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

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

3. Crystals and Crystal Structures
Metal
Semiconductor
Insulator
FCC
HCP

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

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

8. Crystal Structure of Platinum (fcc)

9. Surface Structure
Surface
Bulk Metal
Cleave

10. Miller Indices <001> (100) <010> <100> (110) (111)

11. Surface Structure of Platinum (Ideal)
(100)
(110)
(111)

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

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

14. Surface Energetics Energy is needed to create surface DG > 0
Cleave
Surface
Bulk Metal
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
spontaneous
adsorbent driven

16. Surface Reconstruction
Surface Relaxation and Reconstruction
Surface Reconstruction
spontaneous
adsorbent driven
Normal (100) Surface
Reconstructed Surface

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 ( 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

20. LEED Device grid screen electron gun L = d sinq

21. LEED Theory

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

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

24. Surface Structure Determination
Low Energy Electron Microscopy (LEEM)
Objective lense

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

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

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
open/usr/hono/apfim/tutorial.html
Work function

29. Real Catalyst Surface
Catalyst has been annealed in hydrogen at 873 K for 60 h

30. 55 atom cluster surface energy
Supported Catalyst
highest
Nickel clusters
SiO2
Highly dispersed metal on metal oxide
lowest
55 atom cluster surface energy
minimization

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

32. Influence of support substrate
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. Metal-Support Interaction
Electronic effects of SMSI
Metal-metal oxide junction
Metal catalyst e-
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
SiO2 Support
MoO2 catalyst

38. Surface Structure
Surface usually refers to the to 2-8 monolayer of atoms at the interface of
a solid
Viewed along [010]
Viewed along [100]
[010] (
Straight channel)
[001]
[100]
Sinusoidal channel)
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. Molecules in Zeolite Cages and Frameworks
+ p-xylene
ZSM-5
Y-zeolite
Paraffins

41. Genesis of Catalyst Crystallites
Pt cluster (< 50 nm)
High temperature annealing in hydrogen
High temperature annealing in nitrogen

42. Genesis of Catalyst Crystallites
Pt cluster (< 50 nm)
Surface structural sites
Rough surface
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. Molecules on Surface H3C CH3 CH = CH Pt C CH - CH
2-butene molecule adsorption on Platinum
Ordered Adsorbate layer
cinchonidine on Platinum

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

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

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

48. Dispersed Catalysts Crystal size  then NS/NT  Truncated Octahedron

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

50. Supported Catalysts
Metal supported on metal oxide
Coarsening

51. Supported Truncated Octahedron
Supported Catalysts
Truncated Octahedron
Supported Truncated Octahedron
Support

52. 55 atom cluster surface energy
Supported Catalyst
highest
Nickel clusters
SiO2
Highly dispersed metal on metal oxide
lowest
55 atom cluster surface energy
minimization

53. X-ray in Catalyst Characterization
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511
Lecture 3

54. X-ray Analysis X-ray Diffraction (XRD) Elemental Composition
Catalyst Structure
Particle Size
X-ray Absorption Spectroscopy (XAS)
Phase Structure
Atomic environment: atomic coordination
bond angle
bond distance

55. X-ray Diffractometer

56. X-ray Source
X-ray Emission
Black body
Metal foil
X-ray Gun

57. Characteristic X-ray Lines
M  K: Kb
L  K: Ka
X-ray e- e- e- K L M e- e- e-

58. X-ray Absorption dI/I = - mdx Energy used to eject K-electrons
K-edge lK
Energy used to eject K-electrons
excess energy converted to kinetic
energy of e-  X-ray photoelectron
spectroscopy (XPS)
Atomic relaxation occurs through:
X-ray emission (Fluorescence)  X-ray fluorescence
Auger electron emission  Auger electron spectroscopy

