Metal Catalyst Particles Smaller than 8 nm Have Properties That Depend Strongly on Size

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Metal Catalyst Particles Smaller than 8 nm Have
PropertiesThat Depend Strongly on Size  

• Charles T. Campbell, D. E. Starr, J. T. Ranney,
  J. Larsen, S. C. Parker, L. Ngo, A. W. Grant, S.
  Tait, H. Ihm, H. Ajo
• Department of Chemistry
• University of Washington
• Seattle, WA 98195-1700 USA
• and
• Z. Dohnalek and B. D. Kay
• EMSL, Pacific Northwest Natl. Labs

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FIRST I WILL SHOW THAT Calorimetric
measurements of metal adsorption energies on
oxide surfaces prove that tiny (2 nm diameter)
metal nanoparticles are dramatically less stable
(60 kJ/mol metal atom) than predicted by the
Gibbs-Thompson relation. This is because the
surface energy of such tiny particles is much
larger than for large particles, due to the lower
coordination number of their surface atoms.
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OXIDE-SUPPORTED METAL CATALYSTS Real Ag
catalyst on Al2O3 (0.4m2/g of Ag)
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Model Oxide-Supported Metal
Catalysts Vapor-deposited metals onto
single-crystal oxides Simpler, structurally
well-defined samples clean surfaces,
controlled particle sizes. Issues Effect of
metal particle dimensions on turnover
frequency, selectivity, chemisorption of
intermediates. Electronic effects due to
interaction with underlying oxide. Effect of
oxide or crystal face on activity, resistance to
sintering. Sintering mechanisms,
kinetics. Strength of metal - oxide
bonding. Reviews H J Freund, Faraday Disc. 114
(1999) 1. C R Henry, Surface Sci. Rept. 31
(1998) 231. D W Goodman, D Ranier, J. Mol.
Catal. 131 (1998) 259. C T Campbell, Surface
Sci. Rept. 27 (1997) 1.
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STM Images from Bäumer and Freund group
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Single Crystal Adsorption Microcalorimeter
Stuckless et al., J. Chem. Phys. 107 (1997)
5547. Rev. Sci. Instr. 69 (1998) 2427. Follows
the design of D. A. King But different method of
detection
Detector 9mm thick, 4 mm wide pyroelectric
ribbon, (b-PVDF), flexible, w/ 50 nm NiAl
coating on both sides for measuring V.
PVDF Ribbon
Sample
UHV Chamber with AES, LEED and QMS
Pyroelectric Ribbon
Pulsed Molecular Beam
Thin Sample
Approaching Contact
Thermal Reservoir
In Contact
Quadrupole Mass Spectrometer
Advantages Can use thicker single crystal
samples (up to 8 ?m so far). Works also at low
temperatures (published down to 170 K, but colder
possible). Can pretreat samples to 2000 K.
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MgO(100) thin film (4.0 nm thick) grown on 1
?m-thick Mo(100)
Following recipe from D. W. Goodman, Chem. Phys.
Lett. 182 (1992) 472.
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Cu J. T. Ranney et al., Faraday Discussions,
114, 195, 1999. Ag J. H. Larsen et al., Phys.
Rev. B 63, 195410, 2001. Pb D. E. Starr et al.,
J. Chem. Phys. 114, 3752-64, 2001.
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Campbell et al., JACS 124 (2002) 9212 .
Suggests that covalent metal-Mg bonding
dominates the interaction for 2D
particles. Probably due to very strong bonding at
defects to coordinatively unsaturated Mg atoms.
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Gibbs-Thompson ?(R) - ?(8) 2??/R ? 59 ?J/cm2
Influence of particle size on the energy of the
metal atoms in the particle is dramatically
stronger than predicted by Gibbs-Thompson
relation when diameter Starr, Science 298 (2002) 811.
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NEXT I WILL SHOW THAT Metal atoms on tiny (nm diameter) nanoparticles are dramatically less
stable than in big particles (less than half the
bonding energy)! Much more aggressive
chemisorption reactivity for such nanoparticles
is observed, and attributed to this same effect.
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Implications of Particle Size wrtChemisorptiona
ndCatalytic Reactivity

• Atoms of same element which are bonded more
  weakly in a complex tend to bind next species
  more strongly. Example
• Bond energy between 2 C atoms increase as the
  number of H or R neighbors decreases
• H3C CH3 380 kJ/mol
• H2C CH2 730 kJ/mol
• HC CH 970 kJ/mol
• Metal surface atoms in particles diameter have fewer neighbors and should be much
  less noble, and behave more like elements up
  and to left in periodic table.
• Should be able to tune catalytic properties with
  particle size rather strongly below 4 nm.

