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.