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Reforming petroleum-based fuels for fuel cell vehicles : composition-performance relationships.

Description: Onboard reforming of petroleum-based fuels, such as gasoline, may help ease the introduction of fuel cell vehicles to the marketplace. Although gasoline can be reformed, it is optimized to meet the demands of ICEs. This optimization includes blending to increase the octane number and addition of oxygenates and detergents to control emissions. The requirements for a fuel for onboard reforming to hydrogen are quite different than those for combustion. Factors such as octane number and flame speed are not important; however, factors such as hydrogen density, catalyst-fuel interactions, and possible catalyst poisoning become paramount. In order to identify what factors are important in a hydrocarbon fuel for reforming to hydrogen and what factors are detrimental, we have begun a program to test various components of gasoline and blends of components under autothermal reforming conditions. The results indicate that fuel composition can have a large effect on reforming behavior. Components which may be beneficial for ICEs for their octane enhancing value were detrimental to reforming. Fuels with high aromatic and naphthenic content were more difficult to reform. Aromatics were also found to have an impact on the kinetics for reforming of paraffins. The effects of sulfur impurities were dependent on the catalyst. Sulfur was detrimental for Ni, Co, and Ru catalysts. Sulfur was beneficial for reforming with Pt catalysts, however, the effect was dependent on the sulfur concentration.
Date: December 4, 2001
Creator: Kopasz, J. P.; Miller, L. E.; Ahmed, S.; Devlin, P. R. & Pacheco, M.
Partner: UNT Libraries Government Documents Department

Dynamics of intermediate-length alkane films absorbed on graphite

Description: The dynamics of flexible chain molecules near a solid interface is of fundamental interest in polymer science. This paper makes a preliminary report on a quasielastic neutron scattering study of the dynamics of monolayer films of C{sub 24}H{sub 66} adsorbed on graphite. Quasielastic scattering was observed at temperatures as low as {approximately}180 K, well below the monolayer melting temperature of 340 K. Preliminary analysis of the data indicates that at 230 K the entire molecule executes a rolling motion about its long axis while the terminal methyl groups exhibit relatively rapid reorientation.
Date: January 28, 2000
Creator: Herwig, K. W.; Fuhrmann, D.; Criswell, L.; Taub, H.; Hansen, F. Y.; Dimeo, R. et al.
Partner: UNT Libraries Government Documents Department

Safety testing of 18650-style Li-Ion cells

Description: To address lithium-ion cell safety issues in demanding power applications, electrical and thermal abuse tests were performed on 18650 sized cells. Video and electrically monitored abuse tests in air included short circuit, forced overcharge, forced reversal, and controlled overheating (thermal) modes. Controlled overheating tests to 200 C were performed in a sealed chamber under a helium atmosphere and the gases released from the cell during thermal runaway were analyzed at regular intervals using gas chromatography and mass spectrometry. In addition to alkane and alkene solvent breakdown fragments, significant H{sub 2} was detected and evidence that HF was evolved was also found.
Date: June 8, 2000
Partner: UNT Libraries Government Documents Department

{sup 17}O NMR investigation of oxidative degradation in polymers under gamma-irradiation

Description: The {gamma}-irradiated-oxidation of pentacontane (C{sub 50}H{sub 102}) and the polymer polyisoprene was investigated as a function of oxidation level using {sup 17}O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using {sup 17}O labeled O{sub 2} gas during the {gamma}-irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the {sup 17}O NMR spectra. Production of carboxylic acids is the primary oxygen-containing functionality during the oxidation of pentacontane, while ethers and alcohols are the dominant oxidation product observed for polyisoprene. The formation of ester species during the oxidation process is very minor for both materials, with water also being produced in significant amounts during the radiolytic oxidation of polyisoprene. The ability to focus on the oxidative component of the degradation process using {sup 17}O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches.
Date: March 8, 2000
Partner: UNT Libraries Government Documents Department

