Surface Structure of Catalysts
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511
Lecture 3
Heterogeneous Catalysis
Langmuir-Hinshelwood reaction
Eley-Rideal reaction
adsorption, surface diffusion, surface reaction, desorption
Crystals and Crystal Structures
Metal
Semiconductor Insulator
FCC
HCP
Face Centered Cubic (FCC) Crystal
Number of Atoms per Unit Cell
Coordination Number
Atomic Packing Factor (APF)
Hexagonal Close Packed (HCP) Crystal
Number of Atoms per Unit Cell
Coordination Number
Atomic Packing Factor
Bulk Structure (Crystalline Solid)
Cubic
Simple
bcc
fcc
Diamond
Crystal Structure
http://ece-www.colorado.edu/~bart/book/bravais.htm
Crystal Structure of Platinum (fcc)
Surface Structure
Surface
Bulk Metal
Cleave
Miller Indices
<001>
(100)
<010>
<100>
(111)
(110)
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_1b.htm
Surface Structure of Platinum (Ideal)
(100)
(110)
(111)
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_2.htm
Surface Structure
Surfaces are usually rough consisting of high miller index planes
Surface Structure
terrace
step
Surface Sites
Planar atoms
Edge atoms
Corner atoms
Adatoms
Kinks
Defect
Surface Energetics
Surface
Cleave
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)
Surface Relaxation and Reconstruction
Surface Relaxation
spontaneous
adsorbent driven
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm
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
Surface Structure is Dynamic
UHV
W(001) c(2x2)
H2 chemisorption
W(001) c(2x2)
Surface Structure is Dynamic
Effect of Oxygen
Adsorbent
W(110)
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
LEED Device
grid
screen
electron gun
L = d sinq
http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm
LEED Theory
http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm
LEED Theory
LEED patterns are reciprocal net of surface structure
a1*  a2
a2*  a1
a1*   a1
a2*   a2
 a1*  =1/  a1 
 a2*  =1/  a2 
Low Energy Electron Diffraction
FCC LEED Patterns
BCC LEED Patterns
Surface Structure Determination
Low Energy Electron Microscopy (LEEM)
Objective lense
http://www.research.ibm.com/journal/rd/444/tromp.html
Surface Structure (LEEM)
Si (001)
LEEM
LEED Pattern
Other LEEM imaging
Phase Contrast
(terraces and steps)
Higher vertical resolution,
lateral resolution ~ 5 nm
Photoelectron emission
microscopy (PEEM)
UV-excitation, work
function contrast
Reflection High Energy Electron Diffraction (RHEED)
Advantages
better sample geometry
atom-by-atom growth
Disadvantages
sampling of two alignment
needed
Surface Structure
(Field electron and Field ion microscopy)
FEM
FIM
Tip
Nickel
http://www.nrim.go.jp:8080/
open/usr/hono/apfim/tutorial.html
Work function
Surface structure
Real Catalyst Surface
Catalyst has been annealed in hydrogen at 873 K for 60 h
http://ihome.ust.hk/~ke_lsy/yeung/
Supported Catalyst
Highly dispersed metal on metal oxide
highest
Nickel clusters
lowest
SiO2
55 atom cluster surface energy
minimization
http://brian.ch.cam.ac.uk/~jon/PhD2/node19.html
Supported Molybdenum Sulfide
Formation of stable raft or island structure with geometrical shape
Supported Catalyst
Influence of support substrate
Unrolling carpet
Defect diffusion
Surface wetting and
spreading mechanism
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
Metal-Support Interaction
Metal-Support Interaction
Experimental evidence of encapsulation
Model SIMS
SIMS
Metal-Support Interaction
Electronic effects of SMSI
Metal-metal oxide junction
Metal catalyst
eMetal oxide
partially reduced
metal oxide
This can change the electronic
properties of the metal catalyst
by either pulling away or adding
electrons from metal to oxide
support
Supported Metal Oxide Catalyst
SiO2 Support
MoO2 catalyst
Surface Structure
Surface usually refers to the to 2-8 monolayer of atoms at the interface of
a solid
[010]
(Straight channel)
[001]
Viewed along [010]
[100]
(Sinusoidal channel)
Nanoporous materials
Molecular sized pores
Viewed along [100]
Zeolite Catalysts
p-xylene
Pore size
= 5.5 Å
External surface area = 50 m2/g
Total surface area = 400 m2/g
m-xylene
Molecules in Zeolite Cages and Frameworks
+ p-xylene
ZSM-5
Paraffins
Y-zeolite
Genesis of Catalyst Crystallites
Pt cluster (< 50 nm)
High temperature annealing in hydrogen
High temperature annealing in nitrogen
http://www.lassp.cornell.edu/sethna/CrystalShapes
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
rough surface,
high miller index planes,
low-coordinated surface
atoms
Surface Structure = Adsorption/Catalytic Sites
Molecules on Surface
H3C
CH3
CH = CH
CH3
CH = CH
Pt
CH3
H3 C
CH - CH
Pt
Ordered Adsorbate layer
cinchonidine on Platinum
Pt
Pt
Pt
C
Pt
Pt Pt
2-butene molecule
adsorption on
Platinum
Surface Structure = Adsorption/Catalytic Sites
Surface structural sites serves
as adsorption and catalytic sites
for molecules
Crystal Morphology
Equilibrium-shaped Au
Crystallite
Calculated crystal
shape based on
thermodynamics
calculation
Possible Crystallite Morphologies
ARCHIMEDEAN SOLIDS
Crystal facets will correspond to (111), (100) and (110)
planes of a cubic crystal
Dispersed Catalysts
NS/NT
Truncated Octahedron
Crystal size  then NS/NT 
