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Controlled Growth of Highly Ordered Carbon Nanofibers from Y Zeolite Supported Nickel Catalysts Colin Park† and Mark A. Keane*,‡ Department of Chemical Engineering, The University of Leeds, Leeds LS2 9JT, United Kingdom, and Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046 Received July 9, 2001. In Final Form: October 17, 2001
The use of Y zeolite supported Ni to grow ordered carbon nanofibers from an ethylene/hydrogen feedstock has been investigated where Ni was incorporated into the aluminosilicate framework by ion exchange and impregnation; the action of the Ni/NaY zeolites is compared with that of Ni/SiO2 of comparable Ni loading prepared by impregnation. The reaction of ethylene with hydrogen over these catalysts generated ethane via hydrogenation and solid carbon as a result of ethylene decomposition; hydrogenolysis to methane and the formation of C3 and C4 hydrocarbons were promoted to a lesser degree. The Ni loading, method of catalyst preparation, and nature of the support all have a bearing on the size distribution of the supported Ni crystallites which, in turn, impacts on the dimensions of the carbon nanofibers. The deposited carbon has been characterized by scanning electron microscopy, transmission electron microscopy (TEM), and temperature programmed oxidation. High-resolution TEM has revealed an orientation of graphite platelets that was generally parallel to the fiber axis with Ni metal particles located at the tips and Ni fragments dispersed throughout the carbon structure. The influence of reaction temperature, hydrogen content, and time on-stream was investigated with respect to both the carbon yield and structure. While the zeolite microporous structure limited the extent of carbon deposition, Ni/SiO2 continued to promote carbon production over prolonged reaction times. The Ni metal particles associated with the impregnated zeolite are characterized by a narrower distribution of smaller diameters when compared with the ion-exchanged samples. The former exhibited a greater control over the dimensions of the carbon fibers and produced a more uniform and narrower graphitic product.
Introduction Interest in carbonaceous materials has gained momentum with the discovery of buckminsterfullerene,1 multi-/ single-walled carbon nanotubes,2-5 carbon nanofibers,6-17 and related materials.18-20 The commercial significance * Corresponding author. Tel: (859) 257 5857. Fax: (859) 323 1929. E-mail:
[email protected]. † University of Leeds. ‡ University of Kentucky. (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Iijima, S. Nature 1991, 354, 56. (3) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (4) Zhang, M.; Wu, D. H.; Xu, C. L.; Xu, Y. F.; Wang, W. K. Nanostruct. Mater. 1998, 10, 1145. (5) Saito, Y.; Tani, Y.; Miyagawa, N.; Mitsushima, K.; Kasuya A.; Nishina, Y. Chem. Phys. Lett. 1998, 294, 593. (6) Fryer, J. R.; Paal, Z. Carbon 1973, 11, 665. (7) Baker, R. T. K. Catal. Rev.sSci. Eng. 1979, 19, 161. (8) Baker, R. T. K. Carbon 1989, 27, 315. (9) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233. (10) Chambers, A.; Baker, R. T. K. J. Catal. 1996, 158, 356. (11) Krishnankutty, N.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1996, 158, 217. (12) Chambers, A.; Baker, R. T. K. J. Phys. Chem. B 1997, 101, 1621. (13) Krishnankutty, N.; Park, C.; Rodriguez, N. M.; Baker, R. T. K. Catal. Today 1997, 37, 295. (14) Park, C.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1997, 169, 212. (15) Nemes, T.; Chambers, A.; Baker, R. T. K. J. Phys. Chem. B 1998, 102, 6323. (16) Park, C.; Baker, R. T. K. J. Catal. 1998, 179, 361. (17) Park, C.; Baker, R. T. K. J. Catal. 2000, 190, 104. (18) Govindaraj, A.; Sen, R.; Venkata Nagaraju, B.; Rao, C. N. R. Philos. Mag. Lett. 1997, 76, 363. (19) Martel, R.; Shea, H. R.; Avouris, P. J. Phys. Chem. B 1999, 103, 7551. (20) Cabioc’h, T.; Thune, E.; Jaouen, M. Chem. Phys. Lett. 2000, 320, 202.
