Electrochemical reduction electrochemistry

Electrochemical reduction of carbon dioxide has found no extensive application so far, yet it is of great interest for scientists in the fields of theoretical and applied electrochemistry. To a certain extent, it is analogous to the photochemical carbon dioxide reduction, but it involves no chlorophyll and yields simpler products. In recent years some books and reviews on this topic have been published (e.g., Taniguchi, 1989 Sullivan et al., 1993 Bagotsky and Osetrova, 1995). 

Under potential deposition is a much-studied phenomenon in electrochemistry and is the electrochemical reduction of a metal cation to form a monolayer or submonolayer of the corresponding metal at the surface of an electrode. The critical point is that deposition occurs at a potential higher than that dictated by the reversible potential of the metal/metal cation couple, suggesting that such a upd layer is energetically quite different from the bulk metal. However, subsequent deposition on a upd monolayer occurs at the expected potential, and the resulting surface is typical of the bulk metal. 

It has long been accepted that the most difficult step in the reduction of C02 to CH3OH, etc. is the initial activation of the molecule itself. Electrochemical reduction provides a relatively simple means of activating the molecule and hence has been investigated in depth almost ever since the birth of electrochemistry as a branch of applied science. However, a cheap and efficient electrochemical method of reducing C02 to useful products continues to elude scientists, primarily as a result of several major problems ... 

Lee, C.-L., et al., Preparation of Pt nanoparticles on carbon nanotubes and graphite nanofibers via self-regulated reduction of surfactants and their application as electrochemical catalyst. Electrochemistry Communications, 2005. 7(4) p. 453-458. 

The electrochemistry of derivatized Cjq has also been widely investigated [8, 23-28], As observed by electrochemical reduction, derivatization usually decreases the electron affinity of the CgQ-sphere. Typically, cathodicaUy (more negative) shifted waves have been observed by cyclovoltammetry and other methods. Depending on the addend, the shifts range from 30 to 350 mV per adduct with respect to those of pure Cgfl. Reduction of some derivatives resulted in the loss of the addend. In some cases, like the retro-Bingel-reaction (Section 3.2.2), this can also be advantageous. 

Electrochemical reduction and oxidation processes offer several advantages over conventional methods in their application to organic synthesis. For example, selective transformations can be carried out on specific groups in a multifunctional, valuable compound under the usually mild reaction conditions. Independence of a reagent will result in drastically diminished environmental problems by spent reagents. Electrochemistry also allows the application of alternative feedstocks and better use of raw materials. Product isolation and continuous processing are simplified. 

In both polarographic and preparative electrochemistry in aptotic solvents the custom is to use tetraalkylammonium salts as supporting electrolytes. In such solvent-supporting electrolyte systems electrochemical reductions at a mercury cathode can be performed at —2.5 to —2.9 V versus SCE. The reduction potential ultimately is limited by the reduction of the quaternary ammonium cation to form an amalgam, (/ 4N )Hg , n = 12-13. The tetra-n-butyl salts are more difficult to reduce than are the tetraethylammonium salts and are preferred when the maximum cathodic range is needed. On the anodic side the oxidation of mercury occurs at about +0.4 V versus SCE in a supporting electrolyte that does not complex or form a precipitate with the Hg(I) or Hg(II) ions that are formed. 

Our own interest in applying electrochemistry grew from the highly successful process developed for the manufacture of the third-generation cephalosporin antibiotic, Ceftibuten.93 The electrochemical reduction component of the process and the product extraction step are, in outline, as follows ... 

Figures 5.4 and 5.5 summarize results of a recent study of P. versicolor laccase electrochemistry based on cyclic and rotating disk voltammetry [60]. Figure 5.4 shows unequivocally that this laccase is voltammetrically active and gives a kinetically controlled, unpromoted four-electron peak at edge-plane pyrolytic graphite. Electrochemical reduction of 02 catalyzed by an immobilized laccase monolayer is close to reversible, and unrestricted by mass transport. The electrocatalysis follows, moreover, a Michaelis-Menten pattern (Fig. 5.5). Finally, there is a characteristic bell-shaped functional pH-profile with a pronounced maximum at pH 3.1.

Polyhaloacetic acids and their partially hydrodehalogenated products represent a second important family of herbicide-/pesticide-derived substrates. In their review on the environmental applications of industrial electrochemistry, Juttner and co-authors (Juttner et al. 2000) documented the electroreductive dechlorination of dichloroacetic acid (a by-product of monochloroacetic acid), a way to recover the valuable compound and avoid wastes. The electrochemical reduction of polychloro- and polybromo-derivatives was performed by Korshin and Jensen (2001) on Cu and Au cathodes. Complete dehalogenation was obtained for all substrates, but for monochloroacetic acid. To overcome the intrinsic poor reactivity of the monochloro-derivative the photoelectrochemical properties of a p-doped SiC electrode were investigated (Schnabel et al. 2001) however, the dehalogenation stopped at monochloroacetic acid. 

Catalytic reactions in electrochemistry — When the product of an electrochemical reduction reaction is regenerated by a chemical reoxidation, or when the product of an electrochemical oxidation is regenerated by a re-reduction, the regeneration reaction is called a catalytic reaction. For thermodynamic reasons the chemical oxidant (or the reductant) has to be electro-chemically irreversible in the potential range where the catalyst is electroactive. The reduction of Ti(IV) in the presence of hydroxylamine is an example for an oxidative regeneration [i, ii] ... 

These early observations serve to introduce a subject—the formation of mobile ions in solution—that is as basic to electrochemistry as is the process often considered its fundamental act the transfer of an electron across the double layer to or from an ion in solution. Thus, in an electrochemical system (Fig. 2.1), the electrons that leave an electronically conducting phase and cross the region of a solvent in contact with it (the interphase) must have an ion as the bearer of empty electronic states in which the exiting electron can be received (electrochemical reduction). Convo sely, the filled electronic states of these ions are the origin of the electrons that ente the metal in the... 

The anion [Re(CO)5] (13) is formed by reduction of (1) with Na/Hg. This extremely reactive species yields a variety of Re carbonyl clusters (see Polynuclear Organometallic Cluster Complexes). Further reduction to [Re(CO)4] is possible with Na in HMPA. The electrochemistry of M2(CO)io shows that these species oxidize at the same potential and, in MeCN, give [M(CO)s(MeCN)]+. However, (1) is more difficult to reduce than Mn2(CO)io and electrochemical reduction leads to the typical clusters obtained from (13) under chemical conditions. 

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