Polymorph‒dependent defect chemistry of MnO2 nanorods: grain boundary and oxygen vacancy effects on burn rate enhancement in solid propellants

The controlled synthesis of nanomaterials with tailored morphologies and crystal structures has emerged as a key strategy for optimizing catalytic performance through defect and interface engineering. Manganese dioxide (MnO₂), a transition-metal oxide, existing in multiple polymorphs with distinct atomic frameworks and tunable defect chemistries, is an ideal compound to investigate structure–property–performance correlations. In this study, α- and β-MnO₂ nanorods were synthesized and systematically evaluated to elucidate the influence of the crystal structure, grain boundaries, and oxygen vacancies on the catalytic behavior. The β-MnO₂ phase demonstrated superior catalytic activity, which was attributed to its abundant high-energy grain boundary interfaces, and elevated concentrations of surface oxygen vacancies, as revealed by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) analyses. Phase-dependent structural features and surface defects modulate the Mn oxidation environment and generate unsaturated coordination sites that facilitate reactant adsorption and bond activation. These sites serve as highly active centers that promote reactant adsorption and bond activation, thereby enhancing the catalytic activity and substantially accelerating the combustion dynamics of composite solid propellants. These catalytic enhancements driven by structural modifications resulted in the accelerated decomposition of ammonium perchlorate (AP) and an enhanced burn rate in AP-based composite solid propellants.