Bulk production of bamboo-shaped multi-walledcarbon nanotubes via catalytic decompositionof methane over tri-metallic Ni–Co–Fe catalyst

Ghazaleh Allaedini1 • Siti Masrinda Tasirin1 •

Payam Aminayi2 • Zahira Yaakob1,3 •

Meor Zainal Meor Talib1

Received: 10 April 2015 / Accepted: 18 June 2015

� Akademiai Kiado, Budapest, Hungary 2015

Abstract In this work, bamboo-shaped, multi-walled carbon nanotubes were

synthesized via methane decomposition over a Ni–Co–Fe tri-metallic catalyst at

1000 �C. The nitrogen absorption (BET), X-ray diffraction (XRD), and particle size

analysis results of the catalyst were used to demonstrate the surface area, size

distribution, and crystallinity of the sample. The scanning electron microscopy

(SEM) micrographs of the nanocarbons deposited via methane decomposition

indicated that highly uniform carbon nanotubes were grown on the surface of the tri-

metallic catalyst. The transmission electron microscopy (TEM) images showed that

the carbon nanotubes were multi-walled and bamboo-shaped with a diameter of

*20 nm. Raman spectra revealed the graphitization degree of the CNTs with an ID/

IG of 1.84, indicative of the crystallinity of CNTs with structural defects. The

thermal analysis shows the high oxidation stability of the multi-walled carbon

nanotubes.

Keywords Tri-metallic catalyst � Methane decomposition � Bamboo shape carbon

nanotubes � Multi-walled carbon nanotubes � Graphitization degree

& Ghazaleh Allaedini

jiny_ghazaleh@yahoo.com

& Siti Masrinda Tasirin

masrinda@eng.ukm.my

1 Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia (UKM),

Bangi, Selangor, Malaysia

2 Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI, USA

3 Fuel Cell Institute, Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor, Malaysia

123

Reac Kinet Mech Cat

DOI 10.1007/s11144-015-0897-1

Introduction

Since the discovery of carbon nanotubes (CNTs) in 1991 [1], they have received a

great deal of attention due to their unique properties [2]. CNTs are robust materials

with a Young’s elasticity modulus of up to 2 TPa [3]. The electrical properties of

carbon nanotubes have also made them a good candidate as transistors [4].

Moreover, the carbon nanotubes exhibit superior properties for hydrogen storage

[5]. Bamboo-shaped CNTs are common in the family of carbon nanotubes, which

are structured with separated hollow compartments. This type of CNT has been

frequently investigated to explore its unique structure-associated properties and to

understand the relationship between its growth mechanisms and the continuous

hollow channels. Investigating the growth conditions of bamboo-shaped tubes and

comparing them with the growth of regular tubes would lead to the ability to control

the synthesis process in order to achieve the desired microstructure properties [6]. A

number of different methods have been reported for the production of carbon

nanotubes such as chemical vapor deposition (CVD), plasma enhanced chemical

vapor deposition (PECVD), alcohol catalytic chemical vapor deposition (ACCVD),

hydrothermal or sonochemical, and high-pressure CO conversion (HiPco) [7].

However, the catalytic chemical vapor deposition (CCVD) method has been

considered one of the most successful methods because of its potential to produce

high-quality carbon nanotubes in large-scale at a relatively low cost [8].

The most common catalysts used for the production of CNTs are transition

metals [9] supported on silica, magnesium oxide, zeolite, alumina, and SBA-15

[10]. Early experiments of CNT production were carried out using different metallic

catalysts such as iron (Fe), cobalt (Co), and nickel (Ni) [11]. Seidel et al. [12] had

investigated the growth mechanism of carbon nanotubes using Fe, Co, and Ni

catalysts, individually. They observed that the order of the lowest growth

temperatures agrees with the order of the bulk melting points of the transition

metals (Ni—1450 �C; Co—1490 �C; Fe—1540 �C). However, when used as binary

compounds, the yield of carbon nanotubes has been observed to increase

significantly. It was found that the iron catalyst has a high catalytic activity in

the hydrocarbon decomposition process, which results in a high yield of carbon.

