Hierarchical porous ε-MnO2 from perovskite precursor: Application to the formaldehyde total oxidation

A simple template-free method, based on a mineral acid etching process using manganite perovskite (LaMnO3) as precursor, was successfully developed to obtain a series of 3D meso/macro-porous materials. The LaMnO3 transformation was fully investigated using ICP, XRD, N-2 physisorption, TPR, TPD, SEM, TEM/EDS and XPS. This transformation proceeds through a soft-chemical process involving the dissolution of trivalent lanthanum and manganese from the perovskite structure and the dismutation of Mn3+ cations into MnO2 and Mn2+ species. Strength and oxidizing properties of the acid used as modifying agent strongly impact textural and redox surface properties of the resulting materials. Specifically, extending the acid etching duration promotes the surface area and pore volume of the materials while developing interconnected macro-mesoporous networks. In our case, this soft process allowed us to obtain the epsilon-MnO2 phase with hierarchical porosity without any template. Superior catalytic properties of epsilon-MnO2 were observed toward HCHO oxidation as well as a good catalytic stability with respect to other macro-mesoporous counterparts. In the light of the experimental results, such performances can be related to the formation of a meso/macro-porous structure conferring high surface area and good accessibility of the active surface sites. The latter exhibit greater redox ability of the manganese species and a higher density of active surface oxygen species with respect to the perovskite precursor.