Development of new methods for the reduction of UF6

Methods for the preparation of UF₅ are discussed with respect to the formation of β-UF₅. The reduction of UF₆ by HBr in liquid HF /1/ can be used to synthesize pure β-UF₅ even if greater amounts are required.Details of the direct photolysis of UF₆ without scavenger /2/ are presented. In an advanced version a simplified photo-reactor is used which consists of a stainless steel vessel with a diameter of 100 mm and a volume of about 3 liters. UV-light of a 1000 W super high pressure mercury lamp is used to photolyze about 50 g of high purity UF₆ within 12 h, giving pure β-UF₅ in about 90 % yield without intermediately removing the F₂ formed.Two simple new methods have been developed to synthesize β-UF₅. UF₆ is reduced by H₂ in liquid anhydrous HF at room temperature. This reaction which is hindered kinetically at room temperature can be catalyzed by metallic Au which is applied as a foil in the stirred reaction mixture.In addition, it was found that anhydrous HCl catalyzes the reaction, too. 200 mbar of HCl were added, together with 4 bar H₂, to UF₆ in liquid HF, and the reaction mixture was magnetically stirred for 66 hours. β-UF₅ could be obtained in 91 % yield as a very pure product. This latter method is recommended for large scale production of β-UF₅.Reactions of UF₆ with other reducing agents like HCl, SO₂, and CO in liquid HF were studied. These reactions give poor yields or impure products.UF₆ yields with CO and Au in the presence of HF a carbonyl compound of Au with the very high ν_CO at 2205 cm^?1. Analytical and spectroscopic data suggest the formula Au(CO)₂U₂F₁₁.     

Doping of (CH)x films to the metallic state with metal hexafluorides

When polyacetylene films, (CH)_x, are exposed to the vapours of hexafluorides, the resistances of the films drop rapidly. The following hexafluorides were shown to dope (CM)_x to the metallic state: SeF₆, TeF₆, WF₆, ReF₆, OsF₆, IrF₆, MoF₆, UF₆ and XeF₆. Conductivity $\text{vs}$ degree of doping curves obtained for WF₆, MoF₆ and UF₆ exhibit a shape similar to that observed for AsF₅; namely, an increase in electrical conductivity of several orders of magnitude at low concentrations until a point when additional doping has little further effect. Parallel e.s.r. line-shape measurements confirm metallic behaviour above a critical transition. The highest conductivity observed in the series is 350 Ω^?1 cm^?1 for [CH(WF₆)_0.087]_x. The maximum observed for the XeF₆ doped polyacetylene was about 0.1 Ω^?1 cm^?1. The other hexafluorides gave materials which show intermediate conductivities. The XeF₆ doped polyacetylene is not stable, presumably because of internal fluorination of the (CH)_x by the dopant.     

Redox reactions of uranium hexafluoride. Comparison with phosphorus pentafluoride and preparation of copper(I) hexafluorouranate(V) [1]

Molecular iodine is oxidised by phosphorus pentafluoride in iodine pentafluoride at room temperature giving I₂⁺, PF₆^?, and PF₃. I₂⁺ is formed from uranium hexafluoride under similar conditions, but further oxidation occurs depending on the reaction stoicheiometry used. In all cases uranium pentafluoride is formed. Copper(II) fluoride reacts with UF₅ in acetonitrile at room temperature to give copper(II) hexafluorouranate(V), which is reduced by copper metal to give the copper(I) salt. The latter compound is formed from UF₆ and Cu metal, via the Cu^II salt, only if a fresh Cu surface is used for the reduction step.     

Production of Gas-Phase Uranium Fluoroanions Via Solubilization of Uranium Oxides in the [1-Ethyl-3-Methylimidazolium]<Superscript>+</Superscript>[F(HF)<Subscript>2.3</Subscript>]<Superscript>−</Superscript> Ionic Liquid

