meso-tetrakis(4-N-benzylpyridyl)porphyrin and its supramolecular complexes formed with anionic metal-oxo cluster: Spectroscopy and electrocatalytic reduction of dioxygen

Article abstract of DOI:10.1016/S0020-1693(00)00154-7

The research results show that MTBPyP (meso-tetrakis(4-N-benzylpyridyl)porphyrin, M = H₂, Zn) bearing quadruply positive charges associates with an anionic metal-oxo duster (SiW₁₂O₄₀/^4-) to form a supramolecular complex in aqueous solution. The spectral evolution and Job's plots reveal that the aggregates contain equal numbers of the cationic porphyrins and the anionic SiW₁₂O₄₀/^4-. The association is accompanied by significant changes in the Soret band of the individual porphyrins. The 1:1 supramolecular complex [CoTBPyP][SiW₁₂O₄₀] may serve as an electrocatalyst for electrocatalytic reduction of O₂, and more of O₂ is reduced to H₂O at potential where the SiW₁₂On₄₀/^4- is in reduced state. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00154-7

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Inorganica Chimica Acta 306 (2000) 87–93
meso-Tetrakis(4-N-benzylpyridyl)porphyrin and its supramolecular
complexes formed with anionic metal–oxo cluster: spectroscopy
and electrocatalytic reduction of dioxygen
Shu-Qing Liu, Ji-Qing Xu *, Hao-Ran Sun, Dong-Mei Li
Department of Chemistry, Jilin Uni6ersity, Changchun 130023, People’s Republic of China
Received 13 January 2000; accepted 5 April 2000
Abstract
The research results show that MTBPyP (meso-tetrakis(4-N-benzylpyridyl)porphyrin, M=H , Zn) bearing quadruply positive
2
4−
40
charges associates with an anionic metal-oxo cluster (SiW O ) to form a supramolecular complex in aqueous solution. The
12
spectral evolution and Job’s plots reveal that the aggregates contain equal numbers of the cationic porphyrins and the anionic
4−
40
SiW O . The association is accompanied by significant changes in the Soret band of the individual porphyrins. The 1:1
12
supramolecular complex [CoTBPyP][SiW O ] may serve as an electrocatalyst for electrocatalytic reduction of O , and more of
1
2
40
2
4−
12 40
O is reduced to H O at potential where the SiW O
is in reduced state. © 2000 Elsevier Science S.A. All rights reserved.
2
2
Keywords: Electrocatalysis; Metal complexes; Porphyrin complexes; Oxo complexes; Cluster complexes
1
. Introduction
diporphyrins and the related systems [2–4], cobalt te-
trakisbis(4-pyridyl)porphyrin with three or four
2
+
Interest in fuel cell technology has motivated the
[Ru(NH ) ]
groups appended to the porphyrin pe-
3
5
search for an inexpensive electrode material that can
accomplish the direct four-electron reduction of dioxy-
gen to water at or near the reversible thermodynamic
potential (+1.23 V vs. NHE at pH 0). Many transition
metal complexes with macrocyclic ligands have been
examined as dioxygen reduction catalyst [1]. Most of
these electrocatalysts reduce dioxygen (via the two-elec-
tron pathway) to hydrogen peroxide at relatively nega-
tive potentials. For an oxygen-reducing electrode to
operate at or near the thermodynamic equilibrium po-
tential, hydrogen peroxide cannot be a free intermedi-
ate [2]. Thus, in order to achieve high efficiency, the
catalysts should reduce dioxygen to water directly with-
out the production of hydrogen peroxide. To date, only
a few molecular electrocatalysts have been used to
achieve the four-electron direct reduction of dioxygen
to water. The catalysts include bis(cobalt) cofacial
riphery [5], some iridium porphyrin systems [6],
(5,15,15,20-tetramethylporphyrinato) cobalt (II) [7] and
the simplest systems of the cobalt porphyrins [8]. Re-
cently, D’Souza reported an electrocatalyst formed by
an ion-pair cobalt porphyrin dimer that reduced dioxy-
gen to water by four electrons [9]. We report here the
novel properties of the several supramolecular com-
plexes formed by spontaneous association of the
cationic porphyrins and the anionic metal–oxo clusters
(Scheme 1), including spectral properties of
[ZnTBPyP][SiW O ] and [H TBPyP][SiW O ] sys-
12
40
2
12 40
tems in aqueous solution and electrocatalytic activity
reducing O to H O of [CoTBPyP][SiW O ].
