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 Mn5c/Mn4c sites on the MnO2 surface were found to strongly adsorb NH3. The Ov and Lewis acid sites (Mn5c/Mn4c) 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.