Y Zeolite Based Catalysts

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Growth of Carbon Nanofibers from Ni/Y Zeolite Based Catalysts: Effects of Ni Introduction Method, Reaction Temperature, and Reaction Gas Composition Antonio de Lucas, Agustıń Garrido, Paula Sa´ nchez, Amaya Romero,† and Jose´ L. Valverde* Facultad de Ciencias Quı´micas y Escuela Te´ cnica Agrıćola, Departamento de Ingenierıá Quı´mica, Universidad de CastillasLa Mancha, 13071 Ciudad Real, Spain

Results of thorough studies of the catalytic synthesis of carbon nanofibers (CNFs) by the decomposition of ethylene using Y zeolite as the support and Ni as the active phase were discussed. Experimental results clearly indicated that the metal-incorporation method (ion exchange or impregnation) had very significant effects not only on CNFs growth but also on the deactivation rate, the final yield of CNFs, and the characteristics of the synthesized CNFs. CNFs synthesized from the impregnated catalyst grew from small and well-dispersed Ni particles anchored to the outer surface of the zeolite. Nevertheless, CNFs synthesized from the ion-exchanged catalyst grew from Ni particles (of very small size) lodged inside the pore system of the zeolite. Reaction temperature and C2H4/H2 (v/v) had a considerable effect on both carbon yield and CNFs morphology. Two types of CNFs were observed as a function of the reaction temperature: “fishbone structures” at temperatures below 600° C and “tubular structures” at temperatures above 600° C. On the other hand, as the C2H4/H2 ratio was decreased, the CNFs became slightly more graphitic in nature and the arrangement of graphite sheets changed from the fishbone structure to “octopus carbon”. Introduction Synthesis of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) using catalytic CVD methods has attracted industrial attention because it can be carried out at low temperatures ranging from 500 to 1100 °C1-3 and is easily scaled up for industrial production. These materials have many unique properties, such as high resistance to strong acids and bases, high electric conductivity, high surface area, and high mechanical strength.4,5 These properties result in many potential applications, such as catalyst support,6 selective adsorption agents,7 hydrogen storage,8 composite materials,9 nanoelectronic and nanomechanical devices,10 and field emission devices. Three distinct structural types of filaments have been identified based on the angle of the graphene layer with respect to the filament axis,11 namely, stacked (perpendicular), herringbone (with a specific inclination angle), and nanotubular (parallel). Nevertheless, CNTs can be considered as a special case of CNFs, and therefore, CNFs are used in the present work. Carbon nanofibers and nanotubes are grown by the diffusion of carbon through a metal catalyst and its subsequent precipitation as a graphitic filament.11-13 The most important metals to catalyze the growth of graphite carbon are (alloys of) iron, cobalt, and nickel.5 The metals have been used as both bulk and supported particles. Several methods have been established to synthesize carbon nanomaterials (via catalytic decomposition of carbon containing gases or vaporized carbon * To whom correspondence should be addressed. Tel.: +34926295300. Fax: +34-926295318. E-mail: JoseLuis.Valverde@ uclm.es. † Escuela Tećnica Agrıćola.

from arc discharge or laser ablation). Among these methods, catalytic decomposition of hydrocarbons has been found to be superior for the selectivity to carbon nanotubes/nanofibers.14 The advantages of chemical vapor decomposition (CVD) compared with the arc discharge method are the formation of various nanostructures, such as straight, bent, and also helical, and the much greater lengths.15 Moreover, it was also possible to synthesize aligned carbon nanomaterials in high yields using the CVD method.16-18 The advantages of this method increase if zeolites are applied as catalyst supports.19 The use of metal zeolite catalysts for the efficient synthesis of carbon nanotubes has yet to be fully exploited.20-23 Zeolites with well-defined pore structures and high surface areas have, in general, very good metal-dispersion properties, which is very important in the growth of carbon nanotubes/nanofibers. The role of the catalyst support and the particle size of the metal have been generously discussed.4,21,24 Particularly, the pore diameter of the support in the formation of nanotubes/nanofibers is a problem that has to be thoroughly discussed. Taking into consideration that the metal component, which is the catalytic active center, may situate in the pores and on the external surface, the chance to find the correct answer to the former problem is rather challenging. As the state of the metal component is concerned, the most discussed problem is the influence of the size of the metal clusters generated. On the other hand, the final results of a synthesis of nanotubes/nanofibers should normally depend as much on the feed gas as on the reactor operating conditions.25 In the field of chemical engineering, the reactor operating conditions are considered to be the main factors influencing the chemical reaction, while the detailed features of a given catalyst are secondary. The growth

