In various technological processes using molten salts, molecular chlorine is released or absorbed. When conducting scientific research involving melts, there are often cases, in which an oxidizing atmosphere of chlorine is necessary, or chlorine is a reaction product. This raises the question concerning the mechanism of chlorine solubility and its influence on the processes being studied. Thus, the purpose of this work is to establish the mechanism of chlorine dissolution in molten alkali metal chlorides.
The purpose of this work is to establish the mechanism of chlorine dissolution in molten alkali metal chlorides.
Electronic absorption spectra of saturated chlorine solutions in molten NaCl, NaCl - KCl, KCl, CsCl were recorded in the temperature range from the melting point of salt to 1000 °C with an interval of approximately one hundred degrees. The work was carried out using an SF-26 spectrophotometer (Russia) adapted for operation with high-temperature melts.
In all salts studied, the long-wavelength edge of the absorption band was reliably detected. With increasing temperature, this absorption edge shifted to longer wavelengths in all chlorides. For sodium chloride and an equimolar mixture of sodium and potassium chlorides, it was possible to record a wide plateau with a maximum shifted to the region of higher energies compared to the absorption maximum of chlorine gas. For the other salts, the maximum could not be recorded; this is due to the large absorption of the salt - solvent.
From the data [1 - 7] it is known that the solubility of chlorine in the studied melts is several mole percent. This is significantly higher than the solubility of inert gases, so it is reasonable to assume that the simplest type of polyhalide compounds Cl₃^- is formed in the melt. In addition, we have established a shift in the position of the maximum of the chlorine band in the melt to the high-frequency region [8]. This shift confirms the formation of Cl₃^- complexes in the melt.
In accordance with the theory of molecular orbitals, the sharing of valence electrons in such a complex leads to the formation of axial three-center molecular orbitals. Filling them with electrons in accordance with the principle of minimum energy ensures the stability of such a group of atoms. The stability of such particles at high temperatures can probably be explained by the fact that the anions of the molten salt are not solvated and have high activity.
On the other hand, the close coincidence of the maxima in molten salts with the maximum absorption of chlorine gas, as well as a slight shift of the maximum and an increase in solubility with temperature, allow us to draw a conclusion about the predominant inert gas mechanism of chlorine dissolution. Based on the data presented, it can be concluded that both physical dissolution and chemical interaction with the formation of particles of the Cl₃^- type take place.
 

Electrical conductivity is one of the most important properties required for a competent organization of electrolytic processes in molten salts media; in particular, the processes of the production and refining of metallic hafnium and its separation from zirconium. The separation of zirconium and hafnium is especially important because they have very different thermal neutron capture cross sections: Zr value is ~0.18 barn, Hf value is 115 barns. Both elements are used in nuclear engineering.

Previously, in a series of experimental works, we measured the electrical conductivity of zirconium tetrachloride solutions in various molten alkali metal chlorides [1–5]. In this work, we measured for the first time the electrical conductivity of molten HfCl4 mixtures with a low-melting solvent (LiCl-KCl)eut. A special attention was paid to the purity of the salts used. The measurements were carried out in a capillary-type quartz cell of a special design [4, 5] with a constant within the range of 95.2–91.9 cm^–1. The resistance of the molten mixtures was recorded using an R-5058 AC bridge at a frequency of 10 kHz. The concentration of HfCl4 ranged from 0 to 30 mol.%, and the temperature varied from 780 to1063 K.

With increasing temperature, the values electrical conductivity of molten (LiCl-KCl)_eut.-HfCl₄ mixtures increased from 0.86 to 2.08 S/cm as a result of an increase in the mobility of ions (simple and complex) and a decrease in the viscosity of the melt. As the HfCl4 concentration increased, the electrical conductivity decreased.