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2 nm Au very active!!! 10 nm Au
completely inactive bulk Au
completely inactive
Gold Nanoparticles on TiO2(110) Model of Au /
TiO2 catalysts for Low-temperature CO
oxidation (exhaust cleanup). Selective
oxidations (e.g., of propene). from M. Valden,
X. Lai and D.W. Goodman, Science 281, 1647. (See
also our related work of Murata group referenced
there.)
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2 Ogas
2 Oad
O2,gas
V. Bondzie, S. C. Parker, C. T.
Campbell, Catalysis Letters 63 (1999) 143.
Ea,des and ?Had for Oad 40 larger for
smallest Au islands
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Pd Nanoparticles on MgO(100) Particle Size
Effects in Alkane Activation

• Steven L. Tait, Jr.1, Zdenek Dohnálek2,
• Bruce D. Kay2, Charles T. Campbell3
• 1 Department of Physics, University of
  Washington, Seattle, WA 98195
• 2 William R. Wiley Environmental Molecular
  Sciences Laboratory,
• Pacific Northwest National Laboratory, Richland,
  WA 99352
• 3 Department of Chemistry, University of
  Washington, Seattle, WA 98195

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CH4 Sticking Probability vs. Particle Size
CH4 ? CH3,ad Had
Increasing particle diam. ? 1 nm
3 nm
Ebeam 70 kJ/mol TS 500 K

• Initial sticking prob. for dissoc. ads. of
  methane
• increases as Pd particle size decreases.

Particle sizes from Henry, C. R., C. Chapon, et
al. (1997). Size effects in heterogeneous
catalysis. Chemisorption and Reactivity on
Supported Clusters and Thin Films. R. M. Lambert
and G. Pacchioni, Kluwer Academic Publishers 117.
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NEXT I WILL SHOW THAT The opposite effect is
seem when the metal atoms bind more strongly to
a surface!!!
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Pb adsorption on Mo(100) at 300 K
Stuckless et al., PRB 56 (1997) 13496
Pb/Mo(100)
?Hsub
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Strong bonding of metal to Mo(100) dramatically
weakens that metals chemisorption bond to
adsorbed CO
John M. Heitzinger, Steven C. Gebhard and Bruce
E. Koel Surface Science 275 (1992) 209.
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Trends Observed for Late Transition Metals
1. If metal atom binds more strongly to a
surface than to itself, it grows flat layer which
binds small adsorbates (CO, O, CH3, ) more
weakly than a bulk surface of that pure
metal. 2. If metal atom binds more weakly to a
surface than to itself, it grows 3D islands which
bind small adsorbates more strongly than a bulk
surface of that pure metal, when the islands are
islands. 3. The smaller the metal island,
the more strongly it binds small adsorbates (CO,
O, CH3, ad) unless bonding mechanism is London
dispersion force (e.g. CH4, ad) 4. The weaker
the metal binds to the surface, the fewer and
larger are the 3D islands that grow, resulting
in rougher thin films.
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ONE MUST THEN ASK Why are particle size effects
not more commonly reported in catalysis??
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Pyroelectric Ribbon for Temperature Rise
Detection
Thin single crystal sample
Pulsed Metal Atom Source
Moved into contact with back of thin sample
Metal Nanoparticle
Campbell, Parker Starr, Science 298 (2002) 811.
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Catalyst Sintering

• Nanoparticles are unstable wrt larger particles
• They often sinter (grow in size, decrease in
  number) during use.
• Big problem is catalysis.
• Kills more aggressive particles quickly.
• Slows rate of new catalyst development.

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2 nm Au very active!!! 10 nm Au
completely inactive bulk Au
completely inactive
Gold Nanoparticles on TiO2(110) Model of Au /
TiO2 catalysts for Low-temperature CO
oxidation (exhaust cleanup). Selective
oxidations (e.g., of propene). from M. Valden,
X. Lai and D.W. Goodman, Science 281, 1647. (See
also our related work of Murata group referenced
there.)
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Gibbs-Thompson ?(R) - ?(8) 2??/R ? 59 ?J/cm2
Dramatic influence of particle size on the energy
for diameter accurate kinetic model for sintering!! Campbell,
Parker Starr, Science 298 (2002) 811.
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Recent Calorimeter Improvement

• Now working on crystals that are 70 mm thick!!
• Opens up measurements to almost any sample, since
  single crystals can be mechanically thinned to 70
  mm even over required 1 cm2 area.

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Polymer-backed pyroelectric ribbon