2
2
12 40
2. Experimental
2.1. Materials
*
Corresponding author. Tel.: +86-431-892 2331 ext. 2462; fax:
Pyrolle was distilled at atmospheric pressure before
use. 4-Pyridine aldehyde, benzyl bromide, N,N-
+
86-431-894 9334.
E-mail address: xjq@mail.jlu.edu.cn (J.-Q. Xu).
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00154-7

88
S.-Q. Liu et al. / Inorganica Chimica Acta 306 (2000) 87–93
dimethylformamide (DMF), sodium perchlorate and
tungstosilicic acid (H SiW O ) were used as received
yellow, which showed that the product [H TBPyP]-
2
(ClO ) precipitated completely. Then the precipitate
4
12 40
4 4
from commercial sources. The meso-tetrakis(4-
was isolated by centrifugation and washed for several
times with cold distilled water to remove excess sodium
perchlorate. Deep-red product [H TBPyP](ClO )
pyridyl)porphyrin (H TPyP) was prepared according to
2
the literature method [10].
2
4 4
(
0.357 g) was obtained by drying over P O5 under
2
2.2. Instruments
vacuum. The yield, based on H TPyP, was 80%,
2
m.p.\250°C. Anal. Calc. for [H TBPyP](ClO ) ·3H O
2
4 4
2
UV–Vis absorption spectral measurements were per-
formed on a UV-3100 (Shimadzu, Japan) or a UV-922
Kontron, Italy) spectrometer. FTIR spectra were de-
(
7
C H N O Cl ) (F =1435): C, 56.86; H, 3.97; N,
.80. Found: C, 57.10; H, 3.91; N, 7.60%. H NMR
68 60 8 19 4 w
1
(
(
DMSO-d ): l(ppm) 9.68(d, J=10.24Hz, Hm-pyridyl),
6
termined on a Nicolet Impact 410 spectrophotometer.
9
7
.23 (br, S, Hpyrrole), 9.05 (d, J=10.4Hz, Ho-pyridyl),
.59–7.91 (m, Hphenyl), 6.24 (S, HCH2), −3.12 (S, NH).
1
The proton magnetic resonance ( H NMR) spectrum
was carried out on a Varian Unity-400 NMR system.
Elemental analyses were obtained on a Perkin–Elmer
40C elemental analyzer. And MALDI-TOF-MS (ma-
trix-assisted laser desorption/ionization, time of flight,
mass spectrometry) determination was carried out on
an LDI-1700 mass spectrometer (Linear Scientific Inc.,
USA).
−3
−1
Visable absorption: u nm (in DMSO) (m×10 l mol
cm ) 428.8(210), 518.4(16.6), 554.2(6.91), 589.2(6.49),
−1
2
6
45.0(2.28). FTIR spectral data: 3320.0 (wNꢀH), 3114.2
+
(
w
), 1634.0 (CꢁN , CꢁC), 1086.9 (ClꢁO), 971.5 (p ),
ArꢀH
p
−
1
8
01.1 cm
(lphenylꢀH). MALDI-TOF-MS: M=
+
[H TBPyP](ClO ) , [M−4ClO ] (m/z=983.8), [M−
2 4 4 4
+
3
ClO4] (m/z=1082.3).
