NEOFORMATION OF DLH DURING IMPREGNATION OF GAMMA-ALUMINA

The adsorption of Co(II) or NI(II) ammine complexes from aqueous solut ions onto gamma-alumina at neutral pH and ambient temperature was inve stigated. The formation of coprecipitates including Al (III) ions extr acted from the support was demonstrated by EXAFS 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 hyd roxides and basic nitrates from coprecipitates with a double layer hyd roxide (DLH) structure. Not only is EXAFS shown to be sensitive to the presence of aluminum in the coprecipitates, but in most cases, the M( II)/Al(III) ratio (M = Ni or Co) in the supported coprecipitates can b e estimated. Thus, alumina should not be considered systematically as inert even at pH values close to its isoelectric point. It is suggeste d that a dissolution-precipitation mechanism is involved and that the rate of alumina dissolution is promoted by adsorbed Ni(II) or Co(II) i ons. Site-binding models have a considerable value for the early stage s of impregnation, whereas a geochemical approach involving surface re hydration and coprecipitation have probably a greater validity for the later stages.