VTechWorks Repository :: Browsing by Author "Cox, David F."

Browsing by Author "Cox, David F."

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Adsorption of water and carbon monoxide on Cu₂O(111) single crystal surfaces
Christiaen, Anne-Claire (Virginia Tech, 1994-08-04)
Water and CO adsorptions were studied over the stoichiometric and the oxygen-deficient Cu₂O(111) surfaces, using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). Water is the only desorbing species detected in TDS and the extent of dissociation is unaffected by the surface condition: ≃ 0.25 monolayers of water dissociate on Cu₂O(111) regardless of surface condition. The local defect environment around oxygen vacancies does not play a significant role in the activity of the Cu₂O(111) surface for the dissociation of water. CO is found to bind molecularly to the surface through the carbon atom and with a heat of adsorption of 22 kcal/mol, higher value than that of CO on Cu₂O(100) (16.7 kcal/mol). This suggests that the local geometry of adsorption sites may play an important role in the way CO binds to Cu₂O surfaces. Electronic changes upon CO adsorption and the higher heat of adsorption indicate an increased σ-donor character for CO, with some π-backbonding interactions. The local defect environment around oxygen vacancies does not appear to affect CO adsorption on Cu₂O(111) surfaces.
Aromatization of n-hexane by platinum containing molecular sieves and distribution and motion of organic guest molecules in zeolites
Hong, Suk Bong (Virginia Tech, 1992-04-05)
A vapor phase impregnation technique with Pt(acac)₂ has been developed and used to load Pt into aluminosilicate (KL, BaKL, KBaKL, NaY, CsNaY, FAU, EMT, ZSM-12 and SSZ-24) and aluminophosphate (AIP0₄-5 and VPI-5) molecular sieves. ¹³C MAS NMR, TEM and H₂ chemisorption measurements reveal that Pt can be loaded into the micropores of molecular sieves with both charged and neutral frameworks. Pt containing molecular sieves were tested as catalysts for the aromatization of n-hexane at 460 - 510°C and atmospheric total pressure in order to study the influence of Pt cluster size and support acidity/basicity, microstructure and chemical composition on activity and selectivity. High selectivity to benzene over most of the zeolite samples demonstrates that support acidity/basicity and microstructure do not contribute directly to the aromatization selectivity over Pt catalysts. A clear trend of increasing benzene selectivity with decreasing Pt cluster size is found. These observations suggest that the exceptional reactivity of Pt/KL for the aromatization of n-hexane results from the lack of any acidity in the support and the ability of zeolite L to stabilize the formation of extremely small Pt clusters. Pt/AIP0₄-5 and Pt/VPI-5 show high selectivity to n-hexane with little formation of benzene while opposite is observed for Pt/SSZ-24. The differences in catalytic behavior are attributed to variations in the environment of Pt clusters which are situated in either an aluminophosphate or silicate micropore. See document for rest of abstract.
Catalytic Hydrodeoxygenation of Bio-Oil Model Compounds (Ethanol, 2-Methyltetrahydrofuran) over Supported Transition Metal Phosphides
Bui, Phuong Phuc Nam (Virginia Tech, 2013-01-24)
The objective of this project is to investigate hydrodeoxygenation (HDO), a crucial step in the treatment of bio-oil, on transition metal phosphide catalysts. The study focuses on reactions of simple oxygenated compounds present in bio-oil -- ethanol and 2-methyltetrahydrofuran (2-MTHF). The findings from this project provide fundamental knowledge towards the hydrodeoxygenation of more complex bio-oil compounds. Ultimately, the knowledge contributes to the design of optimum catalysts for upgrading bio-oil. A series of transition metal phosphides was prepared and tested; however, the focus was on Ni2P/SiO2. Characterization techniques such as X-ray diffraction (XRD), temperature-programmed reduction and desorption (TPR and TPD), X-ray photoelectron spectroscopy (XPS), and chemisorption were used. In situ Fourier transform infrared (FTIR) spectroscopy was employed to monitor the surface of Ni2P during various experiments such as: CO and pyridine adsorption and transient state of ethanol and 2-MTHF reactions. The use of these techniques allowed for a better understanding of the role of the catalyst during deoxygenation.
