Molten Media Research Articles

Neodymium(III) cathodic processes in molten fluorides

The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF–CaF 2. A preliminary study based both on thermodynamic and experimental data showed that it is not possible to observe the Nd(III)/Nd(0) system in LiF–KF and LiF–NaF melts; because the K + and Na + cation reduction waves hide the Nd 3+ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 °C using solutions of NdF 3 in fluoride solvents without K + and Na + ions, such as LiF–CaF 2, by cyclic voltammetry, chronopotentiometry and square wave voltammetry. Experimental results show that neodymium trifluoride is reduced in Nd(0) in a one-step process exchanging three electrons (Nd(III) + 3e − → Nd(0)). The electrode process is shown to be diffusion controlled. Nd(III) diffusion coefficient is in the range of 1.1–1.3 × 10 −5 cm 2 s −1 at 810 °C.

Solubilization of rare earth oxides in the eutectic LiCl–KCl mixture at 450 °C and in the equimolar CaCl 2–NaCl melt at 550 °C

Solubilization studies of rare earth oxides and oxohalides have been carried out in the eutectic LiCl–KCl melt at 450 °C and the equimolar CaCl 2–NaCl mixture at 550 °C. First, chlorinating conditions were predicted by comparing the E–pO 2− diagram of the different rare earth-O compounds to that of several chlorinating gaseous mixtures. Then, experimental solubilization tests were performed by using pure HCl and Cl 2 gases and the chlorinating Cl 2(g)+C(s) mixture. In order to build up the potential–acidity diagrams, stability of the oxidation states of rare earths (RE being La, Ce, Nd, Pr and Y), standard potentials of the different redox couples as well as solubility products of oxides and oxychlorides were determined in both molten chlorides at the temperatures studied. Oxidation states (III) and (0) were found to be stable in both molten chlorides for all REs and (II) in the case of neodymium. The standard potential values of the different redox couples were also determined by combining both potentiometry and cyclic voltammetry and then allowed us to obtain the activity coefficients of RECl 3 compounds in the molten media. It was found that RE(III) cations were less solvated by the chloride ions in the calcium melt, which could be explained by formation of CaCl 4 2− ions. Moreover it was demonstrated that at a given working temperature, the rare earth cations with higher polarizability (higher charge to ionic radius values) are also more solvated by the melt. Solubilization tests of RE–O samples showed that in the case using gaseous Cl 2 as chlorinating agent, the kinetics of the chlorinating reaction were much slower than those for gaseous HCl.

Reaction Mechanisms in Thermally Developed Photographic Systems Based on Silver Carboxylate

The chemistry of a thermally developed photographic system based on silver carboxylate is investigated both in the model system and in the film sample. The reaction path is found to consist of the disintegration of the silver carboxylate crystal structure under thermal development (step 1), the formation of silver ion carriers which can diffuse in the molten media (step 2), and the reduction reactions of silver ion to metallic silver by the reducing agent (step 3). In the first reaction step, XRD patterns and FT–IR spectra show that phthalic acid receives silver ion from crystalline silver behenate and becomes di-silver-phthalate. In the second reaction step, the complexes of phthalazine and silver ion are identified as the silver ion carriers in molten media by 13C–NMR, density functional calculations of 13C–NMR chemical shift, and TOF–SIMS. The diffusion coefficients of silver ion carriers are measured by pulsed field gradient 1H–NMR at the development temperature. In the final step, the reduction of silver ion by the bisphenol-type reducing agent is analyzed by the identification of the reaction products. It is found that this reducing agent has eight equivalent reducing power. An overall reaction mechanism for thermally developed photographic systems based on silver carboxylate is proposed.

Dissolution and formation of nuclear materials in molten media

Abstract As innovative means to recover actinides from irradiated fuels, pyrochemical processes are proposed. In the Japan Atomic Energy Research Institute (JAERI), the feasibility of applying molten-salts processes to nitride fuels containing transuranium elements for transmutation is investigated. In the course of study, experiments and analyses have been made on the high-temperature chemistry of M­N­Cl systems, the effects of oxygen impurities, and the implications of the divalent state along with the normal trivalent state of lanthanides and actinides. Thermodynamic calculation and the electrochemical measurements are compared in a few aspects of dissolution and precipitation of nuclear materials in molten halides.

