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D. Bazin
L’interaction Agrégat métallique - Molécule au cœur des processus physicochimiques
environnementaux
Laboratoire de Physique de la Matière CondenséeLaboratoire mixte de l’école Polytechnique et du CNRS (UMR 7643)
3 Novembre 2005
2
Objectiv of the lecture
Cluster BehaviourAdsorption Mode
II
3
PLANI. X-ray Absorption Spectroscopy
II. Interaction between nanometer scale metallic cluster & NO
4
I.1 X-ray Absorption Spectroscopy
x(E) log
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
I
5
I.1 The associated formula
Electronic termsfj(k), j(k),
Structural termsNj , Rj , j.
(k) =j Nj/kRj
2 fj(k)exp(-Rj/)exp(-2j2k2)sin(2kRj +j(k))
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
2 4 6 8 10 12 14 16
k(Å-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(Å)-a
N
Pt
( )R Å 2 3 4 5 6
I
6
I.2 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)
I
7
I.3 Some examples
Modulus of the Fourier Transform or
Pseudo radial distribution function
R(Å)-a
N
Pt
( )R Å 2 3 4 5 6
Cl
Pt
Impregnation
0 1 2 3 4 5 6R(Å)
_ 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(Å)
__ PtO2
_ Pt métal
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,
I
Calcination
0 1 2 3 4 5 6
_ H2PtCl
6
_ PtO2
.. Pt/Y-SiO2
Modules de la T.F. (u.a.)
R(Å)
O
O
PtO
O
8
I.4 Nanometer scale metallic cluster & R.S.
I
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(Å-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)
9
II. Catalyse DeNOx
II.1 IntroductionII.2 Behaviours of the metallic clusters Ex : NO/PtII.3 NO adsorption on metallic surfaceII.4 NO adsorption on metalic clusters (Ru & Pt)II.5 Remarks from Prof. J. Friedel II.6 Other experimental results Ir,Rh,Cu,Pt,PdII.7 Discussion : Support, Preparation, Cluster size, Temp.II.8 Some explanationsII.9 Mechanisms Pt,Cu,Rh,Ru,IrII.10 CO on metallic surface II.11 A bridge between surface science and nanoscience : Implications in heterogeneous catalysis : How to select a catalyst
LMSPC
II
10
Alumina
Ceria
Pt/Rh
II.1 Introduction
The Goal : To obtain CO2 and N2 from CO and NOx
LMSPC
II
11
Pt, Rh, CeO2, Al2O3
Pt :CO+1/2O2 __>CO2 & HC + O2 __> CO2 + H2O
Rh NO+CO__>1/2 N2+CO2 &NO+H2 __>1/2N2+H2O
CeO2 :
Oxygen (Ce3+ ____> Ce4+)Al2O3 : High specific surface (>200m2/g)
PtRh
LMSPC
II
A detailed study of the metallic function of bimetallic PtRh post combustion catalyst by Xas, Met : correlation with their catalytic activity, D. Bazin et al., J. de Phys. IV, C2 - 841, 1997.
NO reaction over nanometer scale Pt clusters deposited on -Al2O3
S. Schneider et al., App. Cat. A 189, 139, 1999.
Analyse par Exafs d'agrégats de platine de taille nanométrique soumis à différents gaz réactifsS. Schneider et al., J. de Phys. IV, Pr10, 299, 2000.
An Exafs study of the interaction of different reactant gases over nanometer scale Pt clusters deposited on -Al2O3.
S. Schneider et al., Cat. Let. 71,155, 2001.
12
II.2 Behaviours of the metallic clusters Ex : NO/Pt
Initial state
II
D. Bazin, L. Guczi, , Recent Res. Dev. Phys. Chem. 3, 387, 1999.D. Bazin, C. Mottet, G. Treglia, J. Lynch, Applied Surf. Sci. 164, 140, 2000.
13
NO/Pt Experimental data
Modulus of the Fourier Transform or
Pseudo radial distribution function
R(Å)-a
N
Pt
( )R Å 2 3 4 5 6
Initial state
14
II.3 NO adsorption on metallic surface
G. Broden et al. Surf. Sci. 59, 593 (1976).
Ability of transition metal surface to dissociate the NO molecule from G. Broden et al.
Sc Ti V Cr Mn Fe Co Ni CuY Zr Nb Mo Tc Ru Rh Pd AgLu Hf Ta W Re Os Ir Pt Au
Metals which are associated with a molecular chemisorption are in red style, the others give rise to a dissociative chemisorption.
