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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)

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

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

repulsive Coulombic interactions

Pauli repulsion

attractive van der Waals interactions

Lennard-Jones Potential

physisorption

-DHads < 35 kJmol-1

potential energy

0

z

DHads

Potential energy profiles

Not activated Activated

Potential energy profiles

For an introduction to PE curves for adsorption see section 2.4 of:

http://www.chem.qmw.ac.uk/surfaces/scc/

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

Close-packed solids

ABA ABC

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)

Miller indices

Atkins & dePaula p. 700

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)

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