292 Matching Results

Search Results

Advanced search parameters have been applied.

A Study of the Effect of Heterogeneous Grouping as Opposed to Homogeneous Grouping for Language Development of Severely Retarded Students

Description: The purpose of the present study was to investigate the effect of heterogeneous grouping and homogeneous grouping in the language development of severely retarded students. A further purpose was to determine how beneficial either of the grouping procedures would be for this level of student.
Date: August 1968
Creator: Burbank, Erwin Russell
Partner: UNT Libraries

The impact of nanoscience on heterogeneous catalysis

Description: Most catalysts consist of nanometer-sized particles dispersed on a high-surface area support. Advances in characterization methods have led to a molecular level understanding of the relationships between nanoparticle properties and catalytic performance. Together with novel approaches to nanoparticle synthesis, this knowledge is contributing to the design and development of new catalysts.
Date: March 3, 2003
Creator: Bell, Alexis T.
Partner: UNT Libraries Government Documents Department

ISHHC XIII International Symposium on the Relations betweenHomogeneous and Heterogeneous Catalysis

Description: The International Symposium on Relations between Homogeneous and Heterogeneous Catalysis (ISHHC) has a long and distinguished history. Since 1974, in Brussels, this event has been held in Lyon, France (1977), Groeningen, The Netherlands (1981); Asilomar, California (1983); Novosibirsk, Russia (1986); Pisa, Italy (1989); Tokyo, Japan (1992); Balatonfuered, Hungary (1995); Southampton, United Kingdom (1999); Lyon, France (2001); Evanston, Illinois (2001) and Florence, Italy (2005). The aim of this international conference in Berkeley is to bring together practitioners in the three fields of catalysis, heterogeneous, homogeneous and enzyme, which utilize mostly nanosize particles. Recent advances in instrumentation, synthesis and reaction studies permit the nanoscale characterization of the catalyst systems, often for the same reaction, under similar experimental conditions. It is hoped that this circumstance will permit the development of correlations of these three different fields of catalysis on the molecular level. To further this goal we aim to uncover and focus on common concepts that emerge from nanoscale studies of structures and dynamics of the three types of catalysts. Another area of focus that will be addressed is the impact on and correlation of nanosciences with catalysis. There is information on the electronic and atomic structures of nanoparticles and their dynamics that should have importance in catalyst design and catalytic activity and selectivity.
Date: June 11, 2007
Creator: Somorjai (Ed.), G.A.
Partner: UNT Libraries Government Documents Department

Wetting of a Chemically Heterogeneous Surface

Description: Theories for inhomogeneous fluids have focused in recent years on wetting, capillary conden- sation, and solvation forces for model systems where the surface(s) is(are) smooth homogeneous parallel plates, cylinders, or spherical drops. Unfortunately natural systems are more likely to be hetaogeneous both in surt%ce shape and surface chemistry. In this paper we discuss the conse- quences of chemical heterogeneity on wetting. Specifically, a 2-dimensional implementation of a nonlocal density functional theory is solved for a striped surface model. Both the strength and range of the heterogeneity are varied. Contact angles are calculated, and phase transitions (both the wetting transition and a local layering transition) are located. The wetting properties of the surface ase shown to be strongly dependent on the nature of the surface heterogeneity. In addition highly ordered nanoscopic phases are found, and the operational limits for formation of ordered or crystalline phases of nanoscopic extent are discussed.
Date: November 20, 1998
Creator: Frink, L.J.D. & Salinger, A.G.
Partner: UNT Libraries Government Documents Department

Microgrids and Heterogeneous Power Quality and Reliability

Description: This paper describes two stylized alternative visions of how the power system might evolve to meet future requirements for the high quality electricity service that modern digital economies demand, a supergrids paradigm and a dispersed paradigm. Some of the economics of the dispersed vision are explored, and perspectives are presented on both the choice of homogeneous universal power quality upstream in the electricity supply chain and on the extremely heterogeneous requirements of end-use loads. It is argued that meeting the demanding requirements of sensitive loads by local provision of high quality power may be more cost effective than increasing the quality of universal homogeneous supply upstream in the legacy grid. Finally, the potential role of microgrids in delivering heterogeneous power quality is demonstrated by reference to two ongoing microgrid tests in the U.S. and Japan.
Date: October 1, 2007
Creator: LaCommare, Kristina & Marnay, Chris
Partner: UNT Libraries Government Documents Department

