Kinetic analysis supporting multielectron reduction of oxygen in bismuth tungstate-photocatalyzed oxidation of organic compounds

Abstract

Light-intensity dependence of the rate of carbon-dioxide liberation in the photocatalytic decomposition of acetic acid by bismuth tungstate particles suspended in an aqueous solution under aerobic conditions was measured by monochromatic photoirradiation using a monochromator (lower intensity <20 mW) and high-intensity UV-LED (higher intensity <300 mW). The light-intensity dependence of both flake ball-shaped micrometer-sized particles (FB-BWO) and wet ball-milled nanometer-sized particles (ML-BWO) seemed to be bimodal, i.e., first and 0.5th orders in the lower and higher intensity ranges, respectively. Approximately 1.5th and second-order light-intensity dependences were also observed for ML-BWO at the lowest intensity range and for FB at the highest intensity range, respectively. The light-intensity dependences could be reproduced by a kinetic model that was derived on the basis of the assumption of oxygen reduction via two-electron (and possibly four-electron at the highest intensity region) transfer and a radical chain mechanism with peroxy radicals as chain carriers. The calculated threshold intensity between the first and 0.5th-order light-intensity dependences for FB-BWO was appreciably lower than that of ML-BWO, suggesting that the higher FB-BWO activity is attributable to the larger effective particle size for accumulation of two (or four) electrons.