molecules (which might be reactants or products) are sorbed within a zeolite or clay catalyst, (ii) the energetics of sorption processes and the dynamics of diffusion, (iii) the stability and structure of existing and hypothetical (new) catalysts and (iv) catalytic reaction mechan- isms.Some recent reviews of related may be useful to the reader. 2. Location of Sorbed Molecules Perhaps the simplest objective that we can address is the prediction of the siting of an adsorbed molecule in a zeolite cavity at low temperatures. Under these circumstances, entropy effects can be ignored and we need only locate the global minimum for the interaction energy between the host and the guest. Several approximations are made: (i) the zeolite is assumed to be rigid and unperturbed by the presence of the solute molecule, (ii) the hydrocarbon is assumed to be rigid and present in very low concentra- tions, (iii) the interaction is described by a simple atom-atom potential, using, for example, the parameterisations (for different Si/Al ratios of the zeolite) of Kiselev' and (iv) the parameterisations are assumed to be transferable from one zeolite to another.We have used a potential of the form: +(tot)=C ( rA x-<+- c:qj) . ij Values of A and B were determined semiempirically' by fitting experimental data for the heat of adsorption of methane in zeolite-Y (one of the faujasite family) and the charges for the electrostatic term were obtained from molecular-orbital calculations on both the hydrocarbons and fragments of the zeolites.Whereas, at ambient temperatures, the molecules are likely to be distributed over several sites, their observed positions at low temperatures should provide a useful test of the computer simulations since they should correspond to the positions of +(min). There have been virtually no single-crystal diffraction studies of zeolite-guest complexes, but several recent studies by high-resolution powder neutron diffraction have been reported. These include the location of Xe in zeolite p,", CO in zeolite-A," benzene in zeolite-Y'* and pyridine in zeolite-L.13 In the last instance, the molecule occupies a single site in the main channel, at 4 K, coordinated to a potassium ion through the nitrogen of the pyridine ring.The molecule is thus able to form an acid-base complex with the cation whilst benefiting from a non-bonding interaction with the cavity wall. Evaluation of the global minimum predicts the location and orientation of the molecule within 0.2 A of the observed position, thus lending credence to the validity of both the experimental result and the simulation. The principal discrepancy is to be found in the N-K distance, which is estimated to be slightly longer than the observed value. This may stem from the neglect of any covalent contribution to the acid-base interaction. 3. The Energetics of Sorption Processes and the Dynamics of Diffusion 3.1. Calculation of Heats of Adsorption The evaluation of the heat of adsorption at, say, room temperature is a more complex problem since it requires us to evaluate eqn ( 1 ) for all orientations of the guest moleculeA.K . Cheetham et al. 81 Table 1. Internal energies of adsorption of hydrocarbons in zeolite-Y (kJrn01-I)’~ calcd exptl CH4 -13.3 -15.2 C2H6 -21.5 -23.3 C3H8 -30.1 -32.3 C4H 10 -35.2 -37.4 and at all positions in the cage. The following integrations, which describe the Boltzmann distribution of the molecule over the available energy levels, can then be performed: I , = \ exp [-$(tot)/RT] du (2) l2 = $(tot) exp [-$(tot)/RT] du. (3 1 V I, The internal energy of adsorption is then given by AU(ads) = $(tot) = ]?/Ii. (4) This treatment accounts for entropy effects that distribute the molecule over an increasing number of higher enthalpy sites as the temperature is raised.For example, though the minimum energy, 4(min), of methane in zeolite-Y is -23.0 kJ mol-’, the molecule would only be expected to occupy the position with this energy at very low temperatures. At room temperature, AU(ads) is only -13.3 kJ mol-I. In table 1 we compare the calculated and experimental adsorption energies for a series of hydrocarbons in zeolite-Y. As the molecules become larger, the energies increase. We emphasise that these values correspond to the energies of isolated molecules and are only valid at infinite dilution when intermolecular interactions can be ignored. Below, we discuss the extension of this treatment to cavities with multiple occupancy. Many of the interesting properties of zeolites are dependent upon cooperative effects between sorbate molecules.The internal energy of adsorption, for instance, will have components due both to the adsorbate/zeolite interactions and to the adsorbate/adsor- bate interactions. In such a case, eqn (2-4) must be evaluated over all configuration space and one must turn to more sophisticated approaches to determine the properties of interest. The techniques of Monte Carlo and molecular dynamics, as used in liquid- state theory, are also applicable here. MD also offers the opportunity to study dynamic processes such as diffusion, a topic that will be addressed in section 3.2. The first use of Monte Carlo simulations in the study of adsorption in zeolites appears to be that by Stroud et al? They studied methane in zeolite 5A and calculated thermodynamic properties such as the isosteric heat of adsorption and the heat capacity.They also calculated the adsorption isotherm, though by a tedious method using coupling parameters and simulations for systems with potentials other than the one of interest. Kretschmer and FiedlerI6 also performed some early Monte Carlo work, simulating alkanes in zeolites. However, their method was restricted to one molecule per cavity and therefore corresponds to the ideal-gas limit. They were particularly interested in the configurations of the sorbate molecules within the zeolite cavities. More recently, Yashonath et al.” found good agreement between MC simulations and experimental results for the heat of adsorption of methane at zero coverage in sodium zeolite-Y, and Smit and den OudenI8 have done zero-coverage MC studies of methane in the zeolites faujasite, mordenite and ZSM-5.Both studies used a potential of the type given in eqn ( 1 ) . A very interesting feature of the work of Smit and den82 Computational-chemical Assessment of Catalysts Ouden was that they varied the Si/AI ratio in mordenite and found a sharp change in the heat of adsorption at Si/Al =r 6.7. They were able to rationalise this result as the blocking of high-energy adsorption sites by sodium cations. This prediction has yet to be verified by experiment. Woods and Rowlinson’’ have recently performed a grand canonical Monte Carlo (GCMC) simulation (in which the chemical potential is held fixed, rather than the number of particles) for xenon and methane adsorbed in zeolites X and Y.They used crystallographically determined coordinates for the zeolites and compared adsorption isotherms and heats of adsorption with experimental data. These workers used fluid-fluid potential parameters available in the literature and fitted the fluid-zeolite potential parameters to zero-coverage data. Although they fitted the potentials at one temperature and used them at a higher temperature, they were able to reproduce all of the qualitative features of the heat of adsorption and adsorption isotherm throughout the range of zeolite cavity occupancies. The potential they used was a particularly simple one, including only the Lennard-Jones terms and a constant “background” correction term. Even with this simple potential, including no explicit polarization or electrostatic terms, many of the features of the experiments were reproduced semi-quantitatively. Soto and Myers?” had previously used GCMC to study hard-sphere and Lennard- Jones fluids in zeolite-13X.They were the first to use this method for zeolite adsorption and were able to demonstrate its usefulness. They did not achieve a level of agreement with experiment comparable with that obtained by Woods and Rowlinson, but instead chose to use simple potentials to study the qualitative effects of including various interactions. Among their conclusions was that the hard-sphere model (used in many earlier theories of adsorption) worked well for the heat of adsorption (because the energy is mostly determined by the fluid-zeolite contributions), but not for the adsorption isotherm (because the fluid-fluid interactions determine the chemical potential).3.2. Dynamics of Sorbate Molecules The MC methods mentioned above are very useful for probing the configuration space of a molecular system, but to obtain information about its time dependence MD must be used. M D has a long history in the study of bulk liquids and gases,” and the methods developed there should easily transfer to the study of adsorption in zeolites. In its simplest form it involves the solution of Newton’s equations of motion for N particles in a specified volume and with a specified total energy. It is very similar to MC except that the system evolves new configurations naturally in time. Quantities such as the diffusivity, which depend on the state of the system at more than one time, can also be calculated.The diffusivity is usually found from the Einstein diffusion equation, ( 5 ) where Ax is the displacement of a particle from its initial position and the brackets denote averages over numbers of particles or over separate experiments. I t will be especially enlightening to compare the values of D calculated from simulations with resu 1 t s from pu 1 s ed - fi e 1 d - gr a d i en t n . in. r . e x pe ri men t s . The only MD study to appear in the literature on diffusion in zeolites is that of Yashonath et al.” They studied methane in Na-Y and investigated the effect of tem- perature on the mobility of the sorbate. A complicated RMKZ3 potential was used to model the methane/methane interactions, and Lennard-Jones plus electrostatic terms to model the methane/zeolite interactions.Only one loading was studied ( 6 molecules per cage), but they studied several temperatures from 50 to 300 K. Yashonath et al. calculated cage- and site-residence times for the methane molecules at the various temperatures. As expected, the methane becomes much more mobile at higher temperatures, with a large drop in the residence times being obtained in the ( S.u2) = 6 DtA. K . Cheetham et al. 83 range 50-150 K. At all of the temperatures studied, the molecules remained close to the walls of the cages and hence the mode of transport is surface diffusion. The trajectory at 300 K was analysed with eqn (5) and D = 2.0 x lop8 m2 s-’ was found, which compares well with an experimental (n.m.r.) value of 1.5 x lo-’ m’ s-l.One feature of the Yashonath et al.’