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(