Catalytic Hydrogenation and Hydrodesulfurization of Model Compounds
Zhao, Haiyan (Virginia Tech, 2009-03-19)
This dissertation describes two related studies on hydrogenation and hydrodesulfurization of heterocyclic S-containing compounds. Alkyl substituted thiophenes are promising candidates for hydrogen carriers as the dehydrogenation reactions are known to occur under mild conditions. Four types of catalysts including supported noble metals, bimetallic noble metals, transition metal phosphides and transition metal sulfides have been investigated for 2-methylthiophene (2MT) hydrogenation and ring opening. The major products were tetrahydro-2-methylthiophene (TH2MT), pentenes and pentane, with very little C5-thiols observed. The selectivity towards the desired product TH2MT follows the order: noble metals > bimetallics > phosphides > sulfides. The best hydrogenation catalyst was 2% Pt/Al2O3 which exhibited relatively high reactivity and selectivity towards TH2MT at moderate temperatures. Temperature-programmed desorption (TPD) of hydrogen indicated that the H2 desorption amount was inversely related to the rate of TH2MT formation. Temperature programmed reaction (TPR) experiments revealed that pentanethiol became the major product, especially with HDS catalysts like CoMoS/Al2O3 and WP/SiO2, which indicates that poisoned or modified conventional HDS catalysts would be good candidates for further 2MT hydrogenation studies. The role of tetrahedral Ni(1) sites and square pyramidal Ni(2) sites in Ni2P hydrotreating catalysts was studied by substitution of Ni with Fe. The Fe component was deemed as a good probe because Ni2P and Fe2P adopt the same hexagonal crystal structure, yet Fe2P is completely inactive for hydrodesulfurization (HDS). For this purpose a series of NiFeP/SiO2 catalysts were prepared with different Ni:Fe molar ratios (1:0, 3:1, 1:1, 1:3, and 0:1) and investigated in the HDS of 4,6-dimethyldibenzothiophene at 300 and 340 oC. The uniformity of the NiFe series was demonstrated by x-ray diffraction analysis and by Fourier transform infrared (FTIR) spectroscopy of adsorbed CO. The position of substitution of Fe was determined by extended X-ray absorption fine structure (EXAFS) analysis. It was found that at 300 oC the HDS activity of the catalysts decreased with increasing Fe content and that this could be explained by the substitution of Fe at the more active Ni(2) sites. As temperature was raised to 340 oC, the activity of the Fe-containing samples increased, although not to the level of Ni2P, and this could be understood from a reconstruction of the NiFe phase to expose more Ni(2) sites. This was likely driven by the formation of surface Ni-S bonds, which could be observed by EXAFS in spent samples.
Catalytic reactions involving chlorofluorocarbons: a review
Berrey, Christopher Lee (Virginia Tech, 1995-02-03)
On December 19, 1994, scientists at NASA's Goddard Space Flight Center announced that they had obtained the first conclusive evidence that the Earth's protective ozone layer is being depleted by anthropogenic chemical products, and not by natural events such as volcanic eruptions (1). Since the early 1970s there has been a controversial debate as to the source of stratospheric chlorine. One view has held that the major portion of chlorine present in the stratosphere has originated, not from chlorofluorocarbons (CFCs), but from natural processes such as evaporation of seawater and volcanic eruptions. In this view, the chlorine originating from the photochemical breakdown of CFCs represents only a minor fraction of the total chlorine present in the stratosphere. General scientific consensus however holds just the opposite.
Catalytic Transformation of Greenhouse Gases in a Membrane Reactor
Prabhu, Anil K. (Virginia Tech, 2003-03-13)
Supported Ni and Rh catalysts were developed for the reforming of two greenhouse gases, methane and carbon dioxide to syngas (a mixture of hydrogen and carbon monoxide). This is an endothermic, equilibrium limited reaction. To overcome the thermodynamic limitations, a commercially available porous membrane (Vycor glass) was used in a combined reactor-separator configuration. This was to selectively remove one or more of the products from the reaction chamber, and consequently shift the equilibrium to the right. However, the separation mechanism in this membrane involved Knudsen diffusion, which provided only partial separations. Consequently, there was some transport of reactants across the membrane and this led to only marginal improvements in performance. To overcome this limitation, a new membrane was developed by modifying the Vycor substrate by the chemical vapor deposition of a silica precursor. This new membrane, termed Nanosil, provided high selectivity to hydrogen at permeabilities comparable to the support material. Application of this membrane in the combined reactor-separator unit provided higher conversions than that obtained using the Vycor membrane.
Characterization and Reactivity of Mo₂C
St. Clair, Todd P. (Virginia Tech, 1998-06-04)
Two types of Mo₂C have been investigated: polycrystalline β-Mo₂C and single crystal α-Mo₂C. The β-Mo₂C material was synthesized via a temperature-programmed method, and then characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), CO chemisorption, and N₂ physisorption. The catalytic activity of the β-Mo₂C was tested for cumene hydrogenation under high pressure conditions, and the effect of sulfur and oxygen poisons on cumene hydrogenation was also investigated. As a complement to the work done on polycrystalline β-Mo₂C, UHV studies of single crystal α-Mo₂C were undertaken to provide fundamental information about a well-characterized Mo₂C surface. The (0001) surface of α-Mo₂C was investigated using XPS and low energy electron diffraction (LEED). It was found that an ion-bombarded surface could be prepared as either Mo-terminated or C-terminated by choosing either low annealing temperatures (~1000 K) or high annealing temperatures (~1500 K), respectively. CO and O₂ adsorption was also studied on α-Mo₂C (0001) using thermal desorption spectroscopy (TDS), XPS, Auger electron spectroscopy (AES), and LEED. Finally, thiophene adsorption was investigated on α-Mo₂C (0001).
