Transcript che 377 lectures - Classnotes For Professor Masel's Classes

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
Hydrogenation
N 2  3H 2  2 NH 3
C2 H 4  H 2 C2 H 6
Dehydrogenation
Pt
C2 H 4  C2 H 2  H 2
Oxidation
Pt
2CO  CO  2CO 2  N 2
Ag
C 2 H 4  1 / 2O 2  C 2 H 4 O
Chemical Production
Crude Oil Upgrade
Essential Oil Upgrade
Octane Enhancement
Monomer Production
Catalytic converters
Monomer Production
Chemicals Production
Pt
2NH 3  4O 2  N 2 O 5  3H 2 O
Chemical Vapor Deposition
Al C 2 H 5  3  Al  3C 2 H 4  3 / 2H 2
Al
Connections on Integrated
Circuits

Fe (s)  Fe 3(Aq)
Corrosion
4
Mechanisms On Surface Similar To Radical Reaction 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)
2O (ad)  H 2  2OH (ad)
H 2  OH  H 2 O  H
H  O 2  OH + O
X  2H  H 2  X
(5.152)
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)
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
+ 1/2 O 2
O O O O O
O O
+1/2 O 2
H H
O O O O O
- H 2O
O O
B
H H
O O O O O
O
+ H2
+H2
- H 2O
A
H
H O
H
H O
Figure 5.10 Catalytic cycles for the production of water a)
via disproportion of OH groups, b) via the reaction
OH(ad)+H)ad)H2O.
7
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 CH3CO+2H2
CH 3 OH
CO
H
O
C
H
H
O
CH
Adsorbed
Methanol
H H -H
H
Carbon
Monoxide
-H
C
O H
C
O
Formyl
H H
O C
-H
Methoxy
-H
Formaldehyde
Figure 5.14 The Mechanism of Methanol Decomposition
on Pt(111).
9
Catalytic Cycles Continued
CH 3 OH
CO
H
HH
C
O
C
H
+H
CH 4
H
O
Adsorbed
Ethanol
Methyl +
CarbonMonoxide
C
H
O C
C
H
H H
C H
O C
Acetyl
-H
-H
H
H H
H
H
C C
HH H
H
H
H
C
O
Ethoxy
-H
Acetaldehyde
Figure 5.15 The Mechanism of Ethanol Decomposition on Pt(111).
10
Notation
S=surface site
CH 3OH (ad)  S  CH 3O(ad)  H (ad)
(5.156)
11
Generic Types Of Surface
Reactions
B
A
B
A
B
A
B
B
A
B
B
B
A
Langmuir-Hinshelwood
A
B
A
B
A
B
A
A
Rideal-Eley
B
A
B
A
B
A
B
A
B
A
B
A
A
A
Precursor
Figure 6.5 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the
reaction A+BAB and ABA+B.
12
Example: Lanmuir Hinshelwood for
C2H4+H2C2H6
H
H
H
C
H
H
C
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
H
H
H
H
H
C
C
H
H
H
H
H
H
C
C
C
H
H
C
C
H
H
H
C
H
H
H
H
H
H
Figure 5.21 A Langmuir-Hinshelwood mechanism for the reaction
C2H4+H2C2H6.
13
Rideal-Eley For Film Growth
CH3
CH4
H H
C
H
H2
H H
C
14
Proposed Precursor Mechanism for
2C0+O22CO2
C
O
O C O
O O
O O
Figure 5.23 A precursor mechanism for the reaction
2CO+O2 CO2.
15
Reactions In Forward And
Reverse
B
A
B
A
B
A
B
B
A
B
B
B
A
Langmuir-Hinshelwood
A
B
A
B
A
B
A
A
Rideal-Eley
B
A
B
A
B
A
B
A
B
A
B
A
A
A
Precursor
Figure 6.5 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c)
precursor mechanism for the reaction A+BAB and ABA+B.
Figure 6.6 Schematic of (a) Langmuir-Hinshelwood; (b) Rideal-Eley; (c)
precursor mechanism for the reaction A-B → A + B.
