Electrooxidation of Fe, Co, Ni and Cu metalloporphyrins on edge-plane pyrolytic graphite electrodes and their electrocatalytic ability towards the reduction of molecular oxygen in acidic media

Article abstract of DOI:10.1002/ejic.200900651

Edge-plane pyrolytic graphite (EPG) electrodes coated with 5,10,15,20-tetrakis(4-hydroxy-3-m.ethoxyphenyl)porphyrin [H₂T3M4HPP] and its Fe^III, Co^II, Ni^II and Cu^II analogs undergo an electrochemical/ch

Full text of DOI:10.1002/ejic.200900651

FULL PAPER
DOI: 10.1002/ejic.200900651
Electrooxidation of Fe, Co, Ni and Cu Metalloporphyrins on Edge-Plane
Pyrolytic Graphite Electrodes and Their Electrocatalytic Ability towards the
Reduction of Molecular Oxygen in Acidic Media
Gregory Richards[a] and Shawn Swavey*[a]
Keywords: Metalloporphyrin / Electrocatalysis / Electrooxidation / Electrochemistry / Oxygen / Reduction / Oxidation
Edge-plane pyrolytic graphite (EPG) electrodes coated with
additional catalytic shift of approximately 100 mV is ob-
5,10,15,20-tetrakis(4-hydroxy-3-methoxyphenyl)porphyrin served for O2 reduction at an EPG electrode coated with the
[H2T3M4HPP] and its FeIII, CoII, NiII and CuII analogs un-
dergo an electrochemical/chemical/electrochemical (ECE)
CoII porphyrin, which has been oxidized in 0.5
M H2SO4. In
addition to the added electrocatalysis, approximately 15% of
the O2 reduced at the oxidized CoII porphyrin EPG electrode
is converted to H2O as determined by rotating-disk electrode
measurements.
reaction when anodically scanned in 0.5
M H2SO4. The new
redox couple formed from this anodic conditioning of the
coated electrode is dependent on the pH of the solution.
Roughened EPG electrodes coated with the metalloporphyr-
ins show a catalytic shift for the reduction of O2 when com-
pared to the reduction of O2 at a bare EPG electrode. An
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
rectly to water but at lower potentials than cobalt(II) por-
phyrins resulting in a lower energy output from the fuel
cells. Studies of metal phthalocyanines for oxygen reduction
in acidic media indicate the order of activity to be Fe Ͼ Co
Ͼ Ni Ͼ Cu.[7] For square-planar geometries this order has
been explained by using the MO theory in which the forma-
tion of the oxygen radical involves a π–σ bond and is fa-
vored by an empty dz2 orbital with filled dxz and dyz orbitals
interacting with the partially filled π* orbitals of oxygen
through π-backbonding. In the case of the Fe2+, Co2+, Ni2+
and Cu2+ metal centers in square-planar geometry all have
filled dxz and dyz orbitals, whereas only Fe2+ has an empty
dz2 orbital, and Co2+ has a half-filled dz2 orbital.[8]
Some ingenious complexes have been synthesized in the
hopes of converting cobalt(II) porphyrins into electrocata-
lysts capable of reducing oxygen directly to water at high
positive potentials. Cofacial dicobalt porphyrins, for exam-
ple, have been shown to catalyze the reduction of oxygen to
water through direct interaction of both metal centers with
the two oxygen atoms of O2.[9–11] However, few examples
exist in which a monomeric cobalt porphyrin has been
shown to catalyze the four-electron reduction of oxygen. In
a series of studies it was shown that by coordination of 3
or 4 substitutionally inert RuII(NH3)5 moieties to the meso-
pyridyl nitrogen atoms of [tetrakis(4-pyridyl)porphyrinato]-
cobalt(II), adsorbed onto pyrolytic graphite electrodes, the
direct four-electron reduction of oxygen to water at catalytic
potentials could be achieved.[12] It was initially thought that
the peripheral RuII redox centers donated electrons to the
CoII metal center leading to the reduction of O2 directly
to H2O. Subsequent studies revealed that the multi-electron
Introduction
One of many alternatives to combustion engines, which
use fossil fuels, under investigation is fuel cells. Fuel cells
work like a galvanic cell (battery) converting chemical en-
ergy into electrical energy capable of doing work. The fuels
used in this process are typically hydrogen and oxygen al-
though methanol/oxygen fuel cells are also receiving much
attention. In each case, oxygen is catalytically reduced to
water at the cathode. This multi-electron process is compli-
cated by high overpotentials, proton dependence and the
formation of deleterious intermediates.[1] Platinum is cur-
rently the catalyst of choice in fuel cells.[2] However, alterna-
tives to this expensive precious metal are continually being
sought.
Macrocyclic complexes have been extensively studied as
electrocatalysts in fuel-cell applications.[3,4] An ideal electro-
catalyst for fuel-cell applications should reduce oxygen by
four electrons directly to water at a potential near the ther-
modynamically allowed value and should avoid formation
of potentially destructive intermediates such as superoxide
or hydrogen peroxide. Within the class of macrocyclic com-
plexes, cobalt and iridium metalloporphyrins have shown
the most promise for the electrocatalytic reduction of oxy-
gen.[5,6] Cobalt(II) porphyrins are known to reduce oxygen
at the most positive potentials; however, in most cases this
reduction is by two electrons to form hydrogen peroxide.
Iron(II) porphyrins electrocatalytically reduce oxygen di-
[a] Department of Chemistry, University of Dayton,
300 College Park, Dayton, OH 45469-2357, USA
E-mail: shawn.swavey@notes.udayton.edu
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G. Richards, S. Swavey
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reduction of O2 to H2O with these complexes is the result trodes in acidic media are presented. The ability of the elec-
of increased electron density placed on CoII through π- trooxidized modified electrodes to reduce molecular oxygen
backbonding of the pyridyl groups with the RuII groups. in acidic media is reported.
