Post on 07-Feb-2018
1
D. Bazin
Comparison between XAS (Xanes-Exafs), AWAXS, ASAXS and DAFS applied to nanometer scale supported
metallic clusters.Definition of a methodology
2
Plan
I. IntroductionI-1. Nanometer scale metallic clusters.I-2. Structural parameters : N & R.
II. X-ray Absorption SpectroscopyII-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 ScatteringIII-1. GeneralitiesIII-2. Monometallic clustersIII-3. Comparison between Xas and AwaxsIII-4. Size distribution & morphologyIII.-5. Bimetallic clusters.
VI. SummaryVII. Future prospects
IV/ Anomalous Small Angle X-ray ScatteringIV-1. Monometallic clustersIV-2. Bimetallic clusters.
V/ Generalities on Diffraction Anomalous Fine StructureV-1. GeneralitiesV-2. First DAFS experimental results at the ESRF on metallic particles.
3
I.1 Nanometer scale metallic clusters
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%)
I
Bimetallic clustersDistribution of the metals inside the cluster : statistical distribution or core/shell distribution
Interaction between the cluster and the support
4
I.2 Structural parameters N & R
Monometallic cluster
Coordinations (N1,N2,N3,N4) --> Size & Morphology
Interatomic distances --> Compression/dilatation
I
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 catalystsEd. 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.
5
I.3 L’outil d’investigation : une source de rayons X
I
Brillance Nb de photons/cône
Tube : 107 Synchrotron : 1020
100000 km/s --> 1000km/s
la fluorescence X (identifier les éléments) ou la diffraction X (identifier les phases)
Temps d’acquisition, Cartographie, Sensibilitéla spectroscopie d’absorption X.
Spéciation
6
1.4 R.S. à travers le monde & Recherche finalisée
Denmark: ISA (Aarhus). France: 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: LNLS (Campinas SP). Canada: CLS (Saskatoon). USA: 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).
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II.1 X-ray Absorption Spectroscopy
µx(E) =log(Ι0/Ι1)
SI0 I1E
1s
2s
2p1/2
2p3/2
Sayers D. A., Lytle F. W. and Stern E. A., Advances in X-ray Analysis, (Ed. Plenum, New-York, 13, 1970).
0
0,5
1
1,5
2
2,5
11400 11600 11800 12000 12200 12400
Absorption
Energy (eV)
Pt, LIII
II
8
II.2 The associated formula
Electronic termsfj(k), φj(k), λ
Structural termsNj , Rj , σj.
χ(k) = Σj Nj/kRj
2 fj(k)exp(-Rj/λ)exp(-2σj2k2)sin(2kRj +φj(k))
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
2 4 6 8 10 12 14 16
k(-1)
Pt, LIII
edge
0
0,5
1
1,5
2
2,5
11400 11600 11800 12000 12200 12400
Absorption
Energy (eV)
Pt, LIII
Modulus of the Fourier Transform or
Pseudo radial distribution function
R(0)-a
N
σ
Πτ
Ρ(0)0 1 2 3 4 5 6
II
9
II.3 Nanometer scale metallic cluster & Xas
• 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).
0
5
10
15
20
25
0 1 2 3 4 5 6 7
N1N2N3N4
Diameter (nm)
II
-50 0 50 100
PtAu
Absorption (a.u.)
LIII
E-E0(eV)
10
II.4a Some examples
Modulus of the Fourier Transform or
Pseudo radial distribution function
R(0)-a
N
σ
Πτ
Ρ(0)0 1 2 3 4 5 6
Cl
Pt
Impregnation
0 1 2 3 4 5 6R(F)
_ H2PtCl
6
.. Pt/Y-SiO2
_ PtO2
Modules de T.F. (u.a.)
Pt, LIII
Pt PtPtPt
PtPt Pt
Reduction
0 1 2 3 4 5 6
Modules de la T.F. (u.a.)
