MECHANISM OF THE PARTIAL OXIDATION OF ETHYLENE ON AN AG SURFACE - DIPPED ADCLUSTER MODEL STUDY

The partial oxidation of ethylene to ethylene oxide catalyzed by silve r is studied by the ab-initio Hartree-Fock and MP2 methods using the d ipped adcluster model (DAM). The active species is the superoxide O-2( -) which is molecularly adsorbed in a bent end-on geometry on the silv er surface. Ethylene reacts with the terminal oxygen atom and the reac tion proceeds smoothly without a large barrier to yield ethylene oxide . The complete oxidation of ethylene involving the superoxide is forbi dden due to the existence of a large energy barrier. This is one origi n of high selectivity. Without the silver surface, the epoxidation rea ction is very unfavorable, showing the catalytic activity of the silve r surface. The atomically adsorbed oxygen seems not to be selective: i t gives both ethylene oxide and complete oxidation products. Therefore , adding to the selectivity due to the superoxide, an overall selectiv ity larger than 6/7 can be possible. In the process yielding ethylene oxide from atomic oxygen, electron transfer and back-transfer from/to the metal are important processes which can be promoted by both electr on donor and acceptor. Detailed electronic mechanisms are clarified an d discussed. (C) 1997 Elsevier Science B.V.