Density Functional Theory Study on Thermal Decomposition Mechanisms of Ammonium Perchlorate

The thermal decomposition characteristics of ammonium perchlorate (AP) have a great influence on the performance of solid propellant. Although a large number of mechanistic studies have been published over the past few decades, there is no unified understanding of the decomposition yet, and the overall mechanism pathway is still unclear. In the present work, the thermal decomposition pathways of AP were studied systematically using broken-symmetry density functional theory method (BS-UB3LYP/6-311+G(d,p). This method can describe the homo-cleavage process of covalent bond well, and locate the transition state with singlet-diradical characteristics. Compared with typical multireference method, broken-symmetry density functional theory (BS-UDFT) can give good results and is much faster, and therefore is highly convenient for practical application. The results show that, the overall thermal decomposition pathway under the experimental conditions is initiated by the proton transfer between NH4+ cation and ClO4- anion, leading to neutral NH3 and HClO4 molecules, which are absorbed on the AP surface and then escape to the gas phase. The second important step is the homolytic cleavage of the Cl-OH bond in HClO4. The energy barrier is 67.5 kJ/mol under 620 K. Then, (OH)-O-. radical and (ClO3)-Cl-. radical react with NH3 molecule, yielding (NH2)-N-. radical. Then the (NH2)-N-. radical react with HClO4, leading to (ClO4)-Cl-. radical, which reacts with NH3, leading to the oxidized species H2NO. The radical species, such as (OH)-O-., (NH2)-N-., (ClO)-Cl-. and so on, abstract the H atom of H2NO, yielding NO. NO reacts with (OH)-O-. radical, leading to NO2; NO reacts with (NH2)-N-. radical and (OH)-O-. radical, leading to N2O. These products are consistent well with the experimental observations. Due to the complexity of the mechanisms, some strategies are used in this study: firstly, we concentrate on the reaction pathways of active species and the NH3 and HClO4 molecules, which exist in large amount; secondly, more reaction pathways involving the newly formed active species are considered.