Comparison between XAS (Xanes-Exafs), AWAXS,
ASAXS and DAFS applied to nanometer scale supported
metallic clusters.
Definition of a methodology
D. Bazin
1
Plan
I. Introduction
I-1. Nanometer scale metallic clusters.
I-2. Structural parameters : N & R.
II. X-ray Absorption Spectroscopy
II-1. K edge of 3d transition metals.
II-2. L edge of 5d transition metals.
II-3. Extended X-ray Absorption Fine Structure (Exafs).
II-3a. Monometallic clusters & II-3b Bimetallic clusters.
II-4 Comparison between Xanes and Exafs.
III. Anomalous Wide Angle X-ray Scattering
III-1. Generalities
III-2. Monometallic clusters
III-3. Comparison between Xas and Awaxs
III-4. Size distribution & morphology
III.-5. Bimetallic clusters.
IV/ Anomalous Small Angle X-ray Scattering
IV-1. Monometallic clusters
IV-2. Bimetallic clusters.
V/ Generalities on Diffraction Anomalous Fine Structure
V-1. Generalities
V-2. First DAFS experimental results at the ESRF on metallic particles.
VI. Summary
VII. Future prospects
2
I.1 Nanometer scale metallic clusters
I
Monometallic clusters


Morphology : Icosahedra are preferred to cubooctahedra,
Relaxation : Large compressions of the central atom are observed
for the icos geometries (6%) while small core compressions are
found for the fcc geometries (1%)
Bimetallic clusters
Distribution
of the metals inside the cluster : statistical
distribution or core/shell distribution
Interaction between the cluster and the support
3
I.2 Structural parameters N & R
I
Monometallic cluster
Coordinations (N1,N2,N3,N4) --> Size & Morphology
Interatomic distances --> Compression/dilatation
Bimetallic cluster
Coordination (NAA, NAB, NBA, NBB) --> Segregation
Very high reactivity --> In situ methods to
preserve the physico chemistry integrity of the
metallic cluster
• In-situ spectroscopy of catalysts
Ed. B.M. Weckhuysen, American Scientific Publishers,2004.
• ”Xas for catalysts and surfaces “,
Ed. Y. Iwasawa,World Scientific, 1996.
• X-ray absorption: principles, applications, techniques of EXAFS, SEXAFS, and XANES.
Ed. DC Koningsberger, R Prins, Wiley, 1988.
4
I.3 L’outil d’investigation : une source de rayons X
I
100000 km/s --> 1000km/s
Brillance Nb de photons/cône
Tube : 107
Synchrotron : 1020
la fluorescence X (identifier les éléments) ou la diffraction X
(identifier les phases)
Temps d’acquisition, Cartographie, Sensibilité
la spectroscopie d’absorption X.
Spéciation
5
1.4 R.S. à travers le monde & Recherche finalisée
Denmark:
France:
ISA (Aarhus).
ESRF(Grenoble),
Soleil (Orsay).
Germany:
ANKA (Karlsruhe),
BESSY (Berlin),
DELTA (Dortmund),
ELSA (Bonn),
HASYLAB (Hamburg).
Italy:
Elettra (Trieste).
Spain:
LLS (Barcelona).
Sweden:
MAX (Lund).
Switzerland: SLS (PSI) (Villigen).
U.K.:
Diamond (Didcot),
SRS (Daresbury).
Brazil:
Canada:
USA:
LNLS (Campinas SP).
CLS (Saskatoon).
ALS (Berkeley CA),
APS (Argonne IL),
CAMD (Baton Rouge LA),
DFELL (Durham NC),
CHESS (Ithaca NY),
SLS (Upton NY), SRC
(Madison WI),
SSRL (Stanford, CA)
,SURF II (Gaithersburg MD).
China (PR): BSRF (Beijing).
India:
INDUS (Indore).
Japan:
Nano-Hana (Ichihara),
Photon Factory (Tsukuba),
SPring-8 (Nishi Harima).
Russia:
SSRC (BINP) (Novosibirsk).
South Korea: Pohang Acc. Lab (Pohang).
Taiwan:
SRRC (Hsinchu).
Australia:
Australian Syn. (Melbourne).
