ChE 553 Lecture 13 Introduction To Surface Reactions 1.
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Transcript of ChE 553 Lecture 13 Introduction To Surface Reactions 1.
Objective
• Start To Discuss Surface Reactions– Overview of mechanism– Typical reactions on metals– Highlight well studied examples– Qualitative trends with changing
composition and structure
2
Topics
• General Mechanisms Of Surface
Reactions– Mechanisms look like gas phase reactions– Need surface site
• Effect of Surface Structure
• Effect of composition
3
Some Examples Of Reactions On Metal Surfaces
4
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
2
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 )
3
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)
5
Catalytic Cycles Needed
All Surface Reactions Occur In Cycles
Where Bare Surface Sites Are Formed And Destroyed
(5.154)
6
Catalytic Cycles Where Needed
7
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.
8
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
9
Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).
Catalytic Cycles Continued
10
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).
Generic Types Of Surface Reactions
12
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
13
Figure 5.21 A Langmuir-Hinshelwood mechanism for the reaction C2H4+H2C2H6.
Proposed Precursor Mechanism for 2C0+O22CO2
O O
O O
15
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
16
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)
17
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.
18
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 .
19
CO Oxidation
2
2
O
+1/2 O
- C
O
+ C
O
OOC
OCO
Figure14.16 The catalytic cycle for CO oxidation
20
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
21
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
22
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
23
Figure 6.7 The geometry of the OH + CH3OH reaction in
the gas phase and in solutions.
Ethylene Decomposition (For H2 Production)
24
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
25
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
26
The Effect Of Surface Structure
Reactions often occur faster on special configurations called active sites
27
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
28
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
29
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].
30
Another Case Of Variation With Surface Structure
31
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
32
(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
33
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
34