in situ x-ray absorption spectroscopy in heterogeneous ... · in situ x-ray absorption spectroscopy...

Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment: In situ XAFS, 17.01.2003 (1/53)

In situ X-ray Absorption Spectroscopy in

Heterogeneous Catalysis

Thorsten Ressler

Abteilung Anorganische Chemie, Fritz-Haber-Institut der MPG

Faradayweg 4-6, 14195 Berlin


• Introduction to XAS

(Synchrotron Radiation, Experimental Set-up,

Data aquisition, data reduction and data analysis)

• XAFS – Examples: Molybdenum oxides and Cu metal

• Data analysis


Introduction


Synchrotron Radiation


X-ray Absorption Spectroscopy


Not accessibleK edgeL edges

Accessible Elements for TR-XAS Studies at 5 – 30 keV

Con

vers

ion

Catalyst composition

+


synchrotron

entrance slits

monochromator

exit slits

I0 I1 I2

sample referencesample

X-rays

Instrumentation for Conventional XAS Studies

Quick-EXAFS (QEXAFS), (Hasylab, X1), ~ min/spec


Mass spectrometer

In situ CellX-ray detector

Monochromator

Experimental Set-up for in situ XAS in Transmission


1 2 3

1'2'

3'

1' 2' 3'

curved Laue monochromator

mirror

sampleholder

diode arraydetector

XAS spectrum

Energy-dispersive XAS (DXAFS), (ESRF, ID24), ~ s/spec

Instrumentation for Time-resolved XAS Studies


Gas flowcontroller

sample Heating wire

X-ray

Al-window

MS Analysis Thermocouple

Al-window

In situ Cell for XAS Studies in Transmission Mode


Gas inGas out

Thermocouple

Sample (pellet)

X-rays

Al window

IoI1

Heating wire

In situ Cell for XAS Studies in Transmission Mode

RT - 600 OC (@ ~ 50 K/min)1 bar (cell volume ~ 4 ml)


Data reduction and analysis


XANES (X-ray absorption near edge structure)

· Valence

· Coordination geometry

· Quantification of phase mixtures (PCA)

· No reliable codes for theoretical calculations available (yet)

EXAFS (Extended X-ray absorption fine structure)

· Nearest neighbour

· Type of nearest neighbours

· Distance of nearest neighbour shells

· Coordination number [and geometry (bond angle)]

· Theoretical calculations become standard for data analysis

Structural Data from XAFS


X-ray Absorption Spectroscopy - Origin

E = hν

Sample

I0 I1

d

Detector

µµµµd = ln (I0 / I1)

0.5

1.0

6.5 6.6 6.7 6.8 6.9 7.0 7.1Ab

sorp

tion

µµ µµd

Photon energy (keV)

XANES

hν

EXAFS


R1

R2

Mo Metal (bcc)

2.0

0.0

-2.0

2 4 6 8 10 12 14 16k [Å-1]

χ(k)

2nd shell(3.14 Å)1st shell

(2.73 Å)0.02

0.04

0 2 4 6 8

R [Å]

FT(χ

(k)*

k2 )

FT �

2nd shell

1st shell

R2R1

X-ray Absorption Spectroscopy - Structure


0.1

0.0

0 2 4 6 8

FT( χχ χχ

(k)*

k3 )

R, (Å)

Experiment

Theory, Mag.

Theory, Imag.

0.1

0.0

-0.1 0 2 4 6 8

FT( χχ χχ

(k)*

k3 )

R, (Å)

Experiment

Theory, Mag.

Theory, Imag.

Tetragonal (P42/mnm) Monoklin (P21/c)

Short-range order from XAFS Data

MoO2


1.0

2.0

0.0

20 20.05

Abso

rptio

n

Photon energy, (keV)

MoO2

MoO3Mo 1s-4d

2nd Deriv.