... Pt/Y-SiO2
R(0)
__ PtO2
_ Pt mŽtal
1%wt Pt/oxide• 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
Reforming …Pt, PtPd, PtRh, PtMo,PtSn,PtRe, PtIr,Fischer–Tropsch, Co, CoPt, CoPd, CoRu,
II
Calcination
0 1 2 3 4 5 6
_ H2PtCl
6
_ PtO2
.. Pt/Y-SiO2
Modules de la T.F. (u.a.)
R(0)
O
O
PtO
O
11
II.4b Experimental set up
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
1 23 345
X-ray beam
Catalyst
Catalyst
5
3
2
Water cooling
Quick disconnect
for gas flow
Sample holder
Thermocouple
Figure 2 : General view of the sample holder
Gas output
Gas input
QuickTimeª et un dŽcompresseurPhoto - JPEG sont requis pour visualiser
cette image.
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II.4c Some examples
II
H2
T=450°C
1%H2S/H2
T=450°C
H2
T=450°C
Pt/Al2O3
PtRe/Al2O3
NPtO= 6.NReO=4.1
NPtO=0.3NPtMet=3.9NReO=0.9NReMet=3.7
NPtS=2.0NPtMet=4.2NReS=2.4NReMet=4.5
NPtS=1.0NPtMet=4.0NReS=2.0NReMet=4.7
NPtO=6
NPtO=0.4NPtPt=4.5
NPtS=2.8
NPtS=2.8NPtPt=1.5
0
0,5
1
1,5
2
2,5
11400 11600 11800 12000 12200 12400
Absorption
Energy (eV)
Pt, LIII
13
II.5a Xanes K edge of 3d transition metals : Size
G. N. Greaves et al., Nature 294,139 (1981).D. Bazin et al. J. de Physique C4-481, 6 (1996).
-10 0 10 20 30 40 50 60
Cu metal
55 Cu
13 Cu
E(eV)
Absorption (u.a.)
Xanes : size effectB C
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.
II
• 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.
14
II.5b Xanes L edge of 5d transition metals
115501156011570115801159011600
43Pt19Pt13Pt
E(eV)
Size effect on the Xanes spectrumfor Pt 13 , Pt 19 and Pt 43.
LIII
00,5
11,5
22,5
33,5
4
0 2 4 6 8 10
D.O.S.
E(eV)E
F
Averaged D.O.S. of 13 Pt, 55 Pt ,147 Pt , 309 Pt , 923 Pt cuboctahedra.
13
923
F. W. Lytle et al. Phys. Rev B 15-4, 2426 (1975).
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
II
16
II.5c Xanes L edge : Dynamical process
Preparation data (%wt )
Samples Pt Re Cl
A 0.97 1.2 1.2
A' 1.0 0.5 1.0
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,
II
B 0.97 1.0 1.2
B' 1.0 1.0 1.0
B" 1.0 1.0 1.0
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.
11400 11540 11680 11820 11960 12100
B
Absorption of the sample (a.u.)
Energy of the photons (eV)
Pt LIII
Fig 1 : Absorption spectrum collected during an in-situreduction of a 1 weight% Pt-1weight%Re/Al
2O
3 sample.
Re LII
Pt-Re/Al2O3
17
II.5c Xanes L edge : Dynamical process
Pt L III
edge
Re L II
edge
0 100 200 300 400 400Temperature (¡C)
Fig 4 : Evolution of the white line intensities ofthe two metals (platinum and rhenium) duringthe reduction of the hydrated sample A.
400
White line area of the two metals (a.u.)
Sample A
0 125 250 375 500
Pt L III
edge
Re LII edge
Region I II
Fig. 6. Evolution of the white line intensities ofthe two metals (platinum and rhenium) during thereduction of the hydrated sample AÕ.
Temperature (¡C)
White line area of the two metals (a.u.)
