Electrocatalytic reduction of oxygen at electrodes coated with a bimetallic cobalt(II)/platinum(II) porphyrin

Article abstract of DOI:10.1039/b614108k

Edge plane pyrolytic graphite (EPG) electrodes coated with 5-(4-pyridyl)-10,15,20-tris(3-methoxy-4-hydroxyphenyl)porphyrin and its Pt(ii) and Co(ii)/Pt(ii) analogs undergo an electrochemical-chemical-electrochemical (ECE) reaction when anodically scanned in 1.0 M HClO₄. 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 Co(ii)/Pt(ii) bimetallic porphyrin show a catalytic shift of 500 mV for the reduction of O₂ when compared to the reduction of O₂ at a bare EPG electrode. An additional catalytic shift of ca. 100 mV is observed for O₂ reduction at an EPG electrode coated with the Co(ii)/Pt(ii) porphyrin which has been oxidized in 1.0 M HClO₄. In addition to the added electrocatalysis a significant percentage of O₂ reduced at the oxidized Co(ii)/Pt(ii) EPG electrode is converted to H₂O as determined by rotating disk electrode measurements. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b614108k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Electrocatalytic reduction of oxygen at electrodes coated with a bimetallic
cobalt(II)/platinum(II) porphyrin
Kalyan Gadamsetti and Shawn Swavey*
Received 28th September 2006, Accepted 17th October 2006
First published as an Advance Article on the web 24th October 2006
DOI: 10.1039/b614108k
Edge plane pyrolytic graphite (EPG) electrodes coated with 5-(4-pyridyl)-10,15,20-tris(3-methoxy-4-
hydroxyphenyl)porphyrin and its Pt(II) and Co(II)/Pt(II) analogs undergo an electrochemical–
chemical–electrochemical (ECE) reaction when anodically scanned in 1.0 M HClO . The new redox
4
couple formed from this anodic conditioning of the coated electrode is dependent on the pH of the
solution. Roughened EPG electrodes coated with the Co(II)/Pt(II) bimetallic porphyrin show a catalytic
shift of 500 mV for the reduction of O
An additional catalytic shift of ca. 100 mV is observed for O
with the Co(II)/Pt(II) porphyrin which has been oxidized in 1.0 M HClO
electrocatalysis a significant percentage of O reduced at the oxidized Co(II)/Pt(II) EPG electrode is
O as determined by rotating disk electrode measurements.
2
when compared to the reduction of O
reduction at an EPG electrode coated
. In addition to the added
2
at a bare EPG electrode.
2
4
2
converted to H
2
Introduction
of the electrocatalyst into a polymeric matrix has increased the
10
stability and lifetime of a number of catalysts. Thin conductive
films formed from the electropolymerization of porphyrins are
Cobalt porphyrins have been extensively studied as catalysts for the
reduction of molecular oxygen. The interest stems from the need
1
11
12
13
used as electrocatalysts, biosensors and in electroanalysis.
to find alternatives to platinum, a costly and easily poisoned cat-
alyst currently used in hydrogen/oxygen and alcohol/oxygen fuel
Synthetically incorporating electroactive organic pendant groups
into the porphyrin 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 have been used to
2
cells. Many transition-metal complexes have been investigated
for their catalytic ability toward O
2
reduction but Co macrocycles
3
have shown the most promise. At issue with Co macrocycles in
general and specifically Co porphyrins is the fact that most are
only capable of reducing molecular oxygen to hydrogen peroxide,
a two-electron/two-proton process, instead of the desired four-
electron/four-proton process which produces water. Reduction
by two electrons lowers the efficiency of the fuel cell in addition to
producing a deleterious product, H
and damaging to the catalyst itself.
To overcome the two electron/two proton reduction of oxy-
gen observed for most cobalt porphyrins cofacial dicobalt bis-
porphyrins have been synthesized. These complexes form a
l-superoxo complex stabilizing the O
14
catalyze the reactions of small molecules.
