Scientific Publications 2006
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
H
2006. "Development and Processing Temperature Dependence of Ferromagnetism in Zn0.98Co0.02O." Journal of Applied Physics 99(8):Art. no. 08M123. doi:10.1063/1.2173956 Abstract We report the development of room-temperature ferromagnetism (FM), with coercivity Hc = 2850 Oe and saturation magnetization Ms ~ 0.01 emu/g, in chemically synthesized powders of Zn0.98Co0.02O processed at 150 °C, and paramagnetism with antiferromagnetic interactions between the Co2+ spins (S = 3/2) in samples processed at higher temperatures 200 ≤ TP ≤ 900 °C. X-ray diffraction data show a decrease in the lattice parameters a and c with TP indicating progressive incorporation of 0.58 Å sized tetrahedral Co2+ at the substitutional sites of 0.60 Å sized Zn2+. Diffuse reflectance spectra show three well defined absorption edges at 660, 615 and 568 nm due to the d – d crystal field transitions 4A2(F) 2E(G), 4A2(F) 4T1(P) and 4A2(F) 2A1(G) of high spin (S = 3/2) Co2+ in tetrahedral crystal field, whose intensities increase with processing temperature. X-ray photoelectron spectroscopy shows that the doped Co2+ ions in the 150 °C processed samples are located mostly on the surface of the particles and they disperse into the entire volume of the particles when processed at higher temperatures. The observations suggest that the FM results from Co2+ attached to the surface sites and it is lost in well dispersed samples formed at TP > 150 °C.
2006. "Measurements of Aerosol Size-Resolved Hygroscopicity at Sub and Supermicron Sizes." Geophysical Research Letters 33(21):L21808. doi:10.1029/2006GL026747 Abstract Airborne measurements of size-resolved aerosol hygroscopicity are presented using an optical particle counting and sizing technique. The measurement range of 0.25 to 3.5 μm is significantly greater, and extends to larger sizes, than previous in situ techniques. Preliminary results reveal a peak in aerosol hygroscopicity in the 0.5 – 1.5 μm diameter size range in both marine and polluted aerosols. Geometric growth factors range from 1.3 to 1.5 and 1.1 to 1.3 for the sub and super-micron particles, respectively.
2006. "Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage." Journal of Catalysis 238(1):153-164. doi:10.1016/j.jcat.2005.12.004 Abstract The selectivity of trimethyl acetate (TMA) photodecomposition on TiO2(110) as a function of O2 pressure and TMA coverage was probed at room temperature (RT) using isothermal mass spectrometry (ISOMS) and scanning tunneling microscopy (STM). The selectivity of TMA photodecomposition on TiO2(110) is sensitive to the initial TMA coverage and the O2 pressure. TMA bridge bonds to the surface via the carboxylate end of the molecule in a manner consistent with the binding of other carboxylate species (e.g., formate and acetate) on TiO2 surfaces. Under all conditions, photodecomposition of TMA was initiated via hole reaction with the electron in carboxylate’s system resulting in opening of the O-C-O bond angle, and formation of CO2 and a t-butyl radical by cleavage of the C-C bond between these groups. The CO2 product desorbs from the surface at RT, but the t-butyl radical has several options for thermal chemistry. In ultrahigh vacuum (UHV), where the O2 partial pressure is <1x10-10 torr, the TMA photodecomposition results in a near 1:1 yield of isobutene (i-C4H8) and isobutane (i-C4H10) from surface chemistry of the t-butyl radicals. STM results show that the reaction occurs fairly homogeneously across the TiO2(110) surface. In the presence of O2, the photodecomposition selectivity switches from initially i-C4H8 to a mixture of i-C4H8 and i-C4H10 and then back to predominately i-C4H8. The latter selectivity change occurs at the point at which void regions form and grow in the TMA overlayer. At this point, the photodecomposition rate accelerates and the reaction occurs preferentially at the interface between the TMA-rich and TMA-void regions on the surface. These results illustrate both the changing dynamics of a typical photooxidation reaction on TiO2, and how factors such as O2 pressure and TMA coverage, impact the photooxidation reaction selectivity. We also present results that suggest the rate of photodecomposition of monodentate carboxylates is greater than that of bidentate (bridging) carboxylates. This implies that the structural arrangement of Ti cation sites on the surface is an important issue that influences photocatalytic rates on TiO2.
