Surface Ammonification Regulates Oxygen Vacancies and Acidic Sites in δ-MnO to Enhance Low-temperature Selective Catalytic Oxidation Activity of Ammonia

Selective catalytic oxidation (NH-SCO) is presently among the foremost technologies for eliminating the malodorous gaseous NH. Simultaneously achieving high NH conversion and high selectivity towards N at low temperatures poses a considerable hurdle. Surface oxygen vacancies and acidic sites, acting as adsorption and active sites in NH-SCO, hold the key to realizing efficient catalytic activity. In this paper, a facile surface ammoniation was implemented in layered birnessite-type MnO (δ-MnO) to enhance its NH-SCO activity. Surface ammoniation is conducive to the generation of oxygen vacancies in δ-MnO, thereby promoting the activation of molecular oxygen and further oxidation of NH; Moreover, ammoniation can also regulate the surface Lewis and Brønsted acidic sites to a reasonable ratio and strength, thereby enhancing the adsorption and activation of NH. Compared with pristine δ-MnO, both the activity and selectivity of NH-modified layered δ-MnO has been greatly enhanced, lowering the complete NH elimination temperature from 150 °C to 130 °C. The results of in situ DRIFTS indicate the layered δ-MnO followed the i-SCR mechanism, and the pristine δ-MnO were dominated by monodentate nitrate, while the NH-modified layered δ-MnO were dominated by bidentate nitrate and supplemented by monodentate nitrate. The further reaction of in situ generated nitrate with NH is the controlling step of the reaction, and the coexistence of monodentate nitrate and bidentate nitrate not only enhances the catalytic oxidation activity of NH but also improves the selectivity of N. This study provides a facile surface engineering approach for designing MnO catalysts for low-temperature NH-SCO.