0040-5795/02/3603- $27.00 © 2002 MAIK “Nauka /Interperiodica” 0287 Theoretical Foundations of Chemical Engineering, Vol. 36, No. 3, 2002, pp. 287–295. Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 36, No. 3, 2002, pp. 317–326. Original Russian Text Copyright © 2002 by Malyshev. A promising method for obtaining refractory com- pounds, including molybdenum carbide, is high-tem- perature electrosynthesis in ionic melts [1, 2]. Molyb- denum carbide was first electrosynthesized by the elec- trolysis of the NaBO 3 –Na 2 CO 3 –LiF–MoO 3 melt by Andrieux and Weiss [3]. Since then, this method has been improved by employing various fluoride- and borate-containing supporting electrolytes (Table 1). However, it is doubtful whether the carbonate ion and the chloride complex K 3 MoCl 6 are compatible in a flu- oride eutectic [5]. An analysis of information concerning the high- temperature electrosynthesis of metal-like refractory compounds demonstrates that the melt composition and the process conditions have usually been optimized heuristically. However, further progress in this method is impossible without a thermodynamically substanti- ated approach to the formulation of the melt composi- tion and to the implementation and control of multi- electron electrode reactions. THERMODYNAMIC FOUNDATIONS OF THE HIGH-TEMPERATURE ELECTROSYNTHESIS OF MOLYBDENUM CARBIDE Theoretical foundations for the high-temperature electrosynthesis of refractory compounds and for opti- mizing the electrolyte composition were laid by our thermodynamic analysis of the electrochemical reac- tions involved in the synthesis and by our calculation of equilibrium decomposition potentials E d for various molybdenum and carbon compounds at high tempera- tures of 900 to 1200 K. In our calculations, we pro- ceeded from thermodynamic data for the compounds involved in the reactions. The equilibrium decomposi- tion potential was calculated from the standard Gibbs energy of the decomposition reaction by the formula (1) E d 0 ∆ G T 0 / nF ( 29 , – = where ∆ is the standard Gibbs energy of decompo- sition of the compound at the temperature T , n is the number of electrons participating in the redox reaction, and F is the Faraday constant. The computational pro- cedure is detailed elsewhere [7]. Initial thermodynamic data were borrowed from [8–10]. Table 2 lists calculated equilibrium potentials for the deposition of molybdenum and carbon from various compounds. It is evident that molybdenum and carbon are deposited from analogous compounds at similar potentials, and, therefore, their simultaneous quasi- equilibrium deposition is possible. The data listed in Table 2 can be represented as plots of the deposition potential versus temperature or, in other terms, as Ellingham diagrams [11] (Fig. 1). The plots intersect for two pairs of compounds, MgMoO 4 –CO 2 (1100 K) and MoO 2 –CO 2 (1050 K); therefore, the compounds of G T 0 Theoretical Foundations and Implementation of the High-Temperature Electrosynthesis of Molybdenum Carbide in Ionic Melts V. V. Malyshev Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, pr. akademika Palladina 32/34, Kiev-142, 03680 Ukraine Received January 31, 2000 Abstract—A thermodynamic analysis and a voltammetric study of molybdenum- and carbon-containing melts were performed. The melt composition and electrosynthesis conditions were optimized to obtain molybdenum carbide powders and coatings. 1 2 3 4 6 5 7 8 9 1.4 1.2 1.0 0.8 9 10 11 12 – E d , V T × 10 –2 , K Fig. 1. Potentials of deposition of carbon and molybdenum from (1) BaMoO 4 , (2) BaCO 3 , (3) SrMoO 4 , (4) Li 2 CO 3 , (5) CO 2 , (6) MgMoO 4 , (7) MoO 2 , (8) MgCO 3 , and (9) MoO 3 as functions of temperature.