Dehydrogenation of acetylene and ethylene studied on clean and oxygen covered palladium surfaces

Abstract

The interaction of acetylene and ethylene with a clean and oxygen covered Pd surface has been studied at a temperature of 473 K. The measurements were performed on a hydrogen sensitive Pd-MOS structure making it possible to obtain direct information on the dissociation of both hydrogen and oxygen containing species on a palladium surface. Desorption studies were also performed as well as ultraviolet photoelectron spectroscopy and work function measurements. The studies show that both acetylene and ethylene adsorb dissociatively at this temperature leaving mainly carbon on the surface. When an oxygen covered Pd surface is exposed to C 2H 2 or C 2H 4 carbon dioxide and water will be formed and desorb until the surface is oxygen free. In the case of acetylene the presence of preadsorbed oxygen does not block or prevent the C 2H 2 dissociation on the surface. For C 2H 4, a large preadsorbed oxygen coverage (⪆ 0.45) will have an impeding effect on the dissociation. The CO 2 desorption is oxygen coverage dependent contrary to the H 2O desorption. This is due to the fact that hydrogen has a large lateral mobility on the surface while carbon has not. Both the CO 2 and H 2O reactions are, however, due to the same type of mechanisms.