Graduate Thesis Or Dissertation
 

Catalytic chemical vapor deposition synthesis of carbon nanotubes from methane on SiO supported Fe and Fe−Ni catalysts

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  • Influences of operating conditions on the production of carbon nanotubes (CNTs) were studied using Fe and Fe-Ni bimetallic catalysts supported on silicon monoxide (SiO). The catalysts were prepared in three steps: (1) impregnation of SiO powders with ferric nitride or combinations of ferric and nickel nitrides, (2) oxidation of nitrides in an air stream, and (3) grinding the powders obtained. CNTs were successfully synthesized by catalytic CVD using NH₃/CH₄ mixtures in a horizontal tubular flow reactor. The following process parameters were varied to investigate their effects on the growth rates of CNTs. The morphologies of catalysts and product CNTs were observed by scanning electron microscope (SEM). • The particle size of SiO, • metal composition, • metal loading, • temperature for catalyst oxidation, • extent of grinding of catalysts, • NH₃ pretreatment time, • reaction temperature for CNT growth, • reaction time, and • NH₃/CH₄ feed ratio. Two different average sizes of SiO particles, 8 μm and 44 μm, were compared based on the growth of CNTs in 5 min. Catalysts supported on 44 μm average sized SiO particles demonstrated higher yields when they were not pretreated in an NH₃ stream. When 1 wt% Fe was loaded, aligned CNTs were formed, and a highest growth rate per unit mass of catalyst was observed. The range of oxidation temperature to achieve highest catalyst activities depended on metals and metal contents: 600 - 750°C for 1 wt% Fe, 450 - 600°C for 3 wt% Fe, and 750 - 900°C for Fe-Ni. Grinding catalysts for at least 3 minutes increased the growth rate of CNTs by approximately 40 percent. The growth of CNTs was enhanced when no NH₃ pretreatment of catalysts was carried out, regardless of metals and metal contents. However, CNTs did not grow appreciably from methane without ammonia. An NH₃/CH₄ feed ratio of 0.15 - 0.25 was observed to yield highest growth rates. The reaction temperature to achieve highest CNT growth rates was found to be in the range between 990 and 1000 °C. The growth of CNTs was not linear but decreased with reaction time.
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