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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 e n e r g y 3 5 ( 2 0 1 0 ) 3 5 0 9 – 3 5 2 1

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Ni/Ce-MCM-41 mesostructured catalysts for simultaneousproduction of hydrogen and nanocarbon viamethane decomposition

J.C. Guevara a, J.A. Wang a,*, L.F. Chen a, M.A. Valenzuela a, P. Salas b, A. Garcıa-Ruiz c,J.A. Toledo d, M.A. Cortes-Jacome d, C. Angeles-Chavez d, O. Novaro e

a ESIQIE, Instituto Politecnico Nacional, Col. Zacatenco, Av. Politecnico s/n, 07738 Mexico D. F., Mexicob Centro de Fısica Aplicada y Tecnologıa Avanzada, Universidad Nacional Autonoma de Mexico, Apartado Postal 1-1010, Queretaro 76000,

Mexicoc UPIICSA, Instituto Politecnico Nacional, Te 950 Col. Granjas-Mexico, 08400 Mexico D.F., Mexicod Programa de Molecular Ingenierıa, Instituto Mexicano del Petroleo, Eje Lazaro Cardenas 152, 07730 Mexico D. F., Mexicoe Instituto de Fisica, Universidad Nacional Autonoma de Mexico, A. P. 20-364, 01000 Mexico D.F., Mexico

a r t i c l e i n f o

Article history:

Received 13 October 2009

Received in revised form

13 January 2010

Accepted 16 January 2010

Available online 19 February 2010

Keywords:

Hydrogen production

Methane catalytic decomposition

Carbon nanotube

Ce-MCM-41

Ni-based catalysts

* Corresponding author. Tel.: þ52 55 5729600E-mail address: [email protected] (J.A

0360-3199/$ – see front matter ª 2010 Profesdoi:10.1016/j.ijhydene.2010.01.068

a b s t r a c t

For the first time, simultaneous production of hydrogen and nanocarbon via catalytic

decomposition of methane over Ni-loaded mesoporous Ce-MCM-41 catalysts was investi-

gated. The catalytic performance of the Ni/Ce-MCM-41 catalysts is very stable and the

reaction activity remained almost unchanged during 1400 min steam on time at temper-

atures 540, 560 and 580 �C, respectively. The methane conversion level over these catalysts

reached 60–75% with a 100% selectivity towards hydrogen. TEM observations revealed that

most of the Ni particles located on the tip of the carbon nanofibers/nanotubes in the used

catalysts, keeping their exposed surface clean during the test and thus remaining active for

continuous reaction without obvious deactivation. Two kinds of carbon materials,

graphitic carbon (Cg) as major and amorphous carbon (CA) as minor were produced in the

reaction, as confirmed by XRD analysis and TEM observations. Carbon nanofibers/nano-

tubes had an average diameter of approximately 30–50 nm and tens micrometers in length,

depending on the reaction temperature, reaction time and Ni particle diameter. Four types

of carbon nanofibers/nanotubes were detected and their formations greatly depend on the

reaction temperature, time on steam and degree of the interaction between the metallic Ni

and support. The respective mechanisms of the formation of nanocarbons were postulated

and discussed.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction regulations require the utilization of clean energy in a variety of

Nowadays, the world is facing two significantly related chal-

lenges: an increasing demand for new energy and a stringently

environmental concern. The increasingly environmental

0x55261.. Wang).sor T. Nejat Veziroglu. Pu

areas. Hydrogen is such a clean fuel that has received a great

attention because of its importance in fuel cell technology,

petroleum refining, food, electronics and metallurgical pro-

cessing industries and many other fields [1–4]. Approximately

blished by Elsevier Ltd. All rights reserved.

mailto:[email protected]

http://www.elsevier.com/locate/he


95% of the hydrogen is produced from carbonaceous raw

materials, primarily fossil in origin and the rest from alterna-

tive resources as biomass and water [5]. There are a number of

emerging and attractive approaches or processes for the

production of hydrogen, as the natural gas catalytic decom-

position, steam reforming, photocatalytic decomposition of

water and biomass gasification, etc. [5–12]. Hydrogen produced

by photocatalytic decomposition of water is very clean and it is

an economical approach. Unfortunately, it still remains at

laboratory stage because of some technical difficulties for

scale-up. Steam reforming of methane is highly endothermic

and produces a large amount of COx (CO2 and CO). Therefore,

additional steps are required to remove these COx by-products

in order to meet the requirements in the fuel cell applications,

which increases the operation cost. Economic analysis has

shown that if the carbon produced in the reaction procedure

can be utilized, it will be financially attractive. It is noted that

both hydrogen and carbon nanotube materials can be simul-

taneously produced through a single technical approach of

methane catalytic decomposition (MCD) that is regarded as

a fully green chemical process. By methane catalytic decom-

position, COx (CO and CO2)-free hydrogen can be obtained with

a moderately endothermic process [13–19]. For hydrogen

production, the MCD process is believed to be superior to steam

reforming from economical points of view, and it is also

superior to the photocatalytic decomposition of water from the

technical point of view [2,20]. Moreover, MCD is easy for

scaling-up for the production of carbon nanotubes, and it is

used to obtain multiwalled nanotubes with high mechanical

strength, differing from the route of arc-discharge evaporation

of graphite where single wall carbon nanotubes could be

produced [20].

