Influence of the Alkaline Earth Metal Cations on the Standard Rate Constants of Charge Transfer for the Redox Couple Ti(IV)/Ti(III) in Chloride-Fluoride Melts

Abstract

The aim of the present investigation was study of charge transfer kinetics for the redox couple Ti(IV)/Ti(III) in the (NaCl-KCl)equimol.-NaF(10 wt.%)-K2TiF6 melt and the estimation of the alkali earth metal cations influence on the standard rate constants of charge transfer (ks ) for this redox couple. Method of cyclic voltammetry was employed for the determination of the standard rate constants of charge transfer. The electrochemical redox process: Ti(IV) + e-↔ Ti(III) (1) was classified as quasi-reversible in the range of scan rate 0.75 V s-1≤ν≤2.0 V s-1. The standard rate constants of charge transfer for the reaction (1) were calculated based on the Nicholson’s equation. It was shown, that k s increase with increasing temperature and the temperature dependence of k swas described by the following empirical equation: log k s=(1.505±0,32)–(3364±675)/Т (2) The activation energy of charge transfer calculated from (2) was found equal to (64.41±13) kJ mol-1. Influence of strongly polarizing cations of Mg2+, Ca2+, Sr2+ and Ba2+on the standard rate constants of charge transfer for the redox couple Ti(IV)/Ti(III) was studied. It was determined that addition of alkali earth metal cations resulted in increasing of ks up to the certain mole ratio of Me2+/Ti(IV), which was inversely proportional to ionic potential of cations. Further addition brought some decreasing of the standard rate constants. Increase of ks is connected with substitution of Na+ and K+ cations by strongly polarizing cations in the second coordination sphere of titanium complexes that led to increasing of the bond distance between Ti and F- ligands and decreasing of titanium fluoride complexes strength. Decrease of ks at the definite ratio of components can be explained by increasing of the viscosity melts, which brings to decreasing of the diffusion coefficients. It was established the linear dependence of ks on ionic potential of alkali earth metal cations and the maximum value was obtained for complexes with outer sphere cations of magnesium. The temperature dependences for the maximum values of ks in the melts containing strongly polarizing cations were fitted by empirical equations and the activation energies of charge transfer were calculated. Values of activation energy for systems with strongly polarizing cations are considerably less than activation energy for initial system. Acknowledgments The work was financially supported by Russian Foundation for Basic Research (15-03-02290_a).