* work reminds us that simulation methods have their limits. At the lowest temperature studied (50 K), the cage residence time was of the same magnitude as the length of a simulation run (ca. 25 ps). When this is the case, the simulation run is not long enough to give a reliable estimate of the diffusivity or the residence time. This will be true whenever the time scales that govern the phenomena of interest are longer than the amount of time than can be afforded for a simulation. This is further emphasized by the fact that they found different results at 50 K, depending on whether the sample was heated from the minimum energy (OK) configuration or cooled from a higher-temperature configuration.If there are metastable states in the vicinity and the lifetime of these states is of the same order as the simulation time, the system can be trapped in them. This problem is particularly acute at low temperatures when mobilities are low. Some of the flexibility offered by the various MC methods can also be obtained by M D. Constant temperature, rather than constant energy, simulations can be performed via a variety of methods.” Also, the chemical potential can be determined (at least for systems with not too high a density) via the potential distribution theory of W i d ~ m . ’ ~ These conveniences, along with its dynamic nature, endow MD with the properties of a very useful tool. Future developments in this area will involve the extension of the treatment to flexible sorbates and non-rigid hosts, thus leading to the possibility of modelling shape-selectivity in microporous catalysts.4. The Stability and Structure of Catalysts 4.1. New Zeolitic Materials Computer-simulation techniques have not only been applied to the study of adsor- bate/adsorbent systems, but also to the investigation of adsorbent structures themselves. Catlow and co-workers have developed lattice simulation techniques that yield informa- tion on the stability of crystal Such techniques are essential in the search for new structures which could be of interest in catalysis, because they allow a comparison of relative stabilities of known and hypothetical structures. Several different approaches have been reported for predicting new structures, but we focus here on the method developed by Akporiaye and Price26 which was used in the investigation of possible zeolitic structures consisting of a mordenite/mazzite intergrowth.*’ The basic assumption is that zeolite structures can be represented as combinations of component layers.The layers are obtained from projections of observed zeolite structures, such as that of zeolite ECR-1A2’ shown in fig. 1, by allowing different topologies of the layer. A combination of these layers in three dimensions using mirror or translational symmetry operations results in a number of hypothetical structures. Fig.2 shows ten of the component layers with the ECR-1A projection, but different topologies. On the basis of the coordination sequence of the tetrahedral atoms we found layer C to result in the most appropriate structure for further investigation [shown in fig.3( a ) ] . The structure, named DF (Davy Faraday), can be described as an intergrowth of two separate structures, ( a ) strings of cancrinite cages showing 6-rings [fig. 3 ( 6 ) ] and ( 6 ) strings of connected 4-, 5-, and 6-rings [fig. 3( c ) ] . Lattice-simulation techniques were then applied to compare the stabilities of DF and ECR-1A at 0 K. The simulations combine two-body short- and long-range potentials and three-body terms in the evaluation of lattice energies and optimum atomic coordin- ates. This approach was applied successfully to various silicate systems. 29~30 Our results84 Compu ta tional-chem ica 1 Assessment of Catalysts MWd I Ma22 Mord I I I I 1 I I I Fig.1. Structure of ECR-1A projected along the a axis. Alternating component strips of mordenite and mazzite are indicated; large spheres denote oxygen atoms, small spheres are T atoms (Si and Al). I I Fig. 2. Ten possible structures with the ECR-1A projection, generated by the methodology of Akporiaye and Price.26 Arrowheads indicate T atoms lying above the plane of atoms with unmarked bonds.A. K . Cheetham et al. 85 Fig. 3. ( a ) Perspective view of the framework of DF, ( b ) the cancrinite cage and ( c ) the layer of 4-, 5- and 6-rings which is found in DF. predict a lower lattice energy for DF than for ECR-lA, the more stable variant of ECR-1, by ca. 0.16 eV per Si02 , thus suggesting the existence of the crystal structure on energetic grounds. Similar predictions of new silicate structures can be made from many different component layers based on more than 50 known zeolite structures.Their use in catalysis, ion-exchange and molecular sieving can also be predicted roughly by studying three- dimensional physical or computer-generated models, or by computer simulations of adsorption and diffusion. A residual difficulty, however, is that it is still not possible to design the synthesis of a new zeolite catalyst from basic principles. 4.2. Pillared Clays Following the successful applications of computer modelling to zeolite structures and their sorbate complexes, we have begun to explore the extension of such simulations to other aluminosilicate catalysts. The pillared clays represent an interesting and chal- lenging group of materials that in many respects are closely related to the zeolites.A typical clay is composed of negatively charged, magnesio-aluminosilicate layers, bound together by hydrated cationic species. Typical examples of naturally occuring clays are vermiculite: [( Mg2.36Fe0.48A10. 16)0ct(S~2.72~~ 1 .28)tet010(OH 121 -0'64[Mg0.32(H20) n 1 +Od4 and montmorillonite, with a somewhat lower layer charge: [ ( Mg0.33A1 1.67)oct( si4)teto 121 -0.33"a0.33(H20) n Several members of the clay family (notably montmorillonite, beidelite, hectorite, and vermiculite) exhibit powerful catalytic properties. Synthetic variants of these naturally occuring solids, such as fluorotetrasilicic mica, labelled FTSM, are also good 'clay' catalysts, and are readily converted to their pillared (more open, accessible) states by the insertion of Keggin ions (such as [Al,,O,(OH),,( H20)12]7+) into their interlamellar regions.So far as the uniform (acidic) catalytic properties of clays are concerned, the active centres are either the quasi-free protons generated by hydrolysis of such cations as A13+, introduced by cation exchange, or the weak, Brprnsted-acidic centres that are attached to the interlamellar macrocations of the Keggin type. A wide variety of organic reactions can be catalysed by modified or synthetic In several of the reactions86 Computational-chemical Assessment of Catalysts ? Fig. 4. The experimental location of the anilinium ion in the interlamellar region of vermiculite, showing hydrogen bonding inferred by the short N - 0 distances.catalysed by sheet silicates, it has been that layer charge, and the density of the charge, are of key importance. From the modelling point of view, several new challenges are presented by pillared clays. First, the pillars are charged and it is therefore essential that our description of the electrostatic interaction between pillar and layer is reliable. Secondly, unlike the zeolites, we can no longer assume that the host is unperturbed by the presence of the guest species; for example, we would certainly expect the interlamellar spacing to be altered. Thirdly, hydrogen bonding is likely to play a key role in the host-sorbate interactions. On the other hand, computer modelling has a great deal to offer in this area because there is frequently a dearth of reliable experimental data.For example, powder X-ray studies have been reported for a large number of organic intercalate^,^^ but only the 001 reflections have been obtained in most cases and, consequently, only one-dimensional projections of the structures are known. Additional structural informa- tion relies upon indirect methods such as m.a.s.n.m.r., infrared and Mossbauer spectros- copies. In the exceptional case of anilinium-pillared vermiculite, Slade and Stone36 have succeeded in obtaining single crystals from which a full, three-dimensional crystal structure, excluding hydrogen atoms, has been obtained by X-ray diffraction. Although later work3’ indicates the presence of a supercell, the salient