Photobleaching of methylene blue sensitised by TiO 2: an ambiguous system?

Abstract

The photobleaching of methylene blue MB, sensitised by TiO 2, in an aqueous solution is studied in the absence and presence of oxygen. In the absence of oxygen and in the presence of a sacrificial electron acceptor (SED), MB is photoreduced to its colourless leuco form, LMB, by the TiO 2 photocatalyst. This same photoreduction process is observed even if an SED is not present, indicating that MB itself can act as an SED. The oxidation of LMB by oxygen to regenerate MB is significantly slower if the aqueous solution is acidified (0.01 mol dm −3 HClO 4) and, at low partial pressures, the rate of reaction depends directly upon the concentration of dissolved oxygen. The TiO 2-sensitised photobleaching of MB is irreversible in an oxygen-saturated aqueous solution, as expected, since the bleaching was due to an oxidative process. However, in an acidified solution (0.01 mol dm −3 HClO 4), the photobleaching process, in an oxygen-saturated solution, generates LMB initially. The latter situation arises because, under acidic conditions, LMB reacts only very slowly with oxygen to form MB. The significance of these findings with respect to the popular use of photobleaching of MB as a demonstration of semiconductor photomineralisation is discussed.