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

One of the major challenges for a high-loading Ni/gamma-Al2O3 catalyst is achieving good metal dispersion and thermal stability. Herein, a Ni/H-gamma-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-gamma-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 gamma-Al2O3 nanosheets. Highly dispersed Ni nanoparticles provided by the LDH precursor are confined on the surface of gamma-Al2O3 nanosheets. The as-synthesized Ni/H-gamma-Al2O3 exhibits high CO2 conversion (68.0%) along with desired selectivity (95.0%) for CH4 at 400 degrees 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 N-2 physisorption, reveals that the interconnected gamma-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.