The partial oxidation of ethylene to ethylene oxide catalyzed by silver is studied by the ab-initio Hartree-Fock and MP2 methods using the dipped adcluster model (DAM). The active species is the superoxide O-2 which is molecularly adsorbed in a bent end-on geometry on the silver surface. Ethylene reacts with the terminal oxygen atom and the reaction proceeds smoothly without a large barrier to yield ethylene oxide. The complete oxidation of ethylene involving the superoxide is forbidden due to the existence of a large energy barrier. This is one origin of high selectivity. Without the silver surface, the epoxidation reaction is very unfavorable, showing the catalytic activity of the silver surface. The atomically adsorbed oxygen seems not to be selective: it gives both ethylene oxide and complete oxidation products. Therefore, adding to the selectivity due to the superoxide, an overall selectivity 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 electron donor and acceptor. Detailed electronic mechanisms are clarified and discussed.
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