Chlorine electrode reactions, mechanisms

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 ... 

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-... 

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. 

Other alkali-metal chlorates are produced by analogous technology while sodium and potassium bromate are produced electrolytically starting both from bromide ion and bromine solutions. The production of bromate is, however, a very small-scale process and the cells have not been optimized to any extent for example while cells with lead dioxide and platinized titanium have been described, some plants still use solid platinum electrodes The mechanism of bromate formation is identical to that described for chlorate by reactions (5.10)—(5.13) the kinetics are, however, different. The hydrolysis of bromine is slower than chlorine but the disproportionation step is much faster (by a factor of 100) and it is therefore advisable to use a more alkaline electrolyte, about pH 11. 

The evolution of chlorine from acidic solutions is certainly an elec-trocatalytic reaction, but experimental difficulties have arisen in fundamental studies, owing to corrosion in the chloride medium. Much work has nevertheless been carried out over the past 12-15 years on the chlorine evolution reaction and its mechanisms. A comprehensive review on this topic, including the relation of electrode performance to chemical composition of oxide films and their band structures, has been given by Novak, Tilak, and Conway in the Modern Aspects of Electrochemistry series. 

Arikado T, Iwakura C, Tamura H. Some oxide catalysts for the anodic evolution of chlorine reaction mechanism and catalytic activity. Electrochim Acta 1978 23 9-15. Alves VA, Da Silva LA, Boodts JFC, Trasatti S. Kinetics and mechanism of oxygen evolution on Ir02-based electrodes containing Ti and Ce acidic solutions. Electrochim Acta 1994 39 1585-9. 

The general concepts of electrocatalysis and the H2 evolution reactions were discussed in some detail in Chapter 1. Here, we should notice the similarity in the chlorine and hydrogen evolution reactions. Both involve the electrode reaction of an ion in solution to give a product which is dimeric and a gas. Hence, the mechanisms which should be considered will be similar, and it is to be expected that a critical property is the strength of adsorption of hydrogen or chlorine atoms on the surface of the electrode material. 

Over recent years, the electrochlorination of a wide range of substrates has been reported. The direct halogenation of benzene has been studied at Pt and Rh electrodes and the in situ spectro-electrochemical monitoring of the process with confocal microprobe Raman methods allowed optimization of the reaction conditions [80]. Toluene has been chlorinated at DSAs and the kinetics of this process have been explored in detail [81]. The electrochlorination of 1,4-dimethoxy-2-tert-butylbenzene has been reported in CCI4 and in acetonitrile environments [82]. A difference in mechanism has been proposed to explain the observation of l,4-dimethoxy-2-tert-butyl-5-chlorobenzene and l,4-dimethoxy-2-tert-butyl-6-chlorobenzene, respectively, as the main products. Succinimide is electrochlo-rinated to give A -chlorosuccinimide at platinum electrodes, but the process has been reported to be relatively inefficient due to side reactions (when compared... 

By diaphragms in a narrow sense we also understand a kind of partition which prevents as much as possible the diffusion of dissolved products of the electrolysis from one electrode to the other but must not hinder the passage of ions migrating under the influence of the electrical field in other words this partition should possess a high diffusion resistance but a low electrical resistance. Unless mutual diffusion and the mechanical mixing of anolyte and catholyte are prevented, i. e. on electrolyzing a solution of common salt, a mutual reaction occurs between the products of electrolysis, namely chlorine and hydroxide in which case hypochlorite ions are formed. They can then be converted, either electrochemically or chemically to chlorate ions. On separating both electrodes... 

The first is that the oxidation mechanism of organics in WEO occurs directly at the electrode surface, without evolving intermediate reactions that form chlorinated compounds, and that can be supported by the fact that free residual chlorine is not detected in the effluent samples for all electrolysis conducted at temperatures higher... 

As far as the chl.e.r. mechanism is concerned, the same, previously described, investigation has been performed and Figures 24 and 25 respectively report the polarization curve and the Tafel plot (currents normalized to the number of active sites at the electrode surface), for the case of a 1 M NaCl/3 M NaC104/0.01 M HCIO4 test solution. The measured Tafel slope has a value of 0.149 V, and the reaction order with respect to CP is about 0.7 the values of b and R both agree well with a Volmer-Heyrovsky mechanism [24], with a rate-determining electrochemical desorption, provided a value of about 0.7 is assumed for the coverage by the intermediate chlorine radicals [28] ... 

The oxidation of chloride and bromide ions is an example of an electroca-taly tic reaction and the mechanism has been the subject of much speculation [23, 24]. The oxidation of chloride ions is also the major industrial route to chlorine gas [25, 26]. Reactions of this family are termed electrocatalytic because the rate of the reaction, as measured by the current or the charge transfer resistance at a given potential, depends on the electrode material. This is a major reaction type in electrode kinetics. 

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