Characterization of palmer drought index as a precursor for drought mitigation
Lohani, Vinod K. (Virginia Tech, 1995-08-15)
Coping with droughts involves two phases. In the first phase drought susceptibility of a region should be assessed for developing proper additional sources of supply which will be exploited during the course of a drought. The second phase focuses on the issuance of drought warnings and exercising mitigation measures during a drought . These kinds of information are extremely valuable to decision making authorities. In this dissertation three broad schemes i) time series modeling, ii) Markov chain analysis, and iii) dynamical systems approach are put forward for computing the drought parameters necessary for understanding the scope of the drought. These parameters include drought occurrence probabilities, duration of various drought severity classes which describe a region's drought susceptibility, and first times of arrival for non drought classes which signify times of relief for a drought-affected region. These schemes also predict drought based on given current conditions. In the time series analysis two classes of models; the fixed parameter and the time varying models are formulated. To overcome the bimodal behavior of the Pallner Drought Severity Index (PDSI), primarily due to the backtracking scheme to reset the temporary index values as the PDSI values, the models are fitted to the Z index in addition to the PDSI for the forecasting of the PDSI.
Chemical vapor deposition of β-SiC thin films on Si(100) in a hot wall reactor
Chiu, Chienchia (Virginia Tech, 1994-01-12)
A systematic method was developed for the deposition of β-SiC thin films on Si(100) substrates in a hot wall reactor, using low pressure chemical vapor deposition (LPCVD). Due to poor adhesion resulting from lattice mismatch and difference in thermal expansion coefficients between the (SiC films and the Si(100) substrates, the feasibility of forming a SiC buffer layer on the Si(100) surface before beginning the chemical vapor deposition (CVD) process was investigated. The SiC buffer layers were formed with either a smooth or porous morphology. A nonporous Si(100) substrate with a 35Å thick SiC buffer layer was formed when the Si surface was heated at 1050°C in an atmosphere of C₂H₂ and H₂. A porous surface was obtained when the Si substrate was heated at 1000°C in C₂H₂ alone. The porous defects were correlated to the out—diffusion of Si in the carburizing process. On smooth Si(100) substrates, polycrystalline and stoichiometric β-SiC thin films with the (111) planes paralleling the Si(100) substrates were grown from a CH₃SiCl₃ (MTS)—H₂ mixture at 1050°C. At high H₂/MTS ratios and/or low deposition pressures, no etching on the Si substrates of the β-SiC films was observed, resulting in a smooth topography. Degradation in film morphology, changes in the preferred orientation, and etching of the Si substrates were observed at higher pressures, temperatures, and H₂/MTS ratios. The etching of the Si substrate was due to the out—diffusion of Si atoms from the substrate and the presence of Cl—containing radicals, which resulted from the decomposition of MTS molecules before arriving at the substrates. A model of the deposition mechanism is proposed which predicts the deposition rates in a hot wall CVD reactor and agrees very well with the experimental data. On the Si(100) substrate with a porous topography, epitaxial β-SiC(100) thin films were grown from MTS—H₂ at 1150°C. The crystallinity of the deposited films was influenced by the deposition time. With increasing deposition time, rotational β-SiC(100) crystals and polycrystalline β-SiC with a highly preferred orientation of (100) and/or (111) were obtained. At a lower temperature of 1100°C, poor morphology and polycrystalline β-SiC thin films were observed. Finally, a new approach to the calculation of the local equilibrium CVD phase diagrams, which represent the most stable phases above the substrates in a hot wall reactor, for SiC deposition from the MTS—H₂ gas mixture by coupling the depletion effects to the equilibrium thermodynamic computer code SOLGASMIX—PV. The calculated CVD phase diagrams were also compared with experimental and the literature data. Although the local equilibrium CVD phase diagrams predicted the deposition of single phase SiC better than established CVD phase diagrams, the experimental regions for depositing single phase SiC are larger than those calculated from local CVD phase diagrams. This may be because of the high linear velocity of the gas flux under low pressure and the polarity of the Si—containing intermediate species.
