Influences of precipitate rinsing solvents on Ni Catalyst for Methane Decomposition to COx-free hydrogen

Article abstract of DOI:10.1021/jp906053h

Influences of precipitate rinsing solvents on Ni for methane decomposition to CO_x-free hydrogen has been investigated in this study. Calcination of nickel hydroxide precipitates rinsed by ethanol leads to the formation of nanosheet needle-like NiO, whereas calcination of those rinsed by deionized water leads to the formation of pure nanosheet NiO. When compared to Ni catalyst (Ni-et) reduced from nanosheet needle-like NiO, Ni catalyst (Ni-etwt), reduced from pure nanosheet-like NiO, exhibits a better catalytic performance for methane decomposition. Among the different rinsing processes, nickel hydroxide precipitates, rinsed first by ethanol and subsequently by deionized water, were calcined to the most suitable nanosheet NiO, which could be reduced to Ni-etwt catalyst with the highest catalytic performance of methane decomposition. A total of 1954 mol_H2/mol_Ni of hydrogen yields could be obtained over Ni-etwt under suitable reaction conditions. Characterization results indicate that Ni-etwt with the higher catalytic activity has the ～34 nm of average particle size.

Full text of DOI:10.1021/jp906053h

3818
J. Phys. Chem. A 2010, 114, 3818–3823
Influences of Precipitate Rinsing Solvents on Ni Catalyst for Methane Decomposition to
COx-free Hydrogen†
W Zhang,‡,§ Qingjie Ge,* and Hengyong Xu‡,*
Laboratory of Applied Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, China
ReceiVed: June 29, 2009; ReVised Manuscript ReceiVed: August 23, 2009
Influences of precipitate rinsing solvents on Ni for methane decomposition to COx-free hydrogen has been
investigated in this study. Calcination of nickel hydroxide precipitates rinsed by ethanol leads to the formation
of nanosheet needle-like NiO, whereas calcination of those rinsed by deionized water leads to the formation
of pure nanosheet NiO. When compared to Ni catalyst (Ni-et) reduced from nanosheet needle-like NiO, Ni
catalyst (Ni-etwt), reduced from pure nanosheet-like NiO, exhibits a better catalytic performance for methane
decomposition. Among the different rinsing processes, nickel hydroxide precipitates, rinsed first by ethanol
and subsequently by deionized water, were calcined to the most suitable nanosheet NiO, which could be
reduced to Ni-etwt catalyst with the highest catalytic performance of methane decomposition. A total of
1954 molH2/molNi of hydrogen yields could be obtained over Ni-etwt under suitable reaction conditions.
Characterization results indicate that Ni-etwt with the higher catalytic activity has the ∼34 nm of average
particle size.
1. Introduction
°C, but also remain stable in a moist stream of normal feed
gas. The results show the importance of morphology control in
the preparation of metal oxides as high efficiency oxidation
catalysts. The morphologies of metal oxides depend on many
preparation parameters,11,22,27-33 such as the preparation medium,
deposition temperatures, rinsing methods, drying techniques, and
so forth. Among the many parameters, the choice of solvent is
a key factor impacting the morphologies of metal oxides during
the preparation processes, such as deposition,11,31,32 rinsing,22
and drying.30,33 There are many studies on the effect of solvents
during the deposition and drying processes.11,27,29-33 Most of
these studies concentrate on the production of highly dispersive
nanoparticles with low agglomeration level by choosing ap-
propriate solvents. However, studies on the influences of rinsing
solvents on the morphologies of metal oxides were rare. As far
as we know, only one work of correlative literature22 reported
that the sizes of zirconia particles could be smaller by rinsing
a conventional ZrO (OH)2 hydrogel with anhydrous ethanol.
But the influences of rinsing solvents on the shapes of metal
oxides have not yet been studied.
