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Transcript of 0581.5271 Electrochemistry for Engineers LECTURE 6 Lecturer: Dr. Brian Rosen Office: 128 Wolfson...
![Page 1: 0581.5271 Electrochemistry for Engineers LECTURE 6 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00.](https://reader031.fdocuments.net/reader031/viewer/2022032015/56649d0d5503460f949e1f68/html5/thumbnails/1.jpg)
0581.5271 Electrochemistry for Engineers
LECTURE 6
Lecturer: Dr. Brian Rosen Office: 128 Wolfson
Office Hours: Sun 16:00
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A Birds Eye View of:Methods of Catalyst Action
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What is a Catalytic Material? A catalyst is a material that increases the rate
of a chemical reaction by reducing the free-energy barrier without being consumed
Pt, Pd, Ag, Sn, Ni, Cu….Pt
e-
O2
Catalytic Converters Fuel Cells, Electrolyzers
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Intelligent Design of Catalysts• Initiate reactions (break bonds, form radicals)• Stabilize intermediates • Hold reactants together in proper configuration• Block side reactions• Stretch bonds (to make them break easier)• Donate or accept electrons • Efficient energy transfer
C
C
C
C
CC
Pd
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Intelligent Design of Catalysts• Initiate reactions (break bonds, form radicals)• Stabilize intermediates • Hold reactants together in proper configuration• Block side reactions• Stretch bonds (to make them break easier)• Donate or accept electrons • Efficient energy transfer
C
C
C
C
CC
Pd
Sabatier’s Principle
In order to optimize catalyst performance, one must design a surface that binds the reactants
neither too strong nor too weak
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Types of Heterogeneous Catalysts Supported Powders (e.g. oxide supported)
Structured (e.g. fibers and cloths)
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Importance Of Catalysis
• 90% of all chemical processes use catalysts• Changes in catalysts have a giant influence on
rates and selectivity’s of reactions. More than anything else
• Most real reactor design associated with optimizing performance of catalyst
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Catalytic Reaction Occurs Via A Catalytic Cycle:
reactants + catalyst complex
complex products + catalyst
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Catalytic Cycles
Rosen et al. J. Phys. Chem. C. 116 (29), 2012.
27
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Large Changes in Rate
Reaction Catalyst EA (kcal/mol)Without Catalyst
EA (kcal/mol)With Catalyst
Enhancement
H2 + I2 2HI Pt 44 14 1013
2N2O 2N2 + O2Au 58 29 1013
(C2H5)2O 2C2H4 + H2O I2 53 34 108
Tk
EAk
b
Aexp
)(sec
kfmols
rate
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The Rate Enhancement In The Presence Of A Catalyst
Reaction Catalyst Rate
EnhancementTemperature
Ortho H2 Para H2 Pt (solid) 300K
2NH3 N2 + 3H2 Mo (solid) 600K
C2 H4 + H2 C2 H6 Pt (solid) 300K
H2 +Br2 2HBr Pt (solid) 1 108 300K
2NO + 2H2 N2 + 2H2 O Ru (solid) 3 1016 500K
CH3COH CH4 + CO I2 (gas) 4 106 500K
CH3CH3 C2H4 +H2 NO2 (gas) 1 109 750K
(CH3)3 COH
(CH3)2CH2CH2+H2O
HBr (gas) 3 108 750K
1040
1020
1042
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Molecular vs Dissociated Adsorption
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Leonard-Jones’ Potential Well
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Leonard-Jones (cont’d)
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Initiation Often Not Enough. Also Need To Stabilize Intermediates
15
-50
0
50
1/2Br2
Ent
halp
y, k
cal/m
ole
of b
rom
ine
atom
s
Br
H+HBr
Br+2HBr
1/2 Br2+2HBr
Reaction Progress
Gas Phase
+H2
+Br2
-50
0
50
+Br2
Ent
halp
y, k
cal/m
ole
of b
rom
ine
atom
s
Br(ad)
H(ad)+HBr
Br(ad)+2HBr
1/2 Br2+2HBr
Reaction Progress
1/2Br2
Surface
+H2
Termination
Figure 12.7 The enthalpy changes during the gas phase reaction H2 + Br2 2 HBr assuming that the reaction terminates after one cycle
Figure 12.8 The enthalpy changes during the Rideal-Eley surface reaction H2 + Br2 2 HBr on Pt(111) assuming that the reaction terminates after one cycle
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Adsorption Sites
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Adsorption Isotherms (Langmuir)
• The surface containing the adsorbing sites is perfectly flat plane with no corrugations (assume the surface is homogeneous) .
• The adsorbing gas adsorbs into an immobile state.• All sites are equivalent.• Each site can hold at most one molecule of A (mono-layer coverage only).• There are no interactions between adsorbate molecules on adjacent sites.
