Autothermal reforming of CH4 over supported Ni catalysts prepared from Mg-Al hydrotalcite-like anionic clay

Article abstract of DOI:10.1016/j.jcat.2003.07.001

spc-Ni/MgAl (spc: solid-phase crystallization method) catalysts were prepared from Mg-Al hydrotalcite-like compounds containing Ni at the Mg site as the precursors and tested for partial oxidation of CH₄ into synthesis gas. The activity of spc-

Full text of DOI:10.1016/j.jcat.2003.07.001

Journal of Catalysis 221 (2004) 43–54
www.elsevier.com/locate/jcat
Autothermal reforming of CH4 over supported Ni catalysts prepared
from Mg–Al hydrotalcite-like anionic clay
Katsuomi Takehira,a,∗ Tetsuya Shishido,a Peng Wang,b Tokuhisa Kosaka,a and Ken Takaki a
a
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1,
Higashi-Hiroshima 739-8527, Japan
b
Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi 332-0012, Japan
Received 21 April 2003; revised 14 July 2003; accepted 21 July 2003
Abstract
spc-Ni/MgAl (spc: solid-phase crystallization method) catalysts have been prepared from Mg–Al hydrotalcite-like compounds con-
taining Ni at the Mg site as the precursors and tested for partial oxidation of CH into synthesis gas. The precursors based on
4
2+
3+
x+
− 2+ 2+
) mH O, in which a ratio of Mg/Al varied and a part of the Mg ions were replaced by Ni ions, were
x
2
[Mg
Al (OH) ] (CO
x
2
3
1−x
prepared by a coprecipitation method, thermally decomposed, and reduced to form spc-Ni/MgAl catalyst. Surface areas of spc-Ni/MgAl
catalysts were around 150 m g . Ni ions first substituted a part of the Mg sites in the Mg–Al hydrotalcite-like compounds and then
−1
2
2+
2+
cat
incorporated in the rock-salt-type Mg–Ni–O solid solutions in the mixed oxide after the decomposition. The dispersion of Ni was thus re-
peatedly enhanced during the spc preparation, resulting in the highly dispersed Ni metal particles after the reduction. The activity of the
spc-Ni/MgAl catalyst was the highest at the ratio of Mg/Al of 1/3. When the catalysts were tested in the partial oxidation of CH , spc-
4
−1
−1
g
cat
5
Ni /Mg Al afforded enough high CH conversion even at the high space velocity (9 × 10 ml h
), exceeding the value obtained
0.5
2.5
4
over 1 wt% Rh/MgO. Ni species on spc-Ni /Mg Al catalysts were stable even under the presence of O , while Ni catalysts prepared
0.5
2.5
2
by the conventional impregnation quickly lost activity due to the surface oxidation of Ni particles. Moreover, the total heat produced by the
reaction was the lowest over the spc-Ni /Mg Al catalyst among the catalysts tested. This strongly suggests that the heat of exothermic
0.5
2.5
CH combustion to H O and CO could be quickly consumed by the following endothermic CH reforming by H O and CO over spc-
4
2
2
4
2
2
Ni /Mg Al. Thus, the spc-Ni /Mg Al catalyst is a hopeful candidate for the autothermal reforming of CH which can be carried out
0.5
2.5
0.5
2.5
4
under the copresence of both H O and O to feed H to the fuel cell economically. Actually the autothermal reforming of CH has been
2
2
2
4
successfully carried out over spc-Ni /Mg Al catalysts.
0.5
2.5
2003 Elsevier Inc. All rights reserved.
Keywords: Autothermal reforming of CH ; Solid-phase crystallization method; Supported Ni catalysts; Mg–Al hydrotalcite; High dispersion; Stable Ni metal
4
particles
1. Introduction
on-board a vehicle but also in a stationary FC system. A suf-
ficient amount of H2 must be continuously produced in a
small reformer and fed to the PEFC and, therefore, the re-
forming catalyst must work under extremely high space ve-
locity. For example, a personal car of 100 horsepower con-
sumes 70 kW of electricity, which requires 70 N m3 h−1
of H2 for driving, if the PEFC in status quo consumes
1 N m3 h−1 of H2 for producing 1 kW. Thus, the catalyst
for the on-board reformer must be exceptionally active com-
pared with that for Fischer–Tropsch and methanol syntheses.
Even in the stationary FC system for home, hospital, and
so on, a small reformer is preferable and therefore the cata-
lyst must be highly active. Moreover, the reformer would be
frequently started up and shut down during domestic use,
Hydrogen production for polymer electrolyte fuel cells
(PEFC) is a current topic in the world. Steam reforming
of hydrocarbons, especially of methane, is the largest and
generally the most economical way to make H2 [1–3]. An
alternative industrial chemical approach includes oxidative
reforming of CH4. However, the H2 production for PEFC
requires enormously high efficiency, taking into account that
the reformer should be compact in the FC system not only
*
Corresponding author.
E-mail address: takehira@hiroshima-u.ac.jp (K. Takehira).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2003.07.001
2003 Elsevier Inc. All rights reserved.

44
K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
and air contamination in the fuel gas could not be avoided.
Recently supported precious metal catalysts have frequently
been reported to solve these problems [4–11], since the pre-
cious metals such as Rh and Ru are highly active and stable
for the reforming reactions. However, the cost of the fabri-
cation of the fuel cell system is also important, since a large
amount of precious metals is required not only for the re-
former but also for the fuel cell. Use of cheaper metals or, at
least, a smaller amount of precious metals is preferable for
the catalyst preparation of the reformer. These restrictions
inevitably require a new concept for the catalyst preparation
and also for its use in the reformer.
Supported metal catalysts have been used in the re-
forming reactions of hydrocarbons and are conventionally
prepared by wet impregnation of different supports. This
method is not fully reproducible and may give rise to some
inhomogeneity in the distribution of the metal on the sur-
face. Moreover, the fine metal particles tend to sinter at high
temperature, resulting in the catalyst deactivation. Use of
the precursors containing metal ions in the crystal struc-
ture, which on further calcination and reduction, may result
in the formation of highly dispersed and stable metal parti-
cles on the surface. The authors have applied this method
for the preparation of stable and highly dispersed metal-
supported catalysts as the “solid-phase crystallization” (spc)
method and tested it by using perovskite as the precursors
[12–15].
2. Experimental
2.1. Preparation of the catalyst
spc-Ni/MgAl catalysts with various Mg/Al ratios were
prepared by the spc method as follows: Mg–Al hydrotal-
2+
cite precursors based on [Mg1−xAl3x+(OH)2]x+(CO3 x) ·
−
mH2O, in which a part of Mg2+ was replaced by Ni2+, were
prepared by the coprecipitation method reported by Miyata
et al. [27] with minor modifications. The loading amount
of Ni was fixed at 16.3 wt% after heating at 1073 K. Both
an aqueous solution containing the nitrates of Mg2+, Ni2+
,
and Al3+ and an aqueous solution of sodium hydroxide were
added slowly and simultaneously with vigorous stirring into
an aqueous solution of sodium carbonate. During the mix-
ing treatment, heavy slurry precipitated. The crystal growth
took place by aging the solution at 333 K for 12 h. After
the solution was cooled to room temperature, the precipitate
was washed with distilled water and dried in air at 353 K.
The Mg(Ni)–Al hydrotalcite-like precursors, thus obtained,
were heated in a muffle furnace in a static air atmosphere
by increasing the temperature from ambient temperature to
1123 K at a rate of 5 K min−1 and kept at 1123 K for 5 h to
form the precursor of spc-Ni/MgAl. Surface areas of the pre-
cursors of spc-Ni/MgAl catalysts were always high around
150 m2 g−ca1t . The catalysts were pressed at 25 tons, crushed,
and sieved to particles of prescribed size, which were used
in the reaction after the prereduction treatment.
The hydrotalcite-like compound is anionic clay, layered
mixed hydroxides containing exchangeable anions and pro-
duces the oxide by heating that shows interesting proper-
ties, such as high surface area, “memory effect,” and ba-
sic character [16]. Moreover, upon heating, the compound
forms a homogeneous mixture of oxides with very small
crystal size, stable against thermal treatments, which by
reduction form small and thermally stable metal crystal-
lites. Preparation of heterogeneous catalysts from Mg–Al
hydrotalcite-like compounds as the precursors and its uti-
lization for the CH4 reforming have been claimed [17] and
reported by Basini et al. [18,19]. They focused their study
on the catalyst application in the CH4 partial oxidation,
and the effects of the residence time and the thermal pro-
file were monitored to know the catalytic reaction mecha-
nism.
We have recently reported that spc-Ni/MgAl catalysts
were prepared starting from Mg–Al hydrotalcite-like com-
pounds containing Ni at the Mg sites as the precursors, and
were successfully applied for partial oxidation [20], steam
reforming [21], and dry reforming of CH4 [22–25]. It was
concluded that the high catalytic performance is uniquely
due to the stable and highly dispersed Ni metal particles on
the catalysts [26]. In the present paper, we report the effects
of Mg/Al ratio and reduction temperature on the Ni disper-
sion on a spc-Ni/MgAl catalyst, and finally its high activity
revealed even at high space velocity in the autothermal re-
forming of CH4.
As the reference, the catalysts were prepared by conven-
tional impregnation (imp) using α-Al2O3, γ -Al2O3, MgO,
and Mg–Al mixed oxides prepared from Mg/Al (3/1) hy-
drotalcite as the carrier. When the Mg–Al mixed oxide was
used as the carrier, water or acetone was used as the solvent
of Ni(NO3)2 ·2H2O and the catalyst obtained was denoted as
imp-Ni/Mg3Al-aq or imp-Ni/Mg3Al-ac. Ni(acac)2 was also
used in acetone solution for impregnation and the catalyst
was denoted as imp-Ni/Mg3Al-acac. The loading amount of
Ni was fixed at 16.3 wt% on all the catalysts after the heat-
ing.
2.2. Characterization of the catalyst
The structures of the catalysts were studied by XRD,
BET, TEM, TPR, and a H2-adsorption method. X-ray dif-
fraction was measured by using a Rigaku RINT 2550VHF
diffractometer with Cu-Kα radiation. The diffraction pat-
terns were identified by a comparison with those included
in the JCPDS data base (Joint Committee of Powder Dif-
fraction Standards). BET measurements were conducted us-
ing N2 at 77 K with a Bell Japan BELSORP18 instru-
ment. Transmission electron micrographs (TEM) were ob-
tained over on a JEOL JEM3000F instrument equipped
with a Hitachi/Kevex H-8100/DeltaIV EDS. Temperature-
programmed reduction (TPR) of the catalyst was performed
with a U-shaped quartz reactor, the inner diameter of which
was 6 mm, at a heating rate of 10 K min−1 using a mixture

