Experimental and theoretical evidence for the temperature-determined evolution of PAH functional groups

Abstract

Elucidating the chemical evolution of various functional groups in polycyclic aromatic hydrocarbons (PAH) and soot aids in understanding soot formation chemistry. In this work, the chemical evolution of various functional groups, including aromatic CH, aliphatic CH, C=O, COH and COC bonds, was experimentally investigated online, rather than with offline diagnostics. Oxidation was performed in a jet-stirred reactor (JSR), fueled with benzene/C2H2/air/N2 and benzene/phenol/C2H2/N2 for a temperature range of 600-1400 K. Kinetic modelling, including ab initio quantum chemistry calculations, reaction rate coefficient calculations and JSR simulations, were conducted to interpret the experimental data and the evolutionary chemistry of the various functional groups. Results show that the formation of functional groups on PAH and oxygenated PAH (OPAH) are highly sensitive to temperature. Aliphatic CH bonds survive mainly in the form of CCH2C, CCH2CH2C or CCH functional groups above 1200 K, and exist in the CHCH2 functional group below 1000 K. For the OPAH, the COC functional group presents stronger thermal stability than COH and C=O functional groups. Simulation results indicate that HACA-like pathway (hydrogen abstraction carbon addition), in which C2H2 attacks the O atom, followed by cyclization and H-atom elimination reactions, qualitatively describe the formation of OPAH with the CO-C functional group at different temperatures. The addition reaction involving PAH radical and C2H4 / C2H3 captures the evolution of PAH with the CHCH2 functional group, but fails to explain the formation of CCH2C and CCH2CH2C functional groups.