Reducibility and toluenehydrogenation activity of nickel catalysts supported on γ-Al2O3 and κ-Al2O3

Abstract

Metastable κ-Al₂O₃ was produced by thermal treatment of γ-Al₂O₃ and then Ni catalysts were prepared on both κ-Al₂O₃ and γ-Al₂O₃. Catalysts containing both low (5 wt.%) and high (20 wt.%) amounts of Ni were tested for activity towards toluene hydrogenation and characterized with X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy. In order to better understand the behaviours of these catalysts, total reducibility, surface oxide reducibility, metal dispersion, and number of adsorption sites were compared with catalytic activity. Of these factors, only surface oxide reducibility was directly correlated with activity – the more easily the surface oxide was reduced the more active the catalyst was. Catalytic activity increased in the following order: Ni5/γ-Al₂O₃<Ni5/κ-Al₂O₃<Ni20/κ-Al₂O₃<Ni20/γ-Al₂O₃. The number of metallic active sites is usually considered as a major factor to determine the corresponding activity, but in this study reducibility changes in surface Ni sites were more influential to the activity than the number of active sites.