Scientific Publications 2005
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2005. "DFT calculations of EPR parameters of transitionmetal complexes: Implications for catalysis." Catalysis Today 105(1):122-133. doi:10.1016/j.cattod.2005.04.018 Abstract Transition metal and ligand hyperfine coupling constants for paramagnetic vanadium and copper model complexes have been calculated using DFT methods that are available in commercial software packages. Variations in EPR parameters with ligand identity and ligand orientation are two of the trends that have been investigated with DFT calculations. For example, the systematic variation of the vanadium hyperfine coupling constant with orientation for an imidazole ligand in a VO2+ complex has been observed experimentally and has also been reproduced by DFT calculations. Similarly, changes in the vanadium hyperfine coupling constant with ligand binding have been calculated using model complexes and DFT methods. DFT methods were also used to calculate ligand hyperfine coupling constants in transition metal systems. The variation of the proton hyperfine coupling constant with water ligand orientation was investigated for [VO(H2O)5]2+ and the results were used to interpret high resolution EPR data of VO2+-exchanged zeolites. Nitrogen hyperfine and quadrupole coupling constants for VO2+ model complexes were calculated and compared with experimental data. The computational results were used to enhance the interpretation of the EPR data for vanadium-exchanged zeolites which are promising catalytic materials. The implications of the DFT calculations of EPR parameters with respect to catalysis will be discussed
2005. "Characterization of Ambient Aerosols in Mexico City during the MCMA-2003 Campaign with Aerosol Mass Spectrometry. Part II: Overview of the Results at the CENICA Supersite and Comparison to Previous Studies." Atmospheric Chemistry and Physics Discussions 5:4183-4221. Abstract An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite during the Mexico City Metropolitan Area field study from March 29-May 4, 2003. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 μm (NR PM1) with high time and size resolution. Measurements of Black Carbon (BC) using an aethalometer, and estimated soil concentrations from Proton-Induced X-Ray Emission (PIXE) analysis of impactor substrates are also presented and combined with the AMS in order to include refractory material and estimate the total PM₂․₅ mass concentration at CENICA during this campaign. In Mexico City, the organic fraction of the estimated PM₂․₅ at CENICA represents 55% of the mass, with the rest consisting of inorganic (mainly ammonium nitrate and sulfate/ammonium salts) compounds, BC, and soil. Inorganic compounds represents 27.5% of PM₂․₅; BC mass concentration is equivalent to about 11%; while soil represents about 7%. The NR species and BC have diurnal cycles that can be qualitatively interpreted as the interplay of direct emissions, photochemical production in the atmosphere followed by condensation and gas-to-particle partitioning, boundary layer dynamics, and/or advection. Bi and trimodal size distributions are observed, with a small primary organic combustion (likely traffic) particle mode and an accumulation mode that contains mainly organic and secondary inorganic compounds. The AMS and BC mass concentrations, size distributions, and diurnal cycles are found to be qualitatively similar to those from other field measurements in Mexico City.
2005. "Ceria Incorporation into YSZ Columnar Nanostructures." Electrochemical and Solid-State Letters 8(10):A525-A527. Abstract We report the growth of porous yttria-stabilized zirconia (YSZ) columnar nanostructures by glancing angle deposition (GLAD) technique. CeO₂ sol solution was incorporated into YSZ creating high interface density columnar nanostructures. Initial experiments suggest higher conductivity in CeO₂/YSZ columnar nanostructures than polycrystalline CeO₂ and lower conductivity than single crystal YSZ in the intermediate temperature range of 600-825 K. GLAD/sol-gel process combination to create high density columnar nanostructures is discussed in the context of solid oxide fuel cells operating at intermediate temperatures.
2005. "Hydrogen Bubbles and Formation of Nanoporous Silicon during Electrochemical Etching." Surface and Interface Analysis 37(6):555-561. Abstract Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etch nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of a heavily n-doped (R = 0.05-0.5 Ω -cm) <100> silicon at a current density of 10 mA/cm² in buffer oxide etch (BOE) electrolyte solution defect craters, containing textured nanopores, were observed to occur in ring shaped patterns of rings. The defect craters apparently originate at the hydrogen-BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current carrying Si-OH species are most likely responsible for the widening in the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the low density of nanopores formed at ring edges are isolated from each other. Overall features observed in photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) intensity strongly emphasize the role of surface oxide that influences these properties.
