SURFACE ENHANCED RAMAN SPECTRA FROM OXYGEN ON SILVER

Abstract

Silver is an important catalyst for the epoxidation of ethylene and the production of formaldehyd from methanol. It is not yet known with certainty which form of adsorbed oxygen on silver is the catalytically active one. Therefore we have started to study the adsorption of oxygen on silver films by Raman spectroscopy. From previous investigations, especially from UPS work /l/, we know that at low exposures of ground state molecular oxygen, oxygen sticks on evaporated silver films only at defect sites. These defect sites are abundant on films which are deposited in UHV on substrates cooled to 120 K. Raman spectra of these oxygen exposed films display vibrational Tines at 1053 cm' and 697 cm', which we assign to superoxidic and peroxidic dioxygen. We are able to follow the dissociation of these dioxygen species to adsorbed oxygen atoms during warming up of the films by multichannel Raman spectroscopy. Exposing the coldly deposited silver films to oxygen with the ion getter pump running leads to additional Raman lines at 815 and 1286 cm. Investigations with 1 8 0 2 show that the responsible adsorbate is composed of or at least comprises oxygen. We tentatively assign the 815 cm line to the antisymmetric stretching vibration of an ozonide species whereas the origin of the 1286 vibration remains unknown. In summary our investigations show that oxygen on a catalytically active silver surface may occur in more states of single and multiatomic adsorption than so far assumed.