Electrode electronic

Alkali and alkaline-earth metals have the most negative standard reduction potentials these potentials are (at least in ammonia, amines, and ethers) more negative than that of the solvated-electron electrode. As a result, alkali metals (M) dissolve in these highly purified solvents [9, 12] following reactions (1) and (2) to give the well-known blue solutions of solvated electrons. 

These reactions proceed to equilibrium when the potential of the solvated-electron electrode equals that of the alkali metal L13] ... 

Electrodes may be classified into the following two categories as shown in Fig. 4-3 one is the electronic electrode at which the transfer of electrons takes place, and the other is the ionic electrode at which the transfer of ions takes place. The electronic electrode corresponds, for instance, to the case in which the transfer of redox electrons in reduction-oxidation reactions, such as Fe = Fe + e,occurs and the ionic electrode corresponds to the case in which the transfer of ions, such as Fe , , = Fe, occiirs across the electrode interface. Usually, the former is found with insoluble electrodes such as platinum electrodes in aqueous solution containing redox particles and the latter is found with soluble metal electrodes such as iron and nickel. In practice, both electron transfer and ion transfer can take place simultaneously across the electrode interface. 

Fig. 4-3. (a) electronic electrode and (b) ionic electrode OX = oxidant particle RED s reductant particle M = ion transferring across the electrode interface... 

The electrode potential can be defined not only by the energy level of electrons (the real potential ofelectrons)butalsoby theener gy/eue/oftons (the real potential of ions) in the electrode. The former maybe called the electronic electrode potenticd and the latter may be called the ionic electrode potential [Sato, 1995]. For instance, the electrode potential of a metal electrode can be defined in terms of the metal ion level (the real potential of metal ions), aM-ai/s/v), in the electrode as... 

For an electronic electrode at which the transfer of redox electrons is in equilibrimn (OX i + e(jj) = RED q), as shown in Fig. 4-17, the Fermi level EpdUEDoxs) of redox electrons e(REDox., s in hydrated redox particles equals the Fermi level cp(M) of electrons e,io in the electrode the energy for the electron transfer across the electrode interface is, then, zero (a M/s) = 0). Consequently, the electron level u M/aAo in the electrode equals the electron level a, s/v) in the aqueous solution, i.e. the redox electron level a KEoax s) of hydrated redox particles. 

Fig. 4-17. Electronic electrode in equilibrium of electron transfer OX = hydrated oxidant particles RED = hydrated reductant partides FWEDQx, s) = Fermi level of redox electrons in hydrated redox partides in solution S p. = electrochemical potential of electrons.

Fig. 4-18. Electron levels of an electronic electrode in equilibrium of redox electron transfer eojenox s> = redox electron at equilibrium e ) = electrons in metal electrode .q = electrode potential in equilibrium of electron transfer.

Controlled one-electron reductions transform l,2,3,4-tetraphenyl-l,3-cyclopentadiene or 1,2,3, 4,5-pentaphenyl-l,3-cyclopentadiene into mixtures of the dihydrogenated products and the corresponding cyclopentadienyl anions (Famia et al. 1999). The anion-radicals initially formed are protonated by the substrates themselves. The latter are thermodynamically very strong acids because of their strong tendency to aromatization. As with the cyclopentadiene anion-radicals, they need two protons to give more or less stable cyclopentadienes. The following equations represent the initial one-electron electrode reduction of l,2,3,4,5-pentaphenyl-l,3-cyclopentadiene (CjHAtj) and explains the ratio and the nature of the products obtained at the expense of the further reactions in the electrolytic pool ... 

It is worth noting that there is a significant difference between the conversion of the same substrate under one-electron (electrode) transfer and charge-transfer complexes (homogeneous medium). 

The theory of electrocatalysis is still in its infancy. It was developed first for the hydrogen evolution reaction in the second half of the 1900s. The grounds can be traced back in a seminal paper by Floriuti and Polanyi [25]. Accordingly, for a simple one-electron electrode reaction ... 

The EC mechanism is a standard departure for discussing electrochemical mechanisms, it represents a one-electron electrode reaction coupled to a chemical reaction in solution ... 

With the initial condition c (t= 0,x) = c °, and the boundary conditions / n = 0 (for an electronic electrode), fam = 0 for an ionic electrode or, dc Idt =0 for a reversible electrode one obtains with the flux equations and the continuity equation the following as solution for the concentration profile ... 

