The effects of Al (111) and Al₂O₃/Al (111) surfaces on the decomposition of ammonium perchlorate: A DFT-D study

Ammonium perchlorate (AP) is a crucial oxidizing agent in composite solid propellants, where aluminum (Al) is used as an additive to increase the energy density of composite solid propellants. The interaction between AP and Al significantly improves the ignition and combustion performance of propellants, but the mechanism of their interactions still lacks essential understanding, especially the interaction between AP and Al, as well as the Al surface containing aluminum oxide (Al₂O₃). To solve this problem, the adsorption and decomposition processes of AP and its decomposition products such as perchloric acid (HClO₄) and ammonia (NH₃) on the pure-Al (111) and Al₂O₃/Al (111) surfaces were studied by using the density functional theory (DFT). The effects of temperature and pressure on the adsorption and reactions were discussed. AP tends to adsorb on the pure-Al (111) surface with O-termination, while under the influence of Al₂O₃, AP is more inclined to adsorb on the surface of Al₂O₃/Al (111). At the low-temperature stage, the Al₂O₃ molecules on the surface of Al (111) promote the disengagement H of AP decomposition and the adsorption and decomposition of NH₃, while at the high-temperature stage, the pure Al surface is more conducive to the AP disengagement H and NH₃ decomposition. The increase in the pressure is in favor of NH₃ adsorption, but the effect gradually decreases. HClO₄ cannot adsorb on the pure-Al (111) and AP (001) surfaces, but it can adsorb on the surface of Al₂O₃/Al (111). In the gas phase, the decomposition of HClO₄ priors to NH₃. At the high temperature and pressure, the pure-Al (111) surface can contribute significantly to the adsorption and activation of NH₃. The findings are very valuable for regulating the combustion of solid propellants.