2
2
.3. Syntheses
2
.3.2. [CoTBPyP](ClO4)4
[
H TBPyP](ClO ) (0.060 g, 0.046 mmol) was dis-
2 4 4
.3.1. [H TBPyP](ClO4)4
2
solved in 100 ml of boiling distilled water. Excess of
Co(OAc) ·4H O (0.060 g, 0.24 mmol) was added to the
H TPyP (0.200 g, 0.324 mmol) was dissolved in 100
2
2
2
ml of hot (ꢀ80°C) N,N-dimethylformamide (DMF),
then excess of benzyl bromide (about 10 ml, 80 mmol)
was added to the solution. The unreacted H TPyP was
monitored by thin-layer chromatography (TLC, Silica,
CHCl –C H OH) during the reaction process. The re-
action was completed after refluxing for about an hour.
The mixture was then allowed to cool to room temper-
ature (r.t.) and was poured into 50 ml of ethyl ether,
crude product [H TBPyP]Br precipitated instantly out
of the solution. The precipitate was isolated by filtra-
tion and dissolved in 250 ml of distilled water, excess of
sodium perchlorate (10.0 g, 80 mmol) was added (it
would be best to pay attention to the explosion hazard
associated with organic perchlorate salts). The solution
was stirred until its color changed from red to light
solution under refluxing. The process of the reaction
was monitored by UV–Vis spectroscopy. It is observed
that the formation of cobalt porphyrin causes a red
shift of the Soret band and reduction of the number of
Q bands. The reaction was completed after refluxing for
2
3
2
5
2
h. The mixture was allowed to cool to r.t. and excess
of sodium perchlorate (1.0 g, 8.0 mmol) was added. The
solution was stirred until its color changed from red to
light yellow, which also showed that the product
2
4
[CoTBPyP](ClO ) precipitated completely. Then the
4 4
precipitate was isolated by centrifugation and washed
for several times with cold distilled water to remove
excess of sodium perchlorate. Brownish black product
[CoTBPyP](ClO4)4 (0.06 g) was obtained with yield
Scheme 1.

S.-Q. Liu et al. / Inorganica Chimica Acta 306 (2000) 87–93
89
9
5%, m.p.\250°C. Visable absorption: u nm (in
ence electrode, and platinum wire was used as the
counterelectrode. The collection coefficient of platinum
ring electrode N=0.2290.01 and the area of glassy
−
3
−1
−1
DMSO) (m×10
l
mol
cm
)
435.8(59.2),
40.2(7.07). FTIR spectral data: 3114.1 (w ), 1632.6
ArꢀH
5
+
−1
2
(CꢁN , CꢁC), 1087.5 (ClꢁO), 1002.2 (p ), 798.4 cm
(l
carbon disk electrode (A=0.15990.05 cm ) were de-
p
).
termined using the ferrocyanide–ferricyanide couple.
Sonication and warming were required for dissolving
the porphyrin-cluster compound in dimethyl sulfoxide
(DMSO). 5.0 ml of solution (0.1 mM) of the porphyrin
and porphyrin-cluster was transferred, respectively, to
the surface of the glassy carbon disk electrode. Then
the surface of the electrode was pumped to dry at r.t.
Most of the porphyrin sample modified on the electrode
was lost when the electrode was subsequently dipped to
acidic aqueous solutions. But the porphyrin-cluster
sample was strongly bound to the surface of the glassy
carbon electrode due to its insolubility in acidic
aqueous solution.
phenylꢀH
2.3.3. [ZnTBPyP](ClO4)4
[
H TBPyP](ClO4)4 (0.096 g, 0.070 mmol) was dis-
2
solved in 100 ml of boiling distilled water. Then excess
of Zn(OAc) ·2H O (0.120 g, 0.55 mmol) was added to
the solution under refluxing. Other procedures were the
same as those of [CoTBPyP](ClO ) . The violet product
of [ZnTBPyP](ClO ) was obtained (yield 0.071 g, 71%).
Visable absorption: u (in H O) 440.0, 566.5, 606.5 nm.
