effect of cr and co contents in the perovskite ... -...

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

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EFFECT OF Cr AND Co CONTENTS IN THE PEROVSKITE TYPE (LaCr(1-x)Co(x)O3) CATALYSTS ON THE CHARACTERISTICS OF CARBON NANOTUBES SYNTHESIZED BY

CVD METHOD

Farshad AKHLAGHI a, Mohammad AGHAZADEH-MESHGI b

a School of Metallurgy and Materials Eng., Faculty of Eng., University of Tehran, P.O.Box 11155-4563,

Tehran, Iran, [email protected] b School of Materials Eng. Islamic Azad University, South Tehran Branch, Ahang Ave., Tehran,

Iran,[email protected]

Abstract

The presence of metallic elements in the catalysts used for synthesis of carbon nanotubes play an important

role due to its direct effect on the size and diameter of the resultant carbon nanotubes. Therefore, by using a

catalyst containing specific amounts of metallic elements, carbon nanotubes with the required size and

distribution can be produced. In this respect, Perovskite type catalysts due to their unique characteristics in

having different metallic contents and their applicability in high temperatures are being considered. In the

present study, the Perovskite type catalysts; LaCr(1-X) Cox O3 (x = 0.02, 0.05 or 0.10) were synthesized by

Pechini route and were used to produce carbon nanotubes (CNT’s) with a narrow distribution of diameters.

The effect of the percentage of cobalt as the active metal in these catalysts on the yield and diameter of

CNT’s synthesized by CVD method was investigated. Some characteristics of the catalysts such as

segregated elements and morphology were considered as the crucial parameters for the catalyst

performance. The results of Raman spectroscopy revealed the increased yield of CNT’s when the cobalt

content was minimized (x=0.02) in the catalyst. TEM studies confirmed the decreased external diameter of

CNT’s with increased percentage of Co in the catalyst.

Keywords: Perovskite type catalysts; Carbon nanotubes; Pechini route; CVD method; Cobalt content;

Raman spectroscopy; TEM studies; Diameter of CNT’s.

1. INTRODUCTION

The carbon nanotubes (CNTs) have found many applications from the time of their discovery by Iijima in

1991 [1]. He discovered the multi-walled CNTs with diameters within the range of 2-20 nano-meters. Then

he noticed the single wall nanotubes in 1993 [2]. The CNTs have exceptional chemical, mechanical and

optical characteristics due to their unique structure and small diameters [3]. There has been many

processing routs for synthesizing single-walled CNTs and in all of them, the role of the catalysts in

determining the diameter of CNTs has been explored [4,5]. It has been found that the diameter of CNTs

decreases with decreasing the size of the catalyst particles. However, the key factor in synthesizing CNTs

with predetermined diameters is inhibiting the agglomeration of the catalyst particles [6,7]. In this respect

various techniques such as using a mixture of 2 or 3 catalysts [7] or impregnation of alumina and active

carbon with metallic solutions have been examined [8,9]. The perovskite type catalysts have been recently

used successfully for synthesizing CNTs [10]. It is possible to synthesize single-walled CNTs by using this

type of catalysts due to their small size that can be about one nanometer [11]. However, despite the initial

target of many researchers in using the perovskite type catalysts for obtaining single-walled nanotubes, they

could only produce multi-walled CNTs [5, 7, 11-14]. The aim of the present study was to investigate the

effect of the percentage of cobalt as the active metal in the Perovskite type catalysts; LaCr(1-X) Cox O3 (x =

0.02, 0.05 or 0.10) on the yield and diameter of CNT’s synthesized by CVD method.


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2. MATERIALS AND EXPERIMENTAL PROCEDURES

The Perovskite type catalysts; LaCr(1-X) Cox O3 (x = 0.02, 0.05 or 0.10) were synthesized by Pechini route by

using different metallic nitrides such as La (NO3)3.6H2O, Cr (NO3)3.9H2O and Co (NO3)3.6H2O together with

citric acid, ethylene glycol and ammonia solution all from Merck Company. A viscose jell made of a mixture

of the required amounts of these compounds was prepared at 95 °C and subsequently fired at 400 °C. It was

then calcinated at 700 °C for 3h to obtain the required catalyst. In order to determine the exact amount of

each element in the catalyst, an X-ray florescent spectrometer (Thermo-ARL advantX, WDS type) was

utilized. The different phases in catalysts were detected by an X-ray diffractometer (Philips-Xpert). In order

to investigate the stability of catalysts in the reducing atmospheres, Temperature Programmed Reduction

