XAFS_course2012_1_Intro

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Introduction to X-ray Absorption Spectroscopy:
an element sensitive method for local
spatial and electronic structure
K. Klementiev, Alba synchrotron - CELLS
After this introductory lecture:
2. Introduction to theory
3. Data analysis (EXAFS, XANES)
4. Experiment (general and @CLÆSS)
5. Hands-on exercises
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K. Klementiev - Introduction to XAS
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X-ray Absorption Spectra
x-rays
I0
sample
transmission experiment
I1
d
I1  I0e
XANES
X-ray Absorption
Near Edge Structure
EXAFS
measured at Alba/CLÆSS
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K. Klementiev - Introduction to XAS
Extended X-ray
Absorption
Fine Structure
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Qualitative Picture of XAFS
M1–M5
L3 2p3/2
L2 2p1/2
L1 2s
EF
destructive interference –
absorption minimum
Here is where
the interference
is important!
Ex-ray
continuum
E photoelectron
constructive interference –
absorption maximum
K 1s
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Theoretical Description
In the MS theory, the expression for 
can be factored in terms of an atomic
background  and the oscillatory part 
In the photoelectron
momentum space,
k = [2m/ħ2(E–EF)]½,
the function is
parameterized as:



f
(
k
)
2
2

2
R
/

(
k
)

2

k
j
j
(
k
)

S
Nsin(
2
kR

(
k
))
ee

j
j
kR
j
2
0 j
2
j
j
For each coordination shell j:
Rj, Nj, 2j are the sought distance, coord. number
and variance of distance
fj(k)=|fj(k)|eij(k) is the scattering amplitude (calculated),
λ is the electron free path (calculated),
S02 accounts for many-electron excitations.
The present theory cannot give reliable 0.
• For XANES region this is a problem, because 0 there is a rapidly changing
function with features comparable with .
• In EXAFS region 0 is a smooth function and can be constructed empirically.
Thus, XANES spectra are mostly interpreted, not analyzed.
EXAFS spectra can be analyzed quantitatively.
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K. Klementiev - Introduction to XAS
See more in the
“Theory” lecture
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What we can learn from XANES: a) Pre-edge Peak
Dipole selection rule (only in central-symmetric case!):
Consider K-absorption for transition metals:
l = ±1
• initial state = 1s (l=0)
• states near EF are formed by nd electrons (l=2)
central-symmetry
no resonance
non-central symmetry
resonance (pre-edge peak)
from [F. Farges, G. E. Brown, J. J. Rehr,
Phys. Rev. B 56 (1997) 1809]
T. Ressler et al. J. Phys. Chem. B 104, 27 (2000) 6360-6370
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What we can learn from XANES: b) Edge Shift
T. Ressler, R. E. Jentoft, J. Wienold, M. M. Günter and O. Timpe:
J. Phys. Chem. B 104, 27 (2000) 6360-6370
I. Arčon, B. Mirtič, A. Kodre,
J. Am. Ceram. Soc. 81 (1998) 222–224
Why does it shift?
1) Electrostatic: it is harder for the photoelectron to leave a positive (oxidized) atom
2) Shorter bonds at higher oxidation states  Fermi energy is higher
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K. Klementiev - Introduction to XAS
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L3 absorption edges for 5d metals:
(transition 2p3/2  5d)
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…white line also depends on particle
size and morphology:
G. Meitzner, G. H. Via, F. W. Lytle, and J. H. Sinfelt,
J. Phys. Chem. 96 (1992) 4960
Intensity is proportional to the number of free 5d
states and also depends on valence state. But…
K. Klementiev - Introduction to XAS
from A. L. Ankudinov, J. J. Rehr, J. J. Low, and S. R. Bare, J. Chem. Phys. 116 (2002) 1911
a) Pt3 triangle (dashes), Pt4 tetrahedron (solid), Pt5 triangular bipyramid (dash-dot), and Pt6 octahedron (dash-dot-dot);
(b) Pt7 and Pt4 clusters of different shape: planar ‘‘honeycomb,(dashes), D5h bipyramid (solid) , single-capped
octahedron (long dashdot), Pt4 tetrahedron (dots), and Pt4 planar rhombus (dash-dot).
What we can learn from XANES: c) White Line
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What we can learn from EXAFS




f
(
k
)
2
2

2
R
/
(
k
)

2
k
j

(
1

),
(
k
)

