Reduction, heteropoly compounds

Hence, to a first approximation, the rate of reduction of these heteropoly compounds by CO expresses the oxidizing ability of the surface, whereas, as described above, the rate of reduction by H2 reflects the oxidizing ability of the catalyst bulk. If the former rate is divided by the surface area and the latter normalized to the catalyst mass, both oxidizing abilities decrease monotonically with the extent of neutralization with alkali (Figs. 53a and 53c). Although it is not shown in Fig. 53, CS2 5H0.5PM012O40, a class B salt that has a high surface area, is reduced exceptionally rapidly. 

The cyclic voltammogram of Fig. 54a is not Hilly symmetrical. The distortion probably originates from the catalytic discharge of protons and evolution of hydrogen in the solid phase. These results suggest the possibility that by using cyclic voltammetry with a single crystal, the reduction potential of solid heteropoly compounds can be measured and that the effects of constituent elements described below can be made clearer. 

The mixed-addenda atoms affect the redox properties mixed-addenda heteropoly compounds are used as industrial oxidation catalysts. For example, the rate of reduction by H2 is slower and less reversible for solid PMO 2-,VJto m+, than for solid PM012O40, although the former are stronger oxidants than the latter in solution (279, 280). The effects of substituting V for Mo on the catalytic activity are controversial (279, 281-284). Differences in redox processes between solutions and solids, the thermal or chemical stability of the heteropoly compounds, and the effects of countercations in solids have been suggested to account for the discrepancies. 

Some heteropoly compounds - and especially heteropolymolybdates - are strong oxidizing agents and can be very readily changed to fairly stable, reduced heteropolymolybdates. The reduction products are colored an intense deep blue. 

The investigation of the oxidation-reduction properties of heteropoly compounds of molybdenum in aqueous and nonaqueous solvents has received increasing attention in recent years. Such knowledge may not only elucidate the redox behavior of such compounds but it could help in the investigation of new preparative procedures for... 

A considerable amount of literature has appeared which describes the use of heteropolymolybdates and their tungsten analogs in various applications. Of these, the main applications have centered on catalysis. Of particular importance to the catalytic behavior of heteropoly compounds is the solubility and solvolytic behavior in both aqueous and organic media and the thermal stability and oxidation-reduction behavior. 

Terminal alkenes can be hydrogenated selectively in the presence of PdCI2 [63] or RhCl(PPhj)3 [64] and heteropoly compounds. The catalytic system is also highly active for the production of urethane or isocyanate compounds by the reductive carbonylation of nitrobenzene. It is considered that polyoxometalate coordinating with Pd2+ in the reduced form is the active species, since easily reducible heteropolyanions are more active [63]. 

Mizuno, N., Watanabe, T. and Misono, M. (1985). Catalysis by Heteropoly Compounds. VIII. Reduction-Oxidation and Catalytic Properties of 12-MoIybdophosphoric Acid and its Alkali Salts. The Role of Redox Carriers in the Bulk, J. Phys. Chem., 89, pp. 80-85. 

Even as little as 20 years ago, arsenic compounds were mainly analyzed by LLE. Organic and inorganic forms were separated by arsenic reduction in its inorganic combinations, leading to formation of halogen metal acids or heteropoly acids extracted with toluene or trichloromethane. Organic arsenic was assayed as the difference between the total quantity and the quantity extracted by performing the... 

Most POM reductions (chemical, photochemical or electrochemical) result in the production of heteropoly blue species that involve reduction of MVI to Mv, M = Mo or W, and some coupling of spins (frequently antiferromagnetic coupling in poly anions with an even number of electrons). Single-electron-reduced species are by far the most common, but two- and four-electron species are not uncommon, and there are several examples of heteropoly blue compounds with six or more electrons. Despite the implication of the name heteropoly blue, there are several examples of reduced isopolyanions (isopoly compounds) that have similar properties. The structural and electronic properties of these reduced complexes are addressed in older reviews cited in the Introduction (Section 4.11.1) and also in the accompanying chapter by M. T. Pope (Chapter 4.10). 

Heteropoly acids [oxygen compounds of Mo(VI), W(VI), Si, P(V), As(V), Ge, and other elements] and their reduction products (molybdenum blues) are extracted into oxygen-containing solvents by a mechanism similar to that above. 

In the spectrophotometric determination of Si, Ge, P(V), As(V), and V(V) the yellow heteropoly acids occurring in acid solutions in the presence of an excess of molybdate or tungstate are important. The yellow heteropoly acids are the basis of less sensitive spectrophotometric methods, but the blue reduction products (e.g., phosphomolybdenum blue) are the basis of very sensitive spectrophotometric methods for determining these elements. The conditions for formation and extraction of these compounds have been investigated [133-135]. 

This type of reaction has been used for the determination of anionic rather than cationic species in inorganic analysis. Thus, halides have been determined using their inhibitory effect on the oxidation of organic compounds by metal ions, as well as in photochemical reactions on the other hand, the oxidizing power of some metal ions such as cerium(IV) and vanadium(V) has been exploited for their own determination. Photochemical reduction reactions have also been used for the determination of anions such as nitrate in effluents. Most of the few reported simultaneous kinetic determinations relying on redox reactions are based on the reduction of heteropoly acids formed between molybdate and silicate, phosphate, or germanate ions. 

The first example of isomerism in the molybdic heteropoly acids was described by STRICKLAND for silicomolybdic acid. In order to compare the chemical properties of the isomers by polarography, the use of a dropping mercury electrode appeared inadequate, since mercury reacts with these compounds. A platinum electrode gives reproducible polarograms displaying perfect reversibility which makes possible the study and characterization of these compounds and of their reduction-products. 

In acid medium, a reduction of the a-isomer carried over the 4 e step results in a destruction of the structure of the heteropoly acid, as is proved through the formation of Mo(V) (which is not extractable in isoamylic acid, in opposition to these compounds). 

All methods for the determination of inorganic phosphate in seawater are based on the reaction of the ions with an acidified molybdate reagent to yield a phosphomolybdate heteropoly acid, which is then reduced to a highly coloured blue compound. In early work, tin(//) chloride was used as the reductant in flow-analysis (Hager et al, 1968). However, this reductant has several disadvantages, including the appreciable temperature dependence of the reduction rate and the pronounced salt error. 

Since both of the yellow silicomolybdic acids have only low intensity colours, several methods have been developed in which the complexes are reduced to intensely coloured blue complexes. These heteropoly acids are well-defined soluble compounds and not colloidal products as are the blue phospho- and arsenomolybdic complexes. The most conunon reducing agents are metol (p-methylaminophenol sulphate) and sulphite Strickland and Parsons, 1968), and ascorbic acid Koroleff, 1971). The manual method and flow-anal5rsis described below use ascorbic acid as the reductant. 

---