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74
Chemistry Letters Vol.34, No.6 (2005)
Direct Oxidation of Methane by Pd–Ni Bimetallic Catalyst over Lanthanum Chromite
Based Anode for SOFC
ꢀ
y
Yuta Nabae, Ichiro Yamanaka, Sakae Takenaka, Masaharu Hatano, and Kiyoshi Otsuka
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
S1-43, Ookayama, Meguro-ku, Tokyo 152-8552
y
Technology Research Laboratory No. 1, Nissan Research Center, Nissan Motor Co., Ltd.,
1, Natsushima-cho, Yokosuka-shi, Kanagawa 237-8526
(Received January 31, 2005; CL-050135)
Steady generation of electricity with dry CH4 fuel,
ꢂ2
with conventional Ni cermet anodes.4,5 A cathode was prepared
ꢂ2
1
50 mWꢁcm at 1073 K and 360 mWꢁcm at 1173 K, was suc-
from La0:8Sr0:2CoO3 (Japan Pure Chemical Co.). The geometric
2
cessfully performed by a low loading of Pd–Ni bimetallic cata-
lyst on a lanthanum chromite based porous-composite anode for
solid oxide fuel cells (SOFC). The amounts of carbon deposition
were quite small under the open and closed circuit conditions.
The electrocatalytic activity of the Pd–Ni catalyst was signifi-
cantly higher than that of Pd or Ni catalysts.
areas of both electrodes were 0.28 cm . Pd and Ni were loaded
7
on the porous-composite anodes by the impregnation method.
ꢂ2
Typical loadings of Pd and Ni were 5.8 mmolꢁcm correspond-
ing to about 9 wt % for Pd and 5 wt % for Ni, respectively.
Au paste (TR-1301, Tanaka Precious Metals Co.) was coat-
ed on the anode to connect a current collector of Au mesh. Pt
paste (TR-7601, Tanaka Precious Metals Co.) and Pt mesh were
used for cathode connection. An activation treatment for the
anode was carried out to reduce Pd and Ni with H2 of 34 kPa
for 30 min at 1073 K before the fuel-cell reaction. SOFC per-
formance experiments were carried out by using dry H2 and
CH4 fuel gases of 101 kPa. Cell voltages were controlled by
an electrochemical instrument system (Electrochemical Inter-
face 1287, Solartron). Oxidation products of CH4 over the anode
were CO, CO2, H2, and H2O. The formation rates of the products
were determined by an online gas chromatograph (GC-8A,
Shimazu) with a TCD and He carrier gas. CO2 and H2O were an-
alyzed by a Gaskuropack 54 column (3ꢀ ꢃ 2 m) and CO was
done by an AC column (3ꢀ ꢃ 2 m). H2 was analyzed by another
GC-8A with a TCD, Ar carrier gas and an AC column.
Direct utilization of hydrocarbon fuels in solid oxide fuel
cells (SOFC) is received a great attention for effective energy
conversion to electric power. In particular, direct oxidation of
hydrocarbons, not through internal steam reforming, is strongly
desired because of an increase in energy conversion efficiency
and a reduction of operation costs. There are, however, major
problems of (i) carbon deposition and (ii) low electrocatalytic
activity to achieve the direct oxidation of hydrocarbons over
the typical (Ni + YSZ) cermet anode. Some attractive works
were reported to suppress carbon deposition using new anode
1
materials, Cu–ceria cermet and lanthanum chromite-based
2
anode. Since these materials have low electrocatalytic activity
3
4
5
for the oxidation, the additions of Rh, Ni, and Pd catalysts
were reported. However, theses electrocatalytic activities are
not enough, especially for the CH4 fuel. Further acceleration
of electrocatalysis is essential to develop active anodes for the
CH4 oxidation. We have investigated active elecrocatalysts for
the CH4 oxidation and found a synergy of Pd and Ni catalysts
over a porous-composite anode (La0:8Sr0:2CrO3 and Ce0:8-
Sm0:2O1:9). We report the performance of the direct oxidation
of CH4 over the Pd–Ni bimetallic catalyst in this work.
We have studied the electrocatalysis of the bimetals (Pd, Rh,
Pt, Ni, Co, Cu, Fe, and Mn) from the view point of the activation
of C–H bond, and found the synergy of Pd and some co-additives
(Ni, Rh, Fe, and Mn). In these combinations, the Pd–Ni catalyst
was the best for the anodic oxidation of CH4. Figure 1 shows
time courses of current density over the Pd–Ni/, Pd/, Ni/ and
1
200
dry CH4
A solid electrolyte used in this work was La0:83Sr0:17-
6
1000
dry H2
Ga0:8Mg0:2O3 electrolyte disk (diameter 14 mm, thickness
Pd + Ni
Pd
0
.5 mm, Japan Fine Ceramics Co.). The porous-composite anode
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6
4
2
00
00
00
00
0
which consists of two layers, first layer: Ce0:8Sm0:2O1:9 (SDC)
and second layer: SDC and La0:8Sr0:2CrO3 (LSCr) (50:50
wt %). LSCr powder was synthesized by the Pechini method
and finally calcined at atmosphere for 8 h at 1423 K. One side
of the LSGM electrolyte disk was painted with the first ink pre-
pared from SDC powder (Toshima MFG Co.), ethyl cellulose
Ni
Bare anode
Cell voltage = 500 mV
(
Aldrich Co.) and benzyl alcohol (Wako Co.). After drying up
at 343 K, the second layer painting was carried out with the sec-
ond ink prepared from SDC powder, LSCr powder, ethyl cellu-
lose and benzyl alcohol. The LSGM painted double layer was
sintered at 1573 K for 5 h. The both thicknesses of the first and
second layer were about 15 mm. LSCr works as an electronic
conductor, therefore loading of metal can be reduced compared
0
1
2
3
4
Process time / h
Figure 1. Time courses of current densities for the Pd–Ni/
(LSCr + SDC)/SDC, Pd/(LSCr + SDC)/SDC, Ni//(LSCr +
SDC)/SDC and (LSCr + SDC)/SDC anodes at 500 mV and
ꢂ2
1
073 K. Pd and Ni loadings; 7.7 mmolꢁcm
.
Copyright ꢀ 2005 The Chemical Society of Japan