ChE 553 Lecture 13 Introduction To Surface

Reactions

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Objective

• Start To Discuss Surface Reactions– Overview of mechanism– Typical reactions on metals– Highlight well studied examples– Qualitative trends with changing

composition and structure

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Topics

• General Mechanisms Of Surface

Reactions– Mechanisms look like gas phase reactions– Need surface site

• Effect of Surface Structure

• Effect of composition

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Some Examples Of Reactions On Metal Surfaces

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H y d r o g e n a t i o n

N 3 H N H

C H H C H2 2 3

2 4 2 2 6

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C h e m i c a l P r o d u c t i o n C r u d e O i l U p g r a d e

E s s e n t i a l O i l U p g r a d e

D e h y d r o g e n a t i o n

C H C H H2 4

P t

2 2 2

O c t a n e E n h a n c e m e n t M o n o m e r P r o d u c t i o n

O x i d a t i o n 2 C O C O 2 C O N

C H 1 / 2 O C H O

2 N H 4 O N O 3 H O

P t

2 2

2 4 2

A g

2 4

3 2

P t

2 5 2

C a t a l y t i c c o n v e r t e r s M o n o m e r P r o d u c t i o n

C h e m i c a l s P r o d u c t i o n

C h e m i c a l V a p o r D e p o s i t i o n

A l C H A l 3 C H 3 / 2 H2 5 3

A l

2 4 2

C o n n e c t i o n s o n I n t e g r a t e d C i r c u i t s

F e F e( s ) ( A q )

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C o r r o s i o n

Mechanisms On Surface Similar To Radical Reaction In Gas Phase – But Radicals Bound To Surface

X+H HX

XO 2OXOH OHH

H OH HOH

HO OH+O

X2H 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 HOO

H OH HO

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)

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Catalytic Cycles Needed

All Surface Reactions Occur In Cycles

Where Bare Surface Sites Are Formed And Destroyed

(5.154)

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Catalytic Cycles Where Needed

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2

2

2O O O O O O O O O O

+ H

H H

O O O O O O O O OH H

- H O

+ 1

/2 O

A 2

2

2

O

+1/2 O

H OH

- H

O

B

+ H

H OH

Figure 5.10 C atalytic cycles for the production of w ater a) via d isproportion of O H groups, b ) via the reaction O H (ad)+H )ad)

H 2O .

General Rules For Overall Reactions On Surfaces

• There must be bare sites on the catalyst to start the reaction.

• Then at least one of the reactants must adsorb on the bare sites.

• Then there are a series of bond dissociation reactions, fragmentations, association reactions and single atom recombinations which convert the adsorbed reactants into products

• Then the products desorb.

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Example CH3CO+2H2

O CH

- H

- H

- H

OC

H

O

CHH H

Methoxy

Formaldehyde

OC

H

CarbonMonoxide

OCH

H

H

H

AdsorbedMethanol

- H

Formyl

CO CH OH3

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Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).

Catalytic Cycles Continued

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O C

+ H

- H

OC

H

HH

Ethoxy

Acetaldehyde

OC

Methyl +CarbonMonoxide

OHH

H

AdsorbedEthanol

- H

Acetyl

CO CH OH

CHH H

CC H

O

HC CH H

H

H

CH

H H

CHH H

4CH

3

- H

Figure 5.15 The Mechanism of Ethanol Decomposition on Pt(111).

Notation

S=surface site

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CH OH S CH O H3 (ad) 3 (ad) (ad)

(5.156)

Generic Types Of Surface Reactions

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B

Langmuir-Hinshelwood Rideal-Eley Precursor

A

B A

A B

A

A B

AB

B AB

AAB

ABA

B

AB

ABA

B

AB

ABA

B

AB

Figure 6.5 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B.

Example: Lanmuir Hinshelwood for C2H4+H2C2H6

HH

HH

HH H

H CC H

H

HH

H

C CH

H H

H

H

C CH

H H

H

H

HC

H

CHH

HH

C

H

CHH

HH

C

H

CHH

HH

H

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Figure 5.21 A Langmuir-Hinshelwood mechanism for the reaction C2H4+H2C2H6.

Rideal-Eley For Film Growth

CH4

HC

H2

H

H

CHH

CH3

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Proposed Precursor Mechanism for 2C0+O22CO2

O O

O O

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Figure 5.23 A precursor mechanism for the reaction 2CO+O2 CO2.

Reactions In Forward And Reverse

B

Langmuir-Hinshelwood Rideal-Eley Precursor

A

B A

A B

A

A B

AB

B AB

AAB

ABA

B

AB

ABA

B

AB

ABA

B

AB

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Figure 6.6 Schematic of (a) Langmuir-Hinshelwood; (b) Rideal-Eley; (c) precursor mechanism for the reaction A-B → A + B.

Figure 6.5 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B.

Next Mechanisms Of Important Reactions: Olefin Hydrogenation

HH

HH

H

C CHH H

HH

HC

H

CHH

HH

HH

HC

H

CHH

HH

C

H

CHH

HH

H

H

C C

HH H

Figure 14.12 The mechanism of ethylene hydrogenation on supported platinum catalysts

Figure 14.13 The mechanism of ethylene hydrogenation on a RhCl(PPh3)3 cluster. (Wilkinson's catalyst)

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Isomerization

+3 H

CCC

HH

H

CC

H

H

H

C

C H

H H

H

H H

H

H

H

CCC

HH

H

CC

H

H

H

C

C H

H H

H

H H

H

H

H

CCC

HH

H

CC

H

HC

C

H H

H

H

H

H

H

CCC

HH

H

CC

H

HC

C

H HH

H

H

H

H

C

HH

C

H

H HH C CH

HH

CCC

HH

H

C

HH

C

H

H HH C CH

HH

CCC

HH

H

-3 H

C

H

H C

H

H HH C CH

HH

CCC

HH

HH

H

H

C

H

H C

H

H HH C CH

HH

CCC

HH

HH

H

H

Figure 14.14 One mechanism of the 3 methyl-hexane isomerization.

