Nitrogen promoted electron transfer to create highly active Cu2+ and Mn3+ sites for efficient catalytic ozonation of Ammonia over copper and nitrogen co-doped Cryptomelane-Type manganese oxide at ambient temperature

Abstract

Eliminating ammonia (NH3) from high-humidity exhaust gases by catalytic oxidation is a great challenge, especially at low temperatures. Herein, Cu and N co-doped cryptomelane-type manganese oxide (Cu-N-OMS-2) catalysts with high active Cu2+ and Mn3+ sites were synthesized and tested for catalytic ozonation of NH3 at 30 °C. The optimal Cu0.05-N2-OMS-2 catalyst exhibited 93.8 % NH3 conversion, 91.7 % O3 conversion, and 92.1 % N2 selectivity when the relative humidity (RH) was 40 %, whereas OMS-2 only demonstrated 39.3 %, 44.0 % and 83.1 %. Additionally, NH3 and O3 conversion at high RH could be recovered with just a basic water-washing, indicating that Cu0.05-N2-OMS-2 catalyst possessed outstanding renewable properties. The results of characterization revealed that N doping increased the amount of Cu fixed on the catalyst surface and promoted the electron transfer from Cu to Mn, resulting in an increase in electron-poor Cu2+ and electron-rich Mn3+ in Cu-N-OMS-2. In dry conditions, the Mn3+ sites provided O3 with electrons to form adsorbed oxygen species and •O2–, which were essential for oxidation of adsorbed NH3. The electron-poor Cu2+ sites combined with H2O and O3 to form Cu-OH and •HO3, which facilitated the generation of •OH and •O2–, thereby improving the stability under humid conditions. This study provides a new theoretical guidance for the efficient removal of NH3 in industrial exhaust gases.