催化裂解CH4制备碳纳米管的影响因素

以柠檬酸法制备的Fe-MgO、Co-MgO和Ni-MgO为催化剂,CH4为碳源气,H2为还原气,在873、973和1073 K制备出碳纳米管,通过TEM和拉曼光谱表征,讨论了催化剂、制备温度、反应时间等因素对碳纳米管形貌、产率和内部结构的影响.结果表明:不同的催化剂在相同的温度下制备的碳纳米管的形态和内部结构有很大的差异.其中Fe-MgO催化剂制备的碳纳米管管径粗,且大小不均匀,而Ni-MgO催化剂制备的碳纳米管管径较细、较均匀.碳纳米管的产率随着裂解温度的变化而改变.Fe-MgO催化剂制备碳纳米管的产率随制备温度的升高而提高,而Ni-MgO催化剂制备碳纳米管的产率随制备温度的升高而降低.Fe-MgO催化剂制备碳纳米管,在1073K甚至更高的制备温度才能达到其最高产率.Co-MgO催化剂制备碳纳米管的产率在973 K左右产率较高,而用Ni-MgO催化剂制备碳纳米管,则在873 K甚至更低的制备温度就能达到最高产率.反应时间与碳纳米管的产率不成正比,有一最佳反应时间,如Ni-MgO催化剂的最佳反应时间为2 h.

The Factors of Preparation of Carbon Nanotubes by Catalytic Decomposition of Methane

Using Fe-MgO、Co-MgO and Ni-MgO catalysts preparated by citric acid method, carbon nanotubes (CNTs) were synthesized from decomposition of methane at 873 K, 973 K and 1013 K, respectively, and were characterized by TEM and Raman spectra. The effects of catalysts, preparation temperature and reaction time on the shape, yield and inter-structure of carbon nanotubes were discussed. TEM results and Raman spectra show that the shapes and inter-structures of carbon nanotubes produced on different catalysts at the same temperature are quite different. Carbon nanotubes synthesized on Fe-MgO incline to be rods, with very thick wall, and the diameter varied greatly from 10 nm to 80 nm. However, the diameter of carbon nanotubes produced on Co-MgO is about 35 nm, and about 20 nm on Ni-MgO. As can see from the Raman spectra, the G peaks of carbon nanotubes produced on the three catalysts shift slightly, which indicate the difference of the inter-structure of nanotubes. The experiments show that the yield of carbon nanotubes varied with decomposition temperature. Using Fe-MgO catalysts, yield of carbon nanotubes grows with the decomposition temperature. However, using Ni-MgO catalysts, the yield drops when the temperature increases. The reaction temperature with the largest yield of carbon nanotubes is higher than 1013 K on Fe-MgO catalysts, about 973 K on Co-MgO catalysts, and lower than 873 K on Ni-MgO catalysts, respectively. The results also show that the yield of carbon nanotubes does not increase with the decomposition time. There is a saturation time of the carbon nanotube growth. The carbon nanotubes do not grow any more when the saturation time reaches. For example, on Ni-MgO catalysts, the carbon nanotubes stop growing after decomposition of CH4for 2 h.