Structure and reactivity of high-surface-area Ni/Mg/Al mixed oxides

High-surface-area Ni/Mg/Al mixed oxides are obtained by calcination up to about 1023 K of hydrotalcite-type (HT) anionic clay precursors. The samples exhibit unusual pore size distributions and low reducibility of the Ni2+ ions. On the basis of the XRD data, TPR profiles and values of the apparent activation energy of the reduction reaction (before and after NaOH treatment), the low reducibility of the Ni-rich samples may be attributed to the presence of a surface spinel-type phase, which decreases the accessibility of the surface to the reducing mixture and/or makes the generation of nickel nuclei at the surface difficult. For Ni/Mg ratios < 1 the reduction of the Ni2+ ions is further hindered by the presence of a fraction of Al3+ ions inside the oxide phases. Higher calcination temperatures destroy any particular properties related to the nature of the precursors, giving rise to segregation of stoichiometric spinel and oxide or oxide solid solution. For the Ni-rich samples, this increases the reducibility of the main fraction of the Ni2+ ions, which behaves like free NiO. The Mg-rich samples show a reduced reducibility due to the formation of NiO/MgO solid solutions. The validity of the structural model previously proposed is discussed as a function of the composition and the calcination temperature.