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Surface Structure of Catalysts Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and

CENG 511 Lecture 3. Surface Structure of Catalysts Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology. Langmuir-Hinshelwood reaction. Heterogeneous Catalysis. Eley-Rideal reaction.

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Surface Structure of Catalysts Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and

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  1. CENG 511 Lecture 3 Surface Structure of Catalysts Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology

  2. Langmuir-Hinshelwood reaction Heterogeneous Catalysis Eley-Rideal reaction adsorption, surface diffusion, surface reaction, desorption

  3. FCC HCP Crystals and Crystal Structures Metal Semiconductor Insulator

  4. Face Centered Cubic (FCC) Crystal Coordination Number Number of Atoms per Unit Cell

  5. Atomic Packing Factor (APF)

  6. Hexagonal Close Packed (HCP) Crystal Number of Atoms per Unit Cell Atomic Packing Factor Coordination Number

  7. Bulk Structure (Crystalline Solid) Cubic Simple bcc fcc Diamond Crystal Structure http://ece-www.colorado.edu/~bart/book/bravais.htm

  8. Crystal Structure of Platinum (fcc)

  9. Surface Structure Surface Bulk Metal Cleave

  10. Miller Indices <001> (100) <010> <100> (110) (111) http://www.chem.qmw.ac.uk/surfaces/scc/scat1_1b.htm

  11. (110) (100) (111) Surface Structure of Platinum (Ideal) http://www.chem.qmw.ac.uk/surfaces/scc/scat1_2.htm

  12. Surface Structure Surfaces are usually rough consisting of high miller index planes

  13. Surface Structure Surface Sites Planar atoms Edge atoms Corner atoms Adatoms Kinks Defect terrace step

  14. Surface Cleave Bulk Metal Surface Energetics Energy is needed to create surface DG > 0 In order to minimize DG (1) smaller surface area (2) expose surface with low DG (3) change atomic geometry (relaxation and reconstruction)

  15. Surface Relaxation and Reconstruction Surface Relaxation spontaneous adsorbent driven http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm

  16. Surface Relaxation and Reconstruction Surface Reconstruction spontaneous adsorbent driven Normal (100) Surface Reconstructed Surface http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm

  17. Surface Structure is Dynamic UHV W(001) c(2x2) H2 chemisorption W(001) c(2x2)

  18. Surface Structure is Dynamic Effect of Oxygen Adsorbent W(110)

  19. Surface Structure Determination Low Energy Electron Diffraction (LEED) Analyzes surface crystallographic structure by bombarding the surface with low energy electrons (10-200 eV) and the diffracted electrons creates patterns on phosphorescent screen. The pattern of spots contains information of surface structure and the spot intensity indicates reconstruction http://electron.lbl.gov/leed/leedtheory.html http://dol1.eng.sunysb.edu/expcht1.html

  20. grid screen electron gun LEED Device L = d sinq http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm

  21. LEED Theory http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm

  22. LEED Theory LEED patterns are reciprocal net of surface structure a1*  a2a2*  a1 a1*   a1 a2*   a2  a1*  =1/  a1   a2*  =1/  a2 

  23. FCC LEED Patterns BCC LEED Patterns Low Energy Electron Diffraction

  24. Surface Structure Determination Low Energy Electron Microscopy (LEEM) Objective lense http://www.research.ibm.com/journal/rd/444/tromp.html

  25. Surface Structure (LEEM) LEED Pattern Si (001) LEEM

  26. Photoelectron emission microscopy (PEEM) Phase Contrast (terraces and steps) UV-excitation, work function contrast Higher vertical resolution, lateral resolution ~ 5 nm Other LEEM imaging

  27. Reflection High Energy Electron Diffraction (RHEED) Advantages better sample geometry atom-by-atom growth Disadvantages sampling of two alignment needed

  28. Surface Structure (Field electron and Field ion microscopy) FEM FIM Tip Nickel Surface structure http://www.nrim.go.jp:8080/ open/usr/hono/apfim/tutorial.html Work function

  29. Real Catalyst Surface Catalyst has been annealed in hydrogen at 873 K for 60 h http://ihome.ust.hk/~ke_lsy/yeung/

  30. Highly dispersed metal on metal oxide Nickel clusters SiO2 Supported Catalyst highest lowest 55 atom cluster surface energy minimization http://brian.ch.cam.ac.uk/~jon/PhD2/node19.html

  31. Supported Molybdenum Sulfide Formation of stable raft or island structure with geometrical shape

  32. Supported Catalyst Influence of support substrate Unrolling carpet Defect diffusion Surface wetting and spreading mechanism

  33. Real Catalyst Surface Catalyst wets support Catalysts are usually small particles or cluster that can exhibit several crystallographic planes of different surface atomic structures Catalyst does not wet support

  34. Metal-Support Interaction

  35. Metal-Support Interaction Experimental evidence of encapsulation Model SIMS SIMS

  36. e- Metal-Support Interaction Electronic effects of SMSI Metal-metal oxide junction Metal catalyst This can change the electronic properties of the metal catalyst by either pulling away or adding electrons from metal to oxide support Metal oxide partially reduced metal oxide

  37. Supported Metal Oxide Catalyst MoO2 catalyst SiO2 Support

  38. [010] ( Straight channel) [001] Viewed along [010] [100] ( Sinusoidal channel) Viewed along [100] Surface Structure Surface usually refers to the to 2-8 monolayer of atoms at the interface of a solid Nanoporous materials Molecular sized pores

  39. Zeolite Catalysts p-xylene m-xylene Pore size = 5.5 Å External surface area = 50 m2/g Total surface area = 400 m2/g

  40. Y-zeolite Molecules in Zeolite Cages and Frameworks + p-xylene ZSM-5 Paraffins

  41. Pt cluster (< 50 nm) High temperature annealing in hydrogen High temperature annealing in nitrogen Genesis of Catalyst Crystallites http://www.lassp.cornell.edu/sethna/CrystalShapes

  42. Rough surface Genesis of Catalyst Crystallites Pt cluster (< 50 nm) Surface structural sites well-defined structure, low miller index planes, high-coordinated surface atoms facets rough surface, high miller index planes, low-coordinated surface atoms

  43. Surface Structure = Adsorption/Catalytic Sites

  44. CH3 CH = CH CH3 CH3 H3C H3C CH = CH CH - CH Pt C H3C Pt Pt 2-butene molecule adsorption on Platinum Pt CH - CH CH3 Pt Pt Pt Pt Molecules on Surface Ordered Adsorbate layer cinchonidine on Platinum

  45. Surface Structure = Adsorption/Catalytic Sites Surface structural sites serves as adsorption and catalytic sites for molecules

  46. Calculated crystal shape based on thermodynamics calculation Equilibrium-shaped Au Crystallite Crystal Morphology

  47. Possible Crystallite Morphologies ARCHIMEDEAN SOLIDS Crystal facets will correspond to (111), (100) and (110) planes of a cubic crystal

  48. NS/NT dc (Å) Dispersed Catalysts Truncated Octahedron Crystal size  then NS/NT 

  49. Increasing stability Icosahedron Random Cubo-octahedron Crystallite Single Facet (111) Amorphous No Facets Crystallite Two Facets (111) and (100) Shape Transformation

  50. Supported Catalysts Metal supported on metal oxide Coarsening

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