GDChKolloquium Technische Universitt Mnchen Mnchen, 8' November 2005

1
Chemistry in the Non-Scalable Size Regime
Ulrich Heiz, Lehrstuhl für Physikalische Chemie
I Technische Universität München
2
Outline

• Introduction- Nanoparticles The scalable size
  regime
• - Clusters The non-scalable size regime
• Why gold is not noble Low temperature reactivity
• Concepts for understanding reactivity in the
  non-scalable size regime
• Recent research developments
• Calorimetric studies of the hydrogenation of
  1,3-butadien
• Cavity ringdown spectroscopy

3
Catalytic Process
? Guncatalyzed (GasPhase)
? Gcatalyzed (Surfaces, Particles, Clusters,)
JABIR IBN HAIYAN (Geber) (776 - 803 C.E., Irak)
CO 1/2 O2
Fritz Haber (Geber) (1868-1934)
? G0
GA
CO2
GB

• Catalysts accelerate a chemical process
• After catalytic process the catalyst is in its
  initial state
• Catalytic processes are characterized by
  turn-over of the reaction (Turn-over
  frequency)

4
Mendelejevs Periodic Table
Mendelejev, Dmitri Ivanovitsj 8. Feb. 1834
(Tobolsk) -2 Feb. 1907 (St.- Petersburg)
In On the Relation of the Properties to the
Atomic Weights of the Elements, received by the
Russian Chemical Society in 1869.
5
The 3rd Dimension of the Periodic Table
Size
Periods
Groups
6
The Scalable Size Regime Nanoparticles of Sodium
(Nan with ngt2000)
E. Schumacher , U. Heiz et al. Chimia 42 (1988)
357-376
T.P. Martin et al. Z. Phys. D 19 (1991) 25
7
Nanoparticles Periodic Table Geometric Shells
1 face added
891-atom octahedron
2 faces added
1156-atom octahedron
N1/3 (10K3-15K211K-3) Mackay
Icosahedra Acta Cryst. 15 (1962) 1916
8
Size Effects of Nanoparticles Some Guiding
Principles

• Various facets with different plane
  densities ? different reactivities
• Different proportions of facets ? influence
  on diffusion barriers
• Different coordination numbers ? changing
  electron densities, edge effects
• Substrate effects ? modification at the
  interface ? lattice mismatch changing lattice
  parameters
• Spill-over and reverse spill-over ? particle
  size and density dependent

9
The Non-Scalable Size Regime Sodium Clusters
(Nan with nlt 150)
de Heer Rev. Mod. Phys. 65 (1993) 611
E. Schumacher, U. Heiz et al. Chimia 42 (1988) 357
10
Clusters Periodic Table Electronic Shells
11
Cluster Properties

• Distinct, strongly size-dependent, electronic
  structures

• Strong impurity doping effects

• Unique and size-dependent, magnetic properties

• Unique structures, non-comparable to crystallites

• Electronic and geometrical structure highly
  dependent on oxidation state

• Manifold of energetically close-lying isomers

• Strong structural fluxionality

12
Non-Scalable Scalable Size Regime
13
Optical Properties Colored Gold
J.-M. Antonietti, U. Heiz et al. Phys. Rev. Lett.
42 (2005) 357
14
Impurity Doping Effects
U. Heiz et al. J. Phys. Chem. 100 (1996) 15033
15
Charged Clusters
Charging
-
-
Na9Au-
Cs9Au-
U. Heiz et al. J. Phys. Chem. 100 (1996) 15033
16
Structural Properties Isomers Structural
Fluxionality
17
Changing Reactivity with Cluster Size
CO ½ O2 ? CO2
Chemical reactivity, R One heating cycle,
temperature programmed reaction experiment.
U. Heiz, A. Sanchez, S. Abbet, W.-D.
Schneider Chemical Physics 262 (2000) 189
18
Tuning Selectivity with Cluster Size
a) C2 H2 decomposition H2 (hydrogen)
b) C2 H2 hydrogenation C2 H4 (ethylene)
c) C2 H2 polymerization C4H4
cyclotrimerization C6H6 (benzene)
hydrogenation C4H6 (butadiene)

