Transfer with Adsorption on the Electrode

When the electrode acts as a source or sink for electrons with the reactant sitting in the outer Helmholtz plane (see Section 1.4.4), reaction rate is not usually dependent on the nature of the electrode material. For some reactions to proceed, however, the reactant or an intermediate must be adsorbed on the electrode surface. Then the nature of the electrode material often has a significant effect on the reaction rate by virtue of its ability or lack of ability to provide the necessary sites. We shall first discuss briefly the phenomenon of adsorption and then derive an example of kinetic expressions when adsorption is taken into account. 

When Gsp G sp the surface concentration of the adsorbed species can be obtained from an adsorption isotherm such as Langmuir s. 

An adsorption isotherm relates the surface concentration of an adsorbed species to the variables which determine it. Langmuir s adsorption isotherm has its roots in the chemical adsorption of gases according to the expression  

Langmuir s analysis, by analogy with chemical reactions, implies that the standard free energy of the adsorption process remains constant and is independent of the number of sites already occupied. Sometimes this assumption is significantly in error and it is advisable to try the Temkin 

The rate of production per unit area of electrode of and that of the adsorbed species A2, denoted by dA /dt and dA2ldt are  

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