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 Atomic Packing Factor Coordination Number

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> (110) (111) 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 Surface Sites Planar atoms Edge atoms terrace Corner atoms Adatoms Kinks Defect terrace step

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)

Surface Relaxation and Reconstruction spontaneous adsorbent driven http://www.chem.qmw.ac.uk/surfaces/scc/scat1_6.htm

Surface Reconstruction 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) LEED Pattern LEEM

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

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 Surface structure http://www.nrim.go.jp:8080/ open/usr/hono/apfim/tutorial.html Work function

Real Catalyst Surface Catalyst has been annealed in hydrogen at 873 K for 60 h http://ihome.ust.hk/~ke_lsy/yeung/

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

Influence of support substrate 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 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

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 Viewed along [010] Viewed along [100] [010] ( Straight channel) [001] [100] Sinusoidal channel) Nanoporous materials Molecular sized pores

Zeolite Catalysts p-xylene m-xylene Pore size = 5.5 Å External surface area = 50 m2/g Total 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 Rough surface well-defined structure, low miller index planes, high-coordinated surface atoms facets rough surface, high miller index planes, low-coordinated surface atoms

Surface Structure = Adsorption/Catalytic Sites

Molecules on Surface H3C CH3 CH = CH Pt C CH - CH 2-butene molecule adsorption on Platinum Ordered Adsorbate layer cinchonidine 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 Crystal size  then NS/NT  Truncated Octahedron

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

Supported Catalysts Metal supported on metal oxide Coarsening

Supported Truncated Octahedron Supported Catalysts Truncated Octahedron Supported Truncated Octahedron Support

55 atom cluster surface energy Supported Catalyst highest Nickel clusters SiO2 Highly dispersed metal on metal oxide lowest 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) 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 M  K: Kb L  K: Ka X-ray e- e- e- K L M e- e- e-

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

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

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 d(100) = a/(1)0.5 sinq = l/2d (111) d(100) = a/(1)0.5 sinq = l/2d (100)

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

Catalyst - Particle Size Rh Particle size (d) dictates catalyst area

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

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 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

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 - 

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 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