Molecular Reactive Force-Field Simulations on the Carbon Nanocavities from Methane Pyrolysis

Hydrocarbon pyrolysis is the main way to achieve carbonaceous materials, while most related conversion mechanisms still remain unclear. This work images pyrolysis of methane at various temperatures and densities by molecular reactive force field (ReaxFF) simulations. First, it is interesting to find that the methane decay is dominated by intermolecular collision displacement instead of direct molecular decomposition. Second, a conversion of 1200 methane molecules into a regular carbon nanocavity (CNC) is realized at 3500 K temperature and 0.1 g/cm(3) density after a simulation lasting for 10 ns, with 923 carbon atoms and a diameter of 3.4 nm. Such CNC is a perfect precursor of carbon nanotubes, which is confirmed by a sequent simulation on a larger system of 2400 methane molecules and in agreement with several experimental observations. It is found that the CNC growth obeys a polyyne model, without any single aromatic ring formed in the growth. Furthermore, the complex CNC growth appears in some successive stages: primary methane decay, chain elongation and branching, cyclization and condensation, and final sheeting and curling. The regular rearrangement of CNC is thought to be attributed to the limited active centers formed at the initial cyclization and condensation stage; that is, it is a key to control the primary active centers to form regular carbonaceous materials. Polyyne is found in the pyrolysis of both methane and acetylene at high temperatures, suggesting that carbyne, a novel valuable carbonaceous material, may be obtained by hydrocarbon pyrolysis.