Electron Spectrometer: HREELS, UHV Surface Chemistry Publications

2008

Eggleston CM, J Voros, L Shi, BH Lower, TC Droubay, and PJ Colberg. 2008. "Binding and Direct Electrochemistry of OmcA, an Outer-Membrane Cytochrome from an Iron Reducing Bacterium, with Oxide Electrodes: A Candidate Biofuel Cell System." Inorganica Chimica Acta 361(3):769-777. doi:10.1016/j.ica.2007.07.015 Abstract Dissimilatory iron-reducing bacteria transfer electrons to solid ferric respiratory electron acceptors. Outer-membrane cytochromes expressed by these organisms are of interest in both microbial fuel cells and biofuel cells. We use optical waveguide lightmode spectroscopy (OWLS) to show that OmcA, an 85 kDa decaheme outer-membrane c-type cytochrome from Shewanella oneidensis MR-1, adsorbs to isostructural Al2O3 and Fe2O3 in similar amounts. Adsorption is ionic-strength and pH dependent (peak adsorption at pH 6.5–7.0). The thickness of the OmcA layer on Al2O3 at pH 7.0 [5.8 ± 1.1 (2r) nm] from OWLS is similar, within error, to that observed using atomic force microscopy (4.8 ± 2 nm). The highest adsorption density observed was 334 ng cm 2 (2.4 · 1012 molecules cm 2), corresponding to a monolayer or 9.9 nm diameter spheres or submonolayer coverage by smaller molecules. Direct electrochemistry of OmcA on Fe2O3 electrodes was observed using cyclic voltammetry, with cathodic peak potentials of 380 to 320 mV versus Ag/AgCl. Variations in the cathodic peak positions are speculatively attributed to redox-linked conformation change or changes in molecular orientation. OmcA can exchange electrons with ITO electrodes at higher current densities than with Fe2O3. Overall, OmcA can bind to and exchange electrons with several oxides, and thus its utility in fuel cells is not restricted to Fe2O3.
Ozensoy E, DR Herling, and J Szanyi. 2008. "NOx Reduction on a Transition Metal-free ?-Al2O3 Catalyst Using Dimethylether (DME)." Catalysis Today 136(1-2):46-54. Abstract NO2 and dimethylether (DME) adsorption as well as DME and NO2 coadsorption on a transition metal-free γ-alumina catalyst were investigated via in-situ transmission Fourier transform infrared spectroscopy (in-situ FTIR), residual gas analysis (RGA) and temperature programmed desorption (TPD) techniques. NO2 adsorption at room temperature leads to the formation of surface nitrates and nitrites. DME adsorption on the alumina surface at 300 K leads to molecularly adsorbed DME, molecularly adsorbed methanol and surface methoxides. Upon heating the DME-exposed alumina to 500-600 K the surface is dominated by methoxide groups. At higher temperatures methoxide groups are converted into formates. At T > 510 K formate decomposition takes place to form H2O(g) and CO(g). DME and NO2 coadsorption at 423 K do not indicate a significant reaction between DME and NO2. However, in similar experiments at 573 K, fast reaction occurs and the methoxides present at 573 K before the NO2 adsorption are converted into formates, simultaneously with the formation of isocyanates. Under these conditions, NCO can further be hydrolyzed into isocyanic acid or ammonia with the help of water which is generated during the formate formation, decomposition and/or NCO formation steps.
Du Y, Z Dohnalek, and I Lyubinetsky. 2008. "Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2 (110)." Journal of Physical Chemistry C 112(7):2649-2653. doi:10.1021/jp077677u Abstract Tracking the same region of the reduced TiO2 (110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. Majority (~81%) of O adatoms are found separated from the original vacancy positions, by up to two lattice constants along [001] direction. Since at room temperature the thermal diffusion of O adatoms has been found to be rather small, with experimentally estimated activation energy of ~1.1 eV, we conclude that observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility in the course of O2 dissociation. Unlike for other known cases of the dissociation of the diatomic molecules where both “hot” adatoms accommodate at the equivalent sites, in the studied system the oxygen atoms filling the vacancies are locked into the bridging oxygen rows and only the O adatoms are relatively free to move. The transient motion of the hyperthermal oxygen adatoms on the TiO2 (110) surface occurs exclusively along the Ti troughs.
