A. Knop-GerickeFritz-Haber-Institut der Max-Planck-Gesellschaft, Depertment of Inorganic Chemistry,

D-14195 Berlin, Germany

Subsurface Species in Heterogeneous Catalytic Reactions: Subsurface Species in Heterogeneous Catalytic Reactions: Insights by in situ Photoelectron SpectroscopyInsights by in situ Photoelectron Spectroscopy

Outline

• Technical aspects

• Methanol oxidation over Cu

• Ethylene Epoxidation over Ag

Collaborators

LBNL & ALS: D.F. Ogletree, G. Lebedev, H. BluhmZ. Hussain, C.S. Fadley, M. Salmeron

FHI: M. Hävecker, K. Ihmann, E. Kleimenov,D. Teschner, S. Zafeiratos, E. Vass, P. Schnörch R. Schlögl

Boreskov Inst. of Catalysis: V.I. Bukhtiyarov

• Very few methods can investigatethe solid-gas interfaceat high pressures- non-linerar optics (SFG, SHG)- scanning probe microscopies- X-ray diffraction

Why in situ XPS ?Why in situ XPS ?

15

10

5

-40 -20 0 20

temperature (ºC)

vapo

rpr e

s sur

e(t o

r r)w

ater Tm

• Photoelectron spectroscopy is very powerful⇒ Goal: XPS at pressures of at least 5 torr

• Many processes cannot beinvestigated in UHV:“Pressure Gap”- environmental chemistry- catalysis- corrosion- electrochemistry- biological samples

Fundamental limit:elastic and inelasticscattering of electronsin the gas phase

Technical issues: - Differential pumping to keep analyzer in high vacuum- Sample preparation and control in a flow reactor

In situ XPS: obstaclesIn situ XPS: obstacles

30x10-21

25

20

15

10

5

0Io

niza

tion

cros

s se

ctio

n (m

2 )

101

102

103

104

105

106

Electron kinetic energy (eV)

in situSEM in situ

TEM

in situEXAFSin situ

XPS

O2 exp. data from Schram et al. (1965) extrapolation of Schram's data

• Photons enterthrough a window

• Electrons and a gasjet escape throughan aperture tovacuum

In situ XPS: basic conceptIn situ XPS: basic concept

H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 319 (1973)2

• H. Siegbahn et al. (1973- )• M.W. Roberts et al. (1979)• M. Faubel et al. (1987) • M. Grunze et al. (1988)• P. Oelhafen (1995)

H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 205 (1981)24

In situ XPS instruments: previous designsIn situ XPS instruments: previous designs

In situ XPS using differentially pumped electrostatic lensesIn situ XPS using differentially pumped electrostatic lenses

D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, Rev. Sci. Instrum. 73 (2002) 3872.

to pump to pump

X-rays from synchrotron

In situ XPS systemIn situ XPS system

Experimental cellsupplied by gas lines (p0)

X-rays enter the cell at 55° incidence through an SiNx window(thickness ~ 1000 Å)

Analyzerinput lens

Focal point of analyzerinput lens

First differentialpumping stage (10-4p0)

Second differentialpumping stage (10-6p0)

Third differentialpumping stage (10-8p0)

Hemisphercalelectronanalyzer(10-9p0)

mass spectrometerand additional pumping

Gas phase composition canbe measured by XPS.gas phase signal: 1 torr·mm ~ a few monolayers

CloseClose--up of sampleup of sample--first aperture regionfirst aperture region

gas

z

d

hν

e-

p0

1.0

0.5

0.0-2 -1 0 1 2

z [d]

p/p 0

z=2 mm

d=1 mm

Gas Flow systemGas Flow system

gas1

gasn

LV /MFCn n

Vg

QMS

TPMS

PTRMS

air

BP TPPTP1

DS1

A0 A1

SCh

LVQMS

LVP

LVm1

LVm2

VP

VSCh

LV /MFC1 1

QMS

1 bar1 bar

1010--66 mbarmbar1010--66 mbarmbar

Gas Phase analysisGas Phase analysis

••Quadrupole Mass Spectrometry (QMS)Quadrupole Mass Spectrometry (QMS)•Simple in use•Real time investigation•Sensitive (10 ppb)•Relatively quantitative•High fragmentation rate at high masses

