Fe(III) photoinduced and Q-TiO2 photocatalysed degradation of naphthalene: comparison of kinetics and proposal of mechanism

Degradation of naphthalene was investigated in two systems producing hydroxyl radical: in homogeneous solution of Fe(III) perchlorate and in heterogeneous colloidal solution of Q-TiO2. The Fe(III) photoinduced degradation of naphthalene approximately followed first order kinetics with rate constant strongly dependent on the content of most photoactive Fe(III) species, Fe(OH)2+. In the case of the degradation photocatalysed by Q-TiO2, more complicated kinetics controlled by a charge-transfer surface complex with naphthalene degradation intermediates was observed. The primary degradation intermediates (2-formylcinnamaldehyde, 1,2-naphthoquinone, 1,4-naphthoquinone, 1-naphthol, 2-naphthol) were the same for both systems. The reaction mechanism of the degradation was proposed assuming the attack of hydroxyl radical followed by the addition of dioxygen, resulting in the direct opening of the aromatic ring.