Reduction of impregnated NiO/α-Al2O3 association of Al3+ ions with NiO

Contrary to common belief that alpha-Al2O3 is an inert support for the impregnation of base metal species, evidence is presented for the incorporation of Al3+ ions into NiO impregnated on alpha-Al2O3, thereby causing the observed increase in reduction temperature. In agreement with this proposed model of support dissolution/incorporation into the impregnated material, temperature programmed reduction measurements on samples of NiO prepared by coprecipitation with small amounts of Al3+ and by multiple impregnation of alpha-Al2O3 gave similar results. As predicted, treating alpha-Al2O3, in the form of rings or powder, with HNO3 extracted small amounts of Al3+ at 30 degrees C and larger amounts at 90 degrees C. During the calcination step following impregnation with Ni(NO3)(2) . 6H(2)O, NiO is formed as the Ni(NO3)(2) crystals decompose and Al3+ ions are extracted from the surface. These Al3+ ions are then retained either in or in the vicinity of evolving NiO crystallites. Successive impregnation steps result in further Al3+ extraction from the support and additional increases in reduction temperature. (C) 1998 Elsevier Science B.V.