Production of pure hydrogen from methane mediated by the redox of Ni- and Cr-added iron oxides

Article abstract of DOI:10.1016/j.jcat.2004.09.015

Methods for the formation of pure hydrogen from methane were proposed based on the reduction of Fe₃O₄ with methane into iron metal followed by the oxidation of iron metal with water vapor into Fe ₃O₄. Iron oxide

Full text of DOI:10.1016/j.jcat.2004.09.015

Journal of Catalysis 228 (2004) 405–416
www.elsevier.com/locate/jcat
Production of pure hydrogen from methane mediated by the redox
of Ni- and Cr-added iron oxides
Sakae Takenaka ∗, Noriko Hanaizumi, Van Tho Dinh Son, Kiyoshi Otsuka ∗
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552, Japan
Received 28 April 2004; revised 7 September 2004; accepted 13 September 2004
Abstract
Methods for the formation of pure hydrogen from methane were proposed based on the reduction of Fe O with methane into iron metal
3
4
followed by the oxidation of iron metal with water vapor into Fe O . Iron oxides without additives were deactivated quickly for the redox
3
4
reaction due to the sintering. Addition of Cr cations to iron oxides prevented the sintering of iron species during the redox, but the reduction
with methane of iron oxides containing Cr cations needed high temperatures (1023 K). Addition of Ni to iron oxides enhanced the reduction
with methane and the subsequent oxidation with water vapor at low temperatures, but Ni species promoted the sintering of iron species during
the redox. In contrast, the iron oxides containing both Ni and Cr species (denoted as Ni–Cr–FeO ) could produce pure hydrogen repeatedly
x
with high reproducibility through the redox at temperatures <923 K. XANES and EXAFS studies showed that Ni species in Ni–Cr–FeO
x
were present as Ni–Fe alloys after the reduction with methane, whereas they were present as Ni metal crystallites after the oxidation with
3+
water vapor. Cr species in Ni–Cr–FeO were always stabilized as octahedral Cr O on the B sites in the ferrites Cr Fe
redox reactions.
O during the
4
x
x
6
3 − x
2004 Elsevier Inc. All rights reserved.
Keywords: Hydrogen; Methane; Redox of iron oxides
1. Introduction
and supply of pure hydrogen based on the redox reactions of
iron oxides as shown in Eqs. (1) and (2) [1]:
Hydrogen is regarded as a candidate for a clean energy
source to be substituted for fossil fuels in order to solve
many environmental problems. H2–O2 fuel cells such as
polymer electrolyte fuel cells (PEFC) are a promising sys-
tem for utilizing hydrogen with high efficiency. PEFC can
convert directly the chemical energy of hydrogen into elec-
tricity through chemical reactions between hydrogen and
oxygen. PEFC does not emit any pollutant gases such as
NOx or SOx, because it operates at low temperatures. Wide-
spread use of PEFCs requires a safe and environmentally
benign technology for the storage and supply of pure hy-
drogen. We have proposed a new technology for the storage
Fe3O4 + 4H2 → 3Fe + 4H2O,
3Fe + 4H2O → Fe3O4 + 4H2.
(1)
(2)
Magnetite, Fe3O4 is reduced with hydrogen into iron metal.
Hydrogen is recovered through the oxidation of iron metal
with water vapor into Fe3O4. Using this method, 1 mol
of iron metal can store and supply 1.33 mol of hydrogen
theoretically. The amount of hydrogen stored and supplied
through the redox of iron oxides corresponds to 4.8 wt%
of iron metal. Our method is very safe because hydrogen
is stored chemically as iron metals. In addition, iron metals
or iron oxides are easily available and inexpensive mater-
ial. However, the method requires the supply of inexpensive
hydrogen for the reduction of magnetite into iron metal. Hy-
drogen is produced through the steam reforming of methane,
the main component of natural gas, followed by the water–
*
Corresponding authors. Fax: +81 3 5734 2879.
E-mail address: stakenak@o.cc.titech.ac.jp (S. Takenaka).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.09.015
2004 Elsevier Inc. All rights reserved.

