The properties of manganese oxide nano-materials are dictated by the structure of the particular phases adopted, and by the morphology of the particles or aggregates formed. In this dissertation a combination of processing and advanced characterization studies has been performed to investigate two key aspects of this system: the role of the surfactant in the synthesis of self-assembled mesoporous manganese oxides, and the effect of post-synthesis heat treatment on the phase stability in manganese oxide nanoparticles.
The first part concerns UCT manganese oxides in which the materials are synthesized via an inverse micelle templating process. The objective was to develop an understanding of the factors that control the morphology, microstructure, and desulfurization behavior of these materials. Samples were synthesized using four different Pluronic surfactants, and these were characterized using a combination of XRD, SAXS, SEM, S/TEM, BET, and sulfur adsorption techniques. While all the as-synthesized samples exhibited similar hierarchical microstructures, consisting of assemblies of Mn3O4 and/or Mn5O8 nanoparticles, the sizes and shapes of these nanoparticles were different for each sample. These effects led to pronounced differences in the pore size distributions. The sulfur adsorption tests revealed a transformation of the mesoporous manganese oxide to a dense manganese sulfide phase, which results in a volumetric expansion, densification, and rapid decay in the sorption capacity after the breakthrough point. The data suggest that the variation in the structure and properties of these materials is related to ratio of the PEO and PPO chain segment lengths in the Pluronic surfactants used. Thus, the PEO:PPO ratio dictates the volume, shape and assembly of the inverse micelles in the synthesis and is a key process design variable in UCT materials.
The focus of the second part is on the phase transformations in manganese oxide nanoparticles upon heating in inert and oxidizing environments. This study involved performing controlled heating experiments on electron-transparent amorphous monodisperse manganese oxide nanoparticles at different temperatures and under different oxygen partial pressures. Samples exposed ex situ were evaluated using X-ray scattering and electron microscopy techniques to reveal the structure, morphology and oxidation states for the manganese oxide phases present. Upon heating in Ar at temperatures up to 1300 ºC, the samples went through a series of phase transformations to Mn2O3, Mn3O4, and MnO corresponding to a gradual reduction of manganese from 3+ to 2+. Upon heating to 1300 ºC in air, the latter transformation did not occur, indicating that Mn3O4 is the stable phase at higher temperatures under oxidizing conditions. In situ TEM heating experiments were used to investigate the dynamic evolution of the microstructure at high spatial resolution under vacuum. In experiments performed at high heating rates, a transformation to MnO was observed, while at lower heating rates the samples transformed to Mn3O4. These observations reveal the roles of temperature, oxygen partial pressure, and heating rate on the phase transformations in manganese oxide upon heating, and serve as a guide for designing thermal processing routes to obtain the manganese oxide phase desired for a particular application
