J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Effects of La2O3, Cu and Fe addition on the catalytic performance of
Ni-SiO2 catalysts for methane decomposition
Heyi Wang,* Wenhua Wang, YongYangˈShuming Peng
Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, PR China(Received: 9 May 2012 / Accepted: 6 November 2012)

Methane decomposition over four kinds of catalysts such as Ni-SiO2, Ni-La2O3-SiO2, Ni-Cu-SiO2 andNi-Fe-SiO2 prepared by co-impregnation method and sol-gel method, respectively, has been studied. The results
indicated that Ni-La2O3-SiO2 catalyst with La/Ni = 0.3 prepared by co-impregnation method showed better catalytic
performance compared with all the other Ni-La2O3-SiO2 catalysts during methane decomposition at 823 K.
However, the temperature resistance of this catalyst was not very good. The introduction of Cu and Fe into Ni-SiO2
enhanced the temperature resistance of the catalysts during methane decomposition at 923 K and the overall
catalytic performance of the catalysts was Ni-Cu-SiO2 (sol-gel method) > Ni-Fe-SiO2 (sol-gel method) >75%Ni-25%SiO2 (sol-gel method) > Ni-La2O3-SiO2 (co-impregnation method). XRD studies of Ni-La2O3-SiO2,
Ni-Cu-SiO2 and Ni-Fe-SiO2 catalysts indicated that Ni species existed in the form of Ni metal in Ni-La2O3-SiO2,
while Ni-Cu alloy and Ni-Fe alloy appeared in Ni-Cu-SiO2 and Ni-Fe-SiO2 catalysts. The structure of the carbon
filaments formed during methane decomposition changed a lot with the introduction of Cu and Fe. At 923 K,“bifurcate carbon filaments” formed over Ni-Cu-SiO2 during methane decomposition, while “bamboo-shaped
carbon filaments” formed over Ni-Fe-SiO2 catalysts.
Keywords: Methane decomposition, Ni-SiO2, Ni-La2O3-SiO2, Ni-Cu-SiO2, Ni-Fe-SiO2

1. Introduction

were hardly investigated. Above all, the unsupported
alloy catalysts in the dynamic systems will deactivate
quickly. Methane decomposition over the supported
catalysts to prepare H2 and carbon filament in the
dynamic system has been studied by many researchersand many achievements have been obtained in recent
years [12-16]. However, the effects of the rare metaloxide, Fe and Cu addition on the temperature resistanceof the Ni-based catalysts were little investigated. In
view of this, methane decomposition over supported
catalysts such as Ni-SiO2, Ni-La2O3-SiO2, Ni-Cu-SiO2
and Ni-Fe-SiO2 catalysts in dynamic systems has beenstudied in this work. The influences of the preparation
method, the reaction temperature and the contents of La,
Cu and Fe in Ni-SiO2 on methane decomposition were
investigated.

If graphite and carbon fiber composite are employed
as plasma facing components for ITER, the deuterated
and tritiated methane may be a considerable constituentof the impurity gas stream [1-6]. Methane decomposition
is a safe and simple method for recovering deuterium andtritium from the deuterated and tritiated methane and an
optional catalyst bed for methane decomposition iscurrently included into the design of the tokamak exhaustprocessing system (TEP) [7,8].
Lots of research on methane decomposition has been
done by many researchers [9-11]. The ZrCo powder has
already been found to decompose methane slowly at
573 K. A product of 0.05 molC/mol ZrCo was obtained
after 80 h when the getter was exposed to 430 torr CH4[9]. Watanabe [10] et al investigated the decompositionof methane on Zr7Ni10, ZrNi, Zr2Ni, Zr4Ni and Zr in thetemperature range 573K–823 K and the alloy of Zr4Ni
removed methane over 99% efficiently in 105 s at 673 K.
Baker [11] et al discovered that a St 909 getter(Zr–Mn–Fe) could effectively remove 99% of NH3, CO2
and O2 at 673 K, CO at 873 K, and CH4 at 973 K from aninert gas stream.
However, methane decomposition for recovering
deuterium and tritium which was studied by the above
researchers all focused on the static systems and theunsupported alloy catalysts, while the dynamic systems