406
S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
gas-shift reaction of CO. Steam reforming of methane is
usually performed at temperatures >1000 K and the highly
exothermic reaction requires a large energy input. Recently,
we demonstrated the formation of hydrogen from methane
based on the reduction of magnetite with methane instead
of hydrogen (Eq. (3)) and the subsequent oxidation of iron
metal with water vapor (Eq. (4)) [2]
2. Experimental
Iron oxide (Fe2O3) without any additives was prepared
from Fe(OH)3 that was precipitated from an aqueous so-
lution of Fe(NO3)3 · 9H2O during the hydrolysis of urea
at 363 K. Iron oxides containing foreign metals were pre-
pared from Fe(OH)3 and the corresponding metal hydrox-
ides that were precipitated from mixed aqueous solutions
of Fe(NO3)3 · 9H2O and added metal cations during the
hydrolysis of urea at 363 K. These precipitates were fil-
tered and dried at 373 K for 10 h. The dried samples were
calcined at 773 K for 10 h under air. The amount of for-
eign metal (M) added to the iron oxides was adjusted to
5 mol% to all the metal atoms, M/(M + Fe) = 0.05. When
two types of metals (M1 and M2) were added to iron ox-
ides, the amount of each metal was adjusted to 5 mol%
(M1/(M1 + M2 + Fe) = M2/(M1 + M2 + Fe) = 0.05). The
iron oxide sample with M or the iron oxide sample with
both M1 and M2 was denoted as M–FeOx or M1–M2–FeOx,
respectively. Al(NO3)3 · 9H2O, (NH4)2TiO(C2O4)2 · 2H2O,
NH4VO3, Cr(NO3)3 · 9H2O, Co(NO3)2 · 6H2O, Ni(NO3)2 ·
6H2O, Cu(NO3)2 ·3H2O, ZrO(NO3)2 ·6H2O, RhCl3, PdCl2,
IrCl3, and H2PtCl6 were used as metal sources. Iron species
in all the fresh iron oxide samples were present as Fe2O3
mainly, regardless of the type of additives. Fe2O3 in the
fresh iron oxide samples was reduced with methane into iron
metal in the first reduction and subsequently the metal was
oxidized with water vapor into Fe3O4 in the first oxidation.
The oxidation of iron metal with water vapor into Fe2O3 is
thermodynamically impossible. The second and subsequent
redox reactions were performed between Fe3O4 and iron
metal.
The reduction with methane and the subsequent oxidation
with water vapor of the iron oxide samples were performed
with a conventional gas-flow system with a fixed catalyst
bed. The iron oxide samples (0.20 g as Fe metal) were
packed in a quartz tube reactor (inner diameter = 1.8 cm and
length = 60 cm). For the reduction, methane (30 ml min−1
and 101 kPa) was passed over the iron oxides at 473 K and
the temperatures increased to 1023 K at 3 K min−1. The
reduction of the iron oxides was continued at a constant
temperature 1023 K until CO or CO2 was not observed in
the effluent gases from the catalyst bed. After Ar was in-
troduced in order to purge out methane remaining in the
reactor, water vapor diluted with Ar (P (H2O) = 18 kPa and
total flow rate = 70 ml min−1) was passed over the reduced
iron oxides at 473 K. The temperatures increased to 823 K
at 4 K min−1. The oxidation of the reduced samples with
water vapor was continued at a constant temperature 823 K
until the formation of hydrogen could not be observed in the
effluent gases from a catalyst bed. During the reaction, a part
of the effluent gases was sampled out and analyzed by a gas
chromatograph. The detection limit of CO and CO2 in the
effluent gases was ca. 50 ppm in the present study. Unless
otherwise noted, the reduction with methane and the subse-
quent oxidation with water vapor were repeated under the
Fe3O4 + CH4 → 3Fe + CO2 + 2H2O,
3Fe + 4H2O → Fe3O4 + 4H2.
(3)
(4)
The principle of our method is analogous to the old steam
iron process for the production of hydrogen-rich gas at high
temperatures >1073 K and high pressures >70 bar, using a
gas mixture of CO, CO2, and hydrogen from coal and bio-
mass for the reduction of iron ores [3,4]. In addition, the
sequential production of synthesis gas and hydrogen was
proposed by the reduction of Fe3O4 with methane (4CH4 +
Fe3O4 → 3Fe + 8H2 + 4CO) and the subsequent oxidation
of iron metal with water vapor [5–7]. The processes must
be performed at temperatures >1000 K, since the forma-
tion of synthesis gas from Fe3O4 and methane is favorable
at temperatures >1000 K according to the thermodynamic
equilibrium. However, the production of hydrogen based on
the reduction of iron oxides with methane, followed by the
oxidation of iron metal with water vapor, should be per-
formed at temperatures as low as possible from an econom-
ical viewpoint. In addition, generally speaking, the sintering
of iron species occurs more easily during the redox reac-
tions at higher temperatures. The iron oxides are deactivated
for the redox reactions by the sintering. However, the re-
duction of pure iron oxides with methane required temper-
atures >1023 K and the pure iron oxides were deactivated
quickly for the redox of Eqs. (3) and (4) due to the sinter-
ing [2]. We have found that the addition of Cr cations to
the iron oxides mitigated the sintering of iron metal and/or
iron oxides during the redox, although the reduction with
methane of the iron oxides with Cr cations required temper-
atures >1023 K [2]. Addition of Cu species to the iron oxide
sample enhanced the redox reactions (Eqs. (3) and (4)) at
low temperatures, but the sample containing only Cu species
was deactivated quickly for the redox due to sintering. In
contrast, the iron oxides containing both Cu and Cr species
could produce pure hydrogen repeatedly with high repro-
ducibility through the redox at low temperatures.
In the present study, we report the preferable effects of
the addition of Ni and Cr species into iron oxides on the re-
dox reactions. The iron oxides containing both Ni and Cr
species could produce pure hydrogen repeatedly with high
reproducibility through the reduction with methane and the
subsequent oxidation with water vapor at lower tempera-
tures, compared to the iron oxides with both Cu and Cr
species. In addition, the role of Ni and Cr species added to
the iron oxide samples on the redox reactions was examined
on the basis of the local structures of these additives.

