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1.
  • Merte, Lindsay R., et al. (author)
  • Structure of the SnO2 (110)- (4×1) Surface
  • 2017
  • In: Physical Review Letters. - 0031-9007. ; 119:9
  • Journal article (peer-reviewed)abstract
    • Using surface x-ray diffraction (SXRD), quantitative low-energy electron diffraction (LEED), and density-functional theory (DFT) calculations, we have determined the structure of the (4×1) reconstruction formed by sputtering and annealing of the SnO2(110) surface. We find that the reconstruction consists of an ordered arrangement of Sn3O3 clusters bound atop the bulk-terminated SnO2(110) surface. The model was found by application of a DFT-based evolutionary algorithm with surface compositions based on SXRD, and shows excellent agreement with LEED and with previously published scanning tunneling microscopy measurements. The model proposed previously consisting of in-plane oxygen vacancies is thus shown to be incorrect, and our result suggests instead that Sn(II) species in interstitial positions are the more relevant features of reduced SnO2(110) surfaces.
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2.
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3.
  • Adams, Emma Catherine, et al. (author)
  • The structure-function relationship for alumina supported platinum during the formation of ammonia from nitrogen oxide and hydrogen in the presence of oxygen
  • 2016
  • In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 18:16, s. 10850-10855
  • Journal article (peer-reviewed)abstract
    • We study the structure-function relationship of alumina supported platinum during the formation of ammonia from nitrogen oxide and dihydrogen by employing in situ X-ray absorption and Fourier transform infrared spectroscopy. Particular focus has been directed towards the effect of oxygen on the reaction as a model system for emerging technologies for passive selective catalytic reduction of nitrogen oxides. The suppressed formation of ammonia observed as the feed becomes net-oxidizing is accompanied by a considerable increase in the oxidation state of platinum as well as the formation of surface nitrates and the loss of NH-containing surface species. In the presence of (excess) oxygen, the ammonia formation is proposed to be limited by weak interaction between nitrogen oxide and the oxidized platinum surface. This leads to a slow dissociation rate of nitrogen oxide and thus low abundance of the atomic nitrogen surface species that can react with the adsorbed hydrogen species. In this case the consumption of hydrogen through the competing water formation reaction and decomposition/oxidation of ammonia are of less importance for the net ammonia formation.
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4.
  • Adams, Emma, 1989, et al. (author)
  • Structure-function relationship for alumina supported platinum during formation of ammonia from nitrogen oxide and hydrogen in presence of oxygen
  • 2016
  • In: Physical Chemistry Chemical Physics. - 1463-9084 .- 1463-9076. ; 18:16, s. 10850-10855
  • Journal article (peer-reviewed)abstract
    • We study the structure-function relationship of alumina supported platinum during forma- tion of ammonia from nitrogen oxide and dihydrogen by employing in situ X-ray absorption and Fourier transformed infrared spectroscopy. Particular focus is directed towards the effect of increased levels of oxygen on the reaction as a model system for emerging technologies for passive selective catalytic reduction of nitrogen oxides. The suppressed formation of ammo- nia observed as the feed becomes net-oxidizing is accompanied by a considerable increase in the oxidation state of platinum as well as enhanced formation of surface nitrates and loss of NH-containing surface species. In the presence of (excess) oxygen, the ammonia formation is proposed to be limited by the weak interaction between nitrogen oxide and the oxidized platinum surface. This leads to slow dissociation rate of nitrogen oxide and thus low abun- dance of atomic nitrogen surface species that can react with adsorbed hydrogen atoms. In this case the consumption of hydrogen through the competing water formation reaction and decomposition/oxidation of ammonia are of less importance for the net ammonia formation.
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5.
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6.
  • Albertin, Stefano, et al. (author)
  • Oxidation and Reduction of Ir(100) Studied by High-Energy Surface X-ray Diffraction
  • 2022
  • In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:11, s. 5244-5255
  • Journal article (peer-reviewed)abstract
    • The oxidation and reduction of an Ir(100) surface using 2.5, 5, and 10 mbar O2 partial pressure and a sample temperature of 775 K have been studied by using high-energy surface X-ray diffraction (HESXRD) which allowed to record large volumes of reciprocal space in short time periods. The complex 3D diffraction patterns could be disentangled in a stepwise procedure. For the 2.5mbar experiment the measurements indicate the formation of an Ir(100)-O c(2 × 2) oxygen superstructure along with the onset of epitaxial IrO2(110) bulk oxide formation. For the 5 and 10 mbar O2 partial pressures the formation of additional IrO2 bulk oxide epitaxies with (100) and (101) orientations as well as of polycrystalline IrO2 was observed. Upon CO reduction, we found the IrO2 islands to be reduced into epitaxial and metallic Ir(111) and (221) oriented islands.
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7.
  • Albertin, S., et al. (author)
  • Surface optical reflectance combined with x-ray techniques during gas-surface interactions
  • 2020
  • In: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 53:22
  • Journal article (peer-reviewed)abstract
    • High energy surface x-ray diffraction (HESXRD), x-ray reflectivity (XRR), mass spectrometry (MS) and surface optical reflectance (SOR) have been combined to simultaneously obtain sub-second information on the surface structure and morphology from a Pd(100) model catalyst during in situ oxidation at elevated temperatures and pressures resulting in Pd bulk oxide formation. The results show a strong correlation between the HESXRD and SOR signal intensities during the experiment, enabling phase determination and a time-resolved thickness estimation of the oxide by HESXRD, complemented by XRR measurements. The experiments show a remarkable sensitivity of the SOR to changes in the surface phase and morphology, in particular to the initial stages of oxidation/reduction. The data imply that SOR can detect the formation of an ultrathin PdO surface oxide layer of only 2-3 angstrom thickness.
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8.
