Significant roles of oxygen and unbound •OH radical in phenol formation during photo-catalytic degradation of benzene on TiO2 suspension in aqueous system

Abstract

Studies on photo-catalytic degradation of benzene using TiO2 photo-catalyst as a suspension in water is reported. Degradation studies have been carried out using 350 nm UV light. Phenol, a photo-catalytic product of benzene, was monitored under varying experimental conditions such as amount of TiO2, concentration of benzene, photolysis time, ambient (air, O2, Ar, N2O and N2O–O2 mixture), etc. The phenol yields in both aerated and O2-purged systems increased with the photolysis time. In contrast to oxygenated systems, the yields of phenol in deoxygenated (viz. Ar-purged and N2O-purged) systems were quite low (~30 μM) and remained steady. H2O2 yields in all these systems were also monitored, and found lower than an order of magnitude as compared to phenol yields for the respective systems. The rate of phenol production in aerated 1 mM benzene solution containing 0.05% TiO2 suspension was evaluated at 12.3 μM min−1 which is lower than the rate obtained in an O2-saturated system (22.4 μM min−1). The low yields of phenol in both Ar- and N2O-purged systems, and also the increasing trends in oxygenated systems, together reveal that, for the phenol formation with an enhanced rate, oxygen is essential. In the present study, it is implied that the photo-generated hole, which is mainly an •OH radical, is either freely available in the aqueous phase or migrates to the aqueous phase from the catalyst surface, to react with benzene to produce HO-adduct radical. Later, following reaction with oxygen, the adduct produces phenol. On the other hand, h+ and surface adsorbed •OH radical, being trapped/bonded due to rigid association with the catalyst surface, were not able to generate phenol under similar experimental conditions. The mechanism of phenol formation with TiO2 photolysis in an aqueous system is rechecked, on the basis of present results on h+/surface adsorbed •OH radical/unbound •OH radical scavenging by benzene, collectively with previous reports on various systems.