Shape dependence and sulfate promotion of CeO2 for selective catalytic reduction of NOx with NH3

Abstract Thermally stable CeO 2 cubes and nanospheres were synthesized and modified by sulfation treatment. Non-sulfated CeO 2 cubes and nanospheres did not exhibit good catalytic performance for selective catalytic reduction of NO x with NH 3 (NH 3 -SCR), but mainly contributed towards an undesired side reaction of NH 3 oxidation above 350 °C. Based on the structural, morphological and physicochemical characterization, it was observed that CeO 2 nanospheres comprising small crystallites could easily release active oxygen species, which resulted in strong NH 3 oxidation. However, sulfation treatment greatly improved the catalytic performance of NH 3 -SCR on both CeO 2 cubes and nanospheres. Sulfated CeO 2 catalysts did not contribute to significant NH 3 oxidation due to the inhibited reducibility of Ce 4+ coordinated with the surface sulfates. The adsorbed ammonia could be activated on Bronsted acid sites generated by the formation of surface Ce 2 (SO 4 ) 3 species, while gaseous NO x could be activated on separate surface sites of Ce 4+ . The presence of separate reaction sites for NH 3 and NO x is believed to be important for the improved catalytic performance of SCR reaction. Sulfated CeO 2 cubes outperformed sulfated CeO 2 nanospheres in the entire test temperature window (200–500 °C). The improved performance of sulfated CeO 2 cubes appears to be related to surfaces with abundant Bronsted acid sites and relatively weak reducibility of Ce 4+ . These fundamental findings contribute to a better mechanistic understanding needed for designing efficient CeO 2 -based NO x reduction catalysts in the future.