Comparison of the Reactivity of Various Mn-Oxides With CrIIIaq: Microscopic and Spectroscopic Observations of Dissolution, Cr-sorption and Cr and Mn Redox Interactions
Weaver, Robert M. (Virginia Tech, 2001-09-19)
Chapter 1 Dynamic Processes Occurring at the Cr^III_aq – Manganite (γ-MnOOH) Interface: Simultaneous Adsorption, Microprecipitation, Oxidation/Reduction and Dissolution The complex interaction between Cr^III_aq and manganite (γ-MnOOH) was systematically studied at room temperature over a pH range of 3 to 6, and within a concentration range of 10⁻⁴ to 10⁻² M CrOH²⁺_aq. Solution compositional changes during batch reactions were characterized by ICP and UVvis. The manganites were characterized before and after reaction with XPS, SEM, high-resolution FESEM, and EDS analysis. Fluid-cell AFM was used to follow these metal-mineral interactions in situ. The reactions are characterized by 1) sorption of Cr^III and the surface-catalyzed microprecipitation of Cr^III-hydroxy hydrate on manganite surfaces, 2) the acidic dissolution of the manganite, and 3) the simultaneous reductive dissolution of manganite coupled with the oxidation of Cr^III_aq to highly toxic Cr^VI_aq. Cr^III-hydroxy hydrate was shown to precipitate on the manganite surface while still undersaturated in bulk solution. The rate of manganite dissolution increased with decreasing pH due both to faster acid-promoted and Mn-reduction- promoted dissolution. Due to direct redox coupling with Mn reduction, Cr oxidation was most rapid in the lower pH range. Neither Mn^II nor Cr^VI were ever detected on manganite surfaces, even at the maximum rate of their generation. At the highest pHs of this study, Cr^III_aq was effectively removed from solution to form Cr^III-hydroxy hydrate on manganite surfaces and in the bulk solution, and manganite dissolution and Cr^VI_aq generation were minimized. All interface reactions described above were heterogeneous across the manganite surfaces. This heterogeneity is a direct result of the heterogeneous semiconducting nature of natural manganite crystals, and is also an expression of the proximity effect, whereby redox processes on semiconducting surfaces are not limited to next nearest neighbor sites. Chapter 2 Comparison of the Reactivity of Various Mn-Oxides with Cr^III_aq: Microscopic and Spectroscopic Observations of Dissolution, Cr-sorption and Cr and Mn Redox Interactions The interaction between Cr^III_aq and seven different Mn-oxides (6 monomineralic, 1 synthetic) have been observed in pH ~4.4 HNO₃ and pH ~4.4 ~10⁴ M Cr^III_aq solutions. For each mineral-solution interaction, the aqueous chemical concentrations (e.g. [Mn]_aq, [Cr]_aq, [Cr^VI_aq]) were measured with time. Reacted samples were examined by XPS to determine if, and to what extent, the surface chemical states of Cr, Mn and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and high-resolution, low-voltage FESEM. The solubility of the Mn-oxides in the acidic, non-Cr bearing solutions varied inversely with the average Mn valence, but did not show systematic behavior with respect to the mineral structure type (e.g. tunnel, layer, framework). This trend was interpreted as resulting from the relative ability of an adsorbed proton to polarize surface Mn-O bonds, with the polarizability being in the order Mn²⁺-O > Mn³⁺-O > Mn⁴⁺-O. For samples reacted with Cr^III_aq, the rate and extent of reductive dissolution was always greater than for acidic dissolution during the initial time period. The measured ratios of the [Mn]_aq : [Cr^VI]_aq were approximately in agreement with the values expected from the proposed stoichiometric reactions. Cr-uptake was observed to occur in undersaturated solutions as a result of adsorption, absorption and surface catalyzed precipitation. The chromium as detected by XPS was predominately Cr^III, however pyrolusite contained both Cr^III and Cr^VI. Previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr^III_aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr-uptake is by isolated site binding, very small (<30 nm) surface clusters or monolayer scale films. Cr-uptake was followed by slow Cr-release on several of the solids (particularly the layered solids) after a substantial portion of the total aqueous Cr had been converted to Cr^VI_aq. The oxidizing ability of the different Mn-oxides for Cr^III_aq is evaluated with regards to the energy level of the redox couple (i.e. the redox potential) as compared with the Fermi energy level of the Mn-oxide. Although these energies were calculated rather than directly measured, the results indicate that electrons originating from adsorbed Cr^III ions may be transferred into the conduction band or more likely, into available surface states. The presence of an initial limited quantity of electron accepting surface states likely explains the observation of a rapid initial Cr^III-oxidation followed by much slower oxidation. The Mn-oxides that exhibited the greatest and longest lasting Cr^III-oxidizing power were the Mn-oxides containing Mn⁺, and in particular those containing Mn³⁺ and Mn⁺. It is believed that the combined presence of a reducible Mn ion (e.g. Mn³⁺) and a highly soluble Mn⁺ ion facilitates a sustained Cr^III-oxidation reaction because fresh surface is exposed during the reaction.
Computational Alchemy: The Rational Design of New Superhard Materials
Teter, David Michael (Virginia Tech, 1998-06-29)
First--principles electronic structure calculations have been performed to help identify and direct the synthesis of new superhard compounds. An improved figure of merit for hardness is identified and used to show that carbon nitrides are not likely to be harder than diamond.
