Structural change of Ni species in Ni/SiO2 catalyst during decomposition of methane

Article abstract of DOI:10.1006/jcat.2002.3523

A study on the structural change of Ni species in a Ni/SiO₂ catalyst during CH₄ decomposition into hydrogen and carbon was carried out. The Ni/SiO₂ catalyst decomposed CH₄ actively into hydrogen and filamentous carbon at initial periods of the reaction. The catalytic activity for the reaction decreased with time on stream of CH₄ and finally the catalyst was deactivated completely. Prior to the contact of CH₄ with the Ni/SiO₂ catalysts, Ni species on the catalyst were present as Ni metal mainly. Ni metal particles were aggregated as soon as the Ni/SiO₂ came into contact with CH₄. After this initial aggregation, the Ni metal structure did not change appreciably when the Ni/SiO₂ was decomposing methane actively. The formation of some nickel carbide species on the catalyst was suggested when significant deactivation of the catalyst was observed.

Full text of DOI:10.1006/jcat.2002.3523

Journal of Catalysis 208, 54–63 (2002)
doi:10.1006/jcat.2002.3523, available online at http://www.idealibrary.com on
Structural Change of Ni Species in Ni/SiO Catalyst
2
during Decomposition of Methane
1
Sakae Takenaka, Hitoshi Ogihara, and Kiyoshi Otsuka
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute
of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received July 26, 2001; revised January 15, 2002; accepted January 15, 2002
Because no CO was contained in the products obtained by
The structural change of Ni species in a Ni/SiO catalyst during methane decomposition, hydrogen can be utilized directly
2
methane decomposition into hydrogen and carbon was investigated as the fuel for the H –O cells. The carbon formed by the
2
2
by X-ray diffraction (XRD), scanning electron microscopy, and Ni
K-edge X-ray absorption near-edge spectroscopy/extended X-ray
absorption fine structure (XANES/EXAFS). The Ni/SiO2 cata-
lyst decomposed methane actively into hydrogen and filamentous
carbon at initial periods of the reaction. The catalytic activity for the
reaction decreased with time on stream of methane and finally the
catalyst was deactivated completely. Prior to the contact of methane
methane decomposition should be used for the synthesis
of useful chemicals through gasification with water (4) or
CO2 (5). The carbons can be used also as functional ma-
terials, such as fibers, graphite, carbon black, composites,
electrodes, and so forth. Recent studies have demonstrated
clean hydrogen production via stepwise steam reforming
of methane, i.e., decomposition of methane over supported
Ni catalyst followed by steam gasification of surface carbon
with the Ni/SiO catalyst, Ni species on the catalyst were present
2
as Ni metal mainly. XRD analyses showed that Ni metal particles
were aggregated as soon as the Ni/SiO2 came into contact with on the catalyst (6, 7).
methane. After this initial aggregation, the structure of the Ni metal
Concerning catalysts for the decomposition of methane,
did not change appreciably when the Ni/SiO was decomposing it is well-known that supported Ni is one of the effective
2
methane actively. Ni K-edge XANES/EXAFS suggested the forma-
tion of some nickel carbide species on the catalyst when significant
catalysts (8–11). We also confirmed the high catalytic activ-
ity and long life of a Ni/SiO2 catalyst for the decomposition
deactivation of the catalyst was observed.
ꢀc 2002 Elsevier Science (USA)
of methane, especially when a fumed silica (Cab–O–Sil)
was utilized as a support (12). In general, hydrogen only is
formed as a gaseous product and carbons deposited on the
catalystgrowwithafilamentousstructureinthedecomposi-
tion of methane over a Ni/SiO2 catalyst. The catalytic activ-
ity of Ni/SiO2 for methane decomposition decreased gradu-
Key Words: silica-supported Ni catalyst; methane decomposition;
hydrogen; carbon filament; nickel metal; nickel carbide.
