Transcript Normalized CH4 saturation

Metal Catalyst Particles Smaller than 8 nm Have
Properties
That 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
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.
OXIDE-SUPPORTED METAL CATALYSTS
“Real” Ag catalyst on Al2O3
(~0.4m2/g of Ag)
Ag
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.
STM Images from Bäumer and Freund group:
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.
Sample
UHV Chamber with AES, LEED and QMS
Pulsed
Molecular
Beam
Thin
Sample
PVDF
Ribbon
Pyroelectric
Ribbon
V
Approaching
Contact
Thermal Reservoir
Quadrupole
Mass Spectrometer
In Contact
Advantages:
• Can use thicker single crystal samples (up to 8 mm so far).
• Works also at low temperatures (published down to 170 K, but colder possible).
• Can pretreat samples to > 2000 K.
MgO(100) thin film (~4.0 nm thick)
grown on 1 mm-thick Mo(100)
Following recipe from:
D. W. Goodman, Chem. Phys. Lett. 182 (1992) 472.
Metal adsorption on MgO(100)/Mo(100)
350
Heat of adsorption [kJ/mol]
Cu on MgO(100)
Cu H Sublimation
300
337 kJ/mol
Ag H Sublimation
250
285 kJ/mol
Ag on MgO(100)
200
Pb on MgO(100)
Pb H Sublimation
150
195 kJ/mol
100
0
1
2
3
4
5
6
7
8
Metal coverage [ML]
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.
Correlation of 2D M-MgO(100) Bond Energy
with Sublimation Enthalpy of Metal
2D M-MgO(100) Bond Energy (kJ / mol M)
200
Cu
150
Ag
100
Pb
50
0
180
230
280
330
Hsublim (kJ / mol M)
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.
Pb on MgO(100)
220
Gibbs-Thompson
m(R) - m(∞) = 2/R
Constant  Model
2
 (
= 59
mJ/cm
= 59
mJ/mol)
Heat of Adsorption (kJ/mol)
200
Hsub = 195.2 kJ/mol
180
160
140
Calorimetry Data
120
100
80
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Metal Island Radius (nm)
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 < 5 nm!!
Campbell, Parker & Starr, Science 298 (2002) 811.
NEXT I WILL SHOW THAT:
Metal atoms on tiny (<4 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.
Implications of Particle Size wrt:
Chemisorption
and
Catalytic 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 <4 nm in 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.
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.)
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
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
CH4 Sticking Probability vs. Particle Size
Initial Sticking Prob.
10-1
CH4  CH3,ad + Had
Increasing particle diam. →
~1 nm
~3 nm
Ebeam = 70 kJ/mol
TS = 500 K
10-2
10-3
0.0
0.3
0.6
0.9
100.0
Pd coverage / ML
• 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.
NEXT I WILL SHOW THAT:
The opposite effect is seem when the metal atoms
bind
more strongly to a surface!!!
-1
Heat of adsorption / (kJ mol )
Pb adsorption on Mo(100) at 300 K
350
Stuckless et al., PRB 56 (1997) 13496
325
300
275
250
Pb/Mo(100)
225
200
Hsub
175
150
0
1
2
3
4
5
6
Pb coverage / ML
7
8
Strong bonding of metal to Mo(100) dramatically weakens
that metal’s chemisorption bond to adsorbed CO
John M. Heitzinger, Steven C. Gebhard and Bruce E. Koel
Surface Science 275 (1992) 209.
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
< 4 nm in diameter. Also true for metastable 2D 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.
ONE MUST THEN ASK:
Why are particle size effects
not more commonly
reported in catalysis??
Pyroelectric
Ribbon for Temperature
Rise Detection
Thin single
crystal sample
V
Pulsed
Metal Atom
Source
Moved into contact with
back of thin sample
Metal Nanoparticle
Energy of added atom / (kJ/mol)
-80
-100
-120
-140
-160
-180
-200
Bulk Energy
-220
0
1
2
3
4
5
Metal particle radius / nanometers
Campbell, Parker & Starr,
Science 298 (2002) 811.
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.
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.)
Pb on MgO(100)
220
Gibbs-Thompson
m(R) - m(∞) = 2/R
Constant  Model
2
 (
= 59
mJ/cm
= 59
mJ/mol)
Heat of Adsorption (kJ/mol)
200
Hsub = 195.2 kJ/mol
180
160
140
Calorimetry Data
120
100
80
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Metal Island Radius (nm)
Dramatic influence of particle size on the energy for diameter < 5 nm
MUST be considered to make accurate kinetic model for sintering!!
Campbell, Parker & Starr, Science 298 (2002) 811.
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.
((
Polymer-backed
pyroelectric ribbon