Synthesis of carbon nanotubes by pyrolysis of acetylene using alloy hydride materials as catalysts and their hydrogen adsorption studies

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1 Chemical Physics Letters 374 (2003) Synthesis of carbon nanotubes by pyrolysis of acetylene using alloy hydride materials as catalysts and their hydrogen adsorption studies M.M. Shaijumon, S. Ramaprabhu * Department of Physics, Alternate Energy Technology and Magnetic Materials Laboratory, Indian Institute of Technology Madras, Chennai , India Received 27 March 2003;in final form 2 May 2003 Abstract The catalytic synthesis of carbon nanotubes by pyrolysis of acetylene over Zr based AB 2 and Mm (Misch metal) based AB 5 alloy hydrides is discussed. The alloy-hydrides have been prepared using hydrogen decrepitation technique. The samples were purified by acid and heat treatment and were characterized by XRD, BET surface area measurements, SEM, TEM and Raman spectroscopy. A maximum adsorption capacity of 3.3 and 3.1 wt% are obtained at 298 K and 100 bar for carbon nanotubes prepared with Mm based AB 5 and Zr based AB 2 hydrogen storage alloy hydride catalysts, respectively. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction An economically viable hydrogen storage medium is an essential component for the hydrogen fueled transportation systems and there is an ongoing research for advanced hydrogen storage materials. Recently, carbon nanotubes (CNTs) and graphitic nanofibers (GNFs) have been reported to be very promising candidates for large amount of hydrogen storage [1 5]. The large empty space inside the single wall nanotubes (SWNTs), the low mass density and their chemical * Corresponding author. Fax: address: ramp@iitm.ac.in (S. Ramaprabhu). stability opens a new application for the hydrogen storage with high capacity. Since their discovery in 1991 [6], various potential applications have been proposed for carbon nanotubes, including high strength composites, sensors, field emission displays, nanometer-sized semiconductor devices, probes and hydrogen storage media [7,8]. Recent advances in the science of carbon nanostructures has initiated numerous studies, both experimental and theoretical on molecular hydrogen adsorption in graphite nanofibers (GNFs) and carbon multi wall and single wall nanotubes [9,10]. Dillon et al. [1] reported a hydrogen storage capacity of 5 10 wt% at 293 K on the SWNTs. Ye et al. [2] were the first to report hydrogen adsorption investigations on purified single wall /03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 514 M.M. Shaijumon, S. Ramaprabhu / Chemical Physics Letters 374 (2003) nanotubes. High hydrogen storage capacities were demonstrated on SWNTs, produced at high yield by a semi continuous arc discharge method [3]. Anyuan Cao et al. [4] has shown that inter nanotube space between densely aligned carbon nanotubes also contribute to the effective uptake of molecular hydrogen. Recently, Gundiah et al. [5] have examined the hydrogen adsorption measurements on well-characterized samples of carbon nanotubes with a maximum storage capacity of 3.7 wt%. Recent studies on interaction of hydrogen with SWNTs, highlight the advancement in the understanding of the phenomenon of hydrogen adsorption in carbon nanotubes [11,12]. Electrochemical hydrogen storage has also been demonstrated for carbon nanotubes. Nutzenadel et al. [13] reported the electrochemical storage of hydrogen in a sample containing wt% of MWNTs and obtained 110 mah/g under 1 atm pressure. A high electrochemical reversible charging capacity up to 800 mah/g, corresponding to a hydrogen storage capacity of 2.9 wt% has been reported by Rajalakshmi et al. [14] for electrodes made of purified SWNTs. Ever since their discovery, there have been tremendous efforts to synthesize CNTs through various routes such as arc discharge, laser ablation and catalytic methods. The catalytic route of pyrolysis has been reported to be much simpler and reproducible [15 19]. More over