Synthesis Of Nano Ce-based Catalysts And Study For CO₂ Hydrogenation

The large-scale use of fossil fules（such as,coal,oil,and natural gas,etc.）has resulted in massive emission of CO₂ and a series of environmental problems such as global warming,which directly threatening human survival and sustainable development.At the same time,CO₂ is the abundant carbon resource in C1 family.The rational use of CO₂ to valuable chemicals has became a hot spot in recent years.The traditional physical application and simple chemical conversion of CO₂ is low utilization rate,high energy consumption and insignificant effects.While CO₂ catalytic hydrogenation to natural gas and clean fuel CH₄ is one of the effective ways to ultilize CO₂,which is low energy and high efficiency,and has been widely concerned in recent years.In this paper,different nano Ce-based catalysts were prepared and studied their CO₂hydrogenation performance.X-ray diffraction（XRD）,N₂ adsorption-desorption（BET）,Transmission electron microscopy（TEM）,In-situ x-ray photoelectron spectroscopy（In-situ XPS）,Inductively coupled plasma（ICP）,Hydrogen-temperature programmed reduction（H₂-TPR）,Hydrogen-temperature programmed desorption（H₂-TPD）and CO₂-temperature programmed desorption（CO₂-TPD）has been conducted to study the physicochemical properties of Ce-based catalysts.And the effect of the pore structure and composition of Ce-based catalysts on its CO₂ hydrogenation performance were investigated.We established the relationship between the structure and composition of Ce-based catalysts and its CO₂hydrogenation performance.The main conclusions were as follows:（1）By examining the effect of Co loading on the CO₂ hydrogenation performance of Co/CeO_2-δcatalyst,it is found that the loading of Co significantly affects the physicochemical properties and CO₂ catalytic hydrogenation performance of Co/CeO_2-δcatalyst.The active metal Co species are highly diepersed on the surface of the CeO₂ support.With the increasing of Co loading,the number of Co species and oxygen vacancies increases,which generated by H₂ reduction.And the ablity to adsorb and activate the reaction molecules H₂ and CO₂ were also enhanced,which effectively promotes the CO₂ hydrogenation reaction.Among the studied Co/CeO_2-δcatalysts,the 9%Co/CeO_2-δcatalyst showed the best CO₂ catalytic hydrogenation activity.In the reaction temperature of 200-400 ^oC,the CO₂ conversion and CH₄ selectivity follow the order:1%Co/CeO_2-δ<3%Co/CeO_2-δ<5%Co/CeO_2-δ<11%Co/CeO_2-δ<9%Co/CeO_2-δ.At 400 ^oC,the CO₂ conversion increases from 30.1%(1%Co/CeO_2-δ)to 59.2%(9%Co/CeO_2-δ).（2）The Ce-O-Co solid solution was formed in the CeCo catalysts prepared by the hard-template method,which promoted the reduction of Co₃O₄ species and the formation of oxygen vacancies in the precursor.The Ce/Co molar ratio significantly affected the pore structure of the CeCo catalysts,the relatively high Ce/Co molar ratio is more likely to form an orderly developed bimodal mesoporous structure.The CeCo05 catalyst（Ce/Co molar ratio is0.5）exhibited the best CO₂ hydrogenation performance:at 400 ^oC,the CO₂ conversion is60.1%,the CH₄ selectivity is 95.3%.At the same time,it showed high stability in CO₂hydrogenation reaction for lang time.In the CO₂ hydrogeation reaction,the formed oxygen vacancies,the highly dispersed metal Co species,and the bimodal mesoporous structure obviously affect the CO₂ catalytic hydrogention performance of the CeCo catalysts.（3）The Ni component was introduced into CeO₂ by soft-template method,and the porous NiCe catalysts were prepared.Compared with single-component Ni and CeO₂catalystss,the low-temperature reducibility of porous NiCe composite catalysts is significantly improved.The NiCe catalysts were samll particle size nanoparticles（⁵-10 nm）and had a certain mesoporous structure.The Ni/Ce molar ratio of the catalyst has a significant effect on the morphology and pore structure,specific surface area,crystallinity and content of metal Ni species of the NiCe catalysts.The interaction between the components Ce and Ni obviously reduced the particle size of the catalyst,the crystallinity of the metal Ni species,and contributed to the formation of mesoporous structure,which effectively improved the CO₂ hydrogenation performance of the NiCe composite catalysts.Among them,the NC3catalyst（Ni/Ce molar ratio is 3/1）showed the excellent CO₂ hydrogenation performance and stability:at 320 ^oC,the CO₂ conversion is 85.2%,and the CH₄ selectivity is 100.%.