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temperature, especially, is crucial for the selective and controlled growth of nanomaterials, which is necessary for many applications. The present work deals with the preparation and characterization of CNFs produced by ethylene decomposition over Ni-supported Y zeolite prepared by both ion exchange and impregnation. The synthesis of carbon nanofibers with various diameters and shapes as a function of the metal incorporation method is described in detail. The investigation has also focused on the effects of the metal loading, reaction temperature, and C2H4/H2 (v/v) ratio on the yield and characteristics of the CNFs synthesized here. Experimental Section Various techniques are available for the introduction of metals into zeolites. Ion exchange and impregnation are often used. The former method preferentially introduces the cation into the zeolite, whereas the latter method also incorporates an equivalent number of anions. In both cases, the introduction of ions has to be followed by calcination and reduction steps. 1. Catalysts Preparation. The parent Na zeolite was a Zeolyst International CBV 100 aluminosilicate which had a unit cell size of 24.6 Å. This zeolite is characterized by two independent yet interconnecting three-dimensional networks of cavities: the more-accessible supercages are linked by sharing tetrahedrons and the less-accessible sodalite cages are linked through adjoining rings of six tetrahedrons that form hexagonal prisms. 1.1. Catalyst Preparation by Impregnation. Impregnation of the zeolite was achieved as follows: several grams of the parent zeolite were placed in a glass vessel and kept under vacuum at room temperature for 2 h in order to remove water and other compounds adsorbed on the structure. A known volume of an aqueous Ni(NO3)2 solution (the minimum amount required to wet the solid) was then poured over the sample. After 2 h, the solvent was removed by evaporation under vacuum at 90° C. 1.2. Catalyst Preparation by Ion Exchange. The ion-exchanged samples were prepared by contacting 20 g of the parent zeolite with 200 cm3 of 0.1 M Ni(NO3)2 solution. The suspension was kept under constant agitation at room temperature for 24 h. Later, the zeolite was separated by filtration, washed with deionized water, and oven-dried at 120° C. Each sample had a different metal loading, depending on the number of steps that were used. Finally, the hydrated catalysts precursors were calcined at 550 °C for 2 h (10 °C min-1). 2. Catalytic Synthesis of Carbon Nanofibers. The carbon nanofibers growth was carried out at atmospheric pressure in a fixed-bed reactor (quartz tube of 2.5 cm diameter and 75 cm length) located in a vertical oven in the temperature range of 400 to 800 °C. In each synthesis run, 100 mg of the prepared catalyst was placed in the center of the reactor and then was activated by heating (10 °C min-1) in 100 cm3 min-1 dry 20% v/v H2/He to the desired reaction temperature. The reduced activated catalyst was thoroughly flushed in dry He (100 cm3/min) for 1 h before introducing the C2H4/H2 (4-0.25 v/v) feed. The growth time was adjusted to allow a uniform growth of carbon. The reactor was subsequently cooled to ambient temperature in He gas. After decomposition of ethylene, the weight of

carbon deposit (Wc) was calculated in each synthesis using the equation

Wc (%) ) (mtot - mcat )/mcat × 100 (gcarbon/gcat) where mcat was the initial amount of the catalyst (before reaction) and mtot was the total weight of the sample after reaction. This yield was also quoted per gram of metal (gcarbon/gNi). The product stream was analyzed by on-line capillary chromatography using a Shimazdu GC17A chromatograph, equipped with a split injector and a flame ionization detector, and a Restek Q-plot (30 m length and 0.32 mm i.d.) capillary column. The yield of solid carbon and the selectivity (in terms of, say, ethane) were calculated using the following expression:

yield to carbon ) ((C2H4)input - (CH4/2 + C2H4 + C2H6)output)/(C2H4)input 3. Purification of Carbon Nanofibers. Zeolite supports were separated from the carbon product by leaching the primary product in hydrofluoric acid (70%) for 15 h under vigorous stirring followed by filtration and washing. Upon treatment of the catalyst-carbon mixture, the metal component of the catalysts is generally transferred into the solution. If not, further treatment in diluted mineral acid solution is recommended (this was not the case in this study). Anyway, metal particles encapsulated into the carbon nanofibers remained intact after this treatment. 4. Catalysts and Nanofibers Characterization. The X-ray diffraction (XRD) patterns were obtained using a Philips model PW 1710 diffractometer with Nifiltered Cu KR radiation. This technique was used to determinate the average nickel particle size of the catalysts and the crystallinity of the carbon nanomaterials. Textural characteristics of both the catalysts and the nanomaterials were determined by using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Samples were outgassed prior to use at 180 °C for 16 h under a vacuum of 6.6 × 10-9 bar. Specific total surface areas were calculated using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at a relative pressure (P/Po) of N2 equal to 0.99. The Horvath-Kawazoe method was used to determine the microporous surface area and micropore volume.26 The Barret, Johner, and Halenda (BJH) method was used to determine the distributions of the mesopores.27 To quantify the total amount of metals into the catalysts, atomic absorption (AA) measurements, with an error of (1%, were made by using a Spectraa 220FS analyzer with simple beam and background correction. The samples were previously dissolved in hydrofluoric acid and diluted to the interval of measurement. The concentration of the acid sites was measured by temperature-programmed desorption of ammonia (TPDA) using a Micromeritics TPD/TPR 2900 analyzer. The sample was first heated at 10 °C min-1 under a flow of helium from room temperature to the calcination temperature, holding this temperature for 30 min. After reducing the catalysts under a hydrogen flow, the system was cooled to 180 °C. Ammonia was then flowed over the sample for 15 min. Later, the sample was purged with helium for 1 h in order to eliminate

Ind. Eng. Chem. Res., Vol. 44, No. 22, 2005 8227 Table 1. Reaction Yields and Weights of Carbon Obtained with Different Ni-Loaded Ion-Exchanged/Impregnated Catalysts at Different Temperatures and H2 Concentrations reaction yield (%) t ) 5 min Ycarbon

YC2H6

t ) 25 min YCH4

Ycarbon

YC2H6

t ) 60 min YCH4

Ycarbon

YC2H6

YCH4

Wc (gc/gcatalyst)

Wc (gc/gNi)

0.3 0.5 0.5 1.4 1.8 2.1

0.0 0.03 0.03 0.8 0.1 0.1

1.6 2.8 3.8 2.9 4.1 4.6

39.4 48.3 59.2 59.2 69.7 78.8

Influence of the Reaction Temperature (reaction conditions: T ) 400-700 °C, C2H4/H2 (v/v) ) 4) 400 °C 27.5 5.4 0.4 8.5 4.0 0.3 7.4 4.2 500 °C 88.8 0.2 10.6 40.2 1.4 7.2 6.5 2.1 550 °C 90.1 0.0 9.8 80.5 0.7 6.5 2.1 0.8 600 °C 93.2 0.0 6.7 63.5 0.1 5.4 0.0 1.6 700 °C 83.4 0.1 2.8 28.2 11.6 1.6 10.1 12.2