Effect of catalyst structure on oxidative dehydrogenation of ethane and propane on alumina-supported vanadia

Description: The catalytic properties of Al2O3-supported vanadia with a wide range of VOx surface density (1.4-34.2 V/nm2) and structure were examined for the oxidative dehydrogenation of ethane and propane. UV-visible and Raman spectra showed that vanadia is dispersed predominantly as isolated monovanadate species below {approx}2.3 V/nm2. As surface densities increase, two-dimensional polyvanadates appear (2.3-7.0 V/nm2) along with increasing amounts of V2O5 crystallites at surface densities above 7.0 V/nm2. The rate constant for oxidative dehydrogenation (k1) and its ratio with alkane and alkene combustion (k2/k1 and k3/k1, respectively) were compared for both alkane reactants as a function of vanadia surface density. Propene formation rates (per V-atom) are {approx}8 times higher than ethene formation rates at a given reaction temperature, but the apparent ODH activation energies (E1) are similar for the two reactants and relatively insensitive to vanadia surface density. Ethene and propene formation rates (per V-atom) are strongly influenced by vanadia surface density and reach a maximum value at intermediate surface densities ({approx}8 V/nm2). The ratio of k2/k1 depends weakly on reaction temperature, indicating that activation energies for alkane combustion and ODH reactions are similar. The ratio of k2/k1 is independent of surface density for ethane, but increase slightly with vanadia surface density for propane, suggesting that isolated structures prevalent at low surface densities are slightly more selective for alkane dehydrogenation reactions. The ratio of k3/k1 decreases markedly with increasing reaction temperature for both ethane and propane ODH. Thus, the apparent activation energy for alkene combustion (E3) is much lower than that for alkane dehydrogenation (E1) and the difference between these two activation energies decreases with increasing surface density. The lower alkene selectivities observed at high vanadia surface densities are attributed to an increase in alkene adsorption enthalpies with increasing vanadia surface density. The highest yield of alkene is obtained for catalysts ...
Date: September 11, 2001
Creator: Argyle, Morris D.; Chen, Kaidong; Bell, Alexis T. & Iglesia, Enrique
Partner: UNT Libraries Government Documents Department

Detailed structural characterization of the grafting of [Ta(=CHtBu)(CH2tBu)3] and [Cp*TaMe4] on silica partially dehydroxylated at 700 C and the activity of the grafted complexes toward alkane metathesis

Description: The reaction of [Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 3}] or [Cp*Ta(CH{sub 3}){sub 4}] with a silica partially dehydroxylated at 700 C gives the corresponding monosiloxy surface complexes [({triple_bond}SiO)Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 2}] and [({triple_bond}SiO)Ta(CH{sub 3}){sub 3}Cp*] by eliminating a {sigma}-bonded ligand as the corresponding alkane (H-CH{sub 2}tBu or H-CH{sub 3}). EXAFS data show that an adjacent siloxane bridge of the surface plays the role of an extra surface ligand, which most likely stabilizes these complexes as in [({triple_bond}SiO)Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 2}({triple_bond}SiOSi{triple_bond})] (1a') and [({triple_bond}SiO)Ta(CH{sub 3}){sub 3}Cp*({triple_bond}SiOSi{triple_bond})] (2a'). In the case of [({triple_bond}SiO)Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 2}({triple_bond}SiOSi{triple_bond})], the structure is further stabilized by an additional interaction: a C-H agostic bond as evidenced by the small J coupling constant for the carbenic C-H (H{sub C-H} = 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [({triple_bond}SiO)Ta({double_bond}CHtBu)-(CH{sub 2}tBu){sub 2}({triple_bond}SiOSi{triple_bond})] (1a') as a catalyst precursor and the inactivity of the surface complex [({triple_bond}SiO)Ta-(CH{sub 3}){sub 3}Cp*({triple_bond}SiOSi{triple_bond})] (2a') show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.
Date: August 30, 2004
Creator: Le Roux, Erwan; Chabanas, Mathieu; Baudouin, Anne; de Mallmann, Aimery; Coperet, Christophe; Quadrelli, E. Allesandra et al.
Partner: UNT Libraries Government Documents Department

