| 1. |
- Beye, Martin, et al.
(författare)
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Chemical Bond Activation Observed with an X-ray Laser
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:18, s. 3647-3651
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Tidskriftsartikel (refereegranskat)abstract
- The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.
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| 2. |
- Casalongue, Hernan G. Sanchez, et al.
(författare)
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Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:50, s. 29252-29259
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Tidskriftsartikel (refereegranskat)abstract
- Molybdenum sulfide structures, particularly amorphous MoS3 nanoparticles, are promising materials in the search for cost-effective and scalable water-splitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS3 to defective MoS2 when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS3 nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoSx systems. We demonstrate that, under operating conditions, surface sites are converted from MoS3 to MoS2 in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS2 edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS2 edge-like sites after the reduction of MoS3 sites.
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| 3. |
- Kaya, Sarp, et al.
(författare)
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Highly Compressed Two-Dimensional Form of Water at Ambient Conditions
- 2013
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- The structure of thin-film water on a BaF2(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to 95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF2 exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF2(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.
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| 4. |
- Ogasawara, Hirohito, et al.
(författare)
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Operando X-Ray Photoelectron Spectroscopy Studies of Aqueous Electrocatalytic Systems
- 2016
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Ingår i: Topics in catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 59:5-7, s. 439-447
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Tidskriftsartikel (refereegranskat)abstract
- Development of efficient fuel cell and electrochemical cell devices to retrieve energy in a renewable manner lies in the molecular level understanding of the conversion processes taking place at surfaces and interfaces. These processes involve complicated bond breaking and formation at the surfaces as well as charge transfer through interfaces which are challenging to track under operational conditions. We address the nature of these interfacial processes using ambient pressure X-ray photoelectron spectroscopy by leveraging both its chemical and surface sensitivity. Herein, we give several examples of fuel cell and electrolysis reactions to demonstrate the importance of probing the surface under operating conditions. Oxygen reduction reaction taking place on the platinum cathode in proton exchange membrane fuel cells, water splitting reactions including oxygen evolution reaction over IrO2 and hydrogen evolution reaction over MoSx reveal that different species dominate on the surface under different operational conditions and surface activities are directly related to the stabilities of those intermediate species and possible structural rearrangements of the catalyst material.
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| 5. |
- Sellberg, Jonas A., et al.
(författare)
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Comparison of x-ray absorption spectra between water and ice : New ice data with low pre-edge absorption cross-section
- 2014
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 141:3, s. 034507-
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Tidskriftsartikel (refereegranskat)abstract
- The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF2(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed.
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| 6. |
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| 7. |
- Sierka, Marek, et al.
(författare)
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Oxygen adsorption on Mo(112) surface studied by ab initio genetic algorithm and experiment
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 126:23
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory in combination with genetic algorithm is applied to determine the atomic models of p(1x2) and p(1x3) surface structures observed upon oxygen adsorption on a Mo(112) surface. The authors’ simulations reveal an unusual flexibility of Mo(112) resulting in oxygen-induced reconstructions and lead to more stable structures than any suggested so far. Comparison of the stabilities of the predicted models shows that different p(1x2) and p(1x3) structures may coexist over a wide range of oxygen pressures. A pure p(1x2) structure can be obtained only in a narrow region of oxygen pressures. In contrast, a pure p(1x3) structure cannot exist as a stable phase. The results of simulations are fully supported by a multitude of experimental data obtained from low energy electron diffraction, x-ray photoelectron spectroscopy, and scanning tunneling microscopy.
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| 8. |
- Soldemo, Markus
(författare)
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Transition metal oxide surfaces : Surface structures and molecular interaction
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metal oxides are both corrosion products and useful materials with a wide range of applications. Two of the most used metals today are iron and copper. In this thesis, surface structures and molecular interaction with surfaces of iron oxides and copper oxides are studied using spectroscopy and microscopy methods. The surface structures of iron oxides grown on the low-index iron (Fe) surfaces (100) and (110) have been studied during the initial oxidation phase. The oxidation condition for both iron surfaces was 400 °C and 1×10−6 mbar of oxygen gas. For the Fe(100)-surface, a Fe3O4(100)-film is formed beyond the oxygen adsorbate structures. For the Fe(110)-surface, a FeO(111)-film is first formed. When the FeO(111)-film grows thicker, it transforms into a Fe3O4(111)-film. The surface structures of Cu2O(100) was studied and the main finding is that the most common surface structure that previously in literature has been described to have a periodicity of (3√2×√2)R45° actually has a periodicity described by the matrix (3,0;1,1). Furthermore, the low-binding energy component in the photoelectron spectroscopy O 1s-spectrum is determined to origin from surface oxygen atoms. Sulfur dioxide, a corrosive molecule that in the environment to large share comes from human activities such as burning of fossil fuels, was studied using photoelectron spectroscopy when interacting with surfaces of iron oxide thin films and bulk Cu2O-surfaces. On the iron oxide thin film surfaces under ultra-high vacuum conditions, sulfur dioxide adsorbs partly as SO4-species and partly dissociates and forms FeS2. On the Cu2O-surfaces under ultra-high vacuum conditions, the adsorption of sulfur dioxide is non-dissociative and forms SO3-species. When interacting with near-ambient pressures of water, it is observed in the photoelectron spectroscopy S 2p-region that the sulfur from SO3-species shifts to Cu2S.
