CHARACTERIZATION OF MICROSTRUCTURES IN NICKEL ALUMINA CATALYSTS BY ANALYTICAL ELECTRON-MICROSCOPY

Evolution of microstructures of Ni on gamma- and alpha-alumina during thermal treatment in air and hydrogen atmospheres was studied using TEM, selected area diffraction and microdiffraction. Oxidation at 775 K led to the formation of nickel aluminate both in the case of gamma- and alpha-alumina. Microdiffraction analysis showed an oriented growth of the NiO microcrystallites with respect to the gamma-alumina substrate. The NiO phase was present even after reduction at 775 K. Nickel crystallites produced during reduction at 775 K were covered by nickel aluminate. The presence of the nickel aluminate phase in the close vicinity of NiO crystallites influences strongly the texture of Ni-Al2O3 microstructures during the reduction of the catalysts. Recrystallization or/and reduction of the nickel aluminate shells during reduction at temperatures greater-than-or-equal-to 875 K leads to the formation of voids between the nickel and the nickel aluminate (alumina) phases. The epitaxial relationship of the Ni microcrystals with the (100), (110), (111) and (112) surfaces of the gamma-alumina has been established from electron diffraction and microdiffraction patterns. An increase of the Ni lattice parameter by 1% after reduction at 1075 K is interpreted by diffusion of aluminum atoms into the Ni lattice to form a solid solution.