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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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slide1

CENG 511

Lecture 3

Surface Structure of Catalysts

Dr. King Lun Yeung

Department of Chemical Engineering

Hong Kong University of Science and Technology

slide2

Langmuir-Hinshelwood reaction

Heterogeneous Catalysis

Eley-Rideal reaction

adsorption, surface diffusion, surface reaction, desorption

slide3

FCC

HCP

Crystals and Crystal Structures

Metal

Semiconductor

Insulator

slide4

Face Centered Cubic (FCC) Crystal

Coordination Number

Number of Atoms per Unit Cell

slide6

Hexagonal Close Packed (HCP) Crystal

Number of Atoms per Unit Cell

Atomic Packing Factor

Coordination Number

slide7

Bulk Structure (Crystalline Solid)

Cubic

Simple

bcc

fcc

Diamond

Crystal Structure

http://ece-www.colorado.edu/~bart/book/bravais.htm

slide9

Surface Structure

Surface

Bulk Metal

Cleave

slide10

Miller Indices

<001>

(100)

<010>

<100>

(110)

(111)

http://www.chem.qmw.ac.uk/surfaces/scc/scat1_1b.htm

slide11

(110)

(100)

(111)

Surface Structure of Platinum (Ideal)

http://www.chem.qmw.ac.uk/surfaces/scc/scat1_2.htm

slide12

Surface Structure

Surfaces are usually rough consisting of high miller index planes

slide13

Surface Structure

Surface Sites

Planar atoms

Edge atoms

Corner atoms

Adatoms

Kinks

Defect

terrace

step

slide14

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)

slide15

Surface Relaxation and Reconstruction

Surface Relaxation

spontaneous

adsorbent driven

http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm

slide16

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

slide17

Surface Structure is Dynamic

UHV

W(001) c(2x2)

H2 chemisorption

W(001) c(2x2)

slide18

Surface Structure is Dynamic

Effect of Oxygen

Adsorbent

W(110)

slide19

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

slide20

grid

screen

electron gun

LEED Device

L = d sinq

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

slide21

LEED Theory

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

slide22

LEED Theory

LEED patterns are reciprocal net of surface structure

a1*  a2a2*  a1

a1*   a1 a2*   a2

 a1*  =1/  a1   a2*  =1/  a2 

slide23

FCC LEED Patterns

BCC LEED Patterns

Low Energy Electron Diffraction

slide24

Surface Structure Determination

Low Energy Electron Microscopy (LEEM)

Objective lense

http://www.research.ibm.com/journal/rd/444/tromp.html

slide25

Surface Structure (LEEM)

LEED Pattern

Si (001)

LEEM

slide26

Photoelectron emission

microscopy (PEEM)

Phase Contrast

(terraces and steps)

UV-excitation, work

function contrast

Higher vertical resolution,

lateral resolution ~ 5 nm

Other LEEM imaging

slide27

Reflection High Energy Electron Diffraction (RHEED)

Advantages

better sample geometry

atom-by-atom growth

Disadvantages

sampling of two alignment

needed

slide28

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

slide29

Real Catalyst Surface

Catalyst has been annealed in hydrogen at 873 K for 60 h

http://ihome.ust.hk/~ke_lsy/yeung/

slide30

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

slide31

Supported Molybdenum Sulfide

Formation of stable raft or island structure with geometrical shape

slide32

Supported Catalyst

Influence of support substrate

Unrolling carpet

Defect diffusion

Surface wetting and

spreading mechanism

slide33

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

slide35

Metal-Support Interaction

Experimental evidence of encapsulation

Model SIMS

SIMS

slide36

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

slide37

Supported Metal Oxide Catalyst

MoO2 catalyst

SiO2 Support

slide38

[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

slide39

Zeolite Catalysts

p-xylene

m-xylene

Pore size = 5.5 Å

External surface area = 50 m2/g

Total surface area = 400 m2/g

slide40

Y-zeolite

Molecules in Zeolite Cages and Frameworks

+ p-xylene

ZSM-5

Paraffins

slide41

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

slide42

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

slide44

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

slide45

Surface Structure = Adsorption/Catalytic Sites

Surface structural sites serves

as adsorption and catalytic sites

for molecules

slide46

Calculated crystal shape based on thermodynamics calculation

Equilibrium-shaped Au Crystallite

Crystal Morphology

slide47

Possible Crystallite Morphologies

ARCHIMEDEAN SOLIDS

Crystal facets will correspond to (111), (100) and (110)

planes of a cubic crystal

slide48

NS/NT

dc (Å)

