University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 8-1973 Thermodynamic and Electrochemical Studies of Niobium in Molten Fluorides and Chloroaluminates Gann Ting University of Tennessee - Knoxville Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Part of the Chemistry Commons Recommended Citation Ting, Gann, "Thermodynamic and Electrochemical Studies of Niobium in Molten Fluorides and Chloroaluminates. " PhD diss., University of Tennessee, 1973. https://trace.tennessee.edu/utk_graddiss/3261 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Gann Ting entitled "Thermodynamic and Electrochemical Studies of Niobium in Molten Fluorides and Chloroaluminates." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Chemistry. G. Mamantov, Major Professor We have read this dissertation and recommend its acceptance: Charles Baes, G. P. Smith, J. O. Chambers Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) July 31, 1973 To the Graduate Council: I am submitting herewith a dissertation written by Gann Ting entitled "Thermodynamic and Electrochemical Studies of Niobium in Molten Fluorides and Chloroaluminates." I recommend that it be accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy , with a major in Chemistry. r Professor fo ma-. We have read this dissertation f!.f. and recommend its acceptance : Accepted for the Council: Vice Chancellor for Graduate Studies and Research THERMODYNAMIC AND ELECTROCHEMICAL STUDIES OF NIOBIUM IN MOLTEN FLUORIDES AND CHLOROALUMINATES A Dissertation Presented to the Graduate Council of The University of Tennessee In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy by Gann Ting August 1973 ACKNOWLEDGMENT The author wishes to express his appreciation to Drs. G. Mamantov and C. F. Baes, Jr. for their guidance and encouragement throughout this investigation. Appreciation is expressed to Drs. c. E. Bamberger and K. w. Fung for their valuable assistance and suggestions in the labora­ tory, to D. L. Manning for making his instruments available for this work and for helpful discussions , to G. M. Begun for assistance with the Raman experiments, and to J. P. Young for performing the UV spectrometric analysis. Thanks are also expressed to the Oak Ridge National Laboratory operated for the United States Atomic Energy Commission by Union Carbide Corporation, especially to its Reactor Chemistry Division, for allowing the author to do this research, for providing equipment, materials, and facilities , and to the members of the Analytical Chemistry Division for sample analysis . Support of the National Science Foundation, Grant GP-32433X, for work carried out at the Department of Chemistry, University of Tennessee, is gratefully acknowledged. I would also like to thank Mrs . J. Wallace for typing this manuscript , and to s. C. Lin for drawing the figures. Financial support from the Institute of Nuclear Energy Research , Chinese Atomic Energy Council, Republic of China, is gratefully acknowledged. ii 11190:1.9 ABSTRACT The oxide chemistry of niobium(V) has been studied in molten LiF-BeF2 mixtures. The stoichiometry of an oxygen-containing niobium(V) F) has been established in molten Li species (Nb02 2BeF4 from the hetero­ geneous equilibrations of NiNb206 and NiO with the melt containing NiF2 , the equilibrium quotient for the reaction was determined. The equili- brium quotient for the heterogeneous reaction of Nb2o5 and BeO with mo lten LiF-BeF2 mixtures (67-33 and 52-48 mole %) was also determined, and the effect of the melt composition on the solubility of Nb2o5 (c) and on the activity coefficient of Nb02F(d) have been examined. The free energies of formation of Nb02F(d) and NiNb206 (c) has been estimated from the experimental results. A Pourbaix diagram for niobium in molten Li2BeF4 at 500° and a phases diagram at 600° involving Nb02F and NiF2 and the equilibrium oxide phases (Nb2o5 , NiNb2o6, NiO, and BeO) in molten Li2BeF4 were constructed. The electrochemical reduction of Nb (V) in molten LiF-BeF2-ZrF4 (65.6-29.4-5.0 mole %) has been studied by linear sweep voltammetry with pyrolytic graphite and platinum electrodes. Potentials were measured with respect to a Ni(II) (saturated)/Ni reference electrode. Nb (V) was found to form Nb02F. The results at the pyrolytic graphite electrode were reproducible; three reduction steps were observed at low scan rates (< 0.5 V/sec) and one reduction step was observed