THE EFFECT OF COADSORBATES IN REVERSE WATER GAS SHIFT REACTION ON ZNO, IN RELATION TO REACTANT-PROMOTED REACTION-MECHANISM

A reactant-promoted reaction mechanism for reverse water-gas shift reaction (H2 + CO2 → H2O + CO; r-WGSR) on ZnO was investigated by FT-IR, MS, and GC. Surface bidentate formate was produced from H2 + CO2 through bidentate carbonate. The decomposition rate of the formate as a reaction intermediate became 1/10 ∼ 1/30 of that in vacuum by the coexistence of CO2. On the contrary, the decomposition rate of the formate was promoted 8-10 times by the coexistence of H2 as compared with that in vacuum. The activation energy decreased from 171 kJ mol-1 in vacuum to 138 kJ mol-1 in H2. About 70% of the surface formates decomposed to H2 and CO2 and about 30% of them decomposed to H2O(-OH) and CO. The decomposition selectivity of the surface formate was not changed by H2. The decomposition selectivity was much different from that for the bidentate formate produced from -OH(H2O) and CO. It was suggested that WGSR and r-WGSR proceeded through bidentate formates, but at different sites on ZnO. The rate of r-WGSR was between the decomposition rates of the formates under CO2 and H2, indicating that the rate of r-WGSR is balanced with H2 promotion and CO2 suppression. The rate determining step is the dissociation of H2 at the (Zn-formate-OH)pair site. The rate constant for the formate decomposition increased linearly with the amount of adsorbed H2. H2 molecules not only act as a reactant to reduce the bidentate carbonate to the bidentate formate, but also promote the decomposition of the bidentate formate to H2O(-OH) + CO or H2 + CO2. © 1993 Academic Press, Inc.