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NieSiO2 and NieFeeSiO2 catalysts for methane decompositionto prepare hydrogen and carbon filaments

Wenhua Wang*, Heyi Wang, Yong Yang, Shubin Jiang

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900 Sichuan, PR China

a r t i c l e i n f o

Article history:

Received 18 December 2011

Received in revised form

29 February 2012

Accepted 1 March 2012

Available online 29 March 2012

Keywords:

Methane decomposition

NieFeeSiO2

Carbon filament

Hydrogen

* Corresponding author. Tel./fax: þ86 816 248E-mail address: Wangwenhua19860610@1

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.03.003

a b s t r a c t

Active and stable NieFeeSiO2 catalysts prepared by solegel method were employed for

direct decomposition of undiluted methane to produce hydrogen and carbon filaments at

823 K and 923 K. The results indicated that the lifetime of NieFeeSiO2 catalysts was much

longer than NieSiO2 catalyst at a higher reaction temperature such as 923 K, however,

a reverse trend was shown when methane decomposition took place at a lower reaction

temperature such as 823 K. XRD studies suggested that iron atoms had entered into the Ni

lattice and NieFe alloy was formed in NieFeeSiO2 catalysts. The structure of the carbon

filaments generated over NieSiO2 and NieFeeSiO2 was quite different. TEM studies showed

that “multi-walled” carbon filaments were formed over 75%Nie25%SiO2 catalyst, while

“bamboo-shaped” carbon filaments generated over 35%Nie40%Fee25%SiO2 catalysts at

923 K. Raman spectra of the generated carbons demonstrated that the graphitic order of

the “multi-walled” carbon filaments was lower than that of the “bamboo-shaped” carbon

filaments.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction the highest carbon yields (491 gC/gNi) were obtained over 40%

In recent years, the catalytic decomposition of methane to

produce high-purified hydrogen and carbon filaments has

drawn lots of attention [1e4]. The hydrogen produced from

methane decomposition can be directly used as the fuel for

H2eO2 fuel cells for it doesn’t contain any CO and CO2 [5e7].

The carbon filaments are also widely used for its high

mechanical strength, high resistance to strong acids and

bases, high electric conductivity and high surface area [8]. The

two products simultaneously obtained during methane

decomposition make this reaction financially attractive.

Among the solid catalysts tested so far, Ni-based catalysts

are believed to be the most effective for methane decompo-

sition. Takenaka et al. [9] examined the catalytic performance

of NieSiO2 catalysts with different Ni loadings and found that

4200.26.com (W. Wang).2012, Hydrogen Energy P

Nie60%SiO2 at 773 K. Coprecipitated 90%Nie10%Al2O3 cata-

lysts also exhibited considerable activity in methane decom-

position at 773e823 K. The amounts of carbon filaments

formed over 90%Nie10%Al2O3 could reach as high as 111 gC/

gNi at 823 K[10].

However, the disadvantage of the Ni-based catalysts is that

they are effective for methane decomposition in the temper-

ature range of 673e873 K but are deactivated immediately at

temperatures above 873 K [11]. While, the conversion of

methane decomposition is not so high at reaction tempera-

tures below 873 K for methane catalytic decomposition is an

endothermic reaction and the conversion rises with the

increase of the reaction temperature [12]. Further studies

indicated that NieCu and PdeNi alloy catalysts showed

a longer lifetime at a higher reaction temperature [13e16]. Fe-

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 6 9059

based catalysts can also decompose methane at a higher

reaction temperature efficiently as the activation temperature

of the Fe-based catalysts is much higher than that of the Ni-

based catalysts [17]. The results of Ermakova et al. [18] sug-

gested that there was no catalytic activity for Fe-based cata-

lysts when reaction temperature was lower than 923 K.

Although the overall catalytic performance of the Fe-based

catalysts is not as good as that of the Ni-based catalysts, it

can also be speculated that the introduction of the iron into

Ni-based catalysts may extend the lifetime of the catalysts

during methane decomposition at a higher reaction temper-

ature. However, the catalytic performance of NieFe alloy

catalysts hasn’t been widely studied.

In this work, NieFeeSiO2 catalysts with different contents

of iron were prepared by solegel method. The influences of

the reaction temperature, the contents of iron and the CH4

flow rates on methane decomposition were studied. The

morphological appearance, the compositions, the reducibility

of NieFeeSiO2 catalysts and the carbon filaments generated

over NieSiO2 and NieFeeSiO2 catalysts during methane

decomposition were investigated.

