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)

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Example Catalytic Mechanisms: Olefin Hydrogenation

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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

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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

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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

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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

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)

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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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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)

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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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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)

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-scission

 R2CDCH2(ad) R2C=CH2 + D(ad)

(14.15)

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Can usually predict mechanism on metal surfaces by just considering the 5 steps

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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?

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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

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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).

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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

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