Concerns over the effect of global emissions of carbon oxides
(COx) and other local air pollutants, such as nitrogen oxides
(NOx) and sulfur oxides (SOx), on the environment have led to
significant research on the use of alternative clean energy
sources. As a clean energy source, hydrogen is an emerging
alternative to conventional fuels.1,2 Among many hydrogen
production reactions, the direct decomposition of methane over
metal-supported catalysts has been receiving widespread atten-
tion.3 Unlike other challenging processes,4,5 which produce a
mixture of hydrogen and carbon oxides, catalytic decomposition
of methane (CDM) produces COx-free hydrogen, which is highly
desirable especially for use in PEM fuel cells applications. Apart
from the principal product hydrogen, the value added byproduct
carbon was also obtained along with unconverted methane.6
Ni-based catalysts were widely used for methane decomposi-
tion due to their relatively higher activities compared with other
transition metals.7-10 Metallic nickels supported on oxides have
been the most commonly used catalysts for methane decomposi-
tion. Recently, pure nickel catalysts with high methane decom-
position activity have been successfully prepared,11 the activity
of which is comparable with the activity of supported nickel
catalysts. Furthermore, pure nickel catalysts could improve the
overall application efficiency of the byproduct carbon nanofibers,
because there is no need to remove or separate the burdensome
support materials, like Al2O3 and SiO2.12-14
It was reported34 that the supersaturation degree was different
for different solvents, which would influence the nucleation
numbers, finally leading to the formation of species with
different morphologies. As for common organic rinsing solvents,
ethanol has a smaller permittivity than water; the supersaturation
degree of precipitates in ethanol could be lowered, and thus
would result in the formation of fine particles. However,
compared with deionized water, ethanol could provide alkoxyl
groups, which would partially substitute the nonbridging hy-
droxyl group and thereby prevent the connection of hydrogen
bonding between the particle surfaces and the nonbridged
hydroxyls, and thus the aggregation of the particles could be
limited due to the steric effect.27,35 Therefore, an investigation
of the effect of rinsing solvents on the morphology of nickel
oxides becomes necessary.
The synthesis of metal oxides with controlled morphologies
has consistently been a hot topic in recent materials science.11,15-26
Lately, Xie et al.20 reported that tricobalt tetraoxide nanorods
not only catalyze CO oxidation at temperatures as low as -77
† Part of the special issue “Green Chemistry in Energy Production
Symposium”.
* To whom correspondence should be addressed. Tel.: 86-411-84581234
(Xu); 86-411-84379229 (Ge); Fax: 86-411-84581234 (Xu); 86-411-
84379152 (Ge); E-mail: xuhy@dicp.ac.cn; geqj@dicp.ac.cn.
‡ Dalian Institute of Chemical Physics.
In this study, pure nickel oxides with different morphologies
were prepared by rinsing the precipitates with different solvents.
§ Graduate University of Chinese Academy of Sciences.
10.1021/jp906053h  2010 American Chemical Society
Published on Web 09/23/2009

Precipitate Rinsing Solvents on Ni Catalyst
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3819
The catalytic behaviors for methane decomposition of the
obtained Ni catalysts were extensively investigated by X-ray
Diffraction (XRD), Temperature-Programmed Reduction (TPR),
and Scanning Electron Microscopy (SEM) techniques.
2. Experimental Section
2.1. Preparation of NiO Catalysts. The nickel oxides were
prepared by precipitation of nickel acetate dissolved in ethylene
glycol with sodium carbonate aqueous solution at 120 °C.11
A
solution containing 0.05 mol of nickel acetate (Ni(OAc)2 ·4H2O)
and 150 mL of ethylene glycol called Ni-EG solution was
gradually heated to 120 °C under stirring and maintained at
steady temperature for 30 min. Then 500 mL of 0.2 M Na2CO3
aqueous solution was slowly added to the Ni-EG solution, with
a final pH value of ∼10. The precipitate was aged in the mother
liquid for another 1 h. After being filtered, the precipitate was
first washed thoroughly with ethanol until conductance of the
Figure 1. XRD patterns of the nickel oxides.
washed solution in the final wash was less than 0.003 Sm-1
,
of 250 mm when we investigated the stability of the catalysts.
The isothermal catalytic tests of methane decomposition were
also carried out with a low gas hourly space velocity of 45 000
mL/ (gcat h). The effluents from the reactor were analyzed by
online gas chromatography. Over all of the catalysts, hydrogen
was detected as the only gaseous product, indicating that
methane decomposition occurs selectively as CH4 f 2H2 + C.
and then washed thoroughly with deionized water until con-
ductance of the washed solution in the final wash was less than
0.003 Sm-1 (Method I). The precipitate was also washed with
deionized water (Method II) and ethanol (Method III) separately
for comparison. The nickel hydroxide precipitate was dried at
100 °C overnight and finally calcined in air at 400 °C for 4 h.
The obtained solids generated from Method I, Method II, and
Method III were designed as NiO-etwt, NiO-wt, and NiO-et,
respectively. Correspondingly, the Ni catalysts obtained from
reduction of NiO-etwt, NiO-wt, and NiO-et were designed as
Ni-etwt, Ni-wt, and Ni-et, respectively.
