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.

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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

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

Pocket

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.

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