A molecular melt of pure hafnium tetrachloride, consisting of HfCl₄ and Hf₂Cl₈ molecules, has a high saturated vapor pressure (45-60 atm) and a very low electrical conductivity (5-7)∙10^-6 S/cm [6]. When interacting with molten alkali metal chlorides, HfCl₄ molecules are ionized and form strong complex anions HfCl₆^2- that displace alkali cations into the second coordination sphere. When the HfCl₄ concentration increased to 33 mol. %, the concentration of relatively weakly mobile complex groups HfCl₆^2- increased. These groups contain 6 chlorine anions and are quite firmly bound to the four-charged metal. The concentrations of the main current carriers: Li⁺, K⁺ and especially mobile Cl- anions decreased more and more. This led to a decrease in the electrical conductivity of the melts. The previously studied molten (LiCl-KCl)-ZrCl₄ mixture illustrated a smaller decrease in the electrical conductivity at the increase in the tetrachloride concentration as opposed to the hafnium-containing mixtures. This indicates a lower strength of the ZrCl₆^2- complexes compared to HfCl₆^2- complexes.
 

During the pyrochemical processing of spent nuclear fuel, multicomponent molten mixtures are formed based on the LiCl-KCl eutectic, containing UCl₃, PuCl₃ and a large number of chlorides of elements - fission fragments (CeCl₃, NdCl₃, SrCl₂, BaCl₂, CdCl₂, CsCl, RbCl, etc.). Electrochemical methods are supposed to be used to separate fission products. Therefore, it is important to know the electrical conductivity of such melts. However, obtaining experimental data for many various multicomponent mixtures is practically insurmountable challenge. Therefore, it is necessary to find a way to reliably estimate their electrical conductivity.

To develop a model and semi-empirical method for assessing the electrical conductivity of multicomponent melts of arbitrary composition with good accuracy, a fairly wide base of experimental data is required. To expand this base, we measured the electrical conductivity of a large number of binary, ternary and various multicomponent, including uranium-containing molten mixtures in wide ranges of temperatures and concentrations, using capillary quartz cells with platinum electrodes and the AC-bridge method. The density of such mixtures was estimated and the molar electrical conductivity was calculated. The results are systematized, and some of the results are published in [1–4].

In all cases, the electrical conductivity of molten mixtures increases with increasing temperature. When heavy cations are added to the molten LiCl-KCl eutectic, the formation of complex chloride anions, which are less mobile than individual ions, occurs. This leads to a decrease in the concentration of current carriers Li⁺, K⁺ and, especially, Cl^–, and as a result, a decrease in the electrical conductivity of the melt.

The electrical conductivity of molten mixtures is a highly non-additive property. Its deviations from the additive sum of electrical conductivities of individual components can reach tens of percent. The stronger the interaction in the system, the greater the deviation of the molar electrical conductivity from additivity.

We propose to evaluate the electrical conductivity of multicomponent mixtures as an additive sum of the electrical conductivities of binary mixtures. We have shown that in this case, the electrical conductivities of molten ternary and quaternary mixtures differ from the experimentally found values by no more than 2%, that is, within the experimental error, since deviations from additivity have been already taken into account in binary mixtures analysis. In other words, we bring the multicomponent molten mixture closer to the ideal one by variation of the subsystems (components of the mixture).
 

Acetamide has a valuable combination of properties which are only beginning to be recognized. Besides low cost and many convenient physical properties, it is also highly polar and has an enthalpy of fusion comparable to that of many inorganic salts which makes it a potential phase change heat storage material. Moreover, in the molten phase it is an excellent solvent for both organics and, particularly, for many inorganic salts. Acetamide melts have found applications as electrolytes for electrochemical treatment of metals [1, 2]. The possibility of electrochemical synthesis of refractory compounds from acetamide melts at 120°С has been examined for Ti-B as an example. In the binary system CH₃CONH₂–(NH₄)₂TiF₆ a new process, which is more electropositive than the decomposition of acetamide and more electronegative than the acetamide discharge has been observed. The scan rate independence of potential characterizes the observed processes as reversible charge transfer. This process corresponds to the one-electron irreversible charge exchange: Ti(IV)/Ti(III). In the binary system CH₃CONH₂–NH₄BF₄ only the decomposition of carbamide has been observed. In the ternary system CH₃CONH₂–(NH₄)₂TiF₆–NH₄BF₄ a new process, which is more electropositive than the decomposition of acetamide and more electronegative than the one-electron charge exchange: Ti(IV)/Ti(III) has been observed. This process corresponds to the electrochemical synthesis of Ti-B compound. Ultra thin coatings of titanium boride have been obtained on nickel by the electrolysis from a CH₃CONH₂–(NH₄)₂TiF₆–NH₄BF₄ melt at 120°C at current densities of 10-20 mA/cm².

