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

Abstract

The reaction mechanism of visible-light responsive photocatalysts was explored by analyzing OH radicals ( • OH) quantitatively by means of a coumarin fluorescence probe method. The photocatalysts investigated were various modified TiO 2, i.e., nitrogen-doped, Pt-complex-deposited, Fe(III)-grafted, and Fe(III)-grafted Ru-doped TiO 2 . The formation rate of • OH was measured to calculate the • OH quantum yield from the absorbed intensity of 470 nm LED light. The highest quantum yield was obtained for Fe(III)-grafted Ru-doped TiO 2 . The • OH yield was increased on the addition of H 2 O 2 for the Fe(III)-grafted TiO 2, indicating that H 2 O 2 is supposedly a reaction intermediate for producing • OH. The photocatalytic activity for each sample was obtained by measuring CO 2 generation rate on the acetaldehyde decomposition in an aqueous suspension system and then it was compared with the • OH formation rate. Although the CO 2 generation rate is positively correlated with the • OH formation rate for each photocatalyst, the values of CO 2 generation were extremely larger than those of • OH. This finding indicates that the oxidation reaction takes place dominantly with surface trapped holes which probably exchange with the • OH in solution.