Scientific Publications 2010
2010. "High resolution infrared spectroscopy of [1.1.1]propellane: the region of the v9 band." Journal of Molecular Spectroscopy 264(1):26-36. doi:10.1016/j.jms.2010.08.008 Abstract The region of the infrared-active ν9 CH2 bending band of [1.1.1] propellane has been recorded at resolution (0.0025 cm-1) sufficient to distinguish individual rovibrational lines. This region includes the partially overlapping bands ν9 (e′) = 1459 cm-1, 2ν18 (l = 2, E′) = 1430 cm-1, ν6 + ν12 (E′) = 1489 cm-1, and ν4 + ν15 (A2″) = 1518 cm-1. In addition, the difference band ν4 - ν15 (A2″) was observed in the far infrared near 295 cm-1 and analyzed to give good constants for the upper ν4 state. The close proximities of the four bands in the ν9 region suggest that Coriolis and Fermi resonance couplings could be significant and theoretical band parameters obtained from Gaussian ab initio calculations were helpful in guiding the band analyses. The analyses of all four bands were accomplished, based on our earlier report of ground state constants determined from combination differences involving more than 4000 pairs of transitions from five fundamental and four combination bands. This paper presents the analyses and the determination of the upper state constants of all four bands in the region of the ν9 band. Complications were most evident in the 2ν18 (l = 2, E′) band, which showed significant perturbations due to mixing with the nearby 2ν18 (l = 0, A1′) and ν6+ν12 (E') levels which are either infrared inactive as transitions from the ground state, or, in the latter case, too weak to observe. These complications are discussed and a comparison of all molecular constants with those available from the ab initio calculations at the anharmonic level is presented. 2
2010. "Achieving 50% ionization efficiency in sub-ambient pressure ionization with nanoelectrospray." Analytical Chemistry 82(22):9344-9349. Abstract Inefficient ionization and poor transmission of the charged species produced by an electrospray from the ambient pressure mass spectrometer source into the high vacuum region required for mass analysis significantly limits achievable sensitivity. Here we present evidence that, when operated at flow rates of 50 nL/min, a new electrospray-based ion source operated at ~20 Torr can deliver ~50% of the analyte ions initially in the solution as charged desolvated species into the rough vacuum region of mass spectrometers. The ion source can be tuned to optimize the analyte signal for readily ionized species while reducing the background contribution.
2010. "Degradation Mechanisms of SOFC Anodes in Coal Gas Containing Phosphorus." Solid State Ionics 181(8-10):430-440. Abstract The interaction of phosphorus in synthetic coal gas with the nickel-based anode of solid oxide fuel cells has been investigated. Tests with both anode-supported and electrolyte-supported button cells were performed at 700 to 800oC in synthetic coal gas containing 0.5 to 10 ppm phosphorus, introduced as phosphine. Two primary modes of degradation were observed. The most obvious was the formation of a series of bulk nickel phosphide phases, of which Ni3P, Ni5P2, Ni12P5 and Ni2P were identified. Phosphorus was essentially completely captured by the anode, forming a sharp boundary between converted and unconverted anode portions. These products partially coalesced into large grains, which eventually affected electronic percolation through the anode support. Thermodynamic calculations predict that formation of the first binary nickel phosphide phase is possible at sub-parts per billion concentrations in coal gas at temperatures relevant to fuel cell operation. A second mode of degradation is attributed to surface diffusion of phosphorus to the active anode/electrolyte interface to form an adsorption layer. Direct evidence for the presence of such an adsorption layer on nickel was obtained by surface spectroscopies on fracture surfaces. Further, cell performance losses were observed well before the entire anode was converted to bulk nickel phosphide. Impedance spectroscopy revealed that these losses were primarily due to growth in electrodic resistance, whereas large ohmic increases were visible when the entire anode was converted to nickel phosphide phases. The rate of resistance growth for anode-supported cells showed a very low dependence on phosphorus concentration, attributed to phosphorus activity control within the anode by bulk nickel phosphide products.
