Indian Journal of Chemistry Vol. 47A, May 2008, pp. 663-668
Synthesis of carbon nanotubes over transition metal ions supported on Al(OH)3 H Kathyayinia, K Vijayakumar Reddya, J B Nagyb & N Nagaraju*, a,b a
Department of Chemistry, St. Joseph’s College P.G. Centre, 46 Langford Road, Shanthinagar, Bangalore 560 027, India b Department of Chemistry, RMN Laboratory, FUNDP, 61 Rue de Bruxelles, Namur B5000, Belgium Email:
[email protected] Received 20 December 2007; revised 14 April 2008
Production of carbon nanotubes on bi-metallic (Fe/Co, Co/Ni, Fe/Ni, Fe/Mo, Co/Mo, Ni/Mo) and trimetallic (Fe/Co/Mo, Co/Ni/Mo, Fe/Ni/Mo) catalysts supported on Al(OH)3 by catalytic chemical vapor deposition is reported. The supportcatalyst systems have been prepared by incipient dry impregnation method and characterized by BET, powder XRD, SEM, and TG/DSC techniques. The carbon nanotubes have been synthesized by allowing C2H2 and N2 to pass over a catalyst bed at 700°C placed on a quartz plate in a tubular furnace. The carbon deposit is examined using TEM. All the catalysts have been found to be active to different extents for the production of multiwall carbon nanotubes of varying thickness (10-15 nm). In the case of catalysts containing Fe/Co or Fe/Ni, a carbon deposit to an extent of 210 to 220% of the initial weight of the catalyst is obtained. This deposit also contains good quantities of carbon nanotubes. The tubes produced on nickel catalysts have been found to be thin and long. The effect of molybdenum on the nature of the carbon nanotubes produced is dependant on whether it is associated with one or two of the other transition metal ions supported over Al(OH)3. To synthesize good quantities of carbon nanotubes, a binary mixture of Fe, Co and Ni without Mo is the best. IPC Code: Int. Cl.8 B01J21/00; B82B3/00
One of the promising methods used for large-scale production of carbon nanotubes1-3 is catalytic chemical vapour deposition (CCVD). Several metals have been investigated for their catalytic activity in the production of carbon nanotubes (CNTs) by this method4-7. Metals mostly belonging to the first row transition series are used either in their elemental form or as their salts supported on different ‘inert’ materials. The quantity and quality of the CNTs produced were found to depend not only on the type of the metal but also the nature of the support used8. Further, in many instances the catalytic performance of metals in pairs has been found to be better than when they were used separately9. Percentage composition of metals, when present in pairs, was found to play an important role10,11. MgO or SiO2 supports containing only Mo as the catalyst did not exhibit any activity for CNTs formation. However, production of CNTs was observed when Mo was associated with either Fe or Co12,13. For instance, growth of SWNT over Mo/Fe catalysts supported on MgO using C2H2 as the carbon source is reported. Yuesleng and co-workers14 reported that production of SWNT’s or MWNT’s on Mo/Co/MgO catalysts depend on the method of preparation of the catalytic
material. It is noteworthy that CO was found to be a good carbon source15,16 for SWNT synthesis on Co/Mo/SiO2. But, acetylene/ethylene12,17 performed better in the production of SWNT on Fe/Mo/MgO. We report herein the catalytic performance of various transition metal oxide-support mixtures in the production of CNTs by CCVD method. Recently, we have reported17 that a commercial sample of aluminium hydroxide serves as an excellent support for Fe/Co in large scale production of MWNTs. It prompted us to investigate the role of Al(OH)3 support in determining the catalytic performance of other bi- and tri-metallic combinations of a few 3d transition metal ions, particularly in the presence of Mo, for the production of CNTs by CCVD method. Materials and Methods Al(OH)3, Fe(NO3)3.9H2O, Ni(NO3)2.6H2O, Co(NO3)2.6H2O, and Molybdenum acetylacetonate (C10H14MoO6) were used as received from different commercial sources. The support-metal ion mixtures were prepared by dry impregnation method18. To prepare a typical sample (20 g) of support-metal ions mixture containing 95 wt% Al(OH)3 and 2.5% each of Fe and
664
INDIAN J CHEM, SEC A, MAY 2008