III
Sample A’
II Samples with water : mobility of the Re oxide species
Preparation data (%wt )
Samples Pt Re Cl
A 0.97 1.2 1.2
A' 1.0 0.5 1.0
18
II.5c Xanes L edge : Dynamical process
50 100 150 200 250 300 350 400
Pt,LIII edge
Re, LII edge
Fig. 8. Evolution of the white line intensities ofthe two metals (platinum and rhenium) duringthe reduction of the dehydrated sample B.
Temperature (¡C)
White line area of the two metals(a.u.)
Sample B
100 150 200 250 300 350 400
Re, LII edge
Pt, LIII
edge
White line area of the two metals (a.u.)
CalcinationN
2 + O
2
ReductionH
2
Temperature (¡C)
Fig. 9. Evolution of the white line intensities ofthe two metals (platinum and rhenium) duringthe reduction of the dehydrated sample B'.
Sample B’
Re, LII edge
Pt, LIII
edge
150 150 200 250 300
white line area of the two metals (a.u.)
350 400
Temperature (¡C)
Cal.N
2 + O
2
ReductionH
2
Fig. 10. Evolution of the white line intensities ofthe two metals (platinum and rhenium) duringthe reduction of the dehydrated sample BÒ.
Sample B”
II Samples without water : Re oxide species “fixed on the support”
B 0.97 1.0 1.2
B' 1.0 1.0 1.0
B" 1.0 1.0 1.0
19
II.5c Xanes L edge : Dynamical process
Pt L III
edge
Re L II
edge
0 100 200 300 400 400Temperature (¡C)
Fig 4 : Evolution of the white line intensities ofthe two metals (platinum and rhenium) duringthe reduction of the hydrated sample A.
400
White line area of the two metals (a.u.)
0 125 250 375 500
Pt L III
edge
Re LII edge
Region I II
Fig. 6. Evolution of the white line intensities ofthe two metals (platinum and rhenium) during thereduction of the hydrated sample AÕ.
Temperature (¡C)
White line area of the two metals (a.u.)
III
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.
PtRe bimetallic clusters
50 100 150 200 250 300 350 400
Pt,LIII edge
Re, LII edge
Fig. 8. Evolution of the white line intensities ofthe two metals (platinum and rhenium) duringthe reduction of the dehydrated sample B.
Temperature (¡C)
White line area of the two metals(a.u.)Monometallic clusters
II
20
II.6a Exafs : Disavantages : Monometallic : polydispersity
J. Moonen et al., Physica B 208, 689 (1995).
Nt N1 N2 N3 N4
13 5.5 1.8 3.7 0.9 147 8.9 4.0 13.0 6.1 1415 10.5 5.0 18.5 9.1 13 & 1415 8.9 4.0 13.8 6.5
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.
0
5
10
15
20
25
0 1 2 3 4 5 6 7
N1N2N3N4
Diameter (nm)
II
21
II.6b Bimetallic clusters : repartition of the two metals
0
2
4
6
8
10
12
0 1 2 3 4 5
NAA
NAB
NBA
NBB
Coordination
Nmono
II
NAA+NAB=12 NAA+NAB>NBA+NBB
■ NA*NAB=NB*NBA
■ σAB=σBA
■ RAB=RBA
Bimetallic system
These relations are no longer valid when monometallic clusters coexist with bimetallic ones. NAB decreases as NMono increases. The distribution of the two metals inside the cluster given by the different coordination is simply false.
A B
BB
B
B B
22
II.7. Comparison between Xanes and Exafs for NSMC
-10 0 10 20 30 40 50 60
Cu metal
55 Cu
13 Cu
E(eV)
Absorption (u.a.)
Xanes : size effectB C
0 1 2 3 4 5 6
N = 13
N = 55
Pt metal
R(�)
F.T. (a.u.)
•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.
II
23
III.1 Anomalous Wide Angle X-ray Scattering
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).
