Oxidation reactions, semiconductor photocatalyzed

The mechanism(s) by which these photocatalyzed oxidations are initiated remain uncertain. Early proposals have included involvement of either the photo-produced holes (h+) arising directly from semiconductor photo-excitation, or the (presumed) derivative hydroxyl radical (OH) which was argued to arise from the hole oxidation of adsorbed hydroxyls (h+ + OH-—> OH ). Recent subambient studies [4] with physisorbed chloromethane and oxygen suggest the dioxygen anion (02 ) as a key active species, and the photocatalytic high efficiency chain destruction of TCE is argued to be initiated by chlorine radicals (Cl) [5]. The chlorine-enhanced photocatalytic destruction of air contaminants has been proposed [1, 2, 6] to depend upon reactions initiated by chlorine radicals. 

Thus, in general, a reduced and an oxidized product are obtained, in complete analogy with a classical electrochemical reaction. Except for a very few examples, almost all reactions photocatalyzed by semiconductors fall within this scheme and we have proposed to classify such reactions as semiconductor photocatalysis type A [45]. 

Mills A., Elliot N., Parkin I.P., O NeiU S.A., Clark R.J. Novel Ti02 CVD films for semiconductor photocatalysis. J. Photochem. Photobiol. A 2002 151 171-179 Minero C., Catozzo F., Pelizzetti E. Role of adsorption in photocatalyzed reactions of organic-molecules in aqueous Ti02 suspensions. Langmuir 1992 8 481 86 Miyauchi M., Nakajima A., Hashimoto K., Watanabe T. A highly hydrophilic thin film under 1 /xW/cm UV illumination. Adv. Mater. 2000 12 1923-1927 Miyauchi M., Nakajima A., Fujishima A., Watanabe T., Hashimoto K. Photoinduced surface reactions on Ti02 and SrTiOa films photocatalytic oxidation and photoinduced hydrophilicity. Chem. Mater. 2000 12 3-5... 

Photocatalysis by semiconductor powders, most usually titanium dioxide, has been applied to the oxidation of several aromatics. In many such photocatalytic reactions, the key intermediates are hydroxyl radicals formed by oxidation of water. With rather good donors such as aromatic compounds, hole transfer on the excited semiconductor surface is a viable alternative in this case, the reaction of the radical cation of the substrate or further intermediates arising from it with oxygen or superoxide anion may have a role. Photocatalyzed oxidation of naphthalene yields 2-formyldrmamaldehyde and 1,4-naphthoquinone as the primary products (Eq. (45.16)), similarly to what occurs upon direct irradiation of naphthalene in water ... 

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