The catalysts used for methane catalytic decomposition

usually consist of transition metals and a support. SiO2

[17,21–23], MgO [22], Al2O3 [20,24,25], SiO2–Al2O3 [26], SiO2–

CeO2 [27], ZnAl2O4 [28] and MgAl2O4 [29], zeolite [5], carbon

[30–32] and hydrotalcite-like materials [33] are the most

common support. The active phases are usually group VIII

transition metals Fe, Co and Ni, others like Cu and Pd are

used as promoters [5,16,25,33–37]. Among these, the

intensely studied catalysts are Ni supported alumina and Ni

supported silica.

The main influence factors on MCD are the reaction

temperature, nature of the support, type and quantity of

active metals. The activity and the lifetime of the Ni supported

catalysts depend significantly on the size of the metal particle

and are sensitive to the textural properties and pore geometry

of the support [29]. It is known that the surfactant-assistant

technique in the materials synthesis may provide materials

with appropriate pore diameter, high surface areas and

a better access to the active sites [38,39]. The application of

mesoporous materials as catalysts support for hydrogen

production has not been reported yet. In addition, in

a preliminary investigation, we found that ceria doped silica

as catalyst support shows a rather stable catalytic activity in

the methane decomposition reaction [27]. In the present work,

we report some new results of the synthesis, structural

characterization and catalytic properties of a series of Ni/Ce-

MCM-41 mesoporous catalysts synthesized by a surfactant-

assisted technique for simultaneous production of hydrogen

and nanocarbons (nanotubes and nanofibers). These catalysts

exhibit high catalytic stability in comparison with the tradi-

tional Ni/SiO2 catalysts. Several kinds of nanocarbons are

formed and their formation mechanisms are discussed and

postulated.

2. Experimental section

2.1. Synthesis of Ce-MCM-41 mesoporous materials

The cerium promoted mesoporous materials (noted as

Ce-MCM-41) were prepared by the use CeCl3$7H2O as cerium

precursor, tetraethyl ortosilicate (TEOS) as silicon precursor

and cetyltrimethylammonium bromide (CTABr) bromide as

synthetic templating agent. A typical preparation of the

sample with a molar relationship of Si/Ce ¼ 20 was described

as follows. Two solutions were prepared, the first solution

was prepared by 1.86 g of CeCl3$7H2O and 22.8 ml of TEOS

diluted in 50 ml of water with agitation; the second solution

was made by addition of 11.4 g of CTABr in 200 ml hot water

(near 50 �C) with stirring, followed by the addition of 145 ml of

NH3$H2O (28 wt.%). Then the first solution was added, drop by

drop, into the second solution to obtain a mixture. During the

addition, the mixture was vigorously agitated for near 2 h

until the gel was formed. The resulting gel was heated at

100 �C by 24 h. The solid was washed with deionized water

and dried for 24 h. Finally the sample was annealed at 600 �C

by 5 h in air with a flow rate of 60 ml/min. The heating rate

was set at 1 �C/min. The other two samples with Si/Ce molar

ratio 10 and 5 were synthesized with the similar method as

described above.

2.2. Preparation of Ni/Ce-MCM-41 catalysts

The nickel supported catalysts were prepared by incipient

impregnation of the calcined supports with Ni(NO3)2 solu-

tion. The Ni loading of the catalysts was 30 wt.% and 50 wt.%,

respectively. After impregnation, the Ni supported materials

were dried at 80 �C for 10 h and then calcined at 400 �C for 4 h.

Before the catalytic evaluation, the oxidized catalysts were

reduced using 99.9% H2 at 500 �C for 2 h to obtain metallic Ni

particles on the catalyst surface. These catalysts are noted as

Ni/Ce-MCM-41-x, where x is Si/Ce molar ratio, x ¼ 5, 10

and 20.

2.3. N2-adsorption–desorption isotherms measurement

The specific surface area, pore volume and pore size distri-

bution of the Ce-MCM-41 samples were measured in a Dig-

isorb 2600 equipment by using low temperature N2

physisorption isotherms. Before the N2 adsorption, the

samples calcined at 600 �C were thermally treated at 400 �C for

2 h. The surface area was determined according to the stan-

dard Brunaur–Emmett–Teller (BET) method in a relative

pressure range of 0.04–0.2 and the total volume was evaluated

from the amount of adsorbed N2 at a relative pressure (P/P0) of

about 0.99. The pore diameter distributions were calculated

based on the desorption isotherms by the Barrett–Joyner–

Halenda (BJH) method.

