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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THE PHENOMENON OF ELECTROCATALYSIS IN THE IMPLEMENTATION OF SUSTAINABLE HIGH-TEMPERATURE ELECTROCHEMICAL SYNTHESIS OF REFRACTORY METAL CARBIDES
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: 33
Topic: 13
Abstract:
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 MxWO4, MxCO3 (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.