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12 CHAPTER1. INTRODUCTION An additional complication when it comes to the selectivity between oxygen evo- lution and chlorine evolution in chlor-alkali production is that electrodes with de- creased oxygen selectivity have been found to instead yield increased chlorate pro- duction rates[8, 31, 56, 73, 115]. Whether this is purely a pH effect (a decreased rate of reaction 1.8 resulting in a higher pH at the anode) or due to direct formation of chlorate at the anode is so far not known. The electrocatalytic properties of electrodes have mostly been studied using elec- trochemical measurements of OER and ClER activities, sometimes with direct measurements of selectivity. However, the possibility that electrodes might cat- alyze chemical decomposition of hypochlorite in the chlorate process has not been studied in much detail. Kuhn and Mortimer [116] suggested this possibility when using unpolarized RuO2 electrodes, while Kokoulina and Bunakova [63] and Ko- towski and Busse [117] did not find that unpolarized RuO2-TiO2 electrodes cat- alyzed the decomposition. To summarize, while the connection between the active surface area of the elec- trode and the selectivity for e.g. ClER is well understood, the connection between the electronic structure of the electrode material and the activity, and selectivity, is lacking. 1.3 Aims As has been made clear in the preceding sections, there are still significant gaps in the understanding of both oxygen-forming reactions in chlor-alkali and sodium chlorate production, and of the hydrogen evolution reaction on cathodes for water electrolysis. The aim of this thesis has been to further the understanding of these technical aspects of electrocatalysis for industrial electrosynthesis. The stability of electrodes has also been examined, primarily by XPS simulation to strengthen the connection between electrode structure and deactivation processes. In this work, both experimental and theoretical methods have been applied. Some key issues that have been examined include: • Why do mixed RuO2-TiO2 coatings exhibit increased chlorine evolution se- lectivity in comparison with pure RuO2? • How are the electrochemical properties of DSAs and of TiO2 altered by doping with other elements? • Which materials catalyze decomposition of hypochlorite? • Which is the mechanism of HER on Mo sulphide cathodes? • What structural changes occur during HER on RuO2?PDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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