S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
407
same conditions. In order to estimate the amount of carbons
deposited on the iron oxide samples during the redox reac-
tions, carbons were oxidized with gaseous oxygen at 823 K
after the redox reactions. The oxidation of carbons with O2
produced CO2 in addition to a trace of CO. The amount of
carbons deposited on the samples was evaluated based on
the amount of CO2 formed during the oxidation with O2.
SEM images of the iron oxide samples were measured
using a Hitachi FE-SEM S-800 (field emission gun scanning
electron microscope).
X-ray absorption spectra (XANES and EXAFS) of the
iron oxide samples were measured on the beam line BL-12C
at the Photon Factory in the Institute of Materials Struc-
ture Science for High Energy Accelerator Research Organi-
zation at Tsukuba in Japan (Proposal Number 2002G108).
Cr and Ni K-edge XANES and EXAFS of the iron ox-
ide samples were measured with a fluorescence mode with
a Si(111) two-crystal monochromator at room temperature.
In order to prepare the iron oxide samples for the measure-
ments of XANES and EXAFS, the reduction with methane
at 923 K and the subsequent oxidation with water vapor at
773 K were performed. After the redox reaction, the iron
oxide samples were cooled to room temperature under an
Ar stream. Then, the sample was packed in a polyethylene
bag under Ar atmosphere. The analysis of EXAFS data
was performed by using an EXAFS analysis program, REX
(Rigaku Co.). For EXAFS analysis, the oscillation was ex-
tracted from the EXAFS data by a spline-smoothing method.
The oscillation was normalized by the edge height around
70–100 eV above the threshold. The Fourier transforma-
tion of k3-weighted EXAFS oscillation was performed over
a k-range of 3.5–14.5 Å−1. Inversely Fourier-transformed
data for each Fourier peak were analyzed by a curve-fitting
method, using theoretically phase-shift and amplitude func-
tions derived by the FEFF8 program [8]. Error bars of the
analysis were estimated by R factor (Rf) defined as the fol-
lowing equation:
Fig. 1. Change in the formation rates of CO, CO , and hydrogen as a
2
function of temperatures during the reduction of different iron oxide sam-
ples with methane. Q, Co–Cr–FeO ; ", Ni–Cr–FeO ; !, Cu–Cr–FeO ;
x
x
x
P, Rh–Cr–FeO ; 2, Pd–Cr–FeO ; 1, Ir–Cr–FeO ; a, Pt–Cr–FeO .
x
x
x
x
Co, Ni, Cu, Rh, Pd, Ir, or Pt was added to Cr–FeOx to en-
hance the redox reaction at low temperatures. During the
reduction with methane, the formation of CO, CO2, H2O,
and hydrogen was observed by G.C. However, quantitative
analysis for the formation of H2O was not satisfactory in
the present study. Therefore, the formation rate of CO and
CO2 during the reduction with methane was used as an in-
dex of the reactivity of each iron oxide for the reduction
with methane. As shown in Fig. 1, the formation of CO2
was observed in the reduction of many iron oxide samples at
temperatures <700 K, where CO was not formed. The for-
mation of CO2 at temperatures <700 K could be ascribed to
the reduction of Fe2O3 into Fe3O4, which was confirmed by
XRD patterns of these samples. The formation rate of CO2
increased again in the temperature range >700 K, where CO
was also formed. The formation of CO and CO2 at tempera-
tures >700 K resulted from the reduction of Fe3O4 into iron
metal. The peaks of the formation rates of CO and CO2 due
to the reduction of Fe3O4 into iron metal were positioned
at higher temperatures in the following order, Rh–Cr–FeOx
< Ir–Cr–FeOx < Pt–Cr–FeOx < Ni–Cr–FeOx < Pd–Cr–
FeOx < Cu–Cr–FeOx ≈ Co–Cr–FeOx. When the formation
ꢃ
ꢀ
ꢀ
ꢁ
ꢂ
ꢁ
ꢂ
2
2
Rf =
k3χ(k)obs − k3χ(k)calc
k3χ(k)obs
.
3. Results and discussion
3.1. Modification of iron oxides with different metal species
Fig. 1 shows the changes in the formation rates of CO,
CO2, and hydrogen as a function of temperatures during the
reduction of different iron oxides with methane. Cr cations
were added in all the iron oxides shown in Fig. 1 in order to
suppress the sintering of iron species during the redox reac-
tions. We have reported that Cr cations prevented the sinter-
ing of iron species during the redox reactions of Eqs. (1) and
(2) [9–11]. Iron oxides containing only Cr cations showed
poor reactivity for the reduction with methane at tempera-
tures <1023 K, as will be demonstrated in Fig. 5. Therefore,

408
S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
Table 1
Total amounts of hydrogen, CO, and CO formed in the oxidation of the
2
reduced iron oxide samples with water vapor
Sample
H /Fe
CO/Fe
CO /Fe
H ∗/Fe
2
2
2
Co–Cr–FeO
Ni–Cr–FeO
Cu–Cr–FeO
Rh–Cr–FeO
Pd–Cr–FeO
1.7
1.7
1.8
1.3
1.5
1.4
1.4
0.02
0.05
0.01
< 0.01
0.01
< 0.01
< 0.01
0.04
0.18
0.08
0.01
0.03
1.4
1.3
1.6
1.3
1.4
1.4
1.4
x
x
x
x
x
Ir–Cr–FeO
Pt–Cr–FeO
0.02
< 0.01
x
x
with temperature. The kinetic curves of the formation rate
of hydrogen for Ni–Cr–FeOx, Cu–Cr–FeOx, Rh–Cr–FeOx,
and Ir–Cr–FeOx were similar; i.e., the formation rate of
hydrogen reached the maximum at around 720–760 K. Pt–
Cr–FeOx showed higher activity for the hydrogen forma-
tion at lower temperatures compared to the other samples.
During the oxidation with water vapor, CO and CO2 were
also produced in addition to hydrogen. CO and CO2 would
be formed by the gasification with water vapor of carbons
which had been deposited on the iron oxides during the re-
duction with methane. Formation of CO or CO2 was not
desirable from a viewpoint of the formation of pure hydro-
gen from methane. The amounts of hydrogen, CO, and CO2
formed during the oxidation with water vapor were roughly
estimated by integrating the formation rates of these prod-
ucts with time. The amounts of these products are summa-
rized in Table 1. The amounts of these products were divided
by moles of iron atoms in the samples, which are denoted
as H2/Fe, CO/Fe, and CO2/Fe. The amounts of hydrogen
formed by the oxidation of Co–Cr–FeOx, Ni–Cr–FeOx, Cu–
Cr–FeOx, and Pd–Cr–FeOx with water vapor were higher
than the theoretical value H2/Fe = 1.33, whereas H2/Fe
values for the other samples were similar to the theoretical
one. The excess amounts of hydrogen would be formed by
the gasification with water vapor of carbons deposited on
the samples during the reduction with methane. The amount
of hydrogen formed by only the redox of iron oxides (de-
noted as H2∗/Fe) was estimated by the subtraction of the
amount of hydrogen formed through the gasification of car-
bons with water vapor from the total amount of hydrogen
formed during the oxidation of the iron oxides with water
vapor, assuming that CO and CO2 were formed accord-
ing to the following reactions; C + H2O → CO + H2 and
C + H2O → CO2 + 2H2. H2∗/Fe values for all the iron
oxides were similar to the theoretical value, suggesting that
the redex between iron metal and Fe3O4 was utilized com-
pletely for all the samples. Pt–Cr–FeOx, Rh–Cr–FeOx, and
Ir–Cr–FeOx are suitable mediators for the hydrogen forma-
tion from methane because these samples could be reduced
with methane at low temperatures and the amounts of CO
and CO2 formed during the oxidation with water vapor were
very small. However, Pt, Rh, and Ir are very expensive metal,
compared to Cu and Ni. Ni–Cr–FeOx is a candidate of the
promising mediators for the formation of hydrogen, because
Fig. 2. Change in the formation rates of hydrogen, CO, and CO as a func-
2
tion of temperatures during the oxidation of different iron oxide samples
with water vapor. Q, Co–Cr–FeO ; ", Ni–Cr–FeO ; !, Cu–Cr–FeO ;
x
x
x
P, Rh–Cr–FeO ; 2, Pd–Cr–FeO ; 1, Ir–Cr–FeO ; a, Pt–Cr–FeO .
x
x
x
x
rates of CO and CO2 crossed over the maxima, the for-
mation of hydrogen was enhanced. It is generally accepted
that iron metal catalyzed the methane decomposition into
hydrogen and carbons (CH4 → C + 2H2) [12,13]. The for-
mation of hydrogen during the reduction of the iron oxide
samples with methane would result from the methane de-
composition catalyzed by iron metal in the samples. Note
that the methane decomposition deposited carbons on the
iron oxides. For example, the amount of carbons deposited
on Cu–Cr–FeOx during the reduction with methane was es-
timated to ca. 5 mol per mole of iron atom in Cu–Cr–FeOx
(C/Fe = 5), which corresponded to 0.72 g per 1 g of Cu–Cr–
FeOx (42 wt% in the Cu–Cr–FeOx after the redox reaction).
The deposition of carbons on the iron oxides would result in
the stoppage of gases in the reactor. Therefore, the amount
of carbons deposited on the iron oxides should be decreased
as low as possible.
After the reduction of the iron oxides with methane
shown in Fig. 1, the oxidation with water vapor was per-
formed. Formation rates of hydrogen, CO, and CO2 during
the oxidation of the iron oxide samples with water vapor
were plotted against temperatures in Fig. 2. Because the
formation rates of any products were very slow in the tem-
perature range <500 K, the results in the temperature range
from 500 to 823 K are shown in Fig. 2. The formation rate
of hydrogen during the oxidation with water vapor increased

