XPS study of the size effect in ethene epoxidation on supported silver catalysts

Abstract

Supported silver catalysts (Ag/α-Al ₂ O ₃ ) with different particle sizes (100–1000 Å) have been prepared and studied by XPS. It has been shown that the increase in the ethene epoxidation rate with silver particle size (size effect) is accompanied by a decrease in the Ag 3d _5/2 binding energy value. The variation in E _b (Ag 3d _5/2 ) cannot be explained in terms of a change in the metallic character of the silver (initial state effect), but seems to be determined by the differential charging of the supported silver. The latter effect originates from the easier screening of surface charge induced by photoemission on the silver particles owing to their internal conductivity, rather than that on the dielectric support. The correlation between the internal conductivity of supported silver particles and their catalytic activity is discussed in the context of possible mechanisms for ethene epoxidation.