••Proton Transfer Reaction Mass Spectrometer (PTRProton Transfer Reaction Mass Spectrometer (PTR--MS)MS)•Very sensitive to volatile organic compounds ( < 1 ppb)•Low fragmentation rate•Selective to substance with proton affinities higher than water

••Micro Gas Chromatography (MicroMicro Gas Chromatography (Micro--GC)GC)•Detection sensitivity (1 ppm)•Quantitative analysis•Not real time detection (> 30 s)

Sample holderSample holder

Sapphire

Stainless steel mask

Si-SiCceramics

Sample pellet

Stainless steel clips

4 electrical contacts

Sample HeatingSample Heating

to the Laser to the Laser

Laser CharacteristicsFibber coupled Diode Laser (Al-GaAs)Output Power (CW) <60 WCentre Wavelength :808 nmFibre Core diameter: 600 μm

Optical fiber Analyzer Aperture

Sample pelletSapphireSample Holder

(RT up to 1100 K)

Experiments done at undulator 49/2, PGM1 at

Electron Energy : 1.7 GeVStorage ring circumference : 240mRing current : 0.25 A

Synchrotron RadiationSynchrotron Radiation

Partial oxidation: CH3OH + ½O2 CH2O + H2O

Total oxidation: CH3OH + O2 CO2 + 2H2O

Cu

Cu32

What is the state of the surface under reaction conditions?

Application of in situ XPS to catalysis: methanol oxidation on CApplication of in situ XPS to catalysis: methanol oxidation on Cuu

Reaction profile isidentical at 1 atm.

Onset of catalyticactivity at T>200 °C

-0,1

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

0 50 100 150 200 250 300 350 400 450

Temperature / °C

Yie

ld

Y CH2O

Y CO2

CO + H O2 2

CH O+ H2 2O

O+ O

CH OH3

CH OH3

CH OH3

CO+ H2

CH O+ H2 2

O2

Cu 0

Oxsurf

Oxbulk

Subox

Ovol

A. Knop-Gericke et al., Topics Catal. 15, 27 (2001).

In situ NEXAFS

I.E. Wachs & R.J. Madix, Surf. Sci. 76, 531 (1978); A. F. Carley et al., Catal. Lett. 37, 79 (1996).UHV XPS

O-(a) + 2CH3OH(g) 2CH3O-(a) + H2O(g)

CH3O-(a) CH2O(g) + H+(a)

Questions for in situ XPS: - Quantitative analysis of surface species- Carbon species on the surface- Depth-dependent analysis

CH3OH + O2 ~ 0.5 mbar

suboxide phase: - only present in situ

Partial oxidation of methanolPartial oxidation of methanol

Experimental conditionsExperimental conditions

sample: polycrystalline Cu foil

Variations of mixing ratios: CH3OH : O2 = 1:2, 3:1, 6:1; T = 400 °C; p = 0.6 mbarTemperature series: gas mixture at room temperature: CH3OH : O2 = 3:1;

p = 0.6 mbar; temperature: 25 °C → 450 °Cflow rates: 10 ... 20 sccm

XPS measurements Beam line U49/2-PGM1 at BessyEnergy range 100...1500 eVtotal spectral resolution 0.1 eV @ 500 eV

O 1s, C 1s, Cu 3p, Cu 2p: KE ~ 180 eVValence Band: KE ~ 260 eV

Depth profiling with KEs 180 eV, 350 eV,500 eV, 750 eV

8 6 4 2 0 -2Binding energy (eV)

VB

Δ ~ 2 eV

160

140

120

100

80

60

40

20

0

Nor

mal

ized

cou

nt ra

te (c

ps/m

A)

542 540 538 536 534 532 530 528 526

Binding energy (eV)

CH

3OH

(g)

CH

2O (g

)H

2O (g

)

CO

2(g

)

O2 (g) Cu2O

?

?

Methanol oxidation on Cu: O1s spectraMethanol oxidation on Cu: O1s spectra

400 °C, 0.5 torr

CH3OH:O2

1:2

3:1

6:1

O 1s

O1s depth profilingO1s depth profiling

1.3

1.2

1.1

1.0

0.9

0.8

0.7529.

9 eV

/ 53

1.5

eV p

eak

area

ratio

14121086Inelastic mean free path (Å)

I529.9/I531.5=n529.9/n531.5·exp[-(z531.5-z529.9)/λ]

Δz = 3 Ǻ, n529.9/n531.5 = 1.6

14

12

10

8

6

Inel

astic

mea

n fre

e pa

th in

Cu

(Å)

800600400200Kinetic energy (eV)

from: Tanuma at al., SIA 17, 911 (1991).

