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

Open tunnel oxide, alpha -MnO2, without any large stabilizing cation has be en successfully synthesized by ozone-oxidation of Mn2+ in H2SO4. Three kind s of acid (H2SO4, HNO3, and HCl) and a wide variety of their concentrations and ozone-oxidation temperatures were tested to produce a well-crystallize d 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 elect ron 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 im ages revealed a needle-like morphology for the alpha -MnO2 crystals; the el ectron diffraction pattern verified the rod axis of the needle-like crystal s in coincident with the direction of the tunnels, The nitrogen adsorption isotherm of alpha -MnO2 exhibited the character of a typical Type IV, accor ding to the IUPAC classification. The chemical, thermogravimetric, and temp erature-programmed-desorption analyses indicated that H2O molecules are occ luded 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