Growth of polycyclic aromatic hydrocarbons (PAHs) by methyl radicals: Pyrene formation from phenanthrene

The formation of polycyclic aromatic hydrocarbons (PAHs) in high temperature environments is an ongoing area of research due to the mismatch between the experimental and simulated concentration profiles of PAHs in several flames. In this work, the role of methyl radicals in the growth of PAHs is identified by developing a detailed reaction mechanism for the conversion of phenanthrene to pyrene by methyl radicals. The reaction energetics are obtained through quantum calculations using B3LYP and M06-2X functionals along with 6-311++G(d,p) basis set, and are used to compare the competing channels for pyrene formation. The transition state theory is used to determine the reaction kinetics. Through kinetic simulations, the most preferred path for pyrene formation from phenanthrene is determined. To quantify the contribution of the newly found reactions in PAH growth in the presence of other competing PAH mechanisms, the reactions are merged with a recently developed and well-validated mechanism for C1C4 hydrocarbons with detailed PAH chemistry. Three premixed laminar aliphatic flames were simulated with the updated mechanism. It was observed that the new reactions did not appreciably influence the computed profiles of phenanthrene and pyrene. Through the rate-of-production analysis, it was found that the phenanthrene radicals created through H-abstraction were more susceptible to attack by C2H2 than by methyl radicals. The indirect role of methyl radicals in PAH formation and growth is shown.