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In-situ Transmission Electron Microscopy for Understanding Heterogenous Electrocatalytic CO2 ReductionAbdellah, Ahmed January 2023 (has links)
This thesis delivers an in-depth investigation into electrochemical carbon dioxide reduction (CO2R), a process with the potential to convert CO2 gas into value-added chemicals and fuels. However, the efficiency and operational durability of current CO2 reduction processes are limited by catalytic performance. To address this, the thesis focuses on gaining a deep understanding of the transformations that CO2R electrocatalysts undergo under realistic conditions, such as morphological, phase structure, and compositional changes. These insights inform the design of next-generation materials by identifying performance descriptors and degradation patterns. A key aspect of this thesis is the development and application of in-situ liquid phase transmission electron microscopy (LP-TEM), an advanced platform that directly correlates nanoscale changes in catalyst materials under the influence of electrode potentials in CO2R reactive environments. Despite its potential, the use of in-situ LP-TEM presents a range of challenges, which this thesis addresses alongside exploring potential advancements for enhancing its accuracy and applicability. With the evolution of nanofabricated liquid cells, dynamic nanoparticle tracking, and high-resolution imaging in a liquid medium, this technology can more accurately mimic realistic exposure conditions. Cumulatively, this thesis systematically navigates the technical hurdles, advancements, and future prospects of in-situ LP-TEM in the context of electrochemical CO2R. The findings not only advance our understanding of the in-situ LP-TEM technical process but also guide new researchers in the field of in-situ TEM of electrocatalyst materials, aiding in the optimization of catalyst design, and paving the way for more sustainable and economically competitive CO2R technologies.
The application of in-situ LP-TEM extends to the examination of two specific catalysts: Palladium (Pd) and a bi-metallic alloy of Copper (Cu) and Silver (Ag). By employing in-situ LP-TEM and selected area diffraction (SAD) measurements, we trace the morphological and phase structure transformations of the Pd catalyst under CO2R conditions. Interestingly, our findings indicate that alterations in reaction energetics, rather than morphological or phase structure changes, chiefly govern catalyst selectivity. This provides invaluable insights for designing more efficient catalysts.
Further, we observe the morphological transformation of a metallic copper catalyst structure into a Cu-Ag bimetallic alloy during a galvanic replacement method. We then investigate the stability of both catalyst structures under operational CO2R conditions. Our results reveal that the metallic Cu structure undergoes significant morphological deformation during CO2R, leading to migration, detachment, and recrystallization of the catalyst surface. Contrarily, the Cu-Ag bimetallic alloy demonstrates notable thermodynamic stability under a similar applied potential. / Thesis / Candidate in Philosophy / This PhD thesis focuses on the development and implementation of cutting-edge technologies to address the climate change implications of CO2 emissions - a potent greenhouse gas. CO2 molecules could be electrochemically converted into various chemical feedstock and fuels. This process involves the development of efficient catalyst designs that can reduce CO2 gas at high conversion rates. Acquiring mechanistic insights on the behavior of the developed catalysts under reaction conditions would significantly assist on producing performance descriptors for catalyst design in CO2 conversion approach. Among a range of different advanced techniques, in-situ liquid phase transmission electron microscopy (LP-TEM) technology is selected for this study. This technique is capable of correlating dynamic nanoscale compositional and morphological changes with the electrochemical response of the catalysts. The primary focus of the thesis is on developing and implementing in-situ LP-TEM techniques to achieve electrochemical CO2 conditions while tracking particle morphology and phase structures as functions of electrochemical potential and time. Furthermore, the thesis investigates the performance of different catalyst designs under CO2 reduction (CO2R) operational conditions, which includes palladium (Pd) nanoparticles and copper–silver (Cu–Ag) bimetallic alloys. On a fundamental level, these studies provide a detailed understanding of the phase transformation and structural changes of these catalysts during CO2R that contributes valuable knowledge to the field and can be used to design next-generation CO2R catalysts.
