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Synthesis of Platelet Carbon Nanofiber/Carbon Felt Composite on in Situ Generated Ni-Cu Nanoparticles† Tiejun Zhao,‡ Ingvar Kvande,‡,§ Yingda Yu,| Magnus Ronning,‡ Anders Holmen,*,‡ and De Chen*,‡ Department of Chemical Engineering, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway, SINTEF Materials and Chemistry, Trondheim, N-7465, Norway, and Department of Material Science and Engineering, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway ReceiVed: July 8, 2010; ReVised Manuscript ReceiVed: October 25, 2010
A platelet carbon nanofiber/carbon felt composite with a high surface area (>250 m2/g) is synthesized over a carbon felt supported Ni-Cu catalyst using ethane-hydrogen mixtures at 923 K. A uniform layer of carbon nanofibers is thus achieved via an in situ generation and dispersion of Ni-Cu nanoparticles induced by the surface reaction of hydrocarbon decomposition. This represents a cost-effective and flexible method without the requirement for presynthesis of uniformly distributed nanoparticles. Both thermodynamic analysis and experimental observation reveal that, in the initial growth period, formation of a molten phase of the nanosized Ni-Cu-C phase promotes the fragmentation of Ni-Cu parent particles of micrometer size. A unique octopuslike growth mode is observed during the formation of graphene layers from this liquid-like Ni-Cu-C system. A comparative study between Ni and Ni-Cu catalysts is performed. It is found that the addition of Cu in the Ni-Cu catalysts can enhance the fragmentation ability of the parent Ni-Cu particles when contacting with hydrocarbon. A treelike growth mode of carbon nanofibers grown on Ni-Cu catalysts is observed due to the further fragmentation of Ni-Cu particles during carbon nanofiber growth. In contrast to Ni-Cu catalysts, two different growth mechanisms on the Ni catalyst are found: a governing tip-growth mechanism by particles smaller than 50 nm and an octopus-like growth mechanism by larger particles (50-100 nm). The graphene sheet orientation in the carbon nanofibers depends on the composition of the metal particle; fishbone carbon nanofibers are obtained with carbon felt supported Ni catalysts, while platelet carbon nanofibers were obtained with Ni-Cu catalysts. The formation of platelet carbon nanofibers from the Ni-Cu catalysts is ascribed to a higher degree of interface wetting between the Ni-Cu particle and the produced graphene sheets. 1. Introduction Carbon nanofibers (CNFs) and carbon nanotubes (CNTs) are made up of nanosized graphene sheets stacked together at different orientations. Three main types of carbon nanofibers can be defined, namely, tubular, fishbone, and platelet CNFs, having their graphene sheets oriented along, at an angle, and perpendicular to the axis of the fibers, respectively.1,2 As expected, the surface reactivity of these nanostructured fibrous carbons is found to be strongly dependent on their edge terminal structures,1 and thereby the different physicochemcial properties. During the last decades, it has been shown that CNFs have great potential as novel catalyst supports in heterogeneous catalysis.1-3 In most reports, these fibrous carbon nanostructures are synthesized as powders, thereby limiting their application in fixed-bed, fluidized bed, and even slurry-bed reactors, where a certain mechanical strength and bulk density are required.4,5 Easy handling and utilization for desired applications can be more readily realized by conceptual production of well-controlled macroshaped carbon nanostructure-carbon composites as also †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed,
[email protected](Anders Holmen) and
[email protected] (De Chen). ‡ Department of Chemical Engineering, Norwegian University of Science and Technology. § SINTEF Materials and Chemistry. | Department of Material Science and Engineering, Norwegian University of Science and Technology.
