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Surface Structure of Catalysts
Dr. King Lun YeungDepartment 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
DiamondCrystal Structure
http://ece-www.colorado.edu/~bart/book/bravais.htm
Crystal Structure of Platinum (fcc)
Surface Structure
Bulk MetalCleave
Surface
Miller Indices
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_1b.htm
(100)
(111) (110)<100>
<001>
<010>
Surface Structure of Platinum (Ideal)
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_2.htm
(100) (110) (111)
Surface Structure
Surfaces are usually rough consisting of high miller index planes
Surface Structure
Surface SitesPlanar atomsEdge atomsCorner atomsAdatomsKinksDefect
terracestep
Surface Energetics
Cleave Surface
Bulk Metal
Energy is needed to create surfaceG > 0
In order to minimize G (1) smaller surface area (2) expose surface with low G (3) change atomic geometry (relaxation and reconstruction)
Surface Relaxation and Reconstruction
Surface Relaxationspontaneous
adsorbent driven
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm
Surface Relaxation and Reconstruction
Normal (100) Surface Reconstructed Surface
Surface Reconstructionspontaneous
adsorbent driven
http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm
Surface Structure is Dynamic
UHV
H2 chemisorption
W(001) c(2x2)
W(001) c(2x2)
Surface Structure is Dynamic
Effect of OxygenAdsorbent
W(110)
Surface Structure Determination
Low Energy Electron Diffraction (LEED)Analyzes surface crystallographic structure by bombarding the surfacewith low energy electrons (10-200 eV) and the diffracted electronscreates patterns on phosphorescent screen. The pattern of spots containsinformation of surface structure and the spot intensity indicates reconstruction
http://dol1.eng.sunysb.edu/expcht1.html
http://electron.lbl.gov/leed/leedtheory.html
LEED Device
http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm
grid
electron gun
screen
L = d sin
LEED Theory
http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm
LEED Theory
LEED patterns are reciprocal net of surface structurea1* 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)
http://www.research.ibm.com/journal/rd/444/tromp.html
Objective lense
Surface Structure (LEEM)
Si (001) LEED Pattern
LEEM
Other LEEM imaging
Photoelectron emission microscopy (PEEM)
UV-excitation, workfunction contrast
Phase Contrast(terraces and steps)
Higher vertical resolution,lateral resolution ~ 5 nm
Reflection High Energy Electron Diffraction (RHEED)
Advantagesbetter sample geometryatom-by-atom growth
Disadvantagessampling of two alignmentneeded
Surface Structure(Field electron and Field ion microscopy)
Tip
FEM FIM
Nickel
Work function
http://www.nrim.go.jp:8080/open/usr/hono/apfim/tutorial.html
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
Nickel clusters
SiO2
Highly dispersed metal on metal oxide
http://brian.ch.cam.ac.uk/~jon/PhD2/node19.html
55 atom cluster surface energyminimization
highest
lowest
Supported Molybdenum Sulfide
Formation of stable raft or island structure with geometrical shape
Supported Catalyst
Influence of support substrate
Surface wetting and spreading mechanism
Unrolling carpet
Defect diffusion
Real Catalyst Surface
Catalysts are usually small particles or clusterthat can exhibit several crystallographic planesof different surface atomic structures
Catalyst wets support
Catalyst does not wet support
Metal-Support Interaction
Metal-Support Interaction
Experimental evidence of encapsulation
SIMSModel SIMS
Metal-Support Interaction
Electronic effects of SMSI
e-
Metal-metal oxide junction
This can change the electronicproperties of the metal catalystby either pulling away or addingelectrons from metal to oxidesupport
partially reduced metal oxide
Metal oxide
Metal catalyst
Supported Metal Oxide Catalyst
SiO2 Support MoO2 catalyst
Surface Structure
Surface usually refers to the to 2-8 monolayer of atoms at the interface ofa solid
Viewed along [010]
Viewed along [100]
[010] (Straight channel)
[001]
[100] (Sinusoidal channel)
Nanoporous materialsMolecular sized pores
Zeolite Catalysts
p-xylene m-xylene
Pore size = 5.5 ÅExternal surface area = 50 m2/gTotal surface area = 400 m2/g
Molecules in Zeolite Cages and Frameworks
+ p-xylene
ZSM-5
Y-zeolite
Paraffins
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
facets
well-defined structure,low miller index planes,high-coordinated surface atoms
rough surface,high miller index planes,low-coordinated surface atoms
Rough surface
