Design of active and stable bimodal nickel catalysts for methane reforming with CO2

Dry reforming of methane (DRM) has been investigated in numerous studies as an attractive process to produce synthesis gas. Although nonprecious-metal catalysts are widely employed in this reaction, their large scale application has been hampered due to difficulties in controlling the metal sintering and coking. The main objective of this research was to improve DRM reaction through highly active and effective nickel catalysts with bimodal structure. In this work, the effect of pore structures (nonporous, monomodal and bimodal structures) on the catalytic performance, stability and coke formation were comparatively discussed. Two series of nickel catalysts were developed by one-step and impregnation methods. By using one-step strategy, three kinds of bases (NH3 center dot H2O, Urea and NaOH) were used to prepare monomodal (Ni-SiO2-NH3 center dot H2O), bimodal (Ni-SiO2-Urea) and non-porous (Ni-SiO2-NaOH) catalysts. By using impregnation method, monomodal (Ni/M-SiO2) and bimodal (Ni/B-SiO2) catalysts were prepared. The pore structure exerted crucial effect on the catalytic performance in DRM. In comparison to monomodal catalyst, in each series bimodal nickel catalyst exhibited higher activity and more stable performance in DRM. The non-porous Ni-SiO2-NaOH exhibited inferior activity to the monomodal or bimodal catalyst due to the significant decrease in surface area. The obtained catalysts prepared by one step method contained Ni nanoparticles with diameter of 3 nm, about 1/8 of the catalysts prepared by impregnation method. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) confirmed the well-dispersed Ni particles were incorporated into the silica framework. Temperature programmed reduction and X-ray photoelectron spectroscopy (XPS) also verified the strengthened interaction between Ni species and silica support. Thus, these structural properties led to higher activity over Ni-SiO2-Urea than the corresponding catalyst with equivalent pore structure. Relative to the initial methane conversion of 74% over Ni/B-SiO2, Ni-SiO2-Urea catalyst showed both stable CH4 and CO2 conversions and a constant H-2/CO ratio close to 1, without significant decay of the activity during 24 h on stream.