INTRODUCTION
Hydrogen is a clean fuel in the sense that no CO2 is ally with time on stream of methane and finally the catalyst
emitted when it is used in a H2–O2 fuel cell. H2–O2 cells was deactivated completely. The reaction mechanisms for
such as phosphoric acid and SPE fuel cells require the thor- methane decomposition over supported Ni catalysts were
ough elimination of carbon monoxide (CO) from the fuel studied by many research groups (13–16). In these stud-
(
H2), because CO poisons strongly the electrocatalysts in ies, the growth mechanisms of the carbon filaments during
the cells. At present, hydrogen is produced mainly through methane decomposition were investigated in detail, but lit-
steam reforming and partial oxidation of natural gas. Be- tle attention was focused on the structural change of Ni
cause the hydrogen produced by these processes inevitably species in the catalysts during methane decomposition. We
contains CO beyond a tolerable range, removal of CO expected that the deactivation of the supported Ni cata-
through a water–gas shift reaction of CO followed by selec- lyst might be caused by the structural change of Ni species
tive CO oxidation should be required. Thus, the purification during methane decomposition.
processes would make the fuel reformer more bulky and
expensive.
In the present study, we investigated the structural
change of Ni species during methane decomposition over
Decomposition of methane into hydrogen and carbon Ni/SiO catalysts by means of X-ray diffraction (XRD)
2
is of current interest from the viewpoint of an alterna- and Ni K-edge X-ray absorption near-edge spectroscopy/
tive route of hydrogen production from natural gas (1–3). extended X-ray absorption fine structure (XANES/
EXAFS). In addition, the appearance of a Ni/SiO2 catalyst
1
during methane decomposition was observed by SEM
and BEI (back-scattering electron image). Especially, we
To whom correspondence should be addressed. Fax: 81-35734-2879.
E-mail: stakenak@o.cc.titech.ac.jp.
54
0
ꢀ
021-9517/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.

STRUCTURAL CHANGE OF Ni SPECIES DURING CH4 DECOMPOSITION
55
focused on the structural change of Ni species during the
deactivation stage of the catalyst and discussed the deacti-
vation mechanism of the catalyst.
EXPERIMENTAL
A silica-supported Ni catalyst (Ni/SiO2) was prepared by
impregnating SiO2 (Cab–O–Sil; fumed silica obtained from
2
−1
CABOT Co.; specific surface area = 200 m · g ) with an
aqueous solution of Ni(NO3)2 · 6H2O at 353 K. The im-
pregnated sample was further dried at 393 K for 12 h and
calcined in air at 873 K for 5 h.
The decomposition of methane over the Ni/SiO2 cata-
lyst was carried out with a gas-flow system at atmospheric
pressure. The catalyst was placed at the bottom of a quartz
reactor (internal diameters: 2.7 cm for vertical, 2 cm from
the bottom, 1.5 cm for upward, 10 cm). Prior to the reac-
tion, the catalyst was reduced with hydrogen at 823 K for
1
h. Decomposition of methane was performed by contact-
−
1
ing methane (P(CH4) = 101 kPa, flow rate = 60 ml · min )
with the catalyst (0.040 g). Hydrogenation of the carbons
FIG. 1. Kinetic curves of the formation rates of hydrogen in methane
deposited on the Ni/SiO2 was carried out successively with decomposition over Ni (5 wt%)/SiO and Ni (10 wt%)/SiO catalysts at
2
2
−
1
8
03 K. P(CH4), 101 kPa; flow rate, 60 ml · min ; catalyst, 0.040 g.
the same apparatus after the Ni/SiO2 catalyst had been
deactivated completely for the methane decomposition.
The hydrogenation of the carbons predeposited on the with loading amount of 5 and 10 wt% as Ni metal. In the
Ni/SiO2 catalyst was initiated by contact with hydrogen reactions, only hydrogen was detected as a gaseous pro-
−
1
(
P(H2) = 101 kPa, 70 ml · min ) at 843 K. During both the duct. In the early period of the reactions, the formation
decomposition of methane and the hydrogenation of car- rates of hydrogen were kept high. The rates decreased
bons, some of the gases in a stream out of the catalyst-bed sharply with time on stream and formation of hydrogen
was sampled out to analyze the gaseous products by gas could not be observed at 240 and 400 min of time on stream
chromatography (GC).
2 2
for Ni (5 wt%)/SiO and Ni (10 wt%)/SiO , respectively.
X-ray absorption experiments were carried out on the The values of C/Ni (the mole of decomposed methane di-
beamline BL-9A at the Photon Factory in the Institute of vided by that of all nickel atoms in the catalyst) during
Materials Structure Science for High Energy Accelerator complete deactivation of the catalysts were estimated to be
Research Organization, Tsukuba, Japan, with a ring energy 1100 and 1280 for Ni (5 wt%)/SiO and Ni (10 wt%)/SiO ,
2
2
of 2.5 GeV and a stored current of 250–450 mA (Proposal respectively.
