摘要:

Date 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Subject Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non- Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Photoelectrochemistry High Resolution Spectroscopy Selectivity in Heterogeneous Catalysis Oxidation 43 1 Volume 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 * 66 67 68 69 70 71 72 * Not available; for current information on prices, etc., of available volumes, please contact the Marketirzg Oficer, Royal Society of Chemistry, Burlington House, London Wl V OBN stating whether or not you are a member of the Society.Date 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Subject Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non- Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Photoelectrochemistry High Resolution Spectroscopy Selectivity in Heterogeneous Catalysis Oxidation 43 1 Volume 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 * 66 67 68 69 70 71 72 * Not available; for current information on prices, etc., of available volumes, please contact the Marketirzg Oficer, Royal Society of Chemistry, Burlington House, London Wl V OBN stating whether or not you are a member of the Society.

ISSN:0301-7249    

DOI:10.1039/DC98172FX001

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

THE SECOND RIDEAL LECTURE What Makes a Catalyst Selective? BY WOLFGANG M. H. SACHTLER Roninklij ke/Shell-Laboratorium (Shell Research B.V.), Amsterdam, The Netherlands Received 14th August, 1981 Selectivity, i.e. the ability to catalyse preferentially one of several possible chemical reactions, is likely to become the most highly valued property of modern heterogeneous catalysts in industry. Once the reaction path of a catalysed reaction is known in every atomic detail, one can define the conditions which the catalyst must fulfil in order to enable the reaction to proceed. Whenever these conditions differ for different reaction paths, the catalyst which meets the requirements of only one of these paths will be selective for this particular reaction. The subject of catalyst selectivity can therefore be discussed in terms of the catalyst requirements for defined reaction mechanisms.Using this approach and modern results, an attempt is made to arrive at a systematic classification of these catalyst requirements. For reactions catalysed by isolated metal atoms in the surface of the catajyst, selectivity can be due to the different numbers of vacant ligand sites required, as appears to be the case with sulphide catalysts. For reactions requiring ensembles of contiguous metal atoms, different ensemble re- quirements control the selectivity of alloy and metaborganic complex catalysts. The stereochemistry of Iigands attached to a catalytic site and acting as '' templates " controls stereosclectivity, including enantioselectivity. Among the various characteristics of a catalyst that determine its suitability and value for use in industry and in the laboratory the following three predominate: activity, stability, selectivity.It is obvious that a catalyst of very low activity is undesirable and often imprac- ticable, as it necessitates either excessively long contact times or high temperatures to achieve the required conversion. Of course, the latter option is not open if the equilibrium constant of the reaction to be catalysed shifts with increasing temperature towards the reactants rather than to the products, as is the case in, for instance, the synthesis of methanol from hydrogen and carbon monoxide, and in ammonia syn- thesis. The stability of the catalyst, i.e. a long life or an easy in situ regenerability, is the next important property.As a catalyst can only operate by forming chemical bonds with at least one of the reactants or its fragments, the danger is always imminent that even stronger bonds are formed with one of the by-products of the reaction. One important reason to strive for high selectivity is, therefore, to avoid the formation of such by-products, which poison the catalyst surface. The problem of catalyst stabi1.i ty, therefore, often reduces to a problem of catalyst sekctivity.' More generally, the selectivity of a catalyst is of interest when out of several chemical reactions compatible with the laws of thermodynamics only one is wanted. The question which of the thermodynamically permitted reactions is considered desirable and which is not, may depend on economic or other factors outside the scope8 CATALYST SELECTIVITY of this lecture.Sometimes different catalysts have been developed, each of which selectively catalyses one particular reaction from a given set of possible routes. This holds e.g. for the oxidation of ethylene: Ethylene oxide is formed with high selectivity over silver-based catalysts.2 Acetalde- hyde is the product obtained with the Wacker catalyst system containing palladium and copper ions as the active ingredient^.^ Total combustion (being the only one of these three reactions which can also proceed at a high rate in the absence of a catalyst) is well catalysed by metallic Pt, as present in e.g. muffler afterburner catalyst^.^ The term selectivity is not only used in a qualitative sense to describe the ability of a catalyst to catalyse preferentially one of several reactions: as a quantitative parameter the selectivity s for forming B from A is defined by (1) number of moles of A ultimately converted to B total number of moles of A converted S = which upon multiplication by 100 is expressed in a percentage.This definition has the advantage of being readily applicable once the product analysis and the stoichio- metry of the individual reactions are known. In order to relate s to other parameters one should consider all the parallel and consecutive reactions which occur in the steady state. This is often simplified by the triangular scheme: in which case one can write Once each of the rates rl, r2, etc. is known as a function of temperature, partial pres- sures and flow rates, they can all be inserted into eqn (2), and s is expressed in terms of the kinetic parameters, in particular the activation energies and the reaction order.An important task of the scientist in catalysis is then to relate these phenomenological properties to the basic physical and chemical characteristics of the catalyst and the reacting molecules. In the present lecture the principles identified as relevant to selectivity will be categorized into four main groups, and each of them will be discussed using results from the recent literature for illustration. The main evidence used in this lecture will be based on metal catalysts following the example set by Sir Eric Rideal,5 whose name has been given to this lecture.ELECTRONIC AND GEOMETRIC FACTORS Some thirty years ago it was conventional to describe the differences in performance between similar catalysts in terms of " electronic " and " geometric "W. M. H. SACHTLER 9 While perfectly correct, this approach led to inconsistencies when the term “ elec- tronic ” factor was tacitly narrowed down to imply only collective parameters as used in e.g. the physics of electric conductivity or magnetism. To avoid such ambiguities we shall use a different approach in the present paper. Let us assume that a molecule interacting with a catalyst surface can react along different reaction paths resulting in different products, and that each of these paths is known in every atomic detail. We should then be able to define for each path the requirements with respect to the chemistry and geometry of the catalysing sites for the particular reaction to proceed.If the catalyst fulfils the requirements of only one of these reaction paths, it is clear hat only this reaction will take place. In generalizing these ideas it appears convenient to classify the requirements of relevance to catalyst selectivity into four main categories : bond strength requirement, coordination requirement, ensemble requirement, template requirement. As the strength of a chemisorption bond is governed by the electronic charac- teristics of the atoms involved, the bond strength requirement is essentially a chemical reformulation of the old electronic factor concept. The other three requirements are, by their very nature, of a geometric character.We define as the coordination requirement of a reaction catalysed by an isolated surface atom the minimum required number of coordination sites per surface atom. For instance, it might be argued that for the H2/D2 equilibration reaction to take place on an isolated metal atom M, the Bonhoeffer-Farkas mechanism requires three vacant coordination sites which can be populated with hydrogen atoms: H a a H D D o o D \I/ M +Dz + M while the Rideal-EleyS mechanism requires D \ H H O M + D z + M \ / ” \ / \I/ - H D + M only two coordination sites: ”\ /” c_t M + H D We define the ensemble requirement of a catalytic reaction as the minimum required number of contiguous surface atoms of the element(s) able to form bonds with the end adsorbate.This definition accepts the conjecture that for a specific reaction mechanism the required active centre on the catalyst surface can be either a single metal atom or an “ ensemble ” of several adjacent atoms. The template requirement describes the stereochemical conditions which a cata- lytic centre must fulfill in order to render the reaction “ stereospecific ” or “ shaFe- selective ”, i.e. capable of preferentially producing one of several isomers. Most papers at this General Discussion focus on one of these requirements. This introductory lecture will briefly describe these principles and provide some illus- trations from the recent literature. THE BOND STRENGTH REQUIREMENT The formation of chemical bonds between atoms of the substrate and the catalyst surface and their rupture at a later stage are essential steps in all heterogeneously10 CATALYST SELECTIVITY catalysed reactions.The theory of the volcano-shaped curves, formulated by Balandin,8 assumes that the activation energy of a given reaction with a given mechan- ism when plotted against the strength of the chemisorption bond passes through a minimum, characterizing the optimum catalyst for the particular reaction. If the bond is too weak, no chemisorption will take place; if it is too strong the desorption step will become too For a given reactant, the main variable on the catalyst side that influences the bond strength is the chemical identity and valence of the adsorbing atom(s). The enormous differences in catalytic performance between platinum and gold (two neighbours in the Periodic Table, with almost identical geometric properties) illustrate the effect of electronic configuration on the bond strength and thus on the activity." The chemist faced with the problem of predicting what catalysts are likely to be selective for a certain reaction will therefore first and foremost look at the chemical reactivity.Taking, again, the oxidation of ethylene as an example, the first question that springs to mind is which chemical bonds in the original molecules have to be broken and which ones have to remain intact. For the three possible products, the answer is given in table 1. TABLE l.-Boms BROKEN IN ETHYLENE OXIDATIONS product 0-0 C-H C=CorC-C coz + HzO yes Yes Y e s CH3CHO Yes Yes no /\ Yes no no 0 CHz--CHZ Combining this with the chemical experience that on all metals with incompletely filled d orbitals ethylene easily undergoes self-hydrogenation and deuterium exchange, both proceeding with cleavage of C-H bonds, we can state that the number of candidates for epoxidation is reduced to the metals with filled d orbitals. Considering further that among the remaining metals in the Periodic Table many will form very strong bonds with oxygen and will therefore not easily release the adsorbed oxygen, we are left with a fairly short list of potential candidates, with silver as one of the prominent members.For a given metal atom and a given adsorbate the bond strength is to some extent also influenced by the environment, in particular the chemical nature of the nearest neighbours.This is the essence of the ligand effectlOpll in catalysis. It appears that in alloys such as Cu-Ni, this ligand effect plays only a minor role.ll This effect is, however, very strong with organic complexes of transition metals. In a recent paper, van Veen et a1.I2 compared four groups of such complexes deposited on carbon for the electrochemical reduction of oxygen and for the chemical decompo- sition of hydrogen peroxide (see fig. 1). The catalysts studied were: (1) phthalocy- anines (PC) of Co and Fe supported on Norit BRX, (2) the same after heating in an inert atmosphere, (3) tetraphenylporphyrins (TPP) of various transition metals (Me) supported on Norit BRX, and (4) the same after heat treatment In fig. 1 the measured rates (for the electrochemical reaction, the current in mA per mg at constant voltage; for the chemical reaction, the first-order rate constant) are plotted against each other.I 7 lo' M E W.M. H. SACHTLER Ir 7 I I / I / / / 11 - 2 9 loo- $ > E 1 o2 I O - ~ 10-2 10-1 100 k(H,Oz)/g- 's- ' FIG. 1.-Oxygen reduction activity of MeTPP and MePc supported on Norit BRX plotted against their rate constant for H202 decomposition. 4 mol dm-3 HzSO4, 23 "C; 0, untreated MePc; 0, untreated MeTPP ; filled symbols : heat-treated samples. It is seen that both rates vary considerably with the catalysing complex; more im- portant is the increase by two orders of magnitude for a given complex owing to the heat treatment. The structural consequences of this procedure were analysed by means of EXAFS, Mossbauer and optical spectroscopy. The results show that the first coordination sphere of the transition metal atom remained intact after heating, but links between the pyrrole rings of the complexes were broken, so that these rings could orient themselves to attain maximum interaction with the carbon surface.It is remarkable that these drastic changes in catalytic performance are due to changes in the molecule at a considerable distance from the transition metal atom. THE COORDINATION REQUIREMENT In homogeneous catalysis Wilkinson's hydrogenation catalyst, for example, exhibits activity by losing a phosphine ligand, as a result of which three coordination sites become available, two for the dissociation of dihydrogen and one for the addi- tion of the substrate.13 The example may be generalized : on a mononuclear complex three vacant coordination sites are required for catalytic hydrogenation.Siege1 l4 applied the concept of the required coordination sites to oxide catalysts and Tanaka15 applied it to the sulphides of nickel and molybdenum. Blakely and Somorjai16 showed with metal single crystals that flat faces have a significantly lower activity than terraces since atoms in step, kink and corner sites exhibit a higher degree of coordinative unsaturation. The principle is elegantly illustrated by Tanaka's work with nickel and molyb- denum sulphide. In this case the adsorbing metal atoms are separated from each other, i.e. the adsorbing " ensemble " is monoatomic (except for heterolytic dissocia- tions which make use of metal and adjacent sulphur atoms), while migration of ad- sorbed atoms from one site to the other is negligible under the conditions chosen.12 CATALYST SELECTIVITY To illustrate the principles we shall concentrate on MoS,.This compound is known to have a layer lattice, each Mo4+ ion in the interior being surrounded by six S2- ions in a trigonal prismatic arrangement. The basal planes of the crystal are close-packed S2- faces, exposing no unsaturated Mo ions. On the edges, however, electroneutrality requires that most Mo ions lack one or more of the six ligands. Writing the number of missing ligands as a superscript, Siegel14 calls these configura- tions lM 2M or 3M. Each of the missing ligand positions is a potential adsorption site for a species capable of forming one adsorption bond with the metal ion.An obvious way to increase the number of these adsorption sites is to start from a macroscopic wafer-shaped crystal and fracture it. This keeps the number of atoms in the close-packed basal planes essentially constant but increases the number of un- saturated atoms in the edge positions. Fig. 2 shows the drastic difference between the 60 - 3 40- W a *g $ s 20- timelmin FIG. 2.-Double-bond migration of but-1-ene on cut (0) and uncut (0) MoS single-crystal catalysts. isomerization rates of but-1-ene at 100 "C in the presence of hydrogen on the intact and the broken wafers.I7 The conclusion is evident that the coordinatively un- saturated edge sites are loci of the observed catalytic activity. The edge structure of MoS, has been extensively discussed by Farragher." The normal habit of MoS2 exposes, besides the basal planes which consist of close-packed sulphur ions only, edge faces of the (1010) type.More precisely, (1010) faces alter- nate with (7010) faces. Farragher19 assumes that upon breaking a crystal in such a way that two electroneutral halves are formed, the newly formed surfaces will stabilize themselves by slight rearrangements of the sulphur ions, such that some S2- ions move from a position where they contact only one Mo4+ ion to a position where they bridge two Mo4+ ions. Fig. 3 shows his model of the freshly cleft and of the rearranged surface." In both structures the exposed Mo4+ ions are present in 'M and ,M con- figurations only, and this will approximately remain true also at temperatures where surface mobility permits the S2- ions to oscillate between these positions.However, 3M configurations can be created by surface imperfections or by partial reduction of the sulphide. Intercalation of ions in the (normally empty) octahedral positions will also modify the site adsorption energies, including the energy to remove sulphur ligands.W. M. H . SACHTLER 13 FIG. 3.-Stoichiometric MoS2 surfaces: (a) (lolo), (6) (iOl0). For the subject of the present lecture the fascinating aspect of the work by Tanaka et al. is the possibility to separate neatly reactions which differ in their coordination requirement. On surfaces which contain virtually no 3M sites, no hydrogenation of olefins is observed, but reactions requiring a smaller number of vacant ligand sites can take place.These include the isomerization of olefins and the protium/deu- terium exchange between olefins, for instance, between C2H4 and C2D4. However, for these reactions the catalysts become active only after exposure to hydrogen,15 as shown in fig. 4. Tanaka et al. assume that the promoting effect of hydrogen is a 0 50 100 150 200 250 time/min FIG. 4.-Hydrogen-promoting effect on the isomerization of but-1-ene on MoSz at room temperature: 0, but-1-ene; a, trans-but-2-ene; A, cis-but-2-ene; 0, butane. heterolytic dissociative adsorption, transforming 2M into 2MH sites. An olefin adsorbed on such a site will react with the hydrogen atom to form an alkyl group i.e. the " half-hydrogenated state '' of the classical Horiutu-Polanyi mechanism.This process is reversible; if, however, the hydrogen atom dissociated is different from the one originally accepted an exchange takes place:14 CATALYST SELECTIVITY \ -c D ( 2 ~ ~ -site) When this process is repeated with a C2D4 molecule, an intermolecular exchange of hydrogen atoms will result. It is remarkable that on these catalysts, unlike simple transition metals, the isotopic exchange is much slower between dihydrogen molecules (H2 + D2 = 2HD) which requires 3MH sites (see section on electronic and geometric factors) than between olefin molecules. In one experiment with 2 Torr C4H8 and 2 Torr C4Ds in the presence of 1.5 Torr of H2 and D2 the authors20 found that after 3 min, >50% of the C4 olefins, but only 3% of the dihydrogen, had undergone exchange.While these results clearly prove the widely different coordination require- ments for hydrogenation and isotope exchange of olefins, they also form an opportunity to study details of the formation, rotation and dissociation of metal alkyl surface complexes on well-defined sites. This is illustrated below. With but-1-ene, the metal-carbon bond of the adsorption complex can be formed with either the primary or the secondary carbon atom. In the former case, the initial product of isotope exchange is [2H,-2]but-l-ene; in the latter case [2H,-l]but-l- ene is formed, a molecule for which a cis (or 2) and trans (or E ) isomer can be dis- cerned. Tanaka identified each of the three products using microwave spectroscopy. The abundancies observed are given in table 2.TABLE 2.-[2HJ ISOMERS FROM BUT-1 -ENE ISOTOPIC EXCHANGE. isomer ['H1-E-l] ['H1-Z-l] r2H1-21 c-c c-c D H c-c c==c \ / C=C / \ \ / \ c=c H D structure \ D abundance (%) 14.5 14.2 71.3 These data show a 70:30 preference for the formation of the Mo-C bond with the primary carbon atom of but-1-ene and equal chances for the formation of the two stereochemical [2H,]but-l-ene molecules, which are both formed by a simple rotation of the -CH2D methyl group before one of the hydrogen atoms is split off. Another type of rotation of the same secondary C-Mo complex can take place around the Mo-C bond: c-c \c4c + D / Mo MO 1 tW. M. H. SACHTLER 15 Cleavage of a carbon-hydrogen bond from the latter structure results in the formation of [2H,-l]but-2-ene; in other words, isotope exchange is accompanied by a double-bond shift.The experimental results l7 show that this isomerisation has an induction period of ca. 30 min, while the isotope exchange starts immediately (see fig. 5). Apparently, 8 a,, timelmin FIG. 5.-Isotope exchange with ['HB]but-l-ene and double-bond shift of [2H,]but-l-ene on 'M-H sites of MoS,. the energy barrier for rotation about the Mo-C bond is initially much higher than for methyl rotation about a C-C axis, but the microgeometry of the catalyst surface is not rigid; after some time both energy barriers have apparently become very similar for these two reactions with identical coordination requirements. THE ENSEMBLE REQUIREMENT While in the preceding section different adsorption complexes and reaction paths could be discerned for a given molecule and one isolated surface atom, the number of possible complexes increases tremendously when a family or " ensemble " of several contiguous surface atoms can form bonds with a molecule.This situation is en- countered, in particular, on surfaces of transition metals. The ensemble requirement for a certain reaction can be studied by diluting the metal in an alloy with a chemically inert metal and thus reducing the concentration of large ensembles of the active metal in the surface. This idea has in recent years been pursued by numerous authors using alloys of Pt-Au, Pt-Sn, Ru-Cu and, in particular, Ni-Cu. Using Ni-Cu Ponec and Sachtler21 reported that of all the reactions observed with hexane, the hydrogenolytic splitting was affected most severely by alloying.Evidently, hydrogenolysis requires the largest ensemble of surface Ni atoms. The same conclusion was reached by Sinfelt et aZ.22 for the hydrogenolysis of ethane and by Ponec and S a ~ h t l e r ~ ~ for cyclopentane. More recently, Dalmon and Martin14*25 combined selectivity measurements for C2Hs, C3H8 and n-C4Hlo on SO,-supported Ni-Cu alloys with magnetic measurements. These authors confirm the purely geometric nature of the striking effect of alloying on selectivity. They have tried to estimate the actual size of the ensembles required for hydrogenolysis and arrived at astoundingly large figures such as 12 contiguous Ni atoms for splitting ethane to16 CATALYST SELECTIVITY 160 120 methane and 17 atoms for propane hydrogenolysis. These results still await con- firmation.For reactions of methylcyclopentane with DZ, fig. 6 shows in a striking manner the large influence of alloying on selectivity.26 While hydrogenolysis to methane is the prevailing reaction on Ni at 150 "C, this is only a small side-reaction on the alloy containing 5% Cu, even at 200 "C, although the activation energy for this reaction is, of course, higher than for the deuterium exchange. Since the effects of alloying Ni with Cu on selectivity have been adequately reported el~ewhere,~' we shall here only mention one last example:28 The methana- tion of CO, as studied by Araki and PoneqZ2 is strongly retarded by this alloying - .,OO 0' - / tlmin 0- 800 - P' 600 200 I tlmin FIG. 6.-Production of C6Dl2 (0) and CD4 (0) from methylcyclopentane and Dz on films of (a) Ni at 150 "C and (b) Cu5Nig5 at 200 "C.(see fig. 7), but the activation energy remains unchanged; the selectivity towards higher hydrocarbons is only little affected. The authors conclude that CO dissocia- tion, which is an essential common step in these reactions, requires a large ensemble. The concentration of such ensembles is low on the alloy surface and therefore the pre-exponential factor of the Arrhenius equation is low. Similar results were reported by Bond and T~rnham,'~ who used Ru-Cu alloys for the Fischer-Tropsch reaction. The smallest adsorption site, consisting of one exposed metal atom, appears to suffice for the rate-determining step of olefin hydrogenation and paraffin dehydro- genation.This follows from the results obtained by Biloen et aL30 for propane dehydrogenation with Pt-Au alloys. Fig. 8 shows that a linear relation exists between reaction rate and Pt concentration for homogeneous alloys with compositions outside the miscibility gap. For these alloys the surface concentration is essentially proportional to the bulk concentration. It is conceivable that many of the isolated Pt atoms in these surfaces have fewer than three vacant ligand sites, the number required for hydrogenation in Tanaka's scheme. However, as the surface mobility of hydrogen atoms is facile on a metal under the conditions of (de)hydrogenation, it is sufficient to have a few Pt atoms with three vacant positions or a few pairs of Pt atoms to serve as portholes for dissociation of dihydrogen molecules to populate all the other Pt atoms on the surface with hydro- gen atoms.When this surface migration is fast the rigid coordination site requirement for olefin hydrogenation ceases to be valid, as was mentioned in the previous section. It is also of interest to observe hydrocarbon conversions of much higher activation energies under more severe conditions. Results of this type obtained with unsup-W. M. H. SACHTLER I 17 b Cu (atom %) FIG. 7.-Effect of alloying Ni with Cu on the initial rates of CHI (0) and C02 (0) formation from H2 + CO at 300 "C. ported Pt-Sn alloys and a programmed temperature rise to study the conversions of n-hexane are summarized in fig. 9 and show the overwhelming ensemble effect in alloy cata1ysis.l Pt and Sn form an ordered solid solution.By consequence, the surface contains Pt atoms isolated from each other by Sn atoms even at moderate dilution. This suffices to suppress completely the hydrogenolysis reactions below / 0 0 0 / / 0 / / / / 0 5 10 15 Pt (atom %) FIG. 8.-Rates of propane dehydrogenation over homogeneous Pt-Au alloys as a function of their composition. y = 0 . 9 ~ E . ~ ~ .18 CATALYST SELECTIVITY 5 c D l Pt S d A I 203 I (cat.D) I I I / 30 I 1.og 300 400 500 300 400 500 T I T FIG. 9.-Catalytic selectivities in n-hexane conversion over Pt(A), unsupported (B, C) and A1203- supported (D) Pt-Sn alloys. (-) C1-Cs products, (- - -) sum of isohexanes, (- - - -) benzene. 450 "C and all the low-temperature isomerization reactions. As a result, the alloys show activity only at high temperature, where dehydrocyclization is favoured by fast aromatization.The similar pattern obtained with this test on Al,O,-supported Pt-Sn catalysts suggests that on these samples, too, Pt and Sn are present in alloyed form. While in the publications quoted, all the isomerizations of a given hydrocarbon are conveniently lumped together, it is also possible, though laborious, to disentangle the multitude of parallel and consecutive isomerization reactions which occur simul- taneously if a paraffin is converted over a transition metal catalyst. In every simple isomerization of an alkane molecule one and only one C-C bond is broken, while one and only one new C-C bond is formed. A convenient classi- fication is obtained by writing the skeleton of C atoms with all the bonds of the original and of the product molecule.We shall call such a fictitious structure a " pseudo- intermediate ". All these pseudointermediates contain a cyclic nucleus. Upon reviewing the literature on this subject it is interesting to note that only pseudo- intermediates with a cyclopropane or a cyclopenfane ring have been reported. For instance, for the isomerization of 2-methylpentane to 3-methylpentane the two pseudo- intermediates which can be visualized are: /c\ Cb c-c I I c- c~-~-c-c and Co- cc where in each case the bond between C, and Cb is to be broken, while a new bond is formed between C, and C,. In the literature, the reaction path characterized by the cyclopropane pseudointermediate is often called " bond-shift mechanism ", while the other path, characterized by the cyclopentane pseudointermediate, is calledW.M. H. SACHTLER 19 " cyclic mechanism ". This historical terminology is confusing and its use should be discouraged. If only a new bond is formed, and no C-C bond is broken, a dehydrocyclization results. Conversely, the ring opening of a cycloalkane is, formally, the second half of an isomerization in that only a C-C bond of the pseudointermediate is to be opened. As numerous cyclopropane pseudointermediates can be formed from a given mole- cule and each of them can be opened by breaking one of the three C-C bonds, a multitude of reactions is possible. This is illustrated in table 3 for the isomerizations TABLE 3 .-FIVE POSSIBLE CYCLOPROPANE PSEUDOINTERMEDIATES AND TEN POSSIBLE PRODUCTS IN THE ISOMERIZATION OF 2-METHYLPENTANE 2-methylpentane cyclo-C3-pseudointermediate products 1 3 5 6 2 4 e 1 2 4 m5 m - ? 1 3 5 IW 3 5 - 6 1 6 m5 6&5 1 1 Jy5 3 1 3 4 1 G4 520 CATALYST SELECTIVITY of 2-methylpentane via the cyclopropane pseudointermediate route.It is clear that isotopic labelling of the carbon atoms is absolutely necessary to determine from the experimental results which bonds have been broken and which have been formed and how the cyclopropane and the cyclopentane routes contribute to the overall isomeriza- tion. This kind of work has been done with admirable ingenuity by the late Prof. FranCois Gault and his coworkers, to whom we owe much of our present understand- ing of these isomeri~ations.~~~~~ Their results show that a variety of reaction paths is used simultaneously, for instance, when pentanes react over Pt/Al,03 catalysts.As shown in table 4, two isomerization routes make use of the C3 pseudointermediate TABLE 4.-C5 HYDROCARBON CONVERSIONS ON Pt/Al,O, (GARIN AND GAULT, 1980) substrate pseudointermediate product E/kJ mol-1 M U M M M m A/ W M A/ /tJ\ r\< A/ %7 X h/ A+ I M A+l 297 230 188 M but they differ in activation energy, one of these paths having an activation energy identical with one of the hydrogenolysis reactions. In the present context two questions are of decisive importance: (1) What are the true intermediates and (2) what is their ensemble requirement ? One can imagine that the cyclopropane pseudointermediate is a, y-diadsorbed, as was assumed by Anderson et aZ.33 in order to rationalize the neopentane isomeriza- tion over platinum :W.M. H. SACHTLER 21 Pt Pt Pt / c ‘c’ ‘ c c c The same type of 1,1,3-triadsorbed species involving two Pt atoms is assumed by Leclerq et aZ.34 to rationalize the hydrolysis of hydrocarbons on Pt. It is also possible, however, to imagine that the reaction characterized by the cyclo-C,-pseudointermediate makes use of one metal atom only. This hypothesis was brought forward by Garin and G a ~ l t , , ~ who assumed that the first step in the isomerization of neopentane over platinum is the formation of a metallocyclobutane : X c c M Y:“ rotation I Here the mechanistic assumptions are supported to be “ real chemistry ” by the very similar intermediate in metathesis, which was first proposed by C h a ~ v i n ~ ~ and is now generally accepted.When classifying such proposals as ensemble requirements one should keep in mind that it is also possible to imagine that neighbouring atoms, not shown in the formulae, are instrumental in forming the intermediate, explicitly written down, for instance, in the case of the metallocyclobutane: M M M22 CATALYST SELECTIVITY which would, formally, increase the required ensemble size to three metal atoms. The metallocyclobutane mechanism, when applied to the isomerization of iso- pentane, predicts that isomerization to a different isopentane molecule (recognizable by labelling) should be accompanied by a parallel reaction resulting from the same intermediate : This is consistent with the experience that substituted metallo-carbenes are rapidly isomerized to n-bonded olefin complexes. Gault’s conclusion that isomerization and hydrogenolysis can arise from the same intermediate is supported by his ob- servation that both reactions have the same activation energy of 188 kJ mol-l.Gault, therefore, strongly advocated this reaction path for one of several “ bond shift ” reactions. He does not comment, however, on the implication that here is a hydrogenolysis reaction which is assumed to require only a monoatomic ensemble! (The ensemble required is larger, however, if sites required to accommodate the dis- sociated hydrogen atoms are also counted, but the usual implication is that hydro- genation-dehydrogenation equilibria are fully established under the conditions of this reaction; C-H bond breaking is therefore assumed not to be part of a rate- limiting step.) Gault claims that the metallocyclobutane can be formed directly on platinum, but not on palladium, where it results from a hydrogen shift via a transient z-ally1 complex : This hypothesis, by its nature a bond-strength requirement, explains the non-occur- rence of neopentane isomerization over palladium. While the metallocyclobutane structure may be visualized with the three carbon atoms and the metal atom in essentially the same plane perpendicular to the surface, Rooney 37 has suggested an intermediate with a cyclopropane ring configuration parallel to the surface, analogous to the well-known non-classical carbenium ion responsible for structural isomerization over acid catalysts :W. M.H . SACHTLER 23 The reactions characterized by the cyclopentune pseudointermediate can also be written down assuming true intermediates with either one or two metal surface atoms. In the case of the monoatomic ensemble a metallocyclohexane structure has been assumed by Muller and G a ~ l t . ~ ~ As their intermediate lacks four hydrogen atoms with respect to metallocyclohexane it can be written as a following scheme for isomerization and related cyclization dicarbene, leading to the of n-hexane : M J M A different intermediate, also using only one surface metal atom, was proposed by Clarke and R00ney.~' These authors assume that first an olefin is formed, x-bonded to a metal atom, and that the subsequent ring closure transforms this into a n-ally1 complex : G a ~ l t ~ ~ admits that this mechanism may be realistic on palladium catalysts, but for platinum he showed that ring closure takes place also with molecules unable to form olefins ; this result therefore confirms the metallocyclohexene formulation.The metallocyclohexene concept has the additional advantage of easily explaining the marked preference of transition metals for five-ring over a six-ring closure. Formation of a six-membered ring on a monoatomic site would require a metallo- cycloheptene intermediate and this is likely to be highly unstable. Experimental data suggest that only five-membered rings arise from direct cyclization, while cyclo- hexanes are formed from cyclopentanes either by ring widening or by hydrogenation of benzene, formed via hexatriene.For the reverse reaction of ring opening also a clear preference for five rings is observed. Miki et uL4* showed that with Ni/A1203 catalysts methylcyclopentane is opened nearly quantitatively at 270 "C while at the same temperature the conversion of cyclohexane is <5%. The easy formation of six-membered rings containing one surface metal atom and the strong reluctance of metals to form the corresponding seven-membered rings24 CATALYST SELECTIVITY emerged from related studies by Groenewegen et aZ.,41 who studied the formation of surface chelates with aminc acids adsorbed on nickel catalysts. In that case it was found by i.r. spectroscopy that a-amino acids readily form the five-membered chelate and /I-amino acids the corresponding six-membered rings : R-C--C/O H I I - HN 0 ‘Ni’ and CH H/ R-C I i - HN .,(0 The y-amino acids, however, failed to form the corresponding seven-membered ring chelates and were only adsorbed on the support.Many of Gault’s results on paraffin isomerization over transition metal catalysts can thus be rationalized in terms of a mono-atomic ensemble requirement as illus- trated by the metallocyclohexane and metallocyclobutane structures of the assumed true intermediates. However, Gault also that more often than not several isomerization mechanisms operate simultaneously on the same surface, and some of these seem to require di-atomic surface ensembles. The reasoning is somewhat indirect : one of the isomerizations of the cyclopentane pseudointermediate family is characterized by a non-statistical chance of breaking the various bonds in the ring intermediate.Conversely, in the formation of rings from larger paraffin molecules a preference for one type of cyclization is observed by the formation of carbon-metal bonds with primary C atoms. To rationalize this, G a ~ l t ~ ~ * ~ ~ assumes that on these catalysts a triple bond M=C must be formed.42 Upon further accepting that a metal atom is unable to form two triple bonds with two carbon atoms, Gault concludes that the active ensemble has to be diatomic. For methylpentane, for example, this is written as follows: c c c ‘c’ ‘c //c-c\ \J \A I 1 --+ M M M M While some of the mechanistic postulates mentioned are still under discussion, there is little doubt that on most transition metals a number of isomerization path- ways coexist. To the extent that they differ in ensemble requirement, their relative contributions can be estimated from selectivity measurements on alloys.This has been done by Ponec et ~ 1 . ~ ~ The interpretation of their data has been complicated by their finding that metals such as Cu, which are inactive for hydrocarbon conversions under the prevailing conditions, can also form part of “ mixed ensembles ” in catalytic reactions requiring several adjacent adsorbing atoms. The general conclusions from this work have been summarized by de Jongste and pone^^^ in three statements: (1) Of hydrogenolysis, isomerization and dehydrocyclization, the first reaction re- quires the largest ensembles.(2) Isomerization decreases faster than the surface concentration of the Group VIII metal in alloys such as Pt-Au. (3) On very diluted Pt-Au alloys some isomerization takes place. Tentatively, the second and third statements can be interpreted by assuming that the rate constant is higher on the larger ensembles, and that a different isomerization route, requiring only a mono- atomic ensemble, prevails on highly diluted alloys. Besides alloys, another class of heterogeneous catalysts has in recent years clearlyW. M. H. SACHTLER 25 shown the dramatic effect of different ensemble requirements on selectivity. These are metal porphyrins, containing one metal atom in a large organic ring skeleton. These mono-atomic ensembles differ from metals, for instance, in their selectivity to oxidize hydrogen and carbon monoxide.While on all metal surfaces, offering large ensembles, the oxidation of hydrogen is faster and inhibited by CO, the situation is reversed on iridium porphyrins, as shown by van Baar et ~ 1 . ~ ~ This drastic re- versal of selectivity is illustrated in fig. 10, which is a double logarithmic plot of the > lo-’ VI 2 6 2 4 2t \ \ \ \ \ \ \ \ I I I I I I , , , I I I I I I I l l 2 4 6 810’ 2 4 6 810’ co (%I FIG. 10.-Electrochemical oxidation of H2 + CO in 4 mol dm-3 HzSOJ at room temperature with Ir/ Norit BRX (5 %w Ir) (0), and with Ir porphyrin/Norit BRX (4 %w Ir) heat-treated at 700 “C (0). current density of electro-oxidizing a mixture of H2 + CO against the CO content of the gas mixture. On metallic iridium H2 is oxidized, and CO is a catalyst poison.Consequently, the current, which represents here the rate of the catalysed reaction, decreases with CO content. On the Ir porphyrin, however, the result is exactly opposite : here only CO is oxidized, and consequently the current increases linearly with CO content. On these electrodes the hydrogen acts as an inert diluent, as was directly proved by replacing it by nitrogen; the current density was found to be typical only for the partial pressure of CO. It seems reasonable to ascribe this total reversal of selectivity to the ensemble requirement of hydrogen, which needs two contiguous metal atoms for dissociation or one with three vacant coordination sites. Neither of these requirements is ful- filled on the Ir porphyrin.Another extremely beautiful example of the catalytic ensemble effect was recently reported by Collman et aZ.,45 who used ring-disc electrodes to study the electrocata- lytic reduction of oxygen. Under their conditions hydrogen peroxide was not reduced or decomposed, so the amount of hydrogen peroxide found was a direct measure of the catalytic selectivity to reduce oxygen either to H202 or to H20. They26 CATALYST SELECTIVITY used as catalysts mono- and di-meric metal porphyrins deposited on a catalytically inert (under the conditions used) graphite support. In the case of the dimeric por- phyrins, the two rings were able to adjust themselves in two parallel planes, the dis- tance being defined by two strings of atoms connecting the rings as shown in fig.11. C02Et H I H H m FIG. 11 .-Cobalt porphyrin catalysts tested for electroreduction of oxygen: I, monomeric porphyrin; 11, dimeric, face-to-face Co porphyrin with six-atom linkage; 111, ditto, with four-atom linkage. They found that monomeric porphyrins and those dimers where the string inter- connecting the rings consisted of six atoms, reduced oxygen predominantly to H202. In sharp contrast to this, with electrodes coated with the bis-cobalt dimer in which the rings were separated by four-atom linkages, oxygen was reduced quantitatively to water, even at more positive potentials. The results are summarized in table 5. TABLE 5.-ELECTROREDUCTION WITH C O PORPHYRIN-COVERED ELECTRODES porphyrin Ea/V O2 reduced to H202 (%) monomer 0.30 69 dimer (six-atom link) 0.51 69 dimer (four-atom link) 0.72 <1 ~~ ~ ~~ * Potential where oxygen reduction current reaches 50% of its limiting value.They strongly suggest that a geometry favourable for forming a Co-0-0-Co com- plex is beneficial to the direct reduction of O2 to H20. This example illustrates not only the importance of an ensemble consisting of two cooperating atoms, but also the critical importance of the distance between them for optimum joint operation. THE TEMPLATE REQUIREMENT In the Encyclopaedia Britannica a template is defined as a pattern or gauge, used as a guide in shaping something. Also on the surface of a heterogeneous catalyst a molecule can be shaped by the action of a template. While zeolites are known to beW. M. H. SACHTLER 27 shape-selective by virtue of their three-dimensional structure, we shall concentrate here on template effects on (essentially) two-dimensional surfaces. For instance, if a molecule is formed with an asymmetric carbon atom, two configurations are possible which, in the absence of such a template, will have identical energies.Steric inter- action with structural features of the catalyst can, however, be the cause of different energies between the two configurations of the adsorbed molecule. Groenewegen et aZ.46 therefore assumed that the two transition states leading to either configuration already differ in energy. A well-known example is the polymerization of propylene. While the polymer is formed on the surface of, for instance, a TiC13 catalyst, every propagation step creates a new asymmetric carbon atom.If all the asymmetric carbon atoms in a given chain have the same absolute configuration, the product is called isotactic. Arlman and C o ~ s e e ~ ~ proposed models to explain the non-equiva- lence of the two adsorption complexes on y-TiC1,. An example where asymmetry is responsible for the high optical activity of the reaction product is encountered in the hydrogenation of methyl acetoacetate to methyl hydroxybutyrate : With a nickel catalyst " modified " with for example, one enantiomeric form of tar- taric acid, the two enantiomeric forms of methyl hydroxybutyrate are formed in unequal quantities. There is a direct relation between the configurations of the tartaric acid " modifier " and the product: upon replacing the modifier by its mirror image, the mirror image of the product is formed.The degree of enantioselectivity depends on the conditions of modification, including the type of alkali metal hydrox- ide used to adjust the optimum pH, and can be further increased by co-modifiers such as potassium bromide. Enantioselectivities >90% have been claimed. The hydrogenation can be carried out in the liquid or gaseous phase. After completely hydrogenating a given batch, the catalyst can be used again without marked loss in enantioselectivity. The phenomena may therefore help us to understand the multi- plicity of asymmetric compounds accidentally formed in earlier stages of the Earth's history. Research on enantioselective hydrogenation with modified metals has been carried out by Izumi et aZ.,4s Yasumori et aZ.,49 Klabunovskii et aZ.,50 Smith et aLS1 and Sachtler et aZ.46952 The details of the mechanism are still under study.The results found by Hoek et aZ.53 strongly suggest that the modification process is a corrosive chemisorption, favoured by an oxidizing atmosphere. In the case of nickel modified with tartaric acid, the surface acts as the stereoselective centre. Hydrogenation takes place on the central metal atom of the complex: the steric interaction of the ligands with the ad- sorbed molecule creates the energetic non-equivalence of the two transition states leading to the two enantiomeric product molecules. In support of this model Hoek et al. found that nickel catalysts modified with asymmetric nickel tartrate induce enantioselectivity of the same sign, and also to very much the same extent, as in the usual modification with tartaric acid.Temperature-programmed desorption data measured by YasumoriS4 further showed that the interaction of substrate and modi- fier is attractive, as would be expected on the basis of hydrogen bonds. When using amino acids as the modifying agents, analogous complexes are expec- ted, resulting from corrosive adsorption. In agreement with this view Friedmann28 CATALYST SELECTIVITY and Klabunovskii 55 found good correlations between the equilibrium constants of these complexes and the enantioselectivity. It is difficult to imagine that a metal ion in a complex with one or two tartrate ligands and sticking to a surface should still have the three free ligand positions available which are required for hydrogenation.It would seem much more realistic to assume that dissociation of dihydrogen takes place on the uncovered part of the metal surface, and that the hydrogen atoms then migrate to the sites where the sub- strate is adsorbed and hydrogenation takes place in two steps following the Horiuti- Polanyi mechanism. The same assumption has been used above by Biloen et aL3* to explain the results on the dehydrogenation of propane over highly diluted Pt-in-Au alloys. While the uncovered part of the metal surface thus plays a beneficial role in dissociating dihydrogen, it will also provide a reaction path for the non-selective hydrogenation of methyl acetoacetate. It is convenient, therefore, to distinguish two types of sites, briefly called the enantioselectivity and racemic sites.Only on the former type will the transition states for the two configurations of the half-hydrogen- ated state possess significantly different free activation energies. The extent of the t + H Rl + H - enantioselectivity of a modified catalyst then depends on: (1) the relative numbers of " enantioselective " and " racemic '' sites, (2) the relative turnover frequencies on each site and (3) the difference between the free activation energies for the R- and the S-configuration of the half-hydrogenated state on the enantioselective sites. Variations of the catalyst which enhance the enantioselectivity are consequently caused by either blocking of the racemic sites or improving the activation energy difference on the enantioselective sites.Until recently it was unknown how, for FIG. 12.-Conversion (0) and enantioselectivity (A) of methyl acetoacetate hydrogenation over tartrate-modified Ni/Si02 catalyst measured under standard conditions (80 "C; 1 bar H2; 20 h 0.1 g Ni) plotted against concentration of alkali metal salt (mmol per g-atom Ni) present.W . M. H. SACHTLER 29 instance, the selectivity increase due to salts of alkali metal halides is to be interpreted. Systematic studies by Bostelaar et aZ.56 have shown that with many of these “ co- modifiers ” the increase in selectivity is accompanied by a decrease in conversion rate. For NaBr, however, present during hydrogenation, it was found that the conversion remains constant, while the enantioselectivity increases with NaBr concentration, later passing through a maximum and decreasing as shown in fig.12. It is, therefore, concluded that with this co-modifier the stereochemistry of the template complex is changed by interaction with the alkali halide. CONCLUSIONS The chemical factors which render a homogeneous catalyst selective for a particular reaction also control the selectivity of heterogeneous catalysts. They are most clearly identified with metal organic complexes which, therefore, provide convenient models for the active sites in surfaces of solid catalysts, the latter usually excelling, however, by a much higher stability under reaction conditions. Ensembles of several contiguous atoms able to interact with substrate molecules are provided both by polynuclear metal organic complexes and by metal surfaces.The large multitude of coexisting reaction paths on surfaces of transition metals is, in part, due to the simultaneous presence of a wide variety of ensembles of different sizes and geometries. As a result, the conversion of hydrocarbons on such catalysts displays a highly complicated pattern. Some reaction paths have been identified by 13C labelling, others by reducing the ensemble sizes, which was achieved by diluting the active metal in the matrix of an inactive metal. Mononuclear metal organic complexes illustrate the catalytic possibilities of monoatomic metal sites in diluted alloys, sulphides and some oxides. For these catalysts the selectivity is dependent on the coordinative unsaturation of the adsorbing atoms, i.e.the number of missing ligands for full coordination, Three such ligand positions appear to be required for olefin hydrogenation, but two positions suffice for H-D exchange of olefins or their double-bond isomerization. In metal organic complexes the ligands have a rather strong influence on the bond strength of the chemical bond between the central atom and a new ligand, as is shown for porphyrins, in particular when they are used as electrocatalysts. The correspond- ing “ ligand effect ” of different metals in an alloy is much weaker. Ligands can, however, be decisive for the geometry of adsorption complexes, as is illustrated by the strong effect on the selectivity of oxygen electroreduction by dimeric porphyrins. More general is the “ template ” action of ligands which create energy differences for adsorption complexes (or adsorbed transition states) differing merely in geometry such as stereoisomers.A very striking illustration of this phenomenon is the high enantioselectivity of nickel catalysts “ modified ” with one of the enantiomers of, for instance, tartaric acid. The examples in this lecture were chosen in order to illustrate each one of these effects. Such a separation of variables is, however, unusual in industrial catalysts. All selectivity requirements have therefore to be considered in designing model catalysts of superior selectivity. F. M. Dautzenberg, J. N. Helle, P. Biloen and W. M. H. Sachtler, J. Catal., 1980,63, 119. R. J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R.Ruttinger and H. Kojer, Angew. Chem., 1959,7, 176. G . J. K. Acres, Platinum Met. Rev., 1970, 14, 78. ’ W. M. H. Sachtler, C. Backx and R. A. van Santen, Catal. Reu. Sci. Eng., 1981,23, 127.30 CATALYST SELECTIVITY ' E. K. Rideal, in Concepts in Catalysis (Academic Press, London, 1968); see also E. K. Rideal, Subutier Lecture, Chem. Ind., 1943, 62, 335. G-M. Schwab, Discuss. Furuday SOC., 1950, 8, 166. ' D. A. Dowden, Discuss. Furaday SOC., 1950, 8, 184. A. A. Balandin, in Multipletnuyu Teoriyu Kutuliza (Moscow University Press, 1970). W. M. H. Sachtler and J. Fahrenfort, Actes Congr. Int. Cutul. 2"" 1960, (Technip, Paris, 1961), W. M. H. Sachtler and P. van der Plank, Surf. Sci., 1969, 18, 62-79. W. M. H. Sachtler, Vide, 1973, 163, 19; V. Ponec, Surf. Sci., 1979, 80, 352.J. A. R. van Veen, J. F. van Baar, C. J. Kroese, J. G. F Coolegem, N. de Wit and H. A. Colijn, Ber. Bunsenges. Phys. Chem., 1981, 85, 693. pp. 831-863. l3 H. Arai and J. Halpern, Chem. Commun., 1971, 1571. I4 S. Siegel, J. Cutul., 1973, 30, 139. l5 A. Takeuchi, K-I. Tanaka, I. Toyoshima and K. Miyahara, J. Cut& 1975, 40, 94; K-I. Tanaka and T. Okuhara, Cutal. Reo. Sci. Eng., 1977, 15, 249; J. Cutul., 1980, 65, 1. D. W. Blakely and G. A. Somorjai, J. Cutul., 1976, 42, 181. at 3rd Int. Conf. Chem. and Uses of Molybdenum, 1979 (Ann Arbor, Michigan, 1980). A. L. Farragher, Adv. Colloid Interface Sci., 1979, 11, 3. Chem., Washington, D.C., 1977). l7 K-I. Tanaka and T. Okuhara, in Anisotropic Properties of M0S2 Single Crystals, paper presented I9 A.L. Farragher, in The Role of Solid State Chemistry in Catalysis (Am. Chem. SOC. Div. Petr. 2o T. Okuhara and K-I. Tanaka, J. Chem. Soc., Furaday Truns. I , 1979,75,7. 21 V. Ponec and W. M. H. Sachtler, Proc. Int. Congr. Cutul., 1970 (Elsevier, Amsterdam, 1971), 22 J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Cutul., 1972, 24, 283. 23 V. Ponec and W. M. H. Sachtler, J. Catul., 1972,24, 250. 24 J. A. Dalmon and G. A. Martin, J. Cutul., 1980, 66, 214. 25 G. A. Martin and B. Imelik, Surf. Sci., 1974, 42, 157. 26 A. Roberti, V. Ponec and W. M. H. Sachtler, J. Catul., 1973, 28, 381. 27 W. M. H. Sachtler and R. A. van Santen, Adu. Cutul., 1977,26,69. 28 M. Araki and V. Ponec, J. Cutul., 1976, 44, 439. 29 G. C. Bond and B. D. Turnham, J. Catal., 1976, 45, 128. 30 P. Biloen, F. M. Dautzenberg and W. M. H. Sachtler, J. Cutul., 1977, 40, 77. 31 F. G. Gault, V. Amir-Ebrahimi, F. Garin, P. Parayre and F. Weisang, Bull. SOC. Chim. Belg., 32 P. Parayre, V. Amir-Ebrahimi, F. G. Gault and A. Frennet, J. Chem. SOC., Faraduy Truns. I , 33 J. R. Anderson and N. R. Avery, J. Cutal., 1966, 5, 446. 34 G. Leclerq, L. Leclerq and R. Maurel, J. Cutal., 1977, 50, 87. 35 F. Garin and F. G. Gault, in: Chemistry and Chemical Engineering of Catalytic Processes, ed. 36 Y. Chauvin and J. L. Herrison, Mukromol. Chem., 1971,142, 161. 37 M. A. McKervey, J. J. Rooney and N. G. Samman, J. Catal., 1973, 30, 330. 38 J. M. Muller and F. G. Gault, J. Cutul., 1972, 24, 361. 39 J. K. A. Clarke and J. J. Rooney, Ado. Cutal., 1976, 25, 125. 40 Y. Miki, S. Yamada and M. Oba, J. Cutul., 1977, 49, 278. 41 J. A. Groenewegen and W. M. H. Sachtler, J. Cutul., 1974, 33, 176. 42 E. 0. Fischer, G. Kreis, G. Kreitner, J. Muller, G. Huttner and H. Lorenz, Angew. Chem., 43 H. C. de Jongste and V. Ponec, Bull. SOC. Chim. Belg., 1979, 88, 453. 44 J. F. van Baar, J. A. R. van Veen and N. de Wit, Electrochim. Actu, 1981, in press. 45 J. P. Collman, M. Marrocco, P. Denisevich, C. Koval and F. C. Anson, J. Electrounul. Chem., 46 J. A. Groenewegen and W. M. H. Sachtler, Proc. 6th Int. Congr. Cutul., 1976 (The Chemical 47 E. J. Arlman and P. Cossee, J. Cutul., 1964, 3, 99. 48 Y. Izumi, Angew. Chem., Int. Ed. Engl., 1971, 10, 871. 49 I. Yasumori, Y. Inoe and K. Okabu, in Catalysis, Heterogeneous and Homogeneous, ed. B. Delmon and G. James (Elsevier, Amsterdam, 1975), p. 41. E. I. Klabunovskii and A. A. Vedenyapin, in Asimetricheskii KatuIiz (Nauka, Moscow, 3980), and numerous papers quoted therein. paper 43, p. 645. 1979, 88, 475. 1980, 76, 1704. R. Prins and G. C. A. Schuit (Sijthoff & Noordhoff, Alphen a.d. Rijn, 1980), p. 351. Int. Ed. Engl., 1973, 12, 564. 1979,101,117. Society, London, 1977), paper B40. 51 G. V. Smith and N. Musoiu, J. Catal., 1979, 60, 184.W. M. H. SACHTLER 31 52 A. Hoek and W. M. H. Sachtler, Proc. 7th In?. Congr. Cutul,, 1980 (Kodansha Ltd, Tokyo; 53 A. Hoek and W. M. H. Sachtler, J. Cutal., 1979, 58, 276. Elsevier, Amsterdam, 1981), paper A25. 54 I. Yasumori, Proc. 6th Int. Congr. Cutal., 1976 (The Chemical Society, London, 1977), p. 1021. 55 Ya.D. Friedmann and E. I. Klabunovskii, Kinet. Katal., 1980, 21, 1199. 56 L. J. Bostelaar and W. M. H. Sachtler, unpublished.

