Browsing by Subject "Catalysts"
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Item An Investigation of the System, Nab2sO-Vb2sOb5s(Texas Tech University, 1962-05) Karimi, HosseinNot Available.Item Creating more effective functional materials: altering the electronics of conducting metallopolymers for different applications.(2014-05) Raiford, Matthew Thomas; Holliday, Bradley J.; Humphrey, Simon M; Anslyn, Eric V; Jones, Richard A; Freeman, Benny DConducting metallopolymers possess attractive electronic properties for use in sensors, photoelectronic devices, catalysts, and other applications. Modification of the conducting polymer backbone, through chemical or electrochemical methods, enables control of catalytic, electronic, and optical properties of the metal via inductive modulation of the electron density. Understanding in detail the relationship between the metal and polymer backbone could lead to more effective metallopolymer materials. We hope to study this relationship by probing the band gaps, excited state energy levels, catalytic activity, and sensor function in four metallopolymer systems. Devices with sub-stochiometric ratios of Cu2ZnSnS4 NPs (CZTS: (Cu2Sn)1-xZn1/xS)(0≥x≥0.75)) grown in Cu(II) conducting metallopolymers were produced to study band gap tuning in hybrid materials. The valence and conductance bands of CZTS (x = 0.60) aligned with the HOMO/LUMO of the Cu(II) metallopolymer. Changing the alignment facilitated charge transfer in the hybrid material, leading to photovoltaic materials with efficiencies of ~0.1%. Chemoresistive ionophore sensors were developed by incorporating selective binding groups, such as thiourea, into conducting polymer backbones. Thiourea monomers and polymers showed high selectivity for Pb(II) ions over many competitive ions. XPS experiments demonstrated that reversible chelation of Pb(II) ions could be achieved through a simple uptake/rinse process. The conductivity of the thiourea polymer increased fifty-fold, from 7.75×10−2 S/cm2 to 3.5 S/cm2, after Pb(II) exposure. Sensitivity measurements indicated the sensors have limits of detection near 10−10 M. Highly conjugated ligands were synthesized to explore effective sensitization of visible and near-IR emitting lanthanides. (3,4-ethylenedioxy)thiophene was appended to dipyridophenazine and dipyridoquinoxaline to introduce a group that could be easily electropolymerized. These bi-functional ligands emitted from π-π* and an inter-ligand charge transfer excited states, and therefore, two distinct triplet states were observed. These separate energy pathways allowed for efficient sensitization of both visible (Tb(III), Eu(III), Dy(III)) and near-IR emitting (Nd(III), Yb(III), Er(III)) ions. Finally, we explored the oxidation of a rhodium-containing conducting metallopolymer and the subsequent effect on the activity of the metal center. Oxidation of the backbone led to ancillary ligand attenuation, allowing for control of the catalytically active species in the conducting metallopolymer. Rh(I,III) monomer and metallopolymer catalytic studies showed potential for new heterogenous/homogeneous hybrid catalysts.Item Deactivation of a commercial reforming catalyst(Texas Tech University, 1969-05) Nunn, Norman PatrickNot availableItem Development of nanofiber protective substrates(Texas Tech University, 2004-08) Subbiah, ThandavamoorthyElectrospinning uses high voltage electric field to produce high surface area fibers in the nanometer range. Polymeric nanofibers were prepared by the electrospinning process and were characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). A study on the relationship between process parameters and fiber characteristics was undertaken. The dependence of fiber morphology on the solvent volatility and collector substrate characteristics was critically analyzed. Results on the self assembling nature of the charged fibers over different collector substrates were obtained and reported in the thesis. Defect free nanofiber webs with high specific surface area and low porosities suitable enough to be used as adsorptive filtration membranes were prepared. Polyurethane nanofibers were used as nano metal oxide catalyst carriers by successfully impregnating the catalyst in a single-step electrospinning process. Aerosol filtration abilities of nanofiber membranes were tested and the results are presented.Item Development of Pd₃Co based catalysts for fuel cell applications and amine based solvents for CO₂ capture : a first principles based modelling of clean energy and clean air technology(2014-12) Manogaran, Dhivya; Hwang, Gyeong S.; Mullins, C. B.With the ever increasing environmental concerns in terms of the need for a vast improvement in clean energy and clean air technologies, this thesis focuses on analyzing the underlying principles that determine the