Kinetic study of photocatalytic oxidation of adsorbed carboxylic acids at TiO2 porous films by photoelectrolysis

A unique transient photoelectrolysis technique was developed to study the photocatalytic oxidation kinetics of adsorbed organic compounds at particulate TiO2 film electrodes. The technique was employed to study the photocatalytic oxidation of a number of adsorbed dicarboxylic acids at TiO2 porous film electrodes. The adsorption of these compounds was found to be heterogeneous, with three major types of surface complexes identified-each having distinctive binding affinities to the TiO2 surface. The three thermodynamically distinctive types of surface complexes exhibit two measurable photocatalytic reaction kinetic characteristics, i.e., a fast process and a slow process. The fast kinetic process can be attributed to the photocatalytic degradation of the strongest adsorbed species at more active sites such as edge and corner titanium ions. The slow kinetic process can be attributed to the photocatalytic degradation of the medium-strength bound complexes and the weakest bound surface complexes. The rate constants for these processes were calculated by curve fitting the photocurrent transient response to a double exponential decay expression. For adsorbed oxalic acid both the fast and the slow processes were shown to be the first-order processes in which both rate constants were independent of surface coverage. For the larger dicarboxylic acids adsorbed, the rate constant for the fast photocatalytic process (k(f)) was found to decrease with an increase in the surface coverage. The rate constant for the slow photocatalytic process, however, was shown to be a true first-order process for all adsorbed dicarboxylic acids. The value of the rate constant (k(s)) was similar for all the adsorbates studied and was independent of the surface coverage. (C) 2004 Elsevier Inc. All rights reserved.