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,Al,-,(OH),](1-2)+ and interlayers containing CO:-, NO, 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 5 0.85, but for hydrothermally aged materials, 0.66 6 x 6 0.75; outside these ranges, boehmite or Ni(OH), form 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.

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Resplts are presented for the total and metallic nickel areas of a series of coprecipitated nickel-alumina catalysts prepared and pretreated in different ways. The total areas of the samples decrease on reduction and with increasing reduction temperature, while the nickel area increases with reduction temperature. Increasing calcination temperature causes a decrease in total and nickel areas, this being particularly marked at high temperatures of calcination. A model for the catalyst system is proposed, based on these results and on complementary structural investigations. The high stability of the catalysts is attributed to the presence in the unreduced catalyst of nickel oxide rich phases containing dissolved aluminium ions; on reduction, alumina crystallises on the surface of the growing nickel crystallites, preventing sintering of the catalysts except under extreme conditions.

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NbOx/SiO2 with a very high catalytic activity for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, was investigated by different spectroscopic methods in order to obtain new insights in the formation and nature of the active sites. FT-IR spectroscopy in combination with pyridine adsorption measurements revealed that the catalyst material contains Lewis-acidic sites, most probably related to the Nb[double bond, length as m-dash]O groups of isolated tetrahedral NbO4 surface species, whereas no Brønsted-acidic sites were observed. Results from in situ Raman and complementary FT-IR measurements strongly suggest that Brønsted-acidic Nb-OH sites can be generated from Nb[double bond, length as m-dash]O groups by reaction with ethanol. This is in agreement with the observation that ethanol is essential for obtaining a very good catalyst performance. However, the Brønsted-acidic sites can be detected in significant amounts in particular in the presence of a Lewis-base, e.g. pyridine, most probably because the formation and/or the stability of these Brønsted-acidic sites are enhanced by a basic molecule. Assuming that cyclohexanone oxime, being a base, can play a similar role as pyridine, we propose on the basis of the spectroscopic findings obtained in this work and our kinetic results published recently, a reaction scheme for the formation of the active site at the Nb[double bond, length as m-dash]O group as well as for the recovery of the Nb[double bond, length as m-dash]O site during the final stage of the gas-phase Beckmann rearrangement.