S
N
sin(
2
kR

(
k
))
e
e

0
j j
kR
j
2
0j 2
j
j
2.5
Cu K-edge for Cu metal foil
4
2
d
k
2
2.0
0d
1.5
0
-2
-4
1.0
-6
2
0.5
4
6
8
10
12
14
-1
photoelectron wave number (Å )
8
9000
9200
9400
2.3
9600
photon energy (eV)
|FT(k2)|
0.0
Unpleasant thing about EXAFS:
FT positions are shifted towards small distances.
More unpleasant: Each FT peak has its own shift.
Because of the phase shifts, extracting the
structural information from EXAFS requires
curve fitting with assumed (calculated) phase shifts.
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2.55 (real RCu-Cu)
6
4
2
0
0
K. Klementiev - Introduction to XAS
2
4
6
8
distance (Å)
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I2
reference
(metal foil)
Experimental Setup (a)
LN2
I1
I0
optics
heating
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
K. Klementiev - Introduction to XAS
source
e+/-
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Experimental Setup (b)
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Example: Application of EXAFS to Pd,Pt/C Catalysts (a)
Supported noble metal catalysts are used in a number of commercial chemical
processes (Pd,Pt/C  toluene and benzene hydrogenation).
Pd,Pt/C catalyst
• Support: graphite-like carbon (sibunit)
• Preparation: mild oxidative treatment of the support followed
by ion exchange with Pd or Pt amine complexes
• Characterization: XPS, TPR, catalytic studies
• Outcome: highly dispersed metal clusters
with ~1.1 wt% of Pd and ~0.9 wt% of Pt
XAFS measurements
• X1 beamline at Hasylab/DESY with a
Si 311 double crystal monochromator, transmission mode
• Samples: non-pressed powders
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Example: Application of EXAFS to Pd,Pt/C Catalysts (b)
Reduction treatment: in 5%H2/He atmosphere, stepwise with T=50°C.
Measurements at LN2 temperature in order not to have interference of two effects: (i) due to different
temperature vibrations and (ii) due to different particle sizes.
photelectronwavenumber k(Å-1)
2
6
8
10
12
14
PdKEXAFS
distancer (Å)
16 0
1
2
3
EXAFS function k2
6
6
increasingreductionT
(200–500°C, T=50°C,
dehydridedinHeat the
sameTafter reduction)
0
hydrided
at 200°C
Pd/Casprepared
-2
increasingreductionT
(200–500°C, T=50°C, de-4 hydridedinHeafter reduction)
Pdmetal foil
Pd/Chydrided
0
Pt metal foil
PtO2
Pt/Cas prepared
Pt/C350°CinH2
Pt/C350°CinHe
Pt/C400, 450,
500°CinH2
Pt metal foil
0
Pt/Cas prepared
Pt/C350°CinH2
Pt/C350°CinHe
Pt/C400, 450, 500°CinH2
-4
2
Pd/Cas
prepared
Pt L3 EXAFS
-2
4
PdO
2
6
4
2
b) Pt/C
0
2
4
6
8
10
12
14
photelectronwavenumber k(Å-1)
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5
metal foil
a) Pd/C
2
4
4
|FT(k2)|
4
4
16 0
1
2
3
4
5
6
distancer (Å)
K. Klementiev - Introduction to XAS
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Example: Application of EXAFS to Pd,Pt/C Catalysts (c)
Reference compound: metallic Pd foil
2
N
12
fcc=
N
12.4(9)
fit=
raw·k2
EXAFS results for Pd metal (measured at LN2 temperature)
Path
r (Å) rcrysta (Å) CN CNcryst 2 (10-3Å2) E (eV)
Pd-Pd 2.742(4) 2.751 12.4(9)
12
3.4(5)
6.2(7)
a
at room temperature [Kittel]
4
0
FT(·k2)
EXAFS function ·k2
4
fitted
2 filtered&
0
Checking your amplitudes and phases
with reference spectra must be always
the first step in every EXAFS study!
-4
data
fit
-2
2
0
4
6 8 10 12 14 16
-1)
w
avenum
berk(Å
0
1 2 3
distancer(Å
)
4
The difference between a simply reduced Pd/C sample (hydrided)
and one subsequently blown out by He flow (dehydrided)
raw·k2
hydrided
-1
0
-1
2
d
tte
i
f
d&
e
r
filte
4
dehydrided
6 8 10 12 14 16
-1)
w
avenum
berk(Å
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2
1
0
0
hydrided
1
dehydrided
0
0
EX
A
FSresultsforPd/Csam
pleduringthereductionprocess
2
reduction
Path r(Å
) C
N 2(10-3Å
) E(eV
)
tem
perature, °C
200, hydrided Pd-Pd 2.83(1) 5.5(8)
6(1)
6(1)
200, dehydrided Pd-Pd2.733(6)5.2(5) 4.4(7)
5.3(7)
250, dehydrided Pd-Pd2.732(6)5.9(5) 4.5(6)
5.4(7)
300, dehydrided Pd-Pd2.733(5)6.6(6) 4.2(6)
5.5(7)
350, dehydrided Pd-Pd2.735(5)7.4(6) 3.8(9)
5.5(7)
400, dehydrided Pd-Pd2.738(4)8.0(6) 3.2(5)
5.7(7)
450, dehydrided Pd-Pd2.740(4)8.4(6) 2.9(4)
6.0(7)
500, dehydrided Pd-Pd2.742(8)8.7(6) 2.8(7)
6.2(7)
data
fit
3
FT(·k2)
EXAFS function ·k2
1
1 2 3
distancer(Å
)
4
K. Klementiev - Introduction to XAS
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Example: Application of EXAFS to Pd,Pt/C Catalysts (d)
[M.Borowski, J. Phys. IV 7, C2-259 (1997)]
2
8
3
=1
cluster radius R, Å
7
6
5
4
3
2
7.5(5)Å
1
N=Nfcc·(1- 4
63)
+1
rPd-Pd
rPt-Pt
= R
 or R

