Browsing by Subject "palladium"
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Item Catalysts for the hydrolysis of thiophosphate triesters(Texas A&M University, 2005-02-17) Picot, AlexandreThe degradation of phosphate triesters is efficiently catalyzed by organophosphate hydrolases (OPH). While a number of recent studies have focused on optimizing the rate of hydrolysis observed with the native enzyme, no dinuclear complexes that mimic the function of OPH have been reported or investigated. Our present research focuses on the synthesis of dinuclear metal complexes and on the study of their catalytic abilities. An important aspect of this research concerns the investigation of the coordination chemistry of dinuclear ligands designed to hold two metal cations in well defined positions. The ability of the different complexes to catalyze the degradation of thiophosphate triester is presented. Out of several complexes studied, ortho-metallated Pd (II) complexes have been found to display the highest catalytic activity for the hydrolysis of parathion.Item Chemisorption of Aromatic Compounds on Well-Defined Palladium Surfaces: Studies by Electron Spectroscopy and Electrochemistry(2010-10-12) Li, DingThe chemisorption of aromatic compounds, derivatized with different functional groups, on well-defined Pd(111) surfaces was studied by a combination of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and electrochemistry (EC). The results of this work led to the following trends and conclusions: (a) At low concentrations, 2,5-dihydroxythiophenol (DHT) chemisorbs on a Pd surface through both diphenolic ring and thiol group. At high concentrations, it chemisorbs only through the thiol group. (b) There is extensive intermolecular attraction between the co-adsorbed thiolated quinone and thiolated hydroquinone molecules. The interaction occurs through the Pd substrate and not through space. (c) The chemisorption properties of Nheteroaromatic compounds are pH-dependent. When the nitrogen heteroatom is protonated, it becomes very weakly surface-active. When the nitrogen heteroatom is deprotonated, surface activity stronger than the diphenolic ring is exhibited. (d) On a palladium surface, the binding strengths of ligands increase in the order: phenyl ring < quinonoid ring, < N-heteroatom < I < -SH.Item Chromium-Catalyzed Homoaldol Equivalent Reaction, Indium-Mediated Cycloisomerization, and Palladium-Catalyzed Cross-Coupling Reaction(2011-10-21) Kang, JunThe homoaldol reaction is one of the most powerful methods for the construction of C?C bonds as well as 1,4-oxygenated compounds yet this reaction remains in challenging tasks due to the instability of homoenolates which spontaneously cyclize to the cyclopropanolate. A regioselective catalytic homoaldol equivalent reaction of 3-bromo vinyl acetate with aldehydes under Cr(III)-Mn(0) redox condition was developed. This homoaldol equivalent reaction allows access to the 1,4-oxygenated compounds that are not possible by a conventional aldol process. Mild hydrolysis of the vinyl acetate and reduction of the homoaldol adducts generated diols and lactols in high yield (99%). Further manipulation including stereoselective epoxidation and cyclopropanation was achieved in an efficient manner. Furans, found in many natural products and utilized in drug discovery, have been well studied but current synthetic methods toward furans have some limitations in functional group tolerance, substrate scope, and low product yield in many cases. A highly efficient and catalytic cycloisomerization reaction that transforms acetylenic ?,?-epoxides to 2,3,5-tri-substituted furans under InCl3 catalysis was developed. This reaction sequence allows access to rapid construction of highly valuable, tri-substituted furan derivatives. Cross-coupling reactions utilizing transition metals and Lewis acids are important synthetic tools for the formation of C?C and C?N bonds and a number of cross-coupling reactions between ?-bromo carbonyl compounds and metal reagents such as aryl metals, alkenyl metals, and alkyl metals have been reported. Transition metal-catalyzed cross-coupling reaction for the construction of ?-alkynyl carbonyl compounds has reported in a limited case. The first approach to secondary ?-alkynyl carbonyl compounds from secondary ?-bromo esters and amides with tributyl(phenylethynyl)stannane under palladium-catalyzed cross-coupling reaction conditions was developed. This synthetic method allows access to secondary ?-alkynyl carbonyl compounds which are valuable precursors in pharmaceuticals and agricultural applications.Item Electrochemical hydrogenation of aromatic compounds chemisorbed at polycrystalline and single-crystal Pd surfaces(2009-06-02) Sanabria-Chinchilla, JeanThe chemisorption and electrochemical hydrogenation of hydroquinone (H2Q) at polycrystalline (pc) Pd, well-ordered Pd(100), and Pd-modified Au(hkl) electrodes were studied using a combination of ultra-high vacuum (UHV) surface spectroscopy, electrochemistry (EC), and electrochemical mass spectrometry (EC-MS). H2Q was found to form a slightly tilted flat-oriented quinone (Q) adlayer, when adsorbed from low concentrations; when chemisorbed from high concentrations, an edgewise-oriented H2Q adlayer was indicated. The hydrogenation of the chemisorbed layer is initiated at potentials before the onset of the hydrogen evolution region. As expected, the kinetics increases as the applied potential is increased, but the hydrogenation pathway appears to be independent of the potential. Hydrogenation in the absence of absorbed hydrogen (sub-surface) was studied at ultra-thin Pd films on Au single-crystal substrates. Hydrogenation and/or potential induced desorption were established, although non-volatile and/or hydrophobic products were detected. In comparison, negative excursions with benzene-coated electrodes resulted in nothing more than potential-induced desorption of the starting material. Negative-potential electro-desorption was more facile at terraces than at steps. Vibrational spectroscopic measurements suggested that hydrogenation occurs one molecule at a time to the fullest extent that resulted in desorption of product; that is, partially hydrogenated species do not exist on the surface.Item Mechanical Evaluation of Electronic Properties of Materials(2011-05-02) Nudo, NicholasThe present research focuses on the coupling of mechanical and