Insight into N2O Formation Over Different Crystal Phases of MnO2 During Low-Temperature NH3–SCR of NO

Different crystal phases of MnO2 were synthesized and tested for NH3–SCR of NO and NH3 oxidation performances during 50–120 °C. Among those catalysts, α-MnO2 showed the most superior SCR performance for NOx conversion and N2 selectivity, and NH3 species on its surface were active to react with the oxygen, while NH2 species were also easily oxidized by the oxygen. For β-MnO2, NH3 (ads, B) species and part of NH3 (ads, L) species on its surface were active to react with oxygen, while NH3 (ads, L) species adsorbed at Lewis sites showed low reactive with O2, thereby producing less N2O and low NO conversion. γ-MnO2 showed the similar NOx conversion rates and N2O amount generated from the NH3 oxidation comparing to α-MnO2, while yielding much more N2O generation ratios from SCR reactions conditions. Besides, NH3 (ads, L), NH3 (ads, B), NO32− and NH2 species adsorbed on γ-MnO2 surface had high reactivity and could all be consumed by oxygen rapidly. And the N2O formation of δ-MnO2 mainly generated from SCR reactions conditions in the temperature range of 50–120 °C, while the adsorbed NH3 (ads, L) species were hard to react with O2.