Comparison between XAS (Xanes-Exafs), AWAXS, ASAXS · PDF fileASAXS and DAFS applied to...

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).

7

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.

12

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

15

II.5c Dynamical processes

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


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)


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


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


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


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


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)


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.

27

III.3 Size distribution & Morphology

W. VOGEL et al. J. Phys. Chem. 97,11611 (1993).

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


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


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.)

37

III.5f Structural proposition

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)

44

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

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