Adsorption, desorption, and diffusion on surfaces. Joachim Schnadt Divsion of Synchrotron Radiation Research Department of Physics

Transcription

1 Adsorption, desorption, and diffusion on surfaces Joachim Schnadt Divsion of Synchrotron Radiation Research Department of Physics

2 Adsorption and desorption

3 Adsorption Desorption Chemisorption: formation of a chemical (covalent, metallic, ionic) bond with the surface Physisorption: formation of a weak (van der Waals) bond with the surface ~ several ev / bond ~ some mev / atom Other types of bonds? Hydrogen bond, dipole/dipole interactions ~ some ten mev / bond

4 Chemisorption: formation of the chemical bond Molecular electronic states in the LCAO approximation: Anderson-Grimley-Newns model: K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience Where is the Fermi level?

5 Chemisorption: formation of the chemical bond K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

6 Task: construct the energy diagram of O 2 Naming convention 1: Naming convention 2: 3σ * 6σ * or 6σ 1π * 2π * 2π 1π 3σ 1π 5σ 1π 5σ 2σ * 4σ * 4σ 2σ 3σ 3σ Missing here: the O 1s core orbitals 1σ, 1σ * 1σ, 2σ * 1σ, 2σ

7 The energy diagram of CO 4σ Lowest unoccupied molecular orbital: LUMO Highest occupied molecular orbital: HOMO More common naming convention : 6σ * 2π * 5σ or 6σ 2π 5σ Energies (ionisation potentials, somewhat simplified): ev 1π 1π ev 4σ 4σ ev 3σ 3σ ~40 ev C 1s O 1s C 1s O 1s ev ev

8 The Blyholder model: adsorption of CO on a metal surface Upon bonding to surface: 6σ LUMO is downshifted, mixes with metal states (hybridises), and straddles the Fermi level, i.e. it becomes partially occupied HOMO is also downshifted and mixes with metal states (hybridises) 4σ 3σ 5σ This implies (Blyholder model): charge donation from 5σ to metal charge back donation from metal to 2π * occupation of the antibonding 2π * leads to weakening of C-O bond extent of C-O bond weakening depends on extent of charge transfer How could you probe this?

9 The Blyholder model: adsorption of CO on a metal surface A. Föhlisch et al., J. Chem. Phys. 112 (2000) 1946

10 The d-band model: oxygen adsorption energies (Hammer & Nørskov, Nature 376 (1995) 238, Adv. Catal, 45 (2000) 71) Two step concept of chemisorption: (a) weak chemisorption, followed by (b) strong chemisorption Location of d-band centre determines reactivity of metal surface: K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

11 Back to adsorption and desorption

12 Sticking coefficient Coverage θ = number of adsorbed molecules / number of molecules in a complete monolayer Sticking coefficient = adsorption rate / collision rate (distinguish between S(0) = lim θ 0 adsorption rate / collision rate and S(θ)) the coverage, The sticking coefficient is a function of surface structure, initial state of the adsorbate. N 2 /metal surfaces O 2 / stepped Pt surfaces O 2 /Pt(111)

13 Different pathways for adsorption (a), (b) scattering events (c) (direct) chemisorption (d) physisorption (a) scattering (b) (direct) chemisorption How can you experimentally distinguish between non-activated and activated adsorption? K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

14 Can you think of another adsorption mechanism? Precursor-mediated adsorption: adsorbate is trapped in a physisorption or dynamic state before it moves into a chemisorption state (only non-activated adsorption!) K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

15 Example for a potential energy surface for dissociative hydrogen adsorption A. Groß: Theoretical Surface Science

16 K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

17 Rate of adsorption and sticking coefficient

18 Rate of adsorption & sticking coefficient Assume non-interacting adsorbates. Rate of adsorption of gas phase atoms: r a =S(θ)F F = flux of incoming atoms, θ = coverage, S = sticking coefficient For direct adsorption: S(θ) = S(0) (1 - θ) What about molecules? The same if they don t dissociate upon adsorption! What if the molecules dissociate? S(θ) = S(0) (1 - θ) α α= 2 for dissociative adsorption of two-atomic molecules Arrhenius behaviour: S(0) = S 0 exp(-e A /kt)