presented elsewhere.6,7 For instance, Downs and Baker prepared carbon nanofiber-carbon fiber mat composites by decomposition of ethylene-hydrogen mixtures over a carbon fiber mat supported Ni-Cu catalyst at 873 K. The surface area and mechanical strength of the composite were significantly enhanced when compared to its parent material.7 However, the use of ethylene may provide difficulties in controlling the nanostructure of CNFs for unsupported catalysts.8 Meanwhile, elsewhere, ethylene has presented itself as an effective carbon source for the growth of carbon nanotube arrays.9 The synthesis of defined and moldable carbon nanofiber/carbon felt (CNF/ CF) composites with good mechanical properties has also been reported by Pham-Huu et al.3 Fishbone CNF/CF composites with excellent mechanical properties were obtained by decomposition of ethane-hydrogen mixtures at 650-680 °C over CF-supported Ni catalysts.10 A composite surface area exceeding 90 m2/g was obtained. Relevant applications of these fishbone CNF/CF composites with good catalytic performance in heterogeneous catalysis were also indicated.11,12 Another interesting approach with regards to the synthesis of CNF/CNT composites is the production of self-supported tubular CNFs with a defined macroshape derived from the decomposition of ethane over powdered Fe/Al2O3 in restricted space. Good mechanical properties and high organics adsorption capacity were demonstrated.3 Macroscopic fishbone CNFs have also been obtained over Ni/SiO2 by decomposition of CO/H2 mixtures,4 as well as on ceramic monoliths,13-16 Ni foam,13,17,18 and carbon paper.19
10.1021/jp106320u 2011 American Chemical Society Published on Web 12/06/2010
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To our knowledge, the synthesis of macroshaped platelet CNFs has not yet been reported. A higher catalytic performance has been observed for platelet CNF-based catalysts compared to tubular and fishbone CNFbased catalysts, such as in fuel cells,20,21 and in hydrogenation and dehydrogenation reactions22,23 and for Fischer-Tropsch synthesis24 etc. A larger amount of surface functional groups24 and a higher proton affinity of platelet CNF surfaces25 as well as a stronger interaction between the metal and the support26,27 have been proposed as reasons for the enhanced catalytic properties found for platelet CNF-based catalysts. Therefore, synthesis of platelet CNFs/CF is of high interest. The alloyed Ni-Cu system is proved to be very effective in the production of carbon nanofibers.28-33 Addition of Cu into Ni enhanced the yield and modified the morphology of grown CNF structures, most probably attributed to changes in the crystallographic orientation and the size of the active particles.30 Recently, Cunha et al.34 have reviewed several pertinent studies on the role of Cu additives in the formation of carbon nanofibers. The yield and stability of CNFs were improved compared to that found for individual Ni catalysts, ascribed to the decrease in encapsulating carbon formation. However, despite the importance of Ni-Cu catalysts in CNF growth, the growth mechanism including CNF nucleation and steady-state growth for this system has not been studied in detail. Pham-Huu et al.5 have studied the growth mechanism of CNF/ CF on Ni/CF and found as the fragmentation of Ni particles takes place during the growth, the diameter of CNF grown on the carbon felt is independent of the size of the initial Ni particles. An octopus-like growth mechanism was found to be dominant in this Ni catalyst system, although the origin of the fragmentation was not revealed. In the present study, we will address the effects of Cu addition on the morphology of metallic particles after reduction, as well as the nanostructures of the CNF obtained. This is realized by a comparative study of CNF growth on Cu-Ni and Ni supported on carbon felt catalysts. Especially, we seek to gain a mechanistic insight to the fragmentation behavior of the large parent metal particles in the reaction atmosphere by a detailed scanning electron microscopy (SEM) study after terminating the process at various stages during the initial growth. 2. Experimental Section Two weight percent Ni-Cu (1:1)/CF catalysts were prepared by incipient wetness impregnation. An ethanol solution containing the proper amount of Ni(NO3)2 · 6H2O and Cu(NO3)2 · 3H2O was impregnated onto a macroscopic CF (Carbone Lorraine Co) disk (26 mm diameter, 6 mm thickness) and subsequently dried overnight at room temperature. Contrary to work reported elsewhere,5 no pretreatment of the CF in concentrated nitric acid was performed. The as-impregnated solid was dried overnight at room temperature. A similar procedure was used to prepare the CF supported Ni (2 wt %) catalyst. The catalytic synthesis of the CNF/CF composite was carried out in a vertical quartz reactor. Five pieces of dried Ni-Cu/CF catalysts were directly reduced by a H2/N2 mixture (40/160 mL/ min) at 923 K (temperature ramp rate 5 K/min) for 2 h. Due to the weak interaction between the inert surface of CF and the metal precursors, and the fact that Ni and Ni-Cu are easily reduced, this procedure is expected to fully reduce the impregnated metal precursors to the corresponding metallic phase. After reduction, CNF growth was performed by using a C2H6/H2 mixture (90/150 mL/min) for 2 h. In order to investigate the effect of the reduction time of catalysts on the yield of carbon
