0581.5271 Electrochemistry for Engineers LECTURE 6 Lecturer: Dr. Brian Rosen Office: 128 Wolfson...
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Transcript of 0581.5271 Electrochemistry for Engineers LECTURE 6 Lecturer: Dr. Brian Rosen Office: 128 Wolfson...
0581.5271 Electrochemistry for Engineers
LECTURE 6
Lecturer: Dr. Brian Rosen Office: 128 Wolfson
Office Hours: Sun 16:00
A Birds Eye View of:Methods of Catalyst Action
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
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
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
Types of Heterogeneous Catalysts Supported Powders (e.g. oxide supported)
Structured (e.g. fibers and cloths)
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
Catalytic Reaction Occurs Via A Catalytic Cycle:
reactants + catalyst complex
complex products + catalyst
Catalytic Cycles
Rosen et al. J. Phys. Chem. C. 116 (29), 2012.
27
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
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
Molecular vs Dissociated Adsorption
Leonard-Jones’ Potential Well
Leonard-Jones (cont’d)
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
Adsorption Sites
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.
Adsorption Isotherms (Langmuir)
“Coverage”
[S]= free surface sites[So] = Total surface sites
N2 adsorption onto W
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!
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.
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)
CO2 conversion on Ag NPs
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
TON for ORR
Very Complex Pore Structure
26Figure 12.4 A diagram of the pore structure in Faugasite.
-Macropores-Micropores-Nanopores
“Mesoporous”
BET Surface Area
Electrochemical Surface Area
420 cm2/C CO desorbed
Comparing ECSA
High Current – Low Overpotential
Volcano Plots on ΔGH
Volcano Plot for ORR
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)
Biomimicry
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
Product Selectivity
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
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)
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]
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.
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.
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
Why are Intermediates Important?CO2 Conversion Example
O2-H2 on Platinum
Thesis of E. Hudak
Thesis of E. Hudak
O2-H2 on Platinum
Pt (hkl) Dependence
ORR
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)
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
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.
49