Molecular electrocatalysis for oxygen reduction by cobalt porphyrins adsorbed at liquid/liquid interfaces

Article abstract of DOI:10.1021/ja908488s

Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE) interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and amphiphilic cobalt porphyrin catalysts adsorbed at the interface. The catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p- amino-phenylporphyrin) cobalt(II) (CoAP), functions like conventional cobalt porphyrins, activating O2 via coordination by the formation of a superoxide structure. Furthermore, due to the hydrophilic nature of the aminophenyl group, CoAP has a strong affinity for the water/DCE interface as evidenced by lipophilicity mapping calculations and surface tension measurements, facilitating the protonation of the CoAP-O₂ complex and its reduction by ferrocene. The reaction is electrocatalytic as its rate depends on the applied Galvani potential difference between the two phases.

Full text of DOI:10.1021/ja908488s

Published on Web 02/04/2010
Molecular Electrocatalysis for Oxygen Reduction by Cobalt
Porphyrins Adsorbed at Liquid/Liquid Interfaces
Bin Su,†,9 Imren Hatay,†,⊥ Anton´ın Troja´nek,‡ Zdeneˇk Samec,‡ Tony Khoury,§
Claude P. Gros,§ Jean-Michel Barbe,§ Antoine Daina,¶ Pierre-Alain Carrupt,¶ and
Hubert H. Girault*,†
Laboratoire d’Electrochimie Physique et Analytique, Ecole Polytechnique Fe´de´rale de
Lausanne, Station 6, CH-1015 Lausanne, Switzerland, J. HeyroVsky´ Institute of Physical
Chemistry of ASCR, V.V.i, DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic, Institut de
Chimie Mole´culaire de l’UniVersite´ de Bourgogne, ICMUB (UMR 5260),
BP 47870, 21078 Dijon Cedex, France, Department of Chemistry, Selcuk UniVersity,
42031 Konya, Turkey, Section des Sciences Pharmaceutiques, Quai Ernest-Ansermet 30,
CH-1211 Gene`Ve 4, Switzerland
Received October 5, 2009; E-mail: hubert.girault@epfl.ch
Abstract: Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE)
interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and amphiphilic cobalt porphyrin
catalysts adsorbed at the interface. The catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-
phenylporphyrin) cobalt(II) (CoAP), functions like conventional cobalt porphyrins, activating O2 via
coordination by the formation of a superoxide structure. Furthermore, due to the hydrophilic nature of the
aminophenyl group, CoAP has a strong affinity for the water/DCE interface as evidenced by lipophilicity
mapping calculations and surface tension measurements, facilitating the protonation of the CoAP-O2
complex and its reduction by ferrocene. The reaction is electrocatalytic as its rate depends on the applied
Galvani potential difference between the two phases.
Introduction
catalytic activity has been investigated either electrochemically
using the modified electrode methodology9-25 or chemically
Molecular oxygen (O2) reduction reaction (ORR) is an
important topic in biological and energy-related chemistry.1,2
However, it is a spin-forbidden process, which is kinetically
slow at ambient temperature unless a catalyst is present. Nature
has made the choice in aerobic organisms by evolving membrane-
bound enzymes that contain a porphyrin substructure as
catalysts.1,3-6 Inspired by their role in nature, a number of
metalloporphyrins have been chemically synthesized.7-9 Their
using molecular electron donors, such as ferrocene (Fc) and its
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† Ecole Polytechnique Fe´de´rale de Lausanne.
‡ J. Heyrovsky´ Institute of Physical Chemistry of ASCR.
§ Institut de Chimie Mole´culaire de l’Universite´ de Bourgogne.
⊥ Selcuk University.
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¶ Section des Sciences Pharmaceutiques.
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9 Present Address: Institute of Microanalytical Systems, Department of
Chemistry, Zhejiang University, Hangzhou 310058, China.
