Some aspects of magneto-chemistry in NiO-Al2O3catalysts

Susceptibility-composition isotherm and thermomagnetic behavior of NiO-Al2O3catalysts, prepared by impregnating α-alumina in nickel nitrate solution and calcining at 450°C. have been studied. The samples were evaluated for ethylene hydrogenation at 200°C and 300°C, respectively. Isothermal reductions at 390°C, with and without magnetic field, for a poorly dispersed catalyst have been reported. In another set, six NiO-Al2O3catalysts, prepared from different nickel salts and having identical composition, were evaluated for thermomagnetic behavior, surface area, crystallite size, and activities for heptane-steam reaction at 700°C, respectively. Impregnated catalysts exhibited widely varying magnetic parameters. At low nickel concentration, NiO ensembles appeared to be bound to alumina by orbital angular moments. The unpaired 3d-electrons of Ni²⁺ion are stabilized by vacant p-orbitals of Al³⁺ions. As the nickel content increased, antiferromagnetism developed with the appearance of Néel point, negative Weiss constant, and spin compensation. At higher nickel concentration magnetic moment increased slightly due to overcoming of the influence of the support and suggested the formation of NiO multilayers. The changes of magnetic susceptibility with time during reduction above paramagnetic Curie point of nickel showed several peaks. The susceptibility increased due to the formation of nickel nuclei and decreased when the unpaired electrons were delocalized in the conduction band of aggregated nuclei. Superparamagnetic nickel showed optimum activities for both ethylene hydrogenation and heptane-steam reaction. So, these may be grouped as "structure-sensitive" reactions.