Topochemistry in Heterogeneous Catalysis

The catalytic activity of a powder catalyst should be proportional to its surface area in case its entire surface is equally effectii-e for the catalytic reaction. On the other hand, this correlation does not apply to catalysts whose active sites are located at crystallographically exceptional positions such as edges and corners of its microcrystallies. Schwab and Rudoloph (99) studied as early as 1934 the Topochemistry in Heterogeneous Catalysis, and concluded that the active sites of several catalysts were located at crystal boundaries in their surfaces. The chief evidence represented by these workers is that in various catalytic reactions, e.g., in the hydrogenation of ethyl cinnamate over different specimens of powdered nickel catalysts, the rates are proportional to a power of the catalyst surface areas lying between 1.8 and 4.0. 

In the preceding section, we have attempted to point out, on the basis of statistical-mechanical interpretations of the adsorption isotherm and of the B.E.T. method for the measurement of the surface areas of cata- 

No convincing experimental evidence of metallic catalysts has been brought forward in this author s opinion that would prove that the boundary lines between surface crystallites are the seats of catalytic action. 

In mixed oxide catalysts of the type Me0 Me203, new phases can be formed, as shown by Ward and Erchak (79), and it is possible that some of these phases have higher activities than any of the components used in preparing the catalyst. 

Morikawa (100) and also Koizumi (101) found that the polymerization of ethylene proceeds rapidly in the presence of a nickel-kieselguhr catalyst containing 15% Ni, while both nickel or kieselguhr alone are extremely poor catalysts of this reaction. This might be interpreted by the presence of active sites localized at the interface of nickel and kieselguhr since the formation of a homogeneous new phase between these two components is improbable. 

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