PHOTOCATALYTIC OXIDATION OF ORGANIC-COMPOUNDS BY POLYOXOMETALATES OF MOLYBDENUM AND TUNGSTEN - CATALYST REGENERATION BY DIOXYGEN

Catalyst regeneration (reoxidation) is key to the effective photocatalvtic oxidation of a variety of organic compounds by polyoxometalates. This paper reports on the reoxidation mechanism of photoreduced polyoxometalates by dioxygen. The overall general reaction, presented generally as M(n+1)- + 1/4O2 + H+ --> M(n-) + 1/2H2O [where M(n-) = P2W18O626- (P2W186-), SiW12O404- (SiW124-), H2W12O406- (H2W126-)], appears to be first order with respect to polyoxometalates and oxygen and zero order with respect to H+ at pH < 1.5 and pH > 3. At the inflection point, the order with respect to H+, is 1.1 +/- 0.4. Rate constants at low pH are 3.8 x 10(-3), 1.0, and > 5.5 s-1 and at high pH are 0.0, 2.7 x 10(-2), and 0.15 s-1 for P2W187-, SiW125-, and H2W127-, respectively (Figure 3). Reoxidation of molybdates is known to be considerably slower; P2Mo18(8-) is not reoxidized by dioxygen, whereas P2Mo1810- reoxidation is from 4 to more than 6 times slower than that of the polyoxotungstates in acid solution. As previously shown with the molybdates, the rates are a function of the degree of reduction; for instance, the rate constants for doubly reduced P2W188- are about 5 times larger than those for the corresponding singly reduced P2W187- anions under identical conditions. The rates parallel the redox potential of the polyoxometalates. Limited work was also done with hydrogen peroxide, which seems to behave differently from dioxygen; the results are compared.