Photolytically and Thermally Initiated Reactions of NH₃ with NO_x (X = 1,2)

Abstract

The kinetics and mechanisms of the NH₃ + NO_x (x = 1, 2) reactions have been systematically studied by pulsed laser photolysis/mass spectrometry (PLP/MS) and pyrolysis/FTIR spectrometry (P/FTIRS). In the PLP/MS experiment, the total rate constants and product branching ratios of the NH₂ + NO_x (x = 1, 2) reactions have been tested by measuring the time-resolved concentrations of H₂O, N₂O, and NO₂ in the laser-initiated reaction of NH₃ with NO in the presence of varying amounts of NO₂ in the temperature range of 300–725 K. The measured concentrations can be quantitatively accounted for by the authors' comprehensive mechanism for the H/N/O system recently established for the ammonium dinitramide decomposition reactions, confirming the authors' reported product branching ratio for NH₂ + NO₂ → N₂O + H₂O (0.19 ± 0.02) over the temperature range investigated. In the thermally initiated reaction of NH₃ with NO_x investigated by the P/FTIRS technique in the temperature range of 575–710 K, the measured kinetic data for the disappearance of the reactants and the formation of products have been modeled with the same mechanism applied to the PLP/MS study. Agreement between the kinetically modeled concentrations and the experimentally measured values was quite satisfactory for the reactants (NH₃ and NO_x) and products (NO and N₂O). Sensitivity analyses showed that the reaction of NO_x with NH₃ is most sensitive to the NH₂ + NO_x reactions, with a slightly lower sensitivity to subsequent reactions involving HONO. The authors' comprehensive mechanism also accounts reasonably for the concentration profiles of NH₃, NO₂, and N₂O reported by Glarborg et al. (1994) in their study of the isothermal reaction of NH₃ with NO₂.

Keywords:

• Kinetics
• NH₃
• NO_x
• Rate constant
• Thermal deNO_x

Notes

Note. Product yields were measured at t = 15 ms at their plateau regions. Typically 3–5 runs were carried out for each temperature.

Note. Rate constants, defined by k = AT ⁿ exp(−E _a /RT), are given in units of cm³, mole, and s; E _a is in units of cal/mole. The complete list of reactions used for kinetic modeling is given in elsewhere (Park & Lin, 2009).