hydrogenation
C4H8 (butene)
19
Tuning Selectivity with Cluster Size
20
When Gold is not Noble
When Gold is not Noble Structural, Electronic,
and Impurity-Doping Effects in Nanoscale
Chemistry Supported Gold Nanoclusters
Charging Effects on Bonding and Catalyzed
Oxidation of CO on Au8 Clusters on MgO Stéphane
Abbet , Ken Judai, Anke Wörz, Jean-Marie
Antonietti and Ueli Heiz Technical University of
Munich, Lehrstuhl für Physikalische Chemie,
D-85747 Garching Hannu Häkkinen, Bokwon Yoon and
Uzi Landman Georgia Institute of Technology,
School of Physics, Atlanta, Georgia
30332-0430 J. Phys. Chem. A 103 (1999) 9573 J.
Am. Chem. Soc.,125 (2003) 10437 Angewandte
Chemie Int. Ed., 115 (2003) 1335 Science 307
(2005) 403
21
Experimental Setup
YAG Laser
Q-mass for size-separation
Analysis chamber
Cluster source
22
Experimental Setup Clusters on Surfaces
23
Methodology Preparation of Support Materials
Characterization
Preparation
LEED
Oxygen
Oxygen
Magnesium
Magnesium
HREELS
EELS, Ep 30eV
001
010
010
001
Heiz et al. J. Phys. D. Appl. Phys. 33 (2000)
R85-R102
Epitaxially grown on Mo(100) or Ag(001) by
evaporation of Mg in 10-6 Torr O2 at RT.
Detection of F-centers
Wu et al. Chem. Phys. Lett. 182 (1991) 472
Schaffner et al. Surf. Sci. 417 (1998) 159
24
Size-Selection and Softlanding
4) Chemical Interaction a) Metal
Clusters/Metal 3-6 eV/Atom b) Metal
Clusters/Oxide 0.2-1.4 eV/Atom
25
Experimental Verification of Soft-Landing

• Absence of defect formation on support upon
  deposition STM studies of Si39 on Ag(111)

1) Counting the number of atoms
Nickel-carbonyl formation
0.02 ML Ni 2 ML CO
0.02 ML Ni2 2 ML CO
Ni2CO
Ni(CO)4
Ni2
Ni
Ni(CO)4
NiCO
Ni3
Temperature K
Temperature K
26
Why is Gold Noble in the Solid State?
Typical DOS of Metals
27
Gold Nanocatalysts
Scalable Size Range
Non-Scalable Size Range
SCIENCE, 14 March 2003
28
Formation of CO2 on Supported Gold Clusters
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1. Guiding Priniciple
Each Atom Counts !
30
Comparison to Reactivity of Free Gold Cluster
Anions
Note No O2 adsorption on neutral and cationic
gold clusters !
31
Reaction Mechanism of the CO Combustion on Free
Au2-
L. Socaciu, J. Hagen, T. Bernhardt, L. Wöste, U.
Heiz, H. Häkkinen, U. Landman J. AM. CHEM. SOC.
2003, 125, 10437
32
Catalytic Turn-Over Frequency (TOF)
TOF ? 0.6 CO2 molecules per gold cluster per
second
2 nm gold particles at 273 K TOF 0.2 s-1 per
Au atom (Haruta et al.) 3.5 nm gold particles at
350 K TOF 4 s-1 per Au atom (Goodman et al.)
33
2. Guiding Priniciple
Steric Effects For Supported Clusters Charging
34
Reactivity of Free and Supported Nanoscale Gold
35
Influence of Defect Sites Au8 on MgO(100)
F-centers
CO
O2
F-center Oxygen vacancy
13CO
B. Yoon, H. Häkkinen, U. Landman, A. Wörz, J.-M.
Antonietti, S. Abbet, K. Judai, U. Heiz, Science
307 (2005) 403
36
Theoretically Proposed Structure
13CO/Au8/MgO(FC) ?theor. 2018 cm-1
(1) ?theor. 1931 cm-1 (2) ?theor. 2004 cm-1 (3)
?CO/MgO 2118 cm-1
37
Influence of Defect Sites Au8 on MgO(100)
Redshift induced by F-center ?? 30-50 cm-1
13CO
38
CO bonding on Au8O2/MgO(FC)
CO on Au8O2/MgO 1.18 e-
1.27 e-
39
CO bonding on Au8O2/MgO(FC)
CO-bonding via backdonation into 2? and
donation of 5? into cluster
40
Effect of F Centers Cluster Charging
CO
O2
Charging ? Frequency shift
? ?exp.(cm-1) 30-50
?Q 0.5
41
Effect of F Centers Cluster Stabilization

• Strong binding between cluster and F-center
  3.4 eV in comparison to 1.2 eV on regular
  terrace sites.
• Charge transfer to the cluster 0.5 e-

