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

Anionic clays with a hydrotalcite-type (HT) structure may be useful pr ecursors of multicomponent catalysts, such as, for instance, Ni/Al cat alysts for steam reforming and methanation reactions. The structure an d 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 t he oxides obtained by thermal decomposition of Ni/Al and Ni/Cr HT anio nic clays at different temperatures, it was found that the HT structur e is necessary, but not sufficient, to obtain well dispersed and stabl e samples; also necessary is the presence of aluminum as the trivalent element. However, for both Ni/Al and Ni/Cr samples the formation of s toichiometric spinel phases, which takes place at different temperatur es depending on the composition, has a negative effect, giving rise to remarkable increases in both NiO crystal size and reducibility. On th e 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 sp inel-type phase. This last phase is thought to be responsible for the thermal stability and low reducibility of the NiO particles.