Two series of NiO/Al2O3 catalysts with various metal loadings were prepared by incipient-wetness impregnation and coprecipitation methods. The reducibilities of NiO over these catalysts were investigated by the temperature-programmed reduction (TPR) technique. For impregnated Ni/Al2O3 catalysts, more than one species was detected by TPR. At low nickel loadings, the NiO-Al2O3 interaction is strong, and at high Ni loadings NiO is observed. NiAl2O3 was not detected at all Ni loadings. For precipitated Ni/Al2O3 catalysts, three NiO species are present in the TPR profiles. NiAl2O4 was detected at all Ni loadings. The rate of reduction of NiO on the support depends not only on the chemical nature of the NiO species but also on the rate of nucleation of metallic nickel. At high nickel loadings, owing to the presence of easily reduced Ni species, the rate of reduction is enhanced.
