TiO₂-catalyzed photodegradation of aromatic compounds: relevance of susceptibility to oxidation and electrophilic attack by hydroxyl radical

The application of nanostructured titanium dioxide (TiO₂) as catalyst for the photodegradation of drugs and dyes is well established. We aimed to evaluate the importance of the reactivity of aromatic compounds submitted to photodegradation. Specifically, we were interested in the correlation between susceptibility to oxidation and/or to electrophilic attack and the efficiency of degradation. We demonstrated that hydroxyl radical (HO˙) is the most relevant species generated in the photodegradation process. Considering that HO˙ has both oxidizing and electrophilic features, the efficiency of degradation of selected aromatic compounds was performed. The choice was based on their susceptibility to oxidation and/or to electrophilic attack. Benzoic acid (C1), salicylic acid (C2), and protocatechuic acid (C3) were compared regarding their oxidability using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay and were ranked as follows: C3 ≫ C2~C1. These compounds were efficiently photodegraded and no significant difference was observed among them. To assess the importance of susceptibility to electrophilic attack, anisole (C4), acetophenone (C5), and nitrobenzene (C6) were selected. Compared to C5 and C6, the higher susceptibility of C4 to electrophilic attack was demonstrated using hypochlorous acid, an electrophilic reagent. The photodegradation showed that C4 was also more susceptible to degradation compared to C5 and C6. In summary, we found that by acting as a powerful oxidant/electrophile agent, HO˙ was able to promote the degradation of aromatic moieties. Considering that the majority of drugs and dyes bear aromatic moieties, our findings explain the great success of photodegradation using metal oxides as catalysts.