2.2. Catalyst Characterization. The X-ray powder diffrac-
tion (XRD) patterns were recorded on a Philips CM-1 powder
X-ray diffractometer, using Cu KR radiation operated at 40 kV
and 40 mA. The mean crystallite sizes of nickel and NiO were
calculated from the Scherrer equation, where the particle shape
factor was taken as 0.89. The SEM images were obtained with
a JEOL JSM-6360LV operated at 22 kV. Temperature-
programmed reduction (TPR) was performed in a conventional
setup equipped with a thermal conductivity detector. Five
milligram NiO samples were heated to 450 °C (10 °C/min)
under Ar flow (30 mL/min), and kept for 0.5 h at this
temperature to remove the adsorbed carbonates and hydrates.
After being cooled to room temperature and introduction of the
reduction agent of 5 vol % H2/Ar (30 mL/min), the temperature
was then programmed to rise from 30 °C to 800 °C at a ramp
rate of 10 °C/min.
CH4 temperature-programmed decomposition was carried out
using the same conventional setup equipped with an online mass
analysis system. Typically, 40 mg of catalyst was initially
reduced in H2 at 500 °C for 30 min and then the catalyst was
flushed with Ar for 20 min. After cooling in Ar, the sample
was tested from 30 °C to 800 °C at 10 °C/min under 20% CH4
in Ar (total 30 mL/min). CH4 (m/e ) 16) and the H2 (m/e ) 2),
CO (m/e ) 28), and H2O (m/e ) 18) produced were recorded
as functions of temperature.
2.3. Catalytic Activity Test. The methane decomposition
reaction was carried out in a continuous-flow fixed-bed quartz
reactor at 500 °C under atmospheric pressure. A 40-mg sample
of NiO (40-60 mesh) was loaded and reduced with pure
hydrogen (20 mL/min) at 500 °C for 1 h before the reaction.
The Ni catalyst was then flushed with Ar for 0.5 h. CH4
(99.995% purity, 60 mL/min) was then introduced with a gas
hourly space velocity of 90 000 mL/ (gcat h) through a mass
flow controller. To eliminate the space limitation for carbon
growth, the reactor had an internal diameter of 28 mm and length
3. Results and Discussion
3.1. Characterization of Nickel Oxides. Figure 1 shows the
XRD patterns of the nickel oxides rinsed by different solvents.
Clearly, all of the samples showed a typical face-center cubic
structure of NiO with crystallite phases (111), (200), and (311).
The crystalline sizes were calculated from the Scherrer equation
according to NiO (200) phases. The NiO-etwt and NiO-wt
samples exhibit similar crystalline sizes of 6.7-6.8 nm, whereas
NiO-et gave larger crystalline sizes of 10.4 nm. The differences
of NiO crystalline sizes are ascribed to the effect of rinsing
solvents with different permittivity. However, the sequence of
crystalline sizes is inconsistent with previous reports22 that the
sizes of zirconia particles could be smaller by rinsing a
conventional ZrO(OH) 2 hydrogel with anhydrous ethanol than
that rinsed by deionized water. Such differences could be
attributed to the different precipitate solubility in ethanol and
water.
Figure 2a-c shows SEM micrographs of NiO-et (Figure 2a),
NiO-etwt (Figure 2b), and NiO-wt (Figure 2c). NiO-etwt
and NiO-wt showed nanosheet shape (nanosheet-NiO), whereas
NiO-et showed a nanosheet needle-like shape (needle-NiO).
Obviously, the morphology of nickel oxides was significantly
related to the rinsing solvents. Controllable synthesis of Co3O4
with a different shape via a hydrothermal process of cobalt
hydroxide precursor and subsequent thermal decomposition for
methane combustion was recently reported.21 Co3O4 nanosheets
showed the highest methane combustion activity among the
different shapes of Co3O4snanosheets, nanobelts, and nanocubes.
Further investigation of HRTEM suggested that the exposed
crystal planes were different for different Co3O4 shapes.
Therefore, the catalytic activities for methane decomposition
of Ni catalysts obtained from the nickel oxides are also expected
to be associated with nickel oxides shapes, of which the exposed
crystal planes could be different. Ni catalysts obtained from
nanosheet-NiO will be anticipated to exhibit better methane
decomposition performance than the catalyst obtained from
needle-NiO.
Figure 3 illustrates the H2-TPR profiles of the nickel oxides.
