Abstract
In the study of hydrocarbon reactions carried out on unsupported and supported alloys, the ‘ensemble size effect’ has been firmly established as the major factor determining the catalytically active surface. Accordingly, the selectivity pattern of a hydrocarbon transformation is determined by the participation of different types of intermediates associated with ensembles of different size. This latter, in turn, is a function of the metal-metal combinations. The effects observed with bimetallic systems with a low metal loading are not as simple as with bulk alloys. Alloy phase formation can be doubted in most cases, and in some instances formation of a ‘metal phase’ is also questionable. Nevertheless, a bimetallic system is frequently superior in its behaviour to a monometallic catalyst. The following effects may be considered to rationalize the existing data: — Formation of bimetallics but segregation occurs; — Particle size effect, dispersion; — Matrix effect, e.g. one component grafted to the support; — Change in hydrogen coverage; — Metal-support interaction (MSI); — Suppression of ageing effects, such as coke formation. These effects are discussed here for a series of examples, e.g. Ru-Cu, Ru-Au, Ru-Pt, Ru-Fe, Pt-Au, Pt-Ir, Pt-Fe bimetallic systems, considering the influence of the factors mentioned for different hydrocarbon transformations as well as for hydrocarbon synthesis.
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