Formation of active chlorine species involving the higher oxide MO_x+1 on active Ti/RuO₂-IrO₂ anodes: A DEMS analysis

It is accepted that the mechanism of electro-generated active chlorine (i.e. chloride oxidation on DSAs) involves water oxidation and hydroxyl radical co-electrosorption to initially form HOCl_ads on the anode surface in a single pathway, and subsequent production of Cl₂ and OCl⁻ ions. The present study comprehensively evaluates the chloride oxidation on Ti/RuO₂-IrO₂ in solutions containing different NaCl concentrations and pH values. Differential Electrochemical Mass Spectroscopy (DEMS) is used to account for the in-situ surface reactions simultaneously occurring during chloride and water oxidations. It is found that HOCl does not lead the chlorine mechanism since OCl^⦁ is detected in considerably larger amounts under some experimental conditions, and its onset overpotential is ~150 mV more negative than HOCl appearance at 0.2 mol L⁻¹ NaCl. Additionally, the mass signals of OCl^⦁ and HOCl are virtually similar at pH = 2 and slightly different at pH = 10, where these species should not respectively exist according to thermodynamic information. Under this premise, an extension of the reaction mechanism is proposed to consider an independent pathway producing adsorbed hypochlorite radical (OCl^•_ads) for electro-generated active chlorine, which could be the preferential one since RuO₂ and IrO₂ belong to the category of “active” anodes towards the Oxygen Evolution Reaction (OER), thus allowing the formation of the so-called higher oxide MO_x(^⦁O)_ads (chemisorbed oxygen). Although the mechanisms involving the productions of HOCl_ads and OCl^•_ads can simultaneously occur on two type of non-identified active sites, promoting the generation of both MO_x(^⦁OH)_ads and MO_x(^⦁O)_ads species, both strongly depend on surface pH.