Mechanism of chlorine evolution on oxide anodes study of pH effects

Chlorine evolution on oxides such as polycrystalline RuO2, IrO2, Co3O4, NiCo2O4, RuO2 + IrO2 and the (110) face of RuO2 single crystals proceeds with the same Tafel slope of 40 mV and the same reaction order of 1 with respect to Cl−. On all oxides but RuO2 (110) the reaction rate is depressed by the acidity, which results in a reaction order with respect to H+ of −1 for IrO2, Co3O4 and NiCO2O4, of 0 for RuO2 (110) and variable for RuO2. Possible reasons for pH effects are discussed, and several plausible reaction mechanisms are examined.