2. Experimental

2.1. Catalysts preparation

75%Nie25%SiO2, 65%Nie10%Fee25%SiO2 and 35%Nie40%

Fee25%SiO2 catalysts (mass fraction) were prepared by a het-

erophase solegel method [19]. The method was based on

mixing the active components such as NiO or mixture of NiO

and Fe2O3 in this case, with alcosol containing silica in

a certain amount. The suspends of the active components and

alcosol were dried in flowing air at room temperature and

finally calcined at 923 K for 3 h. Alcosol was prepared by

mixing 50 mL of TEOS, 40 mL of ethanol, 2 mL of water and

0.5 mL of 40%HCl. Silica content in the alcosol was 0.142 g/mL

and could be diluted by ethanol. Themixture of NiO and Fe2O3

was originated from calcining the mixture of Ni(NO3)2 and

Fe(NO3)3 at 723 K and the Ni(NO3)2 and Fe(NO3)3 mixtures were

obtained by evaporating the solution of the mixed Ni(NO3)2and Fe(NO3)3 at 373 K.

2.2. Activity tests

Methane catalytic decomposition reactions were carried out

in a fixed-bed quartz reactor (10 mm i.d.) with the high-

purified methane (99.99%) as feed stream under atmospheric

pressure. Before the activity tests, all catalysts were subjected

to a reduction pre-treatment using a flow rate of 20 mL/min of

pure hydrogen for 1h at 923 K. The gaseous reaction products

were monitored by gas chromatography (GC) combined with

a 5Amolecular column, using a thermal conductivity detector

(TCD) for hydrogen and methane analysis.

2.3. Catalysts characterization

XRD patterns were recorded on a Bruker D8Advance diffrac-

tometer with Cu Ka radiation at 40 kV and 40 mA.

The morphological appearance of the catalysts and the

deposited carbons were carried out in the FEI InspectF scan-

ning electron microscope (SEM) operated at 20 kV.

The detailed structure of the deposited carbons were

observed using the JEOL JEM-100CX Transmission electro-

nmicroscopy (TEM) and the Tecnai G2F20 Transmission elec-

tronmicroscopy (HRTEM), operated at 80 kV and 200 kV,

respectively.

Raman spectra were recorded on a DXR SmartRaman with

532 nm HeeNe ion laser as excitation source laser.

The reducibility of the catalysts was measured in a quartz

reactor with the temperature-programmed reduction (TPR)

method. The quartz reactor with 30 mg of the catalyst was

heated by an electrical furnace at a heating rate of 10 K/min in

a 40 mL/min gas mixture containing 10%H2 and 90%Ar, at

a maximum temperature of 973 K. Hydrogen consumption

was measured by analyzing the effluent through a thermal

conductivity detector.

3. Results and discussion

3.1. Methane decomposition over NieSiO2 andNieFeeSiO2 catalysts

Fig. 1(a) depicts the kinetic curves of methane decomposition

over NieSiO2 and NieFeeSiO2 catalysts at 923 K. Only

hydrogen was obtained as a gaseous product over all the

catalysts. The lifetime of the 75%Nie25%SiO2 catalyst without

iron was very short at 923 K. After 70 minutes’ reaction, the

methane conversion over 75%Nie25%SiO2 decreased to

around 5%. The performance of this catalyst was a little worse

than that of NieSiO2 which was prepared by La’zaro et al. [19].

Their catalyst prepared through the same method could

catalyze methane decomposition at 973 K for 400 min. The

reason for this difference might be that the amount of the

catalysts they used during methane decomposition was

300 mg, while 50 mg of 75%Nie25%SiO2 was used in this

reaction.

The introduction of the iron improved the lifetime of

NieSiO2 catalysts obviously, although a deactivation period

also occurred for 65%Nie10%Fee25%SiO2 catalyst. When

methane decomposition over 65%Nie10%Fee25%SiO2 catalyst

proceeded to 200 min, the methane conversion decreased

from 44% to 25%. As for methane decomposition over 35%

Nie40%Fee25%SiO2 catalyst, the methane conversion main-

tained at 19% all the time which suggested that 35%Nie40%

Fee25%SiO2 catalyst was very stable. Therefore, iron was

a good modifier for NieSiO2 catalysts to decompose methane

and NieFeeSiO2 could catalyze methane decomposition at

a higher reaction temperature such as 923 K. As the iron

content increased in the catalyst, the catalytic activity of

NieFeeSiO2 catalysts decreased while the stability of the

NieFeeSiO2 catalysts was enhanced.