of these materials has reached new heights as a result of their application as adsorbents, catalyst supports,21-25 and, more recently, as potential hydrogen storage materials.26-29 Moreover, the renewed interest in nanosized carbon materials for electronics applications has prompted a surge in the number of articles devoted to the synthesis and characterization of these structured materials.30-41 The distinction between carbon nanotubes and nanofibers, as (21) Imai, J.; Suzuki, T.; Kaneko, K. Catal. Lett. 1993, 20, 133. (22) Singoredjo, L.; Slagt, M.; van Wees, J.; Kapteijn, F.; Moulijn, J. A. Catal. Today 1990, 7, 157. (23) Ma, J.; Rodriguez, N. M.; Vannice, M. A.; Baker, R. T. K. J. Catal. 1999, 183, 32. (24) Coq, B.; Planeix, J. M.; Brotons, V. Appl. Catal., A 1998, 173, 175. (25) Salman, F.; Park, C.; Baker, R. T. K. Catal. Today 1999, 53, 385. (26) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (27) Chen, P.; Wu, X.; Lin, J.; Tan, T. L. Science 1999, 285, 91. (28) Gordon, P. A.; Saeger, R. B. Ind. Eng. Chem. Res. 1999, 38, 4647. (29) Park, C.; Anderson, P. E.; Chambers, A.; Tan, C. D.; Hidalgo, R.; Rodriguez, N. M. J. Phys. Chem. B 1999, 103, 10572. (30) Fischer, J. E.; Dai, H.; Thess, A.; Lee, R.; Hanjani, N. M.; Dehaas, D. L.; Smalley, R. E. Phys. Rev. B 1997, 56, 9275. (31) Petit, P.; Mathis, C.; Journet, C.; Bernier, P. Chem. Phys. Lett. 1999, 305, 370. (32) Albers, P.; Seibold, K.; Prescher, G.; Mu¨ller, H. Appl. Catal., A 1999, 176, 135. (33) Zaikovskii, V. I.; Chesnokov, V. V.; Buyanov, R. A. Kinet. Catal. 1999, 40, 552. (34) Colomer, J.-F.; Bister, G.; Willems, I.; Ko´nya, Z.; Fonseca, A.; Van Tendeloo, G.; Nagy, J. B. Chem. Commun. 1999, 1343. (35) Avdeeva, L. B.; Kochubey, D. I.; Shaikhutdinov, S. K. Appl. Catal., A 1999, 177, 43. (36) Ermakova, M. A.; Ermakova, D. Y.; Kuvshinov, G. C.; Plyasova, L. M. J. Catal. 1999, 187, 77. (37) Rao, C. N. R. J. Mater. Chem. 1999, 9, 1. (38) Willems, I.; Ko´nya, Z.; Colomer, J.-F.; Van Tendeloo, G.; Nagaraju, N.; Fonseca, A.; Nagy, J. B. Chem. Phys. Lett. 2000, 317, 71.
10.1021/la0110390 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/28/2001
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Table 1. Unit Cell Composition of the Ion-Exchanged Zeolites Used in This Study sample NaY HY Ni/NaY-I Ni/NaY-II Ni/NaY-III
chemical composition
% Ni w/w (hydrated)
% Ni w/w (dehydrated)
58(AlO2)58(SiO2)134.xH2O H+58(AlO2)58(SiO2)134.xH2O Ni2+5.0H+0.5Na+47.5(AlO2)58(SiO2)134.xH2O Ni2+10.4H+1.2Na+36.0(AlO2)58(SiO2)134.xH2O Ni2+14.1H+1.0Na+28.8(AlO2)58(SiO2)134.xH2O
1.9 3.4 6.4
2.5 4.7 8.9
Na+
presented in the recent literature, is far from clear. Carbon fibers, the focus of this study, are generally classified as graphitic structures, characterized by a series of ordered parallel graphene layers arranged in specific conformations42 with an interlayer distance of ca. 0.34 nm. The production of carbon nanotubes/nanofibers can draw on various chemical sources and contrasting methods, such as arc discharge, laser ablation, and catalytic methodologies.2,8,43 With regard to large-scale synthesis, the catalysis route is by far the most feasible option in terms of cost and energy requirements. The catalytic decomposition of carbon-containing gases over heated metal surfaces (normally in the presence of H2) can lead to the formation of copious amounts of carbon.8,11-13,15,25 Baker and coworkers8,11-16 have undertaken extensive studies of carbon nanofiber production using an array of unsupported monoand bimetallic catalyst powders. These authors have ably illustrated the manner in which catalyst structural characteristics can be tailored to obtain desired carbon platelet orientations.44 Unsupported metals, while highly efficient in terms of carbon yield (up to 300 gc gcatalyst-1),9,17,45 exhibit little or no control over the eventual nanofiber diameter. The use of supported metals facilitates the growth of smaller structured carbonaceous material. Moreover, the intrinsic catalytic properties of the metal can be altered by varying the nature of the support and/or the introduction of selected adatoms (such as Cl and S). This, in turn, has been shown to impact on both the yield and chemical/physical properties of the carbonaceous