Converting Homogeneous to Heterogeneous in Electrophilic Catalysis using Monodisperse Metal Nanoparticles

Description: A continuing goal in catalysis is the transformation of processes from homogeneous to heterogeneous. To this end, nanoparticles represent a new frontier in heterogeneous catalysis, where this conversion is supplemented by the ability to obtain new or divergent reactivity and selectivity. We report a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles. These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl{sub 2}, and catalyze a range of {pi}-bond activation reactions previously only homogeneously catalyzed. Multiple experimental methods are utilized to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, our size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared to larger, polymer-capped analogues.
Date: October 15, 2009
Creator: Witham, Cole A.; Huang, Wenyu; Tsung, Chia-Kuang; Kuhn, John N.; Somorjai, Gabor A. & Toste, F. Dean
Partner: UNT Libraries Government Documents Department

CO Oxidation at the Interface of Au Nanoclusters and the Stepped-CeO2(111) Surface

Description: To reveal the richer chemistry of CO oxidation by CeO2 supported Au Nanoclusters NCs)/Nanoparticles, we design a Au12 supported on a stepped-CeO2 model (Au/CeO2-step) and study various kinds of CO oxidation mechanisms at the interface of the Au/CeO2-step: oxygen spillover from the CeO2 to the Au NCs;2 CO oxidation by the O2 bound to the Au-Ce3+ interface;3 and CO oxidation by the Mars-van Krevelen (M-vK) mechanism.4 DFT+U calculations show that lattice oxygen at the CeO2 step edge oxidizes CO bound to Au NCs by the M-vK mechanism. CO2 desorption determines the rate of CO oxidation and the vacancy formation energy (Evac) is a reactivity descriptor for CO oxidation. The maximum Evac that insures spontaneous CO2 production is higher for the Au/CeO2-step than the Au/CeO2-surface suggesting that the CeO2-step is a better supporting material than the CeO2-surface for CO oxidation by the Au/CeO2. Our results also suggest that for CO oxidation by Au NCs supported on nano- or meso-structured CeO2, which is the case of industrial catalysts, the M-vK mechanism accounts for a large portion of the total activity.
Date: January 14, 2013
Creator: Y., Kim H. & Henkelman, G.
Partner: UNT Libraries Government Documents Department

Estimation of hydrologic properties of heterogeneous geologic media with an inverse method based on iterated function systems

Description: The hydrologic properties of heterogeneous geologic media are estimated by simultaneously inverting multiple observations from well-test data. A set of pressure transients observed during one or more interference tests is compared to the corresponding values obtained by numerically simulating the tests using a mathematical model. The parameters of the mathematical model are varied and the simulation repeated until a satisfactory match to the observed pressure transients is obtained, at which point the model parameters are accepted as providing a possible representation of the hydrologic property distribution. Restricting the search to parameters that represent fractal hydrologic property distributions can improve the inversion process. Far fewer parameters are needed to describe heterogeneity with a fractal geometry, improving the efficiency and robustness of the inversion. Additionally, each parameter set produces a hydrologic property distribution with a hierarchical structure, which mimics the multiple scales of heterogeneity often seen in natural geological media. Application of the IFS inverse method to synthetic interference-test data shows that the method reproduces the synthetic heterogeneity successfully for idealized heterogeneities, for geologically-realistic heterogeneities, and when the pressure data includes noise.
Date: May 1, 1996
Creator: Doughty, C.A.
Partner: UNT Libraries Government Documents Department

Neutronic Assessment of Transmutation Target Compositions in Heterogeneous Sodium Fast Reactor Geometries

