Direct measurement of the NH3+OH reaction rate behind incident and reflected shock waves

Abstract

A novel method was used to directly measure the reaction rate, k₁, of NH₃+OH<=>NH₂+H₂O in shock tube experiments behind incident and reflected shock waves from 910–2474 K and 0.23–3.59 atm. NH₃ concentration of test gases was measured prior to each shock with a scanned laser absorption NH₃ diagnostic near 10.36 µm. OH was produced via thermal decomposition of tert‑butyl hydroperoxide behind incident and reflected shock waves, and post-shock OH time-histories were measured via laser absorption at 308.6 nm. Measured OH profiles were fit with a detailed chemical kinetic model to find best-fit values for k₁ at each experimental condition, and results are compared to previous data, calculations, and recommendations for the NH₃+OH reaction rate. To the authors’ knowledge, this is the first direct measurement of the NH₃+OH reaction rate above 1425 K and significantly reduces the uncertainty of k₁ compared to previous indirect determinations at high temperatures. A recommendation is made for continued use of the NH₃+OH rate expression k₁ = 10^6.31 T[K]^2.04 exp(-285/T[K]) cm³/ mol/s suggested by Salimian et al. from 230 < T < 2474 K, which agrees well with the current data and prior low-temperature measurements. The technique used in this work also provides a new strategy for direct measurement of +OH reaction rates at reflected-shock temperatures above ∼1450 K, which has previously been a practical high-temperature limit when using tert‑butyl hydroperoxide as a source of OH radicals.