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Structure Sensitivity of CO Oxidation over Au/TiO 2. 0.30. 2CO + O 2 2CO 2. 0.25. [Data for High Surface Are a Supported Catalysts from: Haruta, e t al., J. Catal. 115 (1989) 301; see also Haruta, Cat. Today, 36 (1997) 153.]. s). 0.20. TOF (1/site. 0.15. 0.10. 0.05. 0.00.

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  1. Structure Sensitivity of CO Oxidation over Au/TiO2 0.30 2CO + O2 2CO2 0.25 [Data for High Surface Are a Supported Catalysts from: Haruta, e t al., J. Catal. 115 (1989) 301; see also Haruta, Cat. Today, 36 (1997) 153.] s) 0.20 TOF (1/site 0.15 0.10 0.05 0.00 1.5 2.5 3.5 4.5 5.5 6.5 Cluster Diameter (nm)

  2. Nanoscience and Catalysis D. Wayne Goodman Texas A&M University Department of Chemstry Introduction to issues Model catalyst preparation and characterization Correlation among structural, chemical and electronic properties of supported metal clusters Thermal stability of metal clusters Grand challenges

  3. Magnetic Moments Versus Metal Cluster Size Ni Co From Gillas, Chatelain, and De Heer, Science (1994) Fe

  4. Ionization Potentials of Ni Clusters Versus Size Knickelbein, Yang, Riley, JCP (1990) ~1.2 nm 0.4 eV ~4.0 nm 0.25 eV

  5. Gold Supported on Titania TEM Image of Gold Supported on Titania (from M. Date, ONRI)

  6. Selective Oxidation of Propylene Over Au/TiO2 (Hayashi, Haruta, Shokubai, Catalysts and Catalysis, 1995) H2/O2/Propylene/Ar = 1:1:1:7 3 Pt = 1 atm T = 350 K Propane 2 Product Yield (%) Propylene Oxide 1 CO2 1.2 1.6 2.0 2.4 2.8 Au Particle Diameter (nm)

  7. C D Model Oxide-Supported Metal Catalysts Single Crystal Oxide Support + Metal Clusters e.g. MgO, TiO2 Oxide Single Crystal TiO2(110) 50 nm Metal Clusters 1.0-50 nm TiO2(110) + 0.25 Au Oxide Single Crystal 50 nm

  8. Morphology of the TiO2(110) Surface 50 nm Xu, Lai, Zajac, and Goodman, Phys. Rev. B (1997) 6.0 nm 6.0 nm

  9. [110] [001] Au Cluster Growth on TiO2(110): Quasi 2-D & 3-D Au clusters 2-D (quasi 2-D) Au clusters can be observed at the initial stages (0.01 - 0.05 ML) of growth. 10 nm Xu, Lai, Zajac, and Goodman, Phys. Rev. B (1997) 10 nm

  10. STM: Au Clusters on TiO2(110) 30 nm 30 nm

  11. Increasing Au coverage Au/TiO2(110) : Cluster Size/Density Vs. Coverage 50 nm 50 nm 50 nm 50 nm 50 nm Lai, St. Clair, Valden and Goodman, Prog. Surf. Sci (1998)

  12. Oxide Thin Film Refractory Single Crystal Oxide Thin Film Refractory Single Crystal Model Oxide-Supported Metal Catalysts Thin Oxide Film Support + Metal Clusters e.g. Mo, Re Ta, W Refractory Single Crystal Re(0001) 400 nm e.g. SiO2, Al2O3, MgO, TiO2 1-10 nm 1.0 ML Al2O3/ Re(0001) 50 nm 0.5 ML Ag Al2O3/ Re(0001) Metal Clusters 1.0 - 50 nm 50 nm

  13. 1.0 MLE Al2O3 on Re(0001) 200 nm 100 nm 16 nm 2.0 nm x 2.0 nm • well-ordered, hexagonal, close-packed O2_ structure 3D surface image • spacings between indentations: 2.6 _ 2.7 Å LEED Luo, Guo and Goodman, Chem. Phys. Letts.(2000)

  14. Model Oxide-supported Metal Catalysts Single Crystal Oxide Support Thin Film Oxide Support 1.0 ML Al2O3/ Re(0001) TiO2(110) 50 nm 50 nm + + 0.5 ML Ag 0.5 ML Au 50 nm 50 nm

