UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item CATALYST DEVELOPMENT FOR NON-OXIDATIVE METHANE UPGRADING TOWARDS HYDROCARBONS AND HYDROGEN PRODUCTION(2024) LIU, ZIXIAO; Liu, Dongxia; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Methane, the main constituent of natural gas and biogas is deemed to be an alternative source to replace crude oil in the production of chemicals and fuel. Non-oxidative methane conversion enables methane coupling or splitting to produce hydrogen and more significant hydrocarbons, but catalyst deactivation has been a challenge in past research. This dissertation addresses catalyst deactivation issues in non-oxidative methane conversion by inventing novel catalyst systems. For direct non-oxidative methane coupling, a pathway for methane upgrading into hydrogen, olefin, and aromatic products, the silica-supported catalysts were synthesized by flame fusion of a mixture of quartz silica and metal silicate precursors. Compared to the cristobalite silica-supported catalysts reported previously, vitreous silica-supported catalysts have disordered Si-O bonds and structural defects, enabling better metal dispersion and more vital metal-support interaction. The as-prepared vitreous silica-supported iron catalyst had a shorter induction period in methane activation and lower coke yield. The increase in iron concentration elongated the catalyst induction period and promoted aromatics and coke formation. Among different transition metal catalysts, the cobalt supported by vitreous silica had the best methane conversion, hydrocarbon product yield, and catalyst stability. For catalytic methane pyrolysis, a pathway producing COx-free hydrogen and valuable carbon product, a siliceous zeolite-supported cobalt catalyst was invented. In comparison to the methane pyrolysis catalysts in literature, the siliceous zeolite support in the invented catalyst has limited Brönsted acidity and increased mesoporosity, which limited the acid-catalyzed deactivation mechanism and facilitated the mass transport, and thus significantly increased the catalyst lifetime. The cobalt sites change the cluster sizes and coordination structures with the loading concentrations in the zeolite support, which leads to carbon products with different properties.Item Effect of Free Chlorine Oxidation on the Deposition Kinetics of Bacteriophage MS2 on a Silica Surface(2015) Stephens, Heungkook Noriomoral; Mi, Baoxia; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the transport kinetics of water contaminates such as pathogenic viruses around solid surfaces is important in controlling groundwater contaminant plumes and optimizing contaminant removal in sand filtration units. The effect of increasing oxidative stress, in the form of small doses of free chlorine, on the deposition behavior of bacteriophage MS2 onto a silica surface was examined using a quartz crystal microbalance with dissipation (QCM-D). MS2 deposition rates were analyzed by reaction time and inactivation level. A statistically significant increase in the deposition rate was identified between MS2 test solutions not exposed to free chlorine and most test solutions that were exposed. However, as exposure to free chlorine was increased, no relationship was able to be deduced from the collected deposition data. Potential explanations based on previous work were discussed. Observations also indicated that more comprehensive purification procedures in comparison to previous studies were necessary to obtain accurate QCM-D data.Item Solvation, Structure and Organization at Liquid Surfaces(2009) Brindza, Michael Ross; Walker, Robert A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the results of nonlinear spectroscopic studies whose goal is to understand how the asymmetric nature of interfaces and intermolecular interactions give rise to interfacial solvation properties and solvent structure. The first part of this thesis uses resonance enhanced second harmonic generation to examine the polarity and hydrogen bonding opportunities at interfaces formed between hydrophilic silica and both weakly and strongly associating organic liquids. Measuring interfacial electronic spectra of probe molecules that exhibit solvatochromic sensitivity to polarity and hydrogen bonding, we saw that small changes in solvent structure affect interfacial polarity, and strongly associating alcohols solvents create a region of heterogeneous polarity at the interface. Silica appears to donate hydrogen bonds to adsorbates no matter what solvent (protic or aprotic) was chosen. The second part of this dissertation uses another nonlinear spectroscopic technique, vibrational sum frequency generation, to determine the structure and orientation of solvent molecules adsorbed to silica/vapor, silica/liquid, and neat liquid/vapor interfaces. By comparing spectral features appearing under different experimental polarization conditions, we have determined average solvent orientations and degree of organization. Our initial studies of alkanes adsorbed to the silica/vapor interface show that despite strong substrate-adsorbate interactions, molecules at the interface show some degree of long range order and organization. In order to examine how the strength of intermolecular forces between adsorbates and either the substrate or neighboring molecules affect interfacial organization, we measured vibrational spectra of octanol isomers as well as different functional group containing n-alkyl molecules at silica/vapor and silica/liquid interfaces. The octanol studies show that strongly associating molecules form ordered monolayers at the silica/vapor interface, but that strength of lateral interactions is important for preserving that order when the liquid is brought into contact. Branched isomers appeared very disordered at solid/liquid interfaces. Further examining this change in order between solvents at silica/vapor and silica/liquid interfaces using equal length but different functional group containing solvents, we see that the energetics of adsorption and solvation are likely to be responsible for the degree of order both at the solid/vapor surface (adsorption) and solid/liquid interface (both adsorption and solvation).