A hierarchical hollow Ni/γ-Al2O3 catalyst derived from flower-like Ni–Al layered double hydroxide with stable catalytic performance for CO2 methanation

Abstract

One of the major challenges for a high-loading Ni/γ-Al₂O₃ catalyst is achieving good metal dispersion and thermal stability. Herein, a Ni/H-γ-Al₂O₃ catalyst with a high loading of Ni (72.3 wt%) and well-dispersed Ni nanoparticles is designed and synthesized for CO₂ methanation. The Ni/H-γ-Al₂O₃ catalyst derived from flower-like Ni–Al layered double hydroxide coated with carbon spheres (CS@Ni–Al LDH) possesses hierarchical hollow structures composed of cross-linked γ-Al₂O₃ nanosheets. Highly dispersed Ni nanoparticles provided by the LDH precursor are confined on the surface of γ-Al₂O₃ nanosheets. The as-synthesized Ni/H-γ-Al₂O₃ exhibits high CO₂ conversion (68.0%) along with desired selectivity (95.0%) for CH₄ at 400 °C, and the stability of the catalyst is confirmed by a 50 h continuous on-stream reaction and by cyclic operation with variable temperature. The characterization, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and N₂ physisorption, reveals that the interconnected γ-Al₂O₃ layers as steric barriers provide the spatial separation effect to inhibit the migration and aggregation of Ni particles. In situ diffuse reflectance infrared Fourier-transform spectra (DRIFT) characterization indicates that the formate route versus the carboxyl (CO*) route is widely detected as the main methanation pathway. Meanwhile, formate is generated by the hydrogenation of gaseous CO₂ rather than HCO₃* species.