Photocatalytic oxidation of organic compounds by polyoxometalates of molybdenum and tungsten. Catalyst regeneration by dioxygen

Photosensitized Degradation of Dyes in Polyoxometalate Solutions Versus TiO2 Dispersions under Visible‐Light Irradiation: Mechanistic Implications

“…Figure 6, curve a) seems somewhat strange, since only a small amount of O 2 C À was detected. In accord with our present results, Papaconstantinou and Hiskia [31] suggested that dioxygen acts as a two-electron-transfer oxidant and that H 2 The effect of superoxide dismutase (SOD) on the RhB degradation rates in both systems is illustrated in Figure 7.…”

“…[40,41] Hence, de-ethylation of RhB in the presence of SiW 12 O 40 4À ions is initiated by excitation of the dye, and is followed by electron transfer to the LUMOs of the SiW 12 O 40 4À ions. This theory is further supported by the following evidence obtained from our experiments: 1) the redox potential of the SiW 12 O 40 4À ion [ + 0.05 V versus normal hydrogen electrode (NHE)], [6,31] which is more positive than that of the excited state of the dye (RhB*, À1.4 V versus NHE), favors electron transfer from the excited dye to the LUMO of the SiW 12 O 40 4À anions; 2) SiW 12 O 40 4À ions can serve as multi-electron relays; 3) interactions between the photocatalyst and the dye make the electron-transfer event possible; and 4) the electron transfer between the dye and SiW 12 O 40 4À ions can be further confirmed by the fact that fluorescence is quenched (Figure 8 B).…”

“…An inner-sphere electron transfer that involves covalently bonded M À O intermediates has been proposed by Hiskia et al [31] and Neumann et al [43] In this mechanism, the dioxygen either becomes associated to the tungsten atom to form a seven-coordinate metal center (Scheme 1, reaction 5, structure A and B), or is added to the terminal oxygen atom to form an ozonide (Scheme 1, reaction 5, structure C). Heteropolyperoxotungstate salts that contain seven-coordinate W atoms linked to oxygen atoms have been synthesized, [44,45] and provide further evidence that inner-sphere coordination between the POM and O 2 is possible.…”

Synthesis, characterization of cesium and cobalt substituted wells–Dawson heteropolyoxotungstates salts and their photocatalytic applications

“…Study Upon absorbance of UV or near-visible light, HPA catalyst is excited and serves as a strong oxidant to oxidize a great variety of organic substances either indirectly via • OH radicals-mediated oxidations, where the photocatalytic degradation of organic pollutants (substrate), in the presence of POM, the oxidation is supposed to be performed by highly oxidizing • OH radicals produced indirectly i.e., substrates react with • OH radicals resulting from reaction of excited POM with water molecules, [23].…”

“…Also, Dioxygen is a very effective oxidant for reduced HPA, thus its main action is the regeneration of the catalyst and may participate further in additive oxidative processes [23,24].…”