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•MnO2 exhibited better NO conversion activity over the whole temperature range.•MnO2 yielded more N2O among the three catalysts at low temperature.•Both E−R and L−H mechanisms were found conducting on the three catalysts.•N2O generation ratios from NH3 oxidation increased with increasing temperature.
Different valence states manganese oxides catalysts (MnO2, Mn2O3 and Mn3O4) were synthesized to investigate their N2O formation pathways during NH3–SCR of NO process. In contrast, the NO conversions of Mn2O3 and Mn3O4 were nearly identical, while MnO2 exhibited better NO conversion activity over the whole temperature range and corresponding to NO conversion of 100% at 150 °C with a space velocity of 36,000 h−1. At low temperature, the majority of N2O was generated from the SCR reactions on the three catalysts. With the increasing temperature, the N2O amounts and the N2O generation ratios from NH3 oxidation of the three catalysts both increased. Besides, NH3 species on MnO2 were easier to be oxidized by gaseous O2, while NH3,ads at Lewis acid sites would partly transfer to NH4+ and NH2 species on Mn2O3 in the presence of O2 and more NH2 species would be formed on the oxygen adsorbed surface of Mn3O4. Both E−R and L−H mechanisms were found conducting on the three catalysts. NH2/NH species on the MnO2 surface would react with gaseous NO to form NH2NO/NHNO and then decomposed to N2/N2O, respectively, while the adsorbed monodentate nitrites combined with NH3,ads and/or NH4+ species to form NH4NO2 that decomposed to N2. Besides the formation and decomposition of NH2NO/NHNO, NH4NO3 was also formed on Mn2O3 and Mn3O4, and then decomposing to N2O.