Scientific Publications 2007
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2007. "3D MRI of Non-Gaussian ³He Gas Diffusion in the Rat Lung." Journal of Magnetic Resonance 188(2):357-366. doi:10.1016/j.jmr.2007.08.014 Abstract In ³He magnetic resonance images of pulmonary air spaces, the confining architecture of the parenchymal tissue results in a non-Gaussian distribution of signal phase that non-exponentially attenuates image intensity as diffusion weighting is increased. Here, two approaches previously used for the analysis of non-Gaussian effects in the lung are compared and related using diffusion-weighted ³He MR images of mechanically ventilated rats. Total lung coverage is achieved using a hybrid 3D pulse sequence that combines conventional phase encoding with sparse radial sampling for efficient gas usage. This enables the acquisition of nine 3D images using a total of only ~ 1 L of hyperpolarized ³He gas. Diffusion weighting ranges from 0 s/cm² to 40 s/cm². Results show that the non-Gaussian effects of ³He gas diffusion in healthy rat lungs are directly attributed to the anisotropic geometry of lung microstructure, and that quantitative analysis over the entire lung can be reliably repeated in time-course studies of the same animal.
2007. "Microsolvation for the Dicyanamide Anion: [N(CN)2-](H2O)n (n=0-12)." Journal of Physical Chemistry A 111(32):7719-7725. Abstract Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)2 -. Photoelectron spectra of [N(CN)2-] (H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n= 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)2-, allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 ±0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)2-](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n= 1 and 2 due to the fact that the N(CN)2- anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)2- anion.
2007. "Influence of Biogenic Fe(II) on the Extent of Microbial Reduction of Fe(III) in Clay Minerals Nontronite, Illite, and Chlorite." Geochimica et Cosmochimica Acta 71(5):1145-1158. doi:10.1016/j.gca.2006.11.027 Abstract Microbial reduction of Fe(III) in clay minerals is an important process that affects properties of clay-rich materials and iron biogeochemical cycling in natural environments. Microbial reduction often ceases before all Fe(III) in clay minerals is exhausted. The factors causing the cessation are, however, not well understood. The objective of this study was to assess the role of biogenic Fe(II) in microbial reduction of Fe(III) in various clay minerals. Bioreduction experiments were performed in a batch system, where lactate was used as the sole electron donor, Fe(III) in clay minerals as the sole electron acceptor, and Shewanella putrefaciens CN32 as the mediator with and without an electron shuttle AQDS. Our results showed that bioreduction activity ceased within two weeks with variable extents of bioreduction of structural Fe(III) in clay minerals. When fresh CN32 cells were added to the old cultures (6 months), bioreduction resumed and extents increased. This result indicated that the previous cessation of Fe(III) bioreduction was not necessarily due to the exhaustion of bioavailable Fe(III) in the mineral structure, and suggested that the changes of cell physiology or solution chemistry, such as Fe(II) production during microbial reduction, affected the extent of bioreduction. To investigate the effect of Fe(II) production on Fe(III) bioreduction, a typical bioreduction process (consisting of lactate, clay, cells and AQDS) was separated into two steps: 1. AQDS was reduced by cells in the absence of clay but in the presence of variable Fe(II) concentrations; 2. reduction of Fe(III) in clays by biogenic AH2DS in the absence of cells. The inhibitory effect of Fe(II) on CN32 activity was confirmed. TEM analysis revealed a thick electron dense halo surrounding the cell surfaces that most likely resulted from Fe(II) sorption/precipitation. Such electron dense materials might have blocked or interfered electron transfers on cell surfaces. The inhibitory effect of Fe(II) was also observed in AH2DS reduction of clay Fe(III). The reduction extent consistently decreased with an increasing concentration of presorbed Fe(II) (onto clay surfaces) at the start of reduction experiments. The relative reduction extent (i.e., reduction extent after normalization to the reduction extent when spiked Fe(II) was zero) was similar for all clay minerals studied and showed a systematic decrease with increasing clay-sorbed Fe(II) concentration. These results suggest a similar inhibitory effect of clay-sorbed Fe(II) on the reduction extent for different clay minerals. An equilibrium thermodynamic model was established with independently estimated parameters to evaluate whether the cessation of Fe(III) reduction by AH2DS was due to the exhaustion of reaction free energy. Model-calculated reduction extents were, however, over 50% higher than experimentally measured, indicating that other factors, such as blockage of the electron transfer chain and mineralogy, restricted the reduction extent. This study also revealed that the relative reducibility of Fe(III) in different clay was as follows: nontronite > chlorite > illite. This order is qualitatively consistent with the differences in crystal chemistry of these minerals.
