Polyoxometalates (POM) are transition-metal oxygen-anion clusters that exhibit interesting catalytic and photoredox properties depending on their composition and architecture. It is well known that some POMs are activated by near-UV photons and photocatalyze a variety of oxidation reactions involving organic molecules. The photoreduced forms of these heteropolyanions are able to reduce protons to molecular hydrogen. According to hydrogen production efficiency the order is correlates with the strength of the C–H bonds in the organic compounds. The polyoxometalate used in my experiments is the Dawson type K₆(P₂W₁₈O₆₂). Experiments were carried in water and in aqueous solutions with different alcohols,the data were recorded with a Clark type electrode. Electrochemical data have showed that the re-oxidation of the photoreduced species (P2W18O62)^7- will occur via reduction of H⁺ when oxygen is not present. Generally, the photochemical reactions proceed until the redox potential of the reduced polyoxometalate is negative enough,as shown in fig., to reduce H⁺. POMred + 2H⁺ → POMox + H₂. The rate of H₂ production is limited by the low quantum yield of the POMred formation. Re-oxidation of POMred by H⁺, presented generally seems to be 100% efficient. The overall reaction proposed for the hydrogen evolution in aqueous solution when an alcohol is added is : R-OH + [P2W18O62]^6- + hυ = R=O + [P2W18O60(OH)2]^6-
[P2W18O60(OH)2]^6- = [P2W18O62]^6- + H₂ .
To improve the rate of hydrogen evolution, Ruthenium-POMs complexes will be synthesize to increase the quantum yield of photoreduction since these clusters absorb light in the visible region.