Phase diagram of the system Ce-Rh-O

Abstract

Although important for catalytic applications, a phase diagram for the ternary system Ce-Rh-O is not found in the literature. Phase diagrams of most Ln-Rh-O systems show LnRhO3 with orthorhombic perovskite structure as the lone stable ternary oxide. Based on systematic trends in experimentally determined thermodynamic properties of eleven LnRhO3 compounds, the Gibbs energy of formation of CeRhO3 is evaluated as a function of temperature. For the reaction, 1/2 Ce2O3 (A-rare earth) + 1/2 Rh2O3 (ortho) → CeRhO3, ∆G°/J mol−1 = ‒69,336 + 5.26(T / K). The estimated data for CeRhO3 along with evaluated data for the binary Ce-Rh and data for binary oxides from the literature are used to compute the phase diagram for the system Ce-Rh-O at high temperature. Because of the significantly higher stability of CeO2-x compared to Ce2O3, the compound CeRhO3 is found to be unstable. A section of the phase diagram and an oxygen potential ‒ composition diagram for the system Ce-Rh-O at 1200 K are computed from thermodynamic data.