Design Of Highly Active Nickel-based Catalysts With Bimodal Porous Structure And Their Performance In Methane Reforming With CO₂

From economic and environmental perspectives,dry reforming of methane（DRM）is a cost-effective and environmentally friendly technology that not only makes rational application of the abundant methane but also converts greenhouse gas CO₂ into useful chemicals.In order to develop active and stable catalyst for the reforming system,herein,the nickel-based catalysts with bimodal structure was emphatically investigated.Based on nickel supported on silica catalysts,the preparation parameter（aging time）,additive（Zr）and preparation methods（one-step strategy and impregnation method）were fully studied.Pore structure,morphology and particular surface information were analyzed by N2adsorption-desorption isotherm,X-ray powder diffraction,transmission electron microscopy,temperature-programmed reduction with H₂,UV-visible diffuse reflectance and thermogravimetric analysis to systematically clarify the key role of the bimodal pore structure,ZrO₂ additives and one-step method in improving the catalytic performance in DRM.Firstly,the bimodal pore silica BS-x（x means aging time,0,48,96,192 h）was synthesized by a chemical,templating-scaffolding method.The aging time significantly changed the pore structure.The increase of the aging time led to the decrease in the first pore amount but the increase in pore size of the second pore.Changes in the pore structure of the supports had a significant effect on the physico-chemical properties of the nickel based catalysts.As the aging time was prolonged,the specific surface area,pore volume and pore size of the catalysts correspondingly increased.Furthermore,the interaction between metal and support was enhanced and the Ni dispersion was as well promoted.When the aging time was 96 h,the above values all reached the maximum,thus Ni/BS-96 catalyst exhibited the highest initial methane conversion（85%）.Secondly,the effect of ZrO₂ addition on the catalytic performance of Ni-Zr/BS-48 was studied in detail.The molar ration of Zr/Si was adjusted to 0.011,0.022,0.044,and 0.088.All Zr-modified Ni-based catalysts exhibited better reactivity and stability than undoped one,which was attributed to the well-dispersed metallic Ni,the stronger metal-support interaction and the improvement of catalysts surface basicity.Moreover,Ni-0.022Zr/BS-48 catalyst showed the optimal catalytic performance because the doping of zirconium played a prominent role in preserving pore structure and improving the sintering resistance.Finally,the preparation strategy was investigated on the nickel-based catalysts.By using one-step strategy,three kinds of bases（NaOH,NH₃·H₂O and Urea）were used to prepare non-porous,monomodal and bimodal catalysts,respectively.By impregnation method,the monomodal（Ni/M-SiO₂）and bimodal catalysts（Ni/B-SiO₂）were synthesized.The pore structure exerted crucial effect on the catalytic performance in DRM.In comparison to monomodal catalyst,bimodal nickel catalysts（Ni/B-SiO2 and Ni-SiO2-Urea）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 severe decrease in surface area.The obtained catalysts prepared by one step method possessed much smaller Ni nanoparticles and the strengthened interaction between Ni species and silica support.Thus,these unique structural properties contributed to higher activity over Ni-SiO₂-Urea than the impregnated Ni/B-SiO2 catalyst,albeit with equivalent pore structure.