10. Catalysis &
Catalytic Reactiono Basic Define
- Catalyst, catalytic mechanism, rate limit step.
o Catalytic Mechanism
- Describe the steps
- Derive a rate law and a mechanism and rate
limiting step consistent with the experimental data
o Use Regression to discriminate between reaction
rate laws and mechanisms
2. Steps in a Catalytic Reaction X
May/26 2011 Spring 2
o Step 4 : Surface Reaction 1
o Rate of adsorption of species A onto a solid surface
o Single site
- Only the site on which the reactant is adsorbed is
involved in the reaction
A
SCOCOAAD
K
CCPkr v
SASA
S
SBSASS
K
CCkr
2. Steps in a Catalytic Reaction XI
May/26 2011 Spring 3
o Dual site
- The adsorbed reactant interacts with another site
(either occupied or unoccupied)
S
SBSASS
K
CCCCkr vv
SSBSSA
SDSCSBSA
S
SDSCSBSASS
K
CCCCkr
2. Steps in a Catalytic Reaction XII
May/26 2011 Spring 4
o Dual site 2
SDS'CS'BSA
S
SDS'C'SBSASS
K
CCCCkr
☞ Langmuir-Hinshelwood Mechanism- Single or dual site mechanism reaction
2. Steps in a Catalytic Reaction XIII
May/26 2011 Spring 5
o Eley-Rideal
- Reaction between an adsorbed molecule and a
molecule in the gas phase
SCB(g)SA
S
SCBSASS
K
CPCkr
2. Steps in a Catalytic Reaction XIV
May/26 2011 Spring 6
o Summary
Single site
a
Dual site
b
c
Eley-Rideal
d
SASA
SCB(g)SA
SSBSSA
SDSCSBSA ''
BBAA
A'
1 PKPK
kPrA
2
BBAA
A'
)1( PKPK
kPrA
2
CCBBAA
BA'
)1( PKPKPK
PkPrA
CCAA
BA'
1 PKPK
PkPrA
2. Steps in a Catalytic Reaction XV
May/26 2011 Spring 7
o Plot 1
2. Steps in a Catalytic Reaction XVI
May/26 2011 Spring 8
o Plot 2
2. Steps in a Catalytic Reaction XVII
May/26 2011 Spring 9
o Plot 3
2. Steps in a Catalytic Reaction XIIIV
May/26 2011 Spring 10
o Plot 4
2. Steps in a Catalytic Reaction XIX
May/26 2011 Spring 11
o Desorption
- Desorption rate constant for C is just reverse of the
adsorption step, rDC = -rADC
- Desorption equilibrium constant for C is just
reciprocal of the adsorption equilibrium constant
SCSC
DC
CSCDDC
K
CPCkr v
C
DC
1
KK
vCPKCkr CCSCDDC
2. Steps in a Catalytic Reaction XX
May/26 2011 Spring 12
o Rate-Limiting Step
- At steady state
- One particular step in the series is found to be rate-
limiting or rate-controlling
☞ Studied by Langmuir-Hinshelwood approach
• adsorption-limited reaction
• surface-limited reaction
• desorption-limited reaction
DSADA rrrr '
2. Steps in a Catalytic Reaction XXI
May/26 2011 Spring 13
o Rate-Limiting Step 2
- Example of adsorption-limited reaction
322 2NHN3H
Rapid S2H2SH2
limitingRateS2NSSN
SNSN
2
22
Rapid
SNHSNH
SSNHSHSNH
SSNHSHSNH
SSHNSHSN
33
32
2
3. Rate Law, Mechanism, and RLS I
May/26 2011 Spring 14
o Modeling Reaction
- Decomposition of cumene to benzene and propylene
63662356 HCHC)CH(CHHC
3. Rate Law, Mechanism, and RLS II
May/26 2011 Spring 15
o Modeling Reaction 2
- Treat each step as an elementary reaction
- The species concentrations in the gas phase are
replaced by their respective partial pressures
- Adsorption of cumeneCC PC
SCACAAD CkCPkr
C
SCCAAD: Adsorption
K
CCPkr v
3. Rate Law, Mechanism, and RLS III
May/26 2011 Spring 16
o Modeling Reaction 3
- Surface reaction
- Desorption
• Propylene is not adsorbed on the surface
SBPSSCSS CPkCkr
S
SBPSCSS:reaction Surface
K
CPCkr
0SPC vCPkCkr BDSBDD
DB
BSBDD: Desorption
K
CPCkr v
3. Rate Law, Mechanism, and RLS IV
May/26 2011 Spring 17
o Modeling Reaction 4
- Desorption 2
- No accumulation of reacting species on the surface
DB
B
1
KK
vCPKCkr BBSBDD: Desorption
DSAD
'
C rrrr
3. Rate Law, Mechanism, and RLS V
May/26 2011 Spring 18
o Modeling Reaction 5
- Adsorption is RLS?
• surface reaction rate is
• surface specific reaction rate is large by comparison
C
SCCAAD
'
CK
CCPkrr v
S
PSBSCSS
K
PCCkr
0S
S k
r
3. Rate Law, Mechanism, and RLS VI
May/26 2011 Spring 19
o Modeling Reaction 5
- Adsorption is RLS? 2
• solving for CC•S
• to obtain CB•S
• desorption rate constant is large by comparison
• solving for CB•S
• solving for CC•S
S
PSBSC
K
PCC
vCPKCkr BBSBDD
0D
D
k
r
vCPKC BBSB
vCK
PPKC
S
PBBSC
3. Rate Law, Mechanism, and RLS VII
May/26 2011 Spring 20
o Modeling Reaction 6
- Adsorption is RLS? 3
• replacing CC•S
• let
• total site
• substituting
vv CK
PPPkC
KK
PPKPkr
P
PBCA
CS
PBBCAAD
P
B
CS KK
KK
SBSC CCCC vt
BBSBPB /1 PKKKPP
CC tv
3. Rate Law, Mechanism, and RLS VIII
May/26 2011 Spring 21
o Modeling Reaction 7
- Adsorption is RLS? 4
• reaction rate = rate of adsorption
• substituting BBSPBB
PBPCAAD
'
C/1
/
PKKPPK
KPPPkCrr t
C
SCCAAD
'
CK
CCPkrr v
vCK
PPK
K
PCC
S
PBB
S
PSBSC
vCPKC BBSB
BBSBPB /1 PKKKPP
CC tv
3. Rate Law, Mechanism, and RLS IX
May/26 2011 Spring 22
o Modeling Reaction 8
- Surface Reaction is RLS?
• reaction rate = rate of surface reaction
- Desorption is RLS?
• reaction rate = rate of desorption
CCBB
PBPCCSS
'
C1
/
PKKP
KPPPKCkrr
k
t
CPCSCCP
PPBCCSDD
'
C
/
PPKKKPP
KPPPKKCkrr
k
t