Enhanced Photocatalytic Degradation of Methyl Orange by Au/TiO2 Nanoparticles under Neutral and Acidic Solutions

A comparative study was carried out on the degradation of methyl orange (MO) by TiO2 and Au/TiO2 photocatalysts in neutral and acidic solutions. Au/TiO2 photocatalysts with an Au : Ti atomic ratio of 1.5 +/- 0.1% : 98.5 +/- 0.1% were prepared by using a microwave-polyol method in the presence of P25 TiO2. Initial degradation rates of MO by TiO2 and Au/TiO2 were 0.13 and 0.22 min(-1) at pH 7, whereas they increased to 0.96 and 3.06 min(-1) at pH 2, respectively. These results indicate that the MO degradation rates are enhanced by loading Au nanoparticles on TiO2 in neutral and acidic solutions by factors of 1.7 and 3.2, respectively. Mass spectroscopic studies lead us to conclude that major reaction products are formed by demethylation and OH addition of benzenoid form of MO in neutral solutions, whereas they are produced through ring opening and carboxylation of quinonoid form of MO after scission of a center N-NH bond in acidic solutions. The relative importance of electron trapping and surface plasmon resonance (SPR) effects of Au nanoparticles is discussed for the enhancement of photocatalytic activity of Au/TiO2.