K10 Montmorillonite Based Catalysts for the Growth of Multiwalled

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Ind. Eng. Chem. Res. 2010, 49, 3242–3249

K10 Montmorillonite Based Catalysts for the Growth of Multiwalled Carbon Nanotubes through Catalytic Chemical Vapor Deposition Candida Milone,*,† Manikandan Dhanagopal,† Saveria Santangelo,‡ Maurizio Lanza,§ Signorino Galvagno,† and Giacomo Messina‡ Department of Industrial Chemistry and Materials Engineering, UniVersity of Messina, Contrada di Dio, I-98166 Messina, Italy, Department of Mechanics and Materials, UniVersity “Mediterranea”, Loc. Feo di Vito, I-89122 Reggio Calabria, Italy, and CNR, Institute for Chemical Physical Processes, Messina Section, Salita Sperone, Contrada Papardo, Faro Superiore I-98158 Messina, Italy

Multiwalled carbon nanotubes (MWCNT) are synthesized by isobutane decomposition at 700 °C over aspurchased and iron-loaded K10-montmorillonite catalysts. The results show that, upon reduction at 500 °C, K10 catalyzes isobutane decomposition. Few carbon fibers accompany the prevailing carbon flakes formation. Upon Na+ exchange or by increasing the reduction temperature, the activity of the clay decreases. Fe-K10 behaves as a bifunctional catalyst: on added metal sites, MWCNT preferentially form, while on the support, carbon flake formation mainly occurs. At a given metal load, the increase of the reduction temperature up to 700 °C or the use of Na+-exchanged K10 as a support generally leads to an enhancement of the selectivity to MWCNT, because of the diminishing of the support active sites. Under the present reaction conditions, Fe supported on Na+ exchanged K10 are the most active among the investigated catalysts. MWCNT copiously form, both at low and high metal load, and exhibit the highest structural order. 1. Introduction The interest in the outstanding mechanical, electrical, and chemical properties of carbon nanotubes has strongly encouraged the research for their synthesis, functionalization, and potential application.1 Low density, associated with excellent mechanical strength, stiffness, and elasticity, and conducting or semiconducting behavior, make carbon nanotubes promising materials for various advanced technology applications including light source,2 nanoelectrodes,3 and high-strength polymer composites.4,5 In the ambit of polymer reinforcing, an attractive strategy might be represented by the use of a new class of composite nanomaterials, consisting in multiwalled carbon nanotubes (MWCNT)/clay hybrid systems prepared by direct growth of MWCNT on the clay. In fact, recent advances in this field have shown that the dispersion in the polymer matrix of small amounts of MWCNT or of smectite clays, alone, leads to final composites with outstanding mechanical, thermal,6-10 and electrical properties.9,10 In the light of these results, it would be expected that the use of MWCNT/clay hybrid systems would guarantee a homogeneous dispersion of two components into the polymer matrixes so as to finally obtain composite materials with even bettered characteristics. Moreover, the use of MWCNT/clay hybrid materials is of great relevance for catalytic applications. A recent paper has demonstrated that MWCNT directly grown on natural clay (bentonite) show a higher efficiency than unsupported MWCNT for the oxidative dehydrogenation of ethylbenzene.11 The great advantage of using CNT dispersed onto the clay matrix lies in the high utilization of carbon active sites owing to the improvement of their dispersion. The use of clay mineral in CNT synthesis has been already reported.11-16 Catalytic chemical vapor deposition (CCVD) or * To whom correspondence should be addressed. E-mail: cmilone@ ingegneria.unime.it. † University of Messina. ‡ University “Mediterranea”. § CNR, Institute for Chemical Physical Processes.

hot filament chemical vapor deposition are the prevailing synthetic methods and acetylene, methane, or ethylene are used as carbon sources. Montmorillonite,12-15 clinoptilolite,15 and laponite14 added with first-row transition metals from Cr to Zn are used as catalysts. Among the added metals, Fe, Co, Ni, and Mn are the most suitable for the growth of MWCNT by CCVD of acetylene at 700 °C,12-14 whereas Cu mainly leads to the formation of C nanospheres and Cr and Zn are not active at all.13 Natural bentonite11 and montmorillonite,16 without the addition of any metal, have been also directly used for the CNT synthesis owing to the presence of Fe ions, 19 and 3.2 wt % respectively, originally present in the minerals. Even if a direct comparison is hard to be done due to the differences in the operating conditions (carbon source, composition of the gaseous mixture and iron load), literature data show that higher selectivity to CNT is obtained with bentonite as catalyst.11 The use of montmorillonite, instead, leads to the formation of carbon fibers having a mean diameter of 50 nm.16 K10 is a commercially available montmorillonite activated with mineral acid before commercialization. The raw montmorillonite is a hydrated 2:1 layered dioctahedral aluminosilicate composed of two tetrahedral (predominantly silicate) sheets sandwiching an octahedral (predominantly aluminate) sheet. Isomorphous substitution of Mg2+ and Fe2+for the octahedral aluminum and of Al3+ for the tetrahedral silicon results in layer charge deficit, which is balanced by exchangeable cations (e.g., Na+, Ca2+). The cations are intercalated between the clay layers, in the so-called interlayer region.17 The acid activation enhances the acidity of clay with respect to that of pristine solid, due to the replacement of the exchangeable cations with protons (H+) and to the partial dissolution of octahedral cations (Al3+, Fe3+)18-20 and also promotes physical changes, such as swelling at the edges of the clay platelets, which open up and separate, while remaining still tightly stacked at the center. As a consequence, the surface area of the final clay increases reaching values even higher than 300 m2/g.20-22

