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6 CHAPTER1. INTRODUCTION containing e.g. platinum, ruthenium, iridium and rhodium were in this way ap- plied on titanium. This change took place in the early 1960s. One coating which was frequently used was a mixture of 70% platinum and 30% iridium. This metal combination had actually been known to be stable in these processes already since the early 1900s[25]. Except for coatings containing platinum, the heating proce- dure that was used to prepare such coated titanium anodes actually converted the noble metal into metal oxide form. Electrodes with this coating were preferable to the old graphite electrodes and to platinum coatings, but the anodes were still not stable enough in the mercury cell process. Furthermore, the prices of irid- ium and platinum were also high in comparison with other noble metals, such as ruthenium[21, 26]. By the middle of the 1960s, Beer proposed that a coating of ruthenium oxide could be preferable to the Pt/Ir coating. It was eventually found that mixing a platinum-group metal (PGM) oxide with a film-forming metal oxide could create a mixed oxide with improved stability. The mixture that was most promising was that of ruthenium oxide and titanium oxide[21]. The first metal oxide patent, for a coating of 50% ruthenium oxide and 50% ti- tanium oxide was filed by Beer in 1965 (“Beer 1”)[27]. Anodes coated with this mixture were claimed to be more durable in mercury cell processes and to have a stronger adhesion of the coating layer to the titanium. Two years later, a second patent was filed (“Beer 2”)[28]. It introduced that the coating should be made up of 30% RuO2 and 70% TiO2. This combination had several advantageous proper- ties. It gave a high activity and was well suited for use in chlor-alkali and chlorate production. The usage of 70% titanium dioxide in the coating reduced the price of the coating. Furthermore, the mixed oxide of ruthenium oxide and titanium ox- ide showed a high stability, enabling operation at higher temperatures and current densities compared with those possible when using graphite electrodes. Titanium oxide has a low electric conductivity but, in the mixed oxide, RuO2 increases the electric conductivity of the coating. Another important advantage, especially of the Beer 2 combination, is that the selectivity for chlorine evolution over oxygen formation is increased when the ruthenium amount in the coating is decreased[29– 35]. Combined with the self-healing properties of the supporting titanium material, these properties made the “Beer 2” DSA a great step forward in anode technology for the chlor-alkali and chlorate industries.[1, 21, 22, 26, 36] The “Beer 2” DSA is considered the standard industrial DSA composition. How- ever, electrode manufacturers usually make further modifications of the electrode, although the details of these modifications are usually kept secret. For exam- ple, coatings containing a reduced amount of ruthenium oxide replaced with tin oxide have been developed to improve the stability of the coating further. In other coatings, some ruthenium is replaced with iridium oxide, also to improve stability.[1, 26] Nevertheless, the standard 30% RuO2 - 70% TiO2 coating has been studied in de- tail, and it is now known that it is made up of small electrode particles of rutilePDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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