7.8 0.1 0.1 0.2 1.6

0.8 4.6 5.5 4.6 2.1

13.0 74.7 88.7 74.7 33.2

Influence of the C2H6/H2 (v/v) Ratio (reaction conditions: T ) 550 °C, C2H4/H2 (v/v) ) 4-0.25) 100% C2H4 91.6 1.3 5.7 29.6 1.9 1.3 1.5 1.0 C2H4/H2 ) 4 90.1 0.0 9.8 80.5 0.7 6.5 2.1 0.8 C2H4/H2 ) 1.85 89.0 0.0 10.9 86.2 0.3 13.0 52.5 3.0 C2H4/H2 ) 1 85.6 0.0 14.4 74.9 0.7 15.3 56.8 2.8 C2H4/H2 ) 0.25 74.1 0.0 25.9 67.1 0.0 32.8 63.7 0

0.1 0.1 14.8 17.6 36.0

5.1 5.5 6.2 4.9 2.1

82.7 88.7 100.0 79.0 33.0

Influence of the Ni Deposition Method (reaction conditions: T ) 500 °C, C2H4/H2 (v/v) ) 4) 4.3 Ni (wt %) ion exchange 76.1 9.1 2.1 8.6 1.1 0.1 5.0 5.8 Ni (wt %) ion exchange 88.9 5.0 4.1 23.1 2.0 0.1 5.0 6.5 Ni (wt %) ion exchange 94.5 2.7 0.3 25.2 2.4 0.2 5.0 4.3 Ni (wt %) impregnation 92.3 0.0 7.1 86.4 0.1 13.2 19.0 5.8 Ni (wt %) impregnation 90.3 0.2 9.0 50.3 0.9 8.0 10.6 6.5 Ni (wt %) impregnation 88.8 0.2 10.6 40.2 1.4 7.2 6.5

physisorbed species. The temperature was ramped at 10 °C min-1 from 180 to 600 °C, and TPDA data were acquired. The total acidity was obtained by integration of the area under the curve. Temperature-programmed reduction (TPR) measurements of the catalysts were carried out with the same apparatus described above. After loading, the sample was outgassed by heating at 10 °C/min in an argon flow up to the calcination temperature of the sample and kept constant at this temperature for 30 min. Next, it was cooled to room temperature and stabilized under an argon/hydrogen flow (g99.9990% purity, 83/17 volumetric ratio). The temperature and detector signals were then continuously recorded while heating at 10 °C/min up to 700 °C. The liquids formed during the reduction process were retained by a cooling trap placed between the sample and the detector. TPR profiles were reproducible, with standard deviations for the temperature of the peak maxima being (2%. The diameter distribution and the morphology of the catalysts and carbon nanomaterials were probed by transmission electron microscopy (TEM) using a Philips Tecnai 20T, operated at an accelerating voltage of 200 keV. Suitable specimens were prepared by ultrasonic dispersion in acetone with a drop of the resultant suspension evaporated onto a holey carbon-supported grid. The diameter distribution was measured by counting ∼200 CNFs on the TEM images. Temperature-programmed oxidation (TPO) was used to determine the crystallinity of the nanomaterials. The analyses were performed on 10 mg samples using a Perkin-Elmer TGA 7 thermogravimetric analyzer under a flow of 50 mL min-1 of 5% v/v O2/He mixture and with a heating rate of 5 °C min-1 up to 900 °C. Results and Discussion 1. Influence of the Metal-Introduction Method. The reaction of C2H4 with H2 over the catalysts considered in this study generated, as the principal product, a solid carbon deposit via ethylene decomposition. Hydrogenolysis to methane, hydrogenation to ethane, and the formation of C3 represented secondary reactions that did not amount to more than 15% conversion of

the inlet C2H4. Table 1 shows reaction yields in terms of the three main competing reactions and the weight of carbon obtained with the different catalysts. The parent NaY exhibited no significant activity in terms of carbon-fiber production, even after extended periods on-stream over the range of conditions that we considered. The activated protonic form (HY) was also largely inactive in terms of structured carbon growth, obtaining an amorphous carbon overlayer, typically associated to the acid-site catalytic conversion of hydrocarbons.28 Incorporation of Ni into the aluminosilicate structure resulted in an appreciable increase in the yield of carbon. Setting an upper limit of 60 min for the reaction time was deemed prudent when assessing the potential of our catalysts as suitable candidates from which to control the growth and characteristics of carbon fibers. As can be observed, both in terms of carbon growth per gram of catalyst and per gram of Ni, the impregnated catalyst was the most efficient, delivering a maximum carbon yield (after 60 min) of 78.8 gc/gNi. To examine how this yield is brought about, the course of the deposition in time was monitored. The results are summarized in Figure 1a. When carbon nanofibers were grown with the use of the impregnated catalyst, a high initial deposition rate was observed which dropped continuously to zero already within 60 min. The same behavior was observed when the ion-exchanged catalyst was used, but the carbon-deposition rate decreases faster, reaching zero within 45 min. Obviously, both types of catalysts deactivated completely. This fact could be related to the selective siting and size of metal particles within the zeolite cage structure in the case of the ion-exchanged samples, as explained below. Table 2 shows the general characterization results obtained with the catalysts reduced according to the same actuation procedure used for the catalytic synthesis of carbon nanofibers. Reduction of Ni ionexchanged Y zeolites has been shown 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.30,31 This can be observed in the TEM pictures in Figure 2 for the ion-exchanged and impregnated samples, and

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Figure 1. Carbon deposition as a function of time: (a) influence of the Ni-introduction method and (b) influence of the reaction temperature (Ni loading ) 6.5 wt %).