2
2
4
4
4
4
2
2.3.4. [CoTBPyP][SiW O ]
12 40
A precipitate appeared immediately when 5 ml of
aqueous solution of H SiW O (0.019 g, 0.007 mmol)
To calculate the Levich limiting currents at rotating
disk electrodes, the following parameters were used:
4
12 40
was added into
5
ml of aqueous solution of
2
−1
[
CoTBPyP](ClO ) (0.010 g, 0.007 mmol). After stirring
kinematic viscosity of water, 0.01 cm s ; diffusion
4
4
−
5
2
−1
for 1/2 h, the solution was centrifuged and the obtained
coefficient for dioxygen, 1.0×10
cm s ; concen-
precipitate was washed for several times with water. A
tration of O in air-saturated solutions, 0.28 mM at
2
brownish powder was obtained by drying over P O5
2092°C.
2
under vacuum (yield 0.020 g, 70%). Elemental analysis
and FTIR spectrum of [CoTBPyP][SiW O ] indicated
12
40
the presence of a number of water molecules. Anal.
3. Results and discussion
Calc. for [CoTBPyP][SiW O ] (CoC H N SiW O )
12
40
68 52
8
12 40
(
Fw=3913): C, 20.85; H, 1.33; N, 2.86; C–N, 7.29.
3.1. The spectra of [MTBPyP][SiW O ] (M=H ,
12
40
2
Found: C, 17.02; H, 1.50; N, 2.50; C–N, 6.81%. Visible
Zn)
absorption: u (in DMSO) 440.2, 539.0 nm. FTIR spec-
−
1
trum: 3435 cm
630.3, 1550.4, 1453.0, 1384.4, 1208.1, 1012.2 cm
attributed to [CoTBPyP], 970.0, 920.1, 883.2, 791.6
(broad and strong) attibuted to H O,
There are numerous examples of supramolecular as-
semblies bound by ion-pair attraction in solution [11].
In most cases, the properties of such complexes differ
notably from those of the parent compounds. It is well
known that charged porphyrins can bind oppositely
charged molecules through electrostatic interaction
2
−
1
1
−
1
cm
.4. Spectroscopic methods
Spectroscopic measurements were performed at r.t. in
attributed to [SiW O ].
12 40
2
[9,11b, 12], and the heteroaggregation between them
can be conveniently studied by spectroscopic methods.
In this paper, we studied the interaction between
quartz cell (1 cm optical pathlength). Titration were
achieved by adding, with a micropipette, directly into
4
+
4−
MTBPyP
(M=H , Zn) and SiW O
in aqueous
2
12 40
−
3
the quartz cell, small aliquots (typical 1 ml) of a 10
M
solution using UV–Vis absorption spectroscopy. Fig.
1a gives the spectral evolution of a solution of the
4−
40
−5
solution of SiW O
to 3 ml of a ca. 10 M solution
1
2
4
+
of porphyrin. Dilution effects were thus negligible. The
added volumes were nevertheless take into account in
the construction of Job’s diagrams.