000G074). The X-ray absorption spectra of the catalysts
After the methane decomposition over Ni/SiO catalysts
2
2
were recorded in a fluorescence mode, and those of the refe- shown in Fig. 1, hydrogenation of the carbons deposited on
rence samples were recorded in a transmission mode with a the catalysts was performed at 843 K. During this hydro-
Si(111) two-crystal monochromator at room temperature. genation, methane only was formed as a gaseous product.
Normalization of XANES and data reduction on EXAFS
were performed as described elsewhere (17).
X-ray diffraction (XRD) patterns of the catalysts were
measured by a Rigaku RINT 2500V diffractometer using
Cu Kα radiation.
SEM image and back-scattering electron image of the
catalyst were measured using a Hitachi FE-SEM S-800
Characterization of the Catalysts by XRD
We measured XRD patterns of the Ni/SiO2 catalysts af-
ter the decomposition of methane in order to investigate
the structural changes of Ni species during the reaction. Ni
(
10 wt%)/SiO2 was used for the measurements of XRD pat-
terns in order to get diffraction patterns due to Ni species
with higher signal-to-noise (S/N) ratios. Prior to measure-
ment of the XRD patterns of the Ni/SiO2 catalyst, methane
decomposition was performed at 803 K over the catalyst.
The results are depicted in Fig. 2. In the XRD pattern
(field emission gun scanning electron microscope).
RESULTS AND DISCUSSION
Decomposition of Methane over Ni/SiO2 Catalyst
of the Ni/SiO2 catalyst after reduction with hydrogen at
◦
Figure 1 shows the kinetic curves of formation rates of H2 823 K (denoted as fresh catalyst), the peaks at 44 and 52
in methane decomposition at 803 K over Ni/SiO2 catalysts are assigned to Ni metal, indicating that Ni metal particles

5
6
TAKENAKA, OGIHARA, AND OTSUKA
of methane decomposition (C/Ni ≤ 14) at 803 K. The mean
particle sizes of Ni metal were estimated from full width at
◦
half maximum of the diffraction line due to Ni(111) at 44.5
using the Scherrer equation. Before contact of methane
with the Ni (10 wt%)/SiO2 catalyst, the mean particle size of
Ni metal was estimated to be 24 nm. On contact of methane
with the catalyst, the mean particle sizes of Ni increased
from 24 to 27 nm in the range of C/Ni < 4, and the particle
size became almost constant in the range of C/Ni > 4. These
results suggest that the aggregation of Ni metal particles on
the catalyst occurs by contact of methane with the Ni/SiO2
catalyst.
Figure 2 shows also the XRD pattern of the Ni (10 wt%)/
SiO2 catalyst after complete deactivation for the methane
decomposition. In the XRD pattern, the peaks due to
graphite structure were observed. It should be noted that
the peaks are very broad. Therefore, the crystallinity of the
carbon deposited on the catalyst should be low (18). In
the XRD pattern of the deactivated Ni/SiO2 catalyst, no
diffraction peak due to Ni species such as Ni metal or some
nickel carbide species was observed. This could be due to
the formation of amorphous nickel species after complete
deactivation of the catalyst or to the low concentration of
Ni species in the Ni/SiO2 catalyst deactivated completely.
That is, in the case of methane decomposition over the Ni
FIG. 2. XRD patterns of graphite and of Ni (10 wt%)/SiO2 catalysts
before and after methane decomposition at 803 K.
(
10 wt%)/SiO2 catalyst, the amount of Ni in the catalyst
are present on the Ni/SiO2 catalyst before contact with was changed from 10 wt% for the fresh catalyst to 0.4 wt%
methane. For the XRD pattern of the catalyst with depo- for the deactivated one due to a large number of deposited
◦
sited carbons of C/Ni = 6, a small peak was found at 26 . carbons.
◦
The peak at 26 is assigned to graphite, indicating that the
carbon deposited on the Ni/SiO2 catalyst has a graphitelike
structure (4).
