Surface Characterization and

Heterogeneous Asymmetric Catalysis

Eugene Kwan

April 2, 2002.

What is Pt-Black? Also called “platinized platinum”, “Adam’s Catalyst”

Electrochemically deposited platinum on platinum

Very high surface area

1x1 um AFM of smooth Pt

SEM (1450x) of Pt-black

images from Ilic, Maclay, et al. J. Mat. Sci. (2000) 35 4337-3457

defect

Why use Pt-Black?- Many reactions are “mass transport limiting”

Reactants and products are formed faster than they can diffuse out

- Catalytic reactions only occur on active surface sites

For example…

OH Oopen circuit oxidationPt-blackH2O, 0.2 N H2SO41 atm O2

Whitesides et al. (MIT)J. Phys. Chem. (1989) 93 768-775

- Found reaction was mass transport limited

- Use of H2O2 to try to go around problem oxidized Pt surface:

2 H2O2Pt O2 + 2H2O

Some DefinitionsROUGHNESS FACTOR

surface area S

geometric area A

e.g. 2rh

hr

takes into account “hills and valleys”

“roughness” in alumina (15x15 um AFM)

image from Ilic, Maclay, et al. J. Mat. Sci. (2000) 35 4337-3457

PRODUCTIVITYmol product

mol of surface PtPROD

- typical roughness: 200-500

- productivity varies

Synthesis Of Pt-Black

- Platinum is electrochemically deposited from chloroplatinic acid (H2PtCl6) onto pre-treated platinum

- Involves three couples:

- in acidic solution PtCl62- is the principal species

PRETREATMENT:

- Start with Pt gauze/metal

- Slight etching with aqua regia/nitric acid

- Removes impurities and improves adherence of deposit

(IV) (II) 26 4

(II) 2 (0)4

(IV) 2 (0)6

Pt Cl + 2e Pt Cl + 2Cl

Pt Cl + 2e Pt + 4Cl

Pt Cl + 4e Pt + 6Cl

Synthesis Of Pt-Black

PRETREATMENT

DEPOSITION

DRYING/STORAGE

- +50 mV (vs. SHE) potentiostatic deposition

- 2% chloroplatinic acid, 1 M HCl

- 20 mA / cm2 for 5 minutes against blackened Pt wire counterelectrode

- Rinsed in distilled water

- Dried under N2 or argon

- Stored in nitric acid

!Pt is oxidized in air and poisoned by CO

Hydrogen Overvoltage- theoretically expect to see hydrogen evolution at cathode at 0 V vs SHE

- never seen due to “kinetic effect” – always see it at higher voltage

- called “overvoltage”

- high overvoltage: mercury, tin, lead, cadmium (first step is slow)

- medium: smooth platinum, nickel, palladium, rhodium, nickel, copper

- low: Pt-black (second step is slow)

ads

ads gas

+solv ads

+ads solv 2

2 2

H H

H H H

H H

e

e

Hydrogen MonolayersHydrogen Evolution Reaction

Cur

rent

(m

A)

- In acid, H2 forms on surface of Pt at –(0.0 + ) V (overvoltage)

- The hydrogen becomes reversibly adsorbed to the surface

- Two peaks correspond to “weak” and “strong” adsorption: complicated analysis

Cyclic Voltammogram of Pt-Black in 0.5 M H2SO4

CV from Bergens et al. J. Phys. Chem. B (1998), 102 1 195

Potential (vs. SHE, V)

zero

H2 evolution

correction for double layer charging

integral is amt. of charge for one H2 monolayer

Determining The Surface Area

Integrate Charge

Obtain the integral from the CV:

Account for Fractional Coverage

- surface is not completely covered at endpoint

- divide by ~0.84 to get charge for readily accessible sites

- divide by ~0.77 to get charge for total sites

!This is the surface for hydrogen, a small molecule. The “hydrogen surface” is not accessible to all molecules.

Conversion of Charge to Real Area

Convention is to define:

1 real cm2 = 1.30 x 1015 surface Pt atoms

210 uC / real cm2

images from Woods, R. Electroanal. Chem. Interfacial Electrochem. (1974) 49 217.

number of surface atoms in 1 cm2 of 100 plane

Different Crystal Planes of a fcc lattice:

7

11

6

11

9

note different coordination numbers

Miller indices specify particular crystal faces (110, 200, etc.)

1. Decide on a basis.

2. Look at the cuts.

- Pick a cut next to the origin

- How many times does it cut

the h unit vector? The k?

3. Label the face. “11”

Miller Indices

2-D lattice. Method applies to 3D.

3rd axis is called “l”

k

h

h, k lattice vectors

red = unit vector

origin

origin

1

“-1”

Fuel Cells

- Chemical batteries: pour fuel in, electricity comes out

polymer: proton exchange membrane

MeOH

CO2, MeOH, H2O H2O, air

air: O2

anodecathode

worke¯

e¯

3 2

+2

CH OH H O

CO 6H 6e

+

2

2

3 O 6 6H23H O

e

Fuel Cells

- high efficiency: not Carnot cycle; real life: 40-70%

- efficient catalysts like Pt needed with high surface area.