However, the graphitization degree of the obtained CNTs was low [13]. It is also

reported that the usage of cobalt catalyst results in better graphitized CNTs but

yields poor efficiency [14]. The presence of the Ni particle has been also reported to

elongate the life-time of the accompanying catalysts [15]. These findings led the

researchers to use a combination of two metals trying to incorporate their individual

advantages. Large volumes of well-graphitized, multi-walled carbon nanotubes

(MWCNTs) were obtained using this approach [16]. One of the main advantages of

using bi-metallic and tri-metallic catalysts is that CNTs could be grown at

significantly lower temperatures. This is possible because the melting point of the

mixed oxides of Fe and Co is lower than their individual melting points [17].

Moreover, alloys are known to be better catalysts than pure metals [17].

Additionally, it is shown that the mixing of two or three metals can enhance the

catalytic activity. It is, therefore, believed that tri-metallic catalysts provide

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interesting results, despite the fact that the interpretation of the results would be

more complicated. However, few reports exist in this field [17].

There are numerous factors to consider when using tri-metallic catalysts. The

effect of added promoters on the activity and stability of tri-metallic catalyst has

been reported. For example, in a work by Wu et al., it was found that the addition of

a small amount of Au and Pt plays an important role in the catalytic performance of

bi-/tri-metallic catalysts and carbon poisoning [18]. This improvement was due to

the formation of highly active Ni–Au–Pt nanoparticles, resulting in the growth of a

small amount of bamboo-like carbon nanotubes. In their work, the bi and tri-

metallic catalysts were supported by Al2O3. In a work by Halonen et al., mono, bi,

and tri-metallic Fe, Co, Ni catalysts have been compared, and it was found that Co

was not active at lower temperatures. In contrast, bi and tri-metallic catalysts

produced MWCNTs with a high graphitization degree at low temperatures [19]. In

addition, it has been reported that using promoters for the tri-metallic catalysts used

in methane reforming increases the coke resistance as well as the performance of the

catalyst [20]. One of the factors that have been studied in the production of carbon

nanotubes using tri-metallic catalysts is the effect of the ratio of the gases employed.

In a work by Han et al., Fe–Ni–Co alloy catalyst dots were deposited into holes,

over which the CNTs were grown. The CNTs were synthesized using the thermal

chemical vapor deposition method, employing a gas mixture of CO and H2 at

500 �C. The obtained CNT lengths were controlled by varying the ratio of CO to H2

[21].

Of the few studies reporting on the variety of nanoparticle catalysts used in the

methane decomposition process [22], most of them report a binary mixture of

catalysts in combination with supports and additional promoters. On the other hand,

Fe-based catalysts can operate at higher temperatures when compared with Co or Ni

based catalysts without suffering from the deactivation, resulting in higher methane

conversions due to the positive shift in the thermodynamic equilibrium. However, in

order to improve the graphitization degree of Fe-based catalysts, several transition

metals such as Co, Ni, Mo, and Cu have been employed as catalyst additives [22]. In

this work, a combination of three catalysts (Ni–Co–Fe), without the addition of

support or other additives, has been used to achieve a superior catalytic performance

by taking advantage of the individual properties of each catalyst.

The carbon nanotubes synthesized via methane decomposition over the supported

catalyst makes the purification a challenge since they always contain a significant

amount of impurities from the support material [23]. There are a number of

complicated methods to purify CNTs obtained in the presence of supports, such as

liquid phase oxidation, centrifugal separation, microfiltration, chromatography, and

intercalation [24]. Therefore, the purification of the carbon nanotubes is a challenge

[25, 26] and the elimination of support materials is preferred in order to facilitate the

purification process. Elimination of the support material simplifies CNT purification

by only requiring they be dipped into HCl. Therefore, in the present study, an

unsupported tri-metallic catalyst was used for production of multi-walled carbon

nanotubes.

It is important to note that the key step in the growth of nanotubes seems to be

the diffusion of carbon in the metal catalysts. The most active metals are iron,

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cobalt, and nickel. However, their catalytic action relies on the type of precursor,

substrate, and reactive gases used. It is suggested that the catalyst is naturally

active if present in the form of metals or as a mixed phase (bi or tri-metallic) [27].