dc (Å)
Shape Transformation
Increasing stability
Random
Cubo-octahedron
Amorphous
No Facets
Crystallite
Two Facets
(111) and (100)
Icosahedron
Crystallite
Single Facet
(111)
Supported Catalysts
Metal supported on metal oxide
Coarsening
Supported Catalysts
Truncated Octahedron
Supported Truncated Octahedron
Support
Supported Catalyst
Highly dispersed metal on metal oxide
highest
Nickel clusters
lowest
SiO2
55 atom cluster surface energy
minimization
http://brian.ch.cam.ac.uk/~jon/PhD2/node19.html
X-ray in Catalyst Characterization
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511
Lecture 3
X-ray Analysis
X-ray Diffraction (XRD)
Elemental Composition
Catalyst Structure
Particle Size
X-ray Absorption Spectroscopy (XAS)
Elemental Composition
Phase Structure
Atomic environment: atomic coordination
bond angle
bond distance
X-ray Diffractometer
http://www.iucr.org/iucr-top/comm/cteach/pamphlets/
X-ray Source
X-ray Emission
Black body
Metal foil
X-ray Gun
Characteristic X-ray Lines
X-ray
ee-
ee-
K L
M
ee-
M  K: Kb
L  K: Ka
X-ray Absorption
dI/I = - mdx
m
K-edge
Energy used to eject K-electrons
excess energy converted to kinetic
energy of e-  X-ray photoelectron
spectroscopy (XPS)
lK l 
Atomic relaxation occurs through:
X-ray emission (Fluorescence)  X-ray fluorescence
Auger electron emission  Auger electron spectroscopy
X-ray Filter/Monochromatic Source
absorption edge
X-ray Absorption
X-ray Diffraction
Bragg’s Law
nl = 2dsinq
for cubic crystals
d = a/(h2 + k2 + l2)0.5
X-ray Diffraction
d(111)
d(1oo)
a
d(111) = a/(3)0.5
sinq = l/2d (111)
d(100) = a/(1)0.5
sinq = l/2d (100)
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
Catalyst - Particle Size
Rh
Rh
Rh
Rh
Particle size (d)
dictates catalyst area
Crystal Size
t = Kl/bcosq
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
t
a-Fe
X-ray Fluorescence
X-ray fluorescence gave elemental
information
X-ray Photoelectron Spectroscopy
Surface composition and chemistry
X-ray Photoelectron Spectroscopy
Electron spectroscopy for chemical analysis (ESCA)
For solid catalyst:
K.E. = hu - B.E. - f
XPS needs monochromatic X-ray source
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
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
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
X-ray Photoelectron Spectrometer
X-ray Sources
Twin Anode (Mg/Al)
- simple and inexpensive
- high flux (1010-1012 photons/s)
- beam size ~ 1 cm
- polychromatic
Monochromatic X-ray
(uses bent SiO2 crystal)
- eliminates satellites
- smaller beam size 50 mm
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
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.
Binding Energy of Electrons
Primary XPS Structure
Spin-Orbit Splitting
Primary XPS Structure
Auger Peaks
- always present in XPS data
- more complex and broader than the
photoemission peaks
- independent of incident hu
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
Primary XPS Structure
Core Level Chemical Shifts for C 1s
Note: the effects of chemical environment
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)
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
Sampling Depth for XPS
Sampling depth ~ 3 l
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
XPS Applications in Catalysis
(1) Analyses of surface composition
provides quantitative information on surface elemental composition
XPS Applications in Catalysis
(2) Oxidation state
provides information on the oxidation state of the catalyst materials
Vanadium catalyst
Tungsten oxide catalyst
XPS Applications in Catalysis
(3) Analyses of surface chemistry
provides quantitative information on chemical states of catalyst
surface
XPS Applications in Catalysis
(4) Surface electronic state
provides information on the electronic properties of catalyst
XPS Applications in Catalysis
(4) Surface electronic state
provides information on the electronic
band-gap structure of the catalyst
material.
Photoemission Electron Microscopy (PEEM)
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.
PEEM - Elemental Contrast
Elemental contrast is achieved by
tuning the incident x-ray wavelength
through absorption edges of elements.
PEEM - Elemental Contrast
X-ray absorption contains
information on local chemical
environment.
Auger Electron Spectroscopy
Auger electrons are generated during the
relaxation of excited atom
Yield of Auger electron is higher for light elements
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 - f
Binding Energy of Electrons
Auger Electron Spectroscopy
AES usually uses electrons for excitation
Simpler and cheaper
Auger Electron Spectroscopy
AES is surface sensitive technique
Also produces many inelastically scattered e-
Auger Electron Spectroscopy
Point analysis (50-200 nm)
Line scan
Elemental mapping
Depth profiling
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
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
AES - Elemental Mapping
Elemental mapping
Using electron excitation source that
could be scanned AES has could
provide elemental mapping
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
AES - Depth Profiling
Depth Profiling
Low carbon steel