Solvated electrons, electrode generation

It follows from Section 4 that solvated electrons are generated during cathodic polarization of inert metal electrodes, e. g., in liquid ammonia and hexamethylphosphotriamide solutions of alkali metals salts. 

In strongly basic solvents like HMPA, amines and liquid ammonia, solvated electrons are relatively stable. In these solvents, if the supporting electrolyte is the salt of Li+ or Na+, blue solvated electrons, esm are generated from the surface of the platinum electrode polarized at a very negative potential ... 

A final topic to be discussed in this section is the direct injection of electrons into a liquid by the use of nanowires. The difference with the discharge processes described above for the point electrodes is twofold the use of nanostructured electrodes and the generation of solvated electrons, instead of the initiation of a discharge. Solvated electrons have a very short... 

These results suggest that the use of nanofiber electrodes in a microreactor environment to generate solvated electrons for chemical synthesis, may offer an interesting new route for reduction reactions. We are currently working on this concept in our laboratory (Agiral et al., 2010). 

Ammonia has been employed mostly for cathodic reactions, but some oxidations [330,333] have been carried out in this medium, although the potential range in the anodic direction is quite small. The anodically limiting reaction is oxidation to nitrogen and protons [340] the cathodically limiting reaction is the transfer of electrons to the solvent, which occurs at about —2.3 V (versus Hg pool electrode) in a saturated solution of TBAI. In the elecltrolytic generation of solvated electrons the potential is determined by the surface concentration of electrons and no external reference electrode is needed. 

LiCl, NaN03, and tetraalkylammonium salts can be used as supporting electrolytes. For the electrolytic generation of solvated electrons mainly LiCl has been employed [343,344]. A reversible reference electrode in EDA is the Zn(Hg)/Zn" electrode [345], but the Hg pool [246] or the aqueous calomel electrode, fitted with a suitable salt bridge, is also applicable. 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

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... 

The studies into the electrochemical kinetics of solvated electrons were to some extent stimulated by the hypothesis put forward in the second half of 6O s (see Sect. 8) for explaining the role of solvated electrons as intermediate products of electrode reactions, and also by the development made at that time in organic synthesis involving the participation of solvated electrons (see Sect. 9). Undoubtedly, knowledge of the mechanism of electrode generation of solvated electrons is of fundamental importance. Electrochemistry is the chemistry of the electron , Professor A. N. Frumkin once said. In fact, electron reactions at the interface of electronic and ionic conductors are inevitably associated with the electron addition or detachment process. In a solvated electron reaction no heavy particle (atom or molecule) acts as electron acceptor, or donor. In this sense, the electrode reactions of solvated electrons are the most simple electrode processes. Therefore, an insight into the solvated electron reaction mechanism is necessary for electrochemical kinetics as a whole. 

In the case of photoelectrochemical generation of solvated electrons, discussed in Section 2, the electrode/electrolyte interface is illuminated with light of quantum energy exceeding the electronic work function for this system and the electrons from the electrode go into the electrolyte — first in the delocalized and then in the solvated state. Since photoelectrochemical generation of electrons takes place at potentials far more positive than the equilibrium electron potential the electrode surface effectively traps (i.e. oxidizes) solvated electrons. This is the reason why in this method the solvated electrons exist near the illuminated electrode only for a limited... 

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... 

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. 

From what has been said in this section it is evident that sufficient information on the electron electrode is now available for a number of solvents. Standard potentials for such an electrode in different solvents are known and this enables one to predict the potential region where the possibility of electrochemical generation of solvated electrons must be reckoned with. 

The results of the studies of this process in different media are summarized in Table 6. When investigated by cyclic voltammetry, one usually starts with solutions that initially do not contain solvated electrons solvated electrons are then obtained during the cathodic sweep of potential. In other methods, the necessary bulk concentration of solvated electrons was attained by dissolving the alkali metal or by preliminary cathodic generation at an auxiliary electrode. 

In some cases the generation of solvated electrons proceeds under diffusion control. A study of the cathodic process under these conditions yielded information on equilibrium standard potentials of the electron electrode (Sect. 5), and for methylamine — on the competition of electron generation and alkali metal deposition processes. Also, information has been obtained on the stoichiometry of the associates formed by electrons and on the tendency of various systems to association. 

Beyond the dependence on solution composition and electrode material the cathodic curve is located in the generation potential range of solvated electrons. It consists of two Tafel portions having different slopes 120-140 mV (lower) and 60 mV (upper)... 

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. 

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. 

Fig. 12. Energy diagram for the process of electron transfer from electrode into solution during electrochemical generation of solvated electrons. 1 thermoemission 2 dissolution of electrons ej delocalized electron e solvated electron Fermi level in metal. Dashed line shows the solvated electron potential well in solution...

At present the formation of passive films on electrodes in the process of generating solvated electrons may be considered to be proved. More precisely, a different degree of electrode passivation explains why different authors obtained qualitatively and quantitatively different results (e.g., diffusion or activation regimes different values of exchange current different values of a — see Table 7). 

Another factor ensuring high current density is the rapid removal of solvated electrons from the electrode due to intense convection in the solution, which is caused by a decrease in its density at the cathode surface. This phenomenon is associated with an increase in solution volume caused by the introduction of electrons into it. Unlike electrostriction that accompagies the solvation of ordinary ions, the formation of solvated electrons increases the volume by 65-96 ml/mol for liquid ammonia and by about 80 ml/mol for hexamethylphosphotriamide . As a result, according to Avaca and Bewick the current densities for the generation of solvated electrons can by 2500 or more times exceed the rate of mass transfer of organic compounds to the electrode. 

Like benzene and toluene, tetralin (tetrahydronaphthalene) is not reduced directly on the electrode, for example on a mercury electrode in ethylenediamine it is however well hydrogenated when using solvated electrons in this solvent (Table 13) and also in hexamethylphosphotriamide 2 , In Ref. it has been shown Ijiat in solutions of polarographically inert benzene and tetralin as well as of directly reducible naphthalene the potentials of the cathode processes coincide. This is also a distinctive indication of reduction of organic substances with the aid of electrochemically generated solvated electrons. The process rate is practically independent of the nature of organic compound. 

In the past two decades, thanks to the work of scientists in the U.K., the USSR, Japan, the USA, and other countries, solvated electrons have become an important subject of electrochemical investigations. These have been obtained in a number of solvents in many of them, they are quite stable. Their typical chemical, physical, and optical properties have been well determined. Also, the fundamental relationships for electrochemical and photoelectrochemical generation of solvated electrons have been found. As is evident from this review, solvated electrons behave as individual chemical reagents, in particular they enter into electrochemical reactions at electrodes, and have their equilibrium potential, etc. 

Both LiCl and KI have been used the former has rather limited solubility in liquid ammonia, whilst the latter, although significantly more soluble, shows a greater tendency to passivate the electrode in the presence of trace water under silent conditions. Electrolysis under galvanos-tatic conditions allows solvated electrons to be generated at the electrode surface. In the presence of power ultrasound, rapid mass transport and mixing produces a deep blue solution stable over a period in excess of an hour. 

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