S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
409
Fig. 4. Change in the formation rates of hydrogen, CO, and CO as
2
a function of temperatures during the oxidation with water vapor of
Ni–FeO containing different cations. !, Ni–Al–FeO ; 1, Ni–Ti–FeO ;
x
x
x
Q, Ni–V–FeO ; ", Ni–Cr–FeO ; 2, Ni–Zr–FeO .
x
x
x
into Fe3O4 was observed for all the samples at around 760 K,
although these peaks were very small. When the temperature
increased to 840 K, CO and CO2 were formed through the
reduction of Fe3O4 into iron metal. The formation rates of
CO and CO2 reached the maxima at around 940 K for all the
samples except for Ni–Ti–FeOx. The maximum formation
rate of CO for the reduction of Ni–Cr–FeOx was apprecia-
bly higher than those for the other samples. These results
suggested that Ni–Cr–FeOx was reduced with methane more
quickly, compared to the other iron oxides shown in Fig. 3.
Fig. 4 shows the changes in the formation rates of hydro-
gen, CO, and CO2 as a function of temperatures during the
oxidation with water vapor of Ni–FeOx containing different
cations. The kinetic curves of the formation rate of hydrogen
for Ni–Al–FeOx, Ni–Cr–FeOx, and Ni–Zr–FeOx were sim-
ilar. The formation rate of hydrogen for the three samples
increased with temperature and it reached the maximum at
around 750 K. In contrast to these samples, Ni–Ti–FeOx and
Ni–V–FeOx showed poor activities for the formation of hy-
drogen. It should be noted that the formation rates of CO and
CO2 during the oxidation of Ni–Cr–FeOx with water vapor
were higher compared to those for the other samples. How-
ever, the formation of CO and CO2 during the oxidation of
Ni–Cr–FeOx with water vapor was negligible in the second
and subsequent redox cycle, as will be described below.
From the results described above, Ni–Al–FeOx, Ni–Cr–
FeOx, and Ni–Zr–FeOx were effective mediators for the for-
mation of hydrogen through the reduction with methane and
Fig. 3. Change in the formation rates of CO, CO , and hydrogen
2
as a function of temperatures during the reduction with methane of
Ni–FeO containing different cations. !, Ni–Al–FeO ; 1, Ni–Ti–FeO ;
x
x
x
Q, Ni–V–FeO ; ", Ni–Cr–FeO ; 2, Ni–Zr–FeO .
x
x
x
Ni–Cr–FeOx can be reduced with methane at lower tempera-
tures than Pd–Cr–FeOx, Cu–Cr–FeOx, and Co–Cr–FeOx. In
addition, the reactivity of Ni–Cr–FeOx for the hydrogen for-
mation through the oxidation with water vapor was almost
the same as those of Rh–Cr–FeOx and Ir–Cr–FeOx.
We have examined the effects of different additives to
the iron oxides on the redox of Eqs. (1) and (2) [9–11].
Al–FeOx, Ti–FeOx, V–FeOx, and Zr–FeOx in addition to
Cr–FeOx also could store and supply hydrogen repeatedly
through the redox of Eqs. (1) and (2). The specific surface
areas of these iron oxide samples remained high after the
redox reactions, whereas the specific surface area of iron
oxide without additives decreased seriously during the re-
dox [9–11]. These results implied that the addition of Al,
Ti, Cr, V, or Zr to the iron oxides prevented the sintering
of iron species during the redox. Therefore, Al, Ti, V, or Zr
species were added to Ni–FeOx. The results of the reduction
with methane and the subsequent oxidation with water vapor
for Ni–Al–FeOx, Ni–Ti–FeOx, Ni–Cr–FeOx, Ni–V–FeOx,
and Ni–Zr–FeOx were shown in Figs. 3 and 4, respectively.
A peak for the CO2 formation due to the reduction of Fe2O3