Co, 3.6 g of iron nitrate and 2.5 g cobalt nitrate were mixed and ground well with a minimum quantity of water. To this, 19 g of Al(OH)3 powder was added and mixed well to get a homogeneous mixture of the metal salts and the support. The material, thus obtained, was dried in an air oven at 120°C for 24 h. The dried sample was further ground well in a pestle and motor to obtain a fine powder of support-metal ion mixture. The other support-metal ion powders prepared were: (i) binary mixtures of metal ions such as Fe/Ni, Co/Ni, Fe/Mo, Ni/Mo, Co/Mo with 2.5 wt% each of metal ions, and (ii) A ternary mixture of metal ions such as Fe/Co/Mo, Fe/Ni/Mo, Co/Ni/Mo with 2:2:1 wt% of the respective ions. In order to determine the effect of calcination of the support-metal ion mixtures on CNTs production, a part of the dried and powdered support-metal ion mixture of each set was calcined at 700°C in an atmosphere of nitrogen (300 mL/min) in a tubular furnace. Both calcined and uncalcined samples were tested for their activity in CNTs production. For the sake of convenience, the support-metal ion mixtures, henceforth, will be represented as catalysts, which will be abbreviated as HK-1 to HK-9. All the catalysts were analysed for their surface area, powder XRD, surface morphology and thermal behavior. The surface area was measured using NOVA (ver-3.7) instrument at liquid nitrogen temperature. Power XRD patterns were recorded in the range of 4-60 degrees on a Phillips X-ray diffractometer with Cu-Kα radiation. SEM images of the catalysts were taken using the Philips instrument. TGA/DSC graphs were recorded on NETZSCH STA machine in the temperature range 30-900°C with a heating rate of 10°C/min in the presence of helium gas. The reactions to synthesize CNTs were conducted in a horizontal tubular furnace at atmospheric pressure using acetylene as the source of carbon. Pyrolysis of the hydrocarbon was conducted at 700°C in the presence of nitrogen as the carrier gas. In a typical experiment, about 1 g of accurately weighed catalyst was spread over a quartz plate as a thin layer and placed inside the furnace which was preheated to 700°C. After purging with nitrogen gas (300 mL/min) for 10 min, the hydrocarbon gas stream (30 mL/min) was opened for 60 min. The reaction vessel was cooled to room temperature and the weight of the carbon deposit along with the catalyst was found. The percentage of carbon deposit was calculated taking
Table 1—Composition and surface area of uncalcined and calcined catalysts Catalyst
Comp.
A
B
C
HK 1 HK2 HK3 HK4 HK5 HK6 HK7 HK8 HK9
Al(OH)3/Fe/Co Al(OH)3/Fe/Ni Al(OH)3/Co/Ni Al(OH)3/Fe/Mo Al(OH)3/Ni/Mo Al(OH)3/Co/Mo Al(OH)3/Fe/Co/Mo Al(OH)3/Fe/Ni/Mo Al(OH)3/Co/Ni/Mo
95/2.5/2.5 95/2.5/2.5 95/2.5/2.5 95/2.5/2.5 95/2.5/2.5 95/2/2/1 95/2/2/1 95/2/2/1 95/2/2/1
2.36 2.33 2.79 2.70 1.56 2.15 2.53 1.05 1.59
117.7 132.5 126.7 129.9 130.6 123.2 111.1 126.0 122.8
A = Wt% of support and metals, B = Surface area (m2/g) of uncalcined catalysts, C = surface area (m2/g) of calcined catalysts
into account the weight loss of the catalyst at 700°C (ref. 8). The synthesized carbon deposits were analysed for their morphology and the nature of the CNTs produced by SEM and TEM techniques. Results and Discussion In general, it is noticed that upon calcination the catalysts become more powdery as compared to their uncalcinied forms. This indicates that there is no sintering of the catalysts on calcination. In Table 1, the chemical composition of the catalyst and the surface area of the calcined and uncalcined forms are given. The surface area of the catalytic materials increased significantly upon calcination. It is also noteworthy that the variation of surface area among the uncalcined as well as the calcined samples is not very significant. The higher surface area of calcined catalysts indicates that there is no sintering of materials upon calcination but become more powdery and porous. The increase in the surface area on calcinations may be attributed to the structural changes of only the support and not due to that of the supported metal ions, which is later confirmed by powder XRD results. Thus, it may be added here that the metal ion species are well dispersed on the support in the percentage range chosen for the present studies. Figures 1 and 2 represent the powder XRD patterns of the uncalcined and calcined catalysts, respectively. The uncalcined samples appear to be more crystalline than the calcined ones, which is evident from sharper diffraction patterns exhibited by the uncalcined samples. Since, no characteristic peaks of any known crystalline phases of the metal species were observed
KATHYAYINI et al.: SYNTHESIS OF CARBON NANOTUBES OVER METAL IONS
Fig. 1—Powder XRD patterns of Uncalcined samples of catalyst– support mixtures (HK 1-9) containing different combinations of Fe/Co/Ni/Mo ions deposited on Al(OH)3 support.