2010. "Interaction of coal-derived synthesis gas impurities with solid oxide fuel cell metallic components." Journal of Power Sources 196(2):636-643. doi:10.1016/j.jpowsour.2010.07.081 Abstract Chromium-containing iron-based alloys Crofer22 APU and SS 441 and nickel-based alloy Inconel600, all commonly used in a solid oxide fuel cell (SOFC) stack as interconnect materials, heat exchanger and gas feeding pipes, were exposed at 700-850oC to a synthetic coal gas containing ≤2 ppm phosphine, arsine, sulfur and antimony. Samples were characterized by SEM/EDS and XRD to monitor the secondary phase formation. Exposure of ferritic stainless steels to P led to the formation of surface Cr-Mn-P-O and Fe-P-O compounds and increased temperatures accelerated the rate of interactions. Fewer interactions were observed after exposures to As and Sb. No sulfur containing compounds were found. Nickel-based alloy exhibited much stronger interactions with As and P in comparison with ferritic steels and the arsenic interactions were particularly strong. The difference between the iron- and nickel-based alloys is explained by the different chemistry and morphology of the scales grown on the alloy surfaces in coal gas. While P and As interactions with the metallic parts in the SOFC are likely to mitigate the nickel/zirconia anode poisoning, the other degradation mechanisms should be taken into consideration to avoid potential stack failures. Manganese spinels were found to be effective as phosphorus getters and could be used in coal gas cleanup.
2010. "Reversible Poisoning of the Nickel/Zirconia Solid Oxide Fuel Cell Anodes by Hydrogen Chloride in Coal Gas." Journal of Power Sources 195(20 SP ISS):7033-7037. Abstract The performance of anode-supported solid oxide fuel cells (SOFC) was evaluated in synthetic coal gas containing HCl in the temperature range 650 to 850oC. Exposure to up to 800 ppm HCl resulted in reversible poisoning of the Ni/zirconia anode by chlorine species adsorption, the magnitude of which decreased with increased temperature. Performance losses increased with the concentration of HCl to ~100 ppm, above which losses were insensitive to HCl concentration. Cell voltage had no effect on poisoning. No evidence was found for long-term degradation that can be attributed to HCl exposure. Similarly, no evidence of microstructural changes or formation of new solid phases as a result of HCl exposure was found. From thermodynamic calculations, solid nickel chloride phase formation was shown to be highly unlikely in coal gas. Further, the presence of HCl at even the highest anticipated concentrations in coal gas would minimally increase the volatility of nickel.
2010. "SOFC Ohmic Resistance Reduction by HCl-Induced Removal of Manganese at the Anode/Electrolyte Interface." Electrochemical and Solid-State Letters 13(6):B63-B67. doi:10.1149/1.3380668 Abstract The ohmic resistance of anode-supported solid oxide fuel cells having a manganese-based cathode was lowered when operated in synthetic coal gas containing hydrogen chloride. This effect was not observed for cells with cathodes that did not contain manganese. Substantial amounts of Mn were found throughout the grain boundaries of the 8 mole% yttria-stabilized zirconia (8YSZ) electrolyte. Exposure to HCl partially removed Mn near the anode/electrolyte interface, presumably by volatilization as MnCl2(g). This work suggests that one of the underlying causes of higher than expected electrolyte resistance in anode-supported SOFCs is a lowering of the ionic conductivity of 8YSZ by incorporation of manganese.
2010. "High-Resolution Infrared Spectra of Bicyclo[1.1.1]pentane." Journal of Molecular Spectroscopy 262(1):42-48. doi:10.1016/j.jms.2010.04.010 Abstract Infrared spectra of bicyclo[1.1.1]pentane (C5H8) have been recorded at a resolution (0.0015 cm-1) sufficient to resolve for the first time individual rovibrational lines. This initial report presents the ground state constants for this molecule determined from the detailed analysis of three of the ten infrared-allowed bands, v14(e′) at 540 cm-1, v17(a2″) at 1220 cm-1, v18(a2″) at 832 cm-1, and a partial analysis of the v11(e′) band at 1237 cm-1. The upper states of transitions involving the lowest frequency mode, v14(e′), show no evidence of rovibrational perturbations but those for the v17 and v18 (a2″) modes give clear indication of Coriolis coupling to nearby e′ levels. Accordingly, ground state constants were determined by use of the combination-difference method for all three bands. The assigned frequencies provided over 3300 consistent ground state difference values, yielding the following constants for the ground state (in units of cm-1): B0 = 0.2399412(2), DJ = 6.024(6) x 10-8, DJK = -1.930(21) x 10-8. For the unperturbed v14(e′) fundamental, more than 3500 transitions were analyzed and the band origin was found to be at 540.34225(2) cm-1. The numbers in parentheses are the uncertainties (two standard deviations) in the values of the constants. The results are compared with those obtained previously for [1.1.1]propellane and with those computed at the ab initio anharmonic level using the B3LYP density functional method with a cc-pVTZ basis set.