features may be seen from the reduced unit cell.Anilinium ions are sandwiched between opposing silicate 6-rings, with the amine group hydrogen bonding to a triangle of oxygen atoms (fig.4). Two anilinium sites are observed, according to whether the molecule points up or down. We38 have simulated this system at the idealised 1 : 1 composition, using the programme THBREL, developed by Catlow and co-w~rkers’~ (see above). Two- and three-bodyA. K . Cheetham et al. 87 Table 2. Results of computer simulations of three structural models for the anilinium-vermiculite complex ~ model lattice energy/eV AE/kJ mol-' a / A b / A c/A PI" exptl" - - 5.330 9.268 14.892 97.02 1 -1184.621 0.0 5.412 9.373 14.97 1 96.86 2 -1184.181 42.4 5.414 9.385 14.923 97.40 - 1 184.360 25.2 5.414 9.384 18.358 1 1 1.24 3 The experimental data of Slade and Stone are shown for comparison. A E represents the difference in energy between the preferred model and the alternatives shown in fig.5. " Ref. (36). Fig. 5. Three possible configurations of the anilinium ion in vermiculite at the high-density limit, ( a ) model 1, ( b ) model 2, (c) model 3. potentials for the aluminosilicate framework are already well known,'9 while those derived from crystal structures of organic molecules4" have been used to complete the force field. The geometry of the anilinium ion was initially held rigid at that determined for anilinium hydrochloride, but the structure of the layers was allowed to relax.41 There has been much debate about the orientation of organic molecules in the interlamellar region of clays. For high-charge-density clays, such as vermiculite, the do,, spacing would be equally well satisfied by two molecules arranged perpendicular to the sheets with either an antiparallel (model 1 ) or parallel (model 2) orientation, or by the molecules lying flat and being stacked one upon the other (model 3, fig.5). The lattice energies and unit cell parameters for the energy-minimised structures correspond- ing to the three starting configurations are given in table 2, together with the experimental unit cell. The two models which contain the cation perpendicular to the layer yield unit cells that agree reasonably well with the experimental results, but the structure with the antiparallel arrangement of anilinium ions gives a markedly lower energy. In the minimisation starting from the third orientation, the anilinium ions partially reorient towards the most favourable packing arrangement before becoming trapped in a local minimum.In nearly all respects, our simulated structure (fig. 6) is in remarkable accord with the experimental findings of Slade and Stone. A further well documented property of clay intercalates is that as the layer charge is decreased, there is an increasing preference for aromatic cations or molecules to orientate themselves parallel to the layers. We have made a preliminary investigation of this phenomenon by reducing the layer charge to half the value used in the previous simulations and correspondingly lowering the number of anilinium ions per unit cell to one. Initial results indicate that this is reproduced by our model.88 Computational-chemical Assessment of Catalysts Fig.6. The calculated minimum-energy packing arrangement, viewed perpendicular to the silicate layers. Our preliminary work in this area has confirmed that computer simulation has the potential to be a powerful method in the structural characterisation of layered silicates and we are now planning to extend the work to a wide variety of pillared clays and other low-dimensional solids. 5. Catalytic Reaction Mechanisms By its very nature, it is difficult to study by direct experiment the critical act of catalytic conversion. It is one thing to derive, by spectroscopic or diffraction procedures, the nature of the bound reactant, but it is quite another to track, on the femtosecond timescale, the rupture and formation of bonds.(One notes, in passing, that only for simple photo-excited, gas-phase reactions has it proved possible, very recently,42 to do just this). In principle, however, a combination of ab initio quantum-mechanical calcula- tions and, say, MD simulations can cope with the rearrangements that are involved in a catalytic reaction within a zeolite, and the quantum-dynamics approach of Car and Parrinel10~~ appears to offer a strategy for future calculations of this type. In the following paragraph, we review the progress that has been achieved to date in this area. Vetrivel et aZ.44*45 have examined the preferred site of binding of methanol in a model ZSM-5 catalyst containing framework Al at the so-called T2 site (for which there is n.m.r. evidence46).After predicting the placement of the physisorbed molecule in the vicinity of the Bronsted-acid site by the energy-minimisation procedure, modified configurations were then explored by using ab initio SCF calculations on the resulting cluster (the GAMESS code developed by Guest and Kendrick4’ at the Cray X-MP at the Rutherford Appleton Laboratory was used for this task). The final configuration arrived at by Vetrivel et al.44 shows one of the methyl hydrogens of the methanol essentially dissociated and re-bound to a framework oxygen of the catalyst. For this dissociation, there appears to be essentially no activation energy. In essence, the mechanism arrived at in this quantum-mechanical fashion signifies that the methanol is activated at the Brgnsted-acid site to yield a CH20H species, leaving the site free for access by other reactant species.It is still not clear4’ precisely which intermediates are critically impli- cated in the catalytic conversion of methanol to gasoline (and especially in carbon-carbonA. K . Cheetham et al. 89 bond formation) so that this computationally derived mechanism cannot yet be tested. Experiments using 13C-enriched methanol, with cross-polarisation m.a.s.n.m.r., recently initiated by Anderson et ~ l . , ~ ~ may help to clarify the situation. 6. Conclusions The foregoing examples serve to illustrate that computer simulations have, in a relatively short time, contributed a great deal towards our understanding of well characterised catalysts, but it is important to stress the future opportunities that are now presenting themselves.We note that it has recently become feasible, on a routine basis, to obtain full, three-dimensional structures from new materials which, although eminently suitable for catalysis, cannot be obtained in single-crystal form. This has been achieved by powder diffraction methods, using, for example, a6 initio techniques, which have recently been applied both to zeolites’” and other inorganic materials. ”,” An alternative strategy was adopted for the zeolite catalyst, ZSM-23.5’ Its structure was solved by augmenting, computationally, the information derived from powder X-ray diffraction (which gave unit-cell dimensions and an indication of the space group), electron diffraction (which confirmed the space group), and from adsorption measurements (which yielded framework density).In effect, this procedure, which could be generalised, arrives at the atomic structure of a catalyst that is so microcrystalline that it could not be solved by a6 initio powder methods. It is significant, then, that armed with the appropriate structural details, computational methods are now capable of assessing the suitability of such new materials for catalysis. The thermodynamic and dynamical behaviour of reactant and product molecules can be interrogated, shape-selectivity can be examined, and we are on the verge of being able to treat the catalytic reactions themselves. A great deal remains to be done and problems have to be solved, especially in relation to force fields; for example, two recent simulations of the siting of p-xylene in silicalite yielded different results, apparently because different parameterisations of the (same) interatomic potential were u~ed.”~” Nevertheless, the successes to date speak for themselves, and the ever-increasing power of the computer augurs well for future progress in this important area.References 1. J . M. Thomas, Proc. 8th Intl. Congre. Catal., 1984, 1, 33 2 A. K. Cheetham, A. K. Nowak and P. W. Betteridge, Proc. Indian Acad. Sci. (Chem. S c i ) , 1986,96,411. 3 R. M. Barrer, in Zeolites and Clay Minerals (Academic Press, London, 1978). 4 W. J. Mortier and R. A. Schoonheydt, Progr. 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Kendrick, CAMESS User Manual, Introductory Guide (University of Manchester Computer Centre, 1986). 48 S. D. Hellring, K. D. Schmith and C. D. Chang, J. Chem. Soc., Chem. Commun., 1987, 1320; see also J. K. A. Clarke, R. Darcey, B. F. Hegarty, E. O’Donogue, V. Amir-Ebrahimi and J. J . Rooney, J. Chem. Soc., Chem. Commun., 1986, 425. 1, 133. New York, 19821, p. 5 5 . A. C. D. Newman (Mineralogical Soc. Monograph no. 6, J. Wiley, Chichester, 1987), p. 317. 49 M. W. Anderson et al., in preparation. 50 P. A. Wright, J. M. Thomas, G. F. Millward, S. Ramdas and S. A. I . Barri, J. Chem. Soc., Chem. Commun., 1985, 11 17. 51 A. K. Cheetham, W. I . F. David, M. M. Eddy, R. J. B. Jakeman, M W. Johnson and C. C. Torardi, Nature (London), 1986, 320, 46. 52 J. P. Attfield, A. W. Sleight and A. K. Cheetham, Nature (London), 1986, 322, 620. 53 L. B. McCusker, J. Appl. Cr?*staIlogr., 1988, 21, 305. 54 S. D. Pickett, A. K. Nowak, A. K. Cheetham and J . M. Thomas, Molecular Simulation, 1989, 2, 353. 55 P. T. Reischman, K. D. Schmitt and D. H. Olson, J. Phys. Chem., 1988, 92, 5165. Paper 8/05061 I; Received 22nd December, 1988

