Correlating the properties of hydrogenated titania to reaction kinetics and mechanism for the photocatalytic degradation of bisphenol A under solar irradiation

Abstract

Hydrogenation of a commercially available TiO2 anatase catalyst was carried out at several annealing temperatures in the range 400–800°C to improve its photocatalytic activity for the degradation of endocrine disruptor bisphenol A (BPA) under simulated solar irradiation. The prepared hydrogenated catalysts, as well as their counterparts calcined in air were characterized with respect to their morphological, optical and electronic properties by means of BET, XRD, XPS, DRS and UPS analyses. Thermal treatment under flowing hydrogen resulted in increased absorption at wavelengths below 400nm, as well as in the appearance of a broad and almost uniform absorption band in the visible region, the intensity of which increased with increase of annealing temperature. The latter was attributed to the creation of gap states in the hydrogenated samples, which was not observed for the samples calcined in air. Interestingly, sodium inherently present in the bulk of the pristine catalyst was found to diffuse at the surface and this was more pronounced for the hydrogenated samples prepared at temperatures above 700°C.The relative catalytic activity was tested to degrade 240μg/L BPA in pure water and it was found that the hydrogenated catalysts were more active than those calcined in air at the same temperatures. The maximum rate (0.0647min−1) was observed for the catalyst hydrogenated at 600°C, i.e. three times greater than the respective calcined catalyst. Higher annealing temperatures had a detrimental effect on photocatalytic activity and this may be associated with a collapse of the specific surface area. Other than the annealing temperature, the rate was also strongly dependent on the water matrix (slower for more complex matrices), BPA and catalyst concentration and the presence of electron acceptors.LC–MS/TOF analysis was employed to identify transformation by-products (TBPs) and elucidate reaction pathways. BPA degradation by hydrogenated catalysts seems to occur mainly through consecutive hydroxylation/oxidation reactions, as evidenced by the various oxygenated TBPs formed; conversely, scission of BPA through the isopropylidene group and further oxidation, yielding different para-substituted phenolic intermediates seems to be the main degradation route in the presence of calcined catalysts.