Polyoxometalates Heteropolyacids

M. Fournier, C. Fuemi-Jantou, C. Rabia, G. Herve, and S. Launay, Polyoxometalates Catalyst Materials X-Ray Thermal Stability Study of Phosphorus-containing Heteropolyacids Hj xPMi2. xVx04o13-14H20 (M = Mo, W x = 0-1), J. Mater. Chem., 2, 971-978 (1992). 

In this context, much effort has also been invested in controlling the nuclearity of the catalyst ensemble through the selection of its precursor. One area in which considerable progress has been made involves the adsorption of polynuclear clusters onto supports [33]. Examples involving the immobilization of small, preformed polynuclear clusters on supports are the reactions of carbonyl clusters of the late metals [16, 34], the binding of polyoxometalates (POMs) and their neutral alkoxy analogues [35] and heteropolyacids such as the Keggin cluster [36, 37]. 

For acid forms, polyacids = polyoxoacids, including heteropolyacids (e.g., H3PW12O40) and isopolyacids (e.g., H2Mo60, ) and for oxoanions, polyanions = polyoxoanions = polyoxometalates, including heteropolyanions (e.g., PW12O40") and isopolyanions (e.g., Mo60 9 ). 

Polyoxometalates (POMs), also known as heteropolyacids (HPA),3 are a class of compounds formed from negatively charged inorganic metal-oxygen building blocks. When charge-balanced with cationic species, POMs self-assemble into unusual 3D structures with specific topological and electronic properties.214 POMs are commonly formed from polyanions of early transition metals such as W, Mo or V. These anions can be substituted with other transition metals. The diversities in POMs composition and structure make them attractive for many applications, particularly as Bronsted acid and redox catalysts. For example,... 

In the literature (and here) the term "polyoxometalate" (POM) is used exchangeable with the term "heteropoly compounds" or heteropolyacids - abbreviated as HPA. 

Zirconium forms heteropolyacids with molybdenum, such as Hg [Zr(M02O7)6], whose alkali metal derivatives have been isolated. In the first member of the series, of composition ZrM02O7(OH)2(H2O)2, the zirconium atom has a pentagonal bipyramidal arrangement see Polyoxometalates). 

Heteropolyacids are polyoxometalates incorporating anions (heteropolyanions) having metal-oxygen octahedra as the basic structural units. Among a wide variety of heteropoly acids those belonging to the so-called Keggin series are the most important for catalysis. They include heteropolyanions where X is the central atoms (P, Si , etc.), y... 

Films of oxides can be produced by anodization of metal electrodes. For example, AI2O3 forms on an aluminum anode immersed in a solution of H3PO4. The thickness of the film can be controlled by the applied potential and the time of anodization. Such a film can be used as a support for other materials, such as poly(vinylpyridine) (PVP). Oxide films of other metals, such as Ti, W, and Ta, can be produced in a similar way. Oxide films can also be produced by CVD, vacuum evaporation and sputtering, and deposition from colloidal solution. Related inorganic films are those of polyoxometallates (iso- and heteropolyacids and their salts) (20). For example, the heteropolyanion P2W17M0O62K6 shows a number of reduction waves at a glassy carbon electrode. A wide variety of metallic polyanionic species (e.g., of W, Mo, V) exist and have a rich chemistry. Films of such materials are interesting for their electrocatalytic possibilities. 

G.M. Maksimov, Advances in synthesis and study of polyoxometallates and heteropolyacids, Russ. Chem. Rev., 64,445,1995. 

Kozhevnikov IV (1998) Catalysis by heteropolyacids and multicomponent polyoxometalates in liquid-phase reaetions. Chem Rev 98 171-198... 

Mansuy D, Bartoli JF, Battioni P, Lyon DK, Finke RG (1991) Highly oxidation resistant inorganic-porphyrin analog polyoxometalate oxidation catalysts. J Am Chem Soc 113 7222-7226 Langpape M, Millet JCM (2000) Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid Part I. An experimental study on isobutane oxidation catalysts. J Mol Catal A Gen 200 89-101... 

Nakato T, Toyoshi Y, Kimura M, Okuhara T (1999) Unique catalysis of an acidic salt of heteropolyacid and consisting of microciystallites. Catal Today 52 23-28 Rong C, Anson FC (1994) Simplified preparations and electrochemical behavior of two chromium-substituted heteropolytungstate anions. Inorg Chem 33 1064-1070 Harrup MK, Hill CL (1994) polyoxometalate catalysis of the aerobic oxidation of hydrogen-sulfide to sulfur. Inorg Chem 33 5448-5455... 

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