ISSN:0301-7249    

DOI:10.1039/DC9898700079

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

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

ISSN:0301-7249    

DOI:10.1039/DC9898700091

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1989, 87, 107-120 Copper-Zinc Oxide Catalysts Activity in Relation to Precursor Structure and Morphology David Waller, Diane Stirling and Frank S. Stone* School of Chemistry, University of Bath, Bath BA2 7AY Michael S. Spencer? ICI Chemicals & Polymers Ltd, Billingham Catalysis Centre, Clerieland TS23 1 L B Cu-Zn hydroxycarbonates have been studied as precursors of Cu-ZnO catalysts, with particular reference to the effect on catalyst activity of ageing the precursor prior to decomposition and reduction. The precursor obtained by precipitation from mixed nitrate solution (Cu/Zn molar ratio 2 : 1 ) at 333 K and pH 7.0 consisted of zincian malachite (Cu/Zn = 8.5: 1.5) and aurichalcite. The precursor was aged in the mother liquor at 333 K for various times.Characterisation by XRD, i.r., DTA, electron microscopy, EDAX and XPS showed that ageing led to loss of the aurichalcite and production of a more finely divided copper-enriched (Cu/Zn = 2 : 1) malachite phase. The unaged precursor yielded a catalyst of low activity for both methanol synthesis (studied at 50 bar and at 1 bar) and the reverse water-gas shift reaction. The aged precursor gave catalysts of much higher activity for both reactions. Increased ageing did not change the selectivity ratio for methanol synthesis cs. reverse shift in the CO,+ H, reaction at normal pressure. The success of the copper-zinc oxide/alumina catalyst for low-pressure (50- 100 bar) methanol synthesis has prompted a great deal of fundamental work on this reaction during the past ten years.The interest was largely triggered by an outstanding series of papers by Klier and co-workers, beginning in 1979.'-3 The combined effect has been to generate a dialogue unmatched for many years in catalysis. As amply documented in several authoritative controversial issues include both the mechanism of the reaction and the nature of the active sites on the catalyst. The principal active site for methanol synthesis over copper-zinc oxide is widely regarded as being a copper centre. There is controversy as to its environment and oxidation state variation during catalysis. Herman er al.' stimulated much interest with their proposal that isolated Cu' in solid solution in the surface of zinc oxide was the key element. However, the opinion of the ICI groupsX.' is that the active copper site is on a particle of copper metal.The activity of copper-zinc oxide is known to depend upon the catalyst precursor. This is the aspect on which the present work is focussed. The significance of characteris- ing the hydroxycarbonate precursor phases of Cu-ZnO catalysts was first highlighted by Herman et al.,' and particular attention has since been accorded to malachite and aurichalcite.'".'' Of the more detailed studies, the most recent are those of Porta et al., ' 2 3 ' 3 with which our own work has partly overlapped. Copper-zinc hydroxycarbonate precursors have been investigated at ten different Cu/Zn molar ratios from 100:O to 0: 100 and the phase analysis results, to be reported elsewhere," agree closely with those i- Present address: School of Chemistry and Applied Chemistry, Uni\ersitc of' U'ales College of C'drditf, Cardiff C F l 3TB.108 Copper- Zinc Oxide Catalysts of Porta et Our work has extended to measurements of the activity for the reverse water-gas shift reaction (CO? + H, -+ CO + H,O) for the Cu-ZnO catalysts derived from these precursors, and we have found that the highest activity is given by a 67:33, i.e.2 : 1, Cu/Zn ratio. On the basis of these results, we have selected the composition Cu/Zn = 2 : 1 for a detailed study of precursor ageing. Ageing of the precursor in the mother liquor after precipitation is often used as a stage in the preparation,” but this does not appear to have been studied systematically hitherto. The characterisation of the precursor can be made precise and the work follows through to the genesis of the catalyst and its activity in methanol synthesis under standard industrial conditions at 50 bar.Activity of the catalysts for the synthesis at 1 bar has also been studied, using in this case C02-H2 mixtures, and parallel measurements of reverse shift activity have been made for comparison. Experimental Catalyst Preparation Cu-Zn nitrate solution with [Cu + Zn] = 1.5 mol dm-’ and Cu/Zn = 2 : 1 was pumped simultaneously with 1.5 mol dm -’ Na2C03 solution into a mixing vessel under conditions such that precipitation occurred at 333 K and a constant pH 7.0, followed by a collector vessel in which the stirred suspension could be aged at 333 K for specified times (0, 30, 75, 140 and 205 min, respectively).The slurry was then filtered and washed with de-ionized water until the Na content of the solid was < 160 ppm. The material was dried for 16 h at 363 K. The solid so formed will be referred to as the precursor. The precursor was heated in air at 623 K for 6 h to form the calcined oxide. This was pelletized, crushed and sieved to give a 600-85Opm fraction. The resulting material, reduced in hydrogen at ca. 520 K, constitutes the reduced catal-yst. Catalyst Characterisation ( a ) X-Ray Powder Diflraction Measurements were made with a Siemens Kristalloflex D500 diffractometer using Fe- filtered Co radiation [ h ( K a ) = 1.7902 A] step-scanning at 0.05 O intervals of 28 with a 4 s count at each step. Some samples wer? studied with a Philips diffractometer using Ni-filtered Cu radiation [ h ( K a ) = 1.5418 A].( b ) Electron Microscopy, Electron Diflraction and EDAX JEOL CXlOO and FX200 transmission electron microscopes equipped with LINK EDAX systems were used. Precursor samples were dispersed in ethanol and deposited on a nitrocellulose film supported on the microscope grid. The samples were carbon-coated before examination in the microscope. Copper grids were used for TEM and selected- area electron diffraction. Nylon grids in a Be holder were used for the energy-dispersive X-ray analysis (EDAX). ( c ) I . R. Spectroscopy A Perkin-Elmer 983 spectrometer equipped with a 3500 data station was used. Precursor samples were wet-ground in acetone, and dried on to NaCl plates. ( d ) U. K- Visible- N. I . R.Reflectance Spectra These were obtained with a Perkin-Elmer 330 spectrometer and a reflectance attachment employing BaSO, as standard. Solids were examined as sieved powders.D. Waller, D. Stirling, E S. Stone and M. S. Spencer 109 ( e ) Diflerential Thermal Analysis A DuPont thermograph was used, the ground sample (10 mg) being heated at 10 K min- ’. ( f) Surface Area Determination B.E.T. surface areas ( N2, 77 K) were obtained from isotherms determined gravimetrically with a Cahn RG microbalance. Copper metal areas (reduced catalyst) were determined using N 2 0 decomposition by the following methods. (i) Pulse method: the sample, H,-reduced (520 K, 16 h), cooled to 333 K, flushed in pure He, was subjected to 0.5 cm3 pulses of N20 in He as carrier until no further decomposition was registered.Integrated areas of N2 g.c. peaks were summed. (ii) Frontal analysis: The procedure of Chinchen et al.” (reactive frontal chromatography) was followed, using 333 K as the reaction temperature with 5% N,O in He as the reacting gas. ( g ) X.P.S. A VG Scientific ESCALAB mark I1 spectrometer was used with A1 X-rays (1486.6 eV). Samples were examined as pressed discs having been pre-outgassed at room temperature overnight in the preparation chamber. Binding energies were referred to the C 1s peak ( E , = 284.8 eV). Catalytic Measurements ( a ) High Pressure Activity testing for methanol synthesis was carried out at ICI Billingham, using a high-pressure microreactor rig equipped with on-line mass-spectrometric analysis. A syngas mixture (2.5% C02, 6.0% CO, 63% H2, 28.5% N2) at 50 bar and 513 K was used for the tests.