Electrochemical desorption

Alternatively the adsorbed hydrogen atom may participate in a second electrochemical reaction, known as electrochemical desorption ... 

For iron it is reasonably well established that the reaction goes by way of chemical recombination under most circumstances, although there is some evidence that electrochemical desorption may take over in very alkaline solutions or at large overpotentials. 

However, when the second stage in the hydrogen evolution reaction is electrochemical desorption, the rate of this reaction is increased as the potential falls, and the adsorbed hydrogen concentration may remain constant or fall, according to the detailed electrochemistry. This results in curves such as that shown in Fig. 8.38 for steel in sodium chloride solution. 

The steady-state coverage, 0h, of the surface by hydrogen atoms can be found from the balance of their rates of formation and elimination. Discharge of the ions occurs only at sites free of adsorbed hydrogen. Hence, the reaction rate will be proportional to the fraction of free surface when considerable amonnts of hydrogen are present on the surface, and it can in brief be written as F/t Ce+Cl -0 /). The rate of electrochemical desorption can be written, similarly, as FhcC + Cg+ 0 In the steady state these two rates will be identical, which implies that... 

These species are adsorbed not only on the bare metal surface but also on surface sections already oxidized. They can undergo further reaction in a variety of steps, both purely chemical ones and steps of the electrochemical desorption type ... 

The mechanism of anodic chlorine evolution has been studied by many scientists. In many respects this reaction is reminiscent of hydrogen evolution. The analogous pathways are possible. The most probable one is the second pathway, in which the adsorbed chlorine atoms produced are eliminated by electrochemical desorption, but sometimes the first pathway is also possible. As a rule the first step, which is discharge of the chloride ion, is the slow step. 

TABLE 15.1 Comparison of Surface Areas Aj and Charges Required for Electrochemical Desorption of Hupd iQn), COads (2co)> and Surface Oxidation (2ox) ... 

C.A. Widrig, C. Chung, and M.D. Porter, The electrochemical desorption of n-alkanethiol monolayers from polycrystalline gold and silver electrodes. J. Electroanal. Chem. 310, 335-359 (1991). 

In the Volmer-Heyrovsky mechanism the second step also involves a charge transfer and is sometimes called electrochemical desorption ... 

Tafel slope for the chlorine evolution reaction follows an electrochemical desorption-type mechanism, it can be expressed [36, 37] in terms of the electrode surface coverage by the adsorbed Cl intermediates, 0aa, as ... 

CI2 evolution reaction, 38 56 electrochemical desorption, 38 53-54 electrode kinetics, 38 55-56 factors that determine, 38 55 ketone reduction, 38 56-57 Langmuir adsorption isotherm, 38 52 recombination desorption, 38 53 surface reaction-order factor, 38 52 Temkin and Frumkin isotherm, 38 53 real-area factor, 38 57-58 regular heterogeneous catalysis, 38 10-16 anodic oxidation of ammonia, 38 13 binding energy quantification, 38 15-16 Haber-Bosch atrunonia synthesis, 38 12-13... 

Alternatively, according to the Volme-Heyrovsky mechanism, the Volmer discharge reaction is followed by the Heyrovsky electrochemical desorption reaction... 

Step 2b Electrochemical desorption step (ED step) or ion-atom combination step ... 

Thus, there are two kinetic paths for the hydrogen evolution. The first path consists of charge transfer (CT) followed by chemical desorption (CD) path CT-CD. The second path consists of charge transfer (CT) followed by electrochemical desorption (ED) path CT-ED. Within each path, either of the consecutive steps can be slow and thus can be the rate-determining step (RDS). Each of these paths has two pKJSsible mechanisms. 

Also, reductive desorption of SAMs of such asymmetrical disulfides as butyl hexadecyl disulfide and decyl-2(perifluoro-hexyl)ethyl disulfide has been studied on Au(lll), using CV [168]. Peak potentials corresponding to electrochemical desorption waves of the adsorbed species were different from those obtained for monolayers... 

Pi 0.5 N NaOH 15.3+0.8 Fast discharge-slow electrochemical desorption... 

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