Complex atomic factorf(q,E) = f0(q) + f'(E) + i f"(E)
Relation between f0(q) & the density ρ(r)
f0(q)=4π∫0∞ρ(r)sin(qr)/qr r2dr
III
10
20
30
40
50
60
70
80
0 5 10 15 20
f0(Pt)
q(� -1)
-30
-20
-10
0
10
20
11000 11500 12000 12500 13000
f'(Pt)
f"(Pt)
E(eV)
Pt LIII
Kramers-Kronig relationf’(E)=2/πP∫0
∞ef”(e)/(e 2-E2 )de
Optical theoremf"(q,E) = E σ(E)/2hcre
Debye equationI(q)=ΣiΣj fi(q)fj(q)sin(qRij)/qRij
24
III.1 Monometallic clusters.
2 3 4 5 6
111200 220
311
222
55 Pt
13 Pt
q(� -1)
Intensity (a.u.)
0 2 4 6 8 10
F.T. (a.u.)
R(�)
... N= 55 Pt
... N= 13 Pt
0
1 107
2 107
3 107
4 107
5 107
2 3 4 5 6 7 8 9 10
N = 2869 PtN = 3871 PtN = 5083 PtN = 6525 PtN = 8217 PtN = 10179 Pt
2 /d (-1)π
Ιντενσιτψ
2 3 4 5 6
N=2057
N=147
N=1415
111
200 220
311
222
N=923
N=561
N=309
N=55
N=13
2 /d(� -1)
Intensity (a.u.)
π
25
Nanotube (111) & Nanotube (001)
2 3 4 5 6 7 8 9 10
Nanotube (111) N=45Nanotube (111) N=69Nanotube (111) N=95Nanotube (111) N=121Nanotube (111) N=235Nanotube (111) N=335
0
1 10 5
2 10 5
3 10 5
4 10 5
5 10 5
6 10 5
7 10 5
8 10 5
111
222
333
2 /d (-1)π
2 3 4 5 6 7 8 9 10
Nanotube (001) N=31Nanotube (001) N=95Nanotube (001) N=147Nanotube (001) N=191Nanotube (001) N=243
0
5 10 4
1 10 5
1,5 10 5
2 10 5
2,5 10 5
3 10 5
3,5 10 5
Intensity
200
400
2 /d (-1)π
26
III.2 Comparison between Xas & Awaxs
0 2 4 6 8 10
F.T. (a.u.)
R(0)
... N= 55 Pt
... N= 13 Pt
0 1 2 3 4 5 6R(0)
F. T. (a.u.)
Pt metal
N= 55 Pt
N = 13 Pt
Awaxs is better than Xas for the determination of the local order after 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 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 change significantly with Z for the latter and thus the chemical sensitivity of Xas is much higher.
28
III.4a Awaxs-Bimetallic Modeling
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).
Equation de Debye
I(q)=ΣiΣj fi(q) fj(q) sin (qRij)/qRij
10
20
30
40
50
60
70
80
0 5 10 15 20
f0(Pt)
q(0 -1)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
-30
-20
-10
0
10
20
11000 11500 12000 12500 13000
f'(Pt)
f"(Pt)
E(eV)
Pt LIII
Relation de Kramers-Kronig
f’(E)=2/πP∫0
∞ef”(e)/(e 2-E2 )de
Théorème optique
f"(q,E) = E σ(E)/2hcre
For cluster A (Pt155-Mo154), We consider a statistical distribution of Pt and Mo.
PtMo
MoMo Pt
Pt
PtFor cluster B (Pt147-Mo162), Pt atoms are in the core and Mo atoms are at the surface.Pt
MoMoMo
MoMoMo
29
III.4b Awaxs-Bimetallic Results
2 3 4 5 6
W.P.S.F. (a.u.)
... Mo K
__ Pt LIII
111
200
k(0 -1)
2 3 4 5 6
__ Pt LIII
... Mo K
W.P.S.F. (a.u.)