We have been interested in combining into a single molecule
the stabilizing affects associated with electropolymerization and
the enhanced catalysis of multi-metallic cobalt porphyrins. We re-
cently described the synthesis and catalytic ability of a Co(II)/Pt(II)
bimetallic porphyrin which undergoes electropolymerization on
4
2
2
O , unsafe for the environment
graphite electrodes. These modified electrodes demonstrated en-
15
hanced electrocatalysis for O
2
reduction in acidic solutions. It was
concluded that the best catalytic behavior required polymerization
of the bimetallic porphyrin, the cobalt(II) metal center and the plat-
inum(II) peripheral metal. Removing any one of these components
decreased the catalytic ability of the modified electrode. To con-
tinue this work we present here a study of Pt(II) and Co(II)/Pt(II)
5
2
reduction intermediate
in acidic media. Other
resulting in a four-electron reduction of O
researchers have chosen a different route to the stabilization of
the Co–peroxo reduction intermediate using multi-metallic cobalt
2
complexes of the previously synthesized porphyrin 5-(4-pyridyl)-
6
porphyrins as the electrocatalyst. For example coordination of
16
1
0,15,20-tris(3-methoxy-4-hyroxyphenyl)porphyrin, 1 (Fig. 1).
Ru(NH
3
)
5
or Os(NH
3
) to the pyridyl nitrogens of cobalt(II)
5
The synthesis, spectroscopic and electrochemical properties of the
compounds in Fig. 1 are presented. Electrode adsorption studies
and the electrocatalysis of molecular oxygen in acidic media using
the Co(II)/Pt(II) bimetallic porphyrin is also discussed.
tetrakis(4-pyridyl)porphyrin converts this electrocatalyst from
a two-electron to a four-electron O
enhanced catalysis is attributed to stabilization of the Co–
peroxo reduction intermediate through p-backbonding from the
7
2
reduction catalyst. The
8
peripheral Ru(II) or Os(II) metal centers. A minimum of three
peripheral metal complexes are required to achieve the four-
electron reduction of oxygen.
A practical aspect of the electrocatalyst is its stability in acidic
solution the medium of choice for many fuel cells. Incorporation
Experimental
9
Preparation of complexes
17
cis-Pt(DMSO)
2
Cl was synthesized as previously described.
2
5
-(4-Pyridyl)-10,15,20-tris(3-methoxy-4-hydroxyphenyl)por-
Department of Chemistry, University of Dayton, 300 College Park, Dayton,
OH, 45469-2357, USA. E-mail: shawn.swavey@notes.udayton.edu
phyrin, 1. A modified procedure to the previous synthetic
5
530 | Dalton Trans., 2006, 5530–5535
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Fig. 1 Complexes in this study.
procedure was used to synthesize this porphyrin. To a solution
of 7.5 g (0.049 mol) of 3-methoxy-4-hydroxybenzaldehyde and
reaction mixture was refluxed under nitrogen for 30 min followed
by reducing the volume to ca. 1 mL under reduced pressure.
The product was precipitated by addition of 10 mL of distilled
water, filtered and washed 3 × 10 mL with distilled water and
1
.6 mL (0.015 mol) of 4-pyridinecarbaldehyde in 100 mL of
propionic acid was added 4.4 mL (0.066 mol) of freshly distilled
pyrrole. The solution was refluxed for 1 h after which it was cooled
to room temperature and cautiously added to 200 mL of a 50 :
◦
dried under vacuum at 100 C for 10 h. 35 mg of a maroon
powder (0.030 mmol, 47% yield) was recovered. UV-Vis (CH
k
2
Cl
max/nm (10 e/M cm ): 418 (23.2), 532 (1.44). Anal. Calc. for
SCl O: C, 46.31; H, 3.97; N, 5.63; S, 2.58.