Analysis was performed on different catalysts in a mixture of hydrogen/Argon gases with 1/10 ratio. A

Raman spectrometer (Thermo Nicolet Almega Dispersive) was used to study the structure of CNTs. A

Transmission Electron Microscope (TEM, Leo 912 AB) together with a Scanning Electron Microscope (SEM,

Camscan MV 2300) were used to study the size and morphology of catalysts and CNTs.

The carbon nanotubes were synthesized in a horizontal tube furnace having a quartz tube with 30 mm

diameter. About 0.2g of catalyst was placed inside the tube furnace and heated to 900 °C at the rate of 5

°C/min while hydrogen gas was passed through the furnace at the rate of 125 Cm3/min. At this stage

methane gas was also passed through the furnace at the same rate with hydrogen gas for 45 min (the ratio

of CH4 to H2 volumes was 1/1). Then the flow of methane gas was terminated and the produced CNTs were

cooled to the ambient temperature under the flow of hydrogen gas. The CNTs were separated from the

residual catalyst particles by washing the mixture with 4 molar nitric acid.

3. RESULTS AND DISCUSSION

The X-ray florescent spectrometry on the produced LaCr(1-X)CoxO3 (x=0.02, 0.05 or 0.10) catalysts confirmed

the presence of the exact amounts of different elements in them. In order to insure the complete solution of

Co in the Perovskite type catalysts, X-ray diffraction analysis was performed on the LaCr0.90 Co0.10 O3 material

having the highest Co content and the results are shown in Fig. 1. The absence of elemental Co or Cr in this

diffraction pattern confirms that these elements have not been separated from their compounds.

Fig. 1 X-ray diffraction pattern of LaCr0.90 Co0.10 O3

The reason for formation of LaCr0.9Co0.1O4 phase together with LaCr0.90Co0.10O3 in this material, as shown in

Fig. 1, is calcination of the material at 700 °C [16].


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100 300 500 700 900

Temperature(.C)

H2 C

onsumption (u.a.)

The SEM images of LaCr0.90Co0.10O3 catalyst at different magnifications are shown in Fig. 2(a,b). It can be

seen that this catalyst consist of agglomerated fine particles in the range of 50-200 nm. Fig. 2-b confirms

that the catalyst particles are dense and have some grooves on their surface. The formation of these

grooves can be attributed to the shrinkage of material during phase transformation [16].

Fig. 2 The SEM images of LaCr0.90Co0.10O3 catalyst at different magnifications; (a):X 40000 and (b): X 20000.

The results of Temperature Programmed Reduction analysis performed on LaCr0.90Co0.10O3 catalyst in a

mixture of hydrogen/Argon gases with 1/10 ratio are shown in Fig.3.

Fig. 3 Temperature Programmed Reduction analysis performed on LaCr0.90 Co0.10 O3 catalyst

The largest peak in this spectrum is related to the reduction of cobalt oxide in the catalyst that started at 400

°C and terminated at 600 °C. However, due to the high stability of chromium oxide, there is not any peak in

relation to reduction of this compound in the reductive atmosphere used in this experiment.

Typical TEM images of carbon nanotubes synthesized by using different Perovskite type catalysts are shown

in Fig. 4. TEM studies conducted on numerous CNTs made during the present study revealed smooth

surfaces without any structural defects and only in rare cases the worm type structure were observed.

The Size characteristics of CNTs synthesized by using different catalysts are shown in Table 1. It can be

seen that the external diameter of CNTs has decreased with increasing the cobalt content in the

(b) (a)


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.

Fig. 4 TEM images of carbon nanotubes synthesized by using different Perovskite type catalysts;

(a): LaCr0.98 Co0.02 O3 , (b): LaCr0.95Co0.05 O3 , (c) and (d): LaCr0.9Co0.1 O3 .

used catalyst. However, the smallest internal diameter was measured for the carbon nanotubes synthesized

by LaCr0.95Co0.05 O3 catalyst. Table 1 shows that the maximum wall thickness of CNTs have decreased with

increasing the cobalt content in the used catalyst. However, the smallest mean wall thickness was measured

for the carbon nanotubes synthesized by LaCr0.90Co0.1O3 catalyst.

