Unraveling the synergistic decomposition mechanisms of AP/RDX pyrolysis using online photoionization mass spectrometry and kinetic calculation

The combustion of composite solid propellants involves highly coupled multistep reactions and complex radical-mediated interactions that challenge mechanistic understanding and kinetic modeling, with the chemical interplay playing a critical role in governing the overall reaction pathways and product distributions. In this work, the pyrolysis behaviors of ammonium perchlorate (AP)/1,3,5-trinitro-1,3,5-triazine (RDX) composites were investigated at 873 K using photoionization time-of-flight mass spectrometry (PI-TOF-MS) with in-situ detection. The cross-reaction specie, ClCN, was detected in the co-pyrolysis products, confirming the existence of substantial chemical interactions between AP and RDX. Furthermore, the potential energy surfaces for the cross-reactions of RDX/INT175 with Cl/ClO/NH 2 and H-abstraction-driven β -scission reactions were calculated at the DLPNO-CCSD(T)/CBS(T-Q)//B3LYP/6-311++G(d,p) level of theory. The pressure-dependent rate constants were computed using RRKM/ME theory at 400–2000 K and 0.1–1000 atm. The Cl radical-initiated H-abstraction exhibits the highest rate constants, exceeding those of NH 2 and NO 2 by nearly three and nine orders of magnitude, respectively, indicating that AP strongly promotes RDX decomposition. The NO 2 elimination reactions of RDX-H and INT174 are markedly faster than the ring-opening channels, with the disparity being particularly pronounced at low temperatures. The mechanistic insights provide a fundamental basis for understanding the synergistic decomposition behavior of AP/RDX systems and for guiding the design of next-generation high-performance propellants. Novelty and significance statement This work provides experimental and mechanistic insights into the synergistic decomposition of AP/RDX composites. Using PI-TOF-MS with online detection, Cl-containing C-N cross-species (ClCN) are identified for the first time in co-pyrolysis products, directly evidencing chemical interactions between AP and RDX. High-level DLPNO-CCSD(T)/CBS//B3LYP/6-311++G(d,p) calculations reveal that Cl-radical-mediated hydrogen abstraction is the dominant initiation pathway, with rate constants exceeding those of NH 2 and NO 2 by up to nine orders of magnitude, underscoring the catalytic role of AP in promoting RDX decomposition. The comprehensive pressure- and temperature-dependent kinetics of radical-induced β -scission reactions provide a robust framework for accurate modeling of composite propellant combustion, offering critical guidance for the rational design of next-generation energetic materials.