Photocatalytic Oxidation of Benzene to Phenol with O₂ over WO₃ Treated by Vacuum-Sealed Annealing

Photocatalytic oxidation of benzene to phenol using molecular O₂ is one of the most promising sustainable approaches for the green synthesis of phenol. Introducing oxygen vacancies (OVs) on semiconductor surfaces by defect engineering is a promising strategy to enhance the efficiency of benzene oxidation to produce phenol due to the unique functions of OVs in facilitating the charge separation and activation of molecular O₂. Herein, a vacuum-sealed annealing strategy has been well developed to generate abundant surface OVs on semiconductors, such as WO₃. The well-sealed quartz vial creates a well-controlled low-pressure condition for the formation of OVs without the need for external energy for maintaining the vacuum state. Moreover, the gaseous species generated during the thermal annealing process help mitigate stress-induced defects, particularly bulk defects. The vacuum-sealed annealed WO₃ with sufficient OVs and reduced bulk defects shows a better photocatalytic performance in the one-step oxidation of benzene to phenol with O₂, compared to the WO₃ synthesized through thermal annealing in Ar and H₂ atmospheres. The present vacuum-sealed annealing strategy is found to be further applicable to engineer a wide range of semiconducting photocatalysts with abundant OVs to optimize their properties for efficient photocatalysis and other OV-promoted systems.