A new methodology for gas-phase uranium ion formation is described in which UO₂ is dissolved in neat N-ethyl,N′-methylimidazolium fluorohydrogenate ionic liquid [EMIm⁺][F(HF)_2.3^?], yielding a blue-green solution. The solution was diluted with acetonitrile and then analyzed by electrospray ionization mass spectrometry. UF₆^? (a U(V) species) was observed at m/z?=?352, and other than cluster ions derived from the ionic liquid, nothing else was observed. When the sample was analyzed using infusion desorption chemical ionization, UF₆^? was the base peak, and it was accompanied by a less intense UF₅^? that most likely was formed by elimination of a fluorine radical from UF₆^?. Formation of UF₆^? required dissolution of UO₂ followed by or concurrent with oxidation of uranium from the +?4 to the +?5 state and finally formation of the fluorouranate. Dissolution of UO₃ produced a bright yellow solution indicative of a U(VI) species; however, electrospray ionization did not produce abundant U-containing ions. The abundant UF₆^? provides a vehicle for accurate measurement of uranium isotopic abundances free from interference from minor isotopes of other elements and a convenient ion synthesis route that is needed gas-phase structure and reactivity studies like infrared multiphoton dissociation and ion-molecule dissociation and condensation reactions. The reactive fluorohydrogenate ionic liquid may also enable conversion of uranium in oxidic matrices into uranium fluorides that slowly oxidize to uranyl fluoride under ambient conditions, liberating the metal for facile measurement of isotope ratios without extensive chemical separations.

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UF6 and UF4 in Liquid Ammonia: [UF7(NH3)]3− and [UF4(NH3)4]

From the reaction of uranium hexafluoride UF₆ with dry liquid ammonia, the [UF₇(NH₃)]^3? anion and the [UF₄(NH₃)₄] molecule were isolated and identified for the first time. They are found in signal‐green crystals of trisammonium monoammine heptafluorouranate(IV) ammonia (1:1; [NH₄]₃[UF₇(NH₃)] ? NH₃) and emerald‐green crystals of tetraammine tetrafluorouranium(IV) ammonia (1:1; [UF₄(NH₃)₄] ? NH₃). [NH₄]₃[UF₇(NH₃)] ? NH₃ features discrete [UF₇(NH₃)]^3? anions with a coordination geometry similar to a bicapped trigonal prism, hitherto unknown for U^IV compounds. The emerald‐green [UF₄(NH₃)₄] ? NH₃ contains discrete tetraammine tetrafluorouranium(IV) [UF₄(NH₃)₄] molecules. [UF₄(NH₃)₄] ? NH₃ is not stable at room temperature and forms pastel‐green [UF₄(NH₃)₄] as a powder that is surprisingly stable up to 147 °C. The compounds are the first structurally characterized ammonia complexes of uranium fluorides.     

The Heat of Formation of the Uranyl Dication: Theoretical Evaluation Based on Relativistic Density Functional Calculations

By using a set of model reactions, we estimated the heat of formation of gaseous UO₂²⁺ from quantum‐chemical reaction enthalpies and experimental heats of formation of reference species. For this purpose, we performed relativistic density functional calculations for the molecules UO₂²⁺, UO₂, UF₆, and UF₅. We used two gradient‐corrected exchange‐correlation functionals (revised Perdew–Burke–Ernzerhof (PBEN) and Becke–Perdew (BP)) and we accounted for spin‐orbit interaction in a self‐consistent fashion. Indeed, spin‐orbit interaction notably affects the energies of the model reactions, especially if compounds of U^IV are involved. Our resulting theoretical estimates for Δ_f (UO₂²⁺), 365±10 kcal mol^?1 (PBEN) and 370±12 kcal mol^?1 (BP), are in quantitative agreement with a recent experimental result, 364±15 kcal mol^?1. Agreement between the results of the two different exchange‐correlation functionals PBEN and BP supports the reliability of our approach. The procedure applied offers a general means to derive unknown enthalpies of formation of actinide species based on the available well‐established data for other compounds of the element in question.     

