fundamental interactions on surfaces. core hole decay core hole life time sum of all decay channels...
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Core Hole Decay
fluoaug
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, 11347 (1993)
The D-band Model
Hammer and Nørskov, Adv. Catal., 2000, 45, 71.
Vacuum
Energy
Adsorbate projected DOS
Coupling to s
Metal projected DOS
sd
Coupling to d
bonding
antibonding
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Probing valence statesPhotoemission and X-ray emission
Cu
Nitrogen 1s resonant x-ray emission
Photoemission
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Cu
Nitrogen 1s resonant x-ray emission
Photoemission
Probing valence statesPhotoemission and X-ray emission
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Cu
Nitrogen 1s resonant x-ray emission
Photoemission
Probing valence statesPhotoemission and X-ray emission
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 spectroscopyOccupied & unoccupied DOS:
sd
N-metal bonding
NiCuNi Cu
N-metal antibonding
Atomic Nitrogen on Ni and Cu
Nilsson et. al, Catal. Lett. 100, 111 (2005)
Bonding Strength
Nitrogen 1s resonant x-ray spectroscopyOccupied & unoccupied DOS:
sd
N-metal bonding
NiCu
Polymer Electrolyte Membrane Fuel Cells – Principle
Cathode
Anode
H2 H+ e-
H+
O2Membrane
e-
e-H2O
O2
Oxygen Reduction (ORR)Hydrogen Oxidation (HOR)
Transforms chemical energy of fuel into electrical energy OHOH 222
12
eHH 222OHeHO 22 222
1
Slow electrode kinetics Cost of catalyst Stability of catalystare most critical issues in fuel cell
research 13
Theoretical Modelling
Weak Pt–O bondStrong Pt–O bond
Nørskov et al., J. Phys. Chem. B, 2004, 108, 46: Greeley et al., Nature Chemistry, 2009, 1, 7
Parameters to control the electronic structure
Coordination # Alloy
step, kink, adatom
Lattice strain
Ligand
flat
Shift in D-bandOccupied Pt-DOS: Photoemission spectroscopy
Anniyev, unpublished
Pt layers on Cu(111)
EF
d-band center
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, NiPt
Due to a fixed substrate the lattice parameter is same so ligand effect can be isolated.
Valence bandhν = 620 eV
Tuning Pt d-band DOS by controlling 3d metal in the second layer
Oxygen/Pt-3d-Pt(111) – Oxygen 1s resonant x-ray spectroscopy results
Antibonding resonance inXAS decreases
Binding Energy
Intensity of the antibondingstates in XES increases
Pt-O
Pt-O*
The d-band center shifts….
O
Pt
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
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 photoemission8000 eV excitation, Spring-8 BL47XUSensitive to Pt
Valence band photoemission1486 eV excitation, BL13-2
Pt 5d DOS is obtainable!Dominated by support and Cu
support (carbon, Nafion)
Anniyev et al- PCCP 12, 5694 (2010)
Atom Selectivity
Selective excitation of inner and outer nitrogen atomsNilsson 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).
LCLS pump-probe experimentsO 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*
dπ
X-ray emission spectroscopy
occupied valence stateOxygen 2p component
X-ray absorption spectroscopy
unoccupied valence stateOxygen 2p component
Ru-CO π-bond
Ru-CO σ-bond
Spatially extended orbital
O1s
CO/Metal CO gas
5σ
1π
Electronic statesCO/MetalEnergy
Nilsson et al., Surf. Sci. Reps. 55, 49 (2004).
Charge Density Differences
gain of charge, attraction
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
loss of charge, repulsion
CO
C
O
s looses charge and p gains charge, but not in a frontier orbital senseAll orbitals are modified and new orbitals appear
We will monitor these orbitals with time-resolved XES and XAS as the CO/Ru bond weakens…
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: LCLSin operation from 2009
ultra short x-ray pulse: <100 fs – sub ps
LCLS
Ultrafast electronic structure probe
Most important catalytic reactionsare 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
Chemisorbedstate Reactive
state
Probing the Reactive State in Catalysis
Pump-Probe
How to initiate the reaction?Probing with adsorbate sensitivity the geometric
and electronic structureWhat intermediate species do we have?
How intermediate species are bonding to the surface?
CO Desorption from Ru(0001):Weakly Bound Precursor State
precursor
~30% ~15%
chemisorbed
En
ergy
/eV
0
-1
1
Precursor state
~15%
rigidquasi free
LCLS pump-probe experimentsX-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
Times scales and temperature
<1ps frustrated rotations>3ps moving to presursor
Hot electron driven Phonon driven
Phys. Rev. Lett. 110 (2013) 186101
New Era in Catalysis
• First surface chemical reaction with LCLS• Proof of principleObservation of two different excitations of COStrong coupling to motion parallel to the surface; early timesPrecursor 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”