Four-electron reduction of dioxygen catalyzed by a decavanadium complex

Article abstract of DOI:10.1016/S0020-1693(02)01158-1

The electroreduction of O₂ in acidic aqueous solutions at an electrode modified with a decavanadium cluster [(V=O)₁₀(μ₂-O)₉(μ₃-O) ₃(C₅H₇O₂)₆] (1) revealed that it produces H₂O with four electrons per O₂ molecule near 0.5 V versus SCE. The usefulness of the complex as a reduction catalyst of O₂ with a high selectivity was demonstrated.

Full text of DOI:10.1016/S0020-1693(02)01158-1

Inorganica Chimica Acta 342 (2003) 316Á318
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Note
Four-electron reduction of dioxygen catalyzed by a decavanadium
complex
Eniya Listiani Dewi, Kenichi Oyaizu, Eishun Tsuchida ꢀ
Advanced Research Institute for Science and Engineering, Department of Polymer Chemistry, Waseda University, Tokyo 169-8555, Japan
Received 24 April 2002; accepted 13 June 2002
Abstract
The electroreduction of O
O) (C ] (1) revealed that it produces H
complex as a reduction catalyst of O with a high selectivity was demonstrated.
2
in acidic aqueous solutions at an electrode modified with a decavanadium cluster [(Vꢀ
/
O)10(m
2
-O)
9
(m
3
-
3
5
7
H O
2
)
6
2
O with four electrons per O molecule near 0.5 V versus SCE. The usefulness of the
2
2
#
2002 Elsevier Science B.V. All rights reserved.
Keywords: Electrochemistry; Four-electron reduction; Dioxygen; Decavanadium
1
. Introduction
tion of O to H O in acid using the modified electrode of
2 2
complex 1 as the electrocatalyst.
The four-electron reduction of O was recently found
2
to be efficiently catalyzed by oxygen bridged complexes,
such as m-oxo divanadium complexes, m-oxo iron-
2
. Experimental
(
III)porphyrins, m-oxo manganese porphyrins, and other
dimeric cobalt porphyrins [1Á12]. Recently, we found
that a novel decavanadium(V) cluster complex [(Vꢀ
O) (m -O) (m -O) (C H O ) ] (1) was very easily pre-
/
The electrochemical behavior of the modified elec-
/
trodes: a Nikko Keisoku DPGS-1 dual potentiogalva-
nostat and a NFG-3 universal programmer were em-
ployed along with a Graphtec WX2400 X-Y recorder to
obtain the voltammogram. The samples and electrolytes
were set up in a conventional two-compartment cell in
which a glassy carbon disk electrode served as the
working electrode. The counter electrode was a plati-
num wire and the reference electrode was a saturated
calomel electrode (SCE).
1
0
2
9
3
3
5
7
2 6
pared from V O (acac) by the spontaneous elimination
2
4
2
of the acac ligand [13,14]. This complex bears ten
oxovanadium(V) cores with good solubility in organic
solvents due to the peripheral ligands, and is capable of
forming a thin layer on a solid surface. The electro-
chemistry of the complex has been examined in organic
electrolyte solutions such as dichloromethane, 1,1,2,2-
tetrachloroethane, nitromethane, and acetonitrile con-
taining a supporting electrolyte, which revealed a
reversible redox wave near 0.6 V versus Ag/AgCl. It
was shown that complex 1 has unique redox chemistry
3
. Results and discussion
ꢁ
as a 10e reservoir in the single redox process [15]. In
order to clarify the multielectron transfer process of the
decavanadium complex, we focused on the electroreduc-
Complex 1 was synthesized and analyzed according to
the procedures described in the literature [13]. The
electrochemical measurements were carried out using a
glassy carbon electrode modified with complex 1 to
determine the four-electron reduction selectivity of O2
under acidic conditions. The 0.1 mM solution of
ꢀ
Corresponding author. Tel.: ꢂ
740
E-mail address: eishun@mn.waseda.ac.jp (E. Tsuchida).
/
813-5286 3120; fax: ꢂ813-3205
/
complex 1 in CH Cl was adsorbed on the surface of a
2
4
0
2
glassy carbon electrode by transferring a measured
020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 5 8 - 1

