Insight into the Mechanism of α-, β-, γ-, and δ-MnO₂ in Catalyzing Ammonium Perchlorate Decomposition: Superoxide Promotion (SOP) Effect

To date, the catalytic mechanism of metal oxides in the thermal decomposition of ammonium perchlorate (AP) remains unclear and hinders the design and manufacture of efficient AP catalysts. Herein, a catalytic mechanism of the superoxide promotion (SOP) effect is reported, and the more oxygen vacancies and the lower band gaps of the catalysts allow deep decomposition of AP by •O₂^-. With the addition of 2 wt % δ-MnO₂, the HTD temperature of AP decreases from 419.4 to 298.8 °C, and the Δ<i>H</i> increases from 585.5 to 1446.0 J g^-1. The SOP effect accelerates the thermal decomposition of AP at low temperatures. Moreover, density functional theory (DFT) calculations confirm that the elongation of the O-O bond (1.34 Å) facilitates the activation of the O₂ adsorbed on these vacancies. δ-MnO₂ shows the most powerful SOP effect; thus, it is one of the best catalysts for AP thermal decomposition. In addition, Δ<i>T</i> and the ratio of NO₂/N₂O are highly correlated with •O₂^-; these results indicate that the diverse catalytic activity exhibited by the different crystal phases of MnO₂ on AP is caused by the differences in the SOP effect.