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The ratio of the amount of carboxymethyl radicals to that of methyl radicals was measured in situ for the photocatalytic decomposition of acetic acid over platinized TiO 2 powder in deoxygenated aqueous solution at room temperature and discussed in connection with the contribution of · OH radicals in the oxidation process. Since carboxymethyl radicals are formed only by indirect oxidation, the radical ratio was used to measure the indirect oxidation via · OH radicals. Thus, ratios of the indirect oxidation were estimated for five commercially available TiO 2 photocatalysts, in addition to the several properties of the powders. The ratios were found to be sensitive to the characteristics of the TiO 2 catalysts. The larger the amount of surface OH group, the more likely the indirect oxidation via · OH radicals occurs . This could be attributed to the difference in the structures of the surface trapped holes elucidated by the ESR measurements of the corresponding TiO 2 powder at 77 K.
Nosaka et al. (Wed,) studied this question.