(6) Normal Pressure Activity testing at 1 bar was carried out in a laboratory-built flow system with g.c. analysis. The principal purpose of these experiments was to determine the activity for reverse shift as well as for methanol synthesis. The conversion has therefore been determined for two mixtures, ( i ) 5% C 0 2 , 55% Hz, 40% He and (ii) 10% C 0 2 , 10% H2, 80% He, the latter being more appropriate for reverse shift studies. In each case the procedure was to load the microreactor with 0.5 cm3 of calcined oxide and pre-reduce in 20% HZ, 80% He for 16 h at 508 K. The copper area of the catalyst was then determined in situ by N 2 0 reactive frontal chromatography at 333 K.After re-reduction at 508 K in the H2-He mixture the stream was switched to one of the two above C02-H2-He mixtures. The conversion was determined at various flow rates in order to ascertain whether conversion was proportional to the residence time. A. Characterisation of Precursors and Effect of Ageing ( a ) X - Ray Diflractometry Under the conditions of preparation described above, hydroxycarbonates may be expected as precipitates, notably zinc-containing malachite (‘zincian malachite’) [(Cu,_,,Zn,)CO,(OH),] and aurichalcite [(Cus~,yZn,)(C0,)2(OH),].1~ X-Ray analysis110 Copper- Zinc Oxide Catalysts 10 20 30 40 50 60 7c 80 201 O Fig. 1. X-Ray diffractograms of precursors (Fe-filtered Co K a radiation: A = 1.7902 A). Ageing times (min) in the mother liquor at 333 K: ( a ) 0, ( b ) 30, ( c ) 75, ( d ) 140, ( e ) 205.of our unaged precipitate [Cu/Zn = 2: 13 after drying confirmed that these two phases were present (fig. 1). Referring to reflections which do not overlap in the two phases, the malachite structure is identified by its (020), (120), (200) and (220) reflections at 28 values of 107.09, 20.34, 22.01 and 28.01 O, respectively (d-spacings of 6.023, 5.069, 4.689 and 3.699 A); correspondingly, the presence of aurichalcite is defined by its characteristic reflections (400), (511), (420) and (901) at 28 values of 15.11, 32.22, 35.81 and 39.81 O, respectively ( d = 6.809, 3.226, 2.909 and 2.629 A). There is no evidence for any other phase. The X-ray diffractograms of precursor which had been aged for 30, 75, 140 and 205 min in the mother liquor at 333 K before drying are also shown in fig.1. There are significant changes, namely: (i) disappearance of the aurichalcite pattern; (ii) broadening of the malachite reflections, e.g. (020), (120); (iii) small displacements in the peak positions, e.g. (020) to 28 = 17.01 O ( d = 6.054 A), (120) to 28 = 20.28 O ( d = 5.084 A), (200) to 28 = 22.13 O ( d =4.664 A ) and (220) to 28 = 28.05 O ( d = 3.694 A) as a result of ageing for 140 min. The largest amount of change occurs during the first 30 min of ageing. ( b ) Infrared Spectra The presence of malachite and aurichalcite phases in the unaged precursor, and the absence of other phases, is confirmed by the i.r. results shown in fig. 2. Furthermore, the disappearance of the aurichalcite on ageing is also clear.Note that the bands characteristic of aurichalcite at 1556, 1201 and 971 cm-' (which are at values where malachite does not absorb significantly) are all destroyed on ageing. There is a change in the OH-stretching region at 3300-3500 cm-' (not shown in fig. 2) whereby on ageing from 75 to 205 rnin the doublet which is a characteristic of pure malachite improves in resolution. ( c ) Diflerential Thermal Analysis DTA results for the precursors are shown in fig. 3. Aurichalcite gives a broad lower- temperature decomposition endotherm compared to malachite and in the unaged precur- sor manifests itself merely as a low-temperature tail (shaded region) on the malachiteFaraday Discuss. Chem. SOC., 1989, Vol. 87 Plate 1. Electron micrographs.( a ) Synthetic malachite ( x 94 000); ( b ) unaged precursor ( x 150 000); ( c ) precursor aged for 75 min ( x 250 000); ( d ) selected area electron diffraction pattern of platelet in centre of field in ( b ) . D. Waller, D. Stirling, F. S. Stone and M. S. Spencer (Facing p. 11 1)D. Waller, D. Stirling, F. S. Stone and M. S. Spencer 1 1 1 Fig. 2. 1.r. spectra. M, malachite [Cu2C03(OH)J; A, aurichalcite [CU~Z~,(CO~)~(OH),] (standard synthetic samples); ( a ) - ( e ) precursors aged for 0, 30, 75, 140 and 205 min, respectively. endotherm, peaking at 600 K. The tail is absent after ageing. The most striking feature of fig. 3 is the pronounced shift of the malachite peak to higher temperatures on ageing. ( d ) Electron Microscopy, Electron Diflraction and EDAX At magnifications of the order of lo5, zinc-free malachite prepared under the same conditions as used for the Cu-Zn precursor has the morphology of ill defined crystallites, as shown in the electron micrograph of plate 1( a ) .The unaged Cu-Zn precursor contains very similar crystallites [plate l(b)], but they are accompanied by well defined platelets of entirely different habit. The platelets disappear on ageing. Plate l ( c ) is an electron micrograph of precursor aged for 75 min: the platelets are now much less evident and the dominant feature is the microcrystalline material characteristic of the malachite phase. Bearing in mind the X-ray and i.r. results, this would be consistent with the platelets being aurichalcite. Selected-area electron diffraction [plate 1 (d)] confirmed that the platelets are well crystallized aurichalcite, the pattern being identical with that illustrated for aurichalcite by Himelfarb et a1.l' Energy-dispersive X-ray analysis of selected areas of samples examined by electron microscopy enabled the relative amounts of copper and zinc in the precursor phases to be assessed.Copper and zinc are revealed at similar sensitivity by EDAX.I6 Fig. 4 ( a ) shows the EDAX pattern for an aurichalcite platelet in unaged precursor, showing K a and KP from Cu and Zn. The Cu KP component overlaps the Zn K a peak and appears as a shoulder on the high-energy side. Making allowance for this contribution to the peak height, it follows that the Zn content of the aurichalcite is lower than that of Cu.112 Copper- Zinc Oxide Catalysts 500 600 700 800 T I K Fig.-3.DTA of precursors. Ageing times (min): ( a ) 0, ( b ) 30, ( c ) 75, ( d ) 140, ( e ) 205. A c c Y .C m CI .- 7 8 9 1 0 7 8 9 1 0 7 8 9 1 0 energy/ keV Fig. 4. Energy-dispersive analysis of X-rays (EDAX) for precursors. ( a ) Platelet in unaged precursor, ( b ) small particles in unaged precursor, ( c ) small particles in precursor aged for 205 min. Fig. 4(6) shows the pattern for the other phase in the unaged precursor, namely that of a cluster of the small crystallites as seen in plate 1 (6). The presence of Zn is directly established, the Cu/Zn ratio being ca. 85: 15. Fig. 4(c) shows the EDAX pattern from the small crystallites present in aged precursor. The Zn content has clearly increased, and the Cu/Zn ratio is now close to the 2 : 1 value required from the known overallD.Waller, D. Stirling, F. S. Stone and M. S. Spencer A B I 1 I I I 930 940 950 96C 970 binding energy/eV 113 binding energy/eV Fig. 5. XPS of precursors. A. Cu 2 ~ , , ~ and Cu 2p,,* spectra of precursors ( a ) - ( e ) aged for 0, 30, 75, 140 and 205 min, respectively. B. Zn 2p,,, spectra of the precursors. composition. Several such patterns were registered from different regions of the sample, and no significant variation in the 2 : 1 Cu/Zn ratio was found. ( e ) X-Ray Photoelectron Spectroscopy X.p. spectra were measured to obtain information on the Cu/Zn ratio in the surface region of the precursor particles. The Cu 2p,/, and Zn2p,/, spectra for unaged and aged precursors are shown in fig.5 : for Cu the 2p region is also included. The satellite peaks for Cu confirm the oxidation state as Cd‘f The surface ratio (Cu/Zn),,, was obtained from the respective intensities of the 2p,/, signals, taking account of both the main and satellite peaks for Cu and noting the sensitivity factor of 0.88 recommended by Wagner et al.:” (Cu/zn)xPs = [ W U 2p,,*)/ Wn2p,,2)1(1/0.88). ( 1 ) The unaged precursor (mixture of zincian malachite and aurichalcite) yielded a (Cu/Zn),,, ratio of 2.6, whilst the aged precursor gave values of 2.0, 2.0, 2.1 and 2.2 for ageing times of 30, 75, 140 and 205 min, respectively. The value for the unaged precursor is not likely to be representative since the two phases present have such different morphology [plate 1 (b)]. The aged precursor (increasingly less aurichalcite) is increasingly free from this problem, and it is significant that for all of these the ratio is close to 2 : 1, the overall composition.The binding energy values and the actual profiles show no particular trend with ageing.114 Copper- Zinc Oxide Catalysts Table 1. Surface areas of calcined oxides and of reduced catalysts derived from unaged and aged precursors precursor ageing surface area of copper surface area of time/ min calcined oxide/m' g-' reduced catalyst/m' g-' 0 30 75 140 205 43 83 65 55 59 13 27 24 25 23 B. Characterisation of Calcined and Reduced Precursor The main aim of the present study is the identification of the changes occurring on precursor ageing and the effect on ultimate activity. Characterisation of the calcined and reduced precursor was therefore limited to XRD analysis, u.v.