Browsing by Subject "Surface science"
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Item Catalysis research using model catalysts(2013-05) Yan, Ting, active 2013; Mullins, C. B.Catalysts are essential for technological advances, because of their indispensable role in chemical and material manufacturing, energy conversion, and pollution control systems. Developing better catalysts is a highly desired goal that is impeded by the complexity of heterogeneous catalysts. This makes it extremely difficult to obtain information regarding active sites and reaction mechanisms, which is critical for improving catalyst design and performance. My research work has led to the understanding of how specific catalytic surface sites affect the performance of catalysts by constructing conceptually simpler planar model catalysts for kinetics and mechanism studies using model surface science tools and batch reaction testing. The work in this dissertation has demonstrated that planar model catalysts are versatile tools to probe reaction mechanisms on industrial catalysts. Supported gold nanoparticles have shown remarkable catalytic activity in a variety of reactions. However, many fundamental aspects of gold catalysts are still unclear, especially about the identity of active sites and oxidizing species. A Au(111) single crystal, the most stable and abundant facet on gold nanoparticles, is utilized to understand the reaction mechanisms of partial oxidation of 2-butanol and allyl alcohol. By controlling oxygen coverage on the surface, 100% selectivity to corresponding ketone and aldehyde, the desirable products, can be achieved. Two model catalysis systems, gold nanoclusters supported on a TiO₂(110) substrate and iron oxide dispersed on a Au(111) surface, were employed to understand the reaction pathways of CO oxidation and probe the role of the oxide/metal interface. The mechanistic and kinetic studies have shown that planar model catalysts are useful tools to probe reactions on industrial catalysts. The mechanistic understanding obtained from model catalyst studies can be used to create better catalysts.Item Determination and Characterization of Ice Propagation Mechanisms on Surfaces Undergoing Dropwise Condensation(2011-08-08) Dooley, Jeffrey B.The mechanisms responsible for ice propagation on surfaces undergoing dropwise condensation have been determined and characterized. Based on experimental data acquired non-invasively with high speed quantitative microscopy, the freezing process was determined to occur by two distinct mechanisms: inter-droplet and intradroplet ice crystal growth. The inter-droplet crystal growth mechanism was responsible for the propagation of the ice phase between droplets while the intra-droplet crystal growth mechanism was responsible for the propagation of ice within individual droplets. The larger scale manifestation of these two mechanisms cooperating in tandem was designated as the aggregate freezing process. The dynamics of the aggregate freezing process were characterized in terms of the substrate thermal di usivity, the substrate temperature, the free stream air humidity ratio, and the interfacial substrate properties of roughness and contact angle, which were combined into a single surface energy parameter. Results showed that for a given thermal di usivity, the aggregate freezing velocity increased asymptotically towards a constant value with decreasing surface temperature, increasing humidity, and decreasing surface energy. The inter-droplet freezing velocity was found to be independent of substrate temperature and only slightly dependent on humidity and surface energy. The intra-droplet freezing velocity was determined to be a strong function of substrate temperature, a weaker function of surface energy, and independent of humidity. From the data, a set of correlational models were developed to predict the three freezing velocities in terms of the independent variables. These models predicted the majority of the measured aggregate, inter- and intra-droplet freezing velocities to within 15%, 10%, and 35%, respectively. Basic thermodynamic analyses of the inter- and intra-droplet freezing mechanisms showed that the dynamics of these processes were consistent with the kinetics of crystal growth from the vapor and supercooled liquid phases, respectively. The aggregate freezing process was also analyzed in terms of its constituent mechanisms; those results suggested that the distribution of liquid condensate on the surface has the largest impact on the aggregate freezing dynamics.Item Methods for modifying the physical and catalytic properties of surfaces(2010-05) Flaherty, David William, 1980-; Mullins, C. B.; Henkelman, Graeme; Hwang, Gyeong S.; Korgel, Brian A.; Sitz, Greg O.Catalysts can be significantly improved by modifying their structure or composition. Simple adaptations of the physical structure of a catalyst can give rise to changes in the chemical behavior, in part, due to alterations in the coordination of active sites. Modifications in the surface or bulk composition of a material have a profound impact on the chemistry that is promoted as a result of electronic and physical factors. Optimizing these qualities may enhance the catalyst’s activity, selectivity or stability. In this dissertation, we explore the application of two distinct approaches for modifying the chemical properties of catalytically active materials. Through the use of a broad array of techniques we quantify changes in critical properties such as physical-crystallographic structure; morphology, surface area and porosity; as well as catalytic activity, selectivity and stability. First, reactive ballistic deposition of metal atoms within a low pressure gas provides a unique opportunity for synthesizing thin films of a wide variety of materials. The morphology, structure, and porosity of the resulting material can be tailored through control of the deposition angle and substrate temperature. By conducting deposition perpendicular to the surface, a film can be grown with a dense, conformal structure. On the other hand, deposition at oblique angles results in high surface area, porous films comprised of regular arrays of nanocolumnar structures. Furthermore, variations in the deposition angle allow for the inclusion of under-coordinated sites which change the chemical activity of the surface. Improvements in the activity, selectivity and stability of transition metal catalysts can be made by alloying the catalyst with a second element. The formation of molybdenum carbide decreases the strength of chemisorption on the surface, with respect to molybdenum, and improves selectivity for the dehydrogenation of formic acid. Platinum is active for the water-gas shift reaction. However, this catalyst cannot operate at low temperatures due to CO poisoning and is susceptible to deactivation due to accumulation of carbonaceous deposits. The formation of a platinum-copper near-surface alloy dramatically modifies the interactions of the surface with CO, H₂O and H₂ which can enhance the performance of this catalyst for the water-gas shift reaction.Item Surface Characterization of Heterogeneous Catalysts Using Low Energy Ion Scattering Spectroscopy Combined with Electrochemistry(2010-07-14) Axnanda, Stephanus R.Fundamental studies of heterogeneous catalysis were performed and presented in this dissertation to gain a better understanding of heterogeneous catalytic reactions at a molecular level. Surface science techniques were employed in achieving the goal. Low energy ion scattering spectroscopy (LEISS) is the main surface science technique which will be used in all the studies discussed throughout this dissertation. The main objectives of LEISS measurements are to: 1) obtain the information of surface composition of heterogeneous catalysts from the topmost layer; 2) observe the effects of reaction conditions on the surface composition of heterogeneous catalysts. The surface composition and morphology of Au-Pd clusters bimetallic model catalysts supported on SiO2 were characterized using LEISS, infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD). It is observed that relative to the bulk, the surface of the clusters is enriched in Au. Ethylene adsorption and dehydrogenation show a clear structure-reactivity correlation with respect to the structure/composition of these Au-Pd model catalysts. Fundamental studies of heterogeneous catalysis were performed and presented in this dissertation to gain a better understanding of heterogeneous catalytic reactions at a molecular level. Surface science techniques were employed in achieving the goal. Low energy ion scattering spectroscopy (LEISS) is the main surface science technique which will be used in all the studies discussed throughout this dissertation. The main objectives of LEISS measurements are to: 1) obtain the information of surface composition of heterogeneous catalysts from the topmost layer; 2) observe the effects of reaction conditions on the surface composition of heterogeneous catalysts. The surface composition and morphology of Au-Pd clusters bimetallic model catalysts supported on SiO2 were characterized using LEISS, infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD). It is observed that relative to the bulk, the surface of the clusters is enriched in Au. Ethylene adsorption and dehydrogenation show a clear structure-reactivity correlation with respect to the structure/composition of these Au-Pd model catalysts.