1
D. Bazin
Bridging Nanoscience and Surface Science to Understand Heterogeneous Catalysis.
Pr. Dr. J. A. Van Bokhoven21-22 May 2007, ETH Zurich
Department of Chemical and BioengineeringHCI E115, 8093 Zurich
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Heterogeneous Catalysis
[a] F. Garin et al. J. of Cat. 77, 323 (1982).[b] A. Dauscher et al. J. of Cat. 105, 233 (1987).
»Using contact reactions of C6 hydrocarbons (isomerization, dehydrocyclization,
hydrocracking), F. Garin et al. [a,b] have studied under the same experimental conditions Pt/Al
2O
3 catalyst of low and high dispersion as well as different well
characterised surfaces : Pt(557), Pt(119), Pt(111). They note that the behaviour of the monometallic catalyst Pt/Al
2O
3 of low
dispersion can be well simulated by such Platinum surfaces. At the opposite, the peculiar properties of the highly dispersed 0.2%Pt/Al
2O
3
catalyst were never simulated by single crystals of Platinum.
The starting point : Is it possible to simulate (understand/predict) the catalytic properties of nanometer scale metallic clusters on the basis of catalytic properties of metallic surfaces (Surface/Bulk) ?
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Heterogenous catalysed procsses : Two distinct mechanistic situations
Two distinct mechanistic situations on surface-catalysed transformation of gas-phase species A and B to a product C
Only one of them is bound and is converted to product when the other impinges upon is from the gas phase : Eley-Rideal mechanism
Both species are attached to the surface and atomic reorganization takes place in the resulting adsorbed layer : Langmuir-Hinshelwood mechanism
A BC
AB
C
Principles and practice of heterogeneous catalysis , J.M. Thomas, W.J. Thomas, Ed. VCH
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Objectiv of the lecture
Cluster BehaviourOxydationSintering
Adsorption ModeDissociative chemisorption Molecular chemisorption
1
2
3
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
PLAN
I. X-ray Absorption Spectroscopy
II. Interaction between nanometer scale metallic cluster & NO
Konigsberger, D. C. & Prins, R. X-ray absorption. Principles,. applications, techniques of EXAFS, SEXAFS and XANES; Wiley:. New York, 1988
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
I.1 X-ray Absorption Spectroscopy
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(Ö)-a
N
σ
Πτ
Ρ(Ö)0 1 2 3 4 5 6
I • Sayers, D. A., Lytle F. W. and Stern E. A., Advances in X-ray Analysis, (Ed. Plenum, New-York, 13, 1970).
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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
-50 0 50 100
PtAu
Absorption (a.u.)
LIII
E-E0(eV)
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
I.3 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(Ö-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.• Soft X-ray absorption spectroscopy at the cutting edge for nanomaterials used in heterogeneous catalysis: the state of the artD. Bazin, J. Rehr, Catalysis Letters 87(1): 85-90, 2003.
• 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.
M. K. Oudenhuijzen et al. J. of Catalysis, 205, 135,( 2002).J.D. Grunwaldt et al., J. of Cat. 213,291 (2003)J.-D. Grunwaldt et al., Cat. Let. 90,221 (2003).
N K edge 400.eV : R. Revel et al. Catalysis Letters (2001) 74, 189.Co L edge 770 .eV : L. Drozdova et al. J. Phys. Chem. B (2002) 106,2240. Al K edge 1560.eV : A. Van Bokhoven et al. J. of Cat. (2002) 11, 540.Fe L edge 708 .eV : W.M. Heijboer et al., Cat. Today (2005) 110, 228.
B. S. Clausen et al., J. of Catalysis 132, 524 (1991)
F. B. Rasmussen et al. , Applied Catalysis A, 267, 165 (2004)
M. G. Samant et al.J. Phys. Chem. 92,3542 (1988 ).
DFT study : A. Travert, et al., J. Phys. Chem. 110,1261 (2006).A.Q. Wang, J. Phys. Chem. B.; 109,18860 (2005).
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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 Other experimental results Ir,Rh,Cu,Pt,PdII.6 Discussion : Support, Preparation, Cluster size, Temp.II.7 Some explanationsII.8 Mechanisms Pt,Cu,Rh,Ru,IrII.9 CO on metallic surface II.10 A bridge between surface science and nanoscience : Implications in heterogeneous catalysis : How to select a catalyst
LMSPC
II
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
LMSPC
II
An Exafs study of the interaction of different reactant gases over nanometer scale Pt clusters deposited on γ-Al2O3.S. Schneider et al., Cat. Let. 71,155, 2001.
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.
NO reaction over nanometer scale Pt clusters deposited on γ-Al2O3
S. Schneider et al., App. Cat. A 189, 139, 1999.
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.