665
in the powder XRD of either the uncalcined or calcined catalysts, it may be inferred that the metal ion species are well dispersed on the support and the observed diffraction patterns are characteristic of only the support. The diffraction peaks corresponding to 2θ = 18.41, 20.33, of uncalcined catalysts represent aluminum oxide hydrate (Al2O3.3H2O), a gibbsite phase. The peaks corresponding 2θ, 35.45, of calcined samples indicate the presence of α-Al2O3. Thus, the transition of the catalysts from more crystalline to amorphous state on calcinations is due to the temperature dependent phase changes of the support. SEM images of uncalcined and calcined catalysts were taken to find the effect of calcination on morphology of the samples. SEM images of the catalysts (Fig. 3) show that the sizes of the particles in the uncalcined samples were larger than the calcined ones. The particles that are noticed in the SEM images are those of Al(OH)3 covered with metal ion species. During the TG studies, a weight loss step in the temperature range 240-330°C was observed (Fig. 4). We attribute this to the removal of most of the
Fig. 3—SEM picture of uncalcined Fe/Ni/Mo-Al(OH)3 catalyst.
Fig. 2—Powder XRD patterns of calcined samples of catalyst– support mixtures (HK 1-9) containing different combinations of Fe/Co/Ni/Mo ions deposited on Al(OH)3 support.
Fig. 4—TG and Al(OH)3 catalyst.
DSC
curves
of
the
catalyst
Fe/Co-
666
INDIAN J CHEM, SEC A, MAY 2008
physisorbed as well as chemisorbed water molecules. DSC pattern shows sharp endotherm between 310-318°C. It is known that sharp endotherms are indicative of crystalline rearrangements, fusions or solid-solid transition for relatively pure materials. Thus, in this temperature range, Al(OH)3 support probably undergoes a phase transition from a crystalline state to an amorphous state. Further, it is observed that there is no weight loss after 500°C, indicating that the catalyst supports are thermally stable above this temperature. Thus, the results from different characterization techniques, viz., BET, XRD and TGA and DSC are consistent and complimentary to each other and indicate that the uncalcined samples are more crystalline than the calcined ones and the later possess larger surface area with the active metal species well dispersed on the support. Quality and quantity of carbon deposit
Different and distinct features of the as synthesised carbon deposit obtained from the CNTs synthesis reactions are noticed. The carbon deposit was found to be voluminous and it was either tough or spongy. These observations gave a wealth of preliminary information on the density and nature of carbon nanotubes in the carbon precipitate. For instance, it is found out from TEM analysis that a voluminous carbon deposit in general exhibited a good density of long CNTs, a tough deposit indicated the presence of CNTs in well packed condition and a spongy deposit had an appreciable amount of amorphous carbon. Table 2 gives information on the wt% of carbon precipitate obtained on various catalysts and shows that all the catalysts are active for acetylene pyrolysis to generate carbon. Bi-metallic catalysts not containing Mo (catalysts HK1, HK2 and HK3) resulted a tough carbon material whereas the one obtained from catalysts containing Mo (HK4, HK5 and HK6) are not tough. Tri-metallic catalyst systems (HK7, HK8 and HK9) yielded a spongy deposit of carbon and hence possibly contain higher percentage of amorphous carbon. Keeping in view these observations, supported by TEM observations made on the physical texture and the percentage of carbon deposit, it may be inferred that in the case of Al(OH)3 as the support (i) 3d series elements offer an excellent combination of bi-metallic catalyst systems for the generation of good quantity of CNT with high density; (ii) Presence of Mo in general, not only
Table 2—The percentage of carbon deposit obtained on HK 1-9 uncalcined and calcined catalysts using acetylene gas Sl. No.