2
)
−
4
−1
−1
5
0 methanol–aqueous ammonia solution cooled in an ice-bath.
The slurry was refrigerated overnight, filtered and washed (3 ×
00 mL) with methanol. The resulting black powder was extracted
C
48
H
39
N
5
O
7
2
PtCo·5H
2
1
Found: C, 46.00; H, 3.60; N, 5.73; S, 3.12%.
in a Soxhlet first with methanol for 24 h to remove polymer
byproduct followed by ethyl acetate for 24 h to extract the
porphyrins. The volume of the ethyl acetate extract was reduced to
ca. 70 mL and chromatographed on silica gel using ethyl acetate
as eluent. The second band from the column was collected giving
a purple powder upon removal of the solvent 130 mg (0.17 mmol,
Materials
All reagents were analytical grade and used without further
purification unless stated otherwise. 3-methoxy-4-hydroxybenz-
aldehyde, 4-pyridinecarbaldehyde, propionic acid, methanol,
ꢀ
−
4
−1
−1
N,N -dimethylformamide (DMF), tetrabutylammonium hexaflu-
1
.15% yield). UV-Vis (CH
16.6), 519 (1.13), 557 (0.78), 594 (0.46), 651 (0.43). H NMR
300 MHz, CDCl , TMS): d 9.03 (2H, d, 2,6-pyridyl), 8.95 (6H,
2
Cl
2
) kmax/nm (10 e/M cm ): 425
1
orophosphate (Bu NPF , used as supporting electrolyte for solu-
tion electrochemistry) and extra dry (<50 ppm H O) DMF for
(
(
4
6
2
3
electrochemistry (ACROS), methylene chloride, perchloric acid,
cobalt(II) acetate, 60–200 mesh silica gel (Fisher) were used as
received. The buffer solutions and their pH values measured
m, pyrrole b), 8.79 (2H, d, pyrrole b), 8.18 (2H, d, 3,5-pyridyl),
.79 (6H, m, 2{3-methoxy-4-hydroxyphenyl} and 6{3-methoxy-4-
hydroxyphenyl}), 4.01 (9H, s, m-methoxy), −2.79 (2H, s, internal
pyrrole). Anal. Calc. for C46 O: C 70.76; H, 4.91; N,
·1.5H
.97. Found: C, 70.97; H, 5.06; N, 8.58%.
7
◦
to ± 0.01 at 25 C using a Denver Instrument UltraBasic pH
H
35
N
6
O
6
2
meter calibrated with standard pH 4.00 and 10.00 buffer solutions
8
(
Fisher), were as follows: 0.2 M NaH
(2.64); 0.24 M CH COOH, 0.05 M CH COONa (3.74); 0.2 M
CH COONa, 0.05 M CH COOH (5.04); 0.2 M NaH PO , 0.05 M
Na HPO (6.67). Elemental analyses were performed by Atlantic
2
PO
4
, 0.05 M H
3
PO
4
cis-Pt(DMSO)[5-(4-pyridyl)-10,15,20-tris(3-methoxy-4-hydroxy-
phenyl)porphyrin]Cl , 2. To a solution containing 70 mg
0.093 mmol) of 1 in 20 mL of CH Cl was added 39 mg
0.093 mmol) cis-Pt(DMSO) Cl and the solution was stirred for
h at ambient temperature. The reaction was chromatographed
3
3
2
3
3
2
4
(
(
2
2
2
4
2
2
Microlabs, Norcross, GA.
4
on silica gel using a 50 : 50 ethyl acetate–chloroform solution
as eluent. The first red band from the column was collected
and the solvent removed resulting in 65 mg of a purple
Procedures and instrumentation
1
H NMR spectra were recorded on a Bruker 300 MHz spec-
trophotometer using deuterated chloroform (CDCl ) referenced
to teteramethylsilane (TMS) as the internal standard. UV-Vis
spectra were recorded at room temperature using a Schimadzu
3
powder (0.056 mmol, 61% yield). UV-Vis (CH
2
Cl
2
) kmax/nm
−
4
−1
−1
(
6
2
10 e/M cm ): 427 (21.2), 521 (1.50), 560 (1.12), 595 (0.56),
1
53 (0.60). H NMR (300 MHz, CDCl
3
, TMS): d 9.19 (2H, d,
1
501 photodiode array spectrophotometer with 2 nm resolution.