1.67

2.33

1.948

OI

OII

MoO2

MoO3

Coordination Geometry from Near-edge Structure


XAFS References – Near-edge Structure and Average Valence

1.0

2.0

3.0

4.0

5.0

6.0

0.0

10 20 30 40 0

Mo

aver

age

vale

nce

Mo K edge, (eV)

MoO2

MoO3

Mo4O11

Mo8O23

Mo18O52

Mo5O14

Mo

0.5

1.0

19.95 20 20.05 20.1 20.15

Nor

mal

ized

abs

orpt

ion


1.0

2.0

3.0

19.95 20 20.05 20.1 20.15

Nor

mal

ized

abs

orpt

ion


MoO2

MoO3

Mo4O11

Mo8O23

Mo18O52

Mo5O14


Phase Analysis from XAFS Data (Factor Analysis)

0.5

1.0

0.0

0

1

5 10 15 200Ph

ase

com

posi

tion

Time, (min)

MS

MoO3 MoO2

Mo4O11

H2 O2

T = 773 K

0.0

1.0

20 20.1 20.2

Nor

mal

ized

abs

orpt

ion


MoO3

MoO2

Mo4O11

Fit + Experiment

Faktoranalyse

MoO3

MoO2

Mo4O11

0.0

0.5

1.0

20 20.1 20.2

5

10

15

Abs

orpt

ion


Time, (m

in)

MoO3

MoO2

T = 773 K

Reduction

Oxidation

H2

O2

O2


Example:

Molybdenum oxides


In situ XAS and XRD

In situ XASDXAFS (ESRF, ID24), ~ 2 s/specQEXAFS (Hasylab, X1), ~ 4 min/specPellet-Reactor in Transmission + MS

In situ Pulver-XRDSTOE STADI P Bragg-Brentano (~ 7 min (10 °))Bühler HDK S1 + MS

Elementspezifisch, Nahordnung, Mittlere Valenz, Phasen (amorph)

Fernordnung, Kristallitgrößen,Verspannungen, Phasen (kristallin)


Reduktion / Oxidation - Teilschritte des “Redox”-Mechanismus

H2, propene

O2

MoO3

Mo8O23

Mo9O26

Mo18O52

Mo4O11

MoO2


Reduktion von MoO3 in 5 % H2 – In situ XRD

50

100

0.0

20 25 30 35 40

Inte

nsity

Diffraction angle 2θ

MoO3

T = 773 K

3 h

27 h

Mo4O11

MoO2

0

50

100

150

200

250

20 25 30 35 40

Inte

nsity

Diffraction angle 2θ

MoO3

T = 673 K

2 h

40 h MoO2

Keine Bildung von Mo4O11

bei T < 700 K

Bei T > 700 K Mo4O11

kein Zwischenprodukt


0.5

0.0

0.5

1.0

0.0

1 2 3 4 5 60

Phas

e co

mpo

sitio

n

Time / t1/2(MoO3)

MoO3

MoO2

Mo4O11

T = 773 KMoO2

MoO3

T = 748 K

Mo4O11

Bildung von Mo4O11 aus MoO3 und MoO2

Keine Bildung von Mo4O11 bei T < 700 K

5 % H2


0.1

0 1 2 3 4 5 6

373

473

573

673

FT( χ

(k)*

k3 )

R, (Å)

Temperature, (K)

MoO3

MoO2

0.04

0.0

-0.04

0 1 2 3 4

FT( χ

(k)*

k3 )

R, (Å)

T = 733 K

ExperimentTheory

66 % MoO3

34 % MoO2

MoO3

MoO2

0.5

1.0

0.0

4.6

4.4

4.2

4.0

373 473 573 673 773

Phas

e co

mpo

sitio

n

Temperature, (K)

Rel. M

o K Edge shift, (eV)

MoO2

MoO3

50 % H2

Change in FT(χχχχ(k)) and Mo K edge position during TPR of MoO3with H2 prior to MoO2 formation


0.02

0.04

0.06

0.0

-0.02

-0.04

373 473 573 673273

0.02

0.04

0.06

0.0

-0.02

-0.04

R = 3.70 Å

R = 3.44 Å

R = 3.96 Å

∆ R

(Mo

- Mo)

, (Å)

∆ R

(Mo

- O),

(Å)

Temperature, (K)

R = 1.67 Å

R = 2.25 Å

R = 1.74 Å

MoO2HxMoO3

TPR of MoO3

in 50 % H2

H0.34MoO3 (Cmcm)

Local structural changes indicate formation of HxMoO3 (x ~ 0.07)


0.5

1.0

0.01.0

0.05 10 15

Exte

nt o

f rea

ctio

n, αα αα

Time, (min)