59. X-ray Filter/Monochromatic Source
X-ray Absorption
absorption edge

60. X-ray Diffraction Bragg’s Law nl = 2dsinq for cubic crystals
d = a/(h2 + k2 + l2)0.5

61. X-ray Diffraction d(111) d(1oo) a d(111) = a/(3)0.5 d(100) = a/(1)0.5
sinq = l/2d (111)
d(100) = a/(1)0.5
sinq = l/2d (100)

62. Powder X-ray Diffraction
Structural Analysis
Powder X-ray Diffraction
Qualitative analysis:
determine the ten most intense
diffraction lines and match
with available diffraction
pattern library.
Quantitative analysis:
relative concentration can be
obtain by measuring the relative the intensities of two strong non-overlapping lines, one belonging to component A, the other to component B

63. Catalyst - Particle SizeRh
Particle size (d)
dictates catalyst area

64. Crystal Size t = Kl/bcosq a-Fe t
where: t is the thickness of crystal  to
diffraction plane
K is a constant that depends on
instrument
b is the full width at half
maximum (FWHM) of the
diffraction peak
a-Fe t

65. X-ray Fluorescence
X-ray fluorescence gave elemental
information

66. X-ray Photoelectron Spectroscopy
Surface composition and chemistry

67. X-ray Photoelectron Spectroscopy
Electron spectroscopy for chemical analysis (ESCA)
For solid catalyst:
K.E. = hu - B.E. - f
where K.E. is the kinetic energy of photoelectron
B.E. is the binding energy
hu is the X-ray energy
f is the work function
Note:
- no photoemission for hu < f
- no photoemission for B.E. + f > hu
- K.E. increases as B.E. decreases
- intensity of photoemission is proportional to the
intensity of the photons
- a range of K.E. can be produced if valence
band is broad
- K.E. can be used as fingerprinting technique
XPS needs monochromatic X-ray source

68. X-ray Fluorescence and Auger Electron Emission
Photoelectron emission lead to formation of core holes
Core holes are eliminated by relaxation that is accompanied by
(1) X-ray fluorescence  X-ray fluorescence spectroscopy
(2) Auger electron emission  Auger electron spectroscopy

69. Koopman’s Theorem B.E. = Efinal(n-1) - Einitial (n)
The slight discrepancy between the experimental
and calculated binding energies arises from:
- electron rearrangement in excited state
- initial state effects  absorption and ionization
- final state effects  response of atom and
photoelectron emission
- extrinsic losses  transport of electron to
surface and escape to vacuum

70. X-ray Photoelectron Spectrometer
X-ray Sources
Twin Anode (Mg/Al)
- simple and inexpensive
- high flux ( photons/s)
- beam size ~ 1 cm
- polychromatic
Monochromatic X-ray
(uses bent SiO2 crystal)
- eliminates satellites
- smaller beam size 50 mm

71. X-ray Photoelectron Spectrometer
Electron Energy Analyzer
Concentric hemispherical Analyzer (CHA)
- the path of electron through the analyzer depends on its K.E. and the applied potentials (V1 and V2)
- changing the applied potential, electrons with different K.E. can be detected using a counter
- a pre-set “pass voltage” is set to fix the resolution of the CHA

72. Primary XPS Structure Stepped Background Intensity
- only electron close to surface can escape without energy loss
(approx. 95% come from 3 l of which 63% are from l)
- electrons deeper in the bulk loss part of its K.E. as it travel towards the surface
- electron deep in the bulk can not escape
more energetic electrons have greater chance of reaching the surface and escaping, thus the “stepped” background effect.

73. Binding Energy of Electrons

74. Primary XPS Structure
Spin-Orbit Splitting

75. Primary XPS Structure Auger Peaks - always present in XPS data
- more complex and broader than the
photoemission peaks
- independent of incident hu

76. Primary XPS Structure Core Level Chemical Shifts
- related to the overall charge on the atom
reduced charge  increased B.E.
- number of substituents
- electronegativity of the substituent
- formal oxidation state
Chemical Shift is important for identifying
• functional group
• chemical environment
• oxidation state
Carbon containing gases

