Structures of Silica-Supported Co Catalysts Prepared Using

Apr 27, 2007 - In addition, the Co catalysts prepared using microemulsion formed bundles of single- or double-walled carbon nanotubes by methane decom...
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J. Phys. Chem. C 2007, 111, 7748-7756

Structures of Silica-Supported Co Catalysts Prepared Using Microemulsion and Their Catalytic Performance for the Formation of Carbon Nanotubes through the Decomposition of Methane and Ethylene Sakae Takenaka,*,† Yoshiki Orita,† Hideki Matsune,† Eishi Tanabe,‡ and Masahiro Kishida*,† Department of Chemical Engineering, Graduate School of Engineering, Kyushu UniVersity, Moto-oka 744, Nishi-ku, Fukuoka, 819-0395, Japan, and Western Hiroshima Prefecture Industrial Institute, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-0046, Japan ReceiVed: January 31, 2007; In Final Form: March 9, 2007

Silica-supported Co catalysts were prepared by using microemulsion. The structure and state of the Co species in the catalysts were characterized by TEM, XRD, Co K-edge XANES/EXAFS, and diffuse reflectance UVvis spectroscopy. Co species in the catalysts prepared using microemulsion were atomically dispersed as Co2+ with tetrahedral symmetry (CoO4), even after reduction of the catalysts with hydrogen at 773 K, while Co metal particles were supported on the silica surface for the catalysts prepared by a conventional impregnation method. The Co catalysts prepared using microemulsion selectively formed multi-walled carbon nanotubes with a uniform diameter by the decomposition of ethylene at 973 K, whereas silica-supported Co catalysts prepared by the impregnation method formed carbon nanofibers with various diameters. In addition, the Co catalysts prepared using microemulsion formed bundles of single- or double-walled carbon nanotubes by methane decomposition at 1073 K. Contact of the Co catalysts with hydrocarbons at the reaction temperatures (973 or 1073 K) resulted in the reduction of atomically dispersed Co oxide species to form small Co metal particles, which grew carbon nanotubes through hydrocarbon decomposition.

Introduction Carbon nanotubes have attracted a great deal of interest since their discovery.1 Nanoscale carbon materials are expected to have a variety of applications for hydrogen storage, chemical sensors, catalytic supports, field emitters in displays, and so on because they have remarkable and unique magnetic, electronic, chemical, and mechanical properties.2-4 Preparation of nanoscale carbon materials usually requires severe reaction conditions such as high temperatures. For example, single-walled carbon nanotubes and multi-walled carbon nanotubes are synthesized by the carbon arc discharge process and laser ablation of graphite elecrodes.5,6 These methods can produce carbon nanotubes with a relatively high selectivity. However, these methods are not adequate for the mass production of nanoscale carbon materials since they require a large energy input and cannot continuously supply carbon sources for carbon nanotubes. Catalytic decomposition of molecules containing carbon atoms such as CO and hydrocarbons (CCVD; catalytic chemical vapor deposition) is a promising method for the production of carbon nanotubes at a lower cost on a larger scale.7-9 Supported Co catalysts are frequently utilized for the production of carbon nanotubes through the decomposition of hydrocarbons.10-15 The structures of nanoscale carbons formed by supported Co catalysts strongly depend on the particle size of the Co metal, which is the catalytically active component for the growth of carbon nanotubes. Co metal particles a few nanometers in size produce carbon nanotubes, while carbon nanofibers are formed from Co metal particles with larger diameters. Thus, the particle * Corresponding author. Tel.: +81-92-802-2752; fax: +81-92-802-2752; e-mail: [email protected]. † Kyushu University. ‡ Western Hiroshima Prefecture Industrial Institute.

