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 KMnO4 by H2O2 under acidic conditions followed by reflux. An acetate-containing buffer solution and HNO3 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 N2 sorption analyses. Both the concentration of H2O2 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.

No abstract

No abstract

In situ X-ray diffraction was used to study the reduction of unsupported CoO, and Co3O4 under different conditions of time and temperature, and the effect of lanthanum, ruthenium, or zinc doping of γ-alumina on the reduction of supported cobalt oxide. The reduction of unsupported cobalt oxide (Co3O4) produced CoO, then hexagonal close-packed Co (Co(hcp)) and at temperatures above 400 °C, only face-centered cubic Co (Co(fcc)) was obtained. However, it was possible to obtain a mixture of Co(hcp) and Co(fcc) at 450 °C only when the reduction was performed stepwise. Metallic Co(fcc) was obtained after reduction at 350 °C of supported Co3O4 on doped alumina; however, the reduction of unsupported CoO under the same conditions produced Co(hcp). Doping alumina with ruthenium oxide favors the reduction of Co3O4 to CoO and then to metallic Co(fcc). The crystal size apparently affects the phase of metallic cobalt obtained.