2. Experimental
2.1. Catalyst preparation

Ni-La2O3-SiO2 catalysts were prepared byco-impregnation method [17] which was done throughimpregnating SiO2 with the mixed aqueous solution ofNi(NO3)2 and La(NO3)3 for 1.5 h, and then drying up the
impregnated samples at 413 K for 12 h, and finallycalcined them at 873 K for 6 h. The Ni content in all thecatalysts was Ni/Si = 0.67 (molar ratio) and the La
contents in the catalysts were La/Ni = 0, La/Ni = 0.1,

author’s e-mail:hywang@caep.ac.cn

42©2013 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H. Wang et al., Effects of La2O3, Cu and Fe Addition on the Catalytic Performance of Ni-SiO2 Catalysts for Methane Decomposition

La/Ni = 0.3, La/Ni = 0.45 and La/Ni = 0.6 (molar ratio),
respectively.
75%Ni-25%SiO2, 65%Ni-10%Fe-25%SiO2, 35%Ni40%Fe-25%SiO2, 65%Ni-10%Cu-25%SiO2, and 35%
Ni-40%Cu-25%SiO2 catalysts (mass fraction) wereprepared by sol-gel method [18,19]. The method wasbased on mixing the active components such as NiO or
the mixtures of NiO and CuO (or Fe2O3) in this case, withalcosol containing silica in a certain amount. The
suspends of the active components with alcosol weredried in flowing air at room temperature and were finallycalcined at 923 K for 3 h. Alcosol was prepared bymixing 50 mL of TEOS, 40 mL of ethanol, 2 mL of waterand 0.5 mL of 40% HCl. Silica content in the alcosol was
0.142 g/mL and could be diluted by ethanol. The mixtureof NiO and CuO (or Fe2O3) was originated from calcining
the mixture of Ni(NO3)2 and Cu(NO3)2 (or Fe(NO3)3) at
723 K and the Ni(NO3)2 and Cu(NO3)2 (or Fe(NO3)3)
mixture was obtained by evaporating the mixed solutionof Ni(NO3)2 and Cu(NO3)2 (or Fe(NO3)3).

35m(catalysts)=60mg

La/Ni=0
La/Ni=0.1
La/Ni=0.3
La/Ni=0.45
La/Ni=0.6

Methane conversion/%

30
25
20
15
10
5

0

50

100

150

200

250

Time/min

300

350

400

450

Fig.1 Kinetic curves of methane decomposition over
Ni-La2O3-SiO2 catalysts at 823 K.
35

773k
823k
873k

m(catalysts)=60mg

Methane conversion/%30

2.2 Activity tests and catalyst characterization

Methane catalytic decomposition reactions werecarried out in a fixed-bed quartz reactor (10 mm i.d.)under atmospheric pressure. Before the activity tests,
Ni-La2O3-SiO2 catalysts were subjected to a reduction
pre-treatment using pure hydrogen of 20 mL/min at 873K
for 1h and Ni-Cu-SiO2 and Ni-Fe-SiO2 catalysts werereduced with pure hydrogen of 20 mL/min at 923 K for 1
h. The feed stream was high-purified methane (99.99%)
of 15 mL/min. The gaseous reaction products weremonitored by an off-line gas chromatography (GC)combined with a 5A molecular column, using a thermal
conductivity detector (TCD) for hydrogen and methane
analysis.
XRD patterns were recorded on a Bruker
D8Advance diffractometer with Cu KD radiation at
40 kV and 40 mA.
The detailed structure of the deposited carbons wasobserved using the FEI Inspect F scanning electron
microscope (SEM) operated at 20 kV and the JEOL
JEM-100CX Transmission electron microscopy (TEM),
operated at 80 kV.

25
20
15
10
5
0

0

100

200

300

400

Time/minFig.2 Effect of reaction temperature on methane decomposition
over Ni-La2O3-SiO2 catalysts with La/Ni = 0.3.

