Experimental Research On The Synthesis Of Carbon Nanotubes Based On The Preparation Of Iron-based Catalysts By Microreactor

As a new type of material,carbon nanotubes have been widely used in all walks of life.There are many methods for preparing carbon nanotubes,of which the chemical vapor deposition method is the most suitable for industrial production.The microreactor has the characteristics of high-efficiency heat and mass transfer,and can be used for the preparation of iron-based catalysts,and then use chemical vapor deposition to prepare carbon nanotubes.At present,there are not a few researches on the use of microreactors to prepare nanoparticles,but there are very few studies on the use of microreactors to prepare CNTs-based catalysts,and there are even more studies on the internal reaction conditions of microreactors to prepare iron-based catalysts.In this paper,a microreactor is used to prepare an iron-based catalyst for the production of carbon nanotubes,and then a chemical vapor deposition method is used to prepare carbon nanotubes in a conical fluidized bed.The catalyst and carbon nanotubes were systematically tested by BET,SEM,EDS,XRD,TEM,Raman,four-probe detection,ash detection and other detection methods.The effects of Fe/Al molar ratio,reaction temperature,initial solution concentration,volume flow rate and cobalt content on the microstructure of the iron-based catalyst prepared by the microreactor and the performance of the carbon nanotubes prepared were studied respectively.In the iron-based catalyst,iron is used as the active element,and alumina is used as the carrier skeleton structure to disperse it.Appropriate Fe/Al molar ratio is the key to the catalyst.Studies have shown that less alumina carrier is not enough to completely disperse the iron element,which makes the diameter of the prepared carbon nanotubes thicker and lowers the quality.Although the addition of too much alumina carrier can ensure the full dispersion of the iron element in the catalyst,the reduction of active components is not conducive to the improvement of the catalytic activity of the catalyst,resulting in a decrease in the production rate.Finally,the catalyst prepared when the Fe/Al molar ratio is 1:2 has a good framework structure and higher catalytic activity,and the prepared carbon nanotubes have higher specific surface area and higher quality.The influence of reaction temperature in the catalyst preparation process is obvious.The lower reaction temperature has little effect on the catalytic activity of the catalyst and the quality of the carbon nanotubes.However,when the reaction temperature is higher than 30?,the specific surface area and catalytic activity of the catalyst are significantly reduced,the elements on the catalyst surface are unevenly distributed,and agglomeration occurs.Furthermore,the specific surface area of the prepared carbon nanotubes is significantly reduced,the tube diameter becomes thicker,and different degrees of folds and fractures appear,the degree of graphitization is reduced,and the quality of the carbon nanotubes is significantly reduced.Finally,the carbon nanotubes prepared when the reaction temperature is 30? have a higher specific surface area and lower resistivity,and the tube diameter distribution is concentrated,mainly 8-10 nm.The initial solution concentration and volume flow have relatively little effect on the microstructure of the catalyst and the performance of the carbon nanotubes produced.A lower initial solution concentration will reduce the supersaturation of the solution,which is not conducive to the preparation of the catalyst and subsequent carbon nanotubes.Too high initial solution concentration will also cause uneven crystal formation and cause particle agglomeration.The increase of the volume flow can make the diameter distribution of the carbon nanotubes more concentrated,which gives full play to the micro-mixing characteristics of the micro-reactor.However,an excessively high volume flow rate did not keep the morphology of the catalyst well,but increased the resistivity of the carbon nanotubes.Finally,the catalyst prepared when the initial solution concentration is 50% and the volume flow rate is 40-60 m L/min can further produce carbon nanotubes with excellent properties.Cobalt can be used as an active site for catalyzing the growth of carbon nanotubes.In this experiment,cobalt is introduced to improve the performance of the catalyst by exerting the bimetallic synergy of cobalt-iron alloy.Studies have shown that the addition of cobalt element improves the catalytic activity of the catalyst and significantly increases the output of carbon nanotubes.The addition of a small amount of cobalt can maintain the specific surface area of the catalyst to prepare carbon nanotubes with excellent performance indicators.However,excessive cobalt will significantly reduce the specific surface area and graphitization degree of the prepared carbon nanotubes,increase the resistivity,and thicken the tube diameter,which is not conducive to the improvement of the various properties of carbon nanotubes.