A high temperature shock tube study of phenyl recombination reaction using laser absorption spectroscopy

H. Jin, B. R. Giri, D. Liu, A. Farooq
Proceedings of the Combustion Institute, In Press, (2020)

Keywords

Phenyl radical, Shock tube, Laser absorption spectroscopy, Absorption cross-sections, Rate coefficients

Abstract

​The chemistry of first aromatic ring, i.e., benzene (C6H6) and phenyl radial (C6H5), plays a key role in the growth of polycyclic aromatic hydrocarbons (PAHs) and ultimately soot formation. In this work, the self-recombination reaction of phenyl radicals was investigated over the temperature range of 950–1300 K and pressures near 1 atm by employing shock tube and laser absorption diagnostics. Phenyl radical was generated by the rapid thermal unimolecular dissociation of nitrosobenzene (C6H5NO), a clean precursor of C6H5 radical. The reaction progress was monitored by detecting C6H5 and NO simultaneously using visible laser absorption near 445 nm and mid-IR laser absorption near 5.517 µm, respectively. For the reaction C6H5NO → C6H5 + NO (R1), our data show an excellent agreement with earlier reports. The high-pressure limiting rate coefficient, by combining all available data, may be expressed as k1∞(T(K))=3.2×1066T−15.2e−37743/T s⁻¹. This work reports the temperature dependence of the absorption cross-section of phenyl radical at 445 nm for first time. Our experiments indicate that the self-reaction of phenyl radicals yielding biphenyl, C6H5 + C6H5 → C6H5C6H5 (R2a), is a major channel. The rate coefficients of reaction (R2a) show a weak temperature dependence with an average value of k2a = (6.91 ± 0.42) × 10¹² cm³ mol⁻¹ s⁻¹ in the temperature range of 950–1300 K. Our measured data, k2a(T, P = 1.1–1.5 atm), are found to be close to the high-pressure limiting rate coefficients. Combining with the literature low-temperature data, the self-recombination reaction of phenyl radicals may be expressed as k2a∞(T=300−1450K)=2.8×1017T−1.44e−540/T cm³ mol⁻¹ s⁻¹. The measurements of this study represent the first high-temperature direct experimental determination of the rate coefficients of this important prototype aromatic radical-radical reaction.

Code

DOI: 10.1016/j.proci.2020.06.164

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