size of Co metal in supported Co catalysts should be small to produce carbon nanotubes selectively. However, Co metal particles on supports are usually seriously aggregated during the decomposition of hydrocarbons because the formation of carbon nanotubes requires temperatures higher than 973 K. To selectively produce carbon nanotubes through hydrocarbon decomposition, serious sintering of Co metal particles on the supports during the reactions should be suppressed. We have studied the preparation of silica-supported metal catalysts using microemulsion.16-18 By this preparation method, metal particles such as Ni, Pt, Rh, and Au with diameters of a few nanometers can be uniformly covered with silica layers. Because each metal particle in these catalysts is covered with silica, the metal particles show a high resistance to sintering at high temperatures.19 Thus, catalysts prepared using microemulsion are expected to selectively produce carbon nanotubes through hydrocarbon decomposition. In the present study, silica-supported Co catalysts were prepared using microemulsion. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) at the Co K-edge, diffuse reflectance UV-vis spectroscopy, and transmission electron microscopy (TEM). In addition, the Co catalysts were utilized for the formation of carbon nanotubes through the decomposition of ethylene and methane. Experimental Procedures Silica-supported Co catalysts were prepared by using a waterin-oil microemulsion. The microemulsion system was prepared by adding aqueous Co(NO3)2 into a surfactant solution in cyclohexane. Poly(oxyethylene) (n ) 15) cetyl ether (denoted

10.1021/jp070826o CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007

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Figure 2. XRD patterns of fresh Imp-Co, ME-Co(NP5), and ME-Co(C15).

Figure 1. TEM images of fresh Imp-Co, ME-Co(NP5), ME-Co(C15), and ME-Pt.

as C15 hereafter) or polyoxyethylene (n ) 5) nonylphenyl ether (denoted as NP5 hereafter) was utilized as the surfactant in the present study. Nanoparticles of some compounds containing Co cations were synthesized by addition of aqueous NH3 into the microemulsion system. Hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) were performed at 323 K by the addition of TEOS and aqueous NH3 into the microemulsion system. Precipitates thus obtained were washed several times with isopropyl alcohol after filtration and calcined at 773 K for 2 h under an air stream. After this treatment, Co loading for these samples was estimated by X-ray fluorescence spectroscopy (XRF). Hereafter, the silica-supported Co catalyst prepared using a microemulsion containing NP5 or C15 as a surfactant is denoted as ME-Co(NP5) or ME-Co(C15), respectively. The silica-supported Co catalyst was also prepared by a conventional impregnation method for comparison (denoted as Imp-Co). The silica supports were prepared in a similar manner to the preparation of ME-Co(C15) and ME-Co(NP5). Decomposition of ethylene or methane over silica-supported Co catalysts was performed with a conventional gas flow system with a fixed catalyst bed at atmospheric pressure. The powder Co catalysts were packed with quartz tubes (inner diameter ) 20 mm and length ) 600 nm). Prior to the decomposition of ethylene or methane, the catalysts were treated with hydrogen at 773 K for 1 h. After hydrogen remaining in the reactor was purged out with Ar at 773 K, the temperature was increased up to 973 or 1073 K under an Ar stream. The formation of carbon nanotubes was performed by contact of ethylene or methane with the reduced catalysts at the required temperatures for 30 min. Measurement of XANES/EXAFS spectra was performed at the Photon Factory (the beamline BL 7C) at the Institute of Materials Structure Science for High Energy Accelerator Research Organization, Tsukuba, Japan. Co K-edge XANES/ EXAFS spectra of silica-supported Co catalysts and reference samples (Co foil, CoO, and Co3O4) were measured in transmission mode with a Si(111) two-crystal monochromator at room temperature with a ring energy of 2.5 GeV and a stored current of 250-450 mA (Proposal 2005G194). To examine the structures of the Co species in silica-supported Co catalysts before the hydrocarbon decomposition (denoted as fresh cata-

