Chlorine evolution reaction

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 ... 

At low coverage, the Tafel slope will be 2RT/3F or c. 40 mV, as observed on high at.% Ru electrodes. As the at.% Ru decreases, the number of Ru sites decrease, resulting in more coverage of the active Ru sites by 0ac]. Hence 0ad will approach 1 and the Tafel slope will tend to reach values of 2RT/F or 120 mV, thus potentially explaining the results in Fig. 5.3. Alternatively, this change in the Tafel slope may arise from an increase in the electrical resistivity of the low at.% Ru electrodes, during the course of the chlorine evolution reaction [35]. 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

The evolution of hydrogen at the cathode leads to the buildup of alkalinity the evolution of oxygen at the anode causes an increase in acidity because the protons generated by Eq. (2) immediately undergo hydration to yield H30+ entities. This is indeed experimentally observed. The chlorine evolution reaction (Eq. 2a) is central and concomitant to the evolution of oxygen. 

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-... 

In a recent study, Harrison et al. [485] used steady-state j-E and Z(co)-E data to characterize the chlorine evolution reaction at Ru02/Ti02 electrodes using a simple redox reaction description of the chlorine evolution process with HOC1 and CR as reactant and product, respectively. The impedance potential data were analyzed by the equivalent circuit method parameter curves such as CiX-E and Rct-E. It has been suggested by the authors [485] that this type of parametric analysis of impedance data can be useful for comparison of the activity of various types of electrodes. 

It is now recognized that anomalously low Tafel slopes can be observed for the chlorine evolution reaction due to rate-limiting transport of gas away from the electrode surface [471, 474], e.g. in concentrated chloride solutions at high temperatures. 

Consider again the chlorine evolution reaction and let us assume first that step 8F is rate determining... 

To show the difference between these two reaction order parameters, we return to the equations derived in Section 14.3 for the chlorine evolution reaction. Consider the case in which the first charge-transfer step (Eq. 13F) is rate determining. Equation 16F in logarithmic form, is ... 

XVIII. Electrode Kinetic Behavior of Chlorine Evolution Reaction, and Role and Identity of Adsorbed Intermediates... 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

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. 

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

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