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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We present a benchmarking study of adsorption energies on transition metal surfaces computed with select functionals across different density functional theory codes. In addition to gradient corrected functionals, we evaluate the accuracies of representative metaGGAs including MS2, SCAN, and SCAN+rVV10 as well as a short-range screened hybrid functional, HSE06. The study shows that the challenge of finding a functional that can simultaneously capture both covalent and noncovalent molecule-surface interactions persists, with no single functional in the benchmarking study with average errors < 0.2 eV. We find that HSE06 on average does not improve the accuracy compared to PBE for the surface chemistry of transition metals. The BEEF-vdW dispersion-corrected GGA and the MS2 metaGGA yield the lowest errors in both chemisorption and dispersion energies, demonstrating that moving up the Jacob's ladder of functionals to screened hybrids does not necessarily improve the description of transition metal surface chemistry.I.

A comprehensive theoretical study of the X-ray photoelectron shifts for RuO 2 during hydrogen evolution has been performed. The shifts have been calculated using firstprinciples density functional theory and are compared with previous theoretical and experimental results to reconsider the proposed structural changes occurring during hydrogen evolution on RuO 2 . We find that during hydrogen evolution hydrogen enters the rutile RuO 2 lattice and converts oxygen groups into hydroxyl groups and that this process explains the experimentally observed increase in unit cell dimensions as well as observed chemical shifts. Furthermore, carbon contamination is the most likely explanation for a set of peaks previously identified as caused by a new RuO(OH) 2 phase. We find that formation of metallic Ru is just one possible explanation for another peak in the X-ray photoelectron spectrum and that explanations including conversion of RuO 2 into Ru(OH) 3 , or removal of oxygen from Ru active surface sites, also can explain the observed shifts.

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