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.