ISSN:0301-7249    

DOI:10.1039/DC9817200007

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Selectivity of Metals and Alloys in Hydrocarbon Reactions BY MARTIJN W. VOGELZANG, MARIAN J. P. BOTMAN AND VLADIMIR PONEC Gorlaeus Laboratoria, Rij ksuniversiteit Leiden, P.O. Box 9502, 2300 RA Leiden, The Netherlands Receiued 1 I th May, 198 1 A classification of selectivity phenomena and factors determining the selectivities is presented in brief. The alloying effects, as established, are also briefly reviewed. It is shown that with neo- hexane it is possible to test the selectivity for various reactions as well as the preference of metals and alloys for the formation of particular chemisorption complexes. The metals Pt, Pd, Ir and Ni as well as the alloys Ni-Cu, Pt-Au and Pt-Ag have been tested by neohexane react ions with hydrogen. It is shown in this paper that alloying with a Group Ib metal causes in most cases a shift from hydrogenolysis to isomerisation and, whereas aj3 complexes are preferred by a pure metal, an ay complex formation is relatively promoted by alloying.Carbon seems to have a similar effect. Several recent review ~ a p e r s l - ~ and Proceedings of conferences4p5 show that a certain progress has been achieved in studies on the selectivities of metals. Some factors determining the selectivity have been identified, or at least their possible role has been indicated. To summarize the present state of affairs and to place this paper into the context of other selectivity studies it is convenient to start with the following classification. Following Bond,6 one can conveniently distinguish three types of selectivities : I.Selectivity SI in the reactions of mixtures (e.g. simultaneous hydrogenation of various alkyl benzenes) or of molecules with several functional groups of comparable reactivities (e.g. hydrogenation of crotonaldehyde) (2). 11. Selectivity SII in consecutive reactions (e.g. C2H2 -+ C2H4 --f C2H6 or C6H6 --f C6H1, -+ C6HI2) (++). 111. Selectivity SIII in parallel reactions (skeletal reactions of hydrocarbons, Within these groups catalysts may also reveal a stereo-selectivity for certain products or starting compounds. Such a separation of various selectivities sometimes appears to be only an abstrac- tion; in reality the various types of selectivities have to operate simultaneously. For example, the hydrogenation of acetylene on Pd is probably a reaction governed by SII selectivity which in this case is determined by thermodynamic factors-the heat of adsorption of reaction components.6 On Ir, in contrast to Pd, a proportion of the ethane is most probably formed by a parallel pathway where the intermediates do not leave the surface before ethane is formed.Then this is rather a case of SIII type selectivity, whereby the degree of dehydrogenation upon adsorption of C2H, is likely to play a decisive ro1e.1*2 The literature on selectivity problems l-’ mentions the following factors as being responsible for the selectivity of metals. (1) Chemisorption bond strength (heat of adsorption). The importance of this factor is particularly well documented for reactions under SI and SII.6 (2) Formation of multiple bonds with the surface.’ (3) Degree of dissociation of the bonds in chemisorbed species.The importance of this oxidations of olefins, various reactions of alcohols, amines, etc.) (,). f34 SELECTIVITY OF METALS factor has been shown for some reactions under SII and SIII.1*2 (4) Number of bonds formed with the surface and the number of metal atoms involved upon chemisorption of reaction components (" ensemble size ") which factor is closely related to factor (3) and probably operating in all three types of selecti~ity.l-~ The last two factors appeared to be the key factors in determining the selectivity in the skeletal (reforming) reactions of hydrocarbons also studied in this paper. How can the selectivity be influenced by alloying? Certainly not to the same extent in the various reactions of different types (1-111 above).The electronic structure of metals changes only moderately on alloying [see ref. (1)-(5) for a review] and so does the heat of adsorption.* Where the thermodynamic factors are most essential (as studied in the SII reactions), not very much can be expected from alloying with respect to the selectivity changes. In this case an application of gas additives or of multi- component catalysts other than alloys may be a more useful tool in influencing the selectivity.8 On the other hand, alloying is a suitable way to modify the selectivity in reactions where factors (3) and (4) are most important. Namely, by alloying the size (alloy : active/inactive metal) and the composition (two active metals) the nature of the ensembles of active sites can be easily be varied.The first effect, the variation in ensemble size, is also the dominating effect with alloys (Pt-Ag, Pt-Au, Pd-Ag, Ni-Cu, up to ca. 60% Cu) studied in this Cu-rich Ni alloys may be an example of the second effect, variation in ensemble composition (whatever the function of Cu may be). All f a c t ~ r s l - ~ mentioned above are in one way or another related to the electronic structure of metals and they may also be influenced by the particle size of the metals (and alloys), since they are more or less all influenced by the coordination of metal atoms. For example, the selectivity in formation of 3-carbon and 5-carbon inter- mediates of hydrocarbon isomerisation illustrates this ~taternent,~'~~ being very sen- sitive to both alloying and particle size. Various reactions reveal a different sensitivity for alloying and for diminishing the ensemble The very " sensitive " reactions are e.g., methanation, Fischer- Tropsch synthesis, skeletal reactions of hydrocarbons, ether formation from alcohols, etc.The " insensitive " ones are exchange reactions with deuterium and hydro/ dehydrogenations on the C-H, C-0 and most probably also on the N-H, S-H, etc. bonds. In work described in this paper a few " sensitive " reactions have been studied. The above-mentioned division of alloying effects reveals a great similarity with Boudart's l3 classification of structure-sensitive and -insensitive reactions, although the parallelism does not hold for all the reactions. Cyclopropane hydrogenolysis and methanation are examples where this parallelism does not hold.14 The alloying effect may also be more subtle than those already mentioned.The data on Pt alloys reveal that some reactions, such as isomerisation and hydrogenolysis, may run in parallel via different intermediates and sometimes with very different rates but to the same Moreover, while hydrogenolysis by a 2C-ap-mechanismf. is apparently the most sensitive reaction for the ensemble size, isomerisation is less sensitive and within isomerisation one of the pathways, namely that employing a 3C- ay-intermediate, is more sensitive than is the route using a 5C-intermediate.9Jo In this paper we intend to demonstrate how the preference of metals for a certain overall * Even such a sensitive tool as i.r.absorption spectra of CO reveals only very modest changes in the electronic properties of adsorption sites due to alloying; the main effect of alloying on the fre- quency of the i.r. CO band is simply due to dilution of a CO layer (as has recently been shown by Toolenaar and Stoop from our laboratory) by not coupling dipoles (different frequency!) residing on the atoms of the second alloy component. t This notation means two carbon atoms in neighbouring (a, /?I positions are involved in the reaction in que~tion.'~M. W. VOGELZANG, M. J . P . BOTMAN A N D V . PONEC 35 reaction (hydrogenolysis, isomerisation) as well as for certain intermediates (2C-@, 3C-ay) can be varied by alloying, particle size and carbon deposition. A suitable molecule to test the preference of metals and alloys for complexes involving either two or three C atoms is neohexane 1 6 9 1 7 (2,2-dimethylbutane) : C6 I c1-c2-c3-c4. I CS Initial tell us product distributions of this reaction (conversions must be kept under 1-2%) immediately how the molecule is attached to the surface upon adsorption and on running the reaction (see Experimental section for details).In addition, the pro- duct patterns also reflect in the overall reaction network, the possible presence of multiple reactions during one single sojourn of molecules on the surface. Using neohexane reactions, such an analysis of the selectivity behaviour has been applied to some metals (Pt, Pd, Ni, Ir) and alloys in this paper. EXPERIMENTAL CATALYSTS The preparation and structural properties of Pt (and Pt-Cu) catalysts on Si02 have already been described in detai1.16-18 The Pd and Pd-Ag catalysts were prepared as follow^.'^ Pd (Drijfhout, Amsterdam) was dissolved in boiling HN03 (a droplet of HC1 is needed) and then mixed with AgN03 (Merck) in the required proportions.This solution was used for impregnation (in a slurry) of SiOz (Merck); the metal loading was 10 wt.%. Pd-Ag alloys were sintered overnight in hydrogen at 650 K and the alloying checked by X-ray diffraction. The Pt-Au and Pd-Ag alloys were described in our earlier paper.20 The same holds for Ni-Cu alloys prepared from mixed carbonates.2' The Ir catalysts were prepared in two forms: Ir in small particles on Si02 and Ir bulk metal. Ir/Si02 was prepared with 1% metal loading from (NH4)21rC16 (Drijfhout, Amsterdam) using Si02 (Merck) as a carrier, by drying a slurry of the impregnated carrier (average Ir particle size ca.1.5 nm). If neces- sary, this catalyst was diluted further (l/lO, 1/1000) by Si02 in order to be able to work at high temperatures. The same dilution technique was applied to the pure Ni catalysts. The Ir catalysts were also prepared by mechanical mixing of crystals of (NH4)21rC16 with inert SiOz (loading 1 wt.%) and reducing afterwards. This procedure led to Ir metal crystals of a larger size. RATE A N D SELECTIVITY MEASUREMENTS The reactions of neohexane (puriss. grade, Fluka, Switzerland) were followed in a tubular continuous flow fixed-bed reactor working under differential conditions already described.21 The neohexane partial pressure was ca.40 Torr and the H2/neohexane ratio was 18. The flow rate was 9 cm3 min- in most experiments. The product analysis (g.1.c.) and calcula- tion of the various parameters characterizing the selectivity were as in our previous papers.16-21 DATA EVALUATION The rates and selectivity parameters were defined in conformity with our previous papers.'6-21 The physical consequence of the appearance of various products is most easily derived when the molecules undergo only one reaction per single sojourn on the surface. This is the case with Pt, Pd, Ni and Ir at the lowest temperatures. Table 1 shows the products derived from complexes which are attached to the surface, always through two carbon atoms (@, ay, "7' complexes). These complexes involve either two (2C-@) or three (3C-ay,36 SELECTIVITY OF METALS TABLE 1 .-RELATION OF PRIMARY PRODUCTS TO STRUCTURE OF COMPLEXES attached as products of single-step conversions ~ isomerisat ion h ydrogenol ysis EY‘, 3 G C 2,3-dimethylbutane methane, 2-methylbutane 2-me t hylpen tane ethane, 2-methylpropane UY, 1c-5c 3 -met h y 1 pent ane methane, 2-met hylbutane U P , 3c-4c methane, neopentane examples of products of some multiple reactions in adsorbed state $3, my’, my’ repeated methane only my’, m y repeated methane > ethane > other products uy’ followed by UP (into 2,2-dimethylbutane much propane without tripod-like complex or ay, my’ repeated or propane, ethane, as the first step) ethane and methane 2-methylpropane, ethane methane 3C-ay’) carbon atoms and one or more metal atoms.However, it is in principle possible and also experimentally observed that in some cases molecules undergo a multiple reaction during one sojourn on the surface. For example, when propane and butane are formed as primary products (i.e. also formed at the lowest conversions and even at contact time z --f 0) this can be due either to consecutive multiple splitting of complexes bound to the surface through two carbon atoms, or to simultaneous or consecutive splitting of two bonds in tripod-like complexes. Similarly, when excessive methane is formed at low conversions, multiple (1C) splitting is evidently taking place during each sojourn of the molecules on the surface, etc. RESULTS Some results representative of the particular metals and their alloys are collected in table 2.It can be seen immediately that the selectivity of metals for isomerisation Siso follows the order Pt > Pd > Ir > Ni, which is also the order of the decreasing ratio 3C-(ay + ccy’)/2C-ap, characterizing the formation of various complexes. It may be further concluded that Siso is also sensitive to the particle size. In the form of very small particles, Pt and Ir catalyse increased hydrogenolysis and less isomerisation. From the metals studied by this paper Pd is most easily self-poisoned by the running reaction. With this metal it is also most clearly observable that self-poisoning increases Siso and the ay’/ay ratio; the same effect is achieved by addition of Ag. Self- poisoning is evidently due to carbon deposition and formation of highly dehydro- genated, firmly bound species; for the sake of simplicity we shall call it “ carbon ”.As already mentioned above, the results for Ir indicate a similar role for carbon. Small Ir particles, which are more difficult to poison, reveal almost pure hydrogenolysis, while in the form of large-particle powder Ir is more easily poisoned and reveals a rather high isomerisation selectivity: not as high as Pt, but nevertheless Ir reminds one of Pt (see table 2) more than of Ni. Also the results on Ni-Cu alloys supply support for the idea that “ carbon ” influences the selectivity in hydrocarbon reactions.M . W. VOGELZANG, M . J . P . BOTMAN A N D V . PONEC 37 TABLE 2.-TYPICAL PRODUCT DISTRIBUTIONS FOR METAL AND ALLOY CATALYSTS total metal types of adsorption complexes loading no.(%) catalyst T/K “Y’iso “Y’hydr “Y’tot “Yiso “Yhydr “Ytot statistical random (I contribution by complexes 0.43 0.43 0.14 metals 1 2 3 4 5 6 alloys 7 8 9 10 9 6 6 9 9 1 12 16 10 Pt/SiOz; impregnation 563 Pt/Si02 (Euro-cat) 503 same as 2 (stable performance) 523 Pd/SiO, (fresh catalyst) 610 same as 3 (after self- poisoning) 610 Ir/SiO, (diluted 1 : 7 by SiO,) 497 Ir( black)b/ SiO, (diluted 1 : 30 by SOz) Ir( black)/ SiOz (after self-poisoning) 623 Ni powder 493 same as 6 (diluted 1 : 14 by SiO,) 603 Pt (82.3%)/ Ag (17.7%) Si02 665 Pt (4%)/ Au (96%) Pd (75%)/ Ag (25%) Ni (65%)/ c u (35%) 598 SiOz 648 SiO, 613 663 carrier-free powder 604 0.52 0.16 0.68 0.22 0.08 0.30 0.02 0.37 0.25 0.62 0.17 0.18 0.35 0.03 0.28 0.23 0.51 0.14 0.22 0.36 0.13 0.33 0.02 0.35 0.13 0.51 0.64 0.02 0.50 0.02 0.52 0.19 0.27 0.46 0.02 0.06 0.06 0.94 0.04 0.56 0.60 0.03 0.04 0.07 0.23 0.13 0.36 0.49 0.11 0.06 0.17 0.26 0.09 0.09 0.12 0.12 0.76 extensive multiple reactions 0.94 0.01 0.95 0.04 0.01 0.05 0.98 (0.01 0.99 0.01 (0.01 0.02 0.46 0.01 0.47 0.25 0.25 0.50 0.01 0.01 0.49 0.50 0.03 0.01 0.04 0.38 a Calculated for the case that the number of C atoms in the respective positions is decisive; Particle size of Pt catalysts from the X-ray line broadening: the rest up to loo%, multiple reactions.no. 1, 5 f 0.5 nm, no. 2, 1.4 f 0.2 nm.38 SELECTIVITY OF METALS With regard to the effect of Ag or Au on Pt or of Ag on Pd, the conclusion is straightforward: addition of a Group Ib metal promotes isomerisation and ay' ad- sorption and correspondingly suppresses other modes of adsorption and other reac- tion pathways.We observe that multiple reactions (i.e. repeated or successive conversions in the adsorbed state) take place to a greater extent than with Pt, Pd and their alloys. However, the general trend observed many times in the past1-6*11*12 is again confirmed: by alloying the activity is quite strongly suppressed but not equally for all reactions. The selectivity in isomerisation increases from 0 for Ni to several percent for alloys between 50 and 60% Cu (fig. 1 and 2). The extent of isomerisation is low but easily measurable at this level. Fig. 3 shows that the changes in the overall selectivity as well as in other para- meters characterizing the catalytic behaviour of these alloys (see below) are accom- panied by changes in the apparent activation energy calculated from the temperature coefficient of the overall reaction rate under standard flow, pressure and gas composi- tion.With Ni-Cu alloys the situation is more complicated. at. % Cu FIG. 1.-Activity of Ni and Ni-Cu alloys as a function of alloy composition. Activity defined as rate under standard conditions, at 571 K, rate in arbitrary units. The existence of multiple reactions makes it more difficult to analyse the preference that Ni and Ni-Cu alloys (or Ir) have for the formation of various complexes at higher temperatures. Only at the lowest temperatures is the conclusion straightforward : Ni (and Ir) strongly prefer to split the molecule at the thermodynamically unfavour- able but evidently kinetically most favourable place between C(3) and C(4), most likely starting from the formation of 2C-olp type complexes.However, when Ni is self-poisoned or when Ni is alloyed with Cu, and measurable rates (still at very low overall conversions, a few percent with alloys and <2% with Pt and Pd) are achieved only at temperatures ca. 80 K higher than those at which pure Ni can be measured,M . W. VOGELZANG, M. J . P . BOTMAN A N D V . PONEC 39 - -\ \ 0 20 40 60 00 at. % Cu FIG. 2.-Selecti~ity'~J~ in hexane and neohexane reactions as a function of alloy composition. T = 573-603 K. n 200 2 150 -= r;l" f 100 ! 1 I I I 0 20 40 60 80 at. % Cu FIG. 3.-Apparent activation energy calculated from the temperature coefficient of the overall conversion as a function of alloy composition.product patterns clearly show that multiple conversion or adsorption takes place before the molecules leave the surface and we also observe that complexes other than ab are being formed. We attempted to characterize the catalytic behaviour under these conditions in the following way. First consider the data obtained with pure Ni for the lowest temperatures. In the low T limit almost equal amounts of methane and neopentane are formed, viz. 41% of each [see fig. 4(a)J. Comparable data for Ni at high T and for the various alloys show a similar low-T limit of equal concentrations of CH, and neo-C5, always around 42%. Let us define a parameter characterizing the deviations from this limit: [CH,] - 42 f= 42 - [neopentane]' If each missing neopentane molecule (due to multiple reactions at higher temperatures) supplied only one CH, and one isobutane molecule, f would be unity; the whole40 SELECTIVITY OF METALS multiple reaction would be under these conditions solely induced by a 3C-ay complex, which at these temperatures apparently reached a reactivity already comparable with that of the 2C-ap complexes.If, on the other hand, each molecule of neopentane which started to react beyond the adsorbed neo-C, stage were completely broken down to CH4, f would be 9 1. For example, if 10% of the neopentane molecules (42 -+ 32%) were all converted into methane,f = 2.4. If other molecules are formed (from T K FIG. 4.-Product patterns as a function of temperature. From left to right: (a) pure Ni measured at low temperatures; (b) pure Ni diluted by SiOz and self-poisoned by the running reactions (notice that methane + neopentane, i.e., multiple reactions are also running at the lowest conversions); ( c ) alloy of indicated composition (increase in methane < decrease in neopentane; this indicates that other than methane molecules are formed, i.e., the role of cry is larger here).For the sake of simplicity, experimental points are shown only for one case, neohexane) than methane and neopentane, then f < 1. In any case, the presence of extensive multiple reaction reveals that, at a certain stage and at least once, an ay type bonding occurs and, in particular, these complexes must be more abundant when We can also use the so-called multifission parameter Mf introduced earlier21 for the characterization of the catalytic behaviour of alloys.The conclusions are the same as withf, as can be seen immediately. Consider the case, first, that each step of the multiple reaction destroying adsorbed neopentane releases just one methane. This means that one may assume that the reaction goes like f < 1. neo-C, -+ neo-C5 + C1 --f isobutane + 2C1 --f propane + 3C1 -+ . . . employing a sequence of complexes like 2C-aP -+ 3C-ay’ -+ (either 2C-ap or 3C-a7 with isobutane) etc. . . . . Again, the mere existence of the multiple reaction in the adsorbed state indicates that the 3C-ay’ and 3C-ay complexes are being formed at a certain stage, in any case. A parameter Mf defined as 6 2 Ci(6 - i) M -- i = 2 - [CH,] (experimental) could be near to unity if the reaction were a pure stepwise splitting off of C1 units. If adsorbed neopentane were broken down to C, species, M , < 1 ; if molecules other than methane were formed by multiple reaction, Mf 9 1.Reactions leading to Mf > 1 like neo-C6 --+ isobutane and ethane, or isomerisation into 2-methylbutane followed by splitting into two propanes, etc., all comprise at least one formation of aM. W. VOGELZANG, M. J . P . BOTMAN AND V . PONEC 41 3C-cry complex. Fig. 5 shows that in the range from 0 to 60-70% Cu the parameter Mf increases; correspondingly f decreases, i.e. the role of ay complexes increases on alloying. At still higher concentrations of Cu, Mf decreases and f increases again. The increase in Mf with Cu% in the region 0-60% Cu has already been observed with hexane and methylcyclopentane.21 The decrease in Mf at Cu > 70% has already been observed with hexane.21 2 .o 0.5 V I I I 1 - 0 20 40 60 at.% Cu FIG. 5.-Fission parameters f and Mf (see text for definitions) as a function of alloy composition. The bars indicate the variations of parameters with temperature for a given alloy. These variations of Mf and f indicate an increasing role for a7 complexes in this region of concentration. DISCUSSION It is easy to see that metals differ in their behaviour in the hydrocarbon, or more specifically neohexane, reactions. It is tempting to correlate this with the position of the metal in the periodic system, but it is most difficult at the moment to identify definitely the reasons leading to the mentioned difference in the catalytic behaviour of metals.Let us postpone this question to the end of the discussion and start with some more obvious conclusions. Alloys of Ni and Cu reveal that alloying considerably suppresses the extent of multiple reactions and simultaneously the role of the my-type of bonding increases. The extent of multiple reactions is inter alia characterized by the divergence of the C, and neo-Cs lines in fig. 4. The analogous divergent lines, when compared with pure Ni, are shifted by alloying by ca. 80 K to higher temperatures. For pure Ni highly diluted by inert SiOz and self-poisoned by a running reaction, the divergent lines show a much wider gap at the low-temperature side. With pure Ni apparently even at the lowest measurable conversions the extent of multiple reactions is high, while it is low42 SELECTIVITY OF METALS on alloys (see fig.4). The data shown in fig. 4 also reveal another interesting feature: with increasing temperature, the contribution of the reaction leading to isobutane (3C-ccy) increases and it is always higher at comparable conversions than on pure Ni. This is another piece of supporting evidence for the conclusion drawn above, that suppression of the 2C-aP splitting by alloying gives more chance for a 3C-ay splitting to show up at temperatures where this becomes possible. The dramatic drop in activity and the selectivity shift towards isomerisation by alloying has already been explained by the role of the ensemble size.'-5 This ex- planation can be applied to the present results on neohexane as well.However, it is necessary to have an additional explanation for the fact that with hexane much higher isomerisation selectivities are reached than with neohexane. We believe that this difference is amongst other effects due to the extent of the carbon layer kept deposited on the surface under a steady-state reaction of, respectively, hexane or neohexane (see below for more support for this). We suggest that in the first case the extent of the carbon deposition is higher and so correspondingly is the value of Siso. Above a Cu concentration of ca. 60% Cu all trends in selectivity shifts (Siw, Mf) appear to turn back to the Ni-like behaviour, only the activity is much lower here, even considerably lower than with the alloys of 10-60% Cu.The catalysts with Cu > 60- 70% are also rather unstable and easily poisoned. Therefore, the points for Cu > 60- 70% were not trusted (see dotted line) in an earlier paper.21 However, we con- sistently find this behaviour with various reaction components and various alloys of Cu. Having now experience with Pt-Cu alloys in neohexane and other hydrocarbon reactions16*18 we are inclined to explain the behaviour of the catalysts in this range of percentage of Cu by assuming the presence of mixed Ni-Cu ensembles. They should isomerize less than the small isolated (by Cu and carbon) ensembles of Ni and thus favour splitting. They split hydrocarbons much more slowly than pure Ni ensembles, but they are active. The picture suggested is as follows.When ca. 10% Cu is added to Ni, the surface concentration of Ni drops to 10 & 5% Ni and varies only margi- nally between (say) 10 and 70% bulk CU.'~ However, Ni atoms in the surface form part of the three-dimensional clusters 1~23-26 decreasing in size with percentage of Cu so that the clusters project smaller and smaller two-dimensional ensembles into the surface. In the above-mentioned region of Cu concentration fewer and fewer Ni clusters make contact with each other, which leads to a decreasing ferromagnetic saturation moment of the Ni-Cu alloy^.^^-^^ At around 60% Cu the alloys become paramagnetic and remain so with further dilution. We suggest that somewhere between 60 and 80% Cu the frequency (and activity) of bigger pure Ni ensembles in the surface required for isomerization and hydrogenolysis are made negligible by that dilution and the mixed ensembles get a chance to manifest their own role in an overall reaction.Between 0 and 60-70% Cu in bulk the average ensemble size of Ni decreases, and isomerization and the role of 3C-ay complexes increase. The transition from a kinetically easier reaction (terminal ZC-oc/3 splitting) to reactions requiring more activation (internal 3C-ccy' splitting of the weakest bond) is reflected in an increasing activation energy, as shown in fig. 3. A similar picture apparently holds for other hydrocarbons. 21 Small particles of metals, as a rule less self-poisoned by running hydrocarbon reaction^,^' reveal a lower isomerization and higher hydrogenolysis selectivity at low temperatures than the same metals in a bulk massive form.Carbon deposited on the surface evidently increases the isomerization selectivity and decreases the selectivity in hydrogenolysis of neohexane. (Note that with this molecule no dehydrocyclization can occur in one reaction step: with hexane2' this is possible.) Carbon also causesM. W. VOGELZANG, M. J . P . BOTMAN AND V . PONEC 43 shifts in the preference for the formation of various complexes from neohexane and it is interesting to see that this is a quite general phenomenon.” Moreover, in a very suggestive way the effects of carbon remind one of the effects of alloying transition metals with a Group Ib metal. What could be the reason? It is known from the i.r. spectra of adsorbed CO that alloying suppresses delocalized binding of carbon (CO) with several transition metal atoms in the s ~ r f a c e ~ ~ - ~ ~ and favours binding to indivi- dual surface atoms.In other words, alloying with Group Ib metals suppresses bind- ing in the multi-coordinated “ valley ” positions and favours the “ summit ” position. Deposition of carbon30 or on Ni (or oxygen on Pt or Rh32) causes the same effect as alloying with Cu. This is summarized by fig. 6 . deactivation the valley position hydrogenolysis -+ isomerisation (dehydrocyclisation) ++ XY activity in multisite reactions + FIG. 6.-Deactivation of the “ valley ” positions leads to the indicated changes by alloying or de- positing carbon or sulphur. The results which we have just mentioned as well as those in table 2 led us to the formulation of the following conclusions.(a) If the structure, state (“ C ”) and composition (alloys) of the surface allow it, then with a molecule like neohexane, Ni and Ir always prefer the 2C-@ complex formation, and splitting of the C-C bond is easier when a valley position is available among Ni or Ir atoms. Alloying with a Group Ib metal invalidates the valley position and in this way promotes the reactions which occur (also) on top of the surface atoms. (b) Pd and Pt easily covered by carbon under the reaction conditions reveal much less of the 2C-ap splitting, although the formation of a/3 complexes as seen at a low temperature (100 “C or more below the temperature of measurable hydrogenolysis) by the exchange reaction with deuterium is possible with these metals.Therefore, the preference of Pd and Pt for 3C-ay complexes over 2C-@ complexes at higher tempera- tures is partially, or even mainly, due to the carbon on the surface of these metals. It may be expected that carbon blocks the sites, which are good for the q? splitting. We identify these as the “ valley ’’ positions. Pd and Pt also show the lowest activity in hydrogenolytic splitting (of all metals) and in other reactions, as pointed out by This fact is additional support for the last statement.44 SELECTIVITY OF METALS (c) Ir shows a high preferential 2C-@ activity with neohexane, but at the same time its Mf is high and with hexane it is still higher at low temperatures (Mf = 7.8-4.9 at 425-525 With this metal 2C-ccp and 3C-ccy complexes most probably are not sharply differentiated as far as the requirements with regard to the activation are concerned.V. Ponec, in Electronic Structure and Reactivity of Metal Surfaces, ed. E. G. Derouane and A. A. Lucas (Plenum Press, New York, 1976), p. 537. ’ V. Ponec, in Progress in Surface and Membrane Science, ed. D. A. Cadenhead and J. F. Daniel (Academic Press, New York, 1979), vol. 13, p. 2. J. K. A. Clarke and J. J. Rooney, Adv. Catal., 1976, 25, 125. Chemistry and Chemical Engineering of Catalytic Processes, Proc. NATO Adv. Studies Inst., ed. R. Prins and G. C. A. Schuit (Sijthoff and Noordhoff, Amsterdam, 1980), pp. 337-349. Proc. Meeting on Catalytic Reactions of Alkanes and Hydrogen on Metals, in Bull. SOC. Chim. Belg., 1979, 88. G. C. Bond, in Catalysis by Metals (Academic Press, London, 1962).C. Kemball, Catal. Reu., 1971, 5, 33. J. A. Don and J. J. F. Scholten, Faraday Discuss. Chem. SOC., 1981, 72, 145. J. M. Dartiques, A. Chambellan and F. G. Gault, J. Am. Chem. Soc., 1976,98, 856. V. Ponec, Int. J. Quantum Chem., 1977, 12, 1. V. Ponec, Surf. Sci., 1979, 80, 352. (The Chemical Society, London, 1976). lo H. C. de Jongste, V. Ponec and F. G. Gault, J. Catal., 1980, 63, 395. l3 M. Boudart, in Proc. 4th Int. Congr. Catal., ed. G. C. Bond, P. B. Wells and F. C. Tompkins l4 J. W. E. Coenen, W. M. T. M. Schats and R. Z. C. van Meerten, in ref. (3, p. 435. l5 H. C. de Jongste and V. Ponec, in ref. (4), p. 337 and ref, (9, p. 453. l6 M. J. P. Botman, H. C. de Jongste and V. Ponec, J. Catal., 1981, 68, 9. I7 V. Ponec, in The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, ed. D. A. I8 H. C. de Jongste, F. J. Kuijers and V. Ponec, in Proc. 6th Int. Congr. Catal. ed. G. C. Bond, l9 E. K. Poels, MSc Thesis (Rijksuniversiteit Leiden, 1979). ‘O H. C, de Jongste, F. J. Kuijers and V. Ponec, in Scientific Basis of the Preparation of Catalysts, ed. B. Delmon, P. A. Jacobs and G. Poncelet (Elsevier, Amsterdam, 1976), p. 207. V. Ponec and W. M. H. Sachtler, in Proc. 5th Int. Congr. Catal., ed. J. Hightower (Miami Beach, 1972), vol. 1, p. 645. King (Elsevier, Amsterdam, 1982), vol. 4, to be published. P. B. Wells and F. C. Tompkins (The Chemical Society, London, 1977), vol. 2, p. 915. ’’ F. J. Kuijers and V. Ponec, Surf. Sci., 1977, 68, 294. 23 J. Vrijen, Thesis (Rijksuniversiteit Utrecht, 1977). 24 J. P. Perrier, B. Tissier and R. Tournier, Phys. Rev. Lett., 1970, 24, 313. 25 C. G. Robbins, H. Claus and P. A. Beck, Phys. Rev. Lett., 1969, 22, 1307. 26 E. Vogt, Phys. Stat. Sol. (B), 1972, 50, 653. ’’ P. P. Lankhorst, H. C. de Jongste and V. Ponec, in Catalyst Deactivation, ed. B. Delmon and 28 Y. Soma-Noto and W. M. H. Sachtler, J. Catal., 1974, 32, 315. 29 Y. Soma-Noto and W. M. H. Sachtler, J. Catal., 1974, 34, 162. 30 W. L. van Dijk, J. A. Groenewegen and W. M. H. Sachtler, J. Catal., 1976, 45, 277. 31 P. T. Rewick and H. Wise, J. Phys. Chem., 1978, 82, 751; J. T. Yates and C. W. Garland, 32 E. K. Poels and F. C. J. M. Toolenaar, personal communication (Rijksuniversiteit Leiden, 33 G. C. Bond, in Mechanisms of Hydrocarbon Reactions, ed. F. Merta and D. Kallo (Elsevier, 34 C. T. J. Wreesman, MSc Thesis (Rijksuniversiteit Leiden, 1980). F. G, Froment (Elsevier, Amsterdam, 1980), p. 43. J. Phys. Chem., 1961,65, 617. 1981). Amsterdam, 1975), p. 49.