Fig. 1(b) depicts the kinetic curves of methane decompo-

sition over NieSiO2 and NieFeeSiO2 at 823 K. At a lower

reaction temperature, 75%Nie25%SiO2 catalysts without iron

also showed a longer lifetime and weren’t deactivated when

methane decomposition proceeded to 400 min. The methane

conversion over 65%Nie10%Fee25%SiO2 (18%) was a little

Fig. 1 e Kinetic curves of methane decomposition over NieSiO2 and NieFeeSiO2 catalysts ((a): 923 K, m(catalyst)[ 50 mg,

v(CH4)[ 15 mL/min; (b):823 K, m(catalyst)[ 30 mg, v(CH4)[ 15 mL/min).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 69060

lower than that of 75%Nie25%SiO2 (22%). While, for the 35%

Nie40%Fee25%SiO2 catalyst, the initial methane conversion

was around 7% which was much lower than that of 75%

Nie25%SiO2 and 65%Nie10%Fee25%SiO2 catalysts, and as the

methane decomposition proceeded, the methane conversion

decreased continuously. Thus, NieSiO2 and NieFeeSiO2

showed a reverse trend during methane decomposition at

823 K and 923 K. The NieFeeSiO2 catalysts weremore suitable

to catalyze methane decomposition at a higher reaction

temperature.

Fig. 2 shows the effect of the CH4 flow rate on methane

decomposition over 65%Nie10%Fee25%SiO2 catalyst at 823 K.

It can be observed that the catalytic activity of 65%Nie10%

Fee25%SiO2 didn’t change apparently during 400 min when

the CH4 flow rate was 15 mL/min. The methane conversion

over 65%Nie10%Fee25%SiO2 with 25 mL/min of CH4 flow rate

0 50 100 150 200 250 300 350 4000

5

10

15

20

25

30

35

40

V(CH4)=15mL/min V(CH4)=25mL/min

Met

hane

con

vers

ion(

%)

t(min)

Fig. 2 e Effect of the CH4 flow rate on methane

decomposition over 65%Nie10%Fee25%SiO2 (823 K,

m(catalyst)[ 30 mg).

was close to that of 65%Nie10%Fee25%SiO2with 15 mL/min of

CH4 flow rate in the first 110 min, however, the catalytic

activity decreased gradually as the reaction continued and

a slight deactivation took place. It can be speculated that the

higher CH4 flow rates can decrease the methane conversion

and reduce the lifespan of the 65%Nie10%Fee25%SiO2 cata-

lyst during methane decomposition and this was in agree-

ment with the results of Suelves et al. [20] and Domınguez

et al. [21] pointed out that higher CH4 flow rates reduced the

contact time between the CH4 molecules and the active

centers present on the catalyst and then the conversion was

impaired.

3.2. Characterization of the catalysts

3.2.1. The morphological appearance of the catalystsThe morphological appearance of the fresh and the reduced

NieSiO2 and NieFeeSiO2 catalysts are displayed in Fig. 3. All

samples appeared as large agglomerates of particles. It could

be clearly observed that the size of the catalyst particles was

highly influenced by the iron content in the catalysts. The

particles of lower content of iron catalyst such as the fresh

and the reduced 65%Nie10%Fee25%SiO2 were smaller and

more homogeneous than those of the fresh and the reduced

75%Nie25%SiO2. While the higher content of iron in the fresh

and the reduced 35%Nie40%Fee25%SiO2 catalysts increased

the particle size of the catalysts and the catalysts turned into

big blocks.

