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

Abstract: 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 NO_x 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, 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 O₂, thereby producing less N₂O and low NO conversion. γ-MnO₂ showed the similar NO_x 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, NH_{3 (ads, L)}, NH_{3 (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 NH_{3 (ads, L)} species were hard to react with O₂. Graphic Abstract: [Figure not available: see fulltext.].