Metal-Catalysed Reactions of Hydrocarbons / 02-Small Metal Particles and Supported Metal Catalysts

character. (iv) Support acidity/basicity may determine the strength of Coulombic interaction across the oxide ion-metal atom interface, causing changes in electronic structure within the metal atoms without actual change transfer (Figure 2.9D).

2.7. PROMOTERS AND SELECTIVE POISONS

We now encounter a semantic problem of considerable size. It has been recognised for a very long time that the activity of metal catalysts can be helped by the presence of quite small amounts of substances that of themselves have no or little activity. This concept first achieved prominence in the development of iron catalysts for ammonia catalysts, and of iron and cobalt catalysts for Fischer-Tropsch synthesis, and the term promoter was applied to these substances. They were of two kinds: (i) structural promoters such as alumina, which acted as grain stabilisers and prevented metal particle sintering and (ii) electronic promoters such as potassium that entered the metallic phase and actually enhanced its activity. In these cases the metal is the major component, so that the catalyst is a promoted metal rather than a supported metal.

The problem of terminology arises when the idea is extended to supported metal catalysts, where a number of tricks have evolved to preserve, protect and defend the activity of the metal, and to direct the reaction into more desirable paths by improving selectivity to the wanted product and eliminating or minimising formation of side products and of species that remain on the surface as catalyst poisons. The term ‘promoter’ can be sensibly used to describe some of these auxiliary agents, but the same or similar effects are produced by other substances to which this term by tradition or convention is not used. Promoters comprise two types: (1) those chiefly associated with the support, e.g. stabilising it for use at very high temperatures (occupying holes in the γ -Al2O3 structure) or neutralising acidic centres that might initiate carbocationic polymerisation of alkenes; and (2) those that mainly interact with metal particles or perhaps the metal-support interface. These need a fuller analysis, which is attempted in Table 2.2, and which for the sake of completeness mentions reactions that are not strictly in our terms of reference. Here we can see two effects at work; one, the less common, where the modifying species245,286,315,326−321 may exert an electronic influence on neighbouring metal atoms, either by tending to donate electrons to the metal or to withdraw them: the consequences are not unlike those shown by altering the acidity or alkalinity of the support. The second, the more common, is where the modifier by occupying places on the surface of the metal eliminates the larger groups of atoms which may be the active centres for dissociative chemisorption or decomposition of a hydrocarbon reactant. There seems to be a general rule applying to hydrocarbon reactions that the reaction most likely to occur is that by which the surface free energy is most greatly

76

TABLE 2.2. Selectivity Promoters for Metal Catalysts

2 CHAPTER

reduced, either because the adsorbed intermediates engage the largest number of metal atoms or because they prefer to react with atoms of low CN: this has thermodynamic logic and explains the tendency of hydrocarbons to dehydrogenate and form strongly held multiply bonded species (see Chapter 12). The corollary is that reactions that are content to proceed on small active centres, even on single atoms or on low index planes, can only do so when larger or more reactive sites are removed by the addition of a partial monolayer of inert atoms or ions. Thus for example Ru/TiO2 becomes quite selective for the skeletal isomerisation of n-butane at 633 K when partially poisoned by sulfur. The widespread interest in and use of bimetallic systems where mutual solubility is low (e.g. Ru-Cu, Pt-Re, Ru-Sn) is largely explained by these effects; species that moderate chain growth in FischerTropsch synthesis, or direct the reaction towards oxygenated products by limiting the dissociation of carbon monoxide, operate in the same way.256 Unfortunately, since such selectivity promoters deactivate part of the surface, some loss of activity has to be accepted as the necessary price to pay for a cleaner reaction: they therefore fall into the category of selective poisons.332−334 Only rarely (as for example with alkaloid-mediated enantioselective hydrogenation) does the modifier actually lead to a faster rate.

The definition of promotion suggested above excludes cases where the support itself provides an essential service (e.g., in bifunctional and spillover catalysis) or where the promotion occurs unintentionally. Carbon deposition can however sometimes result in a selectivity improvement and of course a species may act at the same time as both an electronic and a selectivity promoter. For these reasons it is not always easy to classify what a particular promoter does, or how it works.

2.8. SINTERING AND REDISPERSION145

Sintering is the process whereby at high temperature small metal particles grow into big ones, with consequent loss of metal area and activity (see Further Reading section). It has been widely studied, both experimentally and theoretically, because of its great nuisance value particularly in reactions that need somewhat high temperatures. Two mechanisms have been considered: (i) whole particle migration and (ii) movement of single atom species governed by Ostwald ripening considerations. The thermodynamic driving force for particle growth has already been discussed (Section 2.1.2). Both processes have been observed, but the former is thought to be more important for practical catalysts having rough surfaces. Mobility of intermediate species involved in catalyst preparation (e.g. Pt/(NH3)2O) can be responsible for particle growth during drying and calcination. Sintering rate depends very much on the gas atmosphere; it is much faster under oxidising than under reducing conditions.

Once sintering has taken place, it is no easy task to reverse the process, which is thermodynamically up-hill, unless the metal-support interaction is stronger than that between the metal atoms, in which case sintering ought not to have occurred. It is usually therefore necessary to use a chemical driving force to disaggregate the metal, that is, to convert it via oxidation and reduction back to highly dispersed metal. Most of the work in the open literature concerns the platinum/alumina system because of its relevance to petroleum reforming (Chapter 14), and it mostly appeared in the 1970s and 1980s. In this system, platinum-containing species that are mobile over the surface of the support are formed by treating the used catalyst with oxygen and a chlorine-containing compund (Cl2, HCl or a chlorinated hydrocarbon). The exact composition of this mobile species does not seem to have been established, and is usually represented just as PtOxCly. A procedure of this type also appears to be effective in reconstituting bimetalluc particles (PtIr, PtRe etc), but much of the relevant information is hidden in the patent literature, and does not contribute much to the scientific understanding of the process; neither does the open literature inform us much about means of redispersing other types of catalyst.

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