Mechanism of Visible Light Photocatalytic Oxidation of Methanol in Aerated Aqueous Suspensions of Carbon-Doped TiO2

Visible photolysis of aerated carbon-doped TiO 2 (C-TiO 2 ) aqueous suspensions induces methanol oxidation to formaldehyde. The rate of HCHO formation increases with the concentration of CH 3 OH or C-TiO 2 and is nearly doubled in the presence of catalase or excess of H 2 O 2 . The mechanism involves oxidation of CH 3 OH by surface trapped holes, although these holes have lower energy than those formed upon UV photolysis of undoped TiO 2 . The C-TiO 2 electrons reduce O 2 to H 2 O 2 . CH 3 OH oxidation via reduction of H 2 O 2 by the C-TiO 2 electrons was observed in the presence of added H 2 O 2, where it competes efficiently with O 2 for the C-TiO 2 electrons. At [H 2 O 2 ] > 0.1 mM, the yield of HCHO is about twice that in the absence of added H 2 O 2 . Light absorption measurements in an integrating sphere show that the limiting absorption fraction at high [C-TiO 2 ] 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 H 2 O 2 has been observed, which is not associated with HCHO formation.