Comparison of sulfuric acid- or phosphoric acid-modified CeO2 and the influence of surface acidity and redox property on its activity toward NH3-SCR

Abstract

A series of CeO2 prepared with H2SO4 (C–S), H3PO4 (C–P) and H2SO4 + H3PO4 (C–P–S) were investigated in selective catalytic reduction of NOx with NH3. The sulfates contributed to the improvement of Bronsted (B) acid sites, while phosphates were prone to the enhancement of Lewis (L) acid sites, which could improve the catalytic activity. Furthermore, sulfates existed in the surface of C–P–S and C–S; phosphates occurred on the subsurface region or were highly dispersed on the surface of C–P–S and C–P. Besides, the nitrates adsorbed on the CeO2 and NH3 contacted with the sulfates, which followed the L–H mechanism at 200 °C on C–P–S and C–S. The adsorption and activation of NO and NH3 over C–P occurred at the same active sites, which obeyed the E–R mechanism at 200 °C. C–P–S possessed the best catalytic activity because of the appropriate surface acidity and redox property, and more than 80% conversion of NOx was obtained at 220–450 °C. CeO2 modified by H2SO4 (C–S), H3PO4 (C–P) and H2SO4 + H3PO4 (C–P–S) was prepared and used for selective catalytic reduction of NOx by NH3. The presence of sulfate species over C–S and C–P–S contributed to the formation of surface acidity. CeO2 modified by phosphoric acid favored excellent oxidation ability.