All samples exhibit only one hydrogen consumption peak at

3820 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Zhang et al.
Figure 3. TPR profiles of the nickel oxides.
Figure 4. MS-signal of CH4 in CH4 decomposition over the Ni
catalysts (Reaction conditions: 40 mg catalyst, reduced in H2 at 500
°C for 30 min, 20% CH4/Ar (30 mL/min) and 10 °C/min.)
etwt and Ni-wt, methane conversion starts at about 390 °C, while
starting at a higher temperature of ∼445 °C for Ni-et catalysts.
Large amounts of CH4 are consumed for Ni-etwt and Ni-wt,
while only slight consumption of CH4 could be observed for
Ni-et catalysts. The catalytic activity of Ni-etwt and Ni-wt
reaches the maximum at 610 and 630 °C, respectively. Obvi-
ously, the catalytic activity of Ni-etwt and Ni-wt toward methane
decomposition is much higher than that of Ni-et. Combined with
the SEM micrographs, it could be learned that Ni catalysts
reduced from nanosheet NiO exhibit rather higher methane
decomposition activity than that of Ni-et catalysts reduced from
nanosheet needle-like NiO, similar to the results from SEM
photos that the metallic nickel obtained from nanosheet-NiO is
the active phase for methane decomposition.
3.3. Methane Decomposition. Figure 5 depicts the methane
conversions of different Ni catalysts as a function of time-on-
stream with gas hourly space velocity (GHSV) of 45 000 mL/
(gcat h) (a) and 90 000 mL/ (gcat h) (b), respectively. It is
observed from Figure 5a that Ni-et catalyst exhibited very low
catalytic activity, whereas the Ni-etwt and Ni-wt catalysts show
relatively high catalytic activity. This is in accord with the result
of CH4 temperature-programmed decomposition that the cata-
lytic activity of Ni-etwt and Ni-wt toward methane decomposi-
tion is much higher than that of Ni-et. Meanwhile, during the
tested time, the catalytic activity of Ni-et is observed to decrease
with time, whereas the catalytic activities of Ni-etwt and Ni-wt
become increased with time, which indicates that the stability
Figure 2. SEM images of the nickel oxides: (a) NiO-et, (b) NiO-
etwt, and (c) NiO-wt.
270-500 °C, representing the characteristic reduction of sto-
ichiometric nickel oxide.11 Although the crystalline size and
particle shape of NiO-et sample were different from NiO-etwt
and NiO-wt, similar reduction features with a strong and sharp
hydrogen consumption peak were observed. This indicates that
there is a slight effect of crystalline size (6-10 nm) and
morphology on the reduction behavior of nickel oxides. It is
noted that the consumption peak of NiO-et was at a slightly
higher temperature. Combined with the XRD results, it is found
that the NiO-et sample with larger crystalline sizes corresponds
to a higher reduction temperature. This is consistent with the
results that the reduction temperature of NiO increased with
increasing crystalline size in the range of 6-17 nm.28
3.2. CH4 Temperature-Programmed Decomposition. Fig-
ure 4 shows the results of CH4 temperature-programmed
decomposition. It could be observed that for the catalysts Ni-

Precipitate Rinsing Solvents on Ni Catalyst
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3821
of hydrogen bonds between the particle surfaces and nonbridged
hydroxyls, thus the aggregation of the particles could be limited
due to the steric effect.27,35 In view of this, the NiO-et sample
exhibited a hybrid of needle-like and nanosheet shapes, which
are relatively less aggregated than a pure nanosheet shape due
to the steric effect of ethanol. However, NiO-etwt and NiO-wt
samples finally washed by deionized water exhibited a nanosheet
shape that was more aggregated, possibly resulting from the
weak steric effect of water.
It is known from the results of catalytic performance and SEM
that only the Ni catalysts generated from nanosheet-NiO
exhibited high catalytic activity toward methane decomposition,
so deionized water is a better rinsing solvent of the precipitate
to form NiO with a nanosheet shape, which would show high
methane decomposition activity after reduction. When NiO
samples were reduced by hydrogen, the evolution of the Ni
particles was different, thus resulting in different catalytic
activities of Ni catalysts. Moreover, if the precipitate were rinsed
by Method I, the catalytic activity of the obtained Ni catalyst
for methane decomposition would be higher than that of the Ni
catalyst, whose precursor was rinsed by Method II. If the
precipitates were rinsed by deionized water after being washed
by ethanol, some steric effect might still exist, and this may
separate the formed nanosheet precipitates. Therefore, more
suitable active sites of the reduced nickel for methane decom-
position would be exposed to show higher catalytic activity.