ISSN:0301-7249    

DOI:10.1039/DC9817200033

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Selectivity in Mechanistic Studies of Alkanes on “ Classical ” and “ Inorganic Cluster-derived ” Platinum Catalysts BY ORFAN ZAHRAA, FRANCOIS GARIN AND GILBERT MAIRE Laboratoire de Catalyse et Chimie des Surfaces, E.R.A. 385-C.N.R.S., UniversitC L. Pasteur, 4 rue Blaise Pascal, 67008 Strasbourg, France Received 13th May, 1981 Skeletal isomerization of 2-methylpentane (2MP) and hydrogenolysis of methylcyclopentane (MCP) have been studied over a series of Pt/A1203 catalysts, mono- or bi-metallic “ cluster-derived ” catalysts and well-characterized single-crystal faces of platinum. I3C labelling allowed an estimation of the relative contributions of cyclic or bond-shift mechanisms in isomerization. The bond-shift mechanisms predominate on large crystallites and on single-crystal faces of platinum.The highly dispersed catalysts can be easily simulated by “ inorganic cluster-derived catalysts ”. “ Chini’s cluster-derived catalysts ” showed differences in selectivity due to the number of active sites. “ Parshall and Wilkinson cluster-derived catalysts ” have the same catalytic behaviour as industrial bimetallic Pt-Sn catalysts. New platinum catalysts derived from Chatt clusters led to a very selective demethyl- ation and to a total disappearance of the cyclic mechanisms in spite of the very high dispersion of these catalysts (as z 20 A). Geometric and electronic factors are taken into account to explain the differences in the selectivities obtained on “ classical ” and “ inorganic cluster-derived ” catalysts. In many cases the activity of catalysts even in apparently simple reactions is determined by their selectivity properties.Therefore it is essential to focus attention on selectivity problems. Kinetic measurements of isomerization of pentanes v 2 coupled to studies of 13C labelled pentanes and hexanes on supported platinum- alumina catalysts have shown the existence (i) of two cyclic mechanisms (selective and non-selective) and (ii) of two bond-shift mechanisms involving one or several platinum atoms. The relative amount of each mechanism depends on the metal particle size.3 The bond-shift mechanisms predominate on large crystallites, as seen on 10% Pt- A1203 and on low- and high-index faces of single crystals of platin~rn.~*~ New approaches to the preparation of supported metallic catalysts by the decomposition of surface organometallic complexes have made it possible to prepare highly dispersed particles of transition metals used to study the dependence of the catalytic properties on the size of the metallic particles.The decomposition of surface organometallic complexes also allow the preparation of bimetallic catalysk69 On “ Chini’s cluster- derived ” catalysts differences in selectivity due to the nature and amount of active sites have been observed.’ The cyclic mechanism in the reaction 2-methylpentane + 3-methylpentane increases with the percentage of surface metal particle sizes ( 5 A < ds = %nidi2/Cnidi2 < 10 A). On the other hand the cyclic mechanism in the reaction 2-methylpentane --f n-hexane decreases with the percentage of surface metal particle sizes in the range 5-10 A.By using “ new platinum catalysts ” derived from Chatt clusters deposited on alumina we have been able to isolate a bond-shift mecha- nism via a metallocyclobutane species as the precursor leading either to isomerization or to selective demethylation. Almost all the changes in selectivity observed with “ classical ” catalysts have been correlated with changes in particle-size distribution,46 SELECTIVITY I N ISOMERIZATION OF HEXANES but drastic changes in selectivity occur when using new catalyst systems prepared by the decomposition of surface organometallic complexes. Results given by " classi- cal " mono- or bi-metallic catalysts derived from organometallic complexes for hydro- genolysis, isomerization and demethylation are discussed in the present paper.EXPERIMENTAL CATALYST PREPARATION PREPARATION OF THE CLASSICAL Pt-A1,03 CATALYSTS A series of six Pt-A1,03 catalysts was prepared by impregnating an inert alumina obtained from Woelm with a chloroplatinic solution. In all cases reduction was performed at 100 "C with a hydrogen flow rate of 10 cm3 min-' and completed at 200 "C for 48 h. Three samples of alumina were distinguished, A, A' and B', differing in the number of hydroxyl groups exchangeable with de~terium.~ Besides this series of home-made catalysts, an industrial 5% Pt-A1203 catalyst, interesting for its narrow particle size distribution (around 30 A) was purchased from Matheson- Coleman (MC). PREPARATION OF THE BIMETALLIC Pt-Sn CATALYSTS These were prepared by consecutive impregnation of an alumina A' by stannous chloride and chloroplatinic acid.Two Pt-Sn/A1203 catalysts were prepared by decomposition of Wilkinson or Parshall bimetallic Pt-Sn inorganic complexes : [Pt(SnC13)2C12](NMe4)220 and [Pt3(SnC13)2](C8H12)3,11 respectively. They were deposited on the same alumina followed by reduction under mild conditions as described above. PREPARATION OF ORGANOMETALLIC CLUSTER DERIVED CATALYSTS These were obtained either from Chini's clusters l2 [Pt3(,u2C0)3(CO)3]~- or from Chatt clusters13 Pt3(p2CO),L4 with L = PPh3 or PEt, deposited on B' alumina, previously de- gassed (12 h, 25 "C) and destroyed by air, then reduced by hydrogen for 12 h at 300 "C. CHARACTERIZATION OF THE CATALYSTS The characterization of the catalysts was achieved by hydrogen chemisorption and trans- mission electron microscopy using the extractive replica technique and a Philips microscope, which led, respectively, to the determination of the dispersion a = H/Pt and d,/A and to the particle size distribution dJA.APPARATUS A N D PROCEDURE The differential reactor and the experimental procedure for catalytic experiments have been described elsewhere. The experiments on single crystals at atmospheric hydrogen pressure were performed in a Varian LEED chamber with 4-grid optics as already des~ribed.~ RESULTS AND DISCUSSION In the isomerization of 2-methylpentane two isotopic varieties, labelled on carbon 2 or on carbon 4, are required to determine the contribution of the bond-shift and cyclic mechanisms in the formation of 3-methylpentane and n-hexane, respectively, (fig.1). The I3C labelled hydrocarbons (2-methy1[2-l3C]pentane and 2-methyl- [4-I3C]pentane) were prepared by synthetic methods already described.140. ZAHRAA, F. GARIN A N D G. MAIRE 47 1/2 FIG. 1 ,-Formation of (a) 3-methylpentane and (b) n-hexane. A. ISOMERIZATION O N WELL-CHARACTERIZED PLATINUM SURFACES The contact reactions of 2-methy1[2-l3C]pentane were carried out at 350 "C on the vicinal face Pt(S) [6( 1 1 1) x (loo)] and a platinum polycrystalline foil; the results were compared with those obtained on two low-dispersion catalysts, 5 and 2.25% Pt-Al,O,, with a dispersion a of 0.2 and 0.05, respectively, and a catalyst with a high dispersion, 0.2% Pt-A1203 (a = 1). Table 1 shows the location of the carbon-13 in the 3- methylpentane, the percentage of each C6 isomer and the selectivity factor S defined as the percentage (in moles) of all C6 isomers in the reaction products.From these data the percentages of bond-shift and cyclic mechanisms can be deduced. It is also possible, by using the ratio 3MP/n-H, to distinguish between the selective and the non-selective cyclic-type isomerizations, which involve the selective or the non- selective hydrogenolysis of the methylcyclopentane intermediate. It should be emphasized that the well-characterized Pt surface and the Pt polycrystalline foil have catalytic behaviour quite similar to the low-dispersion catalysts, the predominant mechanism being bond-shift, while on high-dispersion catalysts (1.2% Pt-Al,O,), the non-selective cyclic mechanism at as low temperature as 254 "C predominates.This clearly indicates that the small metal aggregates on high-dispersion platinum catalyst no longer exhibit metallic properties but may rather be considered as molecular particles. B. ISOMERIZATION ON Pt-Sn CATALYSTS AND BIMETALLIC CLUSTER- DERIVED CATALYSTS Alloying Pt with a less active metal such as Sn had the same effect as decreasing the metal particle size.7 On the other hand, when the atomic ratio Pt/Sn is near unity we can see (table 2) that tin blocks the active sites (isomerization and hydrogenolysis reactions were conducted at temperatures up to 360 "C). The number of active sites selectively associated with the cyclic mechanisms of isomerization and the ratio H/Pt decrease simultaneously.Believing that accumulation of tin on the platinum crystal- lites could be due to the formation of a transient Pt-Sn complex, we compared these results with those obtained on Pt-Sn catalysts prepared from clusters. The most striking results are presented in table 2. The " cluster-derived " catalysts simulated very well the Pt-Sn catalysts but not at all the low- or high-dispersed monometallic catalysts. These experiments demonstrate that a transient Pt-Sn complex arises during the preparation of the catalysts when the Pt-Sn ratio reaches unity.TABLE 1 .-ISOMERIZATION OF 2-METHYL[2-13C]PENTANE ON PLATINUM CATALYSTS C6 products (%) mechanisms cat a1 ys t H/Pt ds/A O/"C S 7 3MP n-H MCP NSH BS SC NCS (%) Pt(S) [6(111) x (lOO)] 350 72.1 13.5 86.5 19.7 9.5 42.9 31.5 86.5 9.3 4.2 Pt polycrystalline foil 350 69.9 49 51 15.4 11.0 43.4 36 51 31.5 17.5 Pt-AlzOj 2.25% A 0.05 100 350 94.5 30 70 28.5 12.9 53.1 19 70 24 6 Pt-A120,5% (MC) ' 0.2 34 350 94.2 34 66 31.0 16.5 46.7 26.5 66 25 9 Pt-Al203 0.2% A' 1 20 254 60.5 83 17 20.0 340 6.5 100 17 0 83 MC : Matheson-Coleman catalyst.TABLE 2.-kOMERIZATION OF LABELLED HEXANES ON Pt-Sn CATALYSTS : COMPARISON WITH CLUSTER-DERIVED CATALYSTS Wilkinson Pt-Sn 0.5-0.6 0.5 0.05 80 385 16 Parshall Pt-Sn 8-3.3 1.4 12 360 9 Pt-Sn 0.8-0.7 0.7 - 55 385 15 Pt-Sn 7.5-4.5 1 0.05 45 405 8 - ~~ 22 360 0.7 32 385 0.55 2 385 0.65 Pt 8.5% Pt 0.2 - 0.05 90 254 16 72 220 2.4 - 1 20 254 83 100 220 0.40. ZAHRAA, F. GARIN AND G. MAIRE 49 c. ISOMERIZATION O N Pt CATALYSTS AND MONOMETALLIC CLUSTER- DERIVED CATALYSTS Following the work of Gault et aL3p9 on the correlation between metal particle size and reaction mechanisms, which put forward that in the isomerization of 2- methylpentane to 3-methylpentane the percentage of cyclic mechanism remains constant for catalysts of low and medium dispersions but increases sharply as soon as platinum particles <10 A are present in the catalyst, we focused our attention on the catalytic behaviour when very small particles <10 A are present.Assuming that the aggregates in the range 5-10 A are responsible for the cyclic mechanism we should obtain a correlation between the percentage of cyclic mechanism and the percentage of surface particle sizes in the range 5-10 A, the latter carefully determined by transmission electron microscopy.* In table 3 the more significant catalytic results are reported as a function of the percentage of surface particle sizes in the range 5-10 A.We can see that values of d,/A determined by hydrogen chemisorption and ds/A by t.e.m. are in good agreement except for the catalyst sintered at 400 "C. The following observations must be emphasized : (i) In the reaction 2-methylpen- tane to 3-methylpentane the percentage of cyclic mechanism increased with the percentage of surface particle sizes in the range 5-10 A, and this is true whatever the catalysts used : classical and cluster-derived catalysts from Chini's clusters. (ii) In the reaction 2-methylpentane to n-hexane two distinct phenomena were shown. On classical Pt-A1203 catalysts the cyclic contribution increased with the amount of sur- face particle sizes; on the other hand the cluster-derived catalysts from Chini led to a decrease of the percentage of cyclic mechanism.In view of these results we can first conclude that there is no influence from the support (the Euro-Pt/Si02 6.19% Pt15 fits well in table 3): only metal aggregates in the range 5-10 A are responsible for the cyclic mechanism. In the case of the Pt-Sn catalyst derived from the Parshall cluster we may assume that only 2% of the 26.6% of particles in the range 5-10 A are active for the isomerization, which confirms our hypothesis that the face and edge atoms of the crystallites are blocked by tin, which decreases the number of active sites. Furthermore, note the differences in selecti- vity for the reaction 2-methylpentane -+ n-hexane for catalysts having the same mean particle size but prepared from inorganic clusters.Experimental evidence seems to indicate that electronic rather than geometrical factors should be considered. This invocation of electronic factors is reinforced by the results of a comparative study made with various supported-metal catalysts of similar dispersions. The nature of the predominant mechanism depended upon the metal, being a bond-shift on platinum, a non-selective cyclic mechanism on palladium and a selective cyclic mechanism on iridiurn.I6 D. NEW PLATINUM CATALYSTS DERIVED FROM CHATT CLUSTERS In table 3 we also mentioned the results obtained for a Chatt cluster-derived cata- lyst (29 Chatt, 2-3% Pt).The first observation is that no relationship exists between the percentage of surface particle sizes in the range 5-10 A and the percentages of cyclic mechanisms. However, the selectivity is completely reversed in the case of its very high ability to perform demethylation, as reported in table 4 for the 21 and 28 Chatt catalysts. The products formed by hydrocracking predominate for the con- tact reactions of 2-methylpentane, methylcyclopentane and 1,2-dimethylcyclopentane.TABLE 3.-ISOMEIUZATION OF LABELLED HEXANES ON VARIOUS PLATINUM CATALYSTS catalyst 5% MC Parshall 7.1% Pt 4.1% pt 5.5% Pt 1 co 2.25% Pt EWo-Pt 0.56% Pt 1 Cs 29CH 2-3% Pt 2.5% Pt 1.1% Pt 1c2 = 0.2% Pt 0.4% Pt 1 C3 41 34 254 A’ - 12 360 B’210h 24 36 254 Si02 8.5 19 254 B’210h 15 17 254 B’ - 20 275 A 212 100 275 B’ 20 23 275 B‘ - 22 (25) 300 B’210h 12 15 254 B’ 18 17 275 B’ 16 15 275 A’ 8.5 20 254 0 26.6 2.2 2.2 3.0 4.5 4 .6 5.7 7.6 8.9 9 17.6 39.4 7 9 16 23 30 44 44 57 62 67 80 83 8 (54) 52 90 87 86 92 - 92 90 4 (98) 97 - 82 100 a B’ 210 h is an alumina A’ which has been calcined 210 h at 600 “C. B’ is an alumina similar to the B’ 210 h. “Cluster-derived catalysts” from Chini’s clusters [Pt3(pCO)3(CO)J]2n- with n = 2, 3,4 and 5. “C in HS. ’ Cluster-derived catalyst from Chatt’s clusters (L = PPh3). In brackets are the results after exposure of the catalyst to air 16 h, 200 “C. MC: Matheson-Coleman catalyst. Catalyst previously reduced at 200 “C, sintered at 400 TABLE 4.-sELECTIVITY IN DEMETHYLATION REACTION FOR CHATT CLUSTER-DERIVED CATALYSTS starting molecules 2-met hyl pen tane methylcyclopentane 1 ,2-dimet hylcyclopentane catalysts A1203 demet hylat ion demethylation demeth ylat ion internal fissions hydrogen01 ysis hy drogenol y sis 2.5% Pt B’ 210 h 28 Chatt 1.7% Pt (L = PEt,) 21 Chatt 2-3% Pt (L = PEt,) 1 C3 Chini Pt = 0.4% B’ B’ B’ 1.3 1 .o 2.1 1.6 0 0 3 2 0.01 - - 10.ZAHRAA, F. GARIN AND G. MAIRE 51 The selective demethylation of the methylcyclopentane intermediate is faster than the rupture of the C-C bonds on the ring. This new type of reaction is due to the presence of phosphorus at the surface of the catalyst, which blocks the sites responsible of the cyclic mechanisms.17 This catalyst treated by air at 200 "C for 16 h behaves as do the classical catalysts (see table 3). We again find a correlation between the percentage of cyclic mechanism in the reaction of 2-methylpentane to 3-methylpentane and the number of surface particles in the size range 5-10 A.Furthermore, ESCA results confirm the disappearance of phosphorus on the catalyst surface.17 These " Chatt cluster-derived catalysts " selective for demethylation seem to in- duce either an aay intermediate, triadsorbed on a metal atom in the cracking reactions,* or the selective metallocyclobutane species as precursor, responsible for the bond- shift mechanism instead of the metallo-carbene species (fig. 2). Et Et &-g-ir MJ\ 0 - C ) + C H b ( b ) FIG. 2.-(a) Bond-shift mechanism.z (6) Proposed model for selective demethylation of methyl- cyclopentane,' The bond-shift mechanisms predominate over the cyclic mechanisms (see table 3) : this can be attributed to different electronic and geometric factors due to the environ- ment of the metal particles.CONCLUSION It appears that the single-crystal faces of Pt simulate very well supported catalysts with low dispersion. Moreover, catalysts formed by deposition of a cluster on alumina (mono- or bi-metallic) have the same catalytic properties as industrial cata- lysts. The small metal particles in the range up to 10 A are directly connected to the percentages of cyclic mechanisms as seen from Chini, Wilkinson and Parshall derived catalysts. However, " Chatt cluster-derived catalysts " with the same contribution of small metal particles have a completely different selectivity, which is correlated to the environmental presence of phosphorus, leading to a different charge transfer to the metal.A pure selective cyclic mechanism could be characterized on iridiumI6 and a pure bond-shift mechanism could also be isolated on platinum catalyst while mean particle sizes were of order 22 A. This clearly means that the reactive intermediate species in the various reactions have very different electronic requirements and that electronic factors are very important in determining the nature of the reaction mechanisms. * In this species the carbon a-bonded to the metal should be secondary, as seen on highly dir persed Pt-A1203 catalysts.'52 SELECTIVITY IN ISOMERIZATION OF HEXANES We thank Dr. L. Hilaire for stimulating discussions and Dr. J. L. Schmitt for the t .e.m. and chemisorption measurements.This research has been partially supported by a contract A.T.P. “ Chimie Fine ” from the C.N.R.S. F. Garin, F. G. Gault and G. Maire, Nouu. J. Chim., in press. F. Garin and F. G. Gault, J. Am. Chem. SOC., 1975, 97, 4466. F. G. Gault, F. Garin and G. Maire, in Growth and Properties of Metal Clusters Studies in Sur- face Science and Catalysis, uol. 4, ed. J. Bourdon (Elsevier, Amsterdam, 1980), pp. 451-466. F. Garin and G. Maire, in Soci.te‘ Frunqaise du Vide, E.C.O.S.S.3, ed. D. A. Degras and M. Costa (1980), vol. 1, pp. 490-493. F. Garin, S. Aeiyach, P. Legare and G. Maire, J. Catal., submitted. 5. Maire, 0. Zahraa, F. Garin, C. Crouzet, S. Aeiyach, P. Legare and P. Braunstein, special issue, J. Chim. Phys., Phys. Chim. Biol., 1981. F. G. Gault, 0. Zahraa, J. M. Dartigues, G. Maire, M. Peyrot, E. Weisang and P. A. Engelhard, 7th Int. Congr. Cutal., Tokyo, Japan (1980), paper no. A-11. F. Garin, 0. Zahraa, C. Crouzet, J. L. Schmitt and G. Maire Surf: Sci., 1981,106,466. J. M. Dartigues, A. Chambellan, S. Corolleur, F. G. Gault, A. Renouprez, B. Moraweck, P. Bosch-Giral and G. Dalmai-Imelik, Now. J. Chim., 1979, 3, 591. lo J. F. Young, P. D. Dillard and G. Wilkinson, Chem. Commun., 1964, 5176. l1 R. V. Lindsey, G. W. Parshall and V. G. Stolberg, Int. J. Chem. Eng., 1966,5, 109. P. Chini, G. Longoni and V. G. Albona, Adu. Organomet. Chem., 1976,14,285. l3 G. Booth and J. Chatt, Chem. Commun., 1969, 2131. I4 C. Corolleur, S. Corolleur and F. G. Gault, J. Cutal., 1972, 24, 385. l5 E. G. Derouane and M. H. Moutsy, in Newsletter of the Research Group on Catalysis, July l6 F. G. Gault, V. Amir-Ebrahimi, F. Garin, P. Parayre and F. Weisang, Bull. SOC. Chim. Belg., l7 0. Zahraa, F. Garin and G. Maire, unpublished results; 0. Zahraa, Th2se d’Etat (University 1980, no. 16 (Council of Europe, Strasbourg, 1980). 1979, 88, no. 7-8, 475. of Strasbourg, 1980).

