The Concept of Lewis Acids and Bases Applied to Surfaces

The purpose of this paper is to apply the concept of Lewis acids and bases to provide an understanding of the influence of structure and composition on surface chemistry. The Lewis acid-base concept is briefly reviewed. The factors important for determining the strength of acid-base interactions, including orbital energies and orbital character, are illustrated with the aid of gas phase ion-molecule chemistry. The perturbational molecular orbital theory of acid-base interactions is adapted to the gas phase environment in order to show that changes in the ordering of base strength through a series of Lewis bases must be due to the criterion of maximum orbital overlap. The classification of metal surfaces as acid or base with respect to a molecular adsorbate is determined with the work-function change. The influence of surface composition modification by either electronegative or electropositive elements on CO adsorption is shown to follow an inductive effect analogous to that seen in molecules. On metal oxide surfaces and oxidized metal surfaces, the oxygen anions are assigned to be Lewis base sites and the electron-deficient metal atoms are assigned to be Lewis acid sites. For an oxygen monolayer on a metal surface, two types of structures are distinguished: the overlayer structure and the incorporation structure. Chemical and physical evidence is presented which indicates that the Lewis acid sites on a surface with an incorporation structure are localized. Finally, the chemical evidence from adsorption studies combined with the criterion of maximum orbital overlap indicates that it is the localized vs. delocalized character of the valence electronic states at the surface that determines adsorption selectivity changes.