  15. Oxide-Supported Metal Clusters IRAS H2O, CO, NO, CO/O2, CO/NO, CO/H2 on: Au, Cu, Pd, Ni/MgO, TiO2, SiO2, Al2O3 XPS, UPS Au, Cu, Ag, Pd, Ni/MgO, TiO2, SiO2, Al2O3 REACTION KINETICS CO/O2, CO/NO, CO/H2, C2H6, C2H2 on: Au, Pd, Ni/TiO2, SiO2, Al2O3 HREELS, ELS CH3OH, CO, NO, O2 on Au, Cu, Ag, Pd, Ni/MgO, TiO2, SiO2, Al2O3 TPD H2O, CO, NO, O2 on: Au, Cu, Ag, Pd, Ni/MgO, TiO2, SiO2, Al2O3 ISS Au, Cu, Ag, Pd, Ni/MgO, TiO2, SiO2, Al2O3 STM, STS Au, Cu, Ag, Pd, Ni/MgO, TiO2, SiO2, Al2O3

  16. Apparatus Sample Preparation Chamber AES Ion XPS Gun Gate Valve Sample Manipulator LEED STM Elevated-Pressure Cell

  17. Unique Catalytic Activity of Nanosized Gold Particles 0.30 3.5 2CO + O2 2CO2 0.25 3 CO:O2 = 1:5 PT = 40 Torr 0.20 2.5 TOF (1/site s) TOF (1/site s) 0.15 2 Au/TiO2* Au/TiO2(110) Model Catalyst* 300 K 0.10 1.5 350 K 0.05 1 Valden, Pak, Lai and Goodman, Catal. Lett. (1998) 0.00 0.5 0.0 2.0 4.0 6.0 8.0 10.0 Cluster Diameter (nm) *: The Au/TiO2 catalysts were prepared by a deposition-precipitation method, and the averaged cluster sizes were measured by TEM at 300 K. M. Haruta, et al., Catal. Lett. 44, 83 (1997). *: The Au/TiO2 catalysts were prepared by vapor-depositing Au atoms onto a planar TiO2 film supported on Mo(100). A 1:5 CO:O2 mixture at a total pressure of 40 Torr and 350 K was used for reaction.

  18. Effect of Cluster Size and Morphology on Reactivity of Au/TiO2(110) CO + ½ O2 CO2 CO:O2 = 1:5 PT = 40 Torr 10 nm M. Valden, X. Lai, D.W. Goodman Science 1998 60 10 nm 45 Population (%) 30 15 0 0.0 2.0 4.0 6.0 8.0 10.0 30 nm Cluster Diameter (nm) 30 nm

  19. Scanning Tunneling Spectroscopy (STS) of Supported Metal Clusters: Finite Size Effects bulk metal large clusters small clusters

  20. 2.0 1.6 1.2 0.8 1.5 1.2 0.9 0.6 0.3 0.0 0 2 4 6 8 10 CO Oxidation Activity vs. Electronic Structure Au/Ti(110)-(1x1) CO Catalytic Activity (CO2 molecules per site per sec) 2CO + O2 2CO2 CO:O2 = 1:5 PT = 40 Torr T = 350 K Au/Ti(110)-(1x1) Cluster Band Gap (Volts) as Measured by STS M. Valden, X. Lai, D.W. Goodman Science 1998 Cluster Diameter (nm)

  21. Isosteric Heats of CO Adsorption Vs. Au Cluster Size 20 Clausius-Clapeyron 18 Redhead Appoximation CO Heat of Adsorption (kcal/mol) 16 14 12 BULK GOLD 10 Meier, Bukhtiyarov, and Goodman, J. Phys. Chem, (2002) 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Particle Diameter (nm)

  22. FLAPW Calculations for Au and Au/TiO2(110) Yang, Wu, Goodman, PRB (2000) DOS (states/eV) Au/TiO2(110) Au(001) E(eV)

  23. Theory: FLAPW Calculations of Au/TiO2(110) FLAPW calculated charge density difference obtained by substracting the superposition of the charge densities of a Au monolayer and TiO2(110) from that of Au/TiO2(110) Yang, Wu, Goodman, PRB (2000) Au Predicts an initial state core level shift for Au on TiO2 to a lower binding energy of ~1.2 eV, consistent with the experimental value of 1.0 eV Ti

  24. ~3.0 nm XPS Core Level Shifts: Au/TiO2(110) Core Level Shifts Au/SiO Au/TiO 2 2 2.0 1.8 1.8 1.6 1.5 0.8 BE 1.0 (eV) .5 0 -0.2 - .5 -1.0 -1.0 Final State Initial State Total Contributions St.Clair and Goodman, Topics in Catal (2000)

  25. Au Cluster Height vs. STM Tip Bias

  26. Charge on Au Clusters vs. CO Binding Energy P. Bagus, unpublished data +1 -1 0 Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au +0.03 eV -0.11 eV