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10.1021/ja908488s  2010 American Chemical Society
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A R T I C L E S
Su et al.
derivatives, in homogeneous solutions.26-28 In the latter case,
reduction of O2 by Fc in acidic solutions29-32 proceeds rather
slowly, and the presence of a catalytic amount of metallopor-
phyrins can significantly accelerate the reaction rate.26-28
Scheme 1. Electrochemical Cells Employed
On the other hand, ORR is a proton-coupled electron transfer
(PCET) reaction. When studying ORR on a solid electrode by
amperometry, one measures the electron transfer rate, but it is
not possible to control the proton transfer step by controlling
the electrode potential. Electrochemistry at a liquid/liquid
interface, also called interface between two immiscible elec-
trolyte solutions (ITIES), has manifested itself recently as a
unique approach to study PCET reaction with the possibility of
locating protons in the aqueous phase and electron donors in
the organic phase.33 In this work, proton-coupled oxygen
reduction by Fc involving an amphiphilic cobalt porphyrin
catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-
phenylporphyrin) cobalt(II) (CoAP), at a polarized water/1,2-
dichloroethane (DCE) interface is reported. CoAP serves as a
redox catalyst like conventional cobalt porphyrins, activating
O2 reduction via coordination with the cobalt(II) (CoII) center
by the formation of a superoxide structure. Moreover, since
CoAP is an amphiphilic molecule with a strong interfacial
affinity, the proton-coupled oxygen reduction is an interfacial
process. These two factors play together to account for the
excellent electrocatalytic activity of CoAP, which is better than
those previously reported for cobalt porphine (CoP),33 cobalt
tetraphenylporphyrin (CoTPP),34 and cobalt octaethylporphyrin
Co(OEP)35 that are not amphiphilic. The present system provides
an example of interfacial molecular electrocatalysis for oxygen
reduction combining the advantages of molecular catalyst and
electrocatalysis because the ORR depends on the applied
Galvani potential difference between the two phases.
Electrochemical Measurements. All the electrochemical mea-
surements were performed at ambient temperature (23 ( 2 °C).
Cyclic voltammograms (CVs) at the water/DCE interface were
obtained on a custom-built four-electrode system equipped with a
DS335 synthesized function generator (Stanford Research System).
A three-compartment glass cell featuring a cylindrical vessel was
used, where the water/DCE interface with a geometric area of 1.53
cm2 was formed. Two platinum counter electrodes were respectively
positioned in the aqueous and DCE phases to supply the current
flow. The external potential was applied by means of two silver/
silver chloride (Ag/AgCl) reference electrodes, which were con-
nected respectively to the aqueous and DCE phases by means of a
Luggin capillary. The electrochemical cells used are illustrated in
Scheme 1. The potential was converted to the Galvani potential
difference (∆wo φ), based on cyclic voltammetric measurement of
the reversible half-wave potential of the TMA+ ion transfer (0.160
V).37 Measurements were carried out under aerobic conditions
unless specified otherwise.
Surface Tension Measurements. Surface tension measurements
were performed using the video-image pendant-drop method38,39
in a four-electrode all-glass cell, which was adapted from a
spectroscopic cuvette (10 mm × 10 mm). The cell was connected
to a four-electrode potentiostat (PGSTAT 30, Eco-Chemie, Neth-
erlands). The cell potential was changed stepwise (25 mV per step,
25 s step time), and on each step the drop image was captured (24
s after the potential step change), and analyzed with the help of
CAM101 System (KSV Instruments Ltd., Finland). Potential scale
conversion was based on the reversible half-wave potential of the
TEA+ ion transfer.37 Measurements were carried out with air-
saturated solutions.
Two-Phase Reactions Controlled by the Distribution of a
Common Ion. Two-phase reactions were performed in small glass
flasks with a volume of 10 mL. A flask was filled first with 2 mL
of DCE solution containing reactants (Fc, CoAP), followed by the
addition of 2 mL of aqueous solution containing 10 mM HCl. The
salts of the common ion (TB-), LiTB and BATB, were added in
the same concentration of 5 mM to the aqueous and DCE phases,
respectively. After stirring and further waiting for the clear
separation of two phases, the aqueous and organic solutions were
isolated from each other. The organic phase was directly subjected
to the UV-visible spectroscopic measurement (Ocean Optical
CHEM2000 spectrophotometer, quartz cuvette with a path length
10 mm), while the aqueous phase was first treated by excess NaI
(equivalent to 0.1 M) prior to the UV-visible spectroscopic
measurement.
