A DFT study on the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde in different solvents employing an OH-defected TiO2 cluster model

Density functional theory calculations are employed to study the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde (4-BBA) in acetonitrile and in ethanol solvents. A totally relaxed Ti3O9H6 cluster model is proposed to represent titanium dioxide (TiO2) surfaces. The reduction selectivity of an adsorbed 4-BBA molecule on Ti3O9H6 has been investigated. Owing to the difference in the proton and H atom donating capabilities between explicit CH3CN and C2H5OH solvent molecules, the photocatalytic reduction of 4-BBA is the debromination process in acetonitrile, whereas in ethanol it is the carbonyl reduction process. Therefore 4-BBA can be selectively reduced to benzaldehyde in acetonitrile and 4-bromobenzyl alcohol in ethanol, respectively. Our computational results have verified the reaction mechanism proposed by experiments and show that the debromination of 4-BBA would be efficient if both 4-BBA and Ti3O9H6 have an extra photoelectron. The Ti3O9H6 cluster, playing a role as a hydrogen source and a bridge to transfer photoelectrons from bulk TiO2, would have potential to be an ideal molecular model for understanding photocatalytic reactions on the TiO2 surface.