product.10,12,41,45,46 The supported catalyst route does, however, suffer the drawback of a problematic removal or dissolution of the substrate without damage to the carbon. The latter is normally carried out in an acidic solution, typically by repeated treatment with HNO3 or HF.47 The use of metal/zeolite catalysts to promote the growth of structured carbon has yet to be fully exploited. Hernadi and co-workers47,48 reported that carbon nanotube production is possible from metal-impregnated but not metal ion exchanged zeolites. They concluded that a more graphitic carbon is produced by the zeolite when compared with amorphous supports, as a direct consequence of the narrower zeolite pore structure. The principal disadvantage associated with zeolite catalysts is the rapid blockage of the pore/channel network and associated loss of activity. (39) Egashira, M.; Koura, H.; Korai, Y.; Mochida, I.; Crelling, J. C. Carbon 2000, 38, 615. (40) Jouguelet, E.; Mathis, C.; Petit, P. Chem. Phys. Lett. 2000, 318, 561. (41) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. J. Phys. Chem. B 2000, 104, 2794. (42) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (43) Rodriguez, N. M.; Kim, M.-S.; Baker, R. T. K. J. Phys. Chem. 1994, 98, 13108. (44) Baker, R. T. K.; Kim, M.-S.; Chambers, A.; Park, C.; Rodriguez, N. M. Catalyst Deactivation; Bartholomew, C. H., Fuentes, G. A., Eds.; Elsevier Science: Amsterdam, 1997; p 99. (45) Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31. (46) Owens, W. T.; Rodriguez, N. M.; Baker, R. T. K. Catal. Today 1994, 21, 3. (47) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Rigas, J.; Lucas, A. A. Synth. Met. 1996, 77, 31. (48) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Fudala, A.; Lucas, A. A. Zeolites 1996, 17, 416.
This drawback can be offset somewhat in carbon growth applications if the zeolite generates highly ordered structure(s) with uniform nanoscale dimensions; such is the premise on which this work is based. Separation of the zeolite, as opposed to amorphous substrates, from the deposited carbon is more facile;47 zeolites with low Si/Al ratios are readily attacked by concentrated acid.49 We report herein the growth and characterization of carbon fibers from an ethylene feedstock promoted using Ni supported Y zeolites prepared by ion exchange and impregnation. The effects of varying the reaction conditions and Ni loading are considered, and the nature of the carbon growth is compared with that produced from Ni/ SiO2. The advantages of using a zeolite carrier to influence the characteristics of the deposited carbon nanofibers are examined and discussed. Experimental Section Catalyst Preparation and Activation. The parent zeolite was a Linde LZ-52Y aluminosilicate, which has the nominal unit cell composition given in Table 1. This zeolite is characterized by two independent yet interconnecting three-dimensional networks of cavities: the more accessible supercages of internal diameter 1.3 nm are linked by sharing 12 tetrahedra (free diameter ) 0.7-0.8 nm); the less accessible sodalite cages are linked through adjoining rings of six tetrahedra that form hexagonal prisms (free diameter ) 0.20-0.25 nm). To obtain, as far as possible, the homoionic Na form, the zeolite as received was treated five times with 1 mol dm-3 solutions of NaNO3. The ion-exchanged Ni/NaY samples were prepared by contacting 20 g of the parent NaY with 200 cm3 of 0.1 mol dm-3 Ni(NO3)2 solutions kept under constant agitation (at 600 rpm) at 373 ( 2 K. The system was allowed 48 h to equilibrate, at which point the zeolite was separated by filtration, washed with hot deionized water (3 × 50 cm3), oven-dried at 363 K for 24 h, and stored over saturated NH4Cl at room temperature. The Ni and Na contents were determined (to within (2%) by atomic absorption spectrophotometry (VarianSpectra AA-10). Higher Ni loadings were achieved by repeated exchange, and the unit cell compositions of the three Ni-exchanged zeolite samples employed in this study are given in Table 1. The NH4+-exchanged zeolite was prepared by repeatedly refluxing 20 g of the parent NaY with 200 cm3 0.5 mol dm-3 NH4NO3 solutions as above; the activated protonic form is given in Table 1. The exchange procedure