catalytic growth is one of the viable methods of producing carbon nanotubes in large quantities [15]. Satishkumar et al. [16] and Sen et al. [17] have prepared aligned nanotubes by the pyrolysis of organometallic precursors and hydrocarbons. Carbon nanotubes and nanofibers have been synthesized with various metal catalysts especially Fe, Co, Ni and their alloys [20,21]. In the case of CNTs produced with LaNi 5 alloy catalysts, mechanically grounded to fine powders, the surface treatment of alloy particles with KOH solution was found to be effective in providing catalytic sites for CNT growth [22]. We have investigated the hydrogen storage properties of Zr/Ti based C14 AB 2 alloys, Mm (Misch metal) based AB 5 alloys, RE based AB 3 alloys in order to develop materials for hydrogen storage applications [23 26]. Among these alloys, Zr based C14 AB 2 alloys have almost 20% volume expansion up on hydrogenation and large decrepitation (break down of particle size due to plastic deformation during hydrogen absorption and desorption cycles), while Mm based AB 5 alloys have lesser volume expansion upon hydrogenation [23 25]. In this Letter, we present the formation of carbon nanotubes by the catalytic decomposition of acetylene over Zr based AB 2 and Mm based AB 5 hydrogen storage alloy hydride catalysts, prepared by hydrogen decrepitation route. The presence of transition metals especially Fe, Co, Ni and the novel method of powdering the catalysts into fine particles without getting oxidized, provide effective catalytic sites for carbon nanotubes growth. Hydrogen adsorption studies on these as-grown and purified carbon nanotubes are discussed. 2. Experimetal 2.1. Catalyst preparation Zr based AB 2 and Mm based AB 5 hydrogen storage alloys were prepared by arc-melting the constituent pure elements in an arc furnace under argon atmosphere. They were melted in stoichiometric proportions. The alloy buttons were remelted several times by turning them over to ensure homogeneity. The powder X-ray diffraction of as-formed alloys show single-phase formation. Each alloy was hydrogenated to a maximum storage capacity using a high pressure Seiverts apparatus. Fine powders of alloys were obtained after several cycles of hydrogenation/dehydrogenation and the SEM images of these fine powders show that the particles are in the range of lm Carbon nanotube synthesis Carbon nanotubes have been synthesized by catalytic decomposition of acetylene using Zr based AB 2 and Mm based AB 5 alloy hydrides as the catalysts. About 250 mg of the hydride powder was placed in a quartz boat and then introduced in to a flow reactor (quartz tube with an inner diameter of 30 mm and a length of 50 cm), for the synthesis of carbon nanotubes. The alloy hydride

3 M.M. Shaijumon, S. Ramaprabhu / Chemical Physics Letters 374 (2003) powders were heated at 500 C in flowing hydrogen (50 ml/min) for 1 h. The hydrogen gas flow was stopped and temperature of the furnace was raised to 700 C, for the production of carbon nanotubes. Acetylene was then passed through the quartz reactor (70 ml/min) for 30 min. The deposition was carried out at atmospheric pressure and in an argon flow. After completion of the deposition, reactor was allowed to cool to room temperature in the presence of argon flow. The quartz boat was removed from the reactor and carbon deposits from the quartz boat were taken out. The as-synthesized carbon nanotubes were refluxed with conc HNO 3 for 24 h, followed by washing with de-ionized water several times and then the sample was dried in air for 30 min at 100 C. Carbon samples grown with AB 5 alloy hydride catalysts were further treated with hydrofluoric (HF) acid. This was followed by air oxidation at 500 C for 2 h to remove the amorphous carbon and to open the ends of carbon nanotubes [27]. X-ray diffraction (XRD), BET surface area measurements, Scanning electron microscopy (SEM; JEOL JSM 840 A), Transmissoin electron microscopy (TEM;JEM-200 FX II) and FT-Raman spectroscopy (FRA 106 Bruker) were