Carbon dioxide methanation over Ni catalysts prepared by reduction of Ni_xMg_(3-x)Al hydrotalcite-like compounds: Influence of Ni:Mg molar ratio

摘要

A series of supported Ni catalysts have been prepared from Ni_xMg_(3-x)Al hydrotalcite-like compounds (HTlcs) and the influence of Ni:Mg molar ratio on the structural property and catalytic activity for CO_2 methanation is investigated. The catalysts were characterized by N_2 physical adsorption, X-ray powder diffraction (XRD), temperature-programmed reduction (H_2-TPR), temperature-programmed desorption (CO_2-TPD), H_2 chemisorption, scanning electronic microscopy (SEM), scanning transmission electronic microscopy (STEM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). By reducing HTlcs at 800 ℃, well dispersed Ni particles with average size of 5-10 nm are formed. The Ni crystal size decreases with the decrease of Ni:Mg ratio, attributable to the strong interaction between nickel and magnesium oxides. Among the catalysts, Ni_2Mg_1Al-HT shows the highest activity, giving -93% CO_2 conversion and >99% CH_4 selectivity at 275 ℃ and SV = 5000 mL g~(-1) h~(-1). Meanwhile, this catalyst exhibits good stability without obvious sintering and coking. The high activity is related to the large amount of surface Ni~0 species and medium basic sites. From CO_2-TPD and DRIFTS, it is inferred that CO_2 adsorbs on the medium basic sites, i.e., Ni-Mg(Al)O interface, forming monodentate carbonate. In situ DRIFTS reveals that monodentate carbonate, monodentate formate, and adsorbed CO are the main intermediate species, suggesting that the reaction may proceed via the formate formation route.