Determination of the Quantum Yield for the Photochemical Generation of Hydroxyl Radicals in TiO₂Suspensions

Abstract

The generation of hydroxyl ( • OH) radicals plays a key role in the heterogeneous photocatalytic degradation of organic pollutants in aqueous suspensions of TiO 2 . The quantum yield of this process is thus an important parameter; however, it is not easy to measure in a particulate system arising from problems caused by light scattering from the particles. In this work, a reliable method for the determination of the quantum yield of hydroxyl radical production in heterogeneous systems has been developed, based on measurements of • OH radical generation rates and the photon flux absorbed by TiO 2 suspensions. In this procedure, a modified integrating sphere method was used to determine the true fraction of light absorbed by TiO 2 suspensions. A ferrioxalate chemical actinometer was used to measure the incident photon flux. As a check on the quantum yield method, good agreement with known literature values was obtained for quantum yield measurements of the photochemical generation of the p -benzosemiquinone (BQ •- ) radical in the photolysis of p -benzoquinone and of the • OH radical generation in the photolysis of hydrogen peroxide, respectively. Accordingly, the quantum yield for the • OH radical production in the TiO 2 suspension was determined to be 0.040 ± 0.003 at pH 7. Effects on the quantum yield of suspension loading, photon flux, and electron-acceptor addition (H 2 O 2 and O 2 ) were explored.