lysts hereafter), the catalysts were treated with hydrogen at 773 K for 1 h before the measurement of the Co K-edge XANES/ EXAFS. For the structural analyses of Co species in the catalysts after the hydrocarbon decomposition (denoted as used catalysts hereafter), the fresh catalysts were contacted with ethylene at 973 K for 30 min. After these treatments, the catalysts were packed in a polyethylene bag at room temperature under an Ar atmosphere. The analysis of EXAFS data was performed by using an EXAFS analysis program, REX (Rigaku Co.). Fourier transformation of k3-weighted EXAFS oscillations was performed over the range of k ) 3.5 to ∼15.5 Å-1. Inversely Fourier transformed data for Fourier peaks were analyzed by a curve-fitting method, using phase shift and amplitude functions derived from FEFF 8.0.20 TEM images of silica-supported Co catalysts before and after the hydrocarbon decomposition were measured with a JEOL JEM-3000F. XRD patterns of the fresh catalysts were measured by a Rigaku RINT-2500KS diffractometer using Cu KR radiation at room temperature. Diffuse reflectance UV-vis spectra of the fresh catalysts were measured by a PerkinElmer Lambda95 UV-vis spectrometer. Prior to measurement of the spectra, the catalysts were reduced with hydrogen at 773 K for 1 h, and then the reduced samples were packed into quartz cells for measurement of the spectra without contact with air. Results and Discussion Characterization of the Catalysts. Figure 1 shows TEM images of fresh Imp-Co, ME-Co(NP5), and ME-Co(C15). A TEM image of the silica-supported Pt catalyst prepared using microemulsion (denoted as ME-Pt hereafter) is also shown in Figure 1.21,22 We have prepared silica-supported metal catalysts using microemulsion. By the preparation method, metal particles can be covered uniformly with silica, as shown in Figure 1d. In the TEM image for ME-Pt, a Pt metal particle with a diameter of ca. 3 nm could be observed at the center of a spherical silica particle. In the TEM image of fresh Imp-Co, spherical particles with diameters of 20 to ∼30 nm were observed. Darker spots were also observed in the TEM image. As described next, the spherical particles and the darker spots in this TEM image were assignable to silica particles and Co metal particles, respectively. Co metal particles seemed to be present on the outer surfaces of the silica particles, although the shape of the Co metal particles could not be seen clearly. In contrast, spherical particles could be found in the TEM images for ME-Co(NP5) and MECo(C15), but no darker spots were observed. The diameter of

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Figure 3. Co K-edge XANES spectra of fresh Imp-Co, ME-Co(C15), and ME-Co(NP5) as well as reference samples (Co foil, CoO, and Co3O4).

the spherical particles was ca. 60 nm for ME-Co(NP5) and ca. 20 nm for ME-Co(C15). The spherical particles for MECo(NP-5) and ME-Co(C15) would be composed mainly of silica, judging from the contrast of these TEM images. The loading of Co was estimated by XRF to be 5.0 wt % for ImpCo, 5.4 wt % for ME-Co(NP5), and 3.5 wt % for ME-Co(C15). Therefore, the Co species in fresh ME-Co(C15) and MECo(NP5) would be highly dispersed in silica, although the Co species in Imp-Co were aggregated on the outer surface of silica. Figure 2 shows XRD patterns of fresh Imp-Co, ME-Co(NP5), and ME-Co(C15) catalysts. A sharp peak was observed at ca. 44° in addition to a broad peak at ca. 22° for the XRD pattern of Imp-Co. The peaks at around 44 and 22° were assignable to crystallized Co metal and amorphous silica, respectively. Thus, the Co species in fresh Imp-Co were present as crystallized Co metal. On the other hand, only a broad peak due to amorphous silica was observed in the XRD patterns of fresh ME-Co(NP5) and ME-Co(C15). This result implies that the Co species in fresh ME-Co(C15) and ME-Co(NP5) were highly dispersed in silica. Figure 3 shows Co K-edge XANES spectra of fresh ImpCo, ME-Co(C15), ME-Co(NP5) catalysts, and reference samples (Co foil, CoO, and Co3O4). The XANES spectrum of Imp-Co is very similar to that of Co foil. Thus, the Co species in ImpCo are present as Co metal. On the other hand, XANES spectra of ME-Co(NP5) and ME-Co(C15) are different from those for Co foil, CoO, and Co3O4, while the spectra of these catalysts are very similar to each other. Thus, the Co species in fresh ME-Co(C15) and ME-Co(NP5) are not Co metal, CoO, or Co3O4. However, the position of the threshold at around 7717 eV for the XANES spectra of fresh ME-Co(C15) and MECo(NP5) is reasonably consistent with that for CoO. The position of the threshold of XANES spectra for metals is sensitive to their valence.23,24 These results strongly suggest that the Co species in fresh ME-Co(C15) and ME-Co(NP5) are present as Co2+, even though these catalysts had been reduced with hydrogen at 773 K prior to measurement of the XANES spectra. A strong peak was observed at ca. 7725 eV in the XANES spectra of ME-Co(C15), ME-Co(NP5), CoO, and