So it can be concluded that the appropriate steric effect is
beneficial for the evolution of Ni particles with high catalytic
activity.
3.5. Effect of NiO Morphologies on Ni Particles. Recently,
shape and crystal plane effects of nanocrystal (NC) catalysts
have attracted remarkable attention in different reactions.15-21
The deviation of catalyst reactivity and selectivity was attributed
to the distribution style of surface active atoms and the number
of dangling bonds on different crystal planes.21 As is reported
by Hu,21 the Co3O4 nanosheets, whose dominant exposed planes
are {112}, exhibited higher catalytic activity for methane
combustion than Co3O4 nanobelts, whose dominant exposed
planes are {011}, and Co3O4 nanocubes, whose dominant
exposed planes are {001}. As for the Ni catalysts, the active
component for methane decomposition is metallic nickel, thus
the reason for different catalytic activities of the reduced NiO
samples could be ascribed to the effect of shapes or particle
sizes of the corresponding Ni catalysts. According to previous
reports,20,21 because the dominant exposed planes were different
for NiO samples with different shapes, the produced Ni catalysts
would exhibit different shapes or particle sizes during the
reduction of NiO samples by hydrogen. For the nanosheet NiO
sample, the particle sizes of the obtained Ni catalysts are small
and the active sites of the corresponding shape are suitable for
methane decomposition. Oppositely, the particle sizes of Ni-et
catalyst obtained from needle-NiO are large, and the corre-
sponding active sites are not suitable for methane decomposition.
This was further confirmed by the SEM micrographs and XRD
characterization of the reduced NiO samples.
Figure 5. Methane conversion as a function of time-on-stream over
the Ni catalysts originated by hydrogen reduction of the nickel oxides
at 500 °C, 0.040 g catalyst, 101 kPa methane, and a GHSV of (a) 45
000 mL/ (gcat h) and (b) 90 000 mL/ (gcat h).
for methane decomposition of Ni-etwt and Ni-wt catalysts could
be better than that of Ni-et. To further investigate the H2
production ability of Ni-etwt and Ni-wt catalysts, the stability
of methane decomposition was investigated. Figure 5b shows
that the Ni-wt catalyst exhibited pronounced catalytic activity
for methane decomposition within 37 h, and the total hydrogen
yields were 1356 molH2/molNi. The Ni-etwt sample gave
prolonged catalytic activity for 52 h, and the hydrogen yields
approached 1954 molH2/molNi. In addition, Ni catalysts obtained
from NiO-et samples with small particle sizes (6.9-7.5 nm)
still exhibit very low catalytic activity for methane decomposi-
tion. Thus, it could be indicated that the activity difference of
Ni-etwt and Ni-et is not due to the NiO particle size effect. It
is the rinsing solvents that play crucial roles in determining the
catalytic properties of the Ni catalysts, which corresponds to
the above conclusion from the SEM results.
3.4. Effect of Rinsing Solvents on NiO Morphologies and
Catalytic Activity. Rinsing solvents have an important effect
on the morphologies of nickel oxides according to the said
results, but there are no reports on the morphology-controlled
synthesis of NiO materials by varying rinsing solvents, as far
as we know. As stated above,34 the supersaturation degree was
different for different solvents, which would influence the
nucleation numbers, and this would finally lead to the formation
of species with different morphologies. Taking the above rinsing
process into account, when the rinsing solvent is ethanol, it could
provide alkoxyl groups which would partially substitute the
nonbridging hydroxyl group, and thus prevent the connection
Figure 6a-c shows the SEM micrographs of the reduced NiO
samples. Clearly, the particle sizes of the Ni-et sample (Figure
6a) were larger than those of the Ni-etwt (Figure 6b) and Ni-
wt (Figure 6c) samples. This was also confirmed by XRD
characterization of Ni-et, Ni-etwt, and Ni-wt (Figure 7). It could
be seen from the XRD patterns that all of the NiO samples were
completely reduced to metallic nickel. Furthermore, compared
with their corresponding NiO precursors, the crystalline sizes
of metallic nickel were greatly increased, especially for the Ni-

3822 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Zhang et al.
Figure 7. XRD patterns of the Ni catalysts obtained by hydrogen
reduction of the NiO precursors at 500 °C.
were first separated to produce in situ small nickel crystallites,
which were then active for the growth of carbon nanofibers.
