Electrochemical standard potential

The standard electrochemical potentials of reactants and products may be written in terms of the chemical and electric contributions according to eqn. (36). 

There is an inherent similarity between the spectrum and an electrochemical current-voltage curve that is important from the point of view of chemical selectivity. In both cases, the x-axis (voltage or wavelength) is directly related to the energy. In electrochemistry, this energy corresponds to the transfer of electrons between the analyte and the electrode. It is related to the standard electrochemical potential. In optical interactions, molar absorptivity is probabilistically related to the excitation energy of the molecule. 

In practice the preparation of bimetallic catalysts using direct redox reactions can be extensively used for depositing a noble metal with a high standard electrochemical potential onto a non-noble metal with a lower standard electrochemical potential (eq 3). 

According to the standard electrochemical potentials (Table 1), with Cu/Cu2+ and Ru/Ru3+ couples, the amount of ruthenium deposited on metallic copper will be small, whereas the redox reaction carried out in presence of platinum or gold salts will occur to a large extent. On the other hand, for electrodes of first type (metal immersed in a solution of a salt of that metal), the standard electrochemical potentials as defined by thermodynamics are calculated with regard to a poly-crystalline metallic phase of infinite size. However, in the case of small metallic particles, characterized by metallic atoms of different coordination numbers, the notion of a local potential can be introduced. That no-... 

In summary, the direct redox reactions can be largely used in the preparation of bimetallic catalysts with a close interaction between the metallic constituents. In that case a metal with a high electrochemical potential is deposited on a metal with a lower potential. The applicability of the technique can be extended significantly by using different ligands which, by chelating metallic ions, modify the standard electrochemical potentials. 

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. 

Table 1 Partial List of Standard Electrochemical Potentials...

As shown in Fig. 16.2b, at equal concentrations, the distributions of the reduced and oxidized species are equivalent at the standard electrochemical potential of the redox couple, El redox. By varying the concentrations of the redox species a shift of the Fermi level (where Dreci and Dox are equal) of the redox system is induced according to the Nemst equation (Fig. 16.2c). 

Figure 12. Sketch of the probabiiities Wo (E) and fkRMi( ) to find an empty or filled electron level corresponding to an oxidized and reduced ion, respectively, as a function of the electron free energy (vertical axis). The standard electrochemical potential /i"(Ox/Red) with respect to the vacuum level acts as a reference point.

This, in view of Eq. (36), means that the standard electrochemical potential of in YSZ is... 

The AGo values can be transformed into the corresponding standard electrochemical potentials, (Eq. 4.1). For instance, thc conversion of PTFE (AGr= —365kJ/mol CF2) into polyyne and HI (reaction (4.11b) in a hypothetical cell with standard hydrogen electrode) would have AGo = —71kJ/mol. The corresponding standard redox potential PTFE/polyyne is Eq = 0.74 V, which is just 0.36 V smaller than the standard potential of PTFE/graphite (Eq = 1V) [3]. Apparently, in terms of the reaction thermodynamics, the electrochemical carbyne should be easily accessible via cathodic reduction of PTFE. Analogously, the oxidation of acetylene (AGr = —209.9 kJ/mol) to polyyne (Eq. 4.11a) corresponds to AGo = 2.1kJ/mol, Ea = -0.02 V. 

Trasatti [2] has described the methods used to estimate the absolute electrode potential on the basis of suitable extrathermodynamic assumptions. The method presented here is the one which gives an estimate which can be related to the potential scale used by physicists. Moreover, the resulting estimates of the absolute values of the standard electrochemical potential are based on experimentally measured quantities. The analysis is illustrated here for cell (9.3.30), which contains a hydrogen electrode. An air gap is introduced into the cell, so that the solutions surrounding each electrode are separated. The resulting cell is... 

For this simple kinetic model, the Gibbs energy in the adjacent phases is defined by the standard electrochemical potential, which can be defined as the concentration independent part of the electrochemical potential. 

The kinetic term in Eqn. 46 for the charge transfer resistance may have important consequences. Near the standard electrochemical potential of the couple, rate constants are roughly equal and possibly large enough so that Rct is small the system behaves reversibly. However for very reduced systems, ki is very small, and hence Rct can be large. Similarly for very oxidized systems, k-i is very small, and again Rct can be large. It is important to realize that systems which are kinetically labile close to the standard electrode potential may show kinetic barriers at extremes in reduction or oxidation. 

Iron is a relatively reactive metal - its standard electrochemical potential is -760mV. It reacts with all diluted acids resulting in the salts of iron (II). Chemically pure iron is relatively less prone to corrosion compared to its commonly used alloys. Steels containing various alloying elements have different chemical compositions in material micro-zones. Such micro-zones in contact with the electrolyte solution lead to different electrochemical potentials and are able to create micro-cells, in which iron is most often an anode. As a result of these electrode processes the iron oxidation occurs and the formation of various corrosion products takes place, in which iron occurs primarily at two and three degrees of oxidation. 

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