1. 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
phone
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

2. 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

3. Molecular Beams

4. UHV and single crystals for studying catalysis
Ultrahigh vacuum
~10-10 mbar
Minutes to hours to perform experiments on clean surfaces
Use of photons, electrons and ions in surface sensitive investigations
Technology is readily available, techniques well established
Single Crystals
Models for planes, steps and kinks present on real catalytic particles
Extremely well characterized
Accurate cutting for well oriented planes (<0.1º)
Affordable

5. Problems of 1) Strongly activated adsorption
Honkala et al., Science 307, 555 (2005)
N2/Ru(0001)
Ciobica and van Santen, J.Phys. Chem. B106, 6200 (2002)
CH4/Ru(1120)
How does one perform research on highly activated adsorption using the control and techniques available with UHV technology?

6. 2) high dimensionality of gas-surface reactions
Nieto et al., Science 312, 86 (2006)
H2/Pt(111)
How does one learn about the influence of various d.o.f. in the reactant without loosing the benefits of UHV conditions?

7. 3) coupled degrees of freedom
Henkelman and Jonnson, Phys. Rev. Lett. 86, 664 (2001)
CH4/Ir(111)
How can one separate d.o.f. in gas phase and substrate reactant while allowing for techniques available with UHV conditions?

8. Benefits of (supersonic) molecular beam techniques
Independent control of
substrate temperature
kinetic energy of gas phase reactant
reactant’s impact angles on the surface and, with the appropriate lasers, also over
vibrational and rotational state
impact angle of the vibrational oscillation
Extreme control in localized deposition

9. Molecular beams in general
Effusive sources
Supersonic sources
70 m
nozzle
skimmer
Differentially pumped system with nozzle far away from sample
Pulsed and continuous beams
Localized deposition with kinetic energy and impact angle control
Doser may be suspended in UHV
Localized deposition with a Maxwell distribution of speeds at room temperature

10. Two good introductory reviews
Morse, in Experimental methods in physical sciences, vol 29B
Kleyn, Chem. Soc. Rev. 2003, 23, 87-95

11. The supersonic expansion

12. The supersonic expansion
higher Mave
lower Mave
CH4/He He
300 K
600 K
1000 K
lower Tnozzle
higher Tnozzle
Mach number:
For stationary, adiabatic expansion of a perfect gas:
For a molecule with mass M1, dilutely seeded in an ideal gas with mass M2:
For an monoatomic gas:

13. The supersonic expansion
Efficient cooling of kinetic energy spread
Efficient cooling of rotational energy spread for closely spaced rotational energy levels
Poor cooling of vibrational energy spread
→ very far out of equilibrium
Rotational energy levels:
Vibrational energy levels:

14. kinetic energy and reactivity
Determining
kinetic energy and reactivity

15. Construction of SMB/UHV machines
March 20, 2012
Catalytic Surface Science - JuurlinkMarch 20, 2012
Construction of SMB/UHV machines
11/9/2018

16. Determining the Ekin Time-of-flight spectroscopy
If available, a change of neutral path length
Otherwise, a complete fit of the velocity distribution

17. Determining the S0 and S()
Detection of reacted flux (QMA)
Pdrop ΔP
King & Wells measurements are applicable in the range
S0 = ~0.01 – 1
Complications arise when chamber walls also “pump” the reactant.

18. Determining the S0 and S()
Detection of adsorbed products
Direct measurement (AES, XPS)
CH4/Pt(111) Pt C
S0(E1)
S0(E2)
S0(E3)
S0(E4)
S0(E5)
Oakes, McCoustra, and Chesters, Faraday Discuss. 96, 325 (1993)

19. Determining the S0 and S()
Detection of adsorbed products
Direct measurement (RAIRS)
Chen, Ueta, Bisson, and Beck, Faraday Discuss. 175, 285 (2012)

20. Determining the S0 and S()
Detection of adsorbed products
Indirect measurement (TPD, titration)
CH4/Pt(533)
B. Riedmüller, Ph.D. thesis “Activation barriers in gas-surface reactions” with A.W. Kleyn, Leiden University (2001)

21. Determining the S0 and S()
Detection of scattered flux, e.g. by laser-based techniques
A wealth of information, but extremely tedious.
Gostein, Parhiktheh, and Sitz, Phys. Rev. Lett. 75, 342 (1995)

22. Studying the mechanism of reaction and dependencies on various forms of energy

23. Ekin dependence of reactivity
kinetic energy (kJ/mol)
absolute reactivity 1
200
´direct´
´indirect´
aka:
reaction probability, sticking probability
Indirect:
Direct:

24. Ekin and Tvib dependence of reactivity
CH4/Ni(111) and Ni(100)
Lee, Yang, and Ceyer, J. Chem. Phys. 87, 2724 (1987)
Holmblad, Wambach, and Chorkendorff, J. Chem. Phys. 102, 8255 (1995)

25. Polanyi en Wong, J.Chem.Phys. 51, 1439 (1969)
Polanyi’s rules
Model surface 1:
Model surface 2:
Entrance channel barrier (“early”) of 7.0 kcal/mol
Exit channel barrier (“late”) of 7.0 kcal/mol
Polanyi en Wong, J.Chem.Phys. 51, 1439 (1969)

26. Polanyi en Wong, J.Chem.Phys. 51, 1439 (1969)
Polanyi’s rules
SURFACE 1
SURFACE 2
Ekin= 9.0
Evib= 0.0
Ekin= 1.5 kcal/mol
Evib= 7.5
Early barriers are surpassed more easily by kinetic energy in the reactants.
Late barriers are more easily surpassed by vibrational energy in the reactants.
Ekin= 1.5
Evib= 14.5
Ekin=16.0
Evib= 0.0
Ekin= 1.5
Evib= 14.5
Ekin= 16.0
Evib= 0.0
Polanyi en Wong, J.Chem.Phys. 51, 1439 (1969)

27. Ekin and Tvib dependence of reactivity
CH4/Ni(111) and Ni(100)
Lee, Yang, and Ceyer, J. Chem. Phys. 87, 2724 (1987)
Holmblad, Wambach, and Chorkendorff, J. Chem. Phys. 102, 8255 (1995)

28. Integrating over a barrier distribution
kinetic energy (kJ/mol)
absolute reactivity
barrier distribution 1
S-curve
indirect
direct
Exponential increase
ln(S0) vs Ekin is linear
200

29. Parallel reaction paths
H2/Pt(211)
H2 → 2 Hads
Groot, Schouten, Kleyn, and Juurlink, J. Chem. Phys. 129, (2008)

31. Some early studies

32. Strongly activated dissociation of hydrogen
D2/Cu(111)
Below ~50 kJ/mol, D2 only dissociates appreciably when it is vibrationally excited. Vibrationally exctited states have lower barriers to react.
Rettner, Auerbach, and Michelsen, Phys. Rev. Lett. 68, 1164(1992)

33. Weakly activated dissociation of hydrogen
H2/Ru(0001)
Reactivity for a moderately or non activated system may not reach unity at high Ekin.
There are still significant discrepancies between theory and experiment, even for very ‘simple’ systems.
Groot et al., J. Chem. Phys. 127, (2007)

34. Catalytic Surface Science - JuurlinkMarch 20, 2012
Activated dissociation of nitrogen
11/9/2018
N2/Ru(0001)
N2 dissociation seems completely dominated by defect sites.
Egeberg, Larsen, and Chorkendorff,
PCCP 3, 2007 (2007)
Dahl et al., Phys. Rev. Lett. 83, 1814 (1999)