Investigation of the Mechanism of the Rare-Earth Metal Ions and Nickel Ions Co-Reduction in KCl–NaCl–CsCl Eutectic Melt
摘要
In this work, the electrochemical behavior of dysprosium, neodymium, and lanthanum ions and their co-reduction with nickel ions on tungsten and nickel electrodes in a KCl–NaCl–CsCl eutectic melt at a temperature of 823 K is studied. The electroreduction of Ln3+ ions is found to proceed reversibly in one three-electron stage up to the polarization rates of 0.1 V/s. At the compresence of lanthanide and nickel ions, the voltammograms show reduction waves of nickel ions at potentials of –0.12…–0.3 V; those and of lanthanide ions, at –2.13…–2.18 V vs. silver/silver chloride reference electrode. In addition to these waves, the voltammograms have three more recovery waves in the potential regions: –1.68…–1.77 V; –1.95…–2.0 V; ‒2.13…–2.18 V. The appearance of these waves is connected with the joint electroreduction of lanthanide and nickel ions on the metallic nickel pre-deposited onto the tungsten electrode with a certain depolarization and formation of intermetallic phases of lanthanides and nickel of different compositions (LnxNiy). The open-circuit chronopotentiograms revealed plateaus of potential delay corresponding to the dissolution of separate phases of intermetallides. Electrolysis under potentiostatic condition at potentials –1.7…–2.1 V and at certain concentrations ratio of lanthanide and nickel chlorides produced intermetallic phases LnNi5, Ln2Ni7, Ln2Ni3, LnNi3, and LnNi2. The synthesized samples of lanthanide and nickel intermetallides were characterized by X-ray diffraction analysis and scanning electron microscopy. The e.m.f. for LnxNiy intermetallic compounds in two-phase coexisting states at 823 K is measured. The relative partial molar Gibbs free energy and activity values for lanthanide in the LnxNiy intermetallic compounds were calculated from the measured e.m.f. s.