Photooxidation of Organic Mixtures on Biased TiO₂ Films

Processes that occur in the TiO₂-photocatalysis of binary aqueous solutions containing model photolytes with different affinity for the TiO₂ surface (methanol and oxalic and salicylic acids) are analyzed from the photoelectrochemical response of TiO₂ films under bias in a time window of 1−100 s. Long-lived oxidized intermediates produced upon illumination at 0.6 V_SCE are detected by cathodic sweep run in the dark after irradiation. The main conclusion derived from this work is that a scheme of competitive kinetics describes only those cases in which one of the components is weakly or nonadsorbed on TiO₂, whereas for two photolytes with high affinity for the surface cooperative effects may occur. The methanol−oxalate system is quantitatively modeled by considering that oxalate forms surface complexes with different reactivity and a parallel pathway for hole transfer to −OH and adsorbed oxalate. In this case as well as for electrolytes containing methanol and salicylate photooxidation of methanol (with low affinity for the surface) via intermediates formed by reaction with trapped holes (−^•OH) is partially or fully suppressed. For electrolytes containing oxalic and salicylic acids in which both components chemisorb on TiO₂ the photoelectrochemical response depends on preadsorption, the photooxidation pathways deviates those of single component systems, and there is remotion of salicylate adsorbed byproducts assigned to cooperative effects.