Table 1 – Ni atom position of metallic Ni crystals (spacegroup: Fm3m).

Atom x y z

Ni 0.0000 0.0000 0.0000

Table 3 – C atom position of the amorphous cabonmaterial (space group: P63/mmc).

Atom x y z

C1 0.0000 0.0000 0.0468

C2 0.3333 0.6667 0.0781

C3 0.3333 0.6667 1.1718

C4 0.3333 0.6667 0.7968

Inte

nsid

ad (u

.a.)

(210)(200)(110) Si/Ce - 20

600°C

(100)

Si/Ce - 10

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 e n e r g y 3 5 ( 2 0 1 0 ) 3 5 0 9 – 3 5 2 1 3511

2.4. Low angle X-ray diffraction analysis

The low angle X-ray diffraction patterns of the samples were

measured in a D-500 SIEMENS diffractometer with a graphite

secondary beam monochromator to obtain a monochromatic

Cu Ka1 radiation, and the evaluation of the diffractograms was

made by DIFFRAC/AT software. The scanning was made from

1.5 to 10, 2q step size of 0.02 and step time of 2 s. Position

correction was made using the NIST standard reference

material 675.

2.5. Powder X-ray diffraction and Rietveld refinement

The power X-ray diffraction data were collected at room

temperature in a Siemens D-5000 diffractometer with Cu Ka

radiation and a secondary beam monochromator. The inten-

sities were obtained in the 2q range between 20 and 60� with

a step of 0.02� and a measuring time of 2.67 s at each point.

The crystalline structures were refined using FULLPROF98

code with the Rietveld method [40,41]. The atomic positions

and their coordinates of each structure corresponding to

metallic Ni and graphite carbon and amorphous carbon are

reported in Tables 1–3. The goodness of the refinement fitting

the experimental patterns reported as an Rwp closed to 0.14

was reached.

2.6. Electron microscope

Transmission electron microscopy (TEM) images of Ce-MCM-

41 and Ni/Ce-MCM-41 were carried out in a JEM-2200FS

transmission electron microscope with accelerating voltage of

200 kV. The microscope is equipped with a Schottky-type field

emission gun and an ultra high resolution (UHR) configuration

(Cs ¼ 0.5 mm; Cc 1.1 mm; point-to-point resolution, 0.19 nm)

and in-column energy filter omega-type. The powder samples

were grounded softly in an agate mortar and dispersed in

isopropyl alcohol in an ultrasonic bath for several minutes. A

few drops were then deposited on 200 mesh copper grids

covered with a holey carbon film.

2.7. Raman spectroscopic analysis

The Raman spectra were obtained at room temperature using

a Yvon Jobin Horiba (T64000) spectrometer, equipped with

Table 2 – C atom position of graphite carbon (space group:P63/mmc).

Atom x y z

C1 0.0000 0.0000 0.2500

C2 0.3333 0.6667 0.2500

a CCD camera detector. As a source of excitation the 514 nm

lines of a Spectra Physics 2018 Argon/Krypton Ion Laser

system were focused through an Olympus BX41 microscope

equipped with a 100� magnification objective.

2.8. Catalytic evaluation

The methane catalytic decomposition was carried out in

a microreactor system (Advanced Scientific Design-RXM-100)

with a stainless steel fixed bed reactor (10 mm i.d. and 500 mm

in length) at atmospheric pressure. The reaction temperatures

varied from 500 �C to 540 �C and 580 �C. The catalyst loading

was ca. 50 mg. The feed steam was a mixture of high-purity

methane diluted in argon (20 mol % of methane). The total

flow of the reaction gases was 75 ml min�1. The temperature

increasing rate was controlled at 30 �C min�1. The inlet and

outlet effluents were monitored by an on-line gas chromato-

graph (GC) analyzer with a PE-Molsieve capillary column,

using a thermal conductivity detector (TCD) for hydrogen

analysis and a flame ionization detector (FID) for methane

analysis.

0 2 4 6 8 1 02θ

Si/Ce - 5

Fig. 1 – Low angle of X-ray diffraction patterns of the

calcined Ce-MCM-41 samples.

0,00 0,25 0,50 0,75 1,000

200

400

600

800

1000

0

200

400

600

800

1000

0

200

400

600

800

1000

Vol A

dsor

bed

(cc/

g)

RELATIVE PRESSURE (P/Po)

Si/Ce - 5

Vol A

dsor

bed

(cc/

g)

Si/Ce - 20

Vol A

dsor

bed

(cc/

g)

Si/Ce - 10

Fig. 2 – Loops of N2-adsorption–desportion isotherms of the

Ce-MCM-41 samples.

10 100 1000

Si/Ce - 5

dV/d

log(

D) (

cc/g

)Pore Diameter (Å)

Si/Ce -20

Si/Ce -10

Fig. 3 – Pore diameter distribution of the calcined Ce-MCM-

41 samples.