Co3O4, whereas the peak for Co foil was smaller than those for the former samples. The peak was assignable to a one-electron transition from 1s to the empty state at the Fermi level of Co species.25,26 The peak at 7725 eV was stronger for the oxidized Co species than for the reduced ones, as clarified in the XANES spectra for CoO, Co3O4, and Co foil. The strong peak at around 7725 eV in the XANES spectra for fresh ME-Co(C15) and MECo(NP5) implies that the Co species in these catalysts were not reduced to Co metal. In addition, a pre-edge peak was observed at 7709 eV in the XANES spectra for CoO, Co3O4, ME-Co(C15), and ME-Co(NP5), as shown in the inset of Figure 3b. In the XANES spectrum of Co foil, a shoulder peak was found at almost the same position. Because the shoulder peak was not observed in the XANES spectra of fresh ME-Co(C15) or ME-Co(NP5), most Co species in these catalysts were in the oxidized state. The pre-edge peak at 7709 eV was assignable to a one-electron transition from Co 1s to Co 3d.25-27 The intensity of this pre-edge peak is sensitive to the local symmetry of the Co species (i.e., the pre-edge peak appears for a tetrahedral environment but is forbidden for an octahedral environment).28 Co species in CoO were present as octahedral CoO6, while Co3O4 was composed of tetrahedral CoO4 and octahedral CoO6. Thus, it is reasonable that the intensity of the pre-edge peak for Co3O4 is stronger than that for CoO, as shown in Figure 3b. The intensities of the pre-edge peaks for fresh ME-Co(C15) and ME-Co(NP5) are stronger than that for Co3O4. Therefore, most Co species in fresh ME-Co(C15) and MECo(NP5) are present as tetrahedral CoO4. Figure 4 shows Co K-edge k3-weighted EXAFS spectra for fresh Imp-Co, ME-Co(C15), ME-Co(NP5), Co foil, and Co3O4. The EXAFS spectrum for fresh Imp-Co is very similar in shape to that for Co foil, although the amplitude of the EXAFS oscillation for Imp-Co was smaller than that for Co foil. This result suggests that Co species in Imp-Co are stabilized as Co metal on silica. The features of EXAFS spectra for fresh MECo(NP-5) and ME-Co(C15) are very similar to each other. In the EXAFS spectra for ME-Co(C15) and ME-Co(NP5), the oscillation in the k range >8 Å-1 was very small, whereas a strong oscillation was observed in this k range for the EXAFS

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Figure 4. Co K-edge k3-weighted EXAFS spectra of fresh Imp-Co, ME-Co(C15), and ME-Co(NP5) as well as reference samples (Co foil and Co3O4).

Figure 5. Fourier transforms of Co K-edge k3-weighted EXAFS spectra of fresh Imp-Co, ME-Co(C15), and ME-Co(NP5) as well as reference samples (Co foil and Co3O4).

spectra of Co3O4 and Co foil. As for the Co K-edge EXAFS spectra of silica-supported Co catalysts, electron scattering from relatively light atoms such as oxygen and silicon is the main contributor to EXAFS oscillation in the lower k range, whereas the oscillation in the higher k range is assignable to scattering from heavy atoms such as Co. Thus, Co atoms in fresh ME-Co(NP-5) and ME-Co(C15) are not surrounded by Co atoms, but by oxygen or silicon atoms (i.e., Co atoms in these catalysts are highly dispersed in silica). This result is comparable to the conclusion drawn by the XRD patterns in Figure 2. Figure 5 shows Fourier transforms (RSFs; radial structural functions) for Co K-edge k3-weighted EXAFS spectra of fresh Imp-Co, ME-Co(C15), and ME-Co(NP5) as well as those for reference samples (Co foil and Co3O4). The RSF for Imp-Co was very similar in shape to that for Co foil, although the peak at around 2.2 Å for Imp-Co was smaller than that for Co foil. Thus, most Co species in fresh Imp-Co were present as Co metal. As for the RSF of Co3O4, three peaks were observed at