K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
45
of 3 vol% H2/Ar as reducing gas after passing through a 13X
molecular sieve trap to remove water. A TCD was used for
monitoring the H2 consumption. Prior to the TPR measure-
ments, the sample was heated at 573 K for 2 h in 20 vol%
O2/N2 gas.
The Ni dispersion was determined by static equilibrium
adsorption of H2 at ambient temperature using the pulse
method. A 20 mg of the catalyst was prereduced at 1073 K in
H2/N2 (5/20 ml min−1) flow for 30 min and cooled to am-
bient temperature in Ar atmosphere. The sample was then
used for the measurement by pulsing each 1 ml of 10 vol%
H2/N2. During the pulse experiment, the amount of H2 was
monitored by a TCD-gas chromatograph. Uptake of H2 at
the monolayer coverage of the Ni species was used to esti-
mate Ni metal dispersion, assuming that each surface Ni site
chemisorbs one hydrogen atom (H/Nisurface = 1).
by at a rate of 2 K min−1 from 773 to 1173 K, and finally
by keeping at 1173 for 0.5 h. The thermocouple by which
the reaction temperature was controlled was placed at the
center of the catalyst bed. Product gases were analyzed by
online TCD-gas chromatography. The selectivity to CO2,
CO, or H2 was calculated based on the numbers of carbon
and hydrogen atoms in CH4. The catalytic activity was also
tested by changing the space velocity between 180,000 and
−1
using 5 mg of the catalyst powders of
cat
−1
900,000 ml h
g
size less than 0.15 mm φ as dispersed in quartz wool.
When the distribution of the temperature in the catalyst
bed was measured, 50 mg of the catalyst was dispersed with
500 mg of the quartz beads and packed in the reactor placed
across horizontally by using a catalyst holder made by quartz
and attached to the thermocouple sheath. The length of the
catalyst bed was 18 mm and the temperature was measured
by sliding the thermocouple along the catalyst bed.
Also for testing the autothermal reforming of CH4, O2, or
H2O was added in the CH4/H2O/N2 or CH4/O2/N2 mixed
gas flow, respectively, by changing the flow rate of each gas.
2.3. Catalytic testing
The partial oxidation of CH4 was conducted using a
fixed-bed flow reactor mainly in a mixed gas flow of CH4,
O2, and N2 (1/2/1 vol ratio) at 1073 K at atmospheric pres-
sure. The autothermal reforming was carried out mainly in a
mixed gas flow of CH4, O2, H2O, and N2 (2/1/2/1 vol ra-
tio). N2 gas was used as an internal standard for the analyses
by gas chromatography. A U-shaped quartz tube reactor, the
inner diameter of which was 6 mm, was used with the cata-
lyst bed near the bottom. The catalyst particles of the size
0.35–0.60-mm diameter were used as dispersed in quartz
3. Results and discussion
3.1. Preparation of spc-Ni/MgAl catalyst
Metal composition, surface area, Ni dispersion, and size
of Ni metal particles of the supported Ni catalysts are shown
in Table 1. The composition of metals was determined by
ICP analysis for spc-catalysts, while the value was calcu-
lated for the imp-catalysts from the amount of raw materials,
both after heating at 1123 K. The amount of Ni loading was
almost constant around 16 wt% on spc-catalysts. The surface
−1
.
cat
−1
beads for the space velocity of 4800–290,000 ml h
g
Usually 50 mg of the catalyst was prereduced in a gas flow
of 20 vol% H2/N2 by increasing the temperature at a rate of
10 K min−1 from ambient temperature to 773 K, followed
Table 1
Metal composition, surface area, and Ni dispersion of the catalysts
a
Catalyst
Atomic
ratio
Composition
Ni
loading
BET surface
H
uptake
Ni
Particle size
of Ni (nm)
2
b
b
c
d
for Al = 1.0
area
dispersion
−1
−1
)
cat
2
(Ni + Mg)/Al
Ni
Mg
(wt%)
(m g
)
(µmol g
(%)
XRD
TEM
cat
spc-Ni
spc-Ni
spc-Ni
spc-Ni
spc-Ni
spc-Ni
/Mg
/Mg
/Mg
/Mg
/Mg
/Mg
Al
Al
Al
Al
Al
Al
1
2
3
4
5
6
0.271
0.377
0.488
0.595
0.778
0.824
0.703
1.63
2.52
2.90
3.96
4.67
16.7
16.0
15.8
17.2
17.8
16.3
16.3
16.3
16.3
16.3
16.3
16.3
124.6
156.3
178.6
141.6
125.1
128.5
95.4
54.0
89.2
8.2
106.3
17.2
253.5
247.5
265.1
196.0
203.5
155.5
221.5
66.0
139.3
48.6
122.9
13.8
17.8
18.2
19.7
13.4
13.4
11.2
16.0
4.8
7.8
7.8
7.0
6.9
0.26
0.38
0.50
0.62
0.73
0.85
0.74
1.62
2.50
3.38
4.27
5.15
7.0
6.1
8.3
8.2
6.6
10.2
12.1
9.8
e
imp-Ni/Mg Al-aq
imp-Ni/Mg Al-ac
imp-Ni/Mg Al-acac
imp-Ni/α-Al O
imp-Ni/γ -Al O
imp-Ni/MgO
7.2
8.3
3
f
16.6
16.7
31.1
11.4
−
22.0
18.6
35.5
13.0
8.2
3
g
10.0
3.5
8.9
3
2
3
2
3
1.0
a
−1
Reduced at 1173 K for 0.5 h in H /N (5/20 ml min ).
Determined by ICP analysis for spc-catalysts after calcination at 1123 K for 5 h and calculated for imp-catalysts.
Calcined at 923 K for 14 h and at 1123 K for 5 h.
2
2
b
c
d
e
f
Determined by H pulse method.
2
Impregnated in aqueous solution of Ni(II) nitrate.
Impregnated in acetone solution of Ni(II) nitrate.
Impregnated in acetone solution of Ni(II)-acac.
g

