Factors that Cause a Surface Heterogeneity for Chemisorption

Factors that Cause a Surface Heterogeneity for Chemisorption 

The undeniable fact that the surface may show a dominating heterogeneity for physical adsorption, hence for van der Waals attraction forces or for electrostatic polarization by local fields of the surface (Secs. VIII,2 and 3), does not mean that they should be heterogeneous for chemisorption as well. As was stated in Sec. V,12 the forces between ions and metal surfaces and the covalent forces between chemisorbed atoms or molecules and metal surfaces are far less influenced by active places of the surface than are some of the forces leading to physical adsorption. It is especially the cracks and fissures of the surface, which may give it a pronounced heterogeneous character for physical adsorption, that do not influence the chemisorption bonds very much (274). 

therefore, quite possible that a surface, heterogeneous in character for physical adsorption shows a homogeneous nature for chemisorption. 

The heat of chemisorption also depends on the orientation of the surface. In a qualitative way this is clearly demonstrated by electron emission microscope pictures of inetals on which various atoms are adsorbed. 

The various orientations of the crystallites of a metal specimen show up clearly, and it can be shown that the electron emission depends on the degree of covering with adsorbed atoms, which itself depends on orientation and on temperature (285-287). 

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