6
II.1 X-ray Absorption Spectroscopy
II
E
µx(E) =log(Ι0/Ι1)
I0
S
I1
2,5
Pt, L
III
2
1,5
Absorption
1
0,5
2p3/2
2p1/2
2s
0
11400 11600 11800 12000 12200 12400
Energy (eV)
Sayers D. A., Lytle F. W. and Stern E. A.,
Advances in X-ray Analysis, (Ed. Plenum, New-York, 13, 1970).
1s
7
II.2 The associated formula
II
2,5
Pt, L
III
2
0,3
Pt, L III edge
1,5
0,2
Absorption
1
0,1
0,5
0
-0,1
0
11400 11600 11800 12000 12200 12400
Energy (eV)
-0,2
-0,3
2
4
6
8
10
12
14
16
-1
k(
χ(k) = Σterms
j Nj/kR
Electronic
j
fj(k), φj(k), λ
2
)
Structural jterms
fj(k)exp(-R
/λ)exp(-2σj2k2)sin(2kRj +φj(k))
Nj , Rj , σj.
R(0)-a
Πτ
N
σ
0
1
2
3
4
5
6
Ρ(0
)
Modulus of the Fourier Transform or
Pseudo radial distribution function
8
II.3 Nanometer scale metallic cluster & Xas
II
25
L
N1
N2
N3
N4
20
III
15
Pt
Au
10
Absorption (a.u.)
5
0
0
1
2
3
4
5
6
7
Diameter (nm)
-50
0
50
E-E0(eV)
100
• D. Bazin, D. Sayers, J. Rehr, Comparison between Xas, Awaxs, Asaxs & Dafs applied to nanometer scale
metallic clusters. J. Phys. Chem. B 101, 11040 (1997).
• D. Bazin, D. Sayers, J. Rehr, C. Mottet Numerical simulation of the Pt LIII edge white line relative to
nanometer scale clusters, J. of Phys. Chem. B 101, 5332 (1997).
• D. Bazin, J. Rehr, Limits and advantages of X-ray absorption near edge structure for nanometer scale
metallic clusters. J. Phys. Chem. B 107, 12398 (2003).
9
II.4a Some examples
II
1%wt Pt/oxide
Impregnation
Cl
_ H2PtCl6
.. Pt/Y-SiO 2
Modules de T.F. (u.a.)
_ PtO2
Pt
0
1
2
3
5 R(F) 6
.. Pt/Y-SiO 2
Modules de la T.F. (u.a.)
O Pt
O
O
0
1
Pt Pt
Pt Pt Pt
Pt Pt
Reduction
4
_ H2PtCl6
O
Calcination
Xas studies of bimetallic Pt-Re(Rh)/Al2O3 catalysts in the
first stages of preparation.
D. Bazin et al., J. of Cat. 110, 209 (1988).
• Bimetallic reforming catalysts : Xas of the particle growing
process during the reduction.
D. Bazin et al., J. Cat. 123, 86 (1990).
• In situ high temperature and high pressure Exafs studies of
Pt/ Al2O3 catalysts : Part I.
N. S. Guyot-Sionnest et al., Cat. Let 8, 283 (1991).
• In situ high temperature and high pressure Exafs studies of
Pt/ Al2O3 catalysts : Part II.
N. S. Guyot-Sionnest et al., Cat. Let 8, 297 (1991).
• Investigation of dispersion and localisation of Pt species in
mazzite using Exafs.
A. Khodakov et al. J. of Phys. Chem. 101, 766 (1996).
• In situ study by Xas of the sulfuration of industrial
catalysts : the Pt & PtRe/Al2O3 system.
A. Bensaddik et al., Applied Cat. A 162, 171, (1997).
• Xas of electronic state correlations during the reduction of
the bimetallic PtRe/Al2O3 system.
D. Bazin et al., J. of Synchrotron Radiation 6, 465, 1999.
• Influence of the H2S/H2 ratio and the temperature on the
local order of Pd atoms in the case of a highly dispersed
multimetallic catalyst : Pd-Ni-Mo/Al2O3.
D. Bazin et al., J. de Physique IV, 12-6, 379, (2002).
• Structure & size of bimetallic PtPd clusters in an
hydrotreatment catalyst.