HK 1 HK 2 HK 3 HK 4 HK 5 HK 6 HK 7 HK 8 HK 9
Catalyst on Al(OH)3
% Carbon deposit obtained on Uncalcined Calcined catalyst catalyst
Fe+Co Fe+Ni Co+Ni Fe+Mo Ni+Mo Co+Mo (Fe+Co)Mo (Fe+Ni)Mo (Co+Ni)Mo
212 220 135 78 61 74 201 190 127
130 131 84 116 116 79 139 136 88
decreases the activity of 3d series elements present in pairs for the growth of CNTs but also promotes the formation of amorphous carbon. Effect of precalcination of catalysts
A binary mixture of Fe, Co or Ni irrespective of the presence/absence of Mo, exhibited better activity in their uncalcined form. Since Fe, Co and Ni have CNT growth catalyst function, precalcination seems to be not a necessary step to get better quantities of CNTs. But, presence of Mo may decrease their activity by the formation of catalytically inactive aggregates of metal species. This effect is indicated by the formation of lower percentage of carbon deposit by these systems. A bi-metallic system with Mo as one of the ions showed less activity for acetylene pyrolysis in their uncalcined form. TEM analysis of carbon deposit
Carbon deposit from bi-metallic catalysts without Mo (HK1, HK2 and HK3)—Carbon nanotubes generated on these catalysts are multiwalled and in general are straight and no coiled tubes were noticed (Fig. 5a). The outer diameter of the tubes was found to be in the range 8-50 nm with more than 90% of the tubes with an average diameter of 15 nm. In the case of Co/Ni containing catalysts, bundles of very thin long tubes with an outer diameter of 3-8 nm were noticed (Fig. 5b). Though the quality of nanotubes produced on uncalcined catalysts did not differ much compared with the ones obtained on calcined ones, in the later case many black spots through which CNTs appeared to be gushing out were observed. Thus, calcinations of these catalysts generate aggregates of metallic particles, which are still active. But, due to their association, the number of available active sites decreases resulting in lower yields of the carbon deposit (Table 2).
KATHYAYINI et al.: SYNTHESIS OF CARBON NANOTUBES OVER METAL IONS
667
that the production of high yields of carbon nanotubes with low levels of impurities was related to the promoter character of Mo in the reaction. The TEM results are consistent with the observations reported on the texture of the carbon deposits. The voluminous nature of the carbon deposit is thus due to the presence of long tubes whereas the spongy nature is due to the amorphous carbon in the deposit. Thus, Mo seems to have no advantageous effect on the activity of the bi-metallic catalyst systems studied here, to produce good quality CNTs, however Mo produce thinner tubes when associated with one of the metal ions.
Fig. 5—Low resolution TEM pictures of as synthesized carbon deposit obtained on: (a) Fe/Ni-Al(OH)3, (b) Co/Ni-Al(OH)3 (c) Fe/Mo-Al(OH)3, (d) Co/Mo-Al(OH)3, (e) Fe/Co/Mo-Al(OH)3, and (f) Co/Ni/Mo-Al(OH)3 catalysts.