Crystallization, Morphology, Thermal Stability and Adhesive Properties of Novel High Performance Semicrystalline Polyimides
Ratta, Varun (Virginia Tech, 1999-04-26)
It was the objective of this research to develop high temperature and high performance polyimides that also display (a) thermal stability; (b) crystallinity in the initial material and ability to crystallize from the melt; (c) fast crystallization kinetics and (d) melt processability. This unique combination of properties is presently unavailable in any other polyimide. In this regard, the present work investigates the crystallization, morphology and thermal stability of two novel semicrystalline polyimides based on the same diamine, 1,3-bis (4-aminophenoxy) benzene (TPER), but two different dianhydrides, 3,3',4',4'-biphenyltetracarboxylic dianhydride (BPDA) and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). Phthalic anhydride was used as an endcapper to improve the thermal stability of the polyimides. The BPDA based polyimide was also tested extensively as a structural adhesive using Ti-6Al-4V coupons. Additionally, these polyimides are based on monomers, that are presently commercially available. The bulk thermal stability of the polyimides was first evaluated using dynamic and isothermal thermogravimetric experiments. DSC was utilized to test the ability of the polyimides to crystallize from the melt after exposures to varying melt times and temperatures. Exceptional thermal stability was demonstrated by BPDA based polyimide with no change in the melting behavior after 40 min at 430°C or 30 min at 440°C. The semicrystalline morphology of the material was studied using hot stage polarized optical microscopy (OM) and atomic force microscopy. The spherulitic growth rates were determined as a function of crystallization temperature after quenching from various melt times and temperatures. The effect of crystallization temperature, previous melt time and melt temperature on the morphology was considered. The spherulitic growth rates increased with increasing undercooling in the temperature range studied (nucleation controlled), while the growth rate at a specific crystallization temperature decreased on increasing the previous melt time and temperature. The melting behavior was studied after different crystallization times and temperatures and also as a function of different heating rates. Crystallization kinetics was followed both isothermally and non-isothermally using DSC and OM. Avrami analysis was performed for TPER-BPDA and the obtained results were correlated with microscopic observations. Melt viscosity measurements were carried out as a function of melt temperature, melt time and frequency. The adhesive investigations for TPER-BTDA utilized lap-shear test, wedge test and double cantilever beam tests. The durability of the adhesive and the fracture surface was studied after exposure to various solvents and after high aging and testing temperatures. The polyimide demonstrated very high average room temperature lap-shear strengths (8400 psi or 59 MPa), excellent solvent resistance and durability of strengths at high aging and testing temperatures.
Density Functional Theory Study of Rutile SiO₂ Stishovite: An Electron Pair Description of Bulk and Surface Properties
Muscenti, Thomas Michael (Virginia Tech, 2011-08-06)
The bulk structure and the nonpolar, stoichiometric (110) surface of stishovite, rutile structure type SiO₂, has been studied using a first principles, density functional method. The geometric and electronic structure, including the density of states, charge density, and electron localization function for both the bulk and the surface have been examined. The electron pair properties of both bulk and surface-layer atoms were found to be similar to molecular analogs. The analogs allowed for the description of surface electronic structure using simple molecular models. The adsorption of hydrogen fluoride was studied on the (110) surface. The geometry optimized and electronic structure have been found for various initial geometries. Relaxed structures of certain initial geometries give dissociated hydrogen fluoride upon geometry optimization.
The Design of Stable, Well-Defined Polymer-Magnetite Nanoparticle Systems for Biomedical Applications
Miles, William Clayton (Virginia Tech, 2009-08-10)
The composition and stability of polymer-magnetite complexes is essential for their use as a treatment for retinal detachment, for drug targeting and delivery, and for use as a MRI contrast agent. This work outlines a general methodology to design well-defined, stable polymer-magnetite complexes. Colloidal modeling was developed and validated to describe polymer brush extension from the magnetite core. This allowed for the observation of deviations from expected behavior as well as the precise control of polymer-particle complex size. Application of the modified Derjaguin-Verwey-Landau-Overbeek (DLVO) theory allowed the determination of the polymer loading and molecular weight necessary to sterically stabilize primary magnetite particles. Anchoring of polyethers to the magnetite nanoparticle surface was examined using three different types of anchor groups: carboxylic acid, ammonium, and zwitterionic phosphonate. As assessed by dynamic light scattering (DLS), the zwitterionic phosphonate group provided far more robust anchoring than either the carboxylic acid or ammonium anchor groups, which was attributed to an extremely strong interaction between the phosphonate anchor and the magnetite surface. Coverage of the magnetite surface by the anchor group was found to be a critical design variable for the stability of the zwitterionic phosphonate groups, and the use of a tri-zwitterionic phosphonate anchor provided stability in phosphate buffered saline (PBS) for a large range of polymer loadings. Incorporation of an amphiphlic poly(propylene oxide)-b-poly(ethyelene oxide) (PPO-b-PEO) diblock copolymer attached to the magnetite surface was examined through colloidal modeling and DLS. The relaxivity of the complexes was related to aggregation behavior observed through DLS. This indicated the presence of a hydrophobic interaction between the PPO layers of neighboring complexes. When this interaction was large enough, the complexes exhibited an increased relaxivity and cellular uptake. Thus, we have developed a methodology that allows for design of polymer-magnetite complexes with controlled sizes (within 8% of predicted values). Application of this methodology incorporated with modified DLVO theory aids in the design of colloidally stable complexes with minimum polymer loading. Finally, determination of an anchor group stable in the presence of phosphate salts at all magnetite loadings allows for the design of materials with minimum polymer loadings in biological systems.
