Positive effects of SO2 on NH3-SCR at high temperatures and their relationship with the structure of tungsten on Ce-W oxides.

CeW-based catalysts with W species in different states, i.e., a CeW oxide with only crystalline WO3 (CeW-C) and a CeW oxide with monomeric W species (CeW-M), were prepared by a simple coprecipitation method through changing the mixing time during preparation. SO2 was found to exert a positive effect on NH3-SCR at high temperatures, and CeW-C exhibited higher resistance toward SO2 poisoning. CeW-M with monomeric W exhibited higher activity above 200 °C in the absence of SO2 than CeW-C with crystalline WO3 due to the higher dispersion of W and the resulting increased number of acid sites for NH3 adsorption. CeW-C with crystalline WO3 possessed a higher amount of reducible oxygen species, leading to greater NO and SO2 adsorption, as well as causing higher activity below 200 °C in the absence of SO2. In the presence of SO2, NH3 adsorption was promoted by forming (NH4)2SO4 or NH4HSO4, which could react with NO to form N2 and H2O on CeW-C. Having a large amount of NH3 adsorption was important for high NH3-SCR activity on these Ce-W oxides, and SO2 exerted a positive effect on NH3-SCR by promoting NH3 adsorption at high temperatures. Additionally, more nitrates remained on CeW-C with crystalline WO3 in the presence of SO2. The reasons above led to better SO2 resistance for CeW-C. The different states of W species affect the NH3-SCR activity and ability to resist SO2 and H2O poisoning by changing the redox properties, number of acid sites, and adsorption of NO, NH3 and SO2.