ISSN:0301-7249    

DOI:10.1039/DC9817200045

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Alumina-supported CO Hydrogenation Catalysts Prepared from Molecular Osmium and Ruthenium Clusters BY HELMUT KNOZINGER AND YAPING ZHAO * Institut fur Physikalische Chemie, Universitat Munchen, Sophienstrasse 1 1, 8000 Munchen 2, Federal Republic of Germany AND BERND TESCHE Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, Federal Republic of Germany AND ROGER BARTH, RONALD EPSTEIN,~ BRUCE C . GATES AND JOSEPH P. SCOTT Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 1971 1, U.S.A. Received 4th June, 1981 0 s catalysts on y-A1203 supports have been prepared from 0 s complexes of varying nuclearity, namely HzOsCj6, OS~(CO)~~, H40~4(C0)12 and OS~(CO)~~. Characterization by infrared and X-ray photoelectron spectroscopy and transmission electron microscopy provides evidence of the stabili- zation of well-defined ensembles of 0 s atoms on the support surface, the ensemble size being deter- mined by the nuclearity of the cluster precursor.The catalysts prepared from HZOsCl6 have a dis- persion comparable with that obtained with OS~(CO)~~, although the presence of smaller ensembles and single atoms cannot be excluded. The ensemble size may influence activity and selectivity for Cz and higher hydrocarbons in CO hydrogenation at atmospheric pressure and temperatures between 530 and 610 K, but the results suggest that the heterogeneity of the 0 s species and the chlorine content of the support also influence the catalyst performance.Data obtained with a more highly dis. persed RujA1203 catalyst prepared from RU~(CO)~~ provide the first quantitative comparison between 0 s and another Group VIII metal catalyst for CO hydrogenation. The 0 s was approximately one order of magnitude less active than the Ru catalyst, but it was more selective for formation of C2 and C3 hydrocarbons. Selective conversion of CO and H2 is a goal of much of the current research in catalysis, since CO and H2 may replace petrochemicals as the basic building blocks of the organic chemicals’ industry. Metals are the principal class of CO hydrogenation catalysts, and selectivities vary widely from metal to metal ; Ni, for example, catalyses formation of methane, and Co catalyses a Schulz-Flory distribution of chiefly straight- chain hydrocarbons, including those with many carbon atoms.The product distributions in metal-catalysed CO hydrogenation are strongly in- fluenced by the location of the metal in the periodic table; metals like Fe and Co, which dissociate CO, are active for hydrocarbon formation, whereas metals like Pd and Pt, which do not dissociate CO, are active for methanol formation.’ It has been suggested that CO hydrogenation reactions involving C-C bond * Permanent address : Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, t Present address: Stauffer Chemical Co., Dobbs Ferry, New York, U.S.A. Peoples Republic of China.54 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS formation require catalysts with neighbouring metal centres, i.e.surfaces or metal clusters., The available data are consistent with this hypothesis, but critical evaluation is required. It has further been suggested that the number of metal centres on a sur- face or cluster influences the chain lengths in the hydrocarbon products, and the data obtained with Ru aggregates encaged in zeolites showed a cut-off in chain length near C2, c6 and Cll, depending on the size of the zeolite cage, which determined the size of the Ru aggregate., The presumed requirement of neighbouring metal centres has motivated the application of metal clusters as catalysts for CO hydrogenation, and there is evidence suggesting that Rh clusters in solution are active for formation of ethylene glycol and other product^.^ The soluble metal clusters, like other molecular catalysts in solution, are attractive in prospect because the discreteness of their structures may imply that they will be selective catalysts.This prospect remains largely untested, primarily because of the difficulties of maintaining cluster structures intact during catalysis. One method of stabilization of metal clusters is to bind them to supports, where they may be held apart from each other in a coordinatively unsaturated (and catalytically active) state. A number of supported metal clusters have been tested as catalystss and a triosmium cluster supported on MgO has been suggested to be active for CO hydrogenation to give alkanes.6 Several examples of highly dispersed oxide-supported metals derived from metal clusters have also been reported to be active catalysts for CO hydrogenation, and some of these have unusual selectivities.For example, Rh prepared from carbonyl clusters on La,O,, TiO, and ZrO, is selective for formation of ethanol,' and Fe on A1203 and MgO is selective for formation of olefins.8 Supported-metal catalysts prepared from triosmium carbonyl clusters have been chosen for further study in this work because they may be " ideal " catalysts, having well-defined structures. The triosmium can be bonded directly to metal-oxide supports, giving supported clusters with unique structure^,^'^^ and these decompose into mononuclear surface carbonyl c~mplexes.~*~-'~ The Al,O,-supported catalysts have been shown to be active for CO hydrogenation, giving methane.ll One of the goals of this research was to investigate the activity and selectivity of CO hydrogenation catalysts prepared from triosmium clusters.To provide a basis for relating the catalytic character to the structure of the metal species, other 0 s cata- lysts were also investigated, including those prepared from a mononuclear complex (H,OsCl,) and from tetranuclear and hexanuclear 0 s clusters. Ru clusters are also of interest for the preparation of supported CO hydrogenation catalysts ; like 0 s clusters, these are evidently broken up into mononuclear specie^'^*'^ but the bonding of the Ru to a SiO, or Al,03 support (presumably through Ru-0 bonds) is not as strong as the bonding of 0 s to these supports, and in the presence of H2 the Ru (in contrast to 0s) undergoes aggregation into ~rystallites.~~ In summary, the research plan was to prepare y-Al,O,-supported 0 s catalysts from OS~(CO)~,, H,OsCl,, H40~4(C0)12 and OS~(CO)~~, and supported Ru catalysts from RU,(CO)~~.These were to be characterized structurally (by infrared and X-ray photoelectron spectroscopy and transmission electron microscopy) and as catalysts for CO hydrogenation. The principal goals were to prepare structurally well-defined supported metals and to determine relations between structure and catalytic activity and selectivity.H . KNOZINGER et al. 55 EXPERIMENTAL MATERIALS AND CATALYST PREPARATION OS~(CO)~~ and R U ~ ( C O ) ~ ~ were obtained from Strem and used without further purification. OS~(CO)~~ was prepared by pyrolysis of Os3(CO)12 at 520 K for 100 h in an evacuated glass tube according to the method described by Eady et al.15 The resulting mixture of 0 s clusters was dissolved in boiling ethyl acetate, and the solution was cooled to room tempera- ture to crystallize unreacted OS~(CO)~~.The supernatant solution contained Os3(CO)12, &5(co)13, OS~(CO)~~, Os7(CO)21 and OS~(CO)~~, which were separated by thin-layer chroma- tography using Kieselgel (Macherey-Nagel, art. no. 80901 3) as the stationary phase and a 1 : 99 mixture of ethyl acetate and cyclohexane as the eluent. The OS~(CO)~~ yield was 3.6%. H40~4(C0)12 was prepared by the method of Lewis et aZ.16 H20sC16-6H20 was ob- tained from Colonial Metals, Elkton, MD. TABLE 1 .---CATALYST ANALYSES a estimated metal content catalyst precursor/support metal content determined by (wt %) elemental analysis (wt %) ~~~ ~~ ~ ~ ~ ~~ * Analyses were performed by Analytische Laboratorien, Engelskirchen, F.R.G.Assuming complete uptake of catalyst precursor from the solution. The relative error is <lo%. The A1203 used as support was A1203,P110C1 from Degussa, F.R.G. This material was prepared by flame hydrolysis of AlC13 and consists of small non-porous spherical particles (5-30 nm in diameter). Its B.E.T. surface area was 100 m2 g-l. In experiments with H40~4(C0)12 a different sample of y-A1203 was used. It was obtained from Ketjen (grade D) and had a surface area of ca. 250 m2 g- l. The cluster-derived catalysts were pre- pared by refluxing the parent carbonyl cluster with the thermally pretreated support (02 at 673 K, followed by evacuation at 110 N m-2 at 673 K) in dry octane (b.p. 398 K) under dry N2, as described previously.11*12 A similar procedure was used with R u ~ ( C O ) ~ ~ and A catalyst was prepared from aqueous H20sC16 and AI2O3 by ion exchange, followed by All the catalysts were analysed for the metal. The results are summarized in table 1. y-Al203. drying and reduction in H2 for 16 h at 423 K then 3 h at 523 K. CHARACTERIZATION METHODS I.R. SPECTROSCOPY Thin self-supporting wafers were pressed and placed in a previously described trans- mission infrared cell, which permits in sit^ heat treatment and chemisorption experiments. The i.r. spectra were recorded on a Perkin-Elmer 225 spectrophotometer. To reduce heat effects induced by the infrared beam, the Globar light source was run at only ca.20% of its maximum power. The spectral slit width in the carbonyl stretching region was typically 3 cm-I.56 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS X-RAY PHOTOELECTRON SPECTROSCOPY X-ray source (hv = 1486.6 eV). used as internal standards. low, and repetitive scans were accumulated for the reported spectra. X.p. spectra were recorded on a Leybold Heraeus LHS 10 spectrometer using an A1 Ka Binding energies of 103.0 and 73.5 eV for the Si 2p and A1 2 p levels, respectively, were Because of the low metal loadings, the spectral intensities were TRANSMISSION ELECTRON MICROSCOPY In preparation for examination by high-resolution transmission electron microscopy (TEM), the catalyst powder was subjected to grinding and suspended in hexane. A drop of the suspension was then placed on a 4 nm carbon film mounted on a 1000 mesh Cu grid, and the hexane allowed to evaporate.The micrographs were obtained with a Siemens Elmiskop 102 with an instrumental mag- nification of 377 000 times and an acceleration voltage of 125 keV. To improve the contrast the objective aperture was reduced to ca. 30 pm, which led to a restriction of the spatial frequency spectrum in the electron microscopic image and a restriction of the phase contrast to 10.3 nm. Even with these precautions, there are limitations to be recognized regarding the inter- pretation of the micrographs. The limitations are characteristic of dispersed powder samples, which present variable layer thicknesses and, consequently, varying degrees of defocusing, which produce contrasts which are susceptible to misinterpretation as real structures. Further, local charging of the A1203 is variable, depending on the layer thickness and the contact with the carbon film.Taking these limitations into account, we conclude that the microscopy is capable of detecting 0 s aggregates smaller than 1 nm, but no structural information can be derived for such small species. CATALYTIC REACTION RATE MEASUREMENTS Catalytic reaction experiments were carried out with a steady-state differential flow reactor interfaced to a gas chromatograph. The flow system allowed metering of CO, H2, and He (Matheson UHP grade, further purified by flow through traps to remove traces of oxygen, water and metal carbonyls) to a thermostatted packed-bed reactor which was copper- lined to prevent formation of metal carbonyls.The system was operated at pressures be- tween atmospheric and 3.2 x lo6 N m-2. Pressurereduction downstream of the reactor was achieved by a back-pressure regulator. Details of the system are to be presented elsewhere. l8 Ca. 1 g of catalyst powder (typically) was dispersed between layers of glass wool and loaded into the tubular reactor, which was then packed with glass wool plugs at inlet and exit. The reactor was placed in the flow system and thoroughly purged with helium at lo5 N m-z. The system was then brought to pressure with a continuous helium purge. The reactor was then heated to temperature in a ca. 15 min period, after which CO and Hz reactant gas flows were started. After ca. 1 h steady state was attained and data collection begun.During a catalysis experiment, the reactor temperature, pressure and feed-gas flow rates were held constant, and the product stream (maintained in the vapour phase in a heated exit line) was intercepted periodically with a gas-sampling valve on the gas chromatograph (an Antek 300), which was equipped with a flame-ionization detector and a 3.2 mm x 3 m stainless-steel column packed with 60/80 mesh alumina. The helium carrier-gas flow rate was 18 cm3 min-'. Temperature programming of the column involved an initial 20 min hold at 410 K, heating at 20 K min- ' to 490 K, then a 5 min hold at 490 K. The gas chroma- tograph was calibrated with the following known compounds to determine response factors and elution times : methane, ethane, ethylene, prdpane, propylene and n-butane.Products were identified by their elution times.H. KNOZINGER et al. 57 RESULTS AND DISCUSSION The decomposition of numerous metal carbonyl clusters to form mononuclear carbonyl species on oxide supports has been shown to be typically accompanied by reaction with OH groups of the support and oxidation of the meta1.19*20 The ten- dency of a cluster to undergo decomposition is expected to be indicated by the relative strengths of metal-metal and metal-oxygen bonds, and a summary of relevant litera- ture data (for zero-valent 0 s and Ru) is given in table 2. For 0 s and Ru, the metal- metal bond is the weakest in the cluster carbonyl. 0s-0 bonds are extremely strong, and Ru-0 bonds are significantly weaker.We would therefore expect strong osmium-support interactions in oxide-supported 0 s catalysts. We might expect stable mononuclear osmium species, the presence of which might be associated with the presumably monolayer raft-like structures reported by Sinfelt et al.23*24 on silica supports . TABLE 2.-AVERAGE BOND STRENGTHS IN METAL CARBONYL CLUSTERS (hE) AND BOND DISSOCIATION ENERGIES (D'298) OF DIATOMIC SPECIES AT 298 K (kJ m0l-l) compound bond A B 0'298 ref. - 129 190 - 397 - < 594 117 - 171 - - 292 - 481 - 21 21 21 22 21 21 21 22 The relatively weak interactions between Ru and 0, on the other hand, are ex- pected to allow more facile aggregation of Ru on oxide surfaces. Kuznetsov et CLZ.'~ recently reported the formation of Ru microcrystallites from R U ~ ( C O ) ~ ~ on A1203 after reduction in H2.These microcrystallites, however, could be redispersed into smaller entities when the sample was heated in CO to temperatures >470 K. In contrast, supported 0 s catalysts formed from O S ~ ( C O ) ~ ~ are resistant to metal aggregation and mononuclear carbonyls are the predominant surface species obtained on thermal decomposition of O S ~ ( C O ) ~ ~ on A1203.9*11 Knozinger and Zhao12 recently reported infrared spectra and a model for these surface species. The infrared spectra in the carbonyl region showed two sets of band pairs at 2130 and 2037 cm-l and at 2050 and 1970 ern-', which were assigned to an -OS~~(CO)~ and - O S ~ ~ ( C O ) ~ species, re~pectively.~~~~ These two species could be interconverted without the occurrence of detectable metal aggregation at temperatures up to 770 K.A detailed analysis of the infrared spectra allowed an estimate of bond angles in these species, and it was concluded from the geometries of the complexes and the size of the carbonyl ligand that the minimum 0s-0s distance between two -Os(CO), units was ca. 0.59 nm. This value, compared with the 0s-0s distance of 0.2877 nm25 in O S ~ ( C O ) ~ ~ , clearly confirms the earlier c o n c l ~ s i o n ~ ~ ~ ~ that the original cluster breaks up during heat treatment to give mononuclear carbonyl complexes.58 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS Additional characterization of these complexes is provided by the results of this work, including reactivity of the osmium species in the presence of H2, O2 and CO in the temperature regime in which catalytic hydrogenation of CO occurs.c AT A L Y s TS PREP ARE D FROM O S ~ ( C O ) ~ ~ / A ~ ~ O ~ PHYSICAL CHARACTERIZATION The strength of the 0s-O bonds leads us to expect that the 0 s carbonyl species formed from the clusters may be immobile even at elevated temperatures and may retain their initial positions [with a minimal Os-Os distance of ca. 0.59 nm between --OS~~(CO)~ units], thus forming ensembles of three 0 s atoms on the surface. This expectation is reinforced by the results of Deeba et aZ.,6 who observed the decomposi- tion of osmium clusters on MgO followed by the formation of clusters again in the presence of CO. The estimated diameter of an ensemble is ca. 0.98 nm, and the average distance between ensembles on the A1203 support, estimated from the total metal loading, is ca.7 nm. The transmission electron micrograph of plate 1 confirms the existence of these ensembles. Scattering centres of extremely uniform size (< 1 nm) are clearly evident, and we attribute them to the three-atom ensembles, the average distance between them being roughly the predicted value of 7 MI. The osmium formed by cluster decomposition has been inferred by Smith et aZ.? who measured the stoichiometry of the surface reaction, to be divalent. As mentioned above, this conclusion is consistent with the positions of reported infrared carbonyl stretching Further support is provided by the X-ray photoelectron spectra of fig. 1. The dotted lines in the figure represent the binding energies at 50.0 and 52.7 eV of the 4h12 and 4h12 core levels of zero-valent Os, respectively, which have been measured for a mechanical mixture of OS,(CO)~~ and A1203.Spectrum 1 represents the trinuclear cluster HOS~(CO)~~OS~<, which is bound to a Si02 surface via an edge- bridging oxygen ligand.g-12 This compound was synthesized according to a pre- viously described method and characterized by its infrared carbonyl spectr~m.~-l~ The spectrum can be deconvoluted into a pair of doublets at 49.9 and 53.7 eV and 51.5 and 54.2 eV, which represent, respectively, the 4h12 and 4f12 binding energies of a single osmium bonded to CO ligands only and the two edge osmium atoms bridged by the oxygen in the HOs3(CO)loOSi< cluster. The structureless band (spectrum 2) in fig.1 was obtained with the freshly prepared O S ~ ( C O ) ~ ~ / A ~ ~ O ~ catalyst (after exposure to air), and spectrum 3 is typical of the -OS~~(CO)~ species formed from it. This latter spectrum shows the unresolved 4f doublet (4f,7/2 binding energy at 52.3 ev). The width of these bands (f.w.h.m. - 7.2-7.9 eV for the doublet) is attributed to the surface heterogeneity. The weak shoulders at low binding energies can pre- sumably be explained as A1 2s(Ka) and 0 s 4f(Ka3,J X-ray satellites. The most important result provided by the X.p. spectra is the shift of the 4f712 levels in the oxidized catalyst toward higher binding energies; the shift is 2.3 eV com- pared with the zero-valent 0 s reference value, which is in good agreement with the oxidation state +2 attributed to the mononuclear 0 s species.After treatment in H2 at 650 K, the binding energies were shifted back toward the reference values for zero- valent 0 s (fig. 1). This result clearly indicates a reduction of the original +2 oxidation state, but the true oxidation state of the reduced osmium cannot be determined from these data, since the sample had to be transferred through the atmosphere to the X.P.S. apparatus from the infrared cell where it underwent reduction. (The same wafer was used for X.P.S. and TEM after characterization by infrared spectroscopy.)PLATE 1 .-Transmission electron micrograph of OS~(CO),~/AI~O~ after formation of -OS"(CO)~ species (some 0 s scattering centres are indicated by arrows). [To face page 58PLATE 2.-Transmission electron micrograph of Os3(CO),,/Al,O3 uscd as a catalyst for CO hydro- genation. Some 0 s scattering centres are indicated by arrows.[To face page 59H , KNOZINGER et al. 59 The question now arises whether metal aggregation had occurred during the high- temperature (650 K) reduction. Only scattering centres of uniform size (<I nm), indistinguishable from those observed in the oxidized samples, can be discerned in the electron micrograph (not shown). No larger metal particles appear to have been formed.* Therefore, we infer that the structural model described above for the oxi- dized samples can equally well be applied to the reduced catalysts. FIG. 1.-X.p. prepared, (3) ! I oso 4f5& 4f7* spectra of supported O S ~ ( C O ) ~ ~ : (1) HOS~(CO)~~OS~<, (2) OS~(CO),~/A~~O~ freshly O S ~ ( C O ) ~ ~ / A ~ ~ O ~ after formation of -OS~~(CO)~, (4) sample of spectrum (3) after reduction in Hz at 650 K.The interaction of the supported 0 s with H2, O2 and CO has been studied by infrared spectroscopy in the carbonyl stretching region. The three characteristic bands at 2128-2130, 2038-2050 and 1965-1970 cm-' [which were observed for the oxidized samples containing -OSI~(CO)~ and -OS~I(CO)~ species] were eliminated on reduction in H2 at 650 K, and a new band pair appeared at 2025 and 1925 cm-l (fig. 2, spectrum l), which is typical of the reduced state of the catalyst. The band pair may be assigned to the symmetric and antisymmetric stretching modes of an -Os(CO), species, the 0 s atom being in a low oxidation state.(Note that no CO was admitted after reduction.) Admission of O2 at room temperature led to a slight shift towards higher wavenumbers, while O2 treatment at 470 K led to reoxidation and re- * In contrast, Smith et 0L9 stated that 0 s particles could be obtained by reduction for >24 h at 470 K, but details of their experiment and the metal loading of their catalyst are lacking.60 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS stored the typical band pair at 2045 and 1965 cm-l characteristic of the -OS~~(CO)~ species (fig. 2, spectrum 3). Subsequent exposure to a CO atmosphere at 620 K produced a complete recarbonylation accompanied by the reappearance of the original set of bands at 2128, 2038 and 1965 cm-l (fig. 2, spectrum 4). These results demon- wavenumber/cm - FIG.2.-Infrared spectra of OS~(CO)~Z/AI~O~: (1) after reduction in H2 at 9.3 x lo4 N m-2 and 650 K for 4 h, (2) after subsequent exposure to O2 at 9.3 x lo4 N rn-' and 298 K for 1 h, (3) after heating in O2 at 9.3 x lo4 N m-2 and 473 K, (4) after subsequent exposure to CO at 1.3 x lo4 N m-2 and 625 K for 4 h. strate that the 0 s species can be reduced and reoxidized reversibly without any detect- able metal aggregation ; electron microscopy provided confirmation of this con- clusion. A spectrum representative of -OS"(CO)~ mixed with -OS~~(CO), is shown in fig. 3 (spectrum 1). Spectrum 2 is that of the sample reduced in H,. Exposure of this reduced sample to CO at 1.3 x lo4 N m-2 and room temperature produced band shifts toward higher wavenumbers and a shoulder at 2040 cm-l. Heating the sampleH .KNOZINGER et al. 61 6 I 0 2, I I I I I I I I I 1 4 /' I/ 00 * w 0 0 wc.l wavenumber/cm-' FIG. 3.-Infrared spectra of OS~(CO)~~/AI~O~: (1) sample treated in CO at 9.3 x lo4 N m-2 and 723 K for 15 h, (2) after reduction in H2 at 9.3 X lo4 N m-' and 650 K for 4 h, (3) after exposure of(2) to CO at 1.3 x lo4 N m-' and 298 K for 15 h, (4) after treatment in CO at 1.3 x lo4 N m-2 and 625 K for 4 h. in CO at 1.3 x lo4 N m-2 and 620 K essentially restored the original spectrum characteristic of the -OS~~(CO)~ species. We therefore conclude that, even at room temperature, a partial reoxidation of the 0 s species occurs and that CO treatment at elevated temperatures would lead to a complete reoxidation.Analogous observations have been reported by Primet26 for Rh in zeolites. To account for this observation, one must assume either a simple dissociative chemisorption of CO or CO disproportionation with subsequent dissociation of CO,: CO(g) - C(a> + O(a) 2CO(g) - C(a) + co2 CO&) - CO(g) + O(a>.62 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS 0 s has been classified as a metal that would adsorb CO only ass~ciatively.~~*~* It is clear that this classification does not pertain to our supported 0 s under the condi- tions investigated. Since the formation of a surface carbide via CO dissociation and/ or disproportionation is considered to be a necessary step for catalytic methanation and Fischer-Tropsch ~ y n t h e s i s , ~ * ~ * ~ ~ - ~ ~ we infer that the supported 0 s samples may be appropriate model catalysts for CO hydrogenation, as is discussed further below.The structural model presented above fails to account for dissociative chemisorp- tion of CO on the reduced catalysts, since an 0s-0s distance of 0.59 nm is too great to allow CO dissociation through the following structure: c-0 / \ 0 s 0s. To account for the CO dissociation, we suggest that the 0 s atoms were closer together than 0.59 nm, and we infer that it is important that no spectroscopic evidence was obtained for the formation of tricarbonyl species on the reduced samples. The steric repulsion between carbonyl ligands in neighbouring -OS~~(CO)~ complexes, which is considered to be chiefly responsible for the large separation between them, is expected to be much weaker in the reduced species.We suggest that the 0s-0s distance between neighbouring dicarbonyl species may approach the 0s-0s distance in the OS~(CO)~~ cluster. This is possible if neighbouring 0 s species are placed into adjacent sites bridging two or three surface oxygen ions on (100) and (1 11) faces of the A1203 support (note that the size of the 0 s atom is very nearly the same as that of an oxygen ion), while only every second site can be occupied by the oxidized species.12 If one allows for relatively small local reorganizations within the ensembles, depending on the oxidation state and degree of carbonylation of the 0 s atoms, one can explain how dissociative chemisorption of CO might occur and provide the surface carbide inter- mediate necessary for methanation and Fischer-Tropsch reactions.CATALYTIC HYDROGENATION OF co The catalysts prepared from O S ~ ( C O ) ~ ~ have been found to be active for hydro- genation of CO to give alkenes and alkanes. To simplify the product analysis, reaction conditions were chosen so that only low conversions were observed (< 1 %, assumed to be differential), and only C1-C4 hydrocarbons were found among the organic products. Representative conversion data from an experiment carried out at a pressure of 7.9 x lo5 N me2 are shown in fig. 4. These results indicate a slow loss of catalytic activity (presumably resulting from the formation of carbonaceous deposits on the surface) and a nearly time-invariant product distribution. Similar and more thorough results were obtained at a pressure of 1.0 x lo5 N m-2.The rate data and the product distribution data (giving selectivities for C , and C3 products) are summarized in table 3. These data (consistent with those of fig. 4) show that the supported 0 s catalyst was active for formation of CH4, with lower rates of formation of C2H4, C2H6, C3H6 and C3H8. At the lower pressure, the catalyst deactivation was so slow that reaction rates characteristic of the fresh catalyst could be determined at more than one temperature. Representative data are shown in the Arrhenius plot of fig. 5, where they are compared with data obtained with the catalyst prepared from H20sCl,. The Arrhenius parameters are included in table 3. Infrared spectra of the used catalyst were virtually indistinguishable from those of the fresh catalyst in the oxidized form, confirming an earlier. qualitative rep0rt.l'H.KNOZINGER et al. 63 These results suggest that the ensembles of 0 s on the A1203 were catalytically active for formation of methane and Fischer-Tropsch products. The electron micrograph of the used catalyst (plate 2) is consistent with this interpretation; there is no evidence of larger 0 s aggregates than those present in the fresh catalyst. The only change evident in the micrograph is consistent with the presumed presence of carbonaceous deposits on the A1203 support. - 0 - - _ c3 10-51 I I I I ! I I I 0 10 20 30 40 50' time on stream/h FIG. 4.-CO conversion catalysed by O S ~ ( C O ) ~ ~ / A ~ ~ O ~ : reactant, He:H2:C0 = 1 : 3: 1, total pres- sure: 7.9 x lo5 N m-2, temperature: 573 K, mass of catalyst: 0.1651 g, 0 s content: 0.24 wt %, In summary, all the results point to the ensembles of three.Os atoms as the catalysts for CO hydrogenation.There is no evidence of the existence of larger 0 s aggregates. These results therefore appear to be important in providing the first evidence of CO hydrogenation in the presence of a structurally well-defined supported-metal catalyst. The results suggest that structure-property relations might be obtained by preparation of CO hydrogenation catalysts from 0 s complexes of various nuclearities to produce enembles of various sizes. Such catalysts are reported in the following paragraphs. feed ffow rate: 166 scc min-' (T = total conversion). CATALYSTS PREPARED FROM H40~4(C0)12/A1203 The samples prepared from the tetraosmium cluster have been characterized in a preliminary way with infrared spectroscopy.The A1203-supported 0 s (handled in the absence of air) presented a spectrum with bands at 2123 (vw), 2091 (w), 2058 (s), 2027 (s) and 2012 (s) cm-'; this spectrum is clearly different from that of the sample prepared from O S ~ ( C O ) ~ ~ and A1203, different from that of H40~4(C0)12 itself [2119 (vw), 2065 (vs), 2021 (s), 198O(w)] and different from that of CO adsorbed onTABLE 3. - CATALYTIC ACTIVITIES AND SELECTIVITIES IN CO HYDROGENATION feed composition activity a selectivities apparent activation energies,c Eact molar ratio catalyst lo4 rcH4 rC2H4 rC2H4 + rC2H6 rC3H6 + rC3H8 rC4H10 (CH4) (C2H4) (C2H6) (C3H8) (C4H10) He: H2 : CO YCH4 YCH4 TCHq YCH4 1:3:1 os3(co)i2/ 1:3:1 H20sCl,/ A1203 1.5 0.17 0.20 0.04 - w100 115 - - - A1203 25 0.03 0.05 0.003 - 139 130 - 110 - M 120 A1203 46 0.06 0.08 0.008 0.001 138 - 180 - 7.2 0.05 0.06 - - 161 140 - - - 1:3:1 RU3(C0)12/ 3:l:l H2OSCl6/ A1203 3:l:l RU3(C0)12, - - - - - 120 A1203 8.7 0.07 0.08 - ~~ ,I Rate r of CH, formation, molecules (metal atom s)-l.Data were extrapolated slightly to 606 K. Reaction at 1.0 x lo5 N m-2. ' Rates in molecules of hydrocarbon product (metal atom s)-'. Apparent activation energies in kJ mol-l, from plots of log r against inverse temperature.H . KNOZINGER et al. 65 0 s These results suggest that the osmium was bonded to the support in a highly dispersed form, possibly as tetraosmium clusters. The molecular cluster H40s4(CO)12 is stable in air at room temperature, but the A1203-supported species underwent structural changes in air, the aforementioned car- bony1 bands disappearing and a complex new set of bands appearing.1.7 1.8 1.9 1 0 3 ~ 1 ~ FIG. 5.-Arrhenius plot: initial rates of formation of CH4 from CO + H2 at 1.0 X lo5 N m-2, providing a comparison of the two supported 0 s catalysts. 0, 1:3:1 He:H2:CO; 0, 3 : l : l He: H2: CO; (-) H20sC16/A1203; (- - -) O S ~ ( C O ) ~ ~ / A ~ ~ O ~ . When the initially supported 0 s was heated in vacuum from room temperature to 508 K, the bands disappeared, and a new set of bands formed at ca. 21 15 (w), 2030 (m) and 1960 (w) cm- ; this new set of bands is indicative of a mononuclear 0 s carbonyl and is interpreted as evidence that the initial cluster species broke up to form en- sembles of mononuclear 0 s complexes, perhaps four-atom ensembles.The samples prepared from H40~4(C0)12 were tested as catalysts at 473 K and 3.2 x lo6 N m-2. The catalyst deactivation was negligible for 140 h of operation in the flow reactor. The product distribution was nearly independent of the H2:C0 ratio in the feed (table 4), and the selectivity for C2 formation was large in comparison with that of the sample prepared from O S ~ ( C O ) ~ ~ (table 3), which, however, was investigated under different conditions. The difference in the selectivities of the catalysts prepared from tri- and tetra- nuclear 0 s clusters suggests that the size of the ensemble may influence the product distribution, but this is no more than a speculation because the two catalysts were tested under different conditions.66 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS TABLE 4.<ATALYTIC ACTIVITY OF H~OS~(CO)I~/A~~O~ AT 473 K AND 3.2 X 106 N m-2 H2: CO molar ratio in feed product 3:l 2: 1 1:l CHI 7.9 x lo-6 7.8 x 7.6 x C2H6 + C2H4 4.6 x 4.6 x 4.7 x C3H8 + C3H6 - 1.7 x 1 0 - 7 1.6 x 1 0 - 7 a Reaction rates: molecules of hydrocarbon formed per 0 s atom per second.Conversions of CO varied from 4 x to 9 x lo-'. The catalyst was loaded into the reactor without contacting air, and was brought to the reaction temperature in flowing CO. CAT A LY ST s PREPARED FROM OS~(CO)~~/AI~O~ The thermal decomposition of O S ~ ( C O ) ~ ~ on A1,03 supports has been described by Smith et aZ.,9 whose carbonyl infrared spectra were virtually identical to those charac- teristic of the mononuclear decomposition products of the supported triosmium cluster.We have confirmed this result and we infer that the same mononuclear -OS~~(CO)~ and --OS~~(CO), species mentioned previously were formed from the hexanuclear cluster. However, the ensemble should now be larger, consisting of six atoms. A transmission electron micrograph of the OS~(CO),~/AI~O~ after thermal decomposition (not shown) shows scattering centres of uniform size of ca. 1.2 nm. These results strongly support the proposed model of ensemble formation and demon- strate that unique catalysts containing 0 s ensembles of well-defined size can be tailored using osmium carbonyl clusters of varying nuclearity as the catalyst precursors.CATALYSTS PREPARED FROM H2OsC&/Al,O, A sample prepared by bringing aqueous H2OSCI6 in contact with y-A1,03 had a carbonyl infrared spectrum closely resembling that of the mononuclear species ob- tained from the carbonyl clusters, as observed previous1y.l' A similar SO,-supported sample was studied by Prestridge et aZ.24 by electron microscopy. Some of the 0 s particles appeared to be present in " rafts ", presumably monolayers with an average width of ca. 1.2 nm. EXAFS showed that the average 0s-0s distance was the same as that in metal films or clusters (0.78 nm).33 The 0 s in these samples was presumably zero-valent and had no adsorbed carbonyl ligands. On Al,03 supports the formation of highly dispersed structures might be expected, and these might have a less dense packing on Al,03 than on Si02 because of a stronger 0s-0 interaction.Plate 3 shows an electron micrograph of the catalyst prepared from H,OsCl,. Highly dispersed ensembles are evident, being no larger than the three-atom ensembles shown in plate 1. We recognize that there may also be isolated mononuclear com- plexes too small to be observed by the microscope. This catalyst was also active for CO hydrogenation, giving alkanes and alkenes. The rate data are summarized in table 3, with some data also shown in the Arrhenius plot of fig. 5. The activity of this catalyst for hydrocarbon formation is more than one order of magnitude greater than that of the catalyst prepared from triosmium clusters, whereas the selectivity for C2 and C3 formation is an order of magnitude less.The used catalyst has been characterized by infrared spectroscopy (fig. 6). A comparison with the spectrum of the fresh catalyst (which was nearly indistinguishablePLATE 3.-Transmission electron micrograph of H20sC1,/A1203 after reduction under conditions described in the Experimental section. [To face page 66H. KNOZINGER et al. 67 d ._ M -g C c! Y wavenumber/cm - FIG. 6.-Infrared spectra of catalyst H20sC16/A1203 after use in CO hydrogenation under conditions given in table 3: (I) used catalyst exposed to CO at 8 x lo5 N m-2 and 298 K, (2) after CO treatment at 8 x lo5 N m-’ and 473 K. from spectrum 4 in fig. 3) provides no indication of structural changes during the catalysis. We infer that this catalyst, like the one prepared from OS~(CO)~~, was stable and, in particular,.that no aggregation of the 0 s took place.The reasons for the differences in performance of the two 0 s catalysts are not clear. We recall that the catalyst prepared from H20sC16 contained 3.5 times as much 0 s as the catalyst prepared from Os,(CO),,; the former catalyst also incorporated C1- (derived from H20sC16), which is expected to have increased the acidity (including the Lewis of the support. The analytical values for C1 were 1.23% and 0.25%, respectively, for the two catalysts. A role of the support cannot be ruled out: for example, the support may alter the specific properties of the osmium species, or Lewis acid centres (coordinatively unsaturated A13+ ions) in the support surface could play a role in the catalysis, such as aiding in the dissociation of CO.Since the catalyst prepared from H,0sC16 is expected to be heterogeneous, in- corporating mononuclear surface complexes as well as ensembles, one might speculate68 co HYDROGENATION CATALYSTS FROM OS, Ru CLUSTERS that the mononuclear 0 s complexes are involved in CO dissociation, perhaps with the assistance of AP+ sites, as mentioned above. c A T A L Y s T s PREP A RED FR o M R U ~ ( C O ) ~ ~ / A ~ ~ O ~ and since the thermal decomposition of Ru carbonyl clusters on A1203 has been investigated by ~ t h e r ~ , ~ ~ ~ ~ ~ ~ ~ ~ we discuss the infrared spectra of the RU~(CO)~~/A~,O~ sample only briefly. Fig. 7 shows a typical spectrum of the decomposition product (spectrum 1).Bands can be discerned at 2160, 2140, 2072 and 2002 cm-l. In addition, an asymmetry is evident at ca. 2050 cm-l on the low-wavenumber flank of the Since infrared spectra of CO chemisorbed on supported Ru have been wavenumber/cm-' FIG. 7.-Infrared spectra of RU~(CO)~~/AI~O~ : (1) fresh catalyst (exposed to air) after evacuation (< 1 x lo2 N m-2) for 2 h, (2) after treatment in Hz at 5.3 X lo5 N m-2 and 473 K for 15 h, (3) after treatment in H2 at 5.3 x lo5 N m-z and 573 K for a period of 1 h, (4) for a period of 6 h, (5) for a period of 26 h.H. KNOZINGER et al. 69 band at 2073 cm-', and a shoulder is evident at ca. 1970 cm-'. It can be ~ h o w n ' ~ . ~ ~ that this spectrum must be composed of the contributions from several mononuclear Ru carbonyl species.We assign the band pair at 2140 and 2072 cm-l as a -Ru(CO), species, and two types of -Ru(CO), species are likely to exist with carbonyl stretching bands at ca. 2070 and 2002 cm-l and at 2050 and 1970 cm-l, respectively. By ana- logy with the O S ~ ( C O ) ~ ~ / A ~ ~ O ~ system, we infer that the Ru atoms are in a positive oxidation state in these mononuclear surface species, in agreement with the con- clusions of Smith et aZ.*O This inference is also substantiated by the close similarity of the carbonyl spectra to those of known molecular RulI c~mplexes.'~ The difference between the two proposed dicarbonyl species may be attributed to different oxidation states of Ru and different coordination by surface oxygens. The bands at 2160 and 2140 cm-l vanished, the intensities of the bands at 2072 and 2002 cm-l decreased, the intensity of the band at 1972 cm-l increased and a new band at 2053 cm-l was clearly resolved.This band, together with contributions to the 1970 cm-l band, suggests the formation of a -Ru(CO), species with Ru in a lower oxidation state. However, reduction under more severe conditions (573 K) led to a gradual disappearance of all carbonyl bands with the exception of the band at 1972 cm-l (fig. 7, spectra 3-5). This band could not be eliminated by evacuation or treatment in H2 at 573 K. Only treatment in O2 at 473 K led to complete removal of the band, and subsequent CO adsorption restored the original spectrum of the oxidized sample (fig. 7, spectrum 1). A carbonyl spectrum of this type with a single band at ca.1970 cm-l has not been reported before. The band occurs at a surprisingly low frequency and an unequivocal assignment is not possible. Two explanations are considered plausible : (1) The band is indicative of a terminal CO ligand bonded to a low-valent Ru atom. The low frequency can then be explained only if the Ru atom bears additional electropositive ligands. These may be carbide ligands resulting from CO dissociation, which are known to produce a red-shift of carbonyl stretching bands.36 This explanation implies that somehow CO dissociation occurs on the surface. (2) Alternatively, the band might be assigned to a bridging carbonyl, although it is present at a relatively high frequency for such a species. Both interpretations could be accounted for by ensembles of Ru atoms or Ru particles with metal-like Ru-Ru distances.We might therefore suggest more pronounced aggregation of Ru than of Os, presumably due to the weaker Ru-0 interaction. Kuznetsov et aZ.13 and Ugo et aL3' reported the appearance of Ru micro- crystallites upon reduction of the supported cluster-derived species in H2. The catalytic reaction experiments confirm the activity of Ru for CO hydrogena- tion to give hydrocarbons (table 3). The catalytic activity is represented per Ru atom, even though the dispersion of Ru is not known. We presume that it is nearly unity, since the infrared spectra provide no evidence of CO adsorbed on metal in the used catalysts. Represented in this way, the activity is approximately one order of mag- nitude greater than that of the 0 s catalyst prepared from OS~(CO)'~.These results provide the first quantitative comparison of the CO hydrogenation activities of these two metals." The data of table 3 also show that the Ru catalyst was less selective than the catalyst prepared from OS,(CO),~ for formation of C , and C3 hydrocarbons. * Vannice's tabulati~n~~ of specific rates of methanation on Group VIII metals does not include 0 s . There is no reported value for the enthalpy of adsorption of CO on an 0 s surface, and therefore it is not possible to estimate the specific activity by interpolation of Vannice's volcano plot. Reduction in H2 at 473 K led to drastic spectral changes (fig. 7, spectrum 2).70 co HYDROGENATION CATALYSTS FROM OS, RU CLUSTERS CONCLUSIONS Supported 0 s and Ru catalysts with extremely high dispersion, prepared from O S ~ ( C O ) ~ ~ and RU~(CO)~~, respectively, and y-A1203, are active for CO hydrogenation, giving alkanes and alkenes.The Ru catalysts are about an order of magnitude more active than the 0 s catalysts and are an order of magnitude less selective for synthesis of C2 and higher hydrocarbons. The 0 s catalysts evidently consist of ensembles of three 0 s atoms bonded to near-neighbour oxygens of the A1203 support. We do not have enough evidence to establish a mechanism of the CO hydrogenation reaction on the present catalysts. It is interesting, however, to compare the results with those reported by Steinmetz and G e ~ f f r o y , ~ ~ who investigated the stepwise reduction of triosmium clusters in solution.The present results show that ensembles of varying sizes can be prepared on oxide surfaces. Ensemble size may affect catalytic activity and selectivity, but the data obtained with catalysts prepared from H20sCI, [which are more active and less selective for synthesis of C2 and higher hydrocarbons than those prepared from O S ~ ( C O ) ~ ~ ] suggest that the heterogeneity of the metal species and the composition of the support are also important. The work done in Munich was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie and the Stiftung Volkswagenwerk, the work done in Delaware was supported by the National Science Foundation. A NATO grant provided support for the joint research. Y.Z. was a recipient of a grant from the German Academic Exchange Service.We express our gratitude to Dr. Storp, Bayer AG, Leverkusen, for his help with the X.p. spectroscopy. P. Biloen and W. M. H. Sachtler, Adv. Catal., in press. E. L. Muetterties and J. Stein, Chem. Rev., 1979, 79, 479. H. H. Nijs and P. A. Jacobs, J . Catal., 1980, 65, 328. R. L. Pruett, Science, 1981, 211, 11. B. C. Gates and J. Lieto, Chem. Tech., 1980, 195 and 248. M. Deeba, J. P. Scott, R. Barth and B. C. Gates, J. Catal., in press. M. Ichikawa, Bull. Chem. SOC. Jpn, 1978, 51, 2273. F. Hugues, B. Besson and J. M. Basset, J. Chem. SOC., Chem. Commun., 1980, 719. A. K. Smith, B. Besson, J. M. Basset, R. Psaro, A. Fusi and R. Ugo, J. Organomet. Chem., 1980,192, C31. lo B. Besson, B. Moraweck, A. K. Smith, J. M. Basset, R.Psaro, A. Fusi and R. Ugo, J. Chem. SOC., Chem. Commun., 1980, 569. l1 M. Deeba and B. C. Gates, J. Catal., 1981, 67, 303. l2 H. Knozinger and Y. Zhao, J, Catal., in press. l3 V, L. Kuznetsov, A. T. Bell and Y. I. Yermakov, J. Catal., 1980, 65, 374. l4 J. Robertson and G. Webb, Proc. R. SOC. London, Ser. A, 1974,341, 383. l5 C, R. Eady, B. F. G. Johnson and J. Lewis, J. Chem. Soc., Dalton Trans., 1975, 2606, l6 J. Lewis, personal communication, 1980. H. Knozinger, H. Stolz, H. Buhl, G. Clement and W. Meye, Chem. Ing. Tech., 1970, 42, 548; and H. Knozinger, Acta Cient. Venez., 1973, 24 Suppl. 2, 76. K. J. McQuade, J. P. Scott, R. Barth and B. C. Gates, in preparation. l9 D. A. Hucul and A. Brenner, J. Phys. Chem., 1981,85,496. 2o A. K. Smith, A. Theolier, J. M. Basset, R. Ugo, D. Commereuc and Y. Chauvin, J. Am. Chem. 21 E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker and W. R. Pretzer, Chem. Rev., 1979, 22 L. Brewer and G. M. Rosenblatt, Adv. High Temp. Sci., 1969, 2, 1. 23 J. H. Sinfelt, Rev. Mod. Phys., 1979, 51, 569. 24 E. B. Prestridge, G. H. Via and J. H. Sinfelt, J. Catal., 1977, 50, 115. 25 M. R. Churchill and B. G. de Boer, Inorg. Chem., 1977,16, 878. SOC., 1978, 100, 2590. 79, 91.H . KNOZINGER et al. 71 26 M. Primet, J. Chem. SOC., Faraday Trans. 1, 1978,74, 2570. 21 R. G. Woolley, Plat. Metals Rev., 1980, 24, 25. 28 R. G. Woolley, in Transition Metal Clusters, ed. B. F. G. Johnson (John Wiley & Sons, Chiches- L9 G. G. Low and A. T. Bell, J. Catal., 1979, 57, 397. 30 P. Biloen, J. N. Helle and W. M. H. Sachtler, J. Catal., 1979,58, 95. 31 R. D. Kelley and D. W. 6oodman, in Chemical Physics of Solid Surfaces and Heterogeneous Catalysts, ed. D. A. King and D. P. Woodruff (Elsevier, Amsterdam, to be published), vol. 4. 32 N. Sheppard and T. T. Nguyen, in Advances in Infrared Raman Spectroscopy, ed. R. J. H. Clark and R. E. Hester (Heyden, London, 1978), vol. 5, pp. 67-148. 33 G. H. Via, J. H. Sinfelt and F. W. Lytle, J. Chem. Phys., 1979, 71, 690. 34 Gy. Gati and H. Knozinger, 2. Phys. Chem. (Frankfurt am Main), 1972, 78, 243. 35 A. Zecchina, personal communication. 36 R. A. Dalla Betta and M. Shelef, J. Catal., 1977, 48, 111. 37 R. Ugo, R. Psaro, G. M. Zanderighi, J. M. Basset, A. Theolier and A. K. Smith, in Fundamen- tal Research in Homogeneous Catalysis, ed. M. Tsutsui (Plenum, New York, 1979), pp. 579-601. 38 M. A. Vannice, J. Catal., 1977, 50, 228. 39 G. R. Steinmetz and G. L. Geoffroy, J. Am. Chem. SOC., 1981, 103, 1278. ter, 1980), pp. 545-606.

