Spectroscopic and related techniques in surface science for unravelling heterogeneously catalyzed reaction mechanisms Ludo Juurlink, Ph.D. Leiden Institute of Chemistry Leiden University, Leiden, the Netherlands Office: Gorlaeus Laboratories DE0.01 Email: l.juurlink@chem.leidenuniv.nl phone +31 71 527 4221 Course objectives:  At the this short course students can explain how surface science attempts to understand heterogeneous catalysis can outline how common experimental (spectroscopic) techniques reveal information on surfaces, adsorbates, and chemical reactions Understand why and how (supersonic) molecular beams are useful herein are informed on some recent examples in the field of gas-surface dynamics

Surface Science and Gas-Surface Reaction Dynamics Schedule Date Time Topics 14-Dec-17 11:00 – 11:50 Introduction: Surface Science for Catalysis Surface Crystallography Low Energy Electron Diffraction Scanning Tunneling Microscopy 12:00 – 12:50 Introduction to spectroscopic techniques Auger Electron Spectroscopy X-ray photoelectron spectroscopy Reflection Absorption InfraRed Spectroscopy Temperature Programmed Desorption 15-Dec-17 Controlling molecular impact: supersonic molecular beams Examples of combined use of SMB and Surface Science Examples from the recent literature on CH4 State-selected dissociation Mode-selected dissociation Bond-selected dissociation Stereodynamical effects

Surface Science and Gas-Surface Reaction Dynamics Schedule Date Time Topics 14-Dec-17 11:00 – 11:50 Introduction: Surface Science for Catalysis Surface Crystallography Low Energy Electron Diffraction Scanning Tunneling Microscopy 12:00 – 12:50 Introduction to spectroscopic techniques Auger Electron Spectroscopy X-ray photoelectron spectroscopy Reflection Absorption InfraRed Spectroscopy Temperature Programmed Desorption 15-Dec-17 Controlling molecular impact: supersonic molecular beams Examples of combined use of SMB and Surface Science Examples from the recent literature on CH4 State-selected dissociation Mode-selected dissociation Bond-selected dissociation Stereodynamical effects

Heterogeneous Catalysis Surface Science to help understand Heterogeneous Catalysis

Heterogeneous Catalysis: example MSR CH4 + H2O 3H2 + CO CH4 + H2O  3H2 + CO ~1000 °C 30 bar Simple cubic crystal. Every atom represented by cube or sphere.

Heterogeneous Catalysis: example MSR Al2O3 pellets Also promotors and other chemical to stabilize particles! Simple cubic crystal. Every atom represented by cube or sphere.

Heterogeneous Catalysis: example MSR CH4 + H2O  3H2 + CO Simple cubic crystal. Every atom represented by cube or sphere. Ni Ni Ni

Heterogeneous Catalysis: example MSR CH4 + H2O  3H2 + CO Too much energy required -- reactants don’t react. Intermediate blocks surface – no product formation Simple cubic crystal. Every atom represented by cube or sphere. G. Jones et al. Journal of Catalysis 259, 147 (2008)

Heterogeneous Catalysis: Fischer-Tropsch Simple cubic crystal. Every atom represented by cube or sphere. https://www.youtube.com/watch?v=44OU4JxEK4k

Surface Science approach using single crystals 1970’s onward: Single crystal surfaces: Macroscopic pieces of pure metals, alloys, semiconductors, oxides, …. with a polished surface exposing a single type of ordering of atoms. 2000’s onward: Well-defined particles grown on crystal surfaces: Microscopic single crystalline particles of pure metals, alloys, oxides, …. grown by vapor deposition onto a well-ordered oxide support. Simple cubic crystal. Every atom represented by cube or sphere.

Single crystals Ta single crystal Re single crystal polishing cutting by spark erosion polishing Simple cubic crystal. Every atom represented by cube or sphere.

Single crystals Ag single crystal with a curved surface Simple cubic crystal. Every atom represented by cube or sphere.

