Elucidation of the photocatalytic-mechanism of phenolic compounds

In the present work, phenol and other phenolic compounds (4-chlorophenol, 4-nitrophenol and methyl p-hydroxybenzoate) as typical aromatic pollutants were selected to investigate and elucidate their photocatalytic-mechanism over two commercial TiO2 catalysts: Hombikat UV-100 and Aeroxide (R) P25, with different physico-chemical properties. To try to clarify which pathway is more relevant in the corresponding photodegradation mechanism, the following scavengers were employed: formic acid (scavenger of photogenerated holes), methanol (strong hydroxyl radical scavenger) and cupper (II) nitrate (scavenger of electrons from conduction band). Higher photodegradation rates were always reached with P25, regardless of the studied parent compound. The radiation absorbed by these TiO2 catalysts were calculated by means of Monte Carlo simulations, and volumetric rate of photon absorption (VRPA) values were always higher for P25 because of its greater scattering and absorption coefficients. At the same time, higher values of the quantum efficiency were found in the photocatalytic degradation of phenol, methyl 4-hydroxybenzoate and 4-chlorophenol with P25, whereas in the case of 4-nitrophenol Hombikat catalyst was more photo-efficient despite the drop of the VRPA caused by the 4-nitrophenol light absorption. Regardless of the used photocatalyst, phenol, methyl 4-hydroxybenzoate and 4-chlorophenol were mostly degraded by HO center dot radicals generated from photo-generated holes, whereas an electron mediated mechanism was the most relevant in the case of 4-nitrophenol. Finally, it was demonstrated that the photodegradation rate of this parent compound could be appreciably increased by adding a hole acceptor, such as formic or oxalic acid, which promotes electrons from titania valence band (VB) to the conduction band (CB).