Mechanism of photocatalytic oxidation of 3,4-dichlorophenol on TiO2 semiconductor surfaces

Abstract

In this paper, the results of a detailed study of the kinetics of photocatalytic oxidation of 3,4-dichlorophenol at various concentrations in oxygenated solutions on TiO2 are presented. Electron–hole recombination is often suggested to be decreased by increasing the molecular oxygen concentration; but our study indicates that this is not the primary rate enhancing property of the dissolved oxygen. It is argued here that a hydroxyl radical is ejected from the catalyst surface by photo-excitation onto a repulsive excited electronic state leading to a translationally hot OH(2π) radical. In the presence of molecular oxygen, a simple hydroxyl addition to the dichlorophenol occurs. In the absence of an adsorbed oxygen molecule, the electron transfer to the aromatic ring from a Ti3+ site causes partial dechlorination of the dichlorophenol. Subsequently, hydroxyl radical addition to the aromatic ring may occur. Hence, we find that dissolved molecular oxygen has two important roles in the photo-catalytic oxidation of 3,4-dichlorophenol on the semiconducting TiO2 surfaces. One of these is as a H-atom acceptor required in direct hydroxyl radical addition to the phenyl ring while the other is as an electron transfer inhibitor when adsorbed at defective Ti3+ sites. A theoretical model of the kinetics is proposed which is able to account semi-quantitatively for the overall features of the reaction state space. Significantly, monitoring of the intermediate species produced by these two routes shows that the relative yields can be inverted by changing the dissolved oxygen concentration which significantly is accord with the theoretical predictions.