Oxygen storage dynamics in Pt/CeO2/Al2O3 catalysts

The oxidation; the O-16(2), O-18(2), (OO)-O-16-O-18 isotopic equilibration; the isotope oxygen exchange; and the reduction by CO were studied on Pt/CeO2, Pt/CeO2/Al2O3, Pt/Al2O3, and CeO2 catalysts at 200-600 degrees C. On a 0.1-s residence time scale, the oxidation occurred instantly on all samples. The rates of oxygen exchange and equilibration were closely related, which shows that the rate-determining step in the oxygen storage is the adsorption and desorption of oxygen. The oxygen exchange was faster on the alumina-supported catalysts than on the ceria-supported catalysts; however; the amount of oxygen that could be exchanged was higher on Pt/CeO2 and CeO2. On Pt/Al2O3, the amount of exchange corresponds to the expected number of OH groups. The exchange rate was found to be strongly dependent on the Pt content on the alumina-supported catalysts, but almost independent of the Pt content on the ceria-supported catalysts. This indicates that the oxygen exchange does not proceed via Pt, but occurs directly on ceria. The reduction of ceria by CO at 300-650 degrees C was increased by Pt. A model for the reduction of Pt/ceria with CO is presented. The model contains three types of CO consuming reactions, two on ceria and one which also involves Pt. CO chemisorption on Pt/CeO2 at 25 degrees C resulted in large CO uptakes, a part of which is probably due to CO uptake on ceria. For metal dispersion measurements, a method for subtracting the ceria contribution is suggested. (C) 1998 Academic Press.