CO2 Reforming of CH4 Over Ni - Perovskite Catalysts Prepared by Solid Phase Crystallization Method
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Transcript of CO2 Reforming of CH4 Over Ni - Perovskite Catalysts Prepared by Solid Phase Crystallization Method
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CO2 reforming of CH4 over Ni/perovskite catalysts preparedby solid phase crystallization method
Takashi Hayakawaa, Shu Suzukib, Junji Nakamurab, Toshio Uchijimab, Satoshi Hamakawaa,Kunio Suzukia, Tetsuya Shishidoc, Katsuomi Takehirac,*
aNational Institute of Materials and Chemical Research, Tsukuba Research Center, AIST, Higashi 1-1, Tsukuba, Ibaraki 305, JapanbInstitute of Materials Science, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305, Japan
cDepartment of Applied Chemistry, Hiroshima University, Kagamiyama 1-4-1, Higashi-hiroshima 739, Japan
Received 5 November 1998; received in revised form 25 February 1999; accepted 2 March 1999
Abstract
Ni-supported catalysts on perovskite-type oxides have been prepared by solid phase crystallization (spc) method and
tested for CO2 reforming of CH4 into synthesis gas at 8508C. The Ni catalysts were obtained in situ during the reaction fromthe oxides as the precursors in which nickel species were homogeneously incorporated in the perovskite structure. Ni/
Ca0.8Sr0.2TiO3 and Ni/BaTiO3 catalysts showed high activity as well as high sustainability among the catalysts tested. The
high activity may be due to highly dispersed and stable Ni metal particles (diameter
-
als, into more valuable compounds by catalytic reac-
tions. The CO2 reforming of CH4 (1) has been inten-
sively studied for the purpose of its use in industry
CH4 CO2 ! 2CO 2H2 (1)for the production of synthesis gas [110]. This is
commercialized as the Calcor Process [11] and the
SPARG Process [12], and the catalytic behavior of
LaNiAl mixed oxide [6,7] or the details of the
deactivation of Ni/SiO2 catalyst [9,10] have been
reported. The conversion of CH4 to synthesis gas is
usually carried out by the H2O reforming (2), leading
to the formation of synthesis gas of H2/CO ratio of 3/1
[13].
CH4 H2O! CO 3H2 (2)Since the replacement of H2O by CO2 results in a
lower H2/CO ratio of 1/1 in the product gas, the
combination of these two reforming reactions widens
the utility of synthesis gas, i.e., in methanol or in
FischerTropsch synthesis which requires the H2/CO
ratio of 2/1. This process has also received attention
from a viewpoint of environmental protection because
the emission of CH4 and CO2 in the atmosphere brings
about global warming by the greenhouse effect and
these harmful gases can simultaneously be converted
to useful synthesis gas. Ni or precious metals, such as
Ru, Rh, Pd, Ir and Pt, are reported to be active as the
catalyst for the reaction [2,3]; however, the reaction is
frequently accompanied by coke formation, especially
on Ni catalysts, leading to catalyst deactivation or
plugging of the reactor. Ru or Rh shows high selec-
tivity for coke-free operation, which can be ascribed to
high reforming rates combined with low coke forma-
tion rates. Similar high selectivity can be achieved by
using a sulfur-passivated Ni catalyst [2,4,12] or Ni/
La2O3 catalyst [8].
High dispersion of metal species over catalyst [14]
or use of alkali or alkaline earth metal oxides in
catalyst [15] may reduce coke formation. Metal-sup-
ported catalysts are conventionally prepared by wet
impregnation of different supports. This method is not
fully reproducible and may give rise to some inho-
mogeneity in the distribution of the metal on the
surface. A new concept of the catalyst preparation,
therefore, may be required. Use of the precursors
containing homogeneously distributed metal in the
structure, which on further calcination and reduction,
may result in the formation of well dispersed and
stable metal particles on the surface. We have pro-
posed a new method of the preparation of well dis-
persed and stable metal-supported catalyst, i.e., solid
phase crystallization (spc). This method was suc-
cessfully applied to the preparation of Ni-supported
catalyst for the partial oxidation of CH4 to synthesis
gas [1618]. By using CaTiO3 or BaTiO3 perovskite
containing small amounts of Ni in the Ti sites as the
precursor, highly dispersed and stable Ni metals were
formed in situ on the catalyst, resulting in the high
activity and sustainability against coke formation
during the partial oxidation of CH4 to synthesis gas.
