Surface Enrichment in Catalysts

Abstract

Since heterogeneous catalysis is primarily a surface phenomenon, a knowledge of the exact chemical composition of the surface is a primary requisite for understanding the working of solid catalysts. Whether or not the actual surface composition of a catalyst is the same as its bulk composition has always remained a serious question; until recently there was hardly any experimental technique which could give an unambiguous answer to this. A century ago it was predicted by Gibbs [1] that the equilibrium composition of an alloy surface might not necessarily be its bulk composition and that one of the components could segregate on its surface. With the advent of modern experimental techniques for surface analysis like XPS (ESCA), Auger, SIMS, and vibrational spectroscopy, the elemental composition of the surface layer (up to a few atomic layers) can be determined more or less quantitatively. This is a tremendous advance over the situation of the preceding three or four decades when specific chemisorption of gases was practically the only method to probe the chemical nature of the surface layer of a heterogeneous catalyst.