2010. "Synthesis and Screening of Thin Films in the CeCl3-CeBr3 System for Scintillator Applications." Thin Solid Films 518(12):3194-3198. doi:10.1016/j.tsf.2009.09.015 Abstract Thin film samples of CeCl3, CeBr3, and combinatorial compositions along the CeCl3-CeBr3 join were produced using thermal evaporation, which is being evaluated as a method for rapid screening of new scintillator materials. The combinatorial thin films were shown to be compositionally reproducible from run-to-run within reasonable limitations. Analytical results suggest a continuous variation in the combinatorial samples in terms of their compositions, crystal structures, and luminescence characteristics.
2010. "Magnetic quantum critical point and dimensionality trend in cerium-based heavy-fermion compounds." Physical Review. B, Condensed Matter 82(18):180515(R). doi:10.1103/PhysRevB.82.180515 Abstract We present realistic Kondo-lattice simulation results for the recently-discovered heavy-fermion antiferromagnet CePt2In7 comparing with its three-dimensional counterpart CeIn3 and the less two-dimensional ones, Ce-115’s. We find that the distance to the magnetic quantum critical point is the largest for CeIn3 and the smallest for Ce-115’s, and CePt2In7 falls in between. We argue that thetrend in quasi-two-dimensional materials stems from the frequency dependence of the hybridization between Cerium 4f-electrons and the conduction bands.
2010. "Measuring Diffusivity in Supercooled Liquid Nanoscale Films using Inert Gas Permeation: II. Diffusion of AR, KR, Xe, and CH4 through Methanol." Journal of Chemical Physics 133(17):174505-11. doi:10.1063/1.3497648 Abstract We present an experimental technique to measure the diffusivity of supercooled liquids at temperatures near their Tg. The approach uses the permeation of inert gases through supercooled liquid overlayers as a measure of the diffusivity of the supercooled liquid itself. The desorption spectra of the probe gas is used to extract the low temperature supercooled liquid diffusivities. In the preceding companion paper, we derived equations using ideal model simulations from which the diffusivity could be extracted using the desorption peak times for isothermal or peak temperatures for TPD experiments. Here, we discuss the experimental conditions for which these equations are valid and demonstrate their utility using amorphous methanol with Ar, Kr, Xe, and CH4 as probe gases. The approach offers a new method by which the diffusivities of supercooled liquids can be measured in the experimentally challenging temperature regime near the glass transition temperature.
2010. "H-1 Nuclear Magnetic Resonance Metabolomics Analysis Identifies Novel Urinary Biomarkers for Lung Function." Journal of Proteome Research 9(6):3083-3090. Abstract Chronic obstructive pulmonary disease (COPD), characterized by chronic airflow limitation, is a serious and growing public health concern. The major environmental risk factor for COPD is tobacco smoking, but the biological mechanisms underlying COPD are not well understood. In this study, we used proton nuclear magnetic resonance (1H-NMR) spectroscopy to identify and quantify metabolites associated with lung function in COPD. Plasma and urine were collected from 197 adults with COPD and from 195 adults without COPD. Samples were assayed using a 600 MHz NMR spectrometer, and the resulting spectra were analyzed against quantitative spirometric measures of lung function. After correcting for false discoveries and adjusting for covariates (sex, age, smoking) several spectral regions in urine were found to be significantly associated with baseline lung function. These regions correspond to the metabolites trigonelline, hippurate and formate. Concentrations of each metabolite, standardized to urinary creatinine, were associated with baseline lung function (minimum p-value = 0.0002 for trigonelline). No significant associations were found with plasma metabolites. Two of the three urinary metabolites positively associated with baseline lung function, i.e. hippurate and formate, are often related to gut microflora. This suggests that the microbiome composition is variable between individuals with different lung function. Alternatively, the nature and origins of all three associated metabolites may reflect lifestyle differences affecting overall health. Our results will require replication and validation, but demonstrate the utility of NMR metabolomics as a screening tool for identifying novel biomarkers of lung disease or disease risk.
2010. "Magnetotransport Properties of High Quality Co:ZnO and Mn:ZnO Single Crystal Pulsed Laser Deposition films: Pitfalls Associated with Magnetotransport on High Resistivity Materials ." Review of Scientific Instruments 81(6):Art. No. 063902. doi:10.1063/1.3436648 Abstract The electrical resistivity values for a series of pure and doped (Co, Mn, Al) ZnO epitaxial films grown by pulsed laser deposition were measured with equipment designed for determining the DC resistivity of high resistance samples. Room-temperature resistances ranging from 7x10^1 ohms/square to 4x10^8 ohms/square were measured on vacuum-reduced cobalt-doped ZnO, (Al,Co) co-doped ZnO, pure cobalt-doped ZnO, Mn-doped ZnO, and undoped ZnO. Using a four-point collinear geometry with gold spring-pin contacts, resistivities were measured from 295 to 5 K for resistances of < ~10^12 ohms/square. In addition, magnetoresistance (MR) and Hall effect were measured as a function of temperature for select samples. Throughout the investigation, samples were also measured on commercially available instrumentation with good agreement. The challenges of transport measurements on high resistivity samples are discussed, along with some offered solutions to those challenges.