Henderson MA. 2008. "Effect of Coadsorbed Water on the Photodecomposition of Acetone on TiO2(110)." Journal of Catalysis 256(2):287-292. doi:10.1016/j.jcat.2008.03.020 Abstract The influence of coadsorbed water on the photodecomposition of acetone on TiO2 was examined using temperature programmed desorption (TPD) and the rutile TiO2(110) surface as a model photocatalyst. Of the two major influences ascribed to water in the heterogeneous photocatalysis literature (promotion via OH radical supply and inhibition due to site blocking), only the negative influence of water was observed. As long as the total water and acetone coverage was maintained well below the first layer saturation coverage (‘1 ML’), little inhibition of acetone photodecomposition was observed. However, as the total water+acetone coverage exceeded 1 ML, acetone was preferentially displaced from the first layer to physisorbed states by water and the extent of acetone photodecomposition attenuated. The displacement originated from water compressing acetone into high coverage regions where increased acetone-acetone repulsions caused displacement from the first layer. The immediate product of acetone photodecomposition was adsorbed acetate, which occupies twice as many surface sites per molecule as compared to acetone. Since the acetate intermediate was more stable on the TiO2(110) surface than either water or acetone (as gauged by TPD) and since its photodecomposition rate was less than that of acetone, additional surface sites were not opened up during acetone photodecomposition for previously displaced acetone molecules to re-enter the first layer. Results in this study suggest that increased molecular-level repulsions between organic molecules brought about by increased water coverage are as influential in the inhibiting effect of water on photooxidation rates as are water-organic repulsions.
Epling WS, I Nova, and CHF Peden. 2008. "Preface." Catalysis Today 136(1-2):1-2. doi:10.1016/j.cattod.2008.03.008 Abstract This manuscript is a preface to a special issue of the journal, Catalysis Today, on catalytic diesel emission control edited by the preface authors.
Chorover J, S Choi, P Rotenberg, RJ Serne, N Rivera, CR Strepka, A Thompson, KT Mueller, and PA O'Day. 2008. "Silicon Control of Strontium and Cesium Partitioning in Hydroxide-Weathered Sediments." Geochimica et Cosmochimica Acta 72(8):2024-2047. Abstract Cation partitioning in an aqueous soil suspension depends on the coupling of reaction time, sorbate amount and mineral weathering reactions. These factors were varied in sediment suspension experiments to identify geochemical processes that affect migration of Sr2+ and Cs+ introduced to the subsurface by caustic high level radioactive waste (HLRW). Three glacio-fluvial and lacustrine sediments from the Hanford Site were subjected to hyperalkaline (pH > 13), Na–Al–NO3–OH solution conditions within a gradient field of (i) sorptive concentration (10-5–10-3 M) and (ii) reaction time (0–365 d). Strontium uptake (qSr) exceeded that of cesium at nearly all reaction times. Sorbent affinity for both Cs+ and Sr2+ increased with clay plus silt content at early times, but a prolonged slow uptake process was observed over the course of sediment weathering that erased the texture effect for Sr2+; all sediments showed similar mass normalized uptake after several months of reaction time. Strontium became progressively recalcitrant to desorption after 92 d, with accumulation and aging of neoformed aluminosilicates. Formation of Cs+ and Sr2+-containing cancrinite and sodalite was observed after 183 d by SEM and synchrotron u-XRF and u-XRD. EXAFS data showed ncorporation of Sr2+ into both feldspathoid and SrCO3(s) coordination environments after one year. Adsorption was predominant at early times and low sorbate amount, whereas recipitation, controlled largely by sediment Si release, became increasingly important at longer times and higher sorbate amount. Kinetics of contaminant desorption at pH 8 from one year-weathered sediments showed significant dependence on background cation (Ca2+ versus K+) composition. Results of this study indicate that co-precipitation and ion exchange in neoformed aluminosilicates may be an important mechanism controlling Sr2+ and Cs+ mobility in siliceous sediments impacted by hyperalkaline HLRW.