410
S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
Fig. 6. Change in the formation rates of hydrogen, CO, and CO as a func-
2
tion of time on stream during the oxidation with water vapor of FeO ("),
x
Cr–FeO (!), Ni–FeO (Q), and Ni–Cr–FeO (P).
x
x
x
Fig. 5. Change in the formation rates of CO, CO , and hydrogen as a func-
2
tion of time on stream during the reduction with methane of FeO ("),
effective for the reduction with methane. The peaks of the
formation rates of CO and CO2 in the reduction of Ni–Cr–
FeOx were located at almost the same positions as those for
Ni–FeOx. In addition, the maximum formation rates of CO
and CO2 in the reduction of Ni–Cr–FeOx were significantly
higher than those for Ni–FeOx and Cr–FeOx. These results
strongly implied that Ni species in the iron oxide samples
act as active sites for methane during the reduction.
x
Cr–FeO (!), Ni–FeO (Q), and Ni–Cr–FeO (P).
x
x
x
the subsequent oxidation with water vapor. In particular, Ni–
Cr–FeOx showed a higher reactivity for the reduction with
methane compared to Ni–Al–FeOx and Ni–Zr–FeOx.
3.2. Redox performance of Ni–Cr–FeOx
Fig. 6 shows the change in the formation rates of hydro-
gen, CO, and CO2 with time on stream in the oxidation with
water vapor of FeOx, Cr–FeOx, Ni–FeOx, and Ni–Cr–FeOx.
The oxidation of FeOx with water vapor produced hydro-
gen after 40 min, where the temperature reached 620 K.
Cr–FeOx also formed hydrogen after 40 min, indicating that
the addition of Cr cations into the iron oxide samples did
not enhance the hydrogen formation at low temperatures.
However, the formation rate of hydrogen after 50 min for
Cr–FeOx was higher than that of FeOx. The higher forma-
tion rate of hydrogen for Cr–FeOx might result from the
larger surface area [11]. Table 2 shows the specific surface
areas of FeOx, Cr–FeOx, Ni–FeOx, and Ni–Cr–FeOx before
and after the first redox cycle. The surface areas of FeOx and
Cr–FeOx decreased after the first redox reaction. However,
the surface area of Cr–FeOx after the first cycle was signif-
icantly larger than that of FeOx. Therefore, the reactivity of
Cr–FeOx for the hydrogen formation would be higher than
that of FeOx. On the other hand, the formation rate of hy-
The reduction with methane and the subsequent oxidation
with water vapor for FeOx, Ni–FeOx, Cr–FeOx, and Ni–Cr–
FeOx were performed in order to clarify the role of each
additive on the redox reaction. The results for the reduction
with methane and the subsequent oxidation with water va-
por are shown in Figs. 5 and 6, respectively. In the reduction
of FeOx with methane, the peaks of the formation of CO
and CO2 due to the reduction of Fe3O4 into iron metal were
observed at around 270 min, where the reaction tempera-
ture had reached 1023 K. The addition of Cr cations into
FeOx improved the reactivity slightly for the reduction with
methane, but Cr–FeOx was reduced after the reaction tem-
perature had reached 1023 K. In contrast, the addition of
Ni species to FeOx enhanced the reduction with methane
at low temperatures. The peaks of the formation rates of CO
and CO2 during the reduction of Ni–FeOx were observed at
around 160 min, although the peaks were very small. The
addition of both Ni and Cr species to FeOx was the most

S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
411
Table 2
Specific surface areas of FeO , Cr–FeO , Ni–FeO , and Ni–Cr–FeO
x
x
x
x
2
−1
)
Sample
Composition (as mole ratio)
Specific surface area (m g
a
Before the redox
After the first cycle
FeO
Cr–FeO
Ni–FeO
Fe only
30
46
50
38
3
12
< 1
10
x
Cr:Fe = 5:95
Ni:Fe = 5:95
Ni:Cr:Fe = 5:5:90
x
x
Ni–Cr–FeO
x
a
Specific surface areas of the samples after the first cycle were measured after the removal of carbons deposited on the samples by the oxidation with air at
823 K.
drogen for Ni–FeOx was extremely low even at 823 K. We
have reported that Ni–FeOx was deactivated more quickly
for the redox of Eqs. (1) and (2) compared to FeOx [9]. In
the previous study, it was found that Ni in Ni–FeOx pro-
moted the sintering of iron species during the redox [9]. In
the present case, the surface area of Ni–FeOx (< 1 m2 g−1
)
was smaller than that of FeOx (3 m2 g−1) after the first redox
cycle, as shown in Table 2. Ni–FeOx would show low reac-
tivity for the hydrogen formation through the oxidation with
water vapor because iron metals in the Ni–FeOx had been
aggregated seriously during the reduction with methane. In
contrast, the addition of both Ni and Cr species to FeOx
enhanced the formation of hydrogen at low temperatures;
i.e., hydrogen was formed at temperatures >500 K for Ni–
Cr–FeOx, whereas the formation of hydrogen for FeOx and
Cr–FeOx required temperatures > 620 K. These results im-
plied that Ni species added to the iron oxide sample act as
active sites for the hydrogen formation. The formation rate
of hydrogen for Ni–Cr–FeOx was significantly higher than
that for Ni–FeOx. Table 2 shows that the surface area of Ni–
Cr–FeOx (10 m2 g−1) was significantly larger than that of
Ni–FeOx (< 1 m2 g−1) after the first redox cycle. These re-
sults indicated that Cr cations in Ni–Cr–FeOx prevented the
sintering of iron species during the redox. Therefore, Ni–
Cr–FeOx showed high reactivity for the hydrogen formation
through the oxidation with water vapor.
Ni–Cr–FeOx should be utilized repeatedly for the hydro-
gen formation through the reduction with methane and the
subsequent oxidation with water vapor. Therefore, four re-
dox cycles for Ni–Cr–FeOx were repeated under the same
conditions as those shown in Figs. 1–6. The results for the
reduction with methane and the subsequent oxidation with
water vapor are shown in Figs. 7 and 8, respectively. The
formation rates of CO and CO2 during the first reduction of
Ni–Cr–FeOx with methane increased significantly at tem-
peratures >840 K and their formation was terminated at
around 960 K. The formation rate of hydrogen through the
methane decomposition was accelerated from ca. 900 K,
where the formation rates of CO and CO2 crossed over the
maxima. The kinetic curves of the formation rates of CO and
CO2 during the reduction of Ni–Cr–FeOx did not change
appreciably in four cycles. These results indicated that Ni–
Cr–FeOx could be reduced with methane repeatedly with
high reproducibility. However, carbons were deposited on
Ni–Cr–FeOx by methane decomposition during the reduc-
Fig. 7. Change in the formation rates of CO, CO , and hydrogen as a func-
2
tion of temperatures during the reduction of Ni–Cr–FeO with methane.
x
", the first reduction; !, the second reduction; Q, the third reduction; P, the
fourth reduction.
tion. The amount of carbons deposited on Ni–Cr–FeOx in
the reduction with methane of each redox cycle was esti-
mated to C/Fe = ca. 5–7, which corresponded to carbon de-
position of 0.72–1.01 g per 1 g of Ni–Cr–FeOx in each redox
cycle. After four redox cycles, the total amount of carbons
deposited on Ni–Cr–FeOx reached C/Fe = ca. 26, which
corresponded to 3.74 g per 1 g of Ni–Cr–FeOx (79 wt% in
the sample after four redox cycles).
The formation rate of hydrogen during the first oxidation
of the reduced Ni–Cr–FeOx with water vapor increased at
temperatures higher than 560 K and it reached the maxi-