the analysis of the particle size distribution is shown in Figure 2. The average Ni crystal size is in the range 15-19 nm in the impregnated samples, which was close to the value measured by XRD (13 nm). The average Ni crystal size in the ion-exchanged samples was very high (∼50 nm), corresponding to the big Ni particles located on the zeolite surface and observed by TEM. In our opinion, the majority of the Ni species in the ionexchanged catalysts should be located inside the pore structure of the zeolite (not observed by TEM) as small Ni crystals. This statement is in agreement with the findings of Sachtler and Zhang,29 who asserted that the ion-exchange step could place the ligated metal atoms into the main channel system of zeolites or, in the case of faujasites, in the supercages. Next, the calcination step removes the water, destroys the ligands and, could even cause the migration of the metal ions into smaller zeolite cages. To check it, the TPR profiles of the ionexchanged and impregnated catalysts were examined (Figure 4). These profiles showed the formation of different Ni species during the thermal process. Impregnated catalysts with the lower Ni content showed two peaks at 340 and 480 °C that are located before and after the maximum exhibited by the pure NiO (400 °C, not shown).32 These peaks have been attributed to the

reduction of NiO species that have different interactions with the support.32 At higher Ni contents, the same peaks appear, but the intensity of the peak located at the low temperature is higher. On the other hand, ionexchanged samples showed the same peaks at 340 and 480 °C related to the reduction of different NiO oxides linked to the support with lower or higher intensity. A shoulder centered at 560 °C appears only in the ionexchanged samples, which could be associated with the reduction of stabilized Ni2+ species located in the inner pores of the zeolite that are more difficult to reduce. Additionally, in catalysts prepared by ion exchange, the intensity of the peaks located at 480 and 340 °C were higher and lower, respectively than the intensity of the corresponding ones in samples prepared by impregnation. In general, changes in the maximum reduction temperature reflect the extent of interaction between the metal precursor and the support. When the impregnation was used to insert the metal to the support, most of the Ni particles were located on the external surface of the zeolite, so a weak interaction with the support could be observed. But when ion exchange was used, Ni particles were preferably lodged inside the pore system of the zeolite and, therefore, a strong interaction with the acid sites occurred.4,33 Textural characterization data also confirmed these results. Both BET surface and micropore area were considerably higher over the impregnated catalysts, showing that a high proportion of small Ni2+ ions may exist inside the pore structure of the zeolite when this metal is incorporated by the ion-exchange procedure, leading to a higher pore blockage by Ni particles than that obtained for the impregnated samples. On the other hand, an increase of the metal loading led to a decrease of the surface area of the catalyst due to the partial blocking of the zeolite matrix by the metal species. The parent NaY possesses a small number of weak acid sites.20 A higher degree of acidity was introduced by exchanging the Na+ ions of the zeolite with Ni2+ ions followed by reduction in H2 to generate protons (Bronsted acid sites), as can be confirmed from the acidity values shown in Table 2. The lower surface acidity, associated with the reduced Ni-impregnated catalysts, should serve to stabilize smaller particles according to Law and Kenney.34 These authors 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. Moreover, because the initial Na content was only slightly 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. Therefore, it is possible to assume that the metal particles anchored to the outer surface area are more active for carbon growth,21 and these sites are far more predominant when metal is deposited by impregnation. To explain this, it is necessary to consider both the pore mouth size, 0.7 nm, and the supercage size, 1.3 nm of Y zeolite. The formation of carbon nanostructures with a diameter 750 °C are almost inactive in the carbon formation by the decomposition of ethylene. As has been reported and commented above, the diameter of the nanofibers had a direct relation with the size of metal particles. It is clear that the size of the catalytic metal particles determines the diameter of the CNFs in CVD growth.49 So, at higher reaction temperatures, the migration rate of Ni particles on the zeolite surface is favored, and this fact facilitates the agglomeration of Ni particles.51,52 Therefore, CNFs with larger diameters and lower densities should be grown by raising the reaction temperature. This fact could be confirmed by measuring the pore size distribution by TEM (Figure 6b) and the average diameter (Table 3) of the CNFs synthesized at different temperatures. The average diameter was ∼16.7 nm at 400 °C and rose to ∼29.4 nm at 550 °C. The diameter then started to decrease at 600 °C, the same temperature to which the total weight of carbon started to decrease. Finally, there is the additional possibility of noncatalytic pyrolysis of ethylene on CNFs sidewalls, leading to thickening of the fibers. A similar type of behavior has previously been observed during the synthesis of vapor grown carbon fibers.51-53 We found that noncatalytic reaction of ethylene only occurred at a very high temperature (800 °C). To verify the effect of growth temperature on the morphology and crystallinity of graphitic sheets, TEM images at different levels of magnification were obtained

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Figure 8. TPO profiles of the CNFs: (a) influence of the Ni-introduction method, (b) influence of the time on-stream, (c) influence of the reaction temperature, and (d) influence of the C2H4/H2 (v/v) ratio.

at different temperatures. As ilustrated in Figure 9, two types of CNFs were observed; fishbone-type structures were produced at temperatures 600 °C. The graphitic sheets of CNFs grown at 400 °C (Figure 9a) were coiled and tilted toward the tube axis with an angle of ∼25-30°. CNFs prepared at 400-600 °C showed basically the same structures, but they were less coiled with increasing temperatures. The formation of the coillike shape is probably due to the fact that some surfaces of the metal particles have faster carbon deposition rates than other surfaces.1,54 However, an image of the CNFs grown at 700 °C (Figure 9b) revealed a highly ordered crystalline structure with the graphitic sheets stacked almost parallel to the fiber axis and a hollow core clearly seen along the filament axis, looking like bamboo.55 CNFs prepared at 600-700 °C showed basically the same structures, although it must be remarked that, upon the elevation of the reaction temperature to 600 °C, the stacking angle decreased and the basal graphite planes were becoming almost parallel to the filament axis. Generally, the outer graphitic sheets are usually less crystalline than the inner ones.

