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~  H cond (-  H AD <35 kJ/mol)  Chemisorption: formation of a chemical bond (covalent, ionic, metallic) with exchange of electrons and –  H AD >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. Lennard-Jones Potential physisorption z potential energy  ads < 35 kJmol -1 attractive van der Waals interactions repulsive Coulombic interactions Pauli repulsion  ads 0

5. Potential energy profiles Not activatedActivated

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 1s σ*σ* σ σ* orbitalsσ orbitals MetalAdsorbed molecule Free molecule

8. PHYSISORPTIONCHEMISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions (e.g. London dispersion, dipole-dipole).. STRONG, SHORT RANGE BONDING Chemical bonding involving orbital overlap and charge transfer. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury.  ads = 5 ….. 35 kJ mol -1  ads = 35 ….. 500 kJ mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. No surface reactions.Surface reactions may take place:- Dissociation, reconstruction, catalysis. MULTILAYER ADSORPTION BET Isotherm used to model adsorption equilibrium. MONOLAYER ADSORPTION Langmuir Isotherm used to model adsorption equilibrium.. 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 2 nd and 3 rd year lectures from D. Cunningham on naming crystal and solid structures

12. Overview bccbody-centred cubic fccface-centred cubic hcp hexagonal close packed

13. Close-packed solids ABAABC

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

15. Miller indices

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

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

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

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

27. Surface reconstruction