The effect of a DC voltage on the catalytic combustion of natural gas in air over platinum
Halsey, Andrew C. (Virginia Tech, 1993-04-15)
The effects of a DC voltage on the catalytic combustion rate of natural gas in air over a platinum wire in various configurations is examined. In most cases a high applied voltage reduces and quenches the catalytic reaction. These effects are dependent on the catalyst surface charge density but not on the direction of the field. Greater effects were also observed when the field was applied on the upstream rather than the downstream side of the catalyst. Although the mechanism of this quenching effect is unknown, it is suggested that it involves the generation and diffusion of charged gas-phase species and their interaction with the catalyst surface. Possible quantum effects are also discussed. Further work to verify or refute the mechanisms presented here is suggested.
The Electronic Structure and Reactivity of Sulfide Surfaces: Combining Atomic-Scale Observations with Theoretical Calculations
Rosso, Kevin Michael (Virginia Tech, 1998-06-05)
The electronic structure of clean pyrite {100} and covellite {001} surfaces have been investigated in ultra-high vacuum (UHV) for the purpose of understanding the nature of sulfide surface reactivity. Using primarily scanning tunneling microscopy and spectroscopy (STM/STS), the electronic structure at atomic sites on these surfaces was directly probed, and chemical insight into the results was provided by ab-initio calculations. Pyrite is the most abundant sulfide at the earth's near surface. Its oxidation influences a wide variety of natural and industrial chemical process, but very little is known about the stepwise oxidation reactions involved. For this reason, the first two chapters are directed at understanding the surface electronic structure and fundamental reactivity of pyrite surfaces at the atomic scale. UPS spectra show a characteristic peak at ~ 1 eV forming the top of the valence band for the near surface. Ab-initio calculated densities of states for the bulk crystal suggest that this band is comprised primarily of non-bonding Fe 3d t_2g and lesser S 3p and Fe 3d e_g states. Ab-initio slab calculations predict that the broken bonding symmetry at the surface displaces a Fe 3d_Z2 dangling bond state into the bulk band gap. Evidence confirming the presence of this surface state is found in low bias STM imaging and normalized single-point tunneling spectra, which are in remarkable agreement with calculations of the LDOS at surface Fe and S sites. The results predict that due to the dangling bond surface states, Fe sites are energetically favored for redox interaction with electron donors or acceptor species. STM/STS observations of O₂/H₂O exposed surfaces are consistent with this assertion, as are ab-initio cluster calculations of adsorption reactions between O₂/H₂O derived species and the {100} surface. Furthermore, an enhancement in the "rate" of oxidation was discovered using UPS on pyrite surfaces exposed to a mixture of O₂/H₂O. Cluster calculations of adsorption energies reveal a similar result for the case where both O₂ and H₂O are dissociated on the surface and sorbed to Fe sites. Covellite, similar to pyrite, is a natural semiconducting metal sulfide. In contrast, however, precious metal bearing solutions have a curiously lower affinity for covellite surfaces than for pyrite. At the same time, its unique combination of low resistivity and perfect basal cleavage represented a unique opportunity to improve our ability to interrogate metal sulfide surfaces using STM/STS at the atomic scale. Ab-initio calculations predict that cleaving covellite exposes two slightly different surfaces, one is expected to have dangling bonds, the other is not. Atomic-scale STM images and LEED patterns indicate that the surface structure is laterally unreconstructed. The STM images are predicted to show Cu sites as high tunneling current sites on the dangling bond covered surface, and S sites on the other. Based on tunneling spectra and tip-induced effects therein, reasonable arguments are presented which allow one to uniquely differentiate between the two possible surfaces. For both pyrite and covellite, the combination of experiment and theoretical calculations afforded much more insightful conclusions than either would have alone. The calculations provided the necessary chemical framework with which to make interpretations of the experimental data and, in this sense, contribute information obtainable by no other means. This point is further developed in an investigation of Si-O interactions and the electron density distribution in the model silicate coesite, which is presented in the appendix. In addition, it breaks new ground by delving into differences and similarities between periodic vs. cluster calculations of minerals.
