2024 - Sustainable Industrial Processing Summit
SIPS 2024 Volume 16. Intl. Symp on Electrochemistry, Molten Salts, Corrosion, Recycling and Battery
| Editors: | F. Kongoli, C.A. Amatore, R. Fehrmann, G. Kipouros, I. Paspaliaris, G. Saevarsdottir, R. Singh, R. Gupta, M. Halama, D. Macdonald, F. Wang, M. Barinova, F. Ahmed, C. Gaidau, X. Guo, K. Kolomaznik, H. Ozgunay, K. Tang, N.N. Thanh, S. Yefremova, K. Aifantis, Z. Bakenov, C. Capiglia, V. Kumar, A. U. H. Qurashi, A. Tressaud, R. Yazami |
| Publisher: | Flogen Star OUTREACH |
| Publication Year: | 2024 |
| Pages: | 243 pages |
| ISBN: | 978-1-998384-34-1 (CD) |
| ISSN: | 2291-1227 (Metals and Materials Processing in a Clean Environment Series) |
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ELECTROCHEMICAL SYNTHESIS OF MOLYBDENUM AND TUNGSTEN CARBIDES IN MOLTEN SALTS: INFLUENCE OF ELECTROLYSIS CONDITIONS AND COMPOSITION (ACID-BASE PROPERTIES) OF THE REACTION MEDIUM
Inessa Novoselova1; Anatoliy Omel'chuk1;1V.I. VERNADSKY INSTITUTE OF GENERAL AND INORGANIC CHEMISTRY, NATIONAL ACADEMY OF SCIENCES OF UKRAINE, Kyiv, Ukraine;
Type of Paper: Regular
Id Paper: 46
Topic: 13
Abstract:
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.