  27. TPD: Au from SiO2 as a Function of Coverage Au Coverage Decreasing Cluster Size Heat of sublimation Decreasing Cluster Size Luo, Kim, and Goodman, J. Mol. Catal (2001)

  28. CO Oxidation Over Au/TiO2 as a Function of Reaction Time Au/TiO2(110) 2CO + O2 2CO2 CO:O2 = 1:5 Reaction Rate, CO2 molecules per site per second PT = 40 Torr T = 350 K Reaction Time (minutes) Valden, Lai & Goodman, Science (1998) Figure 7

  29. Morphological Changes of Au/TiO2(110) 50 nm 50 nm Before Rx After Rx Fresh 0.25 ML Au/TiO2(110) After 120 min exposure to 10 Torr CO-O2 (2:1) Lai and Goodman, J. Mol. Catalysis (2000)

  30. UHV-Elevated Pressure STM Apparatus 10-10 - 103 Torr 10-10 Torr MS Sample Storage STM Metal Dosers e-Beam eB load lock AES S Gas 10-10 Torr TMP IP SP SP Microscope: RHK VT - UHV300 Variable Temperature: 100 - 600 K Pressure Range: 10-10 - 103 Torr

  31. STM of Au Clusters on TiO2 at 400K UHV 5.4 Torr CO:O2 Cluster size increase Surface regrowth around Au clusters Surface roughening Cluster size reduction Adhesion of cluster to tip Kolmakov and Goodman, Catal. Letts (2000)

  32. STM Tip as a Mask for Metal Deposition Kolmakov and Goodman, Physical Review B, submitted

  33. Large Scale STM Image of Tip Silhouette for Au Deposition RT Au Deposition Same area after 950K anneal 250 nm 250 nm Kolmakov and Goodman, Physical Review B, submitted

  34. Au/TiO2(110) Before and After Annealing to 950K As deposited After a 950K x 30 min. anneal 100 nm 100 nm Kolmakov and Goodman, Physical Review B, submitted

  35. tip  shadow  d sample Au Evaporation onto Ag/TiO2 at RT Ag Au + Ag

  36. Au and Au-Ag mixed clusters Comparative Stability of Ag and Au-Ag Clusters to 2 x 104 Pa Air Ag particles TiO (110) 2 Kolmakov and Goodman, Physical Review B, submitted

  37. IRAS: Site Differentiation of Mixed-Metal Catalysts IRAS: CO/Cu-Pd/Al2O3 ISS: Cu-Pd/Mo(110) Pd Cu Rainer, Xu, Holmblad, and Goodman, J. Vac. Sci. Technol. A (1997)

  38. Conclusions Catalytic reactivity and selectivity are markedly different for clusters < ~3.0 nm. Core-level shifts, valence band structure, sublimation energies, and adsorbate binding energies are unique for clusters < ~3.0 nm. Nanoclusters are generally unstable to reaction conditions, i.e., understanding and maintaining stability are the keys to technological break-throughs.

  39. What do we need to know about supported nanoclusters? • Methods for the preparation of mono-dispersed clusters with selected sizes • Properties as a function of metal-to-nonmetal transition for a range of transition metals • physical structure (bond lengths, angles, etc.) • electronic properties (valence band, core levels, etc.) • optical properties (plasmon, HOMO-LUMO gap, etc.) • melting temp., sublimation temp., etc. • adsorbate bonding energies, vibrational freq., etc. • catalytic properties for several probe reactions Precise relationship between physical and chemical properties • theory ------- experiment • Detailed sintering kinetics and role of support in altering catalytic activity • Quantitative thermochemical information about metal wetting, nucleation, and particle sintering

  40. Coworkers Past:Dr. Andrei Kolmakov Dr. Charles ChusueiDr. Micha ValdenDr. Xiaofeng Lai Dr. Doug MeierDr. Todd St. ClairDr. Gerry ZajacDr. Jens GuensterDr. Qinlin GuoDr. Valeri BukhtiyarovDr. Kent DavisKai Luo Current: Dr. Ashok Santra Byoun Koun Min Dr. Young Dok Kim Jeff Stultz Emrah Ozensoy Dr. Christian Hess Cheol-Woo Yi Dr. Changmin Kim Dr. Paul Bagus Tushar Choudhary Fan Yang Tao Wei Dheeraj Kumar Dr. Sivadinarayana Chinta

  41. Support Department of Energy, the Office of Basic Energy Sciences, Division of Chemical Sciences The Robert A. Welch Foundation Dow Chemical Company

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