412
S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
Fig. 9. SEM images of Ni–Cr–FeO before the reaction and after four redox
x
cycles.
were similar to the theoretical value. These results strongly
suggested that most of the iron species in Ni–Cr–FeOx were
reduced with methane and subsequently oxidized with water
vapor.
As described earlier, Ni–Cr–FeOx could produce hydro-
gen repeatedly through the reduction with methane and the
subsequent oxidation with water vapor. However, carbons
were deposited on Ni–Cr–FeOx by the methane decompo-
sition during the redox reactions. Fig. 9 shows SEM images
of Ni–Cr–FeOx before the redox and after the four redox
cycles shown in Figs. 7 and 8. SEM image showed that the
fresh Ni–Cr–FeOx was composed of small particles with rel-
atively uniform diameters (20–30 nm). On the other hand,
filamentous carbons were observed in the SEM image of the
Ni–Cr–FeOx after the redox. It is well accepted that Ni metal
and Fe metal catalyze the methane decomposition to form
filamentous carbons [12–14]. Deposition of filamentous car-
bons on the Ni–Cr–FeOx must cause the stoppage of gases
in the reactor. In addition, Ni–Cr–FeOx formed CO and CO2
in addition to hydrogen through the oxidation of carbon with
water vapor. It should be noted that the oxidation of Ni–Cr–
FeOx with water vapor did not form CO or CO2 at al in the
temperature range <760 K in the second oxidation (Fig. 8).
Therefore, the redox reaction of Ni–Cr–FeOx was performed
repeatedly under the following conditions. For the reduction,
methane was passed over Ni–Cr–FeOx at 473 K. The tem-
Fig. 8. Change in the formation rates of hydrogen, CO, and CO as a func-
2
tion of temperatures during the oxidation of Ni–Cr–FeO with water vapor.
x
", the first oxidation; !, the second oxidation; Q, the third oxidation; P, the
fourth oxidation.
Table 3
Total amounts of hydrogen, CO, and CO formed in the oxidation of Ni–
2
Cr–FeO with water vapor
x
Repetition number
H /Fe
CO/Fe
CO /Fe
H ∗/Fe
2
2
2
1
2
3
4
1.7
1.5
1.4
1.3
0.05
0.02
0.02
0.18
0.08
0.09
0.02
1.3
1.3
1.2
1.3
< 0.01
mum at 720 K, as shown in Fig. 8. Reproducible kinetic
curves of the formation rate of hydrogen were observed in
the second and subsequent oxidation. In the oxidation of
Ni–Cr–FeOx with water vapor, the formation rate of hydro-
gen became slow at around 770 K and it increased again
at temperatures higher than 800 K, where the formation
of CO and CO2 was also enhanced. The formation of CO,
CO2, and hydrogen was continuous after the temperature
reached 823 K. These products would be formed by the gasi-
fication of deposited carbons with water vapor. The total
amounts of hydrogen, CO, and CO2 formed during the oxi-
dation of Ni–Cr–FeOx with water vapor are summarized in
Table 3. Total amounts of hydrogen formed in the first, the
second, and the third oxidation were higher than the theo-
retical value (H2/Fe = 1.33). Excess amounts of hydrogen
would be formed through the gasification of deposited car-
bons with water vapor. H2∗/Fe values in the four cycles
perature of the reactor increased to 873 K at 3 K min−1
.
For the oxidation, water vapor balanced with Ar was passed
over the reduced Ni–Cr–FeOx at 473 K and the tempera-
ture increased to 673 K at 4 K min−1. The results for the
reduction with methane and the oxidation with water vapor
are shown in Figs. 10 and 11, respectively. Kinetic curves
of the formation rates of CO and CO2 from the second to
the fourth reduction with methane are similar, as shown in
Fig. 10. In addition, the formation rate of hydrogen through
the methane decomposition became lower by decreasing the
reduction temperature from 1023 K (Fig. 7) to 873 K. The
amount of carbons deposited on Ni–Cr–FeOx in the first re-
duction was estimated to C/Fe = ca. 5, which corresponded
to 0.72 g per 1 g of Ni–Cr–FeOx. After the second reduc-

