Hydrotalcite-type anionic clays as precursors of high-surface-area Ni/Mg/Al mixed oxides

Anionic clays with a hydrotalcite-type (HT) structure may be useful precursors of multicomponent catalysts, such as, for example, Ni/A1 and Ni/Mg/Al catalysts. The flexibility of the HT structure is clearly demonstrated by the fact that pure Ni/Mg/Al precursors may be obtained by coprecipitation regardless of the Ni/Mg ratio. The thermal decomposition of all the precipitates involves an initial loss of interlayer water followed by the elimination of hydroxide and carbonate, with some differences related to sample composition. Regardless of composition, all samples obtained by calcination up to 1073K exhibit a high surface area and stability, and are characterized by a low reducibility of the Ni2+ ions. However, for Ni-rich samples both the lattice parameter of the oxide phases and Rietveld's powder pattern fitting procedure indicate that the low reducibility is not due to the presence of foreign ions, but rather may be attributed to the presence of a surface spinel-type phase. However, with increasing Mg-content, the reduction of the Ni2+ ions is further hindered by the presence of Mg2+ and Al3+ ions in the oxide phases. thigh temperatures, stoichiometric spinel phases form, with segregation of oxide phases and a considerable decrease in surface area. For the Ni-rich samples, this takes place with an increase in the reducibility of the main fraction of Ni2+ ions, which show a behaviour similar to that of free NiO. On the contrary, the Mg-rich samples show a further decrease in reducibility, due the formation of NiO/MgO solid solutions.