Lecture 23 Thursday 4/1/08 OCNB Explosion Noble Prize 2007 Catalytic Steps
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Transcript of Lecture 23 Thursday 4/1/08 OCNB Explosion Noble Prize 2007 Catalytic Steps
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Lecture 23 Thursday 4/1/08
OCNB ExplosionNoble Prize 2007Catalytic Steps
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rxA
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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
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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.
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Catalysts affect both Selectivity and Yeild
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Steps in a Catalytic Reaction
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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.
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The Adsorption Step
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The Surface Reaction Step
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The Rate Limiting Step:Which Step Has the Largest
Resistance?
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Octane Numbers
Catalytic Reforming
Normal Pentane Octane Number = 62
Iso-Pentane Octane Number = 95
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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)
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CV=concentration of vacant sites
Postulate a mechanism: (single site)
In 75% of heterogeneous catalysis, surface reaction is the slow step
SISI
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VIISIVIISIDD
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CkPCCPkCkr
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VDINNSS
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Site Balance:
Is this consistent with the data?
Linear when PNi0 is low
Low slope when PNi0 is large
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Consistent!!
PNi0
rNi0
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