Electrochemical Investigation of the Solvolytic Properties of Ethylammonium Nitrate (EAN) and Propylammonium Nitrate (PAN)

Ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) are unique fused ionic media which have received relatively little attention despite their unusual physical and chemical properties. As a consequence very few applications for these media have been described in the literature. The utility of EAN and PAN as conductive solvents suitable for electrochemical analysis was examined by carrying out cyclic voltammetric analyses of ferrocene, ethylferrocene, and acetylferrocene dissolved in these molten media. Reversibility was observed for the ferrocene redox systems in EAN while those in PAN were not quite as well behaved. A comparison of the diffusion coefficient of ferrocene in EAN and PAN employing both Stokes-Einstein values and values derived from chronocoulometry shows that diffusion is very slow and nearly ideal in these solvents.

Electrochemical Investigation of the Solvolytic Properties of Ethylammonium Nitrate (EAN) and Propylammonium Nitrate (PAN)

Ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) are unique fused ionic media which have received relatively little attention despite their unusual physical and chemical properties. As a consequence very few applications for these media have been described in the literature. The utility of EAN and PAN as conductive solvents suitable for electrochemical analysis was examined by carrying out cyclic voltammetric analyses of ferrocene, ethylferrocene, and acetylferrocene dissolved in these molten media. Reversibility was observed for the ferrocene redox systems in EAN while those in PAN were not quite as well behaved. A comparison of the diffusion coefficient of ferrocene in EAN and PAN employing both Stokes-Einstein values and values derived from chronocoulometry shows that diffusion is very slow and nearly ideal in these solvents.

Pyrite as cathode insertion material in rechargeable lithium/composite polymer electrolyte batteries

The chemical and physical properties of pyrite have been reviewed with reference to cathode material in lithium/polymer electrolyte batteries. The analysis of d Q/d V curves showed that the charge–discharge process in Li/polymer electrolyte/pyrite battery is more complicated than in non-aqueous and molten media. At least seven domains are distinguished on the d Q/d V discharge curve. The low-voltage step on discharge may be associated with the formation of a new phase causing from the reaction of metallic iron with the electrolyte components. This would explain the capacity fading of the cell. The high-voltage 1.85–2.25 V charge region may be attributed to the insertion of lithium into Li 2− x FeS 2. However the de-intercalation of lithium from Li 2FeS 2 in a LiI-CPE cell operating at 130°C is not a pure topotactic one, but rather the Li 2− x FeS 2 undergoes some structural change on cycling. Iron oxides, hydroxides and sulfates contaminate the surface of pyrite from different sources. However the performance characteristics, such as reversible capacity and polarization of the Li/composite polymer electrolyte/FeS 2 cells were found to be independent of the amount of impurities. The thin-cathode cell design has a projected energy density of 130 W h/kg at C/3 rate and specific power of 300 W/kg (on the basis of 5 mA/cm 2 demonstrated in experimental cells). Over 500 100% DOD cycles with a capacity fading rate of less than 0.1%/cycle have been demonstrated in a small laboratory prototype 7 μm-thick modified cathode cell.

Computational and analytical chemistry: Methodology to study chemical reactions between sodium, calcium, and aluminum fluorides in molten cryolite