3.2.2. The compositions of the fresh and the reduced catalystsFig. 4(a) shows the XRD spectra of the fresh NieSiO2 and

NieFeeSiO2 catalysts. In the XRD spectra of the fresh 75%

Nie25%SiO2 catalyst, the diffraction peaks due to NiO species

were observed at 2q¼ 32.21�, 43.25�, 62.82�, 75.34� and 79.32�

indicating that Ni species in the fresh NieSiO2 catalyst were

mainly present as NiO. No diffraction peaks due to SiO2 were

observed which demonstrated that SiO2 in the catalysts

existed in an amorphous state. Although iron was introduced,

no Fe2O3 diffraction peaks were observed in the XRD patterns

Fig. 3 e SEM images of NieFeeSiO2 catalysts with different iron contents (a: fresh 75%Nie25%SiO2; b: fresh 65%Nie10%

Fee25%SiO2; c: fresh 35%Nie40%Fee25%SiO2; d: reduced 75%Nie25%SiO2; e: reduced 65%Nie10%Fee25%SiO2; f: reduced 35%

Nie40%Fee25%SiO2).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 6 9061

of the fresh 65%Nie10%Fee25%SiO2 and 35%Nie40%Fee25%

SiO2 catalysts. The iron in the fresh 65%Nie10%Fee25%SiO2

and 35%Nie40%Fee25%SiO2 catalysts mainly existed in the

form of NiFe2O4 spinel. With the iron content increased from

10% to 40%, the intensity of NiFe2O4 peaks turned stronger

while the intensity of NiO peaks dropped which suggested

that more iron in the catalyst reacted with NiO to generate

more NiFe2O4.

Fig. 4(b) shows the XRD spectra of NieSiO2 and NieFeeSiO2

catalysts reduced by hydrogen. The diffraction peaks at

2q¼ 44.46�, 51.81� and 76.36� in the XRD spectra of 75%Nie25%

SiO2 revealed that Ni species in the reduced 75%Nie25%SiO2

20 30 40 50 60 70 80

Inte

nsity

/cps

75%Ni-25%SiO

2θ/(°)

35%Ni-40%Fe-25%SiO

65%Ni-10%Fe-25%SiO

the fresh catalysts ba

Fig. 4 e XRD spectra of the fresh and the reduced NieSiO2 and

NieFe alloy).

catalyst existed mainly in the form of the metallic Ni. The

diffraction peaks due to the iron were not detected even if the

content of the iron was very high in the NieFeeSiO2 catalysts

for the iron in the catalysts existed in the form of NieFe alloy.

The NieFeeSiO2 catalysts reduced by hydrogen have two

metal phases: the Ni metal phase and the NieFe alloy phase.

The diffraction peaks due to NieFe alloy and Ni metal over-

lapped together which indicated that the NieFe alloy had

identical structure with the Ni metal. Additionally, the NieFe

alloy diffraction peak positions of NieFeeSiO2 catalyst shifted

to a lower angle comparedwith Nimetal although this change

was not very obvious and this suggested that the lattice

20 30 40 50 60 70 80

75%Ni-25%SiO

65%Ni-10%Fe-25%SiO

Inte

nsity

/cps

2θ/(°)

35%Ni-40%Fe-25%SiO

the reduced catalysts

NieFeeSiO2 catalysts (- e NiO, C e NiFe2O4, : e Ni, A e

Table 1 e X-ray diffraction data for NieSiO2 and NieFeeSiO2 catalysts.

Fresh catalysts NiO latticeparameter (nm)

NiFe2O4 latticeparameter (nm)

NiO domainsize (nm)

NiFe2O4 domainsize (nm)

75%Nie25%SiO2 0.4177 w 31.85 w

65%Nie10%Fee25%SiO2 0.4177 0.8377 25.5 10.25

35%Nie40%Fee25%SiO2 0.4177 0.8377 30.6 13.1

Reduced catalysts Ni Lattice

parameter (nm)

NieFe alloy lattice

parameter (nm)

Ni domain

size (nm)

NieFe alloy domain

size (nm)

75%Nie25%SiO2 0.3524 w 25.87 w

65%Nie10%Fee25%SiO2 0.3524 0.3552 13.2 10

35%Nie40%Fee25%SiO2 0.3524 0.3598 25.2 23.87

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 69062

parameters of the Ni crystal turned bigger when the iron

atoms entered into the lattice of the Ni metal.

Table 1 shows the XRD data of the fresh and reduced

NiFe2O4 and NieFe alloy. As the iron content of the fresh

NieFeeSiO2 catalysts increased from0 to 10%, the domain size

of NiO crystallite decreased from 31.85 nm to 25.5 nm, while

as the iron content increased to 40%, the domain size of NiO

particles increased to 30.6 nm. The domain size of NiFe2O4

increased from 10.25 nm to 13.1 nm with the rise of the iron

content from 10% to 40%. Apparently, the NieFe alloy lattice

parameter of the catalysts reduced by H2 was bigger than that

of the Ni metal and with the increase of the iron content, the

NieFe alloy lattice parameter turned even bigger. The Ni

domain size of the NieFeeSiO2 catalysts reduced by hydrogen

also showed a trend of first decrease and then increase with

the increase of the iron content whichwas similar with that of

the NiO in the fresh catalysts.