activity of catalysts/sorbents for fuel cell applications and CO₂ capture using first principles based simulations with a view point to help fabricate efficient catalysts. We attempt to clarify the fuzzy concepts of existing surface-nearsurface interactions in Pd based electrocatalysts with particular attention to Pd₃Co alloy catalysts by presenting a thorough inter and intra-layer orbital analysis and bring forth the crucial role played by the surface-subsurface binding driven by the out of plane d-state interactions in determining the surface reactivity. We first decouple the effects induced by the different Pd-Pd and Pd-Co lattice parameters (lattice strain effect) from the hetero atom induced surface-subsurface interaction (we call it "interlayer ligand effect") and clearly demonstrate how enhanced surface-subsurface d [subscript xz+yz] interaction leads to an increased oxygen hydrogenation to H₂O in Pd₃Co based electrocatalysts. We then extend the concept of hetero atom induced surface-subsurface binding to a series of 3d transition metals and provide guidelines for the right choice of metals that may be potential ORR candidates. Finally, we describe the facet dependence and the effect of surface Au alloying on the surface reactivity of Pd₃Co electrocatalysts. In the second section of the thesis, we emphasize on the underlying principles of CO₂ capture by MEA and study the synergetic interplay of various factors that may lead to better CO₂ capture , also enabling efficient solvent regeneration. Though extensive studies are carried out on the most traditionally used alkanol amine MEA for CO₂ capture, there are several less studied aspects like the molecular orbital redistribution on CO₂ binding that decides the fate of the intermediate species and the role of water arrangement in assisting/hindering the progress of the reaction. We study the fundamental CO₂-amine interactions and highlight the crucial importance of alkanol-amine configuration, water arrangement and protonation/de-protonation tendencies at various basic sites in the development of the reaction. We then analyze the synergetic interplay between the inductive effect, the steric hindrance and the resonance in enhancing efficient CO₂ binding and allowing an alternative O-driven mechanism resulting into easy solvent regeneration. We believe that our efforts may help fabricate better catalysts and sorbents and help improve the existing clean energy and clean air technologies.Item Formaldehyde yields from methanol electrochemical oxidation on platinum and supported catalysts(Texas Tech University, 1999-12) Childers, Christina L.The formation of formaldehyde during methanol electrochemical oxidation is being measured with a fluorescence assay in order to assess the importance of formaldehyde as a reaction intermediate and source of efficiency loss in direct methanol fuel cells. Initial studies have focused on the oxidation of methanol on polycrystalline platinum. The formaldehyde yields approached 30% of the total electrolysis charge at 0.2-0.3 V (vs. a KCI saturated Ag/AgCI reference electrode) for methanol concentrations between 15 mM and 0.3 M in 0.1 M perchloric acid. The formaldehyde yields were lower at more positive potentials, as other oxidation pathways became dominant. However, the rate of formaldehyde production increased up to 0.5 V. These initial studies have demonstrated that formaldehyde, which is often not detectable with modern in situ spectroelectrochemical analysis techniques, can be produced in significant amounts during methanol electrochemical oxidation. More recent work has focused on the formation of formaldehyde during methanol electrochemical oxidation on supported platinum and platinumruthenium catalysts. Solid, polycrystalline platinum-ruthenium alloys have been considered. Other catalysts studied have been suspended in Nafion and supported on glassy carbon. Methanol oxidation on the catalysts has resulted in low formaldehyde yields, below 2% at all potentials studied. The low formaldehyde yields, which result from more complete methanol oxidation, are believed to arise from the ability of partial oxidation products to be transported to an array of active catalyst sites dispersed within the three dimensional Nafion film network. Efforts to eliminate these volume effects through techniques such as electrochemical depositions of catalyst crystallites by reduction of transition metal salts onto solid, glassy carbon electrodes; direct metal nanoparticle deposition onto solid, glassy carbon electrodes using a hydrogen tube furnace; and "sticky" carbon methods for metal/wax/carbon type electrodes have been under investigation.Item Increasing the in vitro digestibility of mesquite with inorganic catalysts and ozone(Texas Tech University, 1982-08) Fish, Steven JeffreyNot availableItem