ELECTROCHEMICAL PRODUCTION OF HYDROXYL RADICAL AT POLYCRYSTALLINE NB-DOPED TIO2 ELECTRODES AND ESTIMATION OF THE PARTITIONING BETWEEN HYDROXYL RADICAL AND DIRECT HOLE OXIDATION PATHWAYS

The use of TiO2 as a photocatalyst for the destruction of organic chem ical pollutants in aqueous systems has been extensively studied. One o bstacle to the effective utilization of these systems is the relativel y inefficient use of the solar spectrum by the photocatalyst. In addit ion, light delivery to the photocatalyst can be impeded by UV-absorbin g components in mixed effluent streams. We present a novel use of TiO2 as a catalyst for the oxidative degradation of organic compounds in w ater that uses a potential source instead of light to generate reactiv e oxidants. Application of an anodic bias of >+2 V vs NHE to titanium electrodes coated with niobium-doped, polycrystalline TiO2 particles e lectrochemically generates hydroxyl radicals at the TiO2 surface. This process has been demonstrated to efficiently degrade a variety- of en vironmentally important pollutants. In addition, these electrodes offe r a unique opportunity to probe mechanistic questions in TiO2 catalysi s. By comparing substrate degradation rates with increases in current density upon substrate addition, the extent of degradation due to dire ct oxidation and OH oxidation can be quantified. The branching ratio f or these two pathways depends on the nature of the organic substrate.; Formate is shown to degrade primarily via a hydroxyl radical mechanis m at these electrodes, whereas the current increase data for compounds such as 4-chlorocatechol indicate that a higher percentage of their d egradation may occur through direct oxidation. In addition, the direct oxidation pathway is shown to be more important for 4-chlorocatechol, a strongly adsorbing substrate, than for 4-chlorophenol, which does n ot adsorb strongly to TiO2.