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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~ D H cond (- D H AD <35 kJ/mol)

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bonding to surfaces
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
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
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

slide4

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
Potential energy profiles

Not activated Activated

potential energy profiles6
Potential energy profiles

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

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

mo diagram for chemisorption
MO diagram for chemisorption

σ orbitals

σ* orbitals

σ*

1s

1s

σ

Free molecule

Metal

Adsorbed molecule

surface science
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 solids14
Close-packed solids

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

hcp ccp (fcc)

bcc

slide16

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)

miller indices
Miller indices

Atkins & dePaula p. 700

fcc 100
fcc (100)

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

Rotate 45o

CN=8 for surface layer (12 for bulk)

fcc 110
fcc(110)

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

Rotate 45o

CN=7 for surface layer (12 for bulk)

fcc 111
fcc(111)

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

Rotate 45o

CN=9 for surface layer (12 for bulk)

surface energetics
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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