X-ray absorption spectroscopy

Jagdeep Singh

Jeroen A. van Bokhoven

Absorption as function of energy of the x-ray

8800 9000 9200 9400 9600 9800-0.5

0.0

0.5

1.0

1.5

2.0

Ab

sorp

tion

(a.u

.)

Energy (eV)

0 200 400 600 8000.0

0.2

0.4

0.6

0.8

1.0

Abso

rptio

n (a

.u.)

Energy (eV)

8950 8975 9000 9025

0.0

0.5

1.0

1.5

2.0

Abso

rptio

n (a

.u.)

Energy (eV)

2 4 6 8 10 12 14-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

χ(k)

k (Å-1)

a.

c. d.

b.

8800 9000 9200 9400 9600 9800-0.5

0.0

0.5

1.0

1.5

2.0

Ab

sorp

tion

(a.u

.)

Energy (eV)

0 200 400 600 8000.0

0.2

0.4

0.6

0.8

1.0

Abso

rptio

n (a

.u.)

Energy (eV)

8950 8975 9000 9025

0.0

0.5

1.0

1.5

2.0

Abso

rptio

n (a

.u.)

Energy (eV)

2 4 6 8 10 12 14-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

χ(k)

k (Å-1)

8800 9000 9200 9400 9600 9800-0.5

0.0

0.5

1.0

1.5

2.0

Ab

sorp

tion

(a.u

.)

Energy (eV)

0 200 400 600 8000.0

0.2

0.4

0.6

0.8

1.0

Abso

rptio

n (a

.u.)

Energy (eV)

8950 8975 9000 9025

0.0

0.5

1.0

1.5

2.0

Abso

rptio

n (a

.u.)

Energy (eV)

2 4 6 8 10 12 14-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

χ(k)

k (Å-1)

a.

c. d.

b.

Pre-edge subtraction Edge energy determination

Background and Normalization EXAFS Function

Data-analysis

4 6 8 10 12 14-60

-40

-20

0

20

40

60

χ(k)

k(Å-1)

1 2 3 4 5 6

-4

-2

0

2

4

Four

ier t

rans

form

(k2 *χ

(k))

R (Å)

Fourier transformation

1st neighbor

2nd neighbor

EXAFS formula

radial distribution function

exp

Scatter power

Damping Disorder

(−2σi k ) ( )( )2

2 202

2R(k) i

i i

SkR

χλ

−∑= NiFi(k) exp sin 2kRi + ϕj k

Scatter power

Damping Disorder

( ) ( )( )2

2 202

2( ) ( ) exp exp 2 sin 2ii i i i j

i i

S Rk N F k k kR kkR

χ σ ϕλ

−⎛ ⎞= − +⎜ ⎟⎝ ⎠

∑

0 5 10 15 200.00

0.05

0.10

0.15

0.20

0.25

Bac

ksca

tterin

gsAm

plitu

de

k (Å)

Pt - O

Pt - Pt

Temperature effect on EXAFS / XANES

Temperature effectScatter power

Damping Disorder

( ) ( )( )2 2exp 2 sin 2i i jk kR kϕ+20

2

2( ) ( ) exp ii i

i i

S Rk N F kkR

χ σλ

−⎛ ⎞= −⎜ ⎟⎝ ⎠

∑

exp(-2σ2k2)

0 5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

Low Tσ2 = 0.001

High Tσ2 = 0.01

K (A-1)

Fj, ϕj, and So2 from reference compound or theory

( ) ( )( )2

2 202

2( ) ( ) exp exp 2 sin 2ii i i i j

i i

S Rk N F k k kR kkR

χ σ ϕλ

−⎛ ⎞= − +⎜ ⎟⎝ ⎠

∑

0 1 2 3 4 5 6

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Four

ier t

rans

form

(k3 )

R (Å)

Au/Al2O3

3.04 < K < 11.5 A-1

0

Coordination number 6.8Au-Au distance 2.76 ÅΔDWF 0.0058C3 9 E-6C4 3E-6

Getting structural information from EXAFS

Added parameter: ΔE0

Abstract

• EXAFS gives local structure

• XANES gives geometry and oxidation state (empty density of states)

exp

Scatter power

Damping Disorder

(−2σi k ) ( )( )2

2 202

2R(k) i

i i

SkR

χλ

−∑= NiFi(k) exp sin 2kRi + ϕj k

EXAFS (χ) Extended X-ray Absorption Fine-StructureSingle scattering

XANES X-ray Absorption Near-Edge StructureMultiple scattering

Single Scattering

Multiple Scattering

A

Pre edge

XANES versus EXAFS

EXAFS- Single scattering dominates

XANES- Multiple scattering- Electronic transitions- Multiple electron transitions

Information- Number & kind of neighbor- Distance- Disorder

- Geometry / subtle distortions- Oxidation state- Electronic information- DOS in final state

Theoretical description of XANES- Detailed electronic information- Aids interpretation of spectra of unknown compounds- Time-consuming - Needs an expert

A

GoalLocal structure of catalysts under well-defined conditions

precursor state

during / after activation

during reaction

deactivation

time-resolution (few msec)

space-resolution (few μm)

… … .

