Abstract
Reactant-promoted reaction mechanism for catalytic water-gas shift reaction (WGSR) on Rh-doped CeO 2, including the activation of intermediate by a reactant, has been investigated by FT-IR. The intermediate of WGSR on Rh/CeO 2 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 H 2 + CO 2 (forward decomposition) in vacuum. In the coexistence of second-adsorbed water molecules, almost 100% of the formate decomposed to H 2 + CO 2. 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/CeO 2 is the decomposition of the bidentate formate to H 2 and unidentate carbonate. The desorption of CO 2 by the decomposition of unidentate carbonate was also promoted by H 2O coexistence. The phenomena observed on Rh/CeO 2 are entirely different from those on pure CeO 2 and similar to the case of ZnO surface. The reactant-promoted reaction mechanism for WGSR is illustrated.
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