Experimental Section
Chemicals. All chemicals were used as received without further
purification. The aqueous solutions were prepared with ultrapure
water (18.2 MΩ cm-1). Ferrocene (Fc, 98%), 1,1′-dimethylferrocene
(DFc, 97%), ferrocenecarboxaldehyde (FcCA, 98%), trifluoroacetic
acid (TFA, 98%), and lithium tetrakis(pentafluorophenyl)borate
(LiTB) diethyl etherate were purchased from Aldrich. Lithium
chloride anhydrous (LiCl, g99%), NaI (>99.5%), starch (from
potatoes), tetramethylammonium chloride (TMACl, >98.0%), tet-
raethylammonium chloride (TEACl, >98.0%), bis(triphenylphos-
phoranylidene)ammonium chloride (BACl, g98%), and 1,2-
dichloroethane(DCE,g99.8%)wereorderedfromFluka.Hydrochloric
acid (HCl, 32%) was bought from Merck. Bis(triphenylphospho-
ranylidene)ammonium tetrakis(pentafluorophenyl)borate (BATB)
was prepared as previously reported.36
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Lipophilicity Mapping. The lipophilicity distribution around
molecules was characterized by the molecular lipophilicity potential
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C34.
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Activation of ORR by CoAP at a Water/DCE Interface
A R T I C L E S
Scheme 2. Molecular Structure of CoAP
(MLP) calculated at any given point in space around a molecule
using two components for the calculation, namely a fragmental
system of lipophilicity and a distance function as expressed by
eq 1:
variations of the fragmental values for the cobalt atom defined as
a polar monocentric atom. Thus, a fragmental value calibrated using
atomic charges was retained for the cobalt atom.
In CLIP representations, the color coding follows a scale starting
from the most polar to the most hydrophobic regions, namely red,
orange, yellow, white, green, green-blue, blue. In MolCAD
representations, the color coding follows a scale starting from the
most polar to the most hydrophobic regions, namely blue, green-
blue, green, brown, dark brown.
N
MLP )
f · fct(d )
(1)
∑
k
i
ik
i)1
where k indicates a given point in space, i a given molecular
fragment, N the total number of fragments in the molecule, fi
the lipophilic increment of fragment i, fct a distance function,
and dik the distance between fragment i and point k.
Two different approaches were used to calculate the MLP,
namely the CLIP software40,41 and MolCAD software42 as imple-
mented in SYBYL 8.0.43 CLIP calculates the MLP on the water
accessible surface area of a compound and displays the result as
colored dots, whereas the MOLCAD calculates the MLP on its
molecular surface and displays the result as a colored surface. The
two approaches used different atomic fragmental systems, namely
the Broto and Moreau system44 for CLIP and the Ghose and
Crippen system45,46 for MOLCAD but a similar distance function
reported in eq 2:
Results and Discussion
Synthesis of CoAP. The synthetic pathway is given in Scheme
2. First, 2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-
phenylporphyrin (H2AP) was synthesized by a method different
from that described previously by Senge et al.47 Indeed, in this
previous report, the authors were reacting H2OEP (OEP:
2,3,7,8,12,13,17,18-octaethylporphyrin) with an excess of the
organolithium reagent of p-bromoaniline at low temperature.
In the present case, the starting material was not the corre-
sponding free base but the 1,19-dideoxy-3,8,12,17-tetraethyl-
2,7,13,18-tetramethylbiladien-a,c-dibromide already described
by our group and readily available in large amounts.48 Reaction
of p-nitrobenzaldehyde with the a,c-biladien gave the corre-
sponding nitroporphyrin which was subsequently converted to
H2AP by reduction with tin(II) dichloride. CoAP was obtained
by reacting H2AP with cobalt acetate tetrahydrate in a refluxing
CHCl3/MeOH mixture. All compounds were fully characterized
N
1 + e-ab
MLP )
f ·
(2)
∑
k
i
1 + ea(d -b)
ik
i)1
where a and b define respectively the decay and the distance cutoff
for the sigmoidal variation of the MLP with respect to the distance
(dik) between the dot k and the fragment i. A fragmental value has
to be assigned to the cobalt atom not parametrized in both
fragmental atomic systems. The qualitative distribution of lipophi-
licity on molecular surfaces was not largely affected by large
1
by HRMS, H NMR, and UV-visible spectroscopy. Detailed
synthetic procedures and spectroscopic data are given in the
Supporting Information.
