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 NH 4 ClO 4 by first-principles calculations, using a generalized gradient approximation with the plane-wave density functional theory. Supercells containing 4, 8, and 16 NH 4 ClO 4 units were used; the predicted enthalpic change for solid NH 4 ClO 4 to gaseous NH 3 and HClO 4 is 45.0 ± 1.5 kcal/mol. The calculated desorption activation energies for NH 3, HClO 4, and H 3 N···HOClO 3 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 H 3 N···HOClO 3 as a pair can be presented by k sub. = 6.53 × 10 12 exp (−28.8 kcal/mol/RT) s −1, which is in reasonable agreement with available experimental data. Expectably, the decomposition of H 3 N···HOClO 3 (g) to NH 3 (g) and HOClO 3 (g) is considerably faster, about 1 × 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 × 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.