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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item FUNDAMENTAL UNDERSTANDING OF SOFC CATHODE DURABILITY; A KINETICS AND CATALYSIS STUDY(2015) Huang, Yi-Lin; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cells (SOFC) have been demonstrated as great prospects for electrochemical conversion of fuels, providing both high efficiency and high power density. Understanding the fundamentals of the oxygen reduction reaction (ORR) mechanisms is necessary to further improve cathode performance. Two different testing systems, gas phase isotopic oxygen exchange and electrical conductivity relaxation, were built to study the kinetics of cathode powders and bulk samples, respectively. A robust strategy was established to extract kinetic parameters from transient response curves for a variety of materials and conditions using numerical solutions. In-situ gas phase isotopic oxygen exchange, which provides real-time information about cathode surface kinetics, was used to determine the ORR mechanisms and the interactions of other gaseous species with the solid surface for two cathode materials: La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3±x (LSM). LSCF has a faster dissociation reaction than LSM, and the limiting step is the surface exchange. Additionally, LSM likely contains different vacancy concentrations in the near surface region and in the bulk. A mathematic model is further established to unify surface exchange rates from different experiments and link solid-state diffusion to surface heterogeneous catalysis. In addition, the long-term durability of these materials is a major challenge. A novel technique called isotope saturated temperature programmed exchange (ISTPX) has been developed to determine the temperature and PO2 range that is preferable for the exchange of water and CO2 on LSM and LSCF. The presence of CO2 and water indicates blocking effects on the LSCF surface from 300°C to 600°C, possibly resulting in two separate degradation mechanisms. On the other hand, CO2 and water exchange with LSM through homoexchange mechanism with a relatively minor impact. Based on isotope exchange results, surface modified LSCF cathodes were fabricated. The surface modification of LSCF through Mn ion implantation enhances the chemical surface exchange coefficient (kchem) from 4.4x10-4 cm/s to 1.9x10-3 cm/s at 800°C. The aims of this study are to increase knowledge and information about the ORR. The results allow us to further investigate the ORR mechanisms as well as to engineer new cathode materials/structures that can improve cathode performance and durability.Item Bimetallic Nanoparticles for Advanced Energy Conversion Technologies(2015) Sims, Christopher; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The increased demand for a more sustainable energy infrastructure has spurred the development of innovative energy conversion processes and devices, such as the proton exchange membrane fuel cell (PEMFC). PEMFCs are highly regarded as a clean alternative energy technology for various applications, such as motor vehicles or power generators. Factors limiting their commercial viability include the poisoning of the hydrogen oxidation reaction (HOR) electrocatalyst at the anode by carbon monoxide (CO), an impurity in the H2 fuel feedstocks derived from hydrocarbons, and the high expense and inefficiency of the oxygen reduction reaction (ORR) electrocatalyst at the cathode. The research described in this dissertation entails the synthesis and characterization of new bimetallic nanoparticle (NP) catalysts with controlled sizes, compositions, and architectures. By varying the NPs' compositions, structures, and electronic environments, we aimed to elucidate the physical and chemical relationships that govern their ability to catalyze chemical reactions pertinent to PEMFC operation. The ongoing research and development of these NP-based catalytic systems is essential to realizing the viability of this energy conversion technology. We describe the development of a simple method for synthesizing monometallic and bimetallic NPs supported on various reduced graphene oxide (rGO) supports. Electrochemical studies illustrate how the chemical nature of the rGO support impacts the catalytic behavior of the NP catalysts through unique metal-support interactions that differ depending on the elemental composition of the NP substrate. In another study, we present the synthesis and characterization of CoxPty NPs with alloy and intermetallic architectures and describe how their inherent characteristics impact their catalytic activities for electrochemical reactions. CoxPty NPs with alloy architectures were found to have improved CO tolerance compared to their intermetallic counterparts, while the performance of the CoxPty NPs for ORR catalysis was shown to be highly dependent on the NPs' crystal structure. Finally, we present the synthesis and characterization of various bimetallic core-shell NPs. Preliminary data for CO oxidation and PrOx catalysis demonstrated how subsurface metals modify the electronic structure of Ni and enhances its catalytic performance for CO oxidation and the PrOx reaction.Item Synthesis, Characterization and Catalytic Properties of Bimetallic Nanoparticles(2009) Dylla, Anthony Greg; Walker, Robert A; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to the ever-increasing desire for catalysts that possess high activities and selectivities for industrially relevant reactions, much effort is being spent on the synthesis of mono and bimetallic nanoparticles with tunable characteristics such as size, shape and bimetallic composition. Understanding how these characteristics influence catalytic performance is the key to rationally designing catalysts for a specific reaction. While significant breakthroughs have been made, particularly in the area of monometallic nanoparticles with regard to shape and size, relating the bimetallic structure, i.e., core@shell or alloy to a specific reactivity remains a difficult task. Work presented in this thesis describes the synthesis, characterization and catalytic properties of mono and bimetallic nanoparticles. Our efforts were motivated by the desire to understand the relationships that exist between metallic nanoparticle structure and their function as catalysts. This work also seeks to better understand the dynamic changes a nanoparticle's structure undergoes during typical catalytic operating conditions. Our approach is to use a wide array of analytical tools including optical methods, electron microscopy, XRD and mass spectrometry to provide an interlocking description of nanoparticle structure, function and durability. We show how the polymer coatings and degraded carbonaceous deposits affect propene hydrogenation catalytic activity of Pt nanoparticles. We also present a unique view of the interplay between thermodynamic and kinetic variables that control bimetallic nanoparticle alloy structures by looking at ordered and disordered PdCu alloy nanoparticles as a function of particle size. In another study we show that Ru@Pt and PtRu alloy nanoparticle catalysts have similar surface structures under oxidizing conditions but completely different surface structures under reducing conditions as probed by vibrational spectroscopy. These differences and similarities in surface composition correlate very well to their catalytic activity for CO oxidation under oxidizing and reducing environments, respectively. Finally, we present the synthesis and characterization of Cu@Pt nanoparticles with a particular focus on the core@shell formation mechanism. We also show how dramatic changes in the surface electronic structure of Cu versus Cu@Pt nanoparticles can affect their ability to transform light into heat by using Raman spectroscopy to observe graphite formation on the surface of these nanoparticles.