ZnTBPyP
in H O upon titration with a solution of
2
4−
40
anionic metal–oxo cluster SiW O . Similarly, the
spectral evolution of a solution of H TBPyP
upon addition of SiW O
12
4
+
in H O
2
is shown in Fig. 1(b). In
2
4−
40
12
2
.5. Electrochemical experiments
above-mentioned both cases, the reaction proceeds with
an isosbestic point (at 454, 436 nm in Fig. 1(a) and (b),
4
−
Cyclic, rotating disk, rotating ring-disk voltam-
respectively, at number equivalents of SiW O
:
12
40
metries and stability of electrocatalytic current were
carried out with a Model HPD-IA potentiostat (Yan-
bian, China) associated with a Type 3086A X-Y -Y
MTBPyPB1) and linear hypochromophores effect in
the Soret band of porphyrins (insert chart of Fig. 1(a)
and (b)), which is completed when the (1:1) stoichiome-
try is nearly reached. At the 1:1 stoichiometric point,
the resulting spectrum of the solution differs markedly
from the sum of those of the reactants (the Soret bands
of porphyrin shift from 440, 423 to 455, 438 nm,
4
1
2
(
Sichuan, China) recorder using a three-electrode sys-
tem. A rotating platinum ring-glassy carbon (GC) disk
electrode (EG&G, Model 636) was used as working
electrode, Ag–AgCl (sta’s KCl) was used as the refer-

90
S.-Q. Liu et al. / Inorganica Chimica Acta 306 (2000) 87–93
−
6
−3
−6
4−
40
Fig. 1. (a) Spectral evolution of a solution of 4.8×10
M ZnTBPyP in H O upon addition of small aliquots of 10
M SiW12
O
(insert chart
2
4−
40
is the plot of A440 nm versus increasing concentration of SiW12O ; (b) Spectral evolution of a solution of 6.1×10
M H TBPyP in H O upon
2
2
−
3
4−
40
4−
40
addition of small aliquots of 10
M SiW12
O
(insert chart is the plot of A423 nm versus increasing concentration of SiW12
O
).
respectively), revealing the formation of at least one
new species, in which the porphyrin chromophores
interact strongly with the metal–oxo clusters. Similar
results, observed by Lipskier [11b] and others [13], were
interpreted as evidence of the formation of 1:1 het-
eroaggregates held by electrostatic attraction, in equi-
librium with the starting monomers. Over the 1:1
stoichiometry, an isosbestic point disappears and the
absorbance at the Soret of the porphyrins decreases
slightly but no new break point occurs (insert chart of
Fig. 1), which probably suggests the formation of
higher unstable aggregates with the stoichiometry other
than 1:1.
The data from the spectroscopic titration experi-
ments were treated using Job’s method, which is con-
structed to ascertain the stoichiometries of the
aggregates [14]. This method relies on the fact that the
optical absorbance of a mixture of chromophores that
do not react on each other is the sum of the absorptions
due to each chromophore separately. Correlatively, de-
partures from additivity as the composition of the
solution is continuously varied can be interpreted as
evidence of the formation of a complex. By using Job’s
method, the stoichiometry of a complex can be deduced
from the mole ratio of parent compounds in solution,
at which the deviation from additivity has a maximum.
In practice, the absorption measured at a given wave-
length for mixtures with various mole ratios of the
4
−
porphyrin and SiW O
has been used to calculate by
12
40
Eq. (1):
DA(x)=A(x)−m cx−m c(1−x)
(1)
where c=[porphyrin]+[SiW O ] is the total concen-
p
w
12
40
tration, x=[porphyrin]/c is the mole fraction of por-
phyrin, m and m are the molar absorptivities of the
p
w
4−
40
porphyrin and the SiW O
respectively, and A(x) is
12
the measured optical absorbance of the solution. Thus,
DA(x) represents a deviation of the absorbance of the

S.-Q. Liu et al. / Inorganica Chimica Acta 306 (2000) 87–93
91
solution from the additivity of absorbance for the
porphyrin and the metal–oxo cluster. Because
3.2. Electrocatalytic reduction of dioxygen
4−
40
SiW O
has no absorption at the wavelength of the
Soret band of porphyrin, Eq. (1) may change to Eq. (2):
A strategy to achieve rapid, multiple-electron trans-
fers (simultaneously avoiding undesired intermediates
formation) has been tested with a variety of substrates,
in which the syntheses of catalysts containing multiple
metal centers and serving as electron donors or accep-
tors are involved [5]. In order to search for highly
efficient electrocatalysts reducing O to H O through a
1
2
DA(x)=A(x)−m cx (2)
p
Job’s diagrams were obtained by plotting DA(x)
against x. Fig. 2 displays the Job’s plots examined at
the wavelength of the Soret band of porphyrins, and it
2
2
suggests the formation of
a
1:1 complex,
four-electron transfer pathway, we designed and syn-
thesized the cationic cobalt porphyrin-anionic metal–
oxo cluster supramolecular complex. It is well known
that cobalt porphyrins have electrocatalytic activity re-
ducing dioxygen [9,15] and metal–oxo clusters can
accept or donate multielectrons but their cluster skele-
tons are not destroyed [16]. Our research results show
that the [CoTBPyP][SiW O ] complex possesses good
[
ZnTBPyP][SiW O ] and [H TBPyP][SiW O ], re-
1
2
40
2
12 40
spectively. However, the existence of higher order ag-
gregates containing equal numbers of the porphyrins
4−
40
and the metal–oxo clusters SiW O
can not be ruled
1
2
out because the dimers of porphyrins can be formed
owing to their p–p interaction and van der Walls force.
