Catalytic performance and mechanistic evaluation of sulfated CeO2 cubes for selective catalytic reduction of NOx with ammonia

Sulfated CeO2 cubes were prepared by the impregnation of CeO2 cubes by ammonium sulfates, and further evaluated in selective catalytic reduction of NOx with ammonia (NH3-SCR). Catalytic activity tests indicated that NOx reduction conversions and N2 selectivity of sulfated CeO2 cubes could be significantly improved compared to pure CeO2 cubes. The synthesized sulfated CeO2 cubes were further characterized by atom-resolved high angle annular dark-field (HAADF) imaging, Fourier-transform infrared spectroscopy (FTIR) by pyridine adsorption, and temperature-programmed reduction by H2 (H2-TPR). The characterization results showed that sulfates were primarily dispersed through the corners, edges, and surfaces of CeO2 cubes, and did not significantly affect the crystal structures of CeO2 cubes. Sulfation treatment could create and strengthen Bronsted acid sites originated from the protons on surface sulfates, further facilitating ammonia adsorption and activation. The kinetic data indicated that the apparent reaction order of NO, O2, and NH3 was 0.95 to 1.01, -0.01 to 0.00, and -0.18 to -0.15, respectively. It could speculate that gaseous phase NO involving in NO catalytic oxidation was the rate-determining step over sulfated CeO2 cubes for NH3-SCR reaction. The presence of NH3 slightly inhibited the SCR reaction rate due to the competitive adsorption blocking NO oxidation sites.