Particle Size Control of Cryptomelane Nanomaterials by Use of H₂O₂ in Acidic Conditions

A new soft-step chemistry method has been developed to prepare pure cryptomelane-type manganese oxide materials (OMS-2) with the smallest particle sizes ever reported. The synthetic procedure is based on the reduction of KMnO 4 by H 2 O 2 under acidic conditions followed by reflux. An acetate-containing buffer solution and HNO 3 are used to control the pH of the reaction mixture. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials have been investigated by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), potentiometric titration, thermogravimetric analysis, and N 2 sorption analyses. Both the concentration of H 2 O 2 and the nature of the acid used affect the crystalline phase formation, microstructure, thermal stability, and the composition of the final product. HRTEM images reveal that the OMS-2 nanofibers are not oriented preferentially and present significant twinning, along with discontinuity in the growth of the tunnel. Catalytic studies of these OMS-2 nanomaterials for oxidation of benzyl alcohol and fluorene have been performed. These nanomaterials show a low performance for the oxidation of benzyl alcohol and a unique catalytic activity for the oxidation of fluorene compared to OMS-2 materials prepared by conventional methods.