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X - ray Absorption Spectroscopy of Oxides and

Oxidation Catalysts

Michael Stockenhuber

299

7

7.1

Principles of EXAFS

A number of modern physical techniques are used to characterize heterogeneous

catalysts. These methods range from techniques probing the interaction of cata-

lysts with probe molecules, to in situ surface characterization techniques as well

as structural elucidation under both in situ and ex situ conditions. In general,

interaction of catalysts with probe molecules is followed using some spectroscopic

property of the probe molecule itself and/or the changes induced by the hetero-

geneous catalyst. The spectroscopic techniques used include vibrational spectros-

copies, NMR spectroscopy, UV - Vis spectroscopy and mass spectrometry to name

a few examples. Similarly, in situ techniques tend to use properties of probe mol-

ecules but also combined with structural techniques such as X - ray diffraction

( XRD ) and X - ray absorption spectroscopy ( XAS ). In recent years XAS has been

widely used in the characterization of catalysts and catalyst surfaces.

XAS is clearly going beyond the capabilities of a number of structural tech-

niques, including XRD, transmission electron microscopy ( TEM ), scanning elec-

tron microscopy ( SEM ), secondary - ion mass spectrometry ( SIMS ) and ion

scattering. The issues with any of the UHV techniques is their apparent incapabil-

ity to operate under real conditions as well as the inherent difﬁ culty in applying

them to highly disordered and/or sub - nanoscale materials. Diffraction techniques

on the other hand are not applicable to very small clusters or materials without

long - range order. XAS is unique in that it can be operated under more or less any

environmental conditions, owing to the very limited experimental complexity of

the spectroscopic technique itself.

Sample environments for X - ray absorption spectroscopy include liquid - phase

reactions, high temperatures and high pressures.

Characterization of oxides and some in situ applications of XAS will be intro-

duced and discussed. Although XAS is relatively widespread in its application, it

is the intention of this short chapter to set out the fundamentals of the technique

as it is important for the practitioner to understand the basic principles. While it

is not necessary to completely understand the quantum chemical background of

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

300 7 X-ray Absorption Spectroscopy of Oxides and Oxidation Catalysts

extended X - ray absorption ﬁ ne structure ( EXAFS ), a brief introduction to the

physical processes is given. Some details of the theory can be found in a few older

textbooks [1, 2] .

7.1.1

Basic Principles of X - ray Absorption Spectroscopy ( XAS )

Photons are absorbed by any form of matter, the degree of absorption being

dependent on the photon energy (which can be calculated from its frequency by

E = h ν ). X - rays are rather high - energy photons, thus the interaction with matter

happens with a “ high - energy part ” of the matter, in this case the inner electrons.

If X - rays interact with atoms, and they are of sufﬁ cient energy, they can eject a core

electron. Scanning a large energy range of X - rays, one observes that the absorption

of the X - rays occurs in a stepwise manner, that is, absorption is quantized.

For XAS, the main edges of interest are the K - and L - edges. K - edge absorption

means that the energy of the X - rays corresponds to the energy of the s electrons,

whereas L - edge absorption corresponds to a p core level. At the absorption edge,

it is found that there is a ﬁ ne structure which is only present for atoms that have

got nearest neighbors. For an isolated atom, the absorption edge would essentially

be featureless.

Figure 7.1 shows a typical absorption edge of a metal oxide. The oscillations

above the edge are due to the backscattering from neighboring atoms.

Physically, the core electron (the s electron for a K - edge) is ejected by the X - ray

photon. This core electron then has a kinetic energy, E

k

which is equal to: E

k

= h ν

− E

b

where E

b

= the binding energy.

Figure 7.1 A full XAS spectrum of aluminum in zeolite H - FER at 423K at P

total

< 1 0

− 6

mbar.

The binding energy is the core - level energy of the electron, and thus it is con-

stant for a given element. This electron (with low kinetic energy) can then interact

with valence electrons of the atoms present. This interaction with valence electrons

is called X - ray Absorption Near - Edge Structure (XANES) or Near - Edge X - ray

Absorption Fine Structure ( NEXAFS ). XANES thus probes the transition of core

electrons into unoccupied electronic states (such as s → p, p → d). The interest in

XANES stems from the fact that the ﬁ nal states may be mixed with other orbitals.

XANES has a very high level of complex multiple scattering paths, which includes

electronic and structural components. In transition metals, pre - edges can be found

that essentially probe transitions from s to d orbitals. Quite often these are indica-

tive of the coordination environment, because the transition is forbidden in octa-

hedral symmetry. Thus, the intensity of the pre - edge is extremely enhanced for

metal compounds with tetrahedral symmetry.

EXAFS is the interaction of higher kinetic energy photoelectrons, which do not

tend to interact with the valence electrons. In this case the process is simply a

scattering process, which results in the variation of the absorption coefﬁ cient at

higher photon energies. In Figure 7.2 a pictorial representation of the process

involving backscattering is shown.