2007. "Kinetic Analysis of Microbial Reduction of Fe(III) in Nontronite." Environmental Science and Technology 41(7):2437-2444. doi:10.1021/es0619399 Abstract Microbial reduction of structural Fe(III) in nontronite was studied in batch cultures under non-growth condition using Shewanella putrefaciens, strain CN32. The rate and extent of structural Fe(III) reduction was examined as a function of electron acceptor [Fe(III)] and bacterial concentration. Fe(II) sorptions onto nontronite and CN32 cells were independently measured and well-described by the Langmuir expression with affinity constant 2.3 and 2.25 for nontronite and cells, respectively. The Fe(II) sorption capacity of nontronite, however, decreased with increasing nontronite concentration, suggesting particulate agglomeration effect. An empirical equation for sorption capacity was derived from the sorption isotherms at different nontronite concentrations and was used to calculate the 'effective' Fe(III) concentration for bioreduction. The initial rate of microbial reduction was found to be first order with respect to the 'effective' Fe(III) concentration. A kinetic biogeochemical model was assembled that incorporated the first order rate expression with respect to the ‘effective’ Fe(III) concentration, rates and extent of Fe(II) sorption to cell and nontronite surfaces, and the empirical equation for sorption capacity. The model successfully described the experimental results of microbial reduction of nontronite with variable nontronite concentrations. The microbial reduction rate, after normalized to cell concentration, however, decreased with increasing cell concentration, indicating that cell concentration did not linearly affect the reduction as commonly assumed in literature. A nonlinear, saturation-type rate expression with respect to cell concentration was needed to model bioreduction at variable cell concentration. Our results indicated that the kinetics of microbial reduction of structual Fe(III) in nontronite can be modeled after consideration of Fe(II) production and sorption, its role in inhibiting further Fe(III) reduction, and nonlinear effect of cell concentration.
2007. "A first principles analysis of the electro-oxidation ofCO over Pt(1 1 1)." Electrochimica Acta 52(18):5517-5528. doi:10.1016/j.electacta.2007.01.060 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. First principle density functional theoretical calculations carried out within a constant potential half-cell formalism were used to model the electro-oxidation of CO over Pt(1 1 1). The method involves tuning the potential by the addition or removal of electrons from the system. The free energy for different adsorbed species within the double-layer is analyzed over a range of different potentials to establish the lowest energy states and the reaction energies that connect these states. The potentials are calculated based on a novel double-reference approach [J.S. Filhol, M. Neurock, Angew. Chem. Int. Ed. 45 (2006) 402] discussed earlier. The potential-dependent reaction energies are reported for the elementary steps of water activation in the presence of co-adsorbed CO and CO oxidation over the model Pt(1 1 1) surface. The potential-dependent activation barriers are computed for the key elementary steps in CO oxidation to develop a detailed reaction energy profile as a function of electrode potential. The results suggest that the coupling of co-adsorbed CO and OH controls the rate. Water activation, however, is necessary to supply a critical coverage of the surface OH oxidant.
2007. "First Principles Analysis of the Electrocatalytic Oxidationof Methanol and Carbon Monoxide." Topics in Catalysis 46(3-4):306-319. doi:10.1007/s11244-007-9004-9 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The strong drive to commercialize fuel cells for portable as well as transportation power sources has led to the tremendous growth in fundamental research aimed at elucidating the catalytic paths and kinetics that govern the electrode performance of proton exchange membrane (PEM) fuel cells. Advances in theory over the past decade coupled with the exponential increases in computational speed and memory have enabled theory to become an invaluable partner in elucidating the surface chemistry that controls different catalytic systems. Despite the significant advances in modeling vapor-phase catalytic systems, the widespread use of first principle theoretical calculations in the analysis of electrocatalytic systems has been rather limited due to the complex electrochemical environment. Herein, we describe the development and application of a first-principles-based approach termed the double reference method that can be used to simulate chemistry at an electrified interface. The simulations mimic the half-cell analysis that is currently used to evaluate electrochemical systems experimentally where the potential is set via an external potentiostat. We use this approach to simulate the potential dependence of elementary reaction energies and activation barriers for different electrocatalytic reactions important for the anode of the direct methanol fuel cell. More specifically we examine the potential-dependence for the activation of water and the oxidation of methanol and CO over model Pt and Pt alloy surfaces. The insights from these model systems are subsequently used to test alternative compositions for the development of improved catalytic materials for the anode of the direct methanol fuel cell.