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Inclusion Detection in Liquid Aluminum Via Laser-Induced Breakdown SpectroscopyHudson, Shaymus W 08 April 2016 (has links)
Aluminum alloy castings are becoming commonplace for critical applications in the automotive and aerospace industries where materials failure is not an option. In order to meet such property demands, tight control over the cleanliness of the melt (mitigation of solid particle inclusions) and microstructure must be achieved. In order to control cleanliness, it must first be well defined and measured. Very few techniques exist in industry that can quantitatively measure inclusion levels in-situ. Laser-induced breakdown spectroscopy (LIBS) is presented as a promising technique to quantify solid particles, desired or undesired, in aluminum melts. By performing LIBS with subsequent statistical analysis on liquid aluminum with varying concentrations of Al2O3, AlB2, TiB2, and SiC particles, calibration curves relating particle concentration and elemental intensity were drawn. Through metallography and automated electron microscopy, it was found that inclusions less than 10 um in size could be detected with LIBS. Concentrations down to at least one part-per-million could be detected and accurately measured, allowing for LIBS to be use as a tool for complete, real-time melt cognition.
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Using Visible and Near Infrared Diffuse Reflectance Spectroscopy to Characterize and Classify Soil ProfilesWilke, Katrina Margarette 2010 August 1900 (has links)
Visible and near infrared diffuse reflectance spectroscopy (VisNIR-DRS) is a
method being investigated for quantifying soil properties and mapping soil profiles.
Because a VisNIR-DRS system mounted in a soil penetrometer is now commercially
available for scanning soil profiles in situ, methodologies for using scans to map soils
and quantify soil properties are needed. The overall goal of this research is to investigate
methodologies for collecting and analyzing VisNIR-DRS scans of intact soil profiles to
identify soil series. Methodologies tested include scanning at variable versus uniform
moistures, using individual versus averaged spectra, boosting an intact spectral library
with local samples, and comparing quantitative and categorical classifications of soil
series. Thirty-two soil cores from two fields, representing three soil series, were
extracted and scanned every 2.5 cm from the soil surface to 1.5 m or to the depth of
parent material at variable field moist conditions and at uniform moist condition.
Laboratory analyses for clay, sand, and silt were performed on each horizon. Soil series
were classified using partial least squares regression (PLS) and linear discriminant
analysis (LDA). A Central Texas intact spectral library (n=70 intact cores) was used for PLS modeling, alone and boosted with the two fields. Because whole-field independent
validation was used, relative percent difference (RPD) values were used to compare
model performance. Wetting soils to uniform moisture prior to scanning improved
prediction accuracy of total clay and RPD improved by 53 percent. Averaging side-by-side
scans of the same soil profile improved prediction accuracy of RPD by 10 percent. When
creating calibration models, boosting a library with local samples improved prediction
accuracy of clay content by 80 and 34 percent for the two fields. Principal component plots
provided insight on the spectral similarities between these datasets. Overall, using PLS
alone performed the same as LDA at predicting soil series. Most importantly, results of
this project reiterate the importance of fully-independent calibration and validation for
assessing the true potential of VisNIR-DRS. Using VisNIR-DRS is an effective way for
in situ characterization and classification of soil properties.
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Solution-Processing of Organic Solar Cells: From In Situ Investigation to Scalable ManufacturingAbdelsamie, Maged 05 December 2016 (has links)
Photovoltaics provide a feasible route to fulfilling the substantial increase in demand for energy worldwide. Solution processable organic photovoltaics (OPVs) have attracted attention in the last decade because of the promise of low-cost manufacturing of sufficiently efficient devices at high throughput on large-area rigid or flexible substrates with potentially low energy and carbon footprints. In OPVs, the photoactive layer is made of a bulk heterojunction (BHJ) layer and is typically composed of a blend of an electron-donating (D) and an electron-accepting (A) materials which phase separate at the nanoscale and form a heterojunction at the D-A interface that plays a crucial role in the generation of charges. Despite the tremendous progress that has been made in increasing the efficiency of organic photovoltaics over the last few years, with power conversion efficiency increasing from 8% to 13% over the duration of this PhD dissertation, there have been numerous debates on the mechanisms of formation of the crucial BHJ layer and few clues about how to successfully transfer these lessons to scalable processes. This stems in large part from a lack of understanding of how BHJ layers form from solution. This lack of understanding makes it challenging to design BHJs and to control their formation in laboratory-based processes, such as spin-coating, let alone their successful transfer to scalable processes required for the manufacturing of organic solar cells. Consequently, the OPV community has in recent years sought out to better understand the key characteristics of state of the art lab-based organic solar cells and made efforts to shed light on how the BHJ forms in laboratory-based processes as