Refinement of thermodynamic properties of ReO2

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 Delta(f)G(o)(ReO2)/J mol(-1) = 451, 510 + 295.011 (T/K) - 14.3261 (T/K) ln (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: Delta(f)H(298.15)(0) K(ReO2)/ kJ mol(-1) =-445.1 (+/-0.2). The uncertainty estimate includes both random (2 sigma) and systematic errors. (C) 2000 Elsevier Science B.V. All rights reserved.