Experimental Adsorption and Reaction Studies on Transition Metal Oxides Compared to DFT Simulations
Chen, Han (Virginia Tech, 2021-06-11)
A temperature-programmed desorption (TPD) study of CO and NH₃ adsorption on MnO(100) with complimentary density functional theory (DFT) simulations was conducted. TPD reveals a primary CO desorption signal at 130 K from MnO(100) in the low coverage limit giving an adsorption energy of -35.6 ±2.1 kJ/mol on terrace sites. PBE+U gives a more reasonable structural result than PBE, and the adsorption energy obtained by PBE+U and DFT-D3 Becke-Johnson gives excellent agreement with the experimentally obtained ΔE_ads for adsorption at Mn²⁺ terrace sites. The analysis of NH₃-TPD traces revealed that adsorption energy on MnO(100) is coverage-dependent. At the low-coverage limit, the adsorption energy on terraces is -58.7±1.0 kJ/mol. A doser results in the formation of a transient NH₃ multilayers that appears in TPD at around 110K. For a terrace site, PBE+U predicts a more realistic surface adsorbate geometry than PBE does, with PBE+U with Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP) provides the best prediction. DFT simulations of the dehydrogenation elementary step of the ethyl and methyl fragments on α-Cr2O₃(101̅2) were also conducted to complement previous TPD studies of these subjects. On the nearly-stoichiometric surface of α-Cr₂O₃(101̅2), CD₃₋ undergoes dehydrogenation to produce CD₂=CD₂ and CD₄. Previous TPD traces suggest that the α-hydrogen (α-H) elimination of methyl groups on α-Cr₂O₃(101̅2) is the rate-limiting step, and has an activation barrier of 135±2 kJ/mol. DFT simulations showed that PBE gives reasonable prediction of the adsorption sites for CH3- fragments in accordance with XPS spectra, while PBE+U did not. Both PBE and PBE+U failed to predict the correct adsorption sites for CH₂=. When the simulation is set in accordance with the experimentally observed adsorption sites for the carbon species, PBE gives very accurate prediction on the reaction barrier when an adjacent I adatom is present, while PBE+U failed spectacularly. When the simulation is set in accordance with the DFT-predicted adsorption sites, PBE is still able to accurately predict the reaction barrier (<1% to 8.7% error) while PBE+U is less accurate. DFT is also used to complement the previous study of the β-H elimination an ethyl group on the α-Cr₂O₃(101̅2) surface. The DFT simulation shows that absent surface Cl adatoms, PBE predicts an activation barrier of 92.6 kJ/mol, underpredicting the experimental activation barrier by 28.7%, while PBE+U predicts a barrier of 27.0 kJ/mol, under-predicting the experimental barrier by 79.2%. The addition of chlorine on the adjacent cation improved the prediction on barrier by PBE+U marginally, while worsened the prediction by PBE marginally. Grant information: Financial support provided by the U.S. Department of Energy through grant DE-FG02 97ER14751.
Experimental and Modeling Studies of the Methane Steam Reforming Reaction at High Pressure in a Ceramic Membrane Reactor
Hacarlioglu, Pelin (Virginia Tech, 2007-11-12)
This dissertation describes the preparation of a novel inorganic membrane for hydrogen permeation and its application in a membrane reactor for the study of the methane steam reforming reaction. The investigations include both experimental studies of the membrane permeation mechanism and theoretical modeling of mass transfer through the membrane and simulation of the membrane reactor with 1-D and 2-D models. A hydrothermally stable and hydrogen selective membrane composed of silica and alumina was successfully prepared on a macroporous alumina support by chemical vapor deposition in an inert atmosphere at high temperature. Before the deposition of the silica-alumina composite, multiple graded layers of alumina were coated on the alumina support with a mean pore size of 100 nm by the sequential application of three boehmite sols with gradually decreasing sol particle sizes of 630, 200 and 40 nm, respectively. The resulting supported composite alumina-silica membrane had high permeability for hydrogen in the order of 10-7 mol m-2 s-1 Pa-1 at 873 K with a H2 /CH4 selectivity of 940 and exhibited much higher stability to water vapor at the high temperature of 873 K. In addition, the same unusual permeance order of Heï¼ H2ï¼ Ne previously observed for the pure silica membrane was also observed for the alumina-silica membrane, indicating that the silica structure did not change much after introduction of the alumina. The permeation of hydrogen and helium through vitreous glass and silica membranes was modeled using ab initio density functional calculations. Comparison of the calculated activation energies to those reported for vitreous glass (20—40 kJ mol -1) indicated the presence of 5- and 6-membered siloxane rings, consistent with the accepted structure of glass as a disordered form of cristobalite. The experimental studies of the steam reforming of methane were examined at various temperatures (773-923 K) and pressures (1-20 atm) with a commercial Ni/MgAl2O4 catalyst in a hydrogen selective silica-alumina membrane reactor and compared with a packed bed reactor. One-dimensional and two-dimensional modeling of the membrane rector and the packed bed reactor were performed at the same conditions and their performances were compared with the values obtained in the experimental study. Improved methane conversions and hydrogen yields were obtained in the membrane reactor compared to the packed bed reactor at all temperatures and pressures. From the two modeling studies, it was also found out that the two-dimensional model performed better in the membrane reactor case especially at higher pressures.
