Can HOCl be superficially stabilized on an electrocatalyst as proposed in the active chlorine mechanism involving •OH radicals?: Beyond the Cl2 evolution

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According to Comninellis’ proposal on water oxidation, anodes physisorbing active oxygen readily form •OH radicals to develop electrocombustion processes, while electrode surfaces forming the so-called higher oxide MO x+1 chemisorb active oxygen, being more prone to perform the O2 evolution reaction (OER). Here, we adopt a similar premise to account for the active chlorine mechanism (ACM), assuming that the capacity of an electrocatalyst to physisorb/chemisorb active oxygen determines the type of chlorine species formed and stabilized. Density Functional Theory (DFT) calculations are conducted to evaluate the stability of HOCl, OCl and OCl --H+ species oxidant species on Boron Doped Diamond (BDD), and rutile-type surfaces (110 plane) of SnO2, TiO2, PbO2, RuO2, and IrO2, in the absence and presence of H2O molecules. The computed relaxed models reveal that SnO2 and PbO2 physisorbing active oxygen are only able to stabilize OCl-ads species on pentacoordinated transition metal atoms (M5c); whereas RuO2, and IrO2 chemisorbing active oxygen produce Clads- on an adjacent M5c active site. The BDD surface is the only surface able to maintain the HOCl stability with and without H2O solvation. Thus, it is proposed that BDD and TiO2, physisorbing active oxygen, preferentially adopt a 2-electron transfer step to generate (HOCl)ads; while SnO2 and PbO2, with moderate physisorption, also follow a 2-electron transfer step but produced (OCl-)ads and (H+)ads, thus dissociating (HOCl)ads. RuO2 and IrO2, preferentially forming the so-called higher oxide MOx+1, adopt a 3-electron transfer step to either produce (OCl)ads, or (O)ads + (Cl)ads, where there is a more equal competition towards the oxygen evolution.