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~ D H cond (- D H AD <35 kJ/mol)
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Enthalpy of chemisorption depends strongly on surface coverage (interactions)
Spectroscopic techniques (see later) are used to confirm chemisorption
(IR or XPS for example)
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
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
attractive van der Waals interactions
-DHads < 35 kJmol-1
For an introduction to PE curves for adsorption see section 2.4 of:
Adsorption and Reaction at Surfaces
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
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
Miller Indices : (100)
Atkins & dePaula p. 700
CN=8 for surface layer (12 for bulk)
CN=7 for surface layer (12 for bulk)
CN=9 for surface layer (12 for bulk)
a high surface atom density
surface atoms of high coordination number
fcc (111) > fcc (100) > fcc (110)