Thermal Decomposition Mechanism of Aqueous Hydroxylammonium Nitrate (HAN): Molecular Simulation and Kinetic Modeling

A detailed mechanism has been developed for thermal decomposition of hydroxylammonium nitrate (HAN) solutions, based on quantum mechanical calculations using the SMD-ωB97X-D method. The mechanism describes multiple kinetic processes, including nitration and nitrosation of hydroxylamine, HNO dimerization, and HONO-regeneration pathways involving H-abstraction reactions. Rate constants of elementary reactions were estimated using transition state theory with consideration of species' diffusion effect. Kinetic modeling was performed to predict species' evolutions in 0.1 m HAN in the temperature range of 463-523 K, and results show reasonable agreement with the experimental data from flow reactor studies. For more concentrated solutions, strong autocatalytic behaviors were predicted with the late emergence of NO₂ and HONO, whose regeneration was previously considered as the major autocatalytic pathway. Sensitivity analysis results suggest an acid-catalyzed nitration-nitrosation pathway, based on which the autocatalysis should be caused by the rise of solution acidity. A linear correlation can be observed in the previously reported apparent Arrhenius parameters, which may be reconciled via a kinetic compensation effect.