II.1 Introduction
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
The Goal : To obtain CO2 and N2 from CO and NOx
Pt, Rh, CeO2, Al2O3
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.2a 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.D. Bazin, Topics in Catalysis 18(1), 79, 2002. D. Bazin, in nanotechnology, Ed. G.A. Somorjai, S. Hermans, B. Zhou, Ed. KAA-PP, 2004.D. Bazin, D. Sayers, J. Lynch, L. Guczi, G. Treglia, C. Mottet, Oil & Gas Science and Technology 61, 5, 677, 2006.D. Bazin, Macromolecular Research, 14, 230-234, 2006.
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.2b Behaviours of the metallic clusters Experiment : NO/Pt
Modulus of the Fourier Transform or
Pseudo radial distribution function
R(Ö)-a
N
σ
Πτ
Ρ(Ö)0 1 2 3 4 5 6
Initial state
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.3 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 γ-Al2O3 .P. Lööf et al., J. Catal. 144 (1993) 60.S. Schneider et al., App. Cat. 189, 139 (1999).
PtInitial state
II
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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 blue, Metals which are associated with 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
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
■ 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.
W. A. Brown , D. A King. J. Phys. Chem. B, 104, 2581 ( 2000).
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
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
Ir
OsReW
Ta
Hf
Lu
Ag
Pd
Rh
RuMo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
3d
4d
5d
Molecular chemisorption
Dissociative chemisorption
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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).
II
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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 N
RhRh= 8 @ 2.68Å. After exposure to
4%NO/He at 313K for 5 s, NRhRh
has significantly decreased (N
RhRh=2). T. Campbell
et al. Chem. Comm. 304-305 (2002)
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.
NO reduction by Cu nanoclusters supported on thin Al2O3 films S. Ha et al., l J. of Catalysis 22 ( 2004) 204.
X. Wang et al.
Cat.Today 96 (2004) 11-20.
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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.
Pd(111)
Pd(110)
Pd(100)
II
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
Pd/CeOx/Al2O3 catalyst,Xafs indicates a Pd oxidation.
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
Os
W
Ta
Hf
Lu
Ag
Pd
Rh
Mo
Nb
Zr
Y
Zn
Ni
Mn
Cr
3d
4d
5d
Ru
Cu
Re
Ir
•Nature of the support
Ru/γ-Al2O3Ir/ Rh/γ-Al2O3, ZrO2, CeO2
Cu/Al2O3 Pt/ SiO2, Al2O3
•Preparation procedure
Ru3(CO)12 IrCl3(H2O)3, Ir(NH3)xCl3(H2O)y
RhCl(CO)2/γ-Al2O3
H2PtCl6 Pt(NH3)4(OH)2Cu : Evaporation
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.7 Discussion : Support, Preparation, Temp., Cluster size
II
Effect on the adsorption mode
NO adsorbs molecularly on Rh at low temperatures and dissociatively at higher temperatures.
Effect on the cluster stability
the oxide decomposes rapidly as the temperature reaches 500°C (this temperature coincides with the desorption temperature of oxygen from rh surfaces.
0
5
10
15
20
25
0
10
20
30
40
50
60
70
0,20,30,40,50,60,70,80,91
N1N2N3N4
Diamtre ()
Dispersion
On Rh/SiO2, 85% of the adsorbed NO is decomposed to N2 on a catalyst with a Rh dispersion of 16.4 %. When the dispersion is of 55% only 51% of NOads is decomposd to N2.
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Preliminary conclusion
Discussion
• Nature of the support• Preparation procedure
• Cluster size• Temperature
• Pressure• Cluster morphology
II
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
A. Khoutami, PhD thesis, Paris XI University (1993).F. Baletto et al., Phys. Rev. Let. 84, 5544 (2000).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.
How change the Brown diagram when we consider metallic cluster (instead of metallic surface) ?
2
2,5
3
3,5
4
4,5
5
5,5
6
0 500 1000 1500
Pt IcosahedraPt CubooctahedraPd IcosahedraPd CubooctahedraRu IcosahedraRu CubooctahedraRh Cubooctahedra
Cohesion Energy (eV)
Number of atoms inside the cluster
Ru ECoh
=4.28eVPd E
Coh=3.91eV
Pt ECoh
=5.86eV
Rh ECoh
=3.03eV
∆Ε/Ε=30%
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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
II
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.9 Mechanisms Pt,Cu,Rh,Ru,Ir
II
High coverage regime
K. Asakura et al. J. Phys. Chem B 101 (1997) 5549.
High temperatureMobility & Decompostion of the nitrosyl species
Sintering ofthe cluster
Pt & Cu
N2 desorption
Oxydation ofthe cluster
Rh,Ru & Ir
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Other molecules
O2 : 5.12 eV, N
2O : 4.4 eV
II
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
NO : 6.50 eV/ Eads(N)/ + / Eads(O)/ > Edis(NO) + /Eads(NO)/
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
N2: 9.76 eV
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
1
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.10 CO on metallic surface
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=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
Effect on the adsorption mode
NO adsorbs molecularly on Rh at low temperatures and dissociatively at higher temperatures.