D. Bazin et al., Accepted in Oil & Gas Science and Technology –
Rev. IFP
•
Pt, L III
_ PtO2
2
3
4
__ PtO
2
5 R(0) 6
Modules de la T.F. (u.a.)
... Pt/Y-SiO 2
_ Pt mŽtal
R(0)-a
Πτ
N
Reforming …Pt, PtPd, PtRh, PtMo,PtSn,PtRe, PtIr,
Fischer–Tropsch, Co, CoPt, CoPd, CoRu,
σ
0
0
1
Modulus of the Fourier Transform or
Pseudo radial distribution function
2
3
4
5
6
Ρ(0)
1
2
3
4
5 R(0
) 6
10
II.4b Experimental set up
4
3
1
2
3
5
Catalyst
Water cooling
X-ray beam
Quick disconnect
for gas flow
5
3
2
Thermocouple
Catalyst
Gas input
Gas output
Figure 1 : Partial view of the sample holder
1, 2 : Boron nitride sample holder and cover plate
3 : Carbon foil for gas leakage
4,5 : Stainless steel mounting block and cover plate
Sample holder
Figure 2 : General view of the sample holder
QuickTimeª et un dŽcompresseur
Photo - JPEG sont requis pour visualiser
cette image.
11
II.4c Some examples
II
Pt/Al2O3
H2
T=450°C
NPtO=6
2,5
Pt, L
III
2
1,5
Absorption
1
0,5
0
11400 11600 11800 12000 12200 12400
Energy (eV)
NPtO= 6.
NReO=4.1
NPtO=0.4
NPtPt=4.5
NPtO=0.3
NPtMet=3.9
NReO=0.9
NReMet=3.7
NPtS=2.8
NPtS=2.0
NPtMet=4.2
NReS=2.4
NReMet=4.5
1%H2S/H2
T=450°C
H2
T=450°C
PtRe/Al2O3
NPtS=2.8
NPtPt=1.5
NPtS=1.0
NPtMet=4.0
NReS=2.0
NReMet=4.7
12
II.5a Xanes K edge of 3d transition metals : Size
II
In
the case of a cluster, it is
essential to consider each kind
of atom since the signal coming
from the surface and the central
atom are definitely not the
same. A 13 Cu environment is
not enough to produce the
resonance C.
Xanes : size effect
B
C
Cu metal
Absorption (u.a.)
55 Cu
13 Cu
-10
0
10
20
30
40
50
60
E(eV)
•
For
nanometer
scale
materials, it is definitely not
possible to simulate their Xanes
part for K edge with a linear
combination of the Xanes of
well crystallised reference
compounds.
G. N. Greaves et al., Nature 294,139 (1981).
D. Bazin et al. J. de Physique C4-481, 6 (1996).
13
II.5b Xanes L edge of 5d transition metals
II
L III
43Pt
19Pt
13Pt
From
an experimental point of
view, the LIII white line is at the
centre of electronic charge transfer
between either the nanometer scale
metallic particle and the support or
between the two metals which are
present inside the cluster.
Ab
initio calculation of DOS
regarding Pt clusters show that the
DOS of clusters is clearly different
from the DOS of the Pt bulk.
Two physical phenomenon can
affect the intensity of the white line :
the size of the cluster which can be
considered as an intrinsic effect and
a possible charge transfer which can
be considered as an extrinsic effect
E(eV)
11550 11560 11570 11580 11590 11600
Size effect on the Xanes spectrum
for Pt 13 , Pt 19 and Pt 43.
4
3,5
3
2,5
D.O.S.2
1,5
1
0,5
00
923
13
6
8 E F 10
E(eV)
Averaged D.O.S. of 13 Pt, 55 Pt ,
147 Pt , 309 Pt , 923 Pt cuboctahedra.
2
4
F. W. Lytle et al. Phys. Rev B 15-4, 2426 (1975).
14
II.5c Dynamical processes
15
II.5c Xanes L edge : Dynamical process
II
B
Pt-Re/Al2O3
Pt LIII
Re LII
bsorption of the sample (a.u.)
11400
11540
11680
11820
11960
Energy of the photons (eV)
12100
Preparation data (%wt )
Samples Pt
Re
Cl
A
0.97 1.2
1.2
A'
1.0
0.5
1.0
B
B'
B"
0.97
1.0
1.0
1.0
1.0
1.0
1.2
1.0
1.0
Fig 1 : Absorption spectrum collected during an in-situ
reduction of a 1 weight% Pt-1weight%Re/Al 2O3 sample.
