6820
J . Phys. Chem. 1990, 94, 6820-6825
Effects of Common Inorganic Anions on Rates of Photocatalytic Oxidation of Organic Carbon over Illuminated Titanium Dioxide Mohammad Abdullah, Gary K.-C. Low, and Ralph W. Matthews* CSIRO Division of Fuel Technology, Lucas Heights Research Laboratories, Private Mail Bag 7 , Menai, N S W 2234, Australia (Received: November 29, 1989; In Final Form: March 7, 1990)
Effects of pH and common inorganic anions on the rates of photocatalytic oxidation of salicylic acid, aniline, and ethanol to CO, over glass-supported near-UV illuminated Ti02 have been investigated by using conductivity detection of evolved C02. In the presence of chlorides and the absence of organics, lowering the pH of the solution below 3.6 caused an increase in conductivity with increasing circulation time. At pH 4.0 0.2, the rate of oxidation decreased with increasing concentration of chlorides. Perchlorates and nitrates have very little effect. Sulfates or phosphates even at millimolar concentrations are rapidly adsorbed by the catalyst and reduce the rates of oxidation by 20-70%. The catalyst can be readily regenerated by washing with 0.1 M NaHC03 solution. The data are considered in terms of kinetic salt effects, surface active sites having discrete adsorption properties, and Langmuir-Hinshelwood kinetic models. The model providing the best fit to the data assumes that inorganic anions may compete with organic solutes for oxidizing sites on the surface to form oxidizing inorganic radical anions. Some of these radical anions also react with surface-adsorbed organics to give CO, but at decreased rates. The implications for water purification processes based on photocatalytic oxidation using titanium dioxide are discussed.
*
Introduction Titanium dioxide when illuminated with light of wavelength