Zhao et al. nanofibers, similar growth procedures are performed on the catalysts reduced at 923 K for 24 h. For Ni catalyst, due to its lower growth rate of CNFs, the growth time is 16 h using a similar reduction procedure. To reveal the growth mechanism of CNFs on these CF supported catalysts, the product after 1 and 5 min of growth was collected. It should be noticed that a flow rate of 2 L/min, N2 was flushed into the reactor to replace the hydrocarboncontaining gases, thus halting the reaction at selected stages. The morphology of the reduced catalyst and the resulting nanocarbon composites was characterized by SEM (Zeiss Ultra 55 Limited Edition). Identification of the graphene sheet orientation of the resulting nanocarbon-based composites was carried out in a high-resolution transmission electron microscope (HRTEM) with a field emission gun (FEM) (JEOL JEM 2010F). The sample used for TEM was crushed into powder in a crucible before it was dissolved in an ethanol solution for 10 min with the help of an ultrasonic bath. One drop of this suspension was deposited onto a copper grid covered by a holey carbon membrane. The BET surface areas were obtained from nitrogen adsorption measurements at about 77 K performed in a Micromeritics Tristar 3000 instrument. Before the measurements, the samples (about 0.1 g) were evacuated at 473 K for 5 h. The mechanical properties of the resulting nanocarbon composites were assessed by ultrasonic treatment. After the samples had been soaked in an ethanol-water (1:1) solution for 2 h, another 1 h ultrasonic treatment was carried out at room temperature. The composites were then dried at 393 K for 2 days. The difference in weight before and after treatment was measured. 3. Results and Discussion 3.1. Reduced CF Supported Ni-Cu and Ni Catalysts. The CF used as support is composed of interwoven carbon macrofibers (the diameter of the carbon macrofiber is about 10 µm), and the carbon macrofiber is composed of bundled microfibers, It is noticed that there are crevices between the adjacent microfibers as shown in Figure 1a. The surface area of the CF in the present study is about 1 m2/g, and the surface of the carbon felt is highly hydrophobic. Figure 1 indicates the typical morphology for the reduced 2 wt % Ni-Cu/CF catalysts at different magnifications. As seen in Figure 1a, loosely dispersed particles are found on the surface of the carbon macrofiber. The size distribution of the reduced Ni-Cu particles on the surface is very broad, and the range is in the order of micrometers. A more detailed image of the reduced Ni-Cu particles is shown in Figure 1b. As indicated by the arrow in Figure 1b, the spherical particles are agglomerates about 200 nm or more in size. A typical high magnification image, indicating pronounced roughness in the microfiber is shown in Figure 1c. However, no nanosized (less than 100 nm) Ni-Cu particles are found to be deposited on/in these surface irregularities. For comparison, Figure 2 exhibits the morphology of reduced CF-supported Ni catalyst obtained using the same reduction conditions. The SEM image with lower magnification of the CF-supported Ni catalyst in Figure 2a is found to be similar to that of the CF-supported Ni-Cu catalyst, also showing loosely dispersed agglomerated particles of micrometer size on the surface of microfibers. However, Figure 2b shows that a large number of Ni particles about 100 nm or less in size are deposited over the entire surface of the carbon microfiber, especially on the irregular sites. In particular, these Ni nanoparticles are enriched in the porous crevices between the microfibers as
Platelet Carbon Nanofiber/Carbon Felt Composite
Figure 1. Morphology of carbon felt supported 2 wt % Ni-Cu (1:1) catalyst reduced by H2/Ar (40/120 mL/min) at 923 K for 2 h. (a) Particle size distribution on the surface of the carbon macrofiber, showing that the particle size is in the range from 200 nm to several micrometers. (b) A higher resolution image of the reduced Ni-Cu particles, indicating that the large particles shown in panel a are composed of spherical particles around 200 nm in size. (c) An image showing the morphology of roughness on the surface of the carbon macrofiber and crevices between the adjacent carbon microfibers. No Ni-Cu particles with comparable size to the roughness are detected.