Ni K-Edge XANES/EXAFS of Ni/SiO2 Catalyst
Figure 3 shows the change in mean particle size of Ni
metal in the Ni (10 wt%)/SiO2 catalyst at a very early period
Figure 4 shows Ni K-edge XANES spectra of the fresh
Ni/SiO2 catalyst, the catalysts after methane decomposition
at 803 K, and a Ni foil. For measurements with Ni K-edge
XANES/EXAFS, Ni (5 wt%)/SiO2 was used. The XANES
spectrum of the fresh Ni/SiO2 (C/Ni = 0) was compatible
with that of a Ni foil, indicating that Ni species in the fresh
catalyst were present as Ni metal mainly (19). The result
was consistent with that deduced from the XRD pattern
shown in Fig. 2. The XANES spectra of Ni/SiO2 catalysts
after methane decomposition did not change in the range
of C/Ni = 49–650 and the spectra were similar to that of the
fresh catalyst. In the range of C/Ni ≤ 650 for methane de-
composition over the Ni (5 wt%)/SiO2 catalyst, the activity
of the catalyst was kept high. Therefore, we consider that
the structure of Ni species did not change appreciably when
the Ni/SiO2 catalyst was decomposing methane actively.
On the other hand, in the XANES spectrum of the
Ni/SiO2 catalyst with deposited carbons of C/Ni = 900, two
shoulder peaks appeared, at 8332 and 8341 eV, and the two
peaks became more intense with an increase in C/Ni. Fur-
thermore, two peaks, at 8349 and 8358 eV, which were ob-
served in the XANES spectra of the fresh Ni/SiO2 cata-
lyst and a Ni foil, became fainter with an increase in C/Ni,
FIG. 3. Change in mean particle size of Ni metal in the Ni (10 wt%)/
SiO2 catalyst during methane decomposition.

STRUCTURAL CHANGE OF Ni SPECIES DURING CH4 DECOMPOSITION
57
spectra became intense compared with that of Ni foil, and
two peaks, at 8349 and 8358 eV, became fainter than those
of Ni foil. As described in Fig. 5, Ni3C was present in the
Ni metal powder treated with CO. The formation of Ni–C
bonds by treating of Ni metal powder with CO would bring
about these changes in the XANES spectra. These changes
in the XANES spectra by treating of Ni metal powder with
CO were similar to those in the XANES spectra observed
during deactivation of the Ni/SiO2 catalyst (C/Ni ≥ 900), as
shown in Fig. 4, suggesting the formation of Ni–C bonds in
Ni metal particles during deactivation of the Ni/SiO2 cata-
lyst for methane decomposition.
We further investigated the structural change of Ni
species during the deactivation of the Ni/SiO2 catalyst for
methane decomposition by means of XANES spectra of
the Ni/SiO2 catalysts treated with hydrogen. After the Ni
(
5 wt%)/SiO2 catalysts had been deactivated completely
for methane decomposition at 803 K, the deactivated cata-
lysts were treated with hydrogen at 843 K, which produced
methane only. The XANES spectra of the Ni/SiO2 catalysts
thus treated are shown in Fig. 7. Treatment of the deacti-
vated Ni/SiO2 catalysts with hydrogen brought about the
change in the Ni K-edge XANES spectra. The feature of
the XANES spectra of the Ni/SiO2 treated with hydrogen
approached gradually that of the fresh catalyst shown in
FIG. 4. Ni K-edge XANES spectra of the fresh Ni (5 wt%)/SiO2 cata-
lyst, the catalysts with carbons deposited by methane decomposition, and Fig. 4, as carbons deposited on the Ni/SiO catalyst were
2
a Ni foil. (a) Ni foil, (b) Ni/SiO2 of C/Ni = 0 (fresh catalyst), (c) C/Ni = 49,
(
d) C/Ni = 300, (e) C/Ni = 650, (f) C/Ni = 900, (g) C/Ni = 1000, and
h) C/Ni = 1100 (complete deactivation).
(
in the range C/Ni ≥ 900. These results strongly suggest
that the structure of Ni species changed significantly in
the range C/Ni ≥ 900, where the conversion of methane
decreased rapidly in methane decomposition over the Ni
(
5 wt%)/SiO2 catalyst. Thus, the structural change of Ni
species implied by the XANES spectra might be related to
the deactivation of the catalyst for methane decomposition.
