Heterogeneous Catalysis: From Imagining to Imaging a Working Surface

Transcription

1 Heterogeneous Catalysis: From Imagining to Imaging a Working Surface D. Wayne Goodman Texas A&M University Department of Chemistry Introduction to issues Studies of model catalysts with surface analytical methods: special properties of metal nanoclusters Thermal stability of metal nanoclusters Strategies for designing sinter resistant catalysts

2 Heterogeneous Catalysis gas phase reaction A B A + B P potential energy catalyst A B catalyst P P catalyst catalyst adsorption reaction desorption Applications Production of transportation fuels (440 oil refineries) Production of chemicals Cleanup of automotive/industrial exhaust gases Green chemistry (unwanted byproducts)

3 Heterogeneous Catalysis A B potential energy catalyst A B catalyst P P catalyst catalyst adsorption reaction desorption Applications Production of transportation fuels (440 oil refineries) Production of chemicals Cleanup of automotive/industrial exhaust gases Green chemistry (unwanted byproducts)

4 Ammonia Synthesis potential energy AN AN BH BH BH catalyst BH AN BH AN BH BH catalyst BH AN BH catalyst catalyst adsorption reaction desorption Applications Production of transportation fuels (440 oil refineries) Production of chemicals Cleanup of automotive/industrial exhaust gases Green chemistry (unwanted byproducts)

5 Carbon Monoxide and Nitric Oxide 2 CO + 2 NO 2 CO 2 + N 2 N C O O N potential energy catalyst C O O catalyst N O C O N catalyst N N O C O catalyst adsorption reaction desorption Applications Production of transportation fuels (440 oil refineries) Production of chemicals Cleanup of automotive/industrial exhaust gases Green chemistry (unwanted byproducts)

6 Carbon Monoxide and Nitric Oxide 2 CO + 2 NO 2 CO 2 + N 2 N C O O N potential energy catalyst C O O catalyst N O C O N catalyst N N O C O catalyst adsorption reaction desorption Applications Production of transportation fuels (440 oil refineries) Production of chemicals Cleanup of automotive/industrial exhaust gases Green chemistry (eliminate unwanted byproducts)

7 Green Chemistry with Catalysts Example: ethylene to ethylene epoxide C 2 H 4 + ½ O 2 C 2 H 4 O Old non-catalytic route (epichlorohydrine process): Step 1: Cl 2 + NaOH HOCl + NaCl Step 2: C 2 H 4 + HOCl CH 2 Cl-CH 2 OH Step 3: CH 2 Cl-CH 2 OH + 1/2Ca(OH) 2 ½CaCl 2 + C 2 H 4 O + H 2 O Total: Cl 2 + NaOH + 1/2 Ca(OH) 2 + C 2 H 4 C 2 H 4 O + 1/2 CaCl 2 + NaCl + H 2 O Catalytic route: Cl promoted Ag catalyst C 2 H 4 + 1/2O > C 2 H 4 O C 3 H 6 + ½ O 2 C 3 H 6 O

8 TOF (1/site s) Unique Catalytic Activity of Nanosized Gold Particles CO Oxidation Propylene Oxidation Au/TiO 2 Au/TiO 2 2CO + O 2 2CO K CO:O 2 = 1:5 P T = 40 Torr Cluster Diameter (nm) Product Yield (%) 3 2 Propane H 2 /O 2 /Propylene/Ar = 1:1:1:7 P t = 1 atm T = 350 K Au/TiO 2 Propylene Oxide 1 CO Au Particle Diameter (nm) Cluster Diameter (nm) from Haruta, et al., Catalysis Letters (1997) from Haruta, et al., Shokubai, Catalysts and Catalysis (1995)

9 Unique Catalytic Activity of Nanosized Gold Particles 0.30 CO Oxidation TOF (1/site s) Au/TiO 2 Au/TiO 2 2CO + O 2 2CO K CO:O 2 = 1:5 P T = 40 Torr Cluster Diameter (nm) from Haruta, et al., Catalysis Letters (1997) TEM Image of Gold Supported on Titania (from M. Date, ONRI)