well as in scalable processes. We take the view that understanding the formation of the solution-processed bulk heterojunction (BHJ) photoactive layer, where crucial photovoltaic processes take place, is the one of the most crucial steps to developing strategies towards the implementation of organic solar cells with high efficiency and manufacturability. In this dissertation, we investigate the mechanism of the BHJ layer formation during solution processing from common lab-based processes, such as spin-coating, with the aim of understanding the roles of materials, formulations and processing conditions and subsequently using this insight to enable the scalable manufacturing of high efficiency organic solar cells by such methods as wire-bar coating and blade-coating. To do so, we have developed state-of-the-art in situ diagnostics techniques to provide us with insight into the thin film formation process. As a first step, we have developed a modified spin-coater which allows us to perform in situ UV-visible absorption measurements during spin coating and provides key insight into the formation and evolution of polymer aggregates in solution and during the transformation to the solid state. Using this method, we have investigated the formation of organic BHJs made of a blend of poly (3-hexylthiophene) (P3HT) and fullerene, reference materials in the organic solar cell field. We show that process kinetics directly influence the microstructure and morphology of the bulk heterojunction, highlighting the value of in situ measurements. We have investigated the influence of crystallization dynamics of a wide-range of small-molecule donors and their solidification pathways on the processing routes needed for attaining high-performance solar cells. The study revealed the reason behind the need of empirically-adopted processing strategies such as solvent additives or alternatively thermal or solvent vapor annealing for achieving optimal performance. The study has provided a new perspective to materials design linking the need for solvent additives or annealing to the ease of crystallization of small-molecule donors and the presence or absence of transient phases before crystallization. From there, we have extended our investigation to small-molecule (p-DTS (FBTTh2)2) fullerene blend solar cells, where we have revealed new insight into the crucial role of solvent additives. Our work has also touched upon modern polymers, such as PBDTTPD, where we have found the choice of additives impacts the formation mechanism of the BHJ. Finally, we have performed a comparative study of the BHJ film formation dynamics during spin coating versus wire-bar coating of p-DTS(FBTTh2)2: fullerene blends that has helped in curbing the performance gap between lab-based and scalable techniques. This was done by implementing a new apparatus that combines the benefits of rapid thin film drying common to spin coating with scalability of wire-bar coating. Using the new apparatus, we successfully attain similar performance of solar cell devices to the ones fabricated by spin coating with dramatically reduced material waste.
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Transparent Conductive Tantalum Doped Tin Oxide as Selectively Solar-Transmitting Coating for High Temperature Solar Thermal ApplicationsLungwitz, F., Escobar-Galindo, R., Janke, D., Schumann, E., Wenisch, R., Gemming, S., Krause, M. 07 May 2019 (has links)
The transparent conductive oxide (TCO) SnO2:Ta is developed as a selectively solar-transmitting coating for concentrated solar power (CSP) absorbers. Upon covering with an antireflective layer, a calculated absorptivity of 95% and an emissivity of 30% are achieved for the model configuration of SnO2:Ta on top of a perfect black body (BB). High-temperature stability of the developed TCO up to 1073 K is shown in situ by spectroscopic ellipsometry and Rutherford backscattering spectrometry. The universality of the concept is demonstrated by transforming silicon and glassy carbon from non-selective into solar-selective absorbers by depositing the TCO on top of them. Finally, the energy conversion efficiencies of SnO2:Ta on top of a BB and an ideal non-selective BB absorber are extensively compared as a function of solar concentration factor C and absorber temperature TH. Equal CSP efficiencies can be achieved by the TCO on BB configuration with approximately 50% lower solar concentration. This improvement could be used to reduce the number of mirrors in a solar plant, and thus, the levelized costs of electricity for CSP technology.
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Understanding Electrode-Electrolyte Interfaces with Metal Dissolution and Redeposition ChemistryHu, Anyang 18 January 2023 (has links)
The fundamental understanding of the dynamic characteristics of metal dissolution and redeposition behavior at the electrode-electrolyte interface is essential, which provides the basis for the development of advanced energy and conversion devices (such as electrochromic devices, electrocatalysts, and batteries) with superior electrochemical performances. We firstly demonstrate the feasibility of resynthesizing the electrode surface chemistry and tuning the electrochemical reactions at the solid-liquid interface by selectively changing the electrolyte composition and electrochemical cycling conditions. Amorphous TiO2 surface layers can be formed on WO3 electrodes by adding exotic Ti cations to the electrolyte, and slow electrochemical cycling. The dissolution and redeposition of electrodes and surface coatings are intertwined, helping to establish a dissolution-redeposition equilibrium at the interface, which can inhibit metal dissolution, stabilize electrode morphology, and promote electrochemical performance.
