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

Different crystal phases of MnO 2 were synthesized and tested for NH 3 –SCR of NO and NH 3 oxidation performances during 50–120 °C. Among those catalysts, α-MnO 2 showed the most superior SCR performance for NO x conversion and N 2 selectivity, and NH 3 species on its surface were active to react with the oxygen, while NH 2 species were also easily oxidized by the oxygen. For β-MnO 2 , NH 3 (ads, B) species and part of NH 3 (ads, L) species on its surface were active to react with oxygen, while NH 3 (ads, L) species adsorbed at Lewis sites showed low reactive with O 2 , thereby producing less N 2 O and low NO conversion. γ-MnO 2 showed the similar NO x conversion rates and N 2 O amount generated from the NH 3 oxidation comparing to α-MnO 2 , while yielding much more N 2 O generation ratios from SCR reactions conditions. Besides, NH 3 (ads, L) , NH 3 (ads, B), NO 3 2− and NH 2 species adsorbed on γ-MnO 2 surface had high reactivity and could all be consumed by oxygen rapidly. And the N 2 O formation of δ-MnO 2 mainly generated from SCR reactions conditions in the temperature range of 50–120 °C, while the adsorbed NH 3 (ads, L) species were hard to react with O 2 . Graphic Abstract