CH3OH : O2 = 3:1

Nor

mal

ized

cou

nt ra

te (a

.u.)

534 532 530 528 526Binding energy (eV)

sub-surfaceoxygen

surfaceoxygen

A

180 eV

350 eV

500 eV

750 eV

Variation of the gas phase compositionVariation of the gas phase composition

160

140

120

100

80

60

40

20

0

Nor

mal

ized

cou

nt ra

te (c

ps/m

A)

542 540 538 536 534 532 530 528 526

Binding energy (eV)

CH

3OH

(g)

CH

2O (g

)H

2O (g

)

CO

2(g

)

O2 (g) Cu2Osub-surface

oxygensurfaceoxygen

400 °C

CH3OH:O2

1:2

3:1

6:1

O 1s

conversionCH3OH

yieldCH2O

yieldCO2

0.68

0.58

0.38

0.46

0.45

0.35 0.03

0.13

0.22

(part. press.CH3OH)

(part. press.CH2O)

(part. press.CO2)

0.030

0.103

0.072

0.173

0.179

0.075

0.307

0.167

0.053

partial pressures in mbar

Correlation of catalytic activity and surface speciesCorrelation of catalytic activity and surface species

CH2O yield vs sub-surface oxygen peak area

0.20

0.15

0.10

0.05

0.00

543210

normalized sub-surface oxygen peak area

1:2

3:1

6:1

CH

2O p

artia

l pre

ssur

e(m

bar) mixing ratio series

(T = 400 °C)

250 °C

300 °C

400 °C

450 °C

400 °C

150 °C

200 °C

temperature series(CH3OH:O2=3:1)

Open questions: What is the nature of thesub-surface oxygenspecies?What is its role in thecatalytic reaction?

Depth profilingDepth profiling

0.6

0.5

0.4

0.3

0.2

0.1

0.0sub-

surfa

ce o

xyge

n : C

u pe

ak ra

tio

14121086Inelastic mean free path (Å)

CH3OH:O2 = 3:1CH3OH:O2 = 6:1

(calculated from Cu 3p and sub-surface O 1s)

Reducing conditions

Open questions: What is the nature of the sub-surface oxygen species?What is its role in the catalytic reaction?

160

140

120

100

80

60

40

20

0

Nor

mal

ized

cou

nt ra

te (c

ps/m

A)

542 540 538 536 534 532 530 528 526

Binding energy (eV)

CH

3OH

(g)

CH

2O (g

)H

2O (g

)

CO

2(g

)

O2 (g) Cu2O

?

?

Methanol oxidation on Cu: C1s spectraMethanol oxidation on Cu: C1s spectra

O 1s

292 288 284 280Binding energy (eV)

CH3OH:O2

1:2

3:1

6:1

CO

2(g

)

CH

3OH

(g)

CH

2O (g

)

Camorph

C 1s

400 °C

Cu L3- NEXAFS

Increased activity forgas flow ratios:O2 / CH3OH ≤ 0.5

Increased activity forgas flow ratios:O2 / CH3OH ≤ 0.5

0.2 0.4 0.6 0.8 1

Flow ratio O 2 / CH3OH

40

60

80

100

Con

vers

ion

CH

3OH

(%)

925 930 935 940 945

Photon Energy / eV

0

0.5

1

1.5

2

2.5

3

3.5

To

tal E

lect

ron

Yie

ld (

a. u

.)

Flow ratio O2 / CH3OH

0.6

0.2

0.2

Reference Cu2O

300 K

570 K

670 K

670 K

Temperature

Cu L3-edge

Catalytic Activity NEXAFS at the Cu L3-edge

Transition from an oxidic copper-phase to the metallic state

Transition from an oxidic copper-phase to the metallic state

•NEXAFS of the activestate is completlydifferent from theNEXAFS of the knowncopper-oxides

• 2 oxidic- and 1 suboxidic species can bedistinguished

•NEXAFS of the activestate is completlydifferent from theNEXAFS of the knowncopper-oxides

• 2 oxidic- and 1 suboxidic species can bedistinguished

520 530 540 550 560 570

Photon Energy / eV

0

2

4

6

8

10

Tota

l Ele

ctro

n Yi

eld

(a. u

.)

