Electrode surfaces adsorption-desorption rates

To illustrate the influence exerted by the energy of adsorption of an intermediate on the rate of an electrocatalytic reaction, consider a very simple two-step reaction of the type A —> X —> B where X, the intermediate, is reversibly adsorbed on the electrode (with a degree of surface coverage 9x). For the sake of simplicity, the electrode surface will be assumed to be homogeneous (i.e., conditions of Langmuir adsorption hold), while the system lacks adsorbed species other than X. The rate, of the adsorption step (the first step) is then proportional to the bulk concentration of the starting material, c, and to the free surface part (1 - 9x) (the part not taken up by species X), while the rate of further transformation of intermediate X, which is tied to its desorption, will be proportional to the surface fraction, 9x, taken up by it ... 

The rate constants of the adsorption and desorption step, and depend on the energy, W, of bonding of the adsorbed species to the electrode surface. Higher bond energies imply that adsorption is facilitated and k increases while k decreases. These functions can be formulated as... 

The extent to which ions, etc. adsorb or experience an electrostatic ( coulombic ) attraction with the surface of an electrode is determined by the material from which the electrode is made (the substrate), the chemical nature of the materials adsorbed (the adsorbate) and the potential of the electrode to which they adhere. Adsorption is not a static process, but is dynamic, and so ions etc. stick to the electrode (adsorb) and leave its surface (desorb) all the time. At equilibrium, the rate of adsorption is the same as the rate of desorption, thus ensuring that the fraction of the electrode surface covered with adsorbed material is constant. The double-layer is important because faradaic charge - the useful component of the overall charge - represents the passage of electrons through the double-layer to effect redox changes to the material in solution. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

Figure 8.4 Cyclic voltammograms of one to five layers of bis-bipyridinum cyclophane (1) cross-linked AuNPs at an ITO electrode showing adsorption (oxidation) and desorption (reduction) of oxygen at the NP surface corresponding to surface area. 1.0 M H2S04, scan rate = 50 mV s 1. Inset Calibration curve is the number of AuNPs cm 2 (n) versus number of layers.4 (Reprinted with permission from A. N. Shipway et al., ChemPhysChem 2000,1, 18-52. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

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]. 

Because the rates of adsorption-desorption at electrode surfaces often are rapid ... 

Since the electrocatalytic reaction implies the existence of an adsorbed species as an intermediate, reactant, or product, the direct interaction with the electrode surface has to be considered first. In this sense, the kinetics of the formation and the stability of the adsorbate are of great importance and may be the determining step for the final value of The slow adsorption kinetics in the case of a reactive adsorbate will make the reaction at the electrocatalyst not fast enough to become operative. However, the same situation can occur in the case of an adsorbed product with a slow desorption kinetics. The most problematic situation can arise due to the stability of an adsorbed intermediate on the surface, which is the rate-determining step of the whole process. In the case of an anodic process, the species desorption can be aided by the presence of a metal oxide on the surface. An interesting example of stable and efficient anodes is the dimensionally stable electrodes (DSE) used in brine... 

The first step is to relate the rate of adsorption to the surface coverage at equilibrium. When the adsorption process is in equilibrium with the solution immediately adjacent to the electrode surface, the adsorption rate 4 should be 0 (since the rate of adsorption equals the rate of desorption, the net change in c02,ads is 0) ... 

Cyclic voltammetry is a widely used electrochemical technique, which allows the investigation of the transient reactions occurring on the electrode surface when the potential applied to the electrode is varied linearly and repetitively at a constant sweep rate between two given suitable limits. The steady-state current-potential curves or voltammograms provide direct information as to the adsorption-desorption processes and allow estimating the catalytic properties of the electrode surface. 

A higher time resolution of 28 ms was used in a rapid-scan time-resolved SEIRA study of adsorption kinetics of uracil on a quasi-Au(lll) electrode surface [15]. Since the adsorption/desorption of uracil is reversible, the S/N of the spectra were enhanced by averaging 64 consecutive repetitions. This study showed that the kinetics is represented by the first-order rate equation, and that the rate constants evaluated from the time-resolved SEIRAS are much smaller than those for overall transition determined by chronoamperometry. The result suggests that the processes observed by the two measurements are not the same. 

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