Role of Hydroxyl, Superoxide, and Nitrate Radicals on the Fate of Bromide Ions in Photocatalytic TiO2 Suspensions

The influence of bromide ions on systems containing highly reactive radical species is of great interest for environmental remediation, atmospheric chemistry, and the synthesis of high-added-value compounds. In this regard, irradiated TiO2 suspensions are simple and suitable systems to highlight some mechanisms of general validity in the aforementioned research fields. In this work the spin-trapping technique has allowed highlighting the concurrent action of hydroxyl and superoxide radicals toward bromide ions. In fact, hydroxyl radicals oxidize bromide ions to bromine atoms, which in turn can be reduced back to bromide ions by superoxide radicals. The results suggest that this relay mechanism, mainly based on secondary redox products (such as reactive oxygen species, ROS) rather than on direct interfacial electron transfer, is responsible for the generally observed constant concentration of bromide ions in irradiated TiO2 suspensions. The presence of nitrate ions in the system significantly alters this equilibrium. In fact, nitrate radicals generated by hole-induced oxidation of nitrate ions selectively oxidize bromide ions to elemental bromine even under anoxic conditions, but in the presence of bromate as an opportune electron scavenger. This result not only reveals the importance of nitrate radicals toward the photocatalytic production of elemental bromine but also confirms the active role of nitrate ions in irradiated photocatalytic suspensions.