is described in greater detail elsewhere.50 Nickel-loaded silica (11% w/w Ni) and Y zeolite (12% w/w Ni) were prepared by impregnation to incipient wetness51 with aqueous Ni(NO3)2 to realize the stated loadings. The hydrated catalyst precursors, sieved in the 200-150 µm range, were activated by heating (10 K min-1) in 100 cm3 min-1 dry 20% v/v H2/He to the reaction temperature. The reduced activated catalysts were thoroughly flushed in dry He (100 cm3 min-1 for 60 min) before introducing the C2H4/H2 feed. Carbon monoxide chemisorption was employed to characterize the supported Ni sites where the catalyst was cooled, following reduction, to 298 K in dry He and a fixed volume (10 µL) of CO was pulsed into the He carrier gas; the concentration of CO exiting the reactor was measured using an on-line thermal conductivity detector (TCD). The injection of CO was repeated until the (49) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (50) Keane, M. A. Microporous Mater. 1995, 3, 93. (51) Coughlan, B.; Keane, M. A. Zeolites 1991, 11, 2.
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downstream peak area was constant, indicating surface saturation with CO. The sample was thoroughly flushed with 100 cm3 min-1 dry He for 60 min to remove any physisorbed CO and then ramped at 25 K min-1 in 75 cm3 min-1 dry He to 1073 K with continual (TCD) monitoring of the exiting gas stream to generate the CO temperature-programmed desorption (TPD) profile; data acquisition and analysis were performed using the JCL 6000 (for Windows) software package. The catalyst bed temperature was independently monitored using an on-line data logging system (Pico Technology, model TC-08). Upon completion of the CO TPD sequence, a series of calibration test peaks were taken at ambient temperature to quantify the CO uptake/release; reproducibility was better than (3%. Catalytic Reactor System. All catalytic reactions were carried out (in situ directly after catalyst activation) under atmospheric pressure in a fixed bed continuous flow silica reactor over the temperature ((1 K) range 673 K e T e 923 K. The catalytic measurements were made at W/Q ) 1.6 × 10-5 g h cm-3, where W represents the weight of activated catalyst and Q is the inlet volumetric feed rate of ethylene; the overall gas hourly space velocity was kept at a constant 1.1 × 104 h-1 while the inlet C2H4/H2 mol ratio was varied from 4/1 to 1/4. A set of standard reaction conditions were chosen to assess the activity/selectivity of each catalyst, that is, T ) 873 K, W/Q ) 1.6 × 10-5 g h cm-3, and C2H4/H2 ) 4/1. The product stream was analyzed by on-line capillary chromatography using an AI Cambridge GC94 chromatograph equipped with a split/splitless injector and a flame ionization detector, employing a DB-1 50 m × 0.20 mm i.d., 0.33 µm capillary column (J&W Scientific) in conjunction with the JCL6000 system. The gaseous stream was sampled at regular intervals by means of a heated gas-sampling valve. The catalyst was typically left on-stream for 60 min to allow for a uniform growth of carbon; in some cases, the reaction had to be terminated sooner due to a plugging of the reactor by deposited carbon. The reactor was subsequently cooled to ambient temperature, and the sample was passivated in a 2% v/v O2/He mixture before any weight changes (carbon deposition) were determined. The yield of solid carbon (YCs) was calculated using the expression
YCs )
(C2H4)input - (CH4/2 + C2H4 + C2H6)output (C2H4)input
and is quoted per gram of catalyst (or gNi-1). The selectivity in terms of (say) ethane is given by
SC2H6(%) )
∑ (CH
4
YC2H6 + Cs + C2H6 + C3 + C4)output
× 100
