Mechanism of Ce-Modified Birnessite-MnO2 in Promoting SO2 Poisoning Resistance for Low-Temperature NH3-SCR

A desirable catalyst for efficiently controlling NOx emissions often demands excellent SO2 poisoning resistance. Here, we introduced Ce to modify birnessite-MnO2 to obtain a Ce-MnO2 catalyst with excellent activity (T90 = 85 °C) for the selective catalytic reduction of NO with NH3 (NH3-SCR) at low temperatures. Compared with the MnO2 catalyst that was severely deactivated after poisoning, the Ce-MnO2 catalyst showed a significant improvement in SO2 resistance with the NO conversion slightly decreasing from 100 to 95% at 150 °C. Physicochemical characterizations combined with density functional theory calculations indicated that the sulfates formed on Mn species and Ce species played different roles in the SCR reaction. The SO32– adsorbed on the surface of the MnO2 catalyst can react with NH3 to form ammonium sulfites, leading to the deactivation of the catalyst. However, the SO42– from Ce2(SO4)3 on the sulfated Ce-MnO2 catalyst existed as new adsorption sites of NH4+, thus providing this catalyst with more acid sites. In addition, SO2 was preferentially adsorbed and oxidized on Ce species, thereby protecting the Mn active centers from sulfation and deactivation. This work reveals the mechanism of Ce in promoting SO2 resistance over birnessite-MnO2 from the perspective of sulfates.