Thermodynamic Investigation of the Redox Properties for Ceria−Hafnia, Ceria−Terbia, and Ceria−Praseodymia Solid Solutions

Ceria-hafnia, ceria-terbia, and ceria-praseodymia solid solutions were prepared by the citric-acid (Pechini) method and characterized using X-ray diffraction (XRD) for structure and coulometric titration for redox thermodynamics. Following calcination at 973 K, XRD showed that both Ce(0.5)Hf(0.5)O2 and Ce(0.8)Hf(0.2)O2 exist as single-phase solid solutions and that the lattice parameters of the cubic fluorite structures shifted linearly with Hf content. Following calcination at 1323 K, Ce(0.8)Hf(0.2)O2 remained single phase but Ce(0.5)Hf(0.5)O2 separated into two phases with compositions of Ce(0.81)Hf(0.19)O2 and Ce(0.15)Hf(0.85)O2. At 973 K, oxidation isotherms for Ce(0.8)Hf(0.2)O2 and Ce(0.5)Hf(0.5)O2 were almost identical to oxidation isotherms for Ce(0.8)Zr(0.2)O2 and Ce(0.5)Zr(0.5)O2. Mixed oxides of ceria with terbia and praseodymia also formed single-phase fluorite structures, but each of the solid solutions was phase stable to calcination at 1323 K. The oxidation isotherms for the solid ceria-terbia and ceria-praseodymia solutions were similar to what would be expected for physical mixtures of these oxides.