Mechanism and kinetics for ammonium perchlorate sublimation: A first-principles study

We have studied for the first time the kinetics and mechanism for the sublimation/decomposition of NH₄ClO₄ by first-principles calculations, using a generalized gradient approximation with the plane-wave density functional theory. Supercells containing 4, 8, and 16 NH ₄ClO₄ units were used; the predicted enthalpic change for solid NH₄ClO₄ to gaseous NH₃ and HClO ₄ is 45.0 ± 1.5 kcal/mol. The calculated desorption activation energies for NH₃, HClO₄, and H₃N⋯ HOClO₃ molecular complexes, individually, from the relaxed surface are 45.3, 43.5, and 28.1 kcal/mol, respectively. The rate constant for the dominant sublimation process desorbing H₃N⋯HOClO₃ as a pair can be presented by k_sub.= 6.53 × 10¹² exp (-28.8 kcal/mol/RT) s^-1, which is in reasonable agreement with available experimental data. Expectably, the decomposition of H ₃N⋯HOClO₃ (g) to NH₃ (g) and HOClO ₃ (g) is considerably faster, about 1 × 10⁷ times greater than that for the sublimation process in the same temperature range. The rate constant for the gas-phase dissociation step can be expressed by 1.20 × 10¹⁵ exp (-14.6 kcal/mol/RT) sec^-1. This study further confirms that the activation energy for the sublimation of an ammonium salt is significantly lower than the enthalpic change and that the molecular complex of acid and base sublimes concurrently as a pair.