around 1.6, 2.5, and 3.1 Å. The peak at around 1.6 Å is assignable to Co-O bonds, and the other peaks are due to neighboring Co atoms in Co3O4. In the RSFs for fresh MECo(NP5) and ME-Co(C15), a strong peak due to Co-O bonds was observed at around 1.6 Å, suggesting that the Co atoms in these catalysts have some oxygen atoms around them. However, the intensity of the peaks in the R range >2 Å for these catalysts was significantly smaller than that for Co3O4. Therefore, most Co oxide species in fresh ME-Co(C15) and ME-Co(NP5) are atomically dispersed. To make the structures of the Co species in the fresh catalysts clearer, curve-fitting analyses of their EXAFS spectra were performed. The structural parameters estimated by the curvefitting analyses are listed in Table 1. The RSFs for the catalysts shown in Figure 5 were inversely Fourier transformed in the R range from 0.8 to 3.1 Å, and the EXAFS spectra thus obtained were fitted in the k range from 4.5 to ∼15.5 Å-1 by using theoretical parameters estimated by FEFF. The EXAFS spectrum for Imp-Co could be fitted by using a shell of a Co-Co bond.

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TABLE 1: Curve-Fitting Results for Co K-edge EXAFS of Fresh Imp-Co, ME-Co(C15), and ME-Co(NP5) Catalysts catalyst

shell

C.N.a

R (Å)b

D.W. (Å)c

Imp-Co ME-Co(C15)

CoCo Co-O CoCo CoCo Co-O CoCo CoCo

9.7 ( 0.3 3.7 ( 0.3 0.3 ( 0.3 1.1 ( 0.3 3.3 ( 0.3 0.3 ( 0.1 1.1 ( 0.2

2.48 1.97 2.47 2.96 1.97 2.47 2.99

0.065 0.067 0.065 0.077 0.059 0.066 0.076

ME-Co(NP5)

a

Coordination number. b Interatomic distance. c Debye-Waller fac-

tor.

The interatomic distance and the coordination number for CoCo in Imp-Co were estimated to be 2.48 Å and 9.7, respectively. This result indicates the presence of Co metal in Imp-Co. In contrast, the EXAFS spectra for fresh ME-Co(NP5) and MECo(C15) were fitted by a shell of a Co-O bond and two shells of Co-Co bonds. Co-Co bonds with interatomic distances of ca. 2.5 and ca. 3.0 Å were assignable to Co metal and aggregated Co oxide species, respectively. The structural parameters of fresh ME-Co(C15) coincided well with those of fresh ME-Co(NP5). Thus, the structures of Co species in these catalysts are very similar to each other. The coordination numbers of the Co-O bonds in ME-Co(C15) and ME-Co(NP5) were estimated to be 3.7 and 3.3, respectively. As described earlier, XANES spectra suggested that the Co species in these catalysts are present as tetrahedral CoO4. The coordination numbers of Co-O bonds estimated by the curve-fitting analyses for these catalysts are very reasonable, taking the XANES results into consideration. On the other hand, the coordination numbers of the Co-Co bonds due to Co metal in the fresh ME-Co(C15) and MECo(NP5) were extremely low. Thus, the fraction of Co metal to all Co species in the fresh ME-Co(C15) and ME-Co(NP5) catalysts was very small. In addition, the coordination numbers of the Co-Co bonds due to aggregated Co oxides for these catalysts were also very small, which indicates that the Co oxide species in ME-Co(C15) and ME-Co(NP5) are highly dispersed in the silica. The structures of Co species in fresh ME-Co(NP5) and MECo(C15) were also characterized by diffuse reflectance UVvis spectra, which are sensitive to the symmetry and electronic state of the Co species on the supports.29-32 Figure 6 shows diffuse reflectance UV-vis spectra for fresh ME-Co(NP5) and ME-Co(C15). A band was observed at around 220 nm in the spectra of both these catalysts. The band at around 220 nm was assignable to charge transfer from 2p of an oxygen atom to 3d of a Co atom. In addition, three bands were observed at around 520, 600, and 660 nm in both spectra. These bands were assignable to the d-d transition of Co2+ in tetrahedral symmetry CoO4, according to previous reports.29,30 No bands other than those due to highly dispersed tetrahedral CoO4 species were observed in the UV-vis spectra, although the spectra were also sensitive to other Co oxide species such as CoO6.29-34 Thus, most Co species in fresh ME-Co(NP5) and ME-Co(C15) were stabilized as Co2+ in the tetrahedral symmetry of CoO4. Diffuse reflectance UV-vis spectra were sensitive to Co species present at the periphery of the catalyst particles, while Co K-edge XANES/EXAFS and XRF spectra gave information on Co species present in the bulk of the catalysts in addition to their periphery. It should be noted that the intensities of the bands observed in UV-vis spectra for ME-Co(C15) were stronger than those for ME-Co(NP5), although Co loading in the fresh MECo(C15) (Co loading ) 3.5 wt %) was smaller than that in the fresh ME-Co(NP5) (Co loading ) 5.4 wt %). These results strongly suggest that more Co species in ME-Co(C15) are

