Surface Characterization and Heterogeneous Asymmetric Catalysis

1
Surface Characterization andHeterogeneous
Asymmetric Catalysis

• Eugene Kwan

April 2, 2002.
2
What is Pt-Black?

• Also called platinized platinum, Adams
  Catalyst
• Electrochemically deposited platinum on platinum
• Very high surface area

defect
SEM (1450x) of Pt-black
1x1 um AFM of smooth Pt
images from Ilic, Maclay, et al. J. Mat. Sci.
(2000) 35 4337-3457
3
Why use Pt-Black?

• Many reactions are mass transport limiting

• Catalytic reactions only occur on active surface
  sites
• For example

Reactants and products are formed faster than
they can diffuse out
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

4
Some Definitions
ROUGHNESS FACTOR
takes into account hills and valleys
h
r
e.g. 2?rh
PRODUCTIVITY

• typical roughness 200-500
• productivity varies

roughness in alumina (15x15 um AFM)
image from Ilic, Maclay, et al. J. Mat. Sci.
(2000) 35 4337-3457
5
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

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Synthesis Of Pt-Black
PRETREATMENT
DEPOSITION

• - 50 mV (vs. SHE) potentiostatic deposition
• 2 chloroplatinic acid, 1 M HCl
• 20 mA / cm2 for 5 minutes against blackened Pt
  wire counterelectrode

DRYING/STORAGE
Pt is oxidized in air and poisoned by CO

• Rinsed in distilled water
• Dried under N2 or argon
• Stored in nitric acid

!
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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)

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Hydrogen Monolayers
Hydrogen Evolution Reaction
Cyclic Voltammogram of Pt-Black in 0.5 M H2SO4
correction for double layer charging
zero
Current (mA)
integral is amt. of charge for one H2 monolayer
Potential (vs. SHE, V)
H2 evolution

• - 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

CV from Bergens et al. J. Phys. Chem. B (1998),
102 1 195
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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.
!
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Conversion of Charge to Real Area
Convention is to define 1 real cm2 1.30 x
1015 surface Pt atoms 210 uC / real cm2
number of surface atoms in 1 cm2 of 100 plane
Different Crystal Planes of a fcc lattice
6
11
9
7
11
note different coordination numbers
images from Woods, R. Electroanal. Chem.
Interfacial Electrochem. (1974) 49 217.
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Miller Indices

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

k
red unit vector
h, k lattice vectors
h
origin
1
-1
2-D lattice. Method applies to 3D. 3rd axis is
called l
origin
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Fuel Cells
- Chemical batteries pour fuel in, electricity
comes out
work
anode
e
cathode
e
CO2, MeOH, H2O
H2O, air
MeOH
air O2
polymer proton exchange membrane
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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
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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 5050
• Found much better PtRuOsIr 4441105
• Dont know why that is better

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Urea Adsorption on Platinum

• Climent, Aldaz, et al. Universitat dAlcant
  (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

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Ligand Accelerated Catalysis
chiral center present - Define ratio rate
with ligand rate without ligand - If ratio gt 1,
ligand acceleration. If ratio lt 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
(DETdiethyl tartrate)
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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.
ethyl pyruvate
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Various Modifier Structures
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Effect of Modifier Structure

• Large aromatic systems give better ees than
  smaller ones of the same type.
• Do not need a nitrogen in the aromatic ring.
• Modifiers containing simple benzene/pyridine ring
  show no chiral induction.
• Aromatic system must be flat.
• 1. Acetic acid gives best ees.
• 2. Fastest rates in EtOH and toluene.

Effect of Solvent
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Inductive Effects

• Electron withdrawing groups increase rate and ee.
• Electron donating groups decreaase rate and ee.
• Steric effects in m and p positions also
  important.

ee up to 92
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Inductive Effects
image from Arx, Baiker, et al. Tet. Asym. 12
3089-3094 (2001)
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Inductive Effects
image from Arx, Baiker, et al. Tet. Asym. 12
3089-3094 (2001)
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Kinetics

• Modifier must be adsorbed on metal surface to be
  effective.
• Modifiers greatly increase reaction rate and ee.
• Linear relationship between ee and 1/rate.

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Chiral Metal Surfaces
Surprise! Metal surfaces can be chiral! Attard,
G. J. Phys. Chem. B. 105, 3158-3167, (2001) If
the surface isnt smooth, you get kink sites.
Edges must be of unequal length
no chirality
100
111
100
100
100
111
111
111
110
110
110
110
R
S
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Observations
1. CV of Glucose Oxidation
a, b D-glucose oxidation on Pt643S,
Pt643R 50 mV/sec c, d L-glucose 0.1 M
H2SO4, 0.005 M glucose
image from Attard, G. J. Phys. Chem. B. 105,
3158-3167, (2001)
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Visualization Pt643S
D-glucose
L-glucose
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Observations
2. Adsorption differs depending on chirality.
Theory predicts energy differences in
adsorptionconfirmed 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 Cu001 with Lysine parallel these
results.