42
CO-Oxidation on Aun on TiO2
43
3. Guiding Priniciple
Cluster-Support-Interaction - Stabilization -
Charging
44
Why is Gold Active at low Temperatures ?
45
Effect of F centers Activation of O2 (peroxo
state)
46
Comparison with Gas Phase Studies
Stolcic, Fischer, Ganteför, Kim, Sun and Jena J.
Am. Chem. Soc. 125, (2003) 2848
47
Experimental Evidence of Molecular O2 Adsorption
Stolcic, Fischer, Ganteför, Kim, Sun and Jena J.
Am. Chem. Soc. 125, 2848 (2003)
48
4. Guiding Priniciple
Unique Activation of Reactants on Clusters !
49
Identification of possible Reaction Mechanisms
50
Oxidation of CO on Au8 Bound to Defect-Poor and
Defect-Rich MgO(100) Surfaces
Langmuir-Hinshelwood-Periphery Mechanism
Adsorption by reverse spill- over
4.5 Å
2.0 Å
Reaction Coordinate d(C-O1)
A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider,
H. Ha1kkinen, R. N. Barnett, Uzi Landman J.
Phys. Chem. A 1999, 103, 9573
51
Oxidation of CO on Au8 Bound to Defect-Poor and
Defect-Rich MgO(100) Surfaces
Langmuir-Hinshelwood-Top Mechanism
Direct Adsorption
3.1 Å
2.0 Å
Reaction Coordinate d(C-O1)
52
Dynamic Structural Fluxionality
Au8 2 CO2
Au8/O2/(CO)2
Au8 O2 2 CO
Au8/O2
Bicapped Bipyramid
Bicapped Bipyramid
Quasi planar Au8
Energy
Reaction Coordinate
53
Dynamic Structural Fluxionality
54
5. Guiding Principle
Dynamic Structural Fluxionality
55
Evolution of Reactivity with Size and Elemental
Composition
Size evolution
56
Activation by Impurity DopingUnderstanding
Size-Evolution of the Reaction
57
Gold Cluster Reactivity
Cluster deposition of FC/MgO(100)

• Aun (nlt8) inert

• Au8 smallest gold catalyst

• Au3Sr smallest doped cluster

• MgO and Aufilm inert

• Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H.
  Häkkinen, R. N. Barnett and U. LandmanWhen gold
  is not noble Nano-scale gold catalyst.J. Phys.
  Chem. A 103 9573-9578 (1999)
• H. Häkkinen, S. Abbet, A. Sanchez, U. Heiz, and
  U. LandmanStructural, electronic, and
  impurity-doping effects in nanoscale chemistry
  Supported gold nanoclusters.Angewandte Chemie
  Int. Ed., 42 1297-1300 (2003)

58
Optimized Atomic Structures of Pure and Mixed
Gold Nanocatalysts
B.E. 3.5 eV ?q0.5
B.E. 2.7 eV ?q0.3
B.E. 4.1 eV ?q0.3
B.E. 0.7 eV rO2 1.44 Å
B.E. 1.9 eV rO2 1.37 Å
B.E. 0.2 eV rO2 1.28 Å
Peroxo
Superoxo
Molecular
59
Au8/Mg(100)/FC LDOS Projected on the O2
Molecule and the Metal Part
60
Au4, Au3Sr/Mg(100)/FC LDOS Projected on the O2
Molecule and the Metal Part
61
6. Guiding Principle
Electronic Structure (Au8, Au4, Au3Sr) Impurity
Doping Effects
62
The Role of Moisture
63
The Effect of Moisture on Gold Catalysts
64
Mechanism Predicted by Theory
Angelo Bongiorno Uzi Landman Phys. Rev. Lett.
95, 1061021 (2005)
65
Cooperative Adsorption of H2O and O2
66
7. Guiding Principle
Cooperative Adsorption and Activation by
Coadsorbants
67
Mechanism 1 Proposed by Landman
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Nanocatalytic Factors

• Each cluster has its characteristic electronic
  structure Intrinsic quantum size effects
• Each cluster size has characteristic
  cluster-support interaction (stability, mobility,
  charging, steric effects ...)
• Clusters are fluxional Low-temperature
  reactivity

