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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. defect. SEM (1450x) of Pt-black. - PowerPoint PPT Presentation
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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.
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