  • Arman, Alif, et al. (author)
  • Co3O4(100) films grown on Ag(100): Structure and chemical properties
  • 2017
  • In: Surface Science. - : Elsevier BV. - 0039-6028. ; 657, s. 90-95
  • Journal article (peer-reviewed)abstract
    • Spinel type Co3O4(100) is successfully grown on Ag(100) at ultrahigh vacuum conditions and its structure, electronic and chemical properties are compared with those of Co3O4(111) grown on Ir(100). We find that the Co3O4(100) is unreconstructed. In contrast to the defect free Co3O4(111) surface the Co3O4(100) surface contains a high concentration of defects that we assign to subsurface cation vacancies analogous to those observed for Fe3O4(100). Our photoemission and absorption spectroscopy experiments reveal a very similar electronic structure of the Co3O4(111) and Co3O4(100) surfaces. The similar electronic structure of the two surfaces is reflected in the CO adsorption properties at low temperatures, as we observe adsorption of molecular CO as well as the formation of carbonate (CO3) species on both surfaces upon CO exposure at 85 K.
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9.
  • Blomberg, Sara, et al. (author)
  • Strain Dependent Light-off Temperature in Catalysis Revealed by Planar Laser-Induced Fluorescence
  • 2017
  • In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 7:1, s. 110-114
  • Journal article (peer-reviewed)abstract
    • Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO2 production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.
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10.
  • Busch, Michael, 1983, et al. (author)
  • Adsorption of NO on Fe3O4(111)
  • 2018
  • In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 693, s. 84-87
  • Journal article (peer-reviewed)abstract
    • Adsorption of NO on Fe 3 O 4 (111) is studied by density functional theory (DFT) calculations. NO is preferably adsorbed atop the octahedral site which has a clearly higher adsorption energy than the tetrahedral site. The difference in adsorption energy correlates with differences in adsorption geometries and N–O stretch vibrations. The results are in good agreement with temperature programmed desorption (TPD) and reflection-adsorption IR spectroscopy (RAIRS) measurements and provide an explanation of the observation of only one vibrational mode despite two distinct TPD features.
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11.
  • Evertsson, Jonas, et al. (author)
  • The thickness of native oxides on aluminum alloys and single crystals
  • 2015
  • In: Applied Surface Science. - : Elsevier BV. - 1873-5584 .- 0169-4332. ; 349, s. 826-832
  • Journal article (peer-reviewed)abstract
    • We present results from measurements of the native oxide film thickness on four different industrial aluminum alloys and three different aluminum single crystals. The thicknesses were determined using X-ray reflectivity, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. In addition, atomic force microscopy was used for micro-structural studies of the oxide surfaces. The reflectivity measurements were performed in ultra-high vacuum, vacuum, ambient, nitrogen and liquid water conditions. The results obtained using X-ray reflectivity and X-ray photoelectron spectroscopy demonstrate good agreement. However, the oxide thicknesses determined from the electrochemical impedance spectroscopy show a larger discrepancy from the above two methods. In the present contribution the reasons for this discrepancy are discussed. We also address the effect of the substrate type and the presence of water on the resultant oxide thickness.
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12.
  • Gajdek, Dorotea, et al. (author)
  • Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O 2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy
  • 2022
  • In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:7, s. 3411-3418
  • Journal article (peer-reviewed)abstract
    • We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O2, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co3O4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.
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13.
  • Gajdek, Dorotea, et al. (author)
  • Sulfidation of Supported Ni, Mo and NiMo Catalysts Studied by In Situ XAFS
  • 2023
  • In: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 66:17-18, s. 1287-1295
  • Journal article (peer-reviewed)abstract
    • Active sites in Mo-based hydrotreating catalysts are produced by sulfidation. To achieve insights that may enable optimization of the catalysts, this process should be studied in situ. Herein we present a comparative XAFS study where the in situ sulfidation of Mo/δ-Al2O3 and Ni/δ-Al2O3 is compared to that of δ-Al2O3 supported NiMo catalysts with different NiMo ratios. The study also covers the comparison of sulfidation of Ni and Mo using different oxide supports as well as the sulfidation conditions applied in the reactor. The XAFS spectra confirms the oxide phase for all catalysts at the beginning of the sulfidation reaction and their conversion to a sulfidized phase is followed with in situ measurements. Furthermore, it is found that the monometallic catalysts are less readily sulfidized than bimetallic ones, indicating the importance of Ni-Mo interactions for catalyst activation. Mo K-edge XAFS spectra did not show any difference related to the support of the catalyst or the pressure applied during the reaction. Ni K-edge XAFS spectra, however, show a more complete sulfidation of the Ni species in the catalyst when SiO2 is used as a support as compared to the Al2O3. Nevertheless, it is believed that stronger interactions with Al2O3 support prevent sintering of the catalyst which leads to its stabilization. The results contribute to a better understanding of how different parameters affect the formation of the active phase of the NiMo catalysts used in the production of biofuel.
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14.
  • Garcia-Martinez, Fernando, et al. (author)
  • Catalytic oxidation of CO on a curved Pt(111) surface : simultaneous ignition at all facets through a transient CO-O complex.
  • 2020
  • In: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 59:45, s. 20037-20043
  • Journal article (peer-reviewed)abstract
    • The catalytic oxidation of carbon monoxide (CO) on transition metals, such as platinum (Pt), is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with oxygen (O). However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. The systematic analysis of carbon and oxygen core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces, as indicated by ab-initio theory: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.
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15.