These complexes were found to be unstable in acidic solu-
tions, the medium by which many fuel cells operate.
Stability of many electrocatalysts has been improved by
their incorporation into polymer films and through electro-
polymerization of the catalysts directly onto electrode sur-
faces.[13–16] Numerous studies on the electropolymerization
of porphyrins onto electrode surfaces has led to the forma-
tion of thin conductive films for use in electrocatalysis,[17]
biosensors[18] and electroanalysis.[19] Synthetically incorpo-
rating electroactive organic pendant groups into the por-
phyrin framework and utilizing the ability of these groups
to undergo either electro-oxidation or electro-reduction has
led to the formation of polymeric arrays on the electrode
surface. Electrodes modified in this manner act both as the
medium and the mediator for electron transfer and have
been used to catalyze the reactions of small molecules.[20–22]
Figure 1. Structure of metalloporphyrins used in this study.
Conductive polymeric films on glassy carbon electrodes
have been produced by continuous anodic cycling in basic
solutions of [5,10,15,20-tetrakis(4-hydroxy-3-methoxyphen-
Results and Discussion
yl)porphyrinato]nickel(II).[23a] These films have been used
to detect trace amounts of nickel. Similar films have also
been used to detect nitric oxide from a single cell.[23b] Stud-
ies of [5,10,15,20-tetrakis(4-hydroxy-3-methoxyphenyl)por-
Synthesis and Characterization
Synthesis of the porphyrin, 5,10,15,20-tetrakis(4-hy-
phyrinato]copper(II) films as p-type semiconductors have droxy-3-methoxyphenyl)porphyrin [H2T3M4HPP], was
shown promise in photoelectrochemical cells converting adapted from a previously reported procedure[24] in which
light into chemical or electrical energy.[23c] Reduction of 4 equiv. of 4-hydroxy-3-methoxybenzaldehyde and 4 equiv.
oxygen to water in buffered pH 7.0 solutions has been ac- of freshly distilled pyrrole were refluxed in propionic acid.
complished by platinum electrodes modified with The crude porphyrin product precipitated from the reaction
[5,10,15,20-tetrakis(4-hydroxy-3-methoxyphenyl)porphyr- mixture upon cooling and was purified by recrystallization
inato]iron(III) formed by cycling the electrode in basic solu- from thf and hot methanol. Insertion of the metal atoms
tions of the iron porphyrin.[23d] To the best of our knowledge, into the core of the porphyrin was accomplished by reaction
electropolymerization of metal complexes of 5,10,15,20-tet- of the free-base porphyrin with the appropriate metal salt
rakis(4-hydroxy-3-methoxyphenyl)porphyrin in acidic solu- in dmf. The product was precipitated by addition of dis-
tions has not been investigated, which may lead to insights tilled water to the reaction mixture; yields of the metall-
into the mechanism of the oxidative process at the electrode. oporphyrins were typically higher than 70%, except for the
In addition, these modified electrodes have not been tested cobalt(II) porphyrin.
for their ability to reduce oxygen in acidic solutions, the
medium by which many fuel cells operate.
Electronic transitions of the free-base porphyrin and the
metalloporphyrins were run in either dmf (due to solubility)
This report describes the synthesis and characterization or acetone in 1 cm quartz cuvettes at room temperature.
by UV/Vis spectroscopy and solution electrochemistry of The free-base porphyrin displays an intense Soret band at
four metalloporphyrins synthesized by insertion of a metal 427 nm and four less intense Q-bands from 500 to 650 nm.
ion into 5,10,15,20-tetrakis(4-hydroxy-3-methoxyphenyl)- Insertion of the metal atom into the porphyrin core results
porphyrin [H2T3M4HPP] (Figure 1). The adsorption of in a slight shift of approximately 3–4 nm for the Soret band
these complexes onto edge-plane pyrolytic graphite elec- to higher energies with a decrease in molar absorptivity,
trodes and the results of oxidation of the modified elec- except in the case of the copper(II) porphyrin (Table 1). The
Table 1. UV/Vis data for the free-base and metal porphyrins run in 1 cm quartz cuvettes at room temperature.
Complex
Soret band [nm] (ε [10–4

–1 cm–1])
Q-bands [nm] (ε [10–4 –1 cm–1])

H2T3M4HPP
FeT3M4HPPCl
CoT3M4HPP
NiT3M4HPP
CuT3M4HPP
427 (28.5)
423 (6.80)
422 (17.4)
423 (23.4)
424 (33.4)
520 (14.4), 556 (10.8), 596 (4.74), 652 (6.10)
broad
534 (1.31)
530 (1.92)
542 (1.84), 580 (0.50)
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Electrooxidation of Fe, Co, Ni and Cu Metalloporphyrins
Q-bands of the free-base porphyrin collapse into one band for 15 min prior to running cyclic voltammograms. Figure 2
upon insertion of the metal atom into the porphyrin core; compares the cyclic voltammograms of the free-base and
however, the Q-band for the iron(III) porphyrin was too metal porphyrins run in dry dmf solutions.
broad to be reported.
When cycled in the anodic direction the only significant
oxidation process occurs for the free-base porphyrin, which
undergoes a broad irreversible oxidation at Epa = 0.84 V
vs. Ag/AgCl. The metallated complexes do not show any
oxidation process within the potential limits of the solvent;
Solution Electrochemistry and Spectroelectrochemistry
Solution electrochemistry was performed by using a therefore, our focus is on the cathodic region of the cyclic
three-electrode cell equipped with a glassy-carbon working voltammograms. In the cathodic direction the free-base
electrode and referenced vs. the Ag/AgCl electrode. Extra- porphyrin displays an irreversible reduction process with
dry dmf was used as the solvent with TBAPF6 serving as Epc = –1.12 V vs. Ag/AgCl due to the one-electron irrevers-
the supporting electrolyte. Solutions were purged with N2 ible reduction of the porphyrin to the anion radical (Fig-
Figure 2. Cyclic voltammograms of 1.0 m solutions of free-base porphyrin and metalloporphyrins measured under nitrogen in 0.1 
TBAPF6 solutions of dry dmf by using a glassy-carbon working electrode. Scan rates 100 mV/s.