This report presents the results of the studies of the influence of the molten reaction medium composition and electrolysis conditions on the composition, properties and yield of the producing molybdenum and tungsten carbides.

Necessary conditions for the implementation of the high-temperature electrochemical synthesis (HES) of refractory metal carbides in molten salts are the joint or sequential deposition of the corresponding metal and carbon on the cathode surface, that is, respectively, “thermodynamic” or “kinetic” synthesis modes [1]; as well as providing conditions for their interaction with each other. A third synthesis mode is also possible, when one of the synthesis components (carbon or metal) is used as a cathode material. In our work, we implemented the first “thermodynamic” mode of the HES, selected and studied systems and conditions for the electrochemical extraction of refractory metal and carbon from molten salts in a close and narrow range of potentials.

Mixtures of molten halides with oxygen-containing compounds of molybdenum (tungsten) and lithium carbonate were used as the initial reaction medium. Carbon dioxide, located at different partial pressures above the molten electrolyte (from 1 to 15 atm.), was also used as a carbon precursor. The research methods used in our studies are: cyclic voltammetry, galvano- and potentio- static modes of electrolysis, chemical and X-ray phase analyses, scanning and transmission microscopy, Raman spectroscopy, and nitrogen adsorption-desorption method.

Metal and carbon oxyanions ([MO₄]^2-; [CO₃]^2-), their oxides in the highest valence state (MO₃ and CO₂) were used as precursors for the synthesis components (where M = Mo; W). The pointed oxyanions are characterized by the presence of acid-base equilibrium in the electrolyte with the formation of a oxide anion. The latter determines the basicity of the melt. These equilibriums have a great influence on the kinetics and the route of the oxyanion electroreduction. We changed the basicity of the molten medium by introducing oxide ion acceptors - acid additives of various types (cations with a high specific charge, anions with a high affinity for the oxide anion, fluoride anions) into the electrolyte. This approach made it possible to form in situ new electrochemically active particles (ECAP) of metal and carbon, which could be reduced at similar values of their deposition potentials.

We used various acidic additives in our studies (magnesium cations, metaphosphate anions, fluoride anions), which bind oxide ions that are released at the cathode from metal and carbon oxyanions during the discharge, and promote the formation of new ECAPs in the electrolyte: cationized oxyanions of metal and carbon; dimeric metal complexes; fluorine-oxide metal complexes. This made it possible to shift the deposition potentials of metals and carbon to the positive potential region up to 0.5 V and obtain a reduction in the consumption of electrical energy for electrolysis. It should be noted that the introduced acid additives do not participate in the electrode processes. This technique is a good example of the use of electrochemical catalysis to realize sustainable electrochemical synthesis.

Five compositions of electrolytic bathes were studied: (1) Na,K|Cl-Na₂МO₄-MgCl₂-CO₂ (7.5-14 atm.); (2) Na,K|Cl-Na₂МO₄-NaPO₃-CO₂ (5-17 atm.); (3) Na,K|Cl,F-Na₃МO₃F₃-CO₂ (10-17 atm.); (4) Na,K|Cl-Na₂W₂O₇-CO₂ (7.5-12.5 atm.); (5) Na,K|Cl-Na₂W₂O₇-Li₂CO₃-CO₂ (5-12.5 atm.). The good solubility of potassium and sodium chlorides in water simplifies the washing of products from residues of the synthesis medium. For each of the five indicated salt mixtures, the sequence of electrochemical transformations occurring during the electrochemical reduction of oxygen-containing metal and carbon compounds introduced into the electrolytic baths separately or together was studied using cyclic voltammetry; the mechanisms and kinetic features of electrode processes were established; the optimal concentrations of each precursor for the synthesis were determined. The results of electrochemical studies and features of the electrolysis for each mixture of salts are presented in [2–5].

