| 1. |
- Dell'Angela, M., et al.
(författare)
-
Real-Time Observation of Surface Bond Breaking with an X-ray Laser
- 2013
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 339:6125, s. 1302-1305
-
Tidskriftsartikel (refereegranskat)abstract
- We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.
|
|
| 2. |
- Beye, M., et al.
(författare)
-
Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)
- 2013
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:18
-
Tidskriftsartikel (refereegranskat)abstract
- We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.
|
|
| 3. |
- Katayama, T., et al.
(författare)
-
Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser
- 2013
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 187, s. 9-14
-
Tidskriftsartikel (refereegranskat)abstract
- We report on an experimental system designed to probe chemical reactions on solid surfaces on a sub-picosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0001) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface.
|
|
| 4. |
|
|
| 5. |
- Schiros, Theanne, et al.
(författare)
-
Unique water-water coordination tailored by a metal surface
- 2013
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 138:23, s. 234708-
-
Tidskriftsartikel (refereegranskat)abstract
- At low coverage of water on Cu(110), substrate-mediated electrostatics lead to zigzagging chains along [001] as observed with STM [T. Yamada, S. Tamamori, H. Okuyama, and T. Aruga, Anisotropic water chain growth on Cu(110) observed with scanning tunneling microscopy Phys. Rev. Lett. 96, 036105 (2006)]. Using x-ray absorption spectroscopy we find an anomalous low-energy resonance at similar to 533.1 eV which, based on density functional theory spectrum simulations, we assign to an unexpected configuration of water units whose uncoordinated O-H bonds directly face those of their neighbors; this interaction repeats over trough sites with enhanced electron density and is analogous to the case of a hydrated electron.
|
|
| 6. |
- Yamamoto, S., et al.
(författare)
-
Water adsorption on α-Fe2O3 (0001) at near ambient conditions
- 2010
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:5, s. 2256-2266
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated hydroxylation and water adsorption on α Fe 2O3(0001) at water vapor pressures up to 2 Torr and temperatures ranging from 277 to 647 K (relative humidity (RH)
|
|
| 7. |
- Andersson, Klas J., et al.
(författare)
-
Preparation, Structure, and Orientation of Pyrite FeS2{100} Surfaces : Anisotropy, Sulfur Monomers, Dimer Vacancies, and a Possible FeS Surface Phase
- 2014
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:38, s. 21896-21903
-
Tidskriftsartikel (refereegranskat)abstract
- Sulfur dimer (S-2(2-)) terminated pyrite FeS2{100} surfaces with a low energy electron diffraction (LEED) pattern of 2 x 1 symmetry are reported. The 2 X 1 symmetry correlates with the orientation of the anisotropic surface structure and external symmetry of macroscopic striations on the pyrite cube face. The basic condition to form these surfaces is a mild 200 V Ne+ sputter-cleaning procedure followed by a 570 K anneal of the sample in a 10(-7) Ton S-2(g) atmosphere. Controlled amounts of surface sulfur monomers (S2-) can be introduced by mild sputtering of the sulfur dimer terminated surfaces. At low monomer concentrations the surface displays the same characteristic 1 x 1 LEED pattern as that for fracture-generated surfaces. With increasing sulfur depletion, a (1/ root 2 x 1/ root 2)R45 degrees LEED pattern emerges, and soft X-ray photoelectron spectroscopy (XPS) results show a sulfur dinner deficient near-surface region and a new high binding energy sulfur spectral component suggesting the presence of local coordination environments where sulfur monomers are coordinated by four Fe ions compared to three as in the pyrite structure. The plausible formation of a defective FeS-like surface phase where monomeric sulfurs are coordinated by four Fe ions, and bond counting energetics favoring surface sulfur monomer recombination around Fe vacancy sites on pyrite FeS2{100}, both imply surface sulfur dimer vacancy sites with unique adsorption and reactivity properties. Taken together, our results suggest a very rich and dynamic defect structural landscape at pyrite FeS2{100} surfaces with direct implications for its surface chemical activity.
|
|
| 8. |
- Casalongue, Hernan G. Sanchez, et al.
(författare)
-
Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction
- 2014
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:50, s. 29252-29259
-
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.
|
|
| 9. |
- Friebel, Daniel, et al.
(författare)
-
In situ x-ray probing reveals the importance of surface platinum oxide formation in fuel cell catalysis
- 2011
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 13:1, s. 262-266
-
Tidskriftsartikel (refereegranskat)abstract
- In situ X-ray absorption spectroscopy (XAS) at the Pt L3 edge is a useful probe for Pt–O interactions at polymer electrolyte membrane fuel cell (PEMFC) cathodes. We show that XAS using the high energy resolution fluorescence detection (HERFD) mode, applied to a well-defined monolayer Pt/Rh(111) sample where the bulk penetrating hard X-rays probe only surface Pt atoms, provides a unique sensitivity to structure and chemical bonding at the Pt-electrolyte interface. Ab initio multiple-scattering calculations using the FEFF code and complementary extended X-ray absorption fine structure (EXAFS) results indicate that the commonly observed large increase of the white-line at high electrochemical potentials on PEMFC cathodes originates from platinum oxide formation, whereas previously proposed chemisorbed oxygen-containing species merely give rise to subtle spectral changes.
|
|
| 10. |
- Miller, Daniel J., et al.
(författare)
-
Low O-2 dissociation barrier on Pt(111) due to adsorbate-adsorbate interactions
- 2010
-
Ingår i: Journal of Chemical Physics. - : American Institute of Physics. - 0021-9606 .- 1089-7690. ; 133:22, s. 224701-
-
Tidskriftsartikel (refereegranskat)abstract
- O2 dissociation on Pt(111) has been followed at low and saturation coverage using temperature-programmed x-ray photoelectron spectroscopy and simulated with mean-field kinetic modeling, yielding dissociation (Ea) and desorption (Ed) barriers of 0.32 and 0.36 eV, respectively. DFT calculations show that Ea is strongly influenced by the O–O interatomic potential in the atomic final state: of the supercells considered, that which maximizes attractive third-nearest-neighbor interactions in the atomic final state yields both the lowest computed dissociation barrier (0.24 eV) and the best agreement with experiment. It is proposed that the effect of adsorbate-adsorbate interactions must be considered when modeling catalytic processes involving dissociative steps.
|
|