3.2.3. The reducibility of the nickel and iron species in thefresh catalystsThe TPR profiles of NieSiO2 and NieFeeSiO2 catalysts are

shown in Fig. 5 to study the reducibility of the catalysts. A

reduction peak at the temperature of 693 K and a shoulder

peak at the temperature of 623 K were observed in the TPR

500 600 700 800 900 1000

35%Ni-40%Fe-25%SiO2

65%Ni-10%Fe-25%SiO2

Hyd

roge

n co

nsum

ptio

n

T/K

75%Ni-25%SiO2

Fig. 5 e Temperature programmed reduction profiles of

NieSiO2 and NieFeeSiO2 catalysts.

profile of 75%Nie25%SiO2 catalyst. Generally, for Ni-based

catalysts the lower temperature peaks were attributed to the

reduction of the bulk NiO particles without interaction with

the support, while the higher temperature peaks were

assigned to the reduction of NiO particles with chemical

interaction with the support or other surface compounds [22].

Thus, the shoulder peak at 623 K of 75%Nie25%SiO2 was

associated with the bulk NiO interacted with SiO2 slightly.

While, the higher temperature peak at 693 K was attributed to

NiO which strongly interacted with SiO2.

Awider reduction peak began at 573 K and finished at 803 K

with amaximumat 683 K for 65%Nie10%Fee25%SiO2 catalyst.

A shoulder peak appeared at around 623 K which was similar

with that of the 75%Nie25%SiO2 catalyst. Another shoulder

peak appeared at 750 K which was attributed to the formation

of NiFe2O4. The 35%Nie40%Fee25%SiO2 catalyst indicated

a very different reduction behaviorda leading peak: the

hydrogen consumption peak began from 593 K to 893 K and

reached the maximum at around 823 K which suggested that

the nickel and iron in the catalyst mainly existed in the form

of NiFe2O4 species and this was in agreement with the XRD

studies.

3.3. Characterization of the catalysts after methanedecomposition

Fig. 6 shows the SEM images of the NieSiO2 and NieFeeSiO2

catalysts after methane decomposition at 823 K and 923 K,

respectively. It could be clearly observed that the surface of

catalyst was completely covered with interlaced carbon fila-

ments comparedwith the clean surfaces of the catalyst before

the reaction (Fig. 3). The structure of the carbon filaments was

seriously influenced by the reaction temperature and the

introduced iron. The carbon filaments generated over 75%

Nie25%SiO2 catalysts at 823 K (Fig. 6(a)) were longer and

thicker than those generated at 923 K (Fig. 6(b)). When the iron

was introduced into the catalysts, the structure of the carbon

filaments also changed. At 823 K, the carbon filaments

generated over 35%Nie40%Fee25%SiO2 catalyst were thinner

and shorter than those generated over 75%Nie25%SiO2 cata-

lyst. While, at 923 K, the carbon filaments generated over 35%

Nie40%Fee25%SiO2 catalyst were much longer and more

homogeneous than those generated over 75%Nie25%SiO2.

Thus, it could be speculated that the carbon capacity of

NieFeeSiO2 catalysts during methane decomposition at 923 K

was higher than that of 75%Nie25%SiO2 and this might be the

Fig. 6 e SEM images of carbon filaments generated over NieSiO2 and NieFeeSiO2 catalysts (a: 75%Nie25%SiO2, 823 K; b: 75%

Nie25%SiO2, 923 K; c: 35%Nie40%Fee25%SiO2, 823 K; d: 35%Nie40%Fee25%SiO2, 923 K).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 6 9063

reason why NieFeeSiO2 catalyst showed a longer lifetime

when methane decomposition took place at 923 K.