Mechanistic studies on yield function of an immobilized multifunctional catalyst(Texas Tech University, 1985-08) Jih, Tsae-jiunnThe purpose of this thesis can be divided into three portions. First of all, initial rates are integrated from differential equations for the single and dual substrate enzymatic reaction by using the GEAR package. The artificial initial rates are used to estimate the limitation of applicability of the Michaelis-Menten equation derived by making the equilibrium assumption. A comparison between Michaelis-Menten and a rate equation based on steady-state assumption is introduced, which is the second portion of the thesis. For the last portion of the thesis a corresponding reactivity with and without immobilization of the enzyme which depends on the orientation of the enzyme is discussed. The formulated model seeks to explain the reaction among plasmin, fibrin, and CYp-antiplasmin from the viewpoint of the orientation of the enzyme in the imm.obilized state.Item Model catalyst studies of the CO oxidation reaction on Titania supported gold nanoclusters(2004) Stiehl, James Daniel; Mullins, C. B.The chemical nature of gold has been determined to be much richer than previously thought. Recent discoveries that properly prepared gold catalysts (i.e. gold particle diameters in 2 – 5 nm size range) can catalyze the CO oxidation reaction at low temperature have spurned a renewed interest in the chemistry of gold. Despite the extensive research that has been performed regarding CO oxidation on Au based catalysts, many issues still remain unresolved. The origin of the particle size dependence of the reaction is not well understood. Also, details concerning the reaction mechanism, specifically identification of the active oxygen species, remain unresolved. In the following studies, ultra high vacuum, molecular beam, surface science techniques are used to study the CO oxidation reaction on titania supported gold nanoclusters (Au/TiO2). Using a radio frequency generated plasma-jet, it is possible to simultaneously populate the Au/TiO2 samples with atomically adsorbed and molecularly chemisorbed oxygen species, allowing for the opportunity to investigate the reactivity of each respective species. The reaction of CO with atomically adsorbed oxygen has been studied over a range of temperatures from 65 – 250 K as a function of gold coverage and oxygen coverage. The reaction is observed to be a strong function of both of the sample temperature and the oxygen coverage. The reaction is also relatively independent of the gold coverage on the sample, in contrast to findings for the reaction employing gas-phase reactants under moderate pressures. The formation and reactivity of molecularly chemisorbed oxygen on the samples following exposure to the plasma-jet was also investigated. Evidence is presented showing that some molecularly chemisorbed oxygen is formed as a result of recombination of impinging atoms on the model catalyst surface. Evidence is also presented showing that adsorption of an oxygen atom on the sample influences the chemisorption of molecular species from the gas phase. Finally, evidence is presented showing that the molecularly chemisorbed oxygen species can participate in the CO oxidation reaction at 77 K. This finding reveals a reaction channel for CO oxidation on Au/TiO2 model catalysts that does not require the dissociation of oxygen.Item Nanoparticles as Reactive Precursors: Synthesis of Alloys, Intermetallic Compounds, and Multi-Metal Oxides Through Low-Temperature Annealing and Conversion Chemistry(2010-07-14) Bauer, John C.Alloys, intermetallic compounds and multi-metal oxides are generally made by traditional solid-state methods that often require melting or grinding/pressing powders followed by high temperature annealing (> 1000 degrees C) for days or weeks. The research presented here takes advantage of the fact that nanoparticles have a large fraction of their atoms on the surface making them highly reactive and their small size virtually eliminates the solid-solid diffusion process as the rate limiting step. Materials that normally require high temperatures and long annealing times become more accessible at relatively low-temperatures because of the increased interfacial contact between the nanoparticle reactants. Metal nanoparticles, formed via reduction of metal salts in an aqueous solution and stabilized by PVP (polyvinylpyrrolidone), were mixed into nanoparticle composites in stoichometric proportions. The composite mixtures were then annealed at relatively low temperatures to form alloy and intermetallic compounds at or below 600 degrees C. This method was further extended to synthesizing multi-metal oxide systems by annealing metal oxide nanoparticle composites hundreds of degrees lower than more traditional methods. Nanoparticles of Pt (supported or unsupported) were added to a metal salt solution of tetraethylene glycol and heated to obtain alloy and intermetallic nanoparticles. The supported intermetallic nanoparticles were tested as catalysts and PtPb/Vulcan XC-72 showed enhanced catalytic activity for formic acid oxidation while Pt3Sn/Vulcan XC-72 and Cu3Pt/y-Al2O3 catalyzed CO oxidiation at lower temperatures than supported Pt. Intermetallic nanoparticles of Pd were synthesized by conversion chemistry methods previously mentioned and were supported on carbon and alumina. These nanoparticles were tested for Suzuki cross-coupling reactions. However; the homocoupled product was generally favored. The catalytic activity of Pd3Pb/y-Al2O3 was tested for the Heck reaction and gave results comparable to Pd/y-Al2O3 with a slightly better selectivity. Conversion chemistry techniques were used to convert Pt nanocubes into Ptbased intermetallic nanocrystals in solution. It was discovered that aggregated clusters of Pt nanoparticles were capable of converting to FePt3; however, when Pt nanocubes were used the intermetallic phase did not form. Alternatively, it was possible to form PtSn nanocubes by a conversion reaction with SnCl2.Item Selective reduction of nitrogen dioxide by ammonia(Texas Tech University, 1972-12) Cheng, Yang-lehNot availableItem The surface chemistry of atomic oxygen pre-covered gold(2008-05) Ojifinni, Rotimi Ayodele, 1975-; Mullins, C. B.Gold used to be regarded as catalytically inert until about 20 years ago when it was shown that supported gold clusters < 5 nm in diameter exhibited some unique catalytic properties. Based on this revelation, several studies have demonstrated the feasibility of reactions previously thought of as impossible on gold. The ability of gold to oxidize CO below ambient temperatures at rates higher than conventional CO oxidation catalysts (Pd and Pt) has been shown to hold potentials for technological applications. Extensive past and on-going research are geared towards elucidating the mechanistic details of this reaction. The nature of the active sites, the effect of the supports and the effect of moisture are still debated in literature. I therefore present some experimental results supported with density functional theory calculations to shed additional light on some of the issues concerning gold catalysis in general, and low temperature CO oxidation in particular. Previous studies of the effect of moisture on oxide-supported gold reported that although water promotes CO oxidation on this surface by as much as two orders of magnitude, it is only a spectator molecule on the surface. I present here evidence for strong water-oxygen interactions when water is co-adsorbed with atomic oxygen on Au(111). Impinging a CO beam on the surface co-adsorbed with oxygen and water produces water-enhanced CO oxidation. Based on these results, I propose that CO reacts with hydroxyls formed from water-oxygen interactions to form CO₂, similar to a previous observation on Pt(111). Exposing a Au(111) surface pre-covered with ¹⁶O to isotopically labeled carbon dioxide (C¹⁸O₂) showed that ¹⁶O¹⁸O (m/e = 34) was produced from carbonate formation and decomposition. Estimates of reaction probability and activation energy gave ~ 10⁻⁴ - 10⁻⁵ and -0.15 eV respectively. The effect of annealing on the reactivity of oxygen pre-covered Au(111) was investigated using water, carbon monoxide and carbon dioxide as probe molecules. Precovering Au(111) with atomic oxygen followed by annealing resulted in surfaces that were less reactive towards water, CO and CO₂. Annealing is believed to stabilize the reactive metastable oxygen thereby increasing the barrier to reaction similar to what is reported on other surfaces.Item The kinetics of methylcyclopentane reactions over a platinum on N-alumina catalyst(Texas Tech University, 1968-06) Vidaurri, Fernando C.Not availableItem The oxidation of ammonia on a supported ruthenium catalyst(Texas Tech University, 1971-05) Johnson, Tiffin ElmoreThe buildup of biologically produced ammonia in a closed spacecabin environment represents a hazard to crewmen making extended voyages. The primary source of ammonia is urine, and although efforts are made to control emissions at this source, traces of ammonia escape into the spacecabin. In order to maintain a safe and breathable atmosphere for life support, the spacecraft's trace contaminant removal system must be able to efficiently remove this ammonia. Potentially the best method of doing this is through catalytic oxidation to water and molecular nitrogen. The National Aeronautics and Space Administration (NASA) has awarded Texas Tech University a two year research contract (NAS 1-9506) to study the changes which ammonia undergoes when passed through the catalytic oxidizer of a spacecraft trace contaminant removal system.