2005. "Oxygen Transport Studies in Nanocrystalline Ceria Films." Journal of Materials Research 20(5):1295-1299. Abstract Oxygen uptake and conductivity were measured by nuclear reaction analysis and ac impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 K and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity are observed in ceria films with ~7 nm grain-size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributes in higher oxygen uptake. However the lower ionic conductivity in the film results from absence of long-range lattice ordering. Relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area and enhanced oxygen mobility.
2005. "Topography of Anatase TiO₂ Film Synthesized on LaAlO₃(001) ." Nanotechnology 16(3):S18–S21. Abstract The surface of an anatase titanium dioxide (TiO₂) film grown on LaAlO₃ was observed by noncontact atomic force microscopy (NC-AFM). After cleaning with a cycle of argon ion sputtering and annealing, (1x4) and (1x5) reconstructed terraces appeared. In addition to the terraces, the sputter-annealed surface included many agglomerations. X-ray photoelectron spectroscopy analysis showed the presence of Tiⁿ⁺ (n≤3). The low diffusivity of the Tiⁿ⁺(n≤3) species generated by argon ion sputtering is responsible for the generation of the agglomerations.
2005. "The Shwachman-Bodian-Diamond Syndrome Protein Family Is Involved in RNA Metabolism." Journal of Biological Chemistry 280(19):19213-19220. doi:10.1074/jbc.M414421200 Abstract A combination of structural, biochemical, and genetic studies in model organisms was used to infer a cellular role for the human protein (SBDS) responsible for Shwachman-Bodian-Diamond syndrome. The crystal structure of the SBDS homologue in Archaeoglobus fulgidus, AF0491, revealed a three domain protein. The N-terminal domain, which harbors the majority of disease-linked mutations, has a novel three-dimensional fold.
2005. "Thiophene Hydrodesulfurization over Nickel Phosphide Catalysts: Effect of the Precursor Composition and Support ." Journal of Catalysis 231(2):300–313. Abstract Silica- and alumina-supported nickel phosphide (NixPy) catalysts have been prepared, characterized by bulk and surface sensitive techniques, and evaluated for the hydrodesulfurization (HDS) of thiophene. Series of 30 wt% NixPy/SiO2 and 20 wt% NixPy/Al2O3 catalysts were prepared from oxidic precursors having a range of P/Ni molar ratios by temperature programmed reduction (TPR) in flowing H2. Oxidic precursors with molar ratios of P/Ni = 0.8 and 2.0 yielded catalysts containing phase-pure Ni2P on the silica and alumina supports, respectively. At lower P/Ni ratios, significant Ni12P5 impurities were present in the NixPy/SiO2 and NixPy/Al2O3 catalysts as indicated by X-ray diffraction. The HDS activities of the NixPy/SiO2 and NixPy/Al2O3 catalysts depended strongly on the P/Ni molar ratio of the oxidic precursors with optimal activities obtained for catalysts containing phase pure Ni2P and minimal excess P. After 48 h on-stream, a Ni2P/SiO2 catalyst was 20 and 3.3 times more active than sulfided Ni/SiO2 and Ni-Mo/SiO2 catalysts, respectively. A Ni2P/Al2O3 catalyst was 2.7 times more active than a sulfided Ni/Al2O3 catalyst but only about half as active as a Ni-Mo/Al2O3 catalyst.
2005. "Global whole-cell FTICR mass spectrometric proteomics analysis of the heat shock response in the radioresistant bacterium Deinococcus radiodurans." Journal of Proteome Research 4(3):709-718. Abstract Despite intense interest in the response to radiation in D. radiodurans, little is known about how the organism responds to other stress factors. Our previous studies indicated that D. radiodurans mounts a regulated protective response to heat shock, and that expression of the groESL and dnaKJ operons are induced in response to elevated temperature. In order to gain greater insight into the heat shock response of D. radiodurans on a more global scale, we undertook the study reported here. Using whole-cell semiquantitative mass spectrometric proteomics integrated with global transcriptome microarray analyses, we have determined a core set of highly induced heat shock genes whose expression correlates well at the transcriptional and translational levels. In addition, we observed that the higher the absolute expression of a given gene at physiological conditions, the better the quantitative correlation between RNA and protein expression levels.