employed to characterize the as-grown and purified carbon nanotubes Hydrogen adsorption measurements The hydrogen adsorption studies of as-synthesized and purified carbon nanotubes were carried out using a high-pressure Seiverts apparatus, in the pressure range bar and at room temperature. Before subjecting to hydrogenation, the CNTs were activated by heating up to a temperature of 175 C under a vacuum of 10 6 Torr for 5 h. Hydrogen was allowed at a temperature of 100 C and then the sample was cooled to room temperature. The experiment was carried out following the procedure given in [28], by taking care to avoid many of the errors generally encountered in such measurements. The pressure-composition relationships were obtained by calculating the hydrogen storage capacity in wt% from the pressure drop during the hydrogen adsorption at constant temperature. After each cycle, the sample was degassed for 5 h at 175 C under a vacuum of 10 6 T. 3. Results and discussions For the catalyst preparation, Zr based AB 2 and Mm (Misch metal) based AB 5 hydrogen storage alloys with a C14 hexagonal and CaCu 5 type of hexagonal structure respectively, were chosen. The alloy was hydrogenated and dehydrogenated several times to bring down the size of the catalyst materials. AB 2 hydrogen storage alloys were found to be finely powdered due to the large volume expansion of about 20%, compared to that of 10 12% in the case of Mm based AB 5 alloys [23,25]. Our novel approach to catalyst preparation, using hydrogen decrepitation, ensures increase in total surface area by providing fresh surfaces, which further increases the catalytic sites for formation of carbon nanotubes. The hydrogen decrepitation was seen more prominent for Zr based AB 2 alloys compared to Mm based AB 5 alloys. The carbon deposit was found to have a mass gain of about 0.45 g, when acetylene was allowed for additional 30 min. Keeping the acetylene flow constant and decreasing the Argon flow rate, more of amorphous carbon were seen to be deposited inside the quartz reactor, which suggests that the Table 1 BET specific surface area and hydrogen adsorption measurements results Sample Specific surface area (m 2 /g) CNTs with AB 2 alloy hydride catalysts (as-grown) CNTs with AB 2 alloy hydride catalysts (purified) CNTs with AB 5 alloy hydride catalysts (as-grown) CNTs with AB 5 alloy hydride catalysts (purified) Hydrogen adsorption (wt%) 298 K, 100 bar

4 516 M.M. Shaijumon, S. Ramaprabhu / Chemical Physics Letters 374 (2003) residence time of acetylene in the reactor is controlling the amount of disorganized carbon coating. We also found a large amount of tar-like liquid by-products being condensed on the cooled part of the reactor outlet, which might be due to a part of acetylene feed, getting converted into higher hydrocarbons by the catalytic action of Ni and Fe, as reported by Soneda et al. [29]. As it is already pointed out, the pre-reduction of the catalytic precursor before nanotube synthesis is already taken care of, with our novel method of catalyst preparation. Further studies need to be carried out, in order to understand the actual role of hydride nanomaterials for the formation of nanotubes. (BET) Specific surface area measurements were performed (Quantachrome, NOVA 1000) for the as-grown and acid treated samples, prepared with AB 2 and AB 5 hydrogen storage alloy catalysts and are tabulated in Table 1. The larger surface area of the purified samples indicates that they have larger porosity compared to as-grown CNTs. The X-ray diffraction pattern of the carbon nanotubes synthesized with Mm based AB 5 Fig. 1. X-ray diffraction pattern of carbon nanotubes synthesized with decomposition of acetylene over AB 5 alloy hydride catalysts. Fig. 2. SEM images of carbon nanotubes obtained from decomposition of acetylene over hydrogen storage alloy hydride catalysts: (a) AB 5 alloy hydride catalysts, (b) AB 2 alloy hydride catalysts, (c) HNO 3 treated carbon nanotubes, prepared with AB 2 hydride catalysts.

