NATURE AND PROPERTIES OF NICKEL-CONTAINING MIXED OXIDES OBTAINED FROM HYDROTALCITE-TYPE ANIONIC CLAYS

Anionic clays with a hydrotalcite-type (HT) structure may be useful precursors of multicomponent catalysts, such as, for instance, Ni/Al catalysts for steam reforming and methanation reactions. The structure and the synthesis of HT anionic clays are briefly reviewed to illustrate the specific properties of these phases and the key factors involved in the preparation of pure compounds. By comparing the properties of the oxides obtained by thermal decomposition of Ni/Al and Ni/Cr HT anionic clays at different temperatures, it was found that the HT structure is necessary, but not sufficient, to obtain well dispersed and stable samples; also necessary is the presence of aluminum as the trivalent element. However, for both Ni/Al and Ni/Cr samples the formation of stoichiometric spinel phases, which takes place at different temperatures depending on the composition, has a negative effect, giving rise to remarkable increases in both NiO crystal size and reducibility. On the basis of bulk and surface analyses (before and after NaOH treatment), a model for Ni/Al oxides obtained from HT anionic clays is proposed, which involves the presence of NiO, a hi-doped alumina phase and a spinel-type phase. This last phase is thought to be responsible for the thermal stability and low reducibility of the NiO particles.