Preparation and Application of Carbon-Nanofiber Based

Apr 21, 2007 - This review deals with the preparation methods and application of the above-mentioned particles and thin layers, especially focusing on...
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Preparation and Application of Carbon-Nanofiber Based Microstructured Materials as Catalyst Supports J. K. Chinthaginjala, K. Seshan, and L. Lefferts* Catalytic Processes and Materials, IMPACT and MESA+, UniVersity of Twente, P.O. Box 217, 7500 AE, Enschede, the Netherlands

In the application of heterogeneous catalysts in liquid phase reactions, the rate of reaction as well as selectivity is often negatively influenced by mass transfer limitations in the stagnant liquid in the pores of the catalyst support. Internal mass transfer limitations can be reduced by maximizing the porosity and lowering the tortuosity of the catalyst support. Particles and layers consisting of carbon nanofibers are promising catalyst supports because of the combination large pore volume (0.5-2 cm3/g) and extremely open morphology, on one hand, and significant high surface area (100-200 m2/g), on the other hand. This review deals with the preparation methods and application of the above-mentioned particles and thin layers, especially focusing on thin layers supported on structured materials like monoliths, metal foams, felts, filters, and fibers. 1. Introduction Catalytic multiphase reactors are commonly used in many fine chemical and petrochemical processes. They allow efficient contact between gas and/or liquid reactant phases with solid catalysts. In order to maximize yield, catalyst activity needs to be enhanced, this is typically achieved by developing catalytic sites having high intrinsic activities and by maximizing the number of active sites, e.g., by using high surface area support materials. In practice, the higher reaction rates can be taken advantage of only under the condition that the transfer of mass (reactant and products), and to a lesser extent the transfer of heat, can keep up with the intrinsic activity of the catalysts used. In the case where mass transfer is relatively slow, concentration gradients will occur, especially in the pore system of a heterogeneous catalyst.1 In many cases concentration gradients induce loss in selectivity and byproducts are usually formed via reaction networks that often contain parallel and consecutive reactions. Concentration gradients may thus prevent optimal operation because the active catalytic sites experience different concentrations and at least a part of the active sites operates under nonoptimal conditions. This problem is most relevant when gases (e.g., hydrogen, carbon monoxide, or oxygen) have to be dissolved in a liquid (e.g., water) resulting in low concentrations. This coupled with the low diffusivity of gases in liquids causes concentration gradients to occur even more easily. Conventional technologies for heterogeneous catalytic reactions involving both liquid and gas phases comprise slurry reactors or trickle bed reactors. Reactions in trickle bed reactors easily end up in diffusion limitations because of the relatively larger catalyst particle sizes used (1-10 mm), implying relatively long diffusion distances inside the catalyst particles. Particle size can be reduced only to a limited extent because this results in increased pressure drop through the reactor. Catalyst particles for slurry reactors are much smaller (30 µm, typically) and thus more suitable for fast reactions; however, additional cost for catalyst separation is an issue. Nevertheless, even slurry catalysts suffer from mass transfer limitations in the case of very active catalysts. * To whom correspondence should be addressed. E-mail address: [email protected]. Phone number: +31(0)534893033. Fax number: +31(0)534894683.

Figure 1. Scanning electron micrographs of carbon nanofibers.53

In principle, internal mass transfer limitations can be reduced by maximizing the porosity and minimizing the tortuosity of the catalyst system. Further, the surface area should be significant in order to be able to host sufficient active sites per unit of volume. In this regard, aggregates of carbon nanofibers (CNFs) are promising because they exhibit (i) high surface area (100-200 m2/g), (ii) large pore volume (0.5-2 cm3/g), and (iii) minimal or no microporosity.2 Figure 1 shows a typical example of CNFs as observed using scanning electron microscopy (SEM), demonstrating the open structure of the aggregate. Schouten et al. suggested that the structure of CNF aggregates, as shown in Figure 2, mimics the inverse structure of a conventional porous support material. The advantages in terms of porosity and tortuosity for the CNF aggregates can be easily recognized from the figure, which can prevent any mass transfer limitations at the outer surface of the aggregates. The goal of this short review is to highlight the promise of CNF based structured catalytic reactors. In this context, the status of the aspects of (i) formation of CNFs and their properties as catalyst supports and (ii) preparation and application of (micro)structured materials based on CNFs, are discussed. The main focus will be on thin layers of entangled CNFs, i.e., a microstructured thin layer based on the fact that the typical diameter of CNFs is sub-micrometer, supported on macrostructured materials, e.g., foams and monoliths. It should

10.1021/ie061394r CCC: $37.00 © 2007 American Chemical Society Published on Web 04/21/2007

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Figure 2. CNFs mimicking the inverse structure of conventional porous structured material. Courtesy of Eindhoven university of Technology, Chemical Reactor Engineering group; internet: www.chem.tue.nl/scr.

