17 Matching Results

Search Results

Advanced search parameters have been applied.


Description: On bulk stoichiometric oxides, SO{sub 2} mainly reacts with the O centers to form SO{sub 3} or SO{sub 4} species that decompose at elevated temperatures. Adsorption on the metal cations occurs below 300 K and does not lead to cleavage of the S-O bonds. In bulk oxides, the occupied cation bands are too stable for effective bonding interactions with the LUMO of SO{sub 2}. The effects of quantum confinement on the electronic properties of oxide nanoparticles and the structural defects that usually accompany these systems in general favor the bonding and dissociation of SO{sub 2}. Thus, nanoparticles of MgO, CaO, SrO, Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3} and CeO{sub 2} are all more efficient for sequestering SO{sub 2} than the corresponding bulk oxides. Structural imperfections in pure or metal-doped ceria nanoparticles accelerate the reduction of SO{sub 2} by CO by facilitating the formation and migration of O vacancies in the oxide surface.
Date: June 30, 2006
Creator: RODRIGUEZ, J.A.
Partner: UNT Libraries Government Documents Department


Description: The experimental and theoretical studies described above illustrate the complex nature of the heteronuclear metal-metal bond. In many cases, bimetallic bonding induces a significant redistribution of charge around the bonded metals. This redistribution of charge is usually linked to the strength of the bimetallic bond, affects the position of the core and valence levels of the metals, and can determine the chemical reactivity of the system under study. New concepts are emerging [22,23,34,36] and eventually the coupling of experiment and theory can be useful for designing more efficient bimetallic catalysts [98,106,107].
Date: September 27, 2001
Creator: Rodriguez, J. A.
Partner: UNT Libraries Government Documents Department

Adsorption of sulfur on bimetallic surfaces: Formation of copper sulfides on Pt(111) and Ru(001)

Description: It has been found that by making specific bimetallic systems (in this case a noble metal deposited on a transition metal surface) one can change the catalytic activity and selectivity of the metals for reactions that lead to the conversion of hydrocarbons. A major problem associated with these catalysts is sulfur poisoning. The interaction of sulfur and copper on Pt(111) and Ru(001) has been examined using X-ray photoelectron spectroscopy (XPS) and thermal desorption mass spectroscopy (TDS). Cu/Pt (111) and Cu/Ru(001) surfaces were exposed to S{sub 2} gas at 300 K. It was found that for both substrates stable copper-sulfide films were formed, of which the Cu-S/Ru(001) system was the more stable. The decomposition of the Cu{sub 2}S films on Pt(111) occurred at 600--850 K and on Ru(001) at 900--1,000 K. Breakdown of the films led to evolution into the gas phase of sulfur (mainly as S{sub 2}) without desorption of copper or the sulfur chemisorbed on the substrate. This chemisorbed sulfur desorbed in a broad feature from 1,000--1,500 K. For the Ru(001) substrate the Cu atoms remained on the surface until they desorbed at 1080 K, while on Pt(111) the Cu atoms migrated into the Pt(111) surface to form a subsurface Cu-Pt alloy and no Cu desorption features were seen at temperatures as high as 1,300 K. On Pt(111), copper-sulfide promoted the formation of a bulk-like platinum sulfide. No sulfidation of ruthenium was detected in the presence of copper-sulfide films. For both substrates the sulfur atoms were found to highly perturb the copper sites, decreasing the ability of the noble-metal ad atoms to adsorb CO.
Date: November 1, 1994
Creator: Kuhn, M. & Rodriguez, J. A.
Partner: UNT Libraries Government Documents Department

Theoretical Studies in Heterogenous Catalysis: Towards a Rational Design of Novel Catalysts for Hydrodesulfurization and Hydrogen Production