Carbon deposit from bi-metallic catalysts with Mo (HK4, HK5 and HK6)—The carbon precipitate made from these catalysts exhibited carbon nanotubes, which were more dispersed, and not in bundles as found in the previous case. Relatively, a larger number of black spots were noticed which probably represented amorphous carbon or the catalyst particles surrounded by not well-grown CNTs. A noteworthy point is that the CNTs formed on calcined catalysts were much thinner. Cobalt is known to form mixed oxide phase with Mo, and hence, formed a good density of very thin carbon nanotubes, whereas Fe and Ni generated thicker tubes (Figs 5c and 5d) Carbon deposit from tri-metallic catalysts with Mo (HK7, HK8 and HK9)—The carbon deposit from the uncalcined catalysts exhibited uneven size CNTs that were mostly long and thick with an outer diameter around 25 nm. The tubes were covered with amorphous carbon (Figs 5e and 5f). However, calcined catalysts produced thinner CNTs with lower amount of amorphous carbon. Recently Benito and co-workers19 have investigated the influence of Mo on the CVD production of carbon nanotubes and inferred
Conclusions Multiwall carbon nanotubes may be synthesised in large quantities by CCVD method using acetylene as the carbon source, at 700°C using a binary mixture of Fe, Co, Ni and Mo catalysts supported on Al(OH)3. The nature and yield of the CNTs generated is influenced by the nature of combination of the metal ions. Fe/Ni/Al(OH)3 catalyst produced up to 220% of crude carbon deposit which contained good quantities of carbon nanotubes. Acknowledgement NN thanks FUNDP university belgium for financial help. HK thanks BCWCC Principal and management for encouragement and permission to do research. References 1
Benito A M, Maser W K & Martinez M T, J Nanotechnol, 2 (2005) 71.
2
Valentin N P, Mater Sci Eng, 43 (2004) 61.
3
Serp P, Corrias M & Kalck P, Appl Catal A Gen, 253 (2003) 337.
4
Hernadi K, Fonseca A, Nagy J B, Bernaerts D, Fudala A & Lucas A, Carbon. 34(19) (1996) 1249.
5
Nagaraju N, Fonseca A, Konya Z & Nagy J B, J Molecul Catal A, 181 (2002) 57.
6
Naresh S, Sidhartha P, Frank H E, Devdas P & Gerald H P, Fuel Process Technol, 83 (2003) 163.
7
Emmanuel F, Alain P, Wolfang B S, Revathi B R & Christophe L, J Mater Chem, 14 (2004) 646.
8
Kathyayini H, Nagaraju N, Fonseca A & Nagy J B, J Molecul Catal A Chem, 223 (2004) 129.
9
Hernadi K, Kónya Z, Siska A, Kiss J, Oszkó A, Nagy J B & Kiricsi I, Mater Chem Phys, 77 (2003) 536.
10 Peigney A, Laurent C, Flahaut E, Bacsa R R & Rousset A, Carbon, 39 (2001) 507.
668
INDIAN J CHEM, SEC A, MAY 2008
11 Buang N A, Najid Z A, Sulaiman Y, Sanip S M & Ismail S F, J Porous Mater, 13 (2006) 331. 12 Lyu S C, Liu B C, Lee S H, Park C Y, Kang H K, Yang C W & Lee C J, J Physical Chem B, 108 (2004) 1613. 13 Serquis A, Liao X Z, Huang J Y, Jia Q X, Peterson D E, Zhu Y T, Carbon, 41 (13), (2003) 2635. 14 Yuesheng N, Xiaobin Z, Youwen W, Yanlin S, Lihua S, Xiaofang Y & Van Tendeloo G, Chemical Phys Lett, 366 (2002) 555.
15 Herrera J E & Resasco D E, J Catalysis, 221(2) (2004) 354. 16 Liu B C, Lyu S C, Jung S I, Kang H K, Yang, Park J W, Park C Y & Lee C J, Chemical Phys Lett, 383 (2004) 104. 17 Herrera J E, Balzano L, Pompeo F & Resasco D E, J Nanosci Nanotechnol, 3 (2003) 133. 18 Kathyayini H, Willems I, Fonseca A, Nagy J B, & Nagaraju N, Catalysis Commun, 7 (2006) 140. 19 Perez-Mendoza M, Valles C, Maser W K, Martinez M T & Benito A M, Nanotecnol, 16 (2005) S224.
