Reactant-Promoted Reaction Mechanism for Water-Gas Shift Reaction on Rh-Doped CeO2

Reactant-promoted reaction mechanism for catalytic water-gas shift reaction (WGSR) on Rh-doped CeO2, including the activation of intermediate by a reactant, has been investigated by FT-IR. The intermediate of WGSR on Rh/CeO2 is bidentate formate produced from CO and terminal (on-top) surface OH groups on Ce ions: 65% of the surface formates decomposed to -OH + CO (backward decomposition) and only 35% of them decomposed to H2 + CO2 (forward decomposition) in vacuum. In the coexistence of second-adsorbed water molecules, almost 100% of the formate decomposed to H2 + CO2. Besides the direct change of decomposition selectivity, the rate constant of the forward decomposition and the activation energy were surprisingly affected by water coadsorption. The rate constant increased one hundred times and the activation energy decreased from 55.6 to 33.3 kJ mol−1. The rate-determining step for catalytic WGSR on Rh/CeO2 is the decomposition of the bidentate formate to H2 and unidentate carbonate. The desorption of CO2 by the decomposition of unidentate carbonate was also promoted by H2O coexistence. The phenomena observed on Rh/CeO2 are entirely different from those on pure CeO2 and similar to the case of ZnO surface. The reactant-promoted reaction mechanism for WGSR is illustrated.