Speciation of Molybdenum and Tungsten in Molten Chlorides: A Spectroelectrochemical Study

Abstract

Speciation of tungsten and molybdenum in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 450 and 750 oC using electronic absorption spectroscopy. Only W(IV) and W(V) chloro- and oxychlorospecies can be stabilised under conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution of W metal at anodic current densities in the range of 0.005-0.1 A/cm2 produces only WCl6 2- ions that can be electrochemically oxidised to WCl6 -. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro-species into W(IV) oxychloro-species. In the presence of O2 WCl6 2- is oxidised into tungsten(V) species WOCl5 2-. Molybdenum forms Mo(III), (IV) and (V) species. MoCl6 2- complex ions in chloride melts can be obtained by reacting MoO2 with HCl, or by electrochemical oxidation of Mo(III) (MoCl6 3- → MoCl6 2-) on a glassy carbon electrode.

Complexation of Mo in FLiNaK Molten Salt: Insight from Ab Initio Molecular Dynamics

“…Molybdenum is a multivalent metal with several available oxidation states, and the stability of Mo in molten salt solution varies with these oxidation states. A similar pattern has been observed in both fluoride and chloride molten salts: Mo(0) precipitates as a solid; Mo­(I) and Mo­(II) are not stable in solution; − Mo­(III) and Mo­(IV) have both been observed to remain stably dissolved within a molten salt melt; , and Mo­(IV), Mo­(V), and Mo­(VI) have been observed to leave solution as a vapor in the form of MoX 4 , MoX 5 , and MoX 6 , with MoX 5 and MoX 6 far more common in the vapor phase. ,,− …”

“…The Mo­(II) mononuclear complex has a roughly square planar geometry often seen in transition metals . This oxidation state is known to be unstable in solution, − and Bader analysis yields a key insight why. Given that the number of unpaired d-orbital electrons in Mo­(II) is four and the coordination number for Mo­(II) is also four, MoF 4 2– is much less a solvated complex and more of a molecule.…”

“…11−13 In 1967, the apparent kinetics of Mo(III) disproportionation in KLiCl led Senderoff and Mellors 12 to speculate that much of the dissolved molybdenum existed in the form of a dinuclear Mo complex (Mo 2 Cl 9 3− ) that is relatively stable in solution and does not undergo disproportionation, in equilibrium with a mononuclear complex (MoCl spectroscopic studies in more recent decades have not identified any polynuclear Mo complexes in KLiCl solution, instead finding MoCl 6 3− to be the only clearly detectable species. 5,6,14,15 This has led some to suggest that the disproportionation reported by earlier studies did not represent a homogenous reaction, with Mo interacting solely with itself, but instead indicated Mo reacting with oxide contaminents. 16 Still, it is difficult for spectroscopic studies of molten salts to definitively rule out the presence of any given Mo species at high temperatures, 5 making dinuclear or even trinuclear Mo solution complexes a possibility.…”

Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy

“…The EDX results show a thin layer of tungsten (W) forming on the surface of the wire. The presence of W is significant as a dissolved W cation, the speciation of this cation is not investigated by this work and this cation has several valence states; however, literature suggests W(IV) is the most likely valence state for the conditions studied 24,43,44 The W(IV) in the salts acts as a significant oxidizer, as it has a less negative redox potential (W/WCl 4 ) than that of Ni (Ni/NiCl 2 ) in this system. 24 This leads to the W oxidizing the Ni 0 to Ni 2+ and the W 4+ depositing onto the wire as W 0 .…”

Molten Salt Reactor Engineering Study for Off-Gas Management

“…Combining spectroscopy with electrochemistry in spectroelectrochemistry is particularly valuable for the interrogation and control of conditions present in the salt. Spectroelectrochemistry has been used in molten salt systems for both its ability to determine speciation and for controlling the oxidation states of specific species within these systems (Nagai et al, 2005;Polovov et al, 2007;Volkovich et al, 2007;Polovov et al, 2008;Abramov et al, 2010;Nagai et al, 2011;Park et al, 2011;Nagai et al, 2013;Schroll et al, 2016). Corrosion products such as Cr, Fe, Ni, Mo, Ti, and Mn have been studied using spectroelectrochemical methods (Volkovich et al, 2007;Abramov et al, 2010).…”