of Ni-SiO2 catalyst obviously. For the Ni-La2O3-SiO2
catalyst with La/Ni = 0.1, the methane conversion
decreased from 16% to 7% after 450 minutes. With theincrease of La2O3 content from La/Ni = 0.1 to La/Ni = 0.3,
the catalytic performance of the catalyst was greatly
improved and the methane conversion decreased from
17% to 13% with only 4% dropped after 450 minutes.
However, when La2O3 content in the catalyst exceeded
La/Ni = 0.3, the catalytic performance of the catalysts
dropped slightly. The methane conversion decreased from
15% to 7.5% for Ni-SiO2 with La/Ni = 0.6 after 450
minutes. Thus, a conclusion can be drawn that an
optimum La2O3 content exists in Ni-La2O3-SiO2 catalysts
which can improve the catalytic performance of Ni-SiO2
the most efficiently, while the introduction of more La2O3
into Ni-SiO2 catalyst will impair the catalytic activity andthe lifetime of the catalysts and this optimum La2O3
content is La/Ni = 0.3.
Fig.2 shows the effect of reaction temperature on
methane decomposition over Ni-La2O3-SiO2 catalyst with
La/Ni = 0.3. The catalytic activity of Ni-La2O3-SiO2
with La/Ni = 0.3 increased remarkably from 773 K to 823
K. When the reaction temperature increased from 823 K
to 873 K, the initial methane conversion increased to 20%,

3. Results and discussion

3.1 The catalytic performance of the catalysts
Fig.1 depicts the kinetic curves of methane
decomposition over Ni-La2O3-SiO2 catalysts withdifferent contents of La2O3 at 823 K. Only hydrogen was
obtained as a gaseous product for all the catalysts. It
could be observed that the catalytic activity of Ni-SiO2
catalyst without La2O3 decreased gradually with the timeon stream and it was completely deactivated after
450 minutes. The addition of La2O3 extended the lifetime

43

H. Wang et al., Effects of La2O3, Cu and Fe Addition on the Catalytic Performance of Ni-SiO2 Catalysts for Methane Decomposition

Ni-Cu-SiO2 didn’t change obviously during methane
decomposition.
The
methaneconversion
of
65%Ni-10%Cu-25%SiO2 catalyst was kept at around40%, while when the Cu content increased to 40%, themethane conversion dropped to about 19%. Therefore, it
was different from Ni-La2O3-SiO2 catalyst that
Ni-Cu-SiO2
catalysts
could
catalyzemethanedecomposition at a higher reaction temperature such as923 K and the introduction of Cu into Ni-SiO2 enhanced
the temperature resistance of the catalysts.
Fig.4(b) shows the kinetic curves of methane
decomposition over Ni-SiO2 and Ni-Fe-SiO2 catalysts at
923 K. The catalytic behaviors of Ni-Fe-SiO2 catalysts
were similar with that of Ni-Cu-SiO2 catalysts. The
introduction of Fe also improved the lifetime of Ni-SiO2
catalysts obviously at 923 K. However, a deactivation
period occurred for 65%Ni-10%Fe-25%SiO2 catalyst andwhenmethanedecomposition
over
65%Ni-10%Fe-25%SiO2
catalyst
proceeded
to200 min, the methane conversion decreased from 44% to25%. While, the methane conversion of 35%Ni-40%Fe-25%SiO2 was maintained at 19% all the time whichindicated that with the increase of the Fe content in the
Ni-Fe-SiO2 catalysts the catalytic activity of the catalyst
dropped and the stability of Ni-Fe-SiO2 catalyst wasenhanced. Therefore, Ni-Fe-SiO2 could also catalyze
methane decomposition at a higher reaction temperaturesuch as 923 K which was similar with Ni-Cu-SiO2.
However,thecatalytic
performanceof
65%Ni-10%Fe-25%SiO2 was a little poorer than that of
65%Ni-10%Cu-25%SiO2 for the poorer stability.
The research on the temperature resistance of thecatalysts is very important for methane decomposition isan endothermic reaction and the conversion rises with theincrease of the reaction temperature [20]. Thus, a catalyst
with high temperature resistance can catalyze methane
decomposition more efficiently at a higher reaction
temperature [21]. According to the above results, a
conclusion can be drawn that the order of the catalytic
performance of the four kinds of catalysts above is
Ni-Cu-SiO2 (sol-gel method) > Ni-Fe-SiO2 (sol-gel
method) > 75%Ni-25%SiO2 (sol-gel method) >Ni-La2O3-SiO2 (co-impregnation method). Also, whenmethane decomposition over Ni-Cu-SiO2 or Ni-Fe-SiO2
catalysts is applied to TEP to recover deuterium andtritium from the deuterated and tritiated methane, a muchhigher reaction temperature can be adjusted to utilize thetemperature resistance of the catalysts to improve thecatalytic efficiency.