2005. "New Effective Core Method (Effective Core Potential and Valence Basis Set) for Al Clusters and Nanoparticles and Heteronuclear Al-Containing Molecules." Journal of Chemical Theory and Computation 1(1):41-53. Abstract In previous work we have shown that the PBE0 hybrid density functional method with the MG3 all-electron basis set is an accurate method for calculating the atomization energies of small aluminum clusters (Al2-Al7). However, the MG3 basis set is very expensive for molecules much larger than Al13; therefore, we have developed a new effective core potential (ECP) method for aluminum to reduce the cost of obtaining accurate results for nanoparticles. Our method involves a hybrid of the Stuttgart semiempirical effective core potential and the compact effective potential (CEP) potential, and it uses a newly optimized polarized valence triple-ζ basis set. The combination of the new ECP and the new polarized valence triple-ζ basis set for Al is called the Minnesota effective core (MEC) method for Al. The method was optimized with a training set of atomization energies and geometries for AlH, AlC, AlO, AlCCH, Al2H, Al2C, Al2O, and Al3 and atomization energies of three Al13 structures, and we tested it on six test sets composed of 20 atomization energies for systems as large as Al13. We also present an improved all-electron polarized triple split basis set for oxygen, called 6-311+G(d*,p). For the test sets, the mean unsigned error (MUE) of the new method with respect to PBE0/MG3 is 0.06 eV for atomization energies and 0.007 Å for bond lengths, which constitutes a very significant improvement over the quality of the results that can be obtained with effective core potentials and valence basis sets in the literature (of the eight methods in the literature, the best previous method had average errors of 0.63 eV and 0.036 Å). We have also tested the MEC method with a variety of hybrid density functional theory, hybrid meta density functional theory, and pure GGA and meta GGA functionals and found that the average MUE, relative to each functional with all-electron basis sets, is 0.04 eV for atomization energies and 0.009 Å for bond lengths for the new effective core method and 0.16-0.20 eV and 0.013-0.033 Å for effective core potential and valence basis sets in the literature.
2005. "Trends in Low Temperature Water Gas Shift Reactivity on Transition Metals." Journal of Catalysis 229(2):265-275. doi:10.1016/j.jcat.2004.10.025 Abstract Low-temperature water–gas shift reactivity trends on transition metals were investigated with the use of a microkinetic model based on a redox mechanism. It is established that the adsorption energies for carbon monoxide and oxygen can describe to a large extent changes in the remaining activation and adsorption energies through linear correlations. In comparisons with experimental data it is found that the model predicts well the order of catalytic activities for transition metals, although it fails to quantitatively describe the experimental data. This discrepancy could be due to the assumption that the redox mechanism dominates and to the neglect of adsorbate interactions, which play an important role at high coverages. The model predicts that the activity of copper can be improved by increasing the strengths with which carbon monoxide and oxygen are bonded to the surface, thus suggesting possible directions for improving the catalyst for low-temperature WGSR.
2005. "Hygroscopic Growth of Self-Assembled Layered Surfactant Molecules at the Interface between Air and Organic Salts." Journal of Colloid and Interface Science 284(1):278-281. Abstract We report here a self-assembly of surfactant molecules at the interface of air/hygroscopic quaternary ammonium salts, tetrabutylammonium acetate (TBAAc). Homogeneously dissolved surfactant molecules at 100oC self-assemble upon contacting air due to high moisture adsorption behavior of the organic salt when cooling down. Highly ordered lamellar phases with different lattice spacings have been observed when surfactants with various lengths of alkyl chains were used. CnTMAB/TBAAc systems showed all trans conformation of interior methylene carbons and interdigited bilayers with an average CH2 increment of 0.119nm, while CnNH2/TBAAc systems showed trans/gauche mixed conformations of interior methylene carbons and bilayers with an average CH2 increment of 0.247nm. CnNH2s in CnNH2/TBAAc formed bilayers through water-mediated intermolecular hydrogen bonds with a water layer thickness of 0.51-0.61nm. In CnTAB/TBAAc, as the head group of CnTAB is bigger, the interdigited bilayer thickness (d-spacing) is smaller because their bigger head groups accommodate enough space for alkyl tails to come in between them.
