The MnFe/TiO2 catalysts were poisoned by metal sulfates and/or ammonium sulfate to understand the SO2 poisoning effect under low-temperature selective catalytic reduction (SCR) of NO with NH3. The results showed that the formation of metal sulfates had a more serious deactivation effect than that of ammonium salts on the MnFe/TiO2 catalysts. After thermal regeneration, the metal sulfates on the catalyst could not be removed so that the NO x conversion was only recovered from 17% to 35%. On the other hand, water washing was capable to remove both ammonium salts and metal sulfates, and the NO x conversion could be recovered to 88% (compared to 99% for the fresh catalyst). The analytical results of the synchrotron-based XRD, BET, NH3-TPD, and XPS revealed that lower crystallinity, lower specific surface area, lower ratio of Mn4+/Mn3+, higher surface acidity, and more chemisorbed oxygen were the main causes for the presence of metal sulfates poisoning, which then resulted in the low NO x conversion at low temperature.

The byproducts of ammonia-selective catalytic reduction (NH3-SCR) process over MnFe/TiO2 catalysts under the conditions of both with and without SO2 poisoning were analyzed. In addition to the NH3-SCR reaction, the NH3 oxidation and the NO oxidation reactions were also evaluated at temperatures of 100–300 °C to clarify the reactions occurred during the SCR process. The results indicated that major byproducts for the NH3 oxidation and NO oxidation tests were N2O and NO2, respectively, and their concentrations increased as the reaction temperature increased. For the NH3-SCR test without the presence of SO2, it revealed that N2O was majorly from the NH3-SCR reaction instead of from NH3 oxidation reaction. The byproducts of N2O and NO2 for the NH3-SCR reaction also increased after increasing the reaction temperature, which caused the decreasing of N2-selectivity and NO consumption. For the NH3-SCR test with SO2 at 150 °C, there were two decay stages during SO2 poisoning. The first decay was due to a certain amount of NH3 preferably reacted with SO2 instead of with NO or O2. Then the catalysts were accumulated with metal sulfates and ammonium salts, which caused the second decay of NO conversion. The effluent N2O increased as poisoning time increased, which was majorly from oxidation of unreacted NH3. On the other hand, for the NH3-SCR test with SO2 at 300 °C, the NO conversion was not decreased after increasing the poisoning time, but the N2O byproduct concentration was high. However, the SO2 led to the formation of metal sulfates, which might inhibit NO oxidation reactions and cause the concentration of N2O gradually decreased as well as the N2-selectivity increased.