A study of the reaction pathways during electrochemical reduction of dense Nb₂O₅ pellets in molten CaCl₂ medium

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• 作者：D. Sri Maha Vishnu ; N. Sanil ; L. Shakila ; G. Panneerselvam ; R. Sudha ; K.S. Mohandas ; K. Nagarajan ; ^{knag@igcar.gov.in}
• 关键词：Niobium pentoxide ; Calcium chloride ; Ternary intermediates ; Electrochemical reduction ; Total solubilised basicity
• 刊名：Electrochimica Acta
• 出版年：2013
• 出版时间：30 June, 2013
• 年：2013
• 卷：100
• 期：Complete
• 页码：51-62
• 全文大小：2910 K

文摘

Electrochemical behaviour of powder compacted and sintered Nb₂O₅ pellet electrodes (~4 mm thick, open porosity ~10 % ) in molten CaCl₂ medium at 1173 K was studied using cyclic voltammetry and polarisation measurements. Similar dense Nb₂O₅ pellets were subsequently electrolysed by using them as cathodes against graphite counter electrode in the molten CaCl₂ electrolyte medium with an aim to gain insight into the electro-reduction mechanism. The chemical changes that are taking place in the cathode and also in the electrolyte melt at different stages of electrolysis were analysed. The electro-reduction experiments were carried out at an applied voltage of 3.1 V and terminated after 2, 9, 22, 26 and 35 h of electrolysis and the visibly distinct layers present in the partially reduced samples were analysed. The analysis showed presence of Ca_xNb_yO_z compounds, sub-oxides of Nb like NbO₂, NbO, etc. and metallic Nb in the samples. A pellet, which was electrolysed for 44 h was found to be reduced completely to Nb metal. The CaO, Ca and CaCO₃ produced and released to the melt during the cathodic polarisation was found to be associated with the electrochemical changes that are taking place on the electrode and this was ascertained by determination of the ¡®total solubilised basicity (TSB)¡¯ and ¡®total insolubilised basicity (TIB)¡¯ of the melt as a function of time during electrolysis. Based on the results, a three-stage mechanism comprising (i) cathodic insertion of Ca²⁺ into the oxide pellet, (ii) reduction of the oxide electrode via formation and decomposition of ternary intermediates and (iii) chemical reduction of the cathode with electro-generated calcium metal has been proposed for electro-reduction of dense Nb₂O₅ pellets.