10 Model Oxide-Supported Metal Catalysts e.g. TiO 2 Oxide Single Crystal TiO 2 (110) 50 nm Metal Clusters nm Oxide Single Crystal TiO 2 (110) Au 50 nm

11 Pd on FeO(111)/Pt(111) S. Shaikhutdinov, R. Meyer, D. Lahav, M. Baeumer, T. Kluener, H.-J. Freund Phys. Rev. Lett. (2003)

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

13 Au/TiO 2 (110):1D 2D 3D 1D 2D 3D M. Valden, X. Lai, and D. W. Goodman, Science 281, 1647 (1998)

14 Unique catalytic activity of Au/TiO 2 (110) Activity CO + 1/2O 2 CO 2 CO:O 2 = 1:5 P T = 40 Torr Population (%) Cluster size (nm) M. Valden, X. Lai, and D. W. Goodman, Science 281 (1998) nm x 30nm

15 Unique properties of Au/TiO 2 (110) Activity Population (%) CO + 1/2O 2 CO 2 CO:O 2 = 1:5 P T = 40 Torr Cluster size (nm) CO Heat of Adsorption (kcal/mol) M. Valden, X. Lai, and D. W. Goodman, Science 281 (1998) D. C. Meier, D. W. Goodman, J. Am. Chem. Soc. 126 (2004) BULK Bulk GOLD Au Particle size (nm)

16 Surface defects on TiO 2 (110) Oxygen vacancies 6.0 nm 6.0 nm

17 Ultraviolet Photoelectron Spectroscopy (UPS): Defects on TiO 2 (110) TiO 2 (110) + O K TiO 2 (110) + O K Ti +3 (3d) Ti +3 (3d) Oxygen Vacancies Top View Oxygen Vacancies Top View [001] 2.96Å [001] 2.96Å [110] 6.49Å Side View [110] 6.49Å Side View Krischok, Guenster, Goodman, Hoefft, and Kempter, submitted for publication

18 Role of Oxygen Defects in Metal Cluster Nucleation and Growth on TiO 2 (110) Bridging oxygen vacancies are the active nucleation sites for Au clusters oxygen vacancies a) STM image of a small Au clusters on TiO 2. Vacancies are marked with squares. b) Simulated STM image of a single Au atom trapped in a oxygen vacancy. c) Line profiles comparing DFT theoretical results and experiment. E. Wahlstroem, N. Lopez, R. Schaub, P. Thostrup, A. Ronnau, C. Africh, E. Laegsgaard, J. K. Norskov, and F. Besenbacher, Phys. Rev. Lett. 90, 101 (2003)

19 Au on TiO 2 (110): Cluster Anchored via Reduced Titania Au bilayer 30 nm DFT shows that Au nanoparticles promote the exchange of oxygen vacancies between the surface and bulk of titania Rodriguez et al, J. Am. Chem. Soc., 124 (2002) 5242 and Single oxygen vacancy can bind 3 Au atoms on average E. Wahlstroem, N. Lopez, R. Schaub, P. Thostrup, A. Ronnau, C. Africh, E. Laegsgaard, J. K. Norskov, and F. Besenbacher, Phys. Rev. Lett. 90, 101 (2003)

20 Reduced Titania Surface: TiO x /Mo(112) STM (8x2) LEED (0 1) (1 1) 50 x 50 nm (1 0) [111] [111] - [110] [110] O O Ti O Mo Mo Chen et al., Surf. Sci. Lett. (2005), & Science 306 (2004) 252. Full-1ML-Ti 3+!!