Reaction constants and composition profiles in molten cryolite have been theoretically investigated. Hartree-Fock and density functional calculations were applied to determine the exact nature (structure and energetic) of complexes existing in molten cryolite. The aim of this work was thus the understanding of chemical processes occurring in the electrowinning of aluminum and was a demonstration of how computational chemistry (based on density functional theory) can help us to determine structures and reaction energies in particularly complex medium such as cryolite. An analytical study, based on mass balance and equilibrium constants has been undertaken. This was performed on molecular liquid entities taking into account the four-, five- and sixfold coordinated aluminum complexes of the AlF3-3NaF melt system. Moreover, the effect of calcium has been studied by substituting two sodium atoms with one calcium atom, thus leading to the CaNaA1F6 system. Two conformers (instead of three for Na) were obtained for this system. They can be described as representing the four- and fivefold coordinated aluminum complexes in molten cryolite. The structurizing effect of calcium was clearly illustrated by the resulting optimized structures, showing that calcium stabilizes the IV and V coordinations of aluminum. By computing reaction constants, we have obtained composition profiles that are presented with those based on experimental data. Comparisons point out that computational chemistry techniques match with experimental results, especially in the case of pure cryolitic melts. For the presence of the fivefold coordinated aluminum complex in cryolite, and the predominance of the fourfold coordinated complex with calcium, it is clear that these computational techniques show us correct trends in predicting the main species in molten media. © 1997 John Wiley & Sons, Inc.

Purely discrete Markovian frequency modulation in molten alkali perchlorates

The vibrational decay of the symmetric stretching v1A1) mode of ClO−4 anion in fused 0.76 LiClO4 - 0.24 KClO4 mixture at 513 K was shown to be arisen from purely discrete Markovian frequency modulation process without any memory caused by ion-induced dipole attraction between the probe anion and adjacent cations. This process is collison-assisted and τO, the modulation time, and τBC, the time between cation-anion collisons in molten media, were found to be in close agreement. Several models for τBC calculation were analysed, and the Enskog model was found to be the most suitable for molten salts.

Comparison of direct esterification and transesterification of fructose by Candida antartica lipase

Fructose oleates synthesis was performed in a batch reactor by trans- or direct esterification. An immobilized lipase from Candida antartica was used. When a solvent was used, 65% and 46% of conversion of fructose were obtained by transesterification and direct esterification, respectively. These two reactions were also compared in a solvent-free melt. Both in molten media and with cosolvent, two isomeric forms of fructose oleates were produced.

Structure and properties of fused materials in the high zirconia region of the ZrO2-SiO2-CaO system

The development of high-temperature processes in the manufacture of technical glasses and sitalls is to a large degree based on the use of new types of refractories. One of the most promising materials is fusion-cast refractory based on zirconia [i]. Most interest is attached to the ZrO=-SiO2-CaO system. Studies of the conditions of synthesis, structure, and properties of fused materials in the high zirconia region (mass proportion of ZrO 2 more than 65%) of the system Zr02-SiO 2 show their prospectiveness for making refractories that are resistant at high temperatures to the corrosive action of gas and molten media [2, 3]. We carried out an investigation of the structure and properties of fused materials in the high zirconia region of the ZrO2-SiO2-CaO system.

Thermogravimetric and spectroscopic studies on trans-[Co(en) 2Cl 2]Cl and cis-[Co(en) 2(NCS)Cl]ClO 4 in molten KSCN and ZnCl 2KCl eutectic

The green trans-dichlorobis(ethylenediamine)cobalt(III) chloride and the red cis-chloroisothiocyanotobis(ethylenediamine)cobalt(III) perchlorate were prepared and their purity was confirmed by measuring their isobestic point spectrophotometrically. Thermal and spectral behaviour of the two Co(III) complexes in the solid state and in molten KSCN and ZnCl 2KCl eutectic was studied. Results of thermogravimetric analysis showed a difference in decomposition temperature of the two complexes alone and in molten media. Moreover, the end product of the complexes alone was found to be Co 3O 4, but in thiocyanate melt CoS 2. Spectral measurements in the melts show characteristic absorption bands of tetrahedrally coordinated Co(II) suggesting the formation of [Co(NCS) 4] 2− and [CoCl 4] 2− species after oxidation—reduction reactions. Spectra in the solid state show the characteristic absorption of Co(III) in octahedral coordination.

