Mechanism development of aqueous hydroxylammonium nitrate under thermal decomposition conditions

A detailed chemical mechanism was recently developed for aqueous hydroxylammonium nitrate (HAN), a potential green propellant material, based on theoretical quantum mechanical (QM) calculations. The ωB97X-D density functional theory (DFT) with the SMD solvation model was implemented to optimize the molecular geometries, locate transition states, and compute the solution-phase free energies. The mechanism includes the nitration reactions between hydroxylamine and nitric acid, the subsequent nitrosation reactions between hydroxylamine and HONO, and the autocatalytic steps of H-abstraction by NO₂. To examine the mechanism, the kinetic modeling was performed with rate constants predicted by the conventional transition-state theory (CTST) and with consideration to effects from diffusion. The kinetic modeling predicted that the activation energy for 0.1 m HAN is 109 kJ/mol, compared to the experimentally reported 103±21 kJ/mol. NO₂, the major H-abstraction agent, only evolves substantially after over 99.9% of the hydroxylamine is consumed. The kinetic simulation also supports that the decomposition can be greatly accelerated with excess nitric acid, but largely suppressed with insufficient acid or excess hydroxylamine.