Description: The sodium fast reactor is under consideration for consuming the transuranic waste in the spent nuclear fuel generated by light water reactors. This work is concerned with specialized target assemblies for an oxide-fueled sodium fast reactor that are designed exclusively for burning the americium and higher mass actinide component of light water reactor spent nuclear fuel (SNF). The associated gamma and neutron radioactivity, as well as thermal heat, associated with decay of these actinides may significantly complicate fuel handling and fabrication of recycled fast reactor fuel. The objective of using targets is to isolate in a smaller number of assemblies these concentrations of higher actinides, thus reducing the volume of fuel having more rigorous handling requirements or a more complicated fabrication process. This is in contrast to homogeneous recycle where all recycled actinides are distributed among all fuel assemblies. Several heterogeneous core geometries were evaluated to determine the fewest target assemblies required to burn these actinides without violating a set of established fuel performance criteria. The DIF3D/REBUS code from Argonne National Laboratory was used to perform the core physics and accompanying fuel cycle calculations in support of this work. Using the REBUS code, each core design was evaluated at the equilibrium cycle condition.
Date: February 1, 2008
Creator: Bays, Samuel E.; Ferrer, Rodolfo M.; Pope, Michael A.; Forget, Benoit & Asgari, Mehdi
Partner: UNT Libraries Government Documents Department

Dynamics of Propane in Silica Mesopores Formed upon PropyleneHydrogenation over Pt Nanoparticles by Time-Resolved FT-IRSpectroscopy

Description: Propylene hydrogenation over Pt nanoparticles supported onmesoporous silica type SBA-15 was monitored by time-resolved FT-IRspectroscopy at 23 ms resolution using short propylene gas pulses thatjoined a continuous flow of hydrogen in N2 (1 atm total pressure).Experiments were conducted in the temperature range 323-413 K. Propanewas formed within 100 milliseconds or faster. The CH stretching regionrevealed distinct bands for propane molecules emerging inside thenanoscale channels of the silica support. Spectral analysis gave thedistribution of the propane product between support and surrounding gasphase as function of time. Kinetic analysis showed that the escape ofpropane molecules from the channels occurred within hundreds ofmilliseconds (3.1 + 0.4 s-1 at 383 K). A steady state distribution ofpropane between gas phase and mesoporous support is established as theproduct is swept from the catalyst zone by the continuous flow ofhydrogen co-reactant. This is the first direct spectroscopic observationof emerging products of heterogeneous catalysis on nanoporous supportsunder reaction conditions.
Date: January 31, 2007
Creator: Waslylenko, Walter & Frei, Heinz
Partner: UNT Libraries Government Documents Department

OH-initiated heterogeneous aging of highly oxidized organic aerosol

Description: The oxidative evolution (“aging”) of organic species in the atmosphere is thought to have a major influence on the composition and properties of organic particulate matter, but remains poorly understood, particularly for the most oxidized fraction of the aerosol. Here we measure the kinetics and products of the heterogeneous oxidation of highly oxidized organic aerosol, with an aim of better constraining such atmospheric aging processes. Submicron particles composed of model oxidized organics—1,2,3,4-butanetetracarboxylic acid (C{sub 8}H{sub 10}O{sub 8}), citric acid (C{sub 6}H{sub 8}O{sub 7}), tartaric acid (C{sub 4}H{sub 6}O{sub 6}), and Suwannee River fulvic acid—were oxidized by gas-phase OH in a flow reactor, and the masses and elemental composition of the particles were monitored as a function of OH exposure. In contrast to our previous studies of less-oxidized model systems (squalane, erythritol, and levoglucosan), particle mass did not decrease significantly with heterogeneous oxidation. Carbon content of the aerosol always decreased somewhat, but this mass loss was approximately balanced by an increase in oxygen content. The estimated reactive uptake coefficients of the reactions range from 0.37 to 0.51 and indicate that such transformations occur at rates corresponding to 1-2 weeks in the atmosphere, suggesting their importance in the atmospheric lifecycle of organic particulate matter.
Date: December 5, 2011
Creator: Kessler, Sean H.; Nah, Theodora; Daumit, Kelly E.; Smith, Jared D.; Leone, Stephen R.; Kolb, Charles E. et al.
Partner: UNT Libraries Government Documents Department

Measurement of fragmentation and functionalization pathways in the multistep heterogeneous oxidation of organic aerosol

