Fundamental Interactions on Surfaces

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)

X-ray Spectroscopy

Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).

The D-band Model

Hammer and Nørskov, Adv. Catal., 2000, 45, 71.

Vacuum

Ener

gy

Adsorbate projected DOS

Coupling to s

Metal projected DOS

sd

Coupling to d

bonding

antibonding

X-ray spectroscopy

X-ray spectroscopy

Additional probing of O and metal core-level shifts with XPS

X-ray Photoelectron Spectroscopy

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- H2OO2

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Anniyev, unpublished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

5σ1π

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

CO

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

Ene

rgy/

eV

0

-1

1

Precursor state

~15%

rigidquasi free

LCLS pump-probe experimentsX-ray emission and X-ray absorption spectrscopy

pump

Time Δt/ps

O1sheat 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 presursorHot 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”