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Browsing by Subject "Mechanism"

Now showing 1 - 5 of 5
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    Characterizing The Mechanism Of Nanocrystalline Anatase To Rutile Phase Transformation
    (2014-10) Sabyrov, Kairat
    Phase transformations are important processes by which materials evolve in both natural and synthetic materials. Understanding the nature, mechanisms, and kinetics of phase transformations, as well as the micro structural changes that accompany them, require comprehensive characterization so as to gain a deeper understanding of atomic scale mechanisms and better control materials properties. A combination of experimental and theoretical techniques has led to improved understanding of how phase transformations are initiated at interfaces and then propagate by growth of the more stable phase at the expense of meta-stable phase. In this work, nanocrystalline TiO2 is used as a model system to systematically explore the atomic level mechanism of particle mediated phase transformation. First, a number of important research studies are highlighted to improve our understanding of this aggregation based phase transformation. Second, the dependence of anatase to rutile phase transformation and crystal growth kinetics on crystallite size and aggregation state of the particles in aqueous suspension is explored by varying reaction conditions. Rates of anatase growth and its transformation to rutile increase with decreasing initial grain size under hydrothermal conditions. Overall, rates are slower at the higher pHs employed. Furthermore, densely aggregated particles show higher transformation and growth rates, compared to loosely aggregated ones. Third, macroscopic modeling was used to characterize the kinetics of anatase to rutile phase transformation. Kinetic data under acidic, hydrothermal conditions are consistent with a two-step phase transformation mechanism: interface-nucleation followed by dissolution-precipitation. Fourth, a kinetic model that enables quantitative assessment of the contribution to the rate of phase transformation by dissolution-precipitation and by interface-nucleation has been developed. Generally speaking, interface-nucleation plays a critical role during the early stages of the transformation, regardless of pH, whereas dissolution-precipitation dominates the later stages of the transformation. Finally, anatase to rutile phase transformation kinetics was exploited to produce nanoporous rutile nanocrystals by controlling the solubility of TiO2 nanocrystals. All in all, the observations and results obtained in this work might enable new insights into the mechanism of particle-mediated phase transformation and better control over the mechanism to produce materials with desired properties.
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    Development of Aryl Decarboxylative Functionalization Reactions
    (2021-05) Daley, Ryan
    New synthetic methods are needed to address the tremendous environmental costs associated with fine chemical production. Towards this goal, my graduate research has focused on designing and developing new decarboxylative cross-coupling methods. These reactions replace high energy, unstable, and expensive organometallic nucleophiles with low energy and readily available (hetero)aryl carboxylic acids. First, I reported an efficient palladium catalyzed salt-free decarboxylative allylation of allylic carbonates, which was designed to generate base in-situ. Next, I advanced a method for the decarboxylative cross-coupling of inexpensive aryl chlorides. The mechanism of this transformation was investigated and gave insights that enabled an increase in catalyst performance. More recently, I developed the first gold catalyzed decarboxylative cross-coupling of aryl iodides. This reaction overcame limitations associated both with decarboxylative couplings and reactions involving redox events at gold. I then investigated the mechanism this reaction with the goal of surmounting the current limitations of this promising technology.
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    Probe reactions of alcohols and alkanes for understanding catalytic properties of microporous materials and alumina oxide solid acid catalysts.
    (2012-08) Chiang, Hsu
    Steady-state chemical reactions of alkanol dehydration and alkane hydroisomerization were employed as probe reactions to evaluate the effects of structure and composition of zeolites and gamma alumina (γ-Al2O3) on measured catalytic rates and selectivity to establish the mechanistic cycles and catalytic site requirements for the corresponding reactions. The measured kinetic effects of ethanol and ethylene pressure on diethyl ether (DEE) formation over three zeolite materials (H-MFI, H-FER and H-MOR) show that ethanol dimers are formed during reaction and that these dimeric species are subsequently dehydrated to form DEE. In zeolites, ethylene formation was only observed on zeolites possessing 8-MR channels (H-MOR). The 8-MR channels protect ethanol monomeric species and prevent the formation of ethanol dimeric species due to size restrictions. The results of ethanol dehydration reactions studied in this research imply that the design and selection of microporous catalysts for performing shape-selective reactions of oxygenates requires us to consider the size and stability of the corresponding surface intermediates as well as the location of Brønsted acid sites. Hydroisomerization reactions on bifunctional metal-acid catalysts (Pt/Al2O3 and acid zeolites) can convert n-hexane into 2-methylpentane (2MP) and 3-methylpentane (3MP). The measured rate of n-hexane isomerization was linearly proportional to the molar ratio of H2 to n-C6H14 over three zeolites (FER, MOR, BEA), consistent with a bifunctional mechanism involving the facile dehydrogenation of n-hexane into n-hexene on the metal catalyst and a kinetically-relevant step involving isomerization of n-hexene to 2MP and 3MP on zeolitic acidic sites. Sodium-exchanged MOR was used to study the rate of n-hexane isomerization in 12MR channel (MOR(12MR) ) and 8MR pockets (MOR(8MR)) in MOR. The measured rate of isomerization over zeolites increase in the order of FER < MOR(12MR) < MOR(8MR) < BEA, showing that pore size cannot be used to accurately predict the occurrence or exclusion of a particular reaction within zeolitic solids. The mechanisms and site requirements for unimolecular and bimolecular dehydration reactions of ethanol on γ-Al2O3 were investigated using steady state and isotopic kinetic studies and in situ titration. The rates of ethylene and DEE formation from ethanol dehydration on γ-Al2O3 at 488K were not inhibited by exposure to CO2 (0-47kPa) while pyridine exposure (3kPa) nearly shut down the rates of formation, showing that acid sites rather than basic sites are required for ethylene and DEE formation. The proposed mechanisms for ethylene and DEE formation proceed through desorption of surface-bound ethoxide species and the activation of surface ethanol dimeric species, respectively. The proposed mechanisms are consistent with the measured pressure dependence in ethanol and water, and the measured kinetic isotope effects using isotopic labeling reactants. The rate and equilibrium constants in the kinetic models derived from these mechanisms were estimated using Athena Visual Studio software to assess the stability of surface species formed during ethanol dehydration reactions.
