Photocatalytic reductive–oxidative degradation of Acid Orange 7 by polyoxometalates

A series of polyoxometalates (POM) PW12O403−, SiW12O404−, P2W18O626− and P2Mo18O626− have been used as photocatalysts for the destruction of the azo dye Acid Orange 7 (AO). There are two ways to consider: reductive and oxidative decomposition. The reductive decomposition involves absorption of light by polyoxometalates, oxidation of an organic substrate, for instance propan-2-ol as sacrificial reducing reagent and reoxidation-recycling of the reduced polyoxometalates by the azo dye via a thermal (dark) reaction. AO is reduced to aromatic amine derivatives in a multi-electron process, as suggested by the analysis of the detected products and the obtained stoichiometry of the electron transfer process, following a first-order dependence for AO. This process takes place within a few minutes. On the other hand, photooxidative decomposition involves again absorption of light by POM followed by direct or OH-mediated oxidation of the dye. This process is an order of magnitude slower than reductive elimination, but leads to mineralization of AO. Several intermediates have been detected prior to evolution of CO2.The nature of the POM catalyst is decisive in the efficiency of reductive AO decoloration, following the order PW12O403− > SiW12O404− > P2W18O626− > P2Mo18O626−, that is their photooxidizing ability, in one-pot photolysis experiments. On the other hand the efficiency of thermal (dark) reaction between reduced POM and AO follows the reductive ability of POM, i.e., SiW12O405− > PW12O404− > P2W18O627− > P2Mo18O628−. The presence of electron scavengers suppresses the reductive degradation rate according to the trend Ag+ > O2 > Cu2+ > Ni2+, while increase of pH retards decoloration.Contrary to TiO2, sensitized photodecomposition, i.e., absorption of light by AO and electron injection to POM has not been observed.