The concept of quasimolecular electrophilic oxygen in ethylene epoxidationover silver

The ethylene-epoxidizing form of oxygen (O-ep) adsorbed on Ag(111) and Ag(1 10) single crystals and polycrystalline silver foil was examined by X-ray a nd UV photoelectron spectroscopy (XPS and UPS). It was shown that O-ep has a quasimolecular nature and exhibits high thermal stability (T > 700 K). Th e quantum-chemical study of associative forms of oxygen on the defects of t he silver surface was carried out by the methods of density functional theo ry (DFT) in a cluster approximation. It was found that the associative (mol ecular) ozone-like species of oxygen on the surface of silver with cationic vacancies (V) is more stable than atomic oxygen (the energy difference is 44 kcal/mol). The identification of emission bands in UV photoelectron spec tra performed on the basis of theoretical calculations of the ozone-like st ructure supported the quasimolecular nature of O-ep. The mechanisms known f or ethylene epoxidation were analyzed. A new mechanism implying the partici pation of quasimolecular electrophilic oxygen O-ep in epoxidation was propo sed. This mechanism describes the major regularities of epoxidation observe d in the experiments on a regular basis.