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

Molecular Beams

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

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?

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?

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?

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

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

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

The supersonic expansion

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:

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:

kinetic energy and reactivity Determining kinetic energy and reactivity

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

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

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.

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)

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

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)

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)

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

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

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)

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)

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)

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)

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

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

Some early studies

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)

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, 244701 (2007)

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)