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8 CHAPTER1. INTRODUCTION current densities (several kAm−2) is yet to be realized[47]. As mentioned above, RuO2 deposited on Ni is used in both alkaline water electrol- ysis and chlor-alkali production[3, 13]. However, RuO2 is not thermodynamically stable versus reduction under normal HER conditions. This has been associated with the decomposition of the cathode, especially under shutdown conditions[50]. Indeed, Trasatti [13] points out that degradation during shutdown is actually a main factor limiting the stability of cathodes. Still, the degree to which RuO2 is reduced is debated in literature, with some even suggesting a bulk conversion of the oxide into metallic form[51]. However, other studies have come to the conclusion that the reduction that occurs is due to incorporation of H into the coating, and that formation of metallic Ru does not occur[52, 53]. This topic has been studied in the present thesis, by comparing calculated XPS shifts with experimental ones of Näslund et al. [51]. 1.2 The present understanding of the selectivity in chlor-alkali and sodium chlorate production The factors that control the selectivity between parasitic oxygen evolution and the desired chlorine gas, or sodium chlorate, production, can essentially be divided into three groups: effects resulting from process parameters (including tempera- ture and concentrations of the main intermediates and reactants), effects resulting from contaminants in the electrolyte (such as those possibly released from the electrodes or construction materials) and finally effects of the electrode structure and composition. In this section, only the more well-understood effects will be presented, and a detailed discussion is provided in [1]. 1.2.1 Process conditions Overall, the effects of process conditions are likely the most well-studied, in com- parison with electrolyte impurity and electrode composition effects. The main anodic reaction is chloride oxidation, reaction 1.1. Oxygen evolution by water oxidation, reaction 1.8, or hypochlorite oxidation under chlorate conditions, are likely important anodic side reactions. Electrochemical chlorate and perchlorate formation will be disregarded for the moment, as these can be controlled by keep- ing the hypochlorite or chlorate concentration at the electrode low[9]. Water oxidation according to reaction 1.8 is clearly pH dependent, and this reaction can be controlled by keeping the pH low. Furthermore, HOCl formation from Cl2 is prevented if the pH is kept low. This is the basis for anolyte acidification, which is practiced in chlor-alkali membrane cells[54]. Still, oxygen does form in industrial cells, although there is some dispute regarding which reaction (orPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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