2005. "Synthesis of SiC ceramics by the carbothermal reduction of mineralized wood with silica." Advanced Materials 17(1):73-77. Abstract This paper describes the synthesis of SiC by the carbothermal reduction of mineralized wood with silica in acidic condition. The biomorphic cellular SiC ceramics were prepared by controlling the amount of silica and the size of SiC nanoparticles. Up to 20wt% of SiO2 was mineralized into wood cellular structures and the hierarchical structures such as cells, lumen, and pits were mainly retained after the thermal treatment at 1400oC.
2005. "Synthesis of supported carbon nanotubes in mineralized silica-wood composites." Carbon 43(5):1096-1098. Abstract Multiwall carbon nanotubes (MWNTs) form spontaneously upon graphitization of organic precursors bound to the internal surfaces of a high porosity mineralized sample of natural wood. Following HF etching, both mesostructured carbon and randomly distributed and intertwined MWNTs were seen throughout the wood cellular structure.
2005. " Multiplet splittings and other properties from Density Functional Theory: An assessment in iron-porphyrin systems." Molecular Physics 103(2-3):273-278. Abstract In transition metal compounds with spin states close in energy, the magnitude and sign of the energy splitting calculated with Density Functional Theory depends strongly on the functional used. Therefore we must turn to additional criteria to assess the level of accuracy and reliability of predictions based on this level of theory. We report optimized geometries, total energies, and Mössbauer quadrupole splitting values for low-spin and high-spin, ferric and ferrous model hemes using a variety of gradient-corrected and hybrid functionals. In one model, the iron-porphyrin is axially ligated by two strong-field imidazole ligands [FeP(Im)2] and has a low-spin ground state. In the other model complex the axial ligands are two weak-field, water molecules [FeP(H2O)2], and has a high-spin ground state. Among all the functionals used (UHF, B3LYP, B3LYP*, BLYP, Half-and-Half, LSDA), the B3LYP hybrid functional most consistently reproduced the experimental geometry, Mössbauer, and spin state data for the two model hemes. Simply gradient-corrected functionals exhibit strong biases towards low spin states, while Hartree-Fock favors strongly high spin states. These findings suggest that for systems with similar characteristics of several accessible electronic spin configurations, it is imperative to include properties other than just the energy in the assessment of the DFT predictions.
2005. "Gold-polyaniline composites: Part II. Effects of nanometer sized particles." Physical Chemistry Chemical Physics. PCCP 7(20):3619-3625. Abstract The amount of electronic charge transferred between gold particles and polyaniline depends not only on the electron affinity of the two materials but also on the size of the gold particles. As measured by X-ray photoelectron spectroscopy, for particles < 5 nm the binding energy of the electrons is size dependent. This nano-effect has its origin in the electrostatics of particles. It is demonstrated as a measurable shift of the binding energy of the Au4f7/2 photoelectrons emitted from Au particles embedded in a polyaniline matrix. Gold nanoparticle size was evaluated by high resolution transmission electron microscopy.
2005. "A Variable-Energy Electron Microbeam: A Unique Modality for Targeted Low-LET Radiation." Radiation Research 164(5):695-700. Abstract We have designed and constructed a low-cost, variable energy low-LET electron microbeam that utilizes energetic electrons to mimic radiation damage produced from y- and X -rays. The microbeam can access lower regions of the LET spectra, analogous to that produced by a 60Co source. The device has two operating modes, as a conventional microbeam targeting single cells or sub-populations of cells or as a pseudo broad beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.
2005. "Low-LET Microbeam Investigation of the Track-End Dependence of Electron-Induced Damage in Normal Human Diploid Fibroblasts." Radiation Research 164(5):677-679. Abstract Using the electron microbeam located at Pacific Northwest National Laboratory, we have investigated the incident electron energy dependence on micronuclei formation in human diploid fibroblasts, AG01522, following non-targeted irradiations at 25 and 80 keV. Examining the dose response, we find that 25 keV electrons are more effective than 80 keV electrons at producing biological damage for a given dose. Our results demonstrating the induction of micronuclei as a function of incident electron energy offer direct support for the hypothesis that the track end is responsible for the biological damage occurring in the cell.
