Formation of active chlorine species involving the higher oxide MOx+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 HOClads on the anode surface in a single pathway, and subsequent production of Cl-2 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 center dot is detected in considerably larger amounts under some experimental conditions, and its onset overpotential is similar to 150 mVmore negative than HOCl appearance at 0.2 mol L-1 NaCl. Additionally, themass signals of OCl center dot 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 (OClads center dot) 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 MOx(O-center dot)(ads) (chemisorbed oxygen). Although the mechanisms involving the productions of HOClads and OClads center dot can simultaneously occur on two type of non-identified active sites, promoting the generation of both MOx((OH)-O-center dot)(ads) and MOx(O-center dot)(ads) species, both strongly depend on surface pH.