Solid state reactions and phase formation in impregnated Ln2O3-Al2O3 (Ln = Sm, Gd, Er)

Solid state reactions and phase formation in impregnated Ln2O3-Al2O3 (Ln = Sm, Gd, Er) were studied in oxidizing and reducing atmosphere by XRD, TEM and SEM-EDS. The temperature phase evolution of Ln2O3-Al2O3 depends strongly on the lanthanide and on its loading at the surface, but is independent on the gas atmosphere (oxidizing or reducing). For all lanthanides studied and low Ln/Al atomic ratios (≤ 0.05) an amorphous surface layer was formed initially at 550 ºC, which then transformed at higher temperature into an intermediate LnAlO3 aluminate with hexagonal structure. For higher loadings (Ln/Al ≥ 0.07), crystalline Ln2O3 was also present after treatment at 550 ºC, which at temperatures at or above 900 ºC reacted with Al2O3 to form an intermediate Ln4Al2O9 aluminate. It has been found that hexagonal LnAlO3 aluminate, not observed during high temperature reaction of coarse crystalline Ln2O3 and Al2O3 oxides, nucleates and is stabilized at the surface of γ-Al2O3.