Adsorption and reaction of methanol and carbon dioxide on model ZnO/Ag catalysts

Abstract

The adsorption and decomposition reactions of carbon dioxide and methanol have been studied on clean Ag(100), O/Ag(100) and ZnO/Ag(100) surfaces. The clean Ag(100) surface itself is unreactive in so far as no low-pressure adsorption of either molecule is observed at ambient temperatures. At lower temperatures a weakly chemisorbed molecular state of methanol may be distinguished. In the presence of pre-adsorbed surface oxygen, however, uptake of both molecules is seen at 300 K to give carbonate and methoxy species. The methoxy species may undergo dehydrogenation to yield formaldehyde or oxidation to surface formate which then decomposes at ca. 390–400 K. The growth of ZnO on the Ag(100) surface leads to the gradual suppression of the chemistry of the silver surface and the appearance of desorption features which relate closely to those seen from polycrystalline ZnO samples or the non-polar faces of ZnO crystals. The uptake of CO₂ is strongly inhibited by annealing the oxide films; this is attributed to the ordering of the oxide surface to generate a polar surface. In the case of methanol adsorption, desorption features also reveal the formation of carbonate-like intermediates on the ZnO surface.