  • Garcia-Martinez, Fernando, et al. (author)
  • CO Chemisorption on Vicinal Rh(111) Surfaces Studied with a Curved Crystal
  • 2020
  • In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:17, s. 9305-9313
  • Journal article (peer-reviewed)abstract
    • Curved crystal surfaces enable the systematic and accurate comparison of physical and chemical processes for a full set of vicinal crystal planes, which are probed in the very same environment. Here, we examine the early stages of the CO chemisorption on vicinal Rh(111) surfaces using a curved Rh crystal that exposes a smoothly variable density of {100} (A-type) and {111} (B-type) steps. We readily identify and quanti step and terrace species by resolving their respective core-level lines using X-ray photoelectron spectroscopy at different locations on the curved surface. Uptake experiments show similar sticking probabilities at all surface planes, subtle asymmetries between A- and B-type steps, and significantly lower saturation coverage at densely stepped surfaces as compared to the (111) plane. The analysis of the C is intensity variation across the curved sample allows us to discuss the adsorption geometry around the step edge.
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16.
  • Garcia-Martinez, Fernando, et al. (author)
  • Reduced Carbon Monoxide Saturation Coverage on Vicinal Palladium Surfaces: The Importance of the Adsorption Site
  • 2021
  • In: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:39, s. 9508-9515
  • Journal article (peer-reviewed)abstract
    • Steps at metal surfaces may influence energetics and kinetics of catalytic reactions in unexpected ways. Here, we report a significant reduction of the CO saturation coverage in Pd vicinal surfaces, which in turn is relevant for the light-off of the CO oxidation reaction. The study is based on a systematic investigation of CO adsorption on vicinal Pd(111) surfaces making use of a curved Pd crystal. A combined X-ray Photoelectron Spectroscopy and DFT analysis allows us to demonstrate that an entire row of atomic sites under Pd steps remains free of CO upon saturation at 300 K, leading to a step-density-dependent reduction of CO coverage that correlates with the observed decrease of the light-off temperature during CO oxidation in vicinal Pd surfaces.
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17.
  • Gericke, Sabrina M., et al. (author)
  • Effect of Different In2O3(111) Surface Terminations on CO2 Adsorption
  • 2023
  • In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 15:38, s. 45367-45377
  • Journal article (peer-reviewed)abstract
    • In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.
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18.
  • Gericke, Sabrina Maria, et al. (author)
  • In Situ H2 Reduction of Al2O3-Supported Ni- and Mo-Based Catalysts
  • 2022
  • In: Catalysts. - : MDPI. - 2073-4344. ; 12:7
  • Journal article (peer-reviewed)abstract
    • Nickel (Ni)-promoted Molybdenum (Mo)-based catalysts are used for hydrotreatment processes in the chemical industry where the catalysts are exposed to high-pressure H2 at elevated temperature. In this environment, the catalyst transforms into the active phase, which involves the reduction of the oxide. Here, we report on the first in situ study on the reduction of alumina supported Ni- and Mo-based catalysts in 1 mbar H2 using ambient-pressure X-ray photoelectron spectroscopy (APXPS). The study confirms that mixing Ni and Mo lowers the reduction temperature of both Ni- and Mo-oxide as compared to the monometallic catalysts and shows that the MoO3 reduction starts at a lower temperature than the reduction of NiO in NiMo/Al2O3 catalysts. Additionally, the reduction of Ni and Mo foil was directly compared to the reduction of the Al2O3-supported catalysts and it was observed that the reduction of the supported catalysts is more gradual than the reduction of the foils, indicating a strong interaction between the Ni/Mo and the alumina support. © 2022 by the authors.
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19.
  • Gustafson, Johan, et al. (author)
  • The Role of Oxides in Catalytic CO Oxidation over Rhodium and Palladium
  • 2018
  • In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 8:5, s. 4438-4445
  • Journal article (peer-reviewed)abstract
    • Catalytic CO oxidation is a seemingly simple reaction between CO and O2 molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen-covered metallic surface is more active than the oxide, while for Pd, thin oxidefilms are at least as active as the metallic surface, but a thicker oxide is less active. Apart from resolving a long-standing debate, our results pinpoint important design principles for oxidation catalysts as to prevent catalytic extinction at high oxygen exposures.
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21.
  • Hagman, Benjamin, et al. (author)
  • Steps Control the Dissociation of CO2 on Cu(100)
  • 2018
  • In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:40, s. 12974-12979
  • Journal article (peer-reviewed)abstract
    • CO2 reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO2 on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO2 dissociation provides an understanding that can be used in the design of future CO2 reduction catalysts.
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22.
  • Johansson, Niclas, et al. (author)
  • Oxidation of Ultrathin FeO(111) Grown on Pt(111) : Spectroscopic Evidence for Hydroxylation
  • 2016
  • In: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 59:5-7, s. 506-515
  • Journal article (peer-reviewed)abstract
    • Using high resolution and ambient pressure X-ray photoelectron spectroscopy we show that the catalytically active FeO $$-{2}$$ 2 trilayer films grown on Pt(111) are very active for water dissociation, in contrast to inert FeO(111) bilayer films. The FeO $$-{2}$$ 2 trilayer is so active for water dissociation that it becomes hydroxylated upon formation, regardless of the applied preparation method. FeO $$-{2}$$ 2 trilayers were grown by oxidation of FeO(111) bilayer films either with molecular oxygen in the mbar regime, or by NO $$-2$$ 2 and atomic oxygen exposures, respectively, in the ultrahigh vacuum regime. Because it was impossible to prepare clean FeO $$-{2}$$ 2 without any hydroxyls we propose that catalytically highly active FeO $$-{2}$$ 2 trilayer films are generally hydroxylated. In addition, we provide spectroscopic fingerprints both for Pt(111)-supported FeO(111) and FeO $$-2$$ 2 films that can serve as reference for future in situ studies.
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23.
  • Knudsen, Jan, et al. (author)
  • Low-Temperature CO Oxidation on Ni(111) and on a Au/Ni(111) Surface Alloy
  • 2010
  • In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 4:8, s. 4380-4387
  • Journal article (peer-reviewed)abstract
    • From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O-2-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO2 production.
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24.