69
Understanding Chemical and Photochemical
Properties in the Non-Scalable Size Regime
Reactivity and Kinetics
Identification of reactants, intermediates and
products p-MBRS, TPR, FTIR Reaction
mechanisms Theory Activation energies
Photochemistry Femtochemistry
Morphology and Structure Thermal and
Photo-Stability
Electronic Structure and Photoabsorption
Size-dependence Optical Spectroscopy (CRDS) Role
of the substrate Electron Spectroscopy
(MIES) Influence of adsorbates Photoelectron
Spectroscopy (UPS)
Geometric Structure and Stability
Morphology and Structure Thermal and
Photo-Stability
Local Probes (AFM/STM)
70
Thermodynamic properties of cluster reactions
OFF
71
Microcalorimetry - Principle
Al
Au
P
Au10nm Cr
Concept Ch. Gerber, J. K. Gimzewski, R.
Schäfer See e.g. J. Chem. Phys. 65 (1999), 10008
72
Microcalorimetry - Principle
Mode of application Thermometer Cantilever in
contact with heat bath, constant
temperature Sensitivity 10-3 K
Calorimeter Cantilever in contact with
heat sink, heat sink at T0, heat from an external
source flows along the cantilever Sensitivi
ty 100 nWatt
73
Clusterdeposition Heats of adsorption
Heat rate during cluster deposition Thermometer
Mode
The heating rate has been corrected by the
heating of the cantilever through the focusing
octopole ion guide.
74
Clusterdeposition Heats of adsorption
Heating rate 6.36 x 10-8 K s-1 pA-1 Heat
capacity of array 0.07 mJ K-1 Q c ? ?T P
?Q/?t c ? ?T/ ?t ? Power released during
cluster deposition 4.45 x 10-12 W pA-1 ?
Average power released by cluster impact 4.45
eV 1 eV originates from kinetic energy, thus
3.5 eV is due to binding and rearrangment of
cluster upon deposition
75
Microcalorimetry Mechanical response
Mass Spectro
76
Microcalorimetry Reaction Heats
Reaction heat of the hydrogenation of
1,3-butadiene on 1 ML Pdn clusters - Isotropic
partial pressure of 1,3-butadiene - H2 pulses
77
Nanocalorimetry Reaction Heats
Hydrogenation of 1,3-butadiene on Pdn
Pressure dependence of the hydrogenation of
1,3-butadiene on palladium cluster model
catalysts. Pdn clusters (mean size n20,
equivalent atomic coverage 1) are deposited on
the cantilever. An isotropic pressure of
1,3-butadiene is introduced in the chamber. H2 is
pulsed at 1 Hz.
Reaction heat of the hydrogenation of
1,3-butadiene on 1 ML Pdn clusters - Isotropic
partial pressure of 1,3-butadiene - H2 pulses
78
Estimation of Reaction Heat
Pdn
Reaction C4H6 H2 ? C4H8 C4H8 H2 ? C4H10
Pdn
Measured heat (peak value) 2,5x10-9 J Cluster
coverage 1 ML (2x1013 clusters/cm2 )
clusters on cantilever (7,5x10-4 cm2) 1,5x1010
clusters Assuming one reaction cycle per cluster
? 1.5x10-19 J ?? 1eV ? 100 kJ/mol
79
Cavity Ringdown Spectroscopy Principle

• ?0 Intrinsic loss of the cavity (transmission of
  the mirror, surface scattering, )
• ?s Additional loss due to the absorption of
  light by the sample

80
Ringdown Time and Sensitivity

• Absorption losses of about 0.5 ppm are detectable
• A typical dipole-allowed transition in small
  noble metal clusters (0.1 Å2) can be detected
  with a coverage of 0.005 ML

81
Integration into Cluster Deposition Machine
82
Size-Dependent Optical Properties
J.-M. Antonietti et al., Phys. Rev. Lett. 94
(2005), 213402 A. Del Vitto et al., accepted for
publication in J. Phys. Chem. B
83
Optical Spectra of Au1 and Au2/a-SiO2
84
Aggregation and Fragmentation
85
Thank You
Present team Dr. M. Arenz (Reactivity of
supported metal clusters) Dr. S. Abbet (formar
collaborator) Dr. K. Judai (formar
collaborator) Dipl. chem. A. Wörz Dipl. chem. M.
Röttgen Dr. J.-M. Antonietti (Microcalorimetry)
Dipl. phys. J. Gong Ms. Sci. V. Habibpour Dr. S.
Gilb (Cavity ring-down spectroscopy,
Microcalorimetry) Dr. M. Michalski Dipl. Chem. J.
Kungl Ms. Sci. A. Kartouzian Dipl. phys. V.
Teslenko (Metastable impact spectroscopy)
Present collaborations Prof. N.
Rösch (Simulations) Prof. U. Landman (Simulati
ons) Dr. H. Häkkinen Prof. G. Pacchioni
coworkers (Simulations) Prof. C. Henry (Pulsed
molecular beams) Prof. L. Wöste (Gas phase
reactivities) Dr. Th. Bernhardt Prof. V.
Kempter (Metastable impact spectroscopy) Prof.
H. Jones (Cavity ring-down spectroscopy) Fundi
ng Deutsche Forschungsgemeinschaft, Sonderforschu
ngsbereich SFB 569 SPP Cluster in Kontakt mit
Oberflächen (1153), Hochschulbau
Förderung Landesstiftung Baden-Württemberg,
Alexander v. Humboldt Stiftung Japanese
Society for the Promotion of Science, Swiss
National Science Foundation