Preparation, Solid-state Characterization, and Surface-chemistry of High Surface-area NixAl2-2xO3-2x Mixed Oxides

High surface area NixAl2-2xO3-2x oxides have been prepared (x = 0, 0.33, 0.44, 0.5, 0.66, 0.85, 1) by a coprecipitation procedure. Both the precipitates and the calcined materials have been characterized structurally by XRD, FT-IR, and TG-DTG analyses. The precipitates are constituted by boehmite (x = 0), by amorphous hydroxy nitrates (x = 0.33, 0.44, and 0.5), by a mixed hydroxy-nitrate similar to Takovite (x = 0.66), and by the compound Ni6Al2CO3(OH)16.4H2O, the Ni analogue of hydrotalcite, for x = 0.85. The calcined mixed oxides are constituted by the spinel type compounds gamma-Al2O3 and NiAI2O4 and their solid solutions for 0 less-than-or-equal-to x less-than-or-equal-to 0.5, while for x = 0.85 a rock salt type NiO phase containing Al3+ in both tetrahedral and octahedral sites is obtained. For x = 0.66 a mixture of the spinel and the rock salt type phase is found. The porosity and the surface structure of the calcined NixAl2-2xO3-x Mixed oxides have been investigated by liquid nitrogen adsorption and FT-IR spectroscopy of surface hydroxy group and of adsorbed carbon monoxide and pyridine. The surface composition of these materials, all mesoporous, reflects their bulk structure. Cation vacancy clusters in defective spinels (x < 0.5) are responsible for characteristic surface structures. The presence of Al3+ in the bulk of rock salt type NiO (sample with x = 0.85) results in a deep modification of its acid-base and electronic properties. Precipitation with Na2CO3 instead of (NH4)2CO3 produces NiAI2O4, whose surface chemical properties are modified by Na+ contamination.