Study On Methanation Catalyst And Reaction Mechanism

In order to protect the environment and make rational use of CO₂ resources,methanation technology has become one of the most potential technology.This paper mainly introduces the research direction of methanation.That is to develop suitable catalysts and explore the reaction mechanism.Among them,methanation catalyst is the focus of research.This paper mainly introduces the noble metal catalysts?Ru,PD,etc.?and transition metal catalysts?Ni,Co,etc.?which are not used in methanation,especially the application of nickel based catalysts.The effects of the support?Al₂O₃,SiO₂,TiO₂,ZrO₂,etc.?,metal promoters?MgO,La₂O₃,etc.?and active components on the physical structure and chemical properties of the catalyst were analyzed.By using N₂ physical adsorption desorption,XRD,H₂-TPR,CO₂-TPD,UV-VISDRS,TEM and in situ drifts,the relationship between the change of intermediate products and the physical structure and surface chemical properties of catalyst was analyzed.In this paper,the reasons for the decline of catalyst efficiency in the use process were also studied.With the help of TG,TPO and on-line mass spectrometry?MS?,the reason and mechanism of the formation of sintering and carbon deposition of the catalyst were described,and the types of carbon deposition were classified.There have been different views on the mechanism of methanation:mainly the mechanism without co intermediate and the mechanism through co intermediate.In this paper,the product changes in the reaction process are analyzed in detail by in situdrifts.It is considered that*CO₂,*CO,formic acid,H₂CO*,*CH₃OH,bicarbonate and carbonate are important reaction intermediates.The main conclusions are as follows:?1?The results showed that the additives significantly affected physiochemical properties of the catalysts.The addition of Sc,Ti,V,Mn,Zr,La and Ce did not lead to the remarkable decrease of the specific areas of the catalysts,while the addition of other additives resulted in the enlarged pores and the decreased surface area,especially for the Na and K additives that led to the sintering of the support.As for the effects on nickel species,the addition of Cr,Mn,Fe,Co,La,Ce showed little effects on promotion of nickel dispersion,while the addition of other additives accelerated the sintering of metallic nickel.The additives also affected the internal structures of the catalysts.The K or Na added reacted with alumina,forming Al?OH?₃phase and leading to the re-structure of the catalysts.Mn oxide reacted with NiO and formed NiMnO₃ structure.In addition,Fe,Ce,Mn species could not highly disperse across alumina as other additives did.The additives also affected the reduction behaviors of nickel oxide.The promotional effects for Mn and Cu on the reduction of nickel oxide were the most significant.?2?The alumina or silica supported nickel catalysts had distinct physiochemical properties and the addition of La significantly impacted the catalytic performances and the formation of the reaction intermediates in CO₂ methanation.For the alumina-based catalysts,the addition of La did not filled much of the pores in alumina and the aggregation of metallic nickel particles could be suppressed.The particle size of nickel over the Ni/Al₂O₃ catalyst was only half to that over the Ni/SiO₂ catalyst.The SiO₂ used mainly contained the microspores,which could not effectively disperse nickel particles.The addition of La to either the alumina or the silica based nickel catalysts led to the reduced particle size of metallic nickel,the reduced reduction degree of nickel oxide and the increased alkalinity number on surface of the catalysts.The alkaline number was even twice over the alumina based catalyst than over the silica based nickel catalysts.These distinct physiochemical properties of the catalysts significantly impacted their catalytic behaviors.?3?The results showed that Co/Al₂O₃ catalyst showed much superior activity to Ni/Al₂O₃catalyst especially in the low temperature regions,even though the Ni/Al₂O₃ catalysts have a higher specific area and reduction degree than Co/Al₂O₃ catalyst.The fundamental reason for this phenomenon was the varied the reaction intermediates formed over Ni/Al₂O₃ and Co/Al₂O₃ catalysts during the methanation process.The in situ DRIFTS studies of methanation of CO₂ showed the distinct distribution of the reaction intermediates formed over monometallic nickel,monometallic cobalt,bare alumina.The supporting of nickel or cobalt on alumina further changed the distribution of the reaction intermediates formed with negative or positive impacts on the catalytic activity.?4?The differences in terms of physicochemical properties of the Ni/Al₂O₃,Ni/SiO₂ and Ni/H?40 catalysts determined the reaction intermediates formed and their catalytic performances in methanation of CO₂.SiO₂ and H?40 contained mainly micropores,which possessed much higher specific areas but could not well disperse nickel species.Al₂O₃ is a mesoporous material,which enhanced the dispersion of nickel species and the nickel particle size was the lowest among the three catalysts.Nevertheless,the nickel particle size was not the dominant factor affecting the catalytic activities of the catalysts,while the distribution of the basic/acidic sites was.Abundances of the medium to strong basic sites followed the orders:Ni/H?40>Ni/SiO₂>Ni/Al₂O₃.Ni/Al₂O₃ mainly contained the weak basic sites.The basic sites on surface of the catalysts aided the absorption/activation of CO₂.The catalytic activities of the catalysts for methanation of CO₂ followed the same order.The Ni/H?40 and Ni/SiO₂were far more active than that of Ni/Al₂O₃ catalyst and they showed the much lower selectivity towards CO,originating from the distinct reaction intermediates formed.With the development of research,the key to methanation technology is to develop and manufacture catalyst with high reaction efficiency and heat resistance.In the aspect of industrial application,it is found that nickel based catalyst has a great advantage in industrial application because of its high activity and relatively low price.