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Adsorption Isotherms (Langmuir)
“Coverage”
[S]= free surface sites[So] = Total surface sites
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N2 adsorption onto W
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Effects Of Surface Area
Consider a platinum catalyzed reaction.You can run the reaction1) Run the reaction on the wire2) Take the wire and smash it with a hammer
and then run the reaction.
The rate will be higher on the wire you smashed with a hammer!
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Why Does Smashing A Wire Change The Rate?
• When you squashed the platinum you created more surface area.
• You also changed the shape of the surface which can affect the rate.
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Turnover Numbers
Rates of catalytic reactions often expressed as turnover number
SNA
N
R = T
RA = Rate per unit area (molecules/cm2-sec)NS = Number of exposed metal atoms / unit area (Atoms/cm2)
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CO2 conversion on Ag NPs
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Turnover Numbers For Some Typical Reactions
200 400 600 800 1000 1200
Reaction Temperature, K
10-6
10-4
10-2
100
102
Tur
nove
r N
um
be
r, s
ec-1 Hydrogenation
OlefinIsomerization
Dehydrogenation
AlkaneHydrogenolysis
Cyclization
SiliconDeposition
GaAsDeposition
1400
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TON for ORR
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Very Complex Pore Structure
26Figure 12.4 A diagram of the pore structure in Faugasite.
-Macropores-Micropores-Nanopores
“Mesoporous”
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BET Surface Area
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Electrochemical Surface Area
420 cm2/C CO desorbed
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Comparing ECSA
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High Current – Low Overpotential
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Volcano Plots on ΔGH
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Volcano Plot for ORR
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Experimental Evidence HCOOHH2+CO2
Heat Of Formation Of Formate
Tem
pera
ture
For
50
% C
onve
rsio
n
Au
Ag
Pt
Pd
Ir
Rh
Ru
Cu
Co
Ni Fe
W
50 60 70 80 90 100 110 120
350
400
450
500
550
600
HCOOH HCOO(ad) Had
H(ad) HCOOad CO2 H2
(12.75)
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Biomimicry
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Y. Tomita, S. Teruya, O. Koga, Y. Hori, J. Elect. Soc. 147, 4164-4167 (2000)
Hori, Y., Electrochemical CO2 reduction on metal electrodes. Modern Aspects of Electrochemistry, (2008). 42: p. 89-189
Aqueous KHCO3
Product Selectivity
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Product Selectivity
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Intelligent Design of Catalysts• Initiate reactions (break bonds, form radicals)• Stabilize intermediates • Hold reactants together in proper configuration• Block side reactions• Stretch bonds (to make them break easier)• Donate or accept electrons • Efficient energy transfer
C
C
C
C
CC
Pd
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Holding Reactants In The Right Configuration
NAD CH3CH2OH NADH CH3CHOH
NAD+
OH
HCC
NH
H
HH
:
Figure 12.16 A cartoon of the reaction of ethanol and NAD+ on the active site of liver alcohol dehydrogenase. Adapted from Oppenheimer and Handlon (1992) (In the Enzyme, vol 20 (1992) 453.
(12.92)
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Catalysts Make Bonds Easier To Break
Active Site
Figure 12.17 A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]
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Transition Metals – Weaken Bond By Attaching To Antibonding Orbitals
Figure 12.20 A diagram of the key interactions during the dissociation of hydrogen on platinum.
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Catalyst For PE Production: Block Side Reactions
Ti
CC
C HHH
CC
C HHH
CC
C HHH
CHH
H
C CC
BindingSite
Blocking Group
Figure 12.24 A diagram of propylene polymerization in a Ziegler-Natta catalyst.
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Supported Metal Catalysts
Figure.12.3 A picture of a
supported metal catalyst.
Use support because platinum very expensive and only the surface is active. Spread platinum out on cheap support. Support also provides strength
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Why are Intermediates Important?CO2 Conversion Example
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O2-H2 on Platinum
Thesis of E. Hudak
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Thesis of E. Hudak
O2-H2 on Platinum
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Pt (hkl) Dependence
ORR
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Effect of crystallographic planesPt has a face centered cubic crystal structure
3 basal planes: (111), (100), (110)Catalytic activity for oxygen reduction reaction
Pt(110) > Pt(100) > Pt(111)
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Key Substitutions
– Noble metals (Re, Ru) Coke resistance due to large EA for carbon formation– Promoters (K, Na) Promotion of CO2 dissociation by e- donation to active
sites. Replenishment of O2 in lattice – Poisons (S, P) Electronegative atoms to remove electrons from
active sites blocking unwanted side reactions
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Summary
• Catalysts can be designed to help initiate reactions.
• Catalysts can be designed to stabilize the intermediates of a reaction.
• Catalysts can be designed to hold the reactants in close proximity.
• Catalysts can be designed to hold the reactants in the right configuration to react.
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