Photocatalytic Degradation of 4-Chlorophenol in Aerated Aqueous Titanium Dioxide Suspensions: A Kinetic and Mechanistic Study

Abstract

The photocatalytic degradation of 4-chlorophenol (4-CP) in aqueous titanium dioxide suspensions has been studied and compared for two different catalysts: Sachtleben Hombikat UV 100 and Degussa P25. The influence of pH and initial 4-CP concentration on reaction rate and product distribution has been investigated in detail. Regarding the initial 4-CP concentration, the degradation kinetics formally can be discribed by a Langmuir−Hinshelwood expression. While the degradation of 4-CP itself is hardly influenced by the pH, the overall mineralization rate (measured in terms of total organic carbon reduction) changes considerably, decreasing strongly at pH > 7. With P25 used as the photocatalyst the mineralization of 4-CP is slightly faster than that with Hombikat UV 100, but smaller concentrations and numbers of intermediates are detected in the latter case. Most of the intermediates observed during this photocatalytic degradation process are identical with those identified during direct photolysis. In acidic solution (pH 3) hydroquinone and benzoquinone are the only intermediates when Hombikat UV 100 is the photocatalyst. Using P25 as a photocatalyst, seven additional intermediates, namely, hydroxyhydroquinone, hydroxybenzoquinone, phenol, 4-chlorocatechol, 4-hydroxyphenylbenzoquinone, 2,5,4‘-trihydroxybiphenyl, and 5-chloro-2,4‘-dihydroxybiphenyl, were found. Hydroquinone (HQ) and benzoquinone (BQ) have been found to be the main intermediates of the photocatalytic 4-CP degradation. It could be shown that the HQ/BQ photosystem acts as an electron relay which effectively short circuits the photocatalyst resulting in a reduced efficiency of the degradation of 4-CP and most probably other aromatic compounds. An electron shuttle mechanism is proposed to explain the highly efficient short circuiting of the photocatalyst in the presence of these compounds. A degradation mechanism for 4-chlorophenol is proposed and discussed to account in particular for the pH dependence of the 4-CP degradation.