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

The chemistry of first aromatic ring, i.e., benzene (C6 H6) and phenyl radial (C6 H5), 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 (C6 H5 NO), a clean precursor of C6 H5 radical. The reaction progress was monitored by detecting C6 H5 and NO simultaneously using visible laser absorption near 445 nm and mid-IR laser absorption near 5.517 μm, respectively. For the reaction C6 H5 NO → C6 H5 + 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 k∞1 (T(K)) = 3.2 × 1066T-15.2 e-37743/Ts-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, C6 H5 + C6 H5 → C6 H5 C6 H5 (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-1s-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 k∞2a (T = 300-1450 K) = 2.8 × 1017 T-1.44 e-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.