Figure 4. Conversion of different carbon sources with a flow of 20 mL/ min (total flow 100 mL/min) at 550 °C and 1 bar over Ni/SiO2.11

Figure 3. Schematic representation of the catalytic growth of a CNF using a carbon-containing gas. Step 1 consists of decomposition of carboncontaining gases on the metal surface. Step 2 is allowing carbon atoms dissolve in and diffuse through the bulk of the metal. Step 3 is precipitation of carbon in the form of a CNF consisting of graphite.11

be stressed that the term “micro” as used here is not related at all to the classical definition of micropores, i.e., pores smaller than 2 nm. Carbon filaments are formed catalytically in metallic catalysts, particularly in Ni, Fe, and Co based catalysts, used for the conversion of carbon-containing gases, e.g., in steam reforming of hydrocarbons and Fischer-Tropsch synthesis.2 In these cases, the carbon filament formation was detrimental for operation as they plugged reactors and deactivated catalysts. This problem was turned into an opportunity in the early work of Robertson5 and Baker et al.4,42 who showed that CNF materials could be prepared on demand from supported Ni, Co, or Fe catalysts. A pioneering work later by Iijima showed that single walled carbon nanotubes could also be produced via arc-discharge synthesis of C60 and other fullerenes.3 This triggered an outburst of the interest in the synthesis of carbon nanofibers and nanotubes via vaporized carbon from arc discharge, laser ablation, catalytic carbon deposition, etc.42-46 Fiber type carbon nanomaterials can be classified into three types, namely, carbon nanofibers (CNFs), carbon nanotube (CNTs), and single walled nanotubes (SWNTs). In CNFs, the graphitic planes are oriented at an angle to the central axis, thus exposing graphite edge planes. In CNTs, the graphitic planes

run parallel to the central axis, in this state only basal planes are exposed. CNTs are also referred to as multiwalled carbon nanotubes (MWNT) or parallel carbon nanofibers. If the fiber consists of only one graphene sheet that is oriented in a parallel direction to the fiber axis, it is called a single-walled carbon nanotube (SWNT). CNFs can be made in high yields at relatively low costs using metal clusters which catalyze their formation from carbon-containing gases.2 This review is limited to this type of synthesis of CNFs. 2. Carbon Nanofibers: Synthesis and Properties. 2.1. Synthesis of CNFs. Iron, cobalt, and nickel particles (10-100 nm) are able to catalyze the formation of CNFs from carboncontaining gases. Examples of carbon sources are methane, carbon monoxide, synthesis gas (CO + H2), ethylene, and ethane. Typically CNF formation occurs in the temperature range 425-925 °C; the reactivity of the carbon source gas has a strong influence on the preferred temperature. Mechanisms proposed by various researchers for the formation of CNF agree, in general, on the sequence of the CNF formation.4,6,7,61 This involves (i) decomposition of the carbon-containing gas on the exposed surface of metal particle, (ii) dissolution of carbon in the metal particle, and (iii) diffusion of the dissolved carbon through the particle and precipitation/growth of CNFs at the other end of the particle. This is schematically shown in Figure 3. Nevertheless, there have also been suggestions that only surface diffusion is involved. Further, there is still debate about the driving force for the carbon to dissolve on the one side of the metal particle and to segregate at the other side. Baker et al.4 have proposed that a temperature gradient over the metal particle accounts for this. The temperature gradient is proposed to be caused by the differences in the thermochemistry of the hydrocarbon decomposition versus the formation of CNF,

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Figure 5. Influence of temperature on carbon deposition rate from methane over nickel.12