The structural characteristics of the catalyst and deposited carbon were probed by transmission electron microscopy (TEM) using a CM200 FEG TEM, operated at an accelerating voltage of 200 keV. Suitable specimens were prepared by ultrasonic dispersion in 2-butanol with a drop of the resultant suspension evaporated onto a holey carbon support grid. Analysis by scanning electron microscopy (SEM) was carried out using a Hitachi S700 field emission SEM, operated at an accelerating voltage of 25 kV; the sample was deposited on a standard aluminum SEM holder and coated with gold. The catalytically grown carbon was also examined by temperature-programmed oxidation (TPO). A 100 mg demineralized (in dilute acid) sample was ramped (10 K min-1) to 1248 K in a 5% v/v O2/He mixture. The effluent gas was analyzed by on-line TCD, and the catalyst bed temperature was again independently monitored, using the Pico data logger, and compared with the programmed reactor temperature. Any deviation between the two temperatures during TPO can be linked to the exothermic carbon gasification. All the gases (He (99.99%), C2H4 (99.95%), H2 (99.99%), and 5% v/v O2/He (99.9%)) were dried by passage through activated molecular sieves before use. Activated carbon (Darco G-60) and graphite (synthetic powder) were obtained from Aldrich Chemicals and also dimineralized, to remove any metallic impurities, before use as model carbons in the TPO studies.
Table 2. CO Uptake at 298 K and Characteristic Desorption Temperature Maxima (Tmax) for the Activated Ion-Exchanged Ni/NaY and the Impregnated Ni/NaY and Ni/SiO2 Catalystsa sample
CO adsorbed µmol gNi-1
Tmax K
Ni/NaY-I Ni/NaY-II Ni/NaY-III Ni/NaY-impreg Ni/SiO2
13.3 7.0 5.7 66.1 104.8
610, 875, 978 670, 909, 973 589, 651, 788, 988 368, 504, 790, 1039 953, 1073
a The T max values given in bold refer to the principal desorption peaks.
Results and Discussion Catalyst Characterization. The CO uptake values for the ion-exchanged and impregnated catalysts are recorded in Table 2 which also contains the characteristic Tmax values from the CO TPD profiles. Carbon monoxide, under the stated conditions, is well documented in the literature to adsorb associatively on Ni in one of two possible configurations, that is, at a bridging site or more commonly in a 1:1 stoichiometry.52 TPD of CO has been used to good effect elsewhere53-55 to probe the electronic properties of supported metal particles. In the case of the ion-exchanged Ni/NaY catalysts, not only is the CO uptake significantly lower (gNi-1) than that recorded for the impregnated catalysts but the majority of the chemisorbed CO is desorbed at relatively low temperatures with a secondary component released at 950-1050 K. The adsorption of CO on supported Ni is strongly dependent on the nature of the support, loading, temperature, pressure, and chemisorption procedure, that is, static or dynamic.56,57 This really renders unfeasible any attempt at an explicit assignment of one particular surface CO/Ni stoichiometry. Nevertheless, the decrease in CO uptake with increasing Ni loading and the lower values recorded for the ion-exchanged samples are diagnostic of the formation of larger metal crystallites. Indeed, the histograms derived from TEM analysis, given in Figure 1, illustrate the wide distribution of Ni sizes that characterize the ion-exchanged zeolites where an increase in Ni loading is accompanied by a significant broadening of the size distribution and a shift in the surface weighted average diameter to higher values. Reduction of Ni-exchanged Y zeolites under similar conditions has been shown elsewhere51,58 to generate a metal phase that exhibits a wide size distribution with particle growth resulting in the formation of larger metal crystallites supported on the outer surface. The increase in crystallite size at higher Ni loadings can be explained by the increased probability of metal atoms being in close proximity to facilitate agglomeration. Alternatively, Law and Kenney59 proposed that a high concentration of protons in the zeolite matrix promotes the counterdiffusion of metal cations and protons, supplying more metal ions to the pore mouths that are subsequently reduced to a zerovalent exterior metal phase. The parent NaY possesses a small number of weak acid sites.60,61 A higher degree of acidity is (52) Zielinski, J. J. Mol. Catal. 1993, 79, 187. (53) Vasquez, N.; Muscat, A.; Madix, R. J. Surf. Sci. 1994, 310, 83. (54) Falconer, J. L.; Schwartz, J. A. Catal. Rev.sSci. Eng. 1983, 25, 141. (55) Arena, F.; Frusteri, F.; Parmaliana, A. Appl. Catal., A 1999, 187, 127. (56) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J. Catal. 1980, 65, 335. (57) Vannice, M. A.; Garten, R. L. J. Catal. 1979, 56, 236. (58) Keane, M. A. Zeolites 1993, 13, 14. (59) Law, P. L.; Kenney, C. N. J. Catal. 1980, 64, 241.