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    Redesign Of The Standard Macintosh Laryngoscope To Improve Glottic Visualization During Endotracheal Intubation
    (2016-08) Sand, Yashovardhan
    Direct laryngoscopy is a procedure which allows for the visualization of the glottis during the process of endotracheal intubation (placement of the endotracheal tube into the trachea). The primary device used for this procedure is called the laryngoscope. The laryngoscope consists of a handle and a blade. During insertion of the blade, the curvature of the tongue is followed and the tip of the blade is inserted into the vallecula. In the vallecula resides the hyoepiglottic ligament, which connects the anterior surface of the epiglottis and the body of the hyoid bone. Different types of laryngoscopes exist, with the two most frequently used being the Macintosh and the Miller laryngoscope. The two laryngoscopes differ in the shape of the blade, one being curved (mac) and the other one straight (miller). During laryngoscopy with the Macintosh laryngoscope, the tip of the blade is used to push on the hyoepiglottic ligament, which lifts the epiglottis up and allows for exposure of glottis. In this procedure, the forces exerted on tissues can be significant (up to 50 N) and there can be complications as a result of the forces exerted on the soft tissues of the oropharynx and larynx. These complications can be esophageal intubation, hypoxia, and nerve damage. Furthermore, difficult intubations result in increased intubation time and a higher number of intubation attempts. The most common hurdle is the inability to successfully complete the endotracheal intubation. One of the reasons why the endotracheal intubation may not be successful is the inability to visualize the glottic opening. The level of intubation difficulty is based on the extent of the view of the glottis obtained during intubation and is classified into four grade views. A new type of laryngoscope blade, which looks like the Macintosh blade, but adds features that would allow to mitigate the aforementioned factors, was designed. The primary goal of the design was to provide a mechanism which could deliver a more anterior direct laryngoscopy view and allow for an enhanced lift of the epiglottis. This would be accomplished by adding a feature which would compress the body of the tongue and also lift the tip of the epiglottis more than the conventional Macintosh blade. The new design, unlike current blades, consists of three blade segments—one fixed and two movable. The fixed blade segment is the largest, with one end fixed to the laryngoscope handle and the other hinged to the first movable segment. The first movable segment is the second largest segment of the blade, and at one end is hinged to the fixed segment and at the other it is hinged to the third segment. The middle segment is designed to compress the tongue and push it out of the way to improve vocal cord visualization. The third (smallest) segment is the blade tip which is hinged at one end to the middle second segment. This is used to enter the vallecula and push against the hyoepiglottic ligament to lift the epiglottis. The design is purely mechanical with no electrical or electronic components.  A prototype was built and tested on manikins by anesthesiologists to primarily assess the change in the grade view of intubation by the flexible design. To evaluate the efficacy of the blade, a poor intubation grade view was forced and then without applying any incremental lifting forces, the mechanism was engaged to produce a final grade view. A qualitative pressure distribution was mapped for each attempt, and was compared to that obtained by a Macintosh blade. The improvement in the view with the flexible design was significant (p=0.000038). The pressures applied on the manikin airway by the flexible laryngoscope was less than that applied by the Macintosh laryngoscope to successfully intubate. The design of the flexible laryngoscope allowed for an improvement in the grade view of intubation and reduced the lifting forces applied on the airway. The built device proved that a flexible design can assist in difficult intubations.
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    Sensory aspects of astringency and changes of the oral environment in the mechanism of astringency.
    (2010-08) Lee, Catherine A.
    Astringency, a tactile sensation felt in the mouth after exposure to various foods, is not well understood. The underlying mechanism is not fully known, and it remains a challenging attribute to assess in sensory tests. Additionally, while most food astringency is caused by polyphenolic compounds, the cause of astringency in some other foods is not known. For the first part of my research, my objective was to improve the understanding of the mechanism of astringency by determining if it was related to a loss of saliva's ability to lubricate, the precipitation of specific classes of salivary proteins, or to the removal of oral lubricating films that coat the inside of the mouth. My results show that while astringency may be related to the latter, astringency is not related to a loss of salivary lubricity or the precipitation of any one class of salivary proteins. The second part of my research aimed to improve palate cleansing strategies for astringent foods. Palate cleansers are often used in an attempt to reduce build-up of astringency intensity that occurs over repeated exposures to astringent foods, but it is unknown how, or if, commonly used palate cleansers affect a person's sensitivity to astringency. Although I did not find that any of the cleansers were superior in their ability to limit the build-up of astringency intensity, it was clear that panelists were better able to discriminate among the astringency of various strength solutions when they used nothing or water to cleanse their palates. The aim of part III of my research was to determine the cause of astringency in acidic whey protein beverages. Although some believe that the whey proteins directly cause the astringency, there was reason to suspect that their high acid concentration was instead responsible. The results of my study found this to be true.