UMD Theses and Dissertations
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Item SOVLENT REACTIVITY AND INTERFACE EVOLUTION AT MODEL ELECTRODES FOR ENERGY APPLICATIONS(2016) Song, Wentao; Reutt-Robey, Janice E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Li-ion rechargeable battery (LIB) is widely used as an energy storage device, but has significant limitations in battery cycle life and safety. During initial charging, decomposition of the ethylene carbonate (EC)-based electrolytes of the LIB leads to the formation of a passivating layer on the anode known as the solid electrolyte interphase (SEI). The formation of an SEI has great impact on the cycle life and safety of LIB, yet mechanistic aspects of SEI formation are not fully understood. In this dissertation, two surface science model systems have been created under ultra-high vacuum (UHV) to probe the very initial stage of SEI formation at the model carbon anode surfaces of LIB. The first model system, Model System I, is an lithium-carbonate electrolyte/graphite C(0001) system. I have developed a temperature programmed desorption/temperature programmed reaction spectroscopy (TPD/TPRS) instrument as part of my dissertation to study Model System I in quantitative detail. The binding strengths and film growth mechanisms of key electrolyte molecules on model carbon anode surfaces with varying extents of lithiation were measured by TPD. TPRS was further used to track the gases evolved from different reduction products in the early-stage SEI formation. The branching ratio of multiple reaction pathways was quantified for the first time and determined to be 70.% organolithium products vs. 30% inorganic lithium product. The obtained branching ratio provides important information on the distribution of lithium salts that form at the very onset of SEI formation. One of the key reduction products formed from EC in early-stage SEI formation is lithium ethylene dicarbonate (LEDC). Despite intensive studies, the LEDC structure in either the bulk or thin-film (SEI) form is unknown. To enable structural study, pure LEDC was synthesized and subject to synchrotron X-ray diffraction measurements (bulk material) and STM measurements (deposited films). To enable studies of LEDC thin films, Model System II, a lithium ethylene dicarbonate (LEDC)-dimethylformamide (DMF)/Ag(111) system was created by a solution microaerosol deposition technique. Produced films were then imaged by ultra-high vacuum scanning tunneling microscopy (UHV-STM). As a control, the dimethylformamide (DMF)-Ag(111) system was first prepared and its complex 2D phase behavior was mapped out as a function of coverage. The evolution of three distinct monolayer phases of DMF was observed with increasing surface pressure — a 2D gas phase, an ordered DMF phase, and an ordered Ag(DMF)2 complex phase. The addition of LEDC to this mixture, seeded the nucleation of the ordered DMF islands at lower surface pressures (DMF coverages), and was interpreted through nucleation theory. A structural model of the nucleation seed was proposed, and the implication of ionic SEI products, such as LEDC, in early-stage SEI formation was discussed.Item Evaluating Surface Mechanisms for Catalytic Combustion Of H2 and CH4 on Pd Catalysts(2005-04-18) Seyed-Reihani, Seyed-Abdolreza; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Applications of fuel-lean catalytic combustion for power generation and exhaust heat recovery have raised the desire for reactor optimization. Such optimization requires adequately detailed surface chemistry models to predict reactor performance over a broad range of conditions relevant to the application. This study presents experimental studies in well-defined micro-reactors for catalytic combustion of CH4 and H2 on g-Al2O3 supported Pd catalysts, which are used to evaluate and refine surface chemistry mechanisms. The experimental results are compared to predictions by a transient numerical model for a catalytic channel flow with intra-phase diffusion in the porous washcoat support. Mechanisms for low temperature (< 250ºC) combustion of H2 under excess O2 and for relatively high temperature (> 400ºC) CH4 combustion under excess O2 have been developed and evaluated by comparison of experimental results in well-defined microreactors with transient model predictions. Low-temperature catalytic combustion over Pd-catalysts of very lean H2/O2 mixture diluted in N2 has been studied in the catalytic washcoat micro-reactor with transient exhaust monitoring using mass spectrometry. Experimental results reveal the important features of the Pd-H2-O2 surface chemistry under excess O2, particularly the effects of competitive adsorption/desorption of both the reactants and H2O product. Results show that H2 conversion depends on equivalence ratio at temperatures £ 125°C and on H2O vapor < 125°C. A proposed multi-step surface chemistry predicts based on detailed elementary reaction steps with thermodynamic reversibility and surface species interaction potentials captures the trends for conversion with respect to inlet temperature and water vapor. Intrinsic low dimensional manifolds (ILDM) were identified for the heterogeneous Pd-H2-O2 kinetics and the results show how specific species equilibration define the slowest modes in the catalytic reaction system. For the fuel lean CH4 oxidation over supported Pd catalysts, isothermal time-on-stream microreactor experiments and heating/cooling cyclic tests from 400°C to 850°C revealed the effects of PdO reduction/reoxidation on CH4 combustion kinetics. Test results with different H2O concentration revealed that competitive CH4 and H2O adsorption impacts on the activity only under catalyst conditions dominated by PdO. A detailed Pd-CH4-O2 surface mechanism predicts the impact of Pd reduction/reoxidation on CH4 oxidation rates. A post-process sensitivity analysis reveals the important reactions steps and provides a means for improving the detailed mechanism for predicting the complex hysteretic kinetics of CH4 oxidation on Pd.