Oxidation of chloride to chlorine by cerium(IV) ions mediated by a microheterogeneous redox catalyst

Abstract

At high chloride concentrations (2 mol dm^–3) the oxidation of chloride to chlorine by Ce⁴⁺ ions, i.e. 2Ce⁴⁺+ 2Cl^–→ 2Ce³⁺+ Cl₂ was found to be catalysed by an activated form of ruthenium dioxide hydrate (RuO₂·x′H₂O^*). A kinetic study of this process revealed that the disappearance of Ce⁴⁺ ions was first-order with respect to the concentrations of Ce⁴⁺ and redox catalyst, and zero-order with respect to the concentrations of chloride and Ce³⁺ ions. On reducing the chloride concentrations below a threshold value, ca. 0.2 mol dm^–3, the Cl₂ and O₂ yields were found to decrease (from 98 to 0%) and increase (from 0 to 95%), respectively, although the rate of disappearance of the Ce⁴⁺ ions remained almost unaffected. At [Cl^–] < 0.01 mol dm^–3 only O₂ evolution was observed. These results were interpreted using an electrochemical model in which the catalyst particles were assumed to be acting as microelectrodes and the current density due to chlorine evolution was proportional to the square of the chloride concentration. In addition to activated ruthenium dioxide hydrate several other materials were tested successfully for catalytic activity, including non-activated ruthenium dioxide hydrate (RuO₂·xH₂O). However, unlike RuO₂·x′H₂O^* these other materials did not catalyse the oxidation of water to oxygen by Ce⁴⁺ ions in the absence of brine.