3. Results

3.1. Ce-MCM-41 supports

3.1.1. Crystalline structureFig. 1 shows the XRD patterns of the calcined cerium-con-

taining solids. All the samples have three diffraction peaks,

which respectively correspond to (100), (110) and (200)

Table 4 – Physical properties of mesoporous Ce-MCM-41.a

SiO2/CeO2(mole ratio) d100 (nm) ao (nm) Dp (nm)

5 4.60 5.31 2.62

10 4.42 4.89 2.62

20 4.10 4.63 2.70

a s ¼ d100 – Dp; ao ¼ 2d100/O3.

reflections of the solid. The XRD patterns indicate that the

long-range order of mesoporous molecular sieves with

hexagonal framework was formed in the materials. These

XRD patterns are very similar to the pure Si-MCM-41 sample,

indicating that a small amount cerium incorporated into the

Si-MCM-41 framework does not strongly modify the structure

of Si-MCM-41. However, it is observed that as the cerium

content increases, intensities of the XRD peaks gradually

diminish, showing a reduction of the structural ordering. For

the sample containing high cerium content, e.g., Si/Ce¼ 5, the

peaks related to (110) and (200) are rather low, which indicates

that the hexagonal lattice structure might partially collapse,

forming wormhole-like pore system in some degree.

3.1.2. Textural propertiesBased on the IUPAC classification, the loops of the N2

adsorption–desorption isotherms belong to type IV profiles

(Fig. 2). The presence of framework-confined mesoporous is

indicated by the adsorption step centered in the relative

pressure P/Po region from 0.25 to 0.45. It is found that the

shape and sharpness of the loops vary with the cerium

Dmp (nm) Dlp (nm) s (nm) S.A.BET (m2/g)

1.98 444

3.5 40–100 1.62 620

3.5 20–100 1.31 820

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

TIME (MIN)

Co

nversio

n (%

)

Ni30%/Ce-MCM-41-5

Ni50%/Ce-MCM-41-5

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

TIME (MIN)

Co

nversio

n (%

)

Ni30%/Ce-MCM-41-10

Ni50%/Ce-MCM-41-10

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

TIME (MIN)

Co

nversio

n (%

)

Ni30%/Ce-MCM-41-20

Ni50%/Ce-MCM-41-20

Fig. 4 – Methane conversion versus time of Nı/Ce-MCM-41 at 580 8C and P [ 1 atm.


content. For example, as the cerium content increases in the

materials, the sharpness of the loop in the second stage

gradually decreases. The lines of the N2 adsorption–

desorption isotherms are almost overlapped in the region

between P/Po < 0.75, indicating that the adsorption and

desorption behaviors in the pores of the samples are very

similar.

The pore diameter distributions of the samples calculated

from the desorption branch of the isotherm by using the BJH

method and the corresponding data are shown in Fig. 3 and

Table 4. In the sample Si/Ce ¼ 20, pore diameter was

concentrated around 2.7 nm. As the cerium content increased,

bimodal pore diameter distributions were observed: the main

peak corresponds to pores having a diameter (Dp) approxi-

mately 2.7 nm and a small one relates to some pores with

larger diameter (Dmp) around 3.5 nm. The structural ordering

is reduced as more cerium ions are incorporated, thus some of

the adjacent pores might collapse at high cerium content

solid, forming some pores with diameter around 3.5 nm. In

addition to this, some large pores (Dlp) with a wide pore

diameter distribution were formed in the range between 40

and 100 nm for the sample Si/Ce ¼ 10 and between 20 and

100 nm for the sample with Si/Ce ¼ 5. The formation of these

pores in the mesoporous and macroporous region can be

explained by the formation of voids due to inter-nanoparticles

in contact.

The surface area (SABET) decreases from 820 to 620 and

444 m2/g as the Si/Ce molar ratio decreases from 20 to 10 and 5,

respectively. Among these samples, the solid with Si/Ce ¼ 5

has the largest lattice cell dimension and smallest surface

area and most pore wall thickness (s). These observations

show that cerium incorporation strongly affects the textural

properties of the resultant materials. High cerium content not

only leads to diminution of the long-range order of the mes-

ostructure and surface area, but also increases the population

of the pores with large diameter.

20 30 40 50

b

a

Inte

nsity

(a.

u.)

c

Two Theta (degree)

Fig. 5 – XRD patterns of the 30 Ni wt%/Ce-MCM-41 catalysts

after 22 h of reaction. This sample contains metallic Ni,

graphite and amorphous carbon. Marks show Ni (upper),

graphite (media), and carbon in a descendent order (lower).

(a) Si/Ce [ 20; (b) Si/Ce [ 10; (c) Si/Ce [ 5.

20 25 30 35 40 45 50 55-20

-10

0

10

20

30

40

50

60

70

80 Cg

CA

Cg

Ni

x103 C

ount

s

Two theta (degree)

50 wt % Ni /Ce-MCM-41-20

Ni

Fig. 6 – A Rietveld refinement plot of the sample 50 wt%

Ni/Ce-MCM-41 after 1400 min of reaction at 580 8C.

