X-ray absorption and photoemission studies of the active oxygen for ethylene epoxidation over silver

Activation of clean polycrystalline silver by a C2H4 + O-2 reaction mixture has been studied by XANES, XPS, and UPS. In situ monitoring of the O K-edge XAS spectrum of the pre-treated silver surface revealed a broad signal at 10-20 eV above the threshold. The comparison of the X-ray absorption spectra with O Is and valence band photoemission data allowed us to attribute this XAS signal to "electrophilic" oxygen (Eb(O Is) = 530.4 eV) which is known to be active in ethylene epoxidation. The complete absence of XAS features in the photon energy range typical for pi* and sigma* transitions of molecular oxygen (530-535 eV) indicates both the atomic origin of the electrophilic oxygen and the absence of molecular species on the catalyst surface under the present reaction conditions.