Study On Large-scale Preparation Of Multi-Walled Carbon Nanotubes Over Aerogel Catalyst

Since their discovery in 1991, carbon nanotubes (CNTs) have attracted many scientists to study them due to their excellent properties. At present, many problems about application and industrialization of CNTs need to be solved, and the technology of large-scale preparation of CNTs has not been well developed. In this dissertation, in order to prepare multi-walled carbon nanotubes (MWNTs) at least one kilogram every day, MWNTs with different morphologies and microstructures were prepared by using composite aerogel NiO/SiO₂ as catalyst, which was synthesized using sol-gel and supercritical fluid drying technology. It was studied that how the aerogel component, decomposition temperature, time and gas flow rate affect the yield, morphology and microstructure, and the influence of purifying process of MWNTs was discussed. On the basis of laboratory work, the author designed a set of product line for large-scale preparing MWNTs. Resultant MWNTs have high purity, low cost and competitive advantage. Some conclusions were draw as below:1. By using tetraethoxysilane, nickel nitrate as starting materials, NiO/SiO₂ aerogel, with high specific surface area in the range of 500¹400m²/g, having micropore and mesopore structure, was synthesized by sol-gel and supercritical fluid drying method. The microstructure and aerogel component could be controlled by adjusting preparing conditions.2. The experimental results indicate that parameters, including decomposition temperature, aerogel component, decomposition time and gas flow rate, directly influence morphology, microstructure and yield of resultant MWNTs.(1) The decomposition temperature influences morphology and yield of MWNTs directly. Proper temperature is helpful for MWNTs' growth. MWNTs prepared at lower temperature can not grow enough and have a low aspect ratio, but MWNTs prepared at higher temperature have much impurity. The yield of MWNTs will be reduced much at too high or low temperature. The temperature range, from 600 to 740℃, is suitable for MWNTs growth, and the yield of MWNTs prepared at 680℃ is the highest (8.6).(2) Aerogel Component is the key factor to influence yield of MWNTs. The yield of MWNTs varies greatly with the different value of n_Ni:in_Si in NiO/SiO₂ aerogel. If value ofri(Ni):n(si) equals 4:6, the yield of MWNTs is the highest.(3) Decomposition time plays an important role on the yield of MWNTs, but little on their morphology. The yield of MWNTs increases with decomposition time within 180min. The length increases but the diameter changes little. MWNTs grow slowly after 180min, and the yield remains constant.(4) The morphology and yield of MWNTs vary with gas flow rate. The MWNTs with good morphology and large aspect ratio can be prepared while the flow rate of CH4 is twice that of H2, and the yield is high. The yield of MWNTs increases with the total gas flow rate, but above 300ml/min, the yield of MWNTs will be constant.(5) Purifying process can remove impurity in MWNTs, enhance their purity and influence their microstructure. Introducing 129Xe NMR to characterization of CNTs for the first time, the experimental results indicate that it is an effective technique to explore microstructure and surface condition of CNTs.3. Possibility of searching for other gas as carbon source is explored. The experimental results show that morphology and structure of MWNTS vary greatly with different carbon sources. MWNTs prepared from CH4 have a large aspect ratio, smooth wall and regular morphology; those from C3H6 have a little amorphous matter, rough wall, and some wrinkle, which means a lower oriented order of carbon atoms. Compared with from CH4, MWNTs prepared from C3H6 have a higher yield and a lower conversion.4. On the basis of above research, the author designed a set of safe, efficient and convenient product line for preparing MWNTs in large scale. The solid distributing style for catalyst can make an area over lm2. At the help of Pd-carbon fiber deoxy catalyst, tittle oxygen in industrial nitrogen can react with hydrogen at low temperature and produce water, and it can be absorbed by zeolite. Flowing route of gases can be prolonged by disturbing tube in pyrolysis furnace, and CH4 and H2 are utilized efficiently by using gas recycling equipment. According to the relationship among distribution area of catalyst, quartz tube volume and gas flow rate in laboratory catalytic decomposition, gas flow rate in large-scale preparation can be calculated. By comparing yield and microstructure of MWNTs, the processing parameters are optimized, and the practical cost of high quality MWNTs is no more than 1.5 yuan RMB.