Use of scavenger agents in heterogeneous photocatalysis: truths, half-truths, and misinterpretations
Even though a gap exists in understanding the behavior of radical scavengers and interfering species, they have been extensively employed to elucidate degradation mechanisms or to improve the degradation efficiency in heterogeneous photocatalysis. Focusing on the influence of different species, such as scavengers (t-butanol, formic acid, methanol, p-benzoquinone, oxalate, superoxide dismutase, and azide), interfering species (sulfite, dichromate, bromate, carbonate, chloride, and iodide) and inorganic ions (nitrate, sulfate, and phosphate), this work investigated the production of hydroxyl radical and singlet oxygen during TiO2/UVA reactions. Electron paramagnetic resonance spectroscopy (EPR) was applied to investigate radicals formed in the presence of each interfering/scavenger species. Some scavengers and interfering species were studied during phenol degradation, chosen as a model substrate. All species, excepting bromate, hindered the degradation. Para-benzoquinone showed an increased hydroxyl radical production, attributed to the photo-reduction of quinones. Radicals other than hydroxyl radicals, such as carbon dioxide, hydroxymethyl, azidyl, and semiquinone, were identified in the presence of oxalate, methanol, azide, and para-benzoquinone, respectively. Some of these radicals can possibly interact with organic substrates due to their reduction potential; as a result, a critical interpretation must be done when these species are added to a heterogeneous photocatalysis process.