Chlorine evolution mechanism

Dr T V Bommaraju 68 Dolphin Drive, Grand Island, NY 14072, USA. Deactivation of Thermally Formed RuO2 + Ti02 Coatings During Chlorine Evolution Mechanisms and Reactivation Measures. E-mail tilak ibm.net... 

Deactivation of Thermally Formed Ru02 + Ti02 Coatings During Chlorine Evolution Mechanisms and Reactivation Measures... 

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. 

To summarise, AC methods have proved most successful where the system is straightforward and can be modelled analytically. By measurement over a wide range of frequencies the constants for the reaction steps constituting the model can be established and, particularly if adsorbed species are involved, AC methods have proved very powerful indeed, with a major area of application being in the study of metal passivation, as discussed in detail elsewhere in the book. An example of this behaviour in practice is provided by the work of Conway s and Hillman s groups on chlorine evolution at platinum. Several mechanisms for this reaction have been proposed, including both Volmer and Heyrovsky types ... 

On the basis of Tafel slope studies and potential relaxation transients, mechanism (1) has been suggested for Cl2 evolution at Pt, whereas chlorine evolution at the technically hydrated Ru02/Ti02 electrode appears to proceed by (3) or, possibly, (4). 

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 ... 

Electrocatalysis of electrochemical chlorine evolution from Ru02-coated titanium anodes rests on the same grounds according to the so-called Krasil shchikov mechanism, which is schematically described by the reactions (8a) and (8b) for anodic chlorine evolution (9-/2) ... 

Owing to the success of Ru02-based DSA electrodes in the chlor-alkali industry, a significant amount of study has been carried out on the kinetics and mechanism of chlorine evolution at Ru02-based electrodes over the past 15 years or so. A considerable body of experimental data has therefore been accumulated regarding the chlorine evolution reaction at Ru02 electrodes, which includes E vs. log j plots, reaction order determinations, pH depen-... 

Inai et al. [483] suggested that an activated complex with a pentagonal bipyramid-type structure was formed in the transition state in the Volmer— Heyrovksy-type mechanism for chlorine evolution at Ru02. A theoretical activation energy for this reaction was calculated by using the difference in the crystal field stabilization energy between the initial and transition... 

C. Mechanism of Chlorine Evolution Reaction on Cobalt Oxide... 

Chlorohydrins and dichloroalkanes are formed in electrogenerative chlorination of olefins on platinum black (50). The former predominate at high currents and low anode potentials, with no side reactions of chlorides, such as chlorine evolution. From the identified products, the following tentative mechanism has been proposed for ethylene chlorination (50) ... 

The kinetics and mechanism of the chlorine evolution reaction in aqueous solutions have been studied on smooth, porous, and impregnated graphite [68, 69], The Tafel slope depends also on the nature and history of carbons. For HOPG and glassy carbon, the anodic Tafel slope is about 0.060 and 0.120 V per decade at 25°C, respectively, whereas for a graphite electrode consisting of a section parallel to the c-axis, three regions in the polarization curve with anodic Tafel slopes from 0.060 to 0.160 V per decade have been observed. 

Janssen, LJ. and Hoogland, J.G. (1970). The electrolysis of an acidic NaCl solution with a graphite anode. III. Mechanism of chlorine evolution. Electrochtm. Acta, 15, 941-51. 

Thus the studying of eleotrode processes has shown that at the potential values close to the chlorine evolution potential, adsorption of olephines considerably decreases, it suggests that the majority of ohlorohydrines forms in the electrolyte volume aooording to the ohemioal mechanism. 

It was noted above that one should seek electrode materials where the overpotential for chlorine evolution is as low as possible but where there is a considerable oxygen overpotential. In addition, for diaphragm and membrane cells there is the requirement for cathodes with a low hydrogen overpotential. Catalysis of hydrogen evolution was discussed in Chapter 1 and it was shown that strong catalysis is promoted by metals where the free energy of adsorption of hydrogen atoms has an intermediate value. This allows mechanisms such as... 

TABLE 4.2.4 Summary of the Reaction Mechanisms and /q Values" for the Chlorine Evolution Reaction as a Function of rZ-Electron Configurations at Various Electrocatalysts... 

As the mol% Ru02 decreases, the mechanism of the chlorine evolution (Fig. 4.5.7) changes from one involving adsorbed intermediates to a slow electron transfer step, presumably because of the limited number of electrocatalytic sites available for the reaction. Note that if adsorbed intermediates participate, the Tafel slope is 30-40 mV, and when the charge transfer step is slow, the Tafel slope is 120 mV [70]. 

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