On the basis of this discussion, it can be inferred that only the
nickel crystallites with small enough particle sizes could catalyze
methane decomposition. So it is easy to understand that Ni
catalysts with smaller particles, shown in Figure 6b,c, will be
more active for methane decomposition. Furthermore, Erma-
kova29 pointed out that the maximal carbon yield was shown to
be attained with the Ni catalyst particles of 10-40 nm average
diameter. Afterward, Chen37 reported that an optimum growth
rate and yield of carbon nanofibers can be achieved on optimally
sized Ni crystals (∼34 nm). Therefore, the Ni catalysts with
particle sizes of ∼34 nm in this work exhibited high catalytic
activities for methane decomposition, while the catalyst with
particle size of 100 nm showed rather low catalytic activity.
3.6. Characterization of the Carbon Deposits. Figure 8
shows the SEM micrographs of deactivated catalysts obtained
on Ni-etwt and Ni-wt samples. The deactivation reason of Ni
catalysts could be speculated from the SEM micrographs. For
both Ni-etwt and Ni-wt samples, interwoven carbon nanofibers
(CNFs) were observed; nickel particles with similar diameter
(>100 nm) were also found in the images, which were shown
as lighter spots. As said above, the catalytic activity of Ni
catalysts for methane decomposition is determined by nickel
particle sizes, and the activity of Ni-et catalyst with particle
sizes of 100 nm is proven to be rather low, as is shown in Figure
5a. Therefore, the Ni catalysts shown in Figure 8 with larger
Ni particle sizes of >100 nm are deactivated.
Figure 6. SEM images of Ni catalysts obtained by hydrogen reduction
of nickel oxides: (a) Ni-et, (b) Ni-etwt, and (c) Ni-wt.
4. Conclusions
et sample (Figure 1). It could be speculated from the SEM
photos and XRD patterns that the particle sizes of Ni catalysts
are dependent on the NiO morphologies. The reduction of NiO
samples with nanosheet shape led to the formation of Ni particles
with smaller sizes (∼34 nm), while the reduction of NiO samples
with hybrid shapes of needle-like and nanosheet led to the
formation of Ni particles with larger sizes (∼100 nm). If nickel
crystalline morphology and size are correlated to the catalytic
behavior of methane decomposition, then it can be readily seen
that the Ni-etwt and Ni-wt samples with similar crystalline sizes
of 34 nm had relatively high hydrogen yields, whereas the Ni-
et sample with the larger crystalline size of 100 nm produced
extremely low hydrogen yields. Methane decomposition activity
was closely correlated with the particle sizes of Ni catalysts as
reported many studies.11,29,36,37 As ref 36 reported, when pure
raney nickel catalysts were contacted with methane, the catalysts
Rinsing solvents play crucial roles in determining the catalytic
activity of Ni catalysts for methane decomposition through
the formation of nickel oxides with different morphologies. The
nickel hydroxide precipitates washed by ethanol led to the
morphology of NiO with a nanosheet needle-like appearance,
whereas the nickel hydroxide precipitation finally washed by
deionized water led to the morphology of NiO with a pure
nanosheet shape. The Ni catalysts generated from nanosheet-
NiO exhibited relatively higher catalytic activity toward methane
decomposition with hydrogen yields of 1356 molH2/molNi-1954
molH2/molNi under suitable reaction conditions, whereas the Ni-
et catalyst obtained from needle-NiO exhibited very low
catalytic activity toward methane decomposition. The high
catalytic activities of Ni catalysts generated from nanosheet-
NiO were the result of the relatively small particle sizes (34

Precipitate Rinsing Solvents on Ni Catalyst
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3823
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Figure 8. SEM images of the completely deactivated Ni catalysts: (a)
NiO-etwt and (b) NiO-wt.
nm) of Ni crystallites, compared with the Ni catalyst obtained
from needle-NiO with a crystalline size of 100 nm. NiO
morphology determines the particle size of the subsequent Ni
catalysts, with the catalyst deactivation occuring when the nickel
particles grew larger. The carbon materials derived from
methane decomposition were carbon nanofibers, which could
be promising materials for various technological applications.
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Acknowledgment. The authors would like to give their
thanks to Prof. Wenzhao Li and Prof. Yanxin Chen for their
helpful discussions, and also thank Mr. Shoufu Hou and Mr.
Junguo Ma for their help with the experimental apparatus
operations.
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References and Notes
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