,6-pyridyl), 8.94 (4H, s, pyrrole b), 8.79 (2H, d, pyrrole b), 8.28
Samples were run in UV-grade CH Cl in 1 cm quartz cuvettes.
2
2
(
4
2H, d, 3,5-pyridyl), 7.76 (6H, m, 3-methoxy-4-hydroxyphenyl),
.01 (9H, s, p-methoxy), 3.62 (6H, s, DMSO), −2.79 (2H, s,
internal pyrrole). Anal. Calc. for C48 SCl O: C,
Pt·3H
0.05; H, 4.11; N, 6.08. Found: C, 50.04; H, 3.83; N, 5.94%.
Solution cyclic voltammograms were recorded using a one-
compartment, three-electrode cell (model 630A Electrochemical
analyzer from CH-Instruments) 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 using 0.30 l followed by 0.05 l
alumina polish (Buehler) and then sonicated for 20 s and tapped
dry with a kimwipe prior to use. Potentials were referenced to
H
41
N
5
O
7
2
2
5
cis-Pt(DMSO)[5-(4-pyridyl)-10,15,20-tris(3-methoxy-4-hydroxy-
phenyl)porphyrinatocobalt(II)]Cl , 3. To a solution of 70 mg
0.064 mmol) of 2 in 20 mL of chloroform was added 32 mg
0.13 mmol) of cobalt(II) acetate in 1.0 mL of methanol. The
2
(
(
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Dalton Trans., 2006, 5530–5535 | 5531

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a saturated calomel electrode (SCE). The supporting electrolyte
was 0.1 M Bu NPF and the measurements were made in extra
dry DMF. Adsorption of the porphyrin and metalloporphyrins
4
6
1–3 onto an edge plane pyrolytic graphite EPG electrode (AFE3T
5
.0 mm diameter, Pine Instrument Co.) which had been roughened
using 600 grit sandpaper was accomplished by placing 5–15 lL
aliquots of 1.0 mM acetone solutions of the porphyrin and
metalloporphyrins onto the electrode surface and allowing the
◦
solvent to evaporate at 20 C. The electrodes were then washed
with distilled water and tapped dry with a kimwipe. Cyclic
voltammetry and rotating disc electrode (RDE) experiments of
the modified electrodes were performed using a Pine AFCBP1
bipotentiostat and an AFMSRX rotator (Pine Instrument Co.) in
1
.0 M perchloric acid solutions.
Fig. 2 Electronic absorption spectra in dichloromethane at room tem-
perature for complex 1 (· · ·), complex 2 (---) and complex 3 (—).
Results and discussion
Characterization of complexes 1–3
Synthesis of porphyrin 1 was complicated by the formation not
only of polymeric material but the inevitable formation of other
porphyrins expected when two different aldehydes are used. Once
the majority of polymeric byproduct was removed from the
reaction mixture by methanol extraction six porphyrins remained.
The use of ethyl acetate to extract the porphyrins from the
remaining polymeric byproduct resulted in good separation of the
porphyrins by column chromatography. Attempts to extract the
porphyrins into ethyl acetate prior to the methanol washing led
to streaking on the column. Integration of the methoxy protons
and 2,6-pyridyl protons (ratio of 9 : 2) in the NMR spectra
helped identify the monopyridyl porphyrin 1. Coordination of
Pt(DMSO)Cl was straight-forward leading to complex 2 in
2
relatively high yield. Comparison of the methoxy protons of the
porphyrin, a singlet at 4.01 ppm, and the methyl protons of
the Pt(II) coordinated DMSO, a singlet at 3.62 ppm, indicated
coordination of the Pt(II) moiety. Repeated attempts using two
different synthetic methods to insert Co(II) into the porphyrin
1
led to an insoluble product each time. For reasons that are
unclear, insertion of Co(II) into the porphyrin–Pt(II) complex 2
was comparatively easy leading to a pure tractable compound, 3
(
Fig. 1).