Interfacial Adsorption and Protonation of CoAP. CVs in the
absence and presence of 50 µM CoAP in DCE are shown in
Figure 1a. The current observed at the negative and positive
ends of the potential window is due to the transfer of Cl- and
H+ from water to DCE, respectively. Compared to the back-
ground current, the presence of CoAP resulted in a large
capacitive current indicating the presence of an adsorbed layer.
To verify this point, surface tension measurements were
(40) Gaillard, P.; Carrupt, P. A.; Testa, B.; Boudon, A. J. Comput. Aided
Mol. Des 1994, 8, 83–96.
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Res. 1996, 13, 335–343.
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9
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A R T I C L E S
Su et al.
Figure 1. (a) CVs in the absence (x ) 0, black line) and presence (x ) 50
µM, red line) of CoAP in DCE using Cell 1: y ) 0 (no ferrocene), pH 2,
scan rate 0.05 V s-1. (b) Electrocapillary curves in the absence and presence
of CoAP using Cell 2: z ) 0 (black) and 50 µM (red). The measurement
started from the negative potential, and the electrocapillary curve obtained
in the reverse direction (blue) is the same.
Figure 2. Lipophilicity maps for CoAP using CLIP (top) and MOLCAD
(bottom).
(Co-O2)AP can be protonated to form [(Co-O2H)AP]+ by
protonation of the bound oxygen, The protonation of the
amino group usually takes place at much more positive
potentials or at much lower pH values.50 The adsorbed
[(Co-O2H)AP]+ will desorb to the organic phase at a positive
potential, leading to a decrease of interfacial coverage of
[(Co-O2H)AP]+ and an increase of the surface tension. The
surface tension data thereby point to the formation of
[(Co-O2H)AP]+ that undergoes an electrochemical ion
transfer reaction from the adsorbed state:
performed using a video-image pendant-drop method,38,39 and
the obtained electrocapillary curves are displayed in Figure 1b.
The parabolic shape of the electrocapillary curve in the absence
of CoAP is characteristic of two back-to-back Gouy-Chapman
diffuse layers without any specific adsorption of electrolyte
ions.49 The addition of CoAP in DCE results in a significant
decrease in the surface tension primarily at potentials negative
to the electrocapillary maximum (potential of zero charge, pzc),
indicating the adsorption of CoAP species. Toward positive
potentials, the surface tension starts to increase and finally
overlaps with the surface tension of the background electrolytes.
It has to be mentioned that the measurements started from
negative potentials, but that surface tension data obtained in
the reverse direction are practically identical.
