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Fundamental Interactions on Surfaces

Fundamental Interactions on Surfaces

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Fundamental Interactions on Surfaces

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  1. Fundamental Interactions on Surfaces

  2. Core Hole Decay XES one electron picture AES two electron interaction; complex Correlation effects Sandell et. al. Phys. Rev. B48, 11347 (1993) Core hole life time Sum of all decay channels

  3. X-ray Spectroscopy Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).

  4. The D-band Model Coupling to s Coupling to d Vacuum antibonding Energy d s bonding Adsorbate projected DOS Metal projected DOS Hammer and Nørskov, Adv. Catal., 2000, 45, 71.

  5. X-ray spectroscopy

  6. X-ray spectroscopy

  7. X-ray Photoelectron Spectroscopy Additional probing of O and metal core-level shifts with XPS

  8. Probing valence statesPhotoemission and X-ray emission Nitrogen 1s resonant x-ray emission Photoemission Cu Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).

  9. Probing valence statesPhotoemission and X-ray emission Nitrogen 1s resonant x-ray emission Photoemission Cu Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).

  10. Probing valence statesPhotoemission and X-ray emission Nitrogen 1s resonant x-ray emission Photoemission Cu Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).

  11. Atomic Nitrogen on Ni and Cu Occupation of antibonding states and bond strength Ni Cu Cu Ni N-metal antibonding d s N-metal bonding Nitrogen 1s resonant x-ray spectroscopy Occupied & unoccupied DOS: Nilsson et. al, Catal. Lett. 100, 111 (2005)

  12. Atomic Nitrogen on Ni and Cu Bonding Strength Cu Ni N-metal antibonding d s N-metal bonding Nitrogen 1s resonant x-ray spectroscopy Occupied & unoccupied DOS: Nilsson et. al, Catal. Lett. 100, 111 (2005)

  13. Polymer Electrolyte Membrane Fuel Cells – Principle Transforms chemical energy of fuel into electrical energy Membrane H2 O2 Cathode H+ e- H+ O2 Anode e- H2O e- Hydrogen Oxidation (HOR) Oxygen Reduction (ORR) • Slow electrode kinetics • Cost of catalyst • Stability of catalyst • are most critical issues in fuel cell research

  14. Theoretical Modelling Nørskovet al., J. Phys. Chem. B, 2004, 108, 46: Greeley et al., Nature Chemistry, 2009, 1, 7 Strong Pt–O bond Weak Pt–O bond

  15. Parameters to control the electronic structure Alloy Coordination # flat Lattice strain step, kink, adatom Ligand

  16. Shift in D-band Occupied Pt-DOS: Photoemission spectroscopy Pt layers on Cu(111) EF d-band center Anniyev, unpublished

  17. Oxygen adsorption on Pt-3d-Pt(111) sandwich structure Pt-3d-Pt sandwich structures are model systems where second layer is exchanged with that of various 3d elements Fe, Co, Ni Pt Tuning Pt d-band DOS by controlling 3d metal in the second layer ligand effect Due to a fixed substrate the lattice parameter is same so ligand effect can be isolated. Valence band hν = 620 eV

  18. Oxygen/Pt-3d-Pt(111) – Oxygen 1s resonant x-ray spectroscopy results O Pt The d-band center shifts…. Pt-O* Binding Energy Pt-O Intensity of the antibonding states in XES increases Antibonding resonance in XAS decreases

  19. Probing the electronic structure of dealloyed nanoparticle catalysts Anniyev et al, PCCP 2010, 12, 5694 support (carbon, Nafion) nanoparticle catalysts are supported on carbon Core-shell structure determined from XPS. Pt shell is compressively strained. Strain induced lowering of the Pt 5d band results in optimized Pt-O bond energy.

