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 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 Al 2 O 3 (~0.4m 2 /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) Rev. Sci. Instr. 69 (1998) Follows the design of D. A. King But different method of detection: Detector : 9  m 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 Pulsed Molecular Beam Thin Sample Pyroelectric Ribbon Quadrupole Mass Spectrometer Sample In Contact Approaching Contact PVDF Ribbon

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.

Cu: J. T. Ranney et al., Faraday Discussions, 114, 195, Ag: J. H. Larsen et al., Phys. Rev. B 63, , Pb: D. E. Starr et al., J. Chem. Phys. 114, , 2001.

2D M-MgO(100) Bond Energy (kJ / mol M)  H sublim (kJ / mol M) Correlation of 2D M-MgO(100) Bond Energy with Sublimation Enthalpy of Metal Pb Ag Cu Campbell et al., JACS 124 (2002) 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.

Gibbs-Thompson  (R) -  (∞) = 2  /R  = 59  J/cm 2 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: H 3 C …… CH kJ/mol H 2 C …… CH 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 TiO 2 (110) Model of Au / TiO 2 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, (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 O gas 2 O ad O 2,gas E a,des and  H ad for O ad : ~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 Dohnálek 2, Bruce D. Kay 2, Charles T. Campbell 3 1 Department of Physics, University of Washington, Seattle, WA William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA Department of Chemistry, University of Washington, Seattle, WA 98195

CH 4 Sticking Probability vs. Particle Size Initial sticking prob. for dissoc. ads. of methane: increases as Pd particle size decreases. E beam = 70 kJ/mol T S = 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. CH 4  CH 3,ad + H ad

NEXT I WILL SHOW THAT: The opposite effect is seem when the metal atoms bind more strongly to a surface!!!

Pb/Mo(100) Pb adsorption on Mo(100) at 300 K  H sub 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 weakens that 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, CH 3, …) 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, CH 3, ad ) unless bonding mechanism is London dispersion force (e.g. CH 4, 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??

V Pulsed Metal Atom Source 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 TiO 2 (110) Model of Au / TiO 2 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, (See also our related work of Murata group referenced there.) 2 nm Au = very active!!! 10 nm Au = completely inactive bulk Au = completely inactive

Gibbs-Thompson  (R) -  (∞) = 2  /R  = 59  J/cm 2 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 cm 2 area. (( Polymer-backed pyroelectric ribbon (