Shock tube determination of the overall rate of NH₂ + NO → products at high temperatures

The rate coefficient of the reaction NH₂ + NO → products (R1) was determined in shock tube experiments using frequency modulation absorption spectroscopy for detection of NH₂. Due to the sensitivity of the diagnostic system, very low reactant concentrations could be employed to reduce the influence of secondary reactions on the NH₂ profiles. The source of the NH₂ radicals in the experiments was the thermal decomposition of CH₃NH₂. To determine κ1, a perturbation strategy was employed that is based on changes in the NH₂ profiles when NO is added to the CH₃NH₂/Ar mixtures. Analysis showed that NH₂ profiles in the CH₃NH₂/NO/Ar mixtures were sensitive primarily to reaction R1, with significantly lower sensitivity to the branching ratio and other NH₂ reactions. The measured NH₂ profiles were interpreted by detailed kinetic modeling to obtain κ₁ values in the temperature range 1716-2507 K. The present κ₁ values are consistent with recent theoretical results of Miller and coworkers. There is no evidence for a positive activation energy for this reaction at elevated temperatures as reported in several other high-temperature experimental studies. Combining the present high-temperature data with lower temperature determinations yields the following simple expression for the overall reaction rate, κ ₁= 2.08 × 10¹⁶ T^-1.34 cm³/mol s for the temperature range 300-2500 K.