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

Abstract

Thermally stable CeO2 cubes and nanospheres were synthesized and modified by sulfation treatment. Non-sulfated CeO2 cubes and nanospheres did not exhibit good catalytic performance for selective catalytic reduction of NOx with NH3 (NH3-SCR), but mainly contributed towards an undesired side reaction of NH3 oxidation above 350 °C. Based on the structural, morphological and physicochemical characterization, it was observed that CeO2 nanospheres comprising small crystallites could easily release active oxygen species, which resulted in strong NH3 oxidation. However, sulfation treatment greatly improved the catalytic performance of NH3-SCR on both CeO2 cubes and nanospheres. Sulfated CeO2 catalysts did not contribute to significant NH3 oxidation due to the inhibited reducibility of Ce4+ coordinated with the surface sulfates. The adsorbed ammonia could be activated on Brønsted acid sites generated by the formation of surface Ce2(SO4)3 species, while gaseous NOx could be activated on separate surface sites of Ce4+. The presence of separate reaction sites for NH3 and NOx is believed to be important for the improved catalytic performance of SCR reaction. Sulfated CeO2 cubes outperformed sulfated CeO2 nanospheres in the entire test temperature window (200–500 °C). The improved performance of sulfated CeO2 cubes appears to be related to surfaces with abundant Brønsted acid sites and relatively weak reducibility of Ce4+. These fundamental findings contribute to a better mechanistic understanding needed for designing efficient CeO2-based NOx reduction catalysts in the future.