-visible-n.i.r.diffuse reflectance spectroscopy and surface area determination. The unaged precursor and also samples of the precursor aged for 30, 75, 140 and 205 min as previously described were converted to calcined oxide by heating at 623 K for 6 h in air. X-Ray diffraction of the products showed the presence of CuO and ZnO, but no other phase. The oxide from aged precursor showed greater line-broadening than that from the unaged precursor, signifying smaller particle size. The effect of ageing in producing a more finely divided calcined oxide was confirmed by surface area measurements (table 1). Interestingly, the 30 min aged precursor gave oxide with the highest specific surface area, almost twice that from the unaged material.The high copper content (Cu/Zn = 2 : 1 ) necessarily resulted in the calcined oxide being very black. However, even without dilution with inert material, u.v.-visible-n.i.r. DRS readily revealed the CuO absorption edge near 850 nm. There was no absorption at 1400-1500 nm, the region characteristic of Cu" in tetrahedral coordination in Zn0.I8 No spectral difference between oxide from unaged and aged precursor could be discerned. Calcined oxide was treated at ca. 520 K in H7 to give the reduced Cu/ZnO used for activity tests. The only significant new characterisation required was the copper surface area. To observe the effect on the metal surface area of ageing the precursor from which the catalyst was derived, the N 2 0 pulse method was applied to the respective samples immediately after reduction under standard conditions.The resulting surface areas are shown in table 1, from which it is seen that the surface area variation observed for calcined oxide is reflected in the copper area of reduced catalyst. C. Catalytic Activity for High pressure Methanol Synthesis The activities of the five reduced Cu/ZnO catalysts listed in table 1 were compared to that of a standard ICI methanol synthesis catalyst (Cu/ZnO/A1,03) and relative activities were determined per unit weight of catalyst. Results are shown in fig. 6. There is a striking increase in activity as a result of deriving the catalyst from a precursor which has been aged. Beyond 30min ageing the activity depends very little on ageing time, the variations being close to those expected for an activity which is proportional to copper surface area.The unaged precursor, by contrast, yields a catalyst whose activity is well below expectations on that basis. D. Catalytic Activity for CO, Conversion at Normal Pressure Substantial evidence has now that the route to methanol synthesis is via C02 hydrogenation. It was of interest, therefore, to study both methanol synthesisD. Waller, D. Stirling, F. S. Stone and M. S. Spencer 1 .o L fi v 0,5 - - I 1 1 I - 0 115 Fig. 6. Relative activity in high-pressure methanol synthesis at 50 bar and 513 K for reduced catalysts as a function of precursor ageing time: data refer to activity after 3 h on stream. 1.0( 0.5 0 0 0.05 0.10 0.15 0.20 0.25 I/(space velocity)- I / s Fig.7. Methanol formation as a function of reciprocal space velocity for reaction of C 0 2 and H2 (C02/H2/He = 1 : 11 : 8) at 453 K and 1 bar for catalysts derived from unaged precursor (a) and precursor aged for different times, uiz. V, 30 min; 0, 75 min; 0, 140 min; A, 205 min.116 Copper- Zinc Oxide Catalysts 0.05 0.10 015 0.20 0.25 (space velocity)-'/s Fig. 8. CO formation by the reverse water-gas shift readion as a function of reciprocal space velocity for reaction of C 0 2 and H2 (C02/H,/He = 1 : 11 : 8) at 453 K and 1 bar. Symbols as for fig. 7. and the reverse shift reaction using C02+ H2 as reactants, and to examine in each case the effect of precursor ageing on the activity of the subsequently derived catalyst.The same set of prepared solids was used as for the high-pressure studies, the samples being loaded into the reactor as calcined oxide and reduced in situ, as described in the Experimental section. Temperature, C02/H2 ratio and flow rate (space velocity) were selected such that it was possible to measure the activity for methanol synthesis and reverse shift in one and the same experiment. Under our experimental conditions, the rate of the reverse shift reaction is not limited by equilibrium approach below 500 K. By contrast, equilibrium for the methanol synthesis reaction C0,+3H, S CH,OH+H,O at normal pressure is achieved at ca. 473 K, the methanol yield being ca. 0.2%. It is therefore necessary to go to lower temperatures in order to measure the rate of methanol formation under conditions which are not equilibrium-limited.However, the problem of sensitivity soon becomes acute: in our case the methanol yield fell below the limit of detectability of the analysis system below 433 K. The temperature of 453 K was therefore chosen as a good compromise. The reaction mixture used was CO,/H,/He = 1:11:8. The basis of the comparison between catalysts for methanol synthesis activity was the determination of an effective rate constant given by the slope of the plot of CH30H concentration in the effluent gas vs. reciprocal space velocity. The results obtained are shown in fig. 7. The catalyst derived from unaged precursor has extremely low activity for methanol synthesis under these conditions. An optimum activity is reached for catalyst produced from precursor aged for 140 min.D.Waller, D. Stirling, E S. Stone and M. S. Spencer 117 0 0.2 0 0.LO 0.60 (space velocity)- ' / s Fig. 9. CO formation by the reverse water-gas shift reaction as a function of reciprocal space velocity for reaction of C02 and H2 (CO,/H,/He = 1 : 1 : 8) at 473 K and 1 bar. Symbols as for fig. 7. The corresponding results for conversion to CO (the reverse shift reaction) in the C02/H2/He 1 : 11 : 8 mixture at 453 K are shown in fig. 8. The maximum activity is attained also in this case for catalyst derived from the precursor which had been aged for 140min. As for methanol synthesis, the unaged precursor gives a catalyst of correspondingly poor activity. Finally, the reverse shift reaction was studied per se using an equimolar C02/H2 mixture (C02/H2/He = 1 : 1 : 8).Fig. 9 shows results obtained at 473 K. The plots of effluent CO concentration us. reciprocal space velocity showed some curvature in this case, but the characteristics were again similar with regard to the relative activities of the five catalysts. The copper surface areas of the catalysts were determined after reduction but prior to the above experiments by the method of N 2 0 frontal analysis using the same flow system as was employed for the catalysis. The specific areas obtained were 15, 26, 27, 21 and 20 m2 g-' for the reduced catalysts whose precursors had been aged at 0, 30, 75, 140 and 205 min, respectively. The values for the last two catalysts are lower than expected (cJ table 1). Discussion Structure and Morphology of the Catalyst Precursor As initially formed, the hydroxycarbonate precursor is made up of two phases, one with malachite structure and the other aurichalcite (fig.1). The d-spacings of the malachite phase show that it contains both copper and zinc. The values are shifted from those of118 Copper- Zinc Oxide Catalysts laboratory-prepared pure-Cu malachite, for which Porta et a1.’