It is important to note that the planes of the fiber were well-exposed, and only a very small amount of amorphous carbon was covering the external surface of the fiber. Figure 10 shows the XRD profiles for the CNFs synthesized at different temperatures. The spectra exhibited a main, sharp graphitic (002) reflection, in which the intensity increased and shifted to a higher 2θ angle when the synthesis temperature increased, indicating progressive graphitization of the CNFs. CNFs prepared at 600 °C showed the highest diffraction angle. Tubular CNFs prepared at temperatures >600 °C showed the graphitic (002) reflection at a lower 2θ angle. The graphitization parameter d(002), and the mean crystallite thickness, deduced from the half-width of the (002) peak, Lc, are shown in Table 3. The degree of graphitization increased with reaction temperatures up to 600 °C and then slightly decreased at higher temperatures. It must be noted that the degree of graphitization at 600 °C was very close to that of the natural graphite (0.335 nm). Such a high degree of graphitization is certainly favorable for applications such as electric and thermal conductivity in composites and for

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Figure 9. Representative TEM images of CNFs synthesized at (a) 400 °C and (b) 700 °C.

Figure 10. XRD patterns of CNFs synthesized at different temperatures.

its use as an intercalation host used without extra graphitization treatment.56,57 Lc values became larger with increasing reaction temperature up to 600 °C and

almost did not change when reaction temperature was further raised. According to Hsieh et al.,58 for Lc , 5 nm, it can be assumed that the CNFs are composed of a short-range order of graphite structure, i.e., a partially graphitized carbon. These results showed that some improvement of stacking and removal of defects was achieved by raising the reaction temperature. This fact can also be checked in Figure 8c, which shows the plots for weight loss vs oxidation temperature, measured by heating the demineralized CNFs in a TGA. This figure demonstrated that it was necessary to use higher temperatures to oxidize CNFs synthesized at higher reaction temperatures. It is important to note that fishbone-type CNFs showed slightly better stacking. Higher and ordered stacking in the fishbone-type CNFs may pose no geometrical hindrance for the graphitization of hexagonal planes stacked in a columnar manner. In contrast, tubular alignment of hexagonal planes forces their stacking distance and helicity to increase because of the absence of planarity.59,60 Such shape, alignment, and degree of graphitization of graphene sheets must be governed by the nature of the catalyst particle.11,60 In the temperature range of 400-600 °C, agglomeration of Ni particles was favored by an increase of the reaction temperature and, in this way, larger CNFs diameters were obtained. Nevertheless, temperatures >600 °C tended to reduce the size of the catalyst particles and, hence, the resulting fibers. The highest graphitization was observed at temperatures ∼600 °C. Above that temperature, the catalyst particle tended to become spherical and arranged the carbon walls to be tubular to give a more cylindrical shape to the fiber, restraining the graphitization geometrically. So far, it is quite difficult to understand completely how the alignment changes from fishbone to tubular types according to the synthesis temperature. Currently, this fact is not clear in the literature. The softening temperature of the metal catalysts, the supply rate of the carbon source, the surface migration of the carbon through the metal particle, and the solidification rate on the fiber must all be influential.60 Other concepts to be considered could be the changes in the crystallographic features of the catalyst particles and the key role of hydrogen in the reconstruction of metal surfaces. Typical isotherms of N2 adsorption on CNFs at 77 K are shown in Figure 11. All of these isotherms were H2-type according to the IUPAC classification, which is characteristic of adsorbents having relatively large pores. Textural data of the purified CNFs synthesized at different temperatures are summarized in Table 3. BET surface area decreased from 207.3 to 73.4 mg-1 with the increase of the reaction temperature. Micropore volume, small in all the samples, increased with the reaction temperature until 550 °C and then started to decrease, and the total pore volume decreased with increasing reaction temperatures. Hence, the carbon surface seems to be predominantly mesoporous (less mesoporous at higher reaction temperatures) with pores of 9 nm, in average size, that increased at temperatures >600 °C. A minor part of the surface is covered by the slitlike micropores of ca. 0.71 nm size.55,61 On the basis of the carbon yield, 550 °C was selected as the optimum reaction temperature for further study. 3. Influence of the C2H4/H2 (v/v Ratio in the Feed. The C2H4/H2 (v/v) ratio has been shown to have a considerable effect on carbon growth.20,43,62 The

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Figure 11. Isotherms of N2 adsorption at 77 K of CNFs synthesized at different temperatures.

C2H4/H2 ratio was varied between 0.25 and 4, and the CNFs were grown for a total time of 60 min at 550 °C. Table 1 shows reaction yields in terms of the three main competing reactions at three representative reaction times and the weights of carbon obtained at different C2H4/H2 ratios. It was apparent that the weight of solid carbon increased as the C2H4/H2 ratio was decreased until C2H4/H2 ) 1.85 and thereafter exhibited a steady decline with a further decrease of this ratio. Several authors have suggested that hydrogen plays a significant role in the formation of CNFs.51,63 First, hydrogen present in the feed could allow the rapid saturation of the valences at the face edges of the graphite planes, preventing the formation of amorphous carbon. Second, the presence of H2 also prevents the carbon buildup on the nickel surface, which would ultimately lead to the catalysts’ deactivation. However, the H2 concentration should be low enough to avoid surface carbon removal during the synthesis, which would lead to a lower CNF yield. As can be observed, in the absence of hydrogen in the feed, the reduced catalyst displayed very little catalytic activity for the decomposition of ethylene, which is indicative that ethylene would only decompose on nickel in appreciable quantities if hydrogen had been coadsorbed onto the metal surface. Inspection of the yield data at times >25 min showed that, while the yields to CH4 and solid carbon increased as the C2H4/H2 ratio decreased, the yields of C2H6 were not affected. It is possible that the significant amount of methane formed under conditions where the reactant mixture contains a large fraction of H2 was generated from the hydrogasification of solid carbon.42 It is clear that C2H4/H2 ratios 60 min to increase the weight of solid carbon (not shown). The ratio of hydrogen to ethylene in the reactant was found to have a considerable effect on the nature and growth of the CNFs. As the C2H4/H2 ratio was decreased, it was evident the CNFs became increasingly more graphitic in nature. This can be observed in Figure 8d, where the TGA profiles are shown. On the other hand, a detailed examination of the TEM pictures (Figure 12) revealed the existence of some major differences with regards to the orientation of the graphite sheets contained within the respective materials. When

