Photolytically and thermally initiated reactions of NH₃ with NO_x (X=1,2)

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 NH ₂+NO_x (x=1, 2) reactions have been tested by measuring the time-resolved concentrations of H₂O, N₂O, and NO ₂ in the laser-initiated reaction of NH₃ with NO in the presence of varying amounts of NO₂ in the temperature range of 300-725K. 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→N ₃O+→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 575-710K, 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 NH ₃ 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 NH ₃ with NO₂.