Quantum Yields And Mechanism In Titanium Dioxide Mediated Photocatalysis

The photocatalytic pathway in TiO{dollar}\sb2{dollar} suspensions was examined using a spin trap/EPR technique within a competition kinetic scheme. Experimental results from competition reactions indicate that there is a marked difference in kinetic behaviours between the systems with (heterogeneous) and without (homogeneous) a TiO{dollar}\sb2{dollar} suspension, which confirms that the reaction pathway of {dollar}\cdot{dollar}OH radicals in the TiO{dollar}\sb2{dollar} suspension is at least partly heterogeneous. A photocatalytic mechanism is proposed.;A method of determining the trapping efficiency of {dollar}\cdot{dollar}OH radicals was developed with the help of a spin trap/EPR method, using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), for the measurement of growth rates of the spin adduct DMPO-OH and HPLC measurement for the {dollar}\cdot{dollar}OH radical generation rates. The agreement of the trapping efficiency (0.33 {dollar}\pm{dollar} 0.04) in the homogeneous ({dollar}\rm H\sb2O\sb2{dollar}) system with the literature value (0.35) confirms the reliability of the measurement method. The trapping efficiency in the heterogeneous (TiO{dollar}\sb2{dollar}) system was found to be 0.28 {dollar}\pm{dollar} 0.03.;A method for quantum yield determinations in heterogeneous systems was developed, based on the measurements of {dollar}\cdot{dollar}OH radical generation rates and the flux of absorbed photons by TiO{dollar}\sb2{dollar} suspensions. In this procedure a modified integrating sphere method was used for the measurement of the absorbance of TiO{dollar}\sb2{dollar} suspensions. A ferrioxalate chemical actinometer was used to measure the flux of absorbed photons. As a check on the method, good agreement with well known literature values was obtained for the quantum yield measurements of BQ{dollar}\cdot{dollar} and {dollar}\cdot{dollar}OH radical and generation in the p-benzoquinone and hydrogen peroxide systems, respectively. Accordingly, the quantum yield of the {dollar}\cdot{dollar}OH radical generation in TiO{dollar}\sb2{dollar} suspensions was determined to be 0.040 {dollar}\pm{dollar} 0.003 at pH 7.;Effects of suspension loading, light intensity, electron acceptor addition, and dissolved oxygen concentration on the quantum yield were observed. The effects of pH and buffer concentration on the formation rate of the DMPO-OH spin adduct are discussed