bridging nanoscience and surface science to understand ...€¦ · principles and practice of...

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

2

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) ?

3


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

4


Objectiv of the lecture

Cluster BehaviourOxydationSintering

Adsorption ModeDissociative chemisorption Molecular chemisorption

1

2

3

5


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

6


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).

7


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)

8


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).

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 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

10


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

11


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.

12


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

13


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

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 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

15


■ 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

16


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.

17


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



Pd/CeOx/Al2O3 catalyst,Xafs indicates a Pd oxidation.

18


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

19


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



20


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



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%

21


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



22


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



23


Other molecules

O2 : 5.12 eV, N

2O : 4.4 eV

II


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



N2: 9.76 eV


1

24


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


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

25


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.

26


Interest of Heterogeneous Catalysis



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



27


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



28


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



29


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



30


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



31


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


II.11d a mixture of CO+O2

2

33


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


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


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


Dissociativ Non-Dissociativ

36


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



37


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



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



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