Table 1. Size characteristics of CNTs synthesized by using different catalysts

Wall Thickness (nm) Mean Internal

Diameter (nm)

Mean External

Diameter (nm) Used catalyst

Minimum Maximum Mean

1 19 8.5 6 23

LaCr0.98 Co0.02

O 4 17 9 3 21

LaCr0.95 Co0.05

O 2 10 6 7 19 LaCr0.90 Co0.10

O

Chena and co-workers [15] noticed the strong effect of the processing temperature on the diameter and

yield of carbon nanotubes synthesized by using LaCoO3 catalyst. They measured the diameters of 13.2 and

28.5 nanometers for CNTs processed at 615 and 705 °C respectively. Kuras and co-workers [14] examined

the TEM images of CNTs synthesized on LaNiO3 catalyst to measure the characteristics of these materials.

They reported the values of 3-6 and 1-6 nanometers for the internal diameter and wall thickness of these

multi-walled nanotubes respectively.

The TEM studies confirmed the formation of multi-walled nanotubes and the presence of some metallic

particles on the surface of catalysts. According to the X-ray diffraction patterns obtained for remained

(b)

(c) (d)


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catalysts after formation of CNTs, these metallic particles were cobalt. In fact, chromium could not be

reduced from the compounds during the synthesizing process. These Co particles were covered by a layer

of carbon during the synthesis of CNTs and could not be solved in the acid during acid washing. The

average diameters of these Co particles were 21, 5 and 15 nanometers for LaCr0.98 Co0.02 O3, LaCr0.95 Co0.05

O3 and LaCr0.90 Co0.10 O3 catalyst respectively. It can be seen that the coarsest metallic particles are formed

on the surface of the catalyst with the minimum Co content. These results seem to be unexpected, since the

increased percentage of the active metal (Co) in the catalyst should be accompanied with increased

diameter of the metallic particles formed after synthesizing the CNTs. It can be concluded that the diameter

of the Co particles formed during synthesizing of CNTs is not related to the Perovskite Co content and is

related to the structural characteristics of the catalysts.

The results of Raman spectrometry performed on carbon nanotubes synthesized by using different

Perovskite type catalyst are shown in Fig. 5. The peaks labeled with IG and ID in this Fig. are related to the

crystalline carbon (CNTs) and amorphous carbon respectively. These results show that the maximum value

for the IG/ ID ratio is obtained for LaCr0.98 Co0.02 O3 catalyst having the minimum Co content. Therefore, this

catalyst was selected as the best one in generating the highest amount of CNTs as compared with the other

two catalysts investigated in this study.

Fig. 5

Raman spectrums of carbon nanotubes synthesized by using different Perovskite type catalysts.

4. CONCLUSIONS

The SEM images of Perovskite type catalysts; LaCr(1-X)CoxO3 (x = 0.02, 0.05 or 0.10) synthesized in the

present study showed that these catalysts are agglomerated fine particles in the range of 50-200 nm.The

IG/ID =1.143

IG/ID =0.966

IG/ID =1.238 LaCr0.98Co0.02O3

LaCr0.95Co0.05O3

LaCr0.90Co0.10O3


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active element in these catalysts is cobalt that can be separated from the compounds in a reductive

atmosphere and facilitate the synthesize of multi-walled carbon nanotubes. The external diameter of CNTs

were in the range of 19-23 nm and decreased with increasing the cobalt content in the used catalyst.

However, the smallest internal diameter was measured for the CNTs synthesized by LaCr0.95Co0.05 O3

catalyst. The maximum wall thickness of CNTs decreased with increasing the cobalt content in the used

catalyst. However, the smallest mean wall thickness was measured for the carbon nanotubes synthesized by

LaCr0.90Co0.1O3 catalyst. The results of Raman spectrometry revealed that the maximum amount of

crystalline carbon (CNTs) as compared with the amorphous carbon was obtained for LaCr0.98 Co0.02 O3

catalyst having the minimum Co content.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financial support of the Iranian Nanotechnology initiative (INI).

