Unraveling the Pyrolysis Kinetics of Ammonium Perchlorate (AP) Using Synchrotron Radiation Photoionization Mass Spectrometry

Understanding the pyrolysis mechanism of ammonium perchlorate (AP) is critical, as it is the major oxidizer of composite solid propellants. In this study, Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry (SVUV-PIMS) was employed to diagnose AP pyrolysis from 250°C to 550°C at photon energies of 11 and 14 eV. The gaseous products formed during the pyrolysis of AP were characterized, covering key intermediates involved in nitrogen, chlorine, and nitrogen-chlorine cooperative reactions, such as NH3, NO, HNO, N2O, NO2, HCl and Cl2. The identification of HN3 and NH2O2 expanded upon the products identified by previous studies. Based on the initial formation temperatures of the products and the dominant reactions, the pyrolysis of AP can be divided into three interrelated stages: the low- (260–330°C), the intermediate- (330–400°C) and the high-temperature decomposition stage (400–550°C). The newly identified species and their associated reaction pathways have been introduced to the existing mechanisms. The updated mechanism was validated, and the results at 550°C and 1.0 atm indicated critical roles of the aforementioned species in the AP pyrolysis. Based on experimental observations and kinetic analyses, the multi-stage pyrolysis reaction pathway of AP has been updated, laying a foundation for the development of a high-precision, experimentally constrained AP reaction kinetic model. Novelty and significance statement: In this study, SVUV-PIMS approach was employed to diagnose the AP pyrolysis process using different photon energies. The pyrolysis process can be divided into three interrelated stages according to temperature evolution of major species, including NH3, NO, HNO, N2O, NO2, HCl and Cl2, which have been previously identified. By introducing newly found species such as HN3 and NH2O2 into the existing AP pyrolysis mechanism, a more complete reaction pathway spanning from the initial proton transfer to the formation of final products was constructed. Building on previous experimental and theoretical studies, this study unravels the pyrolysis kinetics of pyrolysis.