Influences of Adsorption on TiO₂ Photocatalytic One-Electron Oxidation of Aromatic Sulfides Studied by Time-Resolved Diffuse Reflectance Spectroscopy

TiO 2 photocatalytic one-electron oxidation of 4-methyl- p -tolylsulfide (MTS), 4-(methylthio)phenyl methanol (MTPM), 4-(methylthio)benzoic acid (MTBA), and 4-(methylthio)phenyl acetic acid (MTPA) adsorbed on the surface of TiO 2 powder slurried in acetonitrile (CH 3 CN) has been investigated by time-resolved diffuse reflectance spectroscopy. As compared to MTS, high concentrations of the adsorbed MTPM, MTBA, and MTPA determined by UV absorption spectral measurements clearly indicate that −OH and −COOH groups play an important role in being adsorbed on the surface of TiO 2 particles. From Langmuir-type adsorption/desorption plots, the apparent equilibrium constants ( K ad ) of 20, 60, 2.3 × 10 4, and 1.1 × 10 4 M -1 were determined for MTS, MTPM, MTBA, and MTPA, respectively. The difference in the K ad values clearly indicates that −COOH and −CH 2 COOH groups are strongly adsorbed on the TiO 2 surface compared with −CH 2 OH and −CH 3 groups. The initial concentration of the radical cations generated from oxidation reactions with holes significantly depended on the concentration of substrate adsorbed on the TiO 2 surface. The relationship between the amount of adsorbates and the initial intensity was discussed in terms of Marcus theory. The decay kinetics for transient signals of the radical cations was also discussed.