Photocatalytic Decomposition of H2O2 on Different TiO2 Surfaces Along with the Concurrent Generation of HO2 Radicals Monitored Using Cavity Ring Down Spectroscopy

Hydrogen peroxide (H2O2) is an important reactive oxygen species (ROS) involved in photocatalysis. To study the photocatalytic behavior of H2O2, the decomposition of H2O2 on illuminated TiO2 films was investigated using cavity ring down spectroscopy (CRDS). A mixture of H2O2 and O2 gas was flowed through a cavity reactor which contained a TiO2-coated plate. The removal of H2O2 and the accompanying production of HO2 radicals were monitored in the gas phase just above the TiO2 film which was irradiated by a UV light-emitting diode (LED) (375 nm). The TiO2 films tested in this study were mainly Degussa P25 TiO2 (DP), Aldrich anatase (AA), and Aldrich rutile (AR). The photocatalytic production of HO2 was observed only in the presence of H2O2, which indicates that the HO2 radicals were generated from the decomposition of H2O2, not from the photocatalytic reduction of O2. The direct photolysis of H2O2 in the absence of TiO2 was not observed at all under the present irradiation conditions. The degradation of H2O2 and the accompanying production of HO2 was not retarded at all in the absence of O2 (a common electron acceptor), which implies that H2O2 itself should serve as an electron acceptor. Although the HO2 radicals were originated from the decomposition of H2O2, the removal of H2O2 and the production of HO2 were not correlated. H2O2 could be rapidly degraded on illuminated DP with little production of HO2, whereas H2O2 was photodegraded much more slowly over AA and AR but with a marked production of HO2. On illuminated DP, the in situ generated HO2 radicals 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 HO2 radicals, 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 H2O2 with different TiO2 films was investigated with varying the experimental parameters such as light intensity, [H2O2], carrier gas composition (O2 vs N2), and alternative electron donor and acceptor (methanol, EDTA, silver ions). The result implications for photocatalytic mechanism and atmospheric chemistry are discussed.