On the other hand, Ni, Fe, and Co catalysts are reported to be active even at high

temperatures when methane or acetylene is used as the carbon source [28]. It is

also reported that the presence of hydrogen promotes catalyst activity [29].

Therefore, the prepared tri-metallic catalyst is expected to be active in the reaction

temperature employed in this study. Metals in the iron group have a high activity

which promotes the production of carbon nanotubes [30]. The activity of nickel in

the production of carbon nanotubes and its higher efficiency in the conversion of

the carbon feed source have been reported [31]. Reportedly, cobalt has also been

used in oxide form, metallic forms, or as mixed oxides in the production of carbon

nanotubes [32] with high efficiency and quality [33]. Margez et al. [34] has

investigated Fe1-xCox and Fe1-xNix alloys for the growth of carbon nanotubes.

However, the literature still lacks a comprehensive combination of these three

transition metals (Ni–Co–Fe). A few efforts have been made in the direction of

employing tri-metallic catalysts for the CNT production [17]. Therefore, in this

study, highly active iron group metals have been selected to prepare the tri-

metallic catalyst, and its efficiency for the production of carbon nanotubes has

been investigated. To the best of our knowledge, there is no report on the tri-

metallic catalyst to produce bamboo-shaped carbon nanotubes.

Experimental

Materials

Iron(III) nitrate nonahydrate Fe(NO3)3�9H2O, Cobalt(II) nitrate hexahydrate

Co(NO3)2�6H2O, Nickel(II) nitrate hexahydrate Ni(NO3)2�6H2O, and sodium

hydroxide (0.01 M NaOH) were purchased from sigma Aldrich and used as

received.

Catalyst synthesis

The co-precipitation method was employed for the catalyst preparation. A mixture

of weighted amounts of nitrate salts of the metals (8 g Ni(No3)2�6H2O, 8 g

Fe(No3)�9H2O, 8 g Co(No3)3�6H2O) was dissolved in 100 ml of distilled water and

stirred while heated at 50 �C for 1 h. The obtained solution was precipitated using

50 ml sodium hydroxide at 50 �C while stirring for 5 h at room temperature. The

obtained suspension was washed with ethanol three times and dried at 120 �C for

24 h.

Production of carbon nanotubes

For CNT production, 2 g of the tri-metallic catalyst was placed in the middle of

the tubular decomposition reactor heated by an electric muffle furnace. The

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catalyst was pre-heated under the nitrogen atmosphere at 600 �C (1 h), followed

by the reduction at 600 �C (1 h) using a continuous flow of H2 gas (500 ml/min).

After that, the temperature of the reactor was increased gradually to 1000 �C using

N2 (500 ml/min). The gradual increase of the temperature prevents the deacti-

vation of the metallic catalyst [35]. As a matter of fact, the catalysts will show a

higher activity when the temperature is increased gradually. This gradual increase

can be regarded as pre-calcination. Therefore, the calcination step in the synthesis

of the catalyst is not a necessary step to obtain better quality CNTs [36]. Once the

temperature was stabilized at 1000 �C, the methane gas was introduced to the

reaction tube with a flow rate of 600 ml/min for 3 h at atmospheric pressure. It has

been reported that the high flow rate of the carrier gas has a marginal effect on the

production of CNTs with lower amounts of amorphous carbon and higher activity

of the catalyst [37]. After the decomposition reaction, the reactor was cooled down

while passing N2 for 30 min. The obtained powder was collected and purified

using 50 ml of concentrated HCl for further characterization.

Characterization of catalyst and nanocarbons

XRD analysis was conducted using a Bruker D8 Focus advanced powder

diffractometer with a CuKa radiation at the wavelength of 0.15406 nm. The

particle size distribution of the catalyst powder was analyzed using a Malvern

Mastersizer spectrometer. The field emission scanning electron microscopy

(FESEM) pictures were used to visualize the morphology of the prepared samples

in a Zeiss SUPRA55 scanning electron microscope at an operating voltage of

3 kV. Raman spectrophotometer with a laser source operating at 532.230 nm

wavelength (WITec, Model: Alpha 300R) was used to characterize the CNTs.