2007. "Mechanisms controlling soil carbon turnover and their potential application for enhancing carbon sequestration ." Climatic Change 80(1-2):5-23. Abstract Two major mechanisms, (bio)chemical alteration and physicochemical protection, stabilize soil organic carbon (SOC) and thereby control soil carbon turnover. With (bio)chemical alteration, SOC is transformed by biotic and abiotic processes to chemical forms that are more resistant to decomposition and, in some cases, more easily retained by sorption to soil solids. With physicochemical protection, biochemical attack of SOC is inhibited by organomineral interactions at molecular to millimeter scales. Stabilization of otherwise decomposable SOM can occur via sorption to soil surfaces, complexation with soil minerals, occlusion within aggregates, and deposition in pores inaccessible to decomposers and extracellular enzymes. Soil structure (i.e., the arrangement of solids and pores in the soil) is a master integrating variable that both controls and indicates the SOC stabilization status of a soil. To enhance SOC sequestration, the best option is to modify the soil physicochemical environment to favor the activities of fungi. Specific practices that accomplish this include minimizing tillage, maintaining a near-neutral soil pH and an adequate base cation exchange capacity (particularly Ca), ensuring adequate drainage, and minimizing erosion by water and wind. In some soils, amendments with various high-specific-surface micro- and mesoporous sorbents such as fly ash or charcoal can be beneficial.
2007. "Interaction of CO with Surface PdZn Alloys." Surface Science 601(23):5546-5554. doi:10.1016/j.susc.2007.09.031 Abstract The adsorption and bonding configuration of CO on clean and Zn-covered Pd(111) surfaces was studied using Low Energy Electron Diffraction (LEED), Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS). LEED and TPD results indicate that annealing at 520 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(111) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/Pd(111) alloy surfaces compared to clean Pd(111). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al2O3 catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.
2007. "Disordering and Dopant Behaviour in Au+ Ion-Irradiated AlN." Journal of Physics. Condensed matter 19(35):356207, 1-10. doi:10.1088/0953-8984/19/35/356207 Abstract Single-crystal AlN films on SiC were irradiated at 145 K with 1.0 MeV Au+ ions to a wide range of ion fluences. The accumulation of disorder on both the Al and N sublattices in AlN has been investigated in situ using conventional Rutherford backscattering spectrometry (RBS) and non-RBS along the <0001>-axial channelling direction. The results suggest that a disorder saturation stage is attained following an initial disorder increase at intermediate doses (< 10 dpa). A continuously amorphized layer was not formed in AlN for doses up to 208 dpa. Similar disordering behaviour is observed for the Al and N sublattices. The lattice disorder produced at 145 K is thermally stable at room temperature; further irradiation does not induce disorder recovery. The microstructures in the irradiated AlN exhibit both amorphous and crystalline domains at the stage of disorder saturation. The implanted Au does not show significant redistribution during the ion irradiation or room-temperature annealing.
2007. "Variation in lattice parameters of 6H-SiC irradiated to extremely low doses." Applied Physics Letters 91(9):091918, 1-3. doi:10.1063/1.2778630 Abstract Irradiation of 6H-SiC single crystals was performed using 4 MeV H+ ions at 340 and 210 K. The changes in lattice parameters in the basal plane and along the c-axes were measured as a function of dose using high-resolution x-ray diffraction. The c-axis lattice parameter increases monotonically with the increasing dose, while a-axis lattice parameter decreases at extremely low doses. An initial volumetric contraction of the unit cell is observed. The decrease in the a parameter may originate from the irradiation-induced vacancies and the possible formation of antisite defects that cause the lattice structure on the basal plane to shrink.