5 M.M. Shaijumon, S. Ramaprabhu / Chemical Physics Letters 374 (2003) hydride catalysts is shown in Fig. 1. The diffraction peak in the XRD pattern of as-grown CNTs shows the presence of carbon along with catalytic impurities. The partial removal of metallic impurities shows up in the XRD pattern of the HNO 3 treated sample. Carbon nanotubes prepared with AB 5 alloy hydrides were further treated with HF acid for 24 h and the XRD pattern clearly reveals the removal of catalytic impurities. A strong (002) peak in the XRD pattern shows a lower alignment of carbon nanotubes as the intensity of the (002) peak decreases monotonically with better alignment of carbon nanotubes [30]. The quality and nature of carbon deposited on the hydrogen storage AB 2 and AB 5 alloy hydride material surfaces were examined by scanning elctron microscopy (SEM). Figs. 2a,b show the SEM images of carbon nanotubes obtained by the catalytic decomposition of acetylene over hydrogen storage AB 5 and AB 2 alloy hydrides respectively. As we see from the SEM images, though not fully aligned, the packing density of carbon nanotubes is high. As the bottom layer of the deposits inside the quartz tube were left out, not much of the catalytic particles were seen in the SEM images, unlike those reported by Gao et al. [22]. SEM image of the HNO 3 treated carbon nanotubes, produced with AB 2 alloy hydride catalysts is shown in Fig. 2c. Figs. 3a,b show the TEM images of carbon nanotubes synthesized with AB 2 and AB 5 hydride catalysts respectively. Catalyst particle encapsulated in an as-grown carbon nanotube is shown in Fig. 3c. The shape of these particles is seen to match with the hollow core of the nanotube. TEM image of a single carbon nanotube synthesized with AB 5 alloy hydride catalysts is shown in Fig. 3d. The tubes were of approximately nm diameters and with a hollow core of approximately 6 8 nm. Fig. 3. TEM images of: (a) carbon nanotubes synthesized with acetylene over AB 2 alloy hydride catalyst, (b) carbon nanotubes synthesized with acetylene over AB 5 alloy hydride catalysts, (c) carbon nanotube (prepared with AB 5 hydride catalysts) showing catalyst particles encapsulated, (d) a single carbon nanotube synthesized over AB 5 alloy hydride catalyst.

6 518 M.M. Shaijumon, S. Ramaprabhu / Chemical Physics Letters 374 (2003) FT-Raman spectroscopy studies were carried out on as-synthesized and purified carbon nanotubes, using an excitation wavelength of 1064 nm of Nd:YAG laser. Fig. 4 shows the Raman spectra of as-grown and purified CNTs, synthesized with AB 2 and AB 5 hydride catalysts. The spectrum consists of mainly two peaks at 1296 and 1598 cm 1, which are designated as the tangential modes of CNTs [31]. Hydrogen adsorption measurements were carried out at different pressures, and for precise measurement of the hydrogen adsorption capacity, extreme care was taken to eliminate the leakage of the system [28]. Each time, over 200 mg of CNTs were used. The sample was evacuated to 10 6 Torr and activated by heating to 175 C for 5 h, after flushing with H 2 for 2 3 times. It was then cooled to 100 C andh 2 was allowed to interact with the sample subsequently and then allowed to cool to room temperature. The pressure Fig. 4. FT-Raman spectra of carbon nanotubes grown over alloy hydride catalysts. drop at 100 C was found to be very less. The equilibrium pressure drop was observed for about 15 h and was noted. The amount of hydrogen adsorbed by the sample was calculated from the drop in pressure. The carbon nanotubes obtained after the acid treatment and heat treatment was also loaded and hydrogenation studies were carried out. The hydrogen molecules could be physically adsorbed on the external nanotube walls. Hence H 2 may also be adsorbed by the outer walls of the nanotubes and stay in the micro-channels between them. However, sample which was subjected to treatment such as oxidation and acid washing, which could open their ends effectively, showed improved hydrogen sorption capacity, as hydrogen could have entered nanotubes through their ends. The as-grown nanotubes generally have closed ends. In the present