Concurrent formation of γ-alumina and nickel aluminate at the onset of the structural evolution of Ni2+-associated boehmite.

Nickel aluminate emerges concurrently with γ-alumina at the onset of the structural evolution of Ni2+-associated boehmite. Conventionally, spinel-type compounds on γ-alumina surfaces are generated through solid-state reactions between γ-alumina and metal salts at elevated temperatures. In this study, boehmite nanoparticles were generated through the hydrolysis and polycondensation of aluminum-tri-sec-butoxide in an aqueous solution containing Ni2+, producing dispersions in which Ni2+ ions were primarily associated with the particle surfaces rather than incorporated into the boehmite lattice. Upon calcination of the powder obtained by evaporation to dryness of the dispersion, the transformation of boehmite to γ-alumina at 400 °C was accompanied by the concurrent formation of NiAl2O4. Structural analyses using X-ray diffraction, solid-state 27Al nuclear magnetic resonance, and electron microscopy indicate that the NiAl2O4 phase is segregated from the γ-alumina structure and is suggested to be preferentially localized near particle neck regions. This behavior contrasts with conventional impregnation approaches, in which spinel phases form at significantly higher temperatures through reactions between metal oxides and γ-alumina surfaces. In addition, the presence of NiAl2O4 delays the γ-to-θ transformation by approximately 100 °C. These results demonstrate that the structural evolution of Ni2+-associated boehmite provides a previously unrecognized pathway for generating spinel phases at the onset of γ-alumina formation. These findings therefore provide a new strategy for stabilizing the γ-phase and controlling the particle morphology of γ-alumina by introducing additives during precursor evolution rather than through post-impregnation processes.