46
K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
Table 2
Effect of the reduction conditions on the Ni dispersion of the catalysts
Catalyst
Reduction
conditions
H
uptake
Ni
dispersion
(%)
2
a
b
−1
(µmol g
)
cat
spc-Ni /Mg Al
1023 K for 0.5 h
1073 K for 0.5 h
1123 K for 0.5 h
1173 K for 0.5 h
83.8
221.8
244.9
265.1
6.2
16.5
18.2
19.7
0.5
2.5
spc-Ni /Mg Al
0.5
2.5
spc-Ni /Mg Al
0.5
2.5
spc-Ni /Mg Al
0.5
2.5
a
−1
Reduced in H /N (5/20 ml min ).
2
2
b
Determined by H pulse method.
2
area of the catalysts was higher on spc-catalysts than imp-
catalysts, the highest value of 178.6 m2 gc−a1t , among which
was obtained with spc-Ni0.5/Mg2.5Al catalysts. Even when
Mg3Al mixed oxide was prepared by heating the hydrotal-
cite and was used as the support for impregnation, i.e., imp-
Ni/Mg3Al-aq, imp-Ni/Mg3Al-ac, and imp-Ni/Mg3Al-acac,
the lower values of surface area were obtained compared to
the spc-catalysts. Among the other imp-catalysts, imp-Ni/γ -
Al2O3 catalyst alone showed a relatively high surface area
of 106.3 m2 gc−a1t . Ni dispersion obtained by H2 adsorption
was also the highest with spc-Ni0.5/Mg2.5Al, followed by
the other spc-catalysts. By both increasing and decreasing
the Mg/Al ratio, the Ni dispersion decreased. A rather high
value of the Ni dispersion was obtained with imp-Ni/Mg3Al-
aq, even though the surface area was not so high, suggesting
that Ni was highly dispersed during the preparation (vide in-
fra).
Fig. 1. XRD patterns of the precursors of spc-Ni/MgAl catalysts dur-
ing the preparation. (a) spc-Ni /Mg Al after drying; (b) spc-
0.26
0.74
Ni /Mg Al after drying; (c) spc-Ni
/Mg
Al after heating; (d) spc-
0.5
2.5
0.26
0.74
Ni /Mg Al after heating; (e) spc-Ni
/Mg
Al after reduction;
0.74
0.5
2.5
0.26
(f) spc-Ni
/Mg
Al after reduction; (g) spc-Ni /Mg Al after reduc-
1.62 0.5 2.5
0.38
tion.
The conditions of the catalyst prereduction affected sub-
stantially the Ni metal dispersion (Table 2). Increasing tem-
peratures resulted in higher Ni dispersion as well as the
stable activity in the partial oxidation of CH4 (vide infra).
The reduction at 1173 K for 0.5 h afforded the best results
and all the catalysts were prereduced under these conditions.
Further increase in the reduction temperature or in the reduc-
tion time caused the formation of the MgAl2O4 spinel phase
in the catalyst, resulting in the lowering in the Ni dispersion
as well as in the surface area. All catalysts tested for the re-
forming in this paper were prereduced at 1173 K for 0.5 h.
After heating of spc-Ni0.26/Mg0.74Al at 1123 K for 5 h,
both the hydrotalcite and the Al(OH)3 disappeared, and
MgO (including Ni as Mg–Ni–O solid solutions) in turn
appeared together with MgAl2O4 spinel. The line intensi-
ties of the MgAl2O4 spinel phase were weakened with in-
creasing Mg/Al ratios. The lines of MgO (Mg–Ni–O) were
broader in the samples of lower Mg/Al ratios, apparently
suggesting that the size of the MgO crystal decreased or the
Ni2+ incorporation in MgO was enhanced. A lattice con-
stant a calculated from the d value of MgO (200) plane
was smaller than those of MgO and Mg–Ni–O in all spc-
Ni/MgAl catalysts, coinciding well with the results reported
by Fornasari et al. [28] and Ross [29]. This suggests that
Al3+, the ionic radii of which (0.54 Å) is smaller than those
of Mg2+ (0.72 Å) and Ni2+ (0.69 Å) [30], was incorpo-
rated into MgO to form the solid solutions. Olsbye et al. [31]
also reported that Al3+ substitutes into the MgO framework
and inhibits the crystal growth of MgO in the Mg/Al (3/1)
mixed oxide prepared from the hydrotalcite precursor, and a
MgAl2O4 spinel phase was observed after the steaming test.
These coincided well with the results of Al3+ incorporation
and an excess amount of Al3+ was consumed by the forma-
tion of the MgAl2O4 spinel phase in the present work.
3.2. Structure of spc-Ni/MgAl catalyst
XRD patterns of spc-Ni/MgAl catalysts during the prepa-
ration with various Mg/Al ratios are shown in Fig. 1. In the
case of spc-Ni0.26/Mg0.74Al (Mg/Al = 1/1), sharp lines of
Al(OH)3 were observed together with those of Mg–Al hy-
drotalcite just after the drying of the precipitate. When the
Mg/Al ratio was increased above 2/1, no Al(OH)3 appeared
and the hydrotalcite alone was observed after the drying of
the precipitate. After the drying, the highest intensity as well
as the smallest linewidth in the peaks of the hydrotalcite was
observed on spc-Ni/MgAl with the Mg/Al ratio of 2/1–3/1,
and both increasing and decreasing Mg/Al ratios caused the
line broadening in the hydrotalcite peaks.
When the sample was reduced in 20 vol% H2/N2 at
1173 K, Ni2+ in the Mg–Ni–O solid solutions was reduced
to form Ni metal. The peaks of Mg–Ni–O were substan-

K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
47
Fig. 2. TEM images of the supported Ni catalysts after reduction. (a) spc-Ni
/Mg
Al; (b) spc-Ni
/Mg
Al; (c) spc-Ni /Mg Al; (d) spc-
1.62 0.5 2.5
0.26
0.74
0.32
Ni
/Mg
Al; (e) spc-Ni
/Mg
Al; (f) spc-Ni
/Mg
Al; (g) imp-16.3wt%Ni/Mg Al-ac; (h) imp-16.3wt%Ni/Mg Al-acac.
5.15 3 3
0.62
3.38
0.73
4.27
0.85
tially weakened and broadened, while the peaks of Ni metal
appeared, with all the catalyst samples. This suggests that,
during the release of Ni species from the Mg–Ni–O solid
solutions, the MgO crystal structure was collapsed to form
the small size particles or amorphous structure. On the other
hand, the MgAl2O4 spinel phase was still observed after
the reduction in the samples of low Mg/Al ratios, proba-
bly because this phase does not include Ni2+ in the crystal
structure.
3.3. Ni dispersion on the catalysts
TEM images of spc-Ni/MgAl catalysts with various
Mg/Al ratios after the reduction are shown in Fig. 2, to-
gether with those of imp-Ni/Mg3Al-ac and imp-Ni/Mg3Al-
acac. The average size of Ni metal particles of the catalysts
(Table 1) was the smallest with spc-Ni0.5/Mg2.5Al among
the spc-catalysts in both TEM images and XRD calculations.
Interestingly imp-Ni/Mg3Al-aq showed a smaller size of Ni
metal particles compared to the other imp-catalysts, coin-
ciding well with the results of Ni dispersion (Table 1). Ni
metal particles were not clearly observed with imp-Ni/MgO
by XRD and Ni dispersion was very low even after the re-
duction with H2, suggesting that Ni was stable and hardly
reduced in Mg–Ni–O solid solutions. The distributions of the
size of Ni metal particles on spc-Ni/MgAl catalysts after the
reduction are shown in Fig. 3. The smaller ratio of Mg/Al
caused a formation of the smaller size of Ni metal particles
after the reduction (Red.) as well as even after the oxidation
(POM) of CH4 for 5 h at 1073 K. After the POM reaction,
Fig. 3. Distributions of Ni metal particles on spc-Ni/MgAl catalysts. Red.,
after reduction; POM, after partial oxidation.

