Metal electrode reactions

V.A. Sobyanin, and V.D. Belyaev, On the nature ofNon-Faradaic catalysis on metal electrodes, Reaction Kinetics and Catalysis Letters 51(2), 373-382 (1993). 

Oxide-Covered Metal Electrodes Reaction Engineering and Digital Simulation 27... 

Figure 9.3-3 illustrates the superposition of several metal electrode reactions for varions metal ion activities. It is apparent that them are three methods for the reduction of metal ions to metal. By applying an external potential more negative than the half-cell potential, metal reduction occurs, resulting in the deposition or surface layers at the meial-meial ion electrode surface by electrolysis. A second method results whea a metal ion in solution, Mi 4, is contacted by another metal. M2. whose potential is more angalive. This results in the deposition of M, on M, and is known as contact reduction or cementation. In... 

In recent years, our laboratory has sought to apply Aese relationships in a qualitative to semi-quantitative way to transition metal electrode reactions characterize by large structural change. We also have sought to study compounds that are representative of transition elements in biological systems, smce mner-shell effects often have important consequences on reactivity in such systems. In Ae following sections we describe results for a number of metalloporphyrins [3] and for several complexes of Mo Aat model Ae behavior of Ais element m molybdenum-containmg enzymes [4]. 

Reactions c) and e) if fiuitfiil, may be realized onfy at a particular metal electrode. Reaction d), the reduction of paroxy compounds, especially when tried at tranrition metal cathodes, does not seem mcploitable to initiate a polymerization as the radical intermediates are strongly adsorbed and quidd reduced before any possible interaction with a monomer ... 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

The potential of a metallic electrode is determined by the position of a redox reaction at the electrode-solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion. 

A metallic electrode whose potential is a function of the concentration of M"+ in an M"+/M redox half-reaction. 

Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

The semi-consumable electrodes, as the name implies, suffer rather less dissolution than Faraday s law would predict and substantially more than the non-consumable electrodes. This is because the anodic reaction is shared between oxidising the anode material (causing consumption) and oxidising the environment (with no concomitant loss of metal). Electrodes made from silicon-iron, chromium-silicon-iron and graphite fall into this category. 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

Sodium metal is obtained by the electrolysis of molten sodium chloride (Figure 20.3, p. 537). The electrode reactions are quite simple ... 

When a metal is immersed in a solution containing its own ions, say, zinc in zinc sulphate solution, a potential difference is established between the metal and the solution. The potential difference E for an electrode reaction... 

Here p is the coordination number of the complex ion formed, Xb is the ligand and n is the number of electrons involved in the electrode reaction. The concentration of the complex ion does not enter into equation (15), so that the observed half-wave potential will be constant and independent of the concentration of the complex metal ion. Furthermore, the half-wave potential is more negative the smaller value of Kinstabi, i.e. the more stable the complex ion. The half-wave potential will also shift with a change in the concentration... 

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