Experimental pattern is shown by symbols and the

calculated one by a continuous line. This sample contains

metallic Ni, graphite and quasi-amorphous carbon. Marks

show Ni (upper), graphite (media), and carbon in

a descendent order (lower). The quality of the refinements

is shown by the difference between both patterns which

appears as the lowest continuous line. In this case,

Rwp [ 0.14.


3.2. Hydrogen production over Ni/Ce-MCM-41 catalysts

A series of the catalysts of Ni/Ce-MCM-41 with Ni loading

30 wt.% and 50 wt.%, were used for methane catalytic decom-

position. Reaction temperature has a significant influence on

the methane conversion. Over all the catalysts, CH4 conversion

increased with increasing of the reaction temperature (not

shown). In order to study the catalytic stability of the catalysts,

the CH4 decomposition reaction was continuously carried out

for 1400 min. Fig. 4 shows the methane conversion via reaction

time at 580 �C over the different catalysts. During the 1400 min

continuous reaction, the methane conversion over different

catalysts varied between 67 and 74%, no obvious deactivation

was observed, indicating that the catalytic activity of the

catalysts is very stable under experimental conditions.

Compared with Ni/SiO2 catalyst on which complete deactiva-

tion took place after 2 h of reaction [27], the catalytic stability of

the Ni/Ce-MCM-41 catalysts is greatly enhanced by approxi-

mately 10 times. These results show that Ni-based Ce-MCM-41

Table 5 – Lattice cell parameters of metallic Ni, graphite and am

Cell parameter (nm) Cubic Ni

Upper Lower

a 0.35398 0.35288

c

mesostructured catalysts have a great potential for application

in the hydrogen production due to their high stability and good

catalytic activity.

It has been reported that the use of basic or redox

promoters decreases the formation of coke during the

reforming of methane with CeO2 through a mechanism of

Ce4þ/Ce3þ. This means that Ce species promote the adsorption

of CO2 and the elimination of coke species formed on the

surface of the catalyst [42]. Additionally, it has found that Ce

addition increases the thermal stability and affects the

metallic particle size in several catalysts and reactions [43]. In

the case of the methane decomposition reaction, in our

previous work, we have found that addition cerium may

enhance the catalytic stability [27] We speculate that cerium

addition may promote the carbon remove from specific

surface to remain active Ni clean and stabilize the Ni particle,

these roles are quite important for obtaining the catalysts

with long lifetime.

3.3. Carbon nanotube (CNT) formation

3.3.1. XRD analysisThe Fig. 5 shows the XRD patterns of the catalysts after

1400 min of reaction. The XRD reflections at 2q ¼ 44.2� and

orphous carbon materials.

Graphite C Amorphous C

Upper Lower Upper Lower

0.25191 0.23903 0.25406 0.25185

0.68699 0.67900 1.69775 1.63232

Fig. 7 – (a). Nickel unit cell, space group P63/mmc (S. G. 194) (b). Graphite unit cell. All atoms are C and colors are just used to

distinguish the two different atom positions corresponding to the space group P63/mmc (S. G. 194). (c) Amorphous carbon

unit cell (only for simulation purposes). All atoms are C and colors are just used to distinguish the four different positions

corresponding to the space group P63/mmc (S. G. 194).


53.8� correspond to metallic Ni [JCPDS No. 04-850] with an

average particle size approximately 40 nm. The reflections at

2q ¼ 26.1� and 54.0� are assigned to graphitic carbon [ICSD No.

01-0640]. While, a peak located at 42.9� is indicative of the

formation of quasi-amorphous nanocarbons.

The crystalline structures of each phase in the catalysts

were refined with the Rietveld method. Fig. 6 shows a typical

Rietveld refinement plot of the used catalyst 50 wt.% Ni/Ce-

MCM-41 with a Si/Ce molar ratio 10. Ni nanoparticles,

graphitic and quasi-amorphous nanocarbons were formed in

the sample. Quantitative data of the crystalline structures of

metallic Ni, graphite and quasi-amorphous carbon materials

determined from the Rietveld refinements are shown in

Table 5.

800 1000 1200 1400 1600 1800

1585

cm

-1

1352

.5 c

m-1

50wt% Ni/Ce-MCM-41-5

50 wt %NI/Ce-MCM-41-10

50wt % Ni/Ce-MCM-41-20

Raman Shift (cm-1)

Inte

nsity

(a.u

.)

Fig. 8 – Raman Spectra of Ni 50 wt. %/Ce-MCM-41 catalysts

after 1400 min of reaction at 580 8C.


Fig. 7a shows the cubic unit cell used to model the Ni

structure, whose symmetry corresponds to the space group

Fm3m (S. G. 225). The cell parameter a is between 0.35288 and

0.35398 nm. Each cell contains 4 Ni atoms and stacking of this

cell forms particles or cubic crystallites of around 40 nm in

average diameter. The perspective packing drawings of the

structures of carbon arrangements are shown in Fig. 7b and c.

