Preparation Of Small-diameter Carbon Nanotubes By Catalytic Cracking Of Methane Over Fe-based Catalysts

As one of the most attractive nano-carbon materials,carbon nanotubes(CNTs)possesses superior properties such as high electrical conductivity,excellent thermal conductivity and outstanding mechanical strength compared with other materials due to its unique structure.It may notice that the diameter of CNTs is one of the key factors related to its performance.However,at present,controlling the diameter of CNTs is really challenging task and generally incurred high cost and massive energy consumption.Moreover,there is no reported literature on large-scale industrial production of small-diameter CNTs in China,as we know,perhaps only in Russia.In this work,the Chemical Vapor Deposition(CVD)method is used to grow CNTs with small-diameter by preparing different composition and structure of Fe-Mo-based catalyst accompanying with various reaction conditions,aiming to obtain high-quality CNTs with low energy consumption thus low cost.In this study,sol-gel method is used for Fe-Mo-based and Fe-Mo-Pt-based MgO supported catalysts preparation with different Fe/Mo metal ratio and metal loading.Their catalytic performances are evaluated in terms of producing small-diameter CNTs from methane cracking.Upon these,the reaction conditions are further optimized for CNTs growth for the excelled catalysts.Finally,the as-prepared CNTs with small-diameter are also examined as conductive agents applied into the lithium-ion batteries.The typical results of this work are listed as follows.1.By varying Fe/Mo metal ratio for Fe-Mo-based catalysts,the results show that 5Fe5Mo1 catalyst with low Fe/Mo metal ratio is more beneficial for the growth of targeting CNTs.The carbon production yield of such catalyst can reach 20.70%,and the IG/ID(IG/ID refers to the ratio of peak areas of Raman characteristic peaks G and D of CNTs)of as-prepared CNTs was 3.5,higher than that of other catalysts prepared in this work.SEM results show that the catalyst has uniform pores,fluffy texture and small particle size.Moreover,FeMgOx solid solution was formed in 5Fe5Mo1 catalyst,and Mo species dispersed around Fe species according to the results of XRD and TEM.Combined with the corresponding catalytic performance,it is expected that Mo species played an important role in stabilizing and dispersing Fe species,thus shaping favorable environment for the growth of small-diameter CNTs.It is also noted that FeOx species with reduction temperature of 450-500℃ will be preferred to the growth of CNTs with high quality and small diameter,based on the results of TPR.In addition,the growth conditions of CNTs are further optimized.It was found that the carbon yield of 5Fe5Mo1 catalyst can be achieved as high as 37.57%under the conditions of reaction temperature 850℃ and etching gas H2 concentration of 30%.Furthermore,the diameter of CNTs under these conditions is just as small as 9.13 nm with relatively uniform distribution.It can be deduced that the rates of methane cracking,carbon species,migration and deposition on the 5Fe5Mo1 catalyst can reach a perfect balance under the condition asmentioned above.2.The effects of metal loading of Fe-Mo-based catalysts is further explored.The results show that the metal loading hardly affects the catalyst particle size,but the Fe-Mo-based catalysts with high metal loading are prone to agglomerate in the reaction process,which leads to the larger diameter of prepared CNTs.In order to discover the preparation of CNTs with small diameter at lower temperature,a small amount of Pt is tried to add into Fe-Mo based catalyst.It was found that amorphous carbon is easily grown on the trimetallic catalyst,which can be attributed to that Pt has a strong CH4 cracking ability.Although the IG/ID of as-prepared CNTs decreases in some extent for the trimetallic catalyst,the diameter of CNTs prepared under the same reaction conditions is significantly smaller than that of bimetallic catalyst.3.Based on analyzing the growth pattern of CNTs on Fe-Mo-based catalysts,the growth of CNTs can be discussed based on two different ways originated from CH4 catalytic cracking.First,the Fe species can be simultaneously used for cracking CH4 and growing CNTs.Secondly,carbon species cracked can migrate to other Fe species to nucleate and grow CNTs,and the free original Fe species continue to crack CH4.When Fe loading is small,CNTs with small tube wall number can be obtained by Fe-Mo-based catalysts.With the increase of Fe loading,the number of Fe species increases,and more carbon species can be obtained by pyrolysis,meaning more carbon species around Fe species per unit area,so the diameter of CNTs will increase accordingly.In addition,too high metal loading may lead to aggregation of Fe species at high temperature,forming large-diameter catalysts and resulting large-diameter CNTs.4.In this study,CNTs were used as conductive agents to test the performance in lithium batteries.It was found that CNTs with small diameter can significantly improve the conductivity of the batteries,and with the increase of CNTs dosage,the conductivity correspondingly improved.The electrical conductivity of CNTs prepared by 5Fe5Mo1 catalyst at 850℃ is much better than that of the first generation SWCNTs crude products prepared by our research group.Furthermore,after 500 cycles,the conductive agent can retain the cycle stability of the battery at 95 mA·h/g.The conductivity and cycle stability are comparable to OCSiAl-CNTs,the product from Russia.