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-e- O2O2 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 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 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),

Large Changes in Rate ReactionCatalystE A (kcal/mol) Without Catalyst E A (kcal/mol) With Catalyst Enhancement H 2 + I 2  2HIPt N 2 O  2N 2 + O 2 Au (C 2 H 5 ) 2 O  2C 2 H 4 + H 2 OI2I

The Rate Enhancement In The Presence Of A Catalyst

Molecular vs Dissociated Adsorption

Leonard-Jones’ Potential Well

Leonard-Jones (cont’d)

Initiation Often Not Enough. Also Need To Stabilize Intermediates 15

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

N 2 adsorption onto W

Effects Of Surface Area Consider a platinum catalyzed reaction. You can run the reaction 1)Run the reaction on the wire 2)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 R A = Rate per unit area (molecules/cm 2 -sec) N S = Number of exposed metal atoms / unit area (Atoms/cm 2 )

CO 2 conversion on Ag NPs

Turnover Numbers For Some Typical Reactions

TON for ORR

Very Complex Pore Structure 26 Figure 12.4 A diagram of the pore structure in Faugasite. -Macropores -Micropores -Nanopores “Mesoporous”

BET Surface Area

Electrochemical Surface Area 420 cm 2 /C CO desorbed

Comparing ECSA

High Current – Low Overpotential

Volcano Plots on ΔG H

Volcano Plot for ORR

Experimental Evidence HCOOH  H 2 +CO 2 (12.75)

Biomimicry

Y. Tomita, S. Teruya, O. Koga, Y. Hori, J. Elect. Soc. 147, (2000) Hori, Y., Electrochemical CO 2 reduction on metal electrodes. Modern Aspects of Electrochemistry, (2008). 42: p Aqueous KHCO 3 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 Pd

Holding Reactants In The Right Configuration Figure 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 Figure 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 A diagram of the key interactions during the dissociation of hydrogen on platinum.

Catalyst For PE Production: Block Side Reactions Figure 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? CO 2 Conversion Example

O 2 -H 2 on Platinum Thesis of E. Hudak

O 2 -H 2 on Platinum

Pt (hkl) Dependence ORR

Effect of crystallographic planes Pt 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 E A for carbon formation – Promoters (K, Na) Promotion of CO 2 dissociation by e- donation to active sites. Replenishment of O 2 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