(Co-O2)AP(ads) + H+(W) f [(Co-O2H)AP]+(ads)
[(Co-O2H)AP]+(ads) f [(Co-O2H)AP]+(DCE)
(3)
(4)
The adsorption of CoAP stems from its amphiphilicity as
illustrated in Figure 2, with the amino group as the hydrophilic
moiety and the metalated ring as the lipophilic part. The
interfacial activity of CoAP is also related to its acid-base
properties and to the concentration of protons on the aqueous
side of the interface, which increases at potentials positive to
the pzc. Thus, the significant decrease of the surface tension at
very negative potentials can be attributed to the adsorption of
neutral CoAP and oxygenated CoAP, (Co-O2)AP, at the
interface. As shown below, CoAP and (Co-O2)AP coexist in
the stock sample both in solid and in solution. When scanning
the potential more positive than the pzc, the proton concen-
tration at the interface is increased and the adsorbed
Here, the species in water, in DCE and adsorbed at the
interface are denoted as (w), (DCE), and (ads), respectively. It
is well-known that cobalt porphyrins are able to coordinate O2,
thereby forming a superoxide-like structure, which can be
protonated to produce hydroperoxyl radicals.51,52
Figure 3a shows that the surface tension decreases consider-
ably with increasing the concentration of CoAP in DCE. This
dependence can be used to evaluate the surface excess concen-
tration of CoAP and to determine the adsorption isotherm on
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Activation of ORR by CoAP at a Water/DCE Interface
A R T I C L E S
Figure 4. (a) CVs in the presence of only 5 mM Fc (black line, x ) 0, y
) 5 mM) and in the presence of both 5 mM Fc and 25 µM CoAP (x ) 25
µM, y ) 5 mM) in DCE (red line) using Cell 1: pH 2. The blue line was
obtained under anaerobic conditions but others same as the red line, scan
rate 0.05 V s-1; (b) 20 cycles of CVs in the presence of both 5 mM Fc and
25 µM CoAP using Cell 1 (x ) 25 µM, y ) 5 mM) under aerobic (black
Figure 3. (a) Electrocapillary curves at various concentrations of CoAP
using Cell 2: z ) 10 µM (black), 25 µM (red), 50 µM (blue), 100 µM
(green), and 200 µM (brown). The measurements started from the negative
potential. (b) CoAP adsorption isotherm constructed with the surface tension
data at -0.10 V. The red line corresponds to a fit to the Langmuir isotherm.
lines) and anaerobic (red lines) conditions, scan rate 0.05 V s-1
.
The fitting yields Γ* ) (1.03 ( 0.04) × 10-6 mol m-2 and
the basis of the Gibbs adsorption equation (see the derivation
in the Supporting Information):53
CoAP
ꢀ ) (3.29 ( 0.04) × 104 M-1. From Γ*CoAP, the area per molecule
occupied amounts 161 Å2, which suggests that adsorbed CoAP
species take an orientation normal to the interface with the
hydrophilic amino group toward the aqueous phase.
ΓCoAP + Γ(Co-O )AP + Γ[(Co-O H)AP]
)
+
2
2
1
∂γ
-
(5)
Interfacial Proton-Coupled Oxygen Reduction Catalyzed
by CoAP. Figure 4a compares the CVs obtained under different
experimental conditions. First, in the presence of only Fc in
DCE under aerobic conditions a small voltammetric wave with
a half-wave potential at ∆wo φ1/2 ) 0 V was observed, which
corresponds to the transfer of ferrocenium (Fc+) produced by a
slow oxidation of Fc in air of the stock solution.33,34 Also shown
above in Figure 1a is that in the presence of only CoAP in
DCE under aerobic conditions only an overall capacitive
current signal was observed. However, when both Fc and
CoAP were dissolved in DCE, an irreversible positive current
signal appeared in the positive potential range. A control
experiment under anaerobic conditions showed that this
current signal did not appear (blue line in Figure 4a). These
facts suggest that CoAP, O2, and Fc must be present at the
same time to observe this signal. Therefore, similar to that
observed with cobalt porphine,33 this irreversible current
signal corresponds to a proton-coupled oxygen reduction
process catalyzed by CoAP. This process produces Fc+, thus
accounting for the significant increment of the Fc+ transfer
current wave located at 0 V as shown in Figure 4a. Indeed,
the standard redox potential for CoIIIAP+/ CoIIAP is equal
to 0.69 V vs SHE (see Supporting Information, Figure S-1)
more positive than that of Fc equal to 0.64 V vs SHE. In
(
)
RT ∂ ln cCoAP
T,P,∆wo φ,µi*µCoAP
where cCoAP is the bulk concentration of CoAP in DCE and Γi
(i ) CoAP, (Co-O2)AP and [(Co-O2H)AP]+) represent the
respective surface excess concentration. In terms of eq 5, the
surface coverage of CoAP species has three contributions. At
positive potentials [(Co-O2H)AP]+ is formed, the deconvolution
of this contribution from two others is difficult because both
the adsorption and transfer processes are involved. However,
at potentials more negative than -0.1 V, the adsorption of
neutral CoAP and (Co-O2)AP is dominant. Assuming, for
simplicity, that CoAP and (Co-O2)AP have the same molecular
structure and interfacial affinity, the concentration dependence
of the surface excess concentration evaluated at -0.1 V (shown
in Figure 3b), can be fitted to a Langmuir isotherm:
ꢀcCoAP
*
ΓCoAP ) Γ
(6)
CoAP1 + ꢀcCoAP
where Γ*CoAP and ꢀ are the maximum surface excess concentra-
tion and Langmuir adsorption coefficient of CoAP, respectively.
