Polyoxometalates photocatalytic

The applications of polyoxometalates in catalytic dehalogenation of halocar-bons have been succinctly reviewed by Hill and coworkers [188]. This reaction involves the photocatalytic transformation of organic halides coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides) (Eq. (9)) [189, 190] ... 

Hiskia, A., M. Ecke, A. Troupis, A. Kokorakis, H. Hennig, and E. Papaconstantinou (2001). Sonolytic, photolytic, and photocatalytic decomposition of atrazine in the presence of polyoxometalates. Environ. Sci. Technol., 35 2358-2364. 

Gkika E, Troupis A, Hiskia A, Papaconstantinou E. Photocatalytic reduction of chromium and oxidation of organics by polyoxometalates. Appl Catalysis B Environ 2006 62 28-34. 

This activity is particularly useful for degradation of strongly hazardous substances or recalcitrant pollutants that are difficult to remove in chemical or biochemical processes. In this respect any pathway leading to abatement of chromate(VI) pollution arouses interest. One such pathway seems to be created by cooperation between iron and chromium photocatalytic cycles, which were reported as effective in conversion of chromate(VI) into Crm species [20-23,97]. A synergistic photoreduction of CrVI and Cu11 mediated by Ti02 [98], or photocatalytic reduction of Crvl and oxidation of organic matter by environmental polyoxometallates as photocatalysts [99], may constitute alternative possibilities. 

Hill, C. L. Prosser-McCartha, C. M. Photocatalytic and Photoredox Properties of Polyoxometalate Systems. In Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds, Kalyanasundaram, K., Gratzel, M., Eds. Kluwer Academic Publishers Dordrecht, 1993, pp 307-330. 

Hiskia, A. Androulaki, E. Mylonas, A. Troupis, A. Papaconstantinou, E. Photocatalytic Decontamination by Polyoxometalates. In Polyoxometalate Chemistry from Topology Via Self-assembly to Applications, Pope, M. T., Mueller, A., Eds. Kluwer Academic Publishers Dordrecht, 2001 pp 417-424. 

Park H and Choi W (2005), Photocatalytic conversion of benzene to phenol using modified Ti02 and polyoxometalates , Catal. Today, 101,291-297. 

An example of a system for photocatalytic water oxidation with a molecular catalyst based on abundant metals is shown in Fig. Id. The Cobalt polyoxometalate catalyst is oxidized with Ru —trisbipyridine that is generated by quenching of the photo-excited Ru —trisbipyridine sensitizer with peroxodisulfate as sacrificial electron acceptor. The system operates at pH 8 and exhibits a high (30 %) photon-to-02 yield while the stability of the catalyst allowed for turnover numbers >220 that were limited by depletion of electron acceptor only [7]. This performance of the abundant-metal-based catalyst is superior to that of an analogue ruthenium polyoxometalate water oxidation catalyst. 

Troupis A, Gkika E, Hiskia A, Papaconstantinou E (2006) Photocatalytic reduction of metals using polyoxometallates recovery of metals or synthesis of metal nanoparticles. C R Chimie 9 851-857... 

Hill, C. and Prosser-McCartha, C. (1993). Photocatalytic and Photoredox Properties of Polyoxometalates Systems, in M. Gratzel and K. Kalyanasundaram (eds). Photosensitization and Photocatalysis Using Inorganic andOrganometallic Compounds, Kluwer Academic Publishers, Dordrecht, pp. 307-330. 

Mylonas, A., Hiskia, A., Androulaki, E., et al. (1999). New Aspect of the Mechanism of Photocatalytic Oxidation of Organic Compounds by Polyoxometalates in Aqueous Solutions. The Selective Photooxidation of Propan-2-ol to Propanone The Role of OH Radicals, Phys. Chem. Chem. Phys., 1, pp. 437- 440. 

Hiskia, A. and Papaconstantinou, E. (1992). Photocatalytic Oxidation of Organic Compounds by Polyoxometalates of Molybdenum and Tungsten. Catalyst Regeneration by Dioxygen, Inorg. Chem., 31, pp. 163-167. 

Park, H. and Choi, W. (2005). Photocatalytic Conversion of Benzene to Phenol Using Modified Ti02 and Polyoxometalates, Catal. Today, 101, pp. 291-297. 

Fig. 2. Photocatalytic Oxidation/Dehydrogenation of Substrates, SubH2, by Polyoxometalates. Pox represents the oxidized and resting form of the polyoxometalate Pf d represents the reduced form of the polyoxometalate.

A number of reactions involving alkane photooxidation by polyoxometalates have been run under aerobic conditions. Those by PWi2O40 and Wio032 are summarized at the end of Table I (see section 5). Unfortunately these reactions, collectively, are rather uninformative with respect to mechanism. Radical chain autoxidation processes are undoubtedly present and few if any of the reported papers have addressed the requisite experiments to differentiate autoxidation from other oxidation pathways. A similar general limitation exists in the literature on photocatalytic aerobic oxidation of organic compounds by semiconductor materials [66]. 

Schaming D, Allain C, Farha R, Goldmann M, Lobstein S, Giraudeau A, Hasenknopf B, Ruhlmann L (2010) Synthesis and photocatalytic properties of mixed polyoxometalate-porphyrin copolymers obtained from Anderson-type polyoxomolybdates. Langmuir 26 5101-5109... 

In the previous section, transition-metal-substituted polyoxometalates (TMSPs) supported on Si02 [61, 62] have been mentioned. TMSPs have been also tested when immobilized on Ti02- For instance, the H5PW10V2O40 supported on Ti02 was active in the photocatalytic decomposition of dimethyl sulphide (DMS) in gas-... 

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