- byproduct: carbon monoxide. CO sticks to Pt!

SOLUTION:

Reaction deposits a Ru submonolayer on the Pt which cuts off the CO but lets the Pt do the fuel cell oxidations.

See Bergens, et al. J. Phys. Chem. B. (1998) 102 193-199

Ru + 5H2Pt-blackhexane

Ru(0) + + +

Science Article, Tom Malouk

Reddington, Mallouk, et al. Science, 280, 1735-1737 (1998)

- Carried out a combinatorial search for best fuel cell catalysts

- Took salts of Pt, Ru, Os, Ir, and Rh and placed them into an inkjet printer!

- Added fluorescent acid/base indicator that changes color with [H+]

- “Printed” onto carbon paper with subsequent treatment with NaBH4

- Active catalysts became bright

- Previously, a good catalyst was Pt/Ru 50:50

- Found much better: Pt:Ru:Os:Ir 44:41:10:5

- Don’t know why that is better

Urea Adsorption on PlatinumCliment, Aldaz, et al. Universitat d’Alcant (Spain)

- Looked at urea adsorption on Pt(100) and Pt(111)

- Characterization via FTIRS, CV, etc.

Pt(100)

- Saturation coverage = 0.25

- Two electrons transferred per urea molecule

Pt(111)

- Saturation coverage = 0.45

-One electron transferred

per urea molecule

NN

C

O

H H

HH

high coverage

NHC

O

H2N

low coverage

OHN

NH2

R

R'

R''

OH

O

R'R

R''OH

70-90%> 90% ee

Ti(OiPr)4, DETtBu3CO2H, CH2Cl2

Ligand Accelerated Catalysis

* = chiral center present

- Define ratio: rate with ligand : rate without ligand

- If ratio > 1, “ligand acceleration”. If ratio < 1 “ligand deceleration”.

- Lots of asymmetric processes are ligand decelerated (chiral ligands tend to sterically crowd the binding site on the catalyst)

- Asymmetric epoxidation of allylic alcohols is accelerated:

A + B prod* A + B

catk0

cat/ligand*

k1

(DET=diethyl tartrate)

Heterogeneous Asymmetric H2

Only two examples known:

1. Hydrogenation of beta-ketoesters with Nickel/tartaric acid

2. Hydrogenation of alpha-ketoesters with Pt/cinchona alkaloids

- Called “Ciba-Geigy” Process or “Orito Reaction”.

- Discovered by Orito in 1970s.

O

CH3C CO2Et CO2Et

OH

H2, Pt / Al2O3Cinchona Alkaloid

ethylpyruvate

8R/9S R Z 8S/9R

Cinchonidine (Cd) Vinyl H Cinchonine (Cn)

10,11-dihydrocinchonidine (HCd)

Ethyl H 10,11-dihydrocinchonine (HCn)

Quinine (Qn) Vinyl OMe Quinidine (Qd)

10,11-dihydroquinine (HQn)

Ethyl OMe 10,11-dihydroquinidine (HQd)

C8N

CR

C9

N

HO

H

Z

H

Various Modifier Structures

Effect of Modifier Structure

1. Large aromatic systems give better ees than smaller ones of the same type.

2. Do not need a nitrogen in the aromatic ring.

3. Modifiers containing simple benzene/pyridine ring show no chiral induction.

4. Aromatic system must be flat.

1. Acetic acid gives best ees.

2. Fastest rates in EtOH and toluene.

Effect of Solvent

Inductive Effects

1. Electron withdrawing groups increase rate and ee.

2. Electron donating groups decreaase rate and ee.

3. Steric effects in m and p positions also important.

Y

X

O

CF3

Y

X

OH

CF3

ee up to 92%

Inductive Effects

Y

X

O

CF3

image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)

Inductive Effects

Y

X

O

CF3

image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)

Kinetics

1. Modifier must be adsorbed on metal surface to be effective.

2. Modifiers greatly increase reaction rate and ee.

3. Linear relationship between ee and 1/rate.

Chiral Metal Surfaces

Surprise! Metal surfaces can be chiral!

Attard, G. J. Phys. Chem. B. 105, 3158-3167, (2001)

If the surface isn’t smooth, you get “kink” sites. Edges must be of unequal length:

100

110

111

100

110111

“S” “R”

100

110

111

no chirality

100

110

111

Observations

1. CV of Glucose Oxidation

image from Attard, G. J. Phys. Chem. B. 105, 3158-3167, (2001)

a, b: D-glucose oxidation on Pt{643}S, Pt{643}R 50 mV/sec

c, d: L-glucose “ 0.1 M H2SO4, 0.005 M glucose

Visualization: Pt{643}S

D-glucose

L-glucose

Observations

2. Adsorption differs depending on chirality. Theory predicts energy differences in adsorption—confirmed by experiment.

3. Should consider Pt surface as a racemate of R, S kink sites. Preferential adsorption of modifiers, such as the cinchona alkaloid may lead to enantioselective hydrogenation.

4. Experiments by Zhao on Cu{001} with Lysine parallel these results.