Description: Traditionally, knowledge in heterogeneous catalysis has come through empirical research. Nowadays, there is a clear interest to change this since millions of dollars in products are generated every year in the chemical and petrochemical industries through catalytic processes. To obtain a fundamental knowledge of the factors that determine the activity of heterogeneous catalysts is a challenge for modern science since many of these systems are very complex in nature. In principle, when a molecule adsorbs on the surface of a heterogeneous catalyst, it can interact with a large number of bonding sites. It is known that the chemical properties of these bonding sites depend strongly on the chemical environment around them. Thus, there can be big variations in chemical reactivity when going from one region to another in the surface of a heterogeneous catalyst. A main objective is to understand how the structural and electronic properties of a surface affect the energetics for adsorption processes and the paths for dissociation and chemical reactions. In recent years, advances in instrumentation and experimental procedures have allowed a large series of detailed works on the surface chemistry of heterogeneous catalysts. In many cases, these experimental studies have shown interesting and unique phenomena. Theory is needed to unravel the basic interactions behind these phenomena and to provide a general framework for the interpretation of experimental results. Ideally, theoretical calculations based on density-functional theory have evolved to the point that one should be able to predict patterns in the activity of catalytic surfaces. As in the case of experimental techniques, no single theoretical approach is able to address the large diversity of phenomena occurring on a catalyst. Catalytic surfaces are usually modeled using either a finite cluster or a two-dimensionally periodic slab. Many articles have been published comparing the results of these two approaches. An ...
Date: October 1, 2008
Creator: Rodriguez,J.A. & Liu, P.
Partner: UNT Libraries Government Documents Department

Infiltration and Seepage Through Fractured Welded Tuff

Description: The Nopal I mine in Pena Blanca, Chihuahua, Mexico, contains a uranium ore deposit within fractured tuff. Previous mining activities exposed a level ground surface 8 m above an excavated mining adit. In this paper, we report results of ongoing research to understand and model percolation through the fractured tuff and seepage into a mined adit both of which are important processes for the performance of the proposed nuclear waste repository at Yucca Mountain. Travel of water plumes was modeled using one-dimensional numerical and analytical approaches. Most of the hydrologic properly estimates were calculated from mean fracture apertures and fracture density. Based on the modeling results, we presented constraints for the arrival time and temporal pattern of seepage at the adit.
Date: June 20, 2006
Creator: Ghezzehei, T.A.; Dobson, P.F.; Rodriguez, J.A. & Cook, P.J.
Partner: UNT Libraries Government Documents Department


Description: The electronic structure of a solid is affected by size and altered from the continuous electronic levels forming a band, characteristic of bulk or microsized solids, to discrete-like or quantized levels. This is drastically observed when the particle size goes down to the nano-meter range and is the origin of the so-called ''quantum confinement'' terminology referring to this phenomenon. From a solid state point of view, electronic states of confined materials can be considered as being a superposition of bulk-like states with a concomitant increase of the oscillator strength. The valence/conduction band-width and position observables of a solid oxide are functions of the crystal potential and this, in turn, is perturbed by effect of the size in two ways; a short-range effect induced by the presence of ions with a different coordination number and bond distance, and a large-range one, induced by changes in the Madelung potential of the oxide. Theoretical analyses for oxides show a redistribution of charge when going from large periodic structures to small clusters which is roughly considered small for ionic solids and significantly important for covalent ones. Chapter 1 of this book describes the most recent theoretical frameworks employed to deal with these physical phenomena while here we will describe their influence in physico-chemical observables obtained by spectroscopical techniques.
Date: June 30, 2006
Partner: UNT Libraries Government Documents Department


Description: Oxide nanosystems play a key role as components of catalysts used for the production of H{sub 2} via the steam reforming or the partial oxidation of hydrocarbons, and for the water-gas shift reaction. The behavior seen for Cu-ceria and Au-ceria WGS catalysts indicates that the oxide is much more than a simple support. The special chemical properties of the oxide nanoparticles (defect rich, high mobility of oxygen) favor interactions with the reactants or other catalyst components. More in-situ characterization and mechanistic studies are necessary for the optimization of these nanocatalysts. The use of oxide nanomaterials for the fabrication of PEMFCs and SOFCs can lead to devices with a high practical impact. One objective is to build electrodes with low cost conducting oxide nanoarrays. The electron and oxygen-ion conducting capabilities of many oxides improve when going from the bulk to the nanoscale. Furthermore, one can get a more homogeneous surface morphology and an increase of the effective reaction area. Much more fundamental and practical research needs to be done in this area.
Date: June 30, 2006
Creator: Wang, X. & Rodriguez, J. A.
Partner: UNT Libraries Government Documents Department

Toward Photochemical Water Splitting Using Band-Gap-Narrowed Semiconductors and Transition-Metal Based Molecular Catalysts