2005. "Characterization of Medium Conditioned by Irradiated Cells Using Proteome-Wide, High-Throughput Mass Spectrometry." Radiation Research 164(5):651-654. Abstract Shedding, the release of cell surface proteins by regulated proteolysis, is a general cellular response to injury and is responsible for generating numerous bioactive molecules including growth factors and cytokines. The purpose of our work is to determine whether low doses of low-linear energy transfer (LET) radiation induce shedding of bioactive molecules. Using a mass spectrometry-based global proteomics method, we tested this hypothesis by analyzing for shed proteins in medium from irradiated human mammary epithelial cells (HMEC). Several hundred proteins were identified, including transforming growth factor beta (TGFB); however, no changes in protein abundances attributable to radiation exposure, based on immunoblotting methods, were observed. These results demonstrate that our proteomic-based approach has the sensitivity to identify the kinds of proteins believed to be released after low-dose radiation exposure but that improvements in mass spectrometry-based protein quantification will be required to detect the small changes in abundance associated with this type of insult.
2005. "Modeling Water Exchange on an Aluminum Polyoxocation." Journal of Physical Chemistry B 109(50):23771-23775. doi:10.1021/jp0530505 Abstract The abstract for this journal article is not available at this time.
2005. "SubNanosecondTime Resolved XAFS of Laser Excited Thin Ge Films." Physica Scripta T115:1044. Abstract A facility at PNCCAT in the Advanced Photon Source measures with subnanosecond time resolution both XAFS and diffraction on femtosecond laserexcited samples. XAFS measures with relatively high efficiency the time for the laser excitation to couple to the lattice, the sample temperature after reaching thermal equilibrium, any ablation of the sample with time, and, in many cases can distinguish between the amorphous and crystalline states. Preliminary measurements on 200nm thick polycrystal Ge films indicate that the time for transferring the laser excitation to thermal heating of the lattice is less than 2nanoseconds when the initial temperature is 560K.
2005. "Scalable Molecular Dynamics." Journal of Physics: Conference Series 16:287-299. doi:10.1088/1742-6596/16/1/040 Abstract Molecular dynamics simulations enable the study of the time evolution of molecular systems by taking many small successive time steps under atomic forces that are calculated from a parameterized set of interaction functions. These are simple functions describing bonded and non-bonded atomic interactions, so that large molecular systems can be simulated for many time steps. The simulations provide energetic and kinetic properties in the form of statistical ensemble averages. The resulting trajectories can be analyzed for a variety of geometric and kinetic properties and correlations between them. These simulations have been carried out for many different systems, especially in computational biochemistry. NWChem is the computational chemistry software suite developed and maintained by the Molecular Sciences Software Group (MSSG) of the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory. This software has been designed to run efficiently on a variety of massively parallel computer architectures, including distributed memory (e.g. IBM-SP, Linux clusters) and shared memory (e.g. CRAY-T3E) architectures. In NWChem the molecular dynamics simulations module is based on domain decomposition. This allows reduction of memory requirements through the distribution of data across many processors as well as reduction of communication requirements because to the locality of interactions in cut-off based atomic interaction lists. This type of decomposition does require sophisticated load balancing techniques to be implemented, however, because of the heterogeneity of biological systems. In NWChem we have implemented a combination of global and local load balancing techniques, resulting in a significant increase of the parallel scaling that can be achieved. In NWChem we use the AMBER force field with the GLYCAM extensions for simulations of saccharides.
