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

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

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

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

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

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

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

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

(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

[25] A. Parmaliana, F. Arena, F. Frusteri, N. Giordano, J. Chem.

Soc., Faraday Trans. 86 (1990) 2663.

[26] F. Arena, B.A. Horrell, D.L. Cocke, A. Parmaliana, N.

Giordano, J. Catal. 132 (1991) 58.

[27] A. Parmaliana, F. Arena, F. Frusteri, S. Coluccia, L.

Marchese, G. Martra, A.L. Chuvilin, J. Catal. 141 (1993) 34.

[28] F. Arena, F. Frusteri, A. Parmaliana, L. Plyasova, A.N.

Shmakov, J. Chem. Soc., Faraday Trans. 92 (1996) 469.

[29] O. Yamazaki, K. Tomishige, K. Fujimoto, Appl. Catal. A 136

(1996) 49.

[30] K. Tomishige, K. Fujimoto, Catal. Surv. Jpn. 2 (1998) 3.

[31] A. Erdohelyi, J. Cserenyi, E. Rapp, F. Solymosi, Appl. Catal.

A 108 (1994) 205.

[32] T. Osaki, J. Chem. Soc., Faraday Trans. 93 (1997) 643.

[33] P. Meriandeau, O.H. Ellestad, M. Defaux, C. Naccache, J.

Catal. 75 (1982) 243.

[34] R. Burch, A.R. Flambard, J. Catal. 78 (1982) 389.

[35] Denki Kagaku Binran, The Electrochemical Society Japan,

Maruzen, Tokyo, 1988, p. 129.

T. Hayakawa et al. / Applied Catalysis A: General 183 (1999) 273285 285