Changing the reaction pathway in TiO2 photocatalytic dehalogenation of halogenated aromatic pollutants by surface hydroxyl regulation

Abstract

Understanding the photocatalytic reductive dehalogenation mechanism of halogenated aromatic pollutants is of great research value. However, the proton source in the photocatalytic dehalogenation process of representative halogenated aromatic pollutants by TiO₂ is not clear. In this study, the TiO₂ surface was modified by hydrochloric acid, sodium hydroxide, and sodium fluoride to obtain TiO₂ samples with different hydroxyl groups. It was found that the hydroxyl groups on the surface of TiO₂ affects the sequence of proton and electron transfer in dehalogenation. The abundance of hydroxyl groups on the surface of TiO₂ can accelerate the reductive dehalogenation process of representative halogenated aromatic pollutants. The kinetic solvent isotope effect was used to study the proton-coupled electron transfer process in the reaction. It shows that the enriching of protons on TiO₂ bridging oxygen (bridging hydroxyl groups) is conducive to the rapid step of protonation of the reactant, and subsequent proton and electron transfer. On the contrary, the bridging hydroxyl groups can be removed by reacting with strongly basic sodium hydroxide and sodium ions can occupy the bridging oxygen. The substitution of bridging oxygen by fluorine ions can also lead to the destruction of bridge hydroxyl groups. Significantly, the absence of bridging hydroxyl groups on titanium dioxide will lead to the dehalogenation of representative halogenated aromatic pollutants initiated by electron transfer. This study is helpful to understand dehalogenation reaction paths catalyzed by TiO₂.

Keywords: Electron transfer; Halogenated aromatic pollutants; Proton transfer; Surface hydroxyls; Surface modification.