DECORATION OF NICKEL AND MAGNESIUM-OXIDE CRYSTALLITES WITH SPINEL-TYPE PHASES

The structure of the Al promoted magnesium and nickel oxides obtained by calcination of hydrotalcite-type coprecipitates was investigated by X-ray diffraction, Al-27 MAS-NMR, EXAFS, and alkaline leachings. In addition to the reflections of the MgO rock-salt type structure, the diffraction patterns of the Al modified MgO exhibited an additional, well-defined reflection at d = 0.253 nm, attributable to cations on tetrahedral sites. The tetrahedrally coordinated cations were likely to belong to a nonstoichiometric spinel-type phase including an excess of magnesium. The same behavior was observed when Ga(III) were substituted for Al(III) ions in the materials. Furthermore, in the Ga promoted magnesium oxides, EXAFS indicated that only a minor part of the Ga(III) ions was substitutionally dissolved in MgO. The nature of the phases present at the surface of the Al-modified magnesium and nickel oxides was investigated employing alkaline leachings with aqueous sodium hydroxide solutions on Al-doped NiO and with sodium ethoxide in anhydrous ethanol solutions on Al-doped MgO systems. In both cases, the selective dissolution of aluminum-rich phases at the surface of the mixed oxides was observed. Al-27 MAS-NMR before and after leachings confirmed that the surface of the Al modified MgO was enriched in tetrahedrally coordinated aluminum. The presence of an alumina phase at the surface of the Al promoted NiO was also suggested in view of the crystallization of boehmite upon hydrothermal treatments under moderate temperature and pressure, whereas more severe hydrothermal conditions led to hydrotalcite-like structure reconstitution. The aluminum distribution in the magnesium and nickel oxides is not homogeneous and the local composition of the mixed oxides fluctuates between magnesium (or nickel) and aluminum-rich phases. The Al-doped MgO or NiO particles can also be viewed as ''decorated'' by aluminate-type patches, which are thought to be responsible for their surface properties and for their resistance to sintering.