Role of nitrogen-containing species in the structural regulation and growth inhibition of polycyclic aromatic hydrocarbons: A ReaxFF molecular dynamics study

Abstract

This study investigated the role of nitrogen-containing species in the structural regulation and growth inhibition of polycyclic aromatic hydrocarbons (PAHs) under high-temperature conditions through reactive force field molecular dynamics (ReaxFF MD) simulations. The findings revealed that nitrogen-containing species not only effectively inhibit the cyclization process of PAHs but also promote the formation of long branched carbon chains. This regulatory mechanism alters the structural characteristics of PAHs, thereby inhibiting the growth of PAHs. Specifically, during the mass growth of clusters, the nitrogen-containing radicals react with propargyl radical (C₃H₃) to generate nitrogen-containing hydrocarbon precursors that actively participate in clusters formation, facilitating the transition from small clusters to larger clusters, which consequently shortens the growth time of clusters. Moreover, the introduction of nitrogen-containing radicals leads to a 50 % increase in the total number of carbon atoms in C₁₆₊ clusters relative to the C₃H₃ system, while the number of clusters dramatically decreases by 58.3 %. Structural analysis indicates that the total number of rings in PAHs within the nitrogen-containing species system decreases by as much as 80.8 % compared to the C₃H₃ system, and the PAH growth inhibition factor increases by as much as 5.3 times. This suggests that nitrogen-containing species have a significant inhibition effect on the growth of PAHs. Additionally, molecular trajectory analysis further revealed that the instability of nitrogen heterocyclic structures at high temperatures promotes the occurrence of ring-opening reactions, leading to the nitrogen-containing portions of PAHs predominantly existing in chain form. The formation of long carbon chains, compared to cyclic structures, facilitates the combination of carbon-hydrogen small molecules, effectively explaining the promoting effect of nitrogen-containing species on the mass growth of clusters. Overall, nitrogen-containing species exhibit significant reactivity in high-temperature environments that reduce the reaction frequency of the transformation from C₃H₃ to PAHs, effectively inhibit the cyclization of PAHs and promote the formation of branched chains. Furthermore, a looser structure with more branched chains facilitates oxidation reactions, thereby further inhibiting the growth of PAHs. This study offers important insights into the understanding of the mechanism by which ammonia inhibits the growth of PAHs in ammonia-doped hydrocarbon flames.