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 photoelectro chemical response of TiO2 films under bias in a time window of 1-100 s. Lon g-lived oxidized intermediates produced upon illumination at 0.6 V-SCE are detected by cathodic sweep run in the dark after irradiation. The main conc lusion derived from this work is that a scheme of competitive kinetics desc ribes only those cases in which one of the components is weakly or nonadsor bed on TiO2, whereas for two photolytes with high affinity for the surface cooperative effects may occur. The methanol-oxalate system is quantitativel y modeled by considering that oxalate forms surface complexes with differen t reactivity and a parallel pathway for hole transfer to -OH and adsorbed o xalate. In this case as well as for electrolytes containing methanol and sa licylate photooxidation of methanol (with low affinity for the surface) via intermediates formed by reaction with trapped holes (-(OH)-O-.) is partial ly or fully suppressed. For electrolytes containing oxalic and salicylic ac ids in which both components chemisorb on TiO2 the photoelectrochemical res ponse depends on preadsorption, the photooxidation pathways deviates those of single component systems, and there is remotion of salicylate adsorbed b yproducts assigned to cooperative effects.