Oxygen Vacancy Sites Enable Efficient Photocatalytic Oxidation of Nitric Oxide: The Role of W/Mo Valence Transition in Bi2W(Mo)O6-x

Abstract

Oxygen vacancy (O_v) sites play a critical role in the activation and deep oxidation of nitric oxide (NO). However, controlling the concentration and type of O_v remains a significant challenge. In this study, Bi₂W(Mo)O_6-x is investigated as a model system and demonstrates that increasing the concentration of O_v substantially enhances the efficiency of air NO removal. Increasing the O_v concentrations in Bi₂WO_6-x and Bi₂MoO_6-x improves NO removal efficiency ≈12- and 11-fold, respectively, compared to their low-O_v counterparts. This enhancement is attributed to improved adsorption and activation of NO/O₂ molecules, better separation and transfer of photogenerated carriers, and increased visible light absorption. Notably, Bi₂WO_6-x remains highly stable over ten recycling tests for continuous air NO deep photooxidation, while Bi₂MoO_6-x shows a 43.5% decrease in efficiency after ten runs. This sustained performance is attributed to stable O_vs without changes in metal ion valence, unlike Bi₂MoO_6-x, where instability arises from the reduction of Mo⁶⁺ to Mo⁴⁺. In situ DRIFTS reveals possible pathways for the deep photooxidation of NO to nitrate (NO₃⁻). This study provides valuable insights into designing high-performance, durable catalysts by effectively controlling O_v concentration and type, paving the way for efficient photocatalytic air purification technologies.