Growth Of Carbon Nanotubes From Methane By A Catalytic Chemical Vapor Decomposition Method

Nano-carbon materials were obtained from the catalytic decomposition of methane. On account of the selective growth of carbon nanotubes, Fe, Co and Ni catalysts were prepared and characterized. The effects of catalyst and reaction conditions on the carbon morphology and yield were investigated. The formation mechanism of carbon nanotubes was discussed.Mo promoted Fe or Co bimetallic catalysts was prepared by multistep impregnation. Their physical and chemical properties were examined with XRD, TPR, IR, XPS, TG, ICP, TEM, HRTEM and SAD. The results showed that between Mo and Fe, or Mo and Co strong interactions exist, which can be described as follows: ( i) At lower Mo loadings, physical promotion of Mo increases in Fe, Co metal dispersion. (ii) At intermediate levels of Mo loadings, electron transfer property is notable which changes the energy status of active sites, (iii) At high Mo loadings, the indication of the blockage of active sites by MoOx species is significant, (iv) At high temperature, the catalyst system is instable and constituents of the catalyst will sublime. These four effects may overlap within certain ranges of Mo loadings. The predominant effect changes with the preparation and reaction conditions.Monometallic catalysts and bimetallic catalysts were used in the preparation of carbon nanotubes. No single-walled carbon nanotubes were found in the products formed over the monometallic catalysts. While, they were found in thatover the bimetallic catalysts. The growth of single-walled carbon nanotubes is attributed to the synergetic interaction between Mo and Fe, or Mo and Co. The effect (iv) is important for the growth of single-wall carbon nanotubes. Over some bimetallic catalysts, at temperatures without obvious sublimation of Mo components, only multi-wall carbon nanotubes was obtained; at temperatures with obvious sublimation of catalyst components, a large amount of single-wall carbon nanotubes were observed. The mechanism (iii), related to large amounts of Mo loadings, results in a blockage of the active sites for the growth of single-walled carbon nanotubes.Fishbone-type carbon nanotubes were observed over Ni catalysts. Only parallel-type carbon nanotubes grew over Fe and Co catalysts. At low metal loadings, a small amount of La addition is beneficial to the growth of carbon nanotubes with uniform diameters and high yield. At high metal loadings, a small amount of La addition leads to adverse circumstance. The catalyst preparation method and the calcination temperature have a strong effect on the catalytic activity. With the increasing Ha concentration, as an inert gas, the average diameter and the yield of carbon nanotubes became smaller.According to the study of catalyst structure, the growth process of carbon nanotubes and the carbon morphology, it is suggested that the tip growth is the major route for multi-walled carbon nanotubes and the carbon's movement depends on the surface diffusion. The sublimation of Mo constituent is beneficial to the growth of single-walled carbon nanotubes.