Auxiliary basis expansions for large-scale electronic structure calculations

Description: One way to reduce the computational cost of electronic structure calculations is to employ auxiliary basis expansions to approximate 4 center integrals in terms of 2 and 3-center integrals, usually using the variationally optimum Coulomb metric to determine the expansion coefficients. However the long-range decay behavior of the auxiliary basis expansion coefficients has not been characterized. We find that this decay can be surprisingly slow. Numerical experiments on linear alkanes and a toy model both show that the decay can be as slow as 1/r in the distance between the auxiliary function and the fitted charge distribution. The Coulomb metric fitting equations also involve divergent matrix elements for extended systems treated with periodic boundary conditions. An attenuated Coulomb metric that is short-range can eliminate these oddities without substantially degrading calculated relative energies. The sparsity of the fit coefficients is assessed on simple hydrocarbon molecules, and shows quite early onset of linear growth in the number of significant coefficients with system size using the attenuated Coulomb metric. This means it is possible to design linear scaling auxiliary basis methods without additional approximations to treat large systems.
Date: April 4, 2005
Creator: Jung, Yousung; Sodt, Alexander; Gill, Peter W.M. & Head-Gordon, Martin
Partner: UNT Libraries Government Documents Department

Investigation of microwave transitions and nonlinear magneto-optical rotation in anti-relaxation-coated cells

Description: Using laser optical pumping, widths and frequency shifts are determined for microwave transitions between the components of the ground-state hyperfine structure for {sup 85}Rb and {sup 87}Rb atoms contained in vapor cells with alkane anti-relaxation coatings. The results are compared with data on Zeeman relaxation obtained in nonlinear magneto-optical rotation (NMOR) experiments, a comparison important for quantitative understanding of spin-relaxation mechanisms in coated cells. By comparing cells manufactured over a forty-year period we demonstrate the long-term stability of coated cells, which may be useful for atomic clocks and magnetometers.
Date: June 4, 2004
Creator: Budker, D.; Hollberg, L.; Kimball, D.F.; Kitching, J.; Pustclny, S.; Robinson, H.G. et al.
Partner: UNT Libraries Government Documents Department

Recovery of semi-volatile organic compounds during sample preparation: Compilation for characterization of airborne particulate matter

Description: Semi-volatile compounds present special analytical challenges not met by conventional methods for analysis of ambient particulate matter (PM). Accurate quantification of PM-associated organic compounds requires validation of the laboratory procedures for recovery over a wide volatility and polarity range. To meet these challenges, solutions of n-alkanes (nC{sub 12} to nC{sub 40}) and polycyclic aromatic hydrocarbons PAHs (naphthalene to benzo[ghi]perylene) were reduced in volume from a solvent mixture (equal volumes of hexane, dichloromethane and methanol), to examine recovery after reduction in volume. When the extract solution volume reached 0.5 mL the solvent was entirely methanol, and the recovery averaged 60% for n-alkanes nC{sub 12} to nC{sub 25} and PAHs from naphthalene to chrysene. Recovery of higher MW compounds decreased with MW, because of their insolubility in methanol. When the walls of the flasks were washed with 1 mL of equal parts hexane and dichloromethane (to reconstruct the original solvent composition), the recovery of nC{sub 18} and higher MW compounds increased dramatically, up to 100% for nC{sub 22}-nC{sub 32} and then slowly decreasing with MW due to insolubility. To examine recovery during extraction of the components of the High Capacity Integrated Gas and Particle Sampler, the same standards were used to spike its denuders and filters. For XAD-4 coated denuders and filters, normalized recovery was > 95% after two extractions. Recovery from spiked quartz filters matched the recovery from the coated surfaces for alkanes nC{sub 18} and larger, and for fluoranthene and larger PAHs. Lower MW compounds evaporated from the quartz filter with the spiking solvent. This careful approach allowed quantification of organics by correcting for volatility- and solubility-related sample preparation losses. This method is illustrated for an ambient sample collected with this sampler during the Texas Air Quality Study 2000.
Date: May 1, 2002
Creator: Swartz, Erick; Stockburger, Leonard & Gundel, Lara
Partner: UNT Libraries Government Documents Department

