Opposite photocatalytic oxidation behaviors of BiOCl and TiO2 : Direct hole transfer vs. indirect •OH oxidation

Although all the semiconductor-based photocatalysis is initiated by photoinduced conduction band electron and valence band hole, the reaction pathway and activity of the electron and hole would be largely dependent on the type of the photocatalyst. It is essential to distinguish and understand the photocatalyst-dependence of a specific reaction. In the present study, we compared the photocatalytic behaviors of BiOCl and TiO2 for the degradation of different pollutants including perfluorooctanoic acid (PFOA), chloroacetic acids and benzoic acid. We found that the decompositions of perhalocarboxylate acids (PHCAs) such as PFOA and trichloroacetic acid (TCA) were much rapider over BiOCl than on the TiO2 (commercial P25). The surface-area- normalized rate constants for the oxidation of TCA have five orders of magnitude of difference between these two systems. By contrast, the degradation rates of (OH)-O-center dot-sensitive organic pollutants such as dichloroacetic acid, monochloroacetic acid and benzoic acid were much higher on the TiO2. Moreover, much more meta-substituted hydroxylated intermediate was observed during the photocatalytic oxidation of benzoic acid on BiOCl. In addition, PFOA and TCA were degraded efficiently in the BiOCl system even in the presence of other labile organic compounds (such as acetic acid). All the experimental results definitely indicate that BiOCl photocatalyst prefers to directly oxidize the PHCAs and benzoic acid by the hole transfer, while TiO2 tends to oxidize the solvent water molecule to (OH)-O-center dot radical. The mechanism underlying on the different activity of BiOCl and TiO2 are further discussed.