These graphite and quasi-amorphous carbons have different

structures and particle sizes. The graphite carbon contributed

to the formation of CNTs. As shown in Table 5, the lattice cell

parameters corresponding to graphite structure vary within

a small range: a, from 0.23903 nm up to 0.25191 nm, and c from

0.67900 nm up to 0.68699 nm. The primitive rhombohedral unit

cell used for modelling the graphite structure, whose hexag-

onal symmetry corresponds to the space group P63/mmc. Note

that the angle g is 120� and that a hexagonal prism consists of

three unit cells. Every cell contains 4 atoms of C (Z¼ 4). Stacking

of this cell forms polyhedral crystallite particles with shape

hexagonal. Their growth results in the formation of carbon

nanotubes. Another kind of carbon particles with very fine

particle size is of quasi-amorphous, which is usually formed in

the initial stage of reaction, locating on the surfaces of the

metallic Ni particles. This carbon has a reduced local order

forming a hexagonal arrangement, as very small crystallites,

with a symmetry corresponding to the space group P63/mmc. Its

lattice parameters are: a between 0.25185 nm and 0.25406 nm; c

between 1.63232 nm and 1.69775 nm. Each cell contains 16

carbon atoms. Stacking of this arrangement forms small clus-

ters of carbon. The average diameter of this kind carbon clus-

ters is approximately 5.5 nm.

3.3.2. Raman spectroscopyRaman scattering is one of the most useful and powerful tech-

niques to characterize CNT samples. Raman scattering in

carbon nanotubes involves strong resonances of the incoming

and outgoing light and the vibration states with the electronic

energy levels of a nanotube. The Raman spectra, therefore,carry

a wealth of information about the electronic states and the

phonon dispersion of CNTs. Fig. 8 shows the Raman spectra of

the 50 wt.% Ni/Ce-MCM-41 used catalysts. Two bands around

1580 cm�1 and to 1350 cm�1 are observed. The strong peak

around 1580 cm�1 corresponds to G band of the graphitic carbon

arising from the zone-center E2g mode; the peak around

1350 cm�1 is assigned to D band, corresponding to A1g zone-edge

phonon inducedby the disorder due to finite crystalline size [44].

Appearance of D band is a clear indication of the formation of

multiwalled CNTs. The Raman spectrum further confirms that

carbon nanotubes are of multiple walls with a graphitic crys-

talline structure, which reveals the similar information as

shown by XRD analysis and the Rietveld refinement.

3.3.3. TEM imagesFig. 9 shows the bright field TEM images of the tested

Ni/catalysts after 1400 min of time on steam. Large quantities

of nanocarbons were deposited in the catalysts during the

methane decomposition. Most of the nickel particles located

at the tip of the carbon nanofibers/nanotubes. After 1400 min

of reaction, the exposed surfaces of the Ni particles remained

still clean; therefore, they could continuously decompose

methane to produce carbon nanofibers/nanotubes and

hydrogen. It can be observed that the nanocarbons have

multiple walls with length several hundred manometers to

tens micrometers. The outer diameter of the carbon nano-

tubes greatly depends on the size of Ni particles: larger Ni

particle leads to carbon nanotubes with larger diameter, as

shown in the arrowed CNTs in Fig. 9a and inset A in Fig. 9b.

The channel diameter ranged 4–7 nm, about 1/4–1/8 of the

outer diameter of the tubes. Almost all of the carbon nano-

tubes, no matter whether it is thin or thick in wall, they are

strongly curved. Some the CNTs have an open end and some

have a closed end where no Ni particles are observed (Fig. 9b).

As shown in Fig. 9c, some small pieces of Ni particles

embedded inside the carbon tubes were observed.

Most of the carbon nanofibers/nanotubes have multiple

walls parallel with the axis of the carbon nanotubes; however,

some of the nanocarbons have a fish-bone structure (Fig. 10).

At least, four types of nanocarbons were observed by TEM:

(i) Nanocarbons with mouth filled with Ni particle having

pear-shape, which can be divided into two classes: one

has carbon nanotubes with parallel layers in the walls

and another has carbon walls with fish-bone arrange-

ments. These two kinds of nanocarbons are predominant;

(ii) CNTs have open end but without Ni particle on the tip;

(iii) CNTs have closed end but without Ni particle on the tip;

(iv) CNTs embedded small size of Ni particle inside their

tubes. The simplified schematic pictures of these carbon

nanotubes/nanofibers are shown in Fig. 11.

