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

Abstract

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: $Pt-Rh, Rh + Rh_2O_3/(Y_2O_3)ZrO_2/Pd_{1-x} Rh _x + Rh_2O_3, Pt-Rh.$ 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 electro- chemical 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:$\Delta H = X_{Rh}(1 - X_{Rh})(31 130 + 4 585X_{Rh} J/mol and \DeltaS^{ex} = X_{Rh}(1 - X_{Rh})(10.44+ 1.51X_{Rh}) J/mol·K.$ The positive enthalpy of mixing obtained in this study in qualitative agreement with predictions of semiempirical models. The results predict a solidstate miscibility gap with $T_c = 1210(\pm5)K at X_{Rh} = 0.55(\pm0.02)$. 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 $Rh_2O_3$, and PdO coexists with Pd and $Rh_2O_3$. At high temperatures, PdO is unstable and Pd-rich alloys are in equilibrium with diatomic oxygen gas.