Cathodic generation of solvated electrons

General Conditions for Cathodic Generation of Solvated Electrons.167... 

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. 

Electrochemical generation of solvated electrons was first observed in 1897 by Cady who found that when sodium solutions in liquid ammonia are electrolysed the blue coloration intensity increases at the cathode. All information on cathode generation of solvated electrons remained at this qualitative level for over half a century until Laitinen and Nyman made the first attempt to quantitatively investigate the kinetics of this process. This work, however, remained isolated for a long time and only after 20 years, with the awakening of interest in the chemistry of solvated electrons, were systematic studies into the kinetics of electrode reactions of solvated electrons started, almost simultaneously by three groups of researchers in Southampton Tokyo and Moscow In Moscow these studies... 

Another significant discrepancy between the theory and experiment is that even the computed values of the reorganization energy, to say nothing of its experimental estimation, are too small to be used for explaining the rate of the cathodic generation of solvated electrons (see Sect. 7). All this indicates that some factors affecting the electrochemical behaviour of solvated electrons are still unknown. 

Obtaining solvated electrons by dissolving alkali metals and by electrochemical generation is of special interest. Back in the last century, the dissolution of alkali metals in liquid ammonia gave the very first evidence of obtaining solvated electrons. Electrochemical (cathodic) generation of solvated electrons is a process in which electrons are transferred from the electrode into solution under the action of high cathode potentials. 

Cathodic generation of solvated electrons in general competes with other cathodic reactions, say, in alkali metal salt solutions, with electrolytic deposition of the alkah metal on the electrode. Therefore, it is necessary to find a criterion which could be used to estimate how effective the cathodic generation will be, if it is possible at all, for the given system. 

In the systems listed under A-1, thermodynamically it would be more advantageous on solid electrodes if the cathodic generation of solvated electrons proceeds... 

From what has been said it is clear that cathodic generation of solvated electrons is possible in a number of systems hence, account should be taken of the electrochemical formation of solvated electrons in all cases where the cathode potentials are negative enough for this process to proceed. 

Cathodic generation of solvated electrons is a convenient method for studying their association, since it enables the concentration of solvated electrons to be varied independently over a wide range while preserving the cations concentration well in excess. 

Table 5 compares the standard potential of the electron electrode in hexamethylphosphotriamide (5 °C) with the standard potentials of alkali metals (25 °C). Data for liquid ammonia are also given. In both solvents the rubidium electrode potential serves as a reference point since it depends very little on the solvent. It is seen from the Table that in both solvents the standard equilibrium potential of the electron electrode is more positive than that of a lithium electrode and is close to the potentials of other alkali metals. In the course of experiment, cathodic production of dilute solutions (10 — 10 mol/1) of solvated electrons takes place and this makes the electron electrode equilibrium potential more positive compared to the standard value. In case of hexamethylphosphotriamide the same happens when electrons are bound in strong non-paramagnetic associates by the cations of all alkali metals except lithium (see Sect. 4). This enables one to assume that under the conditions of the experiments the electron-electrode equilibrium potential in liquid ammonia and hexamethylphosphotriamide is more positive than the equilibrium potential of all alkali metals. This makes thermodynamically possible primary cathodic generation of solvated electrons in solutions of all alkali metal salts in the two solvents. 

By the beginning of 7O s only one work on the kinetics of electrochemical generation of solvated electrons was known Later, however, a number of papers reflecting the increased interest in this problem appeared. A brief annotation of the works pertaining to the cathodic generation of solvated electrons in liquid ammonia and methylamine and also in hexamethylphosphotriamide is given in Table 7. 

Table 7. Kinetics of cathodic generation of solvated electrons 00... 

In the early days of the electrochemistry of solvated electron it was taken for granted that cathodic generation of solvated electrons proceeds via deposition of alkali metal, M" -I- e (M) -> M with its subsequent dissolution, M - IVT " e". However, besides this secondary process, a basically different way is possible, i. e., direct transition of electrons from electrode to solution, e (M) - e — a primary process... 

If the electrode process is not complicated by adsorption or other phenomena, depending directly on the nature of the electrode, then the rate of the charge transfer stage proper should be independent of the electrode material. Indeed, this type of independence has been earlier ascertained experimentally for the electroreduction of anions and the electron photoemission into solution, fo such processes should belong also the cathodic generation of solvated electrons by the primary mechanism. 

The fact that the process rate under activation control conditions is independent of the nature of the background salt cation can serve as an additional criterion for the primary nature of cathodic generation of solvated electrons. Systematic studies into the effect of this factor were made in hexamethylphosphotriamide and they... 

Thus, it can be unambiguously affirmed that cathodic generation of solvated electrons in a number of cases proceeds by direct transition of electron from electrode into solution, i. e., cathodic generation is a primary process. 

The second mechanism for the cathodic generation of solvated electrons has been suggested in a number of works The term electrochemical dissolution... 

The formation of a passive film and its thickening in systems where cathodic generation of solvated electrons is possible produces various effects ... 

It should be noted that if in calculating the exchange current we had used the experimental value of X, even its upper limit (see Sect. 2), then the discrepancy between the computed and the experimentally determined values of would have increased still further by 2 orders. Thus, we deal not only with insufficient accuracy of theoretical calculation of — the impression arises that experimental data on energy characteristics of solvated electrons poorly correlate with the experimental data on the kinetics of cathodic generation of solvated electrons. 

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