shown in Figure 2c. This is not observed in the case of the Ni-Cu catalyst. The different morphology of reduced particles probably relates to higher susceptibility toward agglomeration of Ni-Cu than Ni particles during reduction. Moreover, the different morphology in the reduced Ni and Ni-Cu supported on CF implies the formation of a Ni-Cu alloy after reduction. It is well-known that Ni and Cu can form a continuous solid solution in the entire composition range. It should be noticed that these two samples were reduced followed by cooling down to room temperature and exposure to air for several minutes prior to the SEM study. Therefore, oxidation of the metal particles will take place. However, the surface oxidation of metal particles will not lead to a significant morphology change since the formation of oxide at the interface between the support and
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Figure 2. Morphology of carbon felt supported 2 wt % Ni catalysts reduced by H2/Ar (40/120 mL/min) at 923 K for 2 h. (a) Large Ni particles loosely dispersed on the surface of the carbon macrofiber. (b) A higher resolution image of the reduced Ni particles, indicating that the nanosized Ni particles (from several 10 to 100 nm) are dispersed on the surface of the carbon macrofibers. (c) An image indicating that the nanosized Ni particles are preferably deposited on the crevices of carbon macrofibers, where the porous structures are rich.
metal particle decreases the surface energy of the metal particles, thus stabilizing the particles on the surface of support.35 It has been reported that preoxidation of CF in concentrated nitric acid overnight could increase the number of anchoring sites, the affinity to metal precursors during impregnation, and thereby the final metal dispersion.5 However, due to the low surface area (about 1 m2/g) and the inert surface of the CF, the treatment in concentrated HNO3 can introduce only a limited amount of anchoring sites on the CF surface. A poor interaction between the metal salt precursor and the surface of CF is expected, leading to a poor dispersion of the metal particles after the high-temperature treatment. It should be pointed out that the CF was received without oxidation in the present work. The anchoring mechanism of the metal particle could be different from the one observed on the preoxidized sample. Bitter et al. have reported that the particle size of Ni on oxidized CNFs can be close to 10 nm after reduction at 673 K if the catalyst is prepared by a homogeneous deposition-precipitation method.36 Oxygen-containing groups can stabilize the metal particles during reduction at certain temperatures. Acidic carboxylic groups (-COOH), typically introduced by nitric acid treatment, start to decompose at temperatures above 523 K in
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inert gas.37 This decomposition process will be accelerated in hydrogen atmosphere at similar temperatures.38 Toebes et al.39,40 investigated the effect of high-temperature treatment of oxidized CNF supported Ru or Pt catalysts prepared by the homogeneous deposition-precipitation method. It was found that no significant changes in particle size of Pt or Ru took place after the removal of acidic oxygen containing groups, demonstrating the higher thermal stability of these Pt and Ru particles. The authors speculate that this thermal stability could be related to the existence of thermally stable carbonyl groups (CdO). In the present case, no preoxidation is carried out for the carbon macrofiber, and the surface area of this material is very low. Therefore, the role of oxygen-containing groups for stabilizing the Ni particles can be excluded on the surface of carbon macrofibers. From SEM images of Figure 2b,c, numerous Ni particles less than 50 nm in size are preferentially deposited on the rough sites on the carbon microfiber surface, and the size of the particles matches well with the size of the rough sites. Therefore, the roughness on the surface of carbon macrofibers plays a key role in the formation of these nanosized Ni particles, even when the catalysts are reduced in H2 at 923 K for 2 h. From this point of view, it seems that both the roughness and the oxygen-containing groups can stabilize the metallic particles. However, due to the weak interaction between the surface of the carbon microfiber and metallic particles, particles larger than 200 nm can still be observed for the reduced catalysts, due to sintering of the metallic phase at high temperatures. Prolonging the reduction