It has been established that the composition and properties of the synthesized carbides are influenced not only by the electrolysis conditions (voltage on the bath, current density, temperature, duration), but also by the ionic composition (qualitative and quantitative) of the initial reaction medium. The highest yield of single-phase stoichiometric metal carbides with minimal specific energy consumption is realized in system (3).

Thus, it was shown that it is possible to change the path and kinetics of the electroreduction reaction of metal and carbon oxy-anions by changing the cationic and anionic composition of the electrolytic bath (thereby forming new ECPs in the reaction medium). This makes it possible to bring together (combine) the deposition potentials of tungsten (molybdenum) and carbon and to carry out the synthesis of both single-phase carbide phases and mixtures based on them with other metals and carbon in a wide range of current densities. 

Two task-specific ionic liquids (TSILs) were encapsulated into the framework of a Zeolite imidazolate framework-8 (ZIF-8) to enhance its CO₂ capture capacity and CO₂/N₂ selectivity at post-combustion conditions. 1-Ethyl-3-methylimidazolium amino-acetate {[EMIM][glycine (Gly)]} and 1-Ethyl-3-methylimidazolium (S)-2-aminopropionate {[EMIM][alanine (Ala)]} were selected as TSILs. TSIL@ZIF-8 composite sorbents were prepared by varying the loading of TSIL, and properties such as sorbent thermal stability, porous structure and crystal nature of the composite were investigated. The incorporation of TSIL into ZIF-8 led to a dramatic rise in CO₂ uptake particularly at pressures lower than 1.0 bar. At this low-pressure range, CO₂ uptake was greater than in pristine ZIF-8 for all TSIL loadings and TSIL@ZIF-8 composites with 30 wt.% [Emim][Gly] reached a CO₂ uptake capacity of 0.76 mmol·g^-1 solid at 0.1 bar, and 0.88 mmol/g-solid at 0.2 bar at 303 K. These values were 13 and 7 times higher that CO₂ uptake in pristine ZIF-8 at identical conditions. TSIL functionalized composites also exhibited much higher selectivity than pristine ZIF-8 at all pressures. For instance, at 30 wt.% [EMIM][Gly] loading, CO₂/N₂ ideal selectivities at 313 K were 28 and 19 at 0.1 and 0.2 bar, respectively. This synthesized composite sorbent, with significantly high CO₂ uptake, better CO₂/N₂ selectivity at the low-pressure region (<1.0 bar), and low isosteric heat of adsorption (Q_st), confirms that TSIL@ZIF-8 composites can be potential candidates for post-combustion CO₂ capture processes and opens the door for the further development of suitable TSIL@MOF composite sorbent to be deployed in the CO₂ capture process.

This state-of-the-art review examines the synergistic effects and applications of binary mixtures of ionic liquids (ILs), delineating their potential as versatile solvents in various fields. Binary mixtures of ILs have gathered compelling attention due to their uncommon properties and interactions, offering tailored solutions for various scientific and industrial applications [1].

The review surveys the synthesis and characterization of binary mixtures of ILs, featuring the diverse combinations of anions and cations employed to obtain desired properties. The physicochemical properties of binary mixtures, including conductivity, viscosity, thermal stability, phase behaviour and solvation behaviour, are examined to expound the synergistic effects of mixing different ILs [2]. In addition, the review analyses the thermodynamic aspects of binary mixtures, investigating miscibility, phase transitions, and phase diagrams to comprehend their complex behaviour under changing conditions.

A detailed analysis of the applications of binary mixtures of ILs reveals their versatility in extraction, separation processes, catalysis, green chemistry, and energy storage. In catalysis, binary mixtures of ILs show enhanced selectivity, catalytic activity, and recyclability compared to individual ILs, effectively synthesizing fine chemicals and organic compounds [3]. In separation and extraction processes, binary mixtures of ILs exhibit better performance in the selective recovery of industrial effluents, electronic waste, and metals from ores, contributing to environmental protection and sustainable resource management.

