Photoreduction of methyl orange sensitized by colloidal titanium dioxide

Abstract

Colloidal TiO₂ can sensitize the photoreduction of Methyl Orange to a hydrazine derivative. In steady-state experiments the maximum rate occurs at pH 4.7 in the absence of O₂, but shifts to pH 3.0 when O₂ is present. Similar behaviour is observed with Methyl Red despite the difference in pK_a values for the two dyes. This suggests that the maximum rate results from changes in the surface composition of TiO₂. Flash-photolysis experiments show that electron transfer from TiO₂ to Methyl Orange depends on the concentration of protonated dye (DH) and the potential of an electron in the conduction band (e_CB) of TiO₂, so the rate of electron transfer = 8.3 × 10⁷[DH][e_CB]/[H⁺]^0.38(mol dm^–3)^–0.62 s^–1. Electron transfer to O₂ was also measured indirectly in this work and showed a similar logarithmic dependence on pH, rate = 5.5 × 10³[O₂][e_CB]/[H⁺]^0.42(mol dm^–3)^–0.58 s^–1. These results can be described by a Taffel relationship when the overvoltage is controlled by the pH of the solution.