at higher temperatures will lead to further sintering of the dispersed nanoparticles on the surface of the carbon macrofibers. The temperature is the key parameter for the sintering.41 With increase in temperature, a higher mobility of the metal particles appears. Generally, the mobility of the metal particles is related to its Tammann temperature, i.e., the temperature corresponding to half of the melting point of the bulk.42 For instance, the Tammann temperature for Ni and Ni-Cu (1:1) is 863 and ∼766 K, respectively. The lower Tammann temperature of Ni-Cu probably leads to higher mobility on the surface of the carbon macrofiber and, hence, larger particles of Ni-Cu after reduction as shown in Figure 1. 3.2. SEM Study of the Initial Periods of CNF Growth. The evolution of the morphology of the catalysts during the initial period of the synthesis of CNF/CF composites gives useful information about the growth mechanism. Samples are collected after exposure of the reduced Ni-Cu catalysts to the C2H6/H2 mixture for 1 min. The corresponding SEM images are shown in Figure 3. A large number of nanosized particles appear on the entire surface of the carbon macrofiber in Figure 3a. This observation is contrary to that in the reduced Ni-Cu catalyst, where only agglomerated particles larger than 100 nm in size can be found in Figure 1b. Support for this in situ generation of smaller spherical particles on the surface of a parent particle is found in Figure 3b, where particles in the range 30-100 nm have been produced from the parent particle. Thus, the introduction of C2H6 to Ni-Cu particles in the micrometer range at certain elevated temperatures will lead to the in situ formation of nanoparticles, which are dispersed on the entire surface of the carbon macrofiber. It is observed in Figure 3a that the main fraction of the nanoparticles is preferentially deposited in the crevices on the surface of the carbon macrofiber. The morphology of the Ni-Cu catalyst exposed to C2H6 for 1 min is very similar to that of the reduced Ni catalyst in Figure 2b. That means, the dispersion of Ni-Cu on the surface of the carbon microfiber is significantly improved after interacting with C2H6.
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Figure 3. SEM images of the morphology of the carbon felt supported Ni-Cu catalyst after 1 min at 923 K in the ethane-hydrogen mixture. (a) Global morphology of metal particles on the surface of a carbon macrofiber, indicating that numerous fresh nanoparticles are formed on the surface of the carbon macrofiber. (b) In situ produced nanoparticles from the parent Ni-Cu particles.
No CNFs could be clearly observed after 1 min of reaction, due to the very short contact time between the active metal and the carbon species. It is suggested that this period is characterized by formation and dispersion of Ni-Cu nanoparticles on the surface of the carbon fibers (induction period). After growth for 5 min, a large number of CNFs is distributed on the entire surface of the carbon macrofiber as shown in Figure 4a. The higher resolution image in Figure 4b shows that predominantly, multiple CNFs are grown from one single particle, indicating an octopus-like growth mechanism.5,31 Due to the limited growth time, the length of the as-grown CNFs is less than 1 µm. Furthermore, Figure 4c indicates that these CNFs grown by an octopus-like growth mechanism can gradually act as scaffolding, supporting and lifting the metallic phase from the surface of the carbon macrofiber. Thus, the as-produced CNFs become the new catalyst support for the fragmented Ni-Cu particles, that is, each fragmented Ni-Cu particle is supported by several CNFs grown from the particle. Although the original dispersion of CF-supported Ni-Cu catalyst is poor, the fragmentation and transportation of Ni-Cu particles and the growth of CNFs significantly improve the Ni-Cu dispersion on the surface of the carbon macrofiber. From this point of view, the initial dispersion of the active metal particles is not crucial in the Ni-Cu case, since the subsequent in situ generation of nanoparticles will compensate for the insufficient initial metal dispersion. The repeated deposition of graphene layers on the interface between the graphene layer of the CNF and the Ni-Cu nanoparticle increases the length of the CNF, thus carrying the Ni-Cu particle further away from the initial location on the surface of the carbon macrofiber. Therefore, the spatial location of the active Ni-Cu particle is highly related to the length of the CNFs grown from it.
