Adsorption on Electrocatalysts

The electrode surface participates actively in an electrochemical reaction sequence by providing adsorption sites for at least one reactant and for the reaction intermediates. Thus, the reaction rate and selectivity depend strongly upon the surface properties and its mode of interaction with reactive species and electrolytes. The existence, however, of the structured double layer interface and of the electric field under which electrosorption takes place distinguishes the latter from gas phase adsorption. Electrolyte ions, solvent molecules, and impurities may adsorb and compete with reactants for surface sites or they may poison the surface or contribute to surface changes under reaction. Despite the wealth of experimental information on the potential dependence of surface coverage and on the nature of some adsorbing species, a fundamental understanding of electrosorption mechanisms is still incomplete. 

Today there is ample evidence in catalysis that the assumption of a homogeneous surface is not valid. Thus, surface irregularities exist such as crystallographic dislocations, defects, and planes with different activity adsorbing molecules may interact with the surface and with each other several adsorption states of a species may exist on a single plane. These 

Although the above models would explain qualitatively the observed multiple adsorption states, there is currently no fundamental treatment to predict such bonding. The possible existence of multiple states deserves attention for other adsorbents as well, including carbonaceous species. The potential dependence of such adsorbed states could explain reactivity and specificity of some reactants. 

A comparison of mechanistic models with observed kinetics is strongly dependent on the adsorption isotherm adopted for reactants, products, or intermediates. Because of simplifying assumptions and of the uncertainties about surface species and catalytic sites, more than one isotherm can yield kinetic expressions in accord with the experimental data. The Langmuir isotherm 94a) is still used extensively because of its simplicity. However, the assumption of a homogeneous catalytic surface with localized adsorption is often not valid, as shown above. Hence, the increasing efforts to use the more realistic Temkin isotherm 88). 

Upon rearrangement, the electrochemical equivalent of the Langmuir isotherm is obtained. If 0 1, Eq. (20) reduces to Henry s isotherm 

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