Description: The competition between the addition of polar, oxygen-containing functional groups (functionalization) and the cleavage of C-C bonds (fragmentation) has a governing influence on the change in volatility of organic species upon atmospheric oxidation, and hence on the loading of tropospheric organic aerosol. However the branching between these two channels is generally poorly constrained for oxidized organics. Here we determine functionalization/fragmentation branching ratios for organics spanning a range of oxidation levels, using the heterogeneous oxidation of squalane (C30H62) as a model system. Squalane particles are exposed to high concentrations of OH in a flow reactor, and measurements of particle mass and elemental ratios enable the determination of absolute elemental composition (number of oxygen, carbon, and hydrogen atoms) of the oxidized particles. At low OH exposures, the oxygen content of the organics increases, indicating that functionalization dominates, whereas at higher exposures the amount of carbon in the particles decreases, indicating the increasing importance of fragmentation processes. Once the organics are moderately oxidized (O/C~;;0.4), fragmentation completely dominates, and the increase in O/C ratio upon further oxidation is due to the loss of carbon rather than the addition of oxygen. These results suggest that fragmentation reactions may be key steps in the atmospheric formation and evolution of oxygenated organic aerosol (OOA).
Date: March 10, 2009
Creator: Kroll, Jesse H.; Smith, Jared D.; Che, Dung L.; Kessler, Sean H.; Worsnop, Douglas R. & Wilson, Kevin R.
Partner: UNT Libraries Government Documents Department

Minor Actinide Recycle in Sodium Cooled Fast Reactors Using Heterogeneous Targets

Description: This paper investigates the plausible design of transmutation target assemblies for minor actinides (MA) in Sodium Fast Reactors (SFR). A heterogeneous recycling strategy is investigated, whereby after each reactor pass, un-burned MAs from the targets are blended with MAs produced by the driver fuel and additional MAs from Spent Nuclear Fuel (SNF). A design iteration methodology was adopted for customizing the core design, target assembly design and matrix composition design. The overall design was constrained against allowable peak or maximum in-core performances. While respecting these criteria, the overall design was adjusted to reduce the total number of assemblies fabricated per refueling cycle. It was found that an inert metal-hydride MA-Zr-Hx target matrix gave the highest transmutation efficiency, thus allowing for the least number of targets to be fabricated per reactor cycle.
Date: April 1, 2009
Creator: Bays, Samuel; Medvedev, Pavel; Pope, Michael; Ferrer, Rodolfo; Forget, Benoit & Asgari, Mehdi
Partner: UNT Libraries Government Documents Department

Evaluation of Heterogeneous Options: Effects of MgO versus UO2 Matrix Selection for Minor Actinide Targets in a Sodium Fast Reactor

Description: The primary focus of this work was to compare MgO with UO2 as target matrix material options for burning minor actinides in a transmutation target within a sodium fast reactor. This analysis compared the transmutation performance of target assemblies having UO2 matrix to those having specifically MgO inert matrix.
Date: March 1, 2008
Creator: Pope, M.; Bays, S. & Ferrer, R.
Partner: UNT Libraries Government Documents Department

Understanding and Controlling Metal-Support Interactions in Nanocrystalline Bimetallic Catalysts

Description: The objectives of this research are to: 1) determine the catalytic behavior of model Pd and Rh catalysts on unpromoted and ceria-promoted supports, for the reduction of NO and N2O with CO, 2) determine the microstructures of the catalysts both before and after reaction in order to understand the catalytic behavior, and 3) understand the role of the metal/support interface in the catalytic process. The research examined the influence of Pd particle size and ceria loading on catalytic reaction for the NO+CO reaction. Dihydrogen chemisorption, temperature-programmed desorption (TPD) of NO, and high-resolution transmission electron microscopy (HRTEM) were used to characterize the catalyst samples. It was found that when ceria is used to promote Pd particles, the activity for NO+CO was a maximum for 2-nm-sized Pd particles. The maximum in activity results from a balance between the Pd/ceria interface, which enhances NO dissociation, and the close-packed planes of the Pd particles that facilitate product formation and/or desorption. The variations in apparent reaction orders and results from TPD were consistent with the idea that NO dissociation is promoted on very small particles (1 nm) and by the addition of ceria. Characterization of the catalysts by HRTEM showed that the ceria was typically present in the form of small crystallites from 3-7 nm in diameter, deposited near Pd particles, rather than as a thin film on the alumina support. This occurred whether the ceria was deposited before or after the Pd particles.
Date: June 28, 2005
Creator: Howe, James M. & Davis, Robert J.
Partner: UNT Libraries Government Documents Department