Fig. 2 illustrates the differences in the electronic spectra of the
complexes 1–3 in CH
2
2
Cl . A Soret band at 425 nm and four Q-
bands at lower energy were observed for complex 1 (Fig. 2, dotted
line). A small perturbation of the porphyrin orbitals was seen upon
coordination of Pt(DMSO)Cl to the peripheral pyridyl groups
2
most noticeably resulting in a shift of the Soret band to 427 nm
and increased intensity of this absorption band (Fig. 2, dashed
line). Insertion of Co(II) into complex 2 increases the symmetry of
the complex leading to a collapse of the four Q-bands into a single
band at 532 nm. In addition the Soret band shifted to 418 nm
Fig. 3 Cyclic voltammograms of complexes 1–3 in 0.1 M Bu
4
NPF
6
in
−1
DMF, scan rate = 100 mV s , referenced to the saturated calomel electrode
(
SCE): (A) complex 1, (B) complex 2 and (C) complex 3.
(
Fig. 2, solid line).
Fig. 3 shows the cyclic voltammograms of complexes 1–3 in
extra dry DMF with a glassy carbon working electrode. All of
the complexes studied display an irreversible oxidation wave at ca.
.00 V vs. SCE suggesting, first that this process is associated with
porphyrin. A quasireversible reduction of the non-metallated
porphyrin at E1/2 = −1.06 V vs. SCE (Fig. 3(A)), becomes
irreversible and shifted to more negative potentials, Epc = −1.18 V,
1
upon coordination of Pt(DMSO)Cl (Fig. 3(B)). An irreversible
2
the porphyrin, most likely the 3-methoxy-4-hydroxyphenyl groups,
and second there is virtually no electronic communication between
the Co(II), Pt(II) metal centers and the peripheral groups of the
shoulder at Epc = −1.03 V vs. SCE is associated with reduction of
18
the Pt(II) center of complex 2 (Fig. 3(B)). The CV of complex 3
shows similar anodic behavior with a single irreversible oxidation
5
532 | Dalton Trans., 2006, 5530–5535
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wave. A reversible redox couple at E1/2 = −0.76 V vs. SCE can
Fig. 4 suggest Co(II) and Pt(II) coordination has little effect on the
redox chemistry of the adsorbed complexes, further evidence to
support the proposition that the irreversible oxidation is localized
on the peripheral groups of the porphyrin. The chemical step which
follows oxidation of the 3-methoxy-4-hydroxyphenyl groups of the
porphyrin is most likely demethylation of the methoxy substituents
leading to a highly reactive quinonoid structure capable of reacting
II/I
be attributed to the Co redox couple while reduction of the
porphyrin and Pt(II) center are shifted to more negative values
III/II
(
Fig. 3(C)). The Co
couple was not observed due to slow
19
electron transfer kinetics in DMF.
Electrode adsorption studies
20
to form a redox active film on the electrode surface.
After scoring the surface of an edge plane pyrolytic graphite
EPG) electrode, using 600 grit sandpaper, 5–15 lL of 1.0 mM
Since the new redox couple is derived from a quinone like
structure its redox chemistry should depend on the pH of
the solution. To test this we examined electrodes coated with
(
solutions of acetone containing the desired complex were placed
onto the electrode surface and the solvent was allowed to evaporate
under ambient conditions. Reproducible results were obtained
in this manner as indicated by CV experiments. Electrochemical
properties of the complexes adsorbed onto an EPG electrode were
complexes 1–3 which had been oxidized in 1.0 M HClO and
4
studied the newly formed redox couple in buffered solutions
of different pH. A linear relation of E1/2 to pH with a slope
−
+
of −0.059 ± 0.005 V/pH unit indicates a 1e /1H process in
studied in 1.0 M HClO
Complexes 1–3 display an irreversible oxidation, I, with Epa
.77, 0.81 and 0.76 V vs. SCE respectively when an EPG coated
electrode was cycled in 1.0 M HClO (Fig. 4(A), (B) and (C)).
