Cryogenic expansion-induced remodeling of pore architecture in δ-MnO2/montmorillonite for enhanced formaldehyde oxidation.

Indoor formaldehyde (HCHO) poses a severe threat to human health, underscoring the need for efficient, sustainable purification solutions. In this work, a novel δ-MnO2/montmorillonite composite was synthesized via an ice bath-assisted co-precipitation method combined with freeze-drying. The ice bath condition effectively controlled the reaction rate and prevented the formation of aggregated δ-MnO2 on the montmorillonite surface, leading to uniform nanosheet dispersion and abundant lattice defects. Furthermore, the freeze-drying process, conducted under low temperature and pressure, causes liquid water to rapidly freeze into ice, expanding in volume during freezing and forming unique pore channels. This results in a specific surface area 2.17 times larger than that of oven-dried samples and 21.5 times larger than that of δ-MnO2, while effectively preserving structural hydroxyl groups and adsorbed water. Moreover, the as-prepared composite materials demonstrated excellent catalytic performance, achieving complete HCHO removal within 120 min, and its mineralization efficiency exceeded 91%. The superior performance of the composite materials from the synergistic effects of high dispersion, an optimal Mn3+/Mn4+ ratio, abundant surface hydroxyl groups, and a hierarchical porous structure. Overall, this study developed a mineral-based catalyst with a porous structure and high catalytic activity, enabling efficient indoor formaldehyde purification at room temperature.