Description: We are carrying out coordinated theoretical and experimental studies of toward photochemical water splitting using band-gap-narrowed semiconductors (BGNSCs) with attached multi-electron molecular water oxidation and hydrogen production catalysts. We focus on the coupling between the materials properties and the H{sub 2}O redox chemistry, with an emphasis on attaining a fundamental understanding of the individual elementary steps in the following four processes: (1) Light-harvesting and charge-separation of stable oxide or oxide-derived semiconductors for solar-driven water splitting, including the discovery and characterization of the behavior of such materials at the aqueous interface; (2) The catalysis of the four-electron water oxidation by dinuclear hydroxo transition-metal complexes with quinonoid ligands, and the rational search for improved catalysts; (3) Transfer of the design principles learned from the elucidation of the DuBois-type hydrogenase model catalysts in acetonitrile to the rational design of two-electron hydrogen production catalysts for aqueous solution; (4) Combining these three elements to examine the function of oxidation catalysts on BGNSC photoanode surfaces and hydrogen production catalysts on cathode surfaces at the aqueous interface to understand the challenges to the efficient coupling of the materials functions.
Date: June 7, 2009
Creator: Muckerman,J.T.; Rodriguez, J.A. & Fujita, E.
Partner: UNT Libraries Government Documents Department

Longitudinal space charge effects near transition

Description: Experimental and numerical studies of the longitudinal beam dynamics in the Small Isochronous Ring (SIR) at Michigan State University revealed a fast, space-charge driven instability that did not fit the model of the negative mass instability. This paper proposes a simple analytical model explaining these results. Also, the paper compares the model to result s of experimental and numerical studies of the longitudinal beam dynamics in SIR.
Date: May 4, 2009
Creator: Pozdeyev,E.; Rodriguez, J.A.; Marti, F. & York, R.C.
Partner: UNT Libraries Government Documents Department

Space charge effect in isochronous rings

Description: Cyclotrons, rings for precise nuclear mass spectrometry, and some light sources with extremely short bunches are operated or planned to be operated in the isochronous or almost isochronous regime. Also, many hadron synchrotrons run in the isochronous regime for a short period of time during transition crossing. The longitudinal motion is frozen in the isochronous regime that leads to accumulation of the integral of the longitudinal space charge force. In low-gamma hadron machines, this can cause a fast growth of the beam energy spread even at modest beam intensities. Additionally, the transverse component of the space charge effectively modifies the dispersion function and the slip factor shifting the isochronous (transition) point. In this paper, we discuss space charge effects in the isochronous regime and present experimental results obtained in the Small Isochronous Ring, developed at Michigan State University specifically for studies of space charge in the isochronous regime.
Date: August 25, 2008
Creator: Pozdeyev,E.; Rodriguez, J.A.; Marti, F. & York, R.
Partner: UNT Libraries Government Documents Department


Description: In recent years, several new interesting phenomena have been discovered when studying the interaction of sulphur with bimetallic surfaces using the modern techniques of surface science. Very small amounts of sulphur can induce dramatic changes in the morphology of bimetallic surfaces. The electronic perturbations associated with the formation of a heteronuclear metal-metal bond affect the reactivity of the bonded metals toward sulphur. This can be a very important issue to consider when trying to minimize the negative effects of sulphur poisoning or dealing with the design of desulfurization catalysts.
Date: October 4, 2001
Creator: Rodriguez, J. A. & Hrbek, J.
Partner: UNT Libraries Government Documents Department

Role of C and P Sites on the Chemical Activity of Metal Carbide and Phosphides: From Clusters to Single-Crystal Surfaces

Description: Transition metal carbides and phosphides have shown tremendous potential as highly active catalysts. At a microscopic level, it is not well understood how these new catalysts work. Their high activity is usually attributed to ligand or/and ensemble effects. Here, we review recent studies that examine the chemical activity of metal carbide and phosphides as a function of size, from clusters to extended surfaces, and metal/carbon or metal/phosphorous ratio. These studies reveal that the C and P sites in these compounds cannot be considered as simple spectators. They moderate the reactivity of the metal centers and provide bonding sites for adsorbates.
Date: July 1, 2007
Creator: Rodriguez, J. A.; Vines, F.; Liu, P. & Illas, F.
Partner: UNT Libraries Government Documents Department


Description: The evolution of our understanding of the behavior of oxide nanostructures depends heavily on the structural information obtained from a wide range of physical methods traditionally used in solid state physics, surface science and inorganic chemistry. In this chapter, we describe several techniques that are useful for the characterization of the structural properties of oxide nanostructures: X-ray diffraction (XRD) and scattering, X-ray absorption fine structure (XAFS), Raman spectroscopy, transmission electron microscopy (TEM), scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The ultimate goal is to obtain information about the spatial arrangement of atoms in the nanostructures with precise interatomic distances and bond angles. This may not be possible for complex systems and one may get only partial information about the local geometry or morphology.
Date: June 30, 2006
Partner: UNT Libraries Government Documents Department