An electrode is called an electronic electrode when the transfer of electrons occurs, while it is called an ionic electrode when the transfer of ions occurs at the electrode interface. Although electrons and ions are in the same category of charged particles, they are different in electrochemical behavior due to a difference in the type of statistics that governs them. Electrons are Fermi particles which obey the Fermi statistics, whereas ions are Boltzmann particles which obey the Boltzmann statistics. 

We may call the electrode potential defined by the ionic energy level the ionic electrode potential, and the electrode potential defined by the electronic energy level may be called the electronic electrode potential. In the case in which the electrode has no electronic level in the energy range of our interest such as certain membrane electrodes, it is convenient to describe the system in terms of the ionic electrode potential rather than the electronic electrode potential [Refs. 4 and 5.]. 

Table 9.1. Standard redox potentials E of electronic electrode reactions is...

The response of SECM is distance-dependent as shown in Fig. 16.8. At large distances from the substrate the current measured is that of the microelectrode tip. The tip is a disc microelectrode, so that far from a substrate in the steady state the diffusion-limited tip current, /T., will be measured. For a simple n-electron electrode reaction this is... 

Electrochemistry is the science of structures and processes at and through the interface between an electronic ( electrode ) and an ionic conductor ( electrolyte ) or between two ionic conductors [vii]. 

The role of the outer solvation shell in mixed solvents was also studied using the CoEn system as a model [302] (En = ethylenediamine). In this system the inner sphere of the substrate (CoEn ) and the product (CoEnl ) was not changed in the course of the one-electron electrode reaction. Therefore, the changes in the rate constant (determined by chronocoulometric method), observed when the composi-... 

A special reference electrode is the electron electrode, which can operate in solvents capable of solvating electrons, such as ammonia. A sheet of platinum suspended in a solution of sodium metal in ammonia may be used as a reference electrode [208]. The concentration of the reference solution can be varied over relatively wide limits without a change in the potential of the electrode. A sodium concentration of 0.001 M was found suitable [208]. 

Over the past 10-15 years a new trend has been developed in theoretical electrochemistry the electrochemistry of solvated electrons. In this review theoretical concepts of the electrochemical properties of solvated electrons and the results of experimental studies are considered from a unified position. Also discussed are energy levels of localized (solvated) and delocalized electrons in solutions and methods for their determination conditions of electrochemical formation of solvated electrons and properties of these solutions equilibrium on an electron electrode . The kinetics and mechanisms of cathodic generation of solvated electrons and of their anodic oxidation are discussed in detail. In the last sections participation of solvated electrons in ordinary electrode reactions is discussed, and the possibilities of cathodic electrosyntheses utilizing solvated electrons are considered. 

The (Gibbs) free energy level of a solvated electron in a solution in equilibrium with the electrode is equal to the level of electrochemical potential of electron in metal p. We shall call this equilibrium electrode the electron electrode. Suppose that we are concerned with a standard solution (1 mol/1) and hence the standard potential E°. In this case, w determined at E° and corrected for the entropy of delocalized electrons (at n conforming to 1 mol/1) is the difference between standard chemical potentials of localized and delocalized electrons. 

Experimental measurements of photoemission currents are generally taken at far more positive potentials compared to the equilibrium potential of the electron electrode. Therefore, even when the solvated electrons are stable in the bulk of the solution, the electrode-emitter surface traps them effectively. For the electrode-to-solution transition of electrons to be irreversible (this is a necessary condition for measuring a stationary photocurrent), readily reducible substances — solvated electron acceptors (so-called scavengers) — are added to the solution. The electron level in a reduced acceptor (A ) is quite low, and this makes this state very stable trapping of electrons by a scavenger is the final transformation an emitted electron undergoes. 

These have been obtained as photoemission work function w (cf. Fig. 1) at the electron electrode equilibrium potential (which for hexamethylphosphotriamide and liquid ammonia was measured by experiment, and, for water, calculated from the thermochemical data ) by making a correction for the ideal gas entropy according to Eq. (3) ri . It should be noted that the aforementioned value, computed in this manner, is independent of the solvated electron concentration (for the same standard concentration of localized and delocalized electrons). 

On placing a metallic electrode in a solution containing solvated electrons, thermodynamic equilibrium is usually established between the metal electrons and the solvated electrons in the solution. Such an electron electrode acts as a reversible electrode of the first kind. 

A knowledge of the electron-electrode equilibrium potential is necessary to form a judgement about the primary or secondary nature of the generation process (see Sect. 7) and the place of this process among other electrode reactions. 

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