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

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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.