Reactant-promoted reaction mechanism for water-gas shift reaction on ZnO, as the genesis of surface catalysis

The behavior of reaction intermediates in the catalytic water-gas shift reaction (WGSR) on ZnO surfaces has been studied by means of FT-IR spectroscopy, and a reactant-promoted mechanism including intermediate-reactant interaction is proposed. On-top (terminal) hydroxyl groups on Zn ions which are formed by the first-adsorbed water molecules react with CO to produce bidentate and bridge formates. Seventy percent of them were decomposed to original CO and surface hydroxyls and only 30% of them were converted to Hz and CO2 (adsorbed) under vacuum. On the contrary, 100% of the formates were converted to the WGSR products, H2, and CO, in coexistence with second-adsorbed water molecules. The rate of the formate decomposition was promoted by a factor of more than 10 by the presence of a second water molecule. The activation energy of the decomposition of the surface formates decreases in the presence of water; 155 kJ mol−1 under vacuum and 109 kJ mol−1 with ambient water. The rate-determining step of the decomposition is the scission of the C-H bond of the formates according to isotope effects. In the absence of ambient water vapor, adsorbed CO, species exist as unidentate carbonate and carboxylate on ZnO surface. At room temperature most of CO2 (ad) exist as carboxylate, and the population of unidentate carbonate increases with an increase in temperature. When the second water molecule coexists on the surface, carboxylate converts into unidentate carbonate. The surface carbonate and carboxylate were thermally stable even at 680 K by themselves, whereas they begin to desorb at room temperature in the coexistence of the adsorbed water. Steady-state rate of catalytic WGSR agree with the decomposition rates of the bidentate formate and the unidentate carbonate; the two decomposition rates are balancing during the steady-state WGSR on ZnO. Water molecules not only act as a reactant to form the formate, but also activate the bidentate formate(ad) to decompose to H2 and unidentate carbonate(ad) and promote the desorption of the carbonate as CO2.