title: Importance of fundamental sp, sp2, and sp3 hydrocarbon radicals in the growth of polycyclic aromatic hydrocarbons.
abstract: The most basic chemistry of products formation in hydrocarbons pyrolysis has been explored via a comparative experimental study on the roles of fundamental sp, sp(2), and sp(3) hydrocarbon radicals/intermediates such as ethyne/ethynyl (C(2)H(2)/C(2)H), ethene/ethenyl (C(2)H(4)/C(2)H(3)), and methane/methyl (CH(4)/CH(3)) in products formations. By using an in situ time-of-flight mass spectrometry technique, gas-phase products of pyrolysis of acetylene (ethyne, C(2)H(2)), ethylene (ethene, C(2)H(4)), and acetone (propanone, CH(3)COCH(3)) were detected and found to include small aliphatic products to large polycyclic aromatic hydrocarbons (PAHs) of mass 324 amu. Observed products mass spectra showed a remarkable sequence of mass peaks at regular mass number intervals of 24, 26, or 14 indicating the role of the particular corresponding radicals, ethynyl (C(2)H), ethenyl (C(2)H(3)), or methyl (CH(3)), in products formation. The analysis of results revealed the following: (a) product formation in hydrocarbon pyrolysis is dominated by hydrogen abstraction and a vinyl (ethenyl, C(2)H(3)) radical addition (HAVA) mechanism, (b) contrary to the existing concept of termination of products mass growth at cyclopenta fused species like acenaphthylene, novel pathways forming large PAHs were found succeeding beyond such cyclopenta fused species by the further addition of C(2)H(x) or CH(3) radicals, (c) production of cyclopenta ring-fused PAHs (CP-PAHs) such as fluoranthene/corannulene appeared as a preferred route over benzenoid species like pyrene/coronene, (d) because of the high reactivity of the CH(3) radical, it readily converts unbranched products into products with aliphatic chains (branched product), and (e) some interesting novel products such as dicarbon monoxide (C(2)O), tricarbon monoxide (C(3)O), and cyclic ketones were detected especially in acetone pyrolysis. These results finally suggest that existing kinetic models of product formation should be modified to include the reported novel species and their formation pathways. It is expected that outcomes of this study will be useful to understand the products formation from reactors to interstellar atmospheres as well as the growth mechanism of carbon nanomaterials.
authors.affiliation: Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453, United States. bikaushu@usc.edu
authors.forename: Bikau
authors.initials: B
authors.lastname: Shukla
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Koshi
mesh_terms: D000096:Acetone / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D005740:Gases; D006358:Hot Temperature; D007700:Kinetics; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D005609:Free Radicals; D005740:Gases; D011084:Polycyclic Aromatic Hydrocarbons; D000096:Acetone