10.1021/ie9018275  2010 American Chemical Society Published on Web 03/10/2010

Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010

The use of high surface area K10 for the direct production of MWCNT could be of interest for the development of catalytic systems with improved CNT dispersion with respect to that prepared on low surface area raw clays, then with improved catalytic performance. Moreover, K10/MWCNT hybrid systems could be considered for polymer reinforcement given that K10 is already used as a component in the synthesis of polymer based composites.23-25 The aim of the present paper is to investigate the use of ironcontaining K10 montmorillonite clay for the synthesis of CNT. This is the first time that K10 based catalysts have been investigated in the synthesis of carbon nanotubes by CCVD. The synthesis of clay-carbon composites is carried out by CCVD over as-purchased and iron-loaded clay K10 catalysts. i-C4H10 is used as cheap, nontoxic, and highly reactive carbon source in the CNT synthesis.26 The influence of iron content and reduction temperature on the yield of carbon deposited, selectivity toward the MWCNT formation, and their crystalline quality is discussed in the light of the results coming out from their characterization by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry (TG), and Raman spectroscopy (RS). 2. Experimental Details 2.1. Preparation and Characterization of Catalysts. In order to investigate the effects of reduction temperature and iron content and to clarify some aspects of the MWCNT growth, three kinds of catalysts were considered: (a) as-purchased (Aldrich) K10 montmorillonite (size: 5-10 µm; surface area: 271 m2/g), below briefly termed K10 (b) Na+ exchanged K10 montmorillonite, indicated as Na+K10 (c) K10 and Na+-K10-based iron loaded catalysts, named Fe/ K10 and Fe/Na+-K10, respectively. Na+-K10 was obtained by the exchange of Na+ ions for other replaceable cations present in K10. For this purpose, 10 g of montmorillonite K10 were dispersed in 300 mL of water and allowed to swell by continuous stirring for about 6 h. Water was then replaced with a 1 M solution of NaCl to enable the ion-exchange reaction. Iron-loaded K10 catalysts were prepared by wet impregnation. In a standard procedure, 5 g of support was impregnated with 8 mL of an aqueous solution of Fe(NO3)3 · 9H2O (Fluka, 99.9%) having an amount of iron salt suitably calculated to obtain a nominal metal content of 5 and 15 g of Fe per 100 g of clay. Catalysts were slowly dried at 80 °C and calcined at 450 °C in air to get iron oxide from its precursor. The solids were wellground to powder and reduced for 2 h under (60 cc/min) hydrogen flow at the desired temperature (500 or 700 °C). The same procedure was adopted for the preparation of iron loaded Na+-K10 catalysts. Codes of catalysts obtained, their reduction temperature and iron content are reported in Table 1. The catalyst code summarizes information relative to clay (K10 or Na+-K10), reduction temperature (in degrees Celsius, subscript) andsif anysthe presence of loaded iron (FeL, where the superscript L is the nominal iron load). For example, Fe5/Na+-K10700 denotes the Na+-substituted clay, reduced at 700 °C, and loaded with 5 g of Fe per 100 g of clay. The total iron content of all the samples, K10 and Na+-K10 included, was calculated as wFe (wt %) ) 100mFe/m0, where m0 is the mass of catalyst and mFe the mass of iron as measured by means of XRF using a Total Reflection Geometry TX2000 (Ital

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Table 1. Properties of Catalysts Employed and Results Attained in Terms of Yield of C Deposits (YC)a catalyst code

wFeb (wt %)

TR (°C)

SA (m2/g)

dFec (nm)

YC (wt %)

K10500 Fe5/K10500 Fe15/K10500 Na+-K10500 Fe5/Na+-K10500 Fe15/Na+-K10500 K10700 Fe5/K10700 Fe15/K10700

2.0 6.3 14.1 1.7 6.3 13.9 2.0 6.3 14.1

500 500 500 500 500 500 700 700 700

229 197 173 222 164 157 201 115 69

ndd nd 21.8 nd nd 22.4 nd 32.0 45.1

11.3 13.2 114.0 2.3 29.3 140.0 0.4 14.7 31.7

a

All the CCVD experiments were carried out under the same reaction conditions. TR denotes the catalyst reduction temperature. The total metal content (wFe) is the sum of iron contained in the clays K10 and Na+-K10 (2.0 and 1.7 wt %, respectively) and iron added by impregnation. b As determined by X-ray fluorescence. c As determined by X-ray diffraction. d nd ) not detected.

