A New Method To Determine the Generation of Hydroxyl Radicals in Illuminated TiO2Suspensions

The generation of hydroxyl (•OH) radicals produced by irradiation of aqueous TiO2 suspensions was investigated by electron paramagnetic resonance (EPR) and product analysis employing a stable free nitroxide radical (3-carboxyproxyl) as a spin trap of •OH. Product analysis demonstrated that the attack of •OH on 3-carboxyproxyl produces a diamagnetic product (proxyl−NH) with a trapping efficiency of ca. 80%. It could also be shown that a small amount of the nitroxides is reduced by conducting band electrons. The rate of formation of •OH could be determined by monitoring the time dependence of the decrease in the concentration of 3-carboxyproxyl monitored by EPR spectroscopy or the time dependence of the increase in the concentration of proxyl−NH monitored by gas chromatography analysis. The rate of formation of •OH serves as a mechanistic parameter to investigate the mechanism of formation of this reactive species by the photoexcitation of TiO2. The dependence of the rate of formation of •OH was shown to be a linear function of light intensity at low intensities. It was observed that molecular oxygen, a good scavenger of conduction band electrons, only slightly influences the rate of formation of •OH and supports the production of •OH from photogenerated holes. Addition of selected anions to the photolysis mixtures results in a diminished rate of disappearance of the spin trap as the result of a competition between the spin trap and the anion for •OH. From a Stern−Volmer analysis of the data, the rate constant for the reaction of hydroxyl radicals with the anions was determined.