MS

MoO3

MoO2

O2H2

Reduction

O2 on

Oxidation

0.0

0.5

1.0

20 20.1 20.2

5

10

15

Abs

orpt

ion


Time, (m

in)

MoO3

MoO2

T = 773 K

Reduction

Oxidation

H2

O2

O2

~ 3 s/scan

Kinetics of the Reduction of MoO3 in H2

αααα ~ t3

nucleation growth kinetics

MoO2

MoO3

Diffusion

Keimwachstum

„Überlapp“


Schematischer Mechanismus der Reduktion von MoO3 in H2R

educ

tion

C

D B

> 700 K

H2

(100)

A

MoO3

(010)

MoO2

MoO2

~ 570 K

~ 600 K

HxMoO3

Mo4O11


0.2

0.4

0.6

0.8

1.0

0.0

373 473 573 673 773 Ph

ase

com

posi

tion

Temperature, (K)

MoO3

MoO2

“Mo18O52”

10 vol-% propene

Bildung von “Mo18O52“ bei der Reduktion von MoO3 in Propen

0.0

1.0

2.0

3.0

373 473 573 673 773

Nor

mal

ized

sig

nal

Temperature, (K)

Acrolein (m/e = 56), 10 vol-% propene

CO2 (m/e = 44), 10 vol-% propene

H2O (m/e = 18), 10 vol-% propene

H2O (m/e = 18), 50 vol-% H2

1st deriv. Mo K edge shift

4 C3H6 + 22 MoO3 � 2 C3H4O + 6 CO2 + 8 H2O + 22 MoO2


0.2

0.4

0.6

0.8

1.0

0.0

20 40 60 80 100 0

Exte

nt o

f red

uctio

n, α

Time, (min)

673 K, 10 %

698 K, 10 %

723 K, 5 %

α ~ t2

α ~ 1-(1-t1/2)3 100

120

140

160

180

0.2 0.4 0.6 0.8 1 0

App.

act

ivat

ion

ener

gy E

α, (k

J/m

ol)

Extent of reduction, α

5 vol-% propene

α ~ t2 α ~ 1-(1-t1/2)3

MoO2

MoO3

Diffusion

Keimwachstum

„Überlapp“

Kinetics of the Reduction of MoO3 in Propene


Schematischer Mechanismus der Reduktion von MoO3 in Propen

Red

uctio

n Oxidation

C

D

MoO2

< 700 K

H2O

B CO2 C3H4O

> 700 K

“Mo18O52”

O2

O2

(100)

A

MoO3

(010)


0.2

0.4

0.6

0.8

0.0

-0.2

-0.4 473 523 573 623 673 723 773

Rel

. Mo

K ed

ge s

hift,

(eV)

Temperature, (K)

1 : 1 (propene : O2) 1 : 3 (propene : O2) 1 : 5 (propene : O2) 20 % O2 0.5

1.0

1.5

0.0 Nor

mal

ized

MS

ion

curre

nts

373 473 573 673 773 Temperature, (K)

273

Acrolein (m/e = 56)

CO2 (m/e = 44)

H2O (m/e = 18)

10 % propene + 10 % O2

(b)

Mittlere Valenz von MoO3 unter Reaktionsbedingungen

0.3 eV ~ 5.94 Reduktion von MoO3 in H2, He, Propen und Propen + O2 bei ~ 620 K

4 C3H6 + 11 O2 � 2 C3H4O + 6 CO2 + 8 H2OMoO3


a

b

Mo+6O-2

a

b c

Mo+5.78O-2

Vergleich der Strukturen von MoO3 und Mo18O52

MoO3 Mo18O52

0

100

200

300

1 2 3 4

Num

ber o

f bon

ds

Distance R, (Å)

MoO3

Mo5O14

Mo18O52


0.05

0.1

0 1 2 3 4 5 6

373

473

573

673

773FT(χ

(k))*

k3

R, (Å)

Temperature, (K)

MoO3

RDF von MoO3 als Funktion von T oder c(Mo18O52)

0.3 eV ~ 5.94 ~ 25% Mo18O52

0.05

0.1

0 1 2 3 4 5

0.3

0.2

0.1

0.0

FT( χ

(k))*

k3 R, (Å)

0.4

Fraction of Mo

18 O52

MoO3


0.05

0.1

0.0

-0.05

0 1 2 3 4 FT

( χ(k

)*k3 )