Since the diffusion layer generated by the dissolution of transition metals is ubiquitous at the electrochemical solid-liquid interface, by combining in situ three-electrode electrochemical reaction cell with advanced spatially resolved synchrotron X-ray fluorescence microscopy and micro-X-ray absorption spectroscopy, we then successfully demonstrate the formation and chemical identification of the diffusion layer. By studying the evolution of diffusion layers(tens of micrometers thick) when using WO3 electrodes in acidic electrolytes, we find that with increasing distance of the dissolved species from the electrode surface, the oxidation state remains largely unchanged, but the local electronic environment of the dissolved W species becomes more distorted.
We subsequently report a systematic experimental approach by collecting a series of twodimensional fluorescence images at the electrodes to study electrode dissolution and redeposition under different electrochemical conditions. The results show that (1) metal dissolution and redeposition behaviors greatly evolve under different electrode polarization and electrolyte compositions; (2) metal dissolution and redeposition behaviors are independent of bulk electrolyte pH but depend on interfacial pH; and (3) the accumulation of interfacial dissolved species promotes the formation of polytungstate interfacial networks, which ultimately manifest as temporal heterogeneity of redeposition.
Lastly, we provide an in-depth study of the underlying mechanism of electrochemicalcycling induced crystallization at the electrode-electrolyte interface through a combination of advanced synchrotron radiation characterization techniques and an in situ electrochemical reaction setup. We have discovered that (1) foreign cations from the electrolyte engender both tensile and compressive strains inside the crystal; (2) repeated electrode dissolution and redeposition promote crystal growth through a non-classical crystallization pathway of particle attachment, but the initial growth of crystals is inhibited by internal strains; and (3) as the strain accumulates, the crystal rotates or moves, which is the fundamental reason for the dynamic structure evolution of the crystal during electrochemical cycling. To our knowledge, this is the first study of electrochemical-cycling-induced crystallization and its strain evolution. These new findings reveal a previously unknown relationship between crystal growth and its internal strain at the electrode-electrolyte interface. / Doctor of Philosophy / Energy drives the entire economy and human civilization. Energy is needed in every aspect of everyday life, and energy is an essential raw material for making and delivering all the products and services that modern society needs, even though it is invisible to us. Since 2000, the global energy demand has increased tenfold and economic growth has spawned a large number of new energy industries, but billions of people are still in urgent need of clean water, sanitation, nutrition, and medical care. Energy is a key factor in meeting these basic requirements for all of humanity. The increasing global energy demand and the increasing impact of climate change have put enormous pressure on the energy market. Therefore, it is necessary to accelerate the relevant actions of energy transition in the world. Among them, the research and innovation of electrochemical energy storage and conversion technology is a major direction. The electrochemical energy storage and conversion technology heavily relies on the various electrochemical reactions in practical devices such as rechargeable batteries, water electrocatalysts, and energy-saving electrochromic smart windows. Within numerous electrochemical reactions under the application, the solid (electrode)-liquid (electrolyte) interface dominates the most important electrochemical reactions. How to understand thephysicochemical reactions at the interface under electrochemical conditions is of great significance. As a major component of research innovations, this research contributes to the design of rational electrode materials, electrolyte compositions, and more efficient and durable electrochemical performance. From a fundamental perspective, my research enriches the understanding of solid-liquid interface reactions under electrochemical conditions, pointing out that electrode dissolution and redeposition and dynamic structural evolution of solid-liquid interfaces are important for further optimizing electrode material design and improving electrochemical performance.
