Visible-light-driven WO3/BiOBr heterojunction photocatalysts for oxidative coupling of amines to imines: Energy band alignment and mechanistic insight

The visible-light-driven WO3/BiOBr heterojunction was for the first time determined for its photocatalytic activity toward oxidative coupling of amines at room temperature using molecular oxygen as a green oxidant. The WO3/BiOBr heterojunction exhibits superior photocatalytic activity and photostability compared with pure BiOBr and WO3 due to an increased oxygen vacancy concentration, an effective separation of photogenerated electron-hole pairs and an efficient interfacial charge transfer. Additionally, the WO3/BiOBr also shows 2.3 and 41.1 times higher activity than that of TiO2 P25 and BiVO4 Alfa Aesar, respectively. Determination of energy band line-up indicates that the WO3/BiOBr is a type II-heterojunction where electron-hole pairs are efficiently separated. Mechanistic studies based on radical quenching experiment, EPR trapping study and Hammett plot reveal that the main reaction pathway is the electron transfer route mediated by superoxide radical. A possible surface reaction mechanism, the insightful information on the structure-activity relationship and the involvement of reactive oxygen species elucidated in this work lay an important background for the material design and encourage a further development of highly efficient photocatalysts toward organic fine chemical syntheses.