Structures) analyzer. For this purpose, 20 mg of solids dissolved in 25 mL of acid solution (HCl and HF) and 800 µL of the solution, added with 50 µL of a standard solution of Ga (1000 ppm), were then analyzed. The specific surface area of catalysts was determined by BET method using a Q-SURF Series surface area analyzer. Nitrogen was adsorbed at -196 °C, after outgassing the sample at 200 °C for 2 h. XRD data were collected with an APD 2000 (Ital Structures) diffractometer using a CuKR radiation source. The patterns were recorded in step scan mode from 10° to 70° (2θ angles), step of 0.02°, counting time of 1 s/step. 2.2. Synthesis and Purification of MWCNT. The CCVD syntheses were carried out, in the presence of i-C4H10-H2, at 700 °C. For this purpose, 0.5 g of reduced catalyst was placed in a quartz boat inside the quartz reactor, located in a horizontal electric furnace, and preliminarily heated up to synthesis temperature under 120 cm3/min 1:1 He-H2 flow. Helium was then replaced with i-C4H10 keeping constant flow ratio and total flow rate. The reactions were stopped after 2 h, and the raw products were subsequently cooled down to room temperature in a He atmosphere. After cooling, support and iron particles were removed by refluxing the composites obtained in a mixture of 12% HCl and 12% HF acids. Finally, C deposits were washed thoroughly with distilled water and dried at 110 °C for 3 h. 2.3. Yield Evaluation and MWCNT Characterization. The yield of C deposits was conventionally calculated as YC (wt %) ) 100(m - m0R)/m0R, where m is the mass of all the materials (reaction products + catalyst) after synthesis and m0R is the mass of catalyst after reduction. The values obtained are reported in Table 1. The selectivity toward nanotubes was evaluated by SEM using a JEOL JSM 5600LV, operating at 20 kV. Purified samples were examined, recording at least 20 different images with diverse magnification factors per specimen, to have a reliable picture of their bulk. The morphology, dimensions, and crystalline structure of the MWCNT obtained were investigated by HRTEM utilizing a JEOL JEM 2010, operating at 200 kV and equipped with a Gatan 794 Multi-Scan CCD camera. The crystalline quality was evaluated by measuring Raman scattering excited by the 514.5 nm line of an Ar+ laser (Coherent Innova 70). The analysis was carried out, at room temperature, in the 800-3350 cm-1 spectral range by using a double monochromator (Jobin Yvon Ramanor U-1000) equipped with