Figure 6. Diffuse reflectance UV-vis spectra of fresh ME-Co(C15) and ME-Co(NP5).

localized at the periphery of silica particles, as compared to in ME-Co(NP5). We have prepared silica-supported metal nanoparticles such as Ni, Rh, and Pt by using microemulsion.16-19 In these samples that had been reduced with hydrogen at high temperatures (573 to ∼773 K), nanoparticles of the metals were covered uniformly with a silica layer, as shown in Figure 1d. In contrast, Co oxide species in the catalysts prepared using microemulsion were atomically dispersed, even after the catalysts were reduced with hydrogen at 773 K. The formation of highly dispersed Co species in the catalysts would be due to the dissolution of nanoparticles containing Co cations at the preparation stage of the catalysts. Nanoparticles containing Co cations, which would be mainly composed of Co(OH)2, were formed by the addition of aqueous NH3 into the microemulsion containing aqueous Co(NO3)2, and then aqueous NH3 was furthermore added into the microemulsion to perform the hydrolysis and polycondensation of TEOS. The addition of excess aqueous NH3 into the microemulsion should dissolve the nanoparticles containing Co cations to form Co-amine complexes.35 The Co-amine complexes would be supported on silica present in the microemulsion. It should be noted that ME-Co(C15) and ME-Co(NP5) have some cavities in their particles, as shown in Figure 1. It is likely that nanoparticles containing Co cations were originally present in these cavities. The dissolution of the nanoparticles to form Co-amine complexes results in the formation of the cavities in ME-Co(C15) and ME-Co(NP5). Nanoscale Carbons Formed on Silica-Supported Co Catalysts. Figure 7 shows TEM images of Imp-Co, MECo(C15), and ME-Co(NP5) after the ethylene decomposition at 973 K. The formation of fibrous carbons on the Imp-Co catalyst is observed in the TEM image (Figure 7a). The carbon yield for Imp-Co was estimated to be 6.0 g of C/g of Co by thermogravimetric analysis under air flow. The diameters of fibrous carbons were widely distributed from 20 to 50 nm. The fibrous carbons seemed to have a hollow structure, but the hollow structure was not complete as shown in Figure 7b. The graphite layers that formed the wall of the fibrous carbons were not parallel to the axis of the fibers, and the structure of graphite was highly disordered. In addition, many knots were observed in the hollow of the fibrous carbons.15,36 Thus, Imp-Co did not form carbon nanotubes through the ethylene decomposition. In

Structures of Silica-Supported Co Catalysts

Figure 7. TEM images of Imp-Co, ME-Co(C15), and ME-Co(NP5) after the ethylene decomposition at 973 K.