4
by cyclic voltammetry.
21
agreement with a quinone like surface structure. Fig. 5 illustrates
=
the results of a pH study performed on an EPG electrode coated
0
with complex 1 after it had been oxidized in 1.0 M HClO
4
. As the
4
+
[
H ] decreases the redox couple shifts to lower potentials. A plot
This oxidation wave can be associated with oxidation of the 3-
methoxy-4-hydroxyphenyl peripheral groups. Upon oxidation of
the adsorbed complex a chemical reaction occurs creating a new
quasireversible redox couple, IIa/IIb (Fig. 4(A), (B) and (C)), with
of E1/2 values vs. pH (Fig. 5, inset) is linear with a slope of −0.064
−
+
V/pH unit consistent with a 1e /1H process. Similar studies of
complexes 2 and 3 give linear E1/2 vs. pH relationships with slopes
of −0.059 and −0.054 V per pH unit, respectively. Further studies
of the electrode surface are needed to allow us to propose an
accurate structure of the surface of the electrochemically modified
electrodes.
E
1/2 = 0.57, 0.60 and 0.57 V for electrodes coated with complexes
1–3 respectively. Similarities between the cyclic voltammograms in
Fig. 5 Cyclic voltammograms of complex 1 adsorbed onto an EPG
electrode after anodic conditioning in 1.0 M HClO
pH 2.64, (· · ·) pH 3.74, (— — —) pH 5.04, (—) pH 6.67.
4
: (---) pH 0.30, (-·-)
2
O Electrocatalysis
The electrocatalytic reduction of O
at an EPG electrode coated with the Co(II)/Pt(II) porphyrin
using cyclic voltammetry and rotating disk electrode (RDE)
voltammetry. Reduction of O at a bare EPG electrode in air
saturated 1.0 M HClO
occurs with an E1/2 = −0.35 V vs. SCE. An
EPG electrode coated with the Co(II)/Pt(II) porphyrin 3 in air
2
in 1.0 M HClO
4
was studied
3
2
4
Fig. 4 Cyclic voltammograms of complexes 1–3 adsorbed on roughened
−
1
saturated 1.0 M HClO reduces O with an E = 0.14 V vs.
EPG electrodes in 1.0 M HClO
the saturated calomel electrode (SCE): (A) complex 1, (B) complex 2 and
C) complex 3.
4
, scan rate = 50 mV s , referenced to
4
2
1/2
SCE (Fig. 6, dotted line) a catalytic shift of 500 mV compared to
the bare EPG electrode. A catalytic shift of nearly 600 mV when
(
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−
5
2
−1
(
(
2.0 × 10 cm s ), m is the kinematic viscosity of the solution
0.01 cm s ), C is the concentration of O in the air-saturated
is the
reduction and can be calculated
2
−1
2
solution (0.25 mM), and x is the rotation rate (rpm). I
kinetic current density for O
k
2
from the intercept of the Koutecky–Levich plot. The slope for
the Koutecky–Levich plot for the experimental data is very near
the slope for the theoretical n = 2 line giving a value for n =
2
.3 (Fig. 7(C)). Enhanced catalytic activity toward O reduction
2
by para-hydroxyphenyl substituted Co(II) porphyrins has been
2
4
attributed to the electron-donating ability of these substituents.