19 Sticking coefficient for direct adsorption For non-ideal adsorption the model has to be adjusted, e.g. if the saturation coverage is different from 1, the existence of different sites on the surface, steric contraints, adsorption at coverages larger 1, etc. Example of sticking coefficient for dissociative adsorption with attractive interaction between the adsorbates K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

20 Adsorption via precursors

21 Adsorption via precursors α = 1: α = 2: α = 1: α = 2: K = k a* /k d *

22 Sticking coefficient for adsorption via a precursor G. A. Somorjai, Introduction to Surface Chemistry and Catalysis

23 Surface reactions

24 Mechanisms of surface reactions Langmuir-Hinshelwood Eley-Rideal

25 Two examples of the Langmuir-Hinshelwood mechanism G. Ertl, Ber. Bunsenges. Phys. Chem. 86 (1982) 425 K. W. Kolasinski: Surface Science Foundations of Catalysis and Nanoscience

26 An example for the Eley-Rideal mechanism: CO oxidation over Au clusters/mgo no barrier towards reaction 0.1 ev barrier 0.5 to 0.8 ev barrier Sanchez et al., J. Phys. Chem. A 103 (1999) 9573

27 Growth processes at surfaces γ s : free surface energy of substrate γ a : free surface energy of adsorbate overlayer γ i : substrate adsorbate interface energy Can you work out the relationship between these entities in the three cases?

28 Growth processes at surfaces γ a + γ i < γ s γ s < γ a + γ i γ a + γ i γ s changes sign at a critical layer thickness This is a purely macroscopic picture!!!

29 Experimental tool for determining the growth mode (no STM or AFM available )

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31 Desorption Rate of desorption for direct desorption: Rate coefficient: α = 1: Desorption via precursor state: α = 2:

32 Temperature-programmed desorption (TPD) β ~ K/s

33 TPD, continued α = 1: coverage-independent! α = 2: coverage-dependent!

34 TPD, continued Task: (a) Interpret this spectrum qualitatively. (b) Interpret the high temperature peak quantitatively. Attard & Barnes, Surfaces

35 Task When CO binds in sites of progressively higher coordination number (on top twofold bridge threefold hollow fourfold hollow) bot the π and σ contributions to bonding increase in magnitude. (a) Predict the trends that are expected in the CO stretching frequency and chemisorption bond energy with change of site. (b) When the π bonding interaction with the surface is weak, which adsorption site is preferred?

36 Diffusion

37 What is diffusion? random thermally activated motion of particles (atoms, molecules, nanoparticles,...) self-diffusion: motion of particles of a particular substance in a solute of the same substance; it is the only diffusion mechanism when no density gradient is present (no net mass transport!) chemical diffusion: diffusion connected with a net mass transport

38 What is diffusion important for? Surface reactions A. Groß, Theoretical Surface Science, Springer-Verlag Growth processes G. Ertl, Lecture at Nobel Symposium Energy in Cosmos, Molecules and Life, July 2005

39 Experimental techniques for studying diffusion Observation of the dynamics of single atoms/molecules: Field ion microscopy Scanning tunnelling microscopy (STM) Characteristics of the dynamics of single atoms/molecules: Helium scattering Observation of the dynamics in a non-equilibrium surface system: Essentially any surface science method!

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41 STM: diffusion of Pt/Pt(110) (1x2) TR Linderoth et al. Phys. Rev. Lett. 82 (1999) 1494 Aarhus Universitet

42 Different adsorbate states on a surface: which adsorbates diffuse? Chemisorption, physisorption, and precursors Image from J. V. Barth, Surf. Sci. Rep. 40 (2000) 75

43 Classical movement Quantum tunnelling T. Ala-Nissila, R. Ferrando, S. C. Ying Adv. Phys. 51 (2002) 949 Tekniska Högskolan, Helsingfors Brown University, Rhode Island Università di Genova only for hydrogen!

