Surface Characterization and Heterogeneous Asymmetric Catalysis
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Transcript of Surface Characterization and Heterogeneous Asymmetric Catalysis
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Surface Characterization and
Heterogeneous Asymmetric Catalysis
Eugene Kwan
April 2, 2002.
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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
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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
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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
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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
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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
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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)
ads
ads gas
+solv ads
+ads solv 2
2 2
H H
H H H
H H
e
e
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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
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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
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
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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”
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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
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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
Ru + 5H2Pt-blackhexane
Ru(0) + + +
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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 50:50
- Found much better: Pt:Ru:Os:Ir 44:41:10:5
- Don’t know why that is better
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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
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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)
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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.
O
CH3C CO2Et CO2Et
OH
H2, Pt / Al2O3Cinchona Alkaloid
ethylpyruvate
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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
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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
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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%
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Inductive Effects
Y
X
O
CF3
image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)
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Inductive Effects
Y
X
O
CF3
image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)
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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.
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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 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
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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
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Visualization: Pt{643}S
D-glucose
L-glucose
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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.