R, (Å)

Mo – O Mo – Mo

Theory

Experiment

Bildung von „Mo18O52“ Defekten unter Reaktionsbedingungen

XAFS Verfeinerung von MoO3 Struktur an experimentelle Daten

350

400

450

1400

1450

1500

0.05 0.1 0.15 0.2 0.250

Deb

ye te

mpe

ratu

re Θ

, (K)

Fraction of Mo18O52

Θ(O)

Θ(Mo)

(a)

200 300 400

1100 1200 1300 1400 1500

373 473 573 673 773

Deb

ye te

mpe

ratu

re Θ

, (K)

Temperature, (K)

Θ(O)

Θ(Mo)

(b)

1.67

2.33

1.948

OI

OII


0.02

0.04

0.06

0.0

0.05 0.1 0.15 0.2 0.250

Rel

ativ

e di

stan

ce R

-R0,

(Å)

Fraction of Mo18O52

1st

Mo – O shells 2nd

3rd

4th

6th

(a)

0.05

0.1

0.0

373 473 573 673 773

Rel

ativ

e di

stan

ce R

-R0,

(Å)

Temperature, (K)

1st

Mo – O shells 2nd

3rd

4th

6th

(a)

1.67

2.33

1.948

OI

OII



H2O

CO2 C3H4O

B

“Mo18O52”

< 720 K

H2O

CO2 C3H4O

A

O2 MoO3

> 720 K

(010)

(100)

Temperature

Reducing potential



Example:

Supported Cu clusters


Cu/ZnO Catalysts for Methanol Synthesis and Steam Reforming

Preparation

Co-precipitation from Cu/Zn nitrate and soda solution. Calcined at 600 K

3 h in air (Cu/Zn 100/0, 90/10, 80/20, 70/30, …, 0/100).

Temperature programmed reduction at 523 K in 2 vol-% H2

Methanol synthesis

CO2+3H2 �� CH3OH+H2O

Methanol steam reforming

CH3OH+H2O �� CO2+3H2


Micro Structural Bulk Properties of Cu/ZnO Catalysts

b

a

εεεε , strain

D, size

Cu-Cluster


Increase in Micro Strain in Cu Clusters Correlates to Increased

Methanol Synthesis Activity

200

400

600

800

1000

0.0

0.6

1.2

1.8

2.4

0 10 20 30 40 50 60 70 800

Cry

stal

lite

size

Cu

(111

) [Å]

mol-% ZnM

icro

stra

in C

u (1

11) [

%]MicrostrainCu

Crystallite size0.5

1.0

0.0

10 20 30 40 50 60 70 80 90

Nor

mal

ized

TO

F

mol-% Zn

Methanol synthesis

CO2+3H2 �� CH3OH+H2O


“Real” Structure of Cu/ZnO Catalysts – Zn in Cu Clusters

0.05

0.0

-0.05

0 1 2 3 4

FT(χ

(k)*

k3 )

R, (Å)

ExperimentXAFS - FitSingle paths

0.05

0.0

-0.05

0 1 2 3 4

FT( χ

(k)*

k3 ) R, (Å)

Experiment XAFS - Fit Single paths

Zn - Cu


Reversible Oxidation - Reduction of Cu/ZnO

0

0.5

1.0

9 9.05 9.18

16

24

Abso

rptio

n

Photon energy [keV]

O2 in feed

Cu

Cu2O

CuOCu

+

Time, (m

in)


“Real” Structure of Cu/ZnO Catalyst – Increase in Micro Strain

b

a

ε

0.015

0.02

0.025

0.03

0.035

3 4 5 6

Cu

Deb

ye-W

alle

r fac

tor σ

2 , (Å2 )

Cu - Cu distance, (Å)

Fresh, SR1st O2, SR2nd O2, SR

0.005

0.01

0.015

0.0fresh reduced 1st O2 cycle

∆ a

[Å]

Cu

under reaction 2nd O2 cycle

0.1

0.0

-0.1

0 1 2 3 4 5 6

FT(χ

(k)*

k3 )

R, (Å)

ExperimentXAFS - FitSS shells

in situ x-ray absorption spectroscopy in heterogeneous ... · in situ x-ray absorption spectroscopy...

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