Moreover, binary mixtures of ILs have exhibited promising uses in energy storage, serving as electrolytes in supercapacitor systems and advanced batteries. Their thermal stability, high ionic conductivity, and wide electrochemical stability window make them excellent candidates for enhancing the safety and performance of energy storage devices, promising the development of next-generation energy technologies [4].

The review also discusses the role of binary mixtures of ILs in fostering green chemistry practices by replacing perilous organic solvents in various chemical processes. Their nontoxicity, low volatility, and recyclability help reduce environmental pollution and minimize waste generation, coinciding with sustainable chemistry and technology principles.

Finally, the review pinpoints future research directions and issues in the field of binary mixtures of ILs, highlighting the need for further investigation of their fundamental characteristics, improvement for specific uses, and incorporation in industrial processes [5-6]. In conclusion, this state-of-the-art review provides valuable perceptivity concerning the synergistic effects and applications of binary mixtures of ILs, emphasizing their immense potential as sustainable solvents for diverse scientific and industrial endeavours.

Electronic absorption spectra (EAS) of rare earth ions in melts are scantily known. Since 4f electrons spend most of their time very near the nucleus, thee are well shielded by all the other electrons of a lanthanide ion from the influence of the crystalline fields of surrounding ligands. This circumstance, coupled with the fact that the spin-orbit interaction is about ten times larger for 4f than 3d electrons, makes the spectra of 4f ions much less sensitive to environmental changes than those of d-ions. However, spectra of the several 4f-cations (Ce(III), Pr(III), Tb(III)) contain not only orbitally-forbidden f↔f transitions, but also 4fⁿ↔4f^n-15d¹ allowed ones. These transitions are almost unexplored. Only very scant data on Ce(III) available [1].

EAS of CeCl₃ and TbCl₃ dilute solutions was recorded in molten NaCl, NaCl-KCl (1:1), KCl and CsCl over the range 40000 to 8000 cm^-1. Quartz 2 mm pathlength cells were used for measurements, temperature region was from 700 (800) up to 1000 ⁰C. Concentrations were about 0.005 mol/l (near 0.03 mol %) in case of CeCl₃ and essentially higher ~0.5 mol/l (~2 mol %) for TbCl₃. By way of example EAS of CeCl₃ and TbCl₃ in molten NaCl-KCl consist of one weakly asymmetrical band with maximum near 31000 and 37000 cm^-1 respectively. This band is 2 to 3 times narrower than the electron-transfer bands. In contrast to f↔f transitions intensity of these bands diminishes as temperature is rises, and rises in the row of the solvents from NaCl to CsCl. However, oscillator strength reduces only slightly with essential increasing of temperature. These changes are result of surroundings influence upon d-sublevel only. By mathematical analysis it was shown band observed consists of two ones. All available data are well compatible with octahedral melt structure – CeCl₆^3- and TbCl₆^3- complex formation.

Chlorine gas is one of the most important reagent and product of many chemical reactions and technological processes involving molten salts, it is also often used in organic chemistry and photochemistry. Data on the optical spectroscopy of gaseous chlorine obtained over a wide temperature range provide additional information about the properties of chlorine and its reactivity.
Electronic absorption spectra (EAS) of chlorine gas were recorded in the range of 8333÷50000 cm-1 (200÷1200 nm). The spectrum consists of a single broad absorption band located mainly in the range 20000÷43500 cm-1 (230÷500 nm). There is no absorption in the range of 500÷1200 nm. The spectra were recorded in the temperature range from 0 to 1000°C approximately every 100 degrees. 

