Probing the Reaction of Propargyl Radical with Molecular Oxygen by Synchrotron VUV Photoionization Mass Spectrometry

Self-reaction of propargyl (C₃H₃) radical is the main pathway to benzene, the formation of which is the rate-controlling step toward the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. Oxidation of C₃H₃ is a promising strategy to inhibit the formation of hazardous PAHs and soot. In the present study, we studied the C₃H₃ + O₂ reaction from 650 to 1100 K in a laminar flow reactor and identified the intermediates and products by synchrotron VUV photoionization mass spectrometry. 2-Propynal, ethenone, formaldehyde, CO, CO₂, C₂H₂, C₂H₄, and C₃O₂ were identified. Among them, 2-propynal, ethenone, and formaldehyde provided direct evidence for the branching reaction of C₃H₃ + O₂ → HCCCHO + OH, C₃H₃ + O₂ → H₂CCO + CHO, and C₃H₃ + O₂ → H₂CO + CHCO, respectively. Potential energy surface calculation and mechanistic analysis of the C₃O₂ formations implied that C₃H₃ + O₂ → CCCHO + H₂O and C₃H₃ + O₂ → HCCCO + H₂O could occur, despite lacking direct observations of CCCHO and HCCCO radicals. The formation of ethenone and CO suggested the occurrence of the two CO elimination channels. We incorporated these validated reactions and the corresponding rate coefficients in the kinetic model of NUIGMech1.3, and the simulation showed obvious improvements toward the measured mole fractions of C₃H₃ and H₂CCO, suggesting that the new C₃H₃ + O₂ reaction channels were crucial in the overall combustion modeling of the important intermediate propyne (C₃H₄).