Redox activity of nonstoichiometric cerium oxide-based nanocrystalline catalysts

Nonstoichiometric cerium oxide-based catalysts were investigated for SO₂ reduction by CO to elemental sulfur, CO oxidation, and complete methane oxidation. Nanocrystalline processing by inert gas condensation was exploited for its unique potential to generate nonstoichiometric ultrahighly dispersed oxides. Nanocrystalline CeO_2-x materials, pure or doped with 10 at.% La or 15 at.% Cu, were generated by magnetron sputtering from pure or mixed metal targets, followed by controlled oxidation. These materials allowed us to investigate the effects of oxide nonstoichiometry and dopants on catalytic activity in oxidation reactions. The nonstoichiometric materials were characterized by X-ray diffraction, nitrogen adsorption porosimetry, and X-ray photoelectron spectroscopy. Catalytic properties were studied in a packed-bed reactor and compared to materials of similar composition prepared by coprecipitation. In general, the nonstoichiometric CeO₂-based materials exhibited greater catalytic activity than precipitated ultrafine materials. The light-off temperatures for SO₂ reduction by CO, CO oxidation, and CH₄ oxidation were 100-180^°C lower for the nanocrystalline pure and La-doped CeO_2-x catalysts than for the respective precipitated materials. The Cu-doped form of both types of catalysts possessed comparable activity. The nonstoichiometric materials did not show a hysteresis behavior in the activity profile for SO₂ reduction by CO, unlike the precipitated catalysts. They further demonstrated a remarkable stability against CO₂ poisoning in this reaction. The differences between the nanocrystalline and the precipitated materials are discussed in terms of the stoichiometry of these oxide catalysts.