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

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


Surface Relaxation and Reconstruction

Normal (100) Surface Reconstructed Surface

Surface Reconstructionspontaneous

adsorbent driven


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


grid

electron gun

screen

L = d sin

LEED Theory


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


Experimental evidence of encapsulation

SIMSModel SIMS


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


Spin-Orbit Splitting


Auger Peaks- always present in XPS data- more complex and broader than the photoemission peaks- independent of incident h


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


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


(2) Oxidation state provides information on the oxidation state of the catalyst materials

Vanadium catalyst

Tungsten oxide catalyst


(3) Analyses of surface chemistry provides quantitative information on chemical states of catalyst surface


(4) Surface electronic state provides information on the electronic properties of catalyst


(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 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


Simpler and cheaper

AES is surface sensitive technique


Also produces many inelastically scattered e-

Point analysis (50-200 nm)


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

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