Reactions at electrodes

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

For thermodynamic reasons, an electrochemical reaction can occur only within a dehnite region of potentials a cathodic reaction at electrode potentials more negative, an anodic reaction at potentials more positive than the equilibrium potential of that reaction. This condition only implies a possibility that the electrode reaction will occur in the corresponding region of potentials it provides no indication of whether the reaction will actually occur, and if so, what its rate will be. The answers are provided not by thermodynamics but by electrochemical kinetics. 

It will be assumed in this review that the reader is familiar with the usual stereochemical concepts employed in organic chemistry. Reactions carried out at electrodes are sometimes complicated by special features, however, which are not commonly encountered in normal organic chemical practice and which one must therefore be aware of. These are all associated with the fact that electrochemical reactions at electrodes are heterogeneous processes. 

A more generally useful measure for the purpose of this book is the (free) energy difference between reactions at electrodes in water at 298 K such as... 

An interesting approach to measuring rates of electron transfer reactions at electrodes is through the study of surface bound molecules (43-451. Molecules can be attached to electrode surfaces by irreversible adsorption or the formation of chemical bonds (461. Electron transfer kinetics to and from surface bound species is simplified because there is no mass transport and because the electron transfer distance is controlled to some degree. 

To use a real example, consider a hydrogen fuel cell. The reaction at electrode A is the hydrogen oxidation reaction,... 

IV. MOLECULAR MODELS OE ELECTRON TRANSFER REACTIONS AT ELECTRODES... 

The difficulty of such treatments are that they do not provide expressions for the rate of die ET reactions at electrodes which can be compared with experiments. They involve complicated computer simulations to determine the free energy profile. Such simulations generally use adjustable parameters to make the results fit experiments. Conversely, these treatments include both short- and long-range ion-solvent interactions and the interaction of the ion and the solvent with the metal electrode at a molecular level. 

However, a very limited number of studies focused on the effect of solvent dynamics on electron transfer reactions at electrodes.Smith and Hynes" introduced the effect of electronic friction (arising from the interaction between the excited electron hole pairs in the metal electrode) and solvent friction (arising from the solvent dynamic [relaxation] effect) in the electron transfer rate at metallic electrodes. The consideration of electron-hole pair excitation in the metal without illumination by light seems unrealistic. 

A term used in electrochemistry to describe facihtated electron transfer, resulting in an increased rate of halfcell reactions at electrode surfaces. 

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. 

The same relationship is plotted in Fig. 32 in a way which is more customary than Fig. 24 for a discussion of reactions at electrodes. The reduction potential of the excited singlet state of rhodamine ° id /2d- has been fixed according to Fig. 31 relative to the valence bands (ionisation energies) at the surface of the organic crystals. The distribution curve for the empty and occupied excited singlet state of rhodamine is also drawn according to Fig. 31. A comparison with the above estimated value of... 

There are two kinds of charge-transfer reactions at electrodes. An electron-transfer reaction is the first kind and is exemplified by the reduction of Fe3 to Fe2+ at the interface. The ions in the layer hardly move while the electron comes from the electrode or leaves the ions in the layer of solution adjacent to the electrode and gpes to the electrode. The charge transfer is dominated by means of electrons transferring from electrode to ions and vice versa. 

There is therefore one essential conclusion from the comparison of electrodic e-i junctions and semiconductor n-p junctions The symmetry factor P originates in the atomic movements that are a necessary condition for the charge-transfer reactions at electrode/electrolyte interfaces. Interfacial charge-transfer processes that do not involve such movements do not involve this factor. By understanding this, ideas on P become a tad less underinformed. Chapter 9 contains more on this subject. 

Carlo approach to bond-breaking reactions at electrodes. 

W. Schmickler, Chem. Phys. Lett. 237 152 (1995). Electron-transfer and ion-transfer reactions at electrodes distinguished. 

Many redox reactions at electrodes involve transfer of more than one electron. It is agreed that such processes usually involve several consecutive one-electron steps rather than a simultaneous multi-electron transfer. The kinetics of the overall reaction (and hence the current flowing) are complicated by such factors as the lifetimes of the transient intermediate species. 

The study of the kinetics of reactions at electrodes is multidisciplinary, involving chemistry, physics, metallurgy, materials science, etc. 

As will be discussed in Sect. 4.1, multielectron transfer reactions at electrodes are most likely to occur in a series of single one-electron steps. For the present discussion, a general single n-electron transfer reaction is considered (only one transition state) with n most probably one... 

Redox electrode reactions on metal electrodes constitute the simpler case for a theoretical approach to the problem. In particular, outer sphere redox electrode reactions not involving specific adsorption interactions have been treated successfully in analogy with homogeneous redox reactions in solution [54, 56], Approximate extension of the theoretical approach to the case of inner sphere redox reactions at electrodes has been done [56, 57b]. 

The kinetics of electron transfer reactions at electrodes can be explained either by surmounting an activation barrier due to the chemical reorganization of the reactants or by tunnelling through the potential barrier across the electrode—solution interface. 

The transition state theory was applied to the proton transfer reaction at electrodes by Horiuti and Polanyi [58] and Eyring et al. [31]. The stretching of the H+—OH2 bond gives rise to the activated complex by a gradual transition in time and space. Details of this model were discussed in Sect. 3.1. 

The transmission coefficient k — 1 for weak overlap of electronic states of reactants and products in the transition state. It is strong enough to be adiabatic but yet weak enough for the free energy of activation not to have an appreciable contribution from the resonance energy. This condition is almost fulfilled by outer sphere redox reactions at electrodes. 

Recent preliminary measurements of the electrokinetics of simple redox reactions at electrodes at low temperatures (— 40 to — 120°C) have shown a decrease of the energy of activation with decreasing temperature, which may indicate the occurrence of nuclear tunnelling [77]. 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

However, the mechanisms of conventional redox reactions and electrochemical reactions maybe quite different. Within the formalism of electron transfer theory, the electron transfer reactions at electrodes are usually of the outer-sphere type, whereas those that involve inorganic ions are often of the inner-sphere type [11]. 

Reversible Electrochemical Reactions at Electrodes and Microelectrodes of Any Geometry... 

As stated in Sect. 7.1, the current in SWV is sampled at the end of each potential pulse and the net response (/fw) is given by the subtraction of the current corresponding to a pulse with odd index (forward current, if) and that of the following pulse with even index (reverse or backward current, if) (see Eq. (7.6)). By using the expression of the current for any pulse of a given sequence (Eq. (5.23)), the expression for the SWV response of a reversible electrode reaction at electrodes of any geometry is immediately derived [21] ... 

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