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 NH4ClO4 units were used; the predicted enthalpic change for solid NH4ClO4 to gaseous NH3 and HClO4 is 45.0 +/- 1.5 kcal/mol. The calculated desorption activation energies for NH3, HClO4, and H3N center dot center dot center dot 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 center dot center dot HOClO3 as a pair can be presented by k(sub.)= 6.53 x 10(12) exp (-28.8 kcal/mol/RT) s(-1), which is in reasonable agreement with available experimental data. Expectably, the decomposition of H3N center dot center dot center dot HOClO3 (g) to NH3 (g) and HOClO3 (g) is considerably faster, about 1 x 10(7) 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 x 10(15) 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.