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Figure 1. Nickel particle size distribution profiles of freshly reduced Ni/NaY-I (hatched bars), Ni/NaY-III (open bars), Ni/NaYimpreg (solid bars), and Ni/SiO2 (crosshatched bars).
introduced by exchanging the indigenous Na+ with Ni2+ ions followed by reduction in H2 to generate protons according to the equilibrium60,62
Ni2+-Z + H2 h Ni0-Z + 2H+-Z where Z represents the zeolite phase; two surface hydroxyl groups (Brønsted acid sites) are generated for each Ni2+ that is reduced. The lower surface acidity associated with activated impregnated Ni/NaY59 should serve to stabilize smaller particles on the basis of the Law and Kenney model. Moreover, as the indigenous Na content is not altered during impregnation, the higher cation density in the migration path of the Ni species may act to retard the aggregation process that leads to particle growth. Although the average Ni particle sizes (8-38 nm) far exceed the free dimensions of the zeolite microporous network, there is a significant internal Ni component in both ionexchanged and impregnated samples that can contribute directly to the catalytic step. On relatively thin areas of samples prepared for TEM, metal particles clustered at pore mouths and within the cage structure can be distinguished, as we have reported elsewhere;61 selected area electron diffraction confirmed that the metallic Ni particles were crystalline. Desorption of CO from the impregnated Ni/NaY catalyst occurred in two broad steps: weakly chemisorbed CO over the range 450-530 K; strongly bound CO in the interval 1010-1090 K. The major difference between the ionexchanged and impregnated Ni/NaY samples is the absence of a predominant lower temperature peak (5 µm) of the fibers as shown in Figure 4c where a relatively small proportion of the zeolite surface is still exposed. After 60 min onstream, the catalyst surface was entirely covered by a carbon overlayer (Figure 4d). It is hoped that this sequence of SEM micrographs gives the reader some indication of the magnitude and time scale of carbon growth. At extended reaction times, the surface is so matted in carbon that little in the way of change to the surface topography can be distinguished.
Figure 5. (a) Low-magnification TEM image depicting typical carbon fiber growth; (b) HRTEM image showing the lattice fringes and carbon platelet orientation; (c) HRTEM micrograph showing the chainlike growth of a carbon nanofiber that incorporates a fragment of the parent Ni particle; Ni/NaY-II; T ) 873 K; C2H4/H2 ) 4/1.
The generally accepted model74 for the growth of carbon fibers assumes a destructive chemisorption of the hydrocarbon on a specific face of a metal particle that precedes diffusion into the metal and precipitation at other faces of the metal particle. The growth continues through further dissolution and diffusion until the metal particle becomes chemically poisoned or completely encapsulated by carbon. The growth process under our stated standard conditions was such that it did not totally suppress the hydrogenation step (see Table 3). The structure of the growing carbon fiber is largely governed by the accessible or exposed crystallographic orientations of metal face(s). The latter is, in turn, determined by the catalyst preparation route, activation conditions, and metal/support interactions. A representative low-magnification TEM image of the carbon fibers grown from Ni/NaY-II is given in Figure 5a wherein it is clear that the Ni particles are positioned at the growing tip of the nanofibers. The pressure exerted on the metal/ support interface due to graphite formation is of sufficient magnitude to extract the Ni particle from the support. Once the metal particle is detached from the zeolite, a fresh surface is exposed to ethylene and growth continues with the Ni particle located on the fiber tip. A wide range of nanofiber diameters is in evidence due to growth from Ni crystallites located within the cage structure, at pore mouths, and on the external surface. The average Ni particle diameters in the freshly activated catalysts are given in Table 4, along with the TEM-derived average (74) Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1972, 26, 51.
