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

Introduction to spectroscopic techniques

Electron spectroscopy Probing of electronic structure of the surface through analysis of energy of secondary electrons emitted from sample. Auger electron spectroscopy (AES) Determines: surface composition Uses: irradiation via electrons Photoelectron spectroscopy (XPS, UPS) Determines: surface composition and electronic structure Uses: irradiation via photons

Spectrum of secondary electrons Ep Elastic peak N(E) electrons photons ions Ep secondary electrons N(E) What is meant by secondary electrons? Does this require a well defined, flat surface? When would this be surface sensititive? Ep = 5-3000 eV with a narrow Ep

Spectrum of secondary electrons photons ions Ep secondary electrons N(E) What is meant by secondary electrons? Does this require a well defined, flat surface? When would this be surface sensititive? Ep = 5-3000 eV with a narrow Ep 6

Electron energy analyzers 4-grid LEED optics: retarding field analyzer electrons with E0<eV0 are repelled applying a sine wave to the potential ramp with LIA detection enhances sensitivity What is meant by secondary electrons? Does this require a well defined, flat surface? When would this be surface sensititive? 7

Electron energy analyzers Cylindrical mirror analyzer (CMA): deflection analyzer electrons in narrow energy window are detected applying a sine wave to the potential ramp with LIA detection enhances sensitivity 8

Electron energy analyzers Concentric hemispherical analyzer (CHA): deflection analyzer electrons in narrow energy window are detected 9

Auger electron spectroscopy Uses an electron beam to create an initial state of a hole in a (surface) atom. Through the Auger process, a second hole is created and a second electron emitted with a specific Ekin. Lise Meitner 1878-1968 Pierre Auger 1899-1993 What type of electron energy analyzer would be used generally for AES? How would one in pactice establish a coverage by AES? Surface composition can be determined as every atom has unique atomic energy levels.

Auger electron spectroscopy Ni(cyl) O S C Dirty Cu sample

Auger electron spectroscopy Quantitative analyses are possible for > 0.01 monolayer of adsorbate or alloy A standard is required (e.g. an adsorbate yielding a known maximum coverage)

Auger electron spectroscopy Uses an electron beam to create an initial state of a hole in a (surface) atom. Through the Auger process, a second hole is created and a second electron emitted with a specific Ekin. Lise Meitner 1878-1968 Pierre Auger 1899-1993 What type of electron energy analyzer would be used generally for AES? How would one in pactice establish a coverage by AES?

Photoelectron spectroscopy Based on photoelectric effect: electron with binding energy Ei absorbs photon with energy ħω, and leaves solid with kinetic energy: Ekin = ħω - Ei – φ where φ = Evacuum-EFermi is the work function of the material. Conditions to detect escaping electron: ħω > Ei + φ Electron velocity is directed towards outer surface Electron does not loose energy due to collisions with other electrons on its way to the surface

Laboratory photoelectron spectroscopy Ekin = ħω - Ei – φ X-ray photoelectron spectroscopy (XPS) Mg K1,2 ħω = 1253.6 eV (9.891 Å) Al K1,2 ħω = 1486.6 eV (8.341 Å) Ultraviolet photoelectron spectroscopy (UPS) discharge lamp He: ħω = 21.22 eV (58.43 nm) 40.81 eV (30.38 nm) X-ray source or UV lamp Alternative: synchrotron facility

X-ray photoelectron spectroscopy Ei = ħω - Ekin- φ Why does the valence band give such a weak signal?

X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy: Core level shifts Due to different environment of surface versus bulk atoms Delta Ed: core level shift of d-band

X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy: Core level shifts Chemical changes lead to large shifts. C2H4/Co(0001) C C2H2 Delta Ed: core level shift of d-band C2H4 Courtesy of: C.J. Weststrate 18

Ultraviolet photoelectron spectroscopy Ultraviolet photon spectroscopy (UPS): low photon energies (< 50 eV), so only valence levels become excited Angle-integrated UPS Angle-resolved UPS (ARUPS)

Vibrational spectroscopies (High Resolution) Electron Energy Loss Spectroscopy (HREELS) and Reflection Absorption Infrared Spectroscopy (RAIRS)