2007. "Behavior of Si and C atoms in ion amorphized SiC." Journal of Applied Physics 101(2):Art. No. 023524. doi:10.1063/1.2431941 Abstract Single crystal 6H-SiC wafers were fully amorphized at room temperature or 200 K using 1.0 or 2.0 MeV Au+ ion irradiation. The thickness of the amorphized layers has been determined using Rutherford backscattering spectrometry under ion channeling conditions. Microstructures of the irradiated SiC have been examined using cross-sectional transmission electron microscopy. The depth profiles of both the Si and C atoms have been studied using both x-ray photoelectron spectroscopy (XPS) and time-of-flight energy elastic recoil detection analysis. Neither Si nor C in the amorphized SiC exhibits a significant mass transport by diffusion during the irradiation and subsequent storage at room temperature. There is no observable phase segregation of either Si or C in the amorphized SiC. Ar+ ion sputtering leads to modifications of the composition, structure and chemical bonding at the 6H-SiC surface. The Si-Si bonds at the sputtered surface (amorphized) do not appear, as suggested by the XPS; however, Raman backscattering data reveals the existence of the Si-Si bonds in the bulk amorphized SiC, in addition to the C-C and Si-C bonds that the XPS also identified.
2007. "Engineered SMR catalysts based on hydrothermally stable, porous, ceramic supports for microchannel reactors." Catalysis Today 120(1):54-62. Abstract A novel engineered, porous, ceramic, catalyst support for stable, high temperature (> 800oC) steam methane reforming operation was demonstrated with a rhodium catalyst. The support was designed for operation in micro-channel reactors. Typically high temperature alloys such as FeCrAlY or 600 series nickel-based alloys are used as structural supports that are wash-coated with catalyst-impregnated, high surface area, ceramic powders. The hydrothermal conditions used for methane steam reforming create several material challenges that interfere with the performance of metallic supports: corrosive degradation of the metal, delamination of the wash-coated catalyst from the metal support, and accelerated sintering of the high-surface area ceramic powder used to disperse the metal catalysts. Additionally, undesirable side reactions such as coke formation promoted by the support metal typically necessitate operating SMR reactions at higher than equilibrium steam to carbon ratios. The engineered, porous, ceramic support with Rh catalyst was tested at a steam to carbon ratio of 1:1, a contact time of 27 ms, and temperatures up to 900oC. Near equilibrium conversion and selectivity were achieved. It was found that there was no degradation or sintering observed in the engineered, porous, ceramic support, the catalyst did not delaminate from the support, nor was any coke formation detected after 100 hr time-on-stream (TOS) under these reaction conditions. Keywords: methane steam reforming, microchannel reactors, engineered catalyst, hydrothemally stable catalyst
2007. "Synthesis and Photoexcited Charge Carrier Dynamics of beta-FeOOH Nanorods." Applied Physics Letters 90(10):Art. No. 103504. doi:10.1063/1.2711395 Abstract Akaganeite(B-FeOOH) nanorods of dimensions 15 nm diameter and 200 nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation displays three timescales. The first is a sub-picosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2 ps. The trapped electrons and holes persist for significantly longer times (at least tens-of-ps), similar to previous results from a-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states.
2007. "Electronic Energy Relaxation and Luminescence Decay Dynamics of Eu3+ in Zn2SiO4:Eu3+." Journal of Luminescence 126(2):491-496. doi:10.1016/j.jlumin.2006.09.004 Abstract Abstract: Electronic energy relaxation and decay dynamics of Eu3+ in Zn2SiO4:Eu3+ phosphors displays evidence of intra-ion energy transfer from the 5D1 to the 5D0 manifold. The energy transfer timescale does not depend on Eu3+ concentration, or the addition of Mn2+ as a co-dopant and is estimated to be about 11 microseconds in Zn2SiO4. Evidence for Eu3+ electronic energy transfer has also been observed in Eu-doped MgS as well as Eu3+ encapsulated in zeolite-Y. The energy transfer timescale in these other materials is shorter than in Zn2SiO4, most likely due to differences in Eu3+ surroundings or site symmetry.
