Abstract
Electronic absorption spectra of vanadium ions were recorded between 450 and 750°C in 3LiCl–2KCl, NaCl–KCl and NaCl–2CsCl based melts. Analysis of the spectra showed that vanadium in the oxidation states of +2 and +3 is present in the melts as complex chloro-ions. Vanadium(II) can form the complex ions in octahedral and tetrahedral coordination, the latter being favored by higher temperatures. Only six-coordinated V(III) ions were observed. Vanadium in the oxidation state of +4 can be present in chloride melts only in the form of vanadyl-based oxochloro-species. Main spectroscopic parameters of vanadium complex ions were determined. Electronic absorption spectroscopy was also applied for studying a variety of the red-ox processes in vanadium containing melts including electrochemical oxidation and reduction of various species, and reaction of vanadium containing melts with oxygen.
Highlights
Vanadium has many attractive properties including high creep strength, excellent strength-to-weight ratio, resistance to corrosion, low neutron cross-section, high thermal conductivity, etc.[1]
Vanadium is a polyvalent metal and, as for many d-element ions with partially filled d-orbitals, electronic absorption spectroscopy was employed for characterization of vanadium species in fused salts
First electronic absorption spectra (EAS) of vanadium-containing melts were recorded over a half of a century ago in LiCl–KCl eutectic, KAlCl4 and Cs2ZnCl4,11–14 followed by KSCN based melts.[15,16]
Summary
In molten salts vanadium is capable of forming the ions containing the element in oxidation states +2, +3, +4 and +5. Vanadium(II) chloro-ions partially reacted with silica (the material of the experimental cell) forming an oxygen-containing phase insoluble in the melt. Anodic dissolution of vanadium in LiCl–KCl, NaCl–CsCl and NaCl–KCl based melts at 450–750◦C resulted in formation of vanadium(II) chloro-ions.[20] Dissolution of vanadium dichloride in molten alkali chlorides produced a mixture of V(II) and V(III) chloro-species due to partial disproportionation of VCl2 to VCl3 and vanadium metal.[20] Vanadium(III) containing melts were obtained upon dissolving VCl3.20 Electrochemical (potentiostatic) reduction of V(III) ions yielded V(II). Reaction of vanadium oxides (V2O5, 99+%, Reachim; V2O4, 99+%, Reahim; and V2O3, 98%, Aldrich) with hydrogen chloride in a molten solvent salt was used to prepare the melts containing vanadium oxo-chloro species. Red-ox titration[22] was used to determine the average oxidation state of vanadium
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