Characterization of dispersion and surface states of NiO/γ-alumina and NiO/La2O3–γ-alumina catalysts

Abstract

A series of NiO/γ-alumina and NiO/La₂O₃–γ-alumina catalysts prepared by impregnation have been investigated by X-ray diffraction, diffuse reflectance spectroscopy and temperature-programmed reduction. For the NiO/γ-alumina catalysts with low nickel oxide content, a strong interaction between NiO and γ-alumina leads to the diffusion of Ni²⁺ ions into vacant surface sites of the support to form surface spinel NiAl₂O₄ and thus the reducibility of NiO was decreased. At high nickel oxide contents, NiO crystal appears after the surface vacancies have been filled. In this case, the counter-diffusion of Ni²⁺ and Al³⁺ ions at the interface was observed during calcination.

For NiO/La₂O₃–γ-alumina catalysts with low La₂O₃ content, Ni²⁺ ions tend to interact with γ-alumina. The addition of the La₂O₃ has little effect on the state of the NiO species interacting directly with γ-alumina. For the catalysts with high La₂O₃ content, the dispersion of NiO was enhanced by the existence of La₂O₃, which can be ascribed to the formation of an La–Ni mixed oxide (or microcrystallites of NiO separated by La₂O₃). Strong interactions between components, such as NiO and Al₂O₃, and La₂O₃ and Al₂O₃, are influenced dramatically by the calcination temperature. As the temperature rises, the increasing interaction leads to the formation of bulk LaAlO₃ and NiAl₂O₄. However, the calcination temperature has little effect on the interaction between NiO and La₂O₃.