Electrochemical and Raman Spectroscopic Investigations on the Speciation and Behavior of Chromium Ions in Fluoride Doped Molten LiCl-KCl

Abstract

The speciation and behavior of chromium ions in LiCl-KCl-CrF 3 and LiCl-KCl-LiF-CrF 3 melts were investigated by using cyclic voltammetry, square wave voltammetry, chronopotentiometry and Raman spectroscopy. Electrochemical reduction of Cr(III) to Cr(0) in LiCl-KCl-CrF 3 is a two-step process mediated by the formation of Cr(II). Most Cr(III) ions exist in the form of CrCl 6 3− while CrCl 6-x F x 3− (x ≤ 3) ions are minor in the molten salt medium. Based on the cyclic voltammetric results obtained at different potential scanning rates, the diffusion coefficients of Cr(III) and Cr(II) in LiCl-KCl-CrF 3 melt were determined between 723 and 873 K, and their dependence on temperature can be described as lnD Cr(III) = −4.90-4322.66/T and lnD Cr(II) = −4.71-4007.09/T respectively. The diffusion coefficients of chromium ions in LiCl-KCl-CrF 3 are higher than those in LiCl-KCl-CrCl 3 , which can be explained by the hopping mechanism. In addition, a series of apparent standard potentials of the Cr(III)/Cr(II) and Cr(II)/Cr(0) couples were obtained over the same temperature range using the open-circuit chronopotentiometry technique. With the addition of LiF into the LiCl-KCl-CrF 3 mixture, the dominant chromium species change from CrCl 6 3− to CrCl 6-x F x 3− (x ≤ 6), which is supported by the appearance of the new peaks in both square wave voltammograms and Raman spectra.

Raman Spectroscopic and Theoretical Study of Scandium Fluoride and Oxyfluoride Anions in Molten FLiNaK

“…It is well-known that group 3 fluorometallates such as ScF 6 3– can be readily synthesized via the reaction of corresponding metal fluorides in excess HF and NH 4 F solutions, and the coordination structures of solid-state fluorometallates are therefore well characterized using a series of spectroscopic methods. , In addition to the aqueous medium, molten fluoride such as FLiNaK is also a kind of appropriate medium that can be employed for the synthesis of group 3 fluorometallates due to the presence of free F – anions. , By varying the ratio of MF 3 and FLiNaK, it is possible to probe the coordination structures of binary fluorometallates that are not isolable in solid state, which not only provide a unique opportunity for a comprehensive understanding of the coordination chemistry of group 3 metal fluorides but also help solve the technical problems such as those involved in the manufacture of aluminum–scandium master alloy, , which is related to the speciation of metal fluoride in a molten salt environment.…”

Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+) in Molten Salts

“…species is expected in accordance with the low polarizing power of these trivalent cations. Although this deduction is supported by some electrochemical studies, additional work on the solution structure of molten salts is needed for further verification [5d,22] …”

Influence of Fluoride Ions on the Speciation and Electrochemical Behavior of Th(IV) in Molten LiCl-KCl with a Copper Electrode

“…Our recent study on the LiCl-KCl-CrF 3 melt revealed that the presence of fluoride ions could cause the appearance of additional feature in the voltammagram due to the formation of metal complexes with mixed fluoride and chloride ions. 31 It is therefore necessary to explore whether the same effect works in the LiCl-KCl-ThF 4 case that could cause the appearance of peak R 2 and its unusual number of exchanged electrons. 4 shows a series of square wave voltammagrams of molten LiCl-KCl-ThF 4 with different concentrations of fluoride ions adjusted by LiF addition.…”

“…This is similar to what was observed when excess LiF was added into the LiCl-KCl-CrF 3 melt, during which CrCl 6 3− was converted to CrCl 6−x F x 3− . 31 As a result, it is possible that the R 2 peak also arises from reduction of Th(IV) to Th(0), but Th(IV) is actually in the form of ThCl x F y z− when the LiF/ThF 4 molar ratio was varied.…”