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Figure 5. A schematic illustration of the growth procedure for the formation of a CNF/CF composite from carbon felt supported Ni-Cu particles. (a) The reduced Ni-Cu/CF catalyst. (b) Dissolution and accumulation of carbon on the surface layer of reduced Ni-Cu particle. With increasing the carbon fraction in Ni-Cu-C system, a molten phase is produced. (c) Reconstruction of molten phase on the surface of the parent Ni-Cu particle, thus producing the island-like or spherical nanoaprticles. (d) Detachment and redispersion of the Ni-Cu-C nanosized compounds on the surface of carbon macrofiber. (e) The octopus-like growth mechanism of carbon nanofibers from these redispersed Ni-Cu-C particles after the graphitic layers are deposited on the surface of a Ni-Cu-C system. Subsequently, several carbon nanofibers grown from one Ni-Cu-C particle act as the scaffolding and enable to lift the active Ni-Cu nanoparticles up from the surface of carbon macrofibers. (f) The relationship between the melting point of the Ni-Cu-C phase and the carbon concentration. Figure 4. SEM images showing carbon nanofibers and Ni-Cu-C particles obtained after 5 min at 923 K in the ethane-hydrogen mixture. The carbon felt supported 2 wt % Ni-Cu (1:1) catalyst first is reduced by H2/Ar (40/120 mL/min) at 923 K for 2 h. Growth was performed in a mixed ethane-hydrogen (90/150 mL/min) flow at 923 K for 5 min. (a) Global morphology of carbon nanofibers on the surface of the carbon macrofiber, indicating the formation of numerous carbon nanofibers. (b) An image indicating an octopus-like growth mechanism for the catalytic synthesis of carbon nanofibers, with several carbon nanofibers being produced from one Ni-Cu particle. (c) An image illustrating the Ni-Cu particles being lifted by the carbon nanofibers from the surface of a carbon microfiber.
The in situ generation of Ni-Cu nanoparticles from the surface of large parent particles and the growth mechanism of CNFs from them in this period is schematically illustrated in Figure 5. In Figure 5a, a reduced Ni-Cu particle is present before the carbon-containing gases are introduced. Due to the large size (in the micrometer range) of the reduced Ni-Cu particle, it will have the same physicochemical properties throughout the entire particle. When the mixed C2H6/H2 gases interact with these particles, the decomposition of C2H6 occurs, thus leading to the formation of carbon species on the metal surface. These carbon species can further diffuse into the bulk of the Ni-Cu particle, producing a C-containing layer near the
gas-solid interface of the parent Ni-Cu particle as shown in Figure 5b in a short time. The carbon-rich layer in the Ni-Cu particle could be presented as a metastable active carbide or a solid solution. Here we denote it as the Ni-Cu-C phase for this carbon-containing Ni-Cu particle. The Ni-Cu-C phase may gradually be converted into a molten phase since the dissolution of C in the Ni-Cu phase can significantly decrease the melting point of this Ni-Cu-C system. If we assume that the heat of fusion for the Ni-Cu (1:1) alloy, ∆Hfusion, is 15.26 kJ/mol (the average heat of fusion for pure Cu and Ni (13.05 and 17.48 kJ/mol, respectively)), the melting point, Tm, is 1532 K, and the behavior of the Ni-Cu-C is that of an ideal solution.43,44 The relationship between the mole fraction of C(x) and the melting point of this Ni-Cu-C solid solution (T), can be described by the Schroeder equation for solubility in an ideal solution45
ln(1 - x) ) -∆Hfusion /R(1/T - 1/Tm)
(1)
where R is the universal gas constant, 8.314 J/(mol K), ∆Hfusion is the molar heat of fusion of the Ni-Cu alloy, and Tm is the melting point of the bulk Ni-Cu alloy solid. Figure 5f gives
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the relationship between the melting point of the Ni-Cu-C system and the C molar fraction using formula 1. Parmon46 has already used this idealized formula (1) to estimate the melting point of a highly dispersed Fe-C system and found that the estimated value fits well with the Fe-carbide composition and the corresponding melting point. In detail, the melting point of the Fe-C system is close to 920 K when the mole fraction of carbon in this Fe-C system is 0.59. Unfortunately, no reference is provided for this Ni-Cu-C system. It is evident from eq 1 that higher values of ∆Hfusion and lower values of Tm will give a lower melting point of the C-metal system. Therefore, the Ni-Cu system will dissolve more carbon as compared to the Fe system at the same temperature (∆Hfusion,Fe 13.8 kJ/mol, and Tm,Fe, 1810 K) due to a slight higher heat of fusion and a considerably lower melting point of the Ni-Cu alloy. For instance, in Figure 5f, a Ni-Cu-C system with a C molecular fraction of 0.17 gives a melting point of 1323 K. This value is more than 200 K lower than the melting point of the bulk phase. According to this, the continuous flux of carbon species from C2H6 decomposition onto the Ni-Cu surface and the dissolution of carbon in the Ni-Cu particle promote melting of this Ni-Cu-C system. The distribution of carbon concentration in the reduced metal particle after switching to the C2H6/H2 flow will obey the unsteady free diffusion equation as follows
∂CC ∂2CC ) DNiCu,eff 2 ∂t ∂d
(2)
where Cc is defined the concentration of carbon in the metal particle, t is the diffusion time, DNiCu,eff is the effective diffusivity of carbon in the Ni-Cu particle, and d is the diffusion distance from the gas side. For semiquantitative analysis, we assume the Ni-Cu particle with relatively large size is a semi-infinite slab, and it is subjected to the following conditions
initial condition, t ) 0, Cc ) 0
(3)
boundary condition, t > 0, Cc ) C0
(4)
if we assume the surface carbon species (C0) is constant due to the steady decomposition of C2H6 at reaction temperature. Therefore, the distribution of carbon concentration is
(√
Cc ) C0 - C0 erf
d
4DNiCu,efft
)
(5)
Where the erf is the error function. Then we can obtain
(
∂Cc C0 -d2 )exp ∂d 4DNiCu,efft √πDNiCu,efft
)
(6)
Therefore, the concentration of carbon in the Ni-Cu particle 2 will decay as a function of e-d , giving a very sharp gradient in the Ni-Cu particle. In a short time, the dissolved carbon will accumulate on the gas side of metal particle. The carbon will then further dissolve into the bulk of the particles. However, there are two processes simultaneously occurring during the interaction between the dissociated carbon species and the Ni-Cu particle: carbon diffusion into the metal particle to form
a Ni-Cu-C phase and the gradual melting of the formed Ni-Cu-C phase. Accumulation of carbon on the metal surface promotes melting of the Ni-Cu-C system and, hence, most likely leading to formation of a molten phase. However, formation of a carbide-like component near the surface cannot be excluded. Krestinin et al. pointed out that this gradual increase of carbon concentration in the metallic phase will lead to an metallic phase enrichment of the surface of this solid solution due to the lower boiling point of Cu or Ni compared to that of carbon.44,47 Segregation of the metallic phase refreshes the active surface, thus promoting more dissolution of carbon into the metal-carbon system. Increasing the carbon concentration in this Ni-Cu-C phase will further decrease the melting point of the solid solution. Ultimately, a subnanosized or nanosized molten layer of Ni-Cu-C on the surface of a Ni-Cu system is produced, leading an interface between this molten layer and the solid bulk phase of Ni-Cu particles. Note that the melting point of this nanosized phase could be 100-300 K lower than the bulk phase as suggested by Parmon.46 Recently, the liquid-like behavior of metal particles during the catalytic synthesis of CNFs or CNTs has been observed by an environmental transmission electron microscopy (ETEM) study, even at low carbon concentrations.48-50 As a result of energy minimization of this subnanosized or nanosized layer, spherical particles with sizes smaller than 100 nm are formed on the surface of the parent particles as shown in Figure 5c. The driving force for the formation of the spherical particles instead of the molten phase liquid-like layer can be due to the lower Gibbs free energy of the particle compared to the layer as suggested by Ruckenstein based on thermodynamic analysis.35 The thin layer covering the surface of Ni-Cu phase will become fragmented and generate the particles since sufficiently high temperature will produce intensive thermal perturbations of the free interface and promote the atom mobility. The transformation from layer to particles occurs only if the particle size exceeds a critical radius rm, which is related to the thickness of the molten layer, the contact angle, and the interaction between the Ni-Cu-C phase and bulk Ni-Cu phase. The presence of numerous spherical particles on the surface of the Ni-Cu phase probably reflects the poor wetting between the molten phase and the bulk phase as shown in Figure 2b, due to the different surface tension energies between this molten phase and bulk phase. The poor wetting property