Atomic oxygen species on silver:: Photoelectron spectroscopy and x-ray absorption studies -: art. no. 235422

Two oxygen species, which are constituents of the active centers for ethylene epoxidation over silver, have been characterized by a number of physical methods sensitive to adsorbate electronic structure such as x-ray photoelectron, ultraviolet photoelectron, Auger, and x-ray-absorption near-edge structure spectroscopy. One of the species denoted as nucleophilic oxygen due to its activity in total oxidation only exhibits spectroscopic characteristics close to those of bulk Ag2O. This allows us to describe this species as atomically adsorbed oxygen in the structure of surface silver(I) oxide. The considerable extent of the covalency in bonding of this oxidelike oxygen with the silver surface due to hybridization of O2p levels with Ag4d and Ag5sp orbitals should be also emphasized. Contrary to this, only 5sp orbitals of silver hybridize with 2p orbitals of oxygen as the other oxygen species forms. As a consequence, this species being also atomic oxygen is characterized by a lower oxygen-silver bonding interaction and a lower charge on the oxygen. The latter causes the activity of this electrophilic species in epoxidation. Possible models of adsorption centers for these oxygen species are discussed.