Thermodynamic mixing properties and solid-state immiscibility in the systems Pd-Rh and Pd-Rh-O

The thermodynamic activity of rhodium in solid Pd-Rh alloys is measured in the temperature range 950 to 1350 K using the solid-state cell: Pt-Rh, Rh + Rh₂O₃/(Y₂O₃)ZrO₂/Pd _1-xRh_x + Rh₂O₃, Pt-Rh. The activity of palladium and the free energy, enthalpy, and entropy of mixing are derived. The activities exhibit strong positive deviation from Raoult's law. The activities obtained by the electrochemical technique, when extrapolated to 1575 K, are found to be significantly lower than those obtained from vapor pressure measurements. The mixing properties can be represented by a pseudosubregular solution model in which excess entropy has the same type of function dependence on composition as the enthalpy of mixing: ΔH = X_Rh(1 - X_Rh)(31 130 + 4585X_Rh) J/mol, and ΔS^ex = X_Rh(1 - X_Rh)(10.44 + 1.51X_Rh) J/mol · K. The positive enthalpy of mixing obtained in this study in qualitative agreement with predictions of semiempirical models. The results predict a solid-state miscibility gap with T_c = 1210 (±5) K at X_Rh = 0.55 (±0.02). The computed critical temperature is approximately 100 K higher than that reported in the literature. The oxygen chemical potential for the oxidation of Pd-Rh alloys under equilibrium conditions is evaluated as a function of composition and temperature. The Gibbs energy of formation of PdO is measured as a function of temperature. At low temperatures, the alloys are in equilibrium with Rh₂O₃, and PdO coexists with Pd and Rh₂O₃. At high temperatures, PdO is unstable and Pd-rich alloys are in equilibrium with diatomic oxygen gas.