Bonding to surfaces

1. Bonding to surfaces • Two classifications distinguished by the magnitude of their enthalpies of adsorption • Physisorption: long-range but weak van der Waals-type interactions with negligible exchange of electrons and enthalpies~DHcond (-DHAD<35 kJ/mol) • Chemisorption: formation of a chemical bond (covalent, ionic, metallic) with exchange of electrons and –DHAD>35 kJ/mol Enthalpy of chemisorption depends strongly on surface coverage (interactions) Spectroscopic techniques (see later) are used to confirm chemisorption (IR or XPS for example)

2. Adsorption kinetics Probability of molecule being associatively adsorbed may defined in terms of a sticking probability, s. We can write: Thus the greater the number of vacant sites, the greater is s. However, s is often not a linear function of θ. Invoke the precursor state

3. Precursor state If adsorbate collides with the surface and doesn’t stick, it may not simply rebound, but rather form a weak bond (physisorption) and diffuse for a period (losing energy) until a vacant site is located for chemisorption to occur. If the weak bond is initially at a vacant site one refers to intrinsic precursor states, whereas if it is at an occupied site this corresponds to extrinsic precursor states. For adsorption to proceed, the gas needs to “dump” energy into the solid, if not it will desorb. The longer the gas molecule resides on the surface, the more likely is energy exchange with the surface. Can write an Arrhenius-type relationship between residence time and the enthalpy of adsorption at the precursor adsorption site. See p.11 Attard & Barnes

4. repulsive Coulombic interactions Pauli repulsion attractive van der Waals interactions Lennard-Jones Potential physisorption -DHads < 35 kJmol-1 potential energy 0 z DHads

5. Potential energy profiles Not activated Activated

6. Potential energy profiles For an introduction to PE curves for adsorption see section 2.4 of: http://www.chem.qmw.ac.uk/surfaces/scc/

7. MO diagram for chemisorption σ orbitals σ* orbitals σ* 1s 1s σ Free molecule Metal Adsorbed molecule

8. Adsorption and Reaction at Surfaces

9. Surface structure

10. Surface defects

11. Surface science In order to ensure reproducible results between experiments precise definition of the chemical and structural state of the surface: well-defined surfaces required. Review 2nd and 3rd year lectures from D. Cunningham on naming crystal and solid structures

12. Overview

13. Close-packed solids ABA ABC

14. Close-packed solids http://www.chem.qmw.ac.uk/surfaces/scc/ hcp ccp (fcc) bcc

15. Miller indices

16. Miller indices Identify the intercepts on the x- , y- and z- axes x = a (at the point (a,0,0) ) parallel to the y- and z-axes Intercepts :    a , ,  Specify the intercepts in fractional co-ordinates In the case of a cubic unit cell each co-ordinate will simply be divided by the cubic cell constant a a/a , /a, /a    i.e.    1 ,  ,  Take the reciprocals of the fractional intercepts yielding Miller Indices :   (100)

17. Try these

18. And this

19. Miller indices Atkins & dePaula p. 700

20. fcc (100) http://www.chem.qmw.ac.uk/surfaces/scc/ Rotate 45o CN=8 for surface layer (12 for bulk)

21. fcc(110) http://www.chem.qmw.ac.uk/surfaces/scc/ Rotate 45o CN=7 for surface layer (12 for bulk)

22. fcc(111) http://www.chem.qmw.ac.uk/surfaces/scc/ Rotate 45o CN=9 for surface layer (12 for bulk)

23. Surface energetics • The most stable solid surfaces are those with : a high surface atom density surface atoms of high coordination number expect fcc (111) > fcc (100) > fcc (110)

24. Intersection of surfaces

25. Surface relaxation

26. Surface relaxation

27. Surface reconstruction