Mechanistic and kinetic study of solar-light induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions

The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO2 suspensions has been investigated with the use of a solar light simulating source. It has been found that, when the full range of emitted photons is used, decolorization and complete mineralization of the solution is achieved with satisfactory rates, depending on initial dye concentration. A reaction pathway is proposed according to which degradation of the dye molecules adsorbed on the photocatalyst surface takes place via a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids, which are further oxidized toward compounds of progressively lower molecular weight. Eventually, mineralization is achieved leading to the formation of gas phase CO2 and inorganic ions in solution. Kinetic results show that the initial rate of decolorization depends strongly on surface coverage and on incident photon energy. Visible light photons contribute to decolorization, via the photosensitization mechanism, but with reaction rates which are more than two orders of magnitude lower than the corresponding ones induced by photons of energy higher than that of the band gap of the semiconductor. In both cases, the initial rate of decolorization is significantly reduced for dye coverages close to monolayer.