however, the lifetime of the catalyst was reduced and themethane conversion decreased to 2% after 300 minutes.
Therefore, a higher reaction temperature reduced thelifetime of the catalyst and another conclusion can also be
drawn that the temperature resistance of Ni-La2O3-SiO2
was not so good even if the introduction of La2O3 into
Ni-SiO2 greatly extended the lifetime of Ni-SiO2 at alower reaction temperature such as 823 K.
Fig.3 compared the catalytic performance of
Ni-La2O3-SiO2 prepared by co-impregnation method withthat of 75%Ni-25%SiO2 prepared by sol-gel method and
it demonstrated that the effect of the preparation methodof the catalysts on methane decomposition was important.
The catalytic activity of Ni-La2O3-SiO2 with La/Ni = 0.3
which showed the best catalytic performance among theNi-La2O3-SiO2 catalysts was a little lower than that of
75%Ni-25%SiO2. The methane conversion of
75%Ni-25%SiO2 catalyst at 823 K remained 21% anddidn’t change during the reaction. However, the methane
conversion of Ni-La2O3-SiO2 with La/Ni = 0.3 decreased
from 17% to 14% after 400 minutes. Therefore, both thecatalytic activity and the stability of 75%Ni-25%SiO2
were better than that of Ni-La2O3-SiO2 with La/Ni = 0.3
which indicated that the Ni-based catalysts prepared bysol-gel method showed better catalytic performance
than that prepared by co-impregnation method during
methane decomposition.
35

Ni-La2O3-SiO2(La/Ni=0.3)
75Ni-25SiO2

Methane conversion/%

30
25
20
15
10
5

0

50

100

150

200

250

300

350

400

Time/minFig.3 Effect of preparation method on methane decompositionat 823 K (m(Ni-La2O3-SiO2) = 60 mg, co-impregnation
method; m(75%Ni -25%SiO2) = 30 mg, sol-gel method).

Fig.4(a) depicts the kinetic curves of methane
decomposition over Ni-Cu-SiO2 catalysts with different
contents of Cu at 923 K. The lifetime of 75%Ni-25%SiO2
catalyst was very short at 923 K unlike the catalytic
behaviors at 823 K of which the methane conversion was
kept at 21% for 400 minutes (Fig.3). After 70 minutes,
the methane conversion of 75%Ni-25%SiO2 decreased toaround 5% and remained at this value. The addition oflower content of Cu improved the lifetime of Ni-SiO2
catalyst apparently and the catalytic activity of

3.2 The characterization of the catalysts before andafter methane decomposition
Fig.5(a) shows the XRD spectra of Ni-La2O3-SiO2

44

H. Wang et al., Effects of La2O3, Cu and Fe Addition on the Catalytic Performance of Ni-SiO2 Catalysts for Methane Decomposition

7070(a) m(catalysts)=30mg

75%Ni-25%SiO2
65%Ni-10%Cu-25%SiO2

60

Methane conversion/%

Methane conversion/%

35%Ni-40%Cu-25%SiO2
50
40
30
20
10
0

(b) m(catalysts)=50mg

75%Ni-25%SiO2
65%Ni-10%Fe-25%SiO2

60

35%Ni-40%Fe-25%SiO2

50
40
30
20
10

50

100

150

200

250

300

0

350

50

100

150

200

Time/min

Time/min250

Fig.4 Kinetic curves of methane decomposition over Ni-Cu-SiO2 and Ni-Fe-SiO2 catalysts at 923 K.

catalysts reduced by hydrogen. The diffraction peaks at
2T = 44.46°, 51.81° and 76.36° in the XRD spectra of
Ni-SiO2 revealed that the Ni species in the Ni-SiO2
catalyst reduced by hydrogen existed mainly in the formof Nimetal. When lower content of La2O3 was introduced
into the catalysts such as Ni-La2O3-SiO2 with La/Ni = 0.1
and La/Ni = 0.3, no La2O3 peaks were detected whichmeant that the lower contents of La2O3 existed in an
amorphous phase. However, as the contents of La2O3
exceeded La/Ni = 0.3, the La2O3 diffraction peaks
appeared. In addition, with the increase of the La2O3
content, the intensity of the diffraction peaks due to Ni
metal turned weaker and weaker and the FWMH of thediffraction peaks turned wider and wider which indicatedthat the Ni crystallite size turned smaller and smaller.
Therefore, the Ni dispersion degree was enhanced with
the increase of La2O3 content.
Fig. 5(b) shows the XRD spectra of Ni-SiO2 andNi-Cu-SiO2 catalysts reduced by hydrogen. The
diffraction peaks at 2T = 44.46°, 51.81° and 76.36° in the