2010. "Why Ozonolysis May Not Increase the Hydrophilicity of Particles." Atmospheric Environment 44(7):939-944. doi:10.1016/j.atmosenv.2009.11.009 Abstract It is commonly assumed that atmospheric oxidation of hydrocarbon particles or hydrocarbon coatings on particles leads to polar products and increased water uptake, altering atmospheric visibility and increasing the likelihood they will act as cloud condensation nuclei (CCN). We show here through laboratory experiments that increased water uptake depends on the 3-dimensional structure of the particles. Laboratory studies of particles formed during ozonolysis of surface-bound alkenes, present as terminally unsaturated self-assembled monolayers (C8¼ SAM) on a silica substrate, were carried out at room temperature and 1 atm pressure. SAMs were exposed to w1013 O3 molecules cm-3 for 40 min and resultant particles were analyzed using single particle Fourier transform infrared micro-spectroscopy (micro-FTIR) and secondary ion mass spectroscopy (SIMS). Spectroscopy results show that eCOOH and other polar groups are formed but are buried inside a hydrophobic shell, consistent with earlier observations (McIntire et al., 2005; Moussa et al., 2009) that water uptake does not increase after reaction of the terminal alkene with O3. These insights into the 3-D structure of particles formed on oxidation have important implications for the ability of secondary organic aerosols to act as CCN. In addition, the nature of the surface of the particles is expected to determine their uptake into biological systems such as the surface of the lungs.
2010. "Comparative Density Functional Study of Methanol Decomposition on Cu4 and Co4Clusters." Journal of Physical Chemistry B 114(45):14458-14466. doi:10.1021/jp101594z Abstract A density functional theory study of the decomposition of methanol on Cu4 and Co4 clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H2 and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu4 cluster, methanol dehydrogenation through hydroxymethyl (CH2OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co4 cluster, the dehydrogenation pathway through methoxy (CH3O) and formaldehyde (CH2O) is slightly more favorable. Each of these pathways results in formation of CO and H2. The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H2 and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd4 and Pd8 clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot.
2010. "Catalyst Size and Morphological Effects on the Interaction of NO2 with BaO/γ-Al2O3 Materials ." Catalysis Today 151(3-4):304-313. doi:10.1016/j.cattod.2010.01.005 Abstract The capability of NOx storage on the supported BaO catalyst largely depends on the Ba loading. With different Ba loadings, the supported BaO component exposes various phases ranging from well-dispersed nanoclusters to large crystalline particles on the oxide support materials. In order to better understand size and morphological effects on NOx storage over -Al2O3 supported BaO materials, the adsorption structures and energetics of single NO2 molecule, as well as NOx+NOy (NO2+NO2, NO+NO3 and NO2+NO3) pairs on the BaO/-Al2O3(100), (BaO)2/-Al2O3(100), and (BaO)5/-Al2O3(100) surfaces were investigated using first-principles density functional theory calculations. A single NO2 molecule prefers to adsorb at basic OBa site forming anionic nitrate species. Upon adsorption, a charge redistribution in the supported (BaO)n clusters occurs. Synergistic effects due to the interaction of NO2 with both the (BaO)n clusters and the Al2O3(100) support enhances the stability of adsorbed NO2. The interaction between NO2 and the (BaO)n/ Al2O3(100) catalysts was found to be markedly affected by the sizes and morphologies of the supported (BaO)n clusters. The adsorption energy of NO2 increases from 0.98 eV on the BaO/-Al2O3(100) surface to 3.01 eV on (BaO)5/ Al2O3(100). NO2 adsorption on (BaO)2 clusters in a parallel configuration on the -Al2O3(100) surface is more stable than on dimers oriented in a perpendicular fashion. Similar to the bulk BaO(100) surface, a supported (BaO)n cluster-mediated electron transfer induces cooperative effects that dramatically increase the total adsorption energy of NOx+NOy pairs on the (BaO)n/-Al2O3(100) surfaces. Following the widely accepted NO2 storage mechanism of , our thermodynamic analysis indicates that the largest energy gain for this overall process of NOx uptake is obtained on the amorphous monolayer-like (BaO)5/-Al2O3(100) surface. This suggests that -Al2O3-supported BaO materials with ~ 6 12 wt% loadings may provide optimum structures for NOx storage. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2010. "Ethanol synthesis from syngas over Rh-based/SiO2 catalysts: A combined experimental and theoretical modeling study." Journal of Catalysis 271(2):325-342. Abstract Catalytic conversion of biomass-derived synthesis gas (CO, CO2 and H2) to ethanol and other ! C 2 + oxygenates has received considerable attention recently due to the strong demands for alternative, renewable energy sources. Combining experimental measurements with first-principles-based kinetic modeling, we investigated the reaction kinetics of ethanol synthesis from CO hydrogenation over silica supported Rh/Mn alloy catalyst. We find that the promoters such as Mn atoms can exist in a binary alloy with Rh and play a critical role in lowering the activation barriers for the C–C bond formation such as CH+CO → CHCO, thus improving catalyst activity and selectivity toward ! C 2 + oxygenates, although the barriers towards methane formation are unaffected. The Rh/Mn alloy nanoparticle model is supported by experimental characterization using X-ray photoelectron spectroscopy, transmission electron microscopy, and X-ray diffraction of practically spent Rh/Mn/SiO2 catalyst material and also by first-principles density functional theory (DFT) calculations. Under the reducing conditions, the binary Rh/Mn alloy is thermodynamically more stable than the mixed metal/metal oxides. Further first-principles studies on the effects of various promoters (Ir, Ga, V, Ti, Sc, Ca, and Li) on the CO+CHx (x = 0~3) coupling reactions that are assumed to be the key reaction steps toward the activity and selectivity of ethanol production indicate that alloying the promoters with the electronegativity difference, Δχ, between the promoter and Rh being 0.7 is the most effective in lowering the C–C bond formation. This conclusion is in excellent accord with the reported catalytic performance of CO hydrogenation over Rh-based catalysts with different promoters. We believe that the electronegativity difference criterion is very useful in improving the catalytic performance using transition metal based catalysts for ethanol synthesis from CO hydrogenation. Finally, the reaction kinetics of CO hydrogenation to ethanol on the Rh/Mn/SiO2 under experimental operating conditions was simulated using kinetic Monte Carlo (KMC) modeling. The simulated reaction kinetics is qualitatively consistent with experimental observations. The ramifications of these findings are discussed and propositions for improving these catalysts are suggested.
2010. "First-principles-based Kinetic Monte Carlo Simulation of Nitric Oxide Reduction over Platinum Nanoparticles under Lean-burn Conditions." Industrial and Engineering Chemistry Research 49(21):10364-10373. doi:10.1021/ie100999e Abstract The kinetics of NO reduction over platinum nanoparticles under lean-burn condition was investigated by first-principles-based kinetic Monte Carlo simulations. Three-dimensional model platinum nanoparticles with diameters ranging from 2.29 to 4.61 nm were represented by a truncated octahedron model consisting of a single (100) facet and eight (111) facets. First-principles density functional theory (DFT) calculations were used to determine the intrinsic kinetic parameters including the binding energies for all of the surface intermediates as well as the activation barriers and reaction energies that comprise the reaction mechanism over the (100) facet, the (111) facet and the (111)/(100) edge sites on the three-dimension nanoparticle. Both intra- and inter- facet diffusion of adsorbates were included to model mass transport over the particle surface. The simulation results show that in the presence of excess oxygen, NO reduction to N2 occurs only on the (100) facets. The oxidation of NO to NO2, while much more favored on the (111) facets, can occur on both (100) and (111) facets. Only small amounts of N2O form over the (100) facet. The apparent activation energies for N2 and NO2 formation over the overall particle are 45 kJ/mol and 42 kJ/mol, respectively. This is in agreement with previous experiments. Particle size effects on the activities of NO reduction and oxidation depend on the atomic fractions of the (100) and the (111) facets exposed on the platinum nanoparticles. For the three-dimensional model platinum nanoparticles examined here, the atomic fraction of the (100) facet is nearly the same while the atomic fraction of the (111) facet increases with the increasing particle size. As a result, the activity of NO reduction is insensitive to the particle size. While the larger particle shows higher activity for NO oxidation thus elucidating previous experimental observations. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2010. "The Unique Role of Anchoring Penta-coordinated Al3+ Sites in the Sintering of γ Al2O3-supported Pt Catalysts ." The Journal of Physical Chemistry Letters 1:2688–2691. doi:10.1021/jz101073p Abstract In this communication, we report on the specific role of the Alp sites in Pt sintering over γ-Al2O3 support. On the basis of HR-STEM and EXAFS results that showed a bimodal distribution of Pt clusters on γ-Al2O3 during Pt sintering under high-temperature calcination conditions, high-level density functional theory (DFT) calculations were performed to investigate the interactions of atomic Pt and PtO on γ-Al2O3 at different coverages. In agreement with experimental observations, DFT calculations indicate that the sintering processes are both thermodynamically and kinetically hindered by strong interactions of atomic Pt and PtO with the Alp sites on the γ-Al2O3 support. Therefore, increasing the quantity of (100) facets of γ-Al2O3, on which the Alp sites reside, may provide a way to control the sintering of γ-Al2O3-supported Pt catalysts. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2010. "Brownian Dynamics Simulation of Protein Solutions: Structural and Dynamical Properties." Biophysical Journal 99(11):3782-3791. doi:10.1016/j.bpj.2010.10.035 Abstract The study of solutions of biomacromolecules provides an important basis for understanding the behavior of many fundamental cellular processes, such as protein folding, self-assembly, biochemical reactions, and signal transduction. Here, we describe a Brownian dynamics simulation procedure and its validation for the study of the dynamic and structural properties of protein solutions. In the model used, the proteins are treated as atomically detailed rigid bodies moving in a continuum solvent. The protein-protein interaction forces are described by the sum of electrostatic interaction, electrostatic desolvation, nonpolar desolvation, and soft-core repulsion terms. The linearized Poisson-Boltzmann equation is solved to compute electrostatic terms. Simulations of homogeneous solutions of three different proteins with varying concentrations, pH, and ionic strength were performed. The results were compared to experimental data and theoretical values in terms of long-time self-diffusion coefficients, second virial coefficients, and structure factors. The results agree with the experimental trends and, in many cases, experimental values are reproduced quantitatively. There are no parameters specific to certain protein types in the interaction model, and hence the model should be applicable to the simulation of the behavior of mixtures of macromolecules in cell-like crowded environments.
2010. "Polarization Resistance of La0.85Ca0.15MnO3 Cathodes for Solid Oxide Fuel Cells (SOFCs) Measured Using Patterned Electrodes." ECS Transactions 28(23):137-146. doi:10.1149/1.3502345 Abstract Patterned cathodes of calcium-doped lanthanum manganite (LCM) were fabricated on polycrystalline yttria-stabilized zirconia (YSZ) substrates by RF magnetron sputtering and photolithographic techniques. Samples were generated with a constant electrode/electrolyte contact area but different three phase boundary (TPB) lengths (lTPB’s). Electrochemical impedance spectroscopy (EIS) was performed in the oxygen partial pressure (pO2) and temperature range of 10-3 atm to 1.00 atm and 600-800°C respectively. The area specific polarization resistance, Rp, was found to scale linearly with the inverse of lTPB, suggesting that the TPB is the active region for oxygen reduction. The resistivity decreased with increased temperature and pO2 and showed an activation energy of 1.17 +/- 0.03 eV.
2010. "Genetic Transformation and Mutagenesis Via Single-Stranded DNA in the Unicellular, Diazotrophic Cyanobacteria of the Genus Cyanothece." Applied and Environmental Microbiology 76(22):7641-7645. doi:10.1128/AEM.01456-10 Abstract We describe a genetic system for producing specific gene knockouts in Cyanothece sp. Strain PCC 7822 using a ssDNA technique (37). The first fully segregated mutant was in the nifK gene (DnifK) and the mutant was unable to grow on a medium lacking combined nitrogen and produced virtually no hydrogen.
2010. "Automated Chemical Analysis of Internally Mixed Aerosol Particles Using X-ray Spectromicroscopy at the Carbon K-Edge." Analytical Chemistry 82(19):7906 - 7904. doi:10.1021/ac1012909 Abstract We have developed an automated data analysis method for atmospheric particles using scanning transmission X-ray microscopy coupled with near edge X-ray fine structure spectroscopy (STXM/NEXAFS). This method is applied to complex internally mixed submicron particles containing organic and inorganic material. Several algorithms were developed to exploit NEXAFS spectral features in the energy range from 278-320 eV for quantitative mapping of the spatial distribution of elemental carbon, organic carbon, potassium, and non-carbonaceous elements in particles of mixed composition. This energy range encompasses the carbon K-edge and potassium L2 and L3 edges. STXM/NEXAFS maps of different chemical components were complemented with a subsequent analysis using elemental maps obtained by scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX). We demonstrate application of the automated mapping algorithms for data analysis and the statistical classification of particles.