Interfacial Behavior of Cysteine between Mild Steel and Sulfuric Acid as Corrosion Inhibitor

Interfacial behavior of cysteine (Cys) between mild steel and sulfuric acid solution as a corrosion inhibitor has been studied with electrochemical AC (alternating current) and DC (direct current) techniques at (25.0±0.1) °C. The AC impedance results were evaluated using equivalent circuits in which a constant phase element (CPE) has been replaced with double layer capacitance (C_dl) to represent the frequency distribution of experimental data. Changes in impedance parameters (charge transfer resistance and double layer capacitance) indicated that cysteine molecules acted by accumulating at the metal/solution interface. The fractional coverage of the metal surface () was determined using AC impedance results and it was found that the adsorption of cysteine on the mild steel surface followed a Langmuir isotherm model with a standard free energy of adsorption (ΔG⁰_ads) of −35.1 kJ·mol⁻¹.To clarify the type of interaction between mild steel surface and cysteine molecules with a molecular orbital approach, electronic properties, such as, the highest occupied molecular orbital (HOMO) energy, the lowest unoccupied molecular orbital (LUMO) energy, and the frontier molecular orbital coefficients have been calculated. Energy gaps for the interaction of mild steel surface and cysteine molecules (E_{LUMO Fe}−E_{HOMO Cys} and E_{LUMO Cys}−E_{HOMO Fe}) were used to determine whether cysteine molecules acted as electron donors or electron acceptors when they interacted with the mild steel surface. The local reactivity was evaluated through the condensed Fukui indices. Theoretical calculations were carried out using the density functional theory (DFT) at B3LYP level with the 6-311++G(d,p) basis set for all atoms by Gaussian 03W program.     

The reaction of uranium hexafluoride with silver fluoride in anhydrous hydrogen fluoride and the chemical properties of the product Ag2UF8

UF₆ reacts with AgF dissolved in anhydrous hydrogen fluoride to precipitate Ag₂UF₈. Ag₂UF₈ has some unexpected properties: On reaction with water it produces O₂ and reduced uranium. No adequate explanation could be found of why UF₆ and AgF combined in this manner should produce a powerful oxidant. Raman spectra and chemical properties of the solid products are given.     

Uranium (V) fluorides

Rhenium and uranium hexafluorides oxidise iodine in iodine pentafluoride at ambient temperature to give the I₂⁺ cation. With UF₆ additional reaction occurs to give β-uranium pentafluoride as one product (J.A. Berry, A. Prescott, D.W.A. Sharp, and J.M. Winfield, $\text{J. Fluorine Chem}$., 1977, $\text{10}$, 247). Further work on the latter reaction together with an electronic spectroscopic study of the oxidation of I₂ by phosphorus pentafluoride in IF₅, suggests that the fate of the I₂⁺ cation depends on the nature and quantity of the oxidising agent. Oxidation of I₂ by PF₅ can be conveniently followed by monitoring its visible spectrum. The reaction occurs over several hours and eventually an apparent equilibrium between I₂ and I₂⁺ results. Formation of I₂⁺UF₆^?is rapid and, with the mole ratio UF₆:I₂ > 10:1, UF₅ is precipitated rapidly from solution, I₂⁺ being oxidised further, apparently to IF₅. With a smaller UF₆:I₂ mole ratio UF₅ is contaminated by I₂, the latter is presumed to result from the disproportion-ation of an I^I or I^III fluoride.β-UF₅ is very soluble in acetonitrile and reacts with thallium(I) fluoride in this solvent to give Tl^IUF₆. It reacts with trimethyl(methoxo)silane to give (CH₃)₃SiF, U(OCH₃)₅, and an insoluble solid, believed to be a mixture of U^V methoxide, fluorides. Both reactions are conveniently followed by near i.r. spectroscopy.     

UF6 plasma chemistry reconstitution experiments using high-temperature fluorine

Prior results indicate techniques have been developed for fluid mechanical confinement of high-temperature uranium hexafluoride (UF₆) plasma for long test times while simultaneously minimizing uranium compound deposition on the walls. Follow-on investigations were conducted to demonstrate a UF₆/argon injection, separation, and reconstitution system for use with rf-heated uranium plasma confinement experiments applicable to UF₆ plasma core reactors. A static fluorine batch-type regeneration test reactor and a flowing preheated fluorine/UF₆ regeneration system were developed for converting all the nonvolatile uranium compound exhaust products back to pure UF₆ using a single reactant. Pure fluorine preheat temperatures up to 1000 K resulted in on-line regeneration efficiencies up to about 90%; static batch-type experiments resulted in 100% regeneration efficiencies but required significantly longer residence times. A custom-built, ruggedized time-of-flight (T.O.F.) mass spectrometer, sampling, and data acquisition system permitted on-line quantitative measurements of the UF₆ concentrations down to 30 ppm at various sections of the exhaust system; this system proved operational after long-time exposure to corrosive UF₆ and other uranium halides.     