XAS in Catalysis

3160 3170 3180 3190 3200-0.5

0.0

0.5

1.0

1.5

Difference

HydrideNaked paticle

2%Pd/SiO2

Energy (eV)

Nor

mal

ized

Abs

orpt

ion

10 nm

3160 3170 3180 3190 3200-0.5

0.0

0.5

1.0

1.5

Palladium hydride formation

Difference

Hydride

Naked paticle

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

2%Pd/Al2O3

<2 nm

Metal d-band reactant level

Fermi level

εd

-

+

Anti bonding state

3160 3170 3180 3190 3200-0.5

0.0

0.5

1.0

1.5

Difference

HydrideNaked paticle

2%Pd/SiO2

Energy (eV)

Nor

mal

ized

Abs

orpt

ion

-15 -10 -5 0 5 10 150.0

0.5

1.0

1.5

2.0

2.5

0.0

0.5

1.0

1.5

2.0

s p d s (H) p (H)

DOS vs XANES for PdH0.64

(93 atoms) M

uX

eV

-15 -10 -5 0 5 10 150.0

0.5

0.0

0.5

1.0

1.5

2.0

s p d s (H) p (H)

DOS vs XANES for PdH0.64 (93 atoms)

MuX

eV

XAS

Partial / complete oxidation of hydrocarbons methane, alkenes, methanol

Hydrogenation / dehydrogenation reactions alkenes, alkynes, alkadienes, (un)saturated ketones

Methanol synthesisCO + 2H2 CH3OHCO2 + 3 H2 CH3OH + H2O

WGSH2O + CO CO2 + H2

Nitric Oxide reduction (with CO, olefins, or H2)

CO oxidation1925: Active in CO oxidationhighly active in presence of H2: Haruta Catal. Today 36 (1997) 153Selective CO removal, air purification, high-purity N2 and O2

Catalysis by Gold

Physical properties• bulk metallic gold is thermodynamically stable • melting point and metallicity of the particle is function of particle size

Catalysis by Gold

Buffat Phys. Rev. A 13 (1976) 2287

CO oxidation: particle-size effect

Physical properties• bulk metallic gold is thermodynamically stable • melting point and metallicity of the particle is function of particle size

Catalysis by Gold

Goodman Science 281 (1998) 1648 Haruta Cattech 3 (2002) 102

CO oxidation: particle-size effect

Large support effects: SiO2: hardly activeAl2O3, MgO: moderately active(TiO2) Fe2O3, CeO2, other reducible supports: very active & less dependent on particle sizes; No clear relation to reducibility of support

Physical properties:• bulk metallic gold is thermodynamically stable • melting point and metallicity of the particle is function of particle size

Active species in gold oxidation catalysis? • Carbonate-mechanism excluded• Small particles become active as soon as they are non-metallic (Goodman)• Oxidic gold (I or III) is active species (Gates)• Theory supports both gold-only and support-aided mechanism• Support supplies oxygen via molecularly (activated) adsorbed oxygen

via Mars van Krevelen

Catalysis by Gold

Geometry / coordination

Density of states

Oxidation state

Time resolved

In situ

How is oxygen activated on the catalyst?

How can the most inert metal be so active?