Figure 12. Representative TEM image of CNFs synthesized using a composition in C2H4 in the feed of (a) 100% and different C2H4/H2 (v/v) ratios of (b) 4, (c) 1.85, and (d) 0.25.

the C2H4/H2 ratio was high, the fishbone structure was observed (Figure 12b), but at lower C2H4/H2 ratios, filaments were non-homogeneous in microstructure and two types of carbon filaments could be classified.55 Some graphite planes had the fishbone structure, but the angle tended to increase with respect to the fiber axis (Figure 12c). Nevertheless, other fractions possessed well-defined graphitic structures in which the sheets were preferentially stacked perpendicularly to the fiber axis (Figure 12d). A filament carbon with the latter structure is typically named “octopus carbon”: the filament originated from one catalyst particle of quasioctahedral shape.55 According to Park and Baker,43 the presence of added hydrogen in the reactant is believed to be responsible for inducing reconstruction of the metal particle surface to generate a set of faces that favor the precipitation of carbon in the form of graphite. Another interesting phenomenon observed by TEM in CNFs synthesized without hydrogen in the reactant gas was the presence of structures of type “bucky-onions” (Figure 12a), that is to say, a metal particle covered by concentric graphitic sheets. Logically, the facts explained above affected the surface area values of the CNFs, as can be observed in Table 3. When the

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C2H4/H2 ratio decreased, the arrangement of graphite sheets shifted from a situation where only a fraction of the edge sites were available for gas adsorption to another in which a large fraction of these sites would be available. As a consequence, the surface area tended to increase. Conclusions Results of thorough studies of the catalytic synthesis of carbon nanofibers by the decomposition of ethylene using Y zeolite as the support and Ni as the active phase were discussed. Experimental results clearly indicated that the metal incorporation method, which reverberates in the crystal size of Ni, has very significant effects on CNFs growth, including the deactivation rate, the final yield of CNFs, and the textural characteristics of the synthesized CNFs. The selective siting and size of Ni particles within the zeolite cage structure was different depending on if the Ni was incorporated by either ion exchange or impregnation. The best results concerning CNFs yield were obtained from the impregnated catalysts, because the Ni particles anchored to the outer surface of the zeolite were smaller and better dispersed in these catalysts. The average diameter was lower in CNFs synthesized from the ion-exchanged catalyst, because the carbon filaments in this catalyst grew up from Ni particles of very small size lodged inside the pore system of the zeolite. Experimental results also demonstrated that the reaction temperature has a considerable effect on carbon growth. So, the total weight of deposited carbon passed through a maximum at 550 °C, which is strongly dependent on the nature of the catalyst and the feedstock. On the other hand, two types of CNFs were observed as a function of the reaction temperature: fishbone structures at temperatures 600° C. In the temperature range of 400-600 °C, agglomeration of Ni particles was favored by an increase of the reaction temperature and so a larger CNFs diameter was obtained. Nevertheless, temperatures >600 °C tended to reduce the size of the catalyst particles and, hence, the resulting fibers. The highest graphitization was observed in CNF synthesized at temperatures ∼600 °C. Above this temperature, the catalyst particles tended to become spherical, and in this situation, the carbon walls tended to be tubular, resulting in a fiber with a more cylindrical aspect. Finally, experimental results have also demonstrated that the C2H4/H2 (v/v) ratio has a considerable effect on both carbon yield and morphology of CNFs. The weight of solid carbon increased as the C2H4/H2 ratio was decreased until C2H4/H2 ) 1.85 and thereafter exhibited a steady decline with further decreasing ratios. As the C2H4/H2 ratio was decreased, the CNFs became slightly more graphitic in nature and the arrangement of graphite sheets changed from the fishbone structure to octopus carbon. The obtained results, in conjunction with the greater ease to dissolve zeolites in acid solutions, makes the zeolite substrate a highly promising catalyst for nanofiber production. Acknowledgment Financial support from the Ministerio de Ciencia y Tecnologıá of Spain (Projects PPQ-2001-1195-C02-01 and CTQ-2004-07350-C02-O) is gratefully acknowledged.