The crystal structure of the perovskite was maintained
and Ni species alone in situ migrated to form ultra fine
particles on the surface during the reaction. The
formation of the highly dispersed and stable Ni metals
may be due to a matrix effect of the stable per-
ovskite crystal structure. Moreover, use of the CaTiO3or BaTiO3 perovskite materials can afford alkaline
earth metals in the catalyst, which may result in a high
resistance against coke formation. A similar idea of in
situ reduction followed by the formation of highly
dispersed metal species has been proposed, using
LaRhO3 [19] or LaNiO3 [20] as the precursor; none-
theless, the perovskite crystal structure was decom-
posed during the reaction. Here we report the results
obtained by using the Ni/perovskite catalysts prepared
by the spc method in the CO2 reforming of CH4.
2. Experimental
2.1. Preparation of the catalyst
The catalysts, spc-Ni/MgTiO3, spc-Ni/CaTiO3, spc-
Ni/Ca0.8Sr0.2TiO3, spc-Ni/SrTiO3, spc-Ni/Ca0.8Ba0.2-TiO3, spc-Ni/BaTiO3 and spc-Ni/TiO2 were obtained
in situ by the spc method from the precursors prepared
by the citrate method [1618]. The precursors were
prepared as follows: an aqueous solution of reagent
grade nickel nitrate, alkaline earth carbonates and
titanium isopropoxide was treated with an excess
amount of citric acid and ethylene glycol; this mixture
was evaporated at 80908C to make a sol of organicmetal complex. This was followed by two-step
decomposition by heating at 2008C for 5 h and5008C for 5 h, and finally calcining at 9008C in air
274 T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285
-
for 10 h. Three catalysts: imp-Ni/Ca0.8Sr0.2TiO3, imp-
Ni/BaTiO3 and imp-Ni/TiO2 were prepared by an
impregnation (imp) method as follows: the calculated
amount of aqueous nickel nitrate was treated with an
equimolar amount of citric acid and ethylene glycol;
this was evaporated at 80908C to make a viscousliquid, and this liquid was then diluted with water. The
solution was then added into a water-suspension of
Ca0.8Sr0.2TiO3, BaTiO3 or TiO2 which had been sepa-
rately prepared by the citrate method. The suspension
was again evaporated at 80908C, and calcined at9008C in air for 5 h. In both the cases of spc andimp, the atomic ratio of Ni/Ti was fixed at 0.2/1.0. The
Ni/a-Al2O3, Ni/MgO, Ni/ZrO2 and Ni/SiO2 catalystswere prepared by the imp method on conventional
supports and finally calcined at 9008C in air for 5 h.The amount of Ni was 10.3 wt% on each support. a-Al2O3 (Taimei, >99.9%], MgO smoke powder
(UBE, Japan, >99.98%; average particle size,
100 nm; pore diameter (BET), 0.108 mm; surface area
(BET), 15.5 m2 g1), ZrO2 (Kanto) and SiO2 (FujiDavison, Syloid 72) were used as the supports.
2.2. Characterization of the catalyst
The structure of the catalysts was studied by using
XRD, TEM, BET, TG/DTA and XPS as follows [16
18]. The powder X-ray diffraction (XRD) patterns of
the catalyst were recorded by using MXP-18 (MAC
Science) with Cu K radiation. Transmission electron
microscopy (TEM) was carried out on a JEM-2000FX
(JEOL) instrument equipped with an energy dispersive
X-ray analyzer (EDX: Northern). Surface area of the
catalyst was measured with a Micromeritics model
2200. Thermal analyses (TGA/DTA) were carried out
by using Shimadzu DTA 50 and TGA 50 containing an
electrobalance. X-ray photoelectron spectra (XPS)
were obtained with a PHI-5000 spectrometer employ-
ing Mg K radiation (1253.6 eV) and an electron flood
gun to provide charge neutralization of the non-con-
ducting samples. All binding energy values were
referenced to C1s (285.0 eV).
2.3. Catalytic reactions
All the catalysts have been tested by using a fixed
bed catalyst in a mixture of CO2 (1.0 l h1), CH4
(1.0 l h1) and N2 (1.4 l h1) at 8508C. The catalytic
activities were also tested by changing the space
velocity from 20 000 to 70 000 (ml h1 g-cat1). Suchreactions for testing the catalyst life were carried out at
8508C for 30 h under the same conditions. A U-shapedquartz reactor was used, with the catalyst bed near the
bottom. 150 mg of the catalyst was dispersed in 2 ml
of quartz wool to avoid sintering and clogging of the
reactor. The thermocouple was introduced from the
top of the reactor, and placed in the middle of the
catalyst bed. Product gases were sampled immediately
after the reactor and injected into a gas chromatograph
for analysis.