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Caracteriza??o in situ e diversidade gen?tica de algodoeiros moc?s (Gossypium hissutum ra?a marie galante) da Regi?o Nordeste do Brasil / In situ characterization of moco cotton plants (Gossypium hirsutum race marie galante) of Brazil s Northeast regionMenezes, Ivandilson Pessoa Pinto de 26 February 2009 (has links)
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Previous issue date: 2009-02-26 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Brazil is one of the major centers of diversity for polyploid cotton plants; these plants belong to the genus Gossypium, which has three known species: G. hirsutum, G. barbadense and G. mustelinum. The Northeast is the only region where the three species occur, the last group being endemic. Northeast s cotton plants can be important sources of variability for genetic breeding. It is believed that great part of local diversity is being lost, due to economic, political, cultural and agricultural problems. In an attempt to mitigate this loss and delineate conservation strategies it is necessary to know how the species are found where they occur. The objective was to characterize and determine how plants are maintained in situ in the states of Maranh?o, Piau?, Cear?, Rio Grande do Norte and Para?ba at the beginning of the XXI century. The in situ characterization of G. hirsutum and G. barbadense was conducted through structured interviews with the cotton plants owners and through the analysis of the environment. The data were collected during expeditions undertaken between the years 2004 to 2005. Twenty-two plants were collected in the state of Para?ba, forty-four in the state of Rio Grande do Norte, one hundred and forty-six in the state of Cear?, forty in the state of Maranh?o and ninety-one plants in the state of Piau?. All plants collected in the states of Para?ba and Rio Grande do Norte belonged to moco type. Moco cotton plants also predominated in the other states, representing 92%, 62% and 78% of plants collected in Cear?, Piau? and Maranh?o, respectively. The other cotton plants collected belong to the species G. barbadense. The cotton plants were found in situ as dooryard plants, roads side, feral populations, cultivation or local varieties. Great part were dooryard plants (45.2%), being major in Piau? and Maranh?o. Cultivation predominated in Cear?; in Rio Grande do Norte feral populations were the most frequent and, in Para?ba, local varieties. The maintenance of moco plants is related, mainly, to the phytotherapic domestic use (20.9%) and to confection of lamp wicks (29.7%). Few inhabitants in Para?ba, Rio Grande do Norte, Piau? and none in Maranh?o used harvest the plants, storage the seeds or gin; however, in Cear?, 40.5% of owners affirmed that they harvested and commercialized the fiber. It was found that the maintenance of species is dependent of the fragile cultural habits of local inhabitants, therefore the maintenance in situ is not a suitable way to conservation of genetic resources. The efforts must be directed to the continuity of collections, maintenance and characterization ex situ / O Brasil ? um dos importantes centros de diversidade de algodoeiros polipl?ides pertencente ao g?nero Gossypium, com 3 esp?cies conhecidas: G. hirsutum, G. barbadense e G. mustelinum. O Nordeste ? a ?nica regi?o com ocorr?ncia das tr?s esp?cies, sendo a ?ltima end?mica. Os algodoeiros desta regi?o podem ser fontes importantes de variabilidade para o melhoramento gen?tico. Acredita-se que grande parte da diversidade local esteja sendo perdida, devido a problemas econ?micos, pol?ticos, culturais e agr?colas. Na tentativa de mitigar tal perda e delinear estrat?gias de conserva??o ? necess?rio conhecer como as esp?cies se encontram no local em que ocorrem. Objetivou-se caracterizar e determinar o modo com que as plantas s?o mantidas in situ nos estados do Maranh?o, Piau?, Cear?, Rio Grande do Norte e Para?ba no inicio do s?culo XXI. A caracteriza??o in situ de G. hirsutum e G. barbadense foi realizada por meio de entrevista estruturada com o propriet?rio e pela an?lise do ambiente. Os dados