Redox potential photocatalysis

There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported (Amadelli et al., 1991). Taking into account the standard reduction potentials, U(VI) can be photocatalytically reduced by Ti02 conduction band electrons to U(V) and then to U(IV) (E° = +0.16 V and +0.58 V, respectively, Bard et al., 1985). However, more reduced U(III) and U(0) forms cannot be generated because of very negative redox potentials (Bard et al., 1985). In addition, U(V) rapidly disproportionates to U(VI) and U(IV), and its chemistry is very complex (Selbin and Ortego, 1969). For example, uranyl... 

The efficiency of the electron transfer reactions illustrated in Fig. 1 governs a semiconductor s ability to serve as a photocatalyst for a redox reaction. This efficiency is, in turn, a function of the positions of the semiconductor s conduction and valence bands (band-edge positions) relative to the redox potentials of the adsorbed substrates. For a desired electron transfer reaction to occur, the potential of the electron acceptor should be below (more positive than) the conduction band of the semiconductor, whereas the potential of the electron donor is preferred to be above (more negative than) the valence band of the semiconductor. For an efficient organic synthesis via oxidative photocatalysis, the substrates must have potentials more negative than the valence band of the semiconductor. For an efficient organic synthesis via reductive photocatalysis, it is the reverse. 

Figure 29.1 Electronic scheme of a photochemical process for heterogeneous photocatalysis (a) and redox potential and band gap energy position of several semiconductors (b).

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. 

In summary, potential improvements could be made to the PUREX process in the following areas (1) separation of Np from U and Pu prior to the U/Pu split and (2) in the requirement to use a large excess of U(IV) reductant to reduce Pu(IV) to Pu(III). The majority of published work on the applications of photo catalysis in actinide redox chemistry has concentrated on solving the first of these difficulties through Np valence control. A smaller volume of literature exists on the applications of photocatalysis in valence state control of U and the radioactive d block metal, technetium. This section will review both of these aspects. 

The main objective of this chapter is to illustrate how fundamental aspects behind catalytic two-phase processes can be studied at polarizable interfaces between two immiscible electrolyte solutions (ITIES). The impact of electrochemistry at the ITIES is twofold first, electrochemical control over the Galvani potential difference allows fine-tuning of the organization and reactivity of catalysts and substrates at the liquid liquid junction. Second, electrochemical, spectroscopic, and photoelectrochemical techniques provide fundamental insights into the mechanistic aspects of catalytic and photocatalytic processes in liquid liquid systems. We shall describe some fundamental concepts in connection with charge transfer at polarizable ITIES and their relevance to two-phase catalysis. In subsequent sections, we shall review catalytic processes involving phase transfer catalysts, redox mediators, redox-active dyes, and nanoparticles from the optic provided by electrochemical and spectroscopic techniques. This chapter also features a brief overview of the properties of nanoparticles and microheterogeneous systems and their impact in the fields of catalysis and photocatalysis. 

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