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Requires at least 5 carbons in the chain so called 5 center isomerization

3-Centered Isomerization Also Possible But May Require A Metallocarbocation

CCC

HH

H

HCH

HH

CH

HH

CCC

HH

H

HCH

H

H HH

CH

HH

CCC

HH

H

HCH

H

H HH

CH

HH

+ 2 H

CCC

HH

H

HCH

H

H

CH

HH

CCC

HH

H

HCH

H

H

CH

HH

CCC

HH

H

HCH

H

H

CH

HHH

H

- 2 H

F ig u re 1 4 .1 5 O n e o f th e p ro p o s e d m e c h a n is m s o f n e o p e n ta n e is o m e r iz a t io n .

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CO Oxidation

2

2

O

+1/2 O

- C

O

+ C

O

OOC

OCO

Figure14.16 The catalytic cycle for CO oxidation

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C O + 1 / 2 O C O 2 2

( 1 4 . 2 7 )

O + 2 S 2 O2 ( a d )

( 1 4 . 2 8 )

C O + S C O ( a d )

( 1 4 . 2 9 )

Partial Oxidation Of Ethylene

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O + 2 S 2 O2 ( a d )

( 1 4 . 3 1 )

C H = C H + S C H = C H2 2 2 2 ( a d )

( 1 4 . 3 2 )

C H C H O C H O

C H2 2 ( a d ) ( a d ) 2 2 ( a d ) `

( 1 4 . 3 3 )

C H O

C H C H O

C H2 2 ( a d ) 2 2 ` `

( 1 4 . 3 4 )

C H C H + 3 O 2 C O + 2 H O2 2 2 2 2 ( 1 4 . 3 5 )

Hydroformulation CO+RCH=CH2+H2RCH2CH2CHO

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Figure 14.17 The catalytic cycle for hydroformylation over a rhodium hydride cluster.

R C H = C H + S R C H = C H2 2 ( a d )

( 1 4 . 3 7 )

C O + R C H = C H + H R C H C H + C O2 ( a d ) ( a d ) 2 2 ( a d ) ( a d )( 1 4 . 3 8 )

R C H C H + C O R C H C H C O2 2 ( a d ) ( a d ) 2 2 ( a d )

( 1 4 . 3 9 )

R C H C H C O + H R C H C H C O H + H2 2 ( a d ) 2 2 2 ( a d )

( 1 4 . 4 0 )

Gas Phase & Solution Show Different Mechanism

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Figure 6.7 The geometry of the OH + CH3OH reaction in

the gas phase and in solutions.

Ethylene Decomposition (For H2 Production)

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Figure 6.8 The mechanism of ethylene decomposition on Pt(111). (Proposed by Kesmodel, Dubois and Somorjai [1979] and confirmed by Iback and Lehwald [1978].)

Methanol Decomposition For H2 Production

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Figure 6.11 The mechanism of methanol decomposition on the hexagonal faces of group VII and 1b metals. (Proposed by Davis and Barteau [1989].)

Summary Of Mechanism Of Reaction On Metals

Mechanisms on metals similar to gas phase-

Key difference – species bound to surface

proximity effect

di-radicals, tri-radicals possible

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The Effect Of Surface Structure

Reactions often occur faster on special configurations called active sites

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Figure 6.15 The multiplet for the reaction of ethyl alcohol (a) to form either ethylene, (b) to form acetaldehyde and hydrogen, as discussed by Balandin [1929]. The asterisks in the figure represent catalytic centers.

The Effects Can Be Huge

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Figure 6.14 The rate on nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. (Adapted from Masel [1983].)

Rates Also Vary With Particular Size

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Figure 6.13 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].)

Mechanisms Vary With Surface Structure

Figure 6.11 The mechanism of methanol decomposition on the hexagonal faces of group VII and 1b metals. (Proposed by Davis and Barteau [1989].)

Figure 6.12 The mechanism of methanol decomposition on(1x1)Pt(110) as proposed by Wang and Masel [1991].

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Another Case Of Variation With Surface Structure

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Figure 10.16 The mechanism of ethylene decomposition on the close packed faces of transition metals. (After Yagasaki and Masel [1994].)

 Figure 10.17 The mechanism of ethylene decomposition on the (100) faces of the transition metals. (After Yagasaki and Masel [1994].)

Notation

Structure sensitive reactions

Structure insensitive reactions

Secondary Structure Sensitivity

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(ad)2CH

Reaction

Largest Variation in Rate with Geometry Observed

Prior to 19962CO + O2 → 2CO2 6

C2H4 + H2 → C2H6 8

CH3OH →

+ H2O

>100

C2H6 + H2 → 2CH4 104

N2 + 3H2 → 2NH3 105

2NO + 2H2 → N2 + 2H2O ≈ 1021

The Effect Of Composition

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Figure 6.19 The relative rate of ethane hydrogenolysis and ethylene hydrogenation over several transition metal catalysts. (Data of Sinfelt [1963].)

Summary

• General Mechanisms Of Surface Reactions– Mechanisms look like gas phase reactions– Need surface site– Multiply bound species– Proximity Effect

• Effect of Surface Structure – Structure Sensitive Reactions– Structure insensitive reactions– Models hard - multiple bonding

• Effect of composition– Rates change– Key effect - stability of intermediates– Dual Sites

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