foram tomados durante expedi??es empreendidas entre os anos de 2004 a 2005. Foram coletadas 22 plantas no estado da Para?ba, 44 no estado do Rio Grande do Norte, 146 no estado do Cear?, 40 no estado do Maranh?o e 91 plantas no estado do Piau?. Todas as plantas coletadas nos estados da Para?ba e Rio Grande do Norte eram do tipo moc?. O algodoeiro moc? tamb?m predominou nos demais estados, representando 92%, 62% e 78% das plantas coletadas no Cear?, Piau? e Maranh?o, respectivamente. Os demais algodoeiros coletados pertencem a esp?cie G. barbadense. Os algodoeiros moc?s foram encontrados in situ na forma de planta de fundo de quintal, beira de estrada, feral, lavoura, variedade local. Em grande parte eram do tipo fundo de quintal (45,2%), sendo maioria no Piau? e Maranh?o. No Cear? predominou o tipo lavoura, no Rio Grande do Norte tipo feral e na Para?ba variedades locais. A manuten??o das plantas do tipo moc? est? ligada, principalmente, ao uso dom?stico fitoter?pico (20,9%) e confec??o de pavios para candeeiro (29,7%). Poucos moradores na Para?ba, Rio Grande do Norte, Piau? e nenhum no Maranh?o apresentaram o h?bito de realizar a colheita, armazenamento e beneficiamento das sementes, entretanto no Cear? 40,5% dos propriet?rios afirmaram realizar a colheita e comercializar a fibra. Verificou-se que a manuten??o da esp?cie ? dependente dos fr?geis h?bitos culturais da popula??o local, portanto a manuten??o in situ n?o ? um meio adequado ? conserva??o dos recursos gen?ticos. Os esfor?os devem ser direcionados para a continuidade das coletas, caracteriza??o e manuten??o ex situ
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Synthesis and Characterization of Mononuclear and Binuclear Copper Species in Cu-Exchanged Zeolites for Redox Reactions including Partial Methane OxidationLaura Wilcox (7534151) 13 October 2021 (has links)
<p>Cu-zeolites have received renewed attention as catalytic materials
that facilitate partial methane oxidation (PMO) to methanol, with a variety of mononuclear,
binuclear, and multinuclear Cu active site motifs that have been proposed in
prior literature. Our approach to more precisely identify and probe the Cu
structures that activate O<sub>2</sub> and reduce in CH<sub>4 </sub>relies on
the synthesis of model supports with varying composition and well-defined Cu
speciation, which also facilitates connections between experimental data and
theoretical models. Chabazite (CHA) zeolites are high-symmetry frameworks that
contain a single lattice tetrahedral site (T-site), in which Cu<sup>2+</sup>
ions exchange at paired Al sites in a six-membered ring (6-MR) while CuOH<sup>+</sup>
species exchange at isolated 6-MR Al sites, the latter of which can react to
form binuclear O/O<sub>2</sub>-bridged Cu structures. In this work, Cu-CHA zeolites
were synthesized to contain predominantly Cu<sup>2+</sup> (Z<sub>2</sub>Cu) or CuOH<sup>+</sup>
(ZCuOH) species of varying density, or a mixture of Z<sub>2</sub>Cu and ZCuOH
sites. Z<sub>2</sub>Cu and ZCuOH sites were quantified by titration of residual
Brønsted acid sites with NH<sub>3</sub>, which respectively exchange with 2:1
or 1:1 H<sup>+</sup>:Cu<sup>2+</sup> stoichiometry. Stoichiometric PMO reaction
cycles on Cu-zeolites involved high-temperature (723 K) activation in O<sub>2</sub>,
and then moderate-temperature (473 K) reduction in CH<sub>4</sub> and treatment
in H<sub>2</sub>O (473 K) to extract CH<sub>3</sub>OH. <i>I</i><i>n-situ</i> UV-Visible spectroscopy under
oxidizing (O<sub>2</sub>, 723 K) and reducing (CO, 523 K; CH<sub>4</sub>, 473
K; He, 723 K) conditions detected the presence of mononuclear and binuclear Cu
site types, while <i>in-situ</i> Cu K-edge X-ray absorption spectroscopy after
such treatments was used to quantify Cu(I) and Cu(II) contents and <i>in situ</i> Raman spectroscopy was used to
identify the Cu structures formed. ZCuOH, but not Z<sub>2</sub>Cu sites, are
precursors to binuclear O/O<sub>2</sub>-bridged Cu sites that form upon O<sub>2</sub>
activation and subsequently produce methanol after stoichiometric PMO cycles,
at yields (per total Cu) that increased systematically with ZCuOH site density.
