Impregnation of .gamma.-Alumina with Ni(II) or Co(II) Ions at Neutral pH: Hydrotalcite-Type Coprecipitate Formation and Characterization

The adsorption of Co(II) or Ni(II) ammine complexes from aqueous solutions onto y-alumina at neutral or near neutral pH (7 < pH < 8.2) and ambient temperature is investigated as a function of the ion concentration in the impregnating solutions, the contact and aging times, and the drying conditions. The formation of coprecipitates including Al(III) ions extracted from the support is demonstrated by EXAFS and IR spectroscopy for contact times and Ni or Co loadings higher than 0.5 h and about 2.0 wt %, respectively. The EXAFS technique makes it possible to distinguish the Ni or Co hydroxides and basic nitrates from coprecipitates with the hydrotalcite-type structure. In other words, EXAFS is shown to be sensitive to the presence of aluminum in the coprecipitates. Furthermore, in most cases, the M(II)/A1(III) ratio (M = Ni or Co) in the supported coprecipitates can be estimated. Infrared spectroscopy can also be used as a fingerprint of coprecipitate formation through lattice octahedral modes characteristic of the NiAl and CoAl hydrotalcite-type compounds in the 400-450 cm-1 region. Thus, alumina should not be considered systematically as inert even at pH values close to its isoelectric point. Possible mechanisms responsible for the formation of coprecipitates at pH values nonaggressive for alumina are discussed. Dialysis experiments leading to the observation of hydrotalcite crystallites at a distance from alumina suggest that a dissolution-precipitation mechanism is possibly involved and that the rate of alumina dissolution is promoted by adsorbed Ni(II) or Co(II) ions. Three phenomena are suggested to occur at the same time at the oxide/water interface; the adsorption of ions, as described by the site binding models; the alumina dissolution, promoted by the adsorbed ions and kinetically limited; the coprecipitation of M(II) with Al(III) ions extracted from the support. Site binding models have a considerable value for the early stages of impregnation, whereas approaches which use geochemistry as a basis and involve surface rehydration and coprecipitate formation have probably a greater validity for the later stages.