Preparative parameters, mechanisms, and kinetics in the syntheses of layered and tunnel manganese oxides

Date of Completion

January 1999

Keywords

Chemistry, Inorganic

Degree

Ph.D.

Abstract

This research concerns the preparative parameters, mechanisms, and kinetics in the syntheses of layered and tunnel manganese oxides, as well as characterization and catalytic applications of these materials. Methods for synthesis include oxidation, reduction, redox, decomposition, pillaring, anchoring, and solid-state conversion, where a series of reactions in aqueous, sol-gel, solvents, and solid-state systems are involved. Catalytic properties of these materials are tested for the total oxidation of volatile organic compounds and partial oxidation of hydrocarbons. ^ Birnessite has been synthesized via oxidation, reduction, and redox syntheses. Influences of preparative parameters, magnesium effects, and anion/cation effects have been investigated in detail on the composition, morphology/crystal size, crystallization rate, crystallinity, ion-exchange properties, thermal stability, adsorptive, and catalytic properties. Kinetics and mechanisms in these processes have been studied, including the calculation of apparent energies of activation for different synthesis systems and elucidation of topotactical conversion from pyrochroite to birnessite via feitknechtite in oxidation syntheses and evolution and growth of birnessite from AMO gel in reduction and redox syntheses. ^ Ion-exchange and stabilization of birnessite to various metal-buserites have been realized via a double-aging method developed here, where metal-buserites have been further converted to various metal-todorokites. Physical and chemical changes via a double-aging process have been investigated. Criteria for the formation of todorokite and hollandite have been obtained. ^ In the presence of long-chain alkyl ammonium hydroxides, mesoporous and large d-spacing lamillar manganese oxides have been prepared. Interconversions between mesoporous and layered structures have been realized via phase transformation or pillaring techniques. Designs for interplanar distances for OL materials and pore size or cation-anchoring have been discussed. ^ Cryptomelane materials prepared via aqueous and solid-state reactions have been found to reversibly evolve lattice oxygen in air below 650°C without loss of structure, to be highly hydrophobic, and of excellent thermal stability. These properties result in the excellent catalytic activity and catalyst stability in the oxidation of volatile organic compounds. ^

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