In particularly, in the case of NO, we can distinguish metals for which there is dissociative chemisorption and
those for which molecular chemisorption occurs.
II
15
W. A. Brown , D. A King. J. Phys. Chem. B, 104, 2581 ( 2000).
The melting points are a direct measure of the cohesive energies of the elements. The higher the metal cohesive energy, the greater is the propensity for NO dissociation.
More recently, W. A. Brown and D. A. King suggest a correlation between the propensity for dissociation of the NO monomer at low
coverage and the melting points of the transition metals.
II
16
II.4 NO adsorption on metalic clusters (Ru & Pt)
In the case of Ru, the adsorption of NO leads to break up the metallic cluster T. Hashimoto et al., Physica B 208& 209, 683 (1995).
Ru
NO adsorption induces a sintering of the Pt clusters deposited on-Al2O3 .P. Lööf et al., J. Catal. 144 (1993) 60.S. Schneider et al., App. Cat. 189, 139 (1999).
PtInitial state
II
17
Correlation adsorption mode & cluster behaviour ?
D. Bazin, Topics in Catalysis 18(1), 79, 2002.
II
18
II.5 Remarks from Prof. J. Friedel
II
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
19
II.6 Other experimental results Ir,Rh,Cu,Pt, Pd
At 500°C, almost all NO in contact with Ir0 was decomposed to N2 and oxidized Ir0 to IrO2 .C. Wögerbauer, et al. J. of Catalysis, 205, 157-167 (2002).
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
II
20
II.6 Other experimental results Ir,Rh,Cu,Pt, Pd
Examination of the effects of the individual gases showed that NO alone disperses Rh over the SiO
2
K. R. Krause et al. J. of Catalysis, 140 (1993) 424.
• In the initial state, the environment of Rh atoms is NRhRh
= 8 @ 2.68Å. After
exposure to 4%NO/He at 313K for 5 s, NRhRh
has significantly decreased
(NRhRh
=2). Note the presence of N (NRhN
[email protected]Å) & O (NRhO
=2 @2.05Å).
T. Campbell et al. Chem. Comm. 304-305 (2002)
Surface studies of supported model catalystsSurface Science Reports 31 (1998) 231-325Claude R. Henry
II
21
II.6 Other experimental results Ir,Rh, Cu, Pt, Pd
The adsorption and reaction of NO on Cu clusters deposited on a 5 Å thick Al2O3 film shows strong similarities to its behaviour on Cu single crystals. The STM results show that the Cu clusters grow according to the Volmer-Weber mechanism.
Nitric oxide reduction by Cu nanoclusters supported on thin Al2O3 films J. of Catalysis, Volume 22 ( 2004) 204.S. Haq, A. Carew and R. Raval
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
II
22
II.6 Other experimental results Ir,Rh, Cu, Pt, Pd
X. Wang et al. Cat.Today 96 (2004) 11-20.
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
II
23
II.6 Other experimental results Ir,Rh,Cu,Pt,Pd
In Xafs of the Pd/MgO catalyst indicates that neither Pd oxidation nor particle sintering occurs during heating in flowing 1%/NO/He to 300°C.
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic clusterPd(111)
Pd(110)
Pd(100)
II
24
II.7 Discussion
Combining Solid State Physics Concepts & X-Ray Absorption Spectroscopy to understand heterogeneous catalysis, D. Bazin, D. Sayers, J. Lynch, L. Guczi, G. Treglia, C. Mottet
II
Discussion
•Nature of the support• Preparation procedure
• Cluster size• Temperature
25
II.7 Discussion : Support, Preparation, Cluster size, Temp.
Ru/-Al2O3Ir/
Rh/-Al2O3, ZrO2, CeO2
Cu/Al2O3 Pt/ SiO2, Al2O3
Support
Ru3(CO)12
Precursor
IrCl3(H2O)3, Ir(NH3)xCl3(H2O)y
RhCl(CO)2/-Al2O3
H2PtCl6 Pt(NH3)4(OH)2
Cu : Evaporation
II
26
II.7 Discussion : Support, Preparation, Temp., Cluster size
Effect on the NO adsorption mode
Rh NO adsorbs molecularly on Rh at low temperatures and dissociatively at higher temperatures. On Rh[100], molecular NO dominates upon adsorption at 100K, but heating leads to N2 and O2 production in TPD, suggesting dissociation.
Ni molecular adsorption takes place on Ni at low temperature and at higher temperatures both molecular and dissociative adsorption are observed.