2007

Droubay TC, KM Rosso, SM Heald, DE Mccready, CM Wang, and SA Chambers. 2007. "Structure, Magnetism and Conductivity in Epitaxial Ti-doped ?-Fe2O3 Hematite: Experiment and density functional theory calculations." Physical Review. B, Condensed Matter and Materials Physics 75(10):, doi:10.1103/PhysRevB.75.104412 Abstract We explore the feasibility of growing epitaxial Ti-doped -Fe2O3 in which Ti(IV) substitutes for Fe(III) preferentially in one magnetic sublattice, but not the other. Such a structure has been predicted by first-principles theory to be energetically likely, and is expected to yield interesting and useful magnetic and electronic properties. However, we find that a majority of Ti dopants disperse and occupy random cation sites in both magnetic sublattices. Density functional theory predicts that the magnetically ordered and magnetically random structures are nearly isoenergetic.
Lower BH, L Shi, R Yongsunthon, TC Droubay, DE Mccready, and SK Lower. 2007. "Specific Bonds between an Iron Oxide Surface and Outer Membrane Cytochromes MtrC and OmcA from Shewanella oneidensis MR-1." Journal of Bacteriology 189(13):4944-4952. doi:10.1128/JB.01518-06 Abstract Shewanella oneidensis MR-1 is purported to express outer membrane cytochromes (e.g., MtrC and OmcA) that transfer electrons directly to Fe(III) in a mineral during anaerobic respiration. A prerequisite for this type of reaction would be the formation of a stable bond between a cytochrome and an iron oxide surface. Atomic force microscopy (AFM) was used to detect whether a specific bond forms between a hematite (Fe2O3) thin film, created with oxygen plasma assisted molecular beam epitaxy (MBE), and recombinant MtrC or OmcA molecules coupled to gold substrates. Force spectra displayed a unique force signature indicative of a specific bond between each cytochrome and the hematite surface. The strength of the OmcA-hematite bond was approximately twice as strong as the MtrC-hematite bond, but direct binding to hematite was twice as favorable for MtrC. Reversible folding/unfolding reactions were observed for mechanically denatured MtrC molecules bound to hematite. The force measurements for the hematite-cytochrome pairs were compared to spectra collected between an iron oxide and S. oneidensis under anaerobic conditions. There is a strong correlation between the whole cell and pure protein force spectra suggesting that the unique binding attributes of each cytochrome complement one another and allow both MtrC and OmcA to play a prominent role in the transfer of electrons to Fe(III) in minerals. Finally, by comparing the magnitude of binding force for the whole cell vs. pure protein data, we were able to estimate that a single bacterium of S. oneidensis (2 x 0.5 μm) expresses ~104 cytochromes on its outer surface.
Xiong G, R Shao, TC Droubay, AG Joly, KM Beck, SA Chambers, and WP Hess. 2007. "Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals." Advanced Functional Materials 17(13):2133-2138. doi:10.1002/adfm.200700146 Abstract Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.
Bondarchuk O, and I Lyubinetsky. 2007. "Preparation of TiO2(110)-(1x1) Surface via UHV Cleavage: An scanning tunneling microscopy study." Review of Scientific Instruments 78(11):Art. No. 113907. doi:10.1063/1.2814160 Abstract TiO2(110) surface was successfully prepared in-situ by UHV cleaving of a commercial TiO2 crystal.. STM imaging revealed atomically flat more than 1 m wide terraces with (110) orientation separated by steps running in [001] direction, with very low kink density. Atomically resolved STM images show periodicity in the [001] and [ ] directions with the unit cell parameters measured to ~3 Å and 6.5 Å respectively which are closed to the expected values of bulk terminated (1x1) surface.

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