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The effect of pH and Cu2+ ions on the heterogeneous photocatalytic oxidation of toluenes by H2O2 formed from dissolved O2 in the presence of illuminated TiO2 powders, i.e. “the photo-Fenton type reaction,” was investigated. At low and high pH, a total amount of products increased drastically compared with that of the additive free system (pH 7). In the acidic region (aqueous H2SO4), side-chain oxidation prevailed over cresol formation and benzaldehyde was formed quite selectively at pH 1. Oxidation of the side chain in preference to hydroxylation of aromatic ring was also observed in the alkaline region (aqueous NaOH). By adding Cu2+ ion to the aqueous H2SO4 (pH 1 and 2), the yield of benzaldehyde increased further and cresols, benzyl alcohol, and bibenzyl were formed newly in high yields. At high Cu2+ concentrations (pH 2), the cresol formation in preference to the side-chain oxidation was attained. The observation was in good agreement with the Fenton reaction reported in which Cu2+ and Fe3+ ions are added as oxidants for the intermediate products, such as hydroxycyclohexadienyl and benzyl radicals, formed by HO· attack to toluene. Cu2+ ion was more preferable to Fe3+ ion as the oxidant, since “short circuiting” by Cu2+ due to the reversible redox reaction at semiconductor particles was much lower than that by Fe3+. By adding a large amount of Cl− ions to the Cu2+–H2SO4 aqueous system, no oxidation of toluene resulted.

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