Photocatalytic degradation of N-heterocyclic aromatics-effects of number and position of nitrogen atoms in the ring.

This study demonstrates the influences of position, number of nitrogen (N) atoms and -C-N- or -N=N- linkage present in the six membered heterocyclic compounds such as pyridine, pyrazine, and pyridazine on their photocatalytic degradation by Au, Ag, and Fe(+2) deposited TiO(2) photocatalyst. The photodegradation rate of these heterocyclic compounds follow the order pyridine > pyrazine > pyridazine due to the different extent of hydroxylation and difference in position and number of N atoms in the aromatic moiety. The Au photodeposition significantly improved the TiO(2) photoactivity as compared to Ag and Fe(+2) loading. The presence of two N atoms in pyrazine and pyridazine as compared to one N atom in pyridine hamper the nucleophilc attack of OH radicals in comparison to easy hydroxylation of pyridine ring. There is 1 N atom, 4C-C, 1C-N and 1C=N bond in pyridine, 2 N atoms in the 1 and 4 positions, 2C-C, 2C-N bonds and 2C=N bonds in pyrazine, and pyridazine ring contains 2 N atoms in the 1 and 2 positions, 3C-C, 1N-N bond and 2C=N bonds. The bond strength/energy decreases gradually as: C=N- (615 KJ/mol) > -N=N- (418 KJ/mol) > -C-C- (347 KJ/mol) > -C-N- (305 KJ/mol) > -N-N- (163 KJ/mol). As pyridine has 1C-N, 1C=N, and no N-N bond, it photodegrades easily as compared to 1 N-N and 2C=N bonds of pyridazine of lowest photodecomposition rate. The improved photoactivity of Au-TiO(2) is explained on the basis of its favorable redox potential, work function, and electron-capturing capacity, etc.