111
200
k(0 -1)
the WPSFs at the Pt and Mo edges are basically the same. The only difference is in the amplitude due to the fact that the atomic scattering factor is larger for Mo than for Pt.
For cluster A (Pt155-Mo154), We consider a statistical distribution of Pt and Mo.
PtMo
MoMo Pt
Pt
Pt
The splitting between the 111 and 200 peaks is less well defined for the WPSFs at the Pt edge than at the Mo edge.
For cluster B (Pt147-Mo162), Pt atoms are in the core and Mo atoms are at the surface.Pt
MoMoMo
MoMoMo
30
III.5 The case of metal oxide
1 1,5 2 2,5 3 3,5 4 4,5
Al2O
3ZnAl
2O
4
IntensitŽ (u.a.)
k (� -1)
111
220 311400
422
511/333
440
331
AB2O4 - CoFe2O4 - decomposition of N2O
Zn/Al2O3 -> ZnAl2O4/Al2O3 : Atomic comp. Zn 2%
31
III.5a AWAXS at the Zn K edge
1 2 3 4 5 6 7 8k (-1)
IntensitŽ (u.a.)
1 2 3 4 5 6 7 8
IntensitŽ diffŽrentielle (u.a.)
k (� -1)
(111)
(440)(511) (333)
(422)(331)
(400)
(311)(220)
Complex atomic factorf(q,E) = f
0(q) + f'(E) + i f"(E)
E= 9660.eVE= 9200.eV
I(9660.)=I(ZnAlO)+I(Al2O3)I(9200.)=I(ZnAlO)+I(Al2O3)∆I=∆I(ZnAlO)
Debye equationI(q)=Σ
iΣ
j f
i(q)f
j(q)sin(qR
ij)/qR
ij
32
III.5c Numerical simulation
1 2 3 4 5 6 7 8
IntensitŽ diffŽrentielle (u.a.)
k (� -1)
x=0.2x=0.3
x=0.4x=0.5
x=0.6
x=0.7
x=0.8x=0.9
x=0x=0.1
(111)
(440)(511) (333)(422)(331)
(400)
(311)(220)
1 2 3 4 5 6 7 8
IntensitŽ diffŽrentielle (u.a.)
k (� -1)
(111)
(440)(511) (333)
(422)(331)
(400)
(311)(220)
Simulations basées sur une substitution sur les sitestétrahédriques : (Zn1-xAlx)Td (Al2)Oh O4
=1/2
33
III.5d Xas at the Zn K edge
0
0,5
1
1,5
2
9620 9640 9660 9680 9700 9720 9740 9760
ZnAl2O
4ZnAl
2O
4/Al
2O
3
log (I
0/I)
E (eV)
0 1 2 3 4 5 6
T.F. (u.a)
R (�)
Zn 2+
Structure spinelle
ZnAl2O4 : ZnO N=4. , R=1.95Å
Zn-O
Zn-AlZn-OZn-Zn
34
III.5e Quantitativ results
Données structurales extraites de l’affinement de Zn/Al2O3
comparées à celles du modèle ZnAl2O4
Chemins N N R(Å) R(Å)Zn-O 4 4 1.95 1.95
Zn-Al 10.9 12 3.34 3.36
Zn-O 10.9 12 3.38 3.40
Zn-Zn 0.9 4 3.49 3.50
Zn-Al-O 23.9 24 3.60 3.62
Zn-O 6.6 12 4.27 4.27
Zn-Al-O 14.3 24 4.35 4.360 1 2 3 4 5 6
T.F. (u.a)
R ( )
35
III.5f Zn K and L edge
0
0,5
1
1,5
2
9650 9660 9670 9680 9690 9700
ZnAl2O
4
Sample 1
Sample 2
E(eV)
Zn, K edge
Absorption (a.u.)
