The Identity and Chemistry of C7H7 Radicals Observed during Soot Formation

James A Rundel; Carles Martí; Judit Zádor; Paul E Schrader; K Olof Johansson; Ray P Bambha; Grant T Buckingham; Jessica P Porterfield; Oleg Kostko; Hope A Michelsen

doi:10.1021/acs.jpca.2c08949

The Identity and Chemistry of C₇H₇ Radicals Observed during Soot Formation

J Phys Chem A. 2023 Apr 6;127(13):3000-3019. doi: 10.1021/acs.jpca.2c08949. Epub 2023 Mar 10.

Authors

Affiliations

¹ Department of Mechanical Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States.
² Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550, United States.
³ Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States.
⁴ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
⁵ Environmental Engineering Program, University of Colorado Boulder, Boulder, Colorado 80309, United States.

PMID: 36897578
DOI: 10.1021/acs.jpca.2c08949

Abstract

We used aerosol mass spectrometry coupled with tunable synchrotron photoionization to measure radical and closed-shell species associated with particle formation in premixed flames and during pyrolysis of butane, ethylene, and methane. We analyzed photoionization (PI) spectra for the C₇H₇ radical to identify the isomers present during particle formation. For the combustion and pyrolysis of all three fuels, the PI spectra can be fit reasonably well with contributions from four radical isomers: benzyl, tropyl, vinylcyclopentadienyl, and o-tolyl. Although there are significant experimental uncertainties in the isomeric speciation of C₇H₇, the results clearly demonstrate that the isomeric composition of C₇H₇ strongly depends on the combustion or pyrolysis conditions and the fuel or precursors. Fits to the PI spectra using reference curves for these isomers suggest that all of these isomers may contribute to m/z 91 in butane and methane flames, but only benzyl and vinylcyclopentadienyl contribute to the C₇H₇ isomer signal in the ethylene flame. Only tropyl and benzyl appear to play a role during pyrolytic particle formation from ethylene, and only tropyl, vinylcyclopentadienyl, and o-tolyl appear to participate during particle formation from butane pyrolysis. There also seems to be a contribution from an isomer with an ionization energy below 7.5 eV for the flames but not for the pyrolysis conditions. Kinetic models with updated and new reactions and rate coefficients for the C₇H₇ reaction network predict benzyl, tropyl, vinylcyclopentadienyl, and o-tolyl to be the primary C₇H₇ isomers and predict negligible contributions from other C₇H₇ isomers. These updated models provide better agreement with the measurements than the original versions of the models but, nonetheless, underpredict the relative concentrations of tropyl, vinylcyclopentadienyl, and o-tolyl in both flames and pyrolysis and overpredict benzyl in pyrolysis. Our results suggest that there are additional important formation pathways for the vinylcyclopentadienyl, tropyl, and o-tolyl radicals and/or loss pathways for the benzyl radical that are currently unaccounted for in the present models.