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 NOx. 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.