Chlorine evolution reaction intermediates

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 ... 

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. 

It should be emphasized that thermodynamically [73,74], Ru02 can oxidize to RuOJ or Ru04 (in alkaline solutions). Ru02 can be leached from the anode during the course of the chlorine evolution reaction via the loss of the unstable adsorbed intermediates or the oxidized surface oxides formed during the discharge of the chloride ions as described by the reaction schemes (13)-(15). 

Ruthenium losses arising from the erosion and the dissolution of the adsorbed intermediates formed during the course of the chlorine evolution reaction, and the surface oxides [75], are depicted in Fig. 4.5.15. These results [76] show the dissolution rate (Vc in g cm hr" ) of Ru to be high initially and decrease rapidly with time (/ in hr). Thus,... 

Ru losses can occur during electrolysis, as well as those due to shorting in mercury cell operations, from erosion, loss of Ru-based intermediates involved during the course of chlorine and oxygen evolution reactions and during shut-downs. 

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]. 

Chlorosulfate groups can be readily removed to give the corresponding hydroxyl groups, with retention of configuration, by treatment of a solution of the carbohydrate chlorosulfate in methanol with sodium iodide in aqueous methanol24 immediate liberation of iodine and evolution of sulfur dioxide occur.34 A possible mechanism for the dechlorosulfation reaction involves displacement by iodide at the chlorine atom the initially formed iodine monochloride would react with iodide ion to give iodine and chloride ion. Alternatively, an unstable iodosulfate could be formed as an intermediate.35... 

The evolution of HBr in the bromination reactions and the uptake of one bromine atom per ring indicate substitution at a secondary allylic carbon atom. The ease of oxidation and cross linking of the polymers and the presence of hydroxyl groups imply the intermediate formation of hydroperoxides on allylic carbon atoms. Treatment of the hydroxyl-containing polymer with benzoyl chloride indicates that the bulk of the hydroxyl groups are on secondary carbon atoms, since tertiary hydroxyl groups would tend to be replaced by chlorine. Although these results do not permit the elimination of structure B, it appears that the bulk of the structural units in the polycyclopentadiene corresponds to 1,2- addition (A). 

Many reactions of industrial importance are electrocatalytic, i.e., they involve the specific adsorption of intermediates, for example hydrogen, chlorine, and oxygen evolution, oxygen reduction, and methanol or ethanol oxidation in fuel cells. Many different electrochemical techniques were used to study these reactions, and EIS is one of them, providing interesting kinetic and surface information. Certain model reactions will be presented in what follows with a detailed method of relating impedance parameters with mechanistic and kinetic equations. 

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