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G. Richards, S. Swavey
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ure 2). A smaller oxidation wave with Epa = –0.55 V is ob- –1.11 V is followed by the beginning of solvent breakdown.
served upon the return scan. Two reversible redox couples Upon reversing the scan anodically, a sharp oxidation wave
are observed upon cathodic scanning of the CoT3M4HPP at –1.10 V is observed. When the switching potential is
solution. The first and less intense couple occurs with an shifted to –1.40 V, prior to solvent breakdown, the sharp
E1/2 = –0.15 V (∆E = 60 mV), most likely the result of re- oxidation wave at –1.10 V is no longer present.
duction of residual amounts of CoIII to CoII. The second
To gain insight into the redox chemistry observed by cy-
redox couple occurs with an E1/2 = –0.82 V (∆E = 70 mV), clic voltammetry, spectroelectrochemistry experiments were
which is consistent with the reduction of CoII to CoI in performed on dilute solutions of the complexes in 0.1 
dmf.[25] Both the CuII and NiII porphyrins give reversible TBAPF6 dmf solutions by using a platinum wire as auxil-
redox couples at E1/2 = –1.20 V (∆E = 70 mV) (Figure 2). iary electrode and a platinum mesh working electrode. The
Electrochemical studies of iron(III) porphyrins have been results of these experiments are illustrated in Figure 3A–E.
the focus of numerous studies.[26] At high concentrations of
The initial spectrum of the free-base porphyrin exhibits
dmf the FeIII porphyrin is believed to be coordinated axially an intense Soret band at 427 nm and four less intense Q-
by two dmf molecules, whereas the reduced FeII porphyrin bands. Controlled-potential electrolysis of the solution at
prefers one axial dmf coordination, which is weakly –1.50 V vs. Ag/AgCl leads to a decrease of the Soret band
bound.[26a] The reduction of Fe(p-CH3)TPPCl to Fe(p- and an increase of bands at 399 nm, 466 nm and 655 nm
CH3)TPP in neat dmf occurs with a peak potential of ap- (Figure 3A). Although, based on the cyclic-voltammetry ex-
proximately –0.29 V vs. SCE.[26b] FeTPPCl is reduced to periments, this reduction is irreversible, approximately 90%
FeTPP in dmf at –0.19 V vs. SCE, whereas the (µ-oxido)- of the original spectrum was obtained upon oxidation of
bis[(tetraphenylporphyrinato)iron] [(FeIIITPP)2O] is re- the more dilute spectroelectrochemical solution at 0.0 V, in-
duced at potentials near –0.9 V in dmf.[26d] In the case of dicating concentration dependence. The initial spectrum of
FeT3M4HPPCl a reversible redox couple is observed in the the CoT3M4HPP solution gave two overlapping Soret
cathodic region with E1/2 = –0.54 V (∆E = 70 mV) vs. Ag/ bands at 422 and 435 nm, consistent with the presence of
AgCl. This is more cathodic than the monomeric tetraphen- both CoIII and CoII porphyrins.[27] Electrolysis at –0.40 V,
ylporphyrin and more anodic than the oxido-bridged tet- approximately 200 mV past the first reduction peak in the
raphenylporphyrin.[26b] This electron is most likely going cyclic voltammogram (Figure 2), shows a decrease in the
into the lowest-lying dπ orbital of the Fe metal reducing band at 435 nm and an increase of the band at 422 nm (Fig-
FeIII to FeII. A subsequent broad reduction with Epc
=
ure 3B). This is consistent with the reduction of CoIII to
Figure 3. Spectroelectrochemical results for the controlled-potential reduction of the free-base porphyrin (A), CoT3M4HPP (B),
CuT3M4HPP (C), FeT3M4HPPCl (D), and NiT3M4HPP (E) in dmf by using a platinum-mesh working electrode referenced to an Ag/
AgCl electrode.
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Electrooxidation of Fe, Co, Ni and Cu Metalloporphyrins
CoII. Continued electrolysis at –1.2 V vs. Ag/AgCl shows penyl)phenol] immobilized on glassy-carbon electrodes in
the disappearance of the Soret band and new bands grow- aqueous solutions show very similar electrochemistry to
ing at 452 nm and 610 nm. The original spectrum, however, that shown in Figure 4. This behavior has been attributed
could not be obtained upon exhaustive oxidative electroly- to a similar ECE mechanism initiated by oxidative demeth-
sis, suggesting that the reduced complex either decomposed ylation.[29]
or underwent reaction with the solvent.[27] Controlled-po-
tential electrolysis of the CuII and NiII porphyrins at
–1.50 V vs. Ag/AgCl (Figure 3C and E, respectively) show
the loss of the Soret band and new bands growing at 396,
472 and 672 nm for the CuII porphyrin and at 390, 467 and
634 nm for the NiII porphyrin. The similarity of the spectro-
electrochemical results of the CuII and NiII porphyrins with
each other and with the spectroelectrochemical results of
the free-base porphyrin lead us to conclude that reduction
of the CuII and NiII porphyrins observed by cyclic voltam-
metry is localized on the porphyrin ring resulting in a cation
radical. Spectroelectrochemical results for the reduction of
FeT3M4HPPCl in dmf (Figure 3D) at –0.80 V vs. Ag/AgCl
indicate a decomposition of the complex as evidenced by a
sharp decrease of the Soret band with only 50% of the orig-
inal spectrum regenerated upon oxidation of the solution at
0.0 V. It is also conceivable that the complex adsorbed onto
the platinum mesh electrode.
