ACTIVATION OF O-2 ON CU, AG, AND AU SURFACES FOR THE EPOXIDATION OF ETHYLENE - DIPPED ADCLUSTER MODEL STUDY

Aiming to clarify why only silver is an effective catalyst for the par tial oxidation of ethylene, we studied theoretically the reactivity an d the stability of oxygen species on Cu, Ag, and Au surfaces. We used the dipped adcluster model (DAM), since electron transfer from metal t o oxygen is important, and the SAC/SAC-CI method, since several electr onic states are involved, We found that if superoxide species exists o n the surface, both Cu and Au surfaces show a reactivity similar to Ag surface, leading smoothly to ethylene oxide, and the barriers leading to complete oxidations should be very high. Therefore, the point is t he relative stability of the various oxygen species and, in particular , the stability of the superoxide species on the metal surface. On Cu, superoxide is much less stable than peroxide, which again is less sta ble than the dissociated species, and no barrier exists for the conver sion From superoxide to peroxide. On Au, our DAM calculations show tha t the electron how from the bulk metal into the adcluster does not occ ur, so that the molecularly adsorbed oxygen species, as well as the di ssociative ones, find it difficult to exist stably on the clean surfac e. On Ag, superoxide should certainly have some life time to react wit h ethylene to give ethylene oxide, which is considered to be the origi n of the unique catalytic activity of silver for the epoxidation of et hylene. This is related to the ability of electron transfer and to the geometry of the Ag surface. We have proposed a basic idea for a new c atalytic design of the epoxidation reaction. (C) 1997 Elsevier Science B.V.