Scientific Publications 2007
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
I
2007. "Ion Funnel Trap Interface for Orthogonal Time-of-Flight Mass Spectrometry ." Analytical Chemistry 79(20):7845-7852. doi:10.1021/ac071091m Abstract A combined electrodynamic ion funnel and ion trap coupled to an orthogonal acceleration (oa)-time-of-flight mass spectrometer was developed and characterized. The ion trap was incorporated through the use of added terminal electrodynamic ion funnel electrodes enabling control over the axial dc gradient in the trap section. The ion trap operates efficiently at a pressure of ~1 Torr, and measurements indicate a maximum charge capacity of ~3 × 107 charges. An order of magnitude increase in sensitivity was observed in the analysis of low concentration peptides mixtures with orthogonal acceleration (oa)-time-of-flight mass spectrometry (oa-TOF MS) in the trapping mode as compared to the continuous regime. A signal increase in the trapping mode was accompanied by reduction in the chemical background, due to more efficient desolvation of, for example, solvent related clusters. Controlling the ion trap ejection time was found to result in efficient removal of singly charged species and improving signal-to-noise ratio (S/N) for the multiply charged analytes.
2007. "Order-disorder Phase Transition of the Cu(001) Surface under Equilibrium Oxygen Pressure." Physical Review. B, Condensed Matter 76(24):241404(R). doi:10.1103/PhysRevB.76.241404 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. In situ surface x-ray scattering is used to determine the stable oxygen-induced surface structure on Cu(001) at high temperatures and under equilibrium oxygen pressure. The structure is composed of a c(2x2)-O adlayer atop a randomly 3/4-filled Cu layer for temperatures between 473 and 1000 K. Below 473 K, the vacancies in the topmost Cu layer order to form a (2x square root 2 x square root 2)R45° missing row structure. This order-disorder transition is confirmed by first-principles calculations.
2007. "Shape of Platinum Nanoparticles Supported on SrTiO₃: Experiment and Theory." Journal of Physical Chemistry C 111(40):14782-14789. doi:10.1021/jp073041r 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. Platinum nanocrystals of about 30-100 nm size were grown on SrTiO3 substrates in different orientations. Their shapes were investigated using first-principles methods and compared to scanning electron microscope images. Analysis of the results for several different surface orientations shows epitaxial growths, uniform size distributions in regular arrays, partial wetting, and several different shapes. In particular, we observe a bimodal distribution of particle shapes, distinguished by the relative size of the (100) facets. This observation is explained by the changes in the surface energy caused by the surface reconstruction.
2007. "Structural Requirements and Reaction Pathways in DimethylEther Combustion Catalyzed by Supported Pt Clusters." Journal of the American Chemical Society 129(43):13201-13212. doi:10.1021/ja073712z 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 identity and reversibility of the elementary steps required for catalytic combustion of dimethyl ether (DME) on Pt clusters were determined by combining isotopic and kinetic analyses with density functional theory estimates of reaction energies and activation barriers to probe the lowest energy paths. Reaction rates are limited by C-H bond activation in DME molecules adsorbed on surfaces of Pt clusters containing chemisorbed oxygen atoms at near-saturation coverages. Reaction energies and activation barriers for C-H bond activation in DME to form methoxymethyl and hydroxyl surface intermediates show that this step is more favorable than the activation of C-O bonds to form two methoxides, consistent with measured rates and kinetic isotope effects. This kinetic preference is driven by the greater stability of the CH3OCH2* and OH* intermediates relative to chemisorbed methoxides. Experimental activation barriers on Pt clusters agree with density functional theory (DFT)-derived barriers on oxygen-covered Pt(111). Measured DME turnover rates increased with increasing DME pressure, but decreased as the O2 pressure increased, because vacancies (*) on Pt surfaces nearly saturated with chemisorbed oxygen are required for DME chemisorption. DFT calculations show that although these surface vacancies are required, higher oxygen coverages lead to lower C-H activation barriers, because the basicity of oxygen adatoms increases with coverage and they become more effective in hydrogen abstraction from DME. Water inhibits reaction rates via quasi-equilibrated adsorption on vacancy sites, consistent with DFT results indicating that water binds more strongly than DME on vacancies. These conclusions are consistent with the measured kinetic response of combustion rates to DME, O2, and H2O, with H/D kinetic isotope effects, and with the absence of isotopic scrambling in reactants containing isotopic mixtures of 18O2-16O2 or 12CH3O12CH3-13CH3O13- CH3. Turnover rates increased with Pt cluster size, because small clusters, with more coordinatively unsaturated surface atoms, bind oxygen atoms more strongly than larger clusters and exhibit lower steadystate vacancy concentrations and a consequently smaller number of adsorbed DME intermediates involved in kinetically relevant steps. These effects of cluster size and metal-oxygen bond energies on reactivity are ubiquitous in oxidation reactions requiring vacancies on surfaces nearly saturated with intermediates derived from O2.
2007. "An experimental investigation of nitrogen gas produced during denitrification." Ground Water 45(4):461-467. Abstract In Situ denitrification relies on the activity of indegenous or introduced denitrifying microorganisms to reduce nitrate to N2 gas. In this study, we investigated the fate of N2 gas produced during denitrification in an intermediate-scale flow cell containing packed sediments. Denitrification was stimulated by a series of nitrate and ethanol additions. Results show limited reduction of hydraulic conductivity in the aquifer material.
