Ludo Juurlink , Ph.D. L eiden Institute of Chemistry Leiden University, Leiden, the Netherlands

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 • 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

2. Surface Science and Gas-Surface Reaction Dynamics Schedule

3. Molecular Beams

4. 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 UHV and single crystals for studying catalysis

5. CH4/Ru(1120) N2/Ru(0001) Ciobica and van Santen, J.Phys. Chem.B106, 6200 (2002) Honkala et al., Science307, 555 (2005) Problems of 1) Strongly activated adsorption How does one perform research on highly activated adsorption using the control and techniques available with UHV technology?

6. H2/Pt(111) Nieto et al., Science312, 86 (2006) 2) high dimensionality of gas-surface reactions How does one learn about the influence of various d.o.f. in the reactant without loosing the benefits of UHV conditions?

7. CH4/Ir(111) Henkelman and Jonnson, Phys. Rev. Lett.86, 664 (2001) 3) coupled degrees of freedom 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. Effusive sources Supersonic sources Molecular beams in general nozzle skimmer 70 m • 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. 300 K CH4/He He 600 K higher Mave lower Mave 1000 K For a molecule with mass M1, dilutely seeded in an ideal gas with mass M2: For stationary, adiabatic expansion of a perfect gas: For an monoatomic gas: Mach number: The supersonic expansion lower Tnozzle higher Tnozzle

13. The supersonicexpansion • 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. Determining kinetic energy and reactivity

15. Construction of SMB/UHV machines

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

17. DeterminingtheS0and 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. DeterminingtheS0and S() Pt C • Detection of adsorbed products • Direct measurement (AES, XPS) CH4/Pt(111) S0(E1) S0(E2) S0(E3) S0(E4) S0(E5) Oakes, McCoustra, and Chesters, Faraday Discuss.96, 325 (1993)

19. Detection of adsorbed products Direct measurement (RAIRS) DeterminingtheS0and S() Chen, Ueta, Bisson, and Beck, Faraday Discuss.175, 285 (2012)

20. Detection of adsorbed products Indirect measurement (TPD, titration) DeterminingtheS0and S() CH4/Pt(533) B. Riedmüller, Ph.D. thesis “Activation barriers in gas-surface reactions” with A.W. Kleyn, Leiden University (2001)

21. DeterminingtheS0and S() • Detection of scatteredflux, e.g. by laser-basedtechniques 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. 1 absolute reactivity 0 kinetic energy (kJ/mol) 200 0 Ekin dependence of reactivity Indirect: Direct: aka: reaction probability, sticking probability ´direct´ ´indirect´

24. EkinandTvibdependence 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’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’s rules SURFACE 1 SURFACE 2 Ekin= 1.5 kcal/mol Evib= 7.5 Ekin= 9.0 Evib= 0.0 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. EkinandTvibdependence 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 barrierdistribution absolute reactivity barrier distribution 1 Exponential increase ln(S0) vs Ekin is linear 0 200 0 kinetic energy (kJ/mol) S-curve indirect direct

29. Parallel reaction paths H2/Pt(211) H2 → 2 Hads Groot, Schouten, Kleyn, and Juurlink, J. Chem. Phys.129, 224707 (2008)

31. Some early studies

32. Stronglyactivateddissociation 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. Weaklyactivateddissociation of hydrogen • H2/Ru(0001) There are still significant discrepancies between theory and experiment, even for very ‘simple’ systems. Reactivity for a moderately or non activated system may not reach unity at high Ekin. Groot et al., J. Chem. Phys. 127, 244701 (2007)

34. Activateddissociation of nitrogen • 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)