48
K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
oxidation between Ni metal and Mg–Ni–O solid solutions
may produce finely dispersed Ni metal particles by redis-
tributing Ni species on the catalyst surface.
3.4. TPR of the catalysts
The results of H2-TPR measurements of spc-Ni/MgAl,
together with imp-Ni catalysts as a comparison, are shown
in Fig. 5. Obviously Mg3–Al mixed oxides prepared by
heating Mg–Al hydrotalcite showed no reduction peak up
to 1300 K. Imp-Ni/γ-Al2O3 showed a reduction peak at
high temperature, probably due to the formation of NiAl2O4
spinel on the catalyst surface. Among imp-Ni/MgAl cata-
lysts, imp-Ni/Mg3Al-aq showed the highest temperature of
the reduction, which is almost the same as that observed
with spc-Ni0.5/Mg2.5Al, followed by imp-Ni/Mg3Al-acac,
imp-Ni/Mg3Al-ac, and imp-Ni/γ-Al2O3. During the prepa-
ration of imp-Ni/Mg3Al-aq, surface reconstitution of Mg–Al
hydrotalcite possibly took place [16], and Ni was simultane-
ously incorporated in the hydrotalcite phase, resulting in the
highest temperature of the reduction among imp-Ni/MgAl
catalysts.
Imp-Ni/MgO showed no peak in the TPR up to 1300 K,
while both spc- and imp-Ni/MgAl catalysts showed a clear
peak of the Ni reduction. It is likely that Ni2+ dissolved
in MgO as the solid solutions is not easily reduced to Ni
metal, while the addition of Al makes the Ni2+ reduc-
tion in both imp- and spc-catalysts easy. All spc-Ni/MgAl
catalysts showed a reduction peak at high temperatures
above 1080 K, and the reduction temperature increased
from 1080 to 1190 K with increasing the Mg/Al ratio in
Fig. 4. Distributions of Ni metal particles on imp-Ni catalysts. Red., after
reduction; POM, after partial oxidation.
the size of Ni metal particles on both spc-Ni0.26/Mg0.74Al
and spc-Ni0.38/Mg1.62Al increased, probably due to the sin-
tering of Ni metal particles during the reaction. On the other
hand, when the Mg/Al ratio exceeded 3/1, very interest-
ingly the average size of Ni metal particles rather decreased
after the POM reaction except spc-Ni0.85/Mg5.15Al.
The distributions of Ni metal particles on imp-Ni cata-
lysts are shown in Fig. 4. When Ni was loaded on Mg3Al
mixed oxides in aqueous solution, a high dispersion of Ni
metal particles was observed as shown in spc-Ni/Mg3Al-
aq. However, use of acetone as the solvent for impreg-
nation resulted in the lower dispersion as shown in both
spc-Ni/Mg3Al-ac and spc-Ni/Mg3Al-acac. Among both cat-
alysts, a considerably higher dispersion was observed by
using Ni(acac)2 than Ni(NO3)2 · 2H2O. Ni(acac)2 can form
a surface-chelating Ni complex on the catalyst, probably re-
sulting in the high dispersion, while Ni(NO3)2 · 2H2O in
acetone cannot afford such an effect. Both imp-Ni/MgO and
imp-Ni/γ-Al2O3 showed a rather sharp distribution com-
posed of smaller size Ni metal particles compared to those
of imp-Ni/α-Al2O3. Imp-Ni/MgO and imp-Ni/γ-Al2O3
formed Mg–Ni–O solid solutions and NiAl2O4 spinel, re-
spectively, also resulting in the high dispersion of Ni metal
particles. Moreover, the size of Ni metal particles decreased
on spc-Ni/MgAl catalysts as already noted in this paragraph
and may be due to the reversible reduction-oxidation of Ni
species during the CH4 oxidation. This reversible reduction-
Fig. 5. Temperature-programmed reduction of the supported Ni catalysts.
(a) NiO; (b) Mg Al mixed oxide; (c) imp-Ni/MgO; (d) imp-Ni/γ -Al O ;
3
2 3
(e) imp-16.3wt%Ni/Mg Al-acac; (f) imp-16.3wt%Ni/Mg Al-ac; (g) imp-
3
3
16.3wt%Ni/Mg Al-aq; (h) spc-Ni
/Mg
/Mg
Al; (i) spc-Ni
Al; (l) spc-Ni
/Mg
/Mg
Al;
Al;
3
0.26
0.62
0.74
3.38
0.32
0.73
1.62
4.27
(j) spc-Ni /Mg Al; (k) spc-Ni
0.5
2.5
(m) spc-Ni
/Mg
Al.
5.15
0.85

K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
49
spc-Ni/MgAl catalysts. Judging from the peak area of H2
consumption, spc-Ni0.26/Mg0.74Al and spc-Ni0.38/Mg1.62Al
showed values of 69 and 88%, respectively, of those of
the other spc-Ni/MgAl catalysts as the amount of Ni2+ re-
duced in TPR. This means that Ni2+ species in both cata-
lysts are not completely reduced. On the other hand, when
the Mg/Al ratio exceeded 3/1, the amount of H2 con-
sumed corresponded well with the amount of Ni2+ in each
catalyst, suggesting that almost all Ni2+ species was re-
duced in TPR of spc-Ni0.5/Mg2.5Al, spc-Ni0.62/Mg3.38Al,
spc-Ni0.73/Mg4.27Al, and spc-Ni0.85/Mg5.15Al.
When the Mg/Al ratio in spc-Ni/MgAl catalysts was in-
creased, the formation of Mg–Ni–O solid solutions might
be enhanced under the MgO-rich conditions. This situation
leads to an increase in the reduction temperature of Ni.
On the contrary, the phase separation into the Mg–Al hy-
drotalcite and Al(OH)3 took place after the coprecipitation,
followed by the formation of the MgAl2O4 spinel phase af-
ter heating, in spc-Ni/MgAl catalysts of low Mg/Al ratios.
These phenomena may make the catalyst phase more com-
plex, resulting in an easy but incomplete reduction of Ni
species in the catalysts.
3.5. Highly dispersed Ni metal particles
On spc-Ni/MgAl catalysts, an increase in the Mg/Al ratio
resulted in an increase in the reduction temperature as well
as the reduction peak area, suggesting that a large amount
of Mg stabilizes a large amount of Ni2+ in the solid solu-
tions. Prmaliana et al. [33–36] reported that the MgO–NiO
system forms “ideal” solid solutions over the whole molecu-
lar fraction range and was used as the catalyst for the steam
reforming of CH4. It is also reported that by Tomishige and
co-workers [37–39] that Mg–Ni–O solid solutions of small
Ni content showed an excellent stability in dry reforming
of CH4 after the reduction treatment. Preliminary heating of
a catalyst or catalyst precursor at a temperature higher than
that of catalytic tests is a common experimental procedure
in catalyst preparation. Such a treatment enhances the inter-
action between an active metal species and an oxide carrier,
resulting in the formation of more stable catalyst [40]. In the
case of Ni catalyst supported on MgO, such treatment, how-
ever, leads to the formation of Mg–Ni–O solid solutions. As
a result, a considerable amount of Ni diffuses from the cat-
alyst surface into the bulk of the carrier, where it becomes
irreducible and therefore ineffective for catalysts [33,36]. An
increasing amount of Mg in spc-Ni/MgAl possibly contains
an increasing amount of Ni, resulting in a stabilization of Ni
species as observed in the increase in the reduction temper-
ature.
Fig. 6. XRD patterns of supported Ni catalysts. (a) After drying, (b) after
heating, (c) after reduction, (d) after reaction (!) Mg–Ni–O; ("): Ni metal;
(2): Mg–Al hydrotalcite; (E): MgAl O .
2
4
lysts is probably due to the substitution of Al3+ in MgO
which affects the stability of Ni2+ in Mg–Ni–O solid solu-
tions. However, the smaller ratio of Mg/Al again resulted
in an incomplete reduction of Ni as observed with spc-
Ni0.26/Mg0.74Al and spc-Ni0.38/Mg1.62Al (Fig. 5). It is likely
that an appropriate value of 3/1 exists in the Mg/Al ratio
to reduce efficiently Ni species to form highly dispersed Ni
metal particles on spc-Ni/MgAl catalysts.
For imp-Ni/Mg3Al catalysts, the use of an aqueous solu-
tion resulted in the highest reduction temperature, followed
by the use of Ni(acac)2 and Ni(NO3)2 · 2H2O in acetone
solution. This may be due to “memory effect” of the hy-
drotalcite structure [32], i.e., the layered structure of the
hydrotalcite was reconstituted, accompanied by the incorpo-
ration of Ni at the Mg sites, when Mg3–Al mixed oxide was
immersed in an aqueous solution of Ni(NO3)2 · 2H2O. In
fact, the diffraction lines of the hydrotalcite were clearly ob-
served in the XRD results after the impregnation (Fig. 6). It
On the other hand, Al3+ is also incorporated into the
MgO framework and inhibits the crystal growth of MgO
(probably of Mg–Ni–O solid solutions, too) in the Mg–Al
mixed oxide, as noted previously [28,29,31]. This probably
makes the Ni reduction in spc-Ni/MgAl catalysts easy. Easy
reduction of Ni2+ observed in TPR of spc-Ni/MgAl cata-