  20. Probing the electronic structure of dealloyed nanoparticle catalysts support (carbon, Nafion) z Valence band photoemission 8000 eVexcitation, Spring-8 BL47XU Sensitive to Pt Valence band photoemission 1486 eV excitation, BL13-2 Core-shell structure determined from XPS. Pt shell is compressively strained. Strain induced lowering of the Pt 5d band results in optimized Pt-O bond energy. Pt 5d DOS is obtainable! Dominated by support and Cu Anniyev et al- PCCP 12, 5694 (2010)

  21. Atom Selectivity Selective excitation of inner and outer nitrogen atoms Nilsson et.al. Phys. Rev. Lett. 78, 2847 (1997) Bennich et. al. Phys. Rev. B57, 9275 (1998) Nilsson et al., Surf. Sci. Reps. 55, 49 (2004).

  22. LCLS pump-probe experiments O 1s X-ray emission and X-ray absorption spectroscopy Electronic states CO/Metal CO/Metal CO gas Energy 2π* Spatially extended orbital “A simple experiment” dπ Ru-CO π-bond 1π 5σ O1s Ru-CO σ-bond X-ray emission spectroscopy occupied valence state Oxygen 2p component X-ray absorption spectroscopy unoccupied valence state Oxygen 2p component Map valence electronic structure changes by measuring x-ray emission spectra as a function of Laser–FEL delay & FEL energies. Data set → Pump-probe XES & XAS Nilsson et al., Surf. Sci. Reps. 55, 49 (2004). photoexcitation resulting in the desorption of CO 400 nm laser pump Hot electron generation in Ru → heat transfer to CO Map valence electronic structure changes by measuring x-ray emission spectra as a function of Laser–FEL delay & FEL energies. Data set → Pump-probe XES & XAS

  23. Charge Density Differences O O C C gain of charge, attraction loss of charge, repulsion s looses charge and p gains charge, but not in a frontier orbital sense All orbitals are modified and new orbitals appear We will monitor these orbitals with time-resolved XES and XAS as the CO/Ru bond weakens… Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).

  24. x-ray free electron laser at SLAC: LCLSin operation from 2009ultra short x-ray pulse: <100 fs – sub ps Ultrafast Surface Chemistry at LCLS This first work: fs-laser (400nm) induced CO desorption from Ru(0001) LCLS SSRL Ultrafast electronic structure probe

  25. Probing the Reactive State in Catalysis Most important catalytic reactions are driven by thermal processes The number of turn-over events at each active site at a given time is extremely low Chemisorbed state Reactive state The Boltzmann energy distribution gives only few molecules to be in a reactive state Ultrafast laser-induced heating leads to orders of magnitude higher population of the reactive state which can now be probed with ultrafast methods

  26. Pump-Probe How to initiate the reaction? Probing with adsorbate sensitivity the geometric and electronic structure What intermediate species do we have? How intermediate species are bonding to the surface?

  27. CO Desorption from Ru(0001): Weakly Bound Precursor State quasi free rigid chemisorbed precursor 1 0 Energy/eV -1 ~15% ~30% Precursor state ~15%

  28. LCLS pump-probe experiments X-ray emission and X-ray absorption spectrscopy Time Δt/ps “A simple experiment” O1s pump heat transfer to CO: ~10ps ? >50% gradual desorption of CO ~30% J. Electron Spectr. 187 (2013) 9 Map valence electronic structure changes by measuring x-ray emission spectra as a function of Laser–FEL delay & FEL energies. Data set → Pump-probe XES & XAS

  29. Times scales and temperature <1ps frustrated rotations >3ps moving to presursor Phonon driven Hot electron driven Phys. Rev. Lett. 110 (2013) 186101

  30. New Era in Catalysis • First surface chemical reaction with LCLS • Proof of principle • Observation of two different excitations of CO • Strong coupling to motion parallel to the surface; early times • Precursor to desorption in a weakened surface chemical bond • CO+O/Ru(0001)  CO2, H+CO  HCO, Fischer-Tropsch,… • Higher pressure (~100 torr), solid-liquid interfaces, photocatalysis • Shorter FEL pulses, THz radiation control (LCLS 2) • “Chemist’s dream”