Degradation of methyl orange by Au/TiO₂, Ag/TiO₂ photocatalytic nanoparticles prepared using microwave heating

Abstract

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.