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

Abstract

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.