2005. "Adsorptive Removal of Organic Sulfur Compounds from Jet Fuel over K-exchanged NiY Zeolites Prepared by Impregnation and Ion Exchange." Industrial and Engineering Chemistry Research 44(15):5740-5749. Abstract NiY zeolites with different Ni loadings were synthesized by incipient wetness impregnation and liquid phase ion-exchange methods using NH4Y and KY zeolites. These Ni-containing Y zeolites were tested as adsorbents for removing organic sulfur compounds from a model jet fuel containing 510 ppmw sulfur and a real JP-8 jet fuel containing 380 ppmw sulfur under ambient conditions either without reduction or after reduction around 600oC. At the adsorption temperature of 80oC, the NiY zeolite containing 30 wt % Ni synthesized by incipient wetness impregnation of NH4Y zeolite was able to clean only about 10 ml of a model jet fuel per g of the adsorbent to produce a desulfurized fuel containing below 1 ppmw sulfur. Under the same experimental conditions, the NiY zeolite prepared using KY zeolite cleaned about 30 ml of the fuel per g of the adsorbent. Better sulfur adsorption performance was observed when the NiY zeolites were synthesized by ion-exchange, and reduced before sulfur adsorption. The reducibility and surface properties of some of the selected NiY zeolites were investigated by temperature-programmed reduction (TPR) and in-situ X-ray photoelectron spectroscopy (XPS). TPR studies indicated that the reducibility of NiY-zeolite was improved when K was present as a co-cation. The in-situ XPS studies of unreduced and reduced samples revealed that presence of K as co-cation in the zeolite matrix helps Ni dispersion at the surface. The promotional effect of K on the sulfur adsorption performance of NiY zeolites was therefore attributed to improved reducibility and surface dispersion of Ni when K was present as a co-cation.
2005. "Partial encapsulation of Pd particles by reduced ceria-zirconia." Applied Physics Letters 87(20):Art. No. 201915. doi:10.1063/1.2132067 Abstract The interaction between metal particles and their oxide support can be strong so as to affect the reactivity of a catalyst system by, for example, encapsulation of the particles by the oxide. Direct observation of metal-oxide interfaces with atomic resolution is a challenge and can only be achieved by cross sectional high-resolution transmission electron microscopy (HRTEM). With this approach, we found partial encapsulation of Pd particles by reduced ceria-zirconia in a model, single-crystal thin film auto catalyst, indicating a strong interaction between Pd and the oxide. Besides obtaining HRTEM images, the valence of cerium was determined by electron energy loss spectroscopy (EELS). The effect of reduction and oxidation conditions on this interaction provides a qualitative explanation for a previously observed reversible reactivation of oxygen storage in model powder auto catalysts. The technique of cross sectional HRTEM can be applied to the study of other metal-particle-on-oxide systems.
2005. "Effect of the Ar–Ni(s) Potential on the Cross Section for Ar+CH₄/ Ni{111} Collision-Induced Desorption and the Need for a More Accurate CH₄ /Ni{111} Potential ." Journal of Chemical Physics 122(4):Art. # 044704. Abstract In a previous paper [L. Sun, P. de Sainte Claire, O. Meroueh, and W. L Hase, J. Chem. Phys. 114, 535 (2001], a classical trajectory simulation was reported of CH₄ desorption from Ni{111} by Ar-atom collisions. At an incident angle θiof 60° (with respect to the surface normal), the calculated collision-induced desorption (CID) cross sections are in excellent agreement with experiment. However, for smaller incident angles the calculated cross sections are larger than the experimental values and for normal collisions, θi=0°, the calculated cross sections are approximately a factor of 2 larger. This trajectory study used an analytic function for the Ar +Ni(s) intermolecular potential which gives an Ar–Ni{111} potential energy minimum which is an order of magnitude too deep. In the work reported here, the previous trajectory study is repeated with an Ar+Ni(s) analytic intermolecular potential which gives an accurate Ar–Ni{111} potential energy minimum and also has a different surface corrugation than the previous potential. Though there are significant differences between the two Ar+Ni(s) analytic potentials, they have no important effects on the CID dynamics and the cross sections reported here are nearly identical to the previous values. Zero-point energy motions of the surface and the CH₄ – Ni(s) intermolecular modes are considered in the simulation and they are found to have a negligible effect on the CID cross sections. Calculations of the intermolecular potential between CH4 and a Ni atom, at various levels of theory, suggest that there are substantial approximations in the ab initio calculation used to develop the CH₄ 1Ni{111} potential. The implication is that the differences between the trajectory and experimental CID cross sections may arise from an inaccurate CH₄ {Ni} potential used in the trajectory simulation.
