Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, concerning the selectivity between chlorine and oxygen evolution in chlorate and chlor-alkali production. It covers experimental as well as theoretical studies and highlights the relationships between process conditions, electrolyte composition, the material properties of the anode, and the selectivity for oxygen formation.

Hypochlorite decomposition has been investigated by the combined measurement of aqueous concentrations of total hypochlorite, chlorate, and chloride, as well as that of evolved oxygen. In all experiments, the initial concentrations of NaOCl and NaCl were 80 mM, and the temperature was 80 °C. The pH was kept constant in the range 5−10.5. The uncatalyzed decomposition of hypochlorite and the formation of chlorate and oxygen were all found to be third order of the form r i = k i [HOCl] 2 [OCl − ], and k O 2 was determined to be 0.046 M −2 s −1 . A reaction mechanism in which oxygen and chlorate formation share an intermediate is proposed. Several compounds were tested for catalytic effects. The addition of chloride salts of cobalt and iridium showed catalytic effects on oxygen formation. The addition of iridium chloride also catalyzed the formation of chlorate with increasing selectivity for chlorate with increasing pH.

Molybdenum sulfide structures, particularly amorphous MoS 3 nanoparticles, are promising materials in the search for cost-effective and scalable watersplitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS 3 to defective MoS 2 when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS 3 nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoS x systems. We demonstrate that, under operating conditions, surface sites are converted from MoS 3 to MoS 2 in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS 2 edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS 2 edge-like sites after the reduction of MoS 3 sites.

The electrocatalytic properties of the (1 1 0) surface of Ru-doped TiO 2 , Ti-doped RuO 2 and the industrially important Dimensionally Stable Anode (DSA) composition Ru 0.3 Ti 0.7 O 2 have been examined using density functional theory. It is found that the oxygen adsorption energy on a Ti site is strongly aected by the presence of small amounts of Ru dopant, whereas oxygen adsorption is relatively unaected by Ti dopants in RuO 2. The calculations also indicate that coordinatively unsaturated Ti sites on Ru-doped TiO 2 and on Ru 0.3 Ti 0.7 O 2 could form active and selective sites for Cl 2 evolution. These results suggest a reason for why DSA shows a higher chlorine selectivity than RuO 2 and propose an experimental test of the hypothesis.

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