Heterogeneous Redox Catalysis

Since the application of POMs is so widespread across areas such as homogeneous and heterogeneous catalysis, as well as acid and redox catalysis, it is not possible to exhaustively review all the applications. Thus the rest of this chapter focuses only on the catalytic properties of the polyoxometallates in heterogeneous gas- or liquid-phase oxidation reactions, and reviews the most recent progress in the knowledge of their properties and working process, underlining both their potential and their limitations. 

Compare the half-factor in Eq. (205) or the half-exponent in Eq. (206.] This effect, which arises from the heterogeneous nature of the electrochemical process (i.e., a surface reaction vis-a-vis a volume reaction in homogeneous phases ), is the basis of the efficiency of redox catalysis or mediated electron transfer (see Sec. III.E.3 and also Chapter 28 mainly devoted to this topic). Thus for a given redox system, as in the sequence in Eqs. (190) and (191), the use of a redox mediator M in Eq. (207) allows the reduction of R to be performed at potentials less cathodic than x/i in Eq. (205) (or the R oxidation at potentials less anodic than E1/2) for the same electrochemical setup (i.e., an identical mass transfer rate). 

Common to all the methods discussed earlier is that B is generated at the electrode surface, that is, by a direct electron exchange between the electrode and the substrate A. This approach is, however, sometimes hampered by the limitations imposed by the heterogeneous nature of the electron transfer reaction. For instance, studies of the kinetics of fast follow-up reactions may be difficult or even impossible owing to interference from the rate of the heterogeneous electron transfer process. In such cases, the kinetics of the follow-up reactions may be studied instead by an indirect method, generally known as redox catalysis [5,124-126]. Another application of redox... 

Chain inorganic reactions have been reviewed several times [9-14] whereas redox catalysis is an extremely large area dealing with metal-catalyzed oxidations [15, 16] (cf. Section 2.4) and bioorganic catalysis [17, 18] (cf. Section 3.2.1). Many important references concern electrochemistry (heterogeneous electron transfer) and therefore are not cited here but interested readers can find them in [4]. 

In addition to the wide range of metal oxide catalysts that can cany out oxidation via redox catalysis, there are a host of other materials that can carry out oxidation over non-reducible metal oxides. The oxidation mechanisms over non-reducible metal oxides are quite different and typically involve the production of free radical intermediates. The mechanisms tend to contain both heterogeneous and homogeneous activation and functionality. The oxide is used to activate a free radical process that can then proceed in the gas phase or at the surface. Li-substituted MgO and the rare earth metal oxides are two classes of materials that are considered non-reducible oxidation catalysts. Here we wiU specifically focus on the activation of alkanes over non-reducible metal oxides. 

BeckF, Gabriel W (1985) Heterogeneous redox catalysis at titanium/titanium dioxide cathodes. Reduction of nitrobenzene. Angew Chem 97 765-767... 

FIGURE 2.1. Schematic representation of various catalytic potentialities, (a) An uncatalyzed reaction at a bare electrode, (b) homogeneous redox catalysis, and heterogeneous redox catalysis at both (c) monolayer and (d) multilayer chemically modified electrodes are shown. 

The EC mechanism is central to the concept of homogeneous redox catalysis, which is used to promote a redox reaction between an electrogenerated mediator and soluble reactant, under conditions where heterogeneous electron transfer to the reactant is restricted on kinetic grounds. Many redox processes between soluble mediators and oxidoreductase enzymes have also been shown to reduce to the simple EC mechanism under limiting conditions. ... 

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