The Concept of Quasimolecular Electrophilic Oxygen in Ethylene Epoxidation over Silver
Kinetics and Catalysis
, E-ISSN: 1608-3210
Boreskov Institute of Catalysis SB RAS
The ethylene-epoxidizing form of oxygen (O-ep) adsorbed on Ag(111) and Ag(110) single crystals and polycrystalline silver foil was examined by X-ray and UV photoelectron spectroscopy (XPS and UPS). It was shown that O-ep has a quasimolecular nature and exhibits high thermal stability (T > 700 K). The quantum-chemical study of associative forms of oxygen on the defects of the silver surface was carried out by the methods of density functional theory (DFT) in a cluster approximation. It was found that the associative (molecular) 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 spectra performed on the basis of theoretical calculations of the ozone-like structure supported the quasimolecular nature of O-ep. The mechanisms known for ethylene epoxidation were analyzed. A new mechanism implying the participation of quasimolecular electrophilic oxygen O-ep in epoxidation was proposed. This mechanism describes the major regularities of epoxidation observed in the experiments on a regular basis.