2006. "Synthesis of Nanodispersed Oxides of Vanadium, Titanium, Molybdenum, and Tungsten on Mesoporous Silica using Atomic Layer Deposition." Topics in Catalysis 39(3-4):245-255. doi:10.1007/s11244-006-0063-0 Abstract The advantages of the atomic layer deposition (ALD) method for preparation of tungsten, vanadium, titanium and molybdenum oxide catalyst supported on mesoporous silica are discussed, with emphasis on the importance of synthesis conditions on dispersion, structure and activity of the resulting materials. A suite of complementary techniques such as DRS-UV/Vis, BET, 1H-NMR, XRD, and TEM were used to study the structural properties of the supported metal oxides, and probe reactions such as 2-butanol dehydration and ethanol partial oxidation were used to demonstrate the potential advantages of the ALD prepared catalysts. Specifically, highly dispersed oxides of titanium, molybdenum, and tungsten oxide on mesoporous silica were synthesized using the ALD method. It is also demonstrated that attainment of high dispersions of vanadium oxide on mesoporous silica requires the presence of at least a single layer of titanium oxide due to the well-known poor interaction between vanadia and silica. The highly dispersed catalysts prepared here by ALD methods exhibited superior catalytic performance relative to those prepared using conventional incipient wetness impregnation.
2006. "Synthesis, Characterization, and Catalytic Function of Novel Highly Dispersed Tungsten Oxide Catalysts on Mesoporous Silica ." Journal of Catalysis 239(1):200-211. Abstract The physical and chemical properties of tungsten oxide supported on SBA-15 mesoporous silica prepared by a controlled grafting process through atomic layer deposition (ALD) were studied using complementary characterization methods. X-ray diffraction, optical absorption and transmission electron microscopy showed that tungsten oxide species are highly dispersed on SBA-15 surfaces, even at 30 wt.% WOx contents (surface density: 1.33 WOx/nm2). ALD methods led to samples with much better thermal stability than those prepared via impregnation. Dehydration reactions of 2-butanol and methanol dehydration were used as probe reactions. Differences in reaction rates between the samples prepared by ALD and conventional impregnation reflect the sintering resistance of catalysts prepared by ALD. Notably, temperature programmed oxidation of spent catalysts showed that carbon formation was not responsible for the different dehydration rates in samples prepared by ALD and impregnation..
2006. "Biomarker Candidate Identification in Yersinia Pestis Using Organism-Wide Semiquantitative Proteomics ." Journal of Proteome Research 5(11):3008-3017. Abstract Yersinia pestis, the causative agent of plague, is listed by the CDC as a level A select pathogen. To better enable detection, intervention and treatment of Y. pestis infections, it is necessary to understand its protein expression under conditions that promote or inhibit virulence. To this end, we have utilized a novel combination of the accurate mass and time tag methodology of mass spectrometry and clustering analysis using OmniViz™ to compare the protein abundance changes of 992 identified proteins under four growth conditions. Temperature and Ca2+ concentration were used to trigger virulence associated protein expression fundamental to the low calcium response. High-resolution liquid chromatography and electrospray ionization mass spectrometry were utilized to determine protein identity and abundance on the genome-wide level. The cluster analyses revealed, in a rapid visual platform, the reproducibility of the current method as well as relevant protein abundance changes of expected and novel proteins relating to a specific growth condition and sub-cellular location. Using this method, 89 proteins were identified as having a similar abundance change profile to 29 known virulence associated proteins, providing additional biomarker candidates for future detection and vaccine development strategies.
2006. "Mode-Locked Yb:KGW Laser Longitudinally Pumped by Polarization-Coupled Diode Bars." Optics Letters 31(18):2719-2721. Abstract Mode-locked (ML) operation of a simple Yb:KGW oscillator is described, providing 12 W at 1038 nm with 290 fs pulse width. A polarization-coupled scheme is used for efficient longitudinal pumping by a pair of reshaped laser diode bars. With changes in cavity dispersion, the pulse width is adjustable from 134 to 433 fs, in a high quality circular mode. A Saturable Absorber Mirror (SAM) provides self-starting operation, and the cavity is stabilized by the Kerr-lens effect.