Figure 4. Cyclic voltammogram of an EPG electrode coated with
FeT3M4HPPCl measured in 0.5  H2SO4 at room temperature.
Scan rate 50 mV/s.
Electrode Adsorption Studies
Because the new redox couple is derived from a quinone-
After scoring the surface of the edge-plane pyrolytic
like structure, its redox chemistry should depend on the pH
graphite (EPG) electrode, by using 600 grit sandpaper, ali-
quots (10 µL) of 0.2–0.5 m solutions of acetone contain-
ing the desired complex were placed onto the electrode sur-
face and the solvent allowed to evaporate under ambient
of the solution. To test this, we examined electrodes coated
with the free-base porphyrin or metalloporphyrins, which
had been oxidized in 0.5  H2SO4, and studied the newly
formed redox couple in buffered solutions of different pH.
conditions. Reproducible results were obtained in this man-
A
linear relation of E1/2 to pH with a slope of
ner as indicated by CV experiments. Electrochemical prop-
erties of the complexes adsorbed onto an EPG electrode
were studied in 0.5  H2SO4 by cyclic voltammetry.
–0.060 VϮ0.010 V/pH unit indicates a 2e–/2H+ process in
agreement with a quinone-like surface structure.[29] Figure 5
Anodic cycling of the coated electrodes reveal an
irreversible oxidation wave between 0.75 and 0.80 V vs. Ag/
AgCl, for all complexes studied. On the reverse scan a new
reduction wave coupled to a new oxidation wave appears
with the disappearance of the initial irreversible oxidation
process. The similarity between the redox chemistry of the
adsorbed complexes suggests that the electrochemical pro-
cess is occurring at the peripheral 4-hydroxy-3-meth-
oxyphenyl groups. Figure 4 illustrates this behavior for an
EPG electrode coated with FeT3M4HPP. A chemical step
follows the initial oxidation of the 4-hydroxy-3-meth-
oxyphenyl groups of the porphyrin (peak I in Figure 4),
most likely demethylation of the methoxy substituents lead-
ing to a highly reactive “quinonoid” structure capable of
reacting to form a redox-active film on the electrode sur-
face.[28] The newly formed quinone undergoes reduction to
the hydroquinone (peak IIa in Figure 4), which is coupled
to the oxidation of the hydroquinone back to the quinone
form (peak IIb in Figure 4). This electrochemical/chemical/
electrochemical ECE-type mechanism explains the ob-
served electrochemical behavior of the adsorbed complexes.
Figure 5. Cyclic voltammograms of CoT3M4HPP, adsorbed onto
an EPG electrode after anodic conditioning in 0.5  H2SO4, mea-
sured in buffered solutions with a pH range of 1.0–6.1. Inset: plot
Electrochemical studies of eugenol [2-methoxy-4-(2-pro- of E1/2 vs. pH. Scan rate 50 mV/s.
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G. Richards, S. Swavey
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illustrates the results of a pH study performed on an EPG concentration, ν is the scan rate in mV/s, F is the Faraday
electrode coated with CoT3M4HPP after it had been oxid- constant (96500 C/mol), R is the gas constant and T is the
ized in 0.5  H2SO4. As [H+] decreases the redox couple temperature in Kelvin. The linear dependence of the peak
shifts to lower potentials. A plot of E1/2 values vs. pH (inset current on the scan rate indicates that the redox couples are
Figure 5) gives a slope of –0.058 V/pH unit, consistent with due to surface-confined species and are not influenced by
a 2e–/2H+ process.
diffusion effects. Figure 6 illustrates the scan-rate depen-
Studies of the free-base porphyrin and the Fe, Ni and Cu dence of the redox couple formed from oxidative cycling of
porphyrins gave similar results with slopes of –0.070 V/pH adsorbed CuT3M4HPP on edge-plane pyrolytic graphite in
unit, –0.063 V/pH unit, –0.063 V/pH unit and –0.063 V/pH acidic solution. The inset shows the linear relationship of
unit, respectively. Further studies of the electrode surface ipa vs. scan rate, with a slope equal to (n2F2/4RT)AΓ.
are needed to allow us to propose an accurate structure of
the surface of the electrochemically modified electrodes.
Surface concentration was determined to be
2.0ϫ10–9 mol/cm2 for the free-base porphyrin and
5.0ϫ10–9 mol/cm2 for the metal complexes. This indicates
multiple layers on the surface of the electrode. Analysis of
the charge under the initial oxidation wave (peak I in Fig-
ure 4) and the charge under the newly formed reduction
and oxidation waves (peaks IIa and IIb, respectively) allow
us to determine the number of electrons transferred per
molecule during each electrochemical process using Equa-
tion (1).
Q = nFAΓ
(1)
where Q is the charge in Coulombs, n is the number of
electrons transferred, F is the Faraday constant and Γ is the
surface concentration in mol/cm2. Assuming that all of the
complexes adsorbed on the electrode are electroactive,
Table 2 gives the results of this calculation.
Figure 6. Scan-rate dependence for adsorbed CuT3M4HPP onto
an EPG electrode after anodic cycling in 1.0  H2SO4. Inset: plot
of ipa vs. scan rate.
