Date post: | 29-Dec-2015 |
Category: |
Documents |
Upload: | abel-floyd |
View: | 214 times |
Download: | 0 times |
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 withunderlying 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 MicrocalorimeterStuckless 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 : 9m thick, 4 mm wide pyroelectric ribbon, (-PVDF), flexible, w/ 50 nm NiAl coating on both sides for measuring V.
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.
UHV Chamber with AES, LEED and QMS
Thermal Reservoir
V
PulsedMolecular
Beam
ThinSample
Pyroelectric Ribbon
Quadrupole Mass Spectrometer
Sample
In Contact
ApproachingContact
PVDFRibbon
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.
Metal adsorption on MgO(100)/Mo(100)
Metal coverage [ML]
0 1 2 3 4 5 6 7 8
Hea
t o
f a
dso
rpti
on
[k
J/m
ol]
100
150
200
250
300
350
Cu on MgO(100)
Ag on MgO(100)
Pb on MgO(100)
Ag H Sublimation
285 kJ/mol
Pb H Sublimation
195 kJ/mol
Cu H Sublimation
337 kJ/mol
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.
2D M
-MgO
(100
) B
ond
En
ergy
(k
J / m
ol M
)
Hsublim (kJ / mol M)
Correlation of 2D M-MgO(100) Bond Energy with Sublimation Enthalpy of Metal
0
50
100
150
200
180 230 280 330
Pb
Ag
Cu
Campbell et al., JACS 124 (2002) 9212 .
Suggests that covalent metal-Mg bonding dominatesthe interaction for 2D particles.Probably due to very strong bonding at defects tocoordinatively unsaturated Mg atoms.
Pb on MgO(100)
Metal Island Radius (nm)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
He
at
of
Ad
so
rpti
on
(k
J/m
ol)
80
100
120
140
160
180
200
220
Hsub = 195.2 kJ/mol
Constant Model( = 59 J/mol)
Calorimetry Data
Gibbs-Thompson(R) - (∞) = 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 < 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
andCatalytic 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.
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 nm Au = very active!!!
10 nm Au = completely inactive
bulk Au = completely inactive
2 Ogas 2 Oad O2,gas
Ea,des and Had for Oad : ~40% larger for smallest Au islands
V. Bondzie, S. C. Parker,C. T. Campbell,Catalysis Letters 63 (1999) 143.
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 981952 William R. Wiley Environmental Molecular Sciences Laboratory,
Pacific Northwest National Laboratory, Richland, WA 993523 Department of Chemistry, University of Washington, Seattle, WA 98195
Pd coverage / ML
0.0 0.3 0.6 0.9 100.0
Init
ial S
tick
ing
Pro
b.
10-3
10-2
10-1
CH4 Sticking Probability vs. Particle Size
• Initial sticking prob. for dissoc. ads. of methane: increases as Pd particle size decreases.
Ebeam = 70 kJ/molTS = 500 K
Increasing particle diam. →~1 nm ~3 nm
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.
CH4 CH3,ad + Had
NEXT I WILL SHOW THAT:
The opposite effect is seem when the metal atoms bind
more strongly to a surface!!!
Pb/Mo(100)
Pb coverage / ML
0 1 2 3 4 5 6 7 8
Hea
t of
ad
sorp
tion
/ (k
J m
ol-1
)
150
175
200
225
250
275
300
325
350 Pb adsorption on Mo(100) at 300 K
Hsub
Stuckless et al., PRB 56 (1997) 13496
John M. Heitzinger, Steven C. Gebhard and Bruce E. Koel Surface Science 275 (1992) 209.
Strong bonding of metal to Mo(100) dramatically weakensthat metal’s chemisorption bond to adsorbed CO
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??
VPulsed
Metal AtomSource
Thin single crystal sample
Pyroelectric Ribbon for Temperature
Rise Detection
Moved into contact with back of thin sample
Metal Nanoparticle
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.
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 nm Au = very active!!!
10 nm Au = completely inactive
bulk Au = completely inactive
Pb on MgO(100)
Metal Island Radius (nm)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
He
at
of
Ad
so
rpti
on
(k
J/m
ol)
80
100
120
140
160
180
200
220
Hsub = 195.2 kJ/mol
Constant Model( = 59 J/mol)
Calorimetry Data
Gibbs-Thompson(R) - (∞) = 2/R = 59 J/cm2
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 m thick!!
•Opens up measurements to almost any sample, since single crystals can be mechanically thinned to 70 m even over required 1 cm2 area.
(
(
Polymer-backedpyroelectric ribbon(