II
27
II.7 Discussion : Support, Preparation, Temp., Cluster size
II
0
5
10
15
20
25
0
10
20
30
40
50
60
70
0,20,30,40,50,60,70,80,91
N1N2N3N4
Diamètre (Å)
Dispersion
28
Preliminary conclusion
Discussion
• Nature of the support• Preparation procedure
• Cluster size• Temperature
• Pressure• Cluster morphology
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
II
29
II.8 Some explanations : Cohesion energy
[a] A. Khoutami, PhD thesis, Paris XI University (1993).[b] F. Baletto et al., Phys. Rev. Let. 84, 5544 (2000).[c] R. A. Guirado-Lopez, PhD thesis, Paris XI University (1999).
As it can be seen, a significant decrease of the cohesive energy, around 30%, is observed independently the nature of the metal and the morphology of the cluster [a-c].
2
3
4
5
6
7
0 500 1000 1500
Pt IcosahedraPt CubooctahedraPd IcosahedraPd CubooctahedraRu IcosahedraRu CubooctahedraRh Cubooctahedra
Cohesion Energy (eV)
Nb of atoms in the cluster
Ru ECoh
=4.28eV
Pd ECoh
=3.91eV
Pt ECoh
=5.86eV
Rh ECoh
=3.03eV
How change the Brown diagram when we consider not metallic surface but metallic cluster ?
II
30
II.8 Some explanations : Validity of the straight line
these adsorption energies are more important for metals which are at the middle of the transition series, metals for which we observed here a dissociative adsorption.
/ Eads(N)/ + / Eads(O)/ > Edis(NO) + /Eads(NO)/
Dissociative chemisorption is the most stable situation 0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
II
31
II.9 Mechanisms Pt,Cu,Rh,Ru,Ir
High coverage regime
Initial state
II
High temperatureMobility & Decompostion of the nitrosyl species
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
32
Initial state
II
N2 desorption
II.9 Mechanisms Pt,Cu,Rh,Ru,Ir
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
High temperatureDecomposition process
33
Other molecules
Sc Ti V Cr Mn Fe Co Ni CuY Zr Nb Mo Tc Ru Rh Pd AgLu Hf Ta W Re Os Ir Pt Au
O2 : 5.12 eV, N
2O : 4.4 eV
II
NO : 6.50 eV
/ Eads(N)/ + / Eads(O)/ > Edis(NO) + /Eads(NO)/
N2: 9.76 eV Sc Ti V Cr Mn Fe Co Ni Cu
Y Zr Nb Mo Tc Ru Rh Pd AgLu Hf Ta W Re Os Ir Pt Au
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
34
II.10 CO on metallic surface
R.B. Anderson ”The fischer-Tropsch Synthesis”, Chap 4, Academic press, New York, 1984
In particularly, in the case of CO, we can distinguish metals for which there is dissociative chemisorption and those for
which molecular chemisorption occurs.
T=20°C T=200°C-300°C
Ability of transition metal surface to dissociate the CO molecule from R.B. Anderson
Sc Ti V Cr Mn Fe Co Ni CuY Zr Nb Mo Tc Ru Rh Pd AgLu Hf Ta W Re Os Ir Pt Au
Dissociativ Non-Dissociativ
II
CO: 11.09 eV
35
II.10 Behaviours of the metallic cluster
Initial state
II
High coverage
High Temperature
On Pt clusters, A. Berko et al. [1] found that CO adsorption induces a significant increase in the initial size of the Pt nanoparticles of 1–2 nm even at room temperature. This agglomeration preceded by disruption of the smaller Pt nanoclusters at lower CO pressures is explained by CO-assisted Ostwald ripening, in which the mass transport proceeds via surface carbonyl intermediates. Similar results have been obtained in the case of Ru [2], Rh[3,4], Cu[5] and Pd [6,7]. [1] A. Berko et al. Surface Science 566568, 337 (2004).[2] T. Mizushima et al. J. Phys. Chem. 94, 4980 (1990).[3] H. F. J. Van't Blik et al. J. Phys. Chem. 87, 2264 (1983).[4] A. Suzuki et al. Angew. Chem. Int. Ed., 42, 4795 (2003).[5] X. Wang et al. J. Phys. Chem. B 2004, 108, 13667.[6] S. L. Anderson et al. J. Phys. Chem., 95, 6603(1991)[7] W. Vogel et al. J. Phys. Chem. B 102, 1750 (1998).
