Efficient photo-oxidation of C(sp3)–H bonds on visible-light-responsive W-doped TiO2 nanocrystals promoted by the photochromic effect†

Abstract

Photocatalytic oxidation of C(sp³)–H bonds using semiconducting catalysts offers a sustainable and promising route for value-added chemical production under milder conditions. However, the photocatalytic performance of semiconductor-based photocatalysts is severely restricted by the limited active sites and low photoredox activity. Herein, we doped redox-active W⁶⁺ ions into TiO₂ to produce W-doped TiO₂ nanocrystals with unique photochromic properties. W-doping effectively optimizes light harvesting, promotes charge carrier separation, and enriches the active sites for efficient photocatalytic oxidation of C(sp³)–H bonds. In particular, 10% W-doped TiO₂ nanocrystals demonstrate exceptional acetophenone production of 235 mmol g⁻¹ under 437 nm light irradiation, which is 16 times higher than that of pure TiO₂ nanocrystals. Furthermore, mechanistic analyses suggest that the photochromic effect-induced W⁵⁺–O_vs–Ti³⁺ (oxygen vacancies denoted by O_vs) active sites in W-doped TiO₂ nanocrystals act as electron trapping centers to capture and store photogenerated electrons, promoting the photogenerated holes to activate C(sp³)–H bonds. Moreover, the O_vs and stored photogenerated electrons on the active sites significantly enhance the adsorption and activation of O₂ to O₂˙⁻. These O₂˙⁻ can directly react with benzylic C(sp³)˙ radicals upon light irradiation, generating acetophenone and thereby enhancing photocatalytic performance. This work showcases a feasible strategy to realize highly efficient ethylbenzene oxidation by designing photochromic photocatalysts.