Preparation Of Carbon Nanotubes Based On A New Hydrotalcite Catalyst Precursor

Carbon nanotubes have attracted much attention due to their unique hollow tubular structure and excellent physical and chemical properties.Chemical vapor deposition is the most widely used method for the preparation of carbon nanotubes due to its advantages of controllable preparation process,low preparation cost,high yield and high applicability,but it also has the disadvantages of low product purity and many defects.Among the various influencing factors,the influence of catalyst on carbon nanotubes cannot be ignored,thus optimizing the preparation process of catalyst becomes an important part of the chemical vapor deposition method for the preparation of carbon nanotubes.Hydrotalcite is the precursor of mixed metal oxides.The mixed metal oxides formed after calcination have high specific surface area and porosity,good thermal stability and high metal dispersion.In this paper,hydrotalcite was used as a catalyst for the preparation of carbon nanotubes.The effects of calcination temperature,reduction temperature and additive amount on the microstructure of the catalyst were analyzed in detail by combining various detection methods,and the growth temperature and growth time of carbon nanotubes were further optimized.The main research contents and conclusions are as follows.（1）LDH was prepared by coprecipitation method as the catalyst precursor and the catalyst was prepared at different calcination temperatures.The catalyst and carbon nanotubes were compared and analyzed by various characterization methods.The results showed that the catalyst activity was strongly influenced by the calcination temperature,and too high calcination temperature would intensify the solid-phase reaction between the active metal and the support to generate more Mg Fe₂O₄ spinel,which makes the reduction of iron oxide difficult and reduces the catalytic activity.The results show that the best calcination temperature is 500℃,the catalyst shows better catalytic performance,and the quality of carbon nanotubes is better.（2）The catalysts and carbon nanotubes prepared at different reduction temperatures were characterized and analyzed by SEM,XRD,XPS,BET and other test technologies.It was found that the catalytic multiplying power of catalysts with close specific surface area was quite different.Compared with the specific surface area,the iron oxide obtained in the reduction process had a more significant impact on the catalytic performance.When the reduction temperature was 660℃,the obtained samples not only had a larger specific surface area,but also the phase structure of the outer layer of Fe_xO and the inner layer of Fe₃O₄ was favorable for the further reduction of Fe and the diffusion of carbon,and the catalytic performance was superior,and the yield and graphitization of the obtained carbon nanotubes were higher.（3）The effect of auxiliaries Cr on the catalyst and the synthesis of carbon nanotubes was studied by using the exchangeable property of anion between the layers of hydrotalcite.The results show that due to Cr is beneficial to dehydrogenation,the addition of an appropriate amount of auxiliaries Cr can improve the catalytic multiplying power of the catalyst.When the addition of Cr reaches 1%,the catalytic multiplying power reaches the maximum,which is 2230%.However,excessive Cr will greatly reduce the specific surface area of the catalyst,reduce the active site,and convert Cr⁶⁺into Cr³⁺,which is not conducive to the preparation of carbon nanotubes.（4）The optimum growth temperature and growth time of carbon nanotubes were explored.It was found that at a growth temperature of 660℃,the rate of carbon atoms produced by the cleavage of propylene matched the rate of carbon atoms consumed by the precipitation of carbon nanotubes,and the catalyst could perform optimally and the yield of carbon nanotubes reached the maximum.When the growth time is less than 30min,the yield of carbon nanotubes can maintain a high growth rate and save energy.After more than 30 min,the activity of catalyst is significantly reduced,and the yield growth is slow.