Literature Cited (1) Zheng, G.-Z.; Kouda, K.; Sano, H.; Uchiyama, Y.; Shi, Y.F.; Quan, H.-J. A model for the structure and growth of carbon nanofibers synthesized by CVD method using nickel as a catalyst. Carbon 2004, 42, 635. (2) Lee, C. J.; Kim, D. W.; Lee, T. J.; Choi, Y. C.; Park, Y. S.; Lee, Y. H. Synthesis of aligned carbon nanotubes using thermal chemical vapor deposition. Chem. Phys. Lett. 1999, 312, 461. (3) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Siska, A.; Kiricsi. I. Production of nanotubes by the catalytic decomposition of different carbon containing compounds. Appl. Catal., A 2000, 199, 245. (4) Chen, D.; Christensen, K. O.; Ochoa-Fernandez, E.; Yu, Z.; Totdal, B.; LaTorre, N.; Monzoń, A.; Holmen. A. Synthesis of carbon nanofibers: Effects of Ni crystal size during methane decomposition. J. Catal. 2004, 229, 82. (5) De Jong, K. P.; Geus, J. W. Carbon Nanofibers: Catalytic synthesis and applications. Catal. Rev.sSci. Eng. 2000, 42, 481. (6) Toebes, M. L.; Zhang, Y.; Ha´jek, J.; Nijhuis, T. A.; Bitter, J. H.; Dillen, A. J.; Murzin, D. Y.; Koningsberger, D. C.; De Jong, K. P. Support effects in the hydrogenation of cinnamaldehyde over carbon nanofiber-supported platinum catalysts: Characterization and catalysis. J. Catal. 2004, 226, 215. (7) Park, C.; Engel, E. S.; Crowe, A.; Gilbert, T. R.; Rodriguez, N. M. Use of Carbon Nanofibers in the Removal of Organic Solvents from Water. Langmuir 2000, 16, 8050. (8) Schimmel, H. G.; Nijkamp, G.; Kearley, G. J.; Rivera, A.; De Jong, K. P.; Mulder, F. M. Hydrogen adsorption in carbon nanostructures compared. Mater. Sci. Eng. 2004, 108, 124. (9) Lozano, K.; Barrera, E. V. Nanofiber-reinforced thermoplastic composites. I. Thermoanalytical and mechanical analyses. J. Appl. Polym. Sci. 2000, 79, 125. (10) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nanotubes as nanoprobes in scanning probe microscopy. Nature 1996, 384, 147. (11) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Catalytic Engineering of Carbon Nanostructures. Langmuir 1995, 11, 3862. (12) Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. J. Catal. 1972, 26, 51. (13) De Jong, K. P.; Geus, J. W. Carbon Nanofibers: Catalytic Synthesis and Applications. Catal. Rev.sSci. Eng. 2000, 42 (4), 481. (14) Kneller, J. M.; Soto, R. J.; Surber, S. E.; Pietrab, T.; Colomer, J.-F.; Fonseca, A.; Nagy, J. B.; Van Tendeloo, G. TEM and Laser-Polarized 129Xe NMR Characterization of Oxidatively Purified Carbon Nanotubes. J. Am. Chem. Soc. 2000, 122, 10591. (15) Teo, K. B. K.; Singh, C.; Chhowalla, M.; Milne, W. I. Encyclopedia of Nanoscience and Nanotechnology; American Scientific Publisher: Stevenson Ranch, CA, 2004; Vol. 10. (16) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Large-scale synthesis of aligned carbon nanotubes. Science 1996, 274, 5293. (17) Fonseca, A.; Hernadi, K.; Piedigrosso, P.; Colomer, J.-F.; Mukhopadhyay, K.; Doome, R.; Lazarescu, S.; Biro, L. P.; Lambin, P.; Thiry, P. A.; Bernaerts, D.; Nagy, J. B. Synthesis of singleand multiwall carbon nanotubes over supported catalysts. Appl. Phys. A 1998, 67, 11. (18) Rao, C. N. R.; Sen, R.; Satishkumar, B. C.; Govindaraj, A. Large aligned-nanotube bundles from ferrocene pyrolysis. Chem. Commun. 1998, 15, 1525. (19) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Riga, J.; Lucas, A. Catalytic synthesis and purification of carbon nanotubes. Synth. Met. 1996, 77, 31. (20) Park, C.; Keane, M. Controlled Growth of Highly Ordered Carbon Nanofibers from Y Zeolite Supported Nickel Catalysts. Langmuir 2001, 17, 8386. (21) Hernadi, K.; Końya, Z.; Siska, A.; Kiss, J.; Oszko´, A.; Nagy, J. B.; Kiricsi, I. On the role of catalyst, catalyst support and their interaction in synthesis of carbon nanotubes by CCVD. Mater. Chem. Phys. 2002, 77, 536. (22) Okamoto, A.; Shinohara, H. Control of diameter distribution of single-walled carbon nanotubes using the zeolite-CCVD method at atmospheric pressure. Carbon 2005, 43, 431. (23) Hulman, M.; Kuzmany, H.; Dubay, O.; Kresse, G.; Li, L.; Tang, Z. K.; Knoll, P.; Kaindl, R. Raman spectroscopy of single wall carbon nanotubes grown in zeolite crystals. Carbon 2004, 42, 1071

8236


(24) Toebes, M. L.; Bitter, H. J.; Dillen, A. J.; De Jong, K. P. Impact of the structure and reactivity of nickel particles on the catalytic growth of carbon nanofibers. Catal. Today 2002, 76, 33. (25) Gommes, C.; Blacher, S.; Bossuot, Ch.; Marchot, P.; Nagy, J. B.; Pirard, J.-P. Influence of the operating conditions on the production rate of multiwalled carbon nanotubes in a CVD reactor. Carbon 2004, 42, 1473. (26) Horvath, G.; Kawazoe, K. Method for the Calculation of Effective Size Distribution in Molecular Sieve Carbon J. Chem. Eng. Jpn. 1983, 16, 470. (27) Barret, R. M.; Joyner, L. G.; Halenda, P. P. The determination of pore volume and area distribution in porous substances. J. Am. Chem. Soc. 1951, 73, 373. (28) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Fudala, A.; Lucas, A. A. Catalytic synthesis of carbon nanotubes using zeolite support. Zeolites 1996, 17, 416. (29) Sachtler, W. M. H.; Zhang, Z. Zeolite-supported transition metal catalysts. Adv. Catal. 1993, 39, 129. (30) Coughlan, B.; Keane, M. A. The effects of the environment on the growth of nickel metal particles supported on a range of Y zeolites prepared by ion exchange and impregnation. Zeolites 1991, 11, 2. (31) Keane, M. A. The liquid-phase hydrogenation of methylacetoacetate using nickel-exchanged Y zeolite catalysts: Part I. Structure sensitivity. Zeolites 1993, 13, 14. (32) Mile, B.; Stirling, D.; Zammit, M. A.; Lovell, A.; Webb, M. The location of nickel oxide and nickel in silica-supported catalysts: Two forms of “NiO” and the assignment of temperatureprogrammed reduction profiles. J. Catal. 1988, 114, 217. (33) Twigg, M. V.; Richardson, J. T. Effects of alumina incorporation in coprecipitated NiO-Al2O3 catalysts. Appl. Catal., A 2000, 190, 61. (34) Law, P. L.; Kenney, C. N. Pentane isomerization kinetics over nickel-loaded Y-type zeolite. J. Catal. 1980, 64, 241. (35) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (36) Anderson, P. E.; Rodriguez, N. M. Influence of the support on the structural characteristics of carbon nanofibers produced from the metal-catalyzed decomposition of ethylene. Chem. Mater. 2000, 12, 823. (37) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Fudala, A.; Lucas, A. A. Catalytic sıńtesis of carbon nanotubes using zeolite support. Zeolites 1996, 17, 416. (38) Cherukuri, L. D.; Yuan, G.; Keane, M. A. Catalytic Growth of Structured Carbon via the Decomposition of Chlorobenzene over Ni/SiO2. Top. Catal. 2004, 29, 119. (39) Walker, P. L., Jr.; Shelef, M.; Anderson, R. A. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1968; Vol. 4. (40) Mackee, D. W. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1981; Vol. 16. (41) Rodriguez, N. M. A review of catalytically grown carbon nanofibers. J. Mater. Res. 1993, 8, 3233. (42) Park, C.; Rodriguez, N. M.; Baker, R. T. K. Carbon Deposition on Iron-Nickel during Interaction with Carbon Monoxide-Hydrogen Mixtures. J. Catal. 1997, 169, 212. (43) Park C.; Baker, R. T. K. Catalytic Behavior of Graphite Nanofiber Supported Nickel Particles. 2. The Influence of the Nanofiber Structure. J. Phys. Chem. B 1998, 102, 5168. (44) Park, C.; Baker, R. T. K. Carbon Deposition on Iron-Nickel During Interaction with Ethylene-Carbon Monoxide-Hydrogen Mixtures. J. Catal. 2000, 190, 104. (45) Chambers, A.; Baker, R. T. K. Influence of the Nature of the Catalyst Precursor on the Carbon Deposition Characteristics during Ethylene Decomposition over Copper-Cobalt. J. Catal. 1996, 158, 356.