2006. "Chromium Phase Behavior in a Multi-Component Borosilicate Glass Melt." Journal of Non-crystalline Solids 352(21-22):2114-2122. Abstract This paper reports the phase behavior of a multicomponent borosilicate glass melt with 03 mass% Cr2O3 at 8001500°C in equilibrium with air. Both upper and lower liquidus temperatures were observed. When the temperature was between the upper and lower liquidus temperatures, eskolaite (Cr2O3) formed in melts with >2 mass% Cr2O3. Below the lower liquidus temperature, a dispersed chromate phase appeared in the melt that eventually became macroscopically segregated. The chemical durability of the glasses was virtually unaffected by chromium concentration. The particular glass studied was prototypic for the vitrification of high-Cr high-level radioactive wastes stored in underground tanks at the Hanford site. The results suggest a significant potential cost benefit for Hanford tank waste cleanup.
2006. "H-1 Relaxation Times of Metabolites in Biological Samples Obtained with Nondestructive Ex-vivo Slow-MAS NMR ." Magnetic Resonance in Chemistry 44(3):269-275. doi:10.1002/mrc.1764 Abstract Methods suitable for measuring 1H relaxation times such as T1, T2 and T1p, in small sized biological objects including live cells, excised organs and tissues, oil seeds etc., were developed in this work. This was achieved by combining inversion-recovery, spin-echo, or spin lock segment with the phase-adjusted spinning sideband (PASS) technique that was applied at slow sample spinning rate. Here, 2D-PASS was used to produce a high-resolution 1H spectrum free from the magnetic susceptibility broadening so that the relaxation parameters of individual metabolite can be determined. Because of the slow spinning employed, tissue and cell damage due to sample spinning is minimized. The methodologies were demonstrated by measuring 1H T1, T2 and T1p of metabolites in excised rat livers and sesame seeds at spinning rates of as low as 40 Hz.
2006. "On The Structure and Chemical Bonding of Tri-Tungsten Oxide Clusters W3On- and W3On (n=7-10): W3O8 As A Potential Molecular Model for O-Deficient Defect Sits in Tungsten Oxides." Journal of Physical Chemistry A 110(1):85-92. doi:10.1021/jp055325m Abstract Electronic and structural properties of a series of tri-tungsten oxide clusters, W3On- and W3On (n = 7-10), are investigated using photoelectron spectroscopy and density functional theory (DFT) calculations. Both W 5d and O 2p detachment features are observed for n = 7-9, whereas only detachment features from O 2p type orbitals are observed for W3O10- at high electron binding energies (>7 eV). A HOMO-LUMO gap (~3.4 eV) is observed for the stoichiometric W3O9 cluster, which already reaches the bulk value, suggesting that W3O9 can be viewed as the smallest molecular model for bulk WO3. DFT calculations are carried out to locate the most stable structures for both the anion and neutral clusters; time-dependent DFT method is used to predict the vertical detachment energies and to compare with the experimental data. It is shown that W3O9 possesses a D3h structure, in which each W atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W3O8 and W3O7 can be viewed as removing one and two terminal O atoms from W3O9, respectively, whereas W3O10 can be viewed as replacing a terminal O in W3O9 by a peroxo O2 unit. We show that W3O8 contain a localized W4+ site, which can readily react with O2 to form the W3O10 clusters with a calculated O2 adsorption energy of -78 kcal/mol. It is suggested that the W3O8 cluster can be viewed as a molecular model for O-deficient site in tungsten oxides.
