The Research On High-yield Synthesis And Characterizaton Of Single-walled Carbon Nanotubes By CVD

Since their discovery in 1993, single-walled nanotubes (SWNTs) promptly became the research hotspot in materials science because of their special characteristic in microstructure and extraordinary mechanical and unique electronic properties. Based on these excellent properties, SWNTs were widely applied in many fields including gas storage, probes, filed emission devices and high-strength composites, et al. At present, three methods to synthesize SWNTs are widely used: arc discharge, laser blazing and chemical vapor deposition (CVD). Among them, arc discharge and laser blazing method are difficult to realize the SWNT production to industrial scale for the complex production equipments. Comparing with the two methods, CVD has the best hope for large-scale production of nanotubes because of the simpler equipments, as well as the possibility to realize the continuous growth of SWNTs. Because the key point is to prepare the high activity catalyst in the procedure of the synthesis of SWNTs by CVD, many research groups adopt various methods to prepare catalysts, but unfortunately the activities of them are not high that the yield of the SWNTs couldn't get a high level. So far, the paper will use two advanced techniques, supercritical drying method and freeze-drying method which always be used to prepare nanoscale powder materials, to prepare high-activity catalysts for the large-scale synthesis of SWNTs. At the same time, we have done some work on the optimization of the growth of SWNTs.The results show that the calcination process deeply affects the activity of catalyst. After investigation, we find the best calcinations temperature for the Fe/Mo/Al2O3 catalyst prepared by supercritical drying method is 600 ℃. But for Fe/Mo/MgO catalyst prepared by freeze-drying method, the calcination process doesn't benefit the improvement of the catalytic activity. That the calcination process will change the graininess, dispersity and acidity-basicity of the catalyst can explain the discrepancy of the above phenomena.The influence parameters in the growth of SWNTs on Fe/Mo/Al₂O₃ catalyst were optimized by a series of check experiments. Based on them, we conclude the bestgrowth process is as follows: The mole ratio of Fe/Mo/Al₂O₃ is 1:0.15:15; Reacting gas: CH₄/H₂=750 /200 v/v; Reaction temperature: 1000 ℃; Reaction time: 30 min. by the above way, the content of synthesized SWNTs in the raw product can highly reach 75 % and the yield can highly reach 140.3 %.To improve the process of the preparation of the catalyst by the supercritical drying, we combined the hydrothermal method with the supercritical drying method. By this way, the preparation of the alcogel was simplified greatly and the cost of the catalyst was largely cut down. The as-prepared SWNTs also have a high content, which is 56 %. By the further optimization, this method will have a large prospect in the preparation of the catalysts for its timesaving and labour saving. At the same time, we also attempt to prepare Fe/Mo/MgO by supercritical drying. Because of the introduction of the acrylamide and N,N'-Methylene bisacrylamide, the C-C network will be collapsed in the calcinations. In this case, the most active component will be buried, so the yield of SWNTs couldn't be very high, which only can reach 15 %. This work provides an approach to get high activity Fe/Mo/MgO catalyst. If we could improve the process of the alcogel and decrease the introduction of the carbonaceous materials, this method can be more propitious to the preparation of the Fe/Mo/MgO catalyst for the large-scale production of SWNTs.In this paper, the freeze-drying method was first used in the preparation of the Fe/Mo/MgO catalyst for the large-scale production of SWNTs. The Fe/Mo/MgO cryogel catalyst has the characters of large specific surface, little graininess, good dispersity, high activity, et al. when growed in the reaction gas, CH₄/H₂=100/200 v/v, the as-prepared SWNTs have the content above 55 %, and the yield of SWNTs highly reach 105.1 %, which is better than the result of the SWNTs synthesized by other research groups using the catalysts with MgO supporter. This method is not only applicable for various wet-chemical systems, but also very simple in the preparation process, so it must have a broad prospect in the preparation of the catalysts for the synthesis of SWNTs.