Photodegradation mechanism of heterocyclic two-nitrogen containing compounds in aqueous TiO2 dispersions by computer simulation

The photocatalytic degradation of six-membered heteroaromatics (pyridazine, pyrimidine and pyrazine) in aqueous TiO2 suspension was investigated. The initial mechanistic sequence(s) in the TiO2-photocatdyzed oxidation of the constituent heterorings was predicted theoretically by molecular orbital (MO) calculations such as frontier electron densities and point charges on all the atoms. The cleavage of the heterocyclic moiety was examined by UV-VIS spectroscopy. The evolution of N-2 and CO2 gas was determined by gas chromatography. The formation of NH4+ and NO3- ions was assessed by ion-HPLC, and that of carboxylic acid intermediates by HPLC. The disappearance of six-membered heteroaromatics and the formation of intermediates were observed by LC-MSD. The heterorings containing an N=N fragment lead to the evolution of Nz gas, and those containing a C-N=C fragment lead to the generation of a large amount of NH4+ ions and a minor amount of NO3- ions. The photodegradation of these heteroaromatics is closely related to the position of the nitrogen in the heterorings. The mechanism of the photodecomposition of the heterocyclic aromatics was revealed through comparison of experimental data with the analytical simulation. (C) 2001 Elsevier Science B.V. Ail rights reserved.