2006. "Experimental and Theoretical Characterization of Superoxide Complexes [W₂O₆(O₂⁻)] and [W₃O₉₋(O₂⁻)]: Models for the Interaction of O₂ with Reduced W Sites on Tungsten Oxide Surfaces." Angewandte Chemie International Edition 45(4):657-660. doi:10.1002/anie.200503652 Abstract Two O-rich tungsten oxide clusters, W2O8- and W3O11-, were produced and investigated by photoelectron spectroscopy and density functional theory calculations. The two anions are best considered as W2O6(O2-) and W3O9(O2-), respectively, each containing a side-on bound superoxide ligand, whereas the neutral clusters W2O8 and W3O11 are shown to involve O2 physisorbed to the W2O6 or W3O9 stoichiometric cluster. The current study indicates that the extra electron in W2O6- and W3O9- are capable of activating dioxygen by non-dissociative electron transfer (W 5d - O2 -*), and the two anionic clusters can be viewed as models for reduced defect sites on tungsten oxide surfaces for the chemisorption of O2.
2006. "Structural Investigations of Silica Polyamine Composites: Surface Coverage, Metal Ion Coordination, and Ligand Modification." Industrial and Engineering Chemistry Research 45(19):6538-6547. doi:10.1021/ie0601448 Abstract Silanization of the silica gel surface in the synthesis of silica gel polyamine composites uses (chloropropyl)-trichlorosilane (CPTCS). It is possible to substitute a molar fraction of reagent CPTCS with methyltrichlorosilane (MTCS), creating a mixed silane surface layer. Two types of silica gels were modified with a seriesof MTCS:CPTCS molar ratios. Solid-state CP/MAS 29Si and 13C NMR spectroscopies were used to evaluate the surface silane composition. Surface silane coverage was markedly improved for the resulting gels. When polyamines were grafted to the resultant MTCS:CPTCS silane layers, it was shown that the decrease in the number of propyl attachments to the polyamine resulted in increased quantities of “free amines”. Optimum MTCS:CPTCS ratios were determined for three polyamines grafted onto one silica gel. A substantial free amine increase was observed for poly(allylamine) (PAA). Metal uptake studies show increases in Cu(II) capacity and/or an improvement in Cu(II) mass-transfer kinetics. The effect of polymer molecular weight upon Cu(II) capacity was investigated for each polyamine. Substantial differences in Cu(II) capacity between 50 000 MW poly(vinylamine) (PVA) and >1000 MW PVA were evident. Similar differences between 25 000 MW poly(ethyleneimine) (PEI) and 1200 MW PEI were found. The mass-transfer kinetics was shown to be improved for composites prepared using a large fraction of MTCS in the reagent silane mixture. This resulted in substantial improvements in the 10% breakthrough Cu(II) capacity for PVA (50 000 MW). PEI composites were further modified to form an amino-acetate ligand. The impact of the MTCS:CPTCS silane ratio on the acetate ligand loading and ultimately on the Cu(II) capacity at pH ) 2 was investigated. A ratio of 12.5:1 was shown to result in an acetate modified PEI composite with a Cu(II) capacity 140% of the Cu(II) capacity of the same composite prepared with “CPTCS only”.
2006. "Silica Polyamine Composites: New Supramolecular Materials for Cation and Anion Recovery and Remediation." Macromolecular Symposia 235(1):161-178. doi:10.1002/masy.200650320 Abstract The surface coverage of amorphous silica gels used in the synthesis of silica polyamine composites has been investigated by 29Si NMR. By diluting the polyamine anchor silane, chloropropyl trichlorosilane, with methyl trichlorosilane it was found that surface coverage could be markedly improved for a range of amine polymers after grafting to the silica surface. The commensurate decrease in the number of anchor points and increase in the number of free amines results in an increase in metal capacity and/or an improvement in capture kinetics. Solid state CPMAS-13C NMR has been employed to investigate the structure and metal ion binding of a series of these composite materials. It is reported that the highly branched polymer, poly(ethyleneimine) (PEI) exhibits much broader 13C NMR resonances than the linear polymers poly(allylamine) (PAA) and poly(vinylamine) (PVA). These results are understood in terms of the low energy conformations calculated from molecular modeling studies. Three new applications of the technology are also presented: 1) separation of lanthanides as a group from ferric ion and all other divalent ions; 2) a multi step process for recovering and concentrating the valuable metals in acid mine drainage; 3) a process for removing low level arsenic and selenium in the presence of sulfate using immobilized cations on the composite materials.