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| 9. |
- Stacchiola, Dario, et al.
(författare)
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Synthese und Struktur eines ultradünnen Alumosilicatfilms
- 2006
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Ingår i: Angewandte Chemie. - : Wiley. - 1521-3757 .- 0044-8249. ; 118:45, s. 7798-7801
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Tidskriftsartikel (refereegranskat)abstract
- Die atomare Struktur eines geordneten Alumosilicatfilms auf einem Mo(112)-Substrat wurde durch hochauflösende Messungen bestimmt und mithilfe der Dichtefunktionaltheorie berechnet. Bei einem niedrigen Al/Si-Verhältnis wird die Struktur dadurch gebildet, dass Al-Atome (blau) die Si-Atome (orange) im Siliciumoxidfilm ersetzen. Der Alumosilicatfilm besteht aus einer Schicht von eckenverknüpften {SiO4}- und {AlO3}-Einheiten.
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| 10. |
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| 11. |
- Tissot, Heloise, et al.
(författare)
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Interaction of Atomic Hydrogen with the Cu2O(100) and (111) Surfaces
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 123:36, s. 22172-22180
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Tidskriftsartikel (refereegranskat)abstract
- Reduction of Cu2O by hydrogen is a common preparation step for heterogeneous catalysts; however, a detailed understanding of the atomic reaction pathways is still lacking. Here, we investigate the interaction of atomic hydrogen with the Cu2O(100):(3,0;1,1) and Cu2O(111):(root 3 x root 3)R30 degrees surfaces using scanning tunneling microscopy (STM), low-energy electron diffraction, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The experimental results are compared to density functional theory simulations. At 300 K, we identify the most favorable adsorption site on the Cu2O(100) surface: hydrogen atoms bind to an oxygen site located at the base of the atomic rows intrinsic to the (3,0;1,1) surface. The resulting hydroxyl group subsequently migrates to a nearby Cu trimer site. TPD analysis identifies H-2 as the principal desorption product. These observations imply that H-2 is formed through a disproportionation reaction of surface hydroxyl groups. The interaction of H with the (111) surface is more complex, including coordination to both Cu+ and O-CUS sites. STM and XPS analyses reveal the formation of metallic copper clusters on the Cu2O surfaces after cycles of hydrogen exposure and annealing. The interaction of the Cu clusters with the substrate is notably different for the two surface terminations studied: after annealing, the Cu clusters coalesce on the (100) termination, and the (3,0;1,1) reconstruction is partially recovered. Clusters formed on the (111) surface are less prone to coalescence, and the (root 3 x root 3)R30 degrees reconstruction was not recovered by heat treatment, indicating a weaker Cu cluster to support interaction on the (100) surface.
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| 12. |
- Wang, Chunlei, et al.
(författare)
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High-Density Isolated Fe1O3 Sites on a Single-Crystal Cu2O(100) Surface
- 2019
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 10:23, s. 7318-7323
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Tidskriftsartikel (refereegranskat)abstract
- Single-atom catalysts have recently been subject to considerable attention within applied catalysis. However, complications in the preparation of well-defined single-atom model systems have hampered efforts to determine the reaction mechanisms underpinning the reported activity. By means of an atomic layer deposition method utilizing the steric hindrance of the ligands, isolated Fe1O3 motifs were grown on a single-crystal Cu2O(100) surface at densities up to 0.21 sites per surface unit cell. Ambient pressure X-ray photoelectron spectroscopy shows a strong metal-support interaction with Fe in a chemical state close to 3+. Results from scanning tunneling microscopy and density functional calculations demonstrate that isolated Fe1O3 is exclusively formed and occupies a single site per surface unit cell, coordinating to two oxygen atoms from the Cu2O lattice and another through abstraction from O-2. The isolated Fe1O3 motif is active for CO oxidation at 473 K. The growth method holds promise for extension to other catalytic systems.
