| Catalytic formation of single-walled carbon nanotubes in diffusion flames is investigated as a potential means of large-scale production. An oxygen-enriched inverse diffusion flame is utilized to control the pathway of carbon at high temperature. This type of flame is shown to minimize polycyclic aromatic hydrocarbon (PAH) and soot formation, while maintaining a high temperature, carbon-rich environment for nanotube formation. Composite catalyst particles for nanotube formation, consisting of iron, silicon, and oxygen, are formed in situ by introducing precursor materials into the flame. These composite catalysts result in significantly higher catalyst yields than iron oxide catalysts, but both catalysts have short lifetimes. Thus, the reaction mechanisms for nanotube nucleation and growth in the flame environment are examined. A differential mobility analyzer is developed and used as an online diagnostic tool. Density functional theory calculations and Car-Parrinello molecular dynamics simulations are employed to determine the structure of the catalysts and how this structure might affect nanotube nucleation. The introduction of silicon into the flame environment results in a non-uniform catalyst surface, which allows for preferential nanotube nucleation on one side of the catalyst. Moreover, the flame environment and the catalyst particle trajectories are modeled. The catalysts are shown to follow isolines in temperature and species, indicating that a change in flame environment does not cause short catalyst lifetimes. A reaction mechanism is postulated for short lifetimes, which is based on catalyst particle reduction and the probability of carbon clustering on the catalyst surface. The results of these studies provide a foundation for future research on improving the catalyst/flame interaction to achieve large-scale production. |
