Two-electron charge transfer reactions

In any two-electron charge transfer reaction, the two steps can be represented as follows  

The value of a is taken to be the same in both of the steps of electron charge transfer and its value is assumed to be close to 0.5. 

the amplitude of the interfacial potential, is considered for the overall reaction. 

If A co, is the rectification potential due to the first step of the reaction and AJECO 1 is the rectification potential contribution due to the second step, then the total rectified potential should be the sum of the rectified potentials for each individual step, i.e., AE + A oo . The combined faradaic rectification change for both the steps of electron charge transfer can be represented as38 

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If a two-electron charge transfer reaction takes place in two separate steps, each being accompanied by transfer of a single electron, the mathematical expression for the determination of kinetic parameters becomes more involved and complicated. 

Very few references are available on the determination of the rate constant for each step of electron charge transfer in the reaction M2+ + 2e -> M(s), i.e., M2+ + e -> M+, M+ + c" -> M(s). Earlier studies are mostly related to two-electron charge transfer reactions either at M2+/Hg(dme), M2+/metal amalgam, or redox couple/Pt interfaces. Even in these studies, the kinetic parameters have been determined assuming that one of the two steps of the reaction is much slower and is in overall control of the rate of reaction in both... 

Some of the two-electron charge transfer reactions which have recently been studied are Cu(II)/Cu(s), Ni(II)/Ni(s), Cd(II)/Cd(s), and Zn(II)/Zn(s). Their kinetic parameters in different supporting electrolytes are given in Tables 1 and 2. 

Kinetic Parameters of Some Two-Electron Charge Transfer Reactions at a Platinum Interface... 

This reaction is found to be stable in sodium acetate and acetic acid buffer (pH 4.65), and so it has only been studied in this medium. The faradaic rectification theory becomes highly complicated when extended to three-electron charge transfer reactions due to the formation of the two intermediate species Al(II) and A1(I). In order to determine the three rate constants and the two unknown concentration terms, C°Rl and C°Ru, corresponding to the two intermediate species formed, it becomes necessary to carry out the experiment at five different concentrations of aluminum ion, each below 2.00 mM. 

The reduction of vanillin is accompanied by a two-electron charge transfer and the mechanism of the reaction can be explained as follows ... 

Earlier studies generally involved the evaluation of kinetic parameters of reactions which are accompanied by single-electron charge transfer.116 Some reactions involving two-electron charge transfer were also studied, assuming either that both electrons are transferred in a single step or that the slower step in the two-step reaction is in overall control of the rate process. As described in this chapter for the first time, the faradaic rectification theory for... 

As for chemical reactions, the oxidation-reduction (redox) reactions in homogenous medium (i.e., in the bulk of the solution) have been experimentally studied with proper intensity only in the last two decades. There has been some development of the bulk reactions. However, as before, a comparison of one and the same compound in chemical and electrochemical electron-charge-transfer reactions is still of current interest. Such a comparison is made in this section. The examples offered are intended to invoke novel interpretations or to discover new colors in pictures that have already been drawn. 

This section presents the solution corresponding to a surface two-electron charge transfer process (EE mechanism) when a sequence of potential pulses H, E2,..., Ep is applied to the reaction scheme (6.II) by assuming that all the species (Oi, 02, and O3) correspond to the oxidation states of a surface-confined molecule O. Under these conditions, Eq. (6.107) has to be replaced by... 

Two different variants of the electrocatalytic process are analyzed here. The first one corresponds to first-order conditions and in this case one-electron and two-electron charge transfers coupled to the chemical reaction are discussed under SWV and Voltcoulometry conditions [19, 83, 95-97], After that, a second-order catalytic scheme is presented in which the mass transport of the substrate of the chemical reaction is considered [98, 99]. 

Recently, Kisza et al. [203] found that the total electrode reaction can be interpreted by a two-step two-electron charge transfer process with an intermediate adsorption ... 

The detailed mechanism of battery electrode reactions often involves a series of chemical and electrochemical or charge-transfer steps. Electrode reaction sequences can also include diffusion steps on the electrode surface. Because of the high activation energy required to transfer two electrons at one time, the charge-transfer reactions are beheved to occur by a series of one electron-transfer steps illustrated by the reactions of the 2inc electrode in strongly alkaline medium (41). 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

The simplest type of complex electrochemical reactions consists of two steps, at least one of which must be a charge-transfer reaction. We now consider two consecutive electron-transfer reactions of the type ... 

Following the early studies on the pure interface, chemical and electrochemical processes at the interface between two immiscible liquids have been studied using the molecular dynamics method. The most important processes for electrochemical research involve charge transfer reactions. Molecular dynamics computer simulations have been used to study the rate and the mechanism of ion transfer across the water/1,2-dichloroethane interface and of ion transfer across a simple model of a liquid/liquid interface, where a direct comparison of the rate with the prediction of simple diffusion models has been made. ° ° Charge transfer of several types has also been studied, including the calculations of free energy curves for electron transfer reactions at a model liquid/liquid... 

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. 

Figure 17. (a) Generic reaction path for charge transfer reactions with both channels of harpooning and electron transfer indicated. Molecular dynamics of the Bz/l2 bimolecular reaction is shown at the bottom, (b) Observed transient for the Bz/l2 reaction (I detection) and the associated changes in molecular structure. Note that we observe the two channels of the reaction, shown in (a), with different kinetic energies and rises of the I atom. 

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