−
The majority of O
2
is electrocatalytically reduced to H
2
O , 2e
2
process, at the EPG electrode coated with 3 prior to oxidation of
the adsorbed complex.
Fig. 8(A) illustrates the results of O
electrode coated with 3, after surface oxidation, in an air saturated
.0 M HClO solution. The Levich plot reveals similar deviation
from linearity related to the kinetics of Co(II)–O bond formation
(Fig. 8(B)). The Koutecky–Levich plot of this data (Fig. 8(C)),
gives an n value of 3.3 suggesting that a significant amount of
2
reduction at an EPG
1
4
Fig. 6
saturated 1.0 M HClO
2
O reduction at an EPG electrode coated with complex 3 in air
2
4
prior to oxidation of the surface adsorbed complex
−
1
(
---) and after anodic conditioning (—), scan rate = 100 mV s , referenced
to the saturated calomel electrode (SCE).
O
2
is reduced directly to H
2
O at this electrochemically modified
compared to the bare EPG electrode is observed for O
2
reduction
surface.
at an EPG electrode coated with 3 after oxidation of the surface
confined complex (Fig. 6, solid line).
Analysis of the Koutecky-Levich intercept before and after oxi-
dation of the adsorbed complex using eqn (2) reveals information
Fig. 7(A) illustrates the results of the RDE experiment per-
formed on an EPG electrode coated with 3 in air-saturated 1.0 M
about the current-limiting formation of the Co–O bond:
2
I
k
= nFkCCO2
(2)
HClO prior to oxidation of the adsorbed complex. A plot of the
4
diffusion-limited current density vs. the square root of the rotation
rate, the Levich plot, indicates a deviation from linearity at higher
rotation rates attributed to a potential independent rate limiting
where C is the surface concentration of catalyst on the electrode
−
2
(
mol cm ) and k is the second-order rate constant associated with
−
1
−1
the current-limiting reaction between Co(II) and O (M s ).
2
22
step (Fig. 7(B)). The Koutecky–Levich plot (Fig. 7(C)), relates
Without a detailed understanding of the structure of the adsorbed
complexes after oxidation it is difficult to determine with any
certainty the surface concentration, therefore a quantitative look
at the rate constant k in eqn (2) is not possible. We can however
23
the plateau current density to the rotation rate using eqn (1):
1
/2
1/I
L
= 1/I
k
+ 1/Bx
(1)
B = 0.2nFCm−1/6
2/3
determine qualitatively the effect of oxidation of complex 3 on the
D
b
rate of interaction between Co(II) and O
2
. The ratio of I
k
/n, where
−
2
a
where I
L
is the current density (A cm ), n is the number
of electrons for the reaction, F is the Faraday constant
96 500 C mol ), D is the diffusion coefficient of O
n = 2.3, before oxidation to I
k
/n, where n = 3.3, after oxidation
b
a
gives a relationship between the rate constants k before and k
−
1
b
a
(
2
in the solution
after oxidation. The value for this ratio is near unity (k = k ).
Fig. 7 Reduction of O
2
4
at a rotating disk electrode coated with complex 3 prior to oxidation. (A) current–potential curves in air-saturated 1.0 M HClO ,
−
1
scan rate = 10 mV s . (B) Levich plot of plateau current vs. square root of the rotation rate for the curves in (A). (C) Koutecky–Levich plot of the inverse
of the plateau current vs. the inverse of the square root of the rotation rate for the curves in (A). The theoretical two- and four-electron lines are marked
n = 2 and 4, respectively.
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Fig. 8 Reduction of O
HClO
2
at a rotating disk electrode coated with complex 3 after anodic conditioning. (A) current-potential curves in air-saturated 1.0 M
, scan rate = 10 mV s . (B) Levich plot of plateau current vs. square root of the rotation rate for the curves in (A). (C) Koutecky–Levich plot of
−
1
4
the inverse of the plateau current vs. the inverse of the square root of the rotation rate for the curves in (A). The theoretical two- and four-electron lines
are marked n = 2 and 4, respectively.
