Structural and morphological evolution of mesoporous α-MnO2 and β-MnO2 materials synthesized via different routes through KMnO4/H2C2O4 reaction

Abstract

Mesoporous α-MnO2 and β-MnO2 were prepared using KMnO4 and H2C2O4 (Oxalate) as precursors via aqueous and hydrothermal routes respectively. The morphological and structural evolution of the MnO2 materials were tracked by Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. The preparation of MnO2 materials was found to proceed through four steps: well-crystallized α-MnO2 was formed first in aqueous solution, and then translated into microcrystalline α-MnO2 microspheres consisting of nanoparticles and nanowires via a dissolution-recrystallization process. Subsequently, the microcrystalline α-MnO2 microspheres collapsed, and γ-MnOOH nanowires were formed. Finally, β-MnO2 nanowires were obtained after calcining the γ-MnOOH nanowires in air at 400°C for 6h. During the process, oxalate acted as an important reagent and played an important role for the structural and morphological evolution of MnO2. Nitrogen sorption analysis showed that the as-prepared α-MnO2 and β-MnO2 materials all exhibited type IV isotherms, indicating mesoporous characters. In addition, the performance of the MnO2 materials as a catalyst was evaluated in the complete catalytic oxidation of a typical volatile organic compound (VOCs), o-xylene.