This structural change of Ni species during the deactiva-
tion of the Ni/SiO2 catalyst could be caused by the forma-
tion of nickel carbide species such as Ni3C (20). In order to
examine this hypothesis, a nickel carbide species was pre-
pared by a method in which Ni metal powder comes into
contact with CO at 540 K (21). The samples were measured
by XRD and Ni K-edge XANES/EXAFS. Figure 5 shows
the XRD patterns of the Ni metal powder samples treated
with CO at 540 K for 50 and 100 h, and a Ni metal powder.
For the XRD patterns of the Ni samples treated with CO,
diffraction peaks due to Ni3C were found at 2θ = 40, 42,
◦
4
5, 58, 72, and 78 , in addition to peaks due to the Ni metal.
The peaks due to Ni3C became intense with the treatment
time with CO. Figure 6 shows the Ni K-edge XANES spec-
tra of the Ni metal powder samples treated with CO, the
Ni/SiO2 catalyst deactivated completely for the methane
decomposition, and a Ni foil. On treatment of Ni metal
FIG. 5. XRD patterns of Ni metal powder and Ni metal powder sam-
ples treated with CO at 540 K. (a) Ni metal powder; (b, c) Ni metal powders
powder with CO, absorption at 8332 eV in the XANES treated with CO at 540 K for 50 and 100 h, respectively.

5
8
TAKENAKA, OGIHARA, AND OTSUKA
For EXAFS spectra of the Ni/SiO2 catalyst with deposited
carbons of C/Ni = 49–650, the intensity and pattern of the
oscillation were almost consistent with those of the EXAFS
spectra of the fresh Ni/SiO2 catalyst. Therefore, the struc-
ture of Ni metal on the Ni (5 wt%)/SiO2 did not change
appreciably in the range of C/Ni = 49–650 of the methane
decomposition. On the other hand, in the range of C/Ni ≥
900, the intensities of the oscillation of the EXAFS spec-
trum became smaller with an increase in C/Ni, although
the pattern of the oscillation did not change with the val-
ues of C/Ni. These results may imply a slight distortion of
the structure of Ni metal during the significant deactivation
of the Ni (5 wt%)/SiO2 for the decomposition of methane
(
C/Ni ≥ 900).
3
Figure 9 shows k -weighted Ni K-edge EXAFS spectra of
the Ni (5 wt%)/SiO2 with deposited carbons. The Ni/SiO2
catalysts, which had been deactivated for the methane de-
composition, were treated with hydrogen at 843 K. The am-
plitude of the EXAFS oscillation of the deactivated Ni/SiO2
catalyst became more intense as the hydrogenation of car-
bons deposited on the catalyst into methane proceeded.
The result suggests that the distortion of the structure of Ni
species observed in the deactivated Ni/SiO2 catalyst was re-
moved by the treatment with hydrogen. This indicates that
FIG. 6. Ni K-edge XANES spectra of Ni foil (a), Ni metal powder
treated with CO at 540 K for 50 (b) and 100 h (c), and the Ni (5 wt%)/
SiO2 catalyst deactivated completely by the methane decomposition at
8
03 K (d).
hydrogenated into methane. The intensities of shoulder
peaks at 8332 and 8341 eV, which had appeared during the
serious deactivation of the catalyst (C/Ni ≥ 900) for the
methane decomposition (Fig. 4), became weak, and two
peaks, at 8349 and 8358 eV, which were observed in the
XANES spectra of the fresh Ni/SiO2 and Ni foil, became
clearer on the hydrogenation of carbons. These results in-
dicate that Ni species on the deactivated Ni/SiO2 catalyst
are recovered to Ni metal by treatment with hydrogen. The
treatment of the deactivated catalyst with hydrogen pro-
duced methane only and brought about the changing of
some Ni species in the deactivated catalyst to Ni metal.
Therefore, we consider that the structural change of Ni
species in the Ni/SiO2 catalyst during the serious deactiva-
tion for methane decomposition was caused by the carbon
atoms deposited, i.e., the formation of some nickel carbide
species.
3
Figure 8 shows k -weighted Ni K-edge EXAFS spectra
FIG. 7. Ni K-edge XANES spectra of Ni (5 wt%)/SiO2 catalysts with
deposited carbons. (a) The Ni/SiO2 catalyst after complete deactivation
for the methane decomposition at 803 K (C/Ni = 1100). (b–d) Carbons
on Ni/SiO2 catalysts which had been deactivated completely for methane
decomposition (C/Ni = 1100) were hydrogenated into methane at 843 K
of the Ni (5 wt%)/SiO2 catalysts before and after methane
decomposition at 803 K. The feature of the EXAFS spec-
tra of the fresh Ni/SiO2 catalyst was compatible with that
of Ni foil, indicating that Ni species on the fresh Ni/SiO2
was present as Ni metal mainly (19), as described in Fig. 4. to C/Ni = 1090, 1070, and 1020, respectively.

