High efficiency carbon nanotube synthesis with Co-Mo/MgO catalyst

Carbon nanotubes, one of the allotropes of carbon, are materials that were discovered in 1991 and have started a new era in material science because of their important properties. Carbon nanotubes are cylindrical structures with a diameter of nanoscale and are materials with various electrical, mechanical and thermal properties. Carbon nanotubes (CNT) have a wide range of industrial applications due to their unique properties such as thermal stability, optical transmittance, electrical conductivity and mechanical strength. They can be used in energy storage and energy conversion mechanisms, composites, electron field emission applications, hydrogen technologies, probes, interconnections and medical applications. Carbon nanotubes are generally produced by three different production methods according to different applications and usages. These methods are Arc Discharge, Laser Evaporation and Chemical Vapor Deposition. In all of these production methods, a carbon source is provided to generate carbon atoms in the production of carbon nanotubes. In the arc discharge method, the current is energized by heating with a furnace in high intensity light and chemical vapor deposition method from a laser in Laser Evaporation method. Chemical vapor deposition method from CNT production methods is more economical than other production methods and it is stated that it is more suitable for large scale production. In addition, controllability is provided by this method, which is the most widely used method because of all these advantages. The supported catalysts prepared in the Chemical Vapor Deposition method are placed in the pipe reactor and heated to high temperatures and the carbon source is fed to the system together with the inert gas at the selected test temperature. In this method, the growth mechanism of carbon nanotubes involves the decomposition of hydrocarbon molecules catalyzed by the transition metal and the dissolution of carbon atoms in the metal nanoparticle. There are various parameters affecting the production efficiency of the carbon nanotubes and the structure of the nanotubes. These parameters affecting the production of carbon nanotube; synthesis temperature, catalyst, carbon source, support material and synthesis time. Diameter controlled and selective productions are possible by changing production parameters. In the literature studies, these production parameters are optimized and compared with carbon nanotube properties for high efficiency production. The catalyst is one of the most important parameter in the production process of carbon nanotubes. Selective and diameter controlled production is achieved with the catalyst used. Therefore, in addition to synthesis conditions, the effects of metals used as catalysts and support materials are investigated in the literature. A large number of studies have been carried out Co and Mo bimetallic catalysts in particular. It is indicated that these two metals together form a synergistic effect and that structures that promote the growth of carbon nanotubes occur during synthesis. Studies have been carried out to observe these effects and to produce highly efficient carbon nanotubes with selective and diameter control by changing various parameters. In the scope of the thesis, different Co/Mo and support material ratios and different catalyst production methods were used for carbon nanotube synthesis. The results were evaluated with Co-Mo catalyst by interpreting the studies in the literature. The master thesis study was carried out at the Materials Production and Preparation Laboratory of the İstanbul Technical University Energy Institute. The chemical vapor deposition method was chosen as the carbon nanotube synthesis and the effects of different Co/Mo and support material ratios on the production efficiency of different catalysts were investigated. Cobalt nitrate (Co(NO3)2·6H2O) and ammonium molybdate ((NH4)6Mo7O24·4H2O) starting materials were used to prepare the Co/Mo catalysts and MgO was used as the support material. Sol-gel and impregnation methods are applied for the catalyst preparation with different Co/Mo ratios (1/1, 1/2.5, 1/3.5, 1/7, 1/14 and 2/1) and 5%, 10% and 15% metal/support material. Carbon nanotube productions were carried out in a fluidized bed reactor system at 800 °C and 45 minutes using acetylene as a carbon source with prepared catalysts. Synthesized carbon nanotubes were characterized by Raman Spectroscopy, Scanning Electron Microscopy, Thermogravimetry and X-Ray Diffractometry. Carbon nanotubes formed in the synthesis using C2H2 as a carbon source with catalysts prepared with Co-Mo catalyst and MgO support for 45 minutes at 800 oC were expected to be single-walled carbon nanotubes. Single-walled carbon nanotubes have a wider range of applications due to their structural properties. In this thesis, the effects of production parameters were compared with the results of FEGSEM, Thermogravimetric analysis, Raman Spectroscopy and XRD characterization methods. Thermogravimetric analysis was also used to determine the amount of metal remaining after carbon nanotube synthesis and then nanotube yields were calculated. Before evaluating the effects of all parameters examined within the scope of the thesis, the structural properties of a CNT sample selected as an example were determined and single-walled carbon nanotubes were clearly identified. When the Raman spectra of the synthesized carbon nanotubes were examined, it was determined that the structures were single walled carbon nanotubes due to the presence of peaks in the RBM band, which is the descriptive of single walled carbon nanotubes. According to XRD analysis of synthesized carbon nanotubes, XRD spectra changed depending on catalyst ratios and support material, but graphitic structures were found in all samples. It was also observed that the yields were changed according to synthesized parameters, such as Co:Mo and MgO support ratios, the catalysts preparation methods. Carbon nanotubes can be used in many applications with their properties. One of the most important applications of carbon nanotubes is lithium ion batteries. Lithium ion batteries are preferred due to their advantages such as high energy density, long life and low cost. In order to see the use of carbon nanotubes in lithium ion batteries, the high yield carbon nanotube was selected and its battery performance was determined as an anode material. When the discharge capacities were examined, the specific capacity was 282 mAh/g at the end of the first cycle. It was found that as the number of cycles increased, specific discharge capacities approached each other.