After 6 h of testing, the reactor was filled with
nitrogen and cooled according to normal procedures.
Finally, a temperature programmed oxidation (TPO)
experiment was performed by heating the reactor from
room temperature to 9508C at a rate of 2.58C min1,with an air flow of 41 ml min1. Off-gases wereanalyzed as usual and the rate of CO2 formation is
plotted against time. The amount of coke formed on
the catalyst was estimated from the amount of CO2formed during the TPO experiment.
Pulse reactions were carried out in order to estimate
the amount of mobile oxygen in the supports, spc-
Ca0.8Sr0.2TiO3, spc-BaTiO3, a-Al2O3, MgO, ZrO2,TiO2 and SiO2 by using the U-shaped quartz reactor
as follows: 150 mg of each support was treated by H2(1 ml5) pulses at 8508C to react with the mobileoxygen, leading to reduction of the support, and then
CO2 was pulsed (1 ml15) over the support, resultingin reoxidation of the reduced support with CO2 to form
CO. The amount of mobile oxygen was thus calculated
from the amount of CO formed.
Self-decoking was also tested by the pulse reactions
over the samples; spc-Ni/Ca0.8Sr0.2TiO3, spc-Ni/
BaTiO3, Ni/a-Al2O3 and spc-Ca0.8Sr0.2TiO3, as fol-lows: coke was deposited over 10 mg of the sample by
CH4 (1 ml10) pulses at 8508C and each samplewas then treated for 4 and 19 h separately under the
atmosphere of He at the same temperature. During
this treatment, coke can be oxidized by the mobile
oxygen species, resulting in the self-decoking. Coke
deposited by the CH4 pulses or that still remained
even after each treatment was converted to CO2 by
oxygen (1 ml10) pulses over the catalyst at 8508C.The amount of remaining coke was estimated from the
amount of CO2 formed by oxygen pulses after each
treatment. Ability of the self-decoking was deduced
T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285 275
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from decrease in the amount of coke during the
treatment.
3. Results and discussions
3.1. Structure of the Ni/perovskite catalysts
X-ray diffraction patterns of powders of spc-Ni/
MgTiO3, spc-Ni/CaTiO3, spc-Ni/SrTiO3 and spc-Ni/
BaTiO3 after the preparation are shown in Fig. 1,
together with those used in the reaction for 6 h.
spc-Ni/MgTiO3 showed the pattern of MgTiO3 (gei-
kielite) (JCPDS: 6-0494) (open squares) together with
that of NiO (JCPDS: 4-835) (open triangles). The
hexagonal geikielite structure was stable and only
NiO was reduced to Ni metal (JCPDS: 4-850) (filled
triangle) during the reaction. spc-Ni/CaTiO3, spc-Ni/
SrTiO3 and spc-Ni/BaTiO3 as prepared showed the
patterns of the perovskite structure (open circles) of
CaTiO3 (JCPDS: 22-153), SrTiO3 (JCPDS: 35-734)
and BaTiO3 (JCPDS: 5-626) as well as that of NiO.
The diffraction lines of NiO were observed most
strongly in spc-Ni/CaTiO3, followed by spc-Ni/
SrTiO3, while traces of the lines were observed in
spc-Ni/BaTiO3. NiO was reduced to Ni metal in each
sample after the reaction. The line strength of Ni metal
well correlated with that of original NiO. XRD pat-
terns of spc-Ni/Ca0.8Mg0.2TiO3, spc-Ni/Ca0.8Sr0.2-TiO3 and spc-Ni/Ca0.8Ba0.2TiO3 after the synthesis
are illustrated in Fig. 2, together with those used in the
reaction for 6 h. spc-Ni/Ca0.8Sr0.2TiO3 showed the
patterns of well crystallized (orthorhombic/cubic)
perovskite together with NiO, suggesting that Sr is
incorporated in the Ca site of CaTiO3 perovskite
structure. On the other hand, spc-Ni/Ca0.8Mg0.2TiO3and spc-Ni/Ca0.8Ba0.2TiO3 showed mixed patterns of
each component, i.e., CaTiO3 (perovskite) and
MgTiO3 (geikielite) in the former and CaTiO3 (per-
ovskite) and BaTiO3 (peorvskite) in the latter, respec-
tively, together with NiO. The latter sample was rather
poorly crystallized compared to the former, and
Fig. 1. X-ray diffractograms for several Ni/perovskites catalysts prepared by the spc method before (a) and after (b) the catalytic testing at
8508C in a mixture of CH4 and CO2 (1:1).