ISSN:0301-7249    

DOI:10.1039/DC9817200053

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年代:1981

数据来源: RSC

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Selectivity in Catalysis by Hydrogen-porous Membranes BY VLADIMIR M. GRYAZNOV P. Lumumba Peoples’ Friendship University, Moscow, U.S.S.R. AND MIKHAIL G. SLIN’KO L. Ya. Karpov Institute of Physical Chemistry, Moscow, U.S.S.R. Received 12th May, 1981 The methods of controlling a catalyst’s selectivity by holding its surface (and in special cases its sub-surface) layer in non-stationary states towards the reagents are discussed. This may be achieved by the use of (a) a membrane catalyst; (6) a fluid bed; (c) a riser reactor; (d) periodical changes of the process parameters or catalyst circulation between the reactor and the regenerator; (e) a chromatographic regime; (f) a self-oscillating or stochastic regime. The mem- brane catalyst produces a non-stationary state of the catalyst surface more easily than do the other methods and for a longer period of time.This propensity of the membrane catalyst is especially im- portant for small-scale industrial installations and for producing ultrapure substances. The transfer of one reagent through the catalyst, for example through a palladium-based septum, increases the selectivity of the hydrogenation of triple bonds into double bonds and of, say, one double bond in a cyclic diene in comparison with normal hydrogenation by the same catalyst. Selectivity of the hydro- genation is a function of hydrogen content in the membrane catalyst. It is shown that hydrogen atoms extracted from the sub-surface layer of the membrane catalyst participate in the hydrogenation process. The rate of a complicated catalytic reaction and the selectivity towards one of its products depend on the compositions of the reagents and of the surface and sub- surface layers of the catalyst.These compositions correlate under stationary con- ditions with the conditions of heat and mass conservation. That is why the equations of heat transfer, adsorption, desorption, solubility in the sub-surface layer of the catalyst, as well as the reaction of the adsorbed species, define the stationary values of concentrations and temperatures. Such stationary concentrations and temperatures are usually non-optimal for the rate of formation of the desired product. New opportunities to achieve optimal selectivities and rates of complicated reactions are presented by dynamic conditions which control by various means the composi- tions of the surface and sub-surface layers of the catalyst and make these compositions non-stationary with respect to the reagents. Processes with non-stationary composi- tion and temperature at the catalyst surface may be realized by the following methods: (1) on membrane catalysts; (2) in fluid catalyst beds; (3) in riser reactors; (4) by periodic changes of the process parameters or by circulation of the catalyst between the reactor and the regenerator; ( 5 ) in the chromatographic regime; (6) in the self-oscillating or stochastic regime.Let us compare these methods of controlling the selectivity and rate of hetero- geneous catalytic reactions.74 HYDROGEN-POROUS MEMBRANES CATALYSIS BY HYDROGEN-POROUS MEMBRANES The possibilities of increasing the selectivity of a catalytic reaction by means of one reagent transfer through a septum have been analysed elsewhere.' The discharge of hydrogen formed during the dehydrogenation or dehydrocyclization reaction through the hydrogen-porous catalyst raises the reaction rate and the selectivity by suppressing the side reactions.2 This method is much more effective than is a decrease in total pressure or partial pressure of the initial reactants.Hydrogen introduction through the membrane catalyst into the zone of hydro- genation permits one independently to control to some extent the surface concentra- tions of hydrogen and hydrogenatable molecules. It is especially important in obtaining incompletely hydrogenated products which are thermodynamically unstable in the presence of hydrogen.From the academic point of view hydrogen transfer through the membrane catalyst provides an opportunity to elucidate the participation of atomic or molecular hydrogen in the hydrogenation reaction. The acetylene hydrogenation study by E. A. Zelyaeva and one of the authors of this paper shows that at temperatures from 100 to 180 "C ethylene was formed on the outer surface of a tube with a wall thickness 0.1 mm made of an alloy of 94.1 wt.% palladium and 5.9 wt. % nickel during hydrogen introduction through the tube walls onIy. Fig. 1 demonstrates that molecular hydrogen input in the glass envelope of the said tube increases the partial pressure of hydrogen (curve 1) but does not change the partial pressure of acetylene (curve 2).Ethylene formation commences immediately after hydrogen introduction through the tube walls at time A (curve 3). The cessation of molecular hydrogen input in the glass envelope at time B does not change the rate of acetylene hydrogenation. However, the halt of hydrogen flow through the mem- brane catalyst at time C stops hydrogenation promptly. These data permit one to A 0 c 300 i 250 t 200 150 ._ 3 e 2 100 v) .m 50 L I 0 50 100 1 2 ooOo0 3 m I 150 200 250 timelmin FIG. 1 .-Mass-spectral peak intensities of hydrogen (curve l), acetylene (curve 2) and ethylene (curve 3) against time during acetylene hydrogenation on a palladium-nickel membrane catalyst at 180 "C (see text).v. M. GRYAZNOV AND M . G . SLIN'KO 75 conclude that under the conditions mentioned hydrogen in molecular form on the surface of palladium-nickel alloy as catalyst does not react with acetylene, whereas hydrogen atoms coming from the sub-surface layer do participate in the hydrogenation of acetylene.The mechanism of acetylene hydrogenation with participation of atomically adsorbed hydrogen has been proposed by Bond and Wells.3 In their experiments palladium on an or-alumina catalyst was very selective for acetylene hydrogenation into ethylene under defined conditions, but the formation of ethane in small quantities hinted at the possibility of complete hydrogenation of a certain proportion of acetylene molecules during one single stay on the catalyst surface. Unlike these data3 ethane was not detected mass-spectrometrically in the products of acetylene hydrogenation by hydrogen diffusing through the palladium-nickel mem- brane catalyst, Such high selectivity of hydrogenation may stem from the use of this catalyst in the or-phase, with a low hydrogen concentration in the alloy.The hydrogen partial pressure inside the palladium-nickel tube was 1 Torr and was thus not high enough for the formation of the hydrogen-rich P-phase. Bond and Wells3 mainly used the palladium catalyst in /?-phase. A marked increase in selectivity of acetylene hydrogenation into ethylene after the transformation of the P-phase of the Pd-H2 system into the a-phase has been found by Palczewska and coworker^.^ The influence of hydrogen concentration in the membrane catalyst on the selecti- vity of triple-bond to double-bond hydrogenation has been investigated by Karava- nov, Maganjuk and one of the authors of this paper.Acetylenic alcohols have been hydrogenated in the liquid phase on thin-walled tubes made of palladium alloy. The tube was immersed into the hydrogenatable liquid and hydrogen was allowed to flow inside the tube. The selectivity of acetylenic alcohol hydrogenation into the corres- ponding ethylenic alcohol was estimated after the establishment of a stationary reaction rate. The amount of hydrogen dissolved in the membrane catalyst at this time was determined by the amount of acetylenic alcohol hydrogenation products obtained after ceasing the hydrogen input inside the membrane catalyst. The resulting value differed by less than 5% from the amount of soluble hydrogen in the membrane catalyst found volumetrically.Palladium alloy with a low concentration of the dissolved hydrogen was highly selective towards triple-bond to double-bond hydrogenation. The increase in the amount of dissolved hydrogen decreased the selectivity from 0.96 for the a-phase to 0.54 for the hydrogen-rich P-phase. The only product of 2-butyne- 1,4-diol hydrogenation by hydrogen diffused through the membrane catalyst during the addition of the first mole of hydrogen was 2-butene- 1,4-diol. In contrast to this result bubbling hydrogen through the liquid butynediol in the presence of the same catalyst at the same temperature produced butenediol and butane-1,4-diol from the beginning of the reaction. This again does confirm that hydrogen input through the membrane catalyst makes the hydrogenation process especially selective. To elucidate the possibility of selective hydrogenation of one ethylenic bond in molecules with two or more double bonds the transformation of cyclopentadiene in the vapour phase was studied.At 102 "C cyclopentadiene (CPD) was passed over palladium-ruthenium alloy at a rate of 7 mol h-l. During hydrogen introduc- tion through the membrane catalyst CPD was converted completely into a mixture of 92% cyclopentene (CPE) and 8% cyclopentane (CPA). The absence of CPD in the products was very important because of its ability to poison the catalyst, causing CPE polymerization into polypentenomer. The presence of hydrogen in the mixture with CPD vapours yielded less CPE in the hydrogenation products at high conversion (see curve 1 of fig.2) than did intake of hydrogen through the membrane catalyst (curve 2). The large excess of hydrogen76 HYDROGEN-POROUS MEMBRANES in the mixture with CPD vapour was not sufficient for the high selectivity of CPE formation which took place during hydrogen intake through the membrane catalyst. The influence of the size of cyclopolyolefin molecules and of the number of double bonds in the ring on the hydrogenation rate and selectivity was investigated by Ermilova, Smirnov and one of the authors. A foil of palladium (90.22 wt.%)- ruthenium (9.78 wt.%) alloy was used as the membrane catalyst as well as in the experiments with cyclopentadiene. Cyclo-octadiene (COD) hydrogenation by hydrogen diffusing through the membrane catalyst yielded cyclo-octene (COE) and cyclo-octane (COA) only.The products of trans,trans,cis-cyclododecatriene (CDT) G 0.5 X 2 1 1 .o FIG. 2.-Product of selectivity towards cyclopentene q with the conversion degree X plotted against X for cyclopentadiene hydrogenation in the mixture with hydrogen (curve 1) and during hydrogen introduction through the membrane catalyst (curve 2). hydrogenation were cyclododecadiene (CDD), cyclododecene (CDE) and cyclodode- cane (CDA). The product yield on adding two hydrogen atoms at the same degree of conversion decreased with the enlargement of the polyolefin ring. This effect did not correlate with the decrease in the number of initial molecules which may be placed on the catalyst surface. The maximum yield of COE from COD was reached at 13 times the decrease in space velocity in comparison with the CPD intake. The relation of the area per molecule in the monolayer on the catalyst surface for COD and CPD is not so high.Hence the decrease in the amount of initial substance adsorbed from CPD to COD was not the unique cause of the decline of the rate and selectivity of hydrogenation. This conclusion was confirmed by the fact that maximum product yield of one double- bond hydrogenation in CDT took place at a 46 times smaller space velocity than for CPD to CPE conversion. The above-mentioned data show that hydrogen transfer through the permselective membrane catalyst increases the selectivity of hydrogenation in comparison with the usual method when the catalyst is in the stationary state uis-2-vis the reagents.v. M.GRYAZNOV AND M. G . SLIN’KO 77 PROCESSES I N FLUID CATALYST BED Catalyst particles in the fluid-bed reactor move in non-uniform fields of con- centration and temperature. The composition of the catalyst particles is a function of the relaxation time of the reaction and the rates of other stages as well as the velocity of the particles. The state of the catalyst, characterized by the density of distribution of the catalyst particles by their composition at a given height in the reactor, depends on the ratio of relaxation times. Let us analyse the simplest reaction A + B -+ C which consists of two irreversible steps with one intermediate compound, for example adsorbed oxygen : A + [ Z l - , C + [ l B + [ l +z. The rate of change of the fraction of oxidized sites is a function of time where kl and k, are rate constants of the reaction steps; C, and C, are concentrations of the reagents in the gas phase; LN is the number of active sites on the unit of surface which is equal to the monolayer coverage of the catalyst surface by the adsorbed oxygen.The motion of catalyst particles in the fluid bed will be found from the diffusion model. The relaxation time of diffusion zD depends on the shape of the reactor and hydrodynamical conditions and is of order L2/D, where L is the expanded height of the fluid bed and D is the effective diffusion coefficient. The catalyst surface state in the reactor is a function of the ratio of the diffusion, zD, and reaction, zR, relaxation times y2 = T,/Z,. The surface composition of the catalyst particles will be stationary at high values of y.This I+Y value depends on the composition of the reaction mixture and on the catalyst activity. y must be >200 in the simplest case.6 The change in I,Y permits one to control the catalyst surface composition. In any event the initial composition of the catalyst particles may be controlled by special treatment outside the reactor using the continuous input and output of the catalyst of the fluid-bed reactor. For the processes of partial oxidation it is important to have a definite degree of catalyst surface oxidation. For example some portion of the oxygen ions is removed from the surface of a vanadium catalyst during SO,, o-xylene or naphthalene oxidation and the valence state of the vanadium ions is also changed. In many cases the activity and selectivity increase with increases in V5 + . Simultaneously a slower process of catalyst surface layer re-construction may take place as a result of the alteration of the metal ion valency.The actual relaxation time of the reaction may be influenced by variation of the reaction mixture composition. If the catalyst properties are functions of oxygen content in the catalyst sub- surface layer and its structure the relaxation times can become much higher than the proper relaxation time of the reaction. These cases require much more precise ana- lysis as well as taking into consideration the processes of oxygen diffusion in the sub- surface layer of the catalyst. Then selectivity control by the corresponding treatment of the catalyst outside the reactor becomes easier because the catalyst remains in a non-stationary state in the reactor for a long time.78 HYDROGEN-POROUS MEMBRANES UP-FLOW REACTORS Up-flow reactors are used with highly active catalysts.The residence time of catalyst particles in the riser is but several seconds. Because of the different velocities of the reagents and the catalyst particles they move in a permanently renewed mixture. The surface-layer composition of the catalyst particles and the selectivity are de- termined by the initial composition and temperature, which are functions of the regeneration conditions and may be changed by the catalyst circulation rate. CHROMATOGRAPHIC REGIME The regime and the catalyst composition are non-stationary with respect to the gas-phase composition in the chromatographic reactor.That is why the optimal com- position of the catalyst can be reached by proper pretreatment or cyclic treatment of the catalyst. SELF-OSCILLATING A N D STOCHASTIC REGIMES In the self-oscillating regime the catalyst surface composition swings in the sim- plest case about an unstable stationary composition. The control of amplitude and frequency of self-oscillations of the heterogeneous catalytic reaction rate permits one under definite conditions to select such a cycle in which the mean rate7 and the selectivity of the complex reaction will be higher than in the stationary unstable state. The solution of the problem of selectivity demands an analysis of dynamical systems of at least the third order.Transition from dimeric systems to dynamic systems of higher order causes stationary regimes of a new type which are judged non- stable by Lyapunov’s criteria and have stochastic character. Such regimes may be realised, for example, in selective ammonia oxidation to nitric oxide on platinum gauze.* The mean values of selectivity and conversion are not functions of time and may be higher than the stationary values. Note that the given stochastic regime is stipulated by the intrinsic properties of the system having not less than three variables but not incidental changes of the initial conditions. As a conclusion we should like to draw attention to the membrane catalysts, which permit holding the surface layer composition in a non-stationary state uis-ci-uis the reaction mixture more easily and for a longer period of time than the other methods discussed. This conclusion is especially true for the production of ultrapure sub- stances. One of the advantages of the membrane catalyst is the realization of a non- stationary catalyst composition in one reactor without using a regenerator or any other device. V. M. Gryaznov, Dokl. Acad. Nauk SSSR, 1969, 189, 794. V. M. Gryaznov, V. S. Smirnov and M. G. Slin’ko, in Proceedings of the Fifth International Congress on Catalysis, ed. J. W. Hightower (North Holland Publishing Co., Amsterdam, G. C. Bond and P. B. Wells, J. Catal., 1965, 5, 65. A. Borodzinski, R. Dus, R. Frak, A. Janko and W. Palczewska, in Proceedings of the Sixth International Congress on Catalysis, ed. G . C . Bond, P. B. Wells and F. C. Tompkins (The Chemical Society, London, 1977), vol. 1, pp. 150-162. V. S. Smirnov, M. M. Ermilova, N. V. Kokoreva and V. M. Gryaznov, Dokl. Acad. Nauk SSSR, 1975, 220, 647. S . A. Pokrovskaya, V. S. Sheplev and M. G. Slin’ko, Dokl. Acad, Nauk SSSR, 1979, 244, 669. 1973), V O ~ . 2, pp. 80-1139-80-1147. ’ M. G. Slin’ko and M. M. Slin’ko, Usp. Khim., 1980, 49, 561. ’ M. Flytzani-Stephanopoulos, L. D. Schmidt and R. Caretta, J. Catal., 1980, 64, 346,

ISSN:0301-7249    

DOI:10.1039/DC9817200073

出版商:RSC    

年代:1981

数据来源: RSC

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GENERAL DISCUSSION Prof. G. C . Bond (Brunel University) said: In addition to those items listed by Prof. Sachtler as requirements for a selective reaction, one should surely add the further stipulation that those factors associated with the catalyst itself should not be time-dependent. Prof. Sachtler knows better than I the strict standards of chemical and mechanical stability which new industrial catalysts must now meet, and research workers should be cautioned that a perfectly selective catalyst of only limited life will remain at best an academic curiosity. One of the principal problems I see in the design of selective metal catalysts to operate at high temperatures is the likelihood of quite rapid migration of atoms in the surface plane, and to a lesser extent normal to this plane: so that for example a well-dispersed bimetallic catalyst at 500 "C may well expose a semi-fl uid surface having no well-defined surface composition or structure.Some attention should be paid to the problem of making catalysts whose surfaces will retain the planned configuration under reaction conditions. Prof. W. M. H. SacMler (Shell, Amsterdam) said: After having dealt with the question " what makes a catalyst selective?" it is certainly of interest to turn to the question " how do we keep our catalysts active and selective?" Dr. 0. Zahraa (L'niuersity of Carbondale) said: While it is obvious that the elec- tronic properties of the faces, edges and corner atoms in a metal crystallite differ, I would like to ask if one considers a catalyst at high dispersion with an average crystal- lite size (20-25 A), which kind of atom could form the " ensembles " ? Are the atoms from the faces or edges? Or perhaps a combination of both? Prof.W. M. H. Sachtler (Shell, Amsterdam) said: For well-reduced transition metals with every atom in the zerovalent state it can safely be assumed that the energy of interaction of the atoms with SOz, or Al,03 support surfaces is smaller than the energy of interaction of these atoms with each other. Consequently the agglomerates will tend to form three-dimensional structures with overall shapes in agreement with the Gibbs-Wulff principle. Such clusters or crystallites can be described as polyhedra with cut-off corners, sometimes approximated by cubo-octahedra as we did in ref. (1).All exposed atoms of these agglomerates can, in principle, be part of ensembles used in chemisorption, but at high dispersion many of the ensembles will have a considerably wider spacing than those on the surface of a close-packed crystal face. W. M. H. Sachtler et al., J. Chim. Phys., 1954, 51, 1954. Dr. R. Burch (Uniuersity of Reading) said : In his lecture Prof. Sachtler discussed the importance of ensemble size in determining the selectivity of alloy catalysts. In the particular case of Ni/Cu alloys it has recently been reported1 that ethane hydro- genolysis requires an ensemble of 12 Ni atoms. It has also been reported2 for these same catalysts that the heat of adsorption of hydrogen at low coverage is decreased when Cu is added to Ni. Since it is difficult to envisage a mechanism in which an ethane molecule is attached to 12 Ni atoms, and since Cu appears to affect the strength80 GENERAL DISCUSSION of chemisorption bonds to Ni, is it possible that in alloys of this type there is a sig- nificant electronic ligand effect operating in addition to an ensemble effect? M.F. Guilleux, J. A. Dalmon and G. A. Martin, J. Catal., 1980, 62, 235, J. J. Prinsloo and P. C. Gravelle, J. Chem. SOC., Faraday Trans. I , 1980, 76, 2221. Prof. W. M. H. Sachtler (Shell, Amsterdam) said: We have addressed ourselves to the problem of the ligand effect in adsorption by alloys in theoretical studies1 and experimental work, including i.r. spectroscopy and temperature-programmed desorption of hydrogen or CO from alloy films.3 While I must refer to the original publications for an extensive analysis of the problem, I conclude from the data at present available that the selectivity by alloy catalysis is much less affected by changes in bond strength due to the electronic ligand effect than by the relative population of different adsorption states due to the geometric ensemble effect.With respect to the model of 12 nickel atoms required for the adsorption of ethane I refer to my remark in the written text of my lecture. R. A. van Santen, W. M. H. Sachtler, Surf. Sci., 1977,63, 358; W. M. H. Sachtler and R. A. van Santen, Adv. Catal., 1977, 26, 69. Y . Soma-Noto and W. M. H. Sachtler, J. Catal., 1974, 32, 315. J. J. Stephan, V. Ponec and W. M. H. Sachtler, Surf. Sci., 1975, 47, 403; J. J. Stephan, P.L. Franke and V. Ponec, J. Catal., 1976, 44, 359. Prof. J. J. Rooney (Qzreen’s Uniuersity, Belfast) said : In order to understand selec- tivities in catalysis we must understand mechanisms. This is quite clear from Prof. Sachtler’s lecture. However, mechanisms are extremely difficult to elucidate. especially as the catalytic centres are often far fewer in number than hitherto generally believed. For hydrocarbon conversions in hydrogen at elevated temperatures on transition-metal surfaces there is the additional complication that many reactions may simultaneously occur. Very often because of the breadth of our ignorance we accept postulated mechanisms for which there is really very little evidence. After these have been written into the literature for a decade or so they acquire the respecta- bility of dogma.I would therefore make an earnest plea to workers in the particular area mentioned to re-examine most of the mechanisms of bond shift and cyclizations, etc., because in the words of Daniel O’Connell one could “ drive a coach and horses ” through most of them. My second point is that most of the non-destructive reactions on metal surfaces also seem to be due to intermediates essentially bonded to only one metal atom. By the term ‘‘ non-destructive ” I am excluding drastic bond fission reactions such as Fischer-Tropsch synthesis and hydrocarbon methanation. The emerging role of a central mononuclear site vindicates a point made almost 20 years ago,‘ that catalysis on surfaces is often a consequence of interconversions of various intermediates as reactive ligands of the same metal atom or ion.If this is true the idea of ensembles, as described, may be a barrier to progress since we should be thinking instead of a complex where the contiguous atoms or ions, metal or metalloid, act as permanent ligands of the one at the centre of the active site. Their number, disposition and electronic properties are then very important considerations as Dr. Burch points out. Mechanisms can then be discussed in the same language as that of the coordination and organometallic chemist such that the geometric and electronic factors are not seen as independent and separate foundations of any theory. I am not disputing that ensembles or clusters exist. I am merely stressing that jargon can be a great hindrance to progress and to the realization of where catalysis theoretically has its true place in chemistry in general.J. J. Rooney and G. Webb, J. Cafal., 1964, 3, 488.GENERAL DISCUSSION 81 Prof. B. C. Gates (University of Delaware) said: It is a common textbook assertion that high selectivities are characteristic of homogeneous catalysis, in contrast to surface catalysis. Some of Prof. Sachtler’s examples appear to be inspired by homo- geneous (molecular) catalysis. May I ask him to be an advocate of solids and surfaces ? What unique opportunities do they present for selectivity in catalysis? Prof. W. M. H. Sachtler (Shell, Amsterdam) said: Three facts of relevance to selectivity might be quoted to illustrate the opportunities of heterogeneous catalysts.(1) Many biocatalysts, renowned for high selectivity, contain active groups attached to protein bodies, while reactants are dissolved in the surrounding liquid. Such biphasic systems are therefore examples of heterogeneous catalysts. (2) Crystal faces and the pore system of zeolites can provide rigid templates, essential for stereoselectivity. Besides shape-selective zeolite catalysts, I would like to mention Ziegler-Natta catalysts, where different crystal modifications of the same compound, TiCl,, yield different (isotactic or atactic) isomers of the same polymer. (3) Numerous catalysts, including biocatalysts, are bi- or multi-functional. By fixing each functional group on a surface one can prevent undesired interactions between these groups, which would occur if they were dissolved in a common solution.Prof. V. M. Gryaznov (Lumumba University, Moscow) said: I would like to ask (1) Does he have an explanation why alloying with a Group Ib metal should (2) Can he explain why Pd is so easily self-poisoned and does he think dissolved Prof. Ponec the following questions. invalidate the valley position? hydrogen could be responsible ? Prof. V. Ponec (Rijksuniversiteit Leiden) said : (1) I think a theoretician could answer Prof. Gryaznov’s first question better than I can. My simple ideas in this respect are as follows. The fully occupied d-orbitals of Cu are spatially more contracted (higher atomic number 2) than the d-orbitals of Ni, which are directed into the same surface hollow (valley position). Both factors -occupation and contraction-diminish the binding strength of Cu atoms and may finally cause the CO molecules to feel more comfortable, on top of the Ni atoms than in the surface holes partially surrounded by Cu.This also means that the neigh- bouring Ni atoms are again covered so that the whole domain of the Ni-Cu alloy surface is finally occupied “ on top ”. (2) Pd is possibly more poisoned because the carbon layer or the layer of carbon- aceous residues (once it is formed) is less reactive than on Ni. In this case dissolved hydrogen may possibly be involved although this has not yet been proved in skeletal rearrangements. Prof. L. Guczi (Institute of Isotopes, Budapest) said : The importance of geometric arrangement could be proved by simple geometric arguments. Assuming the valence angle to be ca.109 O, the 1-3 complex can be accommodated on the Pt surface of Pt(ll1) without serious strain, whereas on Ni(ll1) it can not. This fact may be an explanation for the experimental finding that on Ni, isomerisation has not been ob- served. Concerning the similarity in the role of carbon and sulphur, this can indeed be found in most cases. However, for smaller molecules such as n-butane, we observed that although in both cases the activity decreased, in the case of sulphur isomerisation82 GENERAL DISCUSSION increased, whereas with deposited carbon CH,-formation increased when a Pt-Re/ A1203 catalyst was used.' L. Guczi, Bull. SOC. Chim. Belg., 1978, 88,497. Prof. V. Ponec (Rijksuniversiteit Leiden) said: If one can apply exactly the same " stereo rules " both to free molecules with localized bonds and to adsorbed species where the metal-carbon bond can be a multicentre (delocalized) bond, then interesting conclusions can indeed be made concerning the geometry of the adsorbed species and the consequences of that geometry.However, in that favourable case one should not forget that even if the overlap of the metal and carbon orbitals is not optimal, there is usually some overlap possible and binding cannot be excluded; the same is true for situations which bring about some strain in the adsorbed species. Prof. Bond once made a point that actually " less than optimal " fit is essential for making the adsorbed species an active intermediate and not only a poisoning blocking material.By the way, well-poisoned Ni reveals low activity in the isomerisation of hexane (not neohexane). Prof. Z. Pail (Institute of Isotopes, Budapest) said: (1) In his interesting and excellent paper Prof. Ponec put forward the idea of a " valley position " deactivation by C, S or other impurities. I should like to draw the attention to a special case where valley-position adsorption may enhance an unusual sort of activity, thus causing significant selectivity changes. This is the case with hydrogen adsorbed in valley positions. Hydrogen can thus block the most active sites where multiple hydrocarbon-metal interactions (important for hydrogenolysis and coking) occur. By doing so, the possibility is created that active sites are on the top of metal atoms. These sites form single metal-carbon bonds and we believe that such intermediates are important for C,-cyclization and ring opening.' This is illustrated by fig.1. It is also clear that after the formation of a single metal- FIG. 1.-Schematic representation of hydrogen and carbon effects over the Pt (111) surface. Large empty circles denote Pt atoms, small shaded circles C atoms (to scale). Hydrogen adsorbed in " valleys " and on top of atoms is denoted by " H ". Top adsorption active for C5-cyclic reactions (shown for both 3-methylpentane and methylcyclopentane) is impossible on Pt atoms (light shading) where a C-atom occupies a valley position.GENERAL DISCUSSION 83 carbon bond (preferentially on the tertiary carbon atom having lowest C-H dis- sociation energy) the site of ring opening/closure will be determined by the distance between metal atoms.The active species shown in fig. 1 may give selective ring opening in positions b and c with respect to the substituent. Hydrogen atoms on top of metal atoms may also participate in ring closure/opening, which thus may proceed via an associatively adsorbed surface species such as that proposed by Liberman.’ Such a geometrical agreement between catalyst and reactant is apparently a unique feature of C,-cyclic reactions. We think that this is another important in- herent difference between the so-called “ bond shift ” and “ C,-cyclic ” mechanisms of skeletal rearrangement.3 Selective deactivation experiments support the above mechanistic as~umption.~ For example, the case of a radiotracer study provides evidence that ca.20% of the surface Pt atoms are covered by carbon atoms, the activity drop of C,-cyclization and skeletal isomerization of 3-methylpentane was 38%, whereas only a 3% decrease of aromatization-dehydrogenation activity was observed. Obviously one carbon in a valley position excludes more than one Pt atom from C5-cyclic reactions (shaded atoms in fig. 1) whereas these metal atoms still retain their activity in dehydrogena- tion-aromatization. (2) Prof. Ponec’s finding that Pt and Ir are more liable to catalyse internal C-C bond fission than Pd and Ni is in agreement with our previous results obtained with metal black^.^ Could he agree with an explanation whereby metals in lower rows of the Periodic Table having a second maximum in their photoelectron spectra possess a higher density of delocalized electrons.This facilitates less strong interactions which are more n-type in character.6 The experimental facts indicate that such interactions must favour internal fission, although at present we do not see the reason why this empirical correlation is true. Z . Paiil, Adu. Catul., 1980, 29, 273. A. L. Liberman, Kinet. Katul., 1964, 5, 128. H. C. De Jongste and V. Ponec, Bull. SOC. Chim. Belg., 1979, 88, 453. * Z. Paiil, M. Dobrovolszky and P. TCttnyi, J. Catal., 1977, 46, 65. Z. Padl and P. Ttttnyi, React. Kinet. Catul. Lett., 1979, 12, 131. Z. Knor, Kinet. KutuL, 1980, 21, 17. Prof. V. Ponec (Rijksuniuersiteit Leiden) said : (1) I sympathize with Prof. Pad’s idea, indeed, that hydrogen might also be causing an enforced shift of species from multibound types in the valley to those singly and doubly bound on the summits. I certainly would not exclude the possibility that carbon atoms could be double-bonded to metal atoms on the summits.I think it is conceivable that a double bond reacts with other groups as it should upon ring closure/ opening, but a conversion of two single metal-carbon bonds into a new carbon-carbon bond seems less probable to me; a single bond is much more localized in space, and its electrons do not interact so easily with other groups. Prof. PaXs information on the influence of carbon deposition on the selectivity is very interesting. This finding confirms the idea that 1-6 ring closure and aromatisation is a reaction run- ning via stepwise dehydrogenation (which can proceed even on isolated atoms), as he and his colleagues suggested some years ago.I think that the results of Mr Davis (from Prof. Somorjai’s laboratory) obtained with Pt monocrystals are similar. (2) I do not follow two points of Prof. PaQl’s suggestion. (i) What is meant by “ density of delocalized electrons ”: the number of sp elec- trons per atom (or spd electrons) or does it concern the spatial distribution of elec- trons and density in certain points? Which one, the valley centre? The existence84 GENERAL DISCUSSION of the two X.P.S. maxima with e.g. Pt or Ir is usually explained as a consequence of the more pronounced relativistic behaviour of valence electrons in heavy atoms (spin- orbit interaction), which fact leads to a splitting of the valence band into two sub- bands. Heavy atoms have the (n - 1) d-electron orbitals more contracted so that the density of these (n - 1) d more localized electrons is higher around the nuclei than with light elements.Its relation to the occurrence of two maxima in the X.P.S. spectra is just a coincidence, I think. (ii) The n-type metal-carbon bonding is generally believed to occur more easily when there are binding (but rather localized) metal d-orbitals able to form such a bond. However, I do not see any relation between the propensity of a metal to form n-bonds and the occurrence of two X.P.S. maxima, or a relation to any variation in the density of delocalized conductivity sp electrons. I would prefer an explanation of the exceptional behaviour of Pt and Ir along the lines outlined in our paper: these two metals have an intrinsic preference for binding carbon atoms on top of the surface atoms and this tendency is strengthened by the presence of deposited carbon.Neither Pt nor Ir are good methanation catalysts; once carbon is formed it stays longer than on Ni. If one accepts that isomerisation runs better in species bound on summits, the problem is solved. Dr. R. A. van Santen (Shell, Amsterdam) said: It can be argued that alloying causes a shift in adsorption from hollow sites to top sites for electronic-structure reasons. The electronic structure of a transition metal can be fairly well described by a constant electron density of 1 electron per atom in a broad s-band and a narrower d-band with varying electron density.Alloying with a Ib metal does not appreciably change the s-band electron density and mainly affects the d-electron band density. We simplify the description of chemical bonding by assuming that the interaction of the adsorbate with the metal is additive for s and d electrons. We limit ourselves further to the interaction with d-electrons, which we suspect are responsible for the electronic effect. Note that the s-band contributes significantly to the chemical bond strength; changes in d-band structure (ligand effect) have a small, but certainly definite effect. In the bulk an atom has 12 neighbours, at the (100) face 4 atoms are removed from the first coordination shell. The z-axis is chosen perpendicular to the (100) plane.In this geometry one observes that dzz orbitals have no mutual 0 overlap. The same holds for the ~ , Z - ~ Z orbitals. On the other hand d,,, d,,, d,, have a 0 overlap. As a consequence the d-electron structure can be schematically described as a broad dxy,yz,zx band over- lapping a narrow d , ~ , , 2 - ~ ~ band. Hence for transition metals at the end of the Group VIII period, the d,z and ~ , Z - ~ Z orbitals will be filled, as this sub-band will be under the Fermi level. Upon surface formation the dzz orbital loses overlap with four dx2-y~ orbitals in the upper plane and the d,Z-,z orbital loses overlap with 4dzz orbitals. In this way the d,~, d,2-,~ band narrows upon surface formation (see fig. 2). There is no change in the d,, band, but the dyz and dxz orbitals lose 2 nearest neigh- bour overlaps with dy2, dzz, and dxz-yz and dxz, d , ~ and dx2-yz respectively.Since they maintain their n overlap in the xy plane only a small narrowing of the bands is expected (see fig. 2). So when one moves from left to the right in Group VIII of the periodic system one may expect that at the surface first the d,,, and d,, orbitals will become completely filled and further in the period the d,, band. Of course the validity of this description depends on the assumption that the Consider an atom at the (100) face of a f.c.c. crystal.GENERAL DISCUSSION 85 effect of increased electron-electron repulsion of a completely filled sub-band is small compared with the difference between the maxima of the sub-band energies. How- ever, the order of band filling in the bulk and at the surface that we deduced agrees with that computed by Fassaert and van der Avoirdl for Ni.In Ni the d,, and d,, orbitals turn out to be completely filled. We will illustrate the ligand effect upon the change in the relative bond strengths of an atom in a hollow and top position using adsorption of a hydrogen atom. A hydrogen atom adsorbed on a top position can only interact with the completely filled d,z orbital. So band filling or alloying has little effect. Bonding of an H atom r n(E) E 11 FIG. 2.-Schematic sketch of the electron distribution of a f.c.c. Group VIII metal and its (100) surface : (a) bulk electronic energy distribution ; (b) surface electronic energy distribution. in a bridging position requires overlap with a bonding combination of dz2, dxz and dyz orbitals, Obviously filling of some of the dxz and dyz orbitals will have a negative effect on the band strength of the bridge position. So a change in the electron density affects the bridged position more than the top position.Band narrowing cannot only lead to an increase in electron density, but in addition affects the localization energy. This can lead to a decrease in bond strength of bridge-coordinated hydrogen atoms2 even when there is no direct coordination to an alloying Ib atom. The essential argument for a hydrogen atom is that the ensemble of metal atoms involved in bonding with the adsorbate, if isolated from the rest of the lattice, is found to have electrons distributed over bonding as well as antibonding orbitals, when the metal atoms in the cluster are able to form direct bonds between each other.This occurs even when each of the atomic orbitals contributes only one electron.86 GENERAL DISCUSSION Embedding of the ensemble in a metal can lead to relaxation of the electrons from the antibonding orbitals into states near the Fermi level, when the antibonding orbitals remain higher than the Fermi level after embedding. This favours the adsorption energy of bridged adsorbates in large particles compared with that of particles of the size of the ensembles itself. There is an additional effect of relevance to alloying. Embedding of the ensemble into the metal leads to a decrease between bonding and antibonding levels of the ensemble with adsorbate because of delocaliza- t i ~ n .~ ’ ~ In the case that alloying leads to localization of electrons, this difference between bonding and antibonding levels will increase. As long as an antibonding level before alloying was below the Fermi level, this can lead to a decrease in bond energy of bridge-adsorbed species relative to that of top-adsorbed species. An alternative way to visualize this is to consider bridge bonding as an interaction of the symmetrical H atom orbital with symmetrical bonding combination of the ensemble metal atoms usually of lower energy. Because of symmetry there is no interaction with the antibonding ensemble metal atoms. When the ensemble is embedded in the metal, there will be a broaden- ing of the orbitals. Consequently when bonding of adsorbate to metal is not too strong, the bonding energy will increase since ensemble orbitals higher than in the isolated case become available.In the case of alloying, with an increased localiza- tion of the ensemble orbitals the maximum energy of the available binding ensemble orbitals will decrease, resulting in a lowering of the bonding energy of adsorbate to alloy surface. D. J. M. Fassaert arid A. van der Avoird, Surf. Sci., 1976, 55, 297. R. A. van Santen and W. M. H. Sachtler, Surf. Sci., 1977,63, 358. D. M. Newns, Phys. Rev., 1969, 178, 1123. J. R. Schrieffer, J. Vac, Sci. Technol., 1972, 9, 561. Prof. V. Ponec (Rijksuniversiteit Leiden, The Netherlands) said : In the simplified picture I suggested in my answer to Prof. Gryaznov, the hollow becomes less favour- able for binding CO or hydrocarbons, because Cu itself binds less, and a (1 11) hollow among, say, two Ni atoms and one Cu atom is less binding than the summits of the two Ni atoms.In my picture this is the “ maximum ” effect of the electronic structure difference between pure Ni and a Ni-Cu alloy. I did not recall any effect of Cu on the behaviour of Ni atoms. This is, of course, a simplification, and Dr. van Santen’s suggestion may be considered as a first-order correction to such a simple picture. The question now is how important such a correction is for the adsorption and cataly- sis by Ni atoms in Ni-Cu alloys, etc. My associates, Toolenaar and Stoop addressed themselves to this question. Using i.r. spectra, they investigated how much the summit position properties change with regard to CO adsorption, when Pt is alloyed with Cu.Namely, on alloys the frequency of a stretching CO vibration is lower than on pure Pt and this has al- ways been considered as evidence of the ligand effects of alloying, in the literature. However, Toolenaar and Stoop obtained results which showed that within the error of the method any effect of Cu on the Pt properties was negligible. By using the isotopic dilution (l2C0, 13CO) technique, Stoop and Toolenaar proved that the main effect causing the decrease of v(CO/Pt) was the dilution of the CO layer and the suppression of the CO-CO interaction. The results appeared in J . Chem. Soc., Chem. Commun., 1981, 1027. I think that with alloys like Pt-Cu and other endothermic and less exothermic ones, the ligand effects are not detectable by strong chemisorption. However, the shift in adsorbed species from the valIey to the atom tops caused by alloying a Group VIIIGENERAL DISCUSSION 87 metal with a Ib metal occurs widely and is easily observable.Therefore, I think that an explanation in terms of effects which do not assume changes on Group VIII metal atoms is preferable for the data known up to now. I am convinced that some type of (weak?) adsorption, sensitive to the ligand effect of, e.g., the type Dr. van Santen has suggested, will be found in the future. Prof. J. J. Rooney (Queen's University, Belfast) said : The " metathesis of carbene " mechanism for 172-bond-shift is not correct. There are many good reasons for this conclusion, some of which have been published.' The correct mechanism of neo- pentane rearrangement, etc.on Pt is as follows: or The metallacyclobutane or ay adsorbed species can undergo fission, as in meta- thesis, but fission only gives hydrogenolysis products. 'Pt ' The second mechanism has been published,2 but largely ignored although it is theoretically sound and also explains why there may be a common intermediate for both 1,2-bond-shift and hydrogenolysis. It involves only a simple extension of the m.0. theory for the first mechanism but can be very readily appreciated in terms of canonical forms, e.g. C C M I I - M The first mechanism has already been proven2 for a wide range of model com- pounds, but both may occur for simple alkanes. Indeed it has been found3 that methylcyclopentane undergoes 100% selective bond shift on Au-rich Pt alloys, a result completely in accord with expectation on the basis of the first mechanism.This could well be in general the only important mechanism. The second one via metallacyclobutanes remains to be proven, although there is some evidence for it in the field of metathesis.88 GENERAL DISCUSSION Selective demethylation and substantial hydrogenolysis without much cyclization is a feature of Ni catalysts, so it seems that phosphorus on the surface of Pt (" Chatt clusters ") makes Pt behave like Ni. Since preferential demethylation on Ni is due to preferential attacl; at primary C atoms and fission via map-type triadsorbed species, I would suggest that similar species are responsible for demethylation on the P- modified Pt.If the metallacycle theory for demethylation were correct (my+ m y , fig. 2 of Maire's paper) significant 1,2-bond shift giving ring enlargement and ulti- mately benzene formation should have been noted. Have the authors any comment on this aspect of their results? Finally, catalysts such as Co and Ni, which favour surface reactions involving mxp and ccc@ species, i.e. metallacarbenes and metallacarbynes, do not favour cycliza- t i ~ n . ~ On the other hand Pt is usually not very good at forming surface carbenes and is a poor methanation catalyst, but it is very good at cyclization reactions. There is therefore every reason to rethink all the popular mechanisms of cyclization where carbenes are freely postulated with little or no evidence.C. O'Donohoe, J. K. A. Clarke and J. J. Rooney, J. Chem. SOC., Faraday Trans. I , 1980, 76, 345. J. K. A. Clarke and J. J. Rooney, A h . Catal., 1970, 25, 125. A. F. Kane and J. K. A. Clarke, J. Chem. SOC., Faraday Trans. I , 1980,76, 1640. F. G. Gault and J. J. Rooney, J. Chem. SOC., Faraday Trans. I , 1979, 75, 1320. Prof. G. Maire (Universite' Louis Pasteur, Strasbourg) said: As underlined by Prof. Rooney it seems that the olefin metathesis mechanism is not correct for bond- shift. However, it is interesting to note that the mechanism proposed in our fig. 2 is the same as that proposed by Chauvin and Herrisson for metathesis.' In the temperature range >250 "C homologation is extensive on Pd, Rh and W. Some activity to a lesser extent was also observed with Pt by O'Donohoe et aL2 Olefin metathesis seems to occur on all metals but to different degrees. Dr.F. Luck3 observed in our laboratory: (i) homologation of 2-methylpentane on platinum black at 300 "C under 40 Torr of HZ; (ii) that the bond-shift and homologation reactions of alkanes are directly correlated to the particle sizes of platinum. On low dispersed supported platinum catalysts or platinum black the bond-shift and homologation reactions are favoured in agreement with table 4 of ref. (2) for sintered and unsintered Pt films. To answer more precisely Prof. Rooney's specific questions: (1) On " Chatt cluster-derived catalysts " no formation of benzene was detected from 2-methyl- pentane or methylcyclopentane. (2) Toluene led exclusively to benzene by selective demethylation excluding map species.Y . Chauvin and J. L. Herrisson, Makromol. Chem., 1971, 141, 161, C. O'Donohoe, J. K. A. Clarke and J. J. Rooney, J. Chem. SOC., Faraday Trans I , 1980, 76. 345. F. Luck, Thesis (Strasbourg, 1978). Prof. V. Ponec (Rijksuniversiteit Leiden) said: Prof. Maire and his colleagues men- tion in this and in some earlier papers a novel mechanism of isomerisation. In this mechanism occur-at a certain step-two fragments of the isomerising molecules completely separated (see fig. 2 of Maire's paper). This happens at a temperature when the separated fragments are thermodynamically more stable than the original molecule. My question is: is it then reasonable to assume that the mechanism suggested is really quite general ? One knows that sometimes isomerisation selec- tivity is higher than 90% (see my paper).Tt means that in those cases the reconstitu-GENERAL DISCUSSION 89 tion of carbene and olefine fragments back into the non-destroyed molecule would have been that complete. The mechanism suggested assumes a rather free rotation of n-bonded ethylene (olefin). My question is: is there any example known of such rotation for a Group VIII metal complex which would justify the assumption made for the situation at 600-700 K on the surface of a metal? Does Prof. Maire think this is really possible? Prof. G. Maire (Universite' Louis Pasteur, Strasbourg) said : Prof. Ponec asked first about the general validity of the mechanism of isomerization proposed in fig. 2 of our paper, supposing a metallocyclobutane species as precursor.Such a pro- posed species was chosen by Gault and Garin [see ref. (2) of our paper] because (i) the rotation of the adsorbed olefin allows the methyl migration; (ii) an adsorbed ethylidene formed via such a metallocyclobutane precursor is rapidly isom erized to an adsorbed o1efin;l isomerization is then replaced by hydrogenolysis of the C-C bond ; (iii) the internal fission has the same activation energy as methyl shift [see ref. (1) of our paper], 45 kcal mo1-l compared with 55 kcal mo1-l for chain lengthening or shortening. In our paper (see fig. 2) we suggested the metallocyclobutane species as precursor responsible for the methyl-migration and the selective demethylation on the " Chatt cluster-derived catalysts ". In this case the selectivity for isomerization was 12% leading to CH4, at first sight in agreement with the remark of Prof.Ponec. But to reply straightforwardly to the question we believe more in a carbenoid species as transition state as recently proposed by us to interpret our results on platinum single-crystal surfaces [see ref. (5) of our paper]. Furthermore the selectivity includes isomers formed via bond-shift mechanisms and via cyclic mechanisms which can be higher than 90% on high or low dispersed platinum catalysts. Concerning the second question of Prof. Ponec an example of concerted rotation in the rearrangement of platinacyclobutanes has been proposed by Casey et aL2 which is closely related to Puddephatt's propo~al.~ Chatt4 in 1953 proposed a detailed picture of the orbitals involved in the formation of olefin-platinum complexes corresponding to a back donation of electrons from Pt to the olefin, i.e.Zeise's salts. This back donation of electrons implies a rotation barrier of 15.3 kcal mol-1 in Mo(C,H,),(diphos), at 98 0C.5 C. P. Casey, Org. Chem., 1976, 33, 189. C. P. Casey et al., J. Am. Chem. SOC., 1979, 101, 4233. R. J. Puddephatt, J. Chem. SOC., Chem. Commun., 1976,626. J. Chatt, J. Chem. SOC., 1953, 2939. J. Byrne, H. Blaser and J. Osborn, J. Am. Chem. SOC., 1975, 97, 3871. Prof. L. Guczi (Institute of Zsotopes, Budapest) said : The importance of dispersion in the hydrogenolysis reaction was emphasized earlier where turnover number increases with dispersion for hydrogenolysis of ethane and n-butane. For the latter case, selec- tivity for ethane formation, i.e.the rupture of the middle C-C bond, also increases. This could well be interpreted by the formation of metallocyclopropane intermediate on Pt catalyst. The same is valid for n-pentane., This, however, cannot be confined only to platinum, but it is characteristic of highly dispersed metal. On highly dis- persed Ru the same phenomena are ob~erved.~ L. Guczi and B. S. Gudkov, React. Kinet. Catal., 1978, 9, 343. A. Sirkany et aZ., Proc. 7th Congr. CataZ., Part A, (Kondasha, Tokyo and Elsevier, Amsterdam, 198l), 291. L. Guczi et a!., Bull. Soc. Chem. Belg., 1979, 88, 497. Dr. P. B. Wells (University of Hull) said: Prof. Knozinger proposes breakdown of O S ~ ( C O ) ~ ~ on his supports to give Os(CO), and Os(CO), bound to oxide.We have90 GENERAL DISCUSSION impregnated O S ~ ( C O ) ~ ~ onto silica, alumina and titania and have observed three-band infrared spectra such as he reports. We have also obtained u.v.-visible reflectance spectra which, for OS~(CO)~~ in solution, contain two bands which have been assigned to electronic transitions in the Os,-framework of the cluster molecule.' These bands are retained when OS~(CO)'~ is impregnated onto the supports mentioned above, and when the impregnated materials are rendered catalytically active by thermal activation. Thus we have evidence for the retention or partial retention of cluster nuclearity when the same materials also provide the three-band infrared spectrum. Furthermore, our catalysts are not only active for CO-hydrogenation (as Prof.Knozinger reports) but also for ethane hydrogenolysis, for which the apparent activa- tion energy may be one-quarter of that exhibited by a conventional supported osmium catalyst. Ethane hydrogenolysis is normally considered to require sites consisting of a considerable number of metal atoms, and hence this observation supports our view that multinuclear clusters are present. In view of these observations, I wish to ask: could the three-band infrared spectrum be assigned to some clustered state (possibly involving the dimerisation or trimerisation of the Os,-unit) of high symmetry? Alternatively, may we have a situation here in which the species which is mostly responsible for the infrared spec- trum is not that which is responsible for CO-hydrogenation and ethane hydrogenolysis ? H.B. Gray, R. A. Levenson and D. R. Tyler, J. Am. Chern. Soc., 1978,100,7888. Prof. H. Knozinger (Uniuersitat Miinchen) said : (1) The observed three-band infrared spectrum can certainly not be attributed to a single surface species. Decarbonylation and recarbonylation cycles clearly demon- strated that the three bands must be due to two interconvertible surface carbonyl complexes which we consider as Os(CO), and OS(CO),.~ Dimerisation and tri- merisation reactions seem to be extremely unlikely at the low metal loadings used in our catalysts. (2) We can certainly not absolutely exclude the possible presence of species other than those detected by infrared spectroscopy and TEM. These may indeed be responsible for the catalytic properties of the samples, a situation which can hardly ever be excluded in heterogeneous catalysis.H. Knozinger and Y. Zhao, J. Catal., 1981, 71, 337. Prof. L. Guczi (Institute of Isotopes, Budapest) said: R. C . Baetzoldl has stated In Prof. Knozinger's How can one that ca. 100 metal atoms are needed to form metallic properties. work the number of 0 s atoms which form clusters is between 4 and 6. comprehend the action of those supported metals as metal catalysts? R. C. Baetzold, Surf: Sci., 1981, 106, 243. Prof. H. Knozinger (Uniuersitat Miinchen) said: We have never said that the sup- ported osmium species would have metallic properties. They are to be described as surface carbonyl complexes, the properties of which will be determined by the nature of ligands, the coordination number and formal oxidation state of the 0 s atoms.Prof. J. Cunningharn-(University College, Cork) said: At one point of his paper Prof. Knozinger suggests for catalysts prepared from H,OsCl, on A1203, that " Lewis- acid centres on the surface of the A1,0, support could play a role in the catalysis, such as aiding in the dissociation of CO ',. However, at another point he advances the idea that smaller Os-0s distances (than in osmium tricarbonyl) may be importantGENERAL DISCUSSION 91 for CO dissociation upon ensembles of three osmium atom complexes on A1203. My question is whether it might not be more consistent to envisage upon all the sup- ported osmium catalysts a role in CO dissociation for Lewis-acid centres of the support.The proximity of such sites to osmium atoms seems assured, as also does the different Lewis-acid character towards the oxygen of CO. Prof. H. Knozinger (Universitat Miinchen) said: We have indeed suggested the two alternative possibilities mentioned by Prof. Cunningham to explain the CO dissociation. Although we do not yet have direct experimental evidence, we feel that the CO dissociation with the direct participation of Lewis-acid sites perhaps through intermediate structures such as c--0 / 0 s \ ~ 1 3 + might be the preferred route. The findings of Katzer et al.' regarding the selectivity control via acid-base properties of the support might perhaps be understood in a similar way. J. R. Katzer, A. V. Sleight, P. Gajardo, J. B. Michel, E. F. Gleason and S. McMillan, Faraday Discuss.Chem. Suc., 1981,72, 121. Dr. S. D. Jackson (University of Hull) said: Prof. Knozinger states that there is a slow loss of catalytic activity due to carbon deposition which affects all the products, then also states that electron microscopy gives results consistent with the carbon- aceous deposit being on the support. Could he please explain how carbon on the support affects the activity of the metal? Also, does he see any evidence from X.P.S. for metal carbide on the used catalyst? Prof. H. Knozinger (Universitlit Miinchen) said: The reasons for deactivation dur- ing high-pressure experiments have not been studied in detail. The suggestion of carbonaceous deposits being the reason, came from the observation by TEM that carbon had probably formed on the support.This statement, however, should not necessarily be interpreted in the sense that carbon on the support affects the activity of the metal. Carbonaceous deposits (not directly detected by TEM) might also occur on the metal species. Moreover, if the Lewis sites on the support surface participate actively in CO dissociation, these may be blocked by carbonaceous deposits as well. Photoelectron spectra of the used catalyst have not yet been meas- ured. Prof. M. W. Roberts (University College, Card@): Do the authors have in addi- tion to the Os(4f) spectra shown in fig. 1 of their paper any corresponding C(ls) or O( 1s) spectra ? The C( 1s) and O(ls) spectra would enable comment to be made on the nature of the CO-Ru interaction, since a correlation has been shown to exist' between AH and the O(ls) binding energy.Valence-level spectra might also be helpful to define the CO-0s system. In fig. 1 how did the authors arrive at the conclusion that the Os(4f) spectrum can be deconvoluted to give two peaks attributed to a single 0 s and two edge 0 s atoms ? R. W. Joyner and M. W. Roberts, Chem. Phys. Lett., 1974, 28, 246. Prof. H. Knozinger (Universitdt Miinchen) said: It would certainly be extremely interesting if the correlation between AH and O(1s) binding energy as suggested by92 GENERAL DISCUSSION Prof. Roberts could be applied. C(ls) and O(ls) spectra have indeed been measured. I am afraid, however, that they do not contain much information on the CO-0s interaction. The l'eason is the extremely low concentration of 0 s complexes in the oxide-supported samples; therefore, the O( 1s) signal is essentially produced by the oxygen of the support and relatively small contaminations would also dominate the C( 1s) signal.The conclusion that the Os(4f) spectrum of a silica-supported cluster indicated the superposition of two doublets of two types of chemically different 0 s atoms came from the comparison with the spectra of an authentic molecular compound: Valence-level spectra have not been measured. \I/ The spectra of the supported and unsupported cluster compounds were closely similar with respect to binding energies and full width at half maximum. Prof. J. M. Thomas (Cambridge University) said: The difficulties inherent in imaging small supported clusters of metals or alloys (5-50 A diameter) by conventional transmission, high-resolution electron microscopy (HREM) are well known and are again highlighted in Prof.Knozinger's micrographs. Yet, as he rightly emphasizes, with heavy atoms such as osmium dispersed on light supports such as silica or alumina the chances of success are improved. I would like to suggest that, instead of em- ploying phase contrast or absorption contrast, as is usually done in conventional HREM, one should adopt an approach that uses so-called atomic-number contrast. (With an electron beam of cross-section ca. 3 A sweeping across the surface it is possible to record images from both elastically and inelastically scattered beams. The elastically scattered electrons are deviated through larger angles than the inelastic ones.If a ratio of the elastic to the inelastic signal is recorded, the resulting intensity distribution is directly proportional to the atomic number.) From the results of Wall and others [see ref. (2) and (3) of this comment and plate 11 there is every indication that atomic-number contrast electron microscopy will directly reveal the nuclearity of 0 s clusters. The micrograph reproduced here clearly shows clusters of uranium consisting of two, three and seven atoms, the latter being disposed as in a fragment of a close-packed sheet. J. M. Thomas and D. A. Jefferson, Endeavour, New Ser., 1978, 2, 136. J. Wall, in Nobel Symposium No. 47 (Aug. 1979), published as Direct Imaging of Atoms in Crystals and Mofecules (Royal Swedish Academy, Stockholm, 1979), p.271. .T. M. Thomas, Nature (London), 1979, 281, 523. Prof. H. Knozinger (Universitdt Miinchen) said : In fact, as Prof. Thomas stresses very clearly, resolution was not a problem in our conventional TEM studies. The difficulties in interpretation of contrast arise from the fact that the material used is polydisperse. For this reason we did not want to over-interpret our micrographs. However, it appears to be safe enough to say that TEM results were not in contra- diction to the structural model derived from infrared spectra. The technique which Prof. Thomas suggests would certainly be of great help for elucidation of the structural details of 0s-atom ensembles in our catalysts. Un- fortunately, the technique has not hitherto been available to us.PLATE 1.-Atomic-number contrast image of uranium supported on carbon [from ref. (2)] showing individual atoms of uranium and clusters consisting of two, three and seven atoms.Each white spot represents an atom. [To face page 92GENERAL DISCUSSION 93 Dr. A. P. G. Kieboom (Delft University of Technology) said: I have two short questions for Prof. Gryasnov: (1) Has he tried membrane catalysts for the partial hydrogenation of benzene to cyclohexene ? High selectivity could be expected by regulation of the hydrogen transfer through the catalyst. (2) Please would he give in a nutshell the major advantages and disadvantages of the hydrogen-porous membrane catalysts? Prof. V. M. Gryaznov (Lumumba University, Moscow) said: In reply to Dr. Kieboom's first question I should like to cite the results of a benzene hydrogenation study' on a hydrogen-porous membrane catalyst made of an alloy of 94.1 wt % palladium and 5.9 wt % nickel. The rate of cyclohexene formation was ca.70% of the cyclohexane formation rate at the beginning of the experiments in the flow system. In connection with the second question of Dr. Kieboom I will try to summarize the major advantages of the hydrogen-porous membrane catalysts. They are as follows: (1) the control of atomic hydrogen concentration on the catalyst surface irrespective of partial pressures of hydrogen and other substances in the reaction zone; (2) the increase in hydrogenation rate and selectivity by diminishing the competition in adsorption of hydrogen and the hydrogenatable substance; (3) the enhancement of dehydrogenation rate and selectivity as a result of hydrogen removal through the catalyst ; and (4) the coupling of dehydrogenation and hydrogenation reactions, which are carried out on two surfaces of the membrane catalyst with higher efficiency than during separate courses. The disadvantages of the membrane catalyst are caused by the small fraction of palladium atoms on the surface in comparison with the total amount of palladium atoms in the catalyst. This fraction may be increased by using very thin films of palladium alloys on hydrogen-permeable supports.2 The precious metal losses which are inevitable in the case of other catalysts are eliminated when using membrane catalysts because of their high mechanical strength and corrosion resistivity. That is why the higher initial investments required for membrane catalysts are adequately compensated. V. M. Gryaznov et ul., in Mechanisms of Hydrocarbon Reactions, ed. F. Marta and D. Kallo (Akademiai Kiado, Budapest, 1975), p. 107. V. M. Gryaznov, V. S. Smirnov, V. M. Vdovin et al,, U S . Patent, 4, 132, 668. However, this correlation decreases with time. Prof. V. Ponec (Rijksuniuersiteit Leiden) said : I appreciated very much that Prof. Gryaznov so clearly formulated one important principle : catalysed reactions can be influenced in an important manner when one succeeds in shifting the conditions of the steady state of the working catalyst. He demonstrated what membranes can do in this respect. I would now like to turn attention to the work done at Delft by Prof. H. Van Bekkum. Van Bekkum and associates' use, e.g., zeolites to abstract water from running condensation reactions to achieve higher yields of desirable products, etc. Another example is to use zeolites to remove selectively lower alcohols from a reaction mixture upon an interchange reaction of esters and alcohols, etc. These are other examples of the same approach in manipulating the selectivity. D. P. Roelofsen, J. W. M. De Graaf, J. A. Hagendoorn, H. M. Verschoor and H. Van Bekkum, Rec. Trav. Chinl. Pays-Bas, 1970, 89, 193; D. P. Roelofsen, E. R. J. Wils and H. Van Bekkum, Rec. Trav. Chim. Pays-Bas, 1971, 90, 1141.