21 1.0 monolayers Au: (1x1)-Au/TiO x /Mo(112) 1.33 monolayers Au: (1x3)-Au/TiO x /Mo(112) (1 1) (0 1) (1 0) 3x3nm 2 8x8nm 2 M. S. Chen and D. W. Goodman, Science 306 (2004) 252; STM: Chen et al. (2006)

22 Relative Catalytic Activity of Mono- and Bi-layer Au/TiO x 4 3 TOF: molecules CO 2 produced per Au atom per sec 2 1 (1x3) CO + ½ O 2 CO 2 Au Au Mo Mo Ti 45 times CO:O 2 higher = 1:5 than P T = that 10 Torrfound for the most active Au/TiO 2 high surface (1x1) area supported catalyst Au MoTi Mo Ti Ti Au coverage (ML) Chen, et al., Science, 306, 252 (2004)

23 Similarity of Au nanoparticles & the (1x3) well-ordered bilayer [001] [110] Both form 1D-like chain for the topmost Au atoms!

24 Summary: Properties of Supported Au Nanoclusters Adsorbate binding energies, e.g. CO and O 2, change significantly from the bulk values for clusters < 3.0 nm. DFT calculations show center of Au d-band significantly destabilized for Au/TiO 2 compared to Au. Core-level shifts are markedly non-bulk-like at <ca. 3.0 nm. Surface plasmon not observed for clusters <ca. 3.0 nm. Sublimation energies of clusters < 3.0 nm are markedly lower than the corresponding bulk value. Nanoclusters are generally unstable to reaction conditions, i.e., understanding and maintaining stability is a key to technological break-throughs.

25 CO Oxidation Over Au/TiO 2 as a Function of Reaction Time Reaction Rate, CO 2 molecules per site per second A Before Rx ( ) Au/TiO 2 (110) 2CO + O 2 2CO 2 CO:O 2 = 1:5 P T = 40 Torr T = 350 K D After Rx Reaction Time (minutes)

26 Sintering Mechanisms A P. Wynblatt and N. A. Gjostein, Acta Metallurgica, 24, 1165 (1976) interparticle transport Atom Migration (Ostwald Ripening) : atoms/atom ensembles migrate to adjacent clusters to form larger clusters μ chemical potential; σ surface free energy M/ρ atom volume; r cluster radius Gibbs-Thompson relationship

27 STM: 0.5 MLE Au/TiO 2 (110), CO/O 2 (1:1), K A x 630A Cluster size increase Cluster size decrease Cluster disappears Yang and Goodman, hour Cluster volume (nm ) A x 630A time (minute)

28 Sintering Mechanisms A B P. Wynblatt and N. A. Gjostein, Acta Metallurgica, 24, 1165 (1976) interparticle transport Atom Migration (Ostwald Ripening) : atoms/atom ensembles migrate to adjacent clusters to form larger clusters particle migration/coalescence Cluster Migration : Clusters migrate along the surface, collide with others and coalescence μ chemical potential; σ surface free energy M/ρ atom volume; r cluster radius Gibbs-Thompson relationship

29 Au/TiO 2 (110) Before and After Annealing to 950K As deposited After a 950K x 30 min. anneal a b 100 nm 100 nm Kolmakov and Goodman, Chem. Rev., 2003

30 Role of support in metal activation & cluster sintering: SiO 2 versus TiO 2?

31 Model Oxide-Supported Metal Catalysts Thin Oxide Film Support + Metal Clusters e.g. Mo, Re Ta, W Refractory Single Crystal 400 nm Mo(112) e.g. SiO 2, Al 2 O 3, MgO, TiO nm Oxide Thin Film Refractory Single Crystal SiO 2 / Mo(112) 50 nm Metal Clusters Oxide Thin Film nm Refractory Single Crystal 50 nm 0.5 ML Au SiO 2 / Mo(112)

32 Preparation & Characterization of Ultra-thin, Well-ordered SiO 2 /Mo(112) Schroeder, Adelt, Richter, Naschitzki, Baumer, and Freund. Surf. Rev. Lett. 7 (2000) O 800K 3. K 400 nm 10 nm 0.7 nm thick, sharp hexagonal LEED with a band gap ~8.9 ev (STS)