S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
413
Fig. 11. Change in the formation rates of hydrogen, CO, and CO with time
2
on stream during the oxidation of Ni–Cr–FeO with water vapor. ", the first
x
oxidation; !, the second oxidation; Q, the third oxidation; P, the fourth
oxidation.
maintained at H2/Fe = 1.2 during the four oxidations of
Ni–Cr–FeOx with water vapor. These results indicated that
Ni–Cr–FeOx can form pure hydrogen repeatedly with high
reproducibility through the reduction with methane and the
subsequent oxidation with water vapor. When the reduction
of Ni–Cr–FeOx with methane was terminated compulsorily
at 150 min (Fig. 10), the total amount of carbons deposited
on Ni–Cr–FeOx during the four redox cycles was decreased
to C/Fe = ca. 4, which corresponded to 0.58 g per 1 g of
Ni–Cr–FeOx (37 wt% in Ni–Cr–FeOx after the four redox
cycles). In addition, Ni–Cr–FeOx produced pure hydrogen
of H2/Fe = 1.1 repeatedly with high reproducibility in the
second and subsequent oxidations with water vapor.
Fig. 10. Change in the formation rates of CO, CO , and hydrogen with time
2
on stream during the reduction of Ni–Cr–FeO with methane. ", the first
x
reduction; !, the second reduction; Q, the third reduction; P, the fourth
reduction.
tion, the amount of carbons deposited in each reduction was
estimated to C/Fe = 2–3, which corresponded to carbon
deposition of 0.29–0.43 g per 1 g of Ni–Cr–FeOx in each
redox cycle. The total amount of carbons deposited on Ni–
Cr–FeOx during the four redox reactions in Figs. 10 and 11
(C/Fe = ca. 11; 1.58 g per 1 g of Ni–Cr–FeOx; 61 wt% in
Ni–Cr–FeOx after the four redox cycles) was smaller than
that deposited during the four redox reactions in Figs. 7
and 8 (C/Fe = ca. 26; 3.74 g per 1 g of Ni–Cr–FeOx; 79 wt%
in Ni–Cr–FeOx after the four redox cycles). These results
indicated that deposition of carbons on Ni–Cr–FeOx during
the reduction with methane could be suppressed to some ex-
tent by decreasing the reduction temperatures.
The formation rate of hydrogen during the first oxida-
tion of Ni–Cr–FeOx with water vapor increased with tem-
peratures and it reached the maximum at around 673 K, as
shown in Fig. 11. In the second and subsequent oxidation,
the reproducible kinetic curves of the formation rate of hy-
drogen were observed. CO and CO2 were also formed in
the first oxidation of Ni–Cr–FeOx with water vapor. How-
ever, CO or CO2 were not formed at all in the second and
subsequent oxidation. The amount of hydrogen formed was
3.3. Structures of Ni and Cr species in Ni–Cr–FeOx
As described previously, Ni–Cr–FeOx could form hydro-
gen repeatedly through the reduction with methane and the
subsequent oxidation with water vapor. The structures of Cr
and Ni species in Ni–Cr–FeOx were investigated in order to
clarify the role of these additives on the redox. XRD patterns
of Ni–Cr–FeOx after the redox reactions were measured.
Diffraction peaks due to iron metal and Fe3O4 appeared in
the XRD patterns of Ni–Cr–FeOx after the redox reaction,
but any diffraction lines assignable to compounds containing
Cr or Ni species were not observed. This result implies that
Cr and Ni species in the Ni–Cr–FeOx are highly dispersed.
Therefore, XANES and EXAFS spectra for Ni–Cr–FeOx af-

414
S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
Fig. 13. Cr K-edge XANES spectra of Ni–Cr–FeO after the redox and
x
Fig. 12. Ni K-edge XANES spectra of Ni–Cr–FeO after the redox and
x
of references (Cr foil and Cr O ). (a) Cr foil; (b) Cr O ; (c) Ni–Cr–FeO
x
2
3
2 3
of Ni foil. (a) Ni foil; (b) Ni–Cr–FeO after the first reduction; (c)
x
after the first reduction; (d) Ni–Cr–FeO after the second reduction; (e)
x
Ni–Cr–FeO after the second reduction; (d) Ni–Cr–FeO after the first ox-
x
x
Ni–Cr–FeO after the first oxidation; (f) Ni–Cr–FeO after the second oxi-
x
x
idation; (e) Ni–Cr–FeO after the second oxidation.
x
dation.
ter the redox reactions were measured in order to examine
the local structures of Ni and Cr species. Fig. 12 shows Ni
K-edge XANES spectra of Ni–Cr–FeOx after the reduction
with methane (spectra (b) and (c)) and after the oxidation
with water vapor (spectra (d) and (e)) as well as XANES
spectrum of Ni foil (spectra (a)). Features of Ni K-edge
XANES spectra of the Ni–Cr–FeOx after the oxidation with
water vapor were similar in shape to that of Ni foil, indicat-
ing that Ni species in the Ni–Cr–FeOx after the oxidation
were present as Ni metal crystallites. Threshold of XANES
spectra for the Ni–Cr–FeOx after the reduction with methane
was consistent with that for Ni foil. In general, the threshold
of XANES spectra for metal species is sensitive to the oxida-
tion states of the metal species [15]. Therefore, Ni species in
the Ni–Cr–FeOx after the reduction were of the metallic state
(zero-valent). However, Ni K-edge XANES for the Ni–Cr–
FeOx after the reduction with methane was not similar to that
for Ni foil of fcc structure, but to the spectrum for Fe metal
of bcc structure [13]; i.e., a peak appeared at 8352 eV in the
spectra of the Ni–Cr–FeOx after the reduction, whereas two
peaks were observed at 8350 and 8358 eV in the spectra of
Ni foil and the Ni–Cr–FeOx after the oxidation. It was re-
ported that these peaks in XANES spectra were sensitive to
the crystal structure of metals [16–18]. The XANES spec-
tra of Ni–Cr–FeOx after the reduction were compatible with
that of Fe–Ni alloys of bcc structure [16,17]. Therefore, Ni
species in the Ni–Cr–FeOx after the reduction with methane
would be present as Ni–Fe alloys of bcc structure, whereas
they were present as Ni metal crystallites of fcc structure af-
ter the oxidation with water vapor. The XANES spectrum
of the Ni–Cr–FeOx after the first reduction was similar to
that after the second reduction, and the spectrum of the sam-
ple after the first oxidation resembled that after the second
oxidation. Therefore, the structures of Ni species in Ni–Cr–
FeOx changed reversibly during the redox reactions.
Fig. 13 shows Cr K-edge XANES spectra of Ni–Cr–
FeOx after the reduction with methane (spectra (c) and (d))
and after the oxidation with water vapor (spectra (e) and
(f)), as well as XANES spectra of reference samples, Cr
foil (spectrum (a)), and Cr2O3 (spectrum (b)). The position
of threshold of XANES spectrum for Cr foil was differ-
ent from that for Cr2O3. The threshold of XANES spectra
for all the Ni–Cr–FeOx appeared at the same position irre-
spective of the different treatments (after the oxidation or
after the reduction). The threshold of the spectra for the
Ni–Cr–FeOx was consistent with that for Cr2O3, suggesting
that Cr species in the Ni–Cr–FeOx were always stabilized
as Cr3+. XANES spectra of all the Ni–Cr–FeOx showed a
small preedge peak at 5990 eV, which was characteristic
of octahedral CrO6 of trivalent Cr cations [19,20]. In gen-
eral, when metal cations are introduced into magnetite, the
cations are placed in tetrahedral A sites and in octahedral
B sites with a unit cell written as (Fe38+)A(Fe38+Fe28+)BO32.
It was proposed that Cr cations enter the B site as Cr3+ in

