The influence of applied bias potential on the photooxidation of methanol and salicylate on titanium dioxide films

The mechanism of the photoelectrochemical oxidation of methanol and salicyl ic acid on anatase film electrodes was studied as a function of the applied potential and pollutant concentration at pH 3. The dependencies of the ste ady state photocurrents on substrate concentration reflect the type of surf ace interaction: weak in the case of methanol, that leads to a simple satur ation curve, and strong in the case of salicylate, that shows a steady stat e photocurrent peaking at intermediate concentrations. At 0.6 V vs. SCE the oxidation rate is largely enhanced as compared to open circuit conditions (E-oc = -0.3 V). Even under nitrogen, the reaction proceeds at an appreciab le rate, and the ratio of circulated charge to the number of oxidized salic ylate ions approaches 28 electrons per mol at low salicylate concentration: oxidized salicylate mineralizes almost totally, and the intermediates are rapidly destroyed. At higher substrate concentrations, the ratio decreases, and uv spectral evidence suggests the formation of some undefined oxidatio n products. Under oxygen at 0.6 V, the radicals generated in the initial ph otoelectrochemical step are mostly oxidized by O-2, increasing the amount o f salicylate destroyed for a given total circulated charge; at sufficiently high substrate concentration, the above ratio decreases to values below 4. No evidence of the presence of traces of partially oxidized molecules is f ound. Adequate control of the experimental conditions permits therefore to achieve substantially increased efficiencies of salicylate destruction per absorbed photon, and the build up of uncontrolled intermediates can be prev ented. The results are discussed in terms of the oxidation length Y, define d as the number of oxidation steps that are triggered by a single hole tran sfer event, and of the oxidation efficiency epsilon, defined as the ratio o f the oxidation length to the maximum possible oxidation length (the length achieved when one hole transfer suffices to trigger total mineralization). (C) 1998 Elsevier Science Ltd. All rights reserved.