The kinetics of chlorine evolution and reduction on glassy carbon in molten tetrachloroaluminate at 175°C

Abstract

The chlorine electrode reaction on glassy carbon in sodium tetrachloroaluminate melt (AlCl 3+NaCl) with near equimolar compositions was investigated at 175°C with voltammetric techniques. The kinetic parameters (Tafel slope and exchange current density) measured as functions of chloride ion activity and partial pressure of chlorine, and the reaction orders with respect to Cl − and Cl 2 have been collected extensively, being compared with the theoretical kinetic derivatives deduced from the rate equations solved under three different kinds of adsorption isotherms: Langmuir, non-activated Temkin and activated Temkin isotherms. All the evidence collected in this study indicates that the reaction mechanism for both evolution and dissolution of chlorine consists of a fast electron transfer (Cl −→Cl ad+ e) followed (or preceded) by a slow Heyrovsky-type reaction (Cl −+Cl ad→Cl 2+ e) on glassy carbon surfaces where the adsorbed intermediate obeys the activated Temkin isotherm. The exchange current density was found as 8.6±0.8 μA cm −2 at 175°C in the melt of pCl=1.1 under an atmospheric pressure of Cl 2, and its electrode potential ( E° CΓ/Cl 2 ) was determined as 2.182±0.005 V vs. Al.