O K-edge

Reference Cu2O

300 K

570 K

670 K

Temperature

532.8 eV

531.6 eV

536.6 eV

Flow ratio O2 / CH3OH

670 K

0.6

0.2

0.2

NEXAFS at the O K-edge

less active

very active

Oxygen species on Ag (111)Oxygen species on Ag (111)MotivationMotivation

Important Industrial Applications Important Industrial Applications :

H2C=CH2

H2C--CH2

O

2CO2 + 2H2O

½ O2

3O2

500-600 K

3O2

CH3OH

CO2 + 2H2O

HCH + H2OO

800-900 K

½ O2

““SimpleSimple”” Model adsorption system for theoretical studiesModel adsorption system for theoretical studies

1.1. Physisorbed OPhysisorbed O222.2. Chemisorbed OChemisorbed O223.3. Surface adsorbed atomic OSurface adsorbed atomic O4.4. Subsurface or bulkSubsurface or bulk--dissolved atomic Odissolved atomic O

Literature review :Oxygen species reported on silverLiterature review :Oxygen species reported on silver

no529.2±0.2Atomic oxygen

strongly chemisorbed on Ag

OOγγ

no530.4±0.2Atomic oxygen adsorbed on Ag

surface

OOadsads or or OOelecelec

COCO3,ads3,ads

HH22OOadsads

OHOHadsads

OOββ

OOnuclnucl or or pp(4(4××4)4)--OO

Species

no530.2 ±0.3Surface carbonates

no533.2 ±0.3H2O adsorbed

no531.5 ±0.4Hydroxyl species

no530.5 ±0.5Atomic oxygen

incorporated in the Ag subsurface/bulk

Yes528.3±0.2Atomic oxygen embedded on Ag

Formation of ionic AgBE (eV)Description

1.1. OO22 Pressure Pressure 2.2. TemperatureTemperature3.3. Silver surface crystallographic orientationSilver surface crystallographic orientation4.4. Defect densityDefect density5.5. PrePre--treatment historytreatment historyOβ

Oads

Oβ

Ag+Ag+

Oγ Onucl

Silver

gas phase O2

Oxygen species on Ag (111) at 500 and 600 K in 0.13 mbar OOxygen species on Ag (111) at 500 and 600 K in 0.13 mbar O22

500 K500 K

600 K600 K

534 532 530 528 526 370 369 368 367

367.7368.1528.5

Binding Energy / eVBinding Energy / eV

530.4

Ag 3d5/2

hv=555 eV

O 1shv=720 eV

Inte

nsity

/ a.

u

×5

×5

*Sample was first cleaned (sputtered-annealed) then temp. raised and 0.13 mbar O2 introduced

~0.3 ML~0.3 ML

~0.2 ML~0.2 ML

Total O Total O coveragecoverage

Oxygen species on Ag (111) at 600 K in 0.13 mbar OOxygen species on Ag (111) at 600 K in 0.13 mbar O22Comparison between O 1s spectra recorded using different excitatComparison between O 1s spectra recorded using different excitation energiesion energies

Surface

Bulk

534 532 530 528 526

Inte

nsity

/ a.

u

Binding Energy / eV

Bulk Bulk (1220 eV)(1220 eV)

Surface Surface (720 eV)(720 eV)

O 1sO 1s

SurfaceSurface

BulkBulk

369 368 367

600 K

410 K

Ag 3d hv=555 eV

Inte

nsity

/ a.

u

Binding Energy / eV

Oxygen species on Ag (111) in 0.13 mbar OOxygen species on Ag (111) in 0.13 mbar O22Oxygen Species related with ionic silver formationOxygen Species related with ionic silver formation

534 532 530 528 526

Binding Energy / eV

Inte

nsity

/ a.

u

390 K

430 K

450 K

470 K

520 K

600 K

O 1s hv=720 eV

TT

Related with AgRelated with Ag++

AgAg++

Oxygen species on Ag (111) at 500 and 600 K in 0.13 mbar OOxygen species on Ag (111) at 500 and 600 K in 0.13 mbar O22

500 K500 K

600 K600 K

534 532 530 528 526 370 369 368 367

367.7368.1528.5

Binding Energy / eVBinding Energy / eV

530.4

Ag 3d5/2

hv=555 eV

O 1shv=720 eV

Inte

nsity

/ a.

