Cathodic behaviour and oxoacidity reactions of samarium (III) in two molten chlorides with different acidity properties: The eutectic LiCl-KCl and the equimolar CaCl_2-NaCl melt

摘要

This work presents a study on the chemical and electrochemical properties of Sm(III) solutions in two molten chloride mixtures with different acidity properties: (i) the eutectic LiCl-KCl, in the temperature range 673-823 K and (ii) the equimolar CaCl_2-NaCl melt at 823 K. In both media and on a W inert electrode, the electro-reduction of Sm(III) takes place via one step Sm(III)/Sm(II). The second system Sm(II)/Sm(0) has not been observed within the electrochemical windows, because of the prior reduction of Li(I) and Na(I) from the solvent, which inhibits the electro-extraction of Sm species from the salts on the inert substrate. On the W electrode, the electro-reduction of Sm(III) to Sm(II) takes place in a quasi-reversible electrochemical mode. Accurate values of the reversible half wave potential, the intrinsic rate constant of charge transfer, k~0, and the charge transfer coefficient, α, have been calculated for the first time in both molten chlorides, by simulation of the cyclic voltammograms and logarithmic analysis of the convoluted curves. The diffusion coefficient of Sm(III) has been also calculated by different electrochemical techniques, avoiding the meniscus effect by modification of the immersion dept of the working electrode in stages. The values of the diffusion coefficient indicate that Sm(III) diffuses slower in the equimolar CaCl_2-NaCl than in the eutectic LiCl-KCl. This behaviour can be explained by the different viscosities of both media, which significantly reduces the mobility of Sm(III) in the CaCl_2-NaCl melt. The identification of the Sm-O compounds that are stable in the melts as well as the determination of their solubility products were carried out by potentiometric titration using an yttria stabilised zirconia membrane electrode (YSZME). The results indicated that SmOCl is a solid stable compound in the studied melts and that Sm_2O_3 is a strong oxobase leading to the formation of SmOCl. The best chlorination conditions have been extracted from the comparison of the E-pO~(2-) diagram of samarium and that of some gaseous chlorinating mixtures previously reported.