Analysis of solid uranium samples using a small mass spectrometer

A mass spectrometer for isotopic analysis of solid uranium samples has been constructed and evaluated. This system employs the fluorinating agent chlorine trifluoride (ClF₃) to convert solid uranium samples into their volatile uranium hexafluorides (UF₆). The majority of unwanted gaseous byproducts and remaining ClF₃ are removed from the sample vessel by condensing the UF₆ and then pumping away the unwanted gases. The UF₆ gas is then introduced into a quadrupole mass spectrometer and ionized by electron impact ionization. The doubly charged bare metal uranium ion (U²⁺) is used to determine the U²³⁵/U²³⁸ isotopic ratio. Precision and accuracy for several isotopic standards were found to be better than 12%, without further calibration of the system. The analysis can be completed in 25 min from sample loading, to UF₆ reaction, to mass spectral analysis. The method is amenable to uranium solid matrices, and other actinides.     

Acetonitrile and triphenylphosphine oxide complexes of the uranium pentafluoride - antimony pentafluoride and uranium tetrafluoride oxide - antimony pentafluoride adducts

The new ternary adducts, UF₄O·SbF₅·2CH₃CN, UF₄O·2SbF₅·6L, UF₅·SbF₅· 2L (L = CH₃CN or (C₆H₅)₃PO) and UF₅·2SbF₅·5CH₃CN, have been prepared and studied by infrared, ¹⁹F n.m.r. and e.s.r. spectroscopy, mass spectrometry, X-ray powder diffraction and chemical analysis. The infrared spectra strongly suggest an ionic formulation with the uranium cationic species preferentially coordinated by the organic ligand.     

Relativity and the chemistry of UF6: A molecular Dirac—Hartree—Fock—CI study

The electronic structure and bonding of UF₆ and UF₆⁻ are studied within a relativistic framework using the MOLFDIR program package. A stronger bonding but more ionic molecule is found if one compares the relativistic with the nonrelativistic results. The first peak in the photoelectron spectrum of Karlsson et al. is assigned to the 12γ_8u component of the 4t_1u orbital, in agreement with other theoretical and experimental results. Good agreement is found between the experimental and theoretical 5f spectrum UF₆⁻. Some properties, like the dissociation energy and electron affinity, are calculated and the necessity of a fully relativistic framework is shown. The Breit interaction has an effect on the core spinors and the spin-orbit splitting of these spinors but the influence on the valence spectrum is negligible. © 1996 John Wiley & Sons, Inc.     

Analysis on holdup during processing UF6

Holdup in processing UF₆ is analyzed in the present article. Under normal operation conditions of temperature and pressure, UF₆ stays in gas phase. While if water moisture ingresses, chemical reaction between UF₆ and moisture results in solid or liquid productions that can deposit on structure surfaces. The present report focuses on the chemistry of holdup and how to measure it, and safeguards and criticality safety concerns. An available statistic theoretical model is also discussed.     

Redox reactions involving rhenium and uranium hexafluorides,a convenient synthesis of β-uranium pentafluoride

Rhenium and uranium hexafluorides oxidise elemental iodine in iodine pentafluoride at ambient temperature to give the I₂⁺ cation. With UF₆ an additional reaction occurs to give β-uranium pentafluoride as one product, β-UF₅ is soluble in acetonitrile without disproportionation and is also formed from the reduction of UF₆ by MeCN. Copper, cadmium, and thallium metals are oxidised by ReF₆ in MeCN giving Cu^I, Cd^II, and Tl^I hexafluororhenates(V) but the reactions are complicated by reaction between ReF₆ and the solvent.     

[UF6]2−: A Molecular Hexafluorido Actinide(IV) Complex with Compensating Spin and Orbital Magnetic Moments

The first structurally characterized hexafluorido complex of a tetravalent actinide ion, the [UF₆]^2? anion, is reported in the (NEt₄)₂[UF₆]?2 H₂O salt ( 1 ). The weak magnetic response of 1 results from both U^IV spin and orbital contributions, as established by combining X‐ray magnetic circular dichroism (XMCD) spectroscopy and bulk magnetization measurements. The spin and orbital moments are virtually identical in magnitude, but opposite in sign, resulting in an almost perfect cancellation, which is corroborated by ab initio calculations. This work constitutes the first experimental demonstration of a seemingly non‐magnetic molecular actinide complex carrying sizable spin and orbital magnetic moments.     

