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Fundamental Interactions on Surfaces. Core Hole Decay Core hole life time Sum of all decay channels XES one electron picture AES two electron interaction;

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Presentation on theme: "Fundamental Interactions on Surfaces. Core Hole Decay Core hole life time Sum of all decay channels XES one electron picture AES two electron interaction;"— Presentation transcript:

1 Fundamental Interactions on Surfaces

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

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

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

5 X-ray spectroscopy

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

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

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

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

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

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

13 Polymer Electrolyte Membrane Fuel Cells – Principle Cathode Anode H2H2 H+e-H+e- H+ H+ O2O2 Membrane e-e- e-e- H2OH2O O2O2 Oxygen Reduction (ORR)Hydrogen Oxidation (HOR) Transforms chemical energy of fuel into electrical energy  Slow electrode kinetics  Cost of catalyst  Stability of catalyst are most critical issues in fuel cell research 13

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

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

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

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 ligand effect Fe, Co, Ni Pt Due to a fixed substrate the lattice parameter is same so ligand effect can be isolated. Valence band hν = 620 eV Tuning Pt d-band DOS by controlling 3d metal in the second layer

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

19 Probing the electronic structure of dealloyed nanoparticle catalysts 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. nanoparticle catalysts are supported on carbon support (carbon, Nafion) Anniyev et al, PCCP 2010, 12, 5694

20 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. Probing the electronic structure of dealloyed nanoparticle catalysts z Valence band photoemission 8000 eV excitation, Spring-8 BL47XU Sensitive to Pt Valence band photoemission 1486 eV excitation, BL13-2 Pt 5d DOS is obtainable! Dominated by support and Cu support (carbon, Nafion) Anniyev et al- PCCP 12, 5694 (2010)

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22 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).

23 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 400 nm laser pump Hot electron generation in Ru → heat transfer to CO photoexcitation resulting in the desorption of CO “A simple experiment” LCLS pump-probe experiments O 1s X-ray emission and X-ray absorption spectroscopy 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 2π* dπdπ X-ray emission spectroscopy occupied valence state Oxygen 2p component X-ray absorption spectroscopy unoccupied valence state Oxygen 2p component Ru-CO π-bond Ru-CO σ-bond Spatially extended orbital O1s CO/MetalCO gas 5σ 1π Electronic states CO/Metal Energy Nilsson et al., Surf. Sci. Reps. 55, 49 (2004).

24 Charge Density Differences gain of charge, attraction Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004). loss of charge, repulsion C O C O  looses charge and  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…

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

26 Most important catalytic reactions are driven by thermal processes 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 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 Chemisorbed state Reactive state Probing the Reactive State in Catalysis

27 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?

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

29 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 “A simple experiment” LCLS pump-probe experiments X-ray emission and X-ray absorption spectrscopy pump Time Δt/ps O1s heat transfer to CO: ~10ps ? >50% gradual desorption of CO ~30% J. Electron Spectr. 187 (2013) 9

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

31 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)  CO 2, H+CO  HCO, Fischer-Tropsch,… Higher pressure (~100 torr), solid-liquid interfaces, photocatalysis Shorter FEL pulses, THz radiation control (LCLS 2) “Chemist’s dream”


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