33 Defects on SiO 2 Surfaces Studied by Metastable Electron Impact Spectroscopy: MIES count rate (arb. units) Low defect density High defect density oxygen vacancies binding energy (ev)

34 Au Cluster Nucleation on Low-Defect Versus High-Defect SiO 2 Au + Low Defect SiO 2 Au + High Defect SiO nm 200 nm 0.40 ML of Au ML/min 300 K 0.40 ML of Au ML/min 300 K

35 0.7 ML Au at room T 0.7 ML Au after a 850 K anneal Au Cluster Nucleation on Defective SiO 2 Number Density of Au Clusters (x / cm 2 ) A comparison of Au cluster density after deposition at RT and a subsequent anneal to 850K / Number Density of Au Cluster(x10 / /cm 2 ) room T anneal to 850K 0.2ML 0.7ML 1.3ML Coverage (ML)

36 Sintering of Au Clusters on SiO K anneal 200 nm 200 nm Sintering of Au on SiO 2 more facile than on TiO 2 i.e, Au binds less strongly to SiO 2 than to TiO 2

37 Strategies for a Sinter-Resistant Support: TiO 2 Dispersed onto and into SiO 2 SiO 2 : 1 10 nm Mo(112) TiO 2 islands Substitutional TiO 2 SiO 2 : 1 10 nm Mo(112) or Mo(112) Au clusters SiO 2 : 1 10 nm Mo(112) Mo(112)

38 TiO x Islands Dispersed on SiO ML SiO 2 /Mo(112) 0.2 ML TiO x /SiO 2 /Mo(112) 100 nm 100 nm

39 Au Particles Deposited onto TiO x Islands Dispersed on SiO ML TiO x /SiO 2 /Mo(112) 0.4 ML Au/TiO x /SiO 2 /Mo(112) 100 nm 100 nm

40 Au/SiO 2 versus Au/TiO x /SiO 2 : 850 K Anneal before after SiO ML Au 100 nm TiO x /SiO nm ML Au

41 Ti Point Defects on SiO 2 15 nm 15 nm

42 STM: TiO x -SiO 2 Thin Film with 8% Ti 2.4 nm Scan across Ti defects Height (nm) ~0.1nm Length (nm) Mo (112) Si Ti O

43 Decoration of Ti Point Defects with Gold Ti defect Au/Ti defect Ti/SiO 2 /Mo(112) ML Au Au/Ti/SiO 2 /Mo(112)

44 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 breakthroughs.

45 Coworkers STM Mingshu Chen HREELS Zhen Yan Ming-shu Chen PM-IRAS Yun Cai Hi-press STM Fan Yang Patrick Han IRAS Tao Wei Matt Lundwall ISS Kai Luo Stepahnus Axnanda Rx-XPS Dheeraj Kumar Mingshu Chen Lo-T IRAS Cheol-Woo Yi MIES Sungsik Lee Hi-SA Supported Catalysts Zhen Yan Bo Wang

46 Au Cluster Density After The Indicated Treatment Normalized To The Cluster Density After Nucleation At Room Temperature Normalized Au cluster density Au/TiO 2 (8%)-SiO 2 Au/SiO 2 Au/TiO 2 (17%)-SiO 2 After Anneal 850 K Au/TiO 2 (17%)-SiO 2 After Reaction CO:O 2 = 2:1, 60 Torr, 370 K, 120 minutes

47 XPS Core Level Shifts: Au/SiO 2 vs. Au/TiO 2 Core Level Shift: Bulk Small Cluster Limit XPS Core Level Shifts Binding Energy (ev) Au/SiO 2 Au/TiO Final State Initial State Total Contributions Implications: electron-rich Au on TiO 2!

48 DFT Calculations for Au and Au/TiO 2 (110) Yang, Wu, Goodman, PRB (2000) DOS (states/ev) Au/TiO 2 (110) Au(001) E(eV) 1 ML Au on Au(001) 1 ML Au on TiO 2 (110)