Enhanced selectivity of syngas in partial oxidation of methane: A new route for promising Ni-alumina catalysts derived from Ni/γ-AlOOH with modified Ni dispersion

Partial oxidation of methane was studied over new Ni nanocatalysts prepared from Ni-impregnated γ-AlOOH, which were compared with their counterparts derived from impregnated γ-Al₂O₃ and α-Al₂O₃. The prepared catalysts were characterized by powder X-ray diffraction (XRD), N₂-sorption, H₂-temperatue programmed reduction, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analysis, CO₂- and NH₃-temperature-programmed desorption, Raman spectroscopy, CO chemisorption, and diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO. Employing γ-AlOOH as the support precursor resulted in significantly improved textural properties and catalytic activity. The structure of the support precursor and its surface properties showed a strong influence on the type of Ni active species and their interactions with the support. γ-AlOOH-derived catalysts showed NiO as the dominant Ni species when calcined at 500°C to 650°C, while NiAl₂O₄ became the sole phase at higher temperatures. On the other hand, mixtures of NiO and NiAl₂O₄ formed over γ-Al₂O₃, calcined before impregnation, regardless of the precalcination temperature. All γ-AlOOH-derived catalysts showed higher specific surface areas compared to their counterparts derived from impregnated γ-Al₂O₃. Upon calcination at moderate temperatures, γ-AlOOH-derived catalysts also showed modified textural and morphological properties of the Ni active particles, including higher Ni dispersion, larger metal surface area, and smaller Ni crystallites. The support precursor and the pretreatment conditions showed a strong influence on the catalytic performance, which was referred to their significant effect on the type of the Ni species and interactions with the support. All catalysts showed higher catalytic activity than the α-Al₂O₃-derived catalyst, with CH₄ conversion between 86% and 88.5% at 700°C. While γ-AlOOH- and γ-Al₂O₃-derived catalysts calcined at 650°C showed a stable CH₄ conversion around 88%, and higher syngas selectivity than the other catalysts, Ni/γ-AlOOH-650 showed the highest selectivity to syngas, with a H₂/CO ratio very close to 2.0. The improved Ni dispersion and the enhanced syngas selectivity obtained in the preset work demonstrate that using γ-AlOOH as a support precursor holds a great promise for the development of a new route for more efficient Ni catalysts compared with the widely studied γ-Al₂O₃ and α-Al₂O₃ support precursors.