4. Discussion

4.1. Ni particle shape variation and carbide-like phaseformation

It is noteworthy that in the fresh Ni/Ce-MCM-41 catalysts, most

of the Ni particles have a shape as pseudo-sphere. However,

Fig. 9 – TEM micrographs of the carbon nanotubes found in the catalytic evaluation of 50 wt% Ni/Ce-MCM-41-20 after

1400 min of reaction. Inset A in Fig. 9b shows Ni particles producing carbon nanotube of different external diameter.


after the reaction, the Ni particle shows a shape diamond or

pear-like. The sharp tail of the Ni particle inserts into the

carbon nanotube. It seems that the Ni particle becomes quasi-

liquid during the reaction. As we know that the melting point of

metallic Ni is 1452 �C and its Tamman temperature is around

Tm ¼ 726 �C. It is possible for a Ni particle to be fluidity only at

the temperature reaching its Tamman point. Therefore,

a question is naturally arisen: how can the Ni particle possess

unusual fluidity at a reaction temperature far below the

melting point, herein, 580 �C? It is reported that for a used

Ni/SiO2 catalyst, the C1s level of the XPS spectrum consists of

one bond at 281.9 eV that is attributed to carbide feature (Ni3C)

and another at 284.7 eV assigned to filament carbon [45]. It is

evident that Ni3C has lattice unit like hexagonal Ni crystal and

it is an inactive phase for methane decomposition. Ni3C is

unstable with respect to metal Ni and graphitic carbon; it,

therefore, can be decomposed into a mixture of nickel and

graphite at a relative low temperature, for example, 400 �C,

which is even lower than the Tamman temperature of pure Ni

particle [46,47]. Thus, Ni3C formation provides the possibility of

Ni or/C atoms fluidity. It is proven that carbon atoms produced

from CH4 decomposition over the Ni-based catalysts can form

NixCy solid solution as intermediate due to carbon atoms

dissolution into the bulk Ni; at such condition, C atoms are able

to move within the Ni particles and they are then released from

the NixCy solid solution to form graphitic structure in the

support–metal interface [45]. The Rietveld refinements of our

used catalysts confirm that the largest lattice cell parameter of

metallic Ni in our catalysts is a ¼ 0.35389 nm with a maximal

volume (Vm) of a close-packed arrangement of approximately

0.044354 nm3. It seems to be an insufficient room for carbon

atom (C atom radii is 0.077 nm) locating in the octahedral voids

of Ni crystal. However it is noteworthy that when C atoms

transfer within Ni crystals, the lattice constituents oscillating

Fig. 10 – (a) TEM shows that in the potion contacting the Ni

particle, the graphite layers show as fish-bones

arrangements. After their growth overpasses the Ni

particle dimension, they changed to parallel with the axis

of the tubes in the rest part. (b). A complete fish-bone

structure in the walls of the carbon nanofibers was

observed.

Fig. 11 – The simplified schematic pictures of four types of

carbon nanotubes/nanofibers.


must be produced. Therefore, the temperature of the Ni particle

surface is at an overheat state with respect to its surrounding

temperature because of the continuous release of heat energy

during the CH4 decomposition. Such a lattice constituents

oscillating is equivalent to temperature elevation in the Ni

crystal system to form hot spots. As a result, the Ni-C system

may transfer into a quasi-liquid state at a reaction temperature

even lower than the Tamman temperature inherent in it.

Now, the second question encounters: what is the driving

force to allow the carbon diffusion within the Ni particle?

There must have a gradient of carbide concentration in the Ni

particles during the MCD reaction, therefore, pressure can be

built up at the internal surface of the graphitic envelope, due

to the continuous formation of graphite layers. The mass

transfer of carbon occurred by diffusion through the bulk

particle under the effect of carbon concentration gradient,

drives the Ni particles to be squeezed out. The movement of

the C atoms along the Ni particles through both the surface

and bulk towards the interface induces the Ni sphere

changing to pear-like shape.

4.2. Mechanisms of various nanocarbons formation

4.2.1. Nanocarbons with end filled with Ni particles [48]It is well recognized that in the MCD reaction, C–H bonds in

methane are broken down on the surface of the Ni active

phases of the Ni/Ce-MCM-41 catalysts, producing carbon

atoms and hydrogen. In one hand, some of the carbon atoms

may spread on the surface of Ni particles and move towards

the metal–support interface; in the other hand, some of

carbon atoms may be dissolved into the Ni particle to form

NixCy metastable solid solution as intermediate which may be

decomposed to release Ni and C at the interface between

metal particle and support, where carbon atoms are con-

structed to form nanotubes by precipitation, nucleation and

crystallization. As the reaction proceeds, more carbon atoms

are released in the metal–support interface that forces the

carbon nanotubes to continuously grow, which supports the

Ni particle on the tip of the CNTs. The graphite layers con-

tacting the Ni particle oriented along 40–60� angles parallel

with the surface of the Ni particle (Fig. 10a). In the portion

contacting the Ni particle, the graphite layers show as fish-

bones arrangements. After their growth went beyond the Ni

particle dimension, they changed to parallel with the axis of

the tubes in the rest part. However, some carbon nanofibers

with complete fish-bone structures are also observed

(Fig. 10b). This widely accepted model also explains why Ni

particles are often located at the tip of one end of carbon

nanotubes.