Small gold particles adsorb oxygen(and react)

Nørskov et al. Angew. Chem. 44 (2005) 1824

• Deposition precipitation HAuCl4 adjusted pH• Washing with a base to remove chlorine• Reduction in hydrogen

SupportsAl2O3, SiO2, CeO2, TiO2, ZrO2, Nb2O5

Sample Preparation

Full EXAFS & XANES analyses

Structure of gold catalysts

EXAFS Fitting Results of Reduced Catalysts

2.70

2.72

2.74

2.76

2.78

2.80

2.82

2.84

2.86

2.88

2 4 6 8 10 12

Au-Au CN

Au

Bon

d D

ista

nce,

ÅBulk value

2.88 Å

± 10-20%

± 0.02

0 10 20 30 40 50 60 70 80

2.72

2.76

2.80

2.84

2.88

Al2O3 TiO2 SiO2 Nb2O5 ZrO2 CeO2

Au-A

u D

ista

nce

Particle Size (Å)

EXAFS Fitting Results of Reduced Catalysts

Strong reduction in Au-Au distance with particle sizeNo visible influence of support

30%50%

100%

Electronic structure from LIII XANES

Whitelines reflect number of holes in the d-bandGold whiteline: spd-rehybridization results in 5d10–x6sp1+x

11875 11900 11925 11950 11975 120000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Au(0)Au(III)

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

Mattheiss, L.F. et al. Phys. Rev. B 1980, 22, 1663

11.91 11.92 11.93 11.94 11.950.0

0.2

0.4

0.6

0.8

1.0

Bulk Gold 32 Å 16 Å 11 Å

Nor

mal

ized

Abs

orpt

ion

Energy (keV)11.91 11.92 11.93 11.94 11.95

0.0

0.2

0.4

0.6

0.8

1.0

Bulk Gold 32 nm 12 nm 5 nm

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

11.92 11.930.4

0.6

0.8

Au/ZrO2 Au/Al2O3 Au/TiO2 Au/Al2O3 Bulk Gold

Nor

mal

ized

Abs

orpt

ion

Energy (keV)

Au/Al2O3 Au/TiO2

1.1 nm Au

Whiteline is particle-size dependent

Whitelines reflect number of holes in the d-band

3 4 5 6 7 8 9 10 11 12

0.00

0.15

0.30

0.45

0.60

Au/Al2O3 Bulk Au/ZrO2 CeO2 SiO2 Nb2O5 TiO2

Diff

eren

ce in

WL

Inte

nsity

Au-Au Coordination Number2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90

0.00

0.15

0.30

0.45

0.60

Au/Al2O3 Bulk Au/ZrO2 CeO2 SiO2 Nb2O5 TiO2

Diff

eren

ce in

WL

Inte

nsity

Au-Au Distance (Angstron)

Whiteline intensity versus particle sizeDifference intensity with bulk

Six supports, one trendLarger particles fewer d-electrons

CN R(Å) %Au(III)Reduced 3.6 2.72 0Reox. RT 3.6 / 0.3 2.72 / 2.04 10Reox. 225C 2.7 / 0.5 2.71 / 2.04 15

11900 11920 11940 119600.0

0.2

0.4

0.6

0.8

1.0

Reduced (H2 250C)Reoxidized (20% O2 RT)Reoxidized (20% O2 225C)

Nor

mal

izd

Abs

orpt

ion

Energy (eV)

1.3wt% Au/Al2O3

11875 11900 11925 11950 11975 120000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Au(0)Au(III)

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

Exposure to 20% O2

11900 11910 11920 11930 11940 11950 119600.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

Au/Al2O3

Au whiteline

CN(Au) R(Au)He 6.5 2.771:1 5.3 2.732:1 5.7 2.731:2 5.2 2.77

Small oxygen contribution

11930 11940 11950

0.6

0.8

1.0

XAS during CO Oxidation

CO/O2 2/1CO/O2 1/1CO/O2 1/2HeN

orm

aliz

ed A

bsor

ptio

nEnergy (eV)

119200.4

More intense with more CO:holes in the d-band (anti-bonding states)

11910 11920 11930 11940 119500.0

0.2

0.4

0.6

0.8

1.0

Norm

aliz

ed A

bsor

ptio

n

Energy/eV11910 11920 11930 11940 11950

0.0

0.2

0.4

0.6

0.8

1.0

Norm

aliz

ed A

bsor

ptio

n

Energy/eV11910 11920 11930 11940 11950

0.0

0.2

0.4

0.6

0.8

1.0

Norm

aliz

ed A

bsor

ptio

n

Energy/eV

Gold catalysts and activation of oxygen

• Under (diluted) O2: surface oxidation (Au/Al2O3 & Au/TiO2)

• Switch to CO/O2: CO keeps gold reduced

Au/Al2O3Switch Oxygen to CO2s repetition

Reduced gold is active phase

Gold participates in oxygen activation

Au(0) Au(0)

Aun+

O

OC

O

CO + O2

Au(0)Au(0) Au(0)

Aun+

O

OC

O

CO + O2

Au(0) Au(0)

Aun+

O

OC

O

CO + O2

Au(0)