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| 13. |
- Wang, Chunlei, et al.
(författare)
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Stabilization of Cu2O through Site-Selective Formation of a Co1Cu Hybrid Single-Atom Catalyst
- 2022
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 34:5, s. 2313-2320
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Tidskriftsartikel (refereegranskat)abstract
- Single-atom catalysts (SACs) consist of a low coverage of isolated metal atoms dispersed on a metal substrate, called single-atom alloys (SAAs), or alternatively single metal atoms coordinated to oxygen atoms on an oxide support. We present the synthesis of a new type of Co1Cu SAC centers on a Cu2O(111) support by means of a site-selective atomic layer deposition technique. Isolated metallic Co atoms selectively coordinate to the native oxygen vacancy sites (Cu sites) of the reconstructed Cu2O(111) surface, forming a Co1Cu SAA with no direct Co- Ox bonds. The centers, here referred to as Co1Cu hybrid SACs, are found to stabilize the active Cu+ sites of the low-cost Cu2O catalyst that otherwise is prone to deactivation under reaction conditions. The stability of the Cu2O(111) surface was investigated by synchrotron radiation-based ambient-pressure X-ray photoelectron spectroscopy under reducing CO environment. The structure and reduction reaction are modeled by density functional theory calculations, in good agreement with experimental results.
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| 14. |
- Zhu, Suyun, et al.
(författare)
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HIPPIE : a new platform for ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory
- 2021
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Ingår i: Journal of Synchrotron Radiation. - : INT UNION CRYSTALLOGRAPHY. - 1600-5775 .- 0909-0495. ; 28, s. 624-636
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Tidskriftsartikel (refereegranskat)abstract
- HIPPIE is a soft X-ray beamline on the 3 GeV electron storage ring of the MAX IV Laboratory, equipped with a novel ambient-pressure X-ray photoelectron spectroscopy (APXPS) instrument. The endstation is dedicated to performing in situ and operando X-ray photoelectron spectroscopy experiments in the presence of a controlled gaseous atmosphere at pressures up to 30 mbar [1 mbar = 100 Pa] as well as under ultra-high-vacuum conditions. The photon energy range is 250 to 2200 eV in planar polarization and with photon fluxes >1012 photons s-1 (500 mA ring current) at a resolving power of greater than 10000 and up to a maximum of 32000. The endstation currently provides two sample environments: a catalysis cell and an electrochemical/liquid cell. The former allows APXPS measurements of solid samples in the presence of a gaseous atmosphere (with a mixture of up to eight gases and a vapour of a liquid) and simultaneous analysis of the inlet/outlet gas composition by online mass spectrometry. The latter is a more versatile setup primarily designed for APXPS at the solid-liquid (dip-and-pull setup) or liquid-gas (liquid microjet) interfaces under full electrochemical control, and it can also be used as an open port for ad hoc-designed non-standard APXPS experiments with different sample environments. The catalysis cell can be further equipped with an IR reflection-absorption spectrometer, allowing for simultaneous APXPS and IR spectroscopy of the samples. The endstation is set up to easily accommodate further sample environments.
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| 15. |
- Öberg, Henrik, et al.
(författare)
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Stability of Pt-Modified Cu(111) in the Presence of Oxygen and Its Implication on the Overall Electronic Structure
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:32, s. 16371-16380
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and stability of Cu(111)-hosted Pt overlayers with and without the presence of atomic oxygen have been studied by means of core-level spectroscopy and density functional theory (DFT). Because of lattice mismatch, Pt(111) overlayers grown on Cu(111) are compressively strained, and hard X-ray photoelectron spectroscopy together with Pt L-3-edge X-ray absorption spectroscopy (XAS) reveals a pronounced downshift of the Pt d-band owing to the increased overlap of the d-orbitals, an effect also reproduced theoretically. Exposure to oxygen severely alters the surface composition; the O-Cu binding energy largely exceeds that of O-Pt, and DFT calculations predict surface segregation of Cu atoms. Comparing the adsorbate electronic structure for O on unstrained Pt(111) with that of O on Pt-modified Cu(111) using O K-edge XAS and X-ray emission spectroscopy salient differences are observed and calculations show that Cu-segregation to the topmost layer is required to reproduce the measured spectra. It is proposed that O is binding in a hollow site constituted by at least two Cu atoms and that up to 75% of the Pt atoms migrate below the surface.
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