PHOTOCATALYTIC OXIDATION IN AQUEOUS TITANIUM-DIOXIDE SUSPENSIONS - THE INFLUENCE OF DISSOLVED TRANSITION-METALS

The influence of type, species distribution, standard reduction potential, and concentration of several transition metals on the rate of photocatalytic oxidation of toluene was investigated. A significant increase in reaction rate was observed in the presence of 10(-5) M Cu(II), Fe(III), and Mn(II) at pH 3, with decreased rates at higher concentrations and pH values. There was no clear correlation between reaction rate and aqueous metal species distribution, nor did the oxidation states of Cu or Fe alter their effects on the reaction rate. Neither Ni(II) nor Zn(II) had a significant influence on the rate of organic oxidation. Negligible adsorption of metals onto TiO2 was measured at the metal concentrations and pH values for which the highest reaction rates were observed, indicating that dissolved metals increase the reaction rate via a homogeneous pathway rather than a TiO2 surface reaction. A mechanism involving formation of a reactive complex between the metal, the organic or its oxidation intermediate, and an oxygen-containing species is proposed to explain the experimental data. The rate of the photocatalytic reaction is described by a Langmuir-Hinshelwood rate form, modified to account for homogeneous catalytic pathways and decreased UV transmittance in the presence of dissolved metals.