50
K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
is expected that the effects of spc preparation also take place
on the catalyst surface of imp-Ni/Mg3Al-aq, resulting in the
higher dispersion of Ni metal particles. It is concluded that
the reconstitution of the hydrotalcite affords a substantial ef-
fect on the formation of highly dispersed Ni metal particles
on the supported Ni catalysts prepared by impregnation in
aqueous solution. Such effect was not observed when the
mixed oxide was dipped in acetone solution as revealed by
imp-Ni/Mg3Al-ac (Fig. 6). An increasing temperature in the
reduction peak observed on imp-Ni/Mg3Al-acac may well
coincide with a smaller size of Ni metal particles compared
with imp-Ni/Mg3Al-ac (Table 1 and Figs. 2 and 4). Acetyl-
acetone in Ni(acac)2 may bind to the catalyst surface and
form surface-chelating Ni2+acac species, resulting in the
formation of smaller size Ni metal particles by inhibiting
the cohesion to each other. The largest size of Ni metal
particles was obtained over imp-Ni/α-Al2O3 as well as the
lowest surface area, showing that α-Al2O3 is an inert support
against Ni (Table 1 and Fig. 4). On the other hand, imp-Ni/γ -
Al2O3 showed a smaller size of Ni metal particles as well
as a reduction peak at the higher temperature compared to
imp-Ni/α-Al2O3, probably due to the formation of NiAl2O4
spinel phase on the catalyst surface. On imp-Ni/α-Al2O3,
reduction peaks were observed at low temperature almost
similar to that on NiO, suggesting that Ni was simply loaded
on α-Al2O3 without forming any compound.
Fig. 7. The partial oxidation of CH over supported Ni catalysts.
4
(a) spc-Ni /Mg Al; (b) imp-16.3wt%Ni/Mg Al-aq; (c) imp-16.3wt%
0.5
2.5
3
Ni/Mg Al-acac; (d) imp-16.3wt%Ni/Mg Al-ac; (e) imp-16.3wt%Ni/MgO;
3
3
(f) imp-16.3wt%Ni/γ -Al O ; (g) imp-16.3wt%Ni/α-Al O ; (h) commer-
2
3
2 3
cial Ni/α-Al O . A mixed gas of CH /O /N = 2/1/1 was used at 1073 K
2
3
4
2
2
for 50 and 5 mg of the catalyst diluted by quartz beads at the low (dotted
line) and the high (full line) space velocities, respectively.
even at a high space velocity, followed by imp-Ni/Mg3Al-
aq prepared by impregnation in an aqueous solution of
Ni(NO3)2 · 2H2O.
During the preparation of spc-Ni/MgAl catalysts, Ni was
first replaced at the Mg site in Mg–Al hydrotalcite after dry-
ing and then incorporated in the rock-salt type Mg–Ni–O
solid solutions in the mixed oxide after heating. It is ex-
pected that the dispersion of Ni was thus repeatedly en-
hanced during the spc preparation, resulting in the high dis-
persion of Ni metal particles after the reduction. It must also
be noted that the reducibility of Ni2+ in the catalysts was
moderately controlled by the incorporation of Al3+ in the
Mg–Ni–O solid solutions.
A good performance of imp-Ni/Mg3Al-aq may be ex-
plained by the reconstitution of a hydrotalcite-like structure
in aqueous solution during the preparation, resulting in a cir-
cumstance around the Ni species in the catalyst partly sim-
ilar to that of the spc preparation. Imp-Ni/Mg3Al-acac also
showed a good performance with the powdered catalyst, fol-
lowed by imp-Ni/γ-Al2O3, probably due to the high disper-
sion of Ni metal particles on both catalysts as suggested from
the results of TPR. The chelating agent, i.e., acetylacetone,
may be effective for dispersing Ni metal particles by bind-
ing Ni ions to the catalyst surface. Imp-Ni/Mg3Al-ac showed
rather low CH4 conversion, and both imp-Ni/MgO and imp-
Ni/α-Al2O3 showed much lower CH4 conversion. These
may be due to low dispersion (imp-Ni/Mg3Al-ac and imp-
Ni/α-Al2O3) or small amount (imp-Ni/MgO) of Ni metal
particles on the catalyst surface.
3.6. Catalytic behavior of supported Ni catalysts
The catalytic behavior of spc-Ni0.5/Mg2.5Al in the partial
oxidation of CH4,
CH4 + O2 → CO + 2H2 ꢀH2098 K = −36 kJmol−1, (1)
1
2
XRD observation of imp-Ni/α-Al2O3 after the reaction
showed weak diffraction lines due to NiO, suggesting that
the surface of Ni metal particles was oxidized during the ox-
idation. We have observed a similar phenomenon over imp-
Ni/SrTiO3 catalysts during the partial oxidation of CH4 [15].
When the size of Ni metal particles becomes large on the
catalyst, the interaction between metal and support becomes
weak, resulting in an easy oxidation of the surface of metal
particles during the oxidation reaction. When the space ve-
locity increased, the surface of Ni metal particles can be
quickly oxidized, since a large amount of oxygen attacks the
catalyst surface and the oxidation reaction proceeds more
rapidly than the dissociation of CH4. The oxidized Ni sur-
in a CH4/O2 = 2/1 mixture was tested on the catalyst
by changing the space velocity and compared to the other
imp-Ni catalysts (Fig. 7). Dotted lines were obtained by us-
ing 50 mg of the catalyst of particle size 0.35–0.60 mm φ
at a low space velocity, while full lines were obtained with
5 mg of the catalyst powders of the size <0.15 mm φ dis-
persed in quartz wool at a high space velocity. Generally
a higher CH4 conversion was observed over spc-Ni/MgAl
catalysts than imp-catalysts. When space velocity was in-
creased, the Ni catalysts prepared by impregnation of α-
Al2O3 and MgO, including a commercial Ni/α-Al2O3 cat-
alyst, showed clear declines in the CH4 conversion. On the
contrary, spc-Ni0.5/Mg2.5Al showed high CH4 conversion