  • Larsson, Alfred, et al. (author)
  • The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni–Cr–Mo Alloys
  • 2023
  • In: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 35:39
  • Journal article (peer-reviewed)abstract
    • Corrosion is the main factor limiting the lifetime of metallic materials, and a fundamental understanding of the governing mechanism and surface processes is difficult to achieve since the thin oxide films at the metal–liquid interface governing passivity are notoriously challenging to study. In this work, a combination of synchrotron-based techniques and electrochemical methods is used to investigate the passive film breakdown of a Ni–Cr–Mo alloy, which is used in many industrial applications. This alloy is found to be active toward oxygen evolution reaction (OER), and the OER onset coincides with the loss of passivity and severe metal dissolution. The OER mechanism involves the oxidation of Mo4+ sites in the oxide film to Mo6+ that can be dissolved, which results in passivity breakdown. This is fundamentally different from typical transpassive breakdown of Cr-containing alloys where Cr6+ is postulated to be dissolved at high anodic potentials, which is not observed here. At high current densities, OER also leads to acidification of the solution near the surface, further triggering metal dissolution. The OER plays an important role in the mechanism of passivity breakdown of Ni–Cr–Mo alloys due to their catalytic activity, and this effect needs to be considered when studying the corrosion of catalytically active alloys.
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25.
  • Larsson, Alfred, et al. (author)
  • Thickness and composition of native oxides and near-surface regions of Ni superalloys
  • 2022
  • In: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 895
  • Journal article (peer-reviewed)abstract
    • The surface chemistry and thickness of the native oxide, hydroxide, and modified sub-surface layer of three Ni superalloys (alloy 59, 625, and 718) were determined by synchrotron X-ray Photoelectron Spectroscopy (XPS) and X-ray Reflectivity (XRR). Taking advantage of the synchrotron radiation techniques, a procedure for normalizing the photoelectron intensity was employed, which allowed for accurate quantitative analysis revealing a total oxide thickness for all samples of 12-13 A, a hydroxide layer of 2-3 A, and a thickness of the sub-surface alloy layer of 20-35 A. The thickness results were compared to structural atomic models suggesting that the oxide thickness corresponds to four planes of metal cations in the oxide matrix. The XPS data revealed that the native oxides were enriched in Cr3+, Mo-(4,Mo-5,Mo-6)+, and Nb5+, while no Ni oxide was detected. The hydroxide layer mainly contained Ni2+ and Cr3+ hydroxide. The sub-surface layer was enriched in Ni and depleted in Cr, Fe, Mo, and Nb. The obtained oxide composition can be explained using thermodynamics, and it was found that the oxide composition correlates with the enthalpy of oxide formation for the metal elements in the alloys. Finally, the advantages of synchrotron radiation for composition and thickness determination are discussed.
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26.
  • Linpé, Weronica, et al. (author)
  • Revisiting Optical Reflectance from Au(111) Electrode Surfaces with Combined High-Energy Surface X-ray Diffraction
  • 2021
  • In: Journal of the Electrochemical Society. - : Electrochemical Society. - 0013-4651 .- 1945-7111. ; 168:9
  • Journal article (peer-reviewed)abstract
    • We have combined high-energy surface X-ray diffraction (HESXRD) with 2D surface optical reflectance (2D-SOR) to perform in situ electrochemical measurements of a Au(111) electrode in 0.1 M HClO4 electrolyte. We show that electrochemically induced changes to Au(111) surface during cyclic voltammetry can be simultaneously observed with 2D-SOR and HESXRD. We discuss how small one atom high 1x1 islands, accommodating excess atoms after the lifting of the surface reconstruction, can lead to discrepancies between the two techniques. The use of HESXRD allows us to simultaneously detect parts of the truncation rods from the (1 x 1) surface termination and the p x root 3 electrochemically induced surface reconstruction, during cyclic voltammetry. The presence of reconstruction phenomena is shown to not depend on having an ideally prepared surface and can in fact be observed after going to very oxidizing potentials. 2D-SOR can also detect the oxidation of the Au surface, however no oxide peaks are detected in the HESXRD signal, which is evidence that any Au oxide is X-ray amorphous.
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27.
  • Lundgren, Edvin, et al. (author)
  • Novel in Situ Techniques for Studies of Model Catalysts
  • 2017
  • In: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 50:9, s. 2326-2333
  • Journal article (peer-reviewed)abstract
    • ConspectusMotivated mainly by catalysis, gas-surface interaction between single crystal surfaces and molecules has been studied for decades. Most of these studies have been performed in well-controlled environments and have been instrumental for the present day understanding of catalysis, providing information on surface structures, adsorption sites, and adsorption and desorption energies relevant for catalysis. However, the approach has been criticized for being too far from a catalyst operating under industrial conditions at high temperatures and pressures. To this end, a significant amount of effort over the years has been used to develop methods to investigate catalysts at more realistic conditions under operating conditions. One result from this effort is a vivid and sometimes heated discussion concerning the active phase for the seemingly simple CO oxidation reaction over the Pt-group metals in the literature.In recent years, we have explored the possibilities to perform experiments at conditions closer to those of a technical catalyst, in particular at increased pressures and temperatures. In this contribution, results from catalytic CO oxidation over a Pd(100) single crystal surface using Near Ambient Pressure X-ray Photo emission Spectroscopy (NAPXPS), Planar Laser-Induced Fluorescence (PLIF), and High Energy Surface X-ray Diffraction (HESXRD) are presented, and the strengths and weaknesses of the experimental techniques are discussed. Armed with structural knowledge from ultrahigh vacuum experiments, the presence of adsorbed molecules and gas-phase induced surface structures can be identified and related to changes in the reactivity or to reaction induced gas-flow limitations. In particular, the application of PLIF to catalysis allows one to visualize how the catalyst itself changes the gas composition close to the model catalyst surface upon ignition, and relate this to the observed surface structures. The effect obscures a straightforward relation between the active phase and the activity, since in the case of CO oxidation, the gas-phase close to the model catalyst surface is shown to be significantly more oxidizing than far away from the catalyst. We show that surface structural knowledge from UHV experiments and the composition of the gas phase close to the catalyst surface are crucial to understand structure-function relationships at semirealistic conditions. In the particular case of Pd, we argue that the surface structure of the PdO(101) has a significant influence on the activity, due to the presence of Coordinatively Unsaturated Sites (CUS) Pd atoms, similar to undercoordinated Ru and Ir atoms found for RuO2(110) and IrO2(110), respectively.