Water controls the migration of the Re oxide phase, we distinguish :
■ Hydrated
calcined samples (samples A, A') which were reduced after a
three month exposure to air, implying that they have been largely
rehydrated,
Dehydrated calcined samples (samples B, B', B'') which were reduced
after a new drying operation of 15 hours at 100°C. For samples B' and
B'', an in situ drying at 150°C (120 min for sample B' and 360 min for
sample B") was carried out immediately before reduction.
16
II.5c Xanes L edge : Dynamical process
II
Samples with water : mobility of the Re oxide species
Preparation data (%wt )
Samples Pt
Re
A
0.97
1.2
A'
1.0
0.5
Sample A’
Cl
1.2
1.0
Pt L III edge
Re L II edge
Sample A
White line area of the two metals (a.u.)
Pt L
III
edge
Re L II edge
Region I
III
II
White line area of the two metals (a.u.)
0
125
250
375
500
Temperature (¡C)
Fig. 6. Evolution of the white line intensities of
the two metals (platinum and rhenium) during the
reduction of the hydrated sample AÕ.
0
100
200
300
400
Temperature (¡C)
400
Fig 4 : Evolution of the white line intensities of
the two metals (platinum and rhenium) during
the reduction of the hydrated sample A.
400
17
II.5c Xanes L edge : Dynamical process
II
Samples without water : Re oxide species “fixed on the support”
Pt,L III edge
Sample B
Re, L II edge
B
B'
B"
White line area of the two metals(a.u.)
50
100
150
200
250
300
350
0.97
1.0
1.0
1.0
1.0
1.0
1.2
1.0
1.0
400
Temperature (¡C)
Fig. 8. Evolution of the white line intensities of
the two metals (platinum and rhenium) during
the reduction of the dehydrated sample B.
Sample B”
Sample B’
Calcination
N2 + O 2
Reduction
H2
Reduction
H2
Cal.
N2 + O 2
te line area of the two metals (a.u.)
white line area of the two metals (a.u.)
Pt, L III edge
Re, L II edge
Pt, L III edge
100
150
200
250
300
350
Temperature (¡C)
Fig. 9. Evolution of the white line intensities of
the two metals (platinum and rhenium) during
the reduction of the dehydrated sample B'.
Re, L II edge
400
150
150
200
250
300
350
400
Temperature (¡C)
Fig. 10. Evolution of the white line intensities of
the two metals (platinum and rhenium) during
the reduction of the dehydrated sample BÒ.
18
II.5c Xanes L edge : Dynamical process
II
Pt L III edge
Re L II edge
Pt L III edge
Re L II edge
White line area of the two metals (a.u.)
White line area of the two metals (a.u.)
Region I
0
0
100
200 300
400
Temperature (¡C)
400
Fig 4 : Evolution of the white line intensities of
the two metals (platinum and rhenium) during
the reduction of the hydrated sample A.
400
125
III
II
250
375
500
Temperature (¡C)
Fig. 6. Evolution of the white line intensities of
the two metals (platinum and rhenium) during the
reduction of the hydrated sample AÕ.
PtRe bimetallic clusters
Considering
the
results
obtained on the sample A, we
can assume that the first
regime is linked to the
building of the Pt/Re alloy,
the atomic ratio being equal
to 1.
Pt,L III edge
Re, L II edge
Monometallic clusters
White line area of the two metals(a.u.)
50
100
150
200
250
300
Temperature (¡C)
350
Fig. 8. Evolution of the white line intensities of
the two metals (platinum and rhenium) during
the reduction of the dehydrated sample B.
400
19
II.6a Exafs : Disavantages : Monometallic : polydispersity
II
Nt
13
147
1415
13 &
1415
25
N1
N2
N3
N4
20
15
10
5
N1
5.5
8.9
10.5
N2
1.8
4.0
5.0
N3
3.7
13.0
18.5
N4
0.9
6.1
9.1
8.9
4.0 13.8 6.5
0
0
1
2
3
4
5
6
7
Diameter (nm)
One
of the limitations is the fact that Xas is insensitive to
polydispersity. It is clear that the results given by mixing
clusters which have 13 & 1415 atoms are similar to the
coordination number associated with a cluster of 147 atoms.