XRD spectra of 75%Ni-25%SiO2 also demonstrated that
the Ni species in the Ni-SiO2 catalysts reduced by H2
existed mainly in the form of Ni metal. The diffraction
peaks due to Cu were not detected in Ni-Cu-SiO2
catalysts even if the Cu content was very high. In fact,
when Ni-Cu-SiO2 was reduced by hydrogen, the Ni
species was associated with the Cu species through theformation of Ni-Cu alloy in the catalysts and only Ni-Cu
alloy metal phase existed in the Ni-Cu-SiO2 catalyst.
Fig.5(c) shows the XRD spectra of Ni-SiO2 andNi-Fe-SiO2 catalysts reduced by hydrogen. The Fe in the
catalysts also appeared as Ni-Fe alloy which was similar
with Ni-Cu-SiO2 catalysts. However, the Ni-Fe-SiO2
catalysts reduced by hydrogen had two metal phases: theNi metal phase and the Ni-Fe alloy phase which wasdifferent from Ni-Cu-SiO2. The diffraction peaks due to
Ni-Fe alloy and Ni metal overlapped with each otherwhich indicated that the Ni-Fe alloy had identicalstructure with the Ni metal.

(b) Ni-Cu-SiO2(a): Ni-La2O3-SiO2(c)Ni-Fe-SiO2

65%Ni-10%Cu-25%SiO2

La/Ni=0.3
La/Ni=0.45

30

40

50

2T/ q

60

70

80

75%Ni-25%SiO2

65%Ni-10%Fe-25%SiO2

35%Ni-40%Cu-25%SiO2

La/Ni=0.6
20

Intensity/cps

intensity/cps

La/Ni=0.1

Intensity/cps

75%Ni-25%SiO2
La/Ni=0

20

30

40

50

2T q

60

70

80

35%Ni-40%Fe-25%SiO2
20

30

40

50

2T q

60

70

80

Fig.5 XRD spectra of the Ni-La2O3-SiO2, Ni-Cu-SiO2 and Ni-Fe-SiO2catalysts reduced by hydrogen(Ÿ-Ni, Ƒ-La2O3, Ŷ-Ni-Cu alloy, 佐-Ni-Fe alloy).

Fig.6 shows the SEM images of the Ni-SiO2,
Ni-La2O3-SiO2, Ni-Cu-SiO2 and Ni-Fe-SiO2 catalysts
after methane decomposition at different reaction
temperatures. It could be clearly observed that thesurfaces of all the catalysts were completely covered with
the interlaced carbon filaments and the structure of the
carbon filaments was seriously influenced by the reaction
temperature and the introduced Cu and Fe. The

45

H. Wang et al., Effects of La2O3, Cu and Fe Addition on the Catalytic Performance of Ni-SiO2 Catalysts for Methane Decomposition

catalysts showed better catalytic performance during
methane decomposition.
The carbon filaments formed over 75%Ni-25%SiO2
at 923 K turned shorter and thinner compared with thoseformed at 823 K. Thus, the increase of the reaction
temperature lowered the carbon capacity of
75%Ni-25%SiO2. When Cu was introduced into Ni-SiO2,
the carbon filaments turned much thicker and lesshomogeneous. The introduction of Fe into Ni-SiO2 alsochanged the fundamental structure of the carbon
filaments formed during methane decomposition at 923 K.
They were much longer and thinner.

fundamental structure of the carbon filaments formed
over Ni-La2O3-SiO2 with La/Ni = 0 and La/Ni = 0.3 at
823 K (Fig.6(a) and Fig.6(b)) was similar with that of
75%Ni-25%SiO2 at 823 K (Fig.6(c)) except the thickness
and the thickness of the carbon filaments formed over thethree catalysts was 75%Ni-25%SiO2 > Ni-La2O3-SiO2
with La/Ni = 0.3 > Ni-SiO2 with La/Ni = 0.
From the performance of methane catalytic
decomposition in Fig.1 and Fig.3, the order of thecatalytic performance of the three catalysts was
75%Ni-25%SiO2 > Ni-La2O3-SiO2 with La/Ni = 0.3 >Ni-SiO2 with La/Ni = 0. It could be concluded that whenthicker carbon filaments formed over the catalysts, theFig. 6 SEM images of carbon filaments formed during methane decomposition (a: Ni-SiO2 with La/Ni=0, 823e:K; b:Ni-La2O3-SiO2 with La/Ni=0.3, 823 K; c: 75%Ni-25%SiO2, 823 K; d:75%Ni-25%SiO2, 923 K;
65%Ni-10%Cu-25%SiO2, 923 K; f: 35%Ni-40%Fe-25%SiO2, 923 K).

