Mechanism of visible light photocatalytic oxidation of methanol in aerated aqueous suspensions of carbon-doped TiO₂

Visible photolysis of aerated carbon-doped TiO₂ (C-TiO ₂) aqueous suspensions induces methanol oxidation to formaldehyde. The rate of HCHO formation increases with the concentration of CH₃OH or C-TiO₂ and is nearly doubled in the presence of catalase or excess of H₂O₂. The mechanism involves oxidation of CH ₃OH by surface trapped holes, although these holes have lower energy than those formed upon UV photolysis of undoped TiO₂. The C-TiO ₂ electrons reduce O₂ to H₂O₂. CH₃OH oxidation via reduction of H₂O₂ by the C-TiO₂ electrons was observed in the presence of added H ₂O₂, where it competes efficiently with O₂ for the C-TiO₂ electrons. At [H₂O₂] > 0.1 mM, the yield of HCHO is about twice that in the absence of added H ₂O₂. Light absorption measurements in an integrating sphere show that the limiting absorption fraction at high [C-TiO₂] is 0.5 at 450 nm, the rest of the light being mostly scattered backward. The HCHO quantum yield depends on the square root of the absorbed light density and is only a few percent at the intensities used in this work. A parallel photocatalytic decomposition of H₂O₂ has been observed, which is not associated with HCHO formation.