Figure 6. Effects of NiO crystal size on the final carbon yield of CNF. The reaction conditions are as follows: T ) 580 °C; Ptot ) 100 kPa; PCH4 ) 80 kPa; PH2 ) 5.5 kPa; total flow ) 50 mL/min.13

suggesting that precipitation of excess carbon will occur at the colder zone of the particle resulting in the formation of a carbon nanofiber. On the other hand, Yang et al.6 proposed that the Ni surface decomposing CO exposes (110) lattice planes, which are different from the (111) and (311) planes that are supposed to induce formation of CNFs; these latter planes provide epitaxial fit with graphene layers. Hoogenraad et al.7 postulated a specific mechanism to explain the initiation of CNF formation; diffusion of carbon into the metal particle leads to formation of metal carbide, which then decomposes to regenerate metal and precipitate graphite enveloping the metal particle. The metal particle is squeezed out of graphitic carbon due to pressure buildup during the formation of graphite layers. The ability of Ni, Fe and Co to form graphitic carbon (CNF) is attributed usually to a combination of factors that include their catalytic activity for the decomposition of hydrocarbons, formation of metal carbides, and/or diffusivity of carbon through the metal particles.8 Metals are suggested to undergo surface morphological changes at elevated temperatures in the presence of reactive gases; these changes have also been suggested to facilitate nanofiber growth even though the mechanism is not yet clear.9 Helveg et al.10 presented images of the early stages of nanofiber growth by using in situ high resolution transmission electron microscopy at 500 °C during catalytic reaction between methane and supported nickel particles (5-20 nm). The realtime pictures show that the Ni particle is squeezed out of the hollow core of the growing CNF, similar to the mechanism proposed by Hoogenraad.7 CNF growth continues until the catalyst particle is deactivated or poisoned. Deactivation occurs when carbon deposition is

Figure 7. Different oxygen-containing surface groups on carbon formed by gas or liquid phase oxidation. (a) Carboxyl groups. (b) Carboxylic anhydride groups. (c) Lactone groups. (d) Phenol groups. (e) Carbonyl groups. (f) Quinone groups. (g) Xanthene or ether groups.23

Figure 8. Schematic depiction of experimental procedures used for the immobilization of Rh/anthranilic acid on CNFs.47

Figure 9. Reaction pathway of hydrogenation of cinnamaldehyde.49

faster than carbon diffusion and/or the formation of graphene sheets results in complete coverage of the metal surface with carbon. Nanofiber growth depends on a variety of factors that include the type of carbon source used, temperature, the choice of metal, and the metal particle size. The diameter of the filament produced is generally related to the physical dimensions of the metal particle (CNFs having diameters up to 500 nm are reported);63 on larger Ni particles, multiple fibers of smaller diameters grow as result of decomposition of the metal particle in the initial stages of growth. This aspect will be discussed later. The influence of the type of carbon source on the CNF growth rate is shown in Figure 4. Three different gases, namely, methane, synthesis gas, and ethylene, were decomposed on Ni at 550 °C. The initial rates increased in the order methane < syngas < ethylene, and this order reflects the increasing general chemical reactivity of these molecules in chemical conversions. Ethylene also caused rapid deactivation, attributed to the higher carbon deposition rate. The addition of hydrogen retards carbon formation increasing the lifetime of the metal particle, as expected.12

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Figure 10. Schematic representation of CALCD adsorption explaining the enhanced activity for the CNF supported Ru catalyst after the removal of the majority of the oxygen-containing groups, by support-assisted catalysis.50

Figure 11. Typical structured materials, ceramic Monolith,26 felt,30 and metal (Ni) foam.55

Figure 12. (A) SEM image of the carbon nanofibers composite supported on macroscopic graphite felt. (B) Higher resolution SEM image of the carbon nanofibers with a rather uniform diameter centered at around 30 nm.30

Figure 5 shows the effect of temperature on the rate of CNF growth from methane over Ni. It can be seen that the C-deposition rate at 530 °C is lower, and hence, the CNF formation lasts for a longer time as compared to the C-deposition rate at 590 °C.12 Again, a delicate balance between C deposition and CNF formation needs to be sustained to continue growth. Therefore, every hydrocarbon will have an optimum temperature window to maximize CNF yield. The strong influence of metal particle size on the CNF yield is demonstrated in Figure 6 using methane and Ni.13 It should be noted that the yield is a result of both the growth rate and longevity of the growth. It is obvious that very small particles will not form any CNFs because there is not sufficient metal present to enable CNF formation according the mechanism shown in Figure 3. On the other hand, very large particles would be inactive because carbon diffusion through these particles is too sluggish. 2.2. Properties of CNFs. In addition to the morphological properties discussed so far, CNFs have a number of other characteristics that make them materials of promise as catalyst supports. First CNFs are chemically stable for corrosive attack in acidic and basic environments; this can be a serious advantage in certain liquid phase operations. They are also inert and

