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.