(46) Krishnankutty, N.; Rodriguez, N. M.; Baker, R. T. K. Effect of Copper on the Decomposition of Ethylene over an Iron Catalyst. J. Catal. 1996, 158, 217. (47) Chambers, A.; Baker, R. T. K. Influence of Chlorine on the Decomposition of Ethylene over Iron and Cobalt Particles. J. Phys. Chem. B 1997, 101, 1621. (48) Avdeeva, L. B.; Kochubey, D. I.; Shaikhutdinov, S. K. Cobalt catalysts of methane decomposition: accumulation of the filamentous carbon. Appl. Catal., A 1999, 177, 43. (49) Choi, Y. C.; Shim, Y. M.; Lee, Y. H.; Lee, B. S.; Park, G.S.; Choi, W. B.; Lee, J. M.; Kim, N. S. Controlling the diameter, growth rate, and density of vertically aligned carbon nanotubes synthesized by microwave plasma enhanced chemical vapor deposition. Appl. Phys. Lett. 2000, 76, 2367. (50) Lee, C. J.; Park, J.; Huh, Y.; Lee, J. Temperature effect on the growth of carbon nanotubes using thermal chemical vapor deposition. Chem. Phys. Lett. 2001, 343, 33. (51) Singh, C.; Shaffer, M. S. P.; Windle, A. H. Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method Carbon 2003, 41, 359. (52) Endo, M.; Takeuchi, K.; Kobori, K.; Takahashi, K.; Kroto, H. W.; Sarkar, A. Pyrolytic carbon nanotubes from vapor-grown carbon fibers. Carbon 1995, 33, 873. (53) Oberlin, A.; Endo, M.; Koyama, T. Filamentous growth of carbon through benzene decomposition. J. Cryst Growth 1976, 32, 335. (54) Kawaguchi, M.; Nozaki, K.; Motojima, S.; Iwanaga, H. A growth mechanism of regularly coiled carbon fibers through acetylene pyrolysis. J. Cryst. Growth 1992, 118, 309. (55) Reshetenko, T. V.; Avdeeva, L. B.; Ismagilov, Z. R.; Pushkarev, V. V.; Cherepanova, S. V.; Chuvilin, A. L.; Likholobov, V. A. Catalytic filamentous carbon: Structural and textural properties. Carbon 2003, 41, 1605. (56) Chieu, T. C.; Dressellhaus, M. S; Endo. M. Raman studies of benzene-derived graphite fibers. Phys. Rev. B 1982, 26, 5867. (57) Yang, H. Preparation of carbon anodic materials with high graphitization degree for Li ion secondary battery. Ph.D. Thesis, Kyushu University, Fukuoka, Japan, 2003. (58) Hsieh, C. T.; Chen, J. M.; Kuo, R. R.; Huang, Y. H. Formation and field-emission properties of carbon nanofibers by a simplified thermal growth. Rev. Adv. Mater. Sci. 2003, 5, 459. (59) Charlier, J. C.; Michenaud, J. P. Energetics of multilayered carbon tubules. Phys. Rev. Lett. 1993, 70, 1858. (60) Tanaka, A.; Yoon, S. H.; Mochida, I. Preparation of highly crystalline nanofibers on Fe and Fe-Ni catalysts with a variety of graphene plane alignments. Carbon 2004, 42, 591. (61) Fenelonov, V. B.; Likholobov, V. A.; Derevyankin, A. Y.; Mel’gunov, M. S. Porous carbon materials prepared from C1-C3 hydrocarbons. Catal. Today 1998, 42, 341. (62) Zaera, F. J. Hydrogenation and H-D exchange of chemisorbed ethylene on Ni(100) under vacuum. J. Catal. 1990, 121, 318. (63) Vieira, R.; Ledoux, M. J.; Pham-Huu, C. Synthesis and characterisation of carbon nanofibres with macroscopic shaping formed by catalytic decomposition of C2H6/H2 over nickel catalyst. Appl. Catal., A 2004, 274, 1.

Received for review March 25, 2005 Revised manuscript received August 8, 2005 Accepted August 8, 2005 IE058027K

Y Zeolite Based Catalysts

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