Up in Flames: Phenyl Oxidation Product Distribution

Researchers determine the reaction pathway to how soot and other toxic components form in combustion systems.

Image courtesy of R.I. Kaiser
To help identify routes to mitigate toxic polycyclic aromatic hydrocarbons and soot formation from combustion engines, scientists identified the full list of products in a key reaction between phenyl radicals and oxygen.

The Science

Researchers uncover the full suite of products formed during the reaction of detrimental phenyl radicals with molecular oxygen that make up the toxic polycyclic aromatic hydrocarbons and soot in combustion engines. A hot nozzle simulating combustion conditions was coupled to the Advanced Light Source at Berkeley synchrotron enabling detection of the specific reaction products, from which a reaction mechanism is now elucidated.

The Impact

Understanding how phenyl radicals react with oxygen is expected to lead toward the elimination of carcinogenic, mutagenic, and environmentally hazardous byproducts of combustion systems such as polycyclic aromatic hydrocarbons (PAHs). By knowing the nature of the products made in this fundamental combustion reaction, models can be developed to find ways to mitigate soot formation.


As fossil fuels are burned benzene – an aromatic, ring-shaped molecule - forms a radical, called phenyl, by losing a hydrogen atom. In the hot combustion environment, the phenyl radical reacts with unburnt fuel components to form toxic polycyclic aromatic hydrocarbons (PAHs) and soot.  Despite intense theoretical and experimental scrutiny over half a century, the important role of the phenyl radical (C6H5) reacting with molecular oxygen (O2) in the degradation of poly- and mono- cyclic aromatic radicals in combustion systems reaction have not all been experimentally identified. Researchers at Lawrence Berkeley National Lab and the University of Hawaii have now determined the outcome of this fundamental combustion reaction using tunable vacuum ultraviolet photoionization in conjunction with a combustion simulating chemical reactor.  In the reaction of phenyl radicals and molecular oxygen at 873 K and 1,003 K, ortho-benzoquinone (o-C6H4O2), the phenoxy radical (C6H5O), and cyclopentadienyl radical (C5H5), were identified as primary products formed through emission of atomic hydrogen, atomic oxygen and carbon dioxide. Furan (C4H4O), acrolein (C3H4O), and ketene (C2H2O) were also identified as primary products formed through ring opening and fragmentation of the 7-membered ring 2-oxepinoxy radical. Secondary reaction products para-benzoquinone (p-C6H4O2), phenol (C6H5OH), cyclopentadiene (C5H6), 2,4-cyclopentadienone (C5H4O), vinylacetylene (C4H4), and acetylene (C2H2) were also identified. The pyranyl radical (C5H5O) was not detected, however, electronic structure calculations show that it is formed and isomerizes to 2,4-cyclopentadienone through atomic hydrogen emission. In combustion systems, barrier-less phe­nyl-type radical oxidation reactions could even degrade more complex aromatic radicals.


Ralf I. Kaiser
Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI
(808) 956-5731

Musahid Ahmed
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
(510) 486-6355

Alexander M. Mebel
Department of Chemistry and Biochemistry, Florida International University, Miami, FL


This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Award Number DE-FG02-03ER15411 to the University of Hawaii and Award Number DE-FG02-04ER15570 to Florida International University. The authors MA, OK and TPT, and the Advanced Light Source at Lawrence Berkley National Laboratory are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Contract Number DE-AC02-05CH11231. A.M.M. would like to acknowledge the Instructional & Research Computing Center (IRCC, web: http://ircc.fiu.edu) at Florida International University for providing high performance computing (HPC) resources that have contributed to the research results reported.


D. S. N.  Parker, R. I. Kaiser, T. P. Troy, O. Kostko, M. Ahmed and A. M. Mebel, “Toward the oxidation of the phenyl radical and prevention of PAH formation in combustion systems.” Journal of Physical Chemistry A, Article ASAP.  [DOI: 10.1021/jp509170x]

Related Links

Chemical Dynamics Beamline Website

Reaction Dynamics Group Website at University of Hawaii at Manoa

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