Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

10.2 Kinetics of heterogeneous reactions (surface reactions) 219
— example: Ar on a metal or oxide surface.
• Chemisorption:
— formation of a chemical bond between adsorbate and surface.
— typical bond energy: ∆ ≤−100 − 300 kJ mol −1 ,
— example: CO on Pd:
O
||
C
|
− Pd − Pd −
(10.35)
∗ chemical bond by overlap of orbital of the C atom with free bands of
Pd,
∗ back donation from occupied bands into ∗ orbital of the CO substrate.
∗ Experiment shows that (Pd − C=O)≈ 236 cm −1 .
I
Figure 10.3: Potential curves for physisorption and chemisorption.
10.2.3 The Langmuir adsorption isotherm
I
Reaction scheme:
A() +
−
|
S − 1
À
−1
−
A
|
S − (10.36)
− | S− = “active surface site” (10.37)