The formation of reactive singlet oxygen molecule ( 1 O 2 ), the a 1 D g state of excited O 2 , in the photocatalytic TiO 2 aqueous suspension system was shown to be evident by detecting the nearinfrared phosphorescence at 1270 nm by means of a gated photon counting method. The lifetimes of O 2 ( 1 D g ) in three different media (water, ethanol, 1 : 1 water-ethanol mixture) were elucidated and compared with those reported in corresponding homogeneous media. Based on the comparison of the lifetime with those of the other reactive species such as OH radicals and trapped holes, the contribution of 1 O 2 to the actual photocatalytic procedure was discussed.
Ab initio calculations at the level of CBS-QB3 theory have been performed to investigate the potential energy surface for the reaction of benzyl radical with molecular oxygen. The reaction is shown to proceed with an exothermic barrierless addition of O2 to the benzyl radical to form benzylperoxy radical (2). The benzylperoxy radical was found to have three dissociation channels, giving benzaldehyde (4) and OH radical through the four-centered transition states (channel B), giving benzyl hydroperoxide (5) through the six-centered transition states (channel C), and giving O2-adduct (8) through the four-centered transition states (channel D), in addition to the backward reaction forming benzyl radical and O2 (channel E). The master equation analysis suggested that the rate constant for the backward reaction (E) of C6H5CH2OO-->C6H5CH2+O2 was several orders of magnitude higher that those for the product dissociation channels (B-D) for temperatures 300-1500 K and pressures 0.1-10 atm; therefore, it was also suggested that the dissociation of benzylperoxy radicals proceeded with the partial equilibrium between the benzyl+O2 and benzylperoxy radicals. The rate constants for product channels B-D were also calculated, and it was found that the rate constant for each dissociation reaction pathway was higher in the order of channel D>channel C>channel B for all temperature and pressure ranges. The rate constants for the reaction of benzyl+O2 were computed from the equilibrium constant and from the predicted rate constant for the backward reaction (E). Finally, the product branching ratios forming CH2O molecules and OH radicals formed by the reaction of benzyl+O2 were also calculated using the stationary state approximation for each reaction intermediate.
The important roles of OH radicals for remote oxidation using TiO(2) photocatalysts were evidenced by the in situ detection of OH radicals in the gas phase using the laser-induced fluorescence (LIF) technique. The appearance of OD-LIF intensities after the exposure of D(2)O vapors over TiO(2) powders and the decrease of the time-resolved signals of OH-LIF intensities with increasing calcined temperatures of TiO(2) powders suggested that the exchangeable water at the TiO(2) surface is the origin of the diffused OH radicals.
Diffusion of OH radicals from UV-irradiated TiO 2 surface to the gas phase was successfully detected using a laser-induced-fluorescence technique for various types of TiO 2 powders. The diffusion time of OH radicals was found to vary with the types of TiO 2 powders and to be affected by the heat treatments of these powders, depending on the treatment temperatures. The diffusion mechanism was discussed based on the characteristic OH-LIF intensities for individual TiO 2 powders and the observations of OD-LIF after the exposure of D 2 O vapors over the TiO 2 powders. The quantum yield of OH radicals diffused from the TiO 2 surface was estimated to be about 5 × 10 -5 by comparing the OH-LIF intensities produced by the 266-nm photolysis of HNO 3.
The mechanism of SiO formation in the laser photolysis of SiH 4 /O 2 /CCl 4 mixtures was investigated using a laser-induced fluorescence method. Measured rates for the SiO production corresponded to the decay rates of SiH 3 radical and depended linearly on the O 2 concentration. The yield of SiO was estimated on the basis of LIF intensity, and it was found that SiO was one of the major products in the SiH 3 + O 2 reaction. The bimolecular rate constant for the SiO production was determined to be (1.14 ( 0.18) × 10 -11 cm 3 molecules -1 s -1 . Ab initio molecular orbital calculations were performed for various pathways of the SiH 3 + O 2 reaction. Geometries were optimized at the MP2(full)/6-31G(d) level of theory, and relative energies and barrier heights were calculated at the G2(MP2) level of theory. Silyl radical and O 2 react to form SiH 3 OO, which irreversibly decomposes to various excited products. A new transition state for the production of cyclic H 2 SiO 2 (siladioxirane) + H from SiH 3 OO adduct was found. Possible decomposition channels of the vibrationally excited products of the SiH 3 + O 2 reaction to produce SiO are discussed.
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