Polyoxometalates properties

The development of catalysts for the oxidation of organic compounds by air under ambient conditions is of both academic and practical importance (1). Formaldehyde is an important intermediate in synthetic chemistry as well as one of the major pollutants in the human environment (2). While high temperature (> 120 °C) catalytic oxidations are well known (3), low temperature aerobic oxidations under mild conditions have yet to be reported. Polyoxometalates (POMs) are attractive oxidation catalysts because these extensively modifiable metal oxide-like structures have high thermal and hydrolytic stability, tunable acid and redox properties, solubility in various media, etc. (4). Moreover, they can be deposited on fabrics and porous materials to render these materials catalytically decontaminating (5). Here we report the aerobic oxidation of formaldehyde in water under mild conditions (20-40 °C, 1 atm of air or 02) in the presence of Ce-substituted POMs (Ce-POMs). 

The interest in the redox, catalytic, and electrocatalytic properties of unsubstituted and substituted polyoxometalates arouses much attention [2-15] because they are a versatile family of molecular metal-oxide clusters with applications in catalysis as well as in medicine and material science. Such versatility must be traced to at least two main characteristics. First, the size and mass of these unique molecular oxides place their solution chemistry in an intermediate position between small molecule solution chemistry and infinite lattice solid-state chemistry. Second, their redox behaviors may be very flexible and finely tuned on purpose, by changing smoothly their composition, with a... 

PossibiKties of electrocatalysis of reactions at electrodes are among the powerful incentives for the electrochemical study of POMs. Interesting results were obtained both in electrocatalytic reductions and oxidations, provided the appropriate form of the POM is used. Two recent reviews devoted to the electrochemical properties of polyoxometalates as electrocatalysts are available [8, 9]. The second one focuses more specifically on electrocatalysis on modified electrodes. In the present text, attention will be drawn specially to the basic principles that could be considered to govern most of solution processes. The principles will be illustrated by several recent experimental results, even though earlier achievements will also be described briefly. 

Various polyoxometalates can be reduced electrochemically and reversibly by several electrons at modest potentials (Section VILA), and these properties are exploited in photocatalysis and eiectrocatalysis. In both cases, redox properties of heteropolyanions (Fig. 49) and the organic reactants (Table XXXV) are the principal properties that control the catalytic performance. The selection of the electrode is also important in eiectrocatalysis. Photocatalysis by hereopoly-anions has been reported extensively, but there are only a few reports of eiectrocatalysis by these compounds. 

Polyoxometalates are important catalysts but they are also finding application in optical, electrical, and magnetic devices. Mixed-metal polyoxometalates with vanadium(V) in the polyoxoanion core confers enhanced properties to such structures, principally in their ability to form essentially infinite networks that can be utilized as coatings or as other thin film materials. Additionally, these materials have tunable electromagnetic and photochromic properties. In combination with organic polymers, so-called hybrid polymers, special electrochemical properties are conferred, making possible such electrical storage devices such as capacitors and batteries that utilize the redox properties of the polyoxometalate [7],... 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

Photoluminescence properties of rare-earth polyoxometalates as molecular phosphors Naruke and Yamase (2004)... 

Interest in polyoxometalate complexes of the rare earths has been driven to a large extent by their photophysical and photochemical properties. Table 1 lists several reviews. In general, photoexcitation into LMCT (O -> W, O -> Mo) bands results in intramolecular energy transfer to the rare earth with subsequent emission and luminescence. 

Here we start to examine the pivotal role that polyoxometalate clusters can play in the development of nanoscale devices that utilize POM components, and start to conceptualize some example systems in which POM components could have a crucial role [13, 19]. This is because such functional nanosystems can exploit the building block principle already established in this area of chemistry, coupled with the range of physical properties, and the fact that POM systems can really be seen as molecular metal oxides [20]. To demonstrate this point, a number of examples have been selected across the area of POM chemistry, including our contributions, to help highlight new directions and concepts. It should also be noted that metal oxides already play an important role in the electronics and semiconductor industry today and their solid-state properties have been studied extensively [21, 22]. Many of these concepts are not new in isolation, but the possibility of using molecular design in metal oxides to produce... 

There is little doubt that diverse physical properties common to polyoxometalates places them in an almost unmatched class of materials which could be extremely useful as hybrid materials and nanocomposites [31]. In this section we will focus on POM systems with properties that could be exploited in the development of molecular-scale devices. 

In one such approach we seek to exploit the potential signal transduction properties of the thermochromic Dawson [40] polyoxometalates and combine this with a fluorescent POM-hybrid to produce a device that could respond optically as a function of the local environment, see Figure 2.12. In this example, the clusters would be positioned on a gold surface using SAMs (Self-assembled monolayer) with a cationic tail and local positional control could be aimed using self-assembly, or even by means of an atomic force microscope tip. 

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