catalytic entity is a site on the surface or a complex of the site with a reactant molecule. For example, a chemical reaction, A+B→C, might proceed by the following sequence of steps:

A+S→ A-S

A-S+B→C+S.

The molecule A is adsorbed on a site S to form a surface complex A-S, which then reacts with a molecule of reactant B to form the product molecule C and regenerate the site S. This simple sequence illustrates features common to all catalytic processes, namely, the generation of a reactive intermediate from the reactant, the transformation of the intermediate to a product, and the regeneration of the active catalyst site.

An intriguing aspect of catalysis is the specificity observed. For example, silver catalysts are unique in their ability to catalyze the partial oxidation of ethylene to ethylene oxide:7

2C2H4+O2→2C2H4O.

On other solid catalysts, the ethylene undergoes predominantly complete oxidation to carbon dioxide and water. In this example a change in the catalyst actually leads to a pronounced change in the distribution of reaction products, because the different catalysts have markedly different effects on alternative reaction paths. In some cases the observed reaction product is the same for a number of catalysts, but the specific activity (reaction rate per unit surface area or per surface site) varies widely. A good example is the catalytic hydrogenolysis of ethane to methane, C2H6+H2→2CH4, on metals,8,9 where the specific activity of osmium is almost 8 orders of magnitude higher than that of platinum (Figure 1). These examples demonstrate clearly that the chemical nature of the surface is highly important in heterogeneous catalysis.

Heterogeneous catalysis involves the participation of species chemisorbed on the surface. Maximum catalytic activity is achieved when chemisorption of the reactant is fast but not very strong.6,10 If the adsorption bond is too strong, the catalyst will tend to be highly covered by reactant species that are not readily transformed or by product species that do not desorb readily from the surface. At the other extreme, if the adsorption bond is very weak, the catalytic activity may be severely limited by a low rate of chemisorption, since the activation energy for chemisorption commonly increases as the heat of adsorption decreases. Optimum catalytic activity corresponds in general to some intermediate strength of adsorption between these two extremes.

Many different types of materials have been used as catalysts. In heterogeneous catalysis they are commonly separated into two broad categories— metals and nonmetals. In the first category the most commonly used metals are those in Group VIII and Group IB of the periodic table. In the second



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