Transcript Surface Characterization of Pt

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
1x1 um AFM of smooth Pt
images from Ilic, Maclay, et al. J. Mat. Sci. (2000) 35 4337-3457
Why use Pt-Black?
- Many reactions are “mass transport limiting”
- Catalytic reactions only occur on active surface sites
Reactants and products
are formed faster than
they can diffuse out
For example…
OH
open circuit oxidation
Pt-black
H2O, 0.2 N H2SO4
1 atm O2
O
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 H2O2
Pt
O2 + 2H2O
Some Definitions
ROUGHNESS FACTOR
surface area S

geometric area A
takes into account “hills and valleys”
h
e.g. 2rh
r
PRODUCTIVITY
PROD 
mol product
mol of surface Pt
- 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
Synthesis Of Pt-Black
- Platinum is electrochemically deposited from chloroplatinic acid (H2PtCl6)
onto pre-treated platinum
- Involves three couples:
Pt (IV) Cl6 + 2e
Pt (II) Cl4 2 + 2Cl
Pt (II) Cl4 2 + 2e
Pt (0) + 4Cl
Pt (IV) Cl6 2 + 4e
Pt (0) + 6Cl
- 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
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
Hydrogen Overvoltage
- theoretically expect to see hydrogen evolution at cathode at 0 V vs SHE
+
 e  Hads
Hsolv
+
 e  H 2ads
Hads  Hsolv
H 2ads  H 2gas
- 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)
Hydrogen Monolayers
Hydrogen Evolution Reaction
Cyclic Voltammogram of Pt-Black in 0.5 M H2SO4
Current (mA)
correction for double layer charging
zero
integral is amt. of charge
for one H2 monolayer
H2 evolution
Potential (vs. SHE, V)
- 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
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
number of surface atoms
in 1 cm2 of 100 plane
210 uC / real cm2
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.
Miller Indices
Miller indices specify particular crystal faces (110, 200, etc.)
1. Decide on a basis.
k
red = unit vector
h, k lattice vectors
origin
h
2. Look at the cuts.
- Pick a cut next to the origin
- How many times does it cut
the h unit vector? The k?
1
“-1”
3. Label the face. “11”
2-D lattice. Method applies to 3D.
origin
3rd axis is called “l”
Fuel Cells
- Chemical batteries: pour fuel in, electricity comes out
anode
work
e¯
e¯
CO2, MeOH, H2O
cathode
H2O, air
CH 3OH  H 2O 
CO 2  6H +  6e 
3 O  6e  6H +
2 2
 3H 2O
MeOH
air: O2
polymer: proton exchange membrane
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:
Ru
+ 5H2 Pt-black
hexane
Ru(0) +
+
+
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
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 Platinum
Climent, Aldaz, et al. Universitat d’Alcant (Spain)
- Looked at urea adsorption on Pt(100) and Pt(111)
O
- Characterization via FTIRS, CV, etc.
C
H
Pt(100)
N
- Saturation coverage = 0.25
H
N
H
H
- Two electrons transferred per urea molecule
NH 2
O
Pt(111)
- Saturation coverage = 0.45
-One electron transferred
HN
O
H2N
C
H
N
per urea molecule
low coverage
high coverage
Ligand Accelerated Catalysis
A+B
cat
k0
prod*
cat/ligand*
A+B
k1
* = 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:
R
R
R'
R''
OH
Ti(OiPr)4, DET
t
Bu3CO2H, CH2Cl2
(DET=diethyl tartrate)
R'
O
OH
R''
70-90%
> 90% ee
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
OH
O
ethyl
pyruvate
H2, Pt / Al2O3
Cinchona Alkaloid
C
H3C
CO2Et
CO2Et
- Called “Ciba-Geigy” Process or “Orito Reaction”.
- Discovered by Orito in 1970s.
Various Modifier Structures
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)
R
C
H
H
HO
C8
C9
Z
N
N
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.
Effect of Solvent
1. Acetic acid gives best ees.
2. Fastest rates in EtOH and toluene.
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.
O
X
OH
CF3
Y
X
CF3
Y
ee up to 92%
Inductive Effects
O
X
CF3
Y
image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)
Inductive Effects
O
X
CF3
Y
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:
no chirality
100
100
110
110
“S”
“R”
100
100
110
110
Observations
1. CV of Glucose Oxidation
a, b: D-glucose oxidation on Pt{643}S, Pt{643}R
c, d: L-glucose “
50 mV/sec
0.1 M H2SO4, 0.005 M glucose
image from Attard, G. J. Phys. Chem. B. 105, 3158-3167, (2001)
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.