Chlorine evolution reaction 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 ... 

Elementary steps proposed for the chlorine evolution reaction are formally similar to those for the HER and involve discharge of Cl ion at metal surface or surface oxide sites S, followed by recombination or electrochemical desorption of the SCI -(ads) species to form CI2, as shown below (S representing. 

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. 

In other barrierless reactions, particularly chlorine evolution on graphite, no limiting current in the backward process was observed, the reason being that, in these cases, the slow step of the forward reaction was the transfer of the first electron, followed by that of the second, e.g., in an electrochemical desorption step. In the backward process, the slow activationless step is, in this case, preceded by the transfer of a single electron. The relationship between the rate of this process and the potential masks the limiting current phenomenon. 

The complex reaction of azide oxidation has not been investigated as thoroughly as the chlorine evolution process. However, on the basis of the above data, we can conclude that the most probable mechanism is a slow quasibarrierless reaction, while for higher potentials, it is an ordinary reaction like electrochemical desorption. 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

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