Structure and thermal stability of nanocrystalline Ce1−xRhxO2−y in reducing and oxidizing atmosphere

Nanocrystalline (4–5 nm) Ce1−xRhxO2−y mixed oxides (x = 0.03–0.21) were synthesized using water in oil microemulsion method. Morphology, microstructure and phase composition of the samples subjected to heat treatment in oxidizing and reducing atmosphere were investigated by TEM, SEM-EDS, XRD, Raman and IR spectroscopy. Samples with x ≤ 0.16 were structurally stable in oxidizing atmosphere up to 850 °C. Above this temperature the samples decomposed into Rh deficient, nanosized Ce1−xRhxO2−y and large (few μm) Rh2O3 crystals. In hydrogen atmosphere segregation of small (1–2 nm) Rh crystallites in special epitaxial orientation Rh (1 1 1)||CeO2 (1 1 1) started at 500 °C. The epitaxial orientation of small Rh crystallites was preserved up to 1000 °C indicating their strong bonding to the ceria surface. Partial substitution of Rh for Ce strongly enhanced reducibility of ceria at low temperatures (below 200 °C) and hindered the sintering of ceria at high temperatures. Ce0.89Rh0.11O2−y shows an interesting property of reversible extraction–dissolution of Rh upon reduction–oxidation cycles at 500 °C which is important for potential catalytic applications.