Probing the Conformational Distributions of Sub-Persistence Length DNA

Description: We have measured the bending elasticity of short double-stranded DNA (dsDNA) chains through small-angle X-ray scattering from solutions of dsDNA-linked dimers of gold nanoparticles. This method, which does not require exertion of external forces or binding to a substrate, reports on the equilibrium distribution of bending fluctuations, not just an average value (as in ensemble FRET) or an extreme value (as in cyclization), and in principle provides a more robust data set for assessing the suitability of theoretical models. Our experimental results for dsDNA comprising 42-94 basepairs (bp) are consistent with a simple worm-like chain model of dsDNA elasticity, whose behavior we have determined from Monte Carlo simulations that explicitly represent nanoparticles and their alkane tethers. A persistence length of 50 nm (150 bp) gave a favorable comparison, consistent with the results of single-molecule force-extension experiments on much longer dsDNA chains, but in contrast to recent suggestions of enhanced flexibility at these length scales.
Date: June 8, 2009
Creator: Mastroianni, Alexander; Sivak, David; Geissler, Phillip & Alivisatos, Paul
Partner: UNT Libraries Government Documents Department

Near-Edge X-ray Absorption Fine Structure Spectroscopy of Diamondoid Thiol Monolayers on Gold

Description: Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface-modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 {+-} 0.05 eV and 0.16 {+-} 0.04 eV respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different amounts of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond clusters.
Date: November 27, 2007
Creator: Willey, T M; Fabbri, J; Lee, J I; Schreiner, P; Fokin, A A; Tkachenko, B A et al.
Partner: UNT Libraries Government Documents Department

Temperature control in a 30 stage, 5-cm Centrifugal Contactor Pilot Plant

Description: Temperature profile testing was performed using a 30 stage 5-cm centrifugal contactor pilot plant. These tests were performed to evaluate the ability to control process temperature by adjusting feed solution temperatures. This would eliminate the need for complex jacketed heat exchanger installation on the centrifugal contactors. Thermocouples were installed on the inlet and outlets of each stage, as well as directly in the mixing zone of several of the contactor stages. Lamp oil, a commercially available alkane mixture of C14 to C18 chains, and tap water adjusted to pH 2 with nitric acid were the solution feeds for the temperature profile testing. Temperature data profiles for an array of total throughputs and contactor rpm values for both single-phase and two-phase systems were collected with selected profiles. The total throughput ranged from 0.5-1.4 L/min with rotor speeds from 3500-4000 rpm. Inlet solution temperatures ranging from ambient up to 50 °C were tested. Results of the two-phase temperature profile testing are detailed
Date: September 1, 2009
Creator: Law, Jack D.; Garn, Troy G. & Meikrantz, David H.
Partner: UNT Libraries Government Documents Department

Gamma Ray Radiolysis of the FPEX Solvent

Description: Slide presentation. FPEX contains a calixarene for Cs extraction, a crown ether for Sr extraction, Cs7SB modifier, and TOA to aid in stripping, in Isopar L diluent. The radiation stability FPEX must be evaluated prior to process use. Radiolytic degradation of species in solution are due to reaction with the direct radiolysis products of the diluent. In Isopar L, the reactive species produced include e-, •H and alkane radicals, resulting in a reducing environment. However, in nitric acid, oxidizing hydroxyl (•OH) and nitro (•NO2) radicals dominate system chemistry. Thus, the nature of diluent and the presence of radical scavengers affect the results of irradiation. We report the preliminary results of a new program to investigate the radiolysis of FPEX using the 60Co irradiation of FPEX neat solvent, acid pre-equilibrated solvent and mixed aerated phases. The Cs and Sr distribution ratios were used as metrics.
Date: September 1, 2006
Creator: Mincher, B. J.; Mezyk, S. P. & Peterman, D. R.
Partner: UNT Libraries Government Documents Department