The X - rays generated remove a core electron, which can act as a spherical wave

which moves away from the backscattering atom. As mentioned, this photoelec-

tron has higher kinetic energy than the photoelectron that results in XANES. The

outgoing wave can now be backscattered by any next nearest neighbor atom, which

is in itself the source of a “ returning ” backscattered electron wave. This backscat-

tered electron wave is in phase with the outgoing wave and can show an interfer-

ence pattern. The position of the maxima and minima depend on the energy of

the photoelectron as well as the distance and number of neighbors. The interfer-

ence pattern is the variation of the total mass absorption coefﬁ cient of the energy

of the incoming photon. This can be observed as EXAFS.

Figure 7.2 A pictorial representation of the

processes involved in EXAFS spectroscopy.

The incoming X - ray photon generates a

photoelectron from the core level with

higher energy. The electron wave of the

photoelectron (solid line circles) is

backscattered by the next nearest neighbor.

This backscattering (dashed line circles)

generates an outgoing electron wave that

gives an interference pattern with the

photoelectron wave.

7.1 Principles of EXAFS 301

302 7 X-ray Absorption Spectroscopy of Oxides and Oxidation Catalysts

The intensity of the EXAFS oscillation is dependent on a number of factors.

First, the number and nature of backscattering atoms is a major contributor to the

intensity of the oscillations. Different absorbers have different backscattering

powers so the amplitude is dependent on the element which is the next nearest

neighbor to the absorber atom. Depending on the absorber and the system, not

all ejected electrons take part in the scattering process – up to 30% of the photo-

electrons go into shake - up/down processes and are not available for EXAFS or

XANES spectroscopy (see below).

If there is a spread in the distances (statically) of the next nearest neighbors this

results in a reduction in intensity, which can be observed as an increase of the

Debye – Waller factor. Dynamic changes, such as those induced by changing tem-

perature, result in a reduced amplitude and increased Debye – Waller factor. The

change in the amplitude is also a function of the energy; at higher electron ener-

gies the reduction of the amplitude is more signiﬁ cant than at lower energies.

A simpliﬁ ed form of the EXAFS equation can be written as follows:

χδ

σ

k

Nf k

kR

kR k

jj

k

j

jj

j

()

=

()

+

()

[]

−

∑

e

22

2

2sin

(7.1)

The main parameters in this equation are:

χ ( k ) = EXAFS interference function

k = photoelectron wavevector

k

mE E

=

−

()

2

0

2



N = coordination number of neighboring atom

R = distance to neighboring atom

σ

2

= square disorder of neighboring distance (Debye – Waller factor)

f

j

= scattering amplitude

δ

j

= phase - shift

Equation 7.1 represents a version of the EXAFS function that uses a spherical

wave without any inelastic scattering. However, in a real system inelastic scattering

occurs and the ﬁ nite lifetime of the core level may result in the photoelectron not

being scattered.

A damped wave function can be used in the calculation which leads to the fol-

lowing equation:

χδ

λ

σ

k

Nf k

kR

kR k

jj

Rk

k

j

jj

j

jj

()

=

()

+

()

[]

−

(

)

−

∑

ee

2

22

2

2sin

(7.2)

λ represents the photoelectron mean free path.

Another correction that has to be made is the Amplitude Reduction Factor

( AFAC ) (sometimes called

s

0

2

) This accounts for the relaxation of the other elec-

trons in the absorbing atom to the core hole. It normally has a size of 0.7 < AFAC

< 1. It is directly correlated to the amplitude of the EXAFS, and thus has strong

correlation with the coordination number, as shown in Equation 7.3 :

χδ

λ

σ

k

Nfk

kR

kR k

jj

Rk

k

j

jj

()

=

()

+

()

[]

−

()

−

∑

AFAC e e

2

22

2

2sin

(7.3)

The need to take AFAC and inelastic scattering in to account results in a gener-

ally reduced accuracy of the coordination number determination in EXAFS. This

can in part be overcome by using appropriate standards and a procedure outlined

in ref. [3] . The process is described in detail in Section 7.1.4 .

So far we only have included single scattering pathways. Especially for extended

solids, multiple scattering becomes much more important. Figure 7.3 shows

potential pathways for scattering.

In the EXAFS equation, all scattering from all neighbors is added to form the

total function. In principle it also includes all possible multiple scattering path-

ways. The contribution of the different pathways depends on the angle between

say three different atoms, as indicated in Figure 7.3 . The triangular scattering path

shown has the smallest amplitude, whereas the amplitude of the focused pathways

as illustrated in Figure 7.3 exhibit important and signiﬁ cant contributions. In

general multiple scattering contributions of structures with any angles larger than

150 ° need to be considered as important. Triangular pathways can also become

relevant if there are many of these multiple pathways. Especially for extended

solids such as oxides, multiple scattering has been shown to become crucial for

the calculation of the structure [4] . If there is only one shell considered, the general

rule is that multiple scattering is of limited importance.

7.1.2

Experimental Setup

XAS experiments are in general very simple to perform. For hard X - rays there is

no necessity to provide any special atmosphere. However, quite often the light

Figure 7.3 An illustration of multiple scattering pathways.

7.1 Principles of EXAFS 303

Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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