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

Hydrogen peroxide (H 2 O 2 ) is an important reactive oxygen species (ROS) involved in photocatalysis. To study the photocatalytic behavior of H 2 O 2, the decomposition of H 2 O 2 on illuminated TiO 2 films was investigated using cavity ring down spectroscopy (CRDS). A mixture of H 2 O 2 and O 2 gas was flowed through a cavity reactor which contained a TiO 2 -coated plate. The removal of H 2 O 2 and the accompanying production of HO 2 radicals were monitored in the gas phase just above the TiO 2 film which was irradiated by a UV light-emitting diode (LED) (375 nm). The TiO 2 films tested in this study were mainly Degussa P25 TiO 2 (DP), Aldrich anatase (AA), and Aldrich rutile (AR). The photocatalytic production of HO 2 was observed only in the presence of H 2 O 2, which indicates that the HO 2 radicals were generated from the decomposition of H 2 O 2, not from the photocatalytic reduction of O 2 . The direct photolysis of H 2 O 2 in the absence of TiO 2 was not observed at all under the present irradiation conditions. The degradation of H 2 O 2 and the accompanying production of HO 2 was not retarded at all in the absence of O 2 (a common electron acceptor), which implies that H 2 O 2 itself should serve as an electron acceptor. Although the HO 2 radicals were originated from the decomposition of H 2 O 2, the removal of H 2 O 2 and the production of HO 2 were not correlated. H 2 O 2 could be rapidly degraded on illuminated DP with little production of HO 2, whereas H 2 O 2 was photodegraded much more slowly over AA and AR but with a marked production of HO 2 . On illuminated DP, the in situ generated HO 2 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 HO 2 radicals, which are universally involved in all photocatalytic reactions in the presence of O 2, should be sensitively influenced by and dependent on the kind of TiO 2 . The photocatalytic decomposition of H 2 O 2 with different TiO 2 films was investigated with varying the experimental parameters such as light intensity, [H 2 O 2 ], carrier gas composition (O 2 vs N 2 ), and alternative electron donor and acceptor (methanol, EDTA, silver ions). The result implications for photocatalytic mechanism and atmospheric chemistry are discussed.