Detailed Chemical Kinetic Reaction Mechanisms for Combustion of Isomers of Heptane

Description: Detailed chemical kinetic reaction mechanisms are developed for all nine chemical isomers of heptane (C{sub 7}H{sub 16}), following techniques and models developed previously for other smaller alkane hydrocarbon species. These reaction mechanisms are tested at high temperatures by computing shock tube ignition delay times and at lower temperatures by simulating ignition in a rapid compression machine. Although the corresponding experiments have not been reported in the literature for most of these isomers of heptane, intercomparisons between the computed results for these isomers and comparisons with available experimental results for other alkane fuels are used to validate the reaction mechanisms as much as possible. Differences in the overall reaction rates of these fuels are discussed in terms of differences in their molecular structure and the resulting variations in rates of important elementary reactions. Reaction mechanisms in this study are works in progress and the results reported here are subject to change, based on model improvements and corrections of errors not yet discovered.
Date: March 26, 2001
Creator: Westbrook, C K; Pitz, W J; Curran, H C; Boercker, J & Kunrath, E
Partner: UNT Libraries Government Documents Department

A Rapid Compression Machine Modelling Study of the Heptane Isomers

Description: Previously we have reported on the combustion behavior of all nine isomers of heptane in a rapid compression machine (RCM) with stoichiometric fuel and ''air'' mixtures at a compressed gas pressure of 15 atm. The dependence of autoignition delay times on molecular structure was illustrated. Here, we report some additional experimental work that was performed in order to address unusual results regarding significant differences in the ignition delay times recorded at the same fuel and oxygen composition, but with different fractions of nitrogen and argon diluent gases. Moreover, we have begun to simulate these experiments with detailed chemical kinetic mechanisms. These mechanisms are based on previous studies of other alkane molecules, in particular, n-heptane and iso-octane. We have focused our attention on n-heptane in order to systematically redevelop the chemistry and thermochemistry for this C{sub 7} isomer with the intention of extending our greater knowledge gained to the other eight isomers. The addition of new reaction types, that were not included previously, has had a significant impact on the simulations, particularly at low temperatures.
Date: May 10, 2005
Creator: Silke, E J; Curran, H J; Simmie, J M; Pitz, W J & Westbrook, C K
Partner: UNT Libraries Government Documents Department

Detailed Chemical Kinetic Modeling of Surrogate Fuels for Gasoline and Application to an HCCI Engine

Description: Gasoline consists of many different classes of hydrocarbons, such as paraffins, olefins, aromatics, and cycloalkanes. In this study, a surrogate gasoline reaction mechanism is developed, and it has one representative fuel constituent from each of these classes. These selected constituents are iso-octane, n-heptane, 1-pentene, toluene, and methyl-cyclohexane. The mechanism was developed in a step-wise fashion, adding submechanisms to treat each fuel component. Reactions important for low temperature oxidation (<1000K) and cross-reactions among different fuels are incorporated into the mechanism. The mechanism consists of 1214 species and 5401 reactions. A single-zone engine model is used to evaluate how well the mechanism captures autoignition behavior for conditions corresponding to homogeneous charge compression ignition (HCCI) engine operation. Experimental data are available for both how the combustion phasing changes with fueling at a constant intake temperature, and also how the intake temperature has to be changed with pressure in order to maintain combustion phasing for a fixed equivalence ratio. Three different surrogate fuel mixtures are used for the modeling. Predictions are in reasonably good agreement with the engine data. In addition, the heat release rate is calculated and compared to the data from experiments. The model predicts less low-temperature heat release than that measured. It is found that the low temperature heat-release rate depends strongly on engine speed, reactions of RO{sub 2}+HO{sub 2}, fuel composition, and pressure boost.
Date: January 7, 2005
Creator: Naik, C V; Pitz, W J; Sj?berg, M; Dec, J E; Orme, J; Curran, H J et al.
Partner: UNT Libraries Government Documents Department

