Higher Activity Leading to Higher Disorder: A Case of Four Light Hydrocarbons to Variable Morphological Carbonaceous Materials by Pyrolysis

The subtle and efficient manufacture of high-quality carbonaceous materials dominates their extensive applications. Meanwhile, revealing the underlying mechanism in the formation of carbonaceous materials is crucial to improving their manufacture efficiency. In the present work, we focus upon the pyrolysis mechanism for four light hydrocarbons including methane, CH4, ethane, C2H6, ethylene, C2H4, and acetylene, C2H2, to carbonaceous materials, combined with reactive molecular dynamics (RMD) simulations. The carbonaceous materials with various morphologies are observed in our simulations, and the morphologies are strongly dependent on the initial reactants; i.e., a disorderly C cluster, a crossed C multilayer, and an orderly C monolayer are made from C2H2, C2H4, and C2H6 and CH4, respectively, as ascertained partly in experiments. Tracing the RMD trajectories, we confirm that the pyrolysis of all four light hydrocarbons undergoes three stages, including the C chain elongation with generation of new small carbonaceous molecules or radicals, the formation and growth of polycyclic aromatic hydrocarbons, and the stable growth of C clusters. The morphologic difference of the final C clusters is attributed to the reactant activity and C growth styles. That is, the higher activity and the faster growth by the C-2 addition facilitate the more disorderly arrangement of C atoms, and vice versa. Typically, the dense C2H2 tends to form disorderly C black, while the thin CH4, to orderly C nanotubes. It shows that selecting the reactants in terms of their activities is a key to preparing orderly carbonaceous materials. These findings are expected to be useful to understand the formation mechanism and design techniques for efficiently manufacturing high-quality carbonaceous materials.