Transcript che 551 lectures

ChE 553 Lecture 29
Catalysis By Metals
1
Objective
• Apply what we have learned to
reactions on metal surfaces
2
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
3
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
5
Adsorption On Metals
• Molecular Adsorption
CO + S  COad
• Dissociative adsorption
– oxidative addition
H2 + 2S  2Had
6
Molecular vs Dissociative
Adsorption
Dissociated
CO
300
K
Sc Ti
V Cr Mn Fe
Y Zr Nb Mo Tc
La Hf Ta W
Dissociated
Molecular
Co Ni Cu
Ru Rh Pd Ag
Re Os Ir
N2
300
K
Pt Au
Dissociated
O2
300
K
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
Y Zr Nb Mo Tc Ru Rh
Key
Dissociative
Adsorption
Molecular
Adsorption
Pt Au
O2
100
K
V Cr Mn Fe Co Ni Cu
Y Zr Nb Mo Tc
Ru Rh Pd Ag
La Hf Ta W Re Os Ir Pt Au
Ir
Ni
Cu
Pd Ag
Pt Au
Molecular
And
Dissociative
Adsorption
Sc Ti
Molecular
V Cr Mn Fe Co
Y Zr Nb Mo Tc Ru Rh
La Hf Ta W Re Os Ir
Dissociated
Molecular
Sc Ti V Cr Mn Fe Co
La Hf Ta W Re Os
Sc Ti
Dissociated
Dissociated
NO
300
K
Activated
NO
100
K
Sc Ti
Y Zr Nb Mo Tc
Activated
Dissociative
Adsorption
Pd Ag
Pt Au
Molecular
V Cr Mn
La Hf Ta W
Ni Cu
Fe Co Ni Cu
Ru Rh Pd Ag
Re Os Ir
No
Adsorption
Pt Au
Limited
Data
Figure 5.12 The metals which dissociate CO, NO, H2, O2
and CO at various temperatures.
7
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)
8
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
H
H
H
C
H
H
H
C
H
H
H
H
H
H
H
C
H
H
H
C
C
H
H
H
C
H
H
H
H
H
H
H
H
H
H
H
C
H
H
C
C
H
H
C
H
H
H
H
C
C
H
H
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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
X + H 2  2H  X
H 2  2S  H (ad)
X  O 2  2O  X
O  H 2  OH  H
O 2  2S  2O (ad)
O (ad)  H 2  OH (ad)  H (ad)
H 2  OH  H 2 O  H
H  O 2  OH + O
X  2H  H 2  X
(5.152)
2O (ad)  H 2  2OH (ad)
O (ad)  H (ad)  OH (ad)
2OH (ad)  H 2 O  O (ad)
H (ad)  OH (ad)  H 2 O
H 2  2S  2H (ad)
(5.153)
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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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1.0
-0.5
0.0
-20
-10
0
10
Distance normal to the surface (bohr)
Key:
r s = 2 (Al)
Absolute Electron Density
Scaled Electron Density
Electrons In Metals Solvate
Radicals
1.0
-0.5
0.0
-20
-10
0
10
Distance normal to the surface (bohr)
r s = 5 (Cs)
= Positive Charge
Figure 14.18 The electron density extending out from a metal surface. (Note 1 bohr
=0.52Å)
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Metals Stabilize Intermediates
H
H
C
HC
H
H
HH
H H
C
C
H
H
C
H
H
H
H
C
C
H
C
H
H
C
H
HH
C
H
H
H
H H
H
H
C
C
H
H
H
H
C
C
HH
H
C
H
H
H
H
C
H
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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
Dissociated
CO
300
K
Sc Ti
V Cr Mn Fe
Y Zr Nb Mo Tc
La Hf Ta W
Dissociated
Molecular
Co Ni Cu
Ru Rh Pd Ag
Re Os Ir
N2
300
K
Pt Au
Dissociated
O2
300
K
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
Y Zr Nb Mo Tc Ru Rh
Key
Dissociative
Adsorption
Molecular
Adsorption
Pt Au
O2
100
K
V Cr Mn Fe Co Ni Cu
Y Zr Nb Mo Tc
Ru Rh Pd Ag
La Hf Ta W Re Os Ir Pt Au
Ir
Ni
Cu
Pd Ag
Pt Au
Molecular
And
Dissociative
Adsorption
Sc Ti
Molecular
V Cr Mn Fe Co
Y Zr Nb Mo Tc Ru Rh
La Hf Ta W Re Os Ir
Dissociated
Molecular
Sc Ti V Cr Mn Fe Co
La Hf Ta W Re Os
Sc Ti
Dissociated
Dissociated
NO
300
K
Activated
NO
100
K
Sc Ti
Y Zr Nb Mo Tc
Activated
Dissociative
Adsorption
Pd Ag
Pt Au
Molecular
V Cr Mn
La Hf Ta W
Ni Cu
Fe Co Ni Cu
Ru Rh Pd Ag
Re Os Ir
No
Adsorption
Pt Au
Limited
Data
Figure 5.12 The metals which dissociate CO, NO, H2, O2
and CO at various temperatures.
21
D-bands Help Tear Bonds
Apart
Figure 12.20 A diagram of the key interactions during the
dissociation of hydrogen on platinum.
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Activation Energy For H2
Dissociation
H-H
HH
H 2  2S  / \  | |
S-S
S-S
Metal
Pt
Heat of
dissociative
adsorption of H2
-13 kcal/mole
Intrinsic barrier for
dissociative
adsorption of H2
<6 kcal/mole
Si
-54 kcal/mole
~80 kcal/mole
Al
~-70 kcal/mole
>90 kcal/mole
K
~-70 kcal/mole
>90 kcal/mole
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Redox Chemistry
Co + C6 H5CH 3  Co
3+
(14.53)
2+
H
+ H + C6 H 5 C 
H
+
H
H
C 6 H 5 C  O 2  C 6 H 5 C OO 
H
H
(14.54)
H
H
2
C 6 H 5 C OO  Co  C 6 H 5 C  O  Co 3  OH 
H
(14.55)
OH -  H +  H 2 O
(14.56)
OH - + H +  H 2 O
(14.57)
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Metals Hold Reactants In
Correct Configuration To React
A
H H
HC H
H
*
B
H
C *O
H H
HC H
HC
*
OH
*
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.
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Metals Hold The Reactants In The
Correct Configuration To React
3C2 H 2  C6 H 6
(12.91)
C
C
C
C
C
C
Figure 12.15 The active site for
reaction (12.91) on a palladium catalyst.
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10
1
50
500
30
10 9
10 7
10 5
10 3
10 1
10 -1
10 -3
10 -5
10 -7
10 -9
10 -11
50
(111)
(100)
(410)
(210)
(110)
(111)
1000
Dissociation Temperature (íK)
E A For NO Dissociation
Rate Constant For
NO Dissociation
% Dissociation In TPD
Calculated Orbital Availability
% Dissociation
Dissociation Temperature
Turnover Number Extrapolated To 400íK (sec
100
100
Orbital Availability
Activaton Energy For NO Dissociation (kcal/mole)
2
-1)
Structure Sensitive Reactants
Face
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].
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Structure Sensitive Reactions
Rate (molecules/site/sec)
100
o
300 A
10
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]
1
0.1
0.01
0.001
0
20
40
60
80
100
120
140
o
Particle Size, A
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Different Reactions Show
Different Structure Sensitivity
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
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Practical Catalysts Are Supported
Structures With Multiple Exposed Faces
step
Figure 12.4 A picture of a supported metal
catalyst.
kink
adatom
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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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