at high scan rates (> 5 V/sec). Mechanisms are proposed for the reduction of niobium(V). The results with platinum electrode were complicated and irreproducible. The electrochemical reduction of Nb (V) chloride and Nb (V) oxychloride in molten AlCl3-NaCl mixtures (63-37 to 50-50 mole %) has iii iv been examined by means of chronoamperometry, differential pulse polaro- graphy , chronopotentiometry and linear sweep voltammetry using platinum and tungsten electrodes. Potentials were measured with respect to an Al (III) (AlC13-NaCl, 63-37 mole %) /Al reference electrode. The effect of the melt composition, temperature and Nb (V) concentration on the reduc- tion steps was studied. The results show that the stability of the niobium species and the electrode reduction mechanisms of Nb (V) chloride and oxychloride are very sensitive to the melt composition and to the temperature. The number of reduction steps, the redox potentials, and the stability of the various niobium species at different melt composi- tiona and temperatures are reported. Reduction mechanisms of niobium(V) chloride and oxychloride are proposed. Some vibrational spectroscopic studies of solid Nb20s and NiNb20s were performed; the results indicate that the symmetric stretching fre­ -1 quencies of Nb-0 in Nb20s and NiNb2o6 are 992 and 882 em • respectively. TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION . • • . 1 A. Review of the Literature • • • . 2 1. Oxide chemistry in molten Li2BeF4 and 2 LiF-BeF2-ThF4 ••••••, , • , , • , , , • • 2 , Chemistry of niobium oxides, fluorides, oxyfluorides and oxo-fluoro complexes • • • • • 5 3, Chemistry of niobium chlorides and oxychlorides 10 4. Electrochemistry of niobium in molten chlorides and fluor ides • • • • • • • • • • • • • 14 B, Review of Basic Principles • . 19 1. Acid-base concepts in molten chlorides and fluorides . 19 2 . EMF measurements • • • • • • • • • . 21 3. Representation of equilibria by Pourbaix diagrams . 23 4. Linear sweep voltammetry • • • • . 26 5. Chronopotentiometry • • . 30 6. Chronoamperometry and construction of stationary electrode polarograms • • • • • • • • 43 7. Pulse polarography • • • • • • 43 8, Studies of adsorption by linear sweep voltammetry and chronopotentiometry . 46 c. Proposed Research . • • • • • • • • • • • 49 II. EXPERIMENTAL • 0 Q • 0 0 • • • . • • . 51 v vi CHAPTER PAGE A. Ma terials e • • • • • • • • • • ' • • • • • • • • • • 51 1. Materials for equilibria and electrochemical studies in mo lten fluorides • • • • • • • • • • 51 2. Materials for electrochemical studies in molten chloroaluminates • • • • • • • • • • • • . 52 B. Apparatus 0 • • • • • . 53 1. Apparatus for equilibria studies in molten fluorides . 53 2. Apparatus for electrochemical studies in molten chloroaluminates • • • • • • • • • • • . 58 3. Apparatus for electrochemical studies in molten fluorides • • . • • • • . 61 c. Procedures • . • • • • • • • • • • 64 1. Procedure for equilibria studies • • . 64 2. Procedure for electrochemical studies in molten chloroaluminates . 66 3. Procedure for electrochemical studies in molten fluorides . 68 III. RESULTS AND DISCUSSION • • • • • . 69 A. The Oxide Chemistry of Niobium in Molten LiF-BeF2 Mixtures • • • • • • • • • • • • • • • • • 69 Equilibrations of and BeO with molten 1. Nb20s LiF-BeF 2 mixtures • • • • • • • • • • • 70 2. Equilibrations involving nickel niobates in mo lten Li2BeF4 • • • • • • • • • • • • • • • • • • • • 80 vii CHAPTER PAGE 3. Equilibrations of NiNb o and NiO with molten Li BeF 2 6 2 4 and determination of the stoichiometry of an oxygen-containing niobium species in the melt. 87 4. Estimation of the heat of formation and the lattice energy of NiNb2o6 • • • • • • • • • • 97 B. Studies of Nb (V) in Molten LiF-BeF2-zrF4 (65. 6-29. 4-5.0 Hole %) ••••••••. 102 c. Electrochemistry of Niobium Pentachloride in Holten AlC1 -NaCl Hixtures • • • • • • • • • • • • 114 3 1. Electrochemistry of Nb (V) in molten AlC13-NaCl (63-37 mole %) • • • • • • • • • • • • • • • • 114 2. Electrochemistry of Nb (V) in molten AlC13-NaCl (61-39 mole %) • • • • • • • . • • • • • • • • 127 3. Electrochemistry of Nb (V) in molten AlC13-NaCl (55-45 mole%) ••••••••••• o 0<0 o o 130 4o Electrochemistry of Nb (V) in molten AlC13-NaCl (50-50 mole %) • • • • • o o o oeooae•• 141 So Effect of melt composition and temperature on the reduction of Nb (V) in molten AlC13-NaCl mixtures • o o • • • 0 0