Considering the great changes of the carbon
filaments with the increase of the reaction temperatureand the introduced Cu and Fe, TEM images of thesecarbon filaments were also investigated to analyze thedifferences of the detailed structure of these carbon
filaments.
In the TEM images of 75%Ni-25%SiO2 after
methane decomposition at 823 K (Fig.7(a)), the carbon
filaments with diameters from 70 nm to 110 nm wereobserved. The “pear-shaped” Ni metal particles were
present at the tip of the carbon filaments. While in the
TEM images of 75%Ni-25%SiO2 catalyst formed during
methane decomposition at 923 K (Fig.7(b)), the diameter
range of the carbon filaments was 50 nm-90 nm andspherical Ni metal particles appeared at the tip of the
carbon filaments. The diameters of both types of carbon

filaments were in agreement with their diameters of thecatalyst particles at the tips.
The different shapes of the Ni metal particles atdifferent reaction temperatures reflected the different
carbon diffusion mechanisms during methane
decomposition. Takenaka [22] et al pointed out that theformation of the “pear-shaped” Ni metal particles wasdue to the diffusion of carbon atoms through the bulk of
Ni metal particles, while the formation of the spherical Ni
metal particles resulted from the diffusion of carbon
atoms on the surface of Ni metal particles. Thus, it could
be speculated that the carbon atoms formed during
methane decomposition was preferential to diffuse
through the surface of the Ni metal particles at a higher
temperature.
Another phenomenon which should be paid attention

46

H. Wang et al., Effects of La2O3, Cu and Fe Addition on the Catalytic Performance of Ni-SiO2 Catalysts for Methane Decomposition

Fig.7(c) and Fig.7(d) show the TEM images of the
carbon filaments formed during methane decomposition
over 65%Ni-10%Cu-25%SiO2 at 923 K. It can beobserved that “bifurcate carbon filaments” formed over
65%Ni-10%Cu-25%SiO2 catalysts and the Ni-Cu alloy
particle which was active for methane decomposition was
not at the tip of the carbon filaments as it was in the
carbon filaments formed over Ni-SiO2 catalyst. A few
carbon filaments grew from one Ni-Cu alloy particle andthe Ni-Cu alloy particle was positioned among the carbon
filaments.

to was that when the formed carbons during methane
decomposition diffused through the bulk of the active
metal at 823 K, the catalyst could keep active for methane
decomposition for a long time, while, when the formedcarbons during methane decomposition diffused throughthe surface of the active metal, the catalyst lost its activity
in a short time (Fig.3 and Fig.4). As a result, the diffusionof the formed carbons through the surface of the catalysts
wasn’t beneficial for the long time methane
decomposition.

Fig.7 TEM images of carbon filaments formed over Ni-SiO2, Ni-Cu-SiO2 and Ni-Fe-SiO2 during methane
decomposition (a: 75%Ni-25%SiO2,823 K; b:75%Ni-25%SiO2, 923 K; c,d: 65%Ni-10%Cu-25%SiO2, 923 K;e,f:35%Ni-40%Fe-25%SiO2, 923 K).

