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IN-SITU GAS PHASE CATALYTIC PROPERTIES OF METAL NANOPARTICLES
Title
IN-SITU GAS PHASE CATALYTIC PROPERTIES OF METAL NANOPARTICLES.
Creator
Ono, Luis, Roldan Cuenya, Beatriz, University of Central Florida
Abstract / Description
Recent advances in surface science technology have opened new opportunities for atomic scale studies in the field of nanoparticle (NP) catalysis. The 2007 Nobel Prize of Chemistry awarded to Prof. G. Ertl, a pioneer in introducing surface science techniques to the field of heterogeneous catalysis, shows the importance of the field and revealed some of the fundamental processes of how chemical reactions take place at extended surfaces. However, after several decades of intense research,...
Show moreRecent advances in surface science technology have opened new opportunities for atomic scale studies in the field of nanoparticle (NP) catalysis. The 2007 Nobel Prize of Chemistry awarded to Prof. G. Ertl, a pioneer in introducing surface science techniques to the field of heterogeneous catalysis, shows the importance of the field and revealed some of the fundamental processes of how chemical reactions take place at extended surfaces. However, after several decades of intense research, fundamental understanding on the factors that dominate the activity, selectivity, and stability (life-time) of nanoscale catalysts are still not well understood. This dissertation aims to explore the basic processes taking place in NP catalyzed chemical reactions by systematically changing their size, shape, oxide support, and composition, one factor at a time. Low temperature oxidation of CO over gold NPs supported on different metal oxides and carbides (SiO2, TiO2, TiC, etc.) has been used as a model reaction. The fabrication of nanocatalysts with a narrow size and shape distribution is essential for the microscopic understanding of reaction kinetics on complex catalyst systems ("real-world" systems). Our NP synthesis tools are based on self-assembly techniques such as diblock-copolymer encapsulation and nanosphere lithography. The morphological, electronic and chemical properties of these nanocatalysts have been investigated by atomic force microscopy (AFM), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). Chapter 1 describes briefly the basic principles of the instrumentation used within this experimental dissertation. Since most of the state-of-art surface science characterization tools provide ensemble-averaged information, catalyst samples with well defined morphology and structure must be available to be able to extract meaningful information on how size and shape affect the physical and chemical properties of these structures. In chapter 2, the inverse-micelle encapsulation and nanosphere lithography methods used in this dissertation for synthesizing uniformly arranged and narrow size- and shape-selected spherical and triangular NPs are described. Chapter 3 describes morphological changes on individual Au NPs supported on SiO2 as function of the annealing temperature and gaseous environment. In addition, NP mobility is monitored. Chapter 4 explores size-effects on the electronic and catalytic properties of size-selected Au NPs supported on a transition metal carbide, TiC. The effect of interparticle interactions on the reactivity and stability (catalyst lifetime) of Au NPs deposited on TiC is discussed in chapter 5. Size and support effects on the formation and thermal stability of Au2O3, PtO and PtO2 on Au and Pt NPs supported on SiO2, TiO2 and ZrO2 is investigated in chapter 6. Emphasis is given to gaining insight into the role of the NP/support interface and that played by oxygen vacancies on the stability of the above metal oxides. Chapter 7 reports on the formation, thermal stability, and vibrational properties of mono- and bimetallic AuxFe1-x (x = 1, 0.8, 0.5, 0.2, 0) NPs supported on TiO2(110). At the end of the thesis, a brief summary describes the main highlights of this 5-year research program.
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Date Issued
2009
Identifier
CFE0002940, ucf:47962
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0002940
THE INFLUENCE OF GRAPHIC ORGANIZERS ON STUDENTS' ABILITY TO SUMMARIZE AND COMPREHEND SCIENCE CONTENT REGARDING THE EARTH'S CHANGING SURFACE
Title
THE INFLUENCE OF GRAPHIC ORGANIZERS ON STUDENTS' ABILITY TO SUMMARIZE AND COMPREHEND SCIENCE CONTENT REGARDING THE EARTH'S CHANGING SURFACE.
Creator
Goss, Patricia, Ortiz, Enrique, University of Central Florida
Abstract / Description
The purpose of this action research project was to determine how my practice of using graphic organizers during instruction influenced my students' ability to summarize and comprehend significant fifth grade Earth Science content regarding the Earth's changing surface. A secondary purpose was to determine the students' perceptions of how concept mapping assisted in making connections to understand the fifth grade Earth Science content regarding the Earth's changing surface....
