Determination of the quantum yield for the photochemical generation of hydroxyl radicals in TiO2 suspensions

The generation of hydroxyl ((OH)-O-.) radicals plays a key role in the heterogeneous photocatalytic degradation of organic pollutants in aqueous suspensions of TiO2. 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)-O-. radical generation rates and the photon flux absorbed by TiO2 suspensions. In this procedure, a modified integrating sphere method was used to determine the true fraction of light absorbed by TiO2 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)-O-. radical generation in the photolysis of hydrogen peroxide, respectively. Accordingly, the quantum yield for the (OH)-O-. radical production in the TiO2 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 (H2O2 and O-2) were explored.