Table 2. Values of n determined by analysis of the cyclic voltammo-
grams of adsorbed complexes.[a]
Complex
n (peak I)
n (peak IIa)
n (peak IIb)
O2 Electrocatalysis
H2T3M4HPP
FeT3M4HPP
CoT3M4HPP
NiT3M4HPP
CuT3M4HPP
7
3
3
4
5.5
2
3
3
4
3
[a]
The electrocatalytic reduction of O2 in air-saturated 0.5 
H2SO4 was studied at an EPG electrode coated with the
free-base porphyrin or the metalloporphyrins by using rot-
ating-disk-electrode (RDE) voltammetry. Reduction of O2
at a bare EPG electrode in air-saturated 0.5  H2SO4 occurs
with an E1/2 = –0.35 V vs. Ag/AgCl. When coated with the
free-base porphyrin, no appreciable electrocatalysis over the
bare electrode was observed. An EPG electrode coated with
7
6
3
1.5
[a] Overlapping redox waves prevented a clear determination of
charge.
The data in Table 2 suggest that the assumption that all
adsorbed complexes take part in the redox chemistry is
most likely false. This is not entirely unexpected due to the
multiple layers on the electrode. In nearly every case the
coupled redox waves IIa and IIb transfer the same number
of electrons for oxidation and reduction consistent with the
pH studies. It is also apparent that the first oxidation in-
volves more electrons than the subsequent redox couple
formed suggesting that the newly formed quinone is ac-
companied by a chemical reaction (possibly polymeriza-
tion), which alters the surface structure.
CuT3M4HPP reduces molecular oxygen with an E1/2
=
0.10 V vs. Ag/AgCl, a catalytic shift of 450 mV compared to
the bare electrode. Oxidation of the CuT3M4HPP-modified
electrode to create the quinone-like surface does not affect
the catalytic ability of the electrode. In the case of an EPG
electrode coated with NiT3M4HPP, oxygen reduction oc-
curs with an E1/2 = 0.20 V vs. Ag/AgCl, a catalytic shift of
550 mV compared to the bare electrode. However, no
change in the catalytic ability of this modified electrode was
observed upon oxidation. RDE studies of the
CuT3M4HPP and NiT3M4HPP electrodes did not give re-
producible results, and therefore it was not possible to ana-
lyze the mechanism of oxygen reduction at these electrodes.
When coated with FeT3M4HPPCl, the EPG electrode
For adsorbed electroactive species scan-rate dependence
shows peak currents increasing linearly with increasing scan
rate as governed by Equation (2).[30]
ip = (n2F2/4RT)AΓν
(2)
where n is the number of electrons transferred per adsorbed reduces oxygen with an E1/2 = 0.05 V vs. Ag/AgCl, a cata-
molecule, A is the area of the electrode, Γ is the surface lytic shift of 400 mV compared to the bare EPG electrode.
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Electrooxidation of Fe, Co, Ni and Cu Metalloporphyrins
RDE studies of the iron porphyrin modified electrode did diffusion
coefficient
of
O2
in
the
solution
not give consistent plateau currents and therefore could not (2.0ϫ10–5 cm2 s–1), ν is the kinematic viscosity of the solu-
be further analyzed. However, when the iron porphyrin tion (0.01 cm2 s–1), C is the concentration of O2 in the air-
electrode was first oxidized to create the quinone-like struc- saturated solution (0.2 m), and ω is the rotation rate
ture on the electrode prior to oxygen reduction studies, (rpm). Ik is the kinetic current density for O2 reduction and
more consistent plateau currents were obtained (Figure 7). can be calculated from the intercept of the Koutecky–
It should be mentioned, however, that oxidation of the Levich plot. The slope for the Koutecky–Levich plot for the
coated electrode did not make the surface more catalytic experimental data is the same as the slope for the theoreti-
relative to reduction of oxygen at the un-oxidized coated cal n = 4 line giving a value of n = 4 (Figure 8). This indi-
electrode.
cates that the FeT3M4HPP reduces oxygen directly to water
by a four-electron process.
Figure 9 illustrates the results of oxygen reduction at an
EPG electrode coated with CoT3M4HPP in an air-satu-
rated 0.5  H2SO4 solution. The E1/2 for reduction of oxy-
gen at this electrode is 0.15 V vs. Ag/AgCl, a catalytic shift
of 500 mV compared to the bare EPG electrode. From this
data it can be observed that the reaction kinetics for the
reduction of oxygen at this electrode is relatively slow sug-
gested by the constant rise in the plateau currents.
Figure 7. Reduction of O2 at a rotating-disk electrode coated with
FeT3M4HPPCl after oxidation. Current-potential curves in air-sat-
urated 0.5  H2SO4. Scan rate 10 mV/s.
The Koutecky–Levich plot (Figure 8) relates the plateau-
current density to the rotation rate by using Equa-
tion (3).[31]
(3)
Figure 9. Reduction of O2 at a rotating-disk electrode coated with
CoT3M4HPP prior to oxidation. Current-potential curves in air-
saturated 0.5  H2SO4. Scan rate 10 mV/s.
where IL is the diffusion-limited current, Ik is the current
density (Acm–2), n is the number of electrons for the reac-
tion, F is the Faraday constant (96500 Cmol–1), D is the
Upon oxidation of the CoT3M4HPP-coated EPG elec-
trode in 0.5  H2SO4 the rise in current is much sharper
indicating faster electrode kinetics (Figure 10). The E1/2 for
the reduction of oxygen at this oxidized electrode is 0.25 V
vs. Ag/AgCl, a catalytic shift of 600 mV compared to the
bare electrode and a catalytic shift of 100 mV compared to
the CoT3M4HPP-coated EPG electrode prior to oxidation
(Figure 10).
Comparison of the Koutecky–Levich plots for the
CoT3M4HPP-coated electrodes prior to and after oxi-
dation is illustrated in Figure 11. Prior to oxidation, the
Koutecky–Levich line is identical in slope to the theoretical
n = 2 line indicating that prior to oxidation CoT3M4HPP
reduces oxygen by two electrons to form hydrogen peroxide.
