Structure and thermal stability of nanocrystalline Ce_(1-x)Rh_xO(2_y) in reducing and oxidizing atmosphere

摘要

Nanocrystalline (4-5 nm) Ce_(1-x)Rh_xO(2_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 Ce_(1-x)Rh_xO_(2_y) and large (few μm) Rh_2O_3 crystals. In hydrogen atmosphere segregation of small (1 -2 nm) Rh crystallites in special epitaxial orientation Rh (1 1 l)||CeO_2 (111) started at 500 ℃. 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 ℃) and hindered the sintering of ceria at high temperatures. Ce_(0.89)Rh_(0.11)O_(2-y) shows an interesting property of reversible extraction-dissolution of Rh upon reduction-oxidation cycles at 500 ℃ which is important for potential catalytic applications.