Investigation of the photocatalytic activity of TiO2-polyoxometalate systems

The present study reports the investigation of polyoxometalate catalyzed el ectron transfer from the conduction band of photoexcited TiO2 to molecular oxygen. The oxidation of 1,2-dichlorobenzene (DCB) was used as an index rea ction for evaluating the photocatalyst systems TiO2- PW12O403-, TiO2-SiW12O 40,(4-) and TiO2-W10O324- in oxygenated aqueous solution. Addition of these polyoxometalate (POM) anions to TiO2 suspensions resulted in significant r ate enhancement for DCB oxidation. Photodegradation kinetics exhibited [POM ] dependence, experiencing different maximum (k = 0.0318 min(-1), 0.0108 mi n(-1), and 0.0066 min(-1)) for each POM at different [POM] (0.1 MM PW12O403 -, 0.07 mM SiW12O40,(4-) and 1 mM W10O32,(4-) respectively). The probabilit y that the difference in the adsorption affinity of POMs on TiO2 surface co uld account for the observed ranking of photodegradation rates was ruled ou t by adsorption isotherm experiments that revealed similar binding constant s for each POM (467 M-1, 459 M-1, and 417 M-1 for PW12O403-, SiW12O404-, an d W10O324-, respectively). DCB degradation over TiO2 with O-2 or POM+O-2 sy stems can be modeled by the Langmuir-Hinshelwood (saturation kinetics) mode l. The concentration-independent rate constants (k(L-H)) for TiO2-O-2, TiO2 - W10O324-, TiO2-SiW12O404-, and TiO2-PW12O403- were 0.0818, 0.152, 0.421, and 0.638 min(-1), respectively, An analysis of DeltaG for electron transfe r from the conduction band of TiO2 to POMs in this study shows that the ele ctron transfer takes place even when it is endothermic.