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 TiO₂, i.e., nitrogen-doped, Pt-complex-deposited, Fe(III)-grafted, and Fe(III)-grafted Ru-doped TiO₂. 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 TiO₂. The ^•OH 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. 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 formation rate. Although the CO₂ generation rate is positively correlated with the ^•OH formation rate for each photocatalyst, the values of CO₂ 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.