Effects of surface fluoride modification on TiO₂ for the photocatalytic oxidation of toluene

In the present study, we investigated the influence of surface fluorine (F) on TiO₂ for the photocatalytic oxidation (PCO) of toluene. TiO₂ modified with different F content was prepared and tested. It was found that with the increasing of F content, the toluene conversion rate first increased and then decreased. However, CO₂ mineralization efficiency showed the opposite trend. Based on the characterizations, we revealed that F substitutes the surface hydroxyl of TiO₂ to form the structure of Ti-F. The presence of the appropriate amount of surface Ti-F on TiO₂ greatly enhanced the separation of photogenerated carriers, which facilitated the generation of ·OH and promoted the activity for the PCO of toluene. It was further revealed that the increase of only ·OH promoted the conversion of toluene to ring-containing intermediates, causing the accumulation of intermediates and then conversely inhibited the ·OH generation, which led to the decrease of the CO₂ mineralization efficiency. The above results could provide guidance for the rational design of photocatalysts for toluene oxidation.