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Figure 6. Nickel particle size (crosshatched bars) and carbon nanofiber diameter (solid bars) associated with Ni/NaY-II: T ) 873 K; C2H4/H2 ) 4/1. Inset: Particle and nanofiber diameter distribution associated with Ni/NaY-I; symbols and reaction conditions are as above.
carbon fiber diameters. The weighted mean diameters of the carbon nanofibers (T ) 873 K) generated from Ni/ NaY-II were significantly smaller than the Ni particle size of the freshly activated catalyst. Moreover, the distribution of fiber diameters was quite distinct from the Ni particle size distribution, as illustrated by the histograms given in Figure 6; a bimodal distribution of nanofiber diameters is apparent that is not evident for the Ni particles. While the supported Ni crystallites are characterized by a broad range of sizes (up to 80 nm), the majority (ca. 80%) of the nanofibers possessed diameters of less than 40 nm. In marked contrast, the nanofiber width associated with Ni/NaY-I was appreciably greater than the average particle size, as shown in the inset in Figure 6 and Table 4. In support of the latter, Anderson and Rodriguez66,71 observed that SiO2 supported monoand bimetallics readily underwent sintering and possible reconstruction to generate carbon nanofibers that were wider than the freshly reduced metal crystallites. Nickel sintering during catalysis over zeolites has been noted for other reactions,61,75 and such an effect is to be expected given the elevated reaction temperatures required for ethylene decomposition. There is clear evidence, in Figure 6, of Ni growth in Ni/NaY-II with the appearance of fibers of diameter >60 nm. However, it appears that the smaller particles (from a wide range of sizes) are more active in terms of C2H4 decomposition and contribute to a greater degree to the ultimate carbon growth. Indeed, the width of the majority of the fibers grown from Ni/NaY-II coincides with those from Ni/NaY-I. Carbon growth from the higher loaded Ni/NaY-III showed similar behavior to Ni/NaY-II (see Table 4), while the carbon grown from impregnated silica and zeolite possessed a larger average diameter than the metal in the freshly activated catalyst. The Ni particles supported on the latter two samples were considerably smaller than those associated with Ni/NaY-II or Ni/NaYIII, and Ni particle growth during catalysis, in common with Ni/NaY-I, must have a greater direct impact on the ultimate nanofiber width. (75) Gallezot, P.; Ben Taarit, Y.; Imelik, B. J. Catal. 1972, 26, 295.
High-resolution TEM revealed that the orientation of the graphite platelets in the fibers was, in the main, parallel to the fiber axis, as illustrated in Figure 5b. The fiber bears fragments of Ni that have periodically been lost from the parent metal particle during carbon growth. Whether the metal is entrapped within the central core of the fiber or is merely located on the surface has yet to be ascertained. The lattice spacing of adjacent graphite platelets is 0.34 nm, diagnostic of graphite.76 In addition to these ribbonlike fibers, a small number possessing a chainlike structure were observed, Figure 5c. The latter structures possess an ill-defined central core and also carried fragments of the parent Ni particle. It is unlikely that such carbon fibers can be generated in a continuous fashion; Anderson and Rodriguez reported71 similar structures and attributed their formation to sporadic bursts of growth. In addition, helical shaped fibers were observed where again the carbon lattice ran parallel to the edge of the fiber, but these structures represent a minor component of the overall growth. Taking the results presented in Table 4, it is evident that the dimensions of the carbon fibers are a function not only of Ni loading/ preparation but also of the reaction conditions. The effect of varying the C2H4/H2 ratio on nanofiber diameter distribution, taking Ni/NaY-II as a representative catalyst, is shown in Figure 7. Although a variation in C2H4/H2 from 4/1 to 1/4 did not have a substantial effect on the average fiber width (see Table 4), a broader range of sizes was associated with higher H2 content. An increase in the latter raised the proportion of narrower fibers where, at C2H4/H2 ) 1/4, >90% of the generated nanofibers have diameters of