u

×5

×5Oads

Ag+

Onucl

Silver

gas phase O2

OadsOads

Ag+

Onucl

Ag+

Onucl

Silver

gas phase O2

J. Schnadt et.al PRL 96, 146101 (2006)

Ethylene Epoxidation : Ethylene Epoxidation : Existing reaction modelsExisting reaction models

R. A. van Santen, R. Lambert, C. Campbell (1985)

M. A. Barteau (2004)

AgAg--O O interaction interaction determines determines atomic oxygen atomic oxygen chargecharge

OxametallacycleOxametallacycle(reaction (reaction intermediate)intermediate)

Ethylene epoxidation on SilverEthylene epoxidation on Silver Powder Samples Powder Samples SEM picturesSEM pictures**

*Commercially available samples (Sigma-Aldrich)

Activated Ag Activated Ag NanoNano--powder powder (~100 nm, high defect structure)(~100 nm, high defect structure)

SAMPLE 1 : SAMPLE 1 : AgAg--nanonano

Ag powder Ag powder (<600(<600 μμm)m)

3 3 μμmm

500 500 nmnm

SAMPLE 2 : SAMPLE 2 : AgAg--powderpowder

0 1 2 3 40.00

0.01

0.02

520 K

470 K420 K400 K

370 K

Ag nanopowder (~100 nm)(fresh)

C2H

4O P

artia

l Pre

ssur

e / m

bar

Time /hours

45

0.0 0.2 0.4 0.60.00

0.01

0.02 45

Time /hours

C2H

4O P

artia

l Pre

ssur

e / m

bar

520 K

Ag powder (~30 μm)

Powder Samples : Catalytic resultsPowder Samples : Catalytic resultsDetection of CDetection of C22HH44O (PTRMS) : O (PTRMS) : Reaction conditions : Reaction conditions : CC22HH44/O/O22 (1/2), P=0.5 mbar(1/2), P=0.5 mbar

Ag Ag nanonano

Ag powderAg powder

Silver particles agglomeration Silver particles agglomeration loss of surface arealoss of surface area

Highly active Highly active for ethylene for ethylene epoxidationepoxidation

Inactive for Inactive for ethylene ethylene

epoxidationepoxidationunder our under our conditionsconditions

Powder Samples : Post reaction characterizationPowder Samples : Post reaction characterization(C(C22HH44/O/O22 (1/2), P=0.5 mbar, @300 K)(1/2), P=0.5 mbar, @300 K)

Ag Ag nanonano

Ag powderAg powder

x 103

85

90

95

100

105

110

CPS

536 534 532 530 528Binding Energy (eV)

x 105

2

4

6

8

10

12

CPS

371 370 369 368 367 366 365Binding Energy (eV)

x 103

130

140

150

160

170

180

CPS

536 534 532 530 528Binding Energy (eV)

x 105

2

4

6

8

10

12

14

CPS

371 370 369 368 367 366 365Binding Energy (eV)

O 1sO 1s Ag 3dAg 3d••530.8 eV530.8 eV••532.7 eV532.7 eV

••529.1 eV529.1 eV••530.2 eV530.2 eV••531.6 eV531.6 eV

••368.15 eV368.15 eV

••368.15 eV368.15 eV

x 104

14

16

18

20

22

24

292 290 288 286 284 282Binding Energy (eV)

C 1sC 1s

EK=220 eVEK=220 eV

Intensity ratioIntensity ratio

O/Ag =0.138O/Ag =0.138

O/Ag =O/Ag = 0.0840.084

Powder Samples : Powder Samples : Comparison Post and inComparison Post and in--situ characterizationsitu characterization

O 1sO 1s

x 103

130

140

150

160

170

180

CPS

536 534 532 530 528Binding Energy (eV)

x 103

85

90

95

100

105

110

CPS

536 534 532 530 528Binding Energy (eV)

x 103

120

130

140

150

160

170

180

190

CPS

536 534 532 530 528Binding Energy (eV)

x 104

10

15

20

25

30

CPS

536 534 532 530 528Binding Energy (eV)

Ag powderAg powderAg Ag nanonano

In s

ituIn

situ

Pos

t rea

ctio

nP

ost r

eact

ion

x 105

2

4

6

8

10

12

14

CPS

371 370 369 368 367 366 365Binding Energy (eV)

x 105

2

4

6

8

10

12

CPS

371 370 369 368 367 366 365Binding Energy (eV)

x 104

10

20

30

40

50

60

70

80

CPS

371 370 369 368 367 366 365Binding Energy (eV)

x 104

10

15

20

25

30

35

CPS

371 370 369 368 367 366 365Binding Energy (eV)

