Theoretical Study of PAH Growth by Phenylacetylene Addition

Although there was significant advancement on polycyclic aromatic hydrocarbon (PAH) formation, current mechanisms are still limited in providing an integrated and accurate scheme of PAH yield in combustion conditions; thus, a more detailed and comprehensive understanding is necessary. This work provides a systematic investigation of PAH growth by phenylacetylene addition. A combination of the density functional theory (DFT/B3LYP/6-311+G(d,p)) and the complete basis set method (CBS-QB3) is utilized to calculate the potential energy surfaces. The reaction system is initiated by the H elimination reaction of phenylacetylene + H → o-ethynylphenyl + H₂, and then, the addition reaction of phenylacetylene and o-ethynylphenyl can produce PAHs with one, two, three, and four rings. The temperature- and pressure-dependent reaction rate coefficients are calculated via a combination of conventional transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory with solving the master equation in the temperature range of 500-2500 K and at the pressure range of 0.01-10 atm. There are 263 species and 65 reactions in this reaction system. It shows that the rate constants of this reaction system are highly temperature-dependent and slightly sensitive to the pressure at temperatures lower than 1300 K. To evaluate the yield distributions of various PAH products in the whole reaction network, a closed 0-D batch reactor model in Chemkin is used to calculate the C₆H₅C₂H-C₂H₂-H-Ar reaction system. The results showed that the prevailing products of this system are three-ring PAHs with side chain structures. Compared with the traditional HACA pathways, the investigated reaction system presents higher efficiency in large PAH formations, which could subsequently promote the formation of soot particles. The phenylacetylene and o-ethynylphenyl reaction network emphasizes the importance of species with side chains, and it enriches current PAH growth pathways aside from the addition of small species such as C₂H₂.