SO2-Promoted Molecular Oxygen Activation and NO Oxidation on γ-MnO2.

The activation of molecular oxygen on transition metal oxide surfaces plays a crucial role in atmospheric chemistry and heterogeneous catalysis; however, understanding the intricate mechanisms remains a challenge. In this study, we elucidate for the first time the role of sulfur dioxide (SO2) in enhancing oxygen activation on manganese oxide (γ-MnO2) surface, thereby facilitating the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The theoretical calculation results further demonstrate that the adsorbed SO2 and the formed sulfate can promote the adsorption of O2 and the reactivity of surface reactive oxygen species toward NO oxidation on γ-MnO2 (110) surface, respectively. During SO2 with O2 reaction, the formation of surface sulfate and surface-active oxygen atoms serves as new active sites to facilitate the oxidation of NO to NO2. Comparative analysis of the energy profiles reveals that NO oxidation with SO2 can release more heat energy than that without SO2, indicating enhanced thermodynamic accessibility in the presence of SO2. Therefore, a novel mechanism for the sulfate-mediated activation of O2 is proposed, which sheds light on the synergistic effects of multiple pollutants in heterogeneous reactions and is significant for understanding NO oxidation both in the atmosphere and in exhaust treatment.