Reduction, electrode material effects

In the other examples, the electrode materials are not involved in the reactions chemically, but constitute the source [sink] of electrons. Such electrodes are called nonconsumable. The term inert electrodes sometimes used is unfortunate insofar as the electrode itself is by no means inert rather, it has a strong catalytic effect on the electrode reaction. For reactions occurring at such electrodes, the terms oxidation- reduction... 

A comparison of Figures 46 and 45 shows that the use of a different electrode materials (Pt vs. Hg) leads to a different degree of reversibility in both the reduction processes. As pointed out in Chapter 3, Section 1.1, this proves that the effect is due to surface phenomena connected with the electrode material rather than to the slow rate of the electron transfers. 

The stepwise electron reduction of C02, whether direct or indirect, catalyzed, or by direct transfer on an apparently inert conductive surface, has been the object of considerable attention since the first concise reports of formate anion production. Since then, the list of possible derivatives has grown from formates to carbon monoxide, methane, ethylene, and short-chain saturated hydrocarbons. As noted in Section 12.1, this area of research has been expanded in recent years [8, 80, 83], with information relating to increased yields, to the effect of electrode materials on selectivity, as well as further speculations on possible reaction mechanisms, having been obtained on a continuous basis. Yet, the key to these synthetic processes-an understanding of the relationship between the surface of the electrode and the synthetic behavior of the system-seems no closer to being identified. 

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 ... 

The electrode potential exerts an important influence upon the course of the reduction, but there is undoubtedly a catalytic effect as well the latter is particularly evident with cathodes of copper or platinum. Lob4 and Haber8 consider that electrode potential is the determining factor, but Tafel6 emphasises the importance of the catalytic effect of the electrode material. Undoubtedly the product... 

Electrode Materials and Their Electrochemical Behavior. There is abundant evidence that the rate of electron transfer across an electrode-solution interface is dependent on the physical and chemical properties of the electrode material. The term electrocatalysis has been coined for this effect, and studies of the oxidation of hydrocarbons61 and the reduction of water and hydronium ion62 have provided ample evidence for its existence. 

There is a general view that the electrochemical reduction of 02 in aprotic media is independent of media and electrode materials, but the cyclic voltam-mograms of Figure 9.3 and the electrochemical data of Table 9.9 provide clear evidence that both have a significant effect on the reversibility. Although the... 

The significant effects of media and electrode material on the reduction potentials and voltammetric peak currents for dissolved dioxygen require substantial knowledge of the media and appropriate calibration of the electrochem-... 

A tremendous amount of work has been devoted to ORR on all kinds of surfaces but Bockris and Khan [245] were forced to conclude that no generalizations as to mechanism have been made, and, correspondingly, no key to the treasure chest of fast catalysis... has been found. Despite decades of research and even though the effect of electrode material on the 02 reduction reaction (ORR) has been reviewed extensively, Yang and McCreery [246] concluded even more recently that detailed mechanisms remain elusive and that for carbon surfaces there is no consensus on the mechanism... 

Fig. 4.3 Effect of a change in applied electrode potential on the reduction of O to R (R considered absent in bulk solution and in the electrode material).

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

In reviewing the intrinsic electrochemical behavior of nonaqueous systems, it is important to describe reactions of the most common and unavoidable contaminants. Some contaminants may be introduced by the salts (e.g., HF in solutions of the MFX salts M = P, B, As, etc.). Other possible examples are alcohols, which can contaminate esters, ethers, or alkyl carbonates. We examined the possible effect of alcoholic contaminants such as CH3OH in MF and 1,2-propylenegly-col at concentrations of hundreds of ppm in PC solutions. It appears that the commonly used ester or alkyl carbonate solvents are sufficiently reactive (as described above), and so their intrinsic reactivity dominates the surface chemistry if the concentration of the alcoholic contaminant is at the ppm level. We have no similar comprehensive data for ethereal solutions. However, the most important contaminants that should be dealt with in this section, and which are common to all of these solutions, are the atmospheric ones that include 02, H20, and C02. The reduction of these species depends on the electrode material, the solvent used, and their concentration, although the cation plays the most important role. When the electrolyte is a tetraalkyl ammonium salt, the reduction products of H20, 02 or C02 are soluble. As expected, reduction of water produces OH and... 

The electrode material is important for several reasons. The magnitude of the hydrogen and oxygen overvoltage determines the accessible potential range special surface properties, such as adsorptive and catalytic effects, may determine the course of the reduction. In the electrocatalytic reactions the electrochemical step consists in a reduction of hydrogen ions to adsorbed hydrogen, which then reacts with the substrate as in a catalytic reaction. The study of the influence of the electrode material on the course of the reaction is an area in which further research is very much needed. 

RE2CUO4 perovskites exhibit important and varying magnetic and electrical characteristics, and they are broadly studied as potential high-Tc superconductive materials. At room temperature, they show p-type semiconducting behaviors, and are used as electrode materials in fuel batteries. The catalytic properties of the perovskite oxides also make them effective in various oxidation and reduction reaction, hence they are considered as promising substitutes to the classical Pt/Rh-based catalysts applied to automotive pollution control. 

The catalytic effect of the electrode material on the branching of an electrode reaction is most often apparent at cathodes with low overvoltage but may also be observed at materials with high hydrogen overvoltage. In the reduction of acetone in 6 N H2SO4 at a platinized platinum electrode, two independent paths are accessible One leads to propane and the other to isopropyl alcohol. The rate of formation of these two products depends on the voltage and on the history of the electrode. 

The catalytic effect of high-overvoltage electrode materials may be exemplified by the reduction of ketones in hydrochloric acid at mercury and cadmium or amalgamated zinc electrodes at mercury, alcohols, pinacols, organic mercury compounds, or sometimes hydrocarbons may be formed (Chapter 10), whereas at cadmium, the reaction is to some degree analogous to the Clemmensen reduction and yields methylene compounds [114]. 

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