Thermal decomposition of gaseous ammonium nitrate at low pressure: Kinetic modeling of product formation and heterogeneous decomposition of nitric acid

The thermal decomposition of ammonium nitrate, NH₄NO₃ (AN), in the gas phase has been studied at 423-56 K by pyrolysis/mass spectrometry under low-pressure conditions using a Saalfeld reactor coated with boric acid. The sublimation of NH₄NO₃ at 423 K was proposed to produce equal amounts of NH₃ and HNO₃, followed by the decomposition reaction of HNO₃, HNO₃ + M → OH + NO₂ + M (where M ) third-body and reactor surface). The absolute yields of N₂, N₂O, H₂O, and NH ₃, which can be unambiguously measured and quantitatively calibrated under a constant pressure at 5-6.2 torr He are kinetically modeled using the detailed [H,N,O]-mechanism established earlier for the simulation of NH ₃-NO₂ (Park, J.; Lin, M. C.Technologies and Combustion for a Clean Environment. Proc. 4th Int. Conf. 1997, 34-1, 1-5) and ADN decomposition reactions (Park, J.; Chakraborty, D.; Lin, M. C. Proc. Combust. Inst. 1998, 27, 2351-2357). Since the homogeneous decomposition reaction of HNO₃ itself was found to be too slow to account for the consumption of reactants and the formation of products, we also introduced the heterogeneous decomposition of HNO₃ in our kinetic modeling. The heterogeneous decomposition rate of HNO₃, HNO₃ + (B₂O ₃/SiO₂) → OH + NO₂ + (B₂O ₃/SiO₂), was determined by varying its rate to match the modeled result to the measured concentrations of NH₃ and H ₂O; the rate could be represented by k_2b ) 7.91 × 10⁷ exp(-12 600/T) s^-1, which appears to be consistent with those reported by Johnston and co-workers (Johnston, H. S.; Foering, L.; Tao, Y.-S.; Messerly, G. H. J. Am. Chem. Soc. 1951, 73, 2319-2321) for HNO ₃ decomposition on glass reactors at higher temperatures. Notably, the concentration profiles of all species measured could be satisfactorily predicted by the existing [H,N,O]-mechanism with the heterogeneous initiation process.