Studies of the oxygen release reaction in the platinum-ceria-zirconia system

The relative rates of the CO+oxygen storage material (OSM)double right arro w CO2 reactlion (R-CO2) and the amounts of rate enhancement obtained upon P t promotion were examined for ceria and ceria-zirconia OSM. The effect of P t surface area on R-CO2 was decoupled from metal-oxide surface area by pre- sintering the oxides prior to Pt deposition. We find that R-CO2 is linearly dependent on Pt area over Pt/CeO2, but over Pt/Ce0.75Zr0.25O2, the rate is independent of Pt surface area above a threshold surface area. Although Pt sinters more readily on the Ce0.75Zr0.25O2 support, the dispersion effect is more than compensated by the enhanced availability of "bulk" O2- within the Ce0.75Zr0.25O2 particles, Furthermore, this study demonstrates that, on a unit surface area basis, the ceria-zirconia support is at least two time s more active for the oxygen release reaction than the pure ceria support w hen the materials are slightly reduced. It is also shown that O2- diffusion is not rate-limiting for the ceria reduction reaction when the reaction is carried out in the kinetically limited regime at 500 degrees C with CO as the reductant. (C) 1999 Elsevier Science B.V. All rights reserved.