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 measure d in the temperature range 950 to 1350 K using the solid-state cell: P t-Rh, Rh + Rh2O3/(Y2O3)ZrO2/Pd1-xRhx + Rh2O3, Pt-Rh. The activity of p alladium and the free energy, enthalpy, and entropy of mixing are deri ved. The activities exhibit strong positive deviation from Raoult's la w. The activities obtained by the electrochemical technique, when extr apolated to 1575 K, are found to be significantly lower than those obt ained from vapor pressure measurements. The mixing properties can be r epresented by a pseudosubregular solution model in which excess entrop y has the same type of function dependence on composition as the entha lpy of mixing: Delta H=X-Rh(1 - X-Rh)(31 130 + 4585X(Rh)) J/mol, and D elta S-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 wit h predictions of semiempirical models. The results predict a solid-sta te 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 t hat 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 format ion of PdO is measured as a function of temperature. At low temperatur es, the alloys are in equilibrium with Rh2O3, and PdO coexists with Pd and Rh2O3. At high temperatures, PdO is unstable and Pd-rich alloys a re in equilibrium with diatomic oxygen gas.