276 T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285
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neither Mg nor Ba can be incorporated in the CaTiO3perovskite structure. In both cases, the structures of all
mixed oxides were apparently stable, while NiO was
reduced to Ni metal during the reaction.
Iwahara et al. [21] reported that the solid solution
formation range is limited to x0.1 or less inCaTi1xNixO3. XRD measurements of spc-Ni/CaTiO3 and spc-Ni/Ca0.8Sr0.2TiO3 clearly showed
the peaks of NiO together with CaTiO3 and Ca0.8Sr0.2-TiO3 perovskite, respectively. If we take the ratio of
Ni/Ti as 0.2/1, a part of NiO was separated from the
perovskite structure, while a part of Ni may be incor-
porated in the perovskite. The crystal structure of
CaTiO3 belongs to orthorhombic [22] while SrTiO3has the cubic structure, which is the most symmetric
and contains one formula unit per cell [23]. The
structure transition from orthorhombic CaTiO3 to
cubic SrTiO3 without an intermediate tetragonal phase
may take place around the ratio of Ca/Sr0.8/0.2[17,23].
Ionic radii are as follows: Mg2, 0.89; Ca2, 1.34;Sr2, 1.44; Ba2, 1.61; Ti4, 0.605 and O2, 1.405 A
[24]. Tolerance factors (t rA rO=
2p rB rO,
where rA, rB and rO are ionic radii of A, B and O,
respectively, in ABO3 perovskite) are calculated as
follows: MgTiO3, 0.808; CaTiO3, 0.966, SrTiO3, 1.00
and BaTiO3, 1.06. Among these, MgTiO3 cannot form
perovskite structure because the value of t is too small.
CaTiO3 and BaTiO3 have the values of t which are
enough close to 1.0 to form stable perovskite struc-
tures, belonging to orthorhombic and tetragonal,
respectively. SrTiO3 has the ideal value of t as 1.00,
supporting the most stable cubic crystal structure [23].
The trace of NiO peaks in spc-Ni/BaTiO3 may be due
to good solubility of Ni in the perovskite or because
the crystallites are too small to give a diffraction
signal. After the catalytic testing, the XRD pattern
of spc-Ni/BaTiO3 showed also traces of Ni metal peak.
NiO which was visible in diffractograms after calci-
nation disappeared after the reaction. We believe that a
substantial part of nickel in the structure has been
reduced to its metallic form on the surface during the
reaction. The nickel metal particles are probably too
small to give reasonable signals in XRD.
Fig. 2. X-ray diffractograms for several Ni/mixed oxides catalysts prepared by the spc method before (a) and after (b) the catalytic testing at
8508C in a mixture of CH4 and CO2 (1:1).
T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285 277
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3.2. Activities of the Ni/metal oxide catalysts
Activities of the Ni catalysts supported on conven-
tional metal oxide were tested in the CO2 reforming of
CH4 (1) for 30 h at the space velocity of
22 700 ml h1 g-cat1. The results are shown inFig. 3, where the activity is compared by the conver-
sion of CO2. The catalysts were prepared by imp
method except spc-Ni/TiO2 and the surface area of
each catalyst is shown in Table 1. Among the catalysts
tested, Ni/SiO2 showed the highest activity, followed
by Ni/MgO and Ni/a-Al2O3. The Ni/SiO2 showed thehighest value of surface area among the catalysts
tested, possibly resulting in the highest activity.
XRD measurements of the Ni(10.3 wt%)/MgO cata-
lyst showed the lines of MgO and NiO, which over-
lapped each other, suggesting the formation of a Ni
MgO solid solution. After the reaction, the lines of Ni
metal were observed very weakly, suggesting the
formation of highly dispersed Ni metal particles.
Parmaliana et al. [2528] reported that NiOMgO
system forms ideal solid solutions over the whole
molecular fraction range and was successfully used as
the catalyst for the H2O reforming of CH4. Ni2
diffuses progressively into the MgO matrix during
the air calcination of a 19% Ni/MgO catalyst in the
range 40010008C, resulting in the formation of anNixMg1xO solid solution [28]. The calcination andreduction around 6008C afforded a particle size dis-tribution of Ni metal with the maximum centered at
90130 A, resulting in the high activity in the CH4steam-reforming reaction [27]. Fujimoto and co-
workers [15,29,30] reported that a solid solution
(Ni0.03Mg0.97O, atomic ratio) was reduced at high
temperature (>8008C) to form an active and stablecatalyst for H2O or CO2 reforming of CH4. It is likely
that the formation of a NiMgO solid solution is a
key step for bestowing the high activity on the Ni/
MgO catalyst and that this is the case also in the
present catalyst system.
