Role of Nitrogen Sites in Inhibiting the Growth of Nitrogen-containing Polycyclic Aromatic Compounds: A Theoretical Study and Soot Dynamic Model Improvement

Although nitrogen-containing species are abundant intermediates in ammonia-doped hydrocarbon flame, the inhibitory effects of nitrogen-containing species on the formation of polycyclic aromatic hydrocarbons (PAHs) and soot have not been fully revealed yet. In this work, the reaction pathways for the formation of nitrogen-containing polycyclic aromatic compounds (NPACs) from methylenimine (CH₂NH) with benzene, naphthalene, and biphenyl were constructed to reveal the inhibitory effect of nitrogen sites introduced by nitrogen-containing species on the growth pathways of PAHs and soot. The G3(MP2,CC)//B3LYP method were employed to calculate the electronic structures and potential energy surfaces (PES). The Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory methods were employed to calculate rate constants. PES analysis revealed that the reactions at the nitrogen site exhibit higher energy barriers compared to those at the carbon site. Rate constants analysis further indicated that the substitution and cyclization processes of the nitrogen sites of NPACs usually have a reaction rate constant of 1-2 orders of magnitude lower than that on the carbon sites, that is, the nitrogen sites exhibit significantly low reactivity during the growth process of NPACs. As a unique site in NPACs, the low-reactivity nitrogen site hinders the bonding of hydrocarbon molecules at this site, thereby inhibiting the growth of NPACs. Based on the above conclusions, a new soot dynamic model incorporating nitrogen chemistry was proposed. In this model, nitrogen-containing species present in high concentrations (NH₃ , HCN, and CH₂​NH) generate nitrogen-containing functional groups with low-reactivity nitrogen sites on the soot surface through direct chemical reactions. Simulation results show that the new soot model significantly improves the prediction accuracy of soot reduction, with errors reduced by up to 80%. The predicted nitrogen-containing functional groups and nitrogen content of soot are also within the reasonable range supported by experimental data. This underscores the importance of accounting for the direct chemical effects of nitrogen-containing species in the soot model, and the low reactivity of nitrogen sites observed in this study serves as a theoretical foundation for inserting the inhibitory mechanism of nitrogen-containing functional groups on soot growth in the soot dynamic model.