STRUCTURAL CHANGE OF Ni SPECIES DURING CH4 DECOMPOSITION
59
the Fourier transforms (radial structure function (RSF)) of
3
k -weighted Ni K-edge EXAFS of Ni (5 wt%)/SiO2 cata-
lysts without and with deposited carbons, and Ni foil. A
peak due to Ni–Ni bond was observed at 1.8–2.5 A˚ in all
RSFs in Fig. 10. In the range of C/Ni ≥ 900, the intensities
of the peak due to Ni–Ni became smaller gradually with
an increase in C/Ni, whereas the significant change in the
intensityofthepeakwasnotobservedintherangeofC/Ni≤
6
50. In addition, a peak was found at 1.2–1.8 A˚ in the RSFs
of the Ni/SiO2 catalysts with deposited carbons of more
than C/Ni = 900, and the peak became more intense with
an increase in the C/Ni. This peak should be ascribed to a
Ni–C bond, taking into account the results of the XANES
described above.
Figure 11 depicts the Fourier transforms of the Ni/SiO2
catalyst treated with hydrogen after complete deactivation
for the methane decomposition. It was found that the in-
tensities in the peak due to a Ni–Ni bond at 1.9–2.7 A˚ be-
came stronger as the carbons deposited on the catalyst were
FIG. 8. k3-weighted Ni K-edge EXAFS spectra of Ni (5 wt%)/SiO2
catalysts with deposited carbons. (a) Ni foil, (b) C/Ni = 0 (the fresh cata-
lyst), (c) C/Ni = 49, (d) C/Ni = 300, (e) C/Ni = 650, (f) C/Ni = 900,
(g) C/Ni = 1000, and (h) C/Ni = 1100 (deactivated catalyst).
the carbon atoms deposited on the catalyst cause the distor-
tion of the structure of Ni metal. This could be ascribed to
the formation of some nickel carbide species during the sig-
nificant deactivation of the Ni/SiO2 catalyst for the methane
decomposition, as suggested earlier.
The EXAFS spectra of the Ni metal powder samples
treated with CO for 50 and 100 h are shown in Figs. 9e
and 9f, respectively. In the EXAFS spectra of the Ni metal
powder samples treated with CO, the intensities of the oscil-
−1
lations at 6.0, 7.5, 9.8, and 11.2 A˚ are stronger than those
of the EXAFS spectra of the deactivated Ni/SiO2 catalyst.
−
1
In addition, the intensities at 6.0, 7.5, 9.8, and 11.2 A˚
became stronger as the time for the treatment of Ni metal
powder with CO was lengthened. The feature of these
EXAFS spectra of Ni samples treated with CO, which con-
sist of a mixture of Ni metal and Ni3C, was quite different
from that of the EXAFS spectra of the deactivated Ni/SiO2
catalyst. Therefore, the nickel carbide species present on
FIG. 9. k3-weighted Ni K-edge EXAFS spectra of Ni (5 wt%)/SiO2
the deactivated Ni/SiO2 catalyst was not a Ni3C compound catalysts with deposited carbons, and Ni samples treated with CO
at 540 K. (a) The Ni/SiO2 catalyst after complete deactivation for
itself.
the methane decomposition (C/Ni = 1100). (b–d) Carbons on Ni/SiO2
Data on the structural changes around Ni atoms during
methane decomposition have been strongly supported by
catalysts which had been deactivated for methane decomposition
(
C/Ni = 1100) were hydrogenated at 843 K to C/Ni = 1090, 1070,
3
performing Fourier transform for k -weighted EXAFS of
the Ni/SiO2 catalysts, shown in Figs. 8 and 9. Figure 10 shows 540 K for 50 and 100 h, respectively.
and 1020, respectively. (e, f) Ni metal powders treated with CO at

6
0
TAKENAKA, OGIHARA, AND OTSUKA
sizes of Ni metal particles than that on the fresh catalyst,
before the formation of carbon filaments. The aggregation
of Ni species on contact with methane was also observed in
the case of a nickel–magnesia catalyst (23) and Ni/Al2O3
catalyst (24).
