Photo- and Electrochromism of Polyoxometalates and Related Materials

Publisher Summary This chapter describes the essence of the catalytic chemistry of heteropoly compounds in solution and in the solid state. The catalytic properties of heteropoly compounds have drawn wide attention, owing to the versatility of these compounds as catalysts, which has been demonstrated both by successhl large-scale applications and by promising laboratory results. Heteropolyanions are polymeric oxoanions formed by condensation of more than two different mononuclear oxoanions. Heteropolyanions formed from one kind of polyanion are called isopolyanions. Acidic elements such as Mo, W, V, Nb and Ta, which are present as oxoanions in aqueous solution, tend to polymerize by dehydration at low pH, forming polyanions and water.Heteropoly catalysts can be applied in various ways. They are used as acid as well as oxidation catalysts. They are used in various phases, as homogeneous liquids, in two-phase liquids (in phase-transfer catalysis), and in liquid-solid and in gas-solid combinations, etc.

Catalytic Chemistry of Heteropoly Compounds

Present General Status of Understanding of Heteropoly Electrolytes and a Tracing of Some Major Highlights in the History of Their Elucidation

Ever since the discovery and development of polyoxometalates (POMs), it has been known that they can exist in electron-rich reduced forms of different archetypes, structural flexibilities and functionalities. There are now reliable synthetic strategies for electron-rich POMs — materials that have unique and potentially useful catalytic, electronic and magnetic properties. This Review covers the synthesis and applications of these reduced species, and also highlights their differences and advantages relative to fully oxidized POMs. More than 200 reduced POM structures are described in this Review, with emphasis placed on how reduction influences POM structure, function and properties. The metals in polyoxometalates need not be in their highest oxidation states. Indeed, polyoxometalates can exist in reduced forms, and several different metals can be incorporated into various structural archetypes. This Review describes the synthesis and characterization of these complexes, along with their topical catalytic, electronic and biological properties.

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Synthesis, structures and applications of electron-rich polyoxometalates

Hybrid polyoxometalate materials for photo(electro-) chemical applications

Polymolybdates(VI) have been photochemically reduced to Mov in aqueous solutions containing acetic acid. Carbon dioxide, methane, and succinic acid were isolated as decomposition products of the acetic acid but the quantum yield of their formation was ca. 20 times smaller than that of Mov formation. E.s.r, spin-trapping experiments exhibited the presence of spin-adducts of hydroxyl radical as an oxidation product of water. The photoredox reaction of the polymolybdate(VI) with water was a main process. The reaction mechanism discussed is based on the similarity with the photochemistry for an alkylammonium polymolybdate(VI). The photoredox reaction of the polymolybdate(VI) with water or acetic acid proceeds via the photoinduced formation of a charge-transfer complex between the polymolybdate(VI) and acetic acid, which is preceded by the transfer of an acetic acid proton to an oxygen atom at a photoreducible octahedral site in the polymolybdate(VI).

https://pubs.rsc.org/en/content/articlepdf/1983/dt/dt9830002205

Tadatoshi Kurozumi

Papers

Photoreduction of polymolybdates(VI) in aqueous solutions containing acetic acid

Isopolyanions of molybdenum and tungsten as photocatalysts for hydration of acetylene