Catalytic Reactions on Mixed Electrodes

In order for this mixed electrode reaction shown in Fig. 11-2 to proceed, the Fermi level efod of the iron electrode must be higgler than the Fermi level erh-zhj) of the hydrogen redox reaction and also must be lower than the Fermi level for the transfer reaction of iron ions. In other words, the potential E of the iron electrode must be lower than the equilibrium potential of the 

On the mixed electrode of metallic iron immersed in acidic solutions, the anodic and cathodic charge transfer reactions (the anodic transfer of iron ions and the cathodic transfer of electrons) proceed across the electrode interface, at which the anodic ciurent (the positive charge current) is exactly balanced with the cathodic current (the negative charge current) producing thereby zero net current. 

The presence of a metal surface can catalyze redox reactions which also constitute mixed electrode reactions. For example, the oxidation of hydrogen molecules (2H2, , + 02, , - 2H2O ) does not occur in aqueous solutions but this oxidation catalyticaUy proceeds on platinum electrodes as a coupled process of the anodic and the cathodic reactions shown in Eqn. 11—2 and in Fig. 11—3  

The direct transfer of electrons from the frontier orbital of hydrated hydrogen molecules to the frontier orbital of hydrated o Q n molecules does not take place because its activation energy is high but the indirect transfer of electrons via both the electron level of metallic electrodes and the redox electron level of adsorbed reaction intermediates proceeds at an appreciable rate on metal electrodes. 

For further example, the decomposition of hydrogen peroxide, 2H202, , - 02. j + 2H20, which hardly occurs in aqueous basic solutions, catalytically proceeds on mercury electrodes as a mixed electrode process of anodic and cathodic reactions shown in Eqn. 11-3  

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