ChE 553 Lecture 29Catalysis By Metals
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Objective
• Apply what we have learned to reactions on metal surfaces
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Metals Work By Same Mechanisms As Other Catalysts
• Metal catalysts can help initiate reactions• Metal catalysts can stabilize the intermediates of a
reaction• Metal catalysts can hold the reactants in close
proximity and in the right configuration to react• Metal catalysts can be designed to block side
reactions• Metal catalysts can stretch bonds and otherwise
make bonds easier to break• Metal catalysts can donate and accept electrons• Metal catalysts can act as efficient means for
energy transfer
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Mechanisms Of Reactions On Metals
• Generally metal catalyzed reactions follow catalytic cycle with adsorbtion, reaction, desorption
• Form adsorbed radicals
• Radicals react
• Molecules desorb
4
Typical Reactions On Metals
• Simple molecular adsorption reactions
• Dissociative adsorption reactions
• Bond scission reactions
• Addition reactions
• Recombination reactions
• Desorption reactions
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Adsorption On Metals
• Molecular Adsorption
CO + S COad
• Dissociative adsorption– oxidative addition
H2 + 2S 2Had
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Molecular vs Dissociative Adsorption
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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
O
300 K
Dissociated
O
100 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)
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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)
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Reactions Continued Hydrogen migration
CH2CH2(ad) + H(ad) CH3CH2(ad) + S (14.8)
Molecular desorption:
CO(ad) CO + S (14.9)
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Recombinative Desorption (Reductive Elimination)
CH3CH2(ad) + H(ad) CH3CH3 + 2S
(14.10)
2H(ad) H2 + 2S (14.11)
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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)
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-scission
R2CDCH2(ad) R2C=CH2 + D(ad)
(14.15)
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Example Catalytic Mechanisms: Olefin Hydrogenation
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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 C
HH H
Principles Of Catalytic Reaction
• Metals can help initiate reactions• Metals can stabilize the intermediates
of a reaction• Metals can hold the reactants in close
proximity and in the right configuration to react
• Metals can stretch bonds and otherwise make bonds easier to break
• Metals can donate and accept electrons
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Mechanism On Surface Similar To Radical Reactions In Gas Phase – But Radicals Bound To Surface
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X + H H X
X O 2O XO H OH H
H OH H O H
H O OH + O
X 2H H X
2
2
2
2 2
2
2
2
(5.152)
H S H
O 2S 2OO H OH H
2O H 2OH
O H OH
2OH H O O
H OH H O
H 2S 2H
2 (ad)
2 (ad)
(ad) 2 (ad) (ad)
(ad) 2 (ad)
(ad) (ad) (ad)
(ad) 2 (ad)
(ad) (ad) 2
2 (ad)
2
(5.153)
Electrons In Metals Solvate Radicals
• Metals are solvents for radicals. They lower the energy of radical species which allows initiation-propagation reactions to occur.
• Adsorbed radicals have lower energies than gas phase radicals which leads to higher concentrations.
• Intrinsic barriers of species adsorbed on metals similar to gas phase radicals.
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Electrons In Metals Solvate Radicals
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s
Distance normal to the surface (bohr)
Sca
led
Ele
ctro
n D
ensi
ty
0.0
-0.5
1.0
= 2 (Al)
-20 0-10 100.0
-0.5
1.0
Abs
olut
e E
lect
ron
Den
sity
-20 0-10 10
Distance normal to the surface (bohr)
r s = 5 (Cs)rKey: = Positive Charge
Figure 14.18 The electron density extending out from a metal surface. (Note 1 bohr =0.52Å)
Metals Stabilize Intermediates
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C
H
CHH
C
H
CHH
H H H H H H H HC
H
CHH
HH
C
H
CH H
HH
C
H
CHH
H
Multiple Radicals
One key difference between gas phase and surface is that di & tri radicals are stable on metals
N Gives possibilities
for interesting chemistry
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Metals Initiate Reactions
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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
O
300 K
Dissociated
O
100 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.
D-bands Help Tear Bonds Apart
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Figure 12.20 A diagram of the key interactions during the dissociation of hydrogen on platinum.
Activation Energy For H2 Dissociation
Metal Heat of dissociative adsorption of H2
Intrinsic barrier for dissociative adsorption of H2
Pt -13 kcal/mole <6 kcal/mole
Si -54 kcal/mole ~80 kcal/mole
Al ~-70 kcal/mole >90 kcal/mole
K ~-70 kcal/mole >90 kcal/mole 23
H SH - H/ \S - S
H H| |
S - S2 2
Redox Chemistry
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Co + C H CH Co + H + C HHCH
3+6 5 3
2+ +6 5
(14.53)
C HHCH
O C HHCH
OO6 5 2 6 5
(14.54)
C HHCH
OO Co C HHC O Co OH6 5
26 5
3
(14.55)
OH H H O- +2
(14.56)
OH + H H O- +2
(14.57)
Metals Hold Reactants In Correct Configuration To React
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CC O
H HH
HH
H
**
CC OH H
H
HH
H
**
A B
Figure 14.20 Balandin's suggested multiplet for the decomposition of ethanol a) to form ethylene b)to form acetyladehyde. The asterisks in the figures represent places on the surface where reaction can occur.
Metals Hold The Reactants In The Correct Configuration To React
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C
C
C
C
CC
3C2H2 C6H6
Figure 12.15 The active site for reaction (12.91) on a palladium catalyst.
(12.91)
Structure Sensitive Reactants
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1000
500
100 10
30
5010 -11
10 -9
10 -7
10 -5
10 -3
10 -1
10 1
10 3
10 5
10 7
10 9
Act
ivat
on E
nerg
y F
or N
O D
isso
ciat
ion
(kca
l/mol
e)
Dis
soci
atio
n T
empe
ratu
re (
íK)
% D
isso
ciat
ion
In T
PD
Cal
cula
ted
Orb
ital
Ava
ilab
ilit
y
Tur
nove
r N
umbe
r E
xtra
pola
ted
To
400í
K (
sec
-1)
(111
)
(110
)
(210
)
(410
)
(100
)
(111
)
Face
Orbital Availability
% Dissociation
Dissociation Temperature
EA For NO Dissociation
Rate Constant For NO Dissociation
2
1
100
50
Figure 14.22 The rate of nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. Adapted from Masel[1983].
Structure Sensitive Reactions
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140120100806040200
0.001
0.01
0.1
1
10
100
300
Particle Size,
Rat
e (m
olec
ules
/sit
e/se
c) o
A
Ao
Figure 14.21 The rate of the reaction
N2 + 3H2 2NH3
over an iron catalyst as a function of size of the iron particles in the catalyst. Data of Boudart et al [1975]
Different Reactions Show Different Structure Sensitivity
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Reaction Largest variation in
rate with geometry observed prior to
1999
2CO+O2 2CO2 6
C2H4+H2 C2H6 12
CH3OH CH2(ad)+H2O >100
C2H6+H2 2CH4 104
N2+3H2 2NH3 105
2NO+2H2 N2+2H2O ~1021
Practical Catalysts Are Supported Structures With Multiple Exposed Faces
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Figure 12.4 A picture of a supported metal catalyst.
step
kink
adatom
Summary
• Metals can help initiate reactions - in particular they facilitate bond scission processes
• Metals can stabilize the intermediates of a reaction particularly radical intermediates
• Metals can lower the intrinsic barriers to bond scission. The d-electrons promote bond scission and bond formation. The s-electrons promote redox chemistry.
• D’s convert forbidden reactions to allowed reactions. Produces tremendous rate enhancements.
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