8. Intro to Catalysis
Reforming catalyst –
Algorithm
1) Postulate a mechanism
2) Postulate a rate-limiting step
3) Express Ci·s in terms of partial pressures
4) Site balance (Total number of sites constant)
5) Derive rate law
6) Evaluate parameters
12
5
2
10
5
10
5
10
5
2
10
5
12
5
C
-
c
C
step)
(slow
C
C
-
n
C
C
-
n
pentane
-
c
pentane
-
n
5
2
H
H
H
c
H
c
H
H
H
n
H
Pt
O
Al
Pt
Pt (sputtered on)
Al2O5 (scaffold)
9. CV=concentration of vacant sites
Postulate a mechanism: (single site)
In 75% of heterogeneous catalysis, surface reaction is the slow step
S
I
S
I
S
I
S
N
S
N
S
N
V
I
I
S
I
D
DI
V
I
S
I
D
D
N
S
S
I
S
N
S
S
N
N
A
N
N
S
N
V
V
N
ADN
S
N
N
V
N
N
N
C
P
k
C
k
k
C
P
C
k
r
r
k
C
C
k
r
r
k
k
k
k
C
C
P
k
r
C
k
C
P
k
r
/
/
/
/
'
'
S
I
S
I
S
I
S
N
S
N
S
N
V
I
I
S
I
D
D
S
S
I
S
N
S
S
N
S
N
V
N
N
AD
C
P
K
C
k
r
k
C
C
k
r
k
C
C
P
k
r
/
/
10. V
I
I
S
I
V
I
I
S
I
D
D
V
N
N
S
N
N
S
N
V
N
N
AD
C
k
P
C
C
P
k
C
k
r
C
k
P
C
k
C
C
P
k
r
0
/
/
0
/
V
D
I
N
N
S
S
V
S
I
D
N
N
S
S
N
C
k
P
P
k
k
r
C
k
P
k
P
k
k
r
r
/
/
Site Balance:
Is this consistent with the data?
Linear when PNi0 is low
Low slope when PNi0 is large
0
0
0
1
0
@
Ni
Ni
Ni
I
kP
kP
r
P
Consistent!!
PNi0
rNi0
I
I
N
N
P
I
N
k
T
S
N
S
N
I
I
N
N
V
T
S
I
S
N
V
T
P
k
P
k
k
P
P
C
k
k
r
r
P
K
P
k
C
C
C
C
C
C
1
/
1
'
I
I
N
N
T
V
P
k
P
k
C
C
1
15. Catalysts and Catalysis
A Catalyst is a substance that affects the
rate of chemical reaction but emerges
from the process unchanged.
Catalysis is the occurrence, study, and use
of catalysts and catalytic processes.
Approximately 1/3 of the GNP of materials
produced in the U.S. involves a catalytic
process.
20. Active Site
• Reactions are not catalyzed over the
entire surface but only at certain active
sites or centers that result from
unsaturated atoms in the surface.
• An active site is a point on the surface
that can form strong chemical bonds with
an adsorbed atom or molecule.
