Microscopic Theory of Heterogeneous Catalysis

Abstract

The most important elementary steps of the catalytic reaction are adsorption, dissociation, association, and desorption. The event is essentially a cycle in which the catalytically active sites are regenerated. Adsorption proceeds usually with or without a small activation energy. Dissociation is often activated and requires an ensemble of several surface atoms. The activation energy for dissociation may be lowered by electron back donation into antibonding orbitals of the adsorbate. Association reactions are favored by factors opposite of those favoring dissociation. The activation energy for desorption is usually comparable to the heat of adsorption. According to Sabatier’s principle, an optimum exists in the interaction energy between the adsorbate and the catalytic surface that corresponds to a maximum in the rate of the catalytic reaction. The rate-limiting steps of the reaction cycle are different below or above the optimum interaction energy. Competitive adsorption of reactant and/or product molecules often controls the selectivity and activity of a catalyst.