2010. "Microscopic Characterization of Carbonaceous Aerosol Particle Aging in the Outflow from Mexico City." Atmospheric Chemistry and Physics 10(3):961-976. Abstract The physical and chemical transformations of particles aged in the outflow from Mexico City were investigated for the transport event of 22 March 2006. This study was part of the Megacities Initiative: Local and Global Research Observations (MILAGRO) 2006 field campaign. A detailed chemical analysis of individual particles was performed using a combination of complementary spectro-microscopic techniques. These techniques included scanning transmission X-ray microscopy (STXM) coupled with near edge X-ray absorption fine structure spectroscopy (NEXAFS) and computer controlled scanning electron microscopy with an energy dispersive X-ray analyzer (CCSEM/EDX). As the aerosol plume evolves from the city center, the organic mass per particle increases and the fraction of carbon-carbon double bonds (associated with elemental carbon) decreases. Organic functional groups enhanced with age include: carboxylic acids, alkyl groups, and oxygen bonded alkyl groups. At the city center (T0) the most prevalent aerosol type contained inorganic species (composed of sulfur, nitrogen, oxygen, and potassium) coated with organic material. At the T1 and T2 sites, located northeast of T0 (~29 km and ~65 km, respectively), the fraction of homogenously mixed organic particles increased in both size and number. These observations illustrate the evolution of the physical mixing state and organic bonding in individual particles in a photochemically active environment.
2010. "Microbial Reduction of Uranium under Iron- and Sulfate-reducing Conditions: Effect of Amended Goethite on Microbial Community Composition and Dynamics." Water Research 44(14):4015-4028. Abstract There is a growing need for a better understanding of the biogeochemical dynamics involved in microbial U(VI) reduction due to an increasing interest in using biostimulation via electron donor addition as a means to remediate uranium contaminated sites. U(VI) reduction has been observed to be maximized during iron reducing conditions and to decrease upon commencement of sulfate reducing conditions. There are many unknowns regarding the impact of iron/sulfate biogeochemistry on U(VI) reduction. This includes Fe(III) availability as well as the microbial community changes, including the activity of iron-reducers during the uranium biostimulation period even after the onset of sulfate reduction. Up-flow column experiments were conducted with Old Rifle site sediments containing Fe-oxides, Fe-clays, and sulfate rich groundwater. Half of the columns had sediment that was augmented with small amounts of small-particle 57Fe-goethite to track continuously minute goethite changes, and to study the effects of increased Fe(III) levels on the overall biostimulation dynamics. The addition of the 57Fe-goethite did not delay the onset of sulfate reduction, but slightly suppressed the overall rate of sulfate reduction and hence acetate utilization, it did not affect the bacterial numbers of Geobacter-like species throughout the experiment, but did lower the numbers of sulfate reducers in the sediments. 57Fe-Mössbauer analyses (a 57Fe-specific technique) confirmed that there was bioavailable iron present after the onset of sulfate reduction and that iron was still being reduced during sulfate reduction. Addition of the 57Fe-goethite to the sediment had a noticeable effect on the overall composition of the microbial population. 16S rRNA analyses of biostimulated sediment using TRFLP (terminal restriction fragment length polymorphism) showed that Geobacter sp. (a known Fe-reducer) was still active and replicating during the period of significant sulfate reduction. DNA fingerprints of the sediment-attached microbial communities were dominated by 5 TRFs, that comprised 25-57% of the total profile.
2010. "Atomistic Simulation of Track Formation by Energetic Recoils in Zircon." Journal of Physics. Condensed Matter 22:Art. No. 395008. Abstract We have performed classical molecular dynamics simulations of fission track formation in zircon. We simulated the passage of a swift heavy ion through crystalline zircon using cylindrical thermal spikes with energy deposition (dE/dx) of 2.5 to 12.8 keV/nm and radius of 3 nm. At a low dE/dx of 2.55 keV/nm, the structural damage recovered almost completely and a damage track was not produced. At higher values of dE/dx, tracks were observed and the radius of the track increased with increasing dE/dx. Our structural analysis shows amorphization in the core of the track and phase separation into Si-rich regions near the center of the track and Zr-rich regions near the periphery. These simulations establish a threshold dE/dx for fission-track formation in zircon that is relevant to thermo-chronology and nuclear waste immobilization.
