Selective catalytic ozonation of gaseous ammonia to dinitrogen on cryptomelane-type manganese oxide catalysts: Role of oxygen vacancies and acid sites

Abstract

Two kinds of cryptomelane-type manganese oxide (OMS-2-Ac and OMS-2-SO4) catalysts were prepared with MnAc2·4H2O and MnSO4 as precursors, respectively, and were tested for selective catalytic ozonation of gaseous ammonia (NH3). The results showed that two OMS-2 catalysts exhibited high NH3 conversion and N2 selectivity as well as complete decomposition of ozone. Combining the results of characterization and in-situ DRIFTS, it is revealed that the surface oxygen vacancies and Lewis acid sites of OMS-2 played key roles in excellent catalytic activity and N2 selectivity, while the contribution of Brønsted acid sites to NH3 conversion was much less than that of Lewis acid sites. In-situ DRIFTS studies suggested that catalytic ozonation of NH3 over OMS-2 catalysts followed the imide and i-SCR mechanisms. In addition, the Mn2+ precursor exhibited a certain effect on the catalytic activity of OMS-2. OMS-2-Ac catalysts presented higher NH3 conversion (89%) and N2 selectivity (91%) at 30 °C under dry condition; moreover, the influence of humidity on the conversion of NH3 and O3 over OMS-2-SO4 was greater than OMS-2-Ac at 30 °C, which were mainly due to more surface oxygen vacancies and better hydrophobicity of OMS-2-Ac. This work provided an improved understanding of the role of the oxygen vacancies and acid sites in selective catalytic ozonation of ammonia over cryptomelane-type manganese oxide.