2005. "Kinetic Evidence for Five-Coordination in AlOH(aq)2+ Ion." Science 308(5727):1450-1453. Abstract In dilute aqueous solution, AlIII exists almost entirely as the octahedral Al(H2O)63+ ion at pH <3.0 and as tetrahedral Al(OH)4– ion at pH > 7, while in the biochemically and geochemically critical pH range 4.3-7.0 other hydrolytic species such as AlOH(aq)2+exist that are traditionally assumed to be hexacoordinate. We show, however, from the kinetics of proton- and water exchange on aqueous AlIII, supported by Car-Parrinello simulations, that a five-coordinate Al(H2O)4OH2+ ion is important under ambient conditions. In water, AlIII differs strikingly from other trivalent metal ions.
2005. "Changing Morphology of BaO/AI₂O₃ during NO₂ Uptake and Release." Journal of Physical Chemistry B 109(15):7339-7344. Abstract The changes in the morphology of Ba-oxide-based NOx storage/reduction catalysts were investigated using time resolved x-ray diffraction, transmission electron microscopy and energy dispersed spectroscopy. Large Ba(NO₃)₂ crystallites form on the alumina support when the catalyst is prepared by the incipient wetness method using an aqueous Ba(NO₃)₂ solution. Heating the sample to 873K in a He flow results in the decomposition of the Ba(NO₃)₂ phase and the formation of both a monolayer BaO film strongly interacting with the alumina support, and nano crystalline BaO particles. Upon NO₂ exposure of these BaO phases at room temperature, small (nano-sized) Ba(NO₃)₂ crystals and a monolayer of surface nitrate form. Heating this sample in NO₂ results in the coalescence of the nano crystalline Ba(NO₃)₂ particles into large crystals. The average crystal size in the re-formed Ba(NO₃)₂ layer is significantly smaller than that measured after the catalyst preparation. Evidence is also presented for the existence of a monolayer Ba(NO₃)₂ phase after thermal treatment in NO₂, in addition to these large crystals. These results clearly demonstrate the dynamic nature of the Ba-containing phases that are active in the NOx storage/reduction process. The proposed morphology cycle may contribute to the understanding of the changes observed in the performances of these catalysts during actual operating conditions.
2005. "NO2 Adsorption on BaO/Al2O3: The Nature of Nitrate Species." Journal of Physical Chemistry B 109(1):27-29. doi:10.1021/jp045082i Abstract The nature of nitrate species formed in the Al₂O₃, 8wt%, and 20wt% BaO/Al₂O₃ catalysts was investigated in a combined TPD, FTIR and 15N solid state NMR study. The results strongly suggest the formation of a monolayer bidentate nitrate on the alumina support that forms upon NO₂ exposure. This monolayer nitrate decomposes at lower temperature than bulk Ba(NO₃)₂ and its only decomposition product is NO₂. A bulk-like Ba(NO₃)₂ phase also forms with its characteristic set of TPD, IR and NMR features. The amount of NOx stored in the monolayer nitrate is proportional to the surface area of the catalyst, while that in the bulk nitrate increases with BaO coverage.
2005. "The Catalytic Chemistry of HCN+NO₂ over Na- and Ba-Y, FAU: An In Situ FTIR and TPD/TPR Study." Journal of Physical Chemistry B 109(4):1481-1490. doi:10.1021/jp045671o Abstract The adsorption of HCN and the reaction of HCN with NO₂ over Na-, and Ba-Y,FAU zeolite catalysts were investigated using in situ FTIR and TPD/TPR spectroscopies. Both catalysts adsorb HCN molecularly at room temperature, and the strength of adsorption is higher over Ba-Y than Na-Y. Over Na-Y the reaction between HCN and NO₂ is slow at 473K. On Ba-Y HCN reacts readily with NO₂ at 473K, forming N₂, CO, CO₂, HNCO, NO, N₂O and C₂N₂. The results of this investigation suggest that initial step in the HCN+NO₂ reaction over these catalysts is the hydrogen abstraction from HCN, and the formation of ionic CN⁻ and NC⁻ species. The formation of N2 can proceed directly from these ionic species upon their interaction with NO⁺. Alternatively, these cyanide species can be oxidized to isocyanates which then can be further transformed to N₂, N₂O and COx in their subsequent reaction with NOx.