Ag 3dAg 3d5/25/2 Ag powderAg powderAg Ag nanonano

In s

ituIn

situ

Pos

t rea

ctio

nP

ost r

eact

ion

Differences between Differences between in situin situand and post reactionpost reaction spectraspectra :•Oxygen species•Silver species•Oxygen amount•Adsorbed carbon

Differences between Differences between active active and and nonnon--active active catalystcatalyst :•Oxygen species•Silver species•Oxygen amount

Ag9O

Powder Samples : In situ reaction characterizationPowder Samples : In situ reaction characterization(Reaction conditions : C(Reaction conditions : C22HH44/O/O22 (1/2), P=0.5 mbar, @520 K)(1/2), P=0.5 mbar, @520 K)

Ag Ag nanonano

Ag powderAg powder

x 104

10

15

20

25

30

CPS

536 534 532 530 528Binding Energy (eV)

x 104

10

15

20

25

30

35

CPS

371 370 369 368 367 366 365Binding Energy (eV)

x 103

120

130

140

150

160

170

180

190

CPS

536 534 532 530 528Binding Energy (eV)

x 104

10

20

30

40

50

60

70

80

CPS

371 370 369 368 367 366 365Binding Energy (eV)

O 1sO 1s Ag 3dAg 3d••530.2 eV530.2 eV••531.7 eV531.7 eV

••532.2 eV532.2 eV

••368.15 eV368.15 eV••367.65 eV367.65 eV

••368.15 eV368.15 eV

x 103

64

66

68

70

72

74

76

78

290 289 288 287 286 285 284 283 282Binding Energy (eV)

C 1sC 1s

Intensity ratioIntensity ratio

O/Ag =0.21O/Ag =0.21

O/Ag =O/Ag = 0.850.85

100 200 300 400 500 600 700

10

20

30

40

50

60

70

80

90

%

Inte

nsity

Photoelectron Kinetic Energy / eV

368.15 eV 367.65 eV

100 200 300 400 500 600 70010

20

60

70

80

90

% In

tens

ity

Photoelectron Kinetic Energy / eV

530.2 eV 531.7 eV

Location of ionic SilverLocation of ionic Silver

Location of oxygenLocation of oxygen

SurfaceSurface

Less surfaceLess surface

Less surfaceLess surface

Comparison of oxygen species in all samplesComparison of oxygen species in all samples

536 534 532 530 528 526

Inte

nsity

/ a.

u

Binding Energy / eV

AgAg--powderpowder0.2 mbar O0.2 mbar O2 2 at 520 Kat 520 K

AgAg--nanonano0.2 mbar O0.2 mbar O2 2 at 520 Kat 520 K

Ag (111)Ag (111)pretreatedpretreated0.13 mbar O0.13 mbar O2 2 at 600 Kat 600 K

Ag (111)Ag (111)--freshfresh0.13 mbar O0.13 mbar O2 2 at 600 Kat 600 K

O 1sO 1s

Oads

Ag+

Onucl

Silver

gas phase O2

OadsOads

Ag+

Onucl

Ag+

Onucl

Silver

gas phase O2

Oads

Ag+

Onucl

Silver

gas phase O2

Oβ

Oγ

Ag+

OadsOads

Ag+

Onucl

Ag+

Onucl

Silver

gas phase O2

Oβ

Oγ

Oβ

Oγ

Ag+Ag+

Oads

Ag+

Oβ

Oγ

Silver

gas phase O2

Oads

Ag+

Oβ

Oγ

Oβ

Oγ

Silver

gas phase O2

O

OβSilver

gas phase O2

Ag+

Oβ

Ag+

OO

OβSilver

gas phase O2

Ag+

Oβ

Ag+

O

~0.3 ML~0.3 ML

~0.5 ML~0.5 ML

Total O Total O coveragecoverage

~1.5 ML~1.5 ML

~3 ML~3 ML

Dr. Axel Knop-Gericke

Dr. Michael Hävecker

Peter Schnörch

Dr. Detre Teschner

Dr. Elaine VassDr. Spiros Zafeiratos

Dr. Zoltan Hlavathy