Method for Covalently Attaching and Site-Isolating Molecular
Catalysts on Carbon Electrodes
Stanford University, Department of Chemistry
Anando Devadoss and Christopher E. D. Chidsey
3.0
• Electrochemical oxidation of long-chain hydrocarbons (CH2)n and
reduction of oxygen using electrocatalysts at low temperatures
could be the most effective way to convert fuel to mobile power
• aromatic nitration of graphitic carbon is identified as a method to introduce
nitro groups in a site-isolated fashion.
• Other electrocatalytic processes promise improved energy
efficiency (oxidation of water, transformations of commodity
chemicals etc.)
• conversion of nitro groups to amines permits coupling to carbonyl chlorideterminated molecular catalysts through a robust amide linkage.
Electrocatalyst Efficiency
Fuel
free energy
wasted as heat
of fuel oxidation
heat
best
existing
hydrogen
H2
work
best
existing
methanol
CH3OH
heat
work
0.0
0
E H /H
+
2
free energy
wasted as heat
of O2 reduction
heat
Oxidant
best
existing
oxygen
O2
best
existing
oxygen
O2
heat
oxygen
O2
E O /H O
2
2
0.5
2
2
2
• amine-modified carbon surfaces can be used to couple carbonyl chloride
terminated molecular catalysts as shown in Figure 2.
Oxygen
graphitic
carbon
electrode
polymer
electrolyte
membrane
30
40
50
60
70
80
Conversion of nitro to amine:
H
H
H
H
H
NO2 H
H
H
NO2 H
NH2 H
H
NH2 H
Figure 1: A schematic of
Polymer-Electrolyte-Membrane
(PEM) fuel cell with molecular
electrocatalyst modified carbon
electrodes.
6e-, 6H+
HNO3
H
NH H
• conversion of nitro groups to amine groups can be followed by the absence of
peak at 406 eV (nitro) and appearance of peak at 400 eV (amine) by XPS.
O
O
H
graphite
e-
H+
20
o
ctr st
ele taly
ca
C=
C=
o
ctr st
ele taly
ca
Fuel
10
Figure 4: Plot of variation of nitrogen to carbon ratio obtained from XPS as a
function of mole percentage of acetic anhydride in conc. nitric acid.
• presence of nitro and amine groups can be detected by X-ray photoelectron
spectroscopy (XPS).
C=O
motor
graphitic
carbon
electrode
0
mole % of acetic anhydride
in conc. HNO3
• nitro groups can be converted into amine groups by chemical as well as
electrochemical methods.
electrocatalyst
reduction
catalyst
0.0
-10
• aromatic nitration is self-limiting due to the deactivating effect of aromatic
nitro groups on subsequent nitration and leads to site-isolated introduction of
nitro groups.
H
oxidation
catalyst
1.5
1.0
Polymer-Electrolyte-Membrane Fuel Cell
e-
2.0
• sodium dithionite (Na2S2O4) is used to reduce nitro groups into amine groups.
0.2
0.4
0.6
0.8
1.0
1.2
+
Electrochemical Potential (V vs. H /H2) 0
0
E CO +H O/CH
2.5
Useful features of nitration in surface modfication
How much work can be obtained
and heat avoided
with discrete electrocatalysts?
hydrocarbon
(CH2)n
reversible
potentials
free energy
converted
to work
~ 1 x 1014 molecules/cm2
3.5
Strategy for surface modification:
N/C x 100
Towards More Energy Efficient Conversions
amine
NH H
H
Cl
-HCl
(CH3CO)2O
Figure 2: Schematic diagram of nitration and subsequent modification of
graphitic edges with electrocatalysts.
Nitration of carbon surfaces with nitric acid and acetic anhydride*
mixtures - XPS studies
Molecular Electrocatalysts on Carbon Electrodes
394 396 398 400 402 404 406 408 410 412
nitro
Carbon
• Current PEM-cell electrocatalysts are noble metal
nanoparticles – limited ability to tailor reactivity
binding energy (eV)
Figure 5: XPS data showing the presence of amine groups on the nitrated
carbon surfaces treated with sodium dithionite.
• Our research aims in using coordination complexes of one
or a few metal atoms covalently attached to graphitic carbon.
Oxygen
Nitrogen
Modifying carbon surfaces
390
Important requirements:
• site-isolation of molecular catalysts on the surface
• high surface coverage
• adequate electrical coupling between the carbon surface and the
molecular catalyst
400
410
binding energy (eV)
200
250
300
350
400
450
500
550
600
binding Energy (eV)
Figure 3: XPS data of a carbon surface nitrated using 63:37 mixture of nitric
acid:acetic anhydride. (inset shows the zoomed nitrogen region)
* acetic anhydride is used as a dehydrating agent
420
Conclusion: A method to introduce site-isolated reaction centers on carbon
surfaces to which molecular catalysts can be covalently coupled is developed
by aromatic nitration. The presence of nitro groups and amino groups is
confirmed by XPS technique. Further work on coupling carbonyl chloride
containing electrocatalyst to the amine terminated carbon surfaces are
underway.
Acknowledgements:
• project funded by GCEP
• Prof. Robert Waymouth and Prof. Daniel Stack for discussions