Hydrogen Separation Membranes for Vision 21 Fossil Fuel Plants

Description: Eltron Research and team members CoorsTek, McDermott Technology, Sued Chemie, Argonne National Laboratory, and Oak Ridge National Laboratory are developing an environmentally benign, inexpensive, and efficient method for separating hydrogen from gas mixtures produced during industrial processes, such as coal gasification. This objective is being pursued using dense membranes based in part on Eltron-patented ceramic materials with a demonstrated ability for proton and electron conduction. The technical goals are being addressed by modifying single-phase and composite membrane composition and microstructure to maximize proton and electron conductivity without loss of material stability. Ultimately, these materials must enable hydrogen separation at practical rates under ambient and high-pressure conditions, without deactivation in the presence of feedstream components such as carbon dioxide, water, and sulfur. This project was motivated by the Department of Energy (DOE) National Energy Technology Laboratory (NETL) Vision 21 initiative which seeks to economically eliminate environmental concerns associated with the use of fossil fuels. The proposed technology addresses the DOE Vision 21 initiative in two ways. First, this process offers a relatively inexpensive solution for pure hydrogen separation that can be easily incorporated into Vision 21 fossil fuel plants. Second, this process could reduce the cost of hydrogen, which is a clean burning fuel under increasing demand as supporting technologies are developed for hydrogen utilization and storage. Additional motivation for this project arises from the potential of this technology for other applications. By appropriately changing the catalysts coupled with the membrane, essentially the same system can be used to facilitate alkane dehydrogenation and coupling, aromatics processing, and hydrogen sulfide decomposition.
Date: November 6, 2001
Creator: Roark, Shane E.; Mackay, Richard & Sammells, Anthony F.
Partner: UNT Libraries Government Documents Department

Gas-Phase Reactions of Doubly Charged Lanthanide Cations with Alkanes and Alkenes. Trends in Metal(2+) Reactivity

Description: The gas-phase reactivity of doubly-charged lanthanide cations, Ln2+ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), with alkanes (methane, ethane, propane, n-butane) and alkenes (ethene, propene, 1-butene) was studied by Fourier transform ion cyclotron resonance mass spectrometry. The reaction products consisted of different combinations of doubly-charged organometallic ions?adducts or species formed via metal-ion-induced hydrogen, dihydrogen, alkyl, or alkane eliminations from the hydrocarbons?and singly-charged ions that resulted from electron, hydride, or methide transfers from the hydrocarbons to the metal ions. The only lanthanide cations capable of activating the hydrocarbons to form doubly-charged organometallic ions were La2+, Ce2+, Gd2+, and Tb2+, which have ground-state or low-lying d1 electronic configurations. Lu2+, with an accessible d1 electronic configuration but a rather high electron affinity, reacted only through transfer channels. The remaining Ln2+ reacted via transfer channels or adduct formation. The different accessibilities of d1 electronic configurations and the range of electron affinities of the Ln2+ cations allowed for a detailed analysis of the trends for metal(2+) reactivity and the conditions for occurrence of bond activation, adduct formation, and electron, hydride, and methide transfers.
Date: December 8, 2008
Creator: Gibson, John K.; Marcalo, Joaquim; Santos, Marta; Pires de Matos, Antonio & Haire, Richard G.
Partner: UNT Libraries Government Documents Department