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Objective Start to talk about mechanisms of reactions on solid surfaces
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ChE 553 Lecture 16Mechanisms Of Surface
Reactions
1
Objective• Start to talk about mechanisms of
reactions on solid surfaces
2
Mechanisms Of Reactions On Surfaces
• Generally surface reactions follow catalytic cycle with adsorption, reaction, desorption
• Form adsorbed radicals• Radicals react• Molecules desorb
3
HH
H
H CC H
H
HH
H
C CH
H H
HH
HC
H
CHH
HH
HH
HH
HC
H
CHH
HH
C
H
CHH
HH
H
H
C CH
H H
4
Generic Types Of Surface Reactions
B
Langmuir-Hinshelwood Rideal-Eley Precursor
A
B A
A B
A
A B
A BB AB AA B
ABAB
AB
ABAB
AB
ABAB
AB
Most catalytic reactions Most film growth reactions
Rules Of Thumb• Reactions on solid catalysts (-600C)
usually go by Langmuir-Hinshellwood• Reactions in semiconductor processing
usually Rideal- Eley• Reactions on enzymes often precursor
(can also be Langmuir)
5
Example Catalytic Mechanisms: Olefin Hydrogenation
6
HH
H
H C C H
H
HH
H
C CH
H H
HH
HC
H
CHH
HH
HH
HH
HC
H
CHH
HH
C
H
CHH
HH
H
H
C CH
H H
Transfer one atom at a time
7
Example CH3OHCO+2H2
O CH
- H
- H
- H
OC
H
OCHH H
Methoxy
Formaldehyde
OC
H
CarbonMonoxide
OCH
H
H
H
AdsorbedMethanol
- H
Formyl
CO CH OH3
Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).
Transfer one atom at a time
8
The Mechanism Of Ethanol Decomposition On Pt(111)
O C
+ H
- H
OC
H
HH
Ethoxy
Acetaldehyde
OC
Methyl +CarbonMonoxide
OHH
H
AdsorbedEthanol
- H
Acetyl
CO CH OH
CHH H
CC H
OHC C
H H
HH
CHH H
CHH H
4CH
3
- H
Transfer one atom or one ligand at a time
Typical Reactions On Metals• Simple molecular adsorption reactions• Dissociative adsorption reactions• Bond scission reactions• Addition reactions• Recombination reactions• Desorption reactions
9
Adsorption On Metals• Molecular Adsorption
CO + S COad
• Dissociative adsorption– oxidative addition
H2 + 2S 2Had
10
Molecular vs Dissociative Adsorption
11
2
2 2
DissociativeAdsorption
MolecularAdsorption
NoAdsorption
Molecular And
DissociativeAdsorption
ActivatedDissociativeAdsorption
LimitedData
PtIrOsRe
PdRhRu
NiCo
AuPt
AgPd
CuNi
Ir
Ni
AuPtIrOsRe
AgPdRhRu
CuNi
Sc Ti V Cr Mn Fe
Y Zr Nb Mo Tc
La Hf Ta W
CO300 K
Dissociated
Molecular
Sc Ti V Cr Mn Fe
Y Zr Nb Mo Tc
La Hf Ta W
N
Dissociated Molecular
Ti V Cr Mn Fe Co
Zr Nb Mo Tc Ru
Hf Ta W Re
NO300 K
Dissociated Molecular
Sc
Y
La
Rh
Os
Sc Ti V Cr Mn
Y Zr Nb Mo Tc
La Hf Ta W
NO100 K
Dissociated
Sc Ti V Cr Mn Fe
Y Zr Nb Mo Tc
La Hf Ta W
300 K
Dissociated Activated
Sc Ti V Cr Mn Fe Co Ni Cu
Y Zr Nb Mo Tc Ru Rh Pd Ag
La Hf Ta W Re Os Ir Pt
O300 K
Dissociated
O100 KAu
Key
AuPt
AgPd
Cu
IrOsRe
RhRu
Co
Fe Co
Molecular
Au
Ag
Cu
AuPtIrOsRe
Ag
CuNiCo
PdRhRu
Figure 5.12 The metals which dissociate CO, NO, H2, O2 and CO at various temperatures.
Bond Fragmentation Reactions
CH3CH2OH(ad) + S CH3CH2O(ad) +H(ad) CH3CH3O(ad) + S CH3CHO(ad) + H(ad) CH3CHO(ad) + S CH3CO(ad) + H(ad) CH3CO(ad) + S CO(ad) + CH3(ad)
(14.4)
CH3CH2CO(ad) + S CH3CH2(ad) + CO(ad) (14.5)
12
Association Reactions
CH3CH2(ad) + CO(ad) CH3CH2CO(ad) + S (14.6)
Combined displacement-association reactions
CO + CH3CH2(ad) + CO(ad) CO(ad) + CH3CH2CO(ad)
(14.7)
13
Typical on metal
surfaces
Typical on metal
clusters
Reactions Continued Hydrogen migration
CH2CH2(ad) + H(ad) CH3CH2(ad) + S (14.8)
Molecular desorption:
CO(ad) CO + S (14.9)
14
Recombinative Desorption (Reductive Elimination)
CH3CH2(ad) + H(ad) CH3CH3 + 2S
(14.10)
2H(ad) H2 + 2S (14.11)
15
Ridel-Eley Displacement Reaction
CH3CH2(ad) + H2 CH3CH3 + H(ad)
(14.12)
CO + 2 H(ad) H2 +CO(ad) (14.13)
CO + CH2CH3(ad) + H(ad)
CH3CH3 + CO(ad) (14.14)
16
-scission
R2CDCH2(ad) R2C=CH2 + D(ad)
(14.15)
17
Can usually predict mechanism on metal surfaces by just considering the 5 steps
18
HH
H
H C C H
H
HH
H
C CH
H H
HH
HC
H
CHH
HH
HH
HH
HC
H
CHH
HH
C
H
CHH
HH
H
H
C CH
H H
Dissociative adsorption
Molecular adsorption
Associative hydrogen migration
Associative Desorption
What Happens If We Run The Reaction The Opposite Way?
19
Associative recombination
Molecular desorption
β-elimination
Dissociative Adsorption
HH
H
H C C H
H
HH
H
C CHH H
HH
HC
H
CHH
HH
HH
HH
HC
H
CHH
HH
C
H
CHH
HH
H
H
C CH
H H
20
Example CH3OHCO+2H2
O CH
- H
- H
- H
OC
H
OCHH H
Methoxy
Formaldehyde
OC
H
CarbonMonoxide
OCH
H
H
H
AdsorbedMethanol
- H
Formyl
CO CH OH3
Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).
21
Example: Ethanol Decomposition On Pt(111)
O C
+ H
- H
OC
H
HH
Ethoxy
Acetaldehyde
OC
Methyl +CarbonMonoxide
OHH
H
AdsorbedEthanol
- H
Acetyl
CO CH OH
CHH H
CC H
OHC C
H H
HH
CHH H
CHH H
4CH
3
- H
Transfer one atom or one ligand at a time
Summary• Reactions on surfaces go by Rideal-Eley or
Langmuir Hinschelwood Mechanisms– RE film growth– LH catalysis
• Reactions on metals– Only 5 characteristic reactions– Usually transfer a ligand or atom to or from surface– No direct isomerizations
22