Conversion de UF6 appauvri en U3O8

Uranium hexafluoride (UF₆), to or from isotopic enrichment plants is stored and transported, as a solid, in tanks containing 2 to 12 metric tons of material. Sampling must be carry out after complete melting obtained by heating of the tank. This sampling process is difficult and hazardous by risks of local solidification (sealing), of reaction with air moisture (Fluorhydric Acid, highly corrosive and toxic is formed), of chemical and radioactive contamination (in case of leaking), of loss of expensive material (especialy if enriched UF₆), and of over-filling of sampling pot (possible domage during warming up of itagain).The described new device was concepted and developed by COGEMA Laboratories and is used for two years in sampling facilities of enrichment plant of PIERRELATTE. It permits to warrant sample validity and eliminate all the hereabove risks.It allows seeing and adjusting volume of the samples and their flow, and permits measurement of temperature and pressure, specified for UF₆.This new device is usable for many others materials which present some risks and difficults, as Fluorine and its derivates, chlorine, liquefied inflammable gases etc.     

Density functional studies of AnF6 (An=U,Np, and Pu) and UF6−nCln (n=1–6) using hybrid functionals: geometries and vibrational frequencies

We have compared the performance of widely used hybrid functionals for calculating the bond lengths and harmonic vibrational frequencies of AnF₆ (An=U, Np, and Pu) and UF_6?nCl_n (n=1–6) molecules using “small‐core” relativistic effective core potentials and extended basis sets. The calculated spectroscopic constants compare favorably with experimental data for the bond lengths (average error ≤ 0.01 Å) and vibrational frequencies (average error ≤ 7 cm^?1) of the AnF₆ molecules. The experimental vibrational frequencies of the stretching modes were available for most of the UF_6?nCl_n (n=1–6) molecules. The calculated vibrational frequencies are in good agreement with the experimental data to within 4.6 cm^?1 and 7.6 cm^?1 for selected Becke1 and Lee, Yang, Parr (B1LYP), and Becke3 and Perdew, Wang (B3PW91) functionals, respectively. We conclude that one can predict reliable geometries and vibrational frequencies for the unknown related systems using hybrid density functional calculations with the RECPs. The geometries and vibrational frequencies of the UF_6?nCl_n (n=1–6) molecules that have not been determined experimentally are also presented and discussed. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 2010–2017, 2001     

Composes d'addition entre les pentafluorures d'uranium et d'antimoine,preparation des composes UF5. n SbF5 (n = 1 ou2)

The adduct UF₅.2SbF₅ has been obtained from the reaction of UF₅ with SbF₅ and the reaction of UF₆ with SbF₅ in the presence of freon 114. From this preparation and also from studies on UF₆, SbF₅ solutions, the fluorinating properties of UF₆ were found to be enhanced by the presence of SBF₅. An x-ray diffraction study has shown that crystals of UF₅.2SbF₅ are monoclinic, space group P2₁/c, with unit cell dimensions a = 8.036(3) Å ; b = 14.112(13) Å ; c = 10.028(17) Å ; β = 96.91(7)°.The adduct UF₅.SbF₅ is obtained by thermal decomposition of UF₅.2SbF₅.     

Chemical deposition of smooth nanocrystalline Y2O3 films from solutions of metal-organic precursors

A series of (Y(AcO)₃·4H₂O—Q—Solv) solutions (Q is monoethanolamine (MEA), diethanolamine (DEA), en, dien; Solv = MeOH, EtOH, Pr_iOH, BuOH) was studied to choose the metal-organic precursor for surface smoothing treatment of metallic tapes by chemical deposition of nanocrystalline yttria films. Based on the results of viscosity, wetting angle, and thermal stability measurements, a solution (Y(AcO)₃·4H₂O—dien—PrⁱOH) was proposed as a new metal-organic precursor. After chemical deposition of nanocrystalline yttria films about 300 nm thick on a Hastelloy C-276 metallic tape the surface roughness was reduced by a factor of 11 (from 9.0 to 0.8 nm on a surface area of 5×5 μm²).