withstand most organic solvents. CNFs are also stable toward sintering, for high-temperature gas reactions, unlike oxide supports. However, they are prone to reactions in the presence of oxygen and hydrogen. Temperature programmed oxidation (TPO) is a simple technique to study the resistance of CNFs toward oxidative attack; such studies show that the properties of CNFs are comparable to those of turbostratic graphite.14-16 The mechanical strength of an entangled CNF layer is important for application as a catalyst support.61 Entangled CNF clusters are reported to have a bulk crushing strength of 1 MPa;2 this makes them suitable for use as catalyst supports in fixed bed reactors applications. Hoogenraad et al.7 observed that thin nanofibers (∼12 nm) grow faster and relatively straight, preventing entanglement of the fibers. Thicker fibers (∼35 nm) grow at a low rate and in random and changing directions resulting in highly interweaving structures; this entanglement is responsible for the formation of stronger aggregates. Kushinhov et al.17 and Van der Lee et al.62 reported that increasing the growth time increases the density of CNF agglomerates; this also results in an increased mechanical strength of the aggregates. Finally, it is important to note that CNFs are also conductive.61 Thermal conductivity may be advantageous in strongly

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Figure 13. Thin layer of CNFs on the surface of Al2O3 washcoated monolith.31

exothermic or endothermic reactions where heat transfer may become limiting. On the other hand, electrical conductivity is important in the field of electrocatalysis, e.g., application in fuel cell electrodes. In chemical catalysis, one may wonder whether electrical conductivity is relevant. The conductivity of a CNF may be relevant when the conversion is equivalent to two electrochemical half reactions, and these half reactions take place at different metal particles provided that ions in the liquid phase (like H+ or OH- in the aqueous phase) can contribute to the electrochemical half reactions. A typical candidate for this type of reaction is the hydrogenation of nitrate to either N2 or hydroxylamine. 3. Application of CNFs as Catalyst Supports The properties of CNFs discussed so far indicate the promise these materials show as supports for catalysts. CNFs can be applied as catalyst supports in three ways: (1) Using small aggregates of entangled nanofibers loaded with the catalytic active phase. The typical application would be in slurry reactors with aggregate sizes on the order of 10 µm. (2) Application of larger aggregates (on the order of millimeters) of entangled CNF bodies to form a fixed bed. The fixed bed may be used as such in a single phase operation or as a trickle bed for gas-liquid operation. Advantages of such a system would be its high porosity and low tortuosity (3). The CNFs can form layers on structured materials such as foams, monoliths, or felts; this helps to keep diffusion distances short. The structured materials of choice obviously will also determine the hydrodynamic behavior of the reactor, which will not be discussed here. In this chapter, we will first discuss how CNFs can be used to prepare supported catalysts in general. Then options 1 and 2 will be shortly treated, and finally, option 3 will be dealt with in the most detail. 3.1. Incorporation of Active Catalytic Phase. CNFs are hydrophobic materials with no or hardly any functional groups. To employ carbon nanofibers as catalyst supports, it is important to modify the surface properties. A mild oxidation treatment is often used, and this helps to create oxygen-containing surface groups (see , e.g., Figure 7). Formation of such an oxidized surface helps in two ways. First, small catalyst particles ( e.g., metals such as Pt, Pd, etc.) can be anchored by using these polar functional groups. Second, the presence of the polar functional groups enhances the wettability of the CNFs for polar solvents, e.g., water. However, it is essential to carry out the oxidation in a controlled manner so as not to destroy the CNFs.

Figure 14. Scanning electron micrographs of a side view of a single wall of the monolith after the growth of CNFs at different formation times and nickel loadings (200 mL/min 50% CH4, 10% H2 balance N2 at 570 °C). (a) Al2O3-cord-Ni (1.2 wt % Ni), CH4, 5 h. (b) Al2O3-cord-Ni (3.0 wt % Ni), CH4, 3 h.53 (c) (200 mL/min 50% C2H4, 10% H2 balance N2 at 570 °C), A12O3-cord-Ni (3.0) for 1 min.53

Oxidation in the liquid phase can be controlled much better and proceeds more homogenously than gas phase treatments.18 The process of oxidation proceeds homogeneously along the length of the fiber; however defect sites in the graphene layers enhance the oxidation.18-21 Figure 7 shows the types of oxygen-containing surface groups that can be generated on the surface of CNFs via oxidation. In