ISSN:0301-7249    

DOI:10.1039/DC9817200079

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Variation of Catalyst Selectivity by Control of the Environment of Surface Sites BY ALAN G. BURDEN, JOHN GRANT, J E S ~ S MARTOS," RICHARD B. MOYES AND PETER B. WELLS Department of Chemistry, The University, Hull HU6 7RX Received 15th May, 1981 Previously published models, which have interpreted selectivity characteristics of metallic hydro- genation catalysts in terms of (i) molecular congestion at active sites, (ii) surface contamination and (iii) hydrogen occlusion in metals, are tested and elaborated. The catalyst was prepared by impregnation of RU~C(CO),~ into silica. After a minor displacement of CO it showed behaviour characteristic both of the parent cluster (activity in hydrogen-deuterium exchange) and of conventional supported ruthenium (product composition in the ethene-deuterium reaction).Preferential cis-but-Zene formation from but-1-ene occurred at these CO-congested sites, whereas uncongested sites provided preferential trans-but-Zene formation. Adsorption of sulphur on catalytically active metal by decomposition of H2S modifies selectivity in that it converts the major process in buta-1,3-diene hydrogenation from 1:2-addition to 1:4- addition. Adsorption of H2S has been measured on evaporated films of Cr, Mn, Fe, Coy Ni, Mo, Pd, W, Re and Pt and on Co-powder and Ni/silica. The selectivity changes are recorded and inter- preted in terms of the electronic effect of adsorbed sulphur on metal atoms remaining exposed at the surface and, for Fe, Coy Ni and Yd, the effect of subsequent incorporation of sulphur into the surface. The behaviour of Pt was anomalous, the effect of adsorbed sulphur on selectivity being less than ex- pected.Finally, the cavity theory of hydrogen occlusion in metals correctly predicts the formulation or conditions of preparation required to translate iridium from its traditional position as the least selective Group VIII metal for buta-lY3-diene hydrogenation (to butene) to a new position as one of the most selective. But-1-ene isomerisation has been investigated at ruthenium sites congested by CO. The selectivity exhibited by metal catalysts in the hydrogenation of alkadienes and the isomerisation of alkenes is influenced by (i) molecular congestion at the catalyti- cally active site,' (ii) the presence of non-metal in the neighbourhood of the site2 and (iii) the occlusion of hydrogen in the bulk of the metal.3 This paper presents new information in each of these three areas which tests, confirms and extends the proposed models.In previous work the effect of molecular congestion at sites was inferred from the selectivity observed in CHD=CDC3H7 isomerisation to cis- and trans-pent-Zene catalysed (i) by nickel and other metal complexes in s o l ~ t i o n ~ * ~ (highly congested single- atom sites: three-dimensional congestion by bulky phosphorus-containing ligands), (ii) sodium-activated nickel-phthalocyanine/silica (single-atom sites : two-dimensional congestion) and (iii) nickel/alumina and nickel film * (multiple-atom sites: least congestion). The selectivity in this reaction (i.e. the cis:trans ratio in pent-1-ene or but-1-ene isomerisation) is an indicator of the state of congestion at sites because the most compact conformation of the half-hydrogenated state gives cis-alkene on hydrogen abstraction.In Part 1 of the Results and Discussion section the con- gesting effect of the simpler adsorbate carbon monoxide is examined and we compare * Present address : ENPETROL, Madrid-5, Spain.96 SITE ENVIRONMENT A N D SELECTIVITY the selectivity for but- 1-ene isomerisation at the surface of conventional supported ruthenium with that of a surface derived from the cluster compound Ru,C(CO),~. The presence of S, P, As and Se at cobalt surfaces,2 S, C1 and Br at nickel surfaces,2 and of S at rhenium surfaces5 converts these metals from 1 : 2- to 1 : 4-addition catalysts in buta-l,3-diene hydrogenation. The electronic and geometric properties of the metal sites may be influenced by the presence of the non-metal in such a way that the bonding of surface intermediates is modified and the product composition altered.2 Previous work interpreted experiments the results of which were, in the first instance, totally unexpected.Part 2 of this paper presents our first systematic examination of the effect of chemisorbed sulphur on the selectivities of ten transition metals in buta- 1,3-diene hydrogenation. The formation of alkane in alkadiene or alkyne hydrogenation is normally undesirable. One important factor determining alkane yield over the Group VIII metals appears to be the capacity of the metal crystallites to occlude hydrogen in cavities formed during catalyst preparati~n.~ This model permits of two tests: (i) occlusion should be reduced, and selectivity improved, if the temperature used in catalyst preparation for the reduction of salt or oxide to metal is sufficiently high for metal-atom diffusion to prevent cavity formation, and (ii) catalysts having metal particles too small to contain cavities might be more selective than catalysts consisting of larger particles.The results of these tests are reported in Part 3. EXPERIMENTAL Ru~C(CO)~ was prepared by a literature method6 and R~,C(CO)~~/silica by impregna- tion of a fumed silica with a solution of the cluster compound in toluene. The supported material contained 1.2% by weight Ru. Examination under an optical microscope revealed the presence of crystals, apparently of RU6C(C0)17, covering a small fraction of the support.The designation “ R~,C(CO)~,/silica ” is intended to convey that the catalyst was prepared from the named cluster; evidence presented below suggests but does not prove that Rug- nuclearity was retained, and further characterisation is in progress. Catalytic activity was measured using a grease-free mercury-free high-vacuum system capable of attaining 10- Torr ; the static reactor was connected directly to a modified A.E.I. MS3 mass spectrometer calibrated for the analysis of mixtures containing H2, HD, D2 and CO. Catalyst samples weighed: 0.4 mg [RU~C(CO)~~] or 35 f 5 mg [samples of R~,C(CO)~~/silica]. Evaporated films of Cr, Mn, Fe, Co, Ni, Mo, Pd, W, Re and Pt were prepared by standard methods’ in glass vessels attached to a grease-free stainless-steel high-vacuum system capable of attaining 10- Torr.Fe, Co, Ni, Pd and Pt wires were Specpure grade (Johnson Matthey); Cr chips (Metals Research), Mn chips (Koch Light) and Re wire (Engelhard) were of 99.9% purity; and Mo wire (New Metals) and W wire (Goodfellow Metals) were of 99.95% purity. Film weights were determined by weighing the film source before and after evaporation. Vessel walls were maintained at 273 K during film deposition, and films were not annealed before use. Cobalt powders were prepared2 by thermal decom?osition of CoC03 to COO and subsequent reduction of the oxide to metal in a hydrogen flow at 723 K for 12 h. 10%- Ni/silica was prepared by impregnation of the support with nickel nitrate, calcination at 673 K and reduction at 533 K in flowing hydrogen for 6 h.Metal catalysts were progressively sulphided by exposure to successive doses of H2S. A measured pressure of H2S in a vessel, V, of known volume was expanded into the reactor, also of known volume, and the pressure recorded. When equilibrium was achieved, un- reacted H2S was condensed back into vessel V and the pressure of the non-condensible residue (hydrogen) was recorded. The system was then evacuated and the pressure of unreacted H2S measured in vessel V. The procedure was repeated as required. Five iridium powders (Ir-1 to Ir-5) were prepared by reduction of IrCI, in flowing hydro- gen at 673, 698, 1173 and 1273 (two samples) and one powder (Ir-6) by reduction of IrOz at 673 K.The hydrogen contents of these powders were measured by exchange with deu-BURDEN, GRANT, MARTOS, MOYES A N D WELLS 97 terium3 at 373 or 423 K over 25-90 h and were found to be: Ir-1, TrHo.12; Ir-2, IrHo.r6; Ir-3, IrH0.03; Ir-4 and Ir-5, IrHo.oz; Ir-6, IrH0.17. Five iridium/silicas were prepared by impregnation of silica with aqueous H21rC16 and reduction in flowing hydrogen at 673 K for 6 h. The loadings were 20, 10,0.3,0.1 and 0.01% by weight of iridium. 20%-Ir/silica had a mean particle size of 4.7 nm measured by X-ray line broadening. Particle size distributions were examined by electron microscopy; the distributions showed maxima at 4.0 nm for 20%-Ir/silica and at 2.5 nm for lO%-Ir/silica, and the distributions extended from 2.0 to 10.0 nm in each case.No iridium particles were visible for 0.3, 0.1 and for 0.01%-Ir/silicas (limit of visibility, 0.8 nm). Hydrocarbon reactions over all catalysts were examined by standard methods; pressure fall was measured by use of calibrated pressure transducers or thermal conductivity gauges (LKB), and product analysis was by g.1.c. All reactants were of high purity. Hydrogen and deiiterium were purified by diffusion through heated palladium-silver thimbles. RESULTS AND DISCUSSION PART 1.-SELECTIVITY AND SITE CONGESTION B U T EN E IS 0 ME R I S A T I 0 N CAT A L Y S ED BY RU6C(CO)1,/SI LI C A 0.4 mg of (unsupported) RU,C(CO)~~ catalysed hydrogen-deuterium exchange (H2 + Dz + 2HD) over the range 253-328 K. Fig. 1 shows that the first two experi- ments served to bring the catalyst into a reproducible state after which exchange proceeded with an apparent activation energy of 30 kJ mold'.No CO loss was detected during the first or subsequent evacuations of the catalyst to lo-* Torr. FIG. 1 .-Temperature dependence of the initial rate of hydrogen-deuterium exchange, r/( %HD formed) min- ', catalysed by unsupported RU,C(CO)~, (filled circles) and Ru6C(CO),,/silica (open circles). The numbers denote the experiment sequence. Weight of Ru6C(C0),, = 0.4 mg; weight of Ru,C(C0)17/silica = 37.1 mg. Initial composition, Pressure of H2 + D2 mixture = 10 Torr. H2:DZ = 2: 1. Ru6C(CO),,/silica catalysed hydrogen-deuterium exchange over the same tempera- ture range with the same apparent activation energy and with a closely similar specific activity (fig, 1).Rates were closely reproducible from one catalyst sample to another. Carbon monoxide was released to the gas phase during the first exchange reaction (9 x 1017 molecules CO displaced on treatment with 7.5 Torr of a 2:l H,:D, mixture at 293 K) but none was displaced in subsequent reaction or evacuations. This CO released amounted to one molecule per three Ru atoms present, (or two CO-ligands per Ru,-cluster if indeed such clusters were retained). Exchange was first-order in hydrogen pressure, strongly inhibited by but-1 -ene and by ethene, and completely98 SITE ENVIRONMENT A N D SELECTIVITY poisoned by 5 Torr of carbon monoxide at 293 K. These poisoning effects were completely reversed by subsequent evacuation. When R~~C(CO)~~/silica was heated in hydrogen to 358 K gross CO evolution occurred, hydrogen-deuterium exchange became immeasureably fast even at 195 K, and subsequent characterisation by electron microscopy showed the presence of ruthenium particles 0.8-1.5 nm in size.By contrast, no ruthenium crystallites were visible in electron micrographs of several Ru,C(CO),,/silica samples used for hydrogen exchange, butene isomerisation or ethene hydrogenation which had been used in the range 253-328 K. TABLE 1 .-PRODUCTS OF ETHENE-DEUTERIUM REACTIONS CATALYSED AT 327 K BY RU&(C0)17/ SILICA AND BY 1 %-RUTHENIUM/ALUMINA, AND TWO CALCULATED PRODUCT COMPOSITIONS DETERMINED BY KEMBALL'S METHOD lo RU~C(CO)~ ,/silica a calc. Ru/alumina calc. products 5% 8% A " 1 1% B d conversion conversion conversion 2.8 2.0 2.4 2.6 19.2 9.4 2.8 2.9 0.8 0.3 54.8 1.3 1.4 2.8 4.4 19.0 4.2 5.0 1.8 0.9 4.4 54.8 0.0 0.0 0.4 2.8 17.7 14.5 3.2 0.0 0.6 6.4 54.4 0.0 0.0 0.0 2.4 29.5 10.0 2.1 0.0 0.0 3.7 52.3 0.0 0.0 0.1 2.3 29.6 10.5 0.8 0.0 0.1 3.1 53.5 % chance of ethene(ads) -+ ethene (g) 71 % chance of ethene(ads) + ethyl(ads) 29 % chance of ethyl(ads) -+ ethene(ads) 79 % chance of ethyl(ads) ethane (g) 21 83 17 28 72 Initial pressures: ethene = 10 Torr, deuterium = 19 Torr; Initial pressures: ethene = 27.5 Torr, deuterium = 50 Torr; ' p = 0.4; q = 2.0; r = 3.635; s = 2.0 [see ref.(lo)]; * p = 0.2; q = 9.0; r = 2.5; s = 4.0 [see ref. (lo)]. Ru,C(CO),,/silica catalysts exposed to 6 Torr D2 at 293 K for 0.5 h and sub- sequently pumped for 1 h at lo-' Torr retained 2 x lo1, D-atoms detectable by ex- change with protium at 293 K.This corresponds to one D atom per ten Ru atoms present. The ethene-deuterium reaction provides a useful chemical characterisation of metal surfaces because the reactivities of the adsorbed intermediates, ethene and ethyl, differ considerably on passing from one metal to a n ~ t h e r . ~ Products obtained over Ru,C(CO),,/silica at 327 K are compared with those obtained over a conven- tional Ru/alumina and with the results of calculations in table 1. The patterns of the product compositions are similar, except for the small yields of highly exchanged ethene and ethane obtained over the cluster-derived catalyst. This degree of agree- ment demonstrates that the active sites in Ru,C(CO),,/silica behave like those at a typical ruthenium surface as far as the processes of ethane formation and of exchange of H for D in ethene are concerned.We draw the following conclusions concerning the nature of Ru,C(CO),,/silica. Much of the carbon monoxide originally present in the cluster compound is retainedBURDEN, GRANT, MARTOS, MOYES AND WELLS 99 in the impregnated material, and this appears to be responsible for the characteristic slow catalysis of hydrogen exchange. This exchange activity is so similar to that of the parent cluster compound as to suggest that the ruthenium entities present are closely related to RU,C(CO)~~. Nevertheless, the sites also behave in a manner typical of a conventional ruthenium surface for the exchange of H for D in ethene, a reaction analogous to but-1-ene isomerisation to but-2-ene for which the catalyst is required.This chemical characterisation establishes that the material contains active ruthenium sites having CO in the vicinity, and is therefore an appropriate catalyst for the task in hand. TABLE 2.-cOMPARISON OF PRODUCT COMPOSITIONS IN BUT-1-ENE AND PENT-1-ENE ISOMERI- SATIONS CATALYSED BY VARIOUS RUTHENIUM CATALYSTS alk-2-ene composition (%) cis ref. catalyst reactant temp./K cis trans trans ~~ Ru,C(CO)l,/silica but-1-ene a but-1-ene a but-1-ene a Ru6C(C0)17/silica } but-1-ene a decomposed at 358 K Ru/alumina but-1-ene RuHCI(CO)(PPh3)3 pent-1 -ene pent-1-ene RuHCI(PPh3)3 pent- 1 -ene pent-1-ene f 253 293 313 293 306 298 353 353 353 70 61 55 42 37 71 45 57 23 30 39 45 58 63 29 55 43 77 2.3 1.6 1.2 0.7 0.6 8 2.4 4 0.8 4 1.3 4 0.3 4 ~ ~~~ Initial pressures: but-1-ene, 7.5 Torr; hydrogen, 15 Ton, conversion, 5 %; Initial pressures: but-1-ene, 50 Torr; hydrogen, 50 Torr, conversion 40%; c - f Initial concentrations: 1060, 10, 1200 and 40 pmol dm-3 in benzene; conversions < 5 %.But- 1-ene isomerisation in the presence of hydrogen over Ru,C(CO),,/silica gave the products shown in table 2; reaction was zero-order in but-1-ene, half-order in hydrogen, and no butane was formed. Only a trace of isomerisation occurred in the absence of hydrogen. Remarkably, the activity for but-2-ene formation from but- 1- ene at 293 K was four times that for HD formation from a 2:l H,:D, mixture at the same temperature. R~,C(CO),~/silica clearly gave an excess of cis- but-2-ene over the trans-isomer, and this selectivity differed from the trans-excess obtained (i) over the microcrystalline Ru,C(CO),,/silica decomposed at 358 K and (ii) over ruthenium/ alumina.Table 2 shows for comparison the change in pent-2-ene composition reported for pent- 1-ene isomerisation catalysed by two ruthenium complexes as site congestion was relieved by progressive solvolysis of PPhJigand~.~ The trends are similar. By extension of our previous argument l p 4 we conclude that the cis-excess afforded by Ru,C(CO),,/silica denotes a considerable degree of steric congestion by CO ad- sorbed in the immediate vicinity of these ruthenium sites. By the same token, sites at the surfaces of the small crystallites present in decomposed Ru,C(CO),,/silica and ruthenium-alumina are-not congested in this way.The cis : trans ratio in but-1-ene formed over R~,C(CO),~/silica decreased with increasing temperature primarily because the CO ligands responsible for site con- gestion restrained the conformations of adsorbed secondary-butyl groups less effec- tively as their thermal energy increased.100 SITE ENVIRONMENT AND SELECTIVITY PART 2.-SELECTIVITY AND SULPHUR ADSORPTION BUTADIENE HYDROGENATION CATALYSED BY METALS OF GROUPS VIA, VIIA AND VIII Isotherms for the adsorption of H2S at room temperature were measured using freshly prepared films of Cr, Mn, Fe, Co, Ni, No, Pd, W, Re and Pt, Co-powder, and silica-supported Ni. Typical iso- therms are shown in fig. 2. For all systems the stoichiometry of the dissociation was Multiple determinations were made in each case.equilibrium pressure, p/mTorr FIG. 2.-Typical isotherms obtained at room temperature for adsorption of H2S on Cr film, 18.6 mg; Pd film, 15.2 mg; lO%-Nilsilica containing 17.8 mg metal; and Pt film, 7.3 mg. The quantity a re- presents the number of pmol of H2S adsorbed. H,S(g) -+ S(ads) + H2(g) and there was no volumetric evidence for the formation of adsorbed SH. Plots ofpla against p were linear, and the amounts adsorbed at satura- tion obtained from the gradients corresponded closely with the values obtained by extrapolation of the plateau regions to zero pressure. The number of sulphur atoms chemisorbed per gram of metal at room temperature is shown in column 2 of table 3, and column 3 contains, for comparison, the number of molecules of benzene chemisorbed at the same temperature on similar films, as recorded in our laboratory." The fourth column shows the extent of krypton physisorption on similar films reported by Brennan and GrahamI2 and Frennet and coworkers13 and on this powder by Sing.14 The degree of internal consistency in these results is encouraging.The number of surface metal atoms per gram of metal for film or powder, determined from the krypton adsorption measurements (Kr = 19.5 x m2 molecule-'), is shown in column 5 and the ratio of adsorbed sulphur atoms to surface metal atoms (S :M) at the completion of each H2S adsorption measurement is given in column 6. The structures of ad-layers of sulphur at metal surfaces determined by LEED are many and various,15 but in general most compact arrangements show S:M ratios inBURDEN, GRANT, MARTOS, MOYES AND WELLS 101 the region of 0.45, and this value (hO.10) has also been arrived at by independent methods by Pitkethly and co-workersL6 and by Ro~trup-Nielsen.'~ Thus it is im- mediately apparent that sulphur adsorption at the surfaces of Cr, Mo, W, Mn and Re films differs in extent from that at the surfaces of Co, Ni, Pd, Pt and perhaps Fe films.The high values of the S:M ratio for Co, Ni and Pd films indicate gross uptake of sulphur equivalent to formation of many monolayers, whereas values for the Group VIA and VlIA metals show that chemisorption at these surfaces does not extend much beyond one nominal monolayer. Remarkably, the S:M ratios for Co-powder and Nilsilica indicate that on these surfaces, which were prepared at higher temperatures, sulphur adsorption has not proceeded beyond about one nominal monolayer.TABLE 3.-ADSORPTION MEASUREMENTS, s M RATIOS AND Q VALUES FOR SULPHUR- CONTAMINATED METAL SAMPLES extent of adsorption number of adsorbent at saturation surface metal or catalyst /1020 molecule (g metal)-' atoms/1020 S: M Q H2S C6H6 Kr (g metal)-' Pd-film 0.8 - 0.14 0.3 2.7 850 Fe-film 2.4 1.6 1 .o 3.1 0.8 650 Co-film 2.2 0.5 0.22 0.7 3.1 580 Ni-film 2.1 0.5 0.23 0.7 3 .O 520 Mn-film 2.8 1.7 1.7 4.6 0.6 220 Mo-film 3.8 6.5 3.1 8.4 0.45 150 Cr-film 2.2 2.1 4.6 14.6 0.15 160 W-film 2.6 1.2 1.6 4.2 0.6 150 Re-film 1.6 2.1 0.9 2.8 0.6 100 P t-film 1.5 0.5 0.32 0.8 1.9 70 Co-powder 0.15 - 0.1 1 0.32 0.5 710 - 25 a 0.15 890 10% Ni/silica 3.7 I a Estimated from the particle size distribution obtained by electron microscopy.Further metal films, Co-powder and Ni/silica were prepared in order to examine their selectivities in buta- 1,3-diene hydrogenation and the effect on these selectivities of the action of adsorbed sulphur. Two types of selectivity are distinguished'* (i) the ability of these materials to catalyse butene formation in preference to that of butane and (ii) stereoselectivity within the first stage of reaction, in particular the property of catalysts to promote 1 :Zaddition in preference to 1 :4-addition or vice versa. Selectivity of the first type was examined for sulphur-free surfaces by measuring the values of selectivity S = p(C4H8)/[p(C4HS) + p(C,H,,)] at various conversions using reactions of 50 Torr butadiene with 100 Torr hydrogen at or about room temperature (except Re = 423 K); initial selectivities were determined by extrapolation to zero conversion.Values of S obtained were: Cr, 0.960; Mn, 0.955; Fe, 0.980; Co, 0.990; Ni, 0.990; Mo, 0.830; Pd,0.970; W,0.800; Re, 0.950; andPt,0.570. Those for the Group VIII metals and Re conform to the long-established selectivity pattern l8 which is characterised in part by a diminution in selectivity on passing from the first transition series to the second, and from the second series to the third. This work now shows the same trend for Group VIA, viz. Cr > Mo > W. The situation in102 SITE ENVIRONMENT AND SELECTIVITY Group VIIA is obscured by the necessity to study Re at 423 K on account of its low activity; however, it is well established that values of selectivity, S, increase with increasing temperature,” and hence manganese would almost certainly be more selective than rhenium if they were compared at the same temperature. Thus, the extended form of the selectivity pattern is now: Cr w Mn z Fe w Co z Ni < Cu = 1.00 V V V V ? ? Mo Ru< Rh < Pd V v v v W < Re > 0 s > Ir < Pt < Au = 1.00 H2S adsorption on the ten metals under study caused a diminution in activity and For sulphur-rich surfaces S values were in the With regard to the second type of selectivity, butene compositions at low con- Each sulphur-free sur- an increase in selectivity in all cases.range 0.965-0.998 except in the case of platinum (0.650).version varied with the extent of H,S chemisorption (fig. 3). Cr ,e 0 .C - .*.,..,.* ....................... L i .-- ........................... .’, .”, Mo ..... .m ........ ....... ...... ............. a-. ....... *”.. c ....... .............. ....-..-* c 4 -. ----- .c ................................. .* FIG. 3.-Collected results for the effect on the butene composition of the adsorption of H2S on metal films. Each ordinate represents the percentage of butene and extends from 0 to 80%; each abscissa represents the amount of sulphur adsorbed and extends from zero to the value recorded in table 3. Firm lines represent but-1-em; dashed lines, tram-but-Zene; dotted lines, cis-but-2-ene. Tem- perature/K = Cr, 365; Mn, 388; Fe, 353 ; Co, 323 ; Ni, 363 ; Mo, 423 ; Pd, 273 ; Re, 423 ; pt, 326.Initial pressures: butadiene = 50 Torr, hydrogen = 100 Torr. Conversions variable, but product composition substantially independent of conversion.BURDEN, GRANT, MARTOS, MOYES AND WELLS 103 face catalysed predominantly 1 :2-addition and the trans: cis ratio in the but-Zene was low, usually ca. 2; this is the well-established Type A behaviour.2 As the concentra- tion of sulphur at these surfaces was progressively increased the selectivity changed, so that eventually 1 :4-addition predominated (except over Pt) and the trans: cis ratio in the but-2-ene rose, often to values of 8-10; this is Type B behaviour.2 The absence of butene isomerisation before desorption in both the Types A and B states has been established in deuterium tracer studies over Co,l9 Ni,19 Rh,20 Pd,20 Pt20 and M o .~ ’ ~ Nevertheless, fig. 3 shows that there are differences in the butene compositions ob- tained over the most extensively sulphur-treated samples which may be quantified as follows. The characteristics of Type B behaviour are a high yield of trans-but-2-ene and a high trans: cis ratio in the but-2-ene; accordingly we designate the product of these quantities Q, and list the values in column 7 of table 3. High values of Q represent good Type B selectivity or behaviour, and low values poor Type B behaviour. The metals in table 3 have been listed in decreasing order of Q. The clean film surfaces each gave preferential 1 :2-addition, i.e. Type A selectivity, and the surfaces of the cobalt powder and nickel/silica gave the same product composi- tion, and thus behave as though they are clean.Type A selectivity of clean surfaces is thus well documented, and the mechanism [which involves a-adsorbed half- hydrogenated states and free conformational interconversion of cisoid and transoid C,H,(ads) and C,H,(ads)] is established. Surfaces heavily contaminated by sulphur each gave preferential 1 :4-addition and preferential trans-but-Zene formation, i.e. Type B selectivity, and again the mecha- nism [which involves n-allylic forms of C,H,(ads) and C,H,(ads) and severely restricted conformational interconversion] is established. * A striking feature of the results (fig. 3) is that the smooth transition from Type A to Type B behaviour is a general phenomenon. That so many metals, and different forms of the same metal, show similar behaviour indicates that the surface conditions responsible for Type B selectivity are unlikely to be dependent on the formation of any special surface structure. That other contaminants mimic sulphur supports this view.In previous work with cobalt the transition was sometimes arrested at a point where 1 :2- and 1 :4-addition were of comparable importance (the B’-state).2 We have now shown that this is an artefact and occurs when oxygen is admitted to the surface.21b Suggestions as to the surface conditions which provide sites giving Type B selecti- vity have been made,2 and are re-presented here in a simplified form for the purpose of interpreting the variations in Q values presented in table 3. Sulphur in an ad-layer [shown schematically in (I) and (II)] The smoothness of the transition is also significant.S (1) I /\ -MA-MB~-MB~-MR~-MB~-MBL- S -MA-MBi-M 82-M ~3-M ~3-M B2-M B1- (11) -MBl’-MB2*-MBy-S-M B~’-M B~*-M ~1’- (111) -MB~*-S-MB~~-S-M B3-S-M ~33- (W is considered to polarise the one or more metal atoms to which it is bonded, MB3 and, to a lesser extent, the neighbours MB2 and MB1. The sense of the polarisation is * This mechanism was determined for samples of cobalt which, at the time of publication, were not known to be sulphur-contaminated [ref. (19)].104 SITE ENVIRONMENT A N D SELECTIVITY + at MB3 or MB3t, 66 + at MB2 or MB2e, and 666 + at MB1 or MB1'. Metal atom sites well distant from adsorbed sulphur are unaffected. Polarised metal atoms unobscured by adsorbed sulphur are proposed to facilitate adsorption of intermediates in x-allylic forms and hence promote reaction by the Type B mechanism.' Thus a structure represented by (I) or (11) would provide Type A selectivity at MA, weak Type B selectivity at MB1 and better Type B selectivity at MB2 if that site is obscured by adsorbed sulphur.It is well-known2' that sulphur initially retained in such an ad- layer may become incorporated into the surface [structures (111) and (IV)]; such incorporation may occur either before or after the establishment of a nominal mono- layer. Any condition that permits sulphur incorporation is expected to improve Type B selectivity because the most highly polarised MB3 sites, which were hitherto obscured, now become available for the adsorption of hydrocarbon.[The suffixes (111) and (IV) carry a prime because the number of neighbours to the sulphur atom of a given type has increased]. The best Type B surfaces might contain only MB3t sites Films of Cr, Mn, Mo, W and Re show low values of the S :M ratio in table 3, and low Q values, both of which suggest that sulphur may be predominantly confined to the ad-layer [(I), (II)]. Incorporation of several monolayers of sulphur into films of Co, Ni, Pd and perhaps Fe is evident from the S :M ratios, and the high Q values are consistent with the formation of a surface structure akin to (IV). The formation of some bulk sulphide in the case of cobalt would not be inconsistent with this model, as it is known that Co,S, catalyses butadiene hydrogenation with Type B selectivity under certain conditions.21u However, the low S :M ratios of good Type B cobalt powder and of nickel/silica show that the best Type B selectivities achieved can be ascribed to a structure close to (IV) and that further incorporation of sulphur into the bulk is irrelevant to the selectivity achieved in reactions at the surface.The division of the metal films (except Pt) into these two groups both on the basis of their adsorption properties ( S :M ratios) and their chemical reactivities (Q values) can therefore be given a single interpretation. This division can also be rationalised by consideration of (i) the standard heats of formation of the bulk sulphides (which may provide a measure of ad-layer ~tability)'~ and (ii) the heats of sublimation of the metals [which may provide a measure of the strengths of the metal-metal bonds in (I) and (II)].On this basis, incorporation of sulphur into Fe, Co, Ni and Pd was favoured because these metals have low standard heats of bulk sulphide formation and intermediate heats of sublimation. By the reverse of this argument, Mo, W and Re retained sulphur in an ad-layer because these metals have high heats both of sulphide formation and of sublimation. Manganese might have been expected to incorporate sulphur and show better Type B behaviour because it has the lowest heat of sublimation; however, it also has the highest heat of sulphide formation and hence we conclude that sulphur in an ad-layer on manganese was so stable that its progression towards incorporation was inhibited. Sulphur adsorption was extensive (table 3) but activity, butene composition, and butane yield in butadiene hydrogena- tion were all such as to suggest a much lower surface concentration of sulphur.No interpretation will be offered until independent measurements of sulphur concentra- tion at the surface are available. W). The behaviour of platinum was enigmatic.BURDEN, GRANT, MARTOS, MOYES A N D WELLS 105 PART 3.-SELECTIVITY AND HYDROGEN OCCLUSION VARIATION OF BUTANE YIELDS I N IRIDIUM-CATALYSED BUTADIENE HYDROGENATION The cavity theory of catalyst particle structure associates the yield of butane in catalysed butadiene hydrogenation with the extent of reversible hydrogen occlusion in the catalytically active metale3 The highest butane yield within Group VIII is afforded by iridium (see the selectivity pattern in Part 1) and the cavity theory predicts that higher selectivities will be obtained if the cavity concentration is reduced either by annealing or by reducing iridium particle size to the point where cavities cannot exist.Six iridium powders containing different amounts of occluded hydrogen were prepared as described (see Experimental section) and used as butadiene hydrogenation 0.0 L I I I I 0 40 80 120 temperature/"C FIG. 4.-Temperature dependence of selectivity, S, in butadiene hydrogenation. S = p(C4H8)/ [p(C4H8) + p(C4HIo)]. (a) Reactions catalysed by six iridium powders; (b) reactions catalysed by five iridium/silicas and literature data for 5 %-iridiumlalumina. catalysts.Fig. 4(a) shows that selectivity was very dependent on the extent of hydro- gen occlusion (the variation with temperature is well-known). l8 Ir-2 showed an expected selectivity, but Ir-3, Ir-4 and Ir-5, which were prepared at high temperature and contained but little occluded hydrogen, gave very high values of selectivity such as are normally associated with Co-, Ni- or Pd-catalysed reactions. The selectivities of106 SITE ENVIRONMENT A N D SELECTIVITY Ir-1 and Ir-6 were intermediate, as expected on the basis of the extent of hydrogen occlusion, but were higher than expected. Selectivities afforded by the five iridium/silicas are recorded in fig. 4(b). The 20%- and lO%-Ir/silicas, which contained metal particles of considerable size, gave standard selectivities closely similar to that of 5 %-Ir/alumina (taken from previous and shown for comparison).The 0.3%- 0.1 %- and 0.01 %-Ir/silicas, which contained no iridium particles of sufficient size to be visible by electron microscopy, showed higher selectivities, although values did not approach those given by iridium powders Ir-3, Ir-4 and Ir-5. It is possible that these iridium/silicas contained small numbers of larger cavitated particles which went undetected by electron microscopy and that a high selectivity of the small uncavitated particles was thereby degraded. Alternatively, and more probably, the relative surface concentrations of adsorbed hydrocarbon and hydrogen at the surfaces of particles only a few iingstrom in dia- meter may differ from that at a more extended surface; a shift of the balance in favour of adsorbed hydrogen would be equivalent to an increase in the applied hydrogen pressure and would favour increased butane formation.To summarise, novel high selectivities have been observed for iridium powders prepared at high temperatures in accordance with the prediction of the cavity theory. A considerable improvement in selectivity has also been achieved by reduction in metal particle size. CONCLUSION This paper demonstrates that selectivity in simple reactions of hydrocarbons is much dependent on site environment. These chemical probes can be used in future with some confidence to obtain information about sites in novel catalysts, and novel selectivities can be achieved by intentional modification of site environment.We thank Dr R. Whyman of I.C.I. Corporate Laboratory for Ru6C(C0)17 and Ru,C(CO),,/silica. J. M. thanks his Company for leave of absence during which the work presented in Part 3 was carried out. D. McMunn, R. B. Moyes and P. B. Wells, J. Cutul., 1978,52,472. M. George, R. B. Moyes, D. Ramanarao and P. B. Wells, J. Cutul., 1978,52,486. P. B. Wells, J. Cutul., 1978, 52, 498. D. Bingham, D. E. Webster and P. B. Wells, J. Chem. SOC., Dalton Truns., 1974, 1514, 1519. J. Grant, R. B. Moyes and P. B. Wells, J. Cutul., 1978, 51, 355. B. F. G. Johnson, R. D. Johnston and J. Lewis, J. Chern. SOC. A , 1968, 2865. R. G. James and R. B. Moyes, J. Chem. SOC., Furaday Trans. I , 1978, 74, 1666. J. Grant, R. B. Moyes and P. B. Wells, J. Chern. SOC., Furaday Truns. I , 1973, 69, 1779, and references therein. lo C. Kemball, J. Chem. SOC., 1956, 735; C. Kemball and P. B. Wells, J. Chem. SOC. A, 1968, 444. K. Baron, Ph.D. Thesis (University of Hull, 1971). l2 D. Brennan and M. J. Graham, Philos. Truns. R. SOC. London, Ser. A, 1965, 258, 325; D. Brennan and F. H. Hayes, Philos. Truns. R. SOC. London, Ser. A, 1965, 258, 347; D. Brennan and D. 0. Hayward, Philos. Trans, R. SOC. London, Ser. A, 1965,258, 375. * G. C. Bond, G. Webb and P. B. Wells, Truns. Faruduy SOC., 1968, 64, 3077, l3 Y. Delaunois, A. Frennet and G. Lienard, J. Chem. Phys., 1966, 63,906. l4 K. S. W. Sing, personal communication. l5 E.g. J. Benard, in Catalysis Reviews ed. H. Heinemann (Marcel Dekker, New York, 1970), l6 K. H. Bourne, P. D. Holmes and R. C. Pitkethly, Proc. 5th Int. Congr. Cutul., vol. 2, p. 1400. '' J. R. Rostrup-Nielsen, J. Cutal., 1968, 11, 220. VO~. 3, pp. 93-110.BURDEN, GRANT, MARTOS, MOYES A N D WELLS 107 l8 P. B. Wells, in Surface and Defect PropertiesofMetals, senior reporters J. M. Thomas and M. W. Roberts (Specialist Periodical Report, The Chemical Society, London, 1972), vol. 1, pp. 236- 258. l9 J. J. Phillipson, P. B. Wells and G. R. Wilson, J. Chem. Soc. A , 1969, 1351. 'O A. J. Bates, Z. K. Leszczynski, J. J. Phillipson, P. B. Wells and G. R. Wilson, J. Chem. SOC. A , 21 (a) J. Grant, R. C. Hoodless, R. B. Moyes and P. B. Wells, unpublished work. (b) J. Grant, 22 M. W. Roberts, Prog. Surf. Sci., 1970, 3, 1. 23 M. W. Roberts, Nature (London), 1960,188, 1020. 24 G. C. Bond, G. Webb, P. B. Wells and J. M. Winterbottom, J. Chem. SOC., 1965, 3218. 1970, 2435. Ph.D. Thesis (University of Hull, 1977).

