REDOX ACTIVITY OF NONSTOICHIOMETRIC CERIUM OXIDE-BASED NANOCRYSTALLINE CATALYSTS

Nonstoichiometric cerium oxide-based catalysts were investigated for S O2 reduction by CO to elemental sulfur, CO oxidation, and complete met hane oxidation. Nanocrystalline processing by inert gas condensation w as exploited for its unique potential to generate nonstoichiometric ul trahighly dispersed oxides. Nanocrystalline CeO2-x materials, pure or doped with 10 at.% La or 15 at.% Cu, were generated by magnetron sputt ering from pure or mixed metal targets, followed by controlled oxidati on. These materials allowed us to investigate the effects of oxide non stoichiometry and dopants on catalytic activity in oxidation reactions . The nonstoichiometric materials were characterized by X-ray diffract ion, nitrogen adsorption porosimetry, and X-ray photoelectron spectros copy. Catalytic properties were studied in a packed-bed reactor and co mpared to materials of similar composition prepared by coprecipitation . In general, the nonstoichiometric CeO2-based materials exhibited gre ater catalytic activity than precipitated ultrafine materials. The lig ht-off temperatures for SO2 reduction by CO, CO oxidation, and CH4 oxi dation were 100-180 degrees C lower for the nanocrystalline pure and L a-doped CeO2-x catalysts than for the respective precipitated material s. The Cu-doped form of both types of catalysts possessed comparable a ctivity. The nonstoichiometric materials did not show a hysteresis beh avior in the activity profile for SO2 reduction by CO, unlike the prec ipitated catalysts. They further demonstrated a remarkable stability a gainst CO2 poisoning in this reaction. The differences between the nan ocrystalline and the precipitated materials are discussed in terms of the stoichiometry of these oxide catalysts. (C) 1995 Academic Press, I nc.