Au(0) Au(0)

Aun+

OO

O2

Au(0) Au(0)

Aun+

OO

CO + O2

Au(0)

OC

Au(0)

OC

Fast

Van Bokhoven Angew. Chem. (2006) VIP

Slow

• Rehybridization of spd-orbitals (5d10–x6sp1+x)

• Smaller particles have fewer holes in the d-band

• Particle size dominates support-effect

• Oxygen is activated on gold particle

0 10 20 30 40 50 60 70 80

2.72

2.76

2.80

2.84

2.88

Al2O3 TiO2 SiO2 Nb2O5 ZrO2 CeO2

Au

-Au

Dis

tanc

e

Particle Size (Å)

Boiling point Gold-Gold distance d electrons

2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90

0.00

0.15

0.30

0.45

0.60

Au/Al2O3 Bulk Au/ZrO2 CeO2 SiO2 Nb2O5 TiO2

Diff

eren

ce in

WL

Inte

nsity

Au-Au Distance (Angstron)

Fermi level

d-LDOS

He Traces O21%CO(He)

Pt/Al2O3

Adsorption sites from XAS

bare atop

FEFF8 simulation

HeCO

Experimental

platinum catalystStructure of the active phase?

fuel cell (PEM) catalytic converter

surface rougheningsurface patternssurface reaction

Single Crystals

Langmuir-Hinshelwood

UHV conditions

Single Crystals

Langmuir-Hinshelwood

UHV conditions high pressure

Two reaction regimes

• Low activity – CO poisoning

• High activity ???

Single Crystals

UHV conditions

Real Catalysts

Real Conditions

1.8 nm

1 nm

1.3 nm

pressure gap

material gap

Conversion data

350 400 450 500 5500.00

0.02

0.04

rate

of C

O c

onve

rsio

n (c

m3 /m

in/g

cat)

Temperature (K)

Low activity region

O2/CO = 521

High activity region

2wt%Pt/Al2O3

Conversion dataignition or extinction

temperature (K)O2/CO ratio

heating cooling

1 472 456 yes 340

2 445 440 yes 338

5 433 421 yes 329

hysteresis temperature for

onset ofconversion

(K)

XAS

11560 11570 11580 115900

1

2

3

heating

Energy (eV)

heating

308 K340 K386 K

455 K

422 K

439 K

472 K

O2/CO = 2

350 400 450 500 550 600

Temperature (K)

11560 11570 11580 115900

1

2

3

Energy (eV)

O2

CO

XAS

11560 11570 11580 115900

1

2

3

cooling

Energy (eV)

cooling

308 K340 K386 K

455 K

422 K

439 K

472 K

O2/CO = 2

11560 11570 11580 115900

1

2

3

Energy (eV)

O2

CO

350 400 450 500 550 6000.00

0.02

0.04

Temperature (K)

ratio O2 / CO = 1

11560 11570 11580 115900

1

2

3

heating

(a)

Energy (eV)

heating

ratio O2 / CO = 2

11560 11570 11580 115900

1

2

3

heating

(a)

Energy (eV)

heating

ratio O2 / CO = 5

11560 11570 11580 115900

1

2

3

heating

(a)

Energy (eV)

heating

oxidation parallels ignition

350 400 450 500 5500.00

0.02

0.04

rate

of C

O c

onve

rsio

n (c

m3 /m

in/g

cat)

Temperature (K)

125

Kinetics and XAS

350 400 450 500 5500.00

0.02

0.04

rate

of C

O c

onve

rsio

n (c

m3 /m

in/g

cat)

Temperature (K)

Low activity region

O2/CO = 521

High activity region

adsorbed CO

partially oxidicsurface

Kinetics and XAS

350 400 450 500 5500.00

0.02

0.04

rate

of C

O c

onve

rsio

n (c

m3 /m

in/g

cat)

Temperature (K)

Low activity region

O2/CO = 521

High activity region

adsorbed CO

partially oxidicsurface

Qexafs signalsPt foilL3, L2, L1 edges

XAS (QEXAFS)

470 471 472 473 474 47540

50

60

70

80

90

100

CO

Con

vers

ion

%

Temperature/ K

11580 11600 11620

0.9

1.2

1.5

Energy (eV)

O2/CO = 1

Kinetics and XAS

350 400 450 500 5500.00

0.02

0.04

rate

of C

O c

onve

rsio

n (c

m3 /m

in/g

cat)

Temperature (K)

heating

11560 11570 11580 115900

1

2

3

heating

Energy (eV)

heating

308 K340 K386 K

455 K

422 K439 K

472 KCO poisoning

partially oxidicsurface

J. Singh, et al. Angew. Chem. Int. Ed., accepted

incr

easi

ngly

oxid

ized

Conclusions

• a highly active state of the catalyst is discovered

• the catalyst shows different structure in low- and high-

activity regime; low-activity region : CO adsorbed on

platinum, high-activity region: partially oxidized platinum

• the catalyst increasingly oxidizes during the ignition

• high temperature and a high oxygen concentration

benefit the formation of the more active partially oxidic

catalyst.

