lecture 23 thursday 4/1/08 ocnb explosion noble prize 2007 catalytic steps
TRANSCRIPT
Lecture 23 Thursday 4/1/08
OCNB ExplosionNoble Prize 2007Catalytic Steps
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rxA
CNCNCN
DHVrTTUA
dt
dT
00
0 ))(()(
p
rg
NC
dt
dT
9:55t = 0
10:40 10:50 midnight 12:18
Isothermal Operation
Cooling RestoredQr = 0
200
175
400
Tem
pera
ture
o C
Temperature-time trajectory
fuse
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.
Catalysts affect both Selectivity and Yeild
Steps in a Catalytic Reaction
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.
The Adsorption Step
The Surface Reaction Step
The Rate Limiting Step:Which Step Has the Largest
Resistance?
Octane Numbers
Catalytic Reforming
Normal Pentane Octane Number = 62
Iso-Pentane Octane Number = 95
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
1252105
105105
2105125
C-cC
step) (slow CC-n
CC-n
pentane-c pentane-n
52
HHHc
HcH
HHnH
Pt
OAl
Pt
Pt (sputtered on)
Al2O5 (scaffold)
CV=concentration of vacant sites
Postulate a mechanism: (single site)
In 75% of heterogeneous catalysis, surface reaction is the slow step
SISI
SISN
SNSN
VIISID
DIVISIDDN
SSISNSSN
NANNSNVVNADN
SNNVNNN
CPkCk
kCPCkrr
kCCkrr
kkkkCCPkr
CkCPkr
/
/
/ /
'
'
SISI
SISN
SNSN
VIISIDD
SSISNSS
NSNVNNAD
CPKCkr
kCCkr
kCCPkr
/
/
VIISIVIISIDD
VNNSNNSNVNNAD
CkPCCPkCkr
CkPCkCCPkr
0/
/0/
VDINNSS
VSIDNNSSN
CkPPkkr
CkPkPkkrr
/
/
Site Balance:
Is this consistent with the data?
Linear when PNi0 is low
Low slope when PNi0 is large
0
00 1
0@Ni
NiNiI kP
kPrP
Consistent!!
PNi0
rNi0
IINN
PIN
k
TSNSN
IINNVT
SISNVT
PkPk
kPPCkkrr
PKPkCC
CCCC
1
/
1
'
IINN
TV PkPk
CC
1