(53) Samec, Z.; Trojanek, A.; Girault, H. H. Electrochem. Commun. 2003,
5, 98–103.
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A R T I C L E S
Su et al.
Figure 5. (a) Photographs of two-phase reactions. The composition of the top aqueous phase is the same for three flasks: 5 mM LiTB + 10 mM HCl. The
DCE phase contains: 1 mM Fc + 5 mM BATB (flask 1), 1 mM Fc + 20 µM CoAP + 5 mM BATB (flask 2), and 20 µM CoAP + 5 mM BATB (flask 3).
The inset in (a) shows the colors of three DCE solutions before contact with the water solution; (b) isolated top aqueous solutions with added excess
NaI; (c) further addition of starch to the flasks shown in (b); (d) 4-h two-phase reaction in the glovebox with the same solution composition as that
of flask 2 in (a).
fact, when cycling the potential to the positive values
repeatedly, more Fc+ will be produced, leading to a continu-
ous increase of the Fc+ transfer current scan after scan (black
lines in Figure 4b). In contrast, under anaerobic conditions,
the Fc+ transfer current wave did not increase with repeated
potential scans (red lines in Figure 4b).
The reduction of O2 by Fc was also investigated by so-
called shake-flask experiments performed as reported previ-
ously.33 The Galvani potential difference across the water/
DCE interface was polarized at a very positive value of 0.54
V by two salts, LiTB and BATB, having a common anion
of TB-. At such potential values, protons are transferred from
water to DCE by the lipophilic aqueous TB- ions acting as
a proton pump dragging with them protons as they transfer
to the organic phase. Results of the shake-flask experiments
are shown in Figure 5. As shown in Figure 5a, 10 mM HCl
was present in water in all three flasks, and the DCE phase
contained only 1 mM Fc in flask 1, only 20 µM CoAP in
flask 3, and both in flask 2. It was observed that the DCE
phase in flask 2 changed its color from pink (inset in (a)) to
dark green immediately after being put in contact with the
water solution. In contrast, the DCE phase in the other two
flasks remained the same. The two phases were then separated
from each other for further spectroscopic and colorimetric
tests.
First, UV-visible spectra of the separated DCE solutions
were measured, as shown in Figure 6a. The formation of Fc+
Figure 6. (a) Absorption spectra of the DCE phases shown in Figure 5a:
flask 1 (red), flask 2 (blue), and flask 3 (green), and of a fresh 1 mM Fc
(full black) and a fresh 20 µM CoAP (dotted black); (b) UV-visible spectra
of the 10 times diluted aqueous solutions shown in Figure 5b: flask 2 (blue)
and flask 1 and 3 (green).
in the DCE solution from flask 2 was revealed, and the
absorption band with a maximum at 620 nm represents its
signature (blue line). The color change is thus due to the
oxidation of Fc to Fc+, and the color of DCE phase in the flask
2 reflects a mixed color of green Fc+ and pink CoAP. In contrast,
in the presence of Fc only in DCE (flask 1) no Fc+ was detected,
because no changes in color and in the absorption spectrum
(red line compared to full black line) were observed. On the
other hand, when only CoAP is present in DCE (flask 3),
changes in the absorption spectrum (green line compared to
dotted black line) due to the oxidation of CoIIAP to CoIIIAP+
and the protonation of the amino group (see Supporting
Information, Figure S-2) were observed. As for the isolated
aqueous solutions from three flasks, excess NaI equivalent to
0.1 M was added. It was observed that the color from flask 2
changed from colorless to yellow as illustrated in Figure 5b.