R.B. Anderson ”The fischer-Tropsch Synthesis”, Chap 4,Academic press, New York, 1984
T=20°C CO: 11.09 eV
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.10 Behaviours of the metallic cluster
Initial state
II
High coverage
High Temperature
Pt : 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 [1].Ru [2], Rh[3,4], Cu[5] & Pd [6,7].
1 A. Berko, Surf. Sci. 566, 337, 2004.2 T. Mizushima, J. Phys. Chem. 94, 4980, 1990.3 H. F. J. Van't Blik,J. Phys. Chem. 87, 2264 ,1983.4 A. Suzuki, Angew. Chem. Int. Ed., 42, 4795, 2003.5 X. Wang ,. J. Phys. Chem. B 2004, 108, 13667.6 S. L. Anderson, J. Phys. Chem., 95, 6603(1991)7 W. Vogel, J. Phys. Chem. B 102, 1750 (1998).
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.
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Interest of Heterogeneous Catalysis
Cluster BehaviourOxydationSintering
Adsorption ModeDissociative chemisorption Molecular chemisorption
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.11a the metal-support interaction ?
II
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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.
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
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Improvment in activity by the addition of less noble metals to Pt
Different hypothesis have been proposed to explain the improvment in activity by the addition of less noble metals to Pt including,
- The formation of a new electronic structure (based on higher Pt 5d orbital vacancies)
- decrease in the Pt-Pt distance- special repartition of the two metals inside
bimetallic particls (Pt thin surface)
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
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Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.11c Bimetallic systems
Al2O3CeO2
•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 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.
At the opposite, if we consider a PtCu bimetallic system, the NO adsorption will lead to some large clusters.
NO
N2
t
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
31
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
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.
II
32
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.11d a mixture of CO+O2
2
33
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Gold catalyst - 1/ variation of cohesion energy versus size
2
2,5
3
3,5
4
4,5
5
5,5
6
0 500 1000 1500
Pt IcosahedraPt CubooctahedraPd IcosahedraPd CubooctahedraRu IcosahedraRu CubooctahedraRh Cubooctahedra
Cohesion Energy (eV)
Number of atoms inside the cluster
Ru ECoh
=4.28eVPd E
Coh=3.91eV
Pt ECoh
=5.86eV
Rh ECoh
=3.03eV
∆Ε/Ε=30%
34
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Gold catalyst - 2/ Size effect versus support
Our finding that Au/Al2O
3 is more active than
Au/TiO2 can be explained by the difference in
particle size between these samples. Au/Al2O
3
contains smaller, and thus more reactive, gold clusters compared with Au/TiO
2. Although the
average Au clusters on TiO2
are significantly
larger than those on Al2O
3, Au/TiO
2 shows high
activity. This can be explained by the bimodal particle size distribution found via TEM. Small particles (1–5 nm) cause the activity of this sample, whereas the large particles (20–100 nm) act as spectator species.
0
5
10
15
20
25
0
10
20
30
40
50
60
70
0,20,30,40,50,60,70,80,91
N1N2N3N4
Diamtre ()
Dispersion
On Rh/SiO2, 85% of the adsorbed NO is decomposed to N2 on a catalyst with a Rh dispersion of 16.4 %. When the dispersion is of 55% only 51% of NOads is decomposed to N2.
35
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Gold catalyst - 2/ Size effect versus support
O2 : 5.12 eV, CO : 11.9 eV
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
36
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
Gold catalyst - 3/ effect of a second metal
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster
37
Pr. Dr. J. A. Van Bokhoven, 21-22 May 2007, ETH Zurich
II.12 Conclusion & Perspectives
Structural Characterization @ the atomic scale
Catalytic Activity
• Utilisation des grands instruments analytiques dans l'industrie pétrolière2005 - Vol. 60 - No. 5, C. Pichon (Ed.).
Heterogeneous Catalysis
Cluster BehaviourOxydationSintering
Adsorption ModeDissociative chemisorption Molecular chemisorption
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Melting Point ¡C
Atomic Number
Pt
Au
ReOs
Ir
W
Ta
Hf
Lu
Ag
Pd
Rh
Ru
Mo
Nb
Zr
Y
Zn
Cu
Ni
Mn
Cr
Na + O
a
NOa
Oxydation ofthe metallic cluster
Growth of the metallic cluster