Desorption potential-dependent

Electroneutral substances that are less polar than the solvent and also those that exhibit a tendency to interact chemically with the electrode surface, e.g. substances containing sulphur (thiourea, etc.), are adsorbed on the electrode. During adsorption, solvent molecules in the compact layer are replaced by molecules of the adsorbed substance, called surface-active substance (surfactant).t The effect of adsorption on the individual electrocapillary terms can best be expressed in terms of the difference of these quantities for the original (base) electrolyte and for the same electrolyte in the presence of surfactants. Figure 4.7 schematically depicts this dependence for the interfacial tension, surface electrode charge and differential capacity and also the dependence of the surface excess on the potential. It can be seen that, at sufficiently positive or negative potentials, the surfactant is completely desorbed from the electrode. The strong electric field leads to replacement of the less polar particles of the surface-active substance by polar solvent molecules. The desorption potentials are characterized by sharp peaks on the differential capacity curves. 

Evidence that H20 species also interact with the Ag electrode independent of adsorbed anions comes from the potential dependence of the i/(0H) intensity as compared with the i/(Ag-X) (X-Cl", Br") intensities. The normalized intensities of the i/(Ag-X) (X-Cl", Br") vibrations in 0.1 M KC1 and 0.1 M KBr are shown in Figure la, and the corresponding intensities of the v(0H) vibration shown in Figure lb. The observation that the intensity of the i/(0H) vibration reaches a maximum at more negative potentials than the i/(Ag-X) (X-Cl", Br") vibrations has been interpreted as indication that the H20 molecules can become maximally adsorbed on the surface when the positive charge has decreased to allow partial desorption of the anions.(21) Obviously, the potential at which this occurs depends on the strength of interaction of the anion with the electrode. 

If the adsorption step itself is rate-limiting, one must have available rate expressions for the adsorption and the desorption steps. The flux in (2.108) is then split into two opposing components. Using the notation of Delahay and Mohilner [201,403], there is a forward flux vj, adding to the adsorbate s surface concentration and backward flux tadsorbed substance. These obey rate equations rather analogous to those for electron transfer, the Butler-Volmer equation, in the sense that there are rate constants that are potential dependent. For the forward and backward rates, we have... 

Lorenz and Sali610 also propose an alternative kinetic approach to the determination of / this assumes a potential dependence of the adsorption and desorption rates of the pet process of Eq. (1) which is entirely analogous to that of the Butler-Volmer equation ... 

In this scheme the k values represent rate constants, which are generally potential dependent. It must be emphasized that the four-electron pathway does not imply the transfer of four electrons in a single step rather, it underscores the fact that all intermediate species, such as, but not restricted to peroxide, remain bound to the electrode surface yielding, upon further reduction, water as the sole product. Also depicted in Scheme 3.1 are mass transport processes (diff) responsible for the replenishment of 02 and removal of solution phase peroxide next to the interface, and the adsorption and desorption of the peroxide intermediate, for which the rate constants are labeled as ks and k6, respectively. Not shown, for simplicity, is the one-electron reduction of dioxygen to superoxide, a radical species that exhibits moderate lifetime in strongly alkaline electrolytes [15]. 

Intermediates, if adsorbed on the surface of the electrode substrate, are at low coverages (note, however, that when coverages of intermediates do approach saturation, potential-dependent rates can still arise if an electrochemical desorption type of step is involved then = 118 mV dec at 298 K for P taken as 0.5). 

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