ISSN:0301-7249    

DOI:10.1039/DC9817200095

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Support Effects on Selectivity over Rhodium Bimetallic Cat a1 y st s BY GARY L. HALLER, DANIEL E. RESASCO AND ARMANDO J. Rouco Department of Engineering and Applied Science, Yale University, P.O. Box 2159, Yale Station, New Haven, Connecticut 06520, U.S.A. Received 19th May, 1981 Rhodium catalysts are about one order of magnitude more active for ethane hydrogenolysis when supported on alumina or titania than when supported on silica when reduction is carried out at low temperature. However, the difference in activity for ethane hydrogenolysis between low-(573 K) and high-(773 K) temperature reduced Rh on titania is much greater. When the selectivity between dehydrogenation and hydrogenolysis of cyclohexane is compared at low and high temperature, a modest increase in selectivity is found to accompany the increased reduction temperature.A more substantial effect on selectivity is evident when Rh-Ag on silica and titania, both reduced at low temperature, are compared. The direct effect of rhodium-titania interaction (varied reduction temperature) and the indirect effect of support (changed Rh-Ag interaction) appear to have a com- mon origin. It is proposed that in both the direct and indirect support interactions there may be preferential interaction with the smallest particles in the distribution. The effect of metal-support interaction on the selectivity between dehydrogenation and hydrogenolysis of cyclohexane was first reported by Nehring and Dreyer.' At 773 K, the Pt catalysed selective dehydrogenation to benzene was found to decrease in the order titanium oxide, aluminium oxide, magnesium oxide, silicon oxide, while zinc-oxide-supported Pt was found to be inactive.It was subsequently discovered that Group VIII noble metals supported on titanium oxide and reduced at low temperature (473 K) have distinctly different properties compared with catalysts where the reduction is carried out at high temperature (773 K).2 The high-temperature reduction decreases the hydrogen and carbon monoxide chemisorption to near zero. This effect has been designated by Tauster et al. as a strong metal-support interaction and shown to be a reversible effect when the catalysts were oxidised at 673 K and re-reduced at low temperature (473 K). Meriaudeau et aL3 report that the rates of both benzene dehydrogenation at 273 K and cyclohexane dehydrogenation at 523 K on titania-supported Rh follow the trend in hydrogen chemisorptive capacity, i.e.the activity per unit weight of Rh decreases about one order of magnitude when the reduc- tion temperature of the catalyst is increased from 473 to 773 K. Results on selectivity were not mentioned, probably because the activity for hydrogenolysis is negligible relative to dehydrogenation of cyclohexane over Rh at 523 K. However, the selecti- vity between isomerisation and hydrogenolysis of neopentane over titania-supported Pt has been studied by Foger and Ander~on.~ The percentage neopentane reacting (in the temperature range 473-553 K) to C5 products increased from 21 % to 65% when the reduction temperature was increased from 573 to 723 K.Because there was no evidence from electron microscopy for a change in particle size or morphology with reduction temperature and the activation energy remained constant at ca. 150 kJ mol- l, the increased isomerisation selectivity was ascribed to the decreased concentra- tion of adsorbed hydrogen on the catalyst reduced at 723 K. The frequency factor110 SUPPORT EFFECTS ON SELECTIVITY (rate per surface Pt atom) was lower by a factor of lo2 on the high-temperature reduced catalyst, indicating a much lower activity for both isomerisation and hydro- genolysis. In this work we have investigated two kinds of metal-support interactions. The direct effects of metal-support interaction on the selectivity between dehydrogenation and hydrogenolysis of cyclohexane were studied by comparing silica- and titania- supported Rh reduced at high and low temperature. An indirect effect of metal-support interaction, the influence of the support on the kind of metal-metal interaction between Rh and Ag, was probed by the change in selectivity that accom- panies a change in the support.Previous work has demonstrated two kinds of behaviour of Group VIII-Group IB immiscible bimetallics with regard to hydrogeno- lysis. One kind is exemplified by Ru-Cu (or 0s-Cu) supported on ~ilica.~ For these catalysts the turnover frequency for ethane hydrogenolysis decreases precipita- tely as the Cu/Ru (or Cu/Os) ratio increases. In marked contrast, the turnover frequency of ethane hydrogenolysis on silica-supported Rh-Ag was observed to be nearly constant as the Ag/Rh ratio was increased.6 These catalyst systems exhibit a comparable change in hydrogen chemisorption as the Group IB/Group VIII ratio is increased which suggests that the extent of metal-metal interaction is comparable also.What appears to be different is the manner in which the Group IB metal is distributed on the surface of the Group VIII metal. It has recently been shown that Cu at sub- monolayer coverage on (0001) Ru is uniformly distributed and that the enthalpy change associated with the Ru-Cu interaction is ca. 20 kJ mol-' greater than the Cu- Cu intera~tion.~ However, the Rh-Ag interaction is apparently weaker than the Ag-Ag interaction.s If this is the case, then it may be expected that Ag will form islands or patches on the surface of Rh at submonolayer coverage when the support is silica, a fact that would rationalise the observation that turnover frequency of ethane hydrogenolysis (based on hydrogen chemisorption) is approximately constant on the Ag/Rh catalysts.Our goal was to investigate the Ag-Rh interaction on titanium oxide, a support with which metals are known to interact more strongly than on silica. EXPERIMENTAL MATERIALS USED AND CATALYST PREPARATION The catalysts were prepared by impregnating the support (silica, alumina and titania) with a solution of Rh(N03)3 obtained from Alfa Ventron Corp. The percentage of Rh by weight was 2% in most catalysts. The silica support used was provided by Cabot Corp. (grade HS-5, non-porous, 300 m2 g-I). Two titania supports were used, one provided by Degussa Inc.(grade P-25, SO% anatase/20% rutile, 50 If 15 m2 g-l) and one provided by Cabot Corp. (Cab-0-Ti M-85, 85% anatase/l5% rutile, 50-70 m2 g- I). Additional physical properties of these titanium oxides are given in ref. (2). The alumina support was Cabot Alon C, 90% gamma, 100 m2 g-I. The ratio of impregnating solution to weight of support adjusted to have similar degree of wetness was 3.75 cm3 g-I for SiOl-supported catalysts and 4 cm3 g- for Ti02-supported catalysts. After impregnation the catalysts were dried for two days at room temperature and then 12 h in air at 393 K. A similar prepara- tion was used for the alumina-supported catalyst. The pretreatment was exactly the same for silica-, alumina- and titania-supported catalysts.HYDROGEN CHEMISORPTION MEASUREMENTS Hydrogen adsorption measurements were carried out in a conventional Pyrex volu- An oil diffusion pump provided a dynamic vacuum of lo-' metric adsorption apparatus.G. L , HALLER, D . E. RESASCO AND A. J . ROUCO 111 Torr. Pressures were measured with an absolute pressure gauge (MKS Instruments). After the initial pretreatment the catalysts were reduced in situ at 573 K (LTR) and 773 K (HTR) in a hydrogen flow. Evacuation after reduction was carried out at 573 K for 5 h. Adsorption isotherms at room temperature were measured by admitting a known quantity of gas to the adsorption cell and waiting overnight before reading the equilibrium pressure for the first point. In determining the Rh dispersion, defined as the fraction of Rh atoms present in the surface of the metal crystallites, the isotherms were extrapolated to zero pres- sure.The value of H/Rh obtained in this way is taken as the degree of metal dispersion, i.e. the percentage exposed. ACTIVITY MEASUREMENTS ETHANE HYDROGENOLYSIS The activity measurements for the ethane hydrogenolysis reaction were performed in a microcatalytic pulse reactor. The reactors are Pyrex 6 mm 0.d. glass tubing connected to stainless-steel piping by a Cajon Ultra-torr union with Viton O-ring. The reactor is sus- pended in an electric furnace controlled within 0.5 K by a temperature programme-con- troller (Research Inc. models 7321 1 and 6391 1). Catalysts were activated in situ as described for hydrogen chemisorption. The amount of catalyst used for ethane hydrogenolysis was 50 mg.The 0.5 cm long catalyst bed was preceded by a preheater section of 3.5 g of 60 mesh glass beads that had been washed with sulphuric acid + chromic acid solution and heated to a high temperature in air. The temperature was monitored by an iron-constantan thermo- couple in contact with the catalyst bed. Blank runs indicated that up to 673 K the glass beads, the thermocouple, the supports and supported Ag are not detectably active for ethane hydrogenolysis or cyclohexane dehydrogenation/hydrogenolysis. Purified hydrogen was used as the carrier gas and the hydrocarbon pulse injection was accomplished by a Carle 2015 sampling valve. Each hydrocarbon pulse was 0.05 cm3. The carrier flow was meas- ured with a Hasting mass flowmeter at 30 cm3 min- Analysis was performed by an on-line gas chromatograph (Varian model 3700) by flame ionization detection coupled to Varian CDS 111 electronic integrator following separation on a Chromosorb 104 2m column operated at 348 K.The conversion levels were maintained below 5%. Thus we calculate the differential rate of reaction based on the conversion and a residence time assumed to be the reactor (catalyst bed) volume ratioed to the carrier flowrate and converted to units of molecules converted per surface Rh atom per min based on H/Rh = 1 for hydrogen chemi- sorption. In the case of titanium oxide the percentage exposed after a high-temperature reduction is assumed to be equal to that measured after a low-temperature reduction because of the known depression of hydrogen chemisorption that accompanies the strong metal- support interaction. CYCLOHEXANE REACTIONS The cyclohexane dehydrogenation and hydrogenolysis activity measurements were obtained in the same system used for pulse experiments but operated in steady-state flow mode.The reactant gas was passed over the catalyst for 12 min prior to sampling product for analysis by using a second Carle 2015 sampling valve. The carrier gas was helium. The reactant gas was a mixture of cyclohexane and hydrogen obtained by joining a stream of hydrogen (60 cm3 min-') with another hydrogen stream (2 cm3 min-') passed through a saturator and a condenser thermostatted at 268.7 K. The hydrogen/cyclohexane ratio was 1070. This high hydrogen/cyclohexane ratio was necessary in order to obtain an appreciable amount of hydrogenolysis products which made it possible to measure a change in the selectivity.Initial results performed in the pulse mode showed that the Rh-Ag/SiOz catalysts cracked the whole pulse to methane but Rh-Ag/Ti02 catalysts produced a large amount of benzene. By working at very low cyclohexane partial pressure we could approxi- mate this situation in the flow reactor. The high hydrogen/cyclohexane ratio allowed us to work at higher temperatures with low conversion far from the cyclohexane-benzene equili-112 SUPPORT EFFECTS ON SELECTIVITY brium so that dehydrogenation and hydrogenolysis could be measured simultaneously around 573 K. The differential rates were calculated in the same manner as described for the pulse reactor and the analysis apparatus used was the same as described for ethane hydrogenolysis. RESULTS The rate of ethane hydrogenolysis at 523 K measured in the pulse reactor is Note that the rate is expressed compared for three kinds of oxide supports in table 1.TABLE ET ETHANE HYDROGENOLYSIS AT 523 K ratec /molecule E a catalyst treatment ' CO/Rh (total Rh /kJ mol- atom min)-l 3% Rh/Si02 LTR - 0.40 230 f 10 HTR 0.79 0.33 230 2% Rh/A1203 LTR 0.70 3.52 1 70 HTR 0.72 2.65 180 2% Rh/Ti02 LTR 0.18 0.36 180 HTR 0.007 <0.001 - LTR 0.85 1.9 180 ~~ 11 LTR implies an in situ low-temperature reduction at 473 K following a 723 K reduction and exposure to air at room temperature; HTR is an in situ reduction at 773 K. This is the ratio of chemisorbed CO to total Rh atoms measured by a dynamic pulse method at room temperature.Because hydrogen chemisorption was not measured, the rate is expressed per total Rh atoms; for comparison to results in the figures and other tables the rates can be estimated by dividing by the CO/Rh ratio. This LTR was preceded by an oxygen treatment at 673 K. in molecules converted per total Rh atoms per min because the amount of hydrogen chemisorption was not measured. It should be observed that the low-temperature reduction was the same as used in ref. (2) (473 K), i.e. below the reaction temperature. Tables 2 and 3 compare various silica- and titania-supported catalysts which had TABLE 2.-ETHANE HYDROGENOLYSIS AND CYCLOHEXANE DEHYDROGENATION/HYDROGENOLYSIS AT 573 K FOLLOWING 773 K REDUCTION--673 K OXIDATION-573 K REDUCTION c ycl o hexane ethane rate dehydrogenation 2% Rh on Ag/Rh a H/Rh /molecule rate S " support (surface Rh /molecule atom min) - (surface Rh atom min)- SiO, 0 0.73 6.9 0.60 6 SiOz 1 0.13 0.26 1 .oo 15 TiO, 0 0.33 27.2 3.60 14 Ti02 1 0.22 0.02 6.90 250 ' Atomic ratio of Ag to Rh.Rate of benzene formation. S is the selectivity, the rate of cyclohexane conversion to benzene ratioed to the rate of cyclohexane disappearance to all other hydrogenolysis products.G . L . HALLER, D . E . RESASCO A N D A . J . ROUCO 113 TABLE 3 .-ETHANE HYDROGENOLYSIS AND CYCLOHEXANE DEHYDROGENATION/HYDROGENOLYSIS AT 573 K FOLLOWING 773 K REDUCTION--673 K OXIDATION-773 K REDUCTION cyclohexane ethane rate dehydrogenation 2% Rh on H/Rh /molecule rate a S b support (surface /molecule Rh atom min)- (surface Rh atom min)- SiOz 0.73 6.9 1.15 14 TiOz 0.01 2.7 x lo-” 2.82 60 a Rate of benzene formation.S is the selectivity, the rate of cyclohexane conversion to benzene ratioed to the rate of cyclohexane disappearance to all other hydrogenolysis products. This rate is not based on the H/Rh = 0.01 but on the H/Rh = 0.33 following a low-temperature reduction, see table 2. been reduced just prior to reaction at 573 and 773 K, respectively. Prior to the final reduction the treatment was identical for all catalysts shown in tables 2 and 3, i.e. reduction at 773 K in flowing hydrogen followed by oxidation at 673 K. The final low-temperature reduction used for the results given in table 2 was increased to 573 K (instead of 473 K as in table 1) so that all reactions could be carried out at or below the final reduction temperature.The ethane hydrogenolysis was performed in the pulse reactor while the cyclohexane dehydrogenation/hydrogenolysis was measured in the same reactor in the steady-state flow mode. Fig. 1 presents Arrhenius plots of the rate of benzene formation from cyclohexane at 573 K. All data were collected at times 12 min after cyclohexane was added to the hydrogen stream. This time period was chosen as a compromise, long enough to assure steady-state reaction but short enough so that there was no appreciable decrease in rate due to self-poisoning. The rate decreased by about a factor of 1.5 in the period between 12 and 20 min. This effect is entirely reversible by flowing hydrogen at the reaction temperature, i.e.it is presumably a slow accumulation of less-reactive hydrocarbon intermediates removed as hydrogenolysis products in the presence of pure hydrogen. Fig. 2 is a graphical representation of the dramatic effect of Ag on ethane hydro- genolysis activity. The cyclohexane hydrogenolysis activity is apparently similar but because of the extremely low conversion obtained at 573 K with Ag-Rh/Ti02, e.g. As can be seen in fig. 3, there is a clear correlation between activity and dispersion (percentage exposed), the rate of ethane hydrogenolysis increases by about a factor of 30 as the fraction of Rh atoms at the surface (as measured by hydrogen chemisorption) was increased from 0.3 to 0.8. There is probably an effect of the kind of titania on the dispersion, i.e.the four catalysts of lowest dispersion are on Degussa oxide while the three of highest dispersion are supported on Cab-o-Ti. It may be significant that the activity for the titania supports appears asymptotically to approach that of silica as the dispersion is decreased. it cannot be measured very accurately. DISCUSSION Before turning to the principal question, selectivity of bimetallic Ag-Rh catalysts influenced by the support, it is useful to consider support and preparation effects on pure Rh for the ethane hydrogenolysis reaction. In a previous investigation of114 SUPPORT EFFECTS ON SELECTIVITY silica-supported Rh where the texture of the silica, anion of the impregnating solution and degree of dehydration before reduction were varied, we observed a range of rates at 523 K varying from 0.36 to 4.9 molecules per surface Rh atom per min with no correlation between activity and disper~ion.~ These results were obtained in the same pulse reactor used in the present work and, considering the different reactor used, are in fair agreement with the range of rates reported by Yates and Sinfelt'' when their results are extrapolated to the same temperature and converted to the same units, 4.0 2.0 ' \ 0.\ 0 a '. 1.75 1.80 1.85 103 KIT FIG. 1.-Arrhenius plot of the rate of benzene formation from cyclohexane. The numbers given in parentheses are approximate activation energies in kJ mol-'. Silica support is represented by circles and titania support by squares. Filled symbols indicate pure Rh, 2% by weight; open symbols indicate catalysts with Ag/Rh atomic ratio of one. 0.28-9.7.The rate of ethane hydrogenolysis given in table 1 for silica-supported Rh falls within the range of previous values. However, the rate given in table 2 extra- polated to 523 K with the measured activation energy of 200 kJ mo1-1 is only 0.13 molecules per surface Rh per min. The difference is attributed to the oxidation treatment at 673 K used for all results reported in tables 2 and 3. While it is not possible to rationalise these results at the present time, the observed facts are that oxidation at relatively low temperature, 673 K, and high temperature, 1073 K,1° decreases the rate of ethane hydrogenolysis while an intermediate oxidation tempera- ture, 873-923 K,9 has a relatively small effect on activity.We have previously argued that the effect of oxidation cannot be a particle-size effect alone and should probably be interpreted as a support interaction induced by the ~xidation.~*l~ Two points must be borne in mind in the light of the above discussion: (i) a comparison of supportG . L . HALLER, D. E. RESASCO A N D A . J . ROUCO 0 c, 0 1 . 1 I I 0.5 1 .o 115 FIG. 2.-Rate of ethane hydrogenolysis at 573 K as a function of the AglRh atomic ratio on titania support. I I116 SUPPORT EFFECTS ON SELECTIVITY effects requires an identical preparation and pretreatment, and (ii) an order-of- magnitude difference in activity between two different supports is comparable to observed preparation/pretreatment effects and may not be significant, The rates of ethane hydrogenolysis on silica, alumina and titania are compared in table 1.Alumina and titania, when reduced at low temperature, have considerably greater activity than silica-supported Rh and a significantly lower activation energy. The support effect that arises between titania-supported Rh reduced at low and high temperature is more striking than is the comparison with the other oxides. After the high-temperature reduction, Tauster et al.2 suggested Rh on titania was bonded to Ti cations via electron donationfrom the metal to the cation by analogy to certain barium titanates which contain Group VIII metals. However, the titanates always contain the Group VIII metals as isolated cations instead of small clusters of metal atoms on the surface.A recent theoretical interpretation by Horsely l2 indicates a metal-metal bonding with donationfrom the Ti cation to metal atoms (Pt in Horsely’s calculation) located on surface oxygen-ion vacancies. This picture is consistent with the experi- mental observation of charge transfer from Ti3+ in the surface of (100) SrTiO, to Pt atoms or from Ti3+ in the surface of (110) TiOz to Ni atoms measured by photo- emission.13*14 It should be emphasised that the Ti3+ on the surface of the single crys- tals studied by Chung and co-workers was produced by Ar-ion bombardment and not by reduction in hydrogen at 773 K, the normal procedure for inducing a strong metal- support interaction on dispersed titania-supported catalysts. The hydrogen reduction produces a surface concentration of Ti3+ which is too small to be detected by photo- emission.Moreover, to the extent that there is interaction in the dispersed catalysts, it is an interaction with metal clusters and not individual atoms. This suggests that the strong metal-support interaction of catalytic interest may be more subtle interaction than that described theoretically by Horsely or physically by the experiments of Chung et at. The recent n.m.r. results of Gajardo et al.15 for a 3.8% Rh on a 205 m2 8-l titanium oxide, comparable with the preparation used in this work, suggest an interaction similar to that described by Chung et al. for the single crystals. A sample with a fraction exposed equal to 0.6 (measured by hydrogen chemisorption) had a high-field resonance attributed to hydrogen on or near paramagnetic centres on the surface of titanium oxide when the reduction temperature was 573 K.These paramagnetic centres disappeared upon reduction at 673 K (and are not detected in the absence of Rh). This suggests that the strong metal-support interaction induced by high-temperature reduction may involve electron transfer from a surface Ti3+ to a Rh cluster. We have attempted to probe for the negative charge on Rh using N2 chemisorption.16 Rh has been reported to be unable to chemisorb nitrogen” and only slowly decomposes ammonia at high temperature.18 However, Rh supported on activated carbon or alumina and promoted by K has been shown to chemisorb nitrogen19 and to have a stable activity for ammonia synthesis comparable with Fe.20 The effect of alkali metals on transition metal activity for ammonia synthesis is generally ascribed to its electron- donating nature since chemisorption and activity increase with decreasing ionisation potential of the alkali, i.e.Cs > K > Na.21 Thus, it is reasoned that a high-tempera- ture reduction of Rh on titania should promote N2 chemisorption to some extent if negative charge is transferred to Rh. This has been verified experimentally and shown to be reversible following oxidation at 673 K.16 Most previous investigations have implied that titanium oxide is only quantitatively different from other oxide supports after a high-temperature reduction. Fig. 3, which plots the activity for ethane hydrogenolysis as a function of dispersion after low-temperature reduction, would suggest a support interaction that increases as theG.L . HALLER, D . E. RESASCO A N D A . J . ROUCO 117 particle size decreases, an effect which must be different from the strong metal-support interaction of Tauster et aL2 No comparable correlation between activity and particle size was evident for silica in our own work9 but Yates and Sinfeltlo have claimed a maximum activity for a particle size around 2 nm for ethane hydrogenolysis and Fuentes and Figueras 22 see a similar maximum for cyclopentane hydrogenolysis around 5 nm. However, the maximum selectivity between simple ring opening to pentane and hydrogenolysis to methane occurred at 2 nm. The comparison in table 2 of the effect of Ag on Rh supported on silica and titania also indicates that these two supports are quantitatively different even when reduced at low temperature.There is more than an order of magnitude depression of the ethane hydrogenolysis rate when Ag is added to Rh on a silica support, but the effect is at least three orders of magnitude on titania. Moreover, the effect on silica may have a different origin. In a more detailed investigation of Ag-Rh silica-supported catalysts where no oxida- tion treatment was used and the reduction temperature was 723 K, the specific rate did not vary by more than about a factor of 2 as the Ag/Rh atomic ratio was varied from zero to values >1.6 It may be that Rh-Ag catalysts which have been oxidised at 673 K are not completely reduced at 573 K although the fact that the rate of cyclo- hexane dehydrogenation was greater on the Rh-Ag catalysts than pure Rh catalysts independent of support suggests that incomplete reduction is not a factor for either support.The precipitous decrease in ethane hydrogenolysis activity on titania-supported Rh as a function of increasing Ag/Rh ratio is quite similar to that found on Ru-CuS where the Group IB metal is known to deposit uniformly over the ~urface.~ Because of this similarity we believe that the titania support has altered the Ag-Rh energetics so that Ag deposits uniformly over the Rh surface in contrast to the silica support where the Ag is supposed to form islands or patches below a monolayer.6 If ethane hydrogenolysis requires several Rh atoms to constitute a site as is generally believed5 then the uniform deposition of the Group IB allows a disproportionate number of sites to be poisoned but when islands are formed, that fraction of the surface not covered behaves like normal Rh.Our interpretation requires a linear decrease in hydrogen chemisorption with added Ag up to a monolayer. In fact, this kind of behaviour has now been documented for deuterium on (100) Rh8 and, like the ethane hydrogenolysis, it is in marked contrast to hydrogen coverage dependence on Cu coverage on (0001) R u . ~ The nature of the titania-rhodium interaction that affects the way Ag deposits on small Rh particles is not obvious. It may not involve electron transfer from the support to the Rh since we are considering an effect observed at low-temperature reduction. Whatever the nature of the interaction, the results of fig.3 indicate that it increases as the particle size decreases. Moreover, since the activity for ethane hydrogenolysis apparently increases as the interaction grows on low-temperature reduced titania which was previously oxidised at 673 K and the activity for ethane hydrogenolysis always decreases greatly after a high-temperature reduction, it is conceivable that the low-temperature reduction (or more probably the oxidation that precedes it) involves charge transfer from Rh to titania. This would appear to be consistent with the n.m.r. results that a low-temperature reduction in the presence of small particles of Rh creates paramagnetic centres (probably Ti3+) not formed in the absence of Rh and that these centres disappear when the reduction temperature is increased.15 A simple comparison of the effect of added Ag on Rh activity for ethane hydro- genolysis and cyclohexane dehydrogenation would lead one to surmise that cyclo- hexane dehydrogenation/hydrogenolysis selectivity would be dramatically increased by the addition of Ag on either support.The selectivity does increase but rather118 SUPPORT EFFECTS ON SELECTIVITY modestly even on titania support. If one assumes that the sites for dehydrogenation require only a single Rh atom and ethane hydrogenolysis sites require several, then one would have to conclude that ring opening (and subsequent hydrogenolysis) sites are much more like the dehydrogenation sites than the hydrogenolysis sites. In the terminology of Ander~on,~ the critical reactant site for cyclohexane hydrogenolysis is smaller than for ethane hydrogenolysis.Moreover, these sites may also be distingui- shed by their activation energies. For pure Rh on titania, ethane hydrogenolysis has an activation energy of ca. 180 kJ mo1-I while cyclohexane hydrogenolysis has a lower activation energy, ca. 140 kJ mol-I. A comparison of low- and high-temperature reduced pure Rh on titania is even more striking. Ethane hydrogenolysis decreases by three orders of magnitude, dehydrogenation by ca. 20% but the selectivity only increases by a factor of 4 when low- and high-temperature reductions are compared. As reported by Anderson for Pt,4 the activation energies are apparently not affected as one moves from low- to high- temperature reduction, i.e. it is primarily the number of sites that is changed by the change in reduction. However, all sites cannot be uniformly attenuated or one would not observe any effect on selectivity.If we assume that the strong metal-support interaction (high-temperature reduction) beomes more effective as the particle size decreases, a fact that is demonstrated for the low-temperature reduction in fig. 3, then it may be that the high-temperature reduction selectively deactivates the smallest particles in the distribution. This would lead to the conclusion that the larger parti- cles are more selective for benzene formation than for hydrogenolysis of cyclohexane. Our results would at first glance appear to be in conflict with those of ref. (3) where it is observed that rate of cyclohexane dehydrogenation at 523 K decreases by a factor of 7 when the reduction temperature is increased from 473 to 773 K.How- ever, no oxidation treatment was used in this study. In an experiment where we allow a room-temperature oxidation, the cyclohexane dehydrogenation at 573 K is a factor of 4 lower when the reduction temperature is increased from 573 to 773 K and, if both reductions are preceded by a 673 K oxidation, the increase in the reduction temperature only decreases the dehydrogenation rate by 20% (compare tables 2 and 3). Thus preoxidation of the titania-supported Rh catalysts has a levelling influence with respect to reduction temperature effect for a less demanding reaction like dehydro- genation, but for a demanding, site-structure-sensitive reaction like ethane hydro- genolysis, the effects of reduction temperature remain pronounced. In conclusion, we find that the order of increasing site-structure sensitivity or critical size of sites is cyclohexane dehydrogenation, cyclohexane hydrogenolysis and ethane hydrogenolysis on Rh catalysts and this is the order of increasing influence of support interaction with titanium oxide for the direct Rh-Ti02 interaction.Greater leverage on selectivity can be gained by an indirect support effect for the case of Rh-Ag bimetallic catalysts. In this case the nature of the support, silica or titania, changes the kind of metal-metal interaction but the magnitude of the effect follows the same ordering of reactions as for the direct effect. This suggests that the effect of Ag on Rh sites and the effect of strong metal-support interaction on Rh sites have a common mode of attack.The simplest hypothesis is that the smallest particles are being preferentially deactivated in both the direct and indirect metal-support inter- actions on titanium oxide. We thank the National Science Foundation for support of this research. A. J. R. thanks the Consejo Nacional de Investigaciones Cientificas y Tecnicas de Argentina ; D. E. R. thanks the Bolsa de Comercio de Mar del Plata, Argentina, for scholarships.G . L . HALLER, D . E. RESASCO AND A . J . ROUCO 119 D. Nehring and H. Dreyer, Chem. Tech. (Berlin), 1960,12,343. S . J. Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. SOC., 1978,100,170. P. Meriaudeau, H. Ellestad and C. Naccache, Proc. 7th Int. Congr. Catal., Tokyo, 1980, ed. T. Seiyama and K. Tanabe (Kodansha, Tokyo and Elsevier, Amsterdam, 1981), part B, p. 1464. J. R. Anderson, Am. Chem. SOC., Div. Pet. Chem. Prepr., 1981, 26, 361. J. H. Sinfelt, J. Catal., 1973, 29, 308. ti A. J. Rouco and G. L. Haller, J. Catal., to be published. ’ K. Christmann, G. Ertl and H. Shimizu, J. Catal,, 1980,61, 397. J. M. White, personal communication. A. J. Rouco and G. L. Haller, in Proc. 7th Ibero-American Symposium on Catalysis, La Plata, Argentina, July, 1980. lo D. J. C. Yates and J. H. Sinfelt, J. Catal., 1967, 8, 348. l1 A. J. Rouco and G. L. Haller, J. Chim. Phys., to be published. l2 J. A. Horsely, J. Am. Chem. SOC., 1979, 101, 2870. l3 M. K. Bahl, C. S. Tsai and Y. W. Chung, Phys. Reo. B, 1980,21, 1344. l4 C. C. Kao, S. C. Tsai, M. K. Bahl, Y. W. Chung and W. J. Lo, Surf. Sci., 1980,95, 1. l5 Y. Gajardo, T. M. Apple and C. Dybowski, Chem. Phys. Lett., 1980,74, 306. D. Resasco and G. L. Haller, J. Chem. SOC., Chem. Commun., 1980, 1150. B. M. W. Trapnell, Proc. R. SOC. London, Ser. A , 1953,218, 566. G. C. Bond, Catalysis by Metals (Academic Press, New York, 1962), p. 380. l9 M. Oh-Kita, K-I. Aika, K. Urake and A. Ozaki, J. Catal., 1976, 44, 460. *O K-I. Aika, S. Yamaguchi and A. Ozaki, Chem. Lett., 1973, 161. I. R. Shannon, in Catalysis, senior reporters C. Kemball and D. 0. Dowden (Specialist Periodi- cal Report, The Chemical Society, London, 1978), vol. 2, p. 33. 22 S. Fuentes and F. Figueras, J. Catal., 1980, 61, 443.

