First-principles calculations for determining the mechanism of the photocatalytic selective oxidation of toluene to benzaldehyde on the g-C3N4 catalyst

Abstract

The photocatalytic oxidation of toluene (TL) to benzaldehyde (BAD) has the advantages of mild reaction conditions, green chemistry, and high selectivity of the target products. However, there is a problem on how to improve the selectivity of the product into BAD while ensuring the efficiency of TL conversion. Therefore, it is of great importance to understand the mechanism of the photocatalytic selective oxidation of TL into BAD. In this study, first-principles calculations were performed to investigate the mechanism of the photocatalytic selective oxidation of TL into BAD on a graphitic carbon nitride (g-C₃N₄) catalyst under visible-light irradiation. The calculation results showed that the holes (g-C₃N₄˙⁺) and the hydroperoxyl radicals (˙OOH) were the major reactive oxygen species (ROS). Moreover, the activation energy of the rate-determining step for the dehydration reaction of the C₆H₅CH₂OOH molecule was 40.2 kcal mol⁻¹, which was lower than that of the step in the free sate of about 4.8 kcal mol⁻¹. Moreover, in order to get the maximum selectivity of BAD, the formation of hydroxyl radicals (˙OH) should be avoided. Our work provides theoretical guidance for improving the photocatalytic selective oxidation of TL into BAD.