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330
Chemistry Letters Vol.35, No.12 (2006)
Electrocatalysis of Heat-treated Mn–Porphyrin/Carbon Cathode
for Synthesis of H O Acid Solutions by H /O Fuel Cell Method
2
2
2
2
ꢀ
Ichiro Yamanaka, Takeshi Onizawa, Hirobumi Suzuki, Noriko Hanaizumi, and Kiyoshi Otsuka
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552
(Received August 21, 2006; CL-060953; E-mail: yamanaka@apc.titech.ac.jp)
Mn–porphyrin supported on active carbon, which was
Id / mA cm-2
4 0 20 40 60 80 100
H O conc. / wt %
2
2
activated by heat-treatment in Ar, electrochemically catalyzed
reduction of O2 to H2O2 by the H2/O2 fuel cell method.
The electrocatalytic activities were strongly dependent on the
heat-treatment temperatures. The maximum H2O2 concentration
of 3.5 wt % with 47% current efficiency was obtained for the cat-
0
1
2
3
AC
AC(550°C)
Mn(TPP)Cl
Fe(TPP)Cl
CoTPP
ꢁ
alyst treated at 450 C, and a TON (Mn) for the H2O2 formation
ꢂ1
was over 1000 h
.
NiTPP
Hydrogen peroxide is expected a major oxidant for Green
Sustainable Chemistry. Price of H2O2 is expensive to use for
chemical process. Most of all H2O2 is manufactured by the an-
thraquinone process through multisteps operation with a large
CuTPP
ZnTPP
V(TPP)O
1
amount of energy consumption. Therefore, it has been desired
for development of new direct catalytic synthesis method of
H TTP
2
2
3,4
0
100 200 300 400 0 20 40 60 80 100
H2O2. It is well known that Pd and Au–Pd catalyze H2O2
formation from H2 and O2 in acid solutions. We have recently
reported the effective H2/O2 fuel cell method for H2O2 synthe-
-
2
-1
r(H O ) / µmol cm h
CE / %
2
2
Figure 1. Electrocatalytic activities of metal–porphyrins
supported on AC for the H2O2 formation by the H2/O2 fuel cell
5
–7
sis.
(
Reduction of O2 to H2O2 at the three-phase boundary
gaseous O2, aqueous electrolyte, and solid cathode) is the char-
ꢁ
method at 25 C.
acter of the fuel cell method. O2 gas is directly supplied to the
active site at the three-phase boundary. Therefore, the reduction
of O2 to H2O2 is accelerated to compare with the successive
reduction of H2O2. We have performed the efficient production
ꢂ
efficiency (CE) for H2O2 formation was calculated as 2e
reduction, which was corresponding to a H2O2 selectivity based
on H2.
Figure 1 shows electrocatalytic activities of various M–Por/
AC (550 C) for the H2O2 formation at 25 C. All metal–porphy-
rins loadings were 0.5 wt % based on metal. As mentioned
above, AC functioned as electrocatalyst for the H2O2 synthesis
of H2O2 of 7 wt % with a 94% current efficiency by using the
8
ꢁ
ꢁ
[
(
VGCF + XC72 + PTFE] cathode and NaOH electrolyte
ꢂ1
7
ꢂ1
2 mol L ). When we chose acid electrolyte (0.5 mol L
8
H2SO4) and the [AC + VGCF + PTFE] cathode, the maxi-
6
6
mum concentration of H2O2 was 1.1 wt %. The successive
in acid electrolyte. As you can see clearly, the formation rate
ꢁ
reduction of H2O2 to H2O was competitive to the reduction of
O2 to H2O2, and the two rates were balanced at 1.1 wt % over
the cathode. We have to find and develop a new electrocatalyst
and cathode for accumulation of H2O2 > 1.1 wt % in acid
electrolyte.
of H2O2 over the Mn(TPP)Cl/AC (550 C) was twice higher
than that over the AC. A concentration of H2O2 was 2.0 wt %
ꢁ
at 2 h with 30% CE. Fe(TPP)Cl/AC (550 C) and Co(TPP)Cl/
ꢁ
AC (550 C) were active for the reduction of O2 but not for
the H2O2 formation which was corresponding to the previous
9
Electrocatalysts prepared from Fe– and Co–porphyrin or
phthalocyanine on carbon by heat treatment in inert gas
have been expected as nonprecious metal electrocatalyst for
reports. Other metal–porphyrin did not show significant electro-
catalysis for the reduction of O2. The activated Mn(TPP)Cl/AC
(550 C) shows particular electrocatalysis for the H2O2 forma-
tion.
Various Mn compound /AC (550 C, 0.5 wt % Mn loading)
electrocatalysts were examined for the H2O2 synthesis. The Mn
ꢁ
9
four-electron reduction of O2 to H2O for PEMFC. Our idea
was that uneffective metal–porphyrin derivatives would be
1
0
ꢁ
candidate for the H2O2 formation by our fuel cell method.
Electrocatalysts were prepared from metal–porphyrin
1
0
ꢂ2
loadings on the cathodes were 2.0 mmol cm . Order of electro-
ꢂ2 ꢂ1
h ) >
CH2Cl2 solutions and AC by a conventional impregnation
catalytic activities was Mn(OEP)Cl (356 mmol cm
ꢁ
method, stirring the mixture for 6 h and dried at 50 C. This
metal–porphyrin/AC powder was activated by heat treatment
Mn(TPP)Cl (290) > Mn(TPPS)Cl (265) > Mn(TMPyP)Cl (180)
> AC (116) > Mn(salen) (110), Mn(Pc)Cl (99), Mn(acac)3 (87).
N4-ligand, porphyrin ring was effective for the H2O2 synthesis,
but phthalocyanine ring was not. Mn(OEP)Cl/AC was the most
active electrocatalyst (178 TON h ) for the H2O2 synthesis in
H2SO4 electrolyte (0.5 mol L ) among the Mn compounds
though the CE of 35% was not enough.
ꢁ
at T ¼ 200{800 C in Ar stream for 2 h, omitted as M–Por/
ꢁ
2
AC (T C). The cathode and anode (electrode areas = 1.34 cm )
1
1
ꢂ1
prepared by the hot press-method were attached the two
6
,12
ꢂ1
compartment cell reactor.
A yield of H2O2 was determined
by the KMnO4 titration or the Ce4 titration method. A current
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Copyright ꢀ 2006 The Chemical Society of Japan