Cation catalysis in high-temperature electrochemical synthesis of tungsten carbides in molten salts

摘要

The paper presents a quantum-chemical model and experimental confirmation of cation catalysis phenomenon by the influence of magnesium cation on tungstate anion electroreduction to metallic tungsten in a chloride melt. Ab-initio calculations of the parameters of the {Mg_n~(2+)[WO4]~(2+)}((2n-2)+) type species have been performed. Within the framework of this approximation the compositions for the most stable species in molten salts were obtained. The peculiarities of electroreduction of tungstate anion and carbon dioxide have been studied by the method of cyclic voltammetry in the binary NaCl-KCl melt on platinum electrodes at 750°C separately and jointly, in the presence and without magnesium chloride. It has been shown that addition of Mg~(2+) in the melt is a necessary condition for the electrodeposition of tungsten in the same potential range as carbon from carbon dioxide and for the subsequent synthesis of tungsten carbides. The electrochemical synthesis of tungsten carbides WC and W2C in the form of dispersed powder has been effected in the system NaCl-KCl-Na2WO4-MgCl2-CO2. The optimal electrolysis conditions have been determined for the synthesis of single-phase WC powder. WC product was produced in the studied system with electrodeposition rate 0.3 g/A-h at CO2 pressure over 1.7 MPa and at cathodic current densities 0.05 - 0.2 A/cm~2.