* cite = 5.993 A, d,,, = 5.055 A, dzoo = 4.699 A and d,,, = 3.693 A, in the direction of decreased unit-cell size and by amounts which imply 10-20% Zn in solid solution. The effect of ageing the precursor in the mother liquor at 333 K is rapid disappearance of the X-ray reflections of aurichalcite (fig. l ) , leaving a matured malachite phase with d -spacings suggesting a Zn content increased to 30-40%, which is the solubility limit.12714 The i.r. data (fig. 2) establish that loss of the aurichalcite reflections is not due to fragmentation or deterioration in the quality of the crystals, but is a genuine destruction of the phase. The SOH band at 879cm-I in the Cu-malachite reference spectrum is displaced to 863 cm-’ in the 140 min aged precursor: this band is sensitive to Zn content14 and indicates [Zn] > 20%, consistent with the X-ray results. The peak temperature of the DTA endotherm (fig. 3) also reflects the presence of zinc in the malachite phase. The work of Porta et a1.,I2 confirmed by our own has shown that the malachite endotherm (ca. 575 K in Cu malachite) moves up in temperature on incorporating Zn.Even more significant is the continuing trend with sharpening of the peak. We interpret this as evidence not only that the Zn content increases on ageing but also that Zn confers a higher lattice energy. We recall here the parallel decrease in size of the unit cell and the X.P.S. evidence of Porta et al.” for increased covalency. The electron microscopy and EDAX results (plate 1 and fig. 4) corroborate and extend the above findings. The two phases in the precursor as initially formed are distinguished absolutely (plate 1 ) . The aurichalcite platelets are found by EDAX to have a Cu/Zn ratio rather greater than unity. This is consistent with their Cu content being close to 6O%, the limit found in mineral samples, and that expected if aurichalcite has 60% of its cations in octahedral sites and 40% in tetrahedral sites (by analogy with its close analogue hydrozincite, Zn,( CO,),( OH),), the former being occupied by Cu” and the latter by Zn*+.EDAX (fig. 4) also established that the malachite component of the unaged precursor definitely contains zinc, and moreover that Cu/Zn * 85 : 15, in agreement with XRD. The results illustrated in plate 1 and fig. 4 also provide important insight into the changes induced by ageing. As the relatively zinc-rich aurichalcite platelets disappear, the malachite phase takes on a more microcrystalline appearance [plate 1( c ) at x250 000), consistent with the increased X-ray line-broadening. This speaks against growth of the pre-existing Cu/Zn 85 : 15 crystallites by coating with highly zinc-rich material.The EDAX analysis of the aged malachite microcrystallites, sampled widely, shows the 2 : 1 Cu/Zn ratio consistent with the Zn solubility limit” and with the overall ratio pre-defined by the preparation conditions. The homogeneity of the aged zincian malachite, and confirmation that the particles are not formed of a highly copper-rich core and a highly zinc-rich surrounding is provided by the X.P.S. results (fig. 5 ) . The Cu 2p3,, spectra confirm the oxidation state as CU” in the surface (presence of the satellite and the EB of the main peak at 934.7 eV, invariant with ageing). More importantly, however, the Cu/Zn ratio for the surface region of the aged particles, as derived from the Cu 2p3,, and Zn 2p,/, intensities, is close to 2: 1 , the value which EDAX shows is the overall ratio for the particles.The conclusions about the precursor may now be summarised. Starting from the Cu/Zn 2 : 1 aqueous solution, malachite (Cu/Zn = 85 : 15) and aurichalcite (Cu/Zn = 60:40) are rapidly precipitated. Ageing at 333 K causes solution of the aurichalcite simultaneously with nucleation of Cu/Zn 67 : 33 (i.e. 2 : 1 j malachite, or possibly deposi- tion of this material on the smallest of the 85 : 15 malachite crystallites. This is accom- panied by growth of the 2 : 1 malachite (high lattice energy) as stable small crystallites at the expense of the less stable and significantly larger 85 : 15 crystallites, the extra zinc being continuously supplied by the dissolving aurichalcite, the structure which has the least stability (fig.3).D. Waller, D. Stirling, F. S. Stone and M. S. Spencer 119 On conversion to oxide, the ageing effect manifests itself as textural promotion (decrease in particle size, increase in surface area) and this follows through to reduced catalyst in respect of Cu area (table 1). The short ageing (30min) shows the largest effect on the basis of surface area. However, the real test is catalyst activity, and we address this in the next section. Effect of Precursor Ageing on Ultimate Catalyst Activity The most important single result to emerge is the very poor activity of catalyst derived from unaged precursor, whether for methanol synthesis (fig. 6 and 71, or reverse shift (fig. 8 and 9). It is not simply a matter of smaller copper area: the area-normalised activity relative to catalyst from aged precursor is lower.Thus aurichalcite is an undesired precursor component. Its presence in the precursor affects the normal pressure methanol synthesis (fig. 7) more than the high-pressure synthesis (fig. 6), possibly because of a lowered propensity for keeping copper sufficiently reduced in the steady state. By contrast, precursor consisting solely of a malachite phase which has been stabilised by having zinc incorporated to the solubility limit (the aged precursor) leads to a catalyst of very high relative activity (fig. 6), close to that of standard commercial catalyst. Ageing for more than 30 min has little effect on steady-state activity. The advantage of longer ageing shows up in the activity and selectivity studies for C 0 2 conversion at 1 atmf (fig.7-9). The maximum activity is not observed until the precursor has been aged for 75min or more. The linear plots of fig. 7 and 8 enable direct comparisons of the rates of methanol synthesis and CO formation to be made. The ratio of the rate constants (slopes of the plots) shows the reverse shift reaction to be ca. three times as fast as methanol synthesis, in spite of the unfavourable 1 : 1 1 C02/ H2 ratio for the former. The reactions proceed by different mechanisms,2’ but it is interesting that the pattern of the effect of precursor ageing is the same for both, and also for the reverse shift under conventional 1 : 1 COz/H2 conditions (fig. 9). The dissociative adsorption of C 0 2 , yielding oxidised copper, may be the key to this.The precursor effect on the catalytic reactions does not follow expectations based solely on the copper surface area. The activity for reverse shift is a better guide to methanol synthesis activity than the N,O-determined copper area. The authors acknowledge the support of this work by the S.E.R.C. and by ICI Chemicals & Polymers, Ltd. References 1 R. G. Herman, K. Klier, G. W. Simmons, B. P. Finn, J. B. Bulko and T. P. Kobylinski, J . Catal., 1979, 56, 407. 2 J . B. Bulko, R. G. Herman, K. Klier and G. W. Simmons, J. Phjvs. Chew., 1979, 83, 3118. 3 S. Mehta, G. W. Simmons, K. Klier and R. G. Herman, J. Catal., 1979, 57, 339. 4 H. H. Kung, Catal. Rev. Sci. Eng., 1980, 22, 235. 5 K. Klier, Adu. Catul., 1982, 31, 243. 6 K. Klier, App. Sucf Sci., 1984, 19, 267. 7 G. C . Chinchen, P. J. Denny, J. R. Jennings, M. S. Spencer and K. C . Waugh, Appl. Catal., 1988, 36, 8 S. P. S. Andrew, 7th Int. Congr. Catal., Post-Congr. Symp., Osaka, paper 12, 1980. 9 G. C . Chinchen, K. C. Waugh and D. A. Whan, Appl. Cutal., 1986, 25, 101. 1. 10 P. B. Himelfarb, G. W. Simmons, K. Klier and R. G. Herman, J. Caral., 1985, 93, 442. 1 I M. H . Stacey and M. D. Shannon, in Reactivity of Solids, ed. P. Rarret and L. Dufour (Elsevier, Amsterdam, 1985). p. 713. 12 P. Porta, S. D e Rossi, G. Ferraris, M. Lo Jacono, G. Minelli and G. Moretti, J. Caral., 1988, 109, 367. 13 P. Porta, G. Fierro, M. Lo Jacono a n d G. Moretti, Catal. Today, 1988, 2, 675. -t 1 atm = 101 325 Pa.120 Copper- Zinc Oxide Catalysts 14 F. S. Stone and D. Waller, to be published. 15 G. C. Chinchen, C. M. Hay, H. D. Vandervell and K. C. Waugh, J. Catal., 1987, 103, 79. 16 G. Cliff and G. W. Lorimer, Roc. 5th Eur. Congr. Electron Microscopy (Inst. Physics, London, 1972), 17 C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. H. Raymond and L. H. Gale, SurJ Interface 18 F. H. Chapple and F. S. Stone, Proc. Br. Ceram. SOL:, 1964, I , 45. 19 G. Liu, D. Willcox, M. Garland and H. H. Kung, J. Catal., 1985, 96, 251. 20 G. C. Chinchen, P. J. Denny, D. G. Parker, M. S. Spencer and D. A. Whan, Appl. Caral., 1987,30, 3 3 3 . 21 G. C. Chinchen, M. S. Spencer, K. C. Waugh and D. A. Whan, J. Chem. SOC., Faraday Trans. 1, 1987, p. 140. Anal., 1981, 3, 211. 83. 2193. Paper 9/004761; Received 27th January, 1989

