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

10.2 Kinetics of heterogeneous reactions (surface reactions) 224
b) Surface catalyzed unimolecular dissociation
I
Scheme:
A() +
−
|
S − 3
À
−3
−
A
|
S − 4
→ Products (10.60)
I Example: Decomposition of PH 3 on a W metal surface.
I
Decomposition rate and two limiting cases:
[P]
= 4 Θ = 4 × A
1+ A
(10.61)
(1) A ¿ 1: The reaction becomes first-order in A .
[P]
= 4 A (10.62)
(2) A À 1: The reaction becomes zeroth order, i.e., independent of A !
[P]
= 4 (10.63)
c) Bimolecular reactions
I
Scheme:
A() +
−
|
S −
À
−
−
A
|
S − (10.64)
−
A
|
S − + −
B() +
B
−
|
S − 5
→
|
S −
−
À
−
A − B
−
B
|
S − (10.65)
|
S − S − 6
→ Products (10.66)
I Reaction rate: Assuming that A and B compete for the same active sites, we have
(1 − Θ − Θ )= − Θ (10.67)
y
[P]
(1 − Θ − Θ )= − Θ (10.68)
= 5 Θ Θ =
5
(1 + + ) 2 (10.69)