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28.
  • Martin, Natalia M., et al. (author)
  • Atomic Layer Grown Zinc-Tin Oxide as an Alternative Buffer Layer for Cu2ZnSnS4-Based Thin Film Solar Cells : Influence of Absorber Surface Treatment on Buffer Layer Growth
  • 2022
  • In: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 5:11, s. 13971-13980
  • Journal article (peer-reviewed)abstract
    • Zn1-xSnxOy (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnOx and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-COx species from the CZTS surface, may be useful for the ZTO/CZTS device performance.
  •  
29.
  • Martin, Natalia Mihaela, 1984, et al. (author)
  • Structure-function relationship during CO2 methanation over Rh/Al2O3 and Rh/SiO2 catalysts at atmospheric pressure conditions
  • 2018
  • In: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:10, s. 2686-2696
  • Journal article (peer-reviewed)abstract
    • The effect of support material and chemical state of Rh for Rh/Al2O3 and Rh/SiO2 model catalysts during CO2 hydrogenation were studied by a combined array of in situ characterisation techniques including diffuse reflectance infrared Fourier transform spectroscopy, energy-dispersive X-ray absorption spectroscopy and high-energy X-ray diffraction at 250-350 °C and atmospheric pressure. The CO2 methanation proceeds via intermediate formation of adsorbed CO species on metallic Rh likely followed by their hydrogenation to methane. Linearly-bonded CO species is suggested to be a more active precursor in the hydrogenation compared to the bridge-bonded species, which seems to relate to particle size effects: for larger particles mainly the formation of inactive bridge-bonded CO species takes place. Further, analysis of the chemical state of Rh during reaction conditions reveal a minor formation of RhOx from dissociation of CO2 , which is a consequence of the increased activity observed over Rh/Al2O3 catalyst.
  •  
30.
  • Martin, Natalia M., et al. (author)
  • Structure-function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions
  • 2019
  • In: Catalysis Science & Technology. - : Royal Society of Chemistry. - 2044-4753 .- 2044-4761. ; 9:7, s. 1644-1653
  • Journal article (peer-reviewed)abstract
    • In situ structural and chemical state characterization of Rh/CeO2 and Ni/CeO2 catalysts during atmospheric pressure CO2 methanation has been performed by a combined array of time-resolved analytical techniques including ambient-pressure X-ray photoelectron spectroscopy, high-energy X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy. The ceria phase is partially reduced during the CO2 methanation and in particular Ce3+ species seem to facilitate activation of CO2 molecules. The activated CO2 molecules then react with atomic hydrogen provided from H-2 dissociation on Rh and Ni sites to form formate species. For the most active catalyst (Rh/CeO2), transmission electron microscopy measurements show that the Rh nanoparticles are small (average 4 nm, but with a long tail towards smaller particles) due to a strong interaction between Rh particles and the ceria phase. In contrast, larger nanoparticles were observed for the Ni/CeO2 catalyst (average 6 nm, with no crystallites below 5 nm found), suggesting a weaker interaction with the ceria phase. The higher selectivity towards methane of Rh/CeO2 is proposed to be due to the stronger metal-support interaction.
  •  
31.
  • Martin, R, et al. (author)
  • High-Resolution X-ray Photoelectron Spectroscopy of an IrO2(110) Film on Ir(100)
  • 2020
  • In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:17, s. 7184-7189
  • Journal article (peer-reviewed)abstract
    • High-resolution X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) were used to characterize IrO2(110) films on Ir(100) with stoichiometric as well as OH-rich terminations. Core-level Ir 4f and O 1s peaks were identified for the undercoordinated Ir and O atoms and bridging and on-top OH groups at the IrO2(110) surfaces. Peak assignments were validated by comparison of the core-level shifts determined experimentally with those computed using DFT, quantitative analysis of the concentrations of surface species, and the measured variation of the Ir 4f peak intensities with photoelectron kinetic energy. We show that exposure of the IrO2(110) surface to O2 near room temperature produces a large quantity of on-top OH groups because of reaction of background H2 with the surface. The peak assignments made in this study can serve as a foundation for future experiments designed to utilize XPS to uncover atomic-level details of the surface chemistry of IrO2(110).
  •  
32.
  • Martin, Rachel, et al. (author)
  • Isothermal Reduction of IrO2 (110) Films by Methane Investigated Using In Situ X-ray Photoelectron Spectroscopy
  • 2021
  • In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 11:9, s. 5004-5016
  • Journal article (peer-reviewed)abstract
    • Continuous exposure to methane causes IrO2 (110) films on Ir(100) to undergo extensive reduction at temperatures from 500 to 650 K. Measurements using in situ X-ray photoelectron spectroscopy (XPS) confirm that CH4 oxidation on IrO2 (110) converts so-called bridging oxygen atoms (O-br) at the surface to HObr groups while concurrently removing oxygen from the oxide film. Reduction of the IrO2 (110) film by methane is mildly activated as evidenced by an increase in the initial reduction rate as the temperature is increased from 500 to 650 K. The XPS results show that subsurface oxygen efficiently replaces O-br atoms at the IrO2 (110) surface during CH4 oxidation, even after the reduction of multiple layers of the oxide film, and that metallic Ir gradually forms at the surface as well. The isothermal rate of IrO2 (110) reduction by methane decreases continuously as metallic Ir replaces surface IrO2 (110) domains, demonstrating that IrO2 (110) is the active phase for CH4 oxidation under the conditions studied. A key finding is that the replacement of O-br atoms with oxygen from the subsurface is efficient enough to preserve IrO2 (110) domains at the surface and enable CH4 to reduce the similar to 10-layer IrO2 (110) films nearly to completion. In agreement with these observations, density functional theory calculations predict that oxygen atoms in the subsurface layer can replace O-br atoms at rates that are comparable to or higher than the rates at which O-br atoms are abstracted during CH4 oxidation. The efficacy with which oxygen in the bulk reservoir replenishes surface oxygen atoms has implications for understanding and modeling catalytic oxidation processes promoted by IrO2 (110).
