Date of Award

8-1973

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

G. Mamantov

Committee Members

Charles Baes, G. P. Smith, J. O. Chambers

Abstract

The oxide chemistry of niobium(V) has been studied in molten LiF-BeF2 mixtures. The stoichiometry of an oxygen-containing niobium(V) species (NbO2F) has been established in molten Li2BeF4 from the heterogeneous equilibrations of NiNb2o6 and NiO with the melt containing NiF2, the equilibrium quotient for the reaction was determined. The equilibrium quotient for the heterogeneous reaction of Nb2O5 and BeO with molten 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 NbO2F(d) have been examined. The free energies of formation of NbO2F(d) and NiNb2O6(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 NbO2F 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 NbO2F. 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 been examined by means of chronoamperometry, differential pulse polarography, chronopotentiometry and linear sweep voltammetry using platinum and tungsten electrodes. Potentials were measured with respect to an Al(III) (AlCl3-NaCl, 63-37 mole %)/Al reference electrode. The effect of the melt composition, temperature and Nb(V) concentration on the reduction 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 various niobium species at different melt compositions and temperatures are reported. Reduction mechanisms of niobium(V) chloride and oxychloride are proposed.

Some vibrational spectroscopic studies of solid Nb2O5 and NiNb2O5 were performed; the results indicate that the symmetric stretching frequencies of Nb-O in Nb2O5 and NiNb2O6 are 992 and 882 cm-1, respectively.

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