Dispersed Catalysts

Truncated Octahedron

Crystal size  then NS/NT 

slide49

Increasing stability

Icosahedron

Random

Cubo-octahedron

Crystallite

Single Facet

(111)

Amorphous

No Facets

Crystallite

Two Facets

(111) and (100)

Shape Transformation

slide50

Supported Catalysts

Metal supported on metal oxide

Coarsening

slide51

Supported Catalysts

Supported Truncated Octahedron

Truncated Octahedron

Support

slide52

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

slide53

CENG 511

Lecture 3

X-ray in Catalyst Characterization

Dr. King Lun Yeung

Department of Chemical Engineering

Hong Kong University of Science and Technology

slide54

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

slide55

X-ray Diffractometer

http://www.iucr.org/iucr-top/comm/cteach/pamphlets/

slide56

X-ray Emission

Black body

Metal foil

X-ray Source

X-ray Gun

slide57

Characteristic X-ray Lines

M  K: Kb

L  K: Ka

X-ray

e-

e-

e-

L

K

M

e-

e-

e-

slide58

K-edge

m

lK

l

X-ray Absorption

dI/I = - mdx

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

slide59

X-ray Absorption

absorption edge

X-ray Filter/Monochromatic Source

slide60

X-ray Diffraction

Bragg’s Law

nl = 2dsinq

for cubic crystals

d = a/(h2 + k2 + l2)0.5

slide61

d(111)

d(1oo)

a

X-ray Diffraction

d(111) = a/(3)0.5

sinq = l/2d (111)

d(100) = a/(1)0.5

sinq = l/2d (100)

slide62

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

slide63

Rh

Rh

Rh

Rh

Particle size (d)

dictates catalyst area

Catalyst - Particle Size

slide64

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

a-Fe

t

slide65

X-ray Fluorescence

X-ray fluorescence gave elemental

information

slide66

X-ray Photoelectron Spectroscopy

Surface composition and chemistry

slide67

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

slide68

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

slide69

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

slide70

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

slide71

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

slide72

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.

slide74

Primary XPS Structure

Spin-Orbit Splitting

slide75

Primary XPS Structure

Auger Peaks

- always present in XPS data

- more complex and broader than the

photoemission peaks

- independent of incident hu

slide76

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

slide77

Primary XPS Structure

Core Level Chemical Shifts for C 1s

Note: the effects of chemical environment

slide78

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)

slide79

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

slide80

Sampling Depth for XPS

Sampling depth ~ 3 l

slide81

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

slide82

XPS Applications in Catalysis

(1) Analyses of surface composition

provides quantitative information on surface elemental composition

slide83

XPS Applications in Catalysis

(2) Oxidation state

provides information on the oxidation state of the catalyst materials

Vanadium catalyst

Tungsten oxide catalyst

slide84

XPS Applications in Catalysis

(3) Analyses of surface chemistry

provides quantitative information on chemical states of catalyst

surface

slide85

XPS Applications in Catalysis

(4) Surface electronic state

provides information on the electronic properties of catalyst

slide86

XPS Applications in Catalysis

(4) Surface electronic state

provides information on the electronic band-gap structure of the catalyst material.

slide88

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.

slide89

PEEM - Elemental Contrast

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

slide90

PEEM - Elemental Contrast

X-ray absorption contains information on local chemical environment.

slide91

Auger Electron Spectroscopy

Auger electrons are generated during the relaxation of excited atom

Yield of Auger electron is higher for light elements

slide92

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 - 

slide94

Auger Electron Spectroscopy

AES usually uses electrons for excitation

Simpler and cheaper

slide95

Auger Electron Spectroscopy

AES is surface sensitive technique

Also produces many inelastically scattered e-

slide96

Auger Electron Spectroscopy

Point analysis (50-200 nm)

Line scan

Elemental mapping

Depth profiling

slide97

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

slide98

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

slide99

AES - Elemental Mapping

Elemental mapping

Using electron excitation source that could be scanned AES has could provide elemental mapping

slide100

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

slide101

AES - Depth Profiling

Depth Profiling

Low carbon steel