| Abstrakt: |
Hydrogen peroxide (H2O2) is an important reactive oxygen species (ROS) involved in photocatalysis. To study the photocatalytic behavior of H2O2, the decomposition of H2O2on illuminated TiO2films was investigated using cavity ring down spectroscopy (CRDS). A mixture of H2O2and O2gas was flowed through a cavity reactor which contained a TiO2-coated plate. The removal of H2O2and the accompanying production of HO2radicals were monitored in the gas phase just above the TiO2film which was irradiated by a UV light-emitting diode (LED) (375 nm). The TiO2films tested in this study were mainly Degussa P25 TiO2(DP), Aldrich anatase (AA), and Aldrich rutile (AR). The photocatalytic production of HO2was observed only in the presence of H2O2, which indicates that the HO2radicals were generated from the decomposition of H2O2, not from the photocatalytic reduction of O2. The direct photolysis of H2O2in the absence of TiO2was not observed at all under the present irradiation conditions. The degradation of H2O2and the accompanying production of HO2was not retarded at all in the absence of O2(a common electron acceptor), which implies that H2O2itself should serve as an electron acceptor. Although the HO2radicals were originated from the decomposition of H2O2, the removal of H2O2and the production of HO2were not correlated. H2O2could be rapidly degraded on illuminated DP with little production of HO2, whereas H2O2was photodegraded much more slowly over AA and AR but with a marked production of HO2. On illuminated DP, the in situ generated HO2radicals seem to be rapidly degraded with little chance of desorption into the gas phase, while those on AA and AR are long-lived enough that some desorb into the gas phase. This implies that the fate of HO2radicals, which are universally involved in all photocatalytic reactions in the presence of O2, should be sensitively influenced by and dependent on the kind of TiO2. The photocatalytic decomposition of H2O2with different TiO2films was investigated with varying the experimental parameters such as light intensity, [H2O2], carrier gas composition (O2vs N2), and alternative electron donor and acceptor (methanol, EDTA, silver ions). The result implications for photocatalytic mechanism and atmospheric chemistry are discussed. |
