Refinement of thermodynamic properties of ReO₂

Abstract

The standard Gibbs energy of formation of ReO₂ in the temperature range from 900 to 1200 K has been determined with high precision using a novel apparatus incorporating a buffer electrode between reference and working electrodes. The role of the buffer electrode was to absorb the electrochemical flux of oxygen through the solid electrolyte from the electrode with higher oxygen chemical potential to the electrode with lower oxygen potential. It prevented the polarization of the measuring electrode and ensured accurate data. The Re+ReO₂ working electrode was placed in a closed stabilized-zirconia crucible to prevent continuous vaporization of Re₂O₇ at high temperatures. The standard Gibbs energy of the formation of ReO₂ can be represented by the equation Δ_fG°(ReO₂)/Jmol^-1=-451.510+295.011(T/K)-14.3261(T/K)In(T/K)(±80). Accurate values of low and high temperature heat capacity of ReO₂ are available in the literature. The thermal data are coupled with the standard Gibbs energy of formation, obtained in this study, to evaluate the standard enthalpy of formation of ReO₂ at 298.15 K by the ‘third law’ method. The value of standard enthalpy of formation at 298.15 K is: Δ_fH°_298.15K (ReO₂)/kJ mol⁻¹=−445.1 (±0.2). The uncertainty estimate includes both random (2σ) and systematic errors.