On the mechanism of TiO2-photocatalyzed degradation of aniline derivatives

Interaction between aromatic amines and TiO2 takes place preferentially through the amino group when pH > pKa(BH+) and, as a minor but mechanistically relevant mode, via a π-interaction if pH < pKa(BH+). No significant direct photodegradation of aniline or N,N-dimethyl-aniline is detected in acidic medium using λ > 290 nm, but it is enhanced in alkaline medium. 2-Aminophenol and benzoquinone are the main photoproducts of direct irradiation of aniline. The main photoproducts of photocatalytic degradation of aniline at the pH of maximum adsorption are 2-aminophenol and phenol. Scavenging HO• with t-BuOH shows that adsorbed aniline is oxidized by positive holes (h+), with participation of the anilinium radical cation. In the case of N,N-dimethyl-aniline at the pH of maximum adsorption, N-methyl-aniline is the main photoproduct, formed also via the dimethyl anilinium radical cation. Photocatalytic degradation in acid medium is inhibited due to electrostatic repulsion between the positively charged surface and the protonated amines. Aniline is mainly transformed into phenol and 2-aminophenol, and N,N-dimethyl-aniline into aniline, that undergoes hydroxylation to phenol. In alkaline medium the main photoproduct of degradation of aniline is nitrobenzene, formed with involvement of the anilinyl radical.