Kinetic insights into PAHs growth via C3 Chemistry: the recombination of phenanthryl radicals and propyne

Abstract

Phenanthrene, a key polycyclic aromatic hydrocarbon with three fused aromatic rings, is produced during incomplete combustion of hydrocarbons and serves as a critical precursor to soot formation. A predictive investigation was conducted on recombination kinetics between phenanthryl radicals and propyne. Potential energy surfaces were constructed using PWPB95-D3/def2-QZVPP//M06–2X/6–311+G(d,p) calculations and rate constants were obtained by performing transition state theory-based master equation simulations. The main PAHs products of each phenanthryl/propyne system were identified and the competition between various channels was investigated across a wide range of temperatures and pressures. For 4-phenanthryl radical with “armchair” edge site, its recombination with propyne proceeds predominantly by a sequence of isomerization reactions of initial adduct followed by methyl-elimination, finally leading to pyrene. For 1-phenanthryl radical with “zigzag” edge site, at low temperature the isomerization sequence of initial adduct is primarily followed by H-elimination to generate the alkyl-substituted PAHs, i.e., 5- and 4-methylacephenanthrylene, while at high temperature β-scissions of initial adducts become dominant. For 2-phenanthryl radical with “free” edge site, initial adducts mainly occur via β-scission reactions to produce 2-(prop-1-ynyl)phenanthrene and 2-ethynylphenanthrene. Compared to the well-known HACA mechanism, rate constants of the phenanthryl/propyne recombination are larger than those with acetylene, suggesting the potential importance of propyne in phenanthrene growth. Kinetic modeling simulations were conducted for premixed laminar flames of three gasoline surrogate components (isooctane, n-heptane, and toluene) at an equivalence ratio of 1, to assess the role of propyne in phenanthrene growth. Rate-of-production (ROP) analysis reveals that reaction with propyne contributes more to the growth of phenanthrene than reaction with acetylene in isooctane flame and it exhibits a distinct fuel-structure dependence in flames. This study establishes a foundational kinetic framework for phenanthrene growth via propyne addition and offers valuable insights for further kinetic studies on PAHs formation.