The slope of the Koutecky–Levich line for the data after
oxidation of the CoT3M4HPP-coated electrode lies be-
tween the theoretical n = 2 and n = 4 lines but is closer to
the slope of the n = 2 line. The value for n from this line is
2.3 suggesting that 15% of the oxygen that is reduced at
Figure 8. Koutecky–Levich plot of the inverse of the plateau cur-
rent vs. the inverse of the square root of the rotation rate for the
curves in Figure 7. The theoretical two-electron and four-electron
lines are marked n = 2 and n = 4, respectively.
Eur. J. Inorg. Chem. 2009, 5367–5376
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5373

G. Richards, S. Swavey
FULL PAPER
plex generated by anodic cycling. Further studies with dif-
ferent peripheral metal complexes are underway to investi-
gate this phenomenon.
Conclusions
This paper reports on the synthesis and characterization,
by UV/Vis spectroscopy and cyclic voltammetry, of a series
of metalloporphyrins (Figure 1). Adsorption of these com-
plexes onto EPG electrodes followed by oxidation in 0.5 
H2SO4 results in a new redox-active, pH-dependent modi-
fied electrode. All of the metalloporphyrins catalyze the re-
duction of oxygen relative to the bare EPG electrode prior
to formation of the new redox couple by oxidation. RDE
studies of the FeIII porphyrin, after oxidation, indicate that
this modified electrode reduces oxygen by four electrons di-
rectly to water. When the CoII porphyrin is placed on the
EPG electrode and oxidized in 0.5  H2SO4, its ability to
reduce oxygen is improved relative to the coated electrode
prior to oxidation. It was not possible to analyze the NiII
and CuII porphyrin coated electrodes by RDE, because of
the instability of the complex in acidic solution, even after
oxidation of the electrode.
Figure 10. Reduction of O2 at a rotating-disk electrode coated with
CoT3M4HPP after oxidation. Current-potential curves in air-satu-
rated 0.5  H2SO4. Scan rate 10 mV/s.
this electrode is reduced directly to water by four electrons,
whereas 85% of the oxygen is reduced by two electrons to
hydrogen peroxide.
Experimental Section
Materials: All reagents were analytical grade and used without fur-
ther purification, unless stated otherwise. Acetone, methanol, di-
methylformamide (dmf), tetrabutylammonium hexafluorophos-
phate (TBAPF6, used as a supporting electrolyte for solution elec-
trochemistry) and extra-dry (Ͻ50 ppm H2O) dmf for electrochem-
istry (ACROS), sulfuric acid, cobalt(II) acetate, iron(II) chloride,
copper(II) chloride, and nickel(II) chloride were used as received.
The pH of the buffer solutions were measured to Ϯ0.01 at 25 °C
by using a Denver Instrument UltraBasic pH meter calibrated with
standard pH 4.00 and 10.00 buffer solutions (Fisher): 2.61 (0.2 
NaH2PO4, 0.05  H3PO4), 3.67 (0.24  CH3COOH, 0.05 
CH3COONa), 4.92 (0.2  CH3COONa, 0.05  CH3COOH), 6.07
(0.2  NaH2PO4, 0.05  Na2HPO4). Elemental analyses were per-
formed by Atlantic Microlabs, Norcross, GA.
Figure 11. Koutecky–Levich plots of the inverse of the plateau cur-
rent vs. the inverse of the square root of the rotation rate for the
curves in Figures 9 and 10. The theoretical two-electron and four-
electron lines are marked n = 2 and n = 4, respectively.
5,10,15,20-Tetrakis(4-hydroxy-3-methoxyphenyl)porphyrin [H2T3-
M4HPP]: A solution of propionic acid (70 mL), 4-hydroxy-3-meth-
oxybenzaldehyde (2.85 g, 18.7 mmol) and freshly distilled pyrrole
(1.30 mL, 18.7 mmol) were refluxed for 30 min. The purple precipi-
tate was filtered and washed with cold methanol. The crude prod-
uct was dissolved in thf and crystallized by the addition of hot
methanol. Yield 0.26 g (7.0%). UV/Vis (dmf): λmax [nm] (ε
In a previous study we showed that a similar complex,
cis-Pt(dmso){[5-(4-pyridyl)-10,15,20-tris(4-hydroxy-3-meth-
oxyphenyl)porphyrinato]cobalt(II)}Cl2, in which one of the
4-hydroxy-3-methoxyphenyl substituents is replaced with a
pyridyl group coordinated to a PtII complex could effec-
tively reduce more than 65% of the oxygen directly to water
at potentials similar to those observed for the CoII porphy-
rin in the current study.[33] This is considerably higher than
the 15% observed for the [tetrakis(4-hydroxy-3-meth-
oxyphenyl)porphyrinato]cobalt(II) of this study. It is specu-
lated that the PtII moiety places more electron density on
the CoII center facilitating the CoII–O2 interaction, which
leads to a transfer of four electrons. It is clear that enhanc-
ing the electrocatalytic ability of the CoII complex of this
[10–4

–1 cm–1]) = 427 (28.5), 520 (14.4), 556 (10.8), 596 (4.74), 652
(6.10).
[5,10,15,20-Tetrakis(4-hydroxy-3-methoxyphenyl)porphyrinato]co-
balt(II): Cobalt(II) acetate (0.0560 g, 0.225 mmol) was added to a
solution of H2T3M4HPP (0.149 g, 0.188 mmol) in dmf (10.0 mL).
The reaction mixture was refluxed under nitrogen for 30 min. The
product was precipitated by the addition of distilled water (20 mL),
filtered, washed with distilled water (3ϫ 10 mL) and air-dried.
