Title

I. Syntheses of manganese oxides by using microwave heating and convential heating. II. Syntheses of nanosize materials

Date of Completion

January 2001

Keywords

Chemistry, Inorganic|Engineering, Materials Science

Degree

Ph.D.

Abstract

This research contains two major parts. The first part involves preparation of layered and tunnel structure manganese oxides using microwave heating, along with preparation and characterization of electrochemically active manganese oxide, γ-MnO2. The second part involves preparation and characterization of nanofibrous cryptomelane and nanosize zeolite A. ^ Microwave effects in both aqueous and solid syntheses of manganese oxide materials with layer and tunnel structures have been investigated. Formation of birnessite and cryptomelane is remarkably accelerated by microwave irradiation. In the synthesis of birnessite, no obvious induction period has been observed, which may result from the fast nucleation of birnessite due to the simultaneous interaction between microwaves and MnOx species. In solid synthesis of cryptomelane, high temperature products obtained in microwave syntheses show different properties from those obtained in conventional syntheses, in adsorption of water molecules, recombination with atmospheric oxygen, recovery of cryptornelane structure, which results from the different features of microwave heating and conventional heating. Mechanisms of oxygen evolution of cryptomelane under microwave irradiation and subsequent recovery of cryptomelane structure in oxygen atmosphere have been illustrated. ^ γ-MnO2 materials have been synthesized via redox reactions of Mn 2+ and MnO4 (CMDredox), reduction of MnO4 by melaic acid (CMD reduction), and disproportionation of Mn3O4 (CMD dis-Mn3O4) and Mn2O3 (CMDdis-Mn2O3). γ-MnO 2 materials prepared via different methods show the following sequence for electrochemical activity: EMD > CMDdis-Mn2O3-MnCO3 > CMD dis-Mn2O3-MnOx > CMDreduction ∼ CMDredox > CMDdis-Mn3O4. Correlations of electrochemical properties of γ-MnO2 with defect features and physical properties have been discussed. ^ A novel two-step procedure has been developed to synthesize nanofibrous cryptomelane with a high surface area. Transformation of amorphous manganese oxide is done in acidic media under reflux conditions instead of high temperature calcination method used in the literature. Cryptomelane with particle sizes of ∼5 x 150 nm and a surface area of 216 m2/g is obtained in 10 min. Cryptomelane prepared via our method shows smaller particle size, larger surface area, and higher catalytic activity than those prepared via solid syntheses in the literature. ^ Non-clear solution synthesis method has been developed to produce zeolite A nanocrystals. This method includes two steps. The first step is to synthesize amorphous aluminosilicate under reflux conditions. The second step is to transform amorphous substrate to zeolite A. Optimization of synthetic parameters has been done. Nanocrystals of 50–150 nm have been produced by aging amorphous aluminosilicate at room temperature in TMAOH solution with seeding. Effects of seeding, temperature, preaging, cations, and solvents have been discussed. ^