Show moreThe purpose of this action research project was to determine how my practice of using graphic organizers during instruction influenced my students' ability to summarize and comprehend significant fifth grade Earth Science content regarding the Earth's changing surface. A secondary purpose was to determine the students' perceptions of how concept mapping assisted in making connections to understand the fifth grade Earth Science content regarding the Earth's changing surface. The three processes used to collect data for this research were concept maps, focus groups and the pre- and post-test results. The themes that emerged were the ability to describe, categorize and classify details, the increased accuracy of the use of vocabulary and the memory of the concepts that students' ability to recall information and understand the Earth Science concepts as evidenced through summarization and comprehension through the pre- and post-test.
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Date Issued
2009
Identifier
CFE0002595, ucf:48287
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0002595
First Principles Studies of Pattern Formations and Reactions on Catalyst Surfaces
Title
First Principles Studies of Pattern Formations and Reactions on Catalyst Surfaces.
Creator
Le, Duy, Rahman, Talat, Roldan Cuenya, Beatriz, Schelling, Patrick, Sohn, Yongho, University of Central Florida
Abstract / Description
This dissertation undertakes theoretical research into the adsorption, pattern formation, and reactions of atoms, molecules, and layered materials on catalyst surfaces. These investigations are carried out from first-principles calculations of electronic and geometric structures using density functional theory (DFT) for predictions and simulations at the atomic scale. The results should be useful for further study of the catalytic activities of materials and for engineering functional...
Show moreThis dissertation undertakes theoretical research into the adsorption, pattern formation, and reactions of atoms, molecules, and layered materials on catalyst surfaces. These investigations are carried out from first-principles calculations of electronic and geometric structures using density functional theory (DFT) for predictions and simulations at the atomic scale. The results should be useful for further study of the catalytic activities of materials and for engineering functional nanostructures.The first part of the dissertation focuses on systematic first-principles simulations of the energetic pathways of CO oxidation on the Cu2O(100) surface. These simulations show CO to oxidize spontaneously on the O-terminated Cu2O(100) surface by consuming surface oxygen atoms. The O-vacancy on Cu2O(100) then is subsequently healed by dissociative adsorption of atmospheric O2 molecules.The second part discusses the pattern formation of hydrogen on two and three layers of Co film grown on the Cu(111) surface. It is found that increasing the pressure of H2 changes the hydrogen structure from 2H-(2 x 2) to H-p(1 x 1) through an intermediate structure of 6H-(3 x 3).The third part compares the results of different ways of introducing van der Waals (vdW) interactions into DFT simulations of the adsorption and pattern formation of various molecules on certain substrates. Examinations of the physisorption of five nucleobases on graphene and of n-alkane on Pt(111) demonstrate the importance of taking vdW interactions into account, and of doing so in a way that is best suited to the particular system in question. More importantly, as the adsorption of 1,4 diaminebenzene molecules on Au(111) shows inclusion of vdW interactions is crucial for accurate simulation of the pattern formation.The final part carries out first-principles calculations of the geometric and electronic structure of the Moire pattern of a single layer of Molybdenum disulfide (MoS2) on Cu(111). The results reveal three possible stacking types. They also demonstrate that the MoS2 layer to be chemisorbed, albeit weakly, and that, while Cu surface atoms are vertically disordered, the layer itself is not strongly buckled.
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Date Issued
2012
Identifier
CFE0004224, ucf:48991
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004224
The physical properties and composition of main-belt asteroids from infrared spectroscopy
Title
The physical properties and composition of main-belt asteroids from infrared spectroscopy.
Creator
Landsman, Zoe, Campins, Humberto, Britt, Daniel, Fernandez, Yan, Emery, Joshua, Hernandez, Florencio, University of Central Florida
Abstract / Description
Asteroids are the remnants of planet formation, and as such, they represent a record of the physical and chemical conditions in the early solar system and its evolution over the past 4.6 billion years. Asteroids are relatively accessible by spacecraft, and thus may be a source of the raw materials necessary for future human exploration and settlement of space. Those on Earth-crossing orbits pose impact hazards for which mitigation strategies must be developed. For these reasons, several...
