Electrochemical processes, direct oxidation

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

Oxides of various metals are a broad class of electrode materials useful in many electrochemical processes (Trasatti, 1980-1981). The surfaces of practically all metals (both base and noble) become covered by layers of chemisorbed oxygen upon anodic polarization. The composition and properties of these layers depend on potential, on the electrolyte, and on the electrolysis conditions. They are often rather thick and have a distinct phase character, so that the metal electrode is converted to a typical oxide electrode. One can also make electrodes directly from oxides deposited in some way or other on various conducting substrates. 

There seems to be an opportunity to extend the electrochemical process to direct membrane transport that is, with electrodes plated on either side of a facilitated-transport membrane similar to that of Johnson [24]. The shuttling action of the carrier (Fig. 9) could then be brought about by electrochemical reduction and oxidation instead of pressure difference. 

Fuel cells generate electricity through an electrochemical process in which the energy stored in a fuel is converted directly into electricity. Fuel cells chemically combine the molecules of a fuel and oxidant, without burning, dispensing with the inefficiencies and pollution of traditional combustion. 

Direct electrochemical reduction and oxidation treatment of pollution involving a mass-free reagent - the electron - is a very attractive idea, because it is a uniquely clean process, as (1) the reduction and oxidation take place at inert electrodes and (2) there is no need to add chemicals. The techniques of cathodic reduction/anodic oxidation of wastewaters containing dyes are relatively new and have drawn the attention of investigators in Japan, China, USA and Russia [55]. 

The committee notes that other research in mediated electrochemical processes has shown that the coulom-bic or electrochemical efficiency of the process is directly proportional to the concentration of the material being oxidized and rapidly decreases as the destruction approaches 100 percent—that is, as the concentration of oxidizable material becomes very small (Chiba et al., 1995). 

It has been argued that all steps in the reaction must be electrochemical in nature for the process to be called direct oxidation. According to this definition, any process that involves cracking of the hydrocarbon on the anode material, followed by electrochemical oxidation of the cracking products, should not be considered to be direct oxidation. The primary reason for using this narrow definition for direct oxidation is that the open-circuit voltage (OCV) of the cell will be equal to the theoretical, Nernst potential if there are no other losses and if all steps in the oxidation mechanism are electrochemical. 

Flash rusting exhibited in neutral to alkaline water borne formulations appears to occur through a localised corrosion process probably Involving grit "activity" present from blasting, either directly or indirectly, in an electrochemical process. At such pH the rapid oxidation of ferrous to ferric ion produces... 

In iron production, iron ores are reduced to produce iron metal. The opposite process occurs when iron metals are oxidized to produce iron oxides or rust. Rust is primarily iron(III) oxide. Iron does not combine directly with oxygen to produce rust but involves the oxidation of iron in an electrochemical process. There are two requirements for rust oxygen and water. The necessity of both oxygen and water is illustrated when observing automobiles operated in dry climates and ships or other iron objects recovered from anoxic water. Autos and ships subjected to these conditions show remarkably little rust, the former because of lack of water and the latter because of lack of oxygen. 

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

In several cases, the oxidation of primary alcohols to carboxylic acids is desired as a technical process. Direct and indirect electrochemical oxidations have been developed for this purpose. 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

Wellmann J, Steckhan E. Indirect electrochemical processes 2. Electro-catalytic direct oxidation of aromatic compounds by hydrogen peroxide. Chem Ber 1977 110 3561-3571. 

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. 

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