16
Next Mechanisms Of Important
Reactions: 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
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
H
H
H
C
H H
H
C
C
C C
H
C H CH
H
H
H
H
H
H
H
H
H
C
H H
H
C C C C
H
C H CH
H
H
H
H
H
-3 H
H
H
H
H
H
H
C
H
H
HH
CC
C
H
H
H H
C
H
C
H
C
H
H
C
H
H
HH
CC
C
H
H
H H
C
H
C
H
H
C
H H
C
C
C C
H
H
CH
C
H
H
H
H
H
C
H H
C
C HC HH
CH
H
C C
H
H
H
C
+3 H
H
H
H
C
H
H
HH
H
CC
H
C
H
H
H H
C
C
H
C
H
H
C
H
H
HH
H
CC
H
C
H
H
H H
C
H
C
H
C
H
H
Figure 14.14 One mechanism of the 3 methyl-hexane isomerization.
Requires at least 5 carbons in the chain so called 5
center isomerization
18
3-Centered Isomerization Also Possible But
May Require A Metallocarbocation
H
H
H H
C HC H
H
C CH
HC
H H
H
H
H
HH H H
C C
H
C CH
HC
H H
H
H
H
H
H
H
H
H H
C HC H
H
C CH
HC
H
H
C C
C H
HC H C
H
H H
H
-2H
+2H
H
H
H
H
H
C C
C CH
HC
H
H
H
H
H
H H
C HC H
H
C CH
HC
H
Figure 14.15 One of the proposed mechanisms of neopentane isomerization.
19
CO Oxidation
CO + 1 / 2 O 2  CO 2
(14.27)
+1/2 O2
(14.28)
CO + S  CO (ad)
+ CO
- CO 2
O 2 + 2 S  2O (ad)
O
O
OC
O
O
C
Figure14.16 The catalytic
cycle for CO oxidation
(14.29)
20
Partial Oxidation Of Ethylene
O 2 + 2 S  2O (ad)
(14.31)
CH 2 = CH 2 + S  CH 2 = CH 2(ad)
(14.32)
CH 2  CH 2(ad)  O (ad)
O
 CH 2   `CH 2(ad)
(14.33)
O
O
CH 2   `CH 2(ad)  CH 2   `CH 2
(14.34)
CH 2 CH 2 + 3 O 2  2CO 2 + 2 H 2 O
(14.35)
21
Hydroformulation
CO+RCH=CH2+H2RCH2CH2CHO
RCH = CH 2 + S  RCH = CH 2(ad)
(14.37)
CO + RCH = CH 2(ad) + H (ad)  RCH 2 CH 2(ad) + CO (ad
(14.38)
RCH 2 CH 2(ad) + CO (ad)  RCH 2 CH 2 CO (ad)
(14.39)
Figure 14.17 The catalytic cycle
for hydroformylation over a
rhodium hydride cluster.
RCH 2 CH 2 CO (ad) + H 2  RCH 2 CH 2 COH + H (ad)
(14.40)
22
Gas Phase & Solution Show
Different Mechanism
Figure 6.7 The geometry of the OH + CH3OH reaction in
the gas phase and in solutions.
23
Ethylene Decomposition
(For H2 Production)
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].)
24
Methanol Decomposition For
H2 Production
Figure 6.11 The mechanism of methanol decomposition on the hexagonal
faces of group VII and 1b metals. (Proposed by Davis and Barteau [1989].)
25
Summary Of Mechanism Of
Reaction On Metals
Mechanisms on metals similar to gas
phaseKey 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
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.
27
The Effects Can Be Huge
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].)
28
Rates Also Vary With
Particular Size
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].)
29
Mechanisms Vary With
Surface Structure
30
Another Case Of Variation
With Surface Structure
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].)
31
Notation
Structure sensitive reactions
Structure insensitive reactions
Secondary Structure Sensitivity
2CO + O2 → 2CO2
Largest Variation in Rate
with Geometry Observed
Prior to 1996
6
C2H4 + H2 → C2H6
8
Reaction
CH3OH → CH 2 (ad) + H2O
>100
C2H6 + H2 → 2CH4
104
N2 + 3H2 → 2NH3
105
2NO + 2H2 → N2 + 2H2O
≈ 1021
32
The Effect Of Composition
Figure 6.19 The relative rate of ethane hydrogenolysis and
ethylene hydrogenation over several transition metal catalysts.
(Data of Sinfelt [1963].)
33
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