Fig. 12 – Mechanism of the formation of carbon nanotube with closed end and embedded small Ni pieces.


4.2.2. Carbon nanotubes with open endIt was observed that some of the carbon nanotubes have an

open end. The shape of the open end is the same or very

similar as pear-shape of Ni particle (Fig. 9b). Therefore, the

open end is probably formed due to the Ni releasing from

the carbon tube end in certain reasons, for example,

mechanical disturbing. Formation mechanism of the carbon

nanotubes with open end is assumed to be the same as

those with carbon nanotube filled with Ni particles as

described above.

4.2.3. Carbons nanotubes embedding small pieces of NiparticlesSome of the carbon nanotubes were found to embed small

pieces of metallic Ni particles (Fig. 9c). It is assumed that

some of the Ni particles are strongly interacted with the

support and they are deeply anchored into the support and

are difficult for motion. Methane decomposition reaction

occurs on the exposed surface of such kind Ni particles to

form NixCy metastable solid solution intermediate. As the

NixCy is continuously formed and accumulated, C atoms are

released due to decomposition of the metastable CxNiyintermediate in the other side of the exposed surface of the

Ni particles, but not in the interface between the lower potion

of Ni particle and support as usual. In such case, the carbon

nanotubes are constructed in the surface of the Ni particle

towards gas–solid interface. As a result, the Ni particle is

fixed in the support but leaving carbon tube growing with

a closed end (Fig. 12, Path A).

The tails of the quasi-liquid NixCy particles are easy to be

split into several parts, some of which can be enclosed into

tubes. Similar phenomenon was also observed in the Fe/Al2O3

catalysts [49]. In the methane catalytic decomposition, the

formed Ni carbide in our Ni/Ce-MCM-41 catalysts may be

decomposed into supersaturation of the Ni particle with

carbon that results in the emergence of graphite nucleus and

then layers on the Ni particles. These graphitic layers are

initially oriented in parallel to the particle surface, they are,

however, distorted and their orientation may change to form

a hollow structure. As the carbon nanotubes grow further, its

part in contact with the particle becomes narrowed and the

tail of the metal particle is pulled inside the carbon capsule,

forming a small piece of Ni particle encapsulated in the carbon

tube with diameter the same as the inner diameter of the

carbon tube.

4.2.4. Nanocarbons with closed end without Ni particleon the tipThere are some carbon nanotubes having a closed end

without Ni particle on the tip nor small pieces of Ni particles

embedded inside them (Fig. 12, path B). It is believed that the

formation of these kind carbon nanotubes follows a similar

pathway as the ones encapsulating small pieces of Ni parti-

cles. However, due to interaction between Ni particle and

support is different, the strong interaction between them

makes Ni particle difficult to move, hence, some of metals are

strongly fixed in the support and the carbon tubes have to

grow towards the solid–gas interface. As for whether or not

the carbon nanotubes contain small pieces of Ni particle, it

might depend on the reaction temperature and degree of the

interaction between Ni particle and support. Usually, higher

reaction temperature favors the formation of carbon tubes

embedding small pieces of Ni; if the Ni particle is deeply

anchored into the support, only small part of the surface is

exposed to the reaction atmosphere, the interaction between

Ni particle and support is rather strong, these metal particles

are difficult to be cut off into small pieces and thus the formed

carbon nanotubes do not embed any pieces of small Ni

particle.

5. Conclusions

Ce-MCM-41 mesoporous materials with large surface area and

ordered pore structure system could be synthesized through

a surfactant-assisted approach. In the methane decomposi-

tion reaction, the catalytic activity of the Ni/Ce-MCM-41�x

(x ¼ Si/Ce molar ratio ¼ 20, 10, and 5) catalysts show high

catalytic stability. During the 1400 min of reaction, no obvious

catalytic deactivation was observed. Both nanocarbons with

graphitic structure and quasi-amorphous carbon were formed

in the evaluated catalysts. The carbon nanotubes have 30–

50 nm in diameters and hundred nanometers to tens

micrometers in length and all the carbon nanotubes consist of

multiple layer walls. Most of the metallic Ni particles are

located at the tip of carbon nanotubes and their exposed

surface remain clean after 1400 min of reaction, which

explains the high catalytic stability of these catalysts. Several

types of carbon nanotubes/nanofibiers were formed in the

tested catalysts. Their formation mechanisms greatly


depended on the reaction temperature and degree of the

interaction between the metallic Ni and the support.

Acknowledgments

The authors thank the financial support from CONACyT-

Mexico (Grant No. CONACyT-51007) and Instituto de Ciencia y

Tecnologia de la Ciudad de Mexico (ICyTDF2008) and Instituto

Politecnico Nacional, Mexico (IPN-SIP-20082507 and IPN-SIP-

20090826). J. C. Guevara thanks the Instituto Politecnico

Nacional, Mexico for providing him doctoral scholarship.

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