Other supports(normal XAS)

Pt/Al2O3L, 2 nm

11.55 11.56 11.57 11.58 11.59 11.600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

norm

aliz

ed a

bsor

ptio

n

Energy, keV

125oC, 2% conversion 194oC, 9% conversion 210oC, 20% conversion 218oC, 32% conversion 231oC, 100% conversion

SiO2, 1.5-3 nm

11.55 11.56 11.57 11.58 11.59 11.600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Energy, keV

40oC, 0% conversion 140oC, 1% conversion 240oC, 6% conversion 294oC, 45% conversion 306oC, 100% conversion

norm

aliz

ed a

bsor

ptio

n

TiO2, 1.3 nm

11.55 11.56 11.57 11.58 11.59 11.600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

norm

aliz

ed a

bsor

ptio

n

Energy, keV

68oC, 4% conversion 134oC, 24% conversion 180oC, 67% conversion 190oC, 100% conversion

Extinction plots

100 150 200 250 3000

20

40

60

80

100

perc

ent C

O c

onve

rsio

n

temperature, oC

Pt/TiO2 Pt/Al2O3Pt/SiO2

3 4 5 6 7 8 9 10 11 12

-6

-4

-2

0

2

4

6

o

Chi

x k

3

k (A-1)

Chi FitA

1.5 2.0 2.5 3.0 3.5 4.0

-4

-2

0

2

4

oFT

(A3 )

R (A)

Fourier Transform Fit

o

B

K3 weighted CHI and Fourier transform2.5 < k < 12 Å-1

Pt/Al2O3L reduced, hydrogen removed; He (RT)

EXAFS analysisBelow and above temperature of ignition

Conditions atom N DW R(Å) Eo

RT, He Pt 5.7 0.0042 2.72 1.68

below ignition Pt 6.2 0.0054 2.77 0.58

O 2.4 0.0021 1.99 4.36above ignition

Pt 3.0 0.0065 2.61 7.19

RT, O2/CO Pt 5.6 0.0039 2.76 1.87

EXAFS analysis

350 400 450 500 550 6000.00

0.02

0.04

rate

of C

O c

onve

rsio

n (c

m3 /m

in/g

cat)

Temperature (K)

0.25

0.5125

Pt /Al2O3 small particles

Kinetic oscillations – Single Crystals

T = 470KpCO= 3×10-5mbarpO2= 2.0 2.7×10-4mbar

M. Eiswirth and G. Ertl, Surface Sci. 177 (1986), 90

Pt (110)

Kinetic oscillations – Single Crystals

bulk crystal plane

Pt (110)

Kinetic oscillations – Single Crystals

T. Gritsch et al. Phys. Rev. Lett. 63 (1989) 1086N. Freyer et al. Surface Sci. 166 (1986) 206

higher adsorption energy of CO

lower adsorption energy of CO

sticking coefficient of O on 1x1 is about 1.5 times than on 1x2

Kinetic oscillations - 2wt% Pt/Al2O3

385 380 375 370 365-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

IR P

eak

Inte

nsity

(a

.u.)

Mas

s S

pec.

Sig

nal

(a.u

.)

Temp. (K)

0

5

10

15

20

25

30

35

40

45

50

55

CO2

CO

OCPt

OC

Pt Pt

• storage of CO that is released in sudden spike of CO2

• CO poisoning; activityinversely proportional to adsorbed CO

Kinetic Oscillations and QEXAFS

cata

lyst

par

ticle

s

CO/O2

To exhaust / mass spec

O2 : CO = 192 wt% Pt/Al2O320 mg catalyst30 ml/min

• activity loss due to reduction of active (oxidized) surface

• sudden increase in activity parallels oxidation of surface

kinetic oscillations originate from

the reduction and re-oxidation

of the surface

Best of luck for your exams ☺

EXAFS error margins:N ± 0.5 20%R ± 0.01 – 0.03σ2 ± 20%