Preparation And Properties Of Nickel-based Catalysts For Hydrogenation Of Ccarbon Dioxide To Methane

The overuse of fossil fuels has led to a rapid increase of carbon dioxide in the atmosphere.A series of environmental problems such as global warming,sea level rising and glaciers melting,are caused by a large amount of carbon dioxide emissions,which seriously affects the living environment of human beings.At the same time,CO₂ is also one of precious carbon resources.Therefore,the resource utilization of carbon dioxide as a resource has attracted widespread attention.At present,the hydrogenation of CO₂ to CH₄ is one of the most potential ways,which can not only reduce the carbon dioxide content in the atmosphere,and it can alleviate the imbalance of supply and demand of natural gas in our country.In the reaction of hydrogenation of carbon dioxide to methane,nickel-based catalysts have been widely used because of their high activity and good selectivity.However,nickel-based catalysts tend to deposit carbon during the reaction and cause catalyst deactivation.In this paper,A series of Ni/Al₂O₃-Zr O₂ catalysts with different composition were prepared by different methods（impregnation method,sol-gel method,deposition-precipitation method and co-precipitation method）.The preparati-on methods of catalysts,Ni content,support composition,promoter,reaction conditions（pressure,temperature,space velocity）were systematically investigated.The XRD,TEM,in Situ FTIR,in Situ Raman and other techniques were used to characterize the structure and composition of the catalysts,reveal the real-time interaction mechanism between the reaction molecules（CO₂,H₂）and the surface of the catalysts,and construct the dynamic structure-activity relationship of the active center of the catalysts.The Ni/Al₂O₃-Zr O₂ catalyst was prepared by different methods,and the influence of Ni content and support composition on the performance of the catalyst were investigated.The results of the carbon dioxide methanation reaction show that the preparation method has a significant impact on the performance of the nickel-based catalyst,and the nickel-based catalyst prepared by the co-precipitation method has the highest CO₂ conversion rate.For different Ni content（1%,5%,10%,15%,20%）and different supports composition（Al:Zr=1:0,5:1,2.5:1,1:1,1:2.5,0:1）The methanation performance of Ni/Al₂O₃-Zr O₂ catalyst was systematically investiga-ted.Under normal pressure,reaction temperature of 300℃,CO₂ conversion rate is the highest up to 72%,and methane selectivity is 99%by using the catalyst with Ni content of 10%and supports composition Al:Zr=1:1.The catalyst also shows excellent stability.After 160 h of continuous reaction,its activity remains unchanged.After 280 h of operation（the reaction is stopped,pressure is increased,and hydrogen is reduced）,the CO₂ conversion rate is still maintained at the level of fresh catalyst.The active component nickel is highly dispersed on the Al₂O₃-Zr O₂ composite supports,with an average particle size of 2-4 nm.The formation of Al₂O₃-Zr O₂ solid solution in the catalyst can inhibit the formation of spinel Ni Al₂O₄,so that the surface of the catalyst has more Ni O species that are easy to reduce,which is beneficial to the improvement of its reaction activity and stability.The La-modified Ni/Al₂O₃-Zr O₂ catalyst was prepared by impregnation method,mixing method and co-precipitation method,and the effect of La addition on the performance of Ni-based catalyst was investigated.The results show that the La-Ni/Al₂O₃-Zr O₂ catalyst prepared by the impregnation method with La promoter has the best performance.When the La content is 5%,the CO₂ conversion rate can reach 79%under normal pressure and reaction conditions of 300℃.When the La content is 5%,the CO₂ the performance of Ni-based catalyst was investigated.XRD,TPR,TEM and other characterization results show that the addition of La promoter is beneficial to the reduction of Ni O to metallic Ni,and an appropriate amount of La helps to improve the dispersion of Ni species,which is beneficial to the improvement of catalyst activity.Through In-situ FTIR and In-situ Raman,the catalyst structure changes and the evolution of adsorption intermediates during the methanation of carbon dioxide on Ni-based catalysts were explored.The results show that increasing the Ni content is beneficial to the adsorption of CO₂ on the catalyst surface,and high temperature is beneficial to the formation of formate species of CO₂ on the catalyst surface.Under the CO₂ methanation reaction atmosphere,hydyogencarbonate,carbonate and formate species are formed on the surface of the catalyst.During the reaction process,the hydrogencarbonate and carbonate species on the catalyst surface are gradually hydrogenated to form formate.The formate is then hydrogenated to form methane,which is the main intermediate in the methanation reaction.In-situ Raman spectroscopy shows that during the CO₂ methanation reaction,the supports structure changes with the increase of temperature,and different types of carbon species are formed on the surface of the catalyst during the reaction.These carbon species are unstable in the reaction atmosphere and can partially It is reduced by H₂,so that the catalysts have good activity and stability.