Kinetics of Excited OH Formation in H + N₂O and NH + NO Reactions at High Temperatures

The reactions H + N₂O → N₂ + OH* (R1) and NH + NO → N₂ + OH* (R2) have been proposed as sources of the electronically excited OH(A²Σ⁺) radicals, denoted as OH*, during combustion of nitrogen-containing compounds. The present study investigates the kinetics of these reactions through OH* emission experiments using a shock tube. Time profiles of OH* concentration measured for several shock-heated mixtures are analyzed with the help of kinetic simulations. The rate constants for R1 are determined using the profiles obtained from H₂/N₂O/Ar, NH₃/N₂O/Ar, and C₂H₅I/N₂O/Ar mixtures and can be expressed as k₁ = (8.4 ± 3.8) × 10^-11 exp(-22900 K/T) cm³ molecule^-1 s^-1 over the temperature range of T = 1600-2200 K. For R2, the rate constants are obtained from the measurements for NH₃/NO/Ar and NH₃/N₂O/NO/Ar mixtures and are represented as k₂ = (1.6 ± 0.8) × 10^-13 exp(-10040 K/T) cm³ molecule^-1 s^-1 over 2000-2600 K. Possible mechanisms for the formation of OH* are discussed based on a comparison of the rate constants.