With increasing temperature, the optical density of the absorption band decreases, and the position of the maximum shifts slightly (~ 1 nm per 100 degrees) to the short wavelength region [1]. The shape of the absorption band does not change significantly, although there is a tendency to the band broadening with increasing temperature. As the temperature rises, gaseous chlorine expands and fewer chlorine molecules enter the light beam. To compensate the influence of density change, all further considerations will be carried out in extinction units , dm2/mol.
If the parameters of the potential curves, between which electronic-vibrational transitions occur, are specified, then the question arises what transitions will be more likely and what transitions will be less likely. The excitation of the Cl2 molecule (20277 cm-1) is caused by the transition from the ground state X^1 ∑_g^+▒Cl_2 to the antibonding 1ПuCl2 state [2], which is accompanied by the dissociation of the molecule. Unlike an atom, a molecule consists of two connected subsystems; these subsystems move at significantly different speeds. The set of electrons is a fast subsystem, the set of nuclei is a slow one. Thus, in the process of electronic-vibrational transitions, the molecule finds itself in an excited electronic state at the same value of the internuclear distance r, in which it was caught in the act of absorption. Halogen molecules represent a rare case where r* r. It is obvious that in this case the most probable transitions (v=0  v*=n) are accompanied by a significant increase in the reserve of vibrational energy, and the high-frequency edge of the electronic-vibrational spectrum can become continuous, which corresponds to transitions in the section of the upper potential curve lying above the dissociation boundary. Under such conditions, the chlorine molecule dissociates as a result of a significant relative shift of the stable functions U(r) along the r axis rather than due to a transition to an unstable potential curve. 
In the early 1950s, Sulzer and Wieland [3] proposed an equation describing the temperature dependence of extinction in the Franck-Condon approximation, which, however, did not describe the temperature shift of the absorption band maximum. Later, Golovitsky and Mikhailov [4] proposed an amendment that takes into account the displacement of the maximum with increasing temperature. This shift is apparently a consequence of the manifestation of anharmonic vibrations of the diatomic chlorine molecule [5].
 

Ionic liquids, a novel class of molten salts, exhibit a distinctive array of properties that set them apart from traditional molecular liquids. These properties include negligible vapor pressure, a wide thermal and electrochemical window, non-flammability, high ionic conductivity, and exceptional solvating capabilities for a diverse range of compounds. Their emergence as "organic solvent alternatives" has spurred significant interest in both academic and industrial spheres. The dynamic research landscape surrounding ionic liquids is expanding rapidly, owing to their versatile applicability, which stems from the ease with which their physical properties can be fine-tuned through modifications in cation-anion combinations or attached moieties. This talk will offer an overview of ionic liquids, emphasizing their unique thermophysical attributes crucial for applications such as metal ion extraction, CO₂ capture, fuel desulfurization, and aqueous biphasic systems for extracting value-added products. Furthermore, it will delve into the influence of these thermophysical properties on the efficacy of such applications, while also highlighting current research trajectories exploring ionic liquids as solvents within the chemical industry.

The wide use of refractory metal carbides in industry as structural and tool materials that are able to operate at high temperatures and loads in aggressive media causes great interest for the development of novel sustainable, highly efficient, eco-friendly and safe methods for their production. High-temperature electrochemical synthesis (HTES) from molten salts is one of them. 

The HTES of carbides can be effected in two ways. In the first case, the molten electrolyte contains one synthesis component in the molecular or ionic form, which is discharged at the second synthesis component. The formation of carbide takes place as a result of the reaction diffusion of discharge products deep into the electrode material. Either—alkali (alkaline-earth) metal carbonates, which can be reduced to elemental carbon on the cathode made of a refractory metal (electrochemical carbidization) [1], or refractory metal ions, which are reduced to metal on the graphite cathode [2], are used as discharging component. The above processes occur at a low rate at relatively high temperatures and produce compounds of variable composition in the form of coatings.

In the second case, the electrolyte contains both synthesis components, which can discharge together (thermodynamic or quasi-equilibrium synthesis conditions) or sequentially (kinetic synthesis conditions) at the neutral electrode [3]. After that, a chemical interaction of discharge products takes place at the cathode to from a new carbide phase. The thermodynamic synthesis conditions are undoubtedly more interesting theoretically and promising in practical use. By varying the electrolysis conditions and modes (electrolytic bath composition, current density, and temperature), one can obtain single-phase carbides of a given composition in the form of coatings or ultrafine powders, as well as composite mixtures with other metals and carbon.

To effect electrosynthesis under thermodynamic conditions in a wide current density range, two conditions must be fulfilled:

(1) The synthesis must take place at close values of refractory metal and carbon deposition potential. The theoretical analysis of the electrowinning of alloys, presented in [4], showed that if the deposition potential difference of alloy components (∆E) is not over 0.2 V, the alloy composition will not depend on the current density used, viz the process will take place under quasi-equilibrium conditions.