Yield 0.0803 g (48.9%) of a purple powder. UV/Vis (acetone): λmax
[nm] (ε [10–4

–1 cm–1]) = 422 (17.4), 534 (1.31). C48H36CoN4O8
H2O (873.78): calcd. C 65.98, H 4.38, N 6.41; found C 65.44, H
study requires more than formation of the new surface com- 4.70, N 6.22.
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Eur. J. Inorg. Chem. 2009, 5367–5376

Electrooxidation of Fe, Co, Ni and Cu Metalloporphyrins
[5,10,15,20-Tetrakis(4-hydroxy-3-methoxyphenyl)porphyrinato]iron-
(III): Iron(II) chloride (0.0151 g, 0.0751 mmol) was added to a
solution of H2T3M4HPP (0.0532 g, 0.0626 mmol) in dmf
(10.0 mL). The reaction mixture was refluxed under nitrogen for
30 min. The product was precipitated by the addition of distilled
water (20 mL), filtered, washed with distilled water (3ϫ10 mL) and
air-dried. Yield 0.0395 g (74.0%) of a purple powder. UV/Vis (ace-
[1]
[2]
C. Arana, M. Keshavarz, K. T. Potts, H. D. Abruna, Inorg.
Chim. Acta 1994, 225, 285–295.
J. C. Huang, R. K. Sen, E. Yeager, J. Electrochem. Soc. 1979,
126, 786–792.
J. H. Zagal, Coord. Chem. Rev. 1992, 119, 89–136.
Z. Wei, L. Li, L. I. Li, Z. Tang, H. Guo, Dianyuan Jishu 2004,
28, 116–120.
C. Shi, F. C. Anson, J. Am. Chem. Soc. 1991, 113, 9564–9570.
J. P. Collman, L. L. Chng, D. A. Tyvoll, Inorg. Chem. 1995, 34,
1311–1324.
[3]
[4]
tone): λmax [nm] (ε [10–4 –1 cm–1]) = 423 (6.80). C48H36ClFeN4O8

[5]
[6]
(888.13): calcd. C 64.91, H 4.46, N 6.51; found C 64.80, H 4.46, N
7.15.
[5,10,15,20-Tetrakis(4-hydroxy-3-methoxyphenyl)porphyrinato]cop-
per(II): Copper(II) chloride (0.0110 g, 0.0751 mmol) was added to
a solution of H2T3M4HPP (0.0517 g, 0.0626 mmol) in dmf
(10.0 mL). The reaction mixture was refluxed under nitrogen for
30 min. The product was precipitated by the addition of distilled
water (20 mL), filtered, washed with distilled water (3ϫ10 mL) and
air-dried. Yield 0.0504 g (93.6 %) of a maroon powder. UV/Vis
[7] a) P. Vasudevan, Santosh, N. Mann, S. Tyagi, Transition Met.
Chem. 1990, 15, 81–90; b) M.-S. Liao, S. Scheiner, J. Chem.
Phys. 2002, 117, 205–219.
[8]
[9]
H. Alt, M. Binder, G. Sandstede, J. Catal. 1973, 28, 8–19.
R. R. Durand, S. C. Bencosme, J. P. Collman, F. C. Anson, J.
Am. Chem. Soc. 1983, 105, 2710–2718.
[10]
[11]
[12]
[13]
C. J. Chang, Z. H. Loh, C. Shi, F. C. Anson, D. G. Nocera, J.
Am. Chem. Soc. 2004, 126, 10013–10020.
S. Fukuzumi, K. Okamoto, C. P. Gros, R. Guilard, J. Am.
Chem. Soc. 2004, 126, 10441–10449.
F. C. Anson, C. Shi, B. Steiger, Acc. Chem. Res. 1997, 30, 437–
444.
J. E. Bennett, A. Burewicz, D. E. Wheeler, I. Eliezer, L.
Czuchajowski, T. Malinski, Inorg. Chim. Acta 1998, 271, 167–
173.
(acetone): λmax [nm] (ε [10–4 –1 cm–1]) = 424 (33.4), 542 (1.84), 580

(0.50). C48H36CuN4O8 (860.38): calcd. C 67.01, H 4.22, N 6.51;
found C 67.11, H 4.35, N 6.24.
[5,10,15,20-Tetrakis(4-hydroxy-3-methoxyphenyl)porphyrinato]-
nickel(II): Nickel(II) chloride (0.0189 g, 0.0751 mmol) was added
to a solution of H2T3M4HPP (0.0516 g, 0.0626 mmol) in dmf
(10.0 mL). The reaction mixture was refluxed under nitrogen for
30 min. The product was precipitated by the addition of distilled
water (20 mL), filtered, washed with distilled water (3ϫ10 mL) and
air-dried. Yield 0.0484 g (90.4%) of a purple powder. UV/Vis (ace-
[14]
[15]
T. Abe, M. Kaneko, Prog. Polym. Sci. 2003, 28, 1441–1488.
J. Qu, Y. Shen, X. Qu, S. Dong, Electroanalysis 2004, 16, 1444–
1450.
[16]
[17]
[18]
[19]
[20]
S. M. Chen, Y. L. Chen, J. Electroanal. Chem. 2004, 573, 277–
287.
K. Araki, L. Angnes, C. M. N. Azevedo, H. E. Toma, J. Elec-
troanal. Chem. 1995, 397, 205–210.
J. M. Vago, V. C. Dall’onto, E. Forzani, J. Hurst, I. N. Rezzano,
Sens. Actuators B 2003, 96, 407–412.
G. E. Milczarek, A. Ciszewski, Electroanalysis 2001, 13, 164–
166.
H. Winnischofer, S. D. Lima, K. Araki, H. E. Toma, Anal.
Chim. Acta 2003, 480, 97–107.
tone): λmax [nm] (ε [10–4

–1 cm–1]) = 423 (23.4), 530 (1.92).
