Study On The Production And Utilization Of Carbon Nanofibers From Catalytic Decomposition Of Methane

In the thesis, paracrystalline and highly active nickel-alumina and nickel-copper-alumina catalysts, in which the nickel particle size is between several and several tens nanometers, used for catalytic growth of carbon nanofibers from methane, were prepared from Feitknecht compound precursors by coprecipitation, and consequent calcination and reduction. The feasibility of simultaneous hydrogen and carbon nanofibers production from the catalytic decomposition of methane was discussed. Morphology, structure and their dependence on process conditions of carbon nanofibers were investigated. A mechanism of carbon nanofibers formation and growth on nickel catalyst is proposed. In the last section, the physical-chemical properties and the utilization of carbon nanofibers as an adsorbent were studied. The formation condition and structure of Feitknecht compound precursors during coprecipitation, the structure of mixed oxides after calcination and the nickel catalysts after reduction were investigated with XRD, DTA, TPR, BET and TEM. The results indicate that Feitknecht compound can be formed when M2+/M3+ ratio is in a range 2:1-19:1, however, when the ratio is higher than 3:1 the crystallinity of the coprecipitate becomes weak. After calcination, the mixed oxides resembles the structure of NiO, without showing structural information of Al2O3 or CuO, though these are uniformly solved in the lattice of NiO. A strong interaction between NiO and Al2O3, or CuO leads to a bad crystallinity of NiO lattice, and as a consequence makes the reduction temperature of nickel goes much higher than in other composites. During reduction of the mixed oxides, the existence of irreducible Al2O3 in NiO lattice enhances the formation of paracrystalline nickel particles. The better crystallinity of the Feitknecht compound precursors, the stronger the interaction between Al2O3 and NiO is, the more distortion of the nickel crystal in catalyst. Very high calcination or reduction temperatures may destroy this paracrystalline structure and make the crystallinity of nickel to be better. On the basis of thermodynamics calculation, the feasibility of simultaneous hydrogen and carbon nanofibers production from decomposition of methane was discussed. Compared to the steam reforming of methane, the energy saves amount up to over 40% for producing a unit volume of hydrogen. The solid material carbon nanofibers can be produced in a one step process. In a tubular reactor, simultaneous hydrogen and carbon nanofibers production was investigated experimentally. The results shows that hydrogen is the only gas phase product and the conversion of methane, the amount of carbon nanofibers produced on unit mass of catalyst and the productivity of hydrogen are related to the catalyst composition and reaction conditions. At 773-873 K, the conversion was about 20% and amount of carbon nanofibers produced was 100-600 gC/gNi and a gas with 33 vol% of hydrogen was obtained. With the increase of reaction temperature, the conversion of methane was increased, however, the catalyst was deactivated faster. The amount of carbon nanofibers produced depends on the conversion of methane and the stable time of the catalyst activity. At 1023 K, most of the catalysts were deactivated soon, however, with a catalyst containing 25% copper, a gas mixture with 82% hydrogen and 191 gC/gNi were obtained. In a thermal balance reactor, the process of carbon nanofibers growth was investigated. At temperatures around 773 K, the catalysts were highly active and a large amount of carbon was formed. The growth rate and the amount of carbon nanofibers produced with unit mass of catalyst were depended on the composition and structure of catalyst and reaction conditions. With methane and nitrogen as the feed, the growth of carbon on Ni/Al2O3 was faster than that on Ni-Cu/Al2O3, however, the stability of the former was lower than that of the latter. The total amount of carbon produced on a catalyst was determined by the growth rate and the stable time of its activity. Hydrogen in the feed prohibited the formation of carbon and raised the activation temperature for carbon formation and lowered the growth rate. The morphology and structure of the nanofibers were examined with XRD, SEM, TEM and HREM. The results show that the nanofibers are highly graphitized and the distance of the carbon layer of (002) is uniform and is about 0.34 nm, which is close to that of graphite. The shape and orientation of carbon layers, the morphology of catalyst particles at the tip of a fiber or encapsulated inside a fiber are complex and are related to the composition and structure of the catalyst, reaction conditions, and composition of the feed. Different morphologies of carbon nanofibers were found, such as fishbone-shaped, octopus-shaped, branch-shaped, stick-shaped, tubular, bamboo-shaped and helical and etc. The diameter of them was several nanometers to several tens of nanometers. Carbon nanotubes were found in a reaction condition. These results indicate the possibility of controlling the morphology and structure of nanocarbons by monitoring catalyst and reaction conditions. The mechanism for carbon formation on nickel catalysts was discussed. The growth of carbon naofibers was attributed to the surface diffusion of carbon and its morphology and orientation of carbon layers depended on surface composition and existent state of catalyst particle and reaction condition. The deactivation of catalyst was due to the bad matching of decomposition rate of methane and surface diffusion rate of carbon. The properties of catalytic grown carbon nanofibers were studied. The carbon nanofibers have a high mechanical strength. Their BET specific surface area is about 100m2/g. Stability in CO2 of the carbon nanofibers is better than that of active carbon. Benzene and phenol were used as adsorbates to characterize the adsorption ability of carbon nanofibers. The results show that the amounts of the adsorption of benzene from vapor phase and phenol from water solution on carbon nanofibers depended on their structure. These adsorbed amounts are much lower than that on active carbon, due to its small specific surface area.