Pathways
to Soot Oxidation: Reaction of OH with Phenanthrene
Radicals
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Abstract
Energetics
and kinetics of the oxidation of possible soot surface
sites by hydroxyl radicals were investigated theoretically. Energetics
were calculated by employing density functional theory. Three candidate
reactions were selected as suitable prototypes of soot oxidation by
OH. The first two, OH + benzene and OH + benzene–phenol complex,
did not produce pathways that lead to substantial CO expulsion. The
third reaction, OH attack on the phenanthrene radical, had multiple
pathways leading to CO elimination. The kinetics of the latter reaction
system were determined by solving the master equations with the MultiWell
suite of codes. The barrierless reaction rates of this system were
computed using the VariFlex program. The computations were carried
out over the ranges 1500–2500 K and 0.01–10 atm. At
higher temperatures, above 2000 K, the oxidation of phenanthrene radicals
by OH followed a chemically activated path. At temperatures lower
than 2000 K, chemical activation was not sufficient to drive the reaction
to products; reaction progress was impeded by intermediate adducts
rapidly de-energizing before reaching products. In such cases, the
reaction system was modeled by treating the accumulating adducts as
distinct chemical species and computing their kinetics via thermal
decomposition. The overall rate coefficient of phenanthrene radical
oxidation by OH forming CO was found to be insensitive to pressure
and temperature and is approximately 1 × 10<sup>14</sup> cm<sup>3</sup> mol<sup>–1</sup> s<sup>–1</sup>. The oxidation
of phenanthrene radicals by OH is shown to be controlled by two main
processes: H atom migration/elimination and oxyradical decomposition.
H atom migration and elimination made possible relatively rapid rearrangement
of the aromatic edge to form oxyradicals with favorable decomposition
rates. The reaction then continues down the fastest oxyradical pathways,
eliminating CO