Catalysis and Catalysts - XPS X-Ray Electron Spectroscopy (XPS) Applications: –catalyst composition –chemical nature of active phase –dispersion of active

Catalysis and Catalysts - XPS

X-Ray Electron Spectroscopy (XPS)X-Ray Electron Spectroscopy (XPS)

Applications:– catalyst composition– chemical nature of active phase– dispersion of active phase

Standard technique in catalyst characterisation

levels

3

2

3 Lp2

2

2

1 Lp2

1Ls2

Ks1

h1 h2

-pe

31-A LKLe

XPS XRF AES


Binding EnergyBinding Energy

Conservation of energy

EB depends on chemical environment:• element• valence state• coordination (type of ligands, number, tetrahedral, octahedral. …)

h = EB + Ekin + Ecorr

corrects for potential difference between sample and analyser

kinetic energy of emitted electron

binding energy of emitted electron

energy of photons


XPS EquipmentXPS Equipment

Number of emitted electrons measured as function of their kinetic energy

Al

X-ray source

Electrostatic electron lens Electron

detector

Electron energy analyser

Samplee-

Photon

Slit

Hemispherical electrodes

Slit


XPS Survey of Al2O3XPS Survey of Al2O3

N(E)E

EB

O(KVV)

O 1s

O 2s

Al 2s Al 2p

Ar 2p3/2Ar 2s

C 1sSatellite

O(KVV)

Auger transitions

AUGER

1253 eV

1000 900 800 700 600 500 400 300 200 100 0

Al2O3 Mg K

780.6766.7

745.3

805 765 725


2p 74.7

2p 72.85

Effect of Valence on Chemical Shift - Al and Al2O3Effect of Valence on Chemical Shift - Al and Al2O3

Peak position determined by element and valence

Chemical information on elements

Al2O3Al

‘Binding energy’ (eV)86 76 66


Influence of F Content on XPS Spectrum of Al2O3Influence of F Content on XPS Spectrum of Al2O3

Al (2p)

1.2 at.% F

6.5 %

10.6 %

20.6 %

75.0 % (AlF3)

82 72 eV

75 eV

77 eV


Effect of Mo Valence on Chemical Shift and Multiplet Splitting

Effect of Mo Valence on Chemical Shift and Multiplet Splitting

MoO3

3d3/2

3d5/2

232.65

3.2

Binding energy (eV)

Mo

3d3/2

3d5/2

222.7

3.15

Binding energy (eV)

240 230 220 240 230 220


Satellites in XPS PlotsSatellites in XPS Plots

Causes: non-monochromatic X-Ray source contamination of X-Ray source

excitation of ion by leaving electron: ‘shake-up’

Mg X-ray emission spectrum:

0

20

40

60

80

100

Photon energy (eV)

Inte

nsity

(%

)

1253.6 1262.0 1263.8 1271.1 1273.6 1302.1

84.1

0.6 0.5 0.5


Shake-up Lines in Cu 2p SpectrumShake-up Lines in Cu 2p Spectrum

Shake-up lines

2p3/2

2p1/2

Cu

CuO

CuSO4

Binding energy (eV)

970 960 950 940 930


Charging of Catalyst SamplesCharging of Catalyst Samples

2p74.7

3.3

Al2O3

Binding energy (eV)

Al 2p emission line of Al in alumina: charging results in a shift of 3.3 eV

86 76 66


Where do Electrons come from?Where do Electrons come from?

zzIzI exp0

Intensity of a peak depends on: composition position where electrons are emitted

Electrons interact with the solid only a fraction of the emitted electrons reach detector with

original kinetic energy the longer the distance, the higher the number of “lost” electrons

z = distance travelled by electrons

= escape depth


Inelastic Mean Free PathInelastic Mean Free Path

2 5 10 50 100 500 1000 2000

Electron energy (eV)

Inel

astic

Mea

n F

ree

Pat

h (

nm) 10.0

5.0

1.0

0.5

0.3

Au Au

AuAuAgAu

AgAg

Au

Au

Ag

AgMo Be

AgW

Ag

Be Be

FeBe C

AgMo

Ag

AgBe

Au

AuC

C

Be

Mo

W


Is XPS a Bulk or a Surface Technique?Is XPS a Bulk or a Surface Technique?