Transmission electron microscopy was performed using a Hitachi 477700 m

instrument at an operating voltage of 120 kV. Brunauer–Emmett–Teller and

Barrett–Joyner–Halenda (BET & BJH) analysis were carried out using a

Micrometrics ASAP2010 instrument with nitrogen gas at 77 K to measure the

surface area and porosity of the prepared particles. Thermogravimetric analysis

was performed by Mettler Toledo, Model: TGA/DSC instrument with a heating

rate interval of 10 �C per 5 min.

Fig. 1 SEM images of the Co–Fe–Ni catalyst at (left) 910 K, (center) 910 K, and (right) 915 Kmagnification

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Results and discussion

The SEM images of Ni–Co–Fe catalysts are shown in Fig. 1. The highly porous and

homogenous surface of the tri-metallic catalyst can be observed in this image. The

prepared catalyst has a good particle size distribution which may enhance the

nanocarbon deposition.

The XRD pattern of the tri-metallic catalyst is shown in Fig. 2. The peaks at

2h = 38� and 44� are characteristic of the peak attributed to the face-centered NiO

with hkl planes of (202) and (111), and the peaks at 2h = 46� and 58� are assigned

to the presence of Co3O4 and Fe3O4 with hkl planes of (200) and (333), respectively.

The peak at 2h = 54� is assigned to cobalt ferrite (CoFe2O4) with hkl plane of

(422). The volume-averaged crystal size was calculated using the Debye-Scherrer

formula:

D ¼ Kk=bcosh; ð1Þ

where D is the crystalline size (nm), K is the Scherrer constant equal to 0.89, k is the

radiation wavelength (nm), b is the observed peak width, and h is the diffraction

angle. The average size of the prepared catalyst particles was found to be 42 nm.

The particle size distribution was also analyzed using a Malvern Mastersizer

spectrometer. The average crystal size was found to be 58 nm with the diameter

distribution of ±27 nm.

The BET nitrogen absorption test was performed to measure the surface area of

the nanoparticles (Fig. 3). The surface area of the particles was also measured using

the adsorption isotherm and was found to be 36.8 m2/g. The pore volume

distribution determined by the BJH method was reported to be 0.018 cm3/g.

The SEM micrographs of the grown carbon nanotubes are shown in Fig. 4. The

nanotubes were grown on the surface of the tri-metallic catalyst. It can be proposed

that the tip growth mechanism is the growth mechanism of the carbon nanotubes

30 40 50 60 70

Inte

nsity

2

202 (NiO)

200 (CO3O4)

111 (NiO)

333(Fe3O4)

422 (CoFe2O4)

Fig. 2 XRD pattern of the prepared tri-metallic catalyst (Fe–Co–Ni)

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grown on the homogenous catalysts. The carbon deposit (weight yield) result was

also calculated using the following equation [38, 39]:

Carbon Deposit% ¼ MT � MC

MC

� 100 ð2Þ

where MT is the mass of the sample after the reaction (6.75 g) and MC is the mass of

Fig. 3 BET N2 absorption isotherms of the prepared Co–Fe–Ni tri-metallic catalyst

Fig. 4 SEM micrograph of the grown carbon nanotubes amid the Fe–Ni–Co catalysts

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the catalyst before the reaction (2 g). The amount of the obtained carbon deposit

was calculated to be 237 %. Weight yield ranging from 100 to 2000 % of the

deposited carbon has been obtained depending on the operating conditions [40].

The TEM micrographs of the obtained nanocarbons confirm the formation of

carbon nanotubes. The diameter and the morphology of the obtained CNTs can be

also investigated using the TEM pictures. The bamboo-shaped carbon nanotubes

with average diameter of *20 nm can be observed in Fig. 5. Bamboo-shaped

carbon nanotubes are distinguished in Fig. 5 by their hollow compartments spaced

by curved carbon domes formed from the carbon sheets joining the tubes inner

walls. All the images obtained from the CNT samples showed bamboo-shaped

nanotubes and are indicative of a high fraction of bamboo-shaped nanotubes

obtained through this method.