Figure 13 depicts the SEM images and BEIs of the
Ni/SiO2 catalysts with deposited carbons of C/Ni = 49, 200,
and 600. In the range of C/Ni = 49–600, the activity of the
Ni (5 wt%)/SiO2 catalyst for the methane decomposition
was kept high. In the SEM picture of the Ni/SiO2 cata-
lyst with deposited carbons of C/Ni = 49, it was found that
carbons on the catalyst grew with a filamentous structure (4,
9–16). The filamentous carbons became longer as the value
of C/Ni increased. All the BEIs in Fig. 13 show that a Ni
metal particle is present on the tip of each carbon filament
(
4, 9–16). The Ni metal on the tip of the carbon filament
decomposes methane and grows the length of the carbon
filament successively.
The Ni K-edge XANES spectra of the Ni/SiO2 with de-
posited carbons of C/Ni = 49–650 indicated almost the same
feature irrespective of the C/Ni value (Fig. 4). In addition,
3
FIG. 10. Fourier transforms of k -weighted Ni K-edge EXAFS spec-
tra of Ni (5 wt%)/SiO2 catalysts with deposited carbons. (a) Ni foil,
(b) C/Ni = 0 (the fresh catalyst), (c) C/Ni = 49, (d) C/Ni = 300,
(e) C/Ni = 650, (f) C/Ni = 900, (g) C/Ni = 1000, and (h) C/Ni = 1100
(deactivated catalyst).
hydrogenated into methane (Figs. 11a–11d). These results
indicate that the treatment of the deactivated catalyst with
hydrogen brings about the change of nickel carbide species
to Ni metal.
SEM Images of Ni/SiO2 Catalysts
Figure 12 shows the SEM images and BEIs (back-
scattering electron images) of the Ni (5 wt%)/SiO2 with
deposited carbons of C/Ni = 1 and 10. The BEIs were mea-
sured at the same time as the SEM pictures shown on left
side. The BEIs in Fig. 12 show a number of bright spots.
The spots indicate the presence of Ni atoms on the cata-
lysts. In the SEM pictures in the range of C/Ni ≤ 10, no
carbon filament was observed on the catalyst, while in the
range of larger values of C/Ni, deposited carbons from the
methane decomposition grew with filamentous structure,
as is described later.
3
FIG. 11. Fourier transforms of k -weighted Ni K-edge EXAFS spec-
In the range of C/Ni ≤ 10, it was found that a mean parti-
cle size of Ni metal became larger with an increase in C/Ni,
as shown in Fig. 3. The observation strongly suggests that
Ni metals would be aggregated and crystallized firmly by
tra of Ni (5 wt%)/SiO2 catalysts with deposited carbons, and Ni samples
treated with CO at 540 K. (a) The deactivated catalyst (C/Ni = 1100);
(
b–d) the Ni/SiO2 catalysts after the hydrogenation of deposited carbons
from C/Ni = 1100 to 1090, 1070, and 1020, respectively; (e, f) Ni samples
the contact of methane with Ni/SiO2, forming larger crystal treated with CO at 540 K for 50 and 100 h, respectively.

STRUCTURAL CHANGE OF Ni SPECIES DURING CH4 DECOMPOSITION
61
the tip of a carbon fiber was covered with carbon layers (in
the circle in the SEM). The deposited carbon layers on the
surface of the Ni metal should interfere with the contact of
methane with Ni metal.
FIG. 12. SEM and BEIs of Ni (5 wt%)/SiO2 catalysts with carbons
deposited of C/Ni = 1 and 10. The carbons were deposited on the Ni
(5 wt%)/SiO2 catalyst by methane decomposition at 803 K.
the pattern and amplitude of the EXAFS oscillation of
Ni/SiO2 with deposited carbons did not change in the range
of C/Ni = 49–650 (Fig. 8). These results suggested that the
structure of Ni metal did not change when the catalytic
activity of Ni/SiO2 for the methane decomposition was
kept high.