This seemingly unremarkable finding indicates that the rate of
formation of the Co–O bond is unaffected by electrooxidation of
4 C. Shi and F. C. Anson, J. Am. Chem. Soc., 1991, 113, 9564.
5
J. P. Collman, N. H. Hendricks, K. Kim and S. C. Bencosme, J. Chem.
Soc., Chem. Commun., 1987, 1537; K. M. Kadish, L. Femond, Z. Qu,
J. Shao, C. Shi, F. C. Anson, F. Burdet, C. P. Gros, J.-M. Barbe and R.
Guilard, J. Am. Chem. Soc., 2005, 127, 5625; J. P. Collman, M. Kaplun
and R. A. Decreau, Dalton Trans., 2006, 554.
F. C. Anson, C. Shi and B. Steiger, Acc. Chem. Res., 1997, 30, 437.
C. Shi and F. C. Anson, Inorg. Chem., 1996, 35, 7928.
C. Shi and F. C. Anson, Inorg. Chem., 1992, 31, 5078; B. Steiger, C. Shi
and F. C. Anson, Inorg. Chem., 1993, 32, 2107; H.-Z. Yu, J. S. Baskin,
B. Steiger, F. C. Anson and A. H. Zewail, J. Am. Chem. Soc., 1999, 121,
2
the surface confined complex, 3. Therefore the macrocycle core
seems to be intact after oxidation. The enhanced catalytic ability
resulting from oxidation of the adsorbed complex may result from
formation of a more stable cobalt–peroxo interaction. The role
of the oxidation as well as the resulting structure of the oxidized
complex and the role of Pt(II) in the stabilization of this interaction
is currently being studied.
6
7
8
4
84.
9
B. Steiger and F. C. Anson, Inorg. Chem., 1997, 36, 4138.
Conclusions
10 J. E. Bennett, A. Burewicz, D. E. Wheeler, I. Eliezer, L. Czuchajowski
and T. Malinski, Inorg. Chim. Acta, 1998, 271, 167; T. Abe and M.
Kaneko, Prog. Polym. Sci., 2003, 28, 1441; S. M. Chen and Y. L. Chen,
J. Electroanal. Chem., Interfacial Electrochem., 2004, 373, 277.
1 K. Araki, L. Angnes, C. M. N. Azevedo and H. E. Toma, J. Electroanal.
Chem., 1995, 397, 205.
12 J. M. Vago, V. C. Dall’Orto, E. Farzani, J. Hurst and I. N. Rezzano,
1
To summarize we have synthesized and characterized by
H
NMR, UV-Vis spectroscopy and cyclic voltammetry two new
metalloporphyrins, complexes 2 and 3 (Fig. 1). Adsorption of
these complexes onto EPG electrodes followed by oxidation in
1
Sens. Actuators, B, 2003, 96, 407.
1.0 M HClO
4
results in a new redox active modified electrode
to H and H O. Studies
1
1
3 G. E. Milczarek and A. Ciszewski, Electroanalysis, 2001, 13, 164.
4 H. Winnischofer, S. D. Lima, K. Araki and H. E. Toma, Anal. Chim.
Acta, 2003, 480, 97; A. Ciszewski and G. Milczarek, Talanta, 2003, 61,
11; M. Plonska, K. Winkler, S. Gadde, F. D’Souza and A. L. Balch,
Electroanalysis, 2006, 9, 841.
which electrocatalytically reduces O
are currently underway to probe the effect of the number of
Pt(II) groups coordinated to the porphyrin periphery on the
2
2
O
2
2
electrocatalytic reduction of O in acidic media.
2
1
1
5 S. Swavey, M. Narra and H. Srour, J. Coord. Chem., 2005, 58, 1463.
6 D. Marek, M. Narra, A. Schneider and S. Swavey, Inorg. Chim. Acta,
2
006, 359, 789.
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