Determination of thermodynamic properties of Ca₄PtO₆ and CaPtO₃ using solid state cells with buffer electrodes

Abstract

The standard Gibbs energies of formation of CaPtO₃ and Ca₄PtO₆ have been measured in the temperature range from 940 to 1400 K using solid-state cells with (Y₂O₃) ZrO₂ as the electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Ca-Pt-O were investigated at 1250 and 1350 K. Two ternary oxides, CaPtO₃ and Ca₄PtO₆, compositions of which fall on the join CaO-PtO₂, were found to be stable. Each of the oxides coexisted with pure metal Pt. Therefore, two working electrodes were prepared consisting of mixtures of Pt + CaO + Ca₄PtO₆ and Pt + Ca₄PtO₆ + CaPtO₃. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. A novel apparatus, incorporating a buffer electrode between reference and working electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte, was used for measurement. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. The standard Gibbs energies of formation Δ_fG⁰ of the ternary compounds from elements in their stable form can be obtained from the emfs; the results are represented by the equations: Ca₄PtO₆: Δ_fG⁰/J mol^-1 = -2,774,240 + 582,86 T (±4,500)[940 to 1,115 K] = -2,808,600 + 613,66 T (±4,500)[1,115 to 1,400 K] CaPtO₃: Δ_fG⁰/J mol^-1 = -852,875 + 271,57 T (±1,200) [940 to 1,115 K] = -861,465 + 279,27 T (±1200) [1,115 to 1,260 K] Based on the thermodynamic information, chemical potential diagrams for the Ca-Pt-O system are developed.