Mechanism of ammonium bisulfate deposition on V1M5/Ti catalysts with synergistic effects of V and M (M = Ce, Co, Fe, and Mn) in low-temperature NH3-SCR

Abstract

For industrial flue gas purification at medium and low temperatures, it is highly necessary to promote the denitrification activity of V-based catalysts in a SO₂ atmosphere. Transition metals (Ce, Co, Fe, and Mn) are used to modify V/Ti catalysts to improve SO₂ resistance with ammonium bisulfate deposition on the catalysts. Sulfation on Ce with less ammonium bisulfate deposition considerably improves the SO₂ resistance of V₁Ce₅/Ti, and stable sulfation on TiO₂ promoted by Co provides additional Brønsted acid sites for V₁Co₅/Ti to reduce the deactivation. Serious sulfation on V₁Mn₅/Ti with large ammonium bisulfate deposition on V₁Fe₅/Ti causes deeper deactivation. The formation mechanism of ammonium bisulfate on active metal sites and TiO₂ carriers has been clarified. SO₂ is adsorbed on V sites and transition metal sites followed by oxidation and ammoniation or adsorbed on TiO₂ carriers followed by ammoniation and oxidation. The first pathway is more dominant than the second pathway in the presence of O₂ with the co-oxidation of SO₂ by O₂ and lattice oxygen. The initial formation temperature and stable temperature of dominant intermediates were determined by in situ DRIFTS. SO₂ adsorbed on TiO₂ carriers is oxidized by V sites above 170 °C to form a tridentate sulfate, while SO₂ adsorbed on transition metal sites is initially oxidized by transition metal sites above 50 °C to form monodentate and bidentate sulfates, as well as competitively oxidized by V sites above 170 °C to form tridentate sulfates. The selective adsorption of SO₂ on transition metal sites or TiO₂ carriers and the competitive oxidation of SO₂ by transition metal sites and V sites provide a synergistic effect between V sites and transition metal sites to reduce ammonium sulfate deposition and release more active sites. These findings provide theoretical guidance to design and prepare V-based catalysts with high denitrification activity in the presence of SO₂.