Relation entre oxoacidité(échanges de o2-) et aciditédans des sels fondus. applicationàla solubilisation des oxydes métalliques

A quantitative treatment of oxo-acidity reactions, particularly the solubilization of metal oxides in two low-melting salts — ethylammonium chloride and ethylpyridinium bromide — is described, with the aid of experimental results obtained previously. An “oxo-acidity function”, Ω, is defined, allowing the quantitative classification of oxo-acid/oxo-base strengths in these molten media. The influence of the Brønsted acidity properties (characterized by pH in the fused salts) on the oxo-acidity phenomena is then discussed. A Ω/pH relation is established, the partial pressure of water vapor being introduced. In this way, quantitative estimates of oxo-acidity reactions caused by the action of acids or bases become possible when the moisture content of the fused salts is controlled. The influence of these phenomena on oxidation—reduction properties is also considered, by means of equilibrium potential/Ω diagrams. Their practical exploitation is discussed.

Production of lower olefins by pyrolysis of liquid hydrocarbons in medium of molten heat carrier

1. A process has been developed for the production of lower olefins by decomposition of liquid hydrocarbon feedstocks in molten media; this process makes it possible to operate with continuous removal of carbon from the reaction zone and to achieve high values of the operating indices without using any steam in the cycle. 2. Product yields and compositions were studied in the decomposition of various petroleum products in liquid heat carriers. It was established that these particular feedstocks will yield a gas containing about 30–31% C2H4 by volume and 6.5% C3H6. 3. In the example of decomposition of crude oil, it has been shown that the use of molten media is a promising approach to the processing of heavy feedstocks that cannot be pyrolyzed in tube furnaces.

H2S Promoted Oxidative Dehydrogenation of Hydrocarbons in Molten Media

AbstractThe H2S promoted oxidative dehydrogenation of butane to butadiene can be efficiently carried out if a molten salt medium, e.g. LiCl/KCl, is used to dissipate the heat from the highly exothermic reaction. The reaction is catalyzed by the addition of soluble salts such at Tl2O3, BaCl2 or MnCl2. Optimization of reaction conditions by varying temperature, space velocity and quantity of promoter results in a 53% olefin yield (5% butenes, 48% butadiene) at 80% butane conversion. The system is also shown to be efficient for the dehydrogenation of ethane to ethylene, propane to propylene, butene to butadiene and ethylbenzene to styrene.

A solid silver iodide electrode as silver ion indicator in molten nitrates

Qualitative reasoning suggested the possibility of using silver iodide as indicator electrode for silver ions in molten media. The idea has been confirmed by employing a rotating disk electrode, in which the electroactive part was a AgI-tablet, for the determination of the diffusion coefficient of silver ions in molten potassium and sodium nitrates.

Conductibilite des melanges du bromure d'argent avec les bromures alcalins fondus

The conductivity of binary molten mixtures of (AgAlkali)Br has been measured by a four-electrode method with ac, for (AgNa)Br, (AgK)Br, (AgRb)Br and (AgCs)Br molten mixtures at different compositions and temperatures. Conductance by charge transfer, of Grotthuss type, is considered in molten media that contain a partially covalent compound. The equivalent conductivity variations are connected with the partial covalent of silver bromide and the cation size.

Measurement of potentials in the electrolysis of molten media

Measurement of potentials in the electrolysis of molten media

Anode Effect in Aqueous Electrolysis

A phenomenon which closely resembles “anode effect” in molten electrolysis can be developed in the electrolysis of aqueous solutions at high current density. Normal operation of the electrode ceases and a so‐called “transition period” begins when the electrode temperature reaches the boiling point of the electrolyte. When the applied voltage is increased beyond a critical value the transition behavior suddenly changes to the “aqueous anode‐effect.” During this effect the surface temperature of the anode rises far above the boiling point of the electrolyte. Evidence is provided which indicates that the gaseous envelope surrounding the anode during the aqueous anode‐effect is maintained by the vaporization of the electrolyte against the hot anode surface. An aqueous cathode‐effect was also obtained. The relation between aqueous anode‐effect and anode effect in molten media is discussed.