Formation of nickel, cobalt, copper, and iron aluminates from α- and γ-alumina-supported oxides:: A comparative study

The rates of metal aluminate formation in MeOx/Al2O3 systems (Me = Ni, Co, Cu, Fe) at 500-1000 degrees C in O-2 or N-2 atmospheres were compared, using both alpha-Al2O3 and gamma-Al2O3 substrates. Interfacial reaction to MeAl2O4 was assessed using Rutherford backscattering spectrometry and X-ray diffraction. The lateral homogeneity of the samples was investigated using scanning electron microscopy, The spinel formation rate follows the sequence FeAl2O4 < NiAl2O4 < CoAl2O4 < CuAl2O4. This is explained in terms of thermodynamic limitations (FeAl2O4) and site-preference energies (CoAl2O4, NiAl2O4). The fast reaction to copper aluminate was accompanied by a severe rupture of the top layer (similar to 1 mu m) of the substrate, Penetration of transition metal ions into gamma-Al2O3 substrates is notable at considerably lower temperatures than with alpha-Al2O3 substrates, mainly due to grain boundary diffusion, Annealing conditions that favor the formation of transition metal cations of a low valency (Co-2+, Cu+) are beneficial for reactions to aluminates. (C) 1998 Academic Press.