J. Moonen et al., Physica B 208, 689 (1995).
20
II.6b Bimetallic clusters : repartition of the two metals
II
Bimetallic system
■N
A*NAB=NB*NBA
■σ
B
B
B
■R
B
A
AB=RBA
NAA+NAB=12
NAA+NAB>NBA+NBB
B
B
AB=σBA
12
N AA
N AB
N BA
N BB
10
These relations are no longer
8
valid when monometallic clusters
6
coexist with bimetallic ones. Coordination
4
NAB decreases as NMono
increases. The distribution of the
2
two metals inside the cluster
0
0
given
by
the
different
coordination is simply false.
1
2
3
4
5
Nmono
21
II.7. Comparison between Xanes and Exafs for NSMC
II
Xanes : size effect
B
N = 13
C
N = 55
Pt metal
F.T. (a.u.)
Cu metal
Absorption (u.a.)
55 Cu
13 Cu
0
-10
0
10
20
30
40
50
1
2
3
4
60
5
6
R()
E(eV)
•These two figures show that Xas (Xanes) and (Exafs)
are well adapted to metallic cluster containing a few atoms.
•Xanes as well as Exafs is very sensitive to the size of the particle.
22
III.1 Anomalous Wide Angle X-ray Scattering
III
80
f 0 (Pt)
70
60
Complex
atomic factor
f(q,E) = f0(q) + f'(E) + i f"(E)
between f0(q) & the density ρ(r)
f0(q)=4π∫0∞ρ(r)sin(qr)/qr r2dr
50
40
30
20
10
Relation
Kramers-Kronig
relation
f’(E)=2/πP∫0∞ef”(e)/(e 2-E2 )de
Optical
theorem
f"(q,E) = E σ(E)/2hcre
0
5
10
20 q(
15
-1
)
20
Pt L III
f"(Pt)
10
0
f'(Pt)
-10
-20
-30
11000
11500
12000
12500
13000
E(eV)
Debye
equation
I(q)=ΣiΣj fi(q)fj(q)sin(qRij)/qRij
D. Raoux in “Resonant anomalous X-ray scattering : Theory and applications”
Ed. G. Materlik et al. North Holland.
D. Bazin D. Sayers, Jpn J. Appl. P., Vol. 32, Suppl. 32-2 p 249 (1993).
23
III.1 Monometallic clusters.
111
5 10
311
200
220
7
N = 2869 Pt
N = 3871 Pt
N = 5083 Pt
N = 6525 Pt
N = 8217 Pt
N = 10179 Pt
4 10 7
222
3 10
N=2057
7
Ιντενσιτψ
N=1415
2 10
7
N=923
Intensity (a.u.)
1 10 7
N=561
N=309
0
N=147
2
3
4
5
6
7
3
4
10
-1
N=13
6
5
2π/d(
111
200
9
2 π /d ( )
N=55
2
8
-1
)
... N= 55 Pt
... N= 13 Pt
311
220 222
Intensity (a.u.)
F.T. (a.u.)
55 Pt
13 Pt
2
3
4
5
6
q(
-1
)
0
2
4
6
8
10
R(
)
24
Nanotube (111) & Nanotube (001)
8 10
5
7 10
5
6 10
5
5 10
5
4 10
5
3 10
5
2 10
5
1 10
5
Nanotube
Nanotube
Nanotube
Nanotube
Nanotube
Nanotube
111
(111)
(111)
(111)
(111)
(111)
(111)
N=45
N=69
N=95
N=121
N=235
N=335
222
333
0
2
3
4
5
6
3,5 10
7
8
9
10
2 π /d (-1)
5
Nanotube (001) N=31
Nanotube (001) N=95
Nanotube (001) N=147
Nanotube (001) N=191
Nanotube (001) N=243
200
3 10
2,5 10
2 10
5
5
5
5
Intensity
1,5 10
1 10
5
5 10
4
400
0
2
3
4
5
6
7
8
9
10
2 π /d (-1)
25
III.2 Comparison between Xas & Awaxs
... N= 55 Pt
... N= 13 Pt
Awaxs
is better than Xas for the
determination of the local order after
F.T. (a.u.)
the first shell.