  •  
33.
  • Martin, Rachel, et al. (author)
  • Methane oxidation on an IrO2(110) film
  • 2019
  • In: Abstracts of Papers of the American Chemical Society. - : American Chemical Society (ACS). - 0065-7727. ; 258
  • Journal article (other academic/artistic)
  •  
34.
  • Mehar, Vikram, et al. (author)
  • Adsorption of NO on FeOx films grown on Ag(111)
  • 2016
  • In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:17, s. 9282-9291
  • Journal article (peer-reviewed)abstract
    • We used temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on crystalline iron oxide films grown on Ag(111), including a Fe3O4(111) layer, an FeO(111) monolayer, and an intermediate FeOx multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeOx surfaces, and provides evidence that NO binds more strongly on Fe2+ sites than Fe3+ sites. The NO TPD spectra obtained from the Fe3O4(111) layer exhibit a dominant peak at 380 K, attributed to NO bound on Fe2+ sites, as well as a broad feature centered at ∼250 K that is consistent with NO bound on Fe3+ sites of Fe3O4(111) as well as NO adsorbed on a minority FeO structure. The NO TPD spectra obtained from the monolayer FeO(111) film exhibits a prominent peak at 269 K. After growing FeOx multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at ∼200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur because the multilayer FeOx islands expose Fe3+ sites that bind NO more weakly than the Fe2+ sites of the FeO monolayer. RAIR spectra obtained from the NO-covered FeOx surfaces exhibit an N-O stretch band that blueshifts over a range from about 1800 to 1840 cm-1 with increasing NO coverage. The measured N-O stretching frequency is only slightly red-shifted from the gas-phase value, and lies in a range that is consistent with atop, linearly bound NO on the Fe surface sites. In contrast to the NO binding energy, we find that the N-O stretch band is relatively insensitive to the NO binding site on the FeOx surfaces. This behavior suggests that π-backbonding occurs to similar extents among the adsorbed NO species, irrespective of the oxidation state and local structural environment of the Fe surface site.
  •  
35.
  • Mehar, Vikram, et al. (author)
  • Formation of Epitaxial PdO(100) During the Oxidation of Pd(100)
  • 2023
  • In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 14:38, s. 8493-8499
  • Journal article (other academic/artistic)abstract
    • The catalytic oxidation of CO and CH4 can be strongly influenced by the structures of oxide phases that form on metallic catalysts during reaction. Here, we show that an epitaxial PdO(100) structure forms at temperatures above 600 K during the oxidation of Pd(100) by gaseous O atoms as well as exposure to O2-rich mixtures at millibar partial pressures. The oxidation of Pd(100) by gaseous O atoms preferentially generates an epitaxial, multilayer PdO(101) structure at 500 K, but initiating Pd(100) oxidation above 600 K causes an epitaxial PdO(100) structure to grow concurrently with PdO(101) and produces a thicker and rougher oxide. We present evidence that this change in the oxidation behavior is caused by a temperature-induced change in the stability of small PdO domains that initiate oxidation. Our discovery of the epitaxial PdO(100) structure may be significant for developing relationships among oxide structure, catalytic activity, and reaction conditions for applications of oxidation catalysis.
  •  
36.
  • Mehar, Vikram, et al. (author)
  • Understanding the Intrinsic Surface Reactivity of Single-Layer and Multilayer PdO(101) on Pd(100)
  • 2018
  • In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 8:9, s. 8553-8567
  • Journal article (peer-reviewed)abstract
    • We investigated the intrinsic reactivity of CO on single-layer and multilayer PdO(101) grown on Pd(100) using temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments, as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (∼119 kJ/mol vs 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101), whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge-Pdcusand atop-Pdcussites (coordinatively unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pdcussites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101), as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity of multilayer PdO(101)/Pd(100). We show that the O atom below the Pdcussite, present only on multilayer PdO(101), modifies the electronic states of the Pdcusatom in a way that enhances the CO-Pdcusbonding. Lastly, we show that a precursor-mediated kinetic model, with energetics determined from the present study, predicts that the intrinsic CO oxidation rates achieved on both single-layer and multilayer PdO(101)/Pd(100) can be expected to exceed the gaseous CO diffusion rate to the surface during steady-state CO oxidation at elevated pressures, even though the intrinsic reaction rates are 4-5 orders of magnitude lower on single-layer PdO(101)/Pd(100) than on multilayer PdO(101)/Pd(100).
  •  
37.