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45 Some theory... Following the treatment in A. Groß, Theoretical Surface Science, Springer-Verlag

46 A. Zangwill, Physics at Surfaces, Cambridge University Press

47 Initial state transition state final state T. Ala-Nissila, R. Ferrando, S. C. Ying Adv. Phys. 51 (2002) 949 Single jump diffusion on a fcc(100) metal surface. A. Groß, Theoretical Surface Science, Springer-Verlag.

48 What would you expect for the diffusion on a stepped surface? Ehrlich-Schwoebel-barrier G. Ehrlich, F. G Hudda, J. Chem. Phys. 44 (1966) 1039 R. K. Schwoebel, J. Appl. Phys. 40 (1969) 614

49 Single (nearest neighbour) jumps Long jumps Depends on the energy dissipation rate and the temperature! Images from T. Ala-Nissila, R. Ferrando, S. C. Ying Adv. Phys. 51 (2002) 949

50 Another mechanism: Exchange diffusion Initial state Transition state Final state Exchange diffusion on a fcc(100) metal surface. A. Groß, Theoretical Surface Science, Springer-Verlag.

51 A different view on downward diffusion at steps Image from T. Ala-Nissila, R. Ferrando, S. C. Ying Adv. Phys. 51 (2002) 949

52 Island diffusion of Pt(110) (2x1) T.R. Linderoth et al. Phys. Rev. Lett. 82 (1999) 1494 Aarhus Universitet

53 Single-particle diffusion coefficient and chemical diffusion coefficient

54 Experimental techniques for studying surface diffusion Local probe techniques Field ion microscopy Image from A. Zangwill, Physics at Surfaces. Cambridge University Press Originally from G. Ehrlich, CRC Crit. Rev. Sol. State Mat. Sci. 10 (1982) 391 Scanning tunnelling microscopy M. Schunack et al., University of Aarhus More STM movies on

55 Experimental techniques for studying surface diffusion Integrating methods Laser-induced thermal desorption

56 Experimental techniques for studying surface diffusion Integrating methods Quasielastic helium scattering Graphs from A.P. Graham, A. Menzel, J.P. Toennies, J. Chem. Phys. 111 (1999) 1676

57 Experimental techniques for studying surface diffusion Integrating methods Photoelectron emission microscopy [1-10] O/Pt(110) 50 µm x 190 µm Time between images: 120 s A. von Oertzen, H. H. Rotermund, S. Nettesheim, Surf. Sci. 311 (1994) 322

58 Growth processes at surfaces γ a + γ i < γ s γ s < γ a + γ i γ a + γ i γ s changes sign at a critical layer thickness This is a purely macroscopic picture!!!

59 At low atom mobility: thermodynamic equilibrium is not obtained at all times, diffusion processes become decisive!

60 Example: growth on Al(111) Activation energies and prefactors fcc (111): two different step edges A. Groß, Theoretical Surface Science, Springer-Verlag D = D 0 exp(-e A /k B T)

61 Simulation of growth of Al on Al(111) A. Groß, Theoretical Surface Science, Springer-Verlag

62 Want to know more? Text books: A. Groß, Theoretical Surface Science, Springer G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley K. W. Kolasinski, Surface Sciece Foundations of Catalysis and Nanoscience, Wiley Diffusion, experiment review (plus some theory): J. V. Barth, Surf. Sci. Rep. 40 (2000) 75 Diffusion, review from an experimentalist s point-of-view: A. G. Naumovets, Physica A 357 (2005) 189 Diffusion, theory review (easy to read!): T. Ala-Nissila, R. Ferrando, S. C. Ying, Adv. Phys. 51 (2002) 949