ISSN:0301-7249    

DOI:10.1039/DC9898700107

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

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. Burch, Specialist Periodical Reports, Catalysis (Royal Society of Chemistry, London, 1982), vol. 6, pp. 27-60. 2 J . Santos, J. Phillips and J . A. Dumesic, J. Caral., 1983, 81, 147. 3 D. E. Resasco and G. L. Hallet, J. Caral., 1983, 82, 279. 4 H. R. Sadeghi and V. E. Heinrich, J. Catal., 1984, 87, 979. 5 S. Sakellson, M. McMillan and G. L. Haller, J. fhjvs. C'hem., 1986, 90, 6811. 6 M. S . Spencer, J. Catal., 1985, 93, 216.132 Metall Metal Oxide Interface 7 J. P. S. Badyal, A. J. Gellman, R. W. Judd and R. M. Lambert, Catul. Lett., 1988, 1, 41. 8 G. Owen, C. M. Hawkes, D. Lloyd, J. R. Jennings, R. M. Nix and R. M. Lambert, Appl. Cutal., 1987, 9 W. E. Wallace, Chemtech., 1982, 752. 33, 405. 10 S. J. Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. Soc., 1978, 100, 170. 11 Y. W. Chung, G. Xiong and C . C. Kao, J. Cutal., 1984, 85, 237. 12 R. A. Demmin, C. S. KO and R. J. Gorte, J. Phys. Chem., 1985, 89, 1151. 13 J. P. S. Badyal, A. J. Gellman and R. M. Lambert, J. 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

ISSN:0301-7249    

DOI:10.1039/DC9898700121

出版商:RSC    

年代:1989

数据来源: RSC