Photocatalytic oxidation of gas-phase BTEX-contaminated waste streams

Description: Researchers at the National Renewable Energy Laboratory (NREL) have been exploring heterogeneous photocatalytic oxidation (PCO) as a remediation technology for air streams contaminated with benzene, toluene, ethyl-benzene, and xylenes (BTEX). This research is a continuation of work performed on chlorinated organics. The photocatalytic oxidation of BTEX has been studied in the aqueous phase, however, a study by Turchi et al. showed a more economical system would involve stripping organic contaminants from the aqueous phase and treating the resulting gas stream. Another recent study by Turchi et al. indicated that PCO is cost competitive with such remediation technologies as activated carbon adsorption and catalytic incineration for some types of contaminated air streams. In this work we have examined the photocatalytic oxidation of benzene using ozone (0{sub 3}) as an additional oxidant. We varied the residence time in the PCO reactor, the initial concentration of the organic pollutant, and the initial ozone concentration in a single-pass reactor. Because aromatic hydrocarbons represent only a small fraction of the total hydrocarbons present in gasoline and other fuels, we also added octane to the reaction mixture to simulate the composition of air streams produced from soil-vapor-extraction or groundwater-stripping of sites contaminated with gasoline.
Date: March 1, 1995
Creator: Gratson, D A; Nimlos, M R & Wolfrum, E J
Partner: UNT Libraries Government Documents Department

Major Successes of Theory-and-Experiment-Combined Studies in Surface Chemistry and Heterogeneous Catalysis.

Description: Experimental discoveries followed by theoretical interpretations that pave the way of further advances by experimentalists is a developing pattern in modern surface chemistry and catalysis. The revolution of modern surface science started with the development of surface-sensitive techniques such as LEED, XPS, AES, ISS and SIMS, in which the close collaboration between experimentalists and theorists led to the quantitative determination of surface structure and composition. The experimental discovery of the chemical activity of surface defects and the trends in the reactivity of transitional metals followed by the explanations from the theoretical studies led to the molecular level understanding of active sites in catalysis. The molecular level knowledge, in turn, provided a guide for experiments to search for new generation of catalysts. These and many other examples of successes in experiment-and-theory-combined studies demonstrate the importance of the collaboration between experimentalists and theorists in the development of modern surface science.
Date: November 21, 2009
Creator: Somorjai, Gabor A. & Li, Yimin
Partner: UNT Libraries Government Documents Department

CO Oxidation mechanism on CeO2-supported Au nanoclusters

Description: To reveal the richer chemistry of CO oxidation by CeO2 supported Au Nanoclusters(NCs)/Nanoparticles, we design Au13 and Au12 supported on a flat and a stepped-CeO2 model (Au/CeO2) and study various kinds of CO oxidation mechanisms at the Au-CeO2 interface and the Au NC as well.
Date: September 8, 2013
Creator: Y., Kim H. & Henkelman, G.
Partner: UNT Libraries Government Documents Department

Understanding of catalytic behaviors of TiO2/CuOx catalysts

Description: Aiming to reveal the catalysis at the metal-oxide and oxide-oxide interfaces,1,2 the water-gas shift reaction (WGS, CO + H2O and #61664; CO2 + H2) and CO oxidation (2CO + O2 and #61664; 2CO2) at the interface of the Cu(111) and Cu2O(111) supported TiO2 clusters were studied based on DFT calculations
Date: September 8, 2013
Creator: Y., Kim H. & Liu, P.
Partner: UNT Libraries Government Documents Department

Methanol Synthesis over Cu/ZnO/Al2O3: The Active Site in Industrial Catalysis

Description: Unlike homogeneous catalysts, heterogeneous catalysts that have been optimized through decades are typically so complex and hard to characterize that the nature of the catalytically active site is not known. This is one of the main stumbling blocks in developing rational catalyst design strategies in heterogeneous catalysis. We show here how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al{sub 2}O{sub 3} methanol synthesis catalyst. Using a combination of experimental evidence from bulk-, surface-sensitive and imaging methods collected on real high-performance catalytic systems in combination with DFT calculations. We show that the active site consists of Cu steps peppered with Zn atoms, all stabilized by a series of well defined bulk defects and surface species that need jointly to be present for the system to work.
Date: March 28, 2012
Creator: Behrens, Malte
Partner: UNT Libraries Government Documents Department