Compared with the Ni particles in Fig.7(b) which
was formed over 75%Ni-25%SiO2 after methane
decomposition at 823 K, the Ni-Cu particles were also
“pear-shaped” after methane decomposition overNi-Cu-SiO2 catalysts at 923 K. Thus, it could bespeculated that the addition of Cu into Ni-SiO2 catalystchanged the diffusion mechanism of the formed carbons.
At a higher reaction temperature such as 923 K, thediffusion of the carbons formed over Ni-Cu-SiO2 catalyst
remained through the bulk of Ni-Cu particles. This might
be the reason why the lifetime of Ni-Cu-SiO2 catalysts
was also very long at a higher reaction temperature suchas 923 K.
Fig.7(e) and Fig.7(f) shows the TEM images of the
carbon filaments formed over Ni-Fe-SiO2 catalysts at 923
K. Carbon filaments of “bamboo-shaped” structure
formed over Ni-Fe-SiO2 catalyst. The walls of the“bamboo-shaped carbon filaments” were uneven and thehollows of these carbon filaments were divided into manycells which was different from those formed over Ni-SiO2
catalyst. The Ni-Fe alloy particles which were active formethane decomposition were not only present at the tip ofthe carbon filaments after methane decomposition, butalso existed in the hollow cells of the “bamboo-shaped”
structure.
Apparently, the addition of Fe also changed thecarbon diffusion mechanism of Ni-SiO2 catalyst during
methane decomposition. The results of Ermakova and
Takenaka [18, 23] et al indicated that the products ofmethane decomposition over Fe-SiO2 catalysts were also“bamboo-shaped carbon filaments”. Thus, the carbon
diffusion mechanism of Ni-Fe-SiO2 catalyst during
methane decomposition was similar with that of Fe-SiO2
catalyst.
Since the carbon diffusion mechanism of Ni-Fe-SiO2
is similar with that of Fe-SiO2 catalyst, it can also be

47

H. Wang et al., Effects of La2O3, Cu and Fe Addition on the Catalytic Performance of Ni-SiO2 Catalysts for Methane Decomposition

Acknowledgements

believed that the characterization of methane
decomposition over Ni-Fe-SiO2 catalysts should be inagreement with that of Fe-based catalysts. As theactivation temperature of the Fe-based catalysts is muchhigher than that of the Ni-based catalysts [23], theintroduction of Fe also enhanced the temperature
resistance of Ni-based catalysts and the lifetime of Ni-Fe
alloy catalysts was much longer than that of the single
Ni-based catalysts at a higher reaction temperature suchas 923 K.
In addition, the temperature resistance of Ni-Cu
alloy and Ni-Fe alloy catalysts was both stronger thanthat of the single Ni-based catalysts, while the stability of
65%Ni-10%Fe-25%SiO2 catalyst during methane
decomposition wasn’t as good as that of
65%Ni-10%Cu-25%SiO2 (Fig.4). The reason for thismight be that not all of the Ni species in
65%Ni-10%Fe-25%SiO2 catalyst existed in the form of
Ni-Fe alloy (Fig.5), while Ni and Cu could form alloys
over a wide composition range and this was proved by
the results of Fig.5 and other researchers [24]. When the
Fe content in Ni-Fe-SiO2 increased to 40%, the stabilityof the catalyst was enhanced for the Ni species in
35%Ni-40%Fe-25%SiO2 mainly existed in the form of
Ni-Fe alloy although the catalytic activity of the catalyst
decreased.

This work was ¿nancially supported by the National
Magnetic Confinement Fusion Science Program of China(2011GB111005).

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4. Summary

1) The introduction of La2O3 into Ni-SiO2 catalysts
obviously extended the lifetime of the catalysts during
methane decomposition at 823 K. However, thetemperature resistance of Ni-La2O3-SiO2 didn’t change
obviously and the catalytic activity of Ni-La2O3-SiO2
with La/Ni = 0.3 disappeared at a very short time at 873
K. While the introduction of Cu and Fe into Ni-SiO2
enhanced the temperature resistance of the catalyst. The
catalytic performance of the catalysts during methane
decomposition was Ni-Cu-SiO2 (sol-gel method) >Ni-Fe-SiO2 (sol-gel method) > 75%Ni-25%SiO2 (sol-gel
method)> Ni-La2O3-SiO2 (co-impregnation method).
2) XRD studies of Ni-La2O3-SiO2, Ni-Cu-SiO2 andNi-Fe-SiO2 catalysts indicated that the Ni species existed
in the form of Ni metal in Ni-La2O3-SiO2 catalyst andNi-Cu alloy and Ni-Fe alloy appeared in Ni-Cu-SiO2 andNi-Fe-SiO2 catalysts, respectively.
3) The fundamental structure of the carbon filaments
formed over Ni-La2O3-SiO2 with La/Ni = 0 and La/Ni =0.3 was similar with that of 75%Ni-25%SiO2 at 823 K.
While the carbon filaments formed over 75%Ni-25%SiO2
at 923 K turned shorter and thinner than those formed at
823 K. “Bifurcate carbon filaments” formed overNi-Cu-SiO2, while “bamboo-shaped carbon filaments”
formed over Ni-Fe-SiO2 catalysts during methane
decomposition at 923 K.

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