Electron energy loss spectroscopy Study of inelastically scattered electrons, which have lost well-defined energies during interaction with surface Different scattering processes: Core level excitation (CLEELS): 100 – 104 eV Excitation of plasmons and electronic interband transitions (EELS): 1 – 100 eV Excitation of vibrations of surface atoms and adsorbates (HREELS): 10-3 – 1 eV Scattering mechanisms Dipole scattering – long range Very sharp features near specular angle Impact scattering – short range Wide angle scattering Negative ion resonances Strong dependence on impact energy Excitation of electronic interband transitions: semiconductor/insulator. Excitation of e- from occupied state in valence band or occupied surface state to normally empty state in conduction band. e- Ekin,out Ekin,in 21

Electron energy loss spectroscopy HREELS: Identification of adsorbed species Identification of adsorption sites Identification of spatial orientation of adsorbed molecule Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector ν(C-O) CO (gas phase ) 2143 cm-1 Terminal CO 2100 - 1920 cm-1 Bridging (2f site) 1920 - 1800 cm-1 Bridging (3f/4f site) < 1800 cm-1 22

Electron energy loss spectroscopy HREELS: Identification of adsorbed species Identification of adsorption sites Identification of spatial orientation of adsorbed molecule Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector 23

Reflection Absorption InfraRed Spectroscopy (RAIRS) Highest sensitivity for observing an absorption feature when p-polarized light grazing incidence molecule with transition dipole arranged along surface normal molecule with large transition moment Selection rule: Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector 24

Reflection Absorption InfraRed Spectroscopy (RAIRS) Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector Chen et al., Faraday Discuss. 157, 285 (2012) 25

Reflection Absorption InfraRed Spectroscopy (RAIRS) 50 ML Amorphous Solid Water Crystalline ice 1 ML ASW on H/Pt(533) Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector free OH group 26

RAIRS vs EELS Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector 27

Temperature Programmed Desorption (TPD) Thermal Desorption Spectroscopy (TDS) Temperature Programmed Reaction Spectroscopy (TPRS)

Potential energy diagram O2/M(hkl) The depth of the invidual well depends on material surface structure adsorption site molecular orientation Dissociation energy in the gas phase O2 (g)  2 O(g) physisorption V(r) molecular chemisorption O2, chem The crossings of the adsorption curves determine whether a process is activated or not. O2, phys O2 (g) dissociative chemisorption adiabatic behavior Oads + Oads r

Potential energy surface O2/Pt(111) from: Groß, Eichler, Hafner, Mehl, and Papaconstantopoulos, Surf. Sci. 539, L542 (2003)

occupied fraction of the surface Desorption Desorption is the reverse process of adsorption Molecular or atomic desorption Recombinative desorption (opposite of dissociative adsorption) V(r) Rate of adsorption: occupied fraction of the surface Rate of desorption: A2 (g) A2, phys A2,chem 2 Achem r

Desorption Desorption is the reverse process of adsorption Molecular or atomic desorption Recombinative desorption (opposite of dissociative adsorption) V(r) Rate of adsorption: Rate of desorption: A2 (g) A2, phys A2,chem 2 Achem r

Langmuir isotherm Langmuir adsorption model Only one type of adsorption site All these sites are equivalent Only one adsorbate per site/no interactions At equilibrium rates add up to zero. non-dissociative dissociative

Temperature Programmed Desorption The experiment cool sample under UHV conditions expose it to the relevant gas heat the sample while monitoring desorption Quadrupole Mass Spectrometer track Aads spectroscopically … Polanyi-Wigner equation Note: and

H2O interaction with Pt[n(111)x(100)] Example H2O interaction with Pt[n(111)x(100)]

Temperature Programmed Desorption H2O/Pt(111) From: Hay et al., Surf. Sci. 505, 171 (2002)

Temperature Programmed Desorption H2O/D/Pt(S) H2O/Pt(S) Pt(111) Pt(533) Pt(755) Pt(977) From: Den Dunnen et al., PCCP 17, 8530 (2015)

‘complete analysis’

Temperature Programmed Desorption D2/Cu(211) vs D2/Cu(111) From: Kao, Kleyn and Juurlink, in preparation

Temperature Programmed Desorption D2/Cu(211) vs D2/Cu(111) From: Kao, Kleyn and Juurlink, in preparation

Temperature Programmed Desorption D2/Cu(211) vs D2/Cu(111) From: Kao, Kleyn and Juurlink, in preparation

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