Coprecipitated nickel-alumina catalysts for methanation at high temperature. Part 1. - Chemical composition and structure of the precipitates

A series of materials containing nickel and aluminium ions has been prepared by coprecipitation with alkaline solutions, under various conditions, from solutions of the nitrates. The composition and structure of the precipitates has been examined using a number of techniques including chemical analysis, X-ray diffraction, thermogravimetry and infrared spectroscopy. The precipitates have structures typical of minerals of the hydrotalcite group and particularly of the nickel-aluminium compound, takovite. They consist of brucite-like layers of composition [Ni_xAl_1-x(OH)₂]^(1-x)+ and interlayers containing CO2-3, NO-3 or OH^- ions, or combinations of these, depending on the preparation conditions, together with molecular water; the anions balance the excess charge of the brucite layers. It is concluded that the aluminium ions are randomly substituted for nickel ions in the brucite layers and that the structure of these layers is independent of the anions in the interlayer. For freshly precipitated samples, 0.5 ≤ x ≲ 0.85, but for hydrothermally aged materials, 0.66 ≤ x ≤ 0.75; outside these ranges, boehmite or Ni(OH)₂ from as separate phases. The results are compared with those for precipitates prepared from ammoniacal solutions and it is concluded that the two types of material are indistinguishable.