The observed similarity in the [ZnTBPyP][SiW O ]
12
40
12
40
and [H TBPyP][SiW O ] systems in aqueous solution
2
12 40
electrocatalytic activity reducing O to H O.
2
2
suggests that the interaction between the porphyrin
cations and the metal–oxo cluster anions is primarily
electrostatic while the central ions do not play an
important role in determining the nature of the interac-
A rotation platinum ring-glassy carbon disk electrode
RRDE) was used to determine the amount of H O
(
2
2
produced during the reduction of O . The glassy carbon
2
4
+
4−
40
(GC) disk was coated with [CoTBPyP](ClO
4 4
) or
tion. Similarly, CoTBPyP
and SiW O
may also
1
2
[
CoTBPyP][SiW O ] and the platinum ring was main-
12
40
form 1:1 complex by electrostatic interaction (Although
we failed to get perfect spectral evolution and Job’s plot
because the absorption of the Soret band of CoTBPyP
is weak and only a little absorption change occurs upon
addition with a solution of anionic metal–oxo cluster
tained at 1.0 V to oxidize H O generated at the GC
2
2
electrode due to the reduction of O . The product of
2
reduction of O is essentially H O when using GC disk
2
2
2
electrode coated with [CoTBPyP](ClO ) (see Fig. 3(a)).
4 4
4
−
The plateau current for the first step is generated by
SiW O ).
1
2
40
reduction of O electrocatalyzed probably by face-to-
Job’s plots do not show the formation of higher
aggregates with the stoichiometry other than 1:1,
which indicate that the further aggregations at higher
anion concentrations is too weak to observe by this
method.
2
face cobalt porphyrin formed through p–p interaction
and van der Waals force, while one for the second step
is produced through reduction of O electrocatalyzed
2
by monoric cobalt porphyrin. By contrast, when the
Fig. 2. (a) Job’s plot for the [ZnTBPyP][SiW12O40] system in H O, u=440 nm; (b) Job’s plot for the [H TBPyP][SiW12O40] system in H O, u=423
2
2
2
nm.

92
S.-Q. Liu et al. / Inorganica Chimica Acta 306 (2000) 87–93
4
−
potentials where the SiW O
40
is in reduced state that
12
is the most active for catalyzing the four-electron reduc-
tion of O2.
In Fig. 4(a), the current–potential curve is a ten-
dency for the second step plateau currents to become
less and less plat as the electrode rotation rate is
increased. This pattern of behavior is a very common
feature for the steady state current–potential curves
recorded at rotating disk electrodes for electrode reac-
tions [17]. The n-value, electrons involved in the reduc-
tion of O , calculated from the slope of Fig. 4(c), is 3.8
2
per O molecule, essentially corresponding to the result
2
of RRDE at the same potential of −0.45 V (n=3.5,
according to n=4−2i /(Ni ), N=0.22).
R
D
Fig. 3. Reduction of O at a rotating platinum ring-glassy carbon disk
2
2
(
0.164 cm ) electrode (RRDE) having a collection efficiency of 23%.
