Preparation and characterization of open tunnel oxide α-MnO2 precipitated by ozone oxidation

Open tunnel oxide, alpha -MnO2, without any large stabilizing cation has been successfully synthesized by ozone-oxidation of Mn2+ in H2SO4. Three kinds of acid (H2SO4, HNO3, and HCl) and a wide variety of their concentrations and ozone-oxidation temperatures were tested to produce a well-crystallized alpha -MnO2 specimen. The ozone-oxidation of Mn2+ in H2SO4 alone provided alpha -MnO2 phase, while gamma -MnO2 phases were always formed in HNO3 or HCl. High reaction temperatures (> 70 degreesC) and high concentrations of H2SO4 (> 2 M) were necessary for the alpha -MnO2 formation, X-ray and electron diffraction patterns confirmed that alpha -MnO2,has the hollandite-type structure with space group of 14/m (No. 87) and lattice constants of a = 9.78 and c = 2.85 Angstrom. Scanning and transmission electron microscopy images revealed a needle-like morphology for the alpha -MnO2 crystals; the electron diffraction pattern verified the rod axis of the needle-like crystals in coincident with the direction of the tunnels, The nitrogen adsorption isotherm of alpha -MnO2 exhibited the character of a typical Type IV, according to the IUPAC classification. The chemical, thermogravimetric, and temperature-programmed-desorption analyses indicated that H2O molecules are occluded within the tunnel of a-MnO2, Although N-2 molecules cannot penetrate the tunnel cavity, H2O molecules can be trapped in the narrow tunnels with strong adsorption potential. (C) 2001 Academic Press