Properties of pure and sulfided NiMoO{sub 4} and CoMoO{sub 4} catalysts: TPR, XANES and time-resolved XRD studies

Description: X-ray absorption near-edge spectroscopy (XANES) was used to characterize the structural and electronic properties of a series of cobalt- and nickel-molybdate catalysts (AMoO{sub 4}.nH{sub 2}O, {alpha}-AMoO{sub 4}, {beta}-AMoO{sub 4}; A=Co or Ni). The results of XANES indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the {alpha}-phases to tetrahedral in the {beta}-phases and hydrate. Time-resolved x-ray diffraction shows a direct transformation of the hydrates into the {beta}-AMoO{sub 4} compounds (following a kinetics of first order) at temperatures between 200 and 350{degrees}C. This is facilitated by the similarities that the AMoO{sub 4}.nH{sub 2}O and H{sub 2} at temperatures between 400 and 600{degrees}C, forming gaseous water oxides in which the oxidation state of Co and Ni remains +2 while that of Mo is reduced to +5 or +4. After exposing {alpha}-NiMoO{sub 4} and {beta}-NiMoO{sub 4} to H{sub 2}S, both metals get sulfided and a NiMoS{sub x} phase is formed. For the {beta} phase of NiMoO{sub 4} the sulfidation of Mo is more extensive than for the {alpha} phase, making the former a better precursor for catalysts of hydrodesulfurization reactions.
Date: December 31, 1998
Creator: Chaturvedi, S.; Rodriguez, J.A.; Hanson, J.C.; Albornoz, A. & Brito, J.L.
Partner: UNT Libraries Government Documents Department


Description: High-resolution synchrotron based photoemission and x-ray absorption spectroscopy have been used to study the interaction of SO{sub 2} with a series of metals and oxides. The chemistry of SO{sub 2} on metal surfaces is rich. At low coverages, the molecule fully decomposes into atomic S and O. At large coverages, the formation of SO{sub 3} and SO{sub 4} takes place. The following sequence was found for the reactivity of the metals towards SO{sub 2}: Pt {approx} Rh < Ru < Mo << Zn, Sn, Cs. Alloying can be useful for reducing the chemical affinity of a metal for SO{sub 2} and controlling S poisoning. Pd atoms bonded to Rh and Pt atoms bonded to Sn interact weakly with SO{sub 2}. In general, SO{sub 2} mainly reacts with the O centers of metal oxides. SO{sub 4} is formed on CeO{sub 2} and SO{sub 3} on ZnO. On these systems there is no decomposition of SO{sub 2}. Dissociation of the molecule is observed after introducing a large amount of Ce{sup 3+} sites in ceria, or after depositing Cu or alkali metals on the oxide surfaces. These promote the catalytic activity of the oxides during the destruction of SO{sub 2}.
Date: July 9, 2000
Partner: UNT Libraries Government Documents Department

Ultrasonic Testing of NIF Amplifier FAU Top Plates

Description: A key component in the National Ignition Facility (NIF) laser optic system is the amplifier frame assembly unit (FAU). The cast aluminum top plate that supports the FAU is required to withstand loads that would occur during an earthquake with a recurrence period of 1000 years. The stringent seismic requirements placed on the FAU top plate induced a study of the cast aluminum material used in the top plate. Ultrasonic testing was used to aid in characterizing the aluminum material used in the plates. This report documents the work performed using contact ultrasonic testing to characterize the FAU top plate material. The ultrasonic work reported here had 3 objectives: (1) inspect the plate material before cyclic testing conducted at the Pacific Earthquake Engineering Research Center (PEER); (2) determine the overall quality of individual plates; and (3) detect large defects in critical areas of individual plates. Section III, ''Pre-cyclic test inspection'', describes work performed in support of Objective 1. Section IV, ''Ultrasonic field measurements'', describes work performed in support of Objectives 2 and 3.
Date: May 7, 2002
Creator: Chinn, D J; Huber, R D; Haskins, J J; Rodriguez, J A; Souza, P R & Le, T V
Partner: UNT Libraries Government Documents Department