STUDIES OF THE WATER-GAS-SHIFT REACTION ON CERIA-SUPPORTED PT, PD, AND RH - IMPLICATIONS FOR OXYGEN-STORAGE PROPERTIES

Steady-state, water-gas-shift kinetics were measured on model, ceria-s upported, Pd, Pt, and Rh catalysts and compared to rates obtained on a lumina-supported catalysts. When ceria was calcined at low temperature s prior to addition of the precious metal, the specific rates were fou nd to be identical for each of the metals, with an activation energy o f 11 +/- 1 kcal/mol and reaction orders of zero and one for CO and H2O respectively. For comparison, specific rates on Rh/alumina were at le ast two orders of magnitude lower. However, ceria structure strongly a ffected the results. When ceria was calcined to high temperatures to i ncrease crystallite size, prior to the addition of Pd, specific rates were a factor of 50 lower at 515 K and the activation energy was found to be much higher, 21 +/- 1 kcal/mol. By comparison with results from an earlier study of CO oxidation [17], we propose that water-gas shif t on ceria-supported metals occurs primarily through a bifunctional me chanism in which CO adsorbed on the precious metal is oxidized by ceri a, which in turn is oxidized by water. Deactivation of the catalyst fo llowing growth in the ceria crystallite size is due to the decreased r educibility of large ceria crystallites. The implications of these res ults for automotive, emission-control catalysts is discussed. (C) 1998 Elsevier Science B.V.