Electronic Absorption Spectra of Vanadium Species in Halide Melts

Abstract

Physico-chemical behavior of vanadium in chloride melts was studied using high temperature electronic absorption spectroscopy and spectroelectrochemistry. Dissolution of VCl 3 in molten chloride leads to the formation V(III) species, VCl 6 3-. Vanadium(II) complex ions VCl 6 4-are formed during electroreduction of vanadium(III)-containing melts and vanadium metal anodic dissolution. V(II) chloro-species slowly react with silica to form an oxygen-containing insoluble phase. Due to high volatility of VCl 4 oxygen-containing vanadyl species, VOCl 4 2-, is the only stable form of V(IV) in fused chlorides. It is formed when V 2 O 5 , V 2 O 4 or V 2 O 3 react with HCl. Electroreduction of vanadyl complexes leads to the formation of insoluble oxygen-containing vanadium(III)species, whereas electrochemical oxidation has no effect on vanadium speciation, VO 3+ /VO 2+ red-ox potential is more positive than Cl 2 /Cl -. Further oxidation VO 2+ species gives vanadate anions. The electronic absorption spectra of soluble vanadium species were measured in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl mixtures at 450-750 0 C.

Vanadium Speciation in Fused Alkali Chlorides

“…Reaction of all common vanadium oxides, i.e., V 2 O 3 , V 2 O 4 and V 2 O 5 , with hydrogen chloride in molten alkali chlorides resulted in formation of vanadium(IV)-containing melts irrespectively of the starting oxide. 20 In the visible region the absorption spectra contained a single band around 14000 cm −1 , Figs. 3, 4.…”

“…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 VCl 2 to VCl 3 and vanadium metal.…”

“…Reaction of various vanadium oxides (V 2 O 3 , V 2 O 4 and V 2 O 5 ) with HCl in chloride melts always produced oxygen containing vanadium(IV) species, VOCl 4 2-, in the melts. 20 Electrochemical reduction of these vanadyl(IV) ions resulted in the formation of an insoluble oxygen-containing vanadium(III) phase, whereas the electrochemical oxidation did not alter the vanadium speciation in the melt.In the present study electronic absorption spectroscopy was employed to study vanadium speciation in a variety of chloride melts. The EAS of vanadium species were resolved into individual bands and the spectroscopic parameters of vanadium complex ions were determined.…”

“…Exposure of NaCl-KCl-VCl 3 melts to an oxygen containing atmosphere did not change total vanadium concentration in the melt but VCl 6 3-ions were oxidized to VOCl 4 2-. 19,20 Chlorination of metallic vanadium by chlorine in a NaCl-KCl melt produced a mixture of vanadium chlorides, the composition of the products depended on excess of Cl 2 used. 19 Vanadium(II) and (III) chloro-species were formed at low Cl 2 flow rates.…”

“…15,16 Subsequent studies included other alkali chloride mixtures. [17][18][19][20] Analysis of the data present in the literature showed that there is a considerable controversy about the stable oxidation states of vanadium and the coordination geometry of its complex ions in the lower oxidation states in molten alkali chlorides. Vanadium in the oxidation states +2 and +3 was said to form complex ions where the metal is coordinated to six or four anions in octahedral or tetrahedral geometry, respectively.…”