Ni/TiO2 showed a clear decrease in the activity
during the reaction in the cases of both spc and
imp. A similar decrease in the activity of spc-Ni/
TiO2 was also observed in the partial oxidation of
CH4 to synthesis gas; nonetheless, no significant
coking took place over the catalyst after the reaction
[18]. Both NiTiO3 (illumenite) and TiO2 (rutile) was
formed in spc-Ni/TiO2 after the calcination and the
former mixed oxide decomposed into NiO and TiO2,
followed by the reduction of NiO into Ni metal
particles (average diameter100 nm) during the reac-tion [18]. XRD measurements of imp-Ni/TiO2 showed
that NiO was deposited on TiO2 (rutile) after the
calcination, and NiO was reduced to Ni metal particles
of rather small size (average diameter 25 nm) afterthe reaction.
3.3. Activities of the Ni/perovskite catalysts
Among the catalysts supported on perovskite by the
imp method, imp-Ni/BaTiO3 was the most active,
followed by imp-Ni/Ca0.8Sr0.2TiO3, while imp-Ni/
SrTiO3 showed a low activity (Fig. 4). When the
Fig. 3. CH4 reforming with CO2 over Ni/metal oxides catalysts (at
8508C in a mixture of CH4 and CO2 (1:1)).
Table 1
Surface area and amount of coke formed
Catalyst Surface areaa
(m2 g1)Cokeb
(wt%)
Ni/ZrO2 8.9 41.8
Ni/a-Al2O3 11.0 21.3Ni/SiO2 232.2 11.5
Ni/MgO 24.0 5.2
spc-Ni/TiO2 1.3 2.3
imp-Ni/TiO2 0.4 0.2
a Before the catalytic testing.b After the catalytic testing for 6 h.
278 T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285
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catalyst was prepared by the spc method (Fig. 5), the
high activity was obtained by spc-Ni/Ca0.8Sr0.2TiO3,
spc-Ni/BaTiO3 and spc-Ni/CaTiO3 at the beginning of
the reaction. spc-Ni/SrTiO3 showed again a low activ-
ity at the beginning of the reaction, while the activity
slowly increased and reached the highest value among
the catalysts tested. Surface areas of some of the
catalysts are shown in Table 2. XRD analyses suggest
that SrTiO3 forms a stable cubic crystal structure and
contains much more Ni in the Ti sites compared to
CaTiO3 [16]. This may result in a slow migration of
nickel species in the structure to the surface during the
reaction, followed by the formation of most highly
dispersed Ni species among the catalysts tested. The
details of the behavior of this catalyst will be studied
further.
In the partial oxidation of CH4 over spc-Ni/
Ca1xSrxTiO3 (x01.0), the highest activity wasobserved over spc-Ni/Ca0.8Sr0.2TiO3 [17]. Also in
the present reaction, the replacement of 20% of Ca
with Sr in spc-Ni/CaTiO3 resulted in an increase as
well as in a stabilization in the activity, the value of
which was higher than Ni/MgO. Conversion of CO2over the catalysts showed a slightly higher value than
that of CH4, and can be put in almost identical order to
that of CH4 from the view point of preparation method
and effect of the support.
3.4. High activity of spc-Ni/perovskite catalysts
No significant difference was observed in the activ-
ities of the effective catalysts; i.e., spc-Ni/BaTiO3,
spc-Ni/Ca0.8Sr0.2TiO3, Ni/SiO2, Ni/a-Al2O3, imp-Ni/Ca0.8Sr0.2TiO3, Ni/MgO and imp-Ni/BaTiO3, selected
by the catalytic screening at 8508C for 30 h at thespace velocity of 22 700 ml h1 g-cat1 (Figs. 35).This may be due to the fact that thermodynamic
equilibrium of the reaction (1) was attained over these
active catalysts in the reaction conditions. The activity
Fig. 4. CH4 reforming with CO2 over Ni/perovskites catalysts
prepared by the imp method (at 8508C in a mixture of CH4 and CO2(1:1)).
Fig. 5. CH4 reforming with CO2 over Ni/perovskites catalysts
prepared by the spc method (at 8508C in a mixture of CH4 and CO2(1:1)).
Table 2
Surface area and amount of coke formed
Catalyst Surface areaa
(m2 g1)Cokeb
(wt%)
spc-Ni/CaTiO3 6.9 2.0
spc-Ni/SrTiO3 19.5 1.3
spc-Ni/BaTiO3 5.8 1.3
spc-Ni/Ca0.8Sr0.2TiO3 8.9 3.4
imp-Ni/Ca0.8Sr0.2TiO3 7.6 3.3
Ca0.8Sr0.2TiO3 19.0 4.0
a Before the catalytic testing.b After the catalytic testing for 6 h.
