Mechanism and reaction kinetics analysis of small-molecule gas formation during thermal decomposition of hydroxylamine nitrate

Hydroxylamine nitrate (HAN) is a new type of high-energy oxidant used in controllable propulsion. The present study explores the reaction mechanism of HAN using density functional theory and constructs reaction mechanism diagrams to understand the mechanism of small-molecule gas formation during the thermal decomposition of HAN. Based on transition state theory, the half-life of each reaction is calculated under standard conditions and the kinetic parameters of each reaction are scanned across the temperature range 298.15–1200 K. The research revealed a vast half-life time scale forreaction of NO generation, NO₂ generation, NO+NO₂ generation, N₂O generation and N₂ generation process, meaning thatthese reaction can not occur. The half-life of RDS5 is very short. However, the reaction is also limited by the concentration of the reactant HNO. However, increasing the temperature rapidly decreases the reaction half-life and the reaction can easily proceed. Taking 300 s as the easily reactive boundary point, the cut-off points of the rate-determining steps of Processes 1 (NO generation), 2 (NO₂ generation), 3 (NO + NO₂ generation), 4 and 5 (N₂O generation) and 6 (N₂ generation) are 466 K, 468 K, 576 K, 587 K and 402 K, respectively. The calculated reaction mechanism revealed two isomeric transformations of HNO₂ and H₂N₂O₂ and three isomeric transformations of H₂N₂O. Both HNO₂ and H₂N₂O₂ are transformed via H-atom transfer, while H₂N₂O is transformed either by H-atom transfer or intramolecular rotation. As the formation reaction of NO₂ does not have the lowest free energy, the NO₂ product is easily converted to other products. When NO₂ coexists with NO, it is also easily converted to the stable products N₂O, N₂ and NH₃.