Acceleration of Fe
2
O
3
Reduction Kinetics by Wet Methane with Calcium Titanate as Support
Syunsuke Isogai,
1
Fumihiko Kosaka,
1
Isao Takimoto,
1
Hiroyuki Hatano,
2
Yoshito Oshima,
1
and Junichiro Otomo*
1
1
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563
2
Department of Integrated Science and Engineering for Sustainable Society, Faculty of Science and Engineering,
Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
(Received July 24, 2013; CL-130624; E-mail: otomo@k.u-tokyo.ac.jp)
To develop new chemical-energy conversion and storage
systems using metal oxides, the kinetics ofiron oxide (Fe
2
O
3
)
reduction by humidified methane using an oxide ion conductor,
CaTi
1¹x
Fe
x
O
3¹¤
(CTFO), as a support was analyzed. Significant
improvements in Fe
2
O
3
reduction rate and lattice oxygen
utilization were observed using CTFO, which may be induced
by rapid ion transport at the interface between Fe
2
O
3
and CTFO.
Efficient chemical-energy conversion and storage are im-
portant technologies to establish a low carbon society. The redox
chemistry ofiron oxides has been widely used in various energy
conversion and storage systems, e.g., high-grade heat generation
in chemicallooping combustion,
1,2
hydrogen production in
chemicallooping reforming (CLR),
2
and in the steam-iron
reaction.
3
More recently, a noveliron-air rechargeable battery
using a solid oxide fuel cell has also been proposed.
4,5
In this
system, the redox process ofiron oxide was cycled using H
2
/
H
2
O as mediator. The redox kinetics of the metal oxide plays an
important role in the efficiency of these systems. Thus, the redox
kinetics ofiron oxide (Fe
2
O
3
) has attracted significant attention
because improved understanding may contribute to advances in
energy technology. In particular, the reduction process of Fe
2
O
3
isoften rate-determining and governs the kinetics of redox
cycles.
1,2
Therefore, improvement of the reduction kinetics of
metal oxides is desired. Reduction of a metal oxide (MO) by
hydrocarbon fuel (C
n
H
2m
) involves the following reaction:
ð2n þ mÞMO þ C
n
H
2m
!ð2n þ mÞM þ nCO
2
þ mH
2
O ð1Þ
An oxide ion conductor can be used as an oxygen carrier
support to enhance the reduction kinetics of metal oxides.
6,7
We
recently showed that the onset temperature of NiO reduction by
hydrogen or methane was significantly lowered when gadolinia-
doped ceria, Ce
0.9
Gd
0.1
O
2¹¤
(GDC), was used as a support.
7
Hedayati et al. have also reported that GDC has the potential to
improve the reduction kinetics performance of Fe
2
O
3
- and CuO-
based oxygen carrier materials.
8
GDC is well known as a fast
oxide ion conductor.
9
Rapid transport of oxide ions at the
interface between an oxygen carrier and oxide ion conductor
may effectively influence the reduction kinetics of a system.
7
From the aspect of the practical use, however, the development
of oxygen carrier materials to avoid the use of rare earth
elements is indispensable. Furthermore, the effect of oxide ion
transport on metal oxide reduction kinetics is still unknown. In
this study, we propose CaTi
1¹x
Fe
x
O
3¹¤
(CTFO) as a support
material and analyze the kinetics of the reduction of Fe
2
O
3
by
humidified methane supposing a CLR process. Previous reports
have revealed that CaTiO
3
(CTO) is an oxide ion conductor.
10,11
Iwahara et al. have reported that Fe-doped CTO has a high
oxide ion conductivity. The conductivity was maximum at
around x = 0.2.
10
Since CTFO is composed of low-cost
materials and has relativelyhigh oxide ion conductivity, it can
be a promising support materialfor practical use in CLR. We
also report the significant enhancement of Fe
2
O
3
reduction
kinetics when CTFO is used as a support (Fe
2
O
3
/CTFO)
compared with that when Fe
2
O
3
/CTO or Fe
2
O
3
/Al
2
O
3
are used.
We provide direct evidence of an improvement in lattice oxygen
utilization in Fe
2
O
3
/CTFO.
Oxygen carrier compounds were prepared by solid-state
synthesis. α-Al
2
O
3
powder supplied from Kanto Kagaku (Tokyo,
Japan), Fe
2
O
3
,TiO
2
, and CaCO
3
powders supplied from Wako
Pure Chemical Industries (Osaka, Japan) were used. CTFO and
CTO were prepared with Fe
2
O
3
,TiO
2
, and CaCO
3
by solid-state
synthesis: the raw materials were calcined inair at 1323 K for
10 h. To prepare pelletized porous cermet samples of Fe
2
O
3
/
CTFO (x = 0.2 and 0.4), Fe
2
O
3
/CTO, and Fe
2
O
3
/Al
2
O
3
, desired
amounts of each powder were mixed by ball-milling using
graphite carbon powder to form pores and ethyl cellulose as a
binder. The mixtures were then pressed at 3 t cm
¹2
to form disks.
The disks were calcined inair at 1223 K for 3 h to form porous
cermet samples, which were characterized by scanning electron
microscopy (SEM, JSK5600, JEOL, Japan) and X-ray diffrac-
tometry (XRD, SmartLab, Rigaku, Japan). The specific surface
areas of the metal oxides were examined using the Brunauer-
Emmett-Teller (BET) method with a surface area analyzer
(Gemini 2360, Shimadzu, Japan and NOVA2200e, Quanta
chrome Instruments, USA). To evaluate the Fe
2
O
3
reduction
process, the weight changes of oxygen carrier compounds were
measured by thermogravimetry (TG-DTA, TG8120, Rigaku,
Japan) in wet methane at ambient pressure. First, a cermet
sample was placed on a holder ina furnace and heated at a
desired temperature (1023-1223 K) in dry air. After flushing out
the airwith Ar, the atmosphere was changed to wet methane to
start measuring the weight change of the sample; i.e., gaseous
mixtures (CH
4
/H
2
O/Ar; typical molar ratio = 1:2:17; steam-to-
carbon ratioS/C = 2) were delivered continuously to the cermet
sample using a water bubbler positioned in a water bath. The
totalflow rate was 200 mL min
¹1
.
Porous cermet samples of 36 wt % Fe
2
O
3
/CTFO, 36 wt %
Fe
2
O
3
/CTO, and 30 wt % Fe
2
O
3
/Al
2
O
3
were prepared, as shown
inFigure 1. Any other compounds in the prepared samples of
Fe
2
O
3
/CTFO, Fe
2
O
3
/CTO, and Fe
2
O
3
/Al
2
O
3
were not observed
by XRD. We also confirmed Fe
2
O
3
regeneration and the stability
of the supports in a redox cycle by XRD. The specific surface
areas of Fe
2
O
3
/CTFO (x = 0.2), Fe
2
O
3
/CTO, and Fe
2
O
3
/Al
2
O
3
were 4.0, 6.2, and 1.1 m
2
g
¹1
, respectively. To evaluate the
reduction rates in Fe
2
O
3
/CTFO, Fe
2
O
3
/CTO, and Fe
2
O
3
/Al
2
O
3
,
the kinetics of reduction of Fe
2
O
3
by wet methane at fixed
Published on the web November 5, 2013 1438
doi:10.1246/cl.130624
© 2013 The Chemical Society of Japan Chem. Lett. 2013, 42, 1438-1440 www.csj.jp/journals/chem-lett/