Fig. 7(a) and (b) shows the TEM images of carbon filaments

generated bymethane decomposition over 75%Nie25%SiO2 at

823 K and 923 K, respectively. In the TEM images of 75%

Nie25%SiO2 after methane decomposition at 823 K (Fig. 7(a)),

the carbon filaments with diameters from 70 nm to 110 nm

were observed. The “pear-shaped” Ni metal particles were

present at the tip of the carbon filaments. While in the TEM

images of 75%Nie25%SiO2 catalyst after methane decompo-

sition at 923 K (Fig. 7(b)), the diameter range of carbon fila-

ments was 50e90 nm and spherical Ni metal particles

appeared at the tip of the carbon filaments. The diameters of

both types of carbon filaments were similar to their diameters

of the catalyst particles at the tips. Takenaka et al. [9] pointed

out that Ni metal particles with smaller diameters were more

effective for methane decomposition into hydrogen and

carbon filaments at a higher reaction temperature. This might

be the reason why the carbon filaments turned thinner as the

reaction temperature become higher.

Moreover, the different shapes of Ni metal particles at

different reaction temperatures reflected the different carbon

diffusion mechanisms during methane decomposition. It was

recognized bymany researchers that methane decomposition

and carbon growth over Ni-based catalysts included three

stages: 1) activation and decomposition of methane on (1 0 0)

and (1 1 0) metal surface planes, 2) carbon dissolution and

diffusion through the metal particles, 3) carbon segregation in

the form of graphite-like phase on (1 1 1) nickel planes due to

crystallographic matching of (1 1 1) nickel surface to (0 0 2)

graphite planes [23]. Takenaka et al. [9] pointed out that the

formation of the “pear-shaped” Ni metal particles was due to

the diffusion of carbon atoms through the bulk of Ni metal

particles and “fish-bone” carbon filaments formed in this way,

while the formationof the spherical Nimetal particles resulted

from the diffusion of carbon atoms on the surface of Ni metal

particles and “multi-walled” carbon filaments formed in this

way. Thus, it could be speculated that the carbon atoms

generated during methane decomposition was preferential to

diffuse through the surface of theNimetal particles at a higher

reaction temperature and the carbon structure changed with

the change of the carbon diffusion mechanism. A clearer TEM

images of “multi-walled” carbon filaments generated over 75%

Nie25%SiO2 at 923 K was shown in Fig. 8(a).

Fig. 7(c) and (d) shows the TEM images of carbon filaments

generated by methane decomposition over 35%Nie40%

Fee25%SiO2 catalyst at 823 K and 923 K, respectively. The

structure of the carbon filaments generated over 35%Nie40%

Fee25%SiO2 catalyst at 823 K was similar with that of the

carbon filaments generated over 75%Nie25%SiO2 at 923 K,

while the diameters of the carbon filaments were a little

smaller. “Bamboo-shaped” carbon filaments formed during

methane decomposition over 35%Nie40%Fee25%SiO2 catalyst

at 923 K$ The wall of the “bamboo-shaped” carbon filaments

Fig. 7 e TEM images of carbon filaments generated over NieSiO2 and NieFeeSiO2 (a: 75%Nie25%SiO2, 823 K; b: 75%Nie25%

SiO2, 923 K; c: 35%Nie40%Fee25%SiO2, 823 K; d: 35%Nie40%Fee25%SiO2, 923 K).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 69064

were uneven and the hollows of these carbon filaments were

divided into many cells which was different from those

generated over 75%Nie25%SiO2 catalyst. The NieFe alloy

particles which were active for methane decomposition were

not only present at the tip of the carbon filaments after

methane decomposition, but also existed in the hollow cells of

the “bamboo-shaped” structure. A clearer “bamboo-shaped”

Fig. 8 e HRTEM images of carbon filaments generated over Nie

Nie40%Fee25%SiO2).

carbon filaments was also shown in Fig. 8(b). Apparently, the

addition of the iron changed the carbon deposition mecha-

nism of NieSiO2. The results of Takenaka et al. [17] indicated

that the products of methane decomposition over FeeSiO2

catalysts were also “bamboo-shaped” carbon filaments. Thus,

the carbon deposition mechanism of NieFeeSiO2 was similar

with that of FeeSiO2 catalyst.

SiO2 and NieFeeSiO2 at 923 K (a: 75%Nie25%SiO2; b: 35%

Table 2 e Changes in ratio of area of the D band to that ofthe G band (ID/IG) in Raman spectra of carbon filamentsgenerated over 75%Nie25%SiO2 and 35%Nie40%Fee25%SiO2.