T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285 279
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was further tested by increasing space velocity (Fig. 6).
At the higher space velocity, the reaction (1) must be
kinetically controlled on the catalyst surface, and
therefore, the catalytic activity may be compared more
precisely. The catalysts were clearly divided into two
groups depending on the preparation method, i.e., the
spc method and the imp method. Upon increasing the
space velocity, those prepared by the spc method,
spc-Ni/BaTiO3 and spc-Ni/Ca0.8Sr0.2TiO3, were still
active enough, while those prepared by the imp
method, imp-Ni/BaTiO3, imp-Ni/Ca0.8Sr0.2TiO3, Ni/
a-Al2O3 and Ni/MgO, quickly lost their activities. Thehighest activity was observed over spc-Ni/BaTiO3. It
is thus very likely that the spc method is quite effective
for the catalyst preparation. The high activity of the
catalyst prepared by the spc method may be due to the
formation of highly dispersed and stable Ni particles
[1618]. The high dispersion of Ni species was clearly
observed in the TEM images as seen later and was also
previously reported with spc-Ni/BaTiO3 [18].
3.5. Coke formation over the catalysts
The amount of coke formed over the Ni catalysts
supported on conventional metal oxide after the reac-
tion for 6 h was measured by the TPO experiment
(Table 1). Ni/ZrO2 showed the highest value, followed
by Ni/a-Al2O3, Ni/SiO2, Ni/MgO and Ni/TiO2. Ni/TiO2 showed the lowest value among the catalysts
tested, corresponding to the result obtained over spc-
Ni/TiO2 in the partial oxidation of CH4. The decrease
in the activity was observed with each spc-Ni/TiO2 or
imp-Ni/TiO2 catalyst in spite of no coking or good
dispersion of Ni metal particles, respectively. This
may be due to some unusual properties of TiO2 as
the support [31,32]. Oxygen migration from TiO2 to
metal or strong metal-support interaction was fre-
quently suggested over TiO2 supported metal cata-
lysts, as seen in the migration of the support onto the
metal particles, resulting in a significant decrease in
the activity [33,34].
The amounts of coke formed over the Ni/perovskite
catalysts after the reaction for 6 h are shown in
Table 2. All the catalysts tested showed lower values
between 1.0 and 4.0 wt% compared to those over
ZrO2, a-Al2O3 and SiO2. As seen in the resultsobtained over spc-Ni/Ca0.8Sr0.2TiO3 and imp-Ni/
Ca0.8Sr0.2TiO3, no clear difference was observed
between the preparation methods, i.e., spc and imp,
in the sustainability of the catalyst against coke for-
mation. It is likely that the perovskite compound is
effective for suppressing the coke formation. This may
be partly due to the presence of alkaline earth metals.
Use of Ca0.8Sr0.2TiO3 alone instead of the Ni/perovs-
kite catalysts in the reaction resulted in a small amount
of coke deposition of 4.0 wt% and negligible forma-
tion of synthesis gas under low conversion of CH4 and
CO2.
3.6. Oxygen mobility in the support
The spc method was definitively effective for
decreasing the coke formation in the partial oxidation
of CH4 [1618], while this was not the case in the
present study. This may be due to much easier for-
mation of coke in the reaction (1) compared to the
partial oxidation.
The low amount of coke formation in spite of the
activity decrease observed over both imp-Ni/TiO2 and
spc-Ni/TiO2 may be due to the presence of mobile
oxygen over TiO2 as seen in the strong metal-support
interaction (SMSI) [32]. Mobile oxygen species may
also exist in the perovskite materials used as the
supports in the present study. The amount of mobile
Fig. 6. CO2 conversion as a function of space velocity in the CH4reforming with CO2 over the selected catalysts (at 8508C in amixture of CH4 and CO2 (1:1)).
280 T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285
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oxygen in the support was measured by the pulse
reactions of CO2. The supports, spc-Ca0.8Sr0.2TiO3,
spc-BaTiO3, TiO2, ZrO2, MgO, a-Al2O3 and SiO2,were treated with H2 pulses. The suffix spc means that
the perovskites were prepared by the same procedure
as the spc-Ni/perovskite catalysts in the absence of Ni.
The mobile oxygen in the supports first reacted with
H2 to form H2O and the oxygen vacancies. The
supports were then treated with CO2 pulses, where
the oxygen vacancies reacted with CO2 to form CO.