Show moreAsteroids are the remnants of planet formation, and as such, they represent a record of the physical and chemical conditions in the early solar system and its evolution over the past 4.6 billion years. Asteroids are relatively accessible by spacecraft, and thus may be a source of the raw materials necessary for future human exploration and settlement of space. Those on Earth-crossing orbits pose impact hazards for which mitigation strategies must be developed. For these reasons, several missions to asteroids are in progress or planned with the support of the National Aeronautics and Space Administration (NASA) and other national space agencies. The study of asteroid composition and physical surface properties is vital to both our scientific understanding of the solar system's formation and evolution and to the development of asteroid missions and resource utilization schemes. This dissertation uses infrared spectroscopy to investigate the composition and physical properties of main-belt asteroid surfaces. Our efforts are focused on two populations that are especially relevant to constraining thermal and collisional processes in the asteroid belt: the "M-type" asteroids and primitive asteroid families.To investigate volatiles in the M-type asteroids, we obtained 2-4 micron spectra of six M-type asteroids using NASA's Infrared Telescope Facility. We find spectral signatures of hydrated minerals on all six asteroids, with evidence for rotational variability of hydration in one target. Diversity in the shape of the 3-micron feature in our sampled asteroids suggests there are different modes of hydration in the M-type population. Next, we carried out a thermal and compositional study of M-type asteroid (16) Psyche using 5-14 micron spectra from the Spitzer Space Telescope. Psyche is suspected to be a remnant iron core, and it is the target of an upcoming NASA mission. Using thermophysical modeling, we find that Psyche's surface is smooth and most likely has a thermal inertia of 5-25 J/m^2/K/s^(1/2), and a bolometric emissivity of 0.9, although a scenario with an emissivity of 0.7 and thermal inertia up to 95 J/m^2/K/s^(1/2) is possible if Psyche is somewhat larger than previously determined. From comparisons with laboratory spectra of silicate and meteorite powders, Psyche's emissivity spectrum is consistent with the presence of fine-grained ((<)75 micron) silicates. These silicates may include a magnesian pyroxene component. We conclude that Psyche is likely covered in a fine silicate regolith, which may also contain iron grains, overlying an iron-rich bedrock.Finally, we compared the mid-infrared properties of two primitive asteroids families, ancient Themis (~2.5 Gyr) and young Veritas (~8 Myr). Visible and near-infrared studies show spectral differences between the two families attributed to different degrees of space weathering. To test whether these differences are apparent in the mid-infrared, we analyzed the 5-14 micron Spitzer Space Telescope spectra of 11 Themis-family asteroids and 9 Veritas-family asteroids. We detect a broad 10-micron emission feature, attributed to fine-grained and/or porous silicate regolith, in all 11 Themis-family spectra and six of nine Veritas-family asteroids, with 10-micron spectral contrast ranging from 1% +/- 0.1% to 8.5% +/- 0.9%. Comparison with laboratory spectra of primitive meteorites suggests these asteroids are similar to meteorites with relatively low abundances of phyllosilicates. We used thermal modeling to derive diameters, beaming parameters and albedos for our sample. Asteroids in both families have beaming parameters near unity and geometric albedos in the range 0.031-0.14. Spectral contrast of the 10-micron silicate emission feature is not correlated with asteroid diameter; however, higher 10-micron contrast may be associated with flatter spectral slopes in the near-infrared. There is a slight trend of increasing 10-micron contrast with decreasing albedo in the Veritas asteroids, but not the Themis asteroids. Overall, our results indicate the Themis and Veritas family members show variation in regolith texture and/or structure within both families that is not directly related to family age.
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Date Issued
2017
Identifier
CFE0007124, ucf:51966
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007124
Theoretical Studies of Nanostructure Formation and Transport on Surfaces
Title
Theoretical Studies of Nanostructure Formation and Transport on Surfaces.
Creator
Aminpour, Maral, Rahman, Talat, Stolbov, Sergey, Roldan Cuenya, Beatriz, Blair, Richard, University of Central Florida
Abstract / Description
This dissertation undertakes theoretical and computational research to characterize and understand in detail atomic configurations and electronic structural properties of surfaces and interfaces at the nano-scale, with particular emphasis on identifying the factors that control atomic-scale diffusion and transport properties. The overarching goal is to outline, with examples, a predictive modeling procedure of stable structures of novel materials that, on the one hand, facilitates a better...
Show moreThis dissertation undertakes theoretical and computational research to characterize and understand in detail atomic configurations and electronic structural properties of surfaces and interfaces at the nano-scale, with particular emphasis on identifying the factors that control atomic-scale diffusion and transport properties. The overarching goal is to outline, with examples, a predictive modeling procedure of stable structures of novel materials that, on the one hand, facilitates a better understanding of experimental results, and on the other hand, provide guidelines for future experimental work. The results of this dissertation are useful in future miniaturization of electronic devices, predicting and engineering functional novel nanostructures. A variety of theoretical and computational tools with different degrees of accuracy is used to study problems in different time and length scales. Interactions between the atoms are derived using both ab-initio methods based on Density Functional Theory (DFT), as well as semi-empirical approaches such as those embodied in the Embedded Atom Method (EAM), depending on the scale of the problem at hand. The energetics for a variety of surface phenomena (adsorption, desorption, diffusion, and reactions) are calculated using either DFT or EAM, as feasible. For simulating dynamic processes such as diffusion of ad-atoms on surfaces with dislocations the Molecular Dynamics (MD) method is applied. To calculate vibrational mode frequencies, the infinitesimal displacement method is employed. The combination of non-equilibrium Green's function (NEGF) and DFT is used to calculate electronic transport properties of molecular devices as well as interfaces and junctions.
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Date Issued
2013
Identifier
CFE0005298, ucf:50504
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005298