Hydrotalcite formation at the alumina/water interface during impregnation with Ni (II) aqueous solutions at neutral pH

The adsorption of Ni(II) ammine complexes from aqueous solutions onto γ-alumina at neutral pH (pH = 7.2) and ambient temperature has been investigated by X-ray diffraction, EXAFS, transmission and analytical electron microscopy. The formation of deposited coprecipitates including Al (111) ions extracted from the support was demonstrated by EXAFS. Indeed, the EXAFS technique was shown to be a useful tool to distinguish the Ni hydroxides or basic salts from coprecipitates with the hydrotalcite-type structure. Transmission electron microscopy showed the presence of fibrillar or platelet-like structures in the dried products while analytical electron microscopy confirmed that nickel was unevenly distributed over the alumina surface. Alumina should not be considered systematically as inert, even when impregnation is performed at near neutral pH, i.e., at pH close to the isoelectric point of alumina. Furthermore, impregnation at neutral pH can be viewed as a convenient method to prepare alumina supported hydrotalcite-type mixed hydroxides.