The fraction of Cu(II) sites that undergo auto-reduction in inert at high
temperatures (He, 723 K) is identical, within experimental error, to the
fraction that reduces in CH<sub>4</sub> at temperatures relevant for PMO (473
K), providing a quantitative link between the binuclear Cu site motifs involved
in both reaction pathways and motivating refinement of currently postulated PMO
reaction mechanisms. These Cu-CHA zeolites were also studied for other redox
chemistries including the selective catalytic reduction (SCR) of NO<sub>x</sub>
with NH<sub>3</sub>. <i>In situ </i>UV-Visible and X-ray absorption
spectroscopies were used to monitor and quantify the transient partial
reduction of Cu(II) to Cu(I) during exposure to NH<sub>3</sub> (473 K), in
concert with titration methods that use NO and NH<sub>3</sub> co-reductants to
fully reduce all Cu(II) ions that remain after treatment in NH<sub>3</sub> alone
to the Cu(I) state, providing quantitative evidence that both Z<sub>2</sub>Cu
and ZCuOH sites are able to reduce in NH<sub>3</sub> alone to similar extents
as a function of time. These findings provide new insight into the reaction
pathways and mechanisms in which NH<sub>3</sub> behaves as a reductant of
mononuclear Cu(II) sites in zeolites, which are undesired side-reactions that
occur during steady-state NO<sub>x</sub> SCR and that often unintendedly result
in Cu(II) reduction prior to spectroscopic or titrimetric characterization. Overall,
the strategy in this dissertation employs synthetic methods to control framework
Al density and arrangement in zeolite supports to emphasize extra-framework Cu site
motifs of different structure and at different spatial densities, and to
interrogate these model materials using a combination of <i>in situ</i>
spectroscopic techniques together with theory, in order to elucidate active
site structure and proximity requirements in redox catalysis. This work
demonstrates how quantitative reactivity and site titration data, brought
together with an arsenal of tools available in contemporary catalysis research,
can provide detailed mechanistic insights into transition metal-catalyzed redox
cycles on heterogeneous catalysts.</p>
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ADVANCED CHARACTERIZATIONS FOR THE IDENTIFICATION OF CATALYST STRUCTURES AND REACTION INTERMEDIATESNicole J Libretto (8953583) 16 June 2020 (has links)
<p>In recent decades, alternatives to traditional coal and
fossil fuels were utilized to reduce carbon emissions. Among these
alternatives, natural gas is a cleaner fuel and is abundant globally. Shale
gas, a form of natural gas that also contains light alkanes (C2-C4), is
presently being employed to produce olefins, which can be upgraded to higher
molecular weight hydrocarbons. This thesis describes efforts to develop new
catalytic materials and characterizations for the conversion of shale gas to
fuels.</p>
<p>In the first half, silica supported Pt-Cr alloys containing
varying compositions of Pt and Pt<sub>3</sub>Cr were used for propane
dehydrogenation at 550°C. Although a change in selective performance was
observed on catalysts with varying promoter compositions, the average
nano-particle structures determined by <i>in situ</i>, synchrotron x-ray
absorption spectroscopy (XAS) and x-ray diffraction (XRD) were identical.
Further, this work presents a method for the characterization of the catalytic
surface by these methods to understand its relationship with olefin selectivity.
From this, we can gain an atomically precise control of new alloys
compositions with tunable surface structures.</p>
<p>Once formed by dehydrogenation, the intermediate olefins are
converted to fuel-range hydrocarbons. In the second half, previously unknown
single site, main group Zn<sup>2+</sup> and Ga<sup>3+</sup> catalysts are shown
to be effective for oligomerization and the resulting products follow a Schutlz
Flory distribution. Mechanistic studies suggest these catalysts form metal
hydride and metal alkyl reaction intermediates and are active for olefin
insertion and b-H elimination elementary steps,
typical for the homogeneous, Cossee-Arlman oligomerization mechanism. Evidence
of metal hydride and metal alkyl species were observed by XAS, Fourier
transform infrared spectroscopy (FTIR), and H<sub>2</sub>/D<sub>2</sub> isotope
exchange. Understanding the reaction intermediates and elementary steps is
critical for identifying novel oligomerization catalysts with tunable product
selectivity for targeted applications. </p>
<p> Through
controlled synthesis and atomic level <i>in situ </i>characterizations, new
catalysts compositions can be developed with high control over the resulting
performance. An atomically precise control of the catalyst structure and
understanding how it evolves under reaction conditions can help shed light on
the fundamental principles required for rational catalyst design. </p>
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Development of Alternative Materials to Replace Precious Metals in Sustainable Catalytic TechnologiesJain, Deeksha January 2019 (has links)
No description available.
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