As shown previously, this color change can be due to the
presence of H2O2 in the solution.54 H2O2 is a strong oxidant
that can oxidize I- to I3-, which appears yellow and displays
two absorption bands (λmax ) 286 and 352 nm) as shown by
(54) Su, B.; Nia, R. P.; Li, F.; Hojeij, M.; Prudent, M.; Corminboeuf, C.;
Samec, Z.; Girault, H. H. Angew. Chem., Int. Ed. 2008, 47, 4675–
4678.
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Activation of ORR by CoAP at a Water/DCE Interface
A R T I C L E S
-
the blue line in Figure 6b. Additionally, I3 changes its color
(Co-O2)AP(DCE) f (Co-O2)AP(ads)
(9)
to very brown when meeting with aqueous soluble starch (Figure
5c). In contrast, no H2O2 was detected at all in the aqueous
solutions from flasks 1 and 3, as seen both from the colorimetric
responses and the UV-visible spectra (green curve in Figure
6b).
(Co-O2)AP(ads) + 2H+(W) + Fc(DCE) f CoAP(+DCE)
+
Fc+(DCE) + H2O2(W) (10)
CoAP+(DCE) + Fc(DCE) f CoAP(DCE) + Fc(+DCE)
(11)
The above experimental facts clearly demonstrated that H2O2
and Fc+ were produced in water and DCE, respectively, only
when both Fc and CoAP were present (flask 2). A control
experiment under anaerobic conditions (Figure 5d) showing that
the DCE solution containing Fc and CoAP did not change its
color even after 3 h, further proving the reduction of O2 to H2O2
by Fc catalyzed by CoAP. Similar to other conventional cobalt
porphyrins, CoAP activates O2 by the formation of a superoxide-
like intermediate with O2 coordinated to cobalt, which can be
seen clearly in the UV-visible spectrum of CoAP. As shown
in Figure 6a, a freshly prepared CoAP solution displays two
Soret bands (λmax ) 397 and 423 nm) and a broad convoluted
Q-band (λmax ) 552 nm). The second Soret band reflects that
the solution contains (Co-O2)AP in which the electron
delocalization from Co(II) to O2 occurs, leading to a
bathochromic shift of both Soret and Q bands in the
UV-visible spectrum.51,52
In terms of this scheme, the Nernst equation for the overall
process is:33
RTln 10
∆wo φet ) ∆wo φ0et +
pH +
aoFc aoCoAP
F
+
+
RT
2F
p0
ln
awH O
(12)
o
o
2
2
(
[
)(
)
( )
fO
]
aFc aCoAP
2
where aRi (R ) o or w) represents the activity of species i. p0
and fO are the standard pressure and the fugacity of oxygen.
2
∆wo φ0et represents the standard Galvani potential difference given
by:33
1
2
1
2
o
o
0
w
∆wo φ0et ) [E0CoAP
]
+
[E0Fc
]
- [E ]
O2/H2O2 SHE
+
+
/Fc SHE
/CoAP SHE
(13)
Mechanism and Thermodynamic Analysis. An overall reac-
tion of oxygen reduction with Fc catalyzed by CoAP is:
0
o
0
o
0
w
+
+
where [ECoAP
]
/CoAP SHE, [EFc
]
/Fc SHE, and [EO /H O
]
SHE denote the
2
2
2
standard redox potentials of the CoAP+/CoAP and Fc+/Fc
couples in DCE and the O2/H2O2 couple in water, respectively,
all with respect to the aqueous standard hydrogen electrode
(SHE). Equation 12 predicts the pH dependence of the redox
Galvani potential difference, which is corroborated by the shift
of the current signal with pH by about 60 mV/pH as shown in
Figure 7. Equations 12 and 13 also indicate a dependence on
the standard redox potential of the organic electron donor. When
Fc is replaced either by a stronger electron donor, DFc, or by
a weaker one, FcCA, the onset potential of the irreversible
current shifts to negative and positive potentials, respectively,
as displayed in Figure 8a. A negative shift of 45 mV was
experimentally observed for DFc with a logarithmic plot (Figure
2Fc(DCE) + 2H+(W) + O2(DCE) CoAP8 2Fc(+DCE) + H2O2(W)
(7)
A corresponding reaction scheme involving the adsorption
of (Co-O2)AP (dashed line), proton-coupled oxygen reduc-
tion (solid line), and regeneration of CoAP+ by Fc (dotted
line) is illustrated in Scheme 3. These steps can be expressed
as follows:
CoAP(DCE) + O2(DCE) f (Co-O2)AP(DCE)
(8)
Scheme 3. Catalysis Mechanism
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A R T I C L E S
Su et al.