K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
51
−1
face catalyzes the combustion of CH4 to form CO2 and
H2O, but does not catalyze the reforming reactions of CH4
with H2O and CO2 to CO and H2, resulting in a substan-
tial decrease in the CH4 conversion. The partial oxidation
of CH4 (1) on Ni catalysts is generally composed of an
exothermic combustion (2) and two endothermic reforming
reactions (3) and (4) under the conditions of CH4/O2 = 2/1:
at the high space velocity (9 × 105 ml h−1 g ) corresponds
cat
to that of 1 wt% Rh/MgO reported as the best Rh catalyst
by Ruckenstein and Wang [4]. Either increasing or decreas-
ing in the Mg/Al ratio from the value of 1/3 resulted in a
decrease in the CH4 conversion with increasing the space
velocity. Lower ratios of the Mg/Al, e.g., 1/1, afforded
segregation of the Al(OH)3 phase from Mg–Al hydrotal-
cite during the co-precipitation, while higher Mg/Al ratio
resulted in the formation of a MgO-rich phase after the
calcination. The former hindered the formation of a well-
crystallized hydrotalcite phase, and the latter strongly held
Ni in the MgO structure as the solid solutions by lacking the
moderation effects due to the Al3+ incorporation. It is likely
that both cases suppressed the formation of highly dispersed
Ni metal particles, resulting in the lowering of the activity at
the high space velocity.
1
1
1
14 CH4 + O2 → CO2 + H2O
2
4
2
ꢀH2098 K = −801 kJmol−1
,
(2)
(3)
(4)
12 CH4 + H2O → CO + H2
1
1
3
2
2
2
ꢀH2098 K = +206 kJmol−1
,
1
1
1
14 CH4 + CO2 → CO + H2
4
2
2
ꢀH2098 K = +247 kJmol−1
.
The combustion proceeds more rapidly than the latter two
reforming reactions and the rates of the latter reactions con-
siderably depend on the activity of the Ni catalyst used. If
the combustion of CH4 alone takes place over the Ni cat-
alyst, the CH4 conversion can be calculated as 1/4 = 25%
under the gas composition of CH4/O2 = 2/1. However, the
CH4 conversion remained at values above 25% over all the
powder catalysts tested at the high space velocity. It is likely
that certain equilibrium between the oxidized and the re-
duced states was attained on the surface of Ni particles of
the catalyst, and the latter surface still catalyzed the reform-
ing reactions, though poorly, over imp-catalysts.
The behaviors of spc-Ni/MgAl catalysts of various
Mg/Al ratios were also tested in the partial oxidation of CH4
by changing the space velocity (Fig. 8), the highest activ-
ity among which was still obtained over spc-Ni0.5/Mg2.5Al
with the highest Ni dispersion (Table 1). High activity of
spc-Ni0.5/Mg2.5Al proved by the high CH4 conversion even
3.7. Temperature of the catalyst bed in partial oxidation of
CH4
When the rates of reforming reactions are slow, the heat
of combustion may be accumulated in the catalyst bed, re-
sulting in the formation of a hot spot. On the other hand, if
we can provide a catalyst of high performance for the re-
forming, the heat of combustion may be quickly eliminated
from the catalyst bed by the following reforming reactions
and therefore no accumulation of the heat appears. The high-
est temperature was observed at the inlet of the catalyst bed,
where the heat accumulation mainly occurred by the com-
bustion. The highest temperatures of the catalyst bed were
measured by ch−an1ging the space velocity from 20,000 to
−1
90,000 ml h
g
on the supported Ni catalysts (Fig. 9).
cat
A blank test with quartz wool alone instead of the catalyst
afforded a constant temperature covering all the space ve-
locities. Actually less than 5% of CH4 was consumed by
the combustion reaction, resulting in no substantial heat ac-
Fig. 8. The partial oxidation of CH over spc-Ni/MgAl cata-
Fig. 9. Temperature at the inlet of the catalyst bed in the partial oxida-
4
lysts. (a) spc-Ni
/Mg
Al; (b) spc-Ni
/Mg
Al; (c) spc-Ni
/
tion of CH . (a) spc-Ni
/Mg
Al; (b) spc-Ni
/Mg
Al; (c) spc-
0.26
0.74
0.32
1.62
0.5
4
0.26
0.74
0.32
1.62
Mg Al; (d) spc-Ni
/Mg
Al; (e) spc-Ni
/Mg
Al; (f) spc-
Ni /Mg Al; (d) spc-Ni
/Mg
Al; (e) spc-Ni
/Mg
Al; (f) spc-
4.27
2.5
0.62
3.38
0.73
4.27
0.5
2.5
0.62
3.38
0.73
Ni
/Mg
Al; (g) thermodynamic equilibrium. A mixed gas of CH /
Ni
/Mg
Al. A mixed gas of CH /O /N = 2/1/1 was used at
0.85
5.15
4
0.85
5.15
4
2
2
O /N = 2/1/1 was used at 1073 K for 5 mg of the catalyst diluted by
1073 K for 50 mg of the catalyst diluted by 500 mg of quartz beads. A length
of the catalyst bed was 18 mm.
2
2
quartz beads.

