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Preparation and Characterization of Open Tunnel Oxide α-MnO2 Precipitated by Ozone Oxidation

✍ Scribed by Norihito Kijima; Hiroyuki Yasuda; Toshio Sato; Yuji Yoshimura

Publisher
Elsevier Science
Year
2001
Tongue
English
Weight
575 KB
Volume
159
Category
Article
ISSN
0022-4596

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✦ Synopsis

Open tunnel oxide, -MnO 2 , without any large stabilizing cation has been successfully synthesized by ozone-oxidation of Mn 2 2؉ ؉ in H 2 SO 4 . Three kinds of acid (H 2 SO 4 , HNO 3 , and HCl) and a wide variety of their concentrations and ozone-oxidation temperatures were tested to produce a well-crystallized -MnO 2 specimen. The ozone-oxidation of Mn 2 ؉ in H 2 SO 4 alone provided -MnO 2 phase, while -MnO 2 phases were always formed in HNO 3 or HCl. High reaction temperatures (> 703C) and high concentrations of H 2 SO 4 (> 2 M) were necessary for the -MnO 2 formation. X-ray and electron di4raction patterns con5rmed that -MnO 2 has the hollandite-type structure with space group of I4/m (No. 87) and lattice constants of a ‫؍‬ 9.78 and c ‫؍‬ 2.85 A > . Scanning and transmission electron microscopy images revealed a needle-like morphology for the -MnO 2 crystals; the electron di4raction pattern veri5ed the rod axis of the needle-like crystals in coincident with the direction of the tunnels. The nitrogen adsorption isotherm of -MnO 2 exhibited the character of a typical Type IV, according to the IUPAC classi5cation. The chemical, thermogravimetric, and temperature-programmeddesorption analyses indicated that H 2 O molecules are occluded within the tunnel of -MnO 2 . Although N 2 molecules cannot penetrate the tunnel cavity, H 2 O molecules can be trapped in the narrow tunnels with strong adsorption potential.

Academic Press

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