The CO formation finished within 10 pulses over spc-
Ca0.8Sr0.2TiO3, while slow CO formation was
observed over spc-BaTiO3 during more than 15 pulses
(Fig. 7). This suggests a slow mobility of oxygen in
spc-BaTiO3 compared to that of spc-Ca0.8Sr0.2TiO3.
The total amount of mobile oxygen was calculated in
each catalyst and is shown in Table 3. Both spc-
Ca0.8Sr0.2TiO3 and spc-BaTiO3 showed quite high
values of the oxygen mobility compared to those of
metal oxides. TiO2 showed the highest mobility
among the single metal oxides tested, followed by
ZrO2, MgO, a-Al2O3 and SiO2.
3.7. Self-decoking of the catalyst
Most likely such mobility of oxygen can affect a
self-decoking over the catalysts. Coke was deposited
over the catalysts by CH4 pulses at 8508C and the
catalysts were then treated in the He atmosphere for a
certain period (4 or 19 h) at 8508C. During theseseparate treatments in He, part of the coke over the
catalysts can be oxidized by the mobile oxygen spe-
cies from the supports. The amounts of remaining
coke are shown in Table 4 together with the amounts
of coke first deposited just after the CH4 pulsing (0 h).
Coke was formed over the support spc-Ca0.8Sr0.2TiO3itself by the CH4 pulses and was oxidized by the
mobile oxygen during the He treatment. Much amount
of coke was formed over the Ni/perovskite catalysts,
and was more easily eliminated from the catalysts. Ni/
a-Al2O3 showed rather low coke formation comparedto spc-Ni/Ca0.8Sr0.2TiO3 after the pulse treatment.
However, no substantial elimination of coke from
Ni/a-Al2O3 was observed during the He treatment,coinciding well with the low amount of mobile oxygen
in the a-Al2O3 in Table 3. It is thus likely that both Ni/Ca0.8Sr0.2TiO3 and Ni/BaTiO3 are effective catalysts
for CO2 reforming of CH4 also from the view point of
high oxygen mobility, endowing the catalysts with
sustainability against coking.
Fig. 7. CO formation in the CO2 pulse reaction over Ca0.8Sr0.2TiO3and BaTiO3 after the H2 reduction (at 8508C and with CO2 pulses(1 ml15)).
Table 3
Mobile oxygen in the supports
Support Mobile oxygena (%)
spc-Ca0.8Sr0.2TiO3 0.423
spc-BaTiO3 0.235
spc-TiO2 0.086
ZrO2 0.019
MgO 0.014
a-Al2O3 0.010SiO2 0.0002
a Percentage in total oxygen in the support.
Table 4
Amount of coke after treatmenta
Catalyst Amount of coke (wt%) with treating time (h)
0 4 19
spc-Ca0.8Sr0.2TiO3 0.29 0.22 0.11
spc-Ni/Ca0.8Sr0.2TiO3 1.01 1.04 0.30
spc-Ni/BaTiO3 0.65 0.59 0.15
Ni/a-Al2O3 0.62 0.56 0.66a Coke was deposited over the catalyst sample by CH4 (1 ml10)pulses at 8508C and then treated under the He atmosphere at8508C.
T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285 281
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3.8. TGA analyses of the spc-Ni/perovskite catalysts
The amount of surface Ni on the catalyst can be
estimated by the weight change during its reduction
oxidation treatment. TGA measurements of the cata-
lysts were carried out by using CH4 or H2 gas as the
reducing agent. Some details of TGA measurements
of the Ni/BaTiO3 are shown in Fig. 8. The catalyst was
put in the sample holder and heated to 8508C under N2atmosphere by increasing the temperature (dotted
line) at a rate of 308C/min. Around 10 min afterattaining the constant temperature of 8508C, 10%CH4 in N2 was introduced at the flow rate of
1.3 l h1. A sharp decrease followed by a slowincrease in the weight (solid line) was observed just
after the introduction of CH4 gas. The sharp decrease
may be due to the reduction of surface Ni2 species onthe catalyst, while the slow increase is due to the
coking over the catalyst. If we assume that all Ni2 isbound to oxygen atom which can be released by the
reduction of Ni2 to Ni0, the weight decrease can becalculated as 1.37 wt%, considering the amount of Ni
(Ni/Ti0.2/1) in the catalyst. The actual value of1.20% suggests that 87.1% of Ni in the catalyst
appeared on the surface. When the surface Ni, i.e.,
87.1% of total Ni in the spc-Ni/BaTiO3 catalyst, was
covered by carbon of 3.01 wt%, the amount of carbon
corresponded to 12.3 mol/mol-Ni (Fig. 8). The
amount of carbon (1.3 wt%) observed over the spc-
Ni/BaTiO3 after the catalytic testing was 1.33 mol/
mol-Ni (Table 2), suggesting that CO2 suppress the
coke formation. A much higher value of the surface Ni
was obtained with the spc-Ni/Ca0.8Sr0.2TiO3 catalyst
from the TGA analyses (Table 5). The imp-Ni/
Ca0.8Sr0.2TiO3 showed the value of 100% suggesting
the presence of all Ni species on the surface. On the
spc-Ni/BaTiO3, the weight increase by coking reached
a saturated value of 3.01% and then decreased quite
slowly under the CH4 atmosphere. By replacing 10%
CH4 in N2 with 10% CO2 in N2 at the same flow rate, a
sharp decrease in the catalyst weight followed by a
slow one were observed. This may be due to Bou-
douard reaction to form CO from surface coke and
CO2.