electrical properties of materials and culminates in a direct connection between applied strain to thin-films, thin-film electron binding energy, the energy loss via plastic deformation provided by an indentation, and the substrate resistance. The methods used in this research include X-ray photoelectron spectroscopy (XPS), nanoindentation, digital optical microscopy, and sputter coat deposition. It is discovered that there is a shift in electron binding energy on the scale of 0.2 eV to 1.4 eV in gold and palladium thin-films sputtered on polyvinylidene fluoride (PVDF) through the application of strain induced by a convex shape. There is a change in the area beneath the load-displacement curve measured via indentation from 5.55 x 10^-10 J to 4.78 x 10^-10 J when the gold-palladium thin-film sputtered on PVDF is changed from the flat arrangement to the convex arrangement. Furthermore, the strain also changed the electrical resistance of aluminum foil, which indicates that the substrate electrical resistance is affected by the induced strain. The internal resistance of a circuit developed for this research changed from 7.76 ohms for flat samples to 8.03 ohms and 8.33 ohms for flat and convex samples, respectively. It is expected that the research can be used to estimate the strain in nanogears and other devices at small length scales.Item Simulation tools for predicting the atomic configuration of bimetallic catalytic surfaces(2012-12) Stephens, John Adam; Hwang, Gyeong S.Transition metal alloys are an important class of materials in heterogeneous catalysis due in no small part to the often greatly enhanced activity and selectivity they exhibit compared to their monometallic constituents. A host of experimental and theoretical studies have demonstrated that, in many cases, these synergistic effects can be attributed to atomic-scale features of the catalyst surface. Realizing the goal of designing -- rather than serendipitously discovering -- new alloy catalysts thus depends on our ability to predict their atomic configuration under technologically relevant conditions. This dissertation presents original research into the development and use of computational tools to accomplish this objective. These tools are all based on a similar strategy: For each of the alloy systems examined, cluster expansion (CE) Hamiltonians were constructed from the results of density functional theory (DFT) calculations, and then used in Metropolis Monte Carlo (MC) simulations to predict properties of interest. Following a detailed description of the DFT+CE+MC simulation scheme, results for the AuPd/Pd(111) and AuPt/Pt(111) surface alloys are presented. These two systems exhibit considerably different trends in their atomic arrangement, which are explicable in terms of their interatomic interactions. In AuPd, a preference for heteronuclear, Au-Pd interactions results in the preferential formation of Pd monomers and other small ensembles, while in AuPt, a preference for homonuclear interactions results in the opposite. AuPd/Pd(100) and AuPt/Pt(100) were similarly examined, revealing not only the effects of the same heteronuclear/homonuclear preferences in this facet, but also a propensity for the formation of second nearest-neighbor pairs of Pd monomers, in close agreement with experiment. Subsequent simulations of the AuPd/Pd(100) surface suggest the application of biaxial compressive strain as a means increasing the population of this catalytically important ensemble of atoms. A method to incorporate the effects of subsurface atomic configuration is also presented, using AuPd as an example. This method represents several improvements over others previously reported in the literature, especially in terms of its simplicity. Finally, we introduce the dimensionless scaled pair interaction, whereby the finite-temperature atomic configuration of any bimetallic surface alloy may be predicted from a small number of relatively inexpensive calculations.Item Well-defined ultrathin Pd films on Pt(111): electrochemical preparation and interfacial chemistry(Texas A&M University, 2005-08-29) Park, Yeon SuWell-defined ultrathin films of palladium, with coverages ranging from submonolayer, ΘPd = 0.5 monolayer (ML), to multilayer, ΘP d = 8 ML, were electrochemically deposited on Pt(111) using potentiostatic and potentiodynamic methods. In both methods, between the coverage regimes studied, the growth of the Pd films follows the Stranski-Krastanov mechanism. The interfacial electrochemical properties associated with the film-to-bulk transition were characterized by conventional voltammetric techniques in combination with low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The voltammetric peaks associated with H-atom adsorption and desorption on terrace sites indicate that the Pd electrodeposit starts to exhibit bulk-like properties at a coverage of 3 ML. Voltammetric cycling, in sulfuric acid solution, between the hydrogen evolution and the double-layer regions, was found to exert minimal influence on the annealing (smoothening) of the electrodeposited Pd films. However, cycling within the same potential region in the presence of bromide anions (at which Br- adsorption/Br desorption takes place) smoothens the initially rough Pd films essentially as well as high-temperature annealing. The influence of chemisorbed bromine on the anodic dissolution of Pd was also studied; this was for comparison with previous work on the anodic dissolution of Pd, in inert electrolyte, catalyzed by chemisorbed iodine. The present studies indicated that a small but measurable amount of bromine was desorbed along with dissolution of the Pd step atoms; bromine at the Pd terrace behaved identically to iodine in that the coverage of iodine is maintained regardless of the amount or origin of the of anodically stripped Pd. Atomically smooth, well-defined ultrathin Pd films were prepared by a constant potential deposition (CPD) method followed by multiple potential cycles, in dilute Brsolution, within the double-layer region and reductive removal of Brads, by simple emersion at a potential just before the hydrogen evolution reaction potential (EHER). A previously adapted method for the same purpose involved the chemisorption of iodine onto ultrathin PdCPD films, from dilute I- solution, followed by reductive desorption of Iads in iodide-free solution at pH 10 and at a potential just before EHER.