Basics of two-dimensional crystallography

Bulk crystal structures 1. triclinic 2. monoclinic 3. orthorhombic 4. rhombohedral 5. tetragonal 6. hexagonal 7 lattice systems 14 Bravais lattices simple base-centered simple base-centered body-centered face-centered 7. cubic simple simple body-centered face-centered body-centered

Bulk crystal structures

Bulk crystal structures examples Body centered cubic (bcc) Fe, W Face centered cubic (fcc) Ni, Cu, Pt Hexagonal close packed (hcp) Ru, Co Diamond lattice Cdiamond, Si, Ge Zinc blende structure ZnS, GaAs, InP Rock salt NaCl, NiO

Atomically flat surfaces as bulk truncations surface layer normal to [100] Example: fcc [100] [010] [001] [111] surface layer normal to [111]

A real single crystal surface on a manipulator Ru(0001) 10 mm 2 mm LN2 LN2

Vicinal surfaces (001) Families of planes e.g. {100}, {111}, {110} (1-11) (001) (1-11) (1-10) [110] [110] (1-10) [111] [001] [110] 54.7° 35.3° 54.7°

Vicinal surfaces stepped stepped and kinked

Do these surfaces represent the surface of particles? Helveg et al., Nature 427, 426 (2004) Behrens et al., Science 336, 893 (2012)

Do these surfaces represent the surface of particles? R.A. Olsen and L.B.F. Juurlink, Hydrogen dissociation on stepped Pt surfaces, in Dynamics of Gas-Surface Interactions: Atomic-level Understanding of scatttering processes at surfaces, Ed. Busnengo and Diez-Muiño, Springer Series in Surface Science, Springer (Berlin, 2013) Honkala et al., Science 307, 555 (2005)

Referring to surfaces: Miller indices Plane with Miller indices h, k and l is (hkl). It is orthogonal to reciprocal lattice vector [hkl]. Determine intercept plane on axes a1, a2, and a3 Take reciprocal of obtained numbers: 1/a1, …. Reduce to smallest integers with same ratio (321) a3 a3 h = 1 / (1/3) k = 1 / (1/2) l = 1 / 1 1/2 of a2 a2 1/3 of a1 a1

Miller indices Plane with Miller indices h, k and l is (hkl). It is orthogonal to reciprocal lattice vector [hkl]. Determine intercept plane on axes a1, a2, and a3 Take reciprocal of obtained numbers: 1/a1, …. Reduce to smallest integers with same ratio

Some important crystal structures

Some important crystal structures

Imaging surface in reciprocal space Low Energy Electron Diffraction

Reciprocal lattice Reciprocal lattice: where h and k are integers and …. Reciprocal lattice vectors: real space lattice reciprocal space lattice

Bragg’s law and the Ewald sphere 3-D crystals: conservation of 3D momentum and (3D) energy

Low Energy Electron Diffraction (LEED) Wavelength, , must be close to interatomic distance, “a” For example: Ekin = 20 eV,  = 2.7 Å Ekin = 200 eV,  = 0.87 Å  Inverse relation between the ‘position’ of the diffraction spot and the distance between atoms = n  for diffraction ‘hot spot’

Low Energy Electron Diffraction (LEED)

Interpretation of a LEED pattern Sharpness: well-ordered surface shows bright, sharp spots Geometry: gives information on crystallographic structure Spot profile: intensity distribution across width of spot -> surface imperfections cause weakening of spot I-V analysis: evaluation of atom positions

Surface and overlayer structures (superstructure)

Notation for surface and overlayer structures Matrix notation: Wood’s notation: Primitive and centered: ‘p’ and ‘c’

Low Energy Electron Diffraction (LEED) Ogletree et al, Surf. Sci. 173, 351 (1986)

Diffraction techniques Electron-based techniques Low energy electron diffraction (LEED: typically 50 eV) Reflection high-energy electron diffraction (RHEED: typically 20 keV) Transmission electron diffraction (TED: typically 200 keV) Auger electron diffraction (AED) X-ray based techniques Grazing-incidence X-ray diffraction (GIXRD; typically 15 keV) Surface X-ray diffraction (SXRD; typically 15 keV) Grazing-incidence small-angle X-ray scattering (GISAXS) Other techniques Photo-electron diffraction (PED) Thermal-energy atom diffraction (typically 15 meV He atoms)

Imaging surface in real space Scanning Tunneling Microscopy

Scanning Tunneling Microscopy F d E sample tip vacuum EF Evac EF + eV

Scanning Tunneling Microscopy V tip specimen piezo element trajectory

Scanning Tunneling Microscopy Forschungszentrum Jülich, Germany

Scanning Tunneling Microscopy Au(111) Leiden Probe Microscopy BV

STM images of adsorbates H2O ‘double strands’ along a Pt(553) edge Diffusion of In atoms in a Cu(100) surface Kolb et al., Phys. Rev. Lett. 116, 136101, (2016) Gastel et al, Phys. Rev. Lett. 86, 1562 (2001)

STM images of adsorbates