C48H36N4NiO8·H2O (873.54): calcd. C 66.00, H 4.38, N 6.72; found
C 65.45, H 4.39, N 6.77.
Procedures and Instrumentation: UV/Vis spectra were recorded at
room temperature by using a Shimadzu 1501 photodiode-array
spectrophotometer with 2 nm resolution. Samples were measured
in reagent-grade acetone in 1 cm quartz cuvettes. Solution cyclic
voltammograms were recorded by using a one-compartment, three-
electrode cell (model 630A electrochemical analyzer from CH-In-
struments) equipped with a platinum-wire auxiliary electrode. The
working electrode was a 2.0 mm diameter glassy-carbon electrode
from CH-Instruments. The working electrode was polished first by
using 0.30 µ followed by 0.05 µ alumina polish (Buehler) and
tapped dry with a Kimwipe prior to use. Potentials were referenced
to an Ag/AgCl electrode. The supporting electrolyte was 0.1 
Bu4NPF6, and the measurements were carried out in extra-dry dmf.
Spectroelectrochemical measurements were conducted according to
a previously described method by using a locally constructed H-
cell, which uses a quartz cuvette as the working compartment.[32]
The working and auxiliary compartments were separated by a fine
porous-glass frit. The working electrode was a high-surface-area
platinum mesh, and the auxiliary electrode was a platinum wire.
The reference electrode was Ag/AgCl (0.50 V vs. SHE). The mea-
surements were made in 0.1  TBAPF6 dmf solutions that were
approximately 1ϫ10–6  in metal complex. The electrolysis poten-
tial was controlled by a CH-Instruments 630A electrochemical ana-
lyzer. Adsorption of the porphyrin and metalloporphyrins onto
edge-plane pyrolytic-graphite EPG electrode (AFE3T 5.0 mm dia-
meter, Pine Instrument Co.), which had been roughened by using
600 grit sandpaper, was accomplished by placing 10 µL aliquots of
0.2 m (porphyrin) or 0.5 m (metalloporphyrins) acetone solu-
tions onto the electrode surface and allowing the solvent to evapo-
rate at room temperature. Cyclic voltammetry and rotating disc
electrode (RDE) experiments of the modified electrodes were per-
formed by using a Pine AFCBP1 bipotentiostat and an AFMSRX
rotator (Pine Instrument Co.) in 0.5  sulfuric acid solutions.
[21]
[22]
A. Ciszewski, G. Milczarek, Talanta 2003, 61, 11–26.
F. Armijo, M. Isaacs, G. Ramirez, E. Trollund, J. Canales, M. J.
Aguirre, J. Electroanal. Chem. 2004, 566, 315–322.
a) T. Malinski, A. Ciszewski, J. Bennett, J. R. Fish, L.
Czuchajowski, J. Electrochem. Soc. 1991, 138, 2008–2015; b) T.
Malinski, Z. Taha, Nature 1992, 358, 676–677; c) T. Malinski,
A. Ciszewski, J. Fish, E. Kubaszewski, L. Czuchajowski, Adv.
Mater. 1992, 4, 354–357; d) X. Wu, Y. Li, B. Gründig, N.-T.
Yu, R. Renneberg, Electroanalysis 1997, 9, 1288–1290.
a) A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J.
Assour, L. Korsakoff, J. Org. Chem. 1967, 32, 476–476; b)
A. D. Adler, F. R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl.
Chem. 1970, 32, 2443–2445.
[23]
[24]
[25]
[26]
C. Araullo-McAdams, K. M. Kadish, Inorg. Chem. 1990, 29,
2749–2757.
a) L. A. Bottomley, K. M. Kadish, Inorg. Chem. 1981, 20,
1348–1357; b) D. Lancon, P. Cocolios, R. Guilard, K. M. Kad-
ish, J. Am. Chem. Soc. 1984, 106, 4472–4478; c) K. M. Kadish,
G. Larson, D. Lexa, M. Momenteau, J. Am. Chem. Soc. 1975,
97, 282–288; d) K. M. Kadish, R. K. Rhodes, Inorg. Chem.
1983, 22, 1090–1094; e) K. M. Kadish, L. A. Bottomley, J. Am.
Chem. Soc. 1977, 99, 2380–2382.
[27]
[28]
X. Q. Lin, B. Boisselier-Cocolios, K. M. Kadish, Inorg. Chem.
1986, 25, 3242–3248.
a) F. Pariente, E. Lorenzo, H. D. Abruna, Anal. Chem. 1994,
66, 4337–4344; b) H.-G. Hong, W. Park, Langmuir 2001, 17,
2485–2492.
[29]
C. Ueda, D. Chi-Sing, T. Kuwana, Anal. Chem. 1982, 54, 850–
856; F. D’Souza, G. R. Deviprasad, J. Org. Chem. 2001, 66,
4601–4609.
Eur. J. Inorg. Chem. 2009, 5367–5376
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5375

G. Richards, S. Swavey
FULL PAPER
[30]
[32] K. J. Brewer, M. Calvin, R. S. Lumpkin, J. W. Otvos, O. L.
Spreer, Inorg. Chem. 1989, 28, 4446–4460.
[33] K. Gadamsetti, S. Swavey, Dalton Trans. 2006, 5530–5535.
Received: July 10, 2009
A. J. Bard, L. R. Faulkner, Electrochemical Methods Funda-
mentals and Applications, 2nd ed., John Wiley & Sons, Inc.,
New York, 2001, chapter 9, p. 339.
[31] J. Koutecky, V. G. Levich, Zh. Fiz. Khim. 1958, 32, 1565–1575.
Published Online: November 6, 2009
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Eur. J. Inorg. Chem. 2009, 5367–5376