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

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

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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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