Characterization of the carbon nanotubes using Raman spectroscopy [41] and

thermogravimetric analysis [42] can also provide useful information regarding the

crystallinity and purity of the nanotubes. Fig. 6 shows the Raman spectrum of the

purified CNTs. Graphitic carbon materials exhibit a Raman band at around

1580 cm-1, which is assigned to the G band, and a peak at 1350 cm-1 is assigned to

the D band. The G band represents the most prominent original graphite features,

whereas the D band is known to depend on the disorder features of the hexagonal

graphitic layers as well as the presence of small crystallites [43–45]. The G band

corresponds to the tangential stretching mode of the graphite C=C bond, which is

attributed to the vibration of sp2-bonded carbon atoms in a two-dimensional

hexagonal lattice [46]. The ID/IG intensity ratio is reported to increase with the

Fig. 5 TEM images of the obtained bamboo-like carbon nanotubes at a, b high magnification and c lowmagnification

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increase in the structural disorder of the CNTs. Lower ID/IG values are

representative of well-graphitized CNTs. The value of (ID/IG) for the CNTs grown

using Ni–Co–Fe catalysts was calculated to be 1.84. The ID/IG ratio strongly

depends on the defect fraction. In this study, the ID/IG ratio indicates a level of

graphitization along with the existence of defects. However, it has been reported

that some process parameters such as temperature, delicate purification processes,

and nitrogen doping can decrease the ID/IG ratio [47, 48].

Fig. 7 illustrates the thermogravimetric analysis of Fe–Ni–Co/C. This figure

shows the weight loss as a function of temperature. The maximum temperature was

set to 800 �C to ensure that all the carbon is evaporated through oxidation [49].

Oxidative temperatures more than 450 �C were reported to be mainly attributed to

800 1300 1800 2300

Inte

nsity

( a.

u)

Raman shift cm-1

D

G

Fig. 6 Raman spectrum of the obtained bamboo-shaped carbon nanotubes

6065707580859095

100

25 110 195 280 365 450 535 620 705 790

Mas

s %

Temprature °C

Fig. 7 TGA oxidative analysis of the obtained bamboo-shaped carbon nanotubes

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crystalline carbon with a high degree of graphitization [50, 51]. As observed in

Fig. 7, no weight loss is noted below 450 �C. The decomposition was started at

450 �C and ended at 650 �C. The TGA graph shows about 35 % weight loss for

temperatures between 450 and 650 �C for the obtained CNTs. The TGA plot

indicates the presence of *65 % metal in the Fe–Ni–Co/C sample [52]. This

residual mass remaining is attributed to metal catalysts as well as the oxidation

products of these catalysts [53, 54]. In addition, considering that the produced CNTs

and metal catalysts may not be homogeneously dispersed, the amount of weight loss

may be highly dependent on the small sample chosen for the analysis. Therefore,

TGA results will not be an accurate comparison for the yield results. Unpurified

CNT samples contain a large amount of residual masses, referred to as ash content

(non-aqueous residues, mostly metal oxides, which remain after burning a sample)

[54].

Conclusion

In this work, a tri-metallic catalyst was synthesized, characterized, and employed

for carbon nanotube production. The carbon nanotubes have been synthesized by the

decomposition of methane over the tri-metallic Ni–Co–Fe catalyst. The SEM and

XRD results confirmed the structural properties, crystallinity, and fine-size

distribution. The surface area of the catalyst has been estimated using BET N2

absorption isotherms, and it was found to be 36.8 m2/g with a pore volume

distribution of 0.018 cm3/g. In contrast to the CNT synthesis methods in which the

catalysts are supported by materials such as silica, alumina, or magnesium oxide,

the prepared catalyst of this study did not require the complicated purification steps

with regards to the removal of supports, which reduced the cost and increased the

efficiency, as the catalyst is the only impurity to remove. The TEM micrographs

confirmed that the obtained carbon nanotubes possessed a bamboo-shape with an

average diameter of 20 nm. The Raman result confirmed the degree of graphiti-

zation for the CNTs prepared using the Ni–Co–Fe tri-metallic catalyst. Moreover,

the oxidation stability observed from the TGA results, makes the CNTs growth over

the prepared tri-metallic catalyst possible.

Acknowledgments We would like to acknowledge financial support provided by the CRIM,PKT6/2012

and DIP-2012-05 and FRGS/2/2013/TK05/UKM/02/3 funds, UKM, Malaysia.

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