S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
415
the magnetite lattice [21]. Taking these results into consid-
eration, Cr cations in Ni–Cr–FeOx were always stabilized
as Cr3+O6 on the B site in the ferrites CrxFe3 − xO4 during
the redox reactions of Ni–Cr–FeOx. Therefore, a small part
of iron species in Ni–Cr–FeOx was present as CrxFe3 − xO4
even after the reduction with methane; i.e., a small part of
iron species was not reduced into iron metal.
the reduction or after the oxidation. Therefore, the local
structure around Cr cations in Ni–Cr–FeOx did not change
appreciably during the repeated redox. A strong peak was
observed at around 1.7 Å in the RSFs for all the Ni–Cr–
FeOx. Inversely Fourier-transformed data for Fourier peak
in a R range of 0.9 to 1.9 Å were analyzed by a curve-
fitting method. The curve-fitting for Cr K-edge EXAFS was
performed on k space in a k range of 4.5 to 14.0 Å−1. The
structural parameters evaluated by the curve-fitting are listed
in Table 4. For the Ni–Cr–FeOx after the first reduction, the
coordination number and the interatomic distance of Cr–O
bonds were estimated to 6.2 and 1.98 Å, respectively. The
coordination number or the interatomic distance of Cr–O
bonds did not change appreciably after the subsequent re-
dox reactions of Ni–Cr–FeOx. These results strongly suggest
Fig. 14 shows Cr K-edge k3-weighted EXAFS for Ni–
Cr–FeOx after the reduction with methane (spectra (a)
and (b)) and after the oxidation with water vapor (spectra (c)
and (d)). Fourier transforms of Cr K-edge k3-weighted EX-
AFS for Ni–Cr–FeOx (RSF; radial structure function) are
shown in Fig. 15. The features of k3-weighted EXAFS and
Fourier transforms of EXAFS for all the Ni–Cr–FeOx were
similar irrespective of the different treatments, i.e., after
3
3
Fig. 15. Fourier transforms of Cr K-edge k -weighted EXAFS of
Fig. 14. Cr K-edge k -weighted EXAFS of Ni–Cr–FeO after the redox.
x
Ni–Cr–FeO after the redox. (a) Ni–Cr–FeO after the first reduction;
(a) Ni–Cr–FeO after the first reduction; (b) Ni–Cr–FeO after the second
x
x
x
x
(b) Ni–Cr–FeO after the second reduction; (c) Ni–Cr–FeO after the first
reduction; (c) Ni–Cr–FeO after the first oxidation; (d) Ni–Cr–FeO after
x
x
x
x
oxidation; (d) Ni–Cr–FeO after the second oxidation.
the second oxidation.
x
Table 4
Structural parameters of Cr species in Ni–Cr–FeO estimated by the curve-fitting for Cr K-edge EXAFS
x
a
b
c
d
Condition
Shell
CN
R
(Å)
DW (Å)
ꢀE (eV)
R (%)
f
After the first reduction
After the first oxidation
After the second reduction
After the second oxidation
Cr–O
Cr–O
Cr–O
Cr–O
6.2 0.4
6.2 0.3
5.7 0.5
6.3 0.5
1.98
1.98
1.98
1.99
0.045
0.045
0.039
0.044
−0.3
−0.5
−0.3
0.4
5.0
5.1
4.5
6.2
a
Coordination number of Cr–O bonds.
Interatomic distance of Cr–O bonds.
Debye–Waller factors.
b
c
d
Threshold energy shift.

416
S. Takenaka et al. / Journal of Catalysis 228 (2004) 405–416
that Cr species in Ni–Cr–FeOx were always present as CrO6
during the redox reactions; i.e., they were stabilized on the
B site in the ferrites CrxFe3 − xO4.
in Ni–Cr–FeOx were always stabilized as octahedral
Cr3+O6 on the B sites in the ferrites CrxFe3 − xO4.
As described previously, the addition of Ni species in iron
oxides enhanced the reduction with methane and the oxida-
tion with water vapor at low temperatures. Ni species in Ni–
Cr–FeOx were present as Ni–Fe alloys after the reduction
with methane and as Ni metal crystallites after the oxida-
tion with water vapor. These Ni species in the Ni–Cr–FeOx
would act as active sites for methane molecules in the reduc-
tion and for water molecules in the oxidation. On the other
hand, the addition of Cr cations to the iron oxides prevented
the sintering of iron metals and/or iron oxides during the re-
dox reactions. XANES and EXAFS studies indicated that Cr
cations in the Ni–Cr–FeOx were always stabilized on the B
site in the ferrites CrxFe3 − xO4. The ferrites CrxFe3 − xO4
may be always localized on the surface of Ni–Cr–FeOx dur-
ing the redox reactions. Iron oxide samples would be ag-
gregated due to the contacts between iron metal particles
during the redox reactions at high temperatures. The ferrites
CrxFe3 − xO4 present on the surface of Ni–Cr–FeOx would
inhibit the contact between iron metal particles.
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We approved the following conclusions on the basis of
the results described previously:
1. Ni–Cr–FeOx can produce pure hydrogen repeatedly
with high reproducibility through the reduction with
methane and the subsequent oxidation with water va-
por.
2. Ni species in Ni–Cr–FeOx were present as Ni–Fe alloys
after the reduction with methane and as Ni metal crys-
tallites after the oxidation with water vapor. Cr species