52
K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
cumulation. Imp-Ni catalysts showed significant increases
in the temperature with increasing the space velocity, imp-
Ni/α-Al2O3 among which was the highest, followed by imp-
Ni/MgO, imp-Ni/γ-Al2O3, and imp-Ni/Mg3Al-aq. The tem-
perature increase was the smallest with spc-Ni0.5/Mg2.5Al,
suggesting that the enhanced endothermic reforming reac-
tions on this catalyst canceled the heat accumulation by the
combustion. On the contrary, on imp-Ni/α-Al2O3, the sur-
face of large size Ni metal particles was partially oxidized at
the high space velocity, resulting in an enhanced combustion
reaction than the reforming reactions followed by the heat
accumulation. Imp-Ni/Mg3Al, imp-Ni/MgO, and imp-Ni/γ -
Al2O3 behaved as the catalyst possessing a medium level
of activity for the reforming reactions probably depending
on each level of Ni metal dispersions. Among these imp-
catalysts, imp-Ni/Mg3Al showed the lowest value of heat
accumulation due to the high Ni dispersion accompanied
by the reconstitution of Mg–Al hydrotalcite containing Ni
during the catalyst preparation. Along the catalyst bed, the
temperature decreased from the inlet to the outlet with the
all the supported Ni catalysts and showed almost constant
temperature around 1090 K at the outlet at the space veloc-
Fig. 10. Autothermal reforming of CH carried out by adding
4
ity of 90,000 ml h
g
−1. Only in the case of a blank test, the
−1
O
(I) or H O (II) to the steam reforming. (a) spc-Ni /Mg Al;
2
2 0.5 2.5
cat
(b) imp-16.3wt%Ni/Mg Al-aq; (c) imp-16.3wt%Ni/γ -Al O ; (d) imp-
3
2 3
temperature increased from the inlet (1084 K) to the outlet
(1090 K) along the catalyst bed at the same space velocity.
16.3wt%Ni/MgO; (e) imp-16.3wt%Ni/α-Al O ; (f) thermodynamic equi-
2
3
−1
librium. (I) A mixed gas of CH /H O/N = 50/50/100 ml min
4
2
2
was used at 1073 K for 50 mg of the catalyst. (II) A mixed gas of
−1
3.8. Autothermal reforming of CH4
CH /O /N = 40/20/80 ml min was used for 25 mg of the catalyst.
4
2
2
Autothermal reforming of CH4 was carried out by adding
O2 into the steam reforming mixture of CH4/H2O/N2 =
50/50/100 ml min−1 over 50 mg of the supported Ni cat-
alysts at 1073 K (Fig. 10I). CH4 conversion calculated
by thermodynamic equilibrium increased with increasing
the amount of O2. spc-Ni0.5/Mg2.5Al showed the highest
activity in the autothermal reforming of CH4 almost fol-
lowing the thermodynamic equilibrium, followed by imp-
Ni/γ -Al2O3 and imp-Ni/Mg3Al-aq. Imp-Ni/MgO showed
the lowest activity, followed by imp-Ni/α-Al2O3, and both
showed decreasing CH4 conversions by the addition of O2
cles on imp-Ni/α-Al2O3 must be oxidized at the first stage,
resulting in a lowering of the activity.
Temperature at the inlet of the catalyst bed increased
with increasing the amount of O2 added in the steam re-
forming mixture of CH4/H2O/N2 = 50/50/100 ml min−1
(Fig. 11I), showing that endothermic steam reforming was
converted to exothermic, i.e., oxidative reforming reaction
by the addition of O2. The reaction temperature was con-
trolled at 1073 K at the center of the catalyst bed. Spc-
Ni0.5/Mg2.5Al showed the lowest temperature again as well
as no significant increase in the temperature even in the
presence of a high amount of O2, suggesting that the ac-
tivity for the reforming of this catalyst was the highest
among the Ni catalysts tested. Judging from the temperature
change, the activity was followed by imp-Ni/γ-Al2O3 and
imp-Ni/Mg3Al-aq. Both imp-Ni/MgO and imp-Ni/α-Al2O3
showed a high temperature of the inlet of the catalyst bed,
in which the temperature change was significant with imp-
Ni/α-Al2O3. Imp-Ni/α-Al2O3 showed enough high activity
at 5 ml min−1
.
When the autothermal reforming was carried out by
adding H2O in the partial oxidation mixture of CH4/O2/
N2 = 40/20/80 ml min−1, CH4 conversion at thermody-
namic equilibrium increased with increasing the amount of
H2O (Fig. 10II). Twenty-five milligrams of the supported
Ni catalysts was used in each reaction. Spc-Ni0.5/Mg2.5Al
again showed the highest activity almost following the ther-
modynamic equilibrium judging from the CH4 conversion,
followed by imp-Ni/γ -Al2O3, imp-Ni/Mg3Al-aq, and imp-
Ni/MgO. The activity of imp-Ni/α-Al2O3 was the lowest,
while imp-Ni/MgO showed rather high activity compared to
the case of autothermal reforming carried out by adding O2
to the steam reforming. This may be due to surface oxidation
of Ni metal particles on imp-Ni/α-Al2O3. The reaction was
carried out by increasing the amount of H2O in the mixture
of CH4 and O2, and therefore the surface of Ni metal parti-
in the absence of O2, while the addition of O2 (5 ml min−1
)
caused a clear decrease in the activity (Fig. 10I). This co-
incided well with the increase in the temperature with the
addition of 5 ml min−1 of O2 (Fig. 11I). The activity of
imp-Ni/MgO for steam reforming was very low (Fig. 10I)
coinciding well with the high temperature in the absence
of O2 (Fig. 11I).

K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
53
was almost constant as +38, +32, +32, +34, +37, and
+37 K, when the amount of H2O added was 0, 20, 40,
60, 80, and 100 ml min−1, respectively. The reaction was
always exothermic and the temperature at the inlet of the
catalyst bed was high, showing no substantial effect of H2O
addition in heat production. This may be again due to that
the oxidation proceeds faster than the reforming reactions.
The distribution of products, i.e., H2, CO, and CO2 as well
as CH4 and H2O as the raw materials during the reaction
over spc-Ni0.5/Mg2.5Al always followed the thermodynamic
equilibrium.
3.9. Stability of spc-Ni/MgAl catalyst
A long-term stability of spc-Ni0.5/Mg2.5Al was tested
in the autothermal reforming of CH4 at 1073 K for 50 h
using 50 mg of the catalyst by feeding the mixed gas
of CH4/H2O/O2 = 2/2/1 rather at high GHSV of 1.5 ×
105 ml h
−1 (Fig. 12). The stability of spc-Ni0.5/Mg2.5Al
−1
g
cat
Fig. 11. Temperature at the inlet of the catalyst bed in the au-
tothermal reforming of CH carried out by adding (I) or
H O (II). (a) spc-Ni /Mg Al; (b) imp-16.3wt%Ni/Mg Al-aq; (c) imp-
was compared with those of spc-Ni0.25/Mg2.75Al, imp-
16.3wt%Ni/Mg3Al-aq, and imp-8.3wt%Ni/Mg3Al-aq. Dur-
ing the reaction for 50 h, both spc-Ni0.5/Mg2.5Al and spc-
Ni0.25/Mg2.75Al showed a quite high stability as well as
a high activity, while imp-16.3wt%Ni/Mg3Al-aq showed a
slow decline and imp-8.3 wt% Ni/Mg3Al-aq revealed a dras-
tic decrease in the activity after 30 h of the reaction. It is
clearly shown that the spc-method afforded a good stability
on supported Ni catalysts. Interestingly, even when the load-
ing amount of Ni was decreased to almost half, amount, i.e.,
8.3 wt% (spc-Ni0.25/Mg2.75Al), the activity was still kept at
the same order of spc-Ni0.5/Mg2.5Al during 50 h of the re-
action. As shown in Table 2, the condition of prereduction
treatment was important for affording the high stability on
spc-Ni/MgAl catalysts and the reduction at 1173 K for 0.5 h
afforded the best results.
O
4
2
2
0.5
2.5
3
16.3wt%Ni/γ -Al O ; (d) imp-16.3wt%Ni/MgO; (e) imp-16.3wt%Ni/
2
3
−1
α-Al O . (I) A mixed gas of CH /H O/N = 50/50/100 ml min
2
3
4
2
2
was used at 1073 K for 50 mg of the catalyst. (II) A mixed gas of
−1
CH /O /N = 40/20/80 ml min was used for 25 mg of the catalyst.
4
2
2
Interestingly, the temperature at the inlet of the cat-
alyst bed did not change significantly when H2O was
added in the partial oxidation mixture of CH4/O2/N2 =
40/20/80 ml min−1 (Fig. 11II). This may be due to the fact
that oxidation proceeds more quickly compared to steam
reforming reaction. As a result, the addition of H2O did not
affect substantially the temperature at the inlet of the catalyst
bed. Spc-Ni0.5/Mg2.5Al showed again the lowest tempera-
ture in the absence of H2O, followed by imp-Ni/γ-Al2O3,
and the temperature gradually decreased with increasing
the amount of H2O added. Also over imp-Ni/Mg3Al-aq and
imp-Ni/MgO, even though the temperature at the inlet was
high, the temperature decreased with increasing the amount
of H2O added, reflecting each activity for the reforming
reactions. Imp-Ni/α-Al2O3 showed a clear increase in the
temperature with increasing the amount of H2O. This may
be due to the surface oxidation of Ni metal particles, well
coinciding with the low activity (Fig. 10II) as discussed al-
ready.
The highest activity was obtained with spc-Ni0.5/Mg2.5Al
in both autothermal reforming reactions. A difference in the
temperature between the inlet and the outlet of the catalyst
bed was −80, −35, −27, +13, +35, and +47 K, when
the amount of O2 added in the steam reforming over spc-
Ni0.5/Mg2.5Al was 0, 5, 10, 15, 20, and 25 ml min−1, re-
spectively. The temperature at the inlet of the catalyst bed
significantly increased with increasing the amount of O2
added, clearly showing that endothermic steam reforming
was converted to exothermic oxidative reforming. On the
other hand, interestingly, the difference in the temperatures
Fig. 12. Autothermal reforming of CH over supported Ni catalysts.
4
(a) spc-Ni /Mg Al; (b) spc-Ni
/Mg
Al; (c) imp-16.3wt%Ni/
0.5
2.5
0.25
2.75
Mg Al-aq; (d) imp-8.3wt%Ni/Mg Al-aq. A mixed gas of CH /H O/O /
3
3
4
2
2
−1
N
= 50/50/25/25 ml min was used at 1073 K for 50 mg of the catalyst
2
−1
−1
g
cat
5
(space velocity = 1.5 × 10 ml h
).

