Electrochemical processes electrocatalytical reduction

Electrochemical and Electrocatalytic Reduction. The one-electron reduction of C02 yields the radical anion C02-, which reacts with an H source to give the formate ion. The reaction, however, is not selective because various other reactions may take place. An alternative and more promising approach is the two-electron reduction of C02 in the presence of a proton source to afford formic acid. The latter process requires a considerably lower potential (—0.61 V) than does the one-electron reduction (—1.9 V) consequently, the electrolysis in the presence of catalysts may be performed at lower voltages. The control of selectivity, however, is still a problem, since other two-electron reductions, most importantly reduction to form CO and H2, may also occur.101127 The reduction of C02 to CO, in fact, is the subject of numerous studies. Electrochemical and electrocatalytic reductions of C02 in aqueous solutions have been studied and reviewed.11,128-130... 

The isomerization of 1-butene to cis- and trans- 2-butene onPd/C/Nafion and Pd-Ru/Nafion electrodes is one of the most remarkable and astonishing electrochemical promotion studies which has appeared in the literature.39,40 Smotkin and coworkers39,40 were investigating the electrocatalytic reduction of 1-butene to butane on high surface area Pd/C and Pd-Ru cathodes deposited on Nafion 117 when, to their great surprise, they observed at slightly negative overpotentials (Fig. 9.31) the massive production of 1-butene isomerization, rather than reduction, products, i.e. cis- and trans-2-butenes. This is extremely important as it shows that electrochemical promotion can be used also to enhance nonredox catalytic reactions such as isomerization processes. 

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. 

Some Ni(II) complexes show catalytic activity for the electrocatalytic reduction of C02 in water, where an intermediate formation of Ni(I) species has been proposed. To obtain a useful electrocatalyst in the electroreduction of C02, the selectivity of the process is highly important. As many electrochemical systems available for reducing C02 require the presence of water, the reduction of molecular hydrogen is always a competing reaction that needs to be avoided. 

Although it is usually referred to electrochemical reduction of C02, it is preferable to discuss electrocatalytic reduction because it is a catalytic process involving reduction through the direct transfer of electrons more than an electrochemical process only. The same is also valid for the water oxidation step, but there is confusion in literature about these aspects and it is not clear whether they are electrocatalytic or electrochemical processes. 

A comparison with gas phase catalysis reveals that the electrocatalytic reduction of organic halides operates between conventional electrochemical and catalytic processes (57). However, an examination of the adsorption characteristics, identification of intermediates, or multiple surface states and kinetic results are necessary for elucidating the role of electrocatalysis in cleavage and double bond reduction. 

The need to convert chemical energy to electricity, efficiently and at low temperature, has increased the development of materials with electrocatalytic activity toward multi-electron charge transfer. Reactions of technical relevance are, for example, cathodic processes, such as the oxygen reduction reaction (ORR),3 13 and anodic processes, such as small organics (R-OH, where R = CH3-3-12 or CH3CH-13-17) and sugars.18-20 These complex electrochemical processes are useful in low-temperature systems such as the direct methanol FC (DMFC) or biofuel cell systems. 

The electrocatalytic reduction of dimethylacetylenyl carbinol at an electrode activated with Raney Ni has been studied and it was found that more activity and selectivity than by the ordinary catalytic process could be achieved. In the catalytic process 150 mg KCNS per g of catalyst is sufficient to deactivate the catalyst completely however, in the electro-catalytic process a ten-fold increase in the poison led to selective poisoning and a 99% yield of dimethylvinyl carbinol was obtained. The authors suggested that an electrochemical process is superimposed on the catalytic process. 

Certain electrocatalytic, or at very least electrochemical, processes correspond to classical preparative techniques in organic chemistry such preparations as those involving sodium-alcohol and zinc-dust-acid reductions, or indeed the many processes using different heterogeneous reductive agents... 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

In principle, the choice of a specific electrochemical technique is not crucial in the study of electrocatalytic processes. However, from a practical and qualitative point of view, cyclic voltammetry is, in most cases, suited for a rapid assessment of an electrocatalytic process triggered by POMs. The interest stems from the following general behavior most POMs undergo a series of reversible one- and two-electron reductions and these reduced forms act usually as the active species, inducing an increase of the corresponding... 

At a high cathodic potential (region II), a sharp transition is observed at the potential referred to as ET. The authors demonstrate that the sudden increase of the electrode kinetics could not be attributed to the sole electrochemical reduction of the electrode material, nor to the electrolyte reduction. They conclude that after the transition, the main electrode process is still an oxygen electrode reaction with a major change of mechanism, leading to the onset of an important electrocatalytic effect. This assertion is sustained by the analysis of ... 

An alternative application of cobalt Mb has been reported by Willner and coworkers (92). They immobilized the reconstituted cobalt Mb on the functionalized electrodes and generated cobalt(I)-Mb by the electrochemical reduction of cobalt(II)-Mb. The hydrogenation of acetylenedicarboxylic acid smoothly occurred on the functionalized electrode, and the electrocatalytic reaction in H20 and D20 reveals a clear isotope effect, kH/kD = 2.7, indicating that the hydride transfer from cobalt(III)-hydride in Mb is the key reactive process. 

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