77. Primary XPS Structure Core Level Chemical Shifts for C 1s
Note: the effects of chemical environment

78. Secondary XPS Structure
1) X-ray Satellites
caused by poor X-ray source and X-ray fluorescence
2) Surface Charging
3) Intrinsic Satellites
caused by atomic relaxation 
(1) excitation of electron to bound state (shake-up satellite)
(2) excitation of electron to continuum state (shake-off satellite)
(3) excitation of hole (shake-down satellite)
4) Multiplet Splitting
splitting of 1s orbital
5) Extrinsic Satellites
caused by energy loss in electrons as it travels towards the surface
(i.e., plasmon)

79. Secondary XPS Structure
2) Surface Charging
caused by accumulation of positive charges due to photoemission of electrons  results in peak shift to higher B.E.
Neutralized using a flux of low energy electron

80. Sampling Depth for XPS
Sampling depth ~ 3 l

81. XPS Data Analysis
Quantitative information requires good background subtraction method
must identify and correct for:
- x-ray satellites
- chemically shifted species
- shake-up peaks
- plasmon and other electron energy losses

82. XPS Applications in Catalysis
(1) Analyses of surface composition
provides quantitative information on surface elemental composition

83. XPS Applications in Catalysis
(2) Oxidation state
provides information on the oxidation state of the catalyst materials
Vanadium catalyst
Tungsten oxide catalyst

84. XPS Applications in Catalysis
(3) Analyses of surface chemistry
provides quantitative information on chemical states of catalyst
surface

85. XPS Applications in Catalysis
(4) Surface electronic state
provides information on the electronic properties of catalyst

86. XPS Applications in Catalysis
(4) Surface electronic state
provides information on the electronic band-gap structure of the catalyst material.

87. Photoemission Electron Microscopy (PEEM)

88. PEEM - Topological Contrast
Photoelectron emission if the energy of the X-ray photons is larger than the work function of the sample. These photo-emitted electrons are extracted into an electronoptical imaging onto a phosphor screen that convertes electrons into visible light, which is detected by a CCD camera.
The topographical contrast is due to distortion of the electric field around surface topolographical features.

89. PEEM - Elemental Contrast
Elemental contrast is achieved by tuning the incident x-ray wavelength through absorption edges of elements.

90. PEEM - Elemental Contrast
X-ray absorption contains information on local chemical environment.

91. Auger Electron Spectroscopy
Auger electrons are generated during the relaxation of excited atom
Yield of Auger electron is higher for light elements

92. Auger Electron Spectroscopy
Auger electrons can be generated by:
(1) X-rays
Auger peaks in XPS
(2) Electrons
free of photoemission peaks
Auger electron
K.E. = EA - EB - EC - 

93. Binding Energy of Electrons

94. Auger Electron Spectroscopy
AES usually uses electrons for excitation
Simpler and cheaper

95. Auger Electron Spectroscopy
AES is surface sensitive technique
Also produces many inelastically scattered e-

96. Auger Electron Spectroscopy
Point analysis ( nm)
Line scan
Elemental mapping
Depth profiling

97. AES - Point Analysis Fingerprint Spectra
Use characteristic spectra for
identifying unknown samples
- chemical shift is complex
- broad peak
- presence of loss features
- difficult to assign
- more difficult to interpret than XPS

98. AES - Line Analysis Line Scan AES has good spatial resolution
- monitors auger peak intensity as
a function of position
Line scan across a cratered sample

99. AES - Elemental Mapping
Using electron excitation source that could be scanned AES has could provide elemental mapping

100. AES - Depth Profiling Depth Profiling Analysis procedure:
(1) surface etching is attained by bombardment with Ar ion
(2) AES is obtained from the crater formed by Ar sputtering
(3) the process is repeated to create an
Precise etching can be achieved:
for example,
Si 9.0 nm/min
SiO2 8.5 nm/min
Pt 22 nm/min
Au 41 nm/min
Al 9.5 nm/min
Cr 14 nm/min

101. AES - Depth Profiling
Depth Profiling
Low carbon steel