as voltammetry only measures interfacial charge transfer reac-
tions. The interfacial affinity of CoAP accounts for the excellent
catalytic activity observed by measuring the PCET current.
Indeed, it is much more efficient than cobalt porphine and
2,3,7,8,12,13,17,18-octaethylporphyrin cobalt(II) (CoOEP) which
are not amphiphilic (see Supporting Information, Figure S-3).33,35
The comparison with CoOEP highlights in particular the role
of the amphiphilicity of the catalyst as both CoAP and CoOEP
have the same redox potential for the first oxidation (0.69 V vs
SHE).
Conclusions
In summary, the catalytic effect of (2,8,13,17-tetraethyl-
3,7,12,18-tetramethyl-5-p-amino-phenylporphyrin) cobalt(II)
(CoAP) on the proton-coupled oxygen reduction by Fc was
studied at a water/DCE interface. CoAP serves as a redox
catalyst like conventional monomeric cobalt porphyrins,
activating O2 for the reduction to H2O2. On the other hand,
since CoAP has a strong interfacial affinity due to the amino
group at a meso position, it presents an excellent electro-
catalytic activity over other cobalt porphyrins that do not
adsorb at the water/DCE interface such as the nonamphiphilic
analogue CoOEP. This study demonstrates the concept of
interfacial molecular electrocatalysis for the reduction of
oxygen combining the use of molecular catalysts under
electrochemical control.
Figure 7. CVs in the presence of both 5 mM Fc and 25 µM CoAP (x )
25 µM, y ) 5 mM) using Cell 1 at various pH, scan rate 0.02 V s-1
.
In a general manner, the advantage of using amphiphilic
molecular catalysts at polarized liquid-liquid interfaces stems
from the fact that catalysts may be oxidized and destroyed during
the course of the reaction by spurious radical side reactions.
The degraded catalysts are likely to lose their amphiphilicity
and to desorb from the interface where new catalysts will be
able to adsorb from the bulk. This is similar to what happens
in natural photosystems, in which the molecular catalysts have
a finite lifetime and are replaced permanently by freshly
synthesized ones.
Acknowledgment. This work was supported by EPFL, the
Swiss Natural Science Foundation (FNRS 200020-116588), Eu-
ropean Union COST Action (D36/007/06), the Grant Agency
of the Czech Republic (No. 203/07/1257), CNRS (UMR 5260),
and the Scientific and Research Council of Turkey (TUBITAK)
under the 2212-PhD Scholarship Program.
Figure 8. (a) CVs in the presence of 25 µM CoAP and 5 mM Fc (black),
DFc (red), and FcCA (blue) using Cell 1 (x ) 25 µM, y ) 5 mM), scan
rate 0.02 V s-1. (b) Logarithmic plots of the catalytic current in the forward
scan for Fc (black) and DFc (red).
Supporting Information Available: Porphyrin synthesis pro-
cedures, measurements of redox potentials of CoAP, math
derivation of surface excess concentration (eq 5), spectropho-
tometric titration of CoAP with TFA, and the comparison of
catalysis of CoAP with other cobalt porphyrins. This material
is available free of charge via the Internet at http://pubs.acs.org.
8b), which is in agreement with the theoretical prediction of
eqs 12 and 13.
Finally, it should be mentioned that the proton-coupled
oxygen reduction catalyzed by CoAP is an interfacial process,
JA908488S
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2662 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010