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Photocatalytic Decomposition of H2O2 Monitored using Cavity Ring Down Spectroscopy

Photocatalytic Decomposition of H2O2 Monitored using Cavity Ring Down Spectroscopy
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Hydrogen peroxide (H2O2) is an important reactive oxygen species (ROS) involved in photocatalysis. It is an intermediate that is in situ generated and degraded on the illuminated photocatalyst surface and is closely involved in the fate of other ROS species such as OH radicals and superoxides. In order to study the photocatalytic behavior of H2O2, the decomposition of H2O2 on illuminated TiO2 films was investigated using cw-CRDS. The mixture of H2O2 and O2 gas was flowed through the cavity reactor which contained a TiO2-coated plate (60 cm x 2.5 cm). 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 UV LED (375 nm). The TiO2 films tested in this study are Degussa P25 TiO2 (PT), Aldrich anatase (AA), Aldrich rutile (AR), and Hombikat UV-100 (HBK). The photocatalytic production of HO2 was observed only in the presence of H2O2, which indicates that the observed HO2 are 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 condition and H2O2 was not degraded on the surface of TiO2 in the dark. Although the HO2 radicals 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 PT with little production of HO2 whereas H2O2 was photodegraded much slowly over AA and AR but with a marked production of HO2. The generation of intermediate HO2 radicals and their subsequent degradation seem to be sensitively dependent on the kind of TiO2 substrate. On illuminated PT, 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 involved in the photocatalytic process should be sensitively influenced by the surface property of TiO2 and that their photocatalytic mechanism cannot be generalized.
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