Phase equilibria and thermodynamics of (silver + rhodium + oxygen)

Abstract

An isothermal section of the phase diagram for (silver + rhodium + oxygen) at T= 1173 K has been established by equilibration of samples representing twelve different compositions, and phase identification after quenching by optical and scanning electron microscopy (s.e.m.), X-ray diffraction (x.r.d.), and energy dispersive analysis of X-rays (e.d.x.). Only one ternary oxide, AgRhO2, was found to be stable and a three phase region involving Ag, AgRhO2and Rh2O3was identified. The thermodynamic properties of AgRhO2were measured using a galvanic cell in the temperature range 980 K to 1320 K. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa was used as the reference electrode. The Gibbs free energy of formation of the ternary oxide from the elements, ΔfGo(AgRhO2), can be represented by two linear equations that join at the melting temperature of silver. In the temperature range 980 K to 1235 K,ΔfGo(AgRhO2)/(J · mol−1)=−249080 + 179.08 T/K(±120).Above the melting temperature of silver, in the temperature range 1235 K to 1320 K,ΔfGo(AgRhO2)/(J · mol−1)=−260400 + 188.24 T/K(±95).The thermodynamic properties of AgRhO2at T= 298.15 K were evaluated from the high temperature data. The chemical potential diagram for (silver + rhodium + oxygen) at T= 1200 K was also computed on the basis of the results of this study.