(2) Electrosynthesis is a many-electron process. Therefore, the second necessary condition for synthesis is effecting partial many-electron reduction reactions of synthesis precursors over a narrow potential range practically in one stage. The sources of metal and carbon are their oxy-compounds M_xWO₄, M_xCO₃ (M = Li, Na, Ca, Ba, Mg) and CO₂. The discharge products react chemically with each other to form carbides.

Electrochemical processes in ionic melts at high temperature differ greatly from low-temperature processes in aqueous electrolytes. At high temperature, the effect of catalytic properties of the electrode material on the electrode kinetics becomes weaker. At the same time, the catalyzing role of the medium (electrolyte composition) becomes more pronounced. 

The effect of the medium on the kinetics of electrode reactions is clearly manifested in the reduction processes of tungstate and molybdate anions (complex coordination compounds of d metals in the higher valent state) and carbonate anions. The specific mechanism of formation of electrochemical active spaces (EASs) and many-electron charge transfer reaction are characteristic peculiarities of the electroreduction of the above compounds.

The essence of cation catalysis is the transformation of complicated complex anionic species into a new active state by the action of cations with strong polarizing effect (Li⁺, Ba²⁺, Ca²⁺, Mg²⁺) [5, 6]. This leads to a change in the electronic and energy state of anion, the formation of new EASs, and a change in their composition, the rate of EASs formation and charge transfer reactions. Ultimately, this leads to the fact that carbide precursors are reduced at close potentials and conditions for the implementation of the synthesis of carbides in a wide range of current densities are created. 

The paper presents the application of the cation catalysis phenomenon for effecting the HTES of nanoscale tungsten and molybdenum carbide powders in molten salts.

The changes in the saturated vapor composition and the volatility of the components of molten mixtures of uranium and some other metal tetrachlorides (ThCl4, HfCl4, ZrCl4, TiCl4) with alkali metal chlorides as functions of the temperature, concentration and cationic composition of the melts are discussed using our experimental data and those obtained by other researchers, mainly employees of the Institute of High-Temperature Electrochemistry (Ural Branch, Russian Academy of Sciences).

Like many other high-valence chemical elements, tetravalent uranium acts as a powerful complexing agent in molten alkali metal chlorides; hence, its dissolution is accompanied by substantial rearrangements of bonds of particles leading to the formation of stable complex anions: MeCl_7^(3-), MeCl_6^(2-), Me_2 Cl_10^(2-) (Me - U, Th) and MeCl_6^(2-) (Me - Hf, Zr, Ti) at the concentrations up to 50 or 33 mol % MeCl4, respectively. The complex formation is manifested as a sharp decrease in the saturated vapor pressure of the components of the molten mixtures due to which not only uranium tetrachloride but also such volatile compounds as ThCl4, HfCl4, ZrCl4, and TiCl4 are retained in solutions even at high temperatures. The strength of complex anions increases with decreasing temperature and concentration of the corresponding tetrachloride and under the counterpolarizing action of alkaline cations in the series from Li+ to Cs+ on chlorine anions in the complex chloride groups. As a result, the volatilities of UCl4, ThCl4, HfCl4, ZrCl4, andTiCl4 and the composition of vapors above the solutions in the ionic melts vary over broad ranges.

A decrease in the volatility of tetrachloride results in a decrease in its content in the saturated vapors over the melts. Hafnium, zirconium, and titanium tetrachlorides (especially TiCl4) are significantly more volatile in the individual state than UCl4 andThCl4 and have higher volatilities and contents in the saturated vapors over the solutions in molten alkali metal chlorides. The vapor over solutions in molten metal chlorides of titanium tetrachloride, which has the highest volatility among the considered tetrachlorides, almost completely consists of its molecules only. 

The temperature, concentration, and composition dependences of the saturated vapor pressures and volatilities of the components of solutions of uranium, thorium, hafnium, zirconium, and titanium tetrachlorides can be used as reference material for the organization of diverse pyrometallurgical and pyrochemical (e.g. electrolytic) processes based on salt melts.