Effect of SO2 poisoning on undoped and doped Mn-based catalysts for selective catalytic reduction of NO

Abstract

In this work, the poisoning effect of SO₂ was investigated in binary MnTi and ternary MnCeTi mixed oxides for the NH₃-SCR reaction under conditions relevant for mobile applications. For the binary MnTi sample, catalytic activity increases up to 250 °C, and then drops due to the oxidation of ammonia to NO_x. The addition of Ce decreases the catalytic activity at 150 °C but widens the optimal operational temperature and reaches high conversion at 350 °C. Upon performing activity test with 100 ppm of SO₂ in the gas stream, catalytic activity drastically decreases in all catalyst samples. The shape of the deactivation curve and SO₂ concentrations at the outlet of the reactor suggest a strong adsorption and poisoning of SO₂ on all the catalysts. Although samples containing large amounts of Ce display a better SO₂ tolerance, this is insufficient to be considered for practical applications. Deactivated samples were investigated by a wide range of characterization tools. N₂ physisorption measurements reveal a drop in the surface area that could partially explain catalyst deactivation. TGA reveals the absence of (NH₄)₂SO₄ on the deactivated samples and suggests the formation of Mn and Ce sulfates on the catalyst surface. XPS results confirm the formation of MnSO₄ and also show a decrease in the Mn and Ce oxidation states. Analysis of the redox function by catalytic NO oxidation and H₂-TPR experiments shows a strong loss of redox function upon SO₂ deactivation, which could explain the decrease of NH₃-SCR catalytic activity. Upon unraveling the effect and cause of deactivation, a doping study was performed. As in the binary MnTi and ternary MnCeTi, catalytic activity strongly decreases upon the introduction of SO₂ in the gas stream. None of the dopants investigated was able to suppress SO₂ deactivation, which suggest that other dopants or strategies should be pursued to commercialize Mn-based catalysts for low-temperature applications.