Vladimir A. Volkovich

Group 15 quaternary alkyl bistriflimides: ionic liquids with potential application in electropositive metal deposition and as supporting electrolytes

We report the electrochemical properties of Group 15 quaternary alkyl bistriflimide salts, which have very wide electrochemical windows (between +2.6 and −3.4 V vs. Fc+/Fc for [(Me)4As][N(SO2CF3)2]) when used as supporting electrolytes in MeCN and which can be used for the electrodeposition of very electropositive metals, including Eu, in the molten state.

A new method for determining oxygen solubility in molten carbonates and carbonate–chloride mixtures using the oxidation of UO2 to uranate reaction

One of the possible pyrochemical reprocessing procedures for spent ceramic nuclear fuels may involve the oxidation of UO2 in alkali metal carbonate and carbonate-based melts, and this is controlled by the level of dissolved oxygen in the melt. A quantitative relationship has been derived between the extent of UO2 oxidation and the concentration of oxygen (peroxide/superoxide) species formed upon oxygen dissolution in carbonates. A novel sensitive method for determining oxygen solubility in molten carbonates and carbonate-based melts has thus been developed. The concentrations of the alkali metal uranates(VI) formed can then be accurately determined without interference from unreacted UO2. Oxygen solubilities at temperatures from 450°C to 800°C have been determined. The solubility of oxygen in a range of carbonate–chloride melts was also determined and found to increase with decreasing radius of the cation of the alkali metal chloride added. Measurements at various partial pressures of oxygen allowed the determination of the predominant oxygen species formed in the melt, and preliminary experiments showed that in the ternary carbonate melt, at 450°C, oxygen dissolves forming mainly superoxide ions. The applicability of Henrys Law in this situation is examined.

Formation of lanthanide phosphates in molten salts and evaluation for nuclear waste treatment

The formation of phosphates of the lighter lanthanides (Lnu2006=u2006La, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Dy) by reaction with alkali metal phosphates has been studied in a LiCl–KCl eutectic melt between 450 and 650u2006°C and in a NaCl–KCl equimolar melt at 750u2006°C and under an inert atmosphere. Alkali metal ortho-, meta- and pyrophosphate precursors have been employed. The reaction results in the formation of single lanthanide or double alkali metal–lanthanide orthophosphates, the former favoured in LiCl–KCl melts and the latter in NaCl–KCl melts. The mean crystallite size of precipitated phosphates was evaluated from X-ray powder diffraction patterns, and found to be within 300–450 A. Increasing the melt temperature results in increasing the crystallite size of phosphate phases. This technique offers an attractive means of removing lanthanide fission products from chloride melts arising from pyrochemical reprocessing of spent nuclear fuels. LiCl–KCl based melts are recommended because the bulk of phosphate waste precipitated is smaller since normal rather than double phosphates are formed therein.

Reactions and speciation of technetium and rhenium in chloride melts: a spectroscopy study

The reactions of rhenium metal, rhenium oxides (ReO2 and ReO3) and technetium dioxide with chlorine and hydrogen chloride were investigated in LiCl–KCl, NaCl–CsCl and NaCl–KCl based melts between 450 and 720u2006°C. Reaction progress was followed using in situ electronic absorption spectroscopy, with the spectra measured between 200 and 1100 nm. Samples of the quenched melts were analysed by EXAFS spectroscopy. Rhenium metal and rhenium oxides react with Cl2 or HCl yielding hexachlororhenate(IV) species, although the reaction of rhenium metal with HCl is extremely slow even at 700u2006°C and no reaction between rhenium metal and Cl2 was observed below 500u2006°C. Chlorination of technetium dioxide in molten salts has been studied here for the first time. TcO2 reacts with HCl producing [TcCl6]2−. However, the reaction of TcO2 with chlorine results in the oxidation of technetium to pertechnetate which is retained in the melt and production of a volatile solid, which is possibly a mixture of Tc(IV) chloride and Tc(VII) oxychloride. This is the first reported investigation into Tc speciation in molten salts.

Raman and infrared spectra of rubidium and caesium uranates(VI) and some problems assigning diuranate site symmetries

Abstract The vibrational spectra of rubidium and caesium mono- and diuranates, M2UO4 and M2U2O7 (M=Rb or Cs), have been measured in the infrared range, 4000–200xa0cm−1, and the Raman shift range of 1100–50xa0cm−1. The Raman spectra of these uranates are reported for the first time. The uranate ion site symmetries were assigned, based on the analysis of the spectra, as D4h for Rb2UO4, D2h for Cs2UO4. Assigning definite site symmetries for the diuranates presented some difficulties because it was not possible to do so unambiguously from spectral data but the likeliest for Rb2U2O7 is C2h and either C2 or Cs for Cs2U2O7. The electronic spectra of molecules with a centre of symmetry are predicted to exhibit a greater temperature effect than acentric species. This is here shown experimentally, for the first time from diffuse reflectance spectra, by comparing the spectra of caesium mono- and diuranates at liquid nitrogen and room temperature; the much greater temperature effect of centrosymmetric over acentric species is clearly revealed by this technique. Crystallographic cell parameters of the uranates are reported, from X-ray powder diffraction studies.

