Characterization of nickel species on several γ-alumina supported nickel samples

Abstract

Studies of the chemical preparation, surface areas, pore distributions, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS) of several γ-alumina supported nickel samples active for the catalytic hydrogenation of hexanedinitrile have been carried out. NiO crystallites showed to be larger than the pore mouths of the support. Pore distributions lie in the mesopore region with diameters between 20 and 100 Å. Powder XRD detects the oxidized and reduced-nickel phases present and does not detect the nickel aluminate phase, due to the lack of crystallinity of the latter. SEM micrographs detect the presence of octahedral NiO crystallites and amorphous shelling nickel aluminate. XPS results show the presence of surface Ni2+ (in the form of stoichiometric and non-stoichiometric NiO, and stoichiometric and non-stoichiometric nickel aluminates) and surface reduced nickel. A deconvolution of the experimental curves was carried out in order to obtain a better assignment of the surface nickel species present. Nickel aluminate is detected at calcination temperatures > 623 K and covers all the surface of the support with layers between zero and several atoms thick, depending on calcination temperature and nickel concentration. Catalytically active reduced nickel (for nitrile hydrogenations) either as naked crystallites or as encapsulated nickel inside voided non-stoichiometric aluminate shells lie on top of the underlying catalytically inactive nickel aluminate when precursor calcination temperatures are higher than 623 K and reduction temperatures of 673 K. NiO transformation into nickel aluminate collapses the cubic NiO surface morphology of the small NiO crystallites giving rise to voided shells of nickel aluminate which may hide encapsulated reducible NiO.