  • Merte, Lindsay, et al. (author)
  • Growth of Ultrathin Iron Oxide Films on Ag(100)
  • 2015
  • In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:5, s. 2572-2582
  • Journal article (peer-reviewed)abstract
    • Ultrathin iron oxide films are useful model materials for fundamental studies of surface processes and exhibit intriguing properties as catalysts, as demonstrated recently in a number of studies utilizing platinum as a substrate. We report a study of the initial stages of iron oxide film growth on an Ag(100) surface using scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, with the goal of elucidating the effects of the substrate material on FeOx film growth and physical properties. We demonstrate that a well-ordered, monolayer-thick FeO(111) film can be prepared which is similar to the well-studied structure formed on Pt(111), though with a significantly expanded lattice constant indicative of smaller FeO buckling and weaker interactions with the substrate. Increased oxygen pressure during deposition leads to formation of a multilayer phase taking the form of well-ordered islands. Although superficially similar to FeO(111), spectroscopic measurements show a substantial proportion of Fe3+ in the phase. FeO(100) grains are observed upon deposition at elevated substrate temperatures, which is proposed to result from formation and oxidation of iron clusters embedded in the surface
  •  
38.
  • Merte, Lindsay R., et al. (author)
  • Oxygen Storage by Tin Oxide Monolayers on Pt3Sn(111)
  • 2023
  • In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 127:6, s. 2988-2994
  • Journal article (peer-reviewed)abstract
    • The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt3Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn2+. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below. 
  •  
39.
  • Merte, Lindsay R., et al. (author)
  • Structure of an Ultrathin Oxide on Pt3Sn(111) Solved by Machine Learning Enhanced Global Optimization
  • 2022
  • In: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 61:25, s. 1-7
  • Journal article (peer-reviewed)abstract
    • Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure-the (4×4) surface oxide on Pt3Sn(111)-based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.
  •  
40.
  • Merte, Lindsay R., et al. (author)
  • Structure of two-dimensional Fe3O4
  • 2020
  • In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:11
  • Journal article (peer-reviewed)abstract
    • We have investigated the structure of an ultrathin iron oxide phase grown on Ag(100) using surface X-ray diffraction in combination with Hubbard-corrected density functional theory (DFT+U) calculations. The film exhibits a novel structure composed of one close-packed layer of octahedrally coordinated Fe2+ sandwiched between two close-packed layers of tetrahedrally coordinated Fe3+ and an overall stoichiometry of Fe3O4. As the structure is distinct from bulk iron oxide phases and the coupling with the silver substrate is weak, we propose that the phase should be classified as a metastable two-dimensional oxide. The chemical and physical properties are potentially interesting, thanks to the predicted charge ordering between atomic layers, and analogy with bulk ferrite spinels suggests the possibility of synthesis of a whole class of two-dimensional ternary oxides with varying electronic, optical, and chemical properties.
  •  
41.
  • Merte, Lindsay R., et al. (author)
  • Tuning the Reactivity of Ultrathin Oxides: NO Adsorption on Monolayer FeO(111)
  • 2016
  • In: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 55:32, s. 9267-9271
  • Journal article (peer-reviewed)abstract
    • Ultrathin metal oxides exhibit unique chemical properties and show promise for applications in heterogeneous catalysis. Monolayer FeO films supported on metal surfaces show large differences in reactivity depending on the metal substrate, potentially enabling tuning of the catalytic properties of these materials. Nitric oxide (NO) adsorption is facile on silver-supported FeO, whereas a similar film grown on platinum is inert to NO under similar conditions. Ab initio calculations link this substrate-dependent behavior to steric hindrance caused by substrate-induced rumpling of the FeO surface, which is stronger for the platinum-supported film. Calculations show that the size of the activation barrier to adsorption caused by the rumpling is dictated by the strength of the metal–oxide interaction, offering a straightforward method for tailoring the adsorption properties of ultrathin films.
  •  
42.
  • Merte, Lindsay, et al. (author)
  • Water clustering on nanostructured iron oxide films
  • 2014
  • In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5
  • Journal article (peer-reviewed)abstract
    • The adhesion of water to solid surfaces is characterized by the tendency to balance competing molecule-molecule and molecule-surface interactions. Hydroxyl groups form strong hydrogen bonds to water molecules and are known to substantially influence the wetting behaviour of oxide surfaces, but it is not well-understood how these hydroxyl groups and their distribution on a surface affect the molecular-scale structure at the interface. Here we report a study of water clustering on a moire-structured iron oxide thin film with a controlled density of hydroxyl groups. While large amorphous monolayer islands form on the bare film, the hydroxylated iron oxide film acts as a hydrophilic nanotemplate, causing the formation of a regular array of ice-like hexameric nanoclusters. The formation of this ordered phase is localized at the nanometre scale; with increasing water coverage, ordered and amorphous water are found to coexist at adjacent hydroxylated and hydroxyl-free domains of the moire structure.
  •  
43.
  • Mittendorfer, Florian, et al. (author)
  • Oxygen-Stabilized Rh Adatoms: 0D Oxides on a Vicinal Surface
  • 2011
  • In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 2:21, s. 2747-2751
  • Journal article (peer-reviewed)abstract
    • We have investigated the initial oxidation of the Rh(113) and Rh(223) vicinal surfaces by STM and ab initio simulations. Upon adsorption of small amounts of oxygen, the surface morphology is completely altered. Surprisingly, oxygen-stabilized Rh adatoms can be observed on the (113) facets, with oxide-like electronic properties. We present models of these "0D oxide" phases and discuss reasons for their stability.
  •  
44.
  • Olsson, Pär, et al. (author)
  • Stability, magnetic order, and electronic properties of ultrathin Fe3O4 nanosheets
  • 2020
  • In: Physical Review B. - : American Physical Society. - 2469-9969 .- 2469-9950. ; 101:15
  • Journal article (peer-reviewed)abstract
    • We study the stability, magnetic order, charge segregation, and electronic properties of a novel three-layered Fe3O4 film by means of Hubbard-corrected density functional theory calculations. The stable film is predicted to consist of close-packed iron and oxygen layers, comprising a center layer with octahedrally coordinated Fe sandwiched between two layers with tetrahedrally coordinated Fe. The film exhibits an antiferromagnetic type I spin order. A charge analysis confirms that the stable structure has distinct charge segregation, with Fe2+ ions in the center layer and Fe3+ in the tetrahedral surface layers. Examination of the electronic band structures and densities of states shows that the bandgap is substantially reduced, from 2.4 eV for the bulk rocksalt to 0.3 eV for the film. The reduction in the bandgap is a consequence of the 2+ to 3+ change in oxidation state of Fe in the surface layers.