A Spectroscopic Study of Uranium Species Formed in Chloride Melts

The chlorination of uranium metal or uranium oxides in chloride melts offers an acceptable process for the head-end of pyrochemical reprocessing of spent nuclear fuels. The reactions of uranium metal and ceramic uranium dioxide with chlorine and with hydrogen chloride were studied in the alkali metal chloride melts, NaCl-KCl at 973 K, NaCl-CsCl between 873 and 923 K, and LiCl-KCl at 873 K. The uranium species formed therein were characterised from their electronic absorption spectra measured in situ. The kinetic parameters of the reactions depend on melt composition, temperature and chlorinating agent used. The reaction of uranium dioxide with oxygen in the presence of alkali metal chlorides results in the formation of alkali metal uranates. A spectroscopic study, between 723 and 973 K, on their formation and their solutions was undertaken in LiCl, LiCl-KCl eutectic and NaCl-CsCl eutectic melts. The dissolution of uranium dioxide in LiCl-KCl eutectic at 923 K containing added aluminium trichloride in the presence of oxygen has also been investigated. In this case, the reaction leads to the formation of uranyl chloride species.

Spectroelectrochemical Study of Neptunium in Molten LiCl-KCl Eutectic

Neptunium behaviour in an LiCl-KCl eutectic melt at 723 K was studied using spectroelectrochemistry. Cathodic reduction of neptunium(IV)-containing melts led to the formation of Np(III) ions and then neptunium metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into individual Gaussian bands. The nature of neptunium complex ions in the melt is discussed.

Chemistry of vanadium chlorides in molten salts: An electronic absorption spectroscopy study

Abstract The chloro and oxy-chloro complex ions of vanadium, containing the metal in different oxidation states (II, III, IV and V), were investigated in the melts NaClue5f8CsCl (at 550–700°C) and NaClue5f8KCl (at 680–980°C) under a variety of conditions. Melts were examined using electronic absorption spectroscopy between 4,000 and 33,000 cm −1 . Anodic dissolution of vanadium was studied in NaClue5f8KCl melts at various anodic current densities. Vanadium dissolves, forming V(II) ions, at current densities of 100–150 mA cm −2 ; increasing the current density above ca. 340 mA cm −2 leads to the formation of predominantly V(III) species. Reactions of vanadium metal and vanadium oxides (V 2 O 3 and V 2 O 4 ) with hydrogen chloride or chlorine were followed by in situ spectroscopy measurements. Depending on the experimental conditions, a range of vanadium chloro- and oxychloro-complexes was formed. Reaction of the V(III) chloro-complex with oxygen leads to the formation of VO(II) complex ions. Preliminary results indicate that the V(IV) chloro-complex, VCl 6 2− , can be formed by oxidising VCl 6 3− with chlorine. Vanadium(V) oxide reacts with hydrogen chloride in NaClue5f8KCl melts to form an oxygen-containing complex of V(III). Direct dissolution of V 2 O 5 in NaClue5f8KCl melts yields sodium polyvanadate, NaV 6 O 15 .

Erratum to “Chemistry of vanadium chlorides in molten salts: An electronic absorption spectroscopy study” [J. Mol. Liqs. 103–104 (2003): 387–394]

Abstract Chloro- and oxychloro complex ions of vanadium, containing the metal in all its oxidation states, II, III, IV and V, have been obtained and examined in NaClue5f8KCl and NaClue5f8CsCl melts between 550 and 980 °C. Vanadium was introduced into the melt by anodic dissolution and by reacting the metal and oxides, V 2 O 3 , V 2 O 4 and V 2 O 5 , with chlorine and hydrogen chloride. The spectra of vanadium(II) were recorded in silica cells as well as by reflection-absorption spectroscopy in a silica-free set up. Melts containing initially V(II) and V(III) chloro species were readily oxidised to V(IV) when exposed to air or when water was generated in the melt by reaction of oxide with HCl. The reaction of vanadium with chlorine was studied at Cl 2 flow rates and various temperatures, and yielded V(III) and V(IV) ions, with V(III)-containing melts prevailing at higher temperatures because the volatile VCl 4 was swept out with the Cl 2 . Lower vanadium oxides (III and IV) in these melts react with HCl to produce mainly vanadyl complex ions, VOCl 5 3− .

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X-ray Diffraction, and Density Functional Theory Calculations

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K5Sc3F14 was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF4, Li3ScF6, KSc2F7, and Na3ScF6 compounds were studied in detail from solid-state 19F and 45Sc NMR experiments. The 45Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The 19F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the 19F and 45Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state 45Sc NMR spectroscopy.