E.L. Dewi et al. / Inorganica Chimica Acta 342 (2003) 316Á
/318
317
volume (10 ml) onto the surface of electrode and
allowing the solvent to evaporate with the rotating at
4.2 for an electrode coated with complex 1. These slopes
were almost similar to that of the broken line calculated
for the theoretical diffusion limited four-electron reduc-
1
0
00 rpm. The modified electrode was then immersed in a
.5 M HClO4 aqueous solution containing 0.1 M
tion of O .
2
NH PF as the supporting electrolyte.
4
The rotating Pt-ring glassy carbon-disk electrode
(RRDE) was used to confirm the four-electron reduc-
tion of O . The disk was glassy carbon (diameter 6 mm),
6
The electrocatalytic behavior of the oxygen reduction
catalyzed by the complex 1 was examined by cyclic
voltammetry as shown in Fig. 1. The curve in Fig. 1(a)
2
and the ring was Pt with 7.3 and 9.3 mm inner and outer
reduc-
shows the initial currentÁ/potential response obtained at
diameters, respectively. The efficiency of the O
2
ꢂ
ꢁ
a glassy carbon electrode adsorbed with complex 1 in an
argon-saturated aqueous acidic solution (Fig. 1(b)
comparison for initial redox to the unmodified elec-
trode). When the electrode was transferred to an oxygen
saturated solution, a large catalytic current appeared
starting at 0.8 V versus SCE with a peak at 0.5 V (Fig.
tion to H
calculated as previously described (%H
(N N); N, collection efficiency (i /i ) of RRDE, N ,
0
2
O (O
2
ꢂ
/
4H ꢂ
/
4e 0/2H
2
O), i.e. %H
2
O, was
Oꢃ(N
/
ꢁN)/
/
2
0
ꢂ
/
0
R
D
collection efficiency of a complex for a normalization)
[10]. Under an acidic condition, the ring potential was
maintained at 1.1 V to collect H
two-electron reduction of O at the disk electrode (O
). In the presence of O (Fig. 2), the
cathodic current iD on the disk electrode appeared near
.8 V. The ring current at potentials higher than 0.6 V
O produced by the
2
2
1
(c)). The limiting current iL in the currentÁ
/
potential
2
2
ꢂ
/
ꢂ
ꢁ
curves was analyzed using the Levich equation (i ꢃ
/
2H ꢂ
/
2e 0
/H
2
O
2
2
L
2
/3 ꢁ1/6
1/2
0
.62nFADo
n
C v ; where D ꢃ
/
diffusion coeffi-
5
o
o
ꢁ
2
cm s ) and
ꢁ1
0
cient of O in the solution (1.5ꢄ
/10
2
ꢁ
6
was very low. At lower potentials than 0.6 V, the disk
current increased and the ring current appeared due to
the concomitant two-electron reduction of O to H O .
C ꢃ
/
concentration of O in the solution (1.3ꢄ10
/
o
2
ꢁ
3
viscosity of solution (0.01 cm sꢁ1))
2
mol cm ), nꢃ
16Á18]. The currentÁ
rotating disk electrode (RDE) were calculated and
/
[
/
/potential curves obtained with the
2
2
2
At potentials where the plateau current is obtained, the
selectivity for the reaction where O was directly reduced
ꢁ
1
ꢁ1/2
plotted as KouteckyÁ
/
Levich plots (iL vs. v
) in
Fig. 1(d). The number of electrons (n) transferred
2
to H O by the four-electron reduction was 98%, in
2
comparison with the intrinsic value for the collection
efficiency of the electrode determined by the
during the O reduction can be determined from the
2
slopes of the plots. The value of n was evaluated to be
3
ꢁ/4ꢁ
Fe(CN)6
couple (N ꢃ0.38).
/
0
In summary, it was demonstrated that the vanadium
multinuclear complex catalyzes the reduction of O at a
2
high potential, which is promising to expand the wide
application of the catalyst. Although the mechanism of
this vanadium-based cluster complex 1 was not yet
clarified, the oxophilic [V(III)] of oxovanadium(V)
cluster complexes were predicted actively reacts with
O to accomplish a four-electron reduction of O in acid
2
2
condition as described in previous literature
ꢂ
ꢂ
(
2[V(III)] ꢂ
/
O ꢃ2[V(V)O] ) [1]. More significant de-
/
2
tails were under investigation.
Fig. 1. (a) Cyclic voltammetry on the glassy carbon electrode modified
complex 1 recorded in a saturated argon aqueous electrolyte contain-
ing 0.5 M HClO
unmodified electrode under O
. Scan rate: 100 mV sꢁ1. (d) KouteckyÁ
current during rotating disk voltammetry for the reduction of O
the electrode modified complex 1. The broken lines correspond to the
diffusion limited two and four electron reductions of O , respectively.
4
and 0.1 M NH
, and (c) repeat of (a) recorded under
Levich plots of the plateau
on
4 6
PF , (b) bare glassy carbon
2
2
Fig. 2. Rotating-ring disk voltammetry for the reduction of O on the
O
2
/
Pt-ring RRDE in which the disk was modified with the decavanadium
complex 1 recorded in aqueous electrolyte solutions saturated with
oxygen. The other conditions are as described in Fig. 1. The scan and
rotation rates were 25 mV sꢁ and 50 rpm, respectively.
2
1
2

318
E.L. Dewi et al. / Inorganica Chimica Acta 342 (2003) 316Á318
/
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