  •  
45.
  •  
46.
  • Schaefer, Andreas, et al. (author)
  • Methanol Adsorption and Oxidation on Reduced and Oxidized TbOx(111) Surfaces
  • 2016
  • In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:50, s. 28617-28629
  • Journal article (peer-reviewed)abstract
    • We used temperature programmed reaction spectroscopy (TPRS) and synchrotron-radiation based photoelectron spectroscopy (PES) to investigate the adsorption and oxidation of methanol (CH3OH) on Tb2O3(111) and TbO2(111) thin films grown on Pt(111). We find that methanol mainly desorbs from the Tb2O3 surface through both molecular and recombinative processes, and that a relatively small amount of adsorbed methanol (< 20%) dehydrogenates to CH2O and H2O with these species desorbing between about 160 and 300 K. Oxidation of the terbia film enhances the surface reactivity as ~50% of the adsorbed methanol on TbO2 oxidizes to mainly CH2O and water as well as CO2 that desorbs near 600 K during TPRS. Quantification of the product yields suggests that all of the excess surface O-atoms, resulting from oxidation of Tb2O3 to TbO2, are removed during TPRS by reaction with adsorbed CH3OH. We did not detect CO or H2 production under any conditions. PES measurements show that several adsorbed intermediates form on the TbOx surfaces at temperatures as low as 140 K, including mainly methoxy (CH3O) as well as smaller quantities of a more oxidized species, thought to be either CH2O2 or CHO2. XPS spectra collected as a function of the surface temperature provide evidence that the adsorbed CH3O groups serve as the main intermediate for both CH2O and CH3OH formation at temperatures below 400 K, while the more oxidized species is a spectator to CH2O formation but undergoes complete oxidation on TbO2 at temperatures above 450 K. The high reactivity of the TbO2 surface correlates with the presence of labile oxygen atoms that are generated during oxidation of the Tb2O3 film.
  •  
47.
  • Schiller, Frederik, et al. (author)
  • Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal : Spatial Variation of Active and Poisoning Phases in Stationary Conditions
  • 2018
  • In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:47, s. 16245-16252
  • Journal article (peer-reviewed)abstract
    • Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point, we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultrahigh vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off.
  •  
48.
  • Shipilin, Mikhail, et al. (author)
  • Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation
  • 2014
  • In: Surface Science. - : Elsevier BV. - 0039-6028. ; 630, s. 229-235
  • Journal article (peer-reviewed)abstract
    • We have performed a quantitative structure determination of the SQRT(5×5)R27° surface oxide, formed on Pd(100) under semi-realistic conditions for catalytic CO oxidation, using in situ high-energy surface X-ray diffraction. We describe the experiment and the extraction of quantitative data in detail. The structural results are in agreement with previous reports of a system consisting of a single layer of PdO(101) formed in pure O2 on top of Pd(100) and studied under ultra high vacuum conditions.
  •  
49.
  • Shipilin, Mikhail, et al. (author)
  • Step dynamics and oxide formation during CO oxidation over a vicinal Pd surface.
  • 2016
  • In: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 18:30, s. 20312-20320
  • Journal article (peer-reviewed)abstract
    • In an attempt to bridge the material and pressure gaps - two major challenges for an atomic scale understanding of heterogeneous catalysis - we employed high-energy surface X-ray diffraction as a tool to study the Pd(553) surface in situ under changing reaction conditions during CO oxidation. The diffraction patterns recorded under CO rich reaction conditions are characteristic for the metallic state of the surface. In an environment with low excess of O2 over the reaction stoichiometry, the surface seems to accommodate oxygen atoms along the steps forming one or several subsequent adsorbate structures and rapidly transforms into a combination of (332), (111) and (331) facets likely providing the room for the formation of a surface oxide. For the case of large excess of O2, the diffraction data show the presence of a multilayer PdO with the [101] crystallographic direction parallel to the [111] and the [331] directions of the substrate. The reconstructions in O2 excess are to a large extent similar to those previously reported for pure O2 exposures by Westerström et al. [R. Westerström et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 155410].
  •  
50.
  • Shipilin, Mikhail, et al. (author)
  • The influence of incommensurability on the long-range periodicity of the Pd(100)-(root 5 x root 5)R27 degrees-PdO(101)
  • 2017
  • In: Surface Science. - : Elsevier BV. - 0039-6028. ; 660, s. 1-8
  • Journal article (peer-reviewed)abstract
    • The structural model of the (root 5 x root 5)R27 degrees PdO(101) surface oxide grown on Pd(100) has been proposed and refined by a number of authors over more than a decade. In the current contribution we discuss the long-range periodicity of this structure arising along one of the crystallographic directions due to its incommensurability with the substrate. Analyzing the results of surface sensitive diffraction studies, we determined a slight distortion of the previously reported perfect (root 5 x root 5)R27 degrees surface oxide unit cell. Considering it, we were able to achieve both qualitatively and quantitatively better fit to the experimental diffraction data than it was possible for the perfect structure. Further, taking into account the experimentally obtained scanning tunneling microscopy data and closely examining high-resolution patterns recorded by means of high-energy surface X-ray diffraction, we developed a qualitative structural model based on a larger non-orthogonal surface unit cell to shed more light on the long-range order of the PdO(101) surface oxide. The model comprises a shift of the atoms of the PdO perpendicularly to the direction of the incommensurability to correct for it. This structural model reproduces the fine details of the high-resolution diffraction patterns and qualitatively explains the periodic stripes of structural distortion observed in the images recorded by a scanning tunneling microscope.
  •  
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