Fabrication of Ultrathin Palladium Composite Membranes by a New Technique and Their Application in the Ethanol Steam Reforming for H₂ Production
Yun, Samhun (Virginia Tech, 2011-03-21)
This thesis describes a new technique for the preparation of ultrathin Pd based membranes supported on a hollow-fiber α-alumina substrate for H₂ separation. The effectiveness of the membranes is demonstrated in the ethanol steam reforming (EtOH SR) reaction in a membrane reactor (MR) for H₂ production. The membrane preparation technique uses an electric-field to uniformly deposit Pd nanoparticle seeds on a substrate followed by deposition of Pd or Pd-Cu layers on the activated surface by electroless plating (ELP). The well distributed Pd nanoparticles allow for enhanced bonding between the selective layer and the substrate and the formation of gas tight and thermally stable Pd or Pd-Cu layers as thin as 1 µm, which is a record in the field. The best Pd membrane showed H₂ permeance as high as 5.0 × 10⁶ mol m²s⁻¹Pa⁻¹ and stable H²/N₂ selectivity of 9000 - 7000 at 733 K for 5 days. The Pd-Cu alloy membrane showed H₂ permeance of 2.5 × 10⁶ mol m⁻²s⁻¹Pa⁻¹ and H₂/N₂ selectivity of 970 at the same conditions. The reaction studies were carried out with a Co-Na/ZnO catalyst both in a packed bed reactor (PBR) and in a MR equipped with the Pd or Pd-Cu membrane to evaluate the benefits of employing membranes. For all studies, ethanol conversion and hydrogen product yields were significantly higher in the MRs compared to the PBR. Average ethanol conversion enhancement and hydrogen molar flow enhancement were measured to be 12 % and 11 % in the Pd MR and 22 % and 19 % in the Pd-Cu MR, respectively. These enhancements of the conversion and product yield can be attributed to the shift in reaction equilibria by continuous hydrogen removal by the Pd based membranes. The comparative low enhancement in the Pd MR was found to be the result of significant contamination of Pd layer by CO or carbon compounds deposition during the reaction. A one-dimensional modeling of the MR and the PBR was conducted using identical conditions and their performances were compared with the values obtained from the experimental study. The model was developed using a simplified power law and the predicted values matched experimental data with only minor deviations indicating that the model was capturing the essential physicochemical behavior of the system. Enhancements of ethanol conversion and hydrogen yield were observed to increase with rise in space velocity (SV), which could be explained by the increase in H₂ flux through the membranes with SV in the MRs.
Gas-Phase Epoxidation of Ethylene and Propylene
Gaudet, Jason (Virginia Tech, 2010-10-21)
Catalysts consisting of silver on α-Al₂O₃, α-SiC, and β-SiC supports were synthesized and tested for catalytic performance in the gas-phase direct oxidation of ethylene to ethylene oxide. For this study, which used no promoters, ethylene oxidation selectivity of SiC-supported catalysts ranged from 10 to 60% and conversion from 0-4.5%. Silicon carbide supported catalysts exhibited poor performance except for a surface-modified β-SiC-supported catalyst, which demonstrated conversion and selectivity similar to that of an α-Al₂O₃-supported catalyst. This Ag/β-SiC catalyst was further investigated with a kinetic study, and the reaction orders were found to be 0.18 with respect to ethylene and 0.34 with respect to oxygen. The kinetic results were consistent with Langmuir-Hinshelwood rate expressions developed from single-site and dual-site reaction mechanisms. Gold nanoparticles on titanium oxide and titania-silica supports are active for the formation of propylene oxide by the oxidation of propylene with hydrogen and oxygen mixtures. This study investigates the effect of cyanide treatment on gold supported on titanosilicate zeolite supports (Au/TS-1). Catalysts treated with weak solutions of sodium cyanide resulted in preferential removal of small gold particles, while catalysts treated with strong solutions resulted in dissolution of the gold and re-precipitation as gold (+1) cyanide. X-ray absorption spectroscopy demonstrated that catalysts which produce propylene oxide in the presence of hydrogen and oxygen mixtures had supported gold (+3) oxide nanoparticles of 3 nm size after synthesis, which were reduced to gold metal at reaction conditions. Samples treated with strong solutions of sodium cyanide resulted in supported gold (+1) cyanide particles of large size, 9-11 nm. These particles did not produce propylene oxide but, surprisingly, showed high selectivity toward propylene hydrogenation. Increasing gold (+1) cyanide particle size resulted in a decrease in hydrogenation activity. TS-1 and Au/TS-1 surfaces were studied with laser Raman spectroscopy. Surface fluorescence was substantially reduced with a low-temperature ozone treatment, allowing observation of titanosilicate framework bands. Hydrocarbon vibrations are observed for TS-1 and Au/TS-1 under propylene. Density functional theory models indicated that propylene adsorbed to a metal site along the Ï bond would show a Raman spectrum very similar to gas-phase propylene except for out-of-plane C-H vibrations, which would be moved to higher energy. This adsorption spectrum, with out-of-plane vibrations shifted to higher energy, was observed for both TS-1 and Au/TS-1. Langmuir adsorption isotherms were generated for both TS-1 and Au/TS-1, and a scaling factor derived from propylene uptake experiments allowed these isotherms to be scaled to propylene coverage of titanium.

Browsing by Author "Cox, David F."

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