Figure 14 shows SEM images and BEIs of the Ni/SiO2
catalysts with deposited carbons of C/Ni ≥ 900. In the range
of C/Ni ≥ 900, the Ni (5 wt%)/SiO2 catalyst was deactivated
significantly. In the SEM images, collisions between the side
wall of a carbon fiber and a Ni particle on the tip of another
carbon fiber were found. In SEM images of the Ni/SiO2
catalysts with deposited carbon of C/Ni < 900, some tips
of carbon fibers were found, as shown in Fig. 13, while the
number of nickel particles seen was decreased in SEM ima-
ges of the samples of C/Ni ≥ 900. This may be the result of
the collision between carbon filaments and the Ni particles
at the tips of the filament. However, we cannot exclude the
possibility that dilution of the Ni particles due to the growth
of carbon fibers also decreases the number of particles in
the visual field of the SEM. The images of the deactivated
FIG. 13. SEM and BEIs of Ni (5 wt%)/SiO2 catalysts with deposited
carbons of C/Ni = 49, 200, and 600. The carbons were deposited on the Ni
Ni/SiO2 shown in Fig. 14 indicate that a nickel metal on (5 wt%)/SiO2 catalysts by methane decomposition at 803 K.

6
2
TAKENAKA, OGIHARA, AND OTSUKA
FIG. 14. SEM and BEIs of Ni (5 wt%)/SiO2 catalysts with deposited carbons of C/Ni = 900 and 1100 (deactivated catalyst). The carbons were
deposited on the Ni (5 wt%)/SiO2 catalysts by methane decomposition at 803 K.
In the XANES spectra of the Ni/SiO2 catalyst with de- results described above, we concluded that some nickel
positedcarbonofC/Ni = 900, twoshoulderpeaksappeared, carbide species were formed on the Ni/SiO2 catalyst during
at 8332 and 8341 eV, and the two peaks became more in- the significant deactivation period in the methane decom-
tense with an increase in C/Ni, as shown in Fig. 4. The ampli- position. The shoulder peaks at 8332 and 8341 eV which ap-
tude of EXAFS oscillations of the Ni/SiO2 catalysts became peared in the XANES spectra of the deactivated Ni/SiO2
weaker with an increase in C/Ni, in the range of C/Ni ≥ catalysts would be assigned to the transition of an elec-
9
00. In addition, a peak which would be due to Ni–C ap- tron from Ni 1s to antibonding Ni–C. The density of car-
peared at 1.4 A˚ in the Fourier transforms of the Ni/SiO2 bon atoms in the Ni metal became higher as the catalyst
with deposited carbons of C/Ni ≥ 900 (Fig. 10). From the was more and more deactivated. Thus, the intensity of the

STRUCTURAL CHANGE OF Ni SPECIES DURING CH4 DECOMPOSITION
63
two shoulder peaks in the XANES should be stronger with
3. Ni species are changed from Ni metal to some Ni car-
the value of C/Ni. The increase in the density of carbon bide species during the deactivation period of the Ni/SiO2
atoms in the Ni metal during the deactivation of Ni/SiO2 catalyst for methane decomposition.
should cause the distortion of the structure of Ni metal.
The decrease in the amplitude of the EXAFS oscillations
of the Ni/SiO2 during the deactivation period can be as-
ACKNOWLEDGMENTS
cribed to interference between the slightly different en-
vironments of the Ni–Ni bonds for Ni metal and nickel
carbide species. The formation of a nickel carbide species
was also suggested from the experiments with the deacti-
vated Ni/SiO2 catalyst treated with hydrogen. On treatment
with hydrogen, the XANES and EXAFS spectra of the de-
activated catalyst were changed to those of the catalysts
before deactivation (Figs. 7, 9, and 11). Methane was the
only product in this hydrogen treatment for the deactivated
catalyst. The reaction between the nickel carbide species
with hydrogen must produce methane, regenerating Ni
metals.
On the other hand, the XANES and EXAFS spectra of
the Ni sample composed of Ni metal and Ni3C were differ-
ent from those of the deactivated Ni/SiO2 catalyst (Figs. 6, 9,
and 11). Therefore, the nickel carbide species formed on
the Ni/SiO2 catalyst during the deactivation period of the
catalyst for the methane decomposition was different from
The X-ray absorption experiment was performed under the approval of
the Photon Factory Program Advisory Committee (Proposal 2000G074).
The authors thank Prof. T. Tanaka in Kyoto University for kind help with
the X-ray absorption spectra.
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larger crystal sizes of Ni metal particles than that on the
fresh catalyst.
2
2
2
. The structure of Ni metal does not change signi-
ficantly when the Ni/SiO2 catalyst decomposes methane
actively.