Insights into the influence of water molecules on selective catalytic ozonation of gaseous ammonia into nitrogen on cryptomelane-type manganese oxide using in-situ DRIFTS

Abstract

Catalytic ozonation is an environmentally friendly technology for the removal of gaseous NH3 due to high NH3 conversion and high N2 selectivity at ambient temperature. However, the influence mechanism of ubiquitous water vapor on catalytic ozonation of NH3 is unclear. In this study, cryptomelane-type manganese oxide (OMS-2) catalyst was prepared and tested for catalytic ozonation of NH3 in different relative humidity. The results showed that water vapor significantly decreased the catalytic activity, which was due to the inhibition of water on NH3 adsorption on Lewis acid sites and O3 decomposition on oxygen vacancies, as well as the combination of water with active oxygen species (O22− and Oatom). And the effect of water vapor on NH3 conversion was more significant than O3 decomposition because more Mn–OH were involved in the O3 decomposition under humid conditions. Combining in-situ DRIFTS results with the performance of NH3 oxidation, it is found that L-2 acid sites (the peak of NH3 adsorption on Lewis acid sites at 1188 cm−1) were the main active sites for adsorption and activation of NH3 in the early stage of catalytic reaction; as the reaction progressed, L-2 acid sites were gradually occupied by water and more Brønsted acid sites participated in the catalytic reaction. This work deepened the understanding of the reaction process for selective catalytic ozonation of NH3, and provided theoretical guidance for the design of efficient hydrophobic catalysts to eliminate gaseous NH3 pollution.