Unveiling the Role of Oxygen Vacancies in Manganese Oxides for the Ammonium Perchlorate Thermal Decomposition

Manganese oxides (MnO₂, Mn₂O₃, and Mn₃O₄) were synthesized as catalysts to promote the ammonium perchlorate (AP) thermal decomposition, and the potential catalytic mechanisms were systematically researched. The catalytic activity followed the order of MnO₂ > Mn₃O₄ > Mn₂O₃. The electron paramagnetic resonance (EPR) results showed that MnO₂ has the highest content of superoxide (·O₂^–), followed by Mn₃O₄ and Mn₂O₃, which is consistent with the oxygen vacancy (Ov) content. Significantly, the decrease in the Ov concentration of MnO₂-T, Mn₂O₃-T, and Mn₃O₄-T after the annealing treatment resulted in a decrease in the catalytic performance and catalytic reaction rate k. The inhibitory effect due to the NH₃ accumulation on the AP surface was well resolved by MnO₂. Density functional theory (DFT) calculation results show that O₂ can be activated to ·O₂^– at the Ov, and the O–O bond is elongated from 1.22 to 1.30 Å. The Mn_5c/Mn_4c sites on the MnO₂ surface were found to strongly adsorb NH₃. The Ov and Lewis acid sites (Mn_5c/Mn_4c) synergistically anchor O₂ and NH₃ on the catalyst surface and shorten the reaction distance through the Langmuir–Hinshelwood (L-H) mechanism, thereby endowing MnO₂ with more favorable conditions for catalyzing AP thermal decomposition.