The reduction with CH4 was followed by the quick
coking, and therefore a question arises about the
accuracy of the amount of surface Ni calculated from
the weight decrease. Use of H2 instead of CH4 resulted
in much simpler behavior. The H2 reduction was
performed by increasing the temperature from room
temperature to 9008C at a rate of 208C/min under 10%H2 in N2 at the flow rate of 1.3 l h
1. Weight decreaseof the catalyst by the reduction of surface Ni occurred
around 4008C and finished around 7008C, and nosubstantial change was then observed. The imp-Ni/
Ca0.8Sr0.2TiO3 clearly showed the presence of all Ni
species on the surface. Both the spc-Ni/Ca0.8Sr0.2TiO3and the spc-Ni/BaTiO3 showed the values of 98.7 and
93.1 wt% in the weight decrease, respectively, sug-
gesting that a substantial part of Ni still appeared on
the surface by the reduction. The difference observed
between the spc-Ni/Ca0.8Sr0.2TiO3 and the spc-Ni/
BaTiO3 well explains the fact that much more Ni is
contained in the latter than in the former. This differ-
ence was again observed in the XRD measurements of
the samples after the treatment in a mixture of H2
Fig. 8. TGA of the spc-Ni/BaTiO3 under 10% CH4 followed by
10% CO2 in N2 gas flow.
Table 5
Amount of Ni on the catalyst surfacea
Catalyst Surface Ni (%)
H2 reduction CH4 reduction
imp-Ni/Ca0.8Sr0.2TiO3 100 100
spc-Ni/Ca0.8Sr0.2TiO3 98.7 97.0
spc-Ni/BaTiO3 93.1 87.1
a Calculated from TGA results assuming that only the surface
Ni2(O) can be reduced to Ni0 during the reduction by H2 or CH4.
282 T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285
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(1.0 l h1) and N2 (1.4 l h1) at 8508C for 1 h, i.e., the
spc-Ni/Ca0.8Sr0.2TiO3 showed sharp lines of Ni metal,
while the Ni lines in the spc-Ni/BaTiO3 were negli-
gibly small.
3.9. Highly dispersed Ni particles on perovskite
supports
We reported the formation of highly dispersed and
stable Ni metal particles on the Ni/perovskite catalyst
prepared by the spc method [1618]. TEM observa-
tions of imp-Ni/Ca0.8Sr0.2TiO3 and spc-Ni/Ca0.8Sr0.2-TiO3 after the reaction for 30 h are shown in Fig. 9. In
both cases, catalyst particles are composed of agglom-
erates of oval-shaped single crystals of Ca0.8Sr0.2TiO3perovskite (100140 nm). The spc-Ni/Ca0.8Sr0.2-TiO3 clearly showed many tiny dark spots (Fig. 9(a),
3 and 4) together with Ni metal particles of diameter
between 10 and 40 nm (Fig. 9(a), Ni 1 and 2), while
the imp-Ni/Ca0.8Sr0.2TiO3 showed no dark spots
(Fig. 9(b), 3 and 4) but only the Ni metal particles
(Fig. 9(b), Ni, Ni 1 and 2). The Ni metal particles of
1040 nm were clearly separated from the perovskite
single crystals in the imp-Ni/Ca0.8Sr0.2TiO3. In the
spc-Ni/Ca0.8Sr0.2TiO3, the dark spots were composed
of highly dispersed Ni metal particles (diame-
ter
-
preparation of the precursor. spc-Ni/BaTiO3 showed
the tiny dark spots and no Ni metal particles, and TEM
image under high magnification suggests that the tiny
spots were composed of the agglomerates of fine
nickel particles (diameter
-
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