The low k range of the absorption
spectrum is dominated by multiple
scattering which lead to a loss of
information in the high R range.
If X-ray scattering factors have
0
2
similar k dependence whatever the
atomic number Z and this is true for
wide as well as for small angle
scattering, it is not the case of the Xas.
Amplitude as well as phase scattering
F. T. (a.u.)
change significantly with Z for the
latter and thus the chemical sensitivity
of Xas is much higher.
0
1
4
6
8
10
R(0
)
Pt metal
N= 55 Pt
N = 13 Pt
2
3
4
5
6
R(0
)
26
III.3 Size distribution & Morphology
27
W. VOGEL et al. J. Phys. Chem. 97,11611 (1993).
III.4a Awaxs-Bimetallic Modeling
Pt Mo
Mo Pt Pt
Pt Mo
For cluster A (Pt155-Mo154), We consider a
statistical distribution of Pt and Mo.
MoMo
Mo Pt Mo
MoMo
For cluster B (Pt147-Mo162), Pt
atoms are in the core and Mo atoms
are at the surface.
80
f 0 (Pt)
70
60
Equation de Debye
I(q)=ΣiΣj fi(q) fj(q) sin (qRij)/qRij
50
40
30
20
Facteur de diff. atom. : f(q,E) = f0(q) + f'(E) + i f"(E)
Relation entre f0(q) & la densité électronique ρ(r)
∞
f0(q) =4π∫0 ρ(r)sin(qr)/qr r2dr
10
0
5
10
20 q(0
15
-1
)
20
Pt L III
f"(Pt)
10
0
f'(Pt)
-10
Relation de Kramers-Kronig
∞
f’(E)=2/πP∫0 ef”(e)/(e 2-E2 )de
Théorème optique
f"(q,E) = E σ(E)/2hcre
-20
-30
11000
11500
12000
12500
13000
E(eV)
Samant M. G. et al. J. Phys. Chem. 92,3542 (1988).
Bazin D., Sayers D., Jpn J. Appl. P., Vol. 32, Suppl. 32-2 p 252 (1993).
28
III.4b Awaxs-Bimetallic Results
Pt Mo
Mo Pt Pt
Pt Mo
For cluster A (Pt155-Mo154), We consider a
statistical distribution of Pt and Mo.
111
the WPSFs at the Pt and Mo
edges are basically the same.
The only difference is in the W.P.S.F. (a.u.)
amplitude due to the fact that
the atomic scattering factor is
larger for Mo than for Pt.
2
MoMo
Mo Pt Mo
MoMo
__ Pt L
200
3
III
... Mo K
4
5
6 k(0
-1
)
For cluster B (Pt147-Mo162), Pt
atoms are in the core and Mo atoms
are at the surface.
111
__ Pt L
The splitting between
the 111 and 200 peaks
is less well defined for
the WPSFs at the Pt
edge than at the Mo
edge.
III
200 ... Mo K
W.P.S.F. (a.u.)
2
3
4
5
6 k(0
-1
)
29
III.5 The case of metal oxide
AB2O4 - CoFe2O4 - decomposition of N2O
Zn/Al2O3 -> ZnAl2O4/Al2O3 : Atomic comp. Zn 2%
Al2O3
ZnAl2O4
440
220 311
400
511/333
IntensitŽ (u.a.)
111
331
422
1 1,5 2 2,5 3 3,5 4 4,5 k ( -1 )
30
III.5a AWAXS at the Zn K edge
Complex
atomic factor
f(q,E) = f (q) + f'(E) + i f"(E)
IntensitŽ (u.a.)