XPS is a surface-sensitive technique

0

1000

2000

3000

4000

5000

0 2 4 6 8 10 12

Thickness of absorbing layer z (nm)

XP

S y

ield

2 nm

0.5 nm

zzIzI exp0


Catalyst Structure and XPS CurvesCatalyst Structure and XPS Curves

Monolayer growth: curve “A”

Particle growth: curve “B”

s

p

I

I

p/s

B - particle growth

A - monolayer (can be modelled theoretically)

particle size

XPS

Bulk


Quantitative Model for XPS Signal IntensitiesQuantitative Model for XPS Signal Intensities

Based on:

• model catalyst (parallel sheets of support with promoter crystals)

• X-ray intensity uniform throughout catalyst particle

• Lambert-Beer type equation for absorption of radiation

Model catalyst c = crystal size (promoter)

t = thickness

1

1

2

21 exp1

exp1

exp1

2

s

p

s

p

support

promoter

ED

ED

s

p

I

I

ratio of detector efficiencies

1

ratio of cross sectionsatomic ratio

pssspp

ttc

211 ;;


Formulation of ModelFormulation of Model

ss

ss

t

sssss

tnAdz

znAI exp1exp

0

0,

• one support layer, thickness t

•one layer of active phase (“promoter”)

• allow for position of layers:

surface area

atoms/volume

cross section (electrons/photons.s.at) 1

pppppp

t

ppppp

cnAfdz

znAfI exp1exp

0

0,

1fractional coverage of support

detector

less intensity then


Quantitative Model for XPS Signal IntensitiesQuantitative Model for XPS Signal Intensities

Dispersion coupled with c

Highest dispersion corresponds to c 0 (“monolayer” catalyst)

Model catalyst c = crystal size (promoter)

t = thickness

1

1

2

21 exp1

exp1

exp1

2

s

p

s

p

support

promoter

ED

ED

s

p

I

I

1

pssspp

ttc

211 ;;

2

21

0 exp1

exp1

2lim

s

p

s

p

monos

p

support

promoter

c ED

ED

s

p

I

I

I

I

1

1exp1

/

/

monosp

sp

II

IIc can be calculated from XPS data


Calculation of Crystallite SizeCalculation of Crystallite Size

2

21

exp1

exp1

2

s

p

s

p

monos

p

ED

ED

s

p

I

I

limit c 0

y = x a

s

p

I

I

p/s

experimental

monolayer calculation

particle size

calculation

1

1exp1

/

/

monosp

sp

II

II


Example: Re2O7/Al2O3Example: Re2O7/Al2O3

escape depth

= 1.3 nm

= 1.8 nm

(Re/Al)bulk

I(Re 4f)

I(Al 2p)

0 0.02 0.04

1.0

0.5


Other Catalyst ModelsOther Catalyst Models

Randomly Oriented Support Layers Inhomogeneous promoter distribution: egg-shell catalyst


WO3/SiO2

MoO3/Al2O3 MoO3/SiO22

1 1

0.5

0.5

1

Al

Mo

II

WO3/Al2O3

1

Al

W

II

2

Si

Mo

II

Si

W

IIbulk

AlMo

bulk

SiMo

bulk

AlW

bulk

SiW

Example: MoO3 and WO3 supported on SiO2 and Al2O3Example: MoO3 and WO3 supported on SiO2 and Al2O3


Pt/SiO2 - XPS intensitiesPt/SiO2 - XPS intensities

0

0.05

0.1

0.15

0.2

0.25

0 0.002 0.004 0.006 0.008 0.01 0.012

I(Pt 4f)

I(Si 2p)

(Pt/Si)bulk

Dispersion 34% ?

Experimental points

Monolayer prediction


Intensity Decreases with Crystallite SizeIntensity Decreases with Crystallite Size

1

Fraction monolayer intensity

''

''

p

m

I

I

pp

c

1 literature


1

Fra

ctio

n o

f m

onol

ayer

inte

nsity


0

2

4

6

8

10

12

0 1 2 3 4 5 6 7

c (n

m)


Example: Fluorinated AluminaExample: Fluorinated Alumina

AlF3

Al2O3

powdered

coarse

80 78 76 74eV


Summary of XPSSummary of XPS

XPS can give valuable information regarding: catalyst composition, i.e. the elements present chemical nature of the elements chemical nature of neighbouring (co-ordinating) atoms dispersion of active phase and support location of active phase in the particle

Documents

Catalysis and Catalysts - XPS X-Ray Electron Spectroscopy (XPS) Applications: –catalyst composition –chemical nature of active phase –dispersion of active