Electrolytes oxidation mechanisms

Zinc Oxidation Mechanism. The oxidation reaction for the 2iac anode (eq. 8) takes place ia several steps (16,17), ultimately resulting ia the 2iacate ions [16408-25-6] Zn(OH) 4, that dissolves ia the electrolyte. 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Adsorbed CO on a metal surface is one of the simplest adsorbates and has attracted significant interest within the areas of fundamental surface science, catalysis, and electrochemistry. An understanding of the oxidation mechanism of adsorbed CO is important to design and develop electrocatalysts for fuel cells [69-73] and the surface dynamics of adsorbed CO on electrode surfaces in electrolyte solutions is, therefore, very important. 

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

A combination of HPLC and amperometric detection was proposed for determination of tributylstannyl oxide in antifouling paint. The detector is of the static hanging Hg drop type in a flow cell, the solvent is CH2CI2/THF, containing tetrabutylammonium perchlorate as supporting electrolyte. The oxidation mechanism depicted in reactions 4... 

Scheme 17. Mechanism for Electrolyte Oxidation Coupled with Spinel Disproportionation and Mn + Dissolution...

Scheme 18. Mechanism for Electrolyte Oxidation on a Eully Charged Cathode Surface...

Electrolytic or anodic oxidation is fast, uniform and best suited to mass production. This process is most widely used for treatment of commercial carbon fibers. The oxidation mechanism of most carbon fibers is characterized by simultaneous formation of CO2 and degradation products that are dissolved in the electrolyte of alkaline solution or adhere onto the carbon fiber surface in nitric acid. Only minor changes in the surface topography and the surface area of the fiber are obtained with a small weight loss, say, normally less than 2%. 

It has been generally observed that the nature of the electrode material, the experimental conditions, and the electrolyte composition strongly influence the oxidation mechanism. 

Non-Reversible Processes. —Reactions of the non-reversible type, i.e., with systems which do not give reversible equilibrium potentials, occur most frequently with un-ionized organic compounds the cathodic reduction of nitrobenzene to aniline and the anodic oxidation of alcohol to acetic acid are instances of this type of process. A number of inorganic reactions, such as the electrolytic reduction of nitric acid and nitrates to hydroxylamine and ammonia, and the anodic oxidation of chromic ions to chromate, are also probably irreversible in character. Although the problems of electrolytic oxidation and reduction have been the subject of much experimental investigation, the exact mechanisms of the reactions involved are still in dispute. For example, the electrolytic reduction of the compound RO to R may be represented by... 

Indirect oxidation mechanisms have been proposed in some cases, such as for the oxidation of aliphatic hydrocarbons [25], for which voltammetric studies do not give unambiguous information about the nature of the electroactive species. Often, they are assumed to involve the oxidation of the anion of the supporting electrolyte to form an inorganic radical [Eq. (16)], which subsequently either attacks a C-H bond in the organic substrate [Eq. (17)] or oxidizes the substrate in an electron transfer process [Eq. (18)] ... 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

An investigation of ceramic coating growth mechanisms in plasma electrolytic oxidation (PEO) processing. Electrochim. Acta, 112, 111-119. 

Wang, L., Chen, L., Yan, Z., and Fu, W. (2010a) Optical emission spectroscopy studies of discharge mechanism and plasma characteristics during plasma electrolytic oxidation of magnesium in different electrolytes. Surf. Coat. Technol., 205, 1651-1658. 

The electrolytic oxidation is performed in a two-compartment cell like that in Part A, but using a larger beaker. The anode solution may either be in the beaker or in the porous pot. The anode is preferably of nickel gauze, but a spiral of thick iron wire is also satisfactory. The cathode is a small spiral of thick iron wire. The cathode solution is IM potassium hydroxide, the anode solution the potassium manganate solution prepared above. The anode solution should be kept stirred during the electrolysis, either by a stream of filtered air or by a mechanical stirrer. The temperature is 30° or below. The lower the current density at the anode, the better the yield, but a current density of 0.1 amp/ cm is satisfactory. Pass a current of about 2 amp. 

From these results, it is obvious that to use an electrolyte with an intermediate pH value, such as 3, is compulsory to answer the double requirement of colour and stability/reversibility. In several respects, the specificity of pH3 was pointed out in the literature it is given as a key-value with regard to the stability of either protonated or unprotonated forms [40]. A change between two different oxidation mechanisms egress of protons or insertion of anions is also assumed to occur in this pH range [44], In spite of the necessity to put the polymer in vacuum, with possible ensuing perturba-... 

Completely different monomers were called for. Before long, three of today s workhorses had been identified pyrrole, aniline and thiophene. In Japan, Yamamoto [38] and in Germany, Kossmehl [39] synthesized polythiophene doped with pentafluoroarsenate. At the same time, the possibilities of electrochemical polymerization were recognized. At the IBM Lab in San Jose, Diaz used oxidative electrochemical polymerization to prepare polypyrrole [40] and polyaniline. [41] Electrochemical synthesis forms the polymer in its doped state, with the counter-ion (usually an anion) incorporated from the electrolyte. This mechanism permits the selection of a wider range of anions, including those which are not amenable to vapor-phase processes, such as perchlorate and tetra-fluoroborate. Electrochemical doping also overcomes an issue associated with dopants... 

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