For all these metals, we are in the case of a non dissociative adsorption mode and it seems that the structural evolution of the nanometer scale metallic atoms follows the same scheme. Based on all these experimental facts, we can suppose that CO adsorption leads to a disruption of metal-metal atoms and to the formation of Metal-(CO)n species. Then, metallic clusters can be observed and as supposed in the case of Pt, mass transport proceeds via surface carbonyl intermediates. This general schema is quite close to the
one proposed in the case of NO adsorption process.
36
II.10 Behaviours of the metallic cluster
Initial state
II
Boudouard reaction2CO --> Cads+CO2
What happens if we consider metals which displays a dissociative adsorption mode? A beginning of the answer is given by the study performed by O. Ducreux et al. [1] on the Co/Al2O3 system. Through in situ X-ray diffraction experiments, the formation of a carbide is pointed out.
[1]O. Ducreux, PhD Thesis, University Paris VI, 1999.
37
II.11 A bridge between surface science and nanoscience
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
Cluster BehaviourAdsorption Mode
II
38
II.11a the metal-support interaction ?
• As we have seen the relationship we have proposed between the adsorption mode of NO and the behaviour of the metallic cluster seems to be independant of the nature of the support, except for Pd. • For this metal, a great difference exists between MgO and CeO2. We have linked this dependency to the position of the metal versus the line which separates the two adsorption modes. • If this assumption is correct, such dependency is not so significant for other metals such Rh or Pt.
the metal-support interaction ?
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
II
39
II.11b NO on monometallic cluster
For metals above the stability line, NO adsorption leads to the formation of a metal oxide. Thus, the catalytic activity tends to decrease.
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
Catalytic activity of metallic clusters ?
For metals below the line, large metallic clusters are finally generated and evolution of the catalytic activity will follow these structural modifications.
II
NO
N2
t
40
II.11c Bimetallic systems
This simple model leads to a complete rejection of some bimetallic systems. For example, if we consider a RhRu bimetallic cluster, the NO adsorption process conducts to the formation of a metal oxide i.e. the dissociation of NO will stop.
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
Al2O3CeO2
At the opposite, if we consider a PtCu bimetallic system, the NO adsorption will lead to some large clusters.
•A guideline for the choice of bimetallic systems is to add to Pt (or Cu) a second metal such Rh, Ru or Ir. If we consider the CeO2 support, the
PtPd bimetallic seems to be acceptable while the PtPd bimetallic system supported on alumina has to be rejected.
II
NO
N2
t
41
II.11d a mixture of NO+O2
For metals above the stability line, NO adsorption leads to the formation of a metal oxide. The presence of O2 will not change significantly this simple scheme. For these metals, it is necessary to add to a NO+O2 mixing, a reductor agent in order to retablish the metallic state of the atoms.
Thus, the presence of NO allowed the Pt particles to conserve a metallic character. In this case, we can probably play with the relative concentration of the two gases NO and O2 to keep a metallic state.
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II.12 Conclusion & Perspectives
D. Bazin, (2003) Solid State Physics and Synchrotron Radiation Techniques to Understand Heterogeneous Catalysis in nanotechnology, Ed. G.A. Somorjai, S. Hermans, B. Zhou.D. Bazin, J. Lynch, M. Ramos-Fernandez, X-ray absorption spectroscopy and Anomalous Wide Angle X-ray Scattering Two basic tools in heterogeneous catalysis, Oil & Gas Science and Technology – Rev. IFP, 58 (2003), No. 6, pp. 667-683
• Nature of the support• Preparation procedure• Cluster size• Temperature
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
3d4d
5d
Melting Point °C
Atomic Number
Pt
Au
Ir
Os
Re
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
Fragmentation processof the metallic cluster
NOa
Sintering process of the metallic cluster
Séminaires
7 Avril 2005 : Modélisation de l’interaction entre le monoxyde d’azote et un agrégat métallique de dimension nanométrique, Centre de Recherche en Matière Condensée et Nanosciences, Marseille, France11 Avril 2005 : L’interaction Agrégat métallique - Molécule au cœur des processus physico-chimiques environnementaux, Laboratoire de Chimie Théorique, Ivry sur Seine, France20 Avril 2005 : L’interaction Agrégat métallique - Molécule au cœur des processus physico-chimiques environnementaux, Groupe de Physique des solides (GPS), Paris, France
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II.13 Remarks from Prof. J. Friedel
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II.12 Conclusion & Perspectives
Structural Characterization @ the atomic scale
Catalytic Activity