Surface electrode reactions lateral interactions

In the last two decades, significant attention has been paid to the study of surface electrode reactions with SWV and various methodologies have been developed for thermodynamic and kinetic characterization of these reactions. In the following chapter, several types of surface electrode processes are addressed, including simple quasireversible surface electrode reaction [76-84], surface reactions involving lateral interactions between immobilized species [85], surface reactions coupled with chemical reactions [86-89], as well as two-step surface reactions [90,91]. 

The theory described in the previous chapter has been developed under provision that no interactions exists between immobilized species. However, for many experimental systems, this prerequisite is not fulfilled [85,104,105]. Hence, it is of interest to consider a case of a surface reaction involving lateral interactions. In a general case, various sorts of interactions can be assumed between O and R forms immobilized on the electrode surface [106]. The following discussion is restricted to the case of uniform interactions between immobilized species. 

This is the equation, the isotherm, we were seeking. It is a generalized isotherm for the adsorption of ionic species on a heterogeneous surface. It considers the adsorption reaction as a substitution process, with the possibility of transfer of charge between the ion and the electrode and also lateral interactions among adsorbed species. 

What information can be obtained from A/ ds One important parameter involved in the enthalpy of the reaction is the ion-metal bond. However, A7 ds includes all the different interactions involved in the adsorption process, e.g., the breaking of the hydration sheet of the electrode and the ion, lateral interactions, and heterogeneity of the surface. One can subtract all these energy terms from AH ds and obtain in this way the energy (or strength) of the ion-metal bond. For the example of the adsorption of bisulfate ions onpolycrystalline platinum (Fig. 6.105), the ion-metal bond was found to be -214 60 kJ mol-1. 

The insoluble salt Hg2X2 is precipitated as a submonomolecular layer on the mercury electrode surface. It is assumed that no lateral interactions between the deposited particles exist. At equilibrium, this redox reaction satisfies the Nemst... 

The theory described above for SWV of surface reversible redox couples was developed assuming that there are no interactions between adsorbed species, i.e., the specific interaction of the species with the electrode surface can be described by the Langmuir isotherm (Adamson, 1990). However, for several experimental systems, this pre-condition is not satisfied. Thus, it is also interesting to analyze surface reactions involving lateral interactions between adsorbed molecules (Mirceski, Lovric Guloboski, 2001). 

In the presence of lateral interactions, cOmt depends on both the frequency and the surface coverage of the electrode. This is the most important difference between a simple surface reaction and those that involve interactions between adsorbed species. In these cases, the position of the "quasi-reversible peak" is associated with a critical value of the interaction product (aco)max> which depends on the value of the ratio kj/f. 

There is a negligible effect of adsorbate-adsorbate interaction on step surfaces. Some lateral repulsion of hydrogen adsorbed on Pt(lll) could be inferred. A strong adsorption of bisulphate and sulphate anions on the (111) oriented terraces and step sites considerably affects both reactions. These data show that each crystallographic orientation of the electrode surfaces gives a different electrochemical entity. 

Although the accuracy of this explanation will be discussed later, it is easily understood that the behavior of the electrode is greatly influenced not only by the instantaneous potential of the electrode (potential dependence ) but also by the history of the electrode (time dependence). As this example shows, the electrochemical oxidation of methanol is a series of reactions in which methanol, water, intermediates and surface adsorbates are interacting with each other in various ways, and are yet to be fully understood. 

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