Supported metal catalysts are widely used because it is possible to prepare them in a highly dispersed form, consisting of very small microcrystals, which provide a maximum available surface area for the catalytic reaction [1]. Superficially, one might expect that the effectiveness of the material as a catalyst would be simply proportional to the surface area. In this case the specific catalytic activity would be independent of the size of the microcrystals. Size-independent specific activity is observed in a number of reactions, such as hydrogenation of benzene on alumina-supported platinum [2]. For other reactions, the specific catalytic activity is dependent on the size of the mircrocrystals. It is possible that the surface of a very small microcrystal is either more or less active than that of a larger crystal. Figure 1 shows the specific activity of the catalyst for deuteration of benzene and for hydrogen exchange with benzene on nickel catalysts [3]. Note that the smaller microcrystals are more active than the larger crystals for the hydrogenation reaction, but the smaller microcrystals are less effective for the exchange reaction. Such observations can be discussed in terms of the number and nature of the surface sites on a microcrystal [3—5].