Temperature(K)

ID/IG (75%Nie25%SiO2)

ID/IG (35%Nie40%Fee25%SiO2)

823 1.25 1.13

923 0.935 0.745

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 9 0 5 8e9 0 6 6 9065

Ermakova et al. [24] pointed out that the carbon growth

over FeeSiO2 catalysts did not need a specific set of edges, as it

did in the case of nickel. The iron particles might be liquid

during methane decomposition. When methane decomposi-

tion over the FeeSiO2 catalyst took place at 873e1073 K, iron

carbide generated rapidly. However, the iron carbide was not

stable and decomposition of the unstable carbide made the

iron particles supersaturation with carbon and the graphitic

nucleus appeared. With the formation and decomposition of

the iron carbide continuously, other carbon generated

assembled around the former nucleus for the nucleationmust

overcome a high activation barrier. Along with the accumu-

lation of the carbon a part of the iron particle covered with

graphite gradually extended and become narrower and nar-

rower. At the same time, the orientation of the graphite layers

also transformed from parallel to the iron particle surface to

perpendicular to the iron particle surface gradually and

a “bamboo” node of the carbon filaments finally formed in this

way.

Fig. 9 shows the raman spectra of the 75%Nie25%SiO2 and

35%Nie40%Fee25%SiO2 catalysts after methane decomposi-

tion at 823 K and 923 K. For all the carbon deposited over

different catalysts at different reaction temperatures, two

bands were clearly observed at 1350 cm�1 (D band) and

1580 cm�1 (G band), respectively. The G band at 1580 cm�1 was

attributed to the in plane carbonecarbon-stretching vibration

of graphitic layers and the D band at 1580 cm�1 was ascribed

to the structural imperfection of the graphite. A shoulder peak

(D0 band) in G band was also found at 1615 cm�1 which was

also assigned to be the imperfect graphite or disordered

carbons [25,26].

Table 2 shows the ratio of the area of D band to that of the G

band (ID/IG) for different carbon filaments generated over

NieSiO2 and NieFeeSiO2 catalysts The ID/IG value could be

regarded as a symbol for the crystalline order of graphite. The

reaction temperature and the introduced iron strongly

1300 1400 1500 1600 1700

35%Ni-40%Fe-25%SiO2,823K

75%Ni-25%SiO2,823K

35%Ni-40%Fe-25%SiO2,923K

75%Ni-25%SiO2,923K

inte

nsity

(a.u

.)

Raman shift(cm-1)

D bandG band D' band

Fig. 9 e Raman spectra of carbon filaments obtained over

NieSiO2 and NieFeeSiO2 catalysts.

affected the crystalline order of the generated graphite. For

the carbon filaments generated over 75%Nie25%SiO2, the ID/IGvalue decreased as the reaction temperature increased, indi-

cating that the graphitic order of the carbon filaments become

higher with the rise in the reaction temperature and this was

in agreement with Echegoyen’s results [9]. Thus, the graphitic

order of the “multi-walled” carbon filaments generated at

a higher reaction temperature was higher than that of the

“fish-bone” type filaments.When the ironwas introduced into

NieSiO2 catalysts, the graphitic order of the carbon filaments

generated at both 823 K and 923 K become higher. It could also

be speculated that the “bamboo-shaped” carbon filaments

should be more perfect than the “multi-walled” carbon

filaments.

4. Conclusions

1) The introduction of the iron into NieSiO2 catalysts obvi-

ously extended the lifetime of the catalysts at 923 K during

methane decomposition. However, a reverse trend was

shown when methane decomposition took place at a lower

reaction temperature such as 823 K.

2) XRD studies of NieFeeSiO2 catalysts reduced by hydrogen

indicated that NieFe alloy was formed when iron was

introduced into NieSiO2 catalyst.

3) The structure of the carbon filaments formed over NieSiO2

and NieFeeSiO2 catalysts was different. At 923 K, “multi-

walled” carbon filaments were generated over 75%Nie25%

SiO2 catalyst, while “bamboo-shaped” carbon filaments

were generated over 35%Nie40%Fee25%SiO2 catalysts. The

graphitic order of “bamboo-shaped” carbon filaments was

higher than that of the “multi-walled” carbon filaments.

Acknowledgements

This work was financially supported by the National Magnetic

Confinement Fusion Science Program of China

(2011GB111005) and National Natural Science Foundation of

China (11075134).

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