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 Investigating the Mechanism of Phenol Photooxidation by Humic Substances(2014) Sikorski, Kelli Ann; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well established that organic pollutants such as phenols are degraded in the presence of chromophoric dissolved organic matter (CDOM) and sunlight in natural waters. Early work attributed the photochemical loss of phenols to the involvement of photoproduced reactive oxygen species (ROS) such as singlet oxygen (1O2), hydroxyl radical (*OH) or peroxy radicals (RO2*). However, evidence for the involvement of triplet excited states of aromatic ketones/aldehydes within CDOM has accumulated in the literature. To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (RTMP) on dioxygen concentration and irradiation wavelength was examined both for a variety of untreated as well as borohydride-reduced HS and C18 extracts from the Delaware Bay and Mid-Atlantic Bight. The effect of [O2] and borohydride-reduction of SRFA was also examined for a series of substituted phenols of varying one-electron reduction potentials. We find that RTMP was inversely proportional to dioxygen concentration at [O2] > 50 μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate constants for TMP reaction, O2 quenching and lifetimes compatible with a triplet intermediate. Borohydride reduction significantly reduced TMP loss, supporting the role of aromatic ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride reduction, as well as the inverse dependence of RTMP on [O2] for these reduced samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones. However, the results of the wavelength dependence reveal that the sensitization is driven primarily by shorter wavelength UV-B and UV-A absorbing moieties, consistent with the involvement of aromatic ketones and aldehydes but appearing to exclude the longer wavelength (visible) absorbing quinones as sensitizers. An inverse dependence of Φ on one-electron reduction potential was observed where DMOP ≈ TMP > 4-MOP > 4-MP > phenol. Similar dependencies were observed for TMP and 4-MOP in the dependence of Rprobe on [O2] whereas DMOP did not exhibit a substantially lower Rprobe at high [O2] as would be expected for a triplet sensitization mechanism. Moreover, that a significant amount of sensitization is observed following borohydride reduction of SRFA for DMOP under high [O2], as well as the very low sensitization observed at low [O2] indicates that a separate pathway, unrelated to triplets, may be important for the mechanism of DMOP photooxidation by chromophoric dissolved organic matter.Item UNDERSTANDING DIRECT BOROHYDRIDE - HYDROGEN PEROXIDE FUEL CELL PERFORMANCE(2013) Stroman, Richard O'Neil; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct borohydride fuel cells (DBFCs) generate electrical power by oxidizing aqueous BH4- at the anode and reducing an oxidizer, like aqueous H2O2 for an all-liquid fuel cell, at the cathode. Interest in DBFCs has grown due to high theoretical energy densities of the reactants, yet DBFC technology faces challenges such as side reactions and other processes that reduce cell efficiency and power generation. Relationships linking performance to cell design and operation will benefit from detailed and calibrated cell design models, and this study presents the development and calibration of a 2D, single-cell DBFC model that includes transport in reactant channels and complex charge transfer reactions at each electrode. Initial modeling was performed assuming ideal reactions without undesirable side reactions. Results were valuable for showing how design parameters impact ideal performance limits and DBFC cell voltage (efficiency). Model results showed that concentration boundary layers in the reactant flow channels limit power density and single-pass reactant utilization. Shallower channels and recirculation improve utilization, but at the expense of lower cell voltage and power per unit membrane area. Reactant coulombic efficiency grows with decreasing inlet reactant concentration, reactant flow rate and cell potential, as the relative reaction rates at each electrode shift to favor charge transfer reactions. To incorporate more realistic reaction mechanisms into the model, experiments in a single cell DBFC were performed to guide reaction mechanism selection by showing which processes were important to capture. Kinetic parameters for both electrochemical and critical heterogeneous reactions at each electrode were subsequently fitted to the measurements. Single-cell experiments showed that undesirable side reactions identified by gas production were reduced with lower reactant concentration and higher supporting electrolyte concentration and these results provided the basis for calibrating multi-step kinetic mechanism. Model results with the resulting calibrated mechanism showed that cell thermodynamic efficiency falls with cell voltage while coulombic utilization rises, yielding a maximum overall efficiency operating point. For this DBFC, maximum overall efficiency coincides with maximum power density, suggesting the existence of preferred operating point for a given geometry and operating conditions.