A reflection-absorption infrared spectroscopic study of the adsorption of ethylene and ethylene oxide on oxygen-covered Ag(111)

The adsorption of ethylene and ethylene oxide has been studied on clean and oxygen-covered Ag(111) using temperature-programmed desorption and reflection-absorption infrared spectroscopy (RAIRS). Ethylene adsorbs weakly on Ag(111) at 80 K with the molecular plane oriented parallel to the surface. The effect of adsorbed oxygen (Theta (O) similar to 0.1) is to increase the heat of adsorption slightly and to cause the ethylene to tilt. Ethylene oxide also adsorbs weakly at 80 K with the molecular plane oriented perpendicularly to the surface, where the heat of adsorption also increases due to the presence of adsorbed oxygen. The RAIR spectra of both ethylene and ethylene oxide adsorbed on oxygen-activated Ag(111) at 300 K under a pressure of 1 Torr show the formation of a number of surface species. An eta (2)(C,O) bonded acetaldehyde species is found, where the infrared features decrease coincident with acetaldehyde/ethylene oxide desorption. A species persists on heating to 450 K which exhibits a single infrared peak at 1004 cm(-1). Based on the frequency shifts observed on isotopic substitution (with D and O-18), it appears to contain C, O and H. This feature disappears on heating to 550 K correlating with the desorption of CO2 in temperature-programmed desorption. Finally, a series of features is detected which may be due to an adsorbed formate or strongly distorted ethylene oxide. These results emphasize that good quality infrared spectra can be collected for adsorbed species formed at high pressures on a model, oxygen-activated Ag(111) catalyst and that the surface chemistry is completely different to that found when dosing at 80 K under ultrahigh vacuum conditions. (C) 2001 Elsevier Science B.V. All rights reserved.