0
E= 9660.eV
E= 9200.eV
1
2
3
4
5
6
7
-1
8
k ( )
I(9660.)=I(ZnAlO)+I(Al2O3)
I(9200.)=I(ZnAlO)+I(Al2O3)
∆I=∆I(ZnAlO)
(220)
(311)
(511) (333)
(440)
(331)
(422)
IntensitŽ diffŽrentielle (u.a.)(400)
(111)
Debye
equation
I(q)=Σ Σ f (q)f (q)sin(qR )/qR
i
1
2
3
4
5
6
7
k(
-1
j
i
j
ij
ij
8
)
31
III.5c Numerical simulation
(220)
(311)
(511) (333)
(440)
(331)
(422)
IntensitŽ diffŽrentielle (u.a.)(400)
(111)
1
2
3
4
5
6
7
k(
-1
8
)
(220)
(311)
(511) (333)
(422)(440)
(331)
(400)
(111)
x=0
x=0.1
x=0.2
x=0.3
x=0.4
x=0.5
IntensitŽ diffŽrentielle (u.a.)
x=0.6
x=0.7
x=0.8
x=0.9
1
2
3
4
5
6
7
k(
-1
=1/2
8
)
Simulations basées sur une substitution sur les sites
tétrahédriques : (Zn1-xAlx)Td (Al2)Oh O4
32
III.5d Xas at the Zn K edge
2
1,5
ZnAl2O4
ZnAl2O4/Al2O3
/I)
0
log (I1
Zn 2+
Structure spinelle
0,5
0
9620 9640 9660 9680 9700 9720 9740 9760
E (eV)
Zn-O
T.F. (u.a)
ZnAl2O4 :
ZnO N=4. , R=1.95Å
Zn-Al
Zn-O
Zn-Zn
0
1
2
3
4
5
6
R ()
33
III.5e Quantitativ results
Données structurales extraites de l’affinement de Zn/Al2O3
comparées à celles du modèle ZnAl2O4
Chemins N
Zn-O
Zn-Al
Zn-O
Zn-Zn
Zn-Al-O
Zn-O
Zn-Al-O
N
4
4
10.9 12
10.9 12
0.9
4
23.9 24
6.6
12
14.3 24
R(Å) R(Å)
1.95
3.34
3.38
3.49
3.60
4.27
4.35
1.95
3.36
3.40
3.50
3.62
4.27
4.36
T.F. (u.a)
0
1
2
3
4
5
6
R ()
34
III.5f Zn K and L edge
2
2
ZnAl2O4
Zn, K edge
Zn, LIII edge
Sample 1
Sample 2
1,5
1,5
Absorption (a.u.)
Absorption (a.u.)
1
1
0,5
0,5
E(eV)
0
9650
ZnAl2O4
Sample 1
Sample 2
9660
9670
9680
9690
E(eV)
0
9700 1020 1025 1030 1035 1040 1045 1050
35
III.5f Zn K and L edge
2
ZnAl2O4
Zn, K edge
Sample 1
Sample 2
1,5
ZnAl2O4
Absorption1 (a.u.)
Zn, L II edge
0,5
E(eV)
0
9650
9660
9670
9680
9690
9700
Zn K edge
Different structural hypothesis
1. Multiple scattering processes
of the photoelectron
2. Dimension of the ZnAl2O4
Absorbance (a.u.)
K Zn edge
L III Zn edge
E (eV)
-20
-10
0
10
20
30
40
50
3. Influence of Zn vacancies
4. Influence of oxygen vacancies
36
III.5f Structural proposition
37
IV-1. Monometallic clusters.
N
N
N
N
N
N
N
For
fcc clusters, we can observe
different modulations after the
(0,0,0) peak and thus information
regarding the crystallographic
network as well as on the size on
the particles is available through ab
initio calculations.
Nevertheless, we have to point out
that physical parameters such the
interatomic distance modify the
position of these modulations.
Also if other kinds of morphology
like
hemicuboctahedron
are
considered, no modulations exist.
W. Vogel et al. J. Phys. Chem. 97, 11611 (1993).
Intensity (a.u.)
0,3
0,4
0,5
0,6
0,7
=
=
=
=
=
=
=
13
55
147
309
561
923
1415
0,8
0,9
1 k(•
-1
)
Central diffusion as calculated using the
Debye equation near the (0,0,0) peak for
cuboctahedra FCC Pt clusters of N atoms.
N
N
N
N
N
=
=
=
=
=
10
37
88
181
320
Intensity (a.u.)
