THEORETICAL-STUDIES ON THE CATALYTIC ACTIVITY OF AG SURFACE FOR THE OXIDATION OF OLEFINS

Systematic theoretical studies for the mechanisms of the epoxidation a nd complete oxidation of ethylene and propylene over silver surface as well as the reactivity and the stability of oxygen species on Cu, Ag, and Au surfaces have been presented. The dipped adcluster model (DAM) combined with the ab initio Hartree-Fock (HF), second-order Moller-Pl esset (MP2), and SAC/SAC CI (symmetry-adapted cluster/configuration in teraction) methods are used. These studies clarify the origin of silve r as a unique effective catalyst for the epoxidation of ethylene and t he different mechanisms for the oxidation of olefins over sih er surfa ce. For the epoxidation of ethylene, the superoxide O-2(-), which is m olecularly adsorbed in bent end-on geometry on the silver surface, is the active species. The origin of the unique catalytic activity of sil ver for the epoxidation of ethylene is due to its ability to adsorb ox ygen as the superoxide species. Such an adsorbed species cannot be sta ble or exist on Cu and Au surfaces. For the oxidation of propylene, bo th reaction mechanisms initiated by the activation of olefinic carbon and by the activation of the allyl hydrogen exist, The activation of t he allyl hydrogen is the origin of the complete oxidation of some olef ins over silver surface. The present results not only let us have a be tter understanding of the reaction mechanisms of olefins over silver s urface but also supply a basic idea for the new catalyst design of the epoxidation reaction. (C) 1997 John Wiley & Sons, Inc.