Study on the thermal stability of Ce0.8Gd0.2O1.9 nanocubes on Al2O3 and SiO2 supports

The structural evolution of the Ce0.8Gd0.2O1.9 nanocubes supported on Al2O3 and SiO2 was investigated in oxidizing and reducing atmosphere over a temperature range up to 1100 °C by XRD, TEM and SEM-EDS methods. The supported Ce0.8Gd0.2O1.9 particles preserved perfect cubic shape to 900 °C in oxidizing atmosphere and to 750 °C in a hydrogen atmosphere. No evidence of crystalline aluminate or silicate formation was observed for both supports during heating in oxidizing atmosphere up to 1100 °C. However, for Ce0.8Gd0.2O1.9/SiO2 segregation of Gd3+ from the mixed oxide and its spreading over the silica surface was noticed at 1000 °C-1100 °C. Crystalline aluminate/silicate occurred in a hydrogen atmosphere already at 900 °C. Apatite type (Ce,Gd)4.67(SiO4)3O mixed silicate was formed after reaction with SiO2 substrate, while monoclinic (Gd,Ce)4Al2O9 and tetragonal (Ce,Gd)AlO3 were formed on Al2O3. The (Gd,Ce)4Al2O9 phase became unstable at 950 °C and transformed into (Ce,Gd)AlO3 aluminate, which was stable up to 1100 °C. Various mechanisms of the solid-state reaction of mixed ceria with Al2O3 and SiO2 supports operate depending on the size of ceria nanocubes. Small particles (10-30 nm) reacted as a whole with the support forming single crystal silicate or aluminate; at the same time, for bigger particles (30-80 nm) reactions happen only near the boundary between the particle and the support.