Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item INTERFACIAL INTERACTIONS OF NANOTUBES: AN IN-SITU STUDY OF STRUCTURE AND REACTIONS WITH THEIR ENVIRONMENTS(2021) Chao, Hsin-Yun; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanotubes have the potential to be a revolutionary material for many different applications. Though often touted as versatile and tunable materials, the difficulty of their reliable large-scale production for any specified property is a hurdle in their wide-scale implementation. Interactions at nanotube interfaces dictate overall performance of their growth, radiation resistance, and nanofluidics properties. In this dissertation, I present in-situ experiments using an environmental transmission electron microscope (ETEM). Numerous aspects of interfacial mechanisms of nanotubes are examined at the atomic scale and models considered for the observed behavior. First, I study the interface between nanotubes and catalyst particles during single-walled carbon nanotube (SWCNT) growth. The structure and phase transformation of cobalt catalysts are elucidated for inactive, active, and deactivated nanoparticles by ETEM imaging. Through in-depth studies of multiple distinct cobalt nanoparticles, I establish the dominant nanoparticle phase for SWCNT growth. I also identify the preferred lattice planes and a threshold for work of adhesion for the anchoring and liftoff of SWCNTs. Second, the nanotubes are tested for their radiation resistance properties. I study the resistance of nanotube degradation in an ionizing environment with oxygen pressure, where the damage initiates at the interface with the gas phase. Observations show boron nitride nanotubes (BNNTs) have a higher resistance to damage than carbon nanotubes (CNTs). By computing knock-on threshold energies for the atoms impacted by incoming electrons, a model can be formulated for the oxygen-assisted radiation damage pathway. I provide further validation to the model with heating experiments that demonstrate a surprising increase in damage resistance. Lastly, interfaces between nanotubes and water are studied. The goal is to capture the ordering dynamics of water at the BNNT interface using in-situ characterization at cryogenic temperatures. Water is hyper-quenched to liquid nitrogen temperatures for the formation of low density amorphous (LDA) ice. High resolution images are then acquired, preserving the original water structure. Crystallization of LDA ice is induced by both environmental heating and electron beam irradiation. I present a comparison of the structural evolutions of LDA ice with and without the presence of BNNT, which indicates the presence of nascent ordering at the interface.Item SYNTHESIS OF ZEOLITE@MOF NANOPOROUS COMPOSITES AS BIFUNCTIONAL CATALYSTS(2014) Zhu, Guanghui; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As nanoporous materials, zeolite and metal organic framework (MOF) share common characteristics of high surface areas and uniform micropores and differ in thermal/mechanical stability and structural flexibility. The integration of MOF and zeolite into composite particles is expected to produce useful hybrid nanoporous materials where inorganic zeolite and organic MOF components impart the advantages of high thermal, mechanical and structural stability of zeolites and specific functionality and high flexibility of MOFs. This thesis work addresses the synthesis of zeolite@MOF composites and the exploration of their applications as bifunctional catalysts in one-pot cascade reaction. Zeolite@MOF core-shell composites have been synthesized by solvothermal growth of MOFs on the surface of ZSM-5 particles. The acidity from framework Al-O(H)-Si sites in ZSM-5 and basicity from amine groups in MOFs obtained by pre-/post-synthetic modification endow zeolite@MOF composites bifunctionality in two-step cascade catalytic reactions.Item NANOSTRUCTURED MATERIALS FOR SOLAR HYDROGEN GENERATION(2010) Ma, Xiaofei; Zachariah, Michael R; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrogen can be considered a nonpolluting and inexhaustible energy carrier for the future. However, hydrogen is not readily available for use as a fuel. It exists in bound form with other elements (e.g. water, hydrocarbons) and as such energy is required to abstract molecular hydrogen from various feedstocks. Solar energy due to its abundance and low cost is being considered as the energy source for environmentally safe hydrogen generation. This dissertation focuses on the development and characterization of nano-structured materials for solar thermochemical hydrogen generation, on the principle that concentrated solar radiation can be employed as the high-temperature energy source for driving an endothermic hydrogen generation process. The reaction mechanism and kinetics of different solar thermochemical processes using those nano-structured materials as reactants or catalysts were investigated. The experimental works in this dissertation can be divided into two main areas. The first area is to study the properties and reactivity of in-situ generated Zn nanocrystals (NCs) for solar thermochemical Zn/ZnO water splitting cycle for hydrogen production. The particle size-resolved kinetics of Zn NCs oxidation, evaporation, and hydrolysis were studied using a tandem ion-mobility method in which the first mobility characterization size selects the NCs, whereas the second mobility characterization measures changes in mass resulting from a chemical reaction of the NCs. The second part of the dissertation is concentrated on the investigation of in-situ generated nano-sized metal particles as catalysts in liquid hydrocarbon decomposition process for hydrogen generation. Catalytic decomposition of liquid fuels (n-octane, iso-octane, 1-octene, toluene and methylcyclohexane) was achieved in a continuous tubular aerosol reactor as a model for the solar initiated production of hydrogen, and easily separable CO free carbonaceous aerosol product. The effects of fuel molecular structure and catalyst concentration on the overall hydrogen yield were studied. Using the similar aerosol catalysis idea, ignition of liquid fuels catalyzed by in-situ generated metal nanoparticles was investigated. The morphological change of catalyst particles during fuel ignition process and the catalytic ignition mechanism are discussed.