54
K. Takehira et al. / Journal of Catalysis 221 (2004) 43–54
4. Conclusion
[6] F. Basile, G. Fornasari, F. Trifiro, A. Vaccari, Catal. Today 64 (2001)
21.
[7] M.C.J. Bradford, Catal. Lett. 66 (2000) 113.
[8] Y. Schuurman, C. Mirodatos, P. Ferreira-Aparicio, I. Rodriguez-Ra-
mos, A. Guerrero-Ruiz, Catal. Lett. 66 (2000) 33.
spc-Ni/MgAl catalysts have been prepared from Mg–Al
hydrotalcite precursors containing Ni at the Mg site and
tested for the partial oxidation and autothermal reforming of
CH4 into synthesis gas. The precursors were prepared by the
coprecipitation method, thermally decomposed, and reduced
to form spc-Ni/MgAl catalysts. Ni2+ ions first substituted a
part of the Mg2+ sites in the Mg–Al hydrotalcite-like com-
pounds, then incorporated in the rock-salt type Mg–Ni–O
solid solutions in the mixed oxide after the decomposition,
and reduced to be used as the catalysts. The dispersion of Ni
was thus repeatedly enhanced during the spc preparation, re-
sulting in the highly dispersed and stable Ni metal particles
after the reduction. The activity of spc-Ni/MgAl catalyst was
the highest at the ratio of Mg/Al of 1/3. The reducibility of
Ni2+ was moderately controlled by the Al3+ incorporation
into the Mg–Ni–O solid solutions. In the partial oxidation
[9] P. Pantu, K. Kim, G.R. Gavalas, Appl. Catal. A 193 (2000) 203.
[10] P.M. Torniainen, X. Chu, L.D. Schmidt, J. Catal. 146 (1994) 1.
[11] T. Ashcroft, A.K. Cheetham, J.S. Foord, M.L.H. Green, C.P. Grey, A.J.
Murrell, P.D.F. Vernon, Nature 344 (1990) 319.
[12] T. Hayakawa, H. Harihara, A.G. Andersen, A.P.E. York, K. Suzuki, H.
Yasuda, K. Takehira, Angew. Chem., Int. Ed. Engl. 35 (1996) 192.
[13] A.G. Shiozaki, R. Andersen, T. Hayakawa, S. Hamakawa, K. Suzuki,
M. Shimizu, K. Takehira, J. Chem. Soc., Faraday Trans. 93 (1997)
3225.
[14] T. Hayakawa, S. Suzuki, S. Hamakawa, K. Suzuki, T. Shishido, K.
Takehira, Appl. Catal. A 183 (1999) 273.
[15] K. Takehira, T. Shishido, M. Kondo, J. Catal. 207 (2002) 307.
[16] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173.
[17] UK Patent 1,442,172, 1973, to BASF AG.
[18] F. Basile, L. Basini, M. D’Amore, G. Fornasari, A. Guarinoni, D. Mat-
teuzzi, G. Del Piero, F. Trifiro, A. Vaccari, J. Catal. 173 (1998) 247.
[19] L. Basini, A. Guarioni, A. Aragano, J. Catal. 190 (2000) 284.
[20] T. Shishido, M. Sukenobu, H. Morioka, M. Kondo, Y. Wang, K. Ta-
kaki, K. Takehira, Appl. Catal. A 223 (2002) 35.
of CH4, spc-Ni0.5/Mg2.5Al afforded enough high CH4 con-
−1
version even at the high space velocity (9×105 ml h−1 g ),
cat
exceeding the value obtained over 1wt%Rh/MgO. Ni species
on spc-Ni0.5/Mg2.5Al catalysts were stable even under the
presence of O2, while Ni catalysts prepared by the con-
ventional impregnation quickly lost activity due to the sur-
face oxidation of Ni particles. Moreover, a heat accumu-
lation during the CH4 oxidation was the lowest over the
spc-Ni0.5/Mg2.5Al catalyst among the catalysts tested. This
clearly suggests that the heat of exothermic CH4 combustion
to H2O and CO2 could be quickly consumed by the follow-
ing endothermic CH4 reforming by H2O and CO2 over spc-
Ni0.5/Mg2.5Al. Actually spc-Ni0.5/Mg2.5Al showed a high
and stable activity for the autothermal reforming of CH4 un-
der the copresence of O2 and H2O. Thus, spc-Ni0.5/Mg2.5Al
catalyst is a hopeful candidate for the autothermal reforming
of CH4 which can feed H2 to fuel cell economically.
[21] P. Shishido, T. Wang, T. Kosaka, K. Takehira, Chem. Lett. (2002) 752.
[22] T. Shishido, M. Sukenobu, H. Morioka, R. Furukawa, H. Shirahase, K.
Takehira, Catal. Lett. 73 (2001) 21.
[23] A.I. Tsyganok, K. Suzuki, S. Hamakawa, K. Takehira, T. Hayakawa,
Chem. Lett. (2001) 24.
[24] A.I. Tsyganok, K. Suzuki, S. Hamakawa, K. Takehira, T. Hayakawa,
Catal. Lett. 77 (2001) 75.
[25] A.I. Tsyganok, T. Tsunoda, K. Suzuki, S. Hamakawa, K. Takehira, T.
Hayakawa, J. Catal. 213 (2003) 191.
[26] K. Takehira, Catal. Surv. Jpn. 6 (2002) 19.
[27] M. Miyata, A. Okada, Clays Clay Miner. 25 (1977) 14.
[28] G. Fornasari, M. Gazzano, D. Matteuzzi, F. Trifiro, A. Vaccari, Appl.
Clay Sci. 10 (1995) 69.
[29] J.H. Ross, in: G.C. Bond, G. Webb (Eds.), Specialist Periodical Re-
ports, Vol. 7, Royal Society Chemistry, London, 1985, p. 1, and refer-
ences therein.
[30] R.D. Shannon, Acta Crystallogr. A 32 (1876) 751.
[31] U. Olsbye, D. Akpriaye, E. Rytter, M. Ronnekleiv, E. Tangstad, Appl.
Catal. A 224 (2002) 39.
[32] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173.
[33] A. Parmaliana, F. Arena, F. Frusteri, N. Giordano, J. Chem. Soc., Fara-
day Trans. 86 (1990) 2663.
[34] F. Arena, B.A. Horrell, D.L. Cocke, A. Parmaliana, N. Giordano,
J. Catal. 132 (1991) 58.
Acknowledgments
The authors sincerely thank the Hiroshima Industrial
Technology Organization for financial support.
[35] A. Parmaliana, F. Arena, F. Frusteri, S. Coluccia, L. Marchese, G.L.
Martra, A.L. Chuvilin, J. Catal. 141 (1993) 34.
[36] F. Arena, F. Frusteri, A. Parmaliana, L. Plyasova, A.N. Shmakov,
J. Chem. Soc., Faraday Trans. 92 (1996) 469.
References
[37] K. Tomishige, K. Fujimoto, Catal. Surv. Jpn. 2 (1998) 3.
[38] K. Tomishige, Y. Chen, K. Fujimoto, J. Catal. 181 (1991) 91.
[39] Y.-G. Chen, K. Tomishige, K. Yokoyama, K. Fujimoto, J. Catal. 184
(1999) 479.
[40] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis,
Wiley–Interscience, New York, 1994.
[1] M.A. Pena, J.P. Gomez, J.L.G. Fierro, Appl. Catal. A 144 (1996) 7.
[2] J.N. Armor, Appl. Catal. A 176 (1999) 159.
[3] J.R. Rostrup-Nielsen, Catal. Today 71 (2002) 243.
[4] E. Ruckenstein, H.Y. Wang, Appl. Catal. A 198 (2000) 33.
[5] K. Nakagawa, N. Ikenaga, T. Kobayashi, T. Suzuki, Catal. Today 64
(2001) 31.