Photocatalytic reductive destruction of azo dyes by polyoxometallates: Naphthol blue black

Various polyoxometallates (POM) PW12O403−, SiW12O404−, P2W18O626− and P2Mo18O626− have been used as photocatalysts for the reductive destruction of the azo dye naphthol blue black (NB). In the process POM absorb light and mediate the electron transfer from a sacrificial donor, propan-2-ol, to the azo dye. NB is rapidly destructed in the presence of POM, while toxic intermediate products, such as aromatic amine derivatives (aniline, p-nitro-aniline and p-phenylene-diamine) are observed in a multi-electron process. Increase of catalyst and/or propan-2-ol concentration accelerates the photodegradation of dye till a saturation value, while an optimum concentration of the added dye is required. The nature of the POM catalyst dictates the efficiency of the reductive decoloration of NB, following the order PW12O403− > SiW12O404− > P2W18O626− according to their photooxidizing ability. No reaction is noticed with the 2-e-reduced molybdate, P2Mo18O628−, which exhibits a more positive redox potential.