of the nearly spherical molten phase particles makes them very mobile. In addition, the dynamic transformation from layer to particle at high-temperature probably also give the driving force for the formed particles to leave the parent particles, and disperse on the surface of carbon macrofiber. Migration and redispersion through the existence of the liquidlike Ni-Cu-C phase are illustrated in Figure 5d. The reaction as described in step a can occur again on the refreshed surface of a parent Ni-Cu particle by the migration step d. The repetition of the process a to d allows for continuous generation and distribution of nanoparticles along the surface of the carbon microfibers. The island-like supersaturated Ni-Cu-C phase starts to nucleate and give rise to the formation of graphene layers. Several graphene layers can be produced on the surface of Ni-Cu-C phase in the nucleation step, thus initiating an octopus-like growth mechanism. At steady state growth, more graphene layers are deposited on the interface between the nucleated graphene layers and the Ni-Cu-C phase, promoting lifting of this Ni-Cu-C particle from the surface of the carbon
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J. Phys. Chem. C, Vol. 115, No. 4, 2011 1129 TABLE 1: Average Growth Rates (Yield) (gcarbon-deposited (gmetal,h)-1) of CNFs on Different Catalysts with Different Reduction Timea 2 h reduction time 24 h reduction time yield on Ni catalysts yield on Ni-Cu catalysts
0.12 0.5
0.02 0.4
a The reaction rates were estimated based on the weight gained after 16 and 2 h on Ni and Ni-Cu catalysts, respectively. Operating conditions: 5 pieces of Ni or Ni-Cu catalysts; C2H6/H2 ) 90/150 mL/min; temperature, 923 K.
Figure 6. A SEM image showing the morphology of the carbon macrofiber supported Ni catalyst after 1 min at 923 K in the ethane-hydrogen mixture. (a) Carbon nanofibers produced from smaller active Ni particles (50 nm) by an octopus-like growth mechanism. (b) An image showing the fragmentation process with smaller Ni particles originating from the parent Ni particle and the selective growth of CNF from these. Larger particles are also originating from the parent particle but in this case no CNF growth can be seen.
microfiber. Over time, these CNFs act as the support, dispersing the Ni-Cu particles on the carbon felt. This octopus-like growth mechanism has also been observed in the case of highly loaded (80-90 wt %) Ni-Cu-Al2O3, in which formation of a molten phase during the interaction with CH4 is also possible as a similar reaction temperature and final morphology of carbon nanostructures were reported. The authors suggested the importance of suitable crystalline faces of Cu-Ni nanoparticles for the octopus-like growth.30 The sample derived from reduced Ni/CF after 1 min of reaction was collected for comparison. The corresponding images are shown in Figure 6. In contrast to the Ni-Cu system in Figure 2c, in Figure 6a, CNFs start to catalytically grow within the first minute, ascribed to the already existing nanosized particles after reduction as shown in Figure 2. It is also found that metal particles of size less than 50 nm are predominantly located at the tips of CNFs, clearly indicating a tip-growth mode, whereas Ni particles with slightly larger diameter (50-100 nm) exhibit an octopus-like growth mode (indicated by the arrow in Figure 6a). From this observation, it is suggested that the growth mode in the Ni system is size-dependent. If the particle size is in the range between 50 and 100 nm, the CNFs are grown by an octopus-like mechanism, while the tip-growth mode is dominating when the particle size of Ni is less than 50 nm. Since the diameter of CNFs (30-50 nm) from octopus-like growth mode is about or less than half of the corresponding
metal particles (50-100 nm), this gives a similar diameter range of CNFs as from the tip-growth mode. As a result, the diameter distribution of CNFs grown from Ni catalysts becomes rather uniform, in the range of 30-50 nm. This observation is in good agreement with what has been previously reported.5 In contrast, only the octopus-like growth mode and a broader diameter distribution are observed on Ni-Cu catalysts in the initial periods. In Figure 6b, small Ni particles are produced from a larger Ni particle by fragmentation. In contrast to the Ni-Cu catalysts, small Ni particles are not transported away from the parent particles, but directly catalyze the CNF growth, as indicated by arrows in Figure 6b. The results clearly demonstrate that CNFs preferentially grow from smaller Ni particles (