Surface Structure = Adsorption/Catalytic Sites
Molecules on Surface
CH = CHCH3H3C
Pt
Pt Pt
CH - CHCH3H3C
CH
= C
H
C
CH3
PtPtPt
Pt Pt
CH - CH CH3
H3 C
2-butene molecule adsorption on Platinum
Ordered Adsorbate layercinchonidine on Platinum
Surface Structure = Adsorption/Catalytic Sites
Surface structural sites servesas adsorption and catalytic sitesfor molecules
Crystal Morphology
Equilibrium-shaped Au Crystallite
Calculated crystal shape based on thermodynamics calculation
Possible Crystallite Morphologies
ARCHIMEDEAN SOLIDSCrystal facets will correspond to (111), (100) and (110)planes of a cubic crystal
Dispersed Catalysts
Truncated Octahedron
NS/N
T
dc (Å)Crystal size then NS/NT
Shape Transformation
AmorphousNo Facets
CrystalliteTwo Facets
(111) and (100)
CrystalliteSingle Facet
(111)
Increasing stability
Random Cubo-octahedron Icosahedron
Supported Catalysts
Metal supported on metal oxide
Coarsening
Supported Catalysts
Truncated Octahedron Supported Truncated Octahedron
Support
Supported Catalyst
Nickel clusters
SiO2
Highly dispersed metal on metal oxide
http://brian.ch.cam.ac.uk/~jon/PhD2/node19.html
55 atom cluster surface energyminimization
highest
lowest
X-ray in Catalyst Characterization
Dr. King Lun YeungDepartment 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 SourceX-ray Emission
Black body
Metal foil
X-ray Gun
Characteristic X-ray Lines
e-
e-
e-
e-
e-
X-ray
K L M
e-
M K: K
L K: K
X-ray Absorption
K-edge
dI/I = - dx
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
X-ray Filter/Monochromatic Source
X-ray Absorptionab
sorp
tion
edg
e
X-ray Diffraction
Bragg’s Law
n = 2dsin
for cubic crystals
d = a/(h2 + k2 + l2)0.5
X-ray Diffraction
d(111)
d(1oo)
a
d(111) = a/(3)0.5
sin = /2d (111)
d(100) = a/(1)0.5
sin = /2d (100)
Structural Analysis Powder X-ray Diffraction
Qualitative analysis:determine the ten most intensediffraction lines and matchwith available diffraction pattern library.
Quantitative analysis:relative concentration can beobtain 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 = K/coswhere: 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
-Fe
t
X-ray Fluorescence
X-ray fluorescence gave elementalinformation
X-ray Photoelectron Spectroscopy
Surface composition and chemistry
X-ray Photoelectron SpectroscopyElectron spectroscopy for chemical analysis (ESCA)
For solid catalyst: K.E. = h - B.E. - where K.E. is the kinetic energy of photoelectron B.E. is the binding energy h is the X-ray energy is the work functionNote:- no photoemission for h < - no photoemission for B.E. + > h- 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
X-ray Fluorescence and Auger Electron EmissionPhotoelectron 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 experimentaland 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 tosurface and escape to vacuum
X-ray Sources
X-ray Photoelectron Spectrometer
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 m
X-ray Photoelectron SpectrometerElectron 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 of which 63% are from )- 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 h
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
Carbon containing gases
Chemical Shift is important for identifying• functional group• chemical environment• oxidation state
Primary XPS Structure
Core Level Chemical Shifts for C 1s Note: the effects of chemical environment
1) X-ray Satellites caused by poor X-ray source and X-ray fluorescence2) Surface Charging3) 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
Secondary XPS Structure
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
XPS Data Analysis
Quantitative information requires good background subtraction methodmust 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 stateprovides information on the electronic band-gap structure of the catalyst material.
Photoemission Electron Microscopy (PEEM)
PEEM - Topological ContrastPhotoelectron 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 ContrastElemental 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 -
Binding Energy of Electrons
AES usually uses electrons for excitation
Auger Electron Spectroscopy
Simpler and cheaper
AES is surface sensitive technique
Auger Electron Spectroscopy
Also produces many inelastically scattered e-
Point analysis (50-200 nm)
Auger Electron Spectroscopy
Line scan
Elemental mapping
Depth profiling
AES - Point Analysis
Fingerprint Spectra
Use characteristic spectra foridentifying 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