ISSN:0301-7249    

DOI:10.1039/DC9817200109

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

The Role of the Support in CO Hydrogenation Selectivity of Supported Rhodium BY JAMES R. KATZER, ARTHUR W. SLEIGHT,? PATRICIO GAJARDO, JOHN B. MICHEL, EDWARD F. GLEASON AND SCOTT MCMILLAN Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Delaware 1971 1, U.S.A. Received 29th June, 1981 Catalytic hydrogenation of CO offers tremendous challenges in catalyst design to control selec- tivity because of the large variety of hydrocarbons and oxygen-containing species that can be formed; Rh can produce the full range of potential products. Rh on SO2, A1203, Ti02, CeOz and MgO has been characterized by a complement of techniques and by CO hydrogenation activity. Catalytic activity varied over 200-fold, dependent on the support; Rh/TiOz was the most active.Selectivity to alcohols us. hydrocarbons and to methanol us. ethanol varied over 50-fold. CO hydrogenation selectivity to alcohols varies with the basicity of the support; MgO, the most basic support, ex- hibited 90% selectivity to methanol. Mononuclear transition-metal complexes, when used as catalysts, can show widely varied activity and selectivity dependent on the ligands associated with the supported metal.' The control of activity and selectivity with the appropriate ligands is due to both steric and electronic factors. The electronic factors can be significant because a mononuclear transition-metal complex involves only a small number of orbitals and e!ectrons, and large gaps exist between energy levels. In this case, small shifts in electron density can have an important effect upon the catalytic properties of the transition-metal atom.On the other hand with bulk metals, which for practical purposes also include small supported metal crystallites (a 25 8, crystallite contains ca. 500 atoms), there are a large number of electrons delocalized over the system, and energy level gaps are very small.' Imposing significant shifts in the electron distribu- tion by ligand effects in such a system appears difficult, and recent alloy studies clearly confirm that most effects on catalytic behaviour are geometric and not elec- tronic in Thus, truly new catalytic chemistry seems unlikely with classical supported metals. However, for ultra-dispersed supported metals with no more than a few metal atoms in a cluster, all of which are in close contact with the support, the support surface or other atoms located thereupon can play the role of ligands, and since we are now dealing with a very small number of metal atoms, the support can take on an important role in determining catalytic activity and selectivity of the metal.This support ligand could play a role similar to that of the ligands in homogeneous catalysts. This allows for the potential of activity-selectivity control similar to that for soluble homogeneous catalysts but without all the problems of corrosion and catalyst separation and recovery associated with soluble catalysts. t Present address: Experimental Station, E. I. DuPont deNemours & Co, Wilmington, Delaware 19898, U.S.A.122 CO HYDROGENATION OVER SUPPORTED Rh Rh has shown some very unique catalytic behaviour in homogeneous catalytic ~ h e m i s t r y , ~ ' ~ ~ as a soluble cluster compound in CO hydrogenation," and as a sup- ported metal.l2-I6 I~hikawa'~-'~ has shown that the selectivity of supported Rh in CO hydrogenation depends markedly on the chemical nature of the support.These observations led us to study CO hydrogenation catalysed by Rh on a series of sup- ports and to characterize these catalysts fully by a complement of spectroscopic techniques to provide a basis for interpretation of their activity and selectivity behaviour. EXPERIMENTAL MATERIALS AND CATALYST PREPARATION Alumina was Conoco Catapal calcined at 550 "C to convert it into y-A1203 (194 m2 g-I); Si02 was Davison Grade 57 Si02 gel (210 mz g-').MgO was Baker Analytical grade (50 m2 g-I). All other supports were synthesized by hydrolysis of the metal compounds or salts to obtain materials of sufficient surface area and purity, all supports had surface areas in excess of 100 m2 g-l. Details are given e1sewhere.l' In order to achieve the highest possible extent of dispersion, the Rh was ion-exchanged onto all the supports; for MgO the pore volume saturation technique was applied. Thus, initially atomic dispersion of Rh was achieved. Sulphate-free Rh(N03)3 (10 wt % solution), diluted to give the desired uptake, was used. Catalysts were reduced in situ, in flowing dry H2 at 473 K for 4 h, and the desired measurements were carried out. A summary of the catalysts studied is given in table 1.TABLE 1 .-PROPERTIES OF SUPPORTED Rh CATALYSTS ~~ ~ B.E.T. surface (wt %) H,b cot support area" /m2 g-' Rh Rh Rh Ti02 (anatase) 105 0.95 0.98 2.17 A1203 194 2.4 1.15 NAd Ce02 99 1.6 3.21 2.68 Si02 210 0.89 0.99 NAd MgO 50 3 .O 0.68" NAd ~ ~~~ ~ ~~ " Measured by N2 adsorption using standard B.E.T. methods; * Determined by extrapolation of Determined by pulse the H2 and CO adsorption isotherms at 298 K to the zero pressure intercept; adsorption at 298 K; NA = not available. APPARATUS AND PROCEDURE Table 2 summarizes the characterization techniques applied to the supported Rh systems. Since the methods and results of application of each technique applied cannot be discussed here, only selected results will be presented. TABLE 2.--CHARACTERIZATION TECHNIQUES APPLIED TO SUPPORTED Rh CATALYSTS CO hydrogenation, activity and selectivity ethylene hydrogenation ethane hydrogenolysis H2 chemisorption (static) CO chemisorption (static) f.t.i.r.of chemisorbed CO proton n.m.r. t.p.d.-Hz e.p.r. electron microscopy X-ray photoelectron spectroscopy EXAFS isotopic tracer X-ray diffraction u.v.-visible laser Raman spectroscopyKATZER, SLEIGHT, GAJARDO, MICHEL, GLEASON, MCMILLAN 123 INFRARED MEASUREMENTS 1.r. measurements were carried out in a quartz cell with water-cooled NaCl windows using a Nicolet 7199 FTIR spectrometer. The cells were attached to a conventional high- vacuum system equipped with a manifold for gas flow which permitted us to perform all pre- treatments and measurements in situ. The cells could be heated to 873 K with flowing gases for pretreatments. Calcined samples were pressed into wafers (16 mg cm-2) and placed in a quartz sample holder.CHEMISORPTION Hydrogen was measured in a standard volumetric apparatus; CO chemisorption was The total chemisorption capacity was determined by extrapola- carried out gravimetrically. ting the total chemisorption us. pressure data to zero pressure. X-RAY PHOTOELECTRON SPECTROSCOPY Calcined catalyst was pressed into a wafer and attached to the end of a probe. Pretreat- ments were done in a prechamber attached to the Physical Electronics 550 X-ray photoelec- tron spectrometer. Samples were translated into the main vacuum chamber and X.P.S. spectra were taken, then retracted into the pretreatment chamber and reduced in flowing H1 at the desired temperature (473 K), reexamined by X.P.S., withdrawn for chemisorption of CO, and again examined by X.P.S.co HYDROGENATION CO hydrogenation studies were carried out in a steady-state differential plug-flow reactor interfaced with two gas chromatographs. A manifold equipped with traps for removal of oxygen, water and carbonyls allowed metering of high-purity gases to the reactor at 1-10 atm. Concentrations and flow rates were set by the feed rates to the manifold; pressure was controlled by a backpressure regulator. Everything downstream from the reactor was heated to prevent condensation, allowing direct injection of the whole product stream into each g.c. Hydrocarbons were separated on an activated A1203 column; alcohols were separated on a THEED on chromosorb column.RESULTS AND DISCUSSION Table 1 summarizes the H2 and CO chemisorption results for the catalysts studied. In all cases total hydrogen chemisorption capacity indicates high degrees of dispersion of the Rh. The Rh/MgO is not as highly dispersed because of the alternate prepara- tion method. For Rh/CeO, the total uptake of H2 was surprisingly high, higher than could be attributed solely to adsorption on Rh. The H2 adsorption on Rh/Ce02 involved a rapid process followed by a slow continued uptake which must have in- volved hydrogen uptake by the support.17 CO adsorption provides additional insight into the dispersion of the Rh. Ratios on the supported Rh catalysts approached or surpassed 2.0 CO molecules per Rh atom. Rh/Ce02 in particular gave a ratio markedly larger than 2.0.In this case a second CO adsorption isotherm was measured after evacuation at 298 K for half an hour, and from this the amount of irreversibly adsorbed CO was determined to be 2.11 CO per Rh. These high values are apparently associated with adsorption on the support as appeared to be the case for H2 adsorption. In comparison, CO adsorption on single-crystal surfaces or on large supported metal crystallites will give no more than ca. 0.7 CO molecules per surface Rh atom at saturation.18-20 This is the steric limit for CO on a two-dimensional metallic surface; on small metal crystal- lites the CO to surface Rh atom ratio can approach 1.0.21,22 Only for very small clusters of Rh atoms having essentially all edge atoms could the CO to Rh ratios be124 CO HYDROGENATION OVER SUPPORTED Rh 2.0.These results support the H2 chemisorption studies and suggest a very high degree of Rh dispersion. Infrared studies of adsorbed CO allow further characterization of supported Rh. The three structures shown below represent the adsorbed species identified on the surface of supported Rh catalysts : dicarbonyl 1 i near bridged In the limit of very high dispersion involving either atomic dispersion or very small clusters containing mainly edge atoms the dicarbonyl species should predominate. As the Rh dispersion decreases the proportion of linear and bridged carbonyl species increases. These are the only species present on larger supported metal crystallites.18 In this work spectra of CO adsorbed on all supported Rh catalysts showed very strong dicarbonyl peaks.Fig. 1 shows the carbonyl spectrum for 2.4% Rh/A1203 34.40 2 3.40 I 2.40 I .40 2201 2111 202 I I931 1841 wavenumberlcm - FIG. 1 .-Infrared spectrum of CO adsorbed on Rh/A1203. Conditions : catalyst prepared by ion exchange of 2.4 wt % Rh on to Al2O3, calcined in O2 at 623 K, reduced in situ at 473 K in flowing dry H2, CO adsorption at 298 K and 120 Torr, evacuated just prior to taking the spectrum. reduced at 473 K. There are two principal bands in the CO region of the spectrum; these are assigned to the symmetric and asymmetric stretches of the dicarbonyl species. There is a very weak band assigned to the bridging CO species. Absorp- tion due to the linear species is completely absent from the spectrum.These results confirm that the ion-exchange preparation technique has given a uniquely high degree of Rh dispersion, i.e. atomically dispersed or very small clusters of Rh atoms which show no two-dimensional metal surface-like properties for CO adsorption. For example, the i.r. frequencies of adsorbed CO were all independent of CO cover- age from Torr up, in contrast to expected behaviour for two-dimensional metal-like surfaces, where large shifts in CO frequency are observed.18 These con-KATZER, SLEIGHT, GAJARDO, MICHEL, GLEASON, MCMILLAN 125 clusions are supported by the H2 chemisorption results. Clearly the ion-exchange preparation technique resulted in ultra-dispersed Rh/A1203. All previous prepara- tions of Rh/A1203 employed impregnation, and the resultant catalysts showed a band for linear CO that was almost as intense as that for the dicarbonyl species, indicating the presence of small metal crystallites or large clusters having two-dimensional metal surface-like proper tie^.^^'^^ The dicarbonyl assignment was initially made based on the dirhodium dichloro- tetracarbonyl compound shown in fig.2.26*27 In this case the Rh atoms have a + 1 OC cO dicarbonyl A , 2090 cm-l; B 2030 cm-' 2073 2026 1800 2068 2042 1883 FIG. 2.-1.r. frequencies of rhodium carbonyl cluster compounds. charge, and the dicarbonyl stretching frequencies are 2090 and 2030 cm-'. These values agree with those for Rh/A1,03 to within 9 cm-'. Also given in fig. 2 are the frequencies observed for Rh,(CO),, and Rh4(C0)', cluster compounds. The di- carbonyl frequencies of Rh/A1203 do not match those of the larger clusters.How- ever, the bridged CO peak of Rh/A1203 is composed of two components, one at ca. 1869 cm-l and the other at ca. 1818 cm-'. These values are similar (within 20 cm- ') to frequencies observed in Rh4(C0)12 and Rh6(C0)',, respectively, where edge and face bridging carbonyls occur. These results clearly show that all the Rh cannot be present as atomically dispersed or dimeric species but is present in clusters having only small numbers of Rh atoms. Fig. 3 shows the carbonyl spectra of Rh/TiO, after 473 K reduction, after 673 K reduction and then following recalcination (of the 673 K reduced sample) and re- reduction at 473 K. Clearly the 473 K reduction results in ultra-dispersed Rh/Ti02 as only the CO stretches of the dicarbonyl species are evident in the infrared spectrum.There is no bridged species evident in this spectrum. Reduction at 673 K results in significant cluster growth as evidenced by the marked reduction in the dicarbonyl peak intensities, the appearance of an intense linear carbonyl peak at ca. 2071 cm-' and a weak bridging carbonyl peak at ca. 1880 cm-l. Similar behaviour was ob- served upon 673 K reduction for the other supported Rh systems studied. The total CO chemisorption on Rh/Ti02 was reduced from 2.17 to 1.71 CO/Rh (Tred = 673 K), consistent with the marked reduction in the intensity of dicarbonyl peaks in the spectra (fig. 3). The total H2 chemisorption capacity was reduced from 0.98 H/Rh to 0.24 H/Rh (Tred = 673 K).The more substantial reduction in the H2 chemisorption appears to be related to the metal-support interaction phenomena reported for the third-row Group VIII metals on reducible oxide^.,^-^* In contrast, the H, chemi-126 CO HYDROGENATION OVER SUPPORTED Rh sorption capacity of Rh/A1,0, remains constant, independent of the reduction temperature, although the carbonyl spectra showed evidence of clustering similar to that observed for Rh/Ti0,.31 Following the 673 K reduction, calcination and rereduction of Rh/Ti02 at 473 K resulted in significant redispersion as indicated by the increased intensity of the dicarbonyl peaks and reduced intensity of the linear carbonyl peak. 0 0 ..4 *g c E Y 2155 2110 2065 2020 1975 1930 wavenumberlcm - FIG. 3.-Infrared spectra of CO adsorbed on Rh/Ti02.Conditions: catalyst prepared by ion exchange of 0.95 wt % Rh on to Ti02 (anatase), calcined in O2 at 623 K, reduced in situ at 473 K in flowing dry H2, CO adsorption at 298 K and 120 Torr, evacuated just prior to taking the spectrum; sample then reduced in situ at 673 K in flowing dry H2, CO spectrum taken as before; sample then calcined in situ in flowing O2 at 623 K, followed by rereduction in situ in flowing dry H2 at 473 K, spectrum taken as before. Temperature of H2 reduction as follows: (a) 473 K, (b) 673 K, (c) 473 K (after calcination of 673 K reduced sample in O2 at 623 K). The infrared and chemisorption data indicate the Rh is present in the form of small clusters, even when reduced at 473 IS; reduction at 673 K increases the size of these clusters.Similar information has been interpreted by others to indicate the presence of atomically dispersed Rh, in particular concluding that the dicarbonyl species formed on atomically dispersed Rh.23-25932 The question of dispersion can be clarified by determining the coordination of Rh atoms to adjacent atoms by EXAFS. Determination of the coordination number can give an indication of the number of Rh atoms in the cluster. EXAFS results are an ensemble average, and without addi- tional information it is difficult to distinguish between a uniform population having a single coordination number and a population having two different properties, i.e. coordination numbers. EXAFS measurements of Rh/A1203 and Rh/Ti02 reduced at 473 K show that these samples, which exhibit CO/Rh E 2.0 and which show onlyKATZER, SLEIGHT, GAJARDO, MICHEL, GLEASON, MCMILLAN 127 dicarbonyl i.r.bands, are not atomically dispersed but exhibit a Rh-Rh coordina- tion number of 4 & 1.33 These results further support our speculation that the Rh is not atomically dispersed but is present as small clusters of Rh atoms having less than ca. 10 atoms per cluster. The EXAFS results particularly do not support earlier speculations about atomic dispersion of even a significant fraction of the Rh.23-25*32 Reduction at 673 K resulted in an increase in the coordination number determined by EXAFS, supporting the suggestion of agglomeration of the small clusters of metal atoms to larger clusters. These clusters are still much smaller than units which could warrant the terminology " small metal crystallites ".TABLE 3.-CO HYDROGENATION OVER SUPPORTED Rh (wt %> product formation rate" support Rh CH4 CHjOH CzH50H Ti02 0.95 39.6 7.0 3.4 A1203 2.4 1.6 1.1 0.04 Ce02 1.6 0.40 1 .o 0.060 Si02 0.89 0.16 0.30 0.001 8 MgO 3 .O 0.40 4.69 0.0 Product formation rate (turnover number) x lo4, (g mol product)/(g atom surface Rh s) turn- over number based on Rh dispersion in table 1 , all dispersions assumed to be 1 .O except for Rh/MgO. Reaction conditions: 473 K, 10 atm total pressure, 4% CO in HZ, differential plug-flow reactor operation, conversion < ca. 7%. The oxidation state of Rh in the Rh(CO), species observed for the supported Rh catalysts has also been c o n t r ~ v e r s i a l . ~ ~ ~ ~ ~ The close similarity of the dicarbonyl stretching frequencies of supported Rh to those of the (Rh)2(C0)4(C1)2 model com- pound (where Rh has a +1 charge) suggests a monovalent Rh'(C0)2 species occurs for supported Rh catalysts.The question of the oxidation state of Rh can be answered by X-ray photoelectron spectroscopy (X.P.S.). Furthermore, X.P.S. can clarify the question of electron transfer between Rh and the support. The Rh 3d5/2 electron binding energy of reduced Rh foil is 306.8 & 0.1 eV. Rh/Ti02 reduced at 473 K has a binding energy of 307.0 & 0.1 eV; Rh/A1203 reduced at 473 K has a binding energy of 307.4 5 0.1 eV.35 These results show that the Rh is reduced to the metallic state on A1203 and on TiO,; it is not present as Rh1.36 They strongly suggest, however, that Rh undergoes a small amount of electron transfer to the A1203, relative to Rh/Ti02.This is consistent with X-ray absorption edge studies for Pt/A1203 relative to Pt/TiO,, which show that significant electron transfer occurs from Pt to Al,03, but that there is a small amount of electron transfer from the TiO, to the Pt.37*38 Addition of CO resulted in a +0.4 eV shift in the Rh 3dS/, electron binding energy for Rh/Ti02. This shift is far short of the shift required for a Rh valence change to +1.36 The shift observed upon adsorption of CO is simply due to the electron-withdrawing properties of the CO ligand. Therefore, we find no evidence for the assertion that dicarbonyl is associated with atomically dispersed Rh in the + 1 oxidation state; the Rh is present as clusters containing very small numbers of Rh atoms in a reduced metallic-like state.Table 3 summarizes the CO hydrogenation rate behaviour of the supported Rh catalysts. The activation energy for methane formation was 134 & 4 kJ mo1-I for Rh supported on SiO,, A1203 and Ti02. The >200-fold rate variation between Rh/Ti02 and Rh/Si02 appears to be due to a variation in the number of active sites128 CO HYDROGENATION OVER SUPPORTED Rh or other geometric factors which would appear in the pre-exponential term, and not due to changes in the energetics of the reaction which should appear as changes in the observed activation energy. CO and H2 chemisorption capacity changes as well as changes in the infrared spectra of chemisorbed CO (fig. 3) as a function of reduction temperature (473 K us.673 K) led us to expect similar dependence of CO hydrogenation activity and selec- tivity on reduction temperature. However, the rate of CO hydrogenation (gmol product per g of catalyst per s) and the product selectivities for Rh/Ti02 reduced at 673 K were essentially the same as for Rh/Ti02 reduced at 473 K. The effects referred to as strong metal-support interactions (SMSI) for Group VIII third-row transition metals supported on reducible oxides such as Ti028-30 are not observed here. The nature of the support rather than the reduction temperature is the most important factor ; the effects of reduction temperature are secondary. TABLE 4.-CO HYDROGENATION SELECTIVITIES OF SUPPORTED Rh product selectivity" total (wt %) c2+ CF+ alcohols C2HSOH support Rh CH4 CH4 CH4 CH30H TiOz 0.95 0.023 0.1 1 0.26 0.49 A1203 2.4 0.034 0.0 0.71 0.046 Ce02 1.6 0.039 0.0016 2.72 0.058 Si02 0.89 0.050 0.0 1.95 0.062 MgO 3.0 0.0 0.0 12.0 0.0 rate of formation of species i (mol/g cat s) rate of formation of species j (mol/g cat s).* Selectivity r , j E Reaction conditions: 473 K, 1 atm total pressure, 4% CO in H2, differential plug flow reactor operation, overall conversion < ca. 7%. Essentially all Rh atoms are exposed, and there are no major differences in the dispersion of the Rh in terms of the number of Rh atoms in each cluster, as indicated by all the characterization techniques applied to these catalysts. Therefore, the rate differences must be due to metal-support interactions which change the number of active sites capable of catalysing the formation of CH4.The rates of methanol and ethanol formation also vary by a large factor; the ethanol formation rate is highest for Rh/Ti02, and the methanol formation rate is highest for Rh/MgO (table 3). The activation energy for methanol formation was typically 88 & 4 kJ mol-I; that for ethanol formation was significantly higher. The activation energies for methanol formation over Rh/MgO, Rh/Ti02 and Rh/ A1203 are indistinguishable, indicating that the reaction mechanism remains the same and that there are no electronic effects that change reaction energetics induced by the support. Therefore, the effect must be due to a change in the number of sites active in methanol formation; again, the metal-support interaction is directly affecting the number of active sites.Selectivity to alcohols us. total hydrocarbons varies by ca. 50-fold; selectivity to ethanol us. methanol varied greatly (table 4). The selectivity to alcohols is related to the acid-base properties of the support, Table 5 correlates the selectivity to al- cohols synthesis with the acidic properties reported in the literature for the supports.KATZER, SLEIGHT, GAJARDO, MICHEL, GLEASON, MCMILLAN 129 The selectivity to alcohols formation increases with the basic character of the support; MgO exhibits ca. 90% selectivity to methanol. The ordering is good except for the Ti02-A1203 pair for which the acidity-selectivity relation appears to be reversed. This could well be due to acidity differences between the oxides used here and those upon which the acidity measurements were made.Oxides used here were calcined TABLE 5 . 4 0 HYDROGENATION SELECTIVITY OF SUPPORTED Rh TO ALCOHOLS (wt %) total alcohols support Rh acidity, Hoa ref. CH4 Ti02 0.95 2 +1.5 39 0.26 A1203 2.4 2 - 3 40,41 0.71 CeOz 1.6 5 +3.3 43 2.72 SiOz 2.89 2 +4.8 44 1.95 MgO 3 .O 2 +14 45 12 H,, = Hammett acidity function. at 623 K following ion exchange, air equilibrated, reduced in situ at 473 K, and ex- posed to water, a reaction product, during reaction. The oxides for which acidity measurements are reported were typically calcined at high temperature, and water was rigorously excluded during acidity measurement. Therefore, tabulated acidities should be used as only a rough guide. Furthermore, it is not clear that titration methods are able to measure the appropriate acid property.V I VII Vlll IB /- ambient synthesis temperatures temperatures FIG. 4.-Regions of dissociative and non-dissociative adsorption of CO at room temperature and at CO hydrogenation reaction temperatures. The alcohol selectivity can be explained on the basis of the CO dissociation activity of transition metals. In fig. 4 the metals to the left-hand side of the heavy line designated " ambient temperature " spontaneously dissociate adsorbed CO at room temperature; those metals to the right of the line adsorb CO non-dissociatively at room temperature. The heavy line marked " synthesis temperature " separates those metals that dissociate adsorbed CO from those that do not dissociate adsorbed CO at temperatures of 473-573 K.Metals that dissociate CO catalyse hydrocarbon synthesis from CO and HZ; metals that adsorb CO non-dissociatively catalyse synthesis of methanol. Rabo et ~ 1 . ~ ~ showed that Pd, Ir and Pt synthesized methanol with 100% selectivity at higher pressures. Rh, which lies between Ru (which produces exclusively hydrocarbons) and Pd (which can produce exclusively methanol), catalyses130 co HYDROGENATION OVER SUPPORTED Rh formation of both alcohols and hydrocarbons from CO + HZ. Since it is capable of synthesizing both types of compounds small effects can markedly alter its selectivity. Thus Rh in the form of bulk metal produces mainly hydrocarbons;47 in the form of very small clusters on acidic supports Rh produces mainly hydrocarbons.On more basic supports the selectivity to alcohols is enhanced. This can be explained in terms of the more basic oxides having a higher Fermi level and thus having greater electron- donating capabilities; the supported Rh is thus forced to behave more like Pd (fig. Consideration of the mechanism of the synthesis reactions leads to better under- standing of the selectivity behaviour (fig. 5 and 6). Methanol formation on Rh 4)- 0 II C II Co + - * ADSORPTION 0 II H K C / Q H C I II II + 2 * - 3s HYDROGEN AT I ON w OH H-C’ H II + 2 l - = It w FIG. 5.-Mechanism of CO hydrogenation occurs by a non-dissociative mechanism such + CH30H to methanol on supported Rh. as shown in fig. 5. This has been demonstrated by 13C160 and 12C1s0 tracer CH30H is not an effective reactant for C2H50H formation under synthesis conditions over Rh/Ti0z.49 Further, the formation of high alcohols followed a Fischer-Tropsch type molecular-weight distribution as should result from the mechanism of fig. 6.Thus the synthesis of methanol is related to the amount of undissociated CO on the surface. This amount may be determined by the fraction of the catalytic sitesathat are unable to dissociate CO (sites behaving like Pd) or if all catalytic sites are equivalent, by the relatively slow CO dissociation reaction on Rh (i.e. hydrogenation of CO competes with CO dissociation). Although we are unable to distinguish between these two possibilities, we speculate that the methanol reaction involves different sites from the hydrocarbon and higher alcohols reactions.We speculate that the synthesis of hydrocarbons relates to the dissociation of CO;50 the synthesis of higher alcohols, mainly ethanol, relates to the relative concentration of surface carbene and undissociated CO. This explains the selectivity behaviour observed. The support is inferred to act through direct ligand bonding to the Rh. This bonding involves electronic interactions with the metal changing the relative numbers of the active sites involved in the various CO hydrogenation reactions.KATZER, SLEIGHT, GAJARDO, MICHEL, GLEASON, MCMILLAN 131 0 II II CO + - C ADSORPTION m 0 c o C II I I ll 2-H-% II C - % - 3W + H20 DISSOCIATION 11 m m C H CHx II + I - I I CARBIDE HYDROGENATION m m m CH3 CH2 I + I t - a € % CHAIN GROWTH, INITIAL STEP )Ic yc R I y 2 R CH2 I II - m m P R 0 PAGAT I 0 N * M R H 1 I CH2 + WC - a€ + R-CH3 TERMINATION: HYDROCARBON FORMATION I m 0 II R R C I I II - m C = O COINSERTION m m I m R I H OH C = O + 3 I - R - C ' TERMINATION: ALCOHOL FORMATION I )Ic I'H m H FIG.6.-Postulated mechanism of CO hydrogenation to hydrocarbons and higher alcohols.132 CO HYDROGENATION OVER SUPPORTED Rh SUMMARY AND CONCLUSIONS Ion-exchange preparation, initially giving atomically dispersed Rh, results upon H2 reduction in unique highly dispersed supported Rh catalysts. Chemisorption capacities, infrared spectra of chemisorbed CO, and EXAFS confirm the ultra-high dispersion achieved. EXAFS results show that after reduction the Rh is not atomic- ally dispersed but forms very small clusters containing only a few Rh atoms per cluster ; similar conclusions were inferred from the infrared results.The supported Rh is reduced to the metallic state, but there is evidence for electron transfer between the support and the Rh. CO hydrogenation activity varied over 200-fold dependent upon the support; selectivity to alcohols 0s. hydrocarbons varied over 50-fold. Activity and selectivity behaviour are inferred to be due to absolute and relative numbers of active sites as determined by the support. Methanol formation involves a non-dissociative mechanism on sites that can hydrogenate adsorbed CO. Hydro- carbon formation appears to involve CO dissociation. 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ISSN:0301-7249    

DOI:10.1039/DC9817200121

出版商:RSC    

年代:1981

数据来源: RSC