work, some of the catalytic particles were seen encapsulated in the interior of the as-grown nanotubes (Fig. 3c), which could be effected only through opening of the tube ends. The problem of tubes remaining open-ended under conditions favorable to closure is one, which a number of researchers have reported, as the open sheets will readily incorporate pentagons to eliminate dangling bonds [32]. A maximum storage capacity of 2.3 wt% at 100 bar and at room temperature was obtained for CNTs synthesized with AB 2 alloy hydride catalysts, while for the same sample purified with HNO 3 treatment and air oxidation, the hydrogen storage capacity was found to be increased to a maximum of 3.1 wt% at 298 K and 100 bar (Fig. 5a). In the case of carbon nanotubes prepared with AB 5 hydride catalysts, the maximum storage capacities of 2.4 and 3.3 wt% were obtained for asgrown and purified CNTs (Fig. 5b). The complete removal of catalytic impurities and amorphous carbon, with effective treatment with HF acid for these samples, would have added to the enhanced hydrogen sorption capacity. Dresselhaus et al. [33] have claimed that hydrogen molecules could be effectively adsorbed with in the interstitial space between the nanotubes and predicted 0.7 wt% adsorption with in interstitial space and a 3.3 wt% of hydrogen adsorption with in the inner core of a (10,10) armchair nanotubes. Hence for the purified samples, we feel that the increase in storage ca-

7 M.M. Shaijumon, S. Ramaprabhu / Chemical Physics Letters 374 (2003) technique. The as-synthesized and purified carbon nanotubes are characterized by XRD, SEM, TEM and Raman spectroscopy measurements. The specific surface area obtained from BET measurements is larger for purified carbon nanotubes. The hydrogen storage capacity in CNTs is found to be increased after subjecting the samples to acid and heat treatments and maximum hydrogen storage capacities of 3.1 and 3.3 wt% at 298 K and 100 bar are obtained for the purified samples synthesized with AB 2 and AB 5 alloy hydride catalysts, respectively. Acknowledgements We thank N. Mani, Department of Physics, IITM, Chennai for many discussions. M.M.S. is grateful to IITM for financial support. References Fig. 5. Hydrogen adsorption isotherms of carbon nanotubes synthesized with decomposition of acetylene over: (a) AB 2 alloy hydride catalysts, (b) AB 5 alloy hydride catalysts. pacity of the adsorbed hydrogen would have resulted from the condensation of hydrogen inside the cavity of the tubes. Further work with reference to the preparation of nanotubes with different alloy hydride materials as catalysts and their hydrogen adsorption studies are in progress. A detailed study of the adsorption and desorption measurements at 77 K in CNTs will also be carried out. 4. Conclusions Carbon nanotubes are synthesized by the pyrolysis of acetylene over AB 2 and AB 5 hydride catalysts, obtained using hydrogen decrepitation [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] Y. Ye, C.C. Ahn, C. Witham, B. Fultz, I. Liu, A.G. Rinzler, D. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (1999) [3] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) [4] A. Cao, H. Zhu, X. Zhang, X. Li, D. Ruan, C. Xu, B. Wei, J. Liang, D. Wu, Chem. Phys. Lett. 342 (2001) 510. [5] G. Gundiah, A. Govindaraj, N. Rajalakshmi, K.S. Dhathathreyan, C.N.R. Rao, J. Mater. Chem. 13 (2003) 209. [6] S. Iijima, Nature 354 (1991) 56. [7] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature (London) 393 (1998) 49. [8] S. Wind, J. Appenzeller, R. Martel, V. Derycke, Ph. Avouris, Appl. Phys. Lett. 80 (2002) [9] F. Darkrim, D. Levesque, J. Phys. Chem. B 104 (2000) [10] Y. Yin, T. Mays, B. Mc Enaney, Langmuir 16 (2000) [11] J.S. Arellano, L.M. Molina, A. Rubio, M.J. Lopez, J.A. Alonso, J. Chem. Phys. 117 (2002) [12] M. Shiraishi, T. Takenobu, M. Ata, Chem. Phys. Lett. 367 (2003) 633. [13] C. Nutzenadel, A. Zuttel, D. Chartouni, Schlapbach, Electrochem. Solid State Lett. 2 (1999) 30. [14] N. Rajalakshmi, K.S. Dhathathreyan, A. Govindaraj, B.C. Satishkumar, Electrochim. Acta 45 (2000) 4511.

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