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Figure 16. Inverse of a porous packed bed into solid foam with a layer of CNFs. Courtesy of Eindhoven University of Technology, Chemical Reactor Engineering group; Internet: www.chem.tue.nl/scr. Figure 15. Stability of carbon nanofiber attachment against ultrasound (40 kHz).31

general, oxidation with concentrated acids results in the formation of hydroxyl and carbonyl groups, whereas oxidation with hydrogen peroxide results in the formation of carboxyl groups. The choice of oxidant also depends on the nature of other phases that may be present together with CNFs, e.g., metal or oxide layers. In the case of pure CNF aggregates, oxidative treatments in acids, e.g., nitric acid, can be used. This also helps to remove any residual metal (Fe, Ni, Co) which was used to form the CNFs.22,23 In the case of structured materials supporting thin layers of CNFs, the supporting structure needs to be protected and milder oxidants such as hydrogen peroxide are preferred.48 Once an oxygenate surface group is formed, the catalyst phase can be anchored on to this site. The scheme in Figure 8 shows a typical example of how Rh/anthranilic acid can be immobilized on CNFs.47 A completely different approach is to use apolar solvents, in combination with catalyst precursors soluble in the apolar solvent. Contacting the CNFs with such a solution followed by drying and reduction results in the deposition of metal catalyst particles as reported by Planeix et al.24 The disadvantage of this method is the relatively low dispersions obtained due to weak interaction of the metal precursor with the CNF surface. 3.2. CNF Aggregates as Catalyst Supports. Considerable research work has been done to apply CNF aggregates as catalyst supports in gas phase reactions such as ethylene hydroformylation25,41 and in a variety of liquid phase hydrogenation reactions.56-58 We will describe only one example in more detail. Toebes et al.49 have reported that the product distribution in hydrogenation of cinnamaldehyde can be manipulated by changing the surface chemistry of CNFs, supporting Ru. The reaction scheme presented in Figure 9 shows that two primary products can be formed, namely, cinnamyl alcohol (CALC) and hydrocinnamldehyde (HALD). Both of these undergo further hydrogenation to hydrocinnamyl alcohol (HALC). The selectivity to HALD on CNF-supported Ru (ranging from 70 to 90%) is much larger than that obtained with Ru on activated carbon (30-40%)60 or Ru/Al2O3 (20-30%).59 The polarity of the support surface is reported to induce changes in the selectivity: the increase in polarity leads to enhanced activation of the CdO bond in CALC. On the other hand, lower polarity causes preferential activation of the CdC bond, thus increasing the formation of HALD. This might explain the differences in selectivities observed toward HALD for Ru over the three supports, as schematically shown in Figure 10. PhamHuu et al.58 reported that palladium supported CNFs showed higher activity toward hydrocinnamaldehyde than when an alumina was used as support. Further, it can be speculated that

the absence of microporosity in CNF aggregates results in an increase in activity due to the reduced external mass transfer limitations. 3.3. CNF Layers on Structured Materials. Thin layers of CNFs on structured materials is an interesting proposition as it would combine the advantages of slurry phase operation (short diffusion length), fixed bed operation (no catalyst separation is required, no catalyst attrition, and no catalyst agglomeration), and application of CNF aggregates (high porosity and low tortuosity). In addition, it allows for taking advantage of the use of structured reactor packing optimizing hydrodynamics and especially gas-liquid mass transfer. The pioneering work of Moulijn et al.26 has clearly demonstrated this advantage in the case of monoliths. Schouten et al.51 have started more recently to explore similar opportunities in the case of foam materials. In Figure 11, three different types of structured materials are shown. Next to these, other systems such as porous composite ceramics,27 sintered metal fibers,33 and silicon glass fibers52 are also available. All these structured materials have very low surface areas. Additional surface area needs to be generated to host a sufficiently extended catalytic surface area. Usually, this is achieved by wash coating, i.e., depositing an oxide layer (alumina, silica, etc) with the same textural properties as traditional particles. The same results can also be achieved by preparing thin layers of entangled CNFs. They offer the additional advantages of high porosity and low tortuosity which may enhance external mass transfer at the L-S interface. We will now describe the results of two different approaches. In the first approach, small Ni particles are deposited on a structured material first, followed by growing CNFs from the structured material using the Ni particles. The second approach uses structured materials that are intrinsically active for CNF formation. 3.4. CNFs on Structured Supports. A typical example of growing CNFs on a structured support is immobilized CNFs on a graphite felt, as proposed by Ledoux et al.28 First, Ni was deposited (1 wt %) on the felt and CNFs were synthesized at 680 °C using a reaction mixture of ethane and hydrogen (volume ratio 1:5). Figure 12A shows the macroscopic morphology of the CNF-containing composite; Figure 12B shows that the formed layer is highly porous, consisting of CNFs of about 30 nm in diameter. The CNFs are strongly anchored to the felt as no loss could be detected during ultrasonic treatment in ethanol solution during several minutes. Vieira et al.29 speculated that the anchoring of CNFs was due to penetration of CNFs into the graphite of the graphite felt. Ledoux et al.30 demonstrated the advantage of this material as a catalyst support for Ir in the catalytic decomposition of hydrazine (N2H4). Hydrazine decomposition to N2 and H2 is relevant as it is used as rocket fuel for satellite maneuvering

