Differences in the Nature of Active Sites for Methane Dry Reforming and Methane Steam Reforming over Nickel Aluminate Catalysts

The Pechini synthesis was used to prepare nickel aluminate catalysts with the compositions NiAl₄O₇, NiAl₂O₄, and Ni₂Al₂O₅. The samples have been characterized by N₂ physisorption, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS). Characterization results indicate unique structural properties and excellent regeneration potential of nickel aluminates. Prepared samples were tested when unreduced and reduced prior to reaction for methane dry reforming and methane steam reforming reactivity. NiAl₂O₄ in the reduced and unreduced state as well as NiAl₄O₇ in the reduced state are active and stable for methane dry reforming due to the presence of 4-fold coordinated oxidized nickel. The limited amount of metallic nickel in these samples minimizes carbon deposition. On the other hand, the presence of metallic nickel is required for methane steam reforming. Ni₂Al₂O₅ in the reduced and unreduced states and NiAl₂O₄ in the reduced state are found to be active for methane steam reforming due to the presence of sufficiently small nickel nanoparticles that catalyze the reaction without accumulating carbonaceous deposits.