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

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.