Influence of SO₃ on the MnO_x/TiO₂ SCR catalyst for elemental mercury removal and the function of Fe modification

Elemental mercury (Hg⁰) is a highly hazardous pollutant of coal combustion. The low-temperature SCR catalyst of MnO_x/TiO₂ can efficiently remove Hg⁰ in coal-burning flue gas. Considering its sulfur sensitivity, the effect of SO₃ on the catalytic efficiency of MnO_x/TiO₂ and Fe modified MnO_x/TiO₂ for Hg⁰ removal was investigated comprehensively for the first time. Characterizations of Hg-TPD and XPS were conducted to explore the catalytic mechanisms of Hg⁰ removal processes under different conditions. Hg⁰ removal efficiency of MnO_x/TiO₂ was inhibited irreversibly from 92% to approximately 60% with the addition of 50 ppm SO₃ at 150 ℃, which resulted from the transformation of Mn⁴⁺ and chemisorbed oxygen to MnSO₄. The existence of H₂O would intensify the inhibitory effect. The inhibition almost disappeared and even converted to promotion as the temperature increased to 250 ℃ and above. Fe modification on MnO_x/TiO₂ improved the Hg⁰ removal performance in the presence of SO₃. The addition of SO₃ caused only a slight inhibition of 1.9% on Hg⁰ removal efficiency of Fe modified MnO_x/TiO₂ in simulated coal-fired flue gas, and the efficiency maintained good stability during a 12 h experimental period. This work would be conducive to the future application of MnO_x/TiO₂ for synergistic Hg⁰ removal.