Quantitative Detection of OH Radicals for Investigating the Reaction Mechanism of Various Visible-Light TiO2 Photocatalysts in Aqueous Suspension

The reaction mechanism of visible-light responsive photocatalysts was explored by analyzing OH radicals ((OH)-O-center dot) quantitatively by means of a coumarin fluorescence probe method. The photocatalysts investigated were various modified TiO2, i.e., nitrogen-doped, Pt-complex-deposited, Fe(III)-grafted, and Fe(III)-grafted Ru-doped TiO2. The formation rate of (OH)-O-center dot was measured to calculate the (OH)-O-center dot quantum yield from the absorbed intensity of 470 nm LED light. The highest quantum yield was obtained for Fe(III)-grafted Ru-doped TiO2. The (OH)-O-center dot yield was increased on the addition of H2O2 for the Fe(III)-grafted TiO2, indicating that H2O2 is supposedly a reaction intermediate for producing (OH)-O-center dot. The photocatalytic activity for each sample was obtained by measuring CO2 generation rate on the acetaldehyde decomposition in an aqueous suspension system and then it was compared with the (OH)-O-center dot formation rate. Although the CO2 generation rate is positively correlated with the (OH)-O-center dot formation rate for each photocatalyst, the values of CO2 generation were extremely larger than those of (OH)-O-center dot. This finding indicates that the oxidation reaction takes place dominantly with surface trapped holes which probably exchange with the (OH)-O-center dot in solution.