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Figure 17. SEM micrographs of (a) Ni foam, (b) CNFs-Ni foam loaded with 25 wt % CNF synthesized at 450 °C for 2 h, and (c and d) details of the CNFs layer.54

Figure 18. Surface SEM micrographs of the reduced Ni foam after exposure to 25% C2H4 in N2 (total flow rate 107 mL/min) at 450 °C for (a) smooth surface after reduction in 20% H2/N2 at 700 °C, (b) strips formed after 1 min of exposure, and (c) formation of small Ni particles after 20 min of exposure.54,55

and path correction. The decomposition is extremely exothermic leading to very hot gases and pressure release, resulting in thrust in small rocket engines. The reaction is extremely fast and is mainly controlled by mass and heat transfer. CNF based Ir catalysts outperformed the alumina based catalyst because of the faster mass transport in the latter which allows conversion of all hydrazine immediately leading to higher thrust with a factor of 3. In addition, the high thermal conductivity of the CNF-graphite composite allows faster heating of the catalyst bed which also contributed to a fast and powerful response to the introduction of hydrazine to the engine. The conductive nature of the CNF composite catalyst allows a rapid and homogeneous dispersion of heat in the catalyst bed, assisting complete decomposition of the reactants. In short, mass and heat transfer limitations were overcome by using a CNF-based catalyst.

Work in our own group focused on the introduction of CNFs in ceramic monoliths. Jarrah et al.31,53 synthesized nanofibers on a Ni loaded cordierite monolith, wash coated with γ-Al2O3. Carbon nanofibers were grown in an atmospheric gas mixture containing 50% CH4 and 10% H2 and balanced with N2. The result was a highly porous, homogeneous, and thin (around 1 µm) layer containing exclusively CNFs, as shown in Figure 13. However, the thin layer contained both CNFs and fragments of the original γ-Al2O3 washcoat. It is observed that the thickness of the washcoat increased by a factor of 2 as shown in Figure 14a, this is due to the fragmentation of the alumina layer combined with the massive presence of CNFs. The fragmentation of the alumina washcoat is rationalized based on the fact that the diameter of the CNFs (10-30 nm) is larger than the average pore size (5-20 nm) of the γ-Al2O3 washcoat; therefore, fragmentation is occurring as soon as the formation

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 3975 Table 1. General Recipe to Grow CNFs on Various Structured Supports surface area (SA), m2/g type of macrosupport

type of pretreatment

Ni on γ-Al2O3 coated Monolith31

reduction in 20% H2/N2 at 700 ˚C for 2 h

Ni on SiO2 coated monoliths32 Ni foam55

reduction in H2 at 500 °C for 1 h oxidation in stagnant air at 700 °C for 1 h

sintered metal fibers (INCONEL), 60.5% Ni33

Ni on silica glass fibers52 Ni on graphite felt29

reduction in 20% H2/N2 at 700 ˚C for 2 h oxidation in air at 650 °C for 3 h reduction in H2 at 600 °C for 2 h reduction in 10% H2/N2 at 550 °C for 3 h reduction in H2 (50 mL/min) at 400 ˚C for 1 h

composite before CNFs growth

composite after CNFs growth

CNFs

temperature ) 570 °C; C-source ) 50% CH4/10% H2/N2; gas flowrate ) 200 mL/min C-source ) CH4 in N2

45

63

190

temperature ) 450 °C; C-source ) 25% C2H4/N2; gas flowrate ) 107 mL/min