2010. "From Ultrafine Thiolate-Capped Copper Nanoclusters toward Copper Sulfide Nanodiscs: A Thermally Activated Evolution Route." Chemistry of Materials 22(1):261-271. doi:10.1021/cm903038w Abstract In this report we show that the size, shape, and composition of pre-synthesized metal nanoparticles can be engineered through exploiting concurrent interparticle coalescence and interfacial copper-thiolate cleavage under a thermally-activated evolution process. This concept is demonstrated by thermally-activated processing of ultrafine (~0.5 nm) copper nanoparticles encapsulated with thiolate monolayer (Cun(SR)m) toward copper sulfide nanodiscs with controllable sizes and shapes. It involved a thermally-activated coalescence of Cun(SR)m nanoclusters accompanied by interfacial Cu-S cleavage towards the formation of Cu2S nanocrystals with well-defined nanodisc shapes with an average diameter and thickness ranging from 10.7 ±1.4 nm and 5.5 ±0.5 nm (aspect ratio ~2) to 31.2 ±4.3 nm and 3.9 ±0.4 nm (aspect ratio ~7) depending on the thermal processing parameters. These nanodiscs are stable and display remarkable ordering upon self-assembly. The abilities to create the ultrafine copper nanoclusters and to enable them to undergo a thermally-activated coalescence and a concurrent Cu-S bond cleavage toward the formation of Cu2S nanodiscs is entirely new. The viability of fine tuning the size and shape of the Cu2S nanocrystals by controlling the relative binding strength of thiolates, the C-S cleavage reactivity, and the interparticle coalescence activity, and their potential applications in electronic, sensing and photochemical devices are also discussed.
2010. "Formation, characterization and reactivity of adsorbed oxygen on BaO/Pt(111)." Journal of Physical Chemistry C 114(47):20195-20206. Abstract The formation of adsorbed O (Oad) species and their reactivities in CO oxidation on BaO/Pt(111) (at two BaO coverages) were studied with temperature programmed desorption (TPD), infrared reflection absorption (IRA) and X-ray photoelectron (XP) spectroscopies. In neither of these two systems was the Pt(111) surface completely covered with BaO. On the system with lower BaO coverage (~45 % of the Pt(111) surface is covered by BaO), two different Oad species form following the adsorption of O2 at 300 K: O adsorbed on BaO-free Pt(111) sites (OPt) and at the Pt-BaO interface (Oint). On the system with higher BaO coverage (~60 % of the Pt(111) surface is covered by BaO), two types of Oint are seen at the Pt-BaO interface. The desorption of OPt from the BaO-free portion of the Pt(111) surface gives an O2 desorption peak with a maximum desorption rate at ~690 K. Migration of Oint to the Pt(111) sites and their recombinative desorption give two explosive desorption features at ~760 and ~790 K in the TPD spectrum. The reactivities of these Oad species with CO to form CO2 follow their sequence of desorption; i.e., the OPt associated with the BaO-free Pt(111) surface, which desorbs at 690 K, reacts first with CO, followed by the Oint species at the Pt-BaO interface (first the one that desorbs at ~760 K and finally the one that is bound the most strongly to the interface, and desorbs at ~790 K). This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2010. "Reactions of NO2 with Ba(OH)2 on Pt(111)." Journal of Physical Chemistry C 114(40):16955-16963. Abstract The interaction of NO2 with amorphous and crystalline Ba(OH)2 supported on Pt(111) was studied in the wide pressure range of 1.0 × 10-9 1.0 × 10-4 Torr and compared to that with a thick (> 20 monolayer equivalent (MLE)) BaO film using infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The amorphous and crystalline Ba(OH)2 layers were prepared by exposing a thick BaO (> 20 MLE) layer on Pt(111) to H2O at 300 and 425 K, respectively. The amorphous and crystalline Ba(OH)2 layers partially convert to Ba(NOx)2 (nitrites and nitrates) following their exposure to elevated NO2 pressure (~1.0 × 10-4 Torr) at 300 K. The exposure of the crystalline-Ba(OH)2/Pt(111) system to NO2 at 425 K, however, leads to the desorption of H2O and the complete conversion of the crystalline Ba(OH)2 layer to Ba(NOx)2, which consists of mainly crystalline nitrates and a small amount of nitrites. The amounts of NOx stored by BaO (> 20 MLE)/Pt(111) and crystalline Ba(OH)2/Pt(111) systems upon their exposure to NO2 at 425 K are comparable. The thus-formed bulk crystalline Ba(NO3)2 phase decomposes in two steps, both releasing NO and O2, in accord with the melting/decomposition scheme for bulk Ba(NO3)2.