Ammonia complex of nickel(II) as precursor for synthesis of highly dispersed Ni/Al₂O₃ catalysts: the effect of preparation conditions on the physicochemical properties and hydrogenating activity

A series of granulated Ni/Al₂O₃ catalysts were prepared by impregnating the γ-alumina support with a nickel (II) ammine complex solution. The study of catalysts at different stages of preparation using IR and UV–Vis spectroscopy, XRD, thermogravimetric analysis, H₂-TPR, TPD-CO₂ and O₂ chemosorption methods made it possible to trace the evolution of the active component. The interaction between the [Ni(NH₃)₅(H₂O)]²⁺ cations and γ-Al₂O₃ results in the partial replacement of NH₃ ligands with hydroxyl groups of the support, forming inner-sphere complexes between Ni²⁺ ions and the Al₂O₃ surface. Drying the catalyst at 140 °C causes the NH₃ ligands in the nickel complexes to be replaced by H₂O molecules and/or carbonate ligands. The calcination temperature of 300 °C is sufficient for decomposition of the active component precursor to NiO. In the calcined samples, almost all of the nickel oxide is strongly bound to the alumina surface. The highest reducibility was achieved for the samples calcined at 300 °C. Increasing the calcination temperature to 400 °C decreases the reduction rate. It was demonstrated that the reduction of oxide precursors (obtained by calcination at 300–400 °C) in an H₂ flow at 400 °C leads to the formation of a highly dispersed nickel phase on the γ-Al₂O₃ surface. The Ni/Al₂O₃ catalysts synthesized using nickel ammine complexes show ~ 40% higher activity in 2-heptanone hydrogenation compared to the reference catalyst prepared using the nickel(II) nitrate. Increasing the nickel content in the samples from ~ 12 to 18–19 wt% does not significantly alter the Ni specific surface area or the catalytic activity (per mol of Ni).