CO₂ Methanation On Rare Earth Nickel Compounds

CO₂ is the burning exhaust gas,the main cause of greenhouse effect,but it is also a non-toxic and cheap carbon source with high thermodynamic stability.The reduction and recycling of carbon dioxide is not only a valid solution to alleviate the greenhouse effect and the energy crisis,but also an effective pathway to synthesize industrial chemicals and fuels.For now,Ni-based catalysts are the most widely investigated materials for CO₂ methanation reaction due to their low prices,high efficiency and selectivity.However,the active components of supported catalysts are easily sintered and coalesced under the high temperature of reforming reaction,which leads to the collapse of porous structure and the aggregation of active components.Furthermore,as the reaction process carries out,the surface of catalyst is also vulnerable to carbon deposition,leading to the blocking of the catalyst pores and thus deactivation.Short cycle life and high reaction temperature limit its industrialization and application.Besides,in the conventional process of CO₂ methanation,a mixutre of CO₂ and H₂ as the starting gas reactants is necessary,which is highly intensive energy comsuption due to preparation and compression processes of hydrogen,leading to the methanation reaction uneconomically and energy inefficiently.Therefore,development of an alternative strategy with efficient and cost-effective fashion for CO₂ methanation is highly desirable.Firstly,we investigated the methanation reaction process on LaNi₅ under high temperature and pressure condition.The effects of temperature,CO₂/H₂ ratio,different hydrogen source and cycling times on catalytic performance were analyzed.Under 200℃,the CO₂ conversion rate could reach to 99.8%,and the CH₄ yield could reach to 83.3%.Moreover,this catalyst exhibited durable stability with 99%conversion rate of CO₂ retained after 400 h cycling test.During the reaction,LaNi₅ was decomposed and generated LaCO₃OH and Ni,which was the active center of methanation reaction.Further investigation of the reaction mechanism revealed that highly active Ni was in situ generated on the surface,which lead to a low reaction temperature and slow particle growth rate.Meanwhile,the existance of La could prevent Ni sintering and segregation.The small grain size and uniform distribution enabled LaNi₅ to exhibit excellent cycling performance.Then,we investigated the methanation reaction process on LaNi₅ under ball-milling condition.Using mechanochemistry,CO₂ methantion at room temperature was realized for the first time.The effects of rotational speed,pressure,ball-to-powder weight ratio,different hydrogen sources and cycling times on catalytic performance were analyzed.As the rotation speed reached to 500 r/min,the CO₂ conversion rate reach to 100%,and CH₄ yield reach to83.8%.However,the cycling performance under ball-milling condition was not as good as it under high temperature and pressure condition.By comparing the differences between these systems and analyzing the characterizations of solid products,we proposed possible reaction mechanisms and further speculated the reasons for the catalyst deactivation and the source of catalytic performance.In order to find out the most suitable catalyst among metal hydrogen storage materials,we tested the catalytic performance of XNi₅（X=Y,Zr,La,Ce,Pr,Nd,Sm）under ball-milling reaction condition.The results show that XNi₅ materials could catalyze methanation reaction,with CO₂ conversion rate all above 95%,among which,CeNi₅ shows the highest CH₄ yiled89.5%,and ZrNi₅ lowest,only 39.2%.However,XNi₅ all showed performance dropping during the cycling reaction,which requires further improvement.