500°C
6.8(5)Å
450°C
6.1(4)Å
400°C
5.5(7
)Å
reduced
5.3(4)Å
350°C Pt/C
4.4(3)Å
300°C
3.9(2)Å
250°C
3.4(2)Å
200°C
Pd/C
Å
)
3
Pt/C
2.5(
aspreparedPt/C
3
4
5
6
7
8
support
9
apparent coordinationnum
ber N
Catalytic properties:
• The catalytic activity in toluene and benzene hydrogenation was found to exceed the activity
of conventional Pd/Al2O3 and Pt/Al2O3 fourfold for Pd/C and almost ten-fold for Pt/C.
• If the reduction temperature exceeds 350oC the activity of both catalysts sharply decreases.
The EXAFS study made it possible to exclude intense metal sintering,
or Pd carbide formation as possible reasons of the activity drop.
Stakheev et al., Russ.Chem.Bull.Int.Ed. 53 (2004) 528
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Conclusions
• XAFS (XANES and EXAFS) spectroscopies:
• suitable under reaction conditions
• does not require long-range order
• element specific
• XANES spectroscopy for:
• symmetry information from the pre-edge peaks
• valence state from the edge shift
• analysis of mixtures using basis spectra
• XANES is experimentally simpler than EXAFS
• signal is stronger (one can measure faster and at lower concentrations)
• does not depend on T (if without phase transitions, of course)
• EXAFS gives:
• inter-atomic distances and coordination numbers
• identification of neighbor atoms
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