Dynamic technique to elucidate the reaction intermediate in surface catalysis. Water-gas shift reaction

Abstract

The mechanism of the water-gas shift reaction on ZnO and MgO was studied by means of infra-red spectroscopy during the course of the reaction. When a mixture of carbon dioxide and hydrogen was introduced over ZnO, formate ion was observed. The rate of decomposition (dehydration) of the surface formate ion was measured at the reaction temperature (230°C) as a function of its concentration, and compared with the rate of the overall reaction on ZnO at the same coverage of the surface formate is the reaction intermediate of the water-gas shift reaction on ZnO and its decomposition is the rate-determining step. When a mixture of carbon monoxide and water vapour was passed over MgO, the formation of surface formate ion was similarly confirmed. It was, however, different from ZnO, in that it was difficult to detect the surface formate ion from a mixture of carbon dioxide and hydrogen. The comparison of the rate of the dehydrogenation decomposition of the surface formate ion with that of the overall reactions at the same formate coverage leads to conclusion that the surface formate ion is the reaction intermediate of the water-gas shift reaction on MgO and the rate determining step is the dehydrogenation of the formate ion.