The potential of the ring electrode was maintained at 1.0 V versus
Ag–AgCl (sat’s KCl). S =5, S =1.25 mA. The supporting elec-
1
2
trolyte, 0.5 M H SO was saturated with air. Rotation rate: 100 rpm,
2
4
−
1
scan rate: 2 mV s . (a) The RRDE of the GC disk electrode coated
with [CoTBPyP](ClO ) ; (b) The RRDE of the GC disk electrode
Fig. 4. Reduction of O2 at a rotating glassy carbon disk electrode
coated with [CoTBPyP][SiW12O40]. (a) Current–potential curves at
electrode rotation rates (from bottom to top) of 100, 144, 225, 400,
4
4
coated with [CoTBPyP][SiW12O40]. The dashed line shows the cyclic
voltammogram obtained for [CoTBPyP][SiW12O40] in argon-satu-
6
25, 900 and 1600 rpm. For clarity, the base line for each successive
−
1
rated 0.5 M H SO4 solution, S =5 mA, scan rate is 10 mV s
.
2
3
−1
curve was adjusted upward. Scan rate=2 mV s , S=5 mA; (b)
Levich plot of the plateau currents at potential of −0.45 V from (a)
for the second step; (c) Koutecky–Levich plot of the data from (b).
The n=2 and 4 lines are the calculated responses for the diffusion–
GC disk was coated with [CoTBPyP][SiW O ], the
1
2
40
ring current became smaller while the disk current
became larger, especially for the plateau current of the
second step (Fig. 3(b)). For the first step, the main
product of reduction of O is H O . For the second
convection-limited reduction of O by two or four electrons. Support-
2
ing electrolyte is 0.5 M H SO aqueous saturated with air.
2
4
2
2
2
step, the ratio of the ring current to that of disk is 22%
and it corresponds to 78% of the disk current resulting
in the four-electron reduction of O to H O, i.e. 64% of
2
2
the O molecules were reduced to H O and 36% to
2
2
H O . In addition, the behavior shown in Fig. 3(b) can
2
2
be understood by comparison with the current–poten-
tial curve for the adsorbed catalyst of [CoTBPyP]-
[
SiW O ] in the absence of O , shown by the dashed
12 40 2
curve in Fig. 3(b). Although the second step of the
catalyzed reduction of O at the GC disk begins before
2
4−
40
the SiW O
in electrocatalyst [CoTBPyP][SiW12O40]
is reduced, the more of O is reduced to H O at this
situation than that after SiW O
1
2
Fig. 5. The stability of the catalytic activity of electrocatalyzing
2
2
2
reduction of O at GC electrode coated with [CoTBPyP][SiW12O40] in
2
4−
40
reduction. That is,
12
air-saturated 0.5 M H SO . Potential was maintained at −0.45 V
2
4
the more of O is reduced to H O at more negative disk
versus Ag–AgCl (sat’s KCl), rotation rate=1600 rpm.
2
2

S.-Q. Liu et al. / Inorganica Chimica Acta 306 (2000) 87–93
93
trochem. 145 (1983) 439. (e) H.Y. Liu, I. Abdalmuhdi, C.K.
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The stability of catalytic activity of [CoTBPyP]-
SiW O ] coated at the GC disk electrode for reduc-
[
1
2
40
tion of dioxygen was examined at −0.45 V. It can be
seen from Fig. 5 that catalytic current increases gradu-
ally in first 5 h then decreases slightly, and about 8 h
later becomes stable. It seems that a desirable catalyst
for electrocatalyzing reduction of O to H O will likely
[
2
2
[
[
[
6] J.P. Collman, K. Kim, J. Am. Chem. Soc. 108 (1986) 7847.
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(
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[
(
Acknowledgements
[
[
This work was supported by the National Natural
Science Foundation of China under grants No.
29733090 and No. 29803003 and the Research Funds
for the Doctoral Program of Higher Education.
(
1997) 8699. (f) K. Kano, T. Nakajima, M. Takei, S. Hashimoto,
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