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10 CHAPTER1. INTRODUCTION having been found to result in increased rates of parasitic oxygen evolution and effects on the decomposition rate of hypochlorite species. F– has been found to result in decreased selectivity for oxygen[58, 78, 81], probably due to competitive adsorption. There is likely no reason to consider F– addition to industrial cells (in fact, it has been claimed that F– might lead to electrode deactivation[82]), since the same effect most probably is achieved by increasing the chloride concentration. As hypochlorite concentrations should be kept low in chlor-alkali production, oxygen evolution due to decomposition of hypochlorite is a less severe problem in that process. The situation is more challenging in chlorate production. Co[83–91], Cu[83, 84, 90, 92, 93], Ni[83, 87–90, 92] and Ir[88, 94, 95] are all metals that have been found to catalyze hypochlorite decomposition, and might thus lead to oxygen evolution in both processes. 1.2.3 Electrode structure and composition The effects of electrode structure and composition can essentially be divided into two parts, as pointed out by Trasatti [36]: effects due to changes in electrode active surface area (the exposed number of active sites) and actual catalytic effects. The first aspect is most likely the one that is most well understood. Increasing the active surface area, though necessary for achieving a high electrode activity, is connected with an increased selectivity for parasitic oxygen formation[43]. Similar effects have been noted when decreasing coating particle sizes[96]. The effect is fun- damentally the same as that of increased current density, as an increased active surface area results in a decreased local (per-site) current density. It is likely that control of the surface area is one aspect that electrode manufacturers make use of already today. Doping a pure oxide often results in increased surface area[97, 98], but not necessarily in clear changes in actual catalytic activity (such as e.g. changes in Tafel slope[13]). Nevertheless, exceptions do exist. One such case is DSA itself, where a reduction in the Ru content of the mixed oxide down to about 20 to 30mol−% has been found to result in increased selectivity for chlorine evolution[29–35]. While this is likely partially an effect of the surface area also decreasing when reducing the Ru content[97], measurements of exchange current densities and Tafel slopes for OER on RuO2-TiO2 electrodes indicate that the Tafel slope increases and the exchange current density decreases as the Ru content is lowered, even at temperatures of 80 ◦C[99], both in alkaline[99, 100] and acidic[32, 39, 100] solutions. While some studies note increased Tafel slopes only below 30% Ru[99, 100], others find a more gradual increase in Tafel slopes when reducing the Ru content in the range 80 % to 20 % Ru[39]. Changes in Tafel slope reflect true electronic effects on cataly- sis, rather than surface area effects, while the exchange current density might be affected by changes in true surface area[13]. In the case of ClER, Tafel slopes have generally been found to be constant down to 20% Ru[100], in some casesPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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