0,3 0,4 0,5 0,6 0,7 0,8 0,9
1 k(•
-1
)
Central diffusion as calculated using the
Debye equation near the (0,0,0) peak for
hemicuboctahedra FCC Pt clusters of N atoms
38
V. Diffraction Anomalous Fine Structure (Dafs)
D. Sayers, H. Renevier, J. L. Hodeau, J. F. Berar, J. M. Tonnerre, D. Raoux,
A. Chester, D. Bazin, C. Bouldin, ESRF NewsLetter, January 1997 N°27.
Complex
atomic factor
f (q,E) = f (q) + f' (E) + i f" (E)
site
0
site
site
Debye
equation
I(q) = Σ Σ f (q)f (q)sin(qR )/qR
i
j
i
j
ij
ij
39
V. Diffraction Anomalous Fine Structure (Dafs)
15
Pt
10
Complex
atomic factor
f (q,E) = f (q) + f' (E) + i
site
0
site
f" (E)
site
◆The
Debye equation is then used to
get the diffraction diagram and
finally by plotting the maximum of
the intensity versus the photon energy
it is easy to build the Dafs spectrum.
5
f"
0
-5
-10
-15
-20
f'
11550
11725 11900 12075 E(eV)
f' & f" associated
to the central atom (0,0,0).
1,3
1,2
55 Pt
1,1
1
13 Pt
0,9
This approach has been
extended to clusters of Pt
containing 13 and 55 atoms. It
is clear that the Exafs
oscillations are bigger than the
Dafs ones.
0,8
0,7
E(eV)
11400 11600 11800 12000 12200
Dafs spectrum as calculated for a 13
Pt and a 55 Pt cluster.
40
First DAFS experimental results @ ESRF on NSMC
1,3
1,2
55 Pt
1,1
1
13 Pt
0,9
0,8
0,7
E(eV)
11400 11600 11800 12000 12200
Dafs spectrum as calculated for a 13
Pt and a 55 Pt cluster.
D. Sayers, H. Renevier, J. L. Hodeau, J. F. Berar, J. M. Tonnerre, D. Raoux,
A. Chester, D. Bazin, C. Bouldin, ESRF NewsLetter, January 1997 N°27.
41
VI. Summary - NSMC & RS
Monometallic cluster
Xas
Awaxs
Asaxs
Network
-
+
-
Size
+
+
+
Size distribution -
+
+
Morphology
+
+
-
Bimetallic cluster
Xas
Distribution of the
+
Awaxs
-
Asaxs
-
metals inside the
cluster
42
II.5 N.S.M.C. & R.S.
II
111
311
200
220
222
• Comparison between Xas & Awaxs applied to monometallic clusters. D.
Bazin, D. Sayers, Jpn J. Appl. Phys. 32-2, 249, 1993.
D.
• Comparison between Xas & Awaxs applied to bimetallic clusters.
Bazin, D. Sayers, Jpn J. Appl. Phys. 32-2, 252, 1993.
D.
• AWAXS in heterogeneous catalysis
Bazin, L. Guczi, J. Lynch, App. Cat. A 226, 87, 2002.
N=2057
N=1415
N=923
Intensity (a.u.)
N=561
N=309
N=147
N=55
2
3
4
5
J.D. Grunwaldt et al., J. of Cat. 213,291 (2003)
N=13
6 k(• -1 )
L. Drozdova et al. J. Phys. Chem. B 106,2240 (2002)
• Real time in situ Xanes approach to characterise electronic state of nanometer scale entities
D. Bazin, L. Guczi, J. Lynch, Rec. Res. Dev. Phys. Chem. 4, 259, 2000.
• Soft X-ray absorption spectroscopy and heterogeneous catalysis.
D.
Bazin, L. Guczi, App. Cat. A 213/2, 147, 2001.
• New opportunities to understand heterogeneous catalysis processes through S.R. studies and theoretical
calculations of density of states : The case of nanometer scale bimetallic particles
D.
Bazin, C. Mottet, G. Treglia, Applied Catalysis A (1-2), 47-54, 2000.
D.
• New trends in heterogeneous catalysis processes on metallic clusters from S.R. & theoretical studies
Bazin, C. Mottet, G. Treglia, J. Lynch, Applied Surf. Sci. 164, 140, 2000.
43
VII. Conclusion - Future Prospects
Characterisation
technics related to synchrotron radiation like
Xas, Awaxs, Asaxs are in situ chemical tools.
Follow the evolution of catalysts during the activation process.
Structural Characterization
@ the atomic scale
Catalytic Activity
44