A comparative study was carried out on degradation of methyl orange (MO) by TiO2, Au/TiO2, and Ag/TiO2 photocatalysts in neutral and acidic solutions. Au/TiO2, and Ag/TiO2 photocatalysts were prepared by microwave-polyol method in the presence of P25 TiO2 . The Au/Ti and Ag/Ti atomic ratio in Au/TiO2 and Ag/TiO2 catalysts was 1.5%. Initial degradation rates of MO by TiO2,Au/TiO2, and Ag/TiO2 at pH=7 were 0.13, 0.22, and 0.12 min-1, whereas those at pH=2 were 0.96, 3.1, 2.9 min-1, respectively.These results indicate that the degradation rates are enhanced for TiO2, Au/TiO2, and Ag/TiO2 by factors of 7.4, 14, and 24 in acidic conditions, respectively. On the basis of effects of additions of isopropyl alcohol, oxalic acid, and p-benzoquinone as scavengers of ･OH, h+, and ･O2-, major active species for the degradation of MO using TiO2, Au/TiO2, and Ag/TiO2 were determined. Results show that all three active species take part in the degradation of MO in neutral conditions and their contribution was ･OH < h+ < ･O2- . On the other hand, major active species in acidic solution was ･O2-. Combining these results with mass spectroscopic studies on the degradation products led us to conclude that the great enhancement of degradation rates using Au/TiO2 and Ag/TiO2 photocatalysts in acidic conditions arises from a great increase in the ･O2- concentration due to electron trapping effects of Au and Ag nanoparticles loaded on TiO2 under UV irradiation. Mass spectroscopic data suggested that major product channels are ring opening and carboxylation in the reactions of ･O2- with MO in acidic conditions.