addition, Co metal particles were observed at the tip or in the body of fibrous carbons, as shown in the TEM images (Figure 7a,b). The Co metal should decompose ethylene to grow fibrous carbons.37-40 On the other hand, the formation of multi-walled carbon nanotubes was observed on ME-Co(C15) and MECo(NP5), as shown in TEM images (Figure 7c,e). The carbon yields were estimated to be 3.61 g of C/g of Co for MECo(NP5) and 2.57 g of C/g of Co for ME-Co(C15). The diameter of the multi-walled carbon nanotubes formed on these catalysts was smaller and more uniform than that on Imp-Co. The graphite layers of the carbon nanotubes formed on MECo(NP5) were parallel to the axes of the tubes, although the graphite layers on ME-Co(C15) were slightly disordered. These results indicate that Co catalysts prepared using microemulsion could form multi-walled carbon nanotubes with a uniform diameter through ethylene decomposition. In the TEM images of multi-walled carbon nanotubes formed on ME-Co(C15) and ME-Co(NP5), no Co species were observed at the tip or in the body of the tubes. Thus, it is likely that ME-Co(C15) and MECo(NP5) formed carbon nanotubes through a different mechanism from that of Imp-Co (i.e., Co metal particles were always present at the base of the carbon nanotubes (the base growth model)).41-43 In general, Co metal particles in the supported Co catalysts catalyzed the hydrocarbon decomposition to grow carbon nanotubes. However, most Co species in fresh ME-Co(C15) and ME-Co(NP5) were present as highly dispersed CoO4. It is likely that the Co oxide species in these catalysts were reduced during the ethylene decomposition to Co metal, which catalyzed

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7753 ethylene decomposition to grow multi-walled carbon nanotubes. Thus, the structures of the catalysts after the reaction (used catalysts) were investigated by Co K-edge XANES/EXAFS. Figure 8 shows Co K-edge XANES spectra of Imp-Co, MECo(C15), and ME-Co(NP5) before and after the ethylene decomposition. The XANES spectrum of the used Imp-Co coincided well with that of the fresh one, suggesting that the structure of the Co species in Imp-Co did not change during the ethylene decomposition. The XANES spectra of Imp-Co were consistent with that for Co foil in Figure 3. Thus, the Co species in Imp-Co were always present as Co metal. In contrast, XANES spectra of ME-Co(C15) and ME-Co(NP5) changed significantly after contact with ethylene at 973 K. In the XANES spectra for used ME-Co(NP5) and ME-Co(C15), a sharp peak was observed at around 7722 eV. As described earlier, the peak was assignable to oxidized Co species. The intensity of this peak was lower for the used catalysts than for the fresh ones. In addition, the contact of ethylene with fresh ME-Co(NP5) and ME-Co(C15) resulted in the disappearance of a pre-edge peak at 7709 eV and the appearance of a shoulder peak at 7712 eV in their XANES spectra. The pre-edge peak at 7709 eV and the shoulder peak at 7712 eV were assignable to Co oxides and Co metal, respectively. These results indicate that some Co oxide species were reduced to Co metal during the ethylene decomposition. It should be noted that the intensity of the shoulder peak at 7712 eV due to Co metal in the XANES spectrum for used ME-Co(C15) was stronger than that for used ME-Co(NP5). This result suggests that the fraction of Co metal to all Co species in used ME-Co(C15) is higher than that in used MECo(NP5). Figure 9 shows Co K-edge k3-weighted EXAFS spectra of Imp-Co, ME-Co(C15), and ME-Co(NP5) before and after ethylene decomposition at 973 K. As shown in Figure 9a, the EXAFS spectrum for used Imp-Co coincided well with that for the fresh one. Thus, the structure of Co species in Imp-Co did not change during the ethylene decomposition. In contrast, the EXAFS spectra for ME-Co(NP-5) and ME-Co(C15) changed significantly after the ethylene decomposition, as shown in Figure 9b. The EXAFS spectra for used ME-Co(C15) and MECo(NP5) were similar to that for Imp-Co in shape, although the amplitude of the oscillations for the former catalysts was smaller than that for the latter one. These results imply that many Co species in the used ME-Co(C15) and ME-Co(NP5) are present as small Co metal particles. XANES spectra of MECo(C15) and ME-Co(NP5) also suggest the reduction of Co oxides species into Co metal during the ethylene decomposition. The amplitude of the EXAFS oscillation for used ME-Co(C15) was stronger than that for used ME-Co(NP5). The oscillation amplitude of EXAFS spectra for any metal depends on the crystallite size of the metal (i.e., the EXAFS oscillation becomes stronger as the crystallite size of the metal increases).44,45 Thus, the average crystallite size of Co metal in used ME-Co(C15) is larger than that in used ME-Co(NP5). Figure 10 shows Fourier transforms for Co K-edge k3weighted EXAFS spectra (RSFs) of Imp-Co, ME-Co(C15), and ME-Co(NP5) before and after ethylene decomposition at 973 K. The RSF for used Imp-Co agreed well with that for the fresh one. Thus, the structure of Co species in Imp-Co did not change during the ethylene decomposition. In contrast, the RSFs for the used ME-Co(C15) and ME-Co(NP5) were different from those for the fresh ones. In the RSFs for ME-Co(C15) and MECo(NP5) shown in Figure 10b, new peaks appeared at around 2.1, 4.0, and 4.6 Å after the ethylene decomposition. The positions of these peaks agree with those for Co foil. Thus, Co