0
0,5
1
1,5
2
1020 1025 1030 1035 1040 1045 1050
ZnAl2O
4
Sample 1Sample 2
E(eV)
Zn, LIII
edge
Absorption (a.u.)
36
III.5f Zn K and L edge
-20 -10 0 10 20 30 40 50
K Zn edge
LIII
Zn edge
Absorbance (a.u.)
E (eV)
ZnAl2O
4Zn, L
II edge
Zn K edgeDifferent structural hypothesis
1. Multiple scattering processes of the photoelectron
2. Dimension of the ZnAl2O4
3. Influence of Zn vacancies4. Influence of oxygen vacancies
0
0,5
1
1,5
2
9650 9660 9670 9680 9690 9700
ZnAl2O
4
Sample 1
Sample 2
E(eV)
Zn, K edge
Absorption (a.u.)
38
IV-1. Monometallic clusters.
W. Vogel et al. J. Phys. Chem. 97, 11611 (1993).
0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
N = 13N = 55N = 147N = 309N = 561N = 923N = 1415Intensity (a.u.)
k(• -1)Central diffusion as calculated using theDebye equation near the (0,0,0) peak forcuboctahedra FCC Pt clusters of N atoms.
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.
0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
N = 10N = 37N = 88N = 181N = 320
Intensity (a.u.)
k(• -1)Central diffusion as calculated using theDebye equation near the (0,0,0) peak forhemicuboctahedra FCC Pt clusters of N atoms
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.
39
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.
Debye equationI(q) = Σ
iΣ
j f
i(q)f
j(q)sin(qR
ij)/qR
ij
Complex atomic factorf
site(q,E) = f
0(q) + f'
site(E) + i f"
site(E)
40
V. Diffraction Anomalous Fine Structure (Dafs)
11400 11600 11800 12000 122000,7
0,8
0,9
1
1,1
1,2
1,3
E(eV)
Dafs spectrum as calculated for a 13Pt and a 55 Pt cluster.
55 Pt
13 Pt
◆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.
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.
-20
-15
-10
-5
0
5
10
15
11550 11725 11900 12075 E(eV)
f"
f'
f' & f" associated to the central atom (0,0,0).
Pt
Complex atomic factorf
site(q,E) = f
0(q) + f'
site(E) + i
f"site
(E)
41
First DAFS experimental results @ ESRF on NSMC
11400 11600 11800 12000 122000,7
0,8
0,9
1
1,1
1,2
1,3
E(eV)
Dafs spectrum as calculated for a 13Pt and a 55 Pt cluster.
55 Pt
13 Pt
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.
42
VI. Summary - NSMC & RS
Monometallic clusterXas Awaxs Asaxs
Bimetallic clusterXas Awaxs Asaxs
Distribution of the + - -metals inside the cluster
Morphology - + +
Size + + +
Size distribution - + +
Network - + -
43
II.5 N.S.M.C. & R.S.
II
2 3 4 5 6
N=2057
N=147
N=1415
111
200 220
311
222
N=923
N=561
N=309
N=55
N=13
k(• -1)
Intensity (a.u.)
• 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.
• Comparison between Xas & Awaxs applied to monometallic clusters. D. Bazin, D. Sayers, Jpn J. Appl. Phys. 32-2, 249, 1993.• Comparison between Xas & Awaxs applied to bimetallic clusters. D. Bazin, D. Sayers, Jpn J. Appl. Phys. 32-2, 252, 1993.• AWAXS in heterogeneous catalysis D. Bazin, L. Guczi, J. Lynch, App. Cat. A 226, 87, 2002.
• 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.• New trends in heterogeneous catalysis processes on metallic clusters from S.R. & theoretical studies D. Bazin, C. Mottet, G. Treglia, J. Lynch, Applied Surf. Sci. 164, 140, 2000.
J.D. Grunwaldt et al., J. of Cat. 213,291 (2003) L. Drozdova et al. J. Phys. Chem. B 106,2240 (2002)