We are especially grateful to Dr. Kazem Saidnejad and Dr. Matin Parvari form Irandelco Co.

LITERATURE

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[2] O’Connell, M., Carbon Nanotubes - Properties and Applications, 1st Ed., Taylor & Francis Group,2006.

[3] A.E. Agboola, Development and Model Formulation of Scalable Carbon Nanotube Processes: HiPCO and CoMoCAT process models, PhD Thesis, Louisiana State University, The Department of Chemical Engineering, 2005, page 10.

[4] RUMMELI, M.H., KRAMBERGER, C., SCHAFFEL, F., BOROWIAK-PALEN, E., GEMMING, T., RELLINGHAUS, B., JOST, O., LOFFLER, M., AYALA, P., PICHLER, T., KALENCZUK, R.J., Catalyst Size Dependencies for Carbon Nanotube Synthesis, Physica status solidi, 2007,Vol.224, Issue 11, pages 3911-3915.

[5] KURAS, M., ROUCOU, R., PETIT, C., Studies of LaNiO3 Used as a Precursor for Catalytic Carbon Nanotubes Growth, Journal of Molecular Catalysis A: Chemical, 2007, Vol.265, page 209.

[6] MORTAZAVI, S.Z., REYHANI, A. IRAJI ZAD, A., The effect of Pd Addition to Fe as Catalysts on Growth of Carbon Nanotubes by TCVD Method, Applied Surface Science, 2008, Vol.254, pages 6416-6421.

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[8] AN, J.W. LiIMA, D.S., Synthesis and Characterization of Alumina/Carbon Nanotube Composite Powders, Journal of Ceramic Processing Research, 2002, Vol.3, pages 174&177.

[9] VEZIRI, Ch.M., PILATOS, G., KARANIKOLOS, G.N., LABROPOULOS, A., KORDATOS, K., KASSELOURI-RIGOPOULOU, V. KANELLOPOULOS, N.K., Growth and Optimization of Carbon Nanotubes in Activated Carbon by Catalytic Chemical Vapor Deposition, Microporous and Mesoporous Materials, 2008, Vol.110, pages 41-50.

[10] GALLEGO, G.S., MONDRAGON, F., TATIBOUET, J.M., BARRAULTt, J., DUPEYRAT, C.B., Carbon Dioxide Reforming of Methane Over La2NiO4 as Catalyst Precursor-Characterization of Carbon Deposition, Catalysis Today, 2008, Vol.133,pages 207-208.

[11] TANAKA, H., UENISHI, M., TANIGUCHI, M., TAN, I., NARITA, K., KIMURA, M., KANEKO, K., NISHIHATA, Y., MIZUKI, J.,The Intelligent Catalyst Having the Self-Regenerative Function of Pd, Rh and Pt for Automotive Emissions Control, Catalysis Today, 2006, Vol.117, page 327.

[12] LIANG, Q., GAO, L.Z., LI, Q., TANG, S.H., LIU, B.C., YU, Z.L, Carbon Nanotube Growth on Ni-Particles Prepared in Situ by Reduction of La2NiO4, Carbon, 2001, Vol.39, pages 897–903.

[13] LIANG, Q., LI, Q., GAO, L., YU, Z., Carbon Nanotubes Prepared From CO on Pre-Reduced La2NiO4 Perovskite Precursor, Materials Research Bulletin, 2001, Vol.36, pages 471-477.

[14] KURAS, M., ZIMMERMANN, Y., PETIT, C., Reactivity of Perovskite-type Precursor in MWCNTs synthesis, Catalysis Today, 2008, Vol.138, Issues 1-2, pages 55-61.

[15] CHENA, L., LIUA, H., YANGA, K., WANGA, J., WANG, X., The Effect of Reaction Temperature on the Diameter Distribution of Carbon Nanotubes Grown from Ethylene Decomposition Over a Co-La-O Catalyst, Materials Chemistry and Physics, 2008, Vol.112, pages 407-411.

[16] ORLOVSKAYA, N., CORATOLO, A., LUGOVY, M., JOHNSON, C., GEMMEN, R., Structural Evolution of La–Cr–O Thin Films: Part I. Microstructure and Phase Development, Thin Solid Films, 2006, Vol.515, pages 1741 – 1747.

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