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Figure 8. Co K-edge XANES spectra of Imp-Co, ME-Co(C15), and ME-Co(NP5) before and after ethylene decomposition.

Figure 9. Co K-edge k3-weighted EXAFS spectra of Imp-Co, ME-Co(C15), and ME-Co(NP5) before and after ethylene decomposition.

oxide species in the fresh ME-Co(C15) and ME-Co(NP5) were reduced to Co metal during the ethylene decomposition. The peak due to Co-Co at around 2.1 Å for used ME-Co(C15) was more intense than that for used ME-Co(NP-5), suggesting that the average crystallite size of Co metal in used ME-Co(C15) is larger than that in used ME-Co(NP-5).44,45 To further clarify the structural changes of Co species in the catalysts during the ethylene decomposition, curve-fitting analysis for the RSF of each catalyst was performed. The structural parameters for Co species in the used catalysts are shown in Table 2. The peaks in the R range from 1.0 to 3.2 Å in the RSF for each catalyst were inversely Fourier transformed, and the EXAFS spectra thus obtained were fitted using phase shift and amplitude functions derived from FEFF. The EXAFS spectrum for the used Imp-Co could be fitted by the shell of a Co-Co bond. The coordination number and interatomic distance of Co-Co bonds for the used Imp-Co were estimated to be 9.6 and 2.48 Å, respectively. These values for the used Imp-Co are very similar to those for the fresh one in Table 1. Thus, the structure of the Co metal in Imp-Co did not change during the ethylene decomposition. On the other hand, the EXAFS spectra for used ME-Co(C15) and ME-Co(NP5) could be fitted by the

shell of a Co-O bond and two shells of Co-Co bonds, due to Co metal and aggregated Co oxides. Thus, the Co species in used ME-Co(C15) and ME-Co(NP5) were composed of Co metal and Co oxide. After the ethylene decomposition over MECo(C15) and ME-Co(NP5), the coordination numbers of CoCo bonds with interatomic distances of ca. 2.9 Å and of Co-O bonds became smaller, whereas the coordination numbers of Co-Co bonds with interatomic distances of ca. 2.5 Å became larger. The Co-Co bonds with interatomic distances of ca. 2.5 and 2.9 Å were assignable to Co metal and aggregated Co oxides, respectively. These results indicate that many Co oxide species in fresh ME-Co(C15) and ME-Co(NP5) were reduced to Co metal during the ethylene decomposition. The coordination number of Co-Co bonds due to Co metal in used ME-Co(NP5) and ME-Co(C15) was significantly smaller than that for used Imp-Co. As described earlier, fibrous carbons with diameters from 20 to 50 nm were formed by the ethylene decomposition over Imp-Co, whereas ME-Co(C15) and ME-Co(NP5) formed multi-walled carbon nanotubes with diameters