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a microscope (Olympus BX40, X50 objective) and a photomultiplier (Hamamatsu R943-02) operating in photon-counting mode. The use of a low laser-power (3 mW at the sample surface) prevented annealing effects. In order to reliably describe the sample bulk, several different locations of each specimen were sampled on account of the possible structural nonhomogeneity. A 30 s long acquisition time was used to improve the S/N ratio. Before decomposition, spectra recorded were normalized and averaged. The thermal stability of C deposits in the temperature range 200-1000 °C was investigated by TG using a TA Instruments SDTQ 600 (balance sensitivity: 0.1 µg). A multipoint calibration was preliminarily carried out by means of metallic standards of very high purity. In order to avoid thermal capacity effects, only 2-3 mg of mass were analyzed for each sample and a low scan rate (1 °C/min) was used. 3. Results and Discussion 3.1. Properties of Unloaded and Iron-Loaded K10 Catalysts. Table 1 reports total amount, wFe, of iron and specific surface area (SA) of all the samples, as respectively inferred by XRF analysis and BET measurements. As can be seen, aspurchased montmorillonite (sample K10) contains 2 wt % iron cations. The ion exchange reaction with an NaCl solution (sample Na+-K10) produces only a very small reduction (∆wFe ) -0.3 wt %) of the pristine iron content. The low removal of iron suggests that Fe cation is not in an exchangeable position, and it is mainly located as a structural ion and/or as iron oxide formed from the dissolution of octahedral iron sites upon acid treatment. Upon reduction at 500 °C, the clay’s SA decreases from 271 to 229 m2/g; a further increase of the reduction temperature to 700 °C causes only a slight SA decreasing (Table 1). At a given reduction temperature, no relevant change in SA is introduced, upon Na+-exchange, with respect to the unexchanged clay. The addition of iron (samples FeL/K10 and FeL/Na+-K10) causes a decrease of the specific surface area. For a given support, SA diminishes with increasing total iron content. At fixed wFe, the decrease is more marked for higher reduction temperature (Table 1). Figure 1 displays the XRD powder spectra of the clay catalysts considered. Several 2θ peaks are observed in the diffractogram of as-purchased K10 (pattern a in Figure 1); the peaks located at 20°, 35°, 54°, and 62° are assigned to the diffraction of (110), (105), (210), and (300) reflections of montmorillonite, respectively. The spectrum totally agrees with that reported in the literature for the K10 clay.22 Muscovite and quartz diffraction lines are also present, mainly as clay’s impurities.22 The diffraction patterns of samples K10500, K10700, and Na+-K10500 (not shown) do not significantly differ from that of the pristine clay, indicating that the reduction process and the Na+ exchange do not introduce relevant structural changes in unloaded catalysts. Among the Fe-loaded catalysts reduced at 500 °C, only the XRD spectra of Fe15/K10500 and Fe15/Na+-K10500 show, in addition to the support lines, the signal at 2θ ) 44.7°, peculiar of metallic iron (patterns c and e in Figure 1). Contrarily, upon reduction at 700 °C, the diffraction peak of metallic iron is visible in all Fe-loaded samples (patterns f and g in Figure 1). These findings suggest that Fe(III) fully reduces already upon lower reduction temperature; the absence of the signal in samples Fe5/K10500 and Fe5/Na+-K10500 (patterns b and d in Figure 1)

Figure 1. XRD spectra of as purchased K10 and iron loaded catalysts. The diffraction patterns shown refer respectively to catalysts K10 (a); Fe5/K10500 (b); Fe15/K10500 (c); Fe5/Na+-K10500 (d); Fe15/Na+-K10500 (e); Fe5/K10700 (f); and Fe15/K10700 (g). Symbols: (•) montmorillonite; (*) muscovite; (9) quartz; (2) metallic iron.

may be due to the insufficient amount of iron and/or to the smaller particle size. The mean iron-particle size (dFe), as estimated from XRD patterns by applying the Scherrer equation, is reported in Table 1. No relevant iron size change seems to be introduced by the Na+ exchange (compare samples Fe15/K10500 and Fe15/Na+K10500). Instead, the reduction temperature has a stronger influence: actually, dFe doubles going from Fe15/K10500 to Fe15/ K10700. A minor variation is produced by the increase of wFe: dFe enlarges by nearly a factor of 3/2 going from sample Fe5/ K10700 to Fe15/K10700. 3.2. Growth Mechanism and Selectivity toward MWCNT. 3.2.1. Behavior of Unloaded Catalysts. As iron is active toward the MWCNT formation via isobutane decomposition,26 CCVD is first carried out in an i-C4H10-H2 atmosphere, at 700 °C, on the as-purchased K10 catalysts previously reduced at 500 or 700 °C (samples K10500 and K10700, respectively). K10500 is able to decompose isobutane with a yield to C deposits of 11.3 wt %, while contrarily K10700 is almost inactive in the reaction (YC < 1 wt %), indicating that the thermal treatment at higher temperature promotes the deactivation of sites present in the montmorillonite or, at least, a diminishing in their number. A strong decrease of catalytic activity of K10 also occurs upon Na+-exchange reaction; as shown in Table 1 yield is only 2.3 wt % over Na+-substituted montmorillonite reduced at 500 °C (sample Na+-K10500). This finding clearly indicates that the exchangeable cations are the most active clay’s sites toward the hydrocarbon decomposition. This rules out the possibility that catalytic activity exclusively pertains to iron present in the pristine clay. Therefore, it can be reasonably hypothesized that the catalytic activity of K10500 substantially originates from the exchangeable Bro¨nsted acid sites (H+) introduced in the clay, as substitutes of Ca2+, Na+, K+, upon the precommercialization acid treatment. Indeed, it is known that acid catalysts, such as acid activated bentonite and kaolinite and Y-zeolites catalyze hydrocarbon cracking and that the initial cracking activity is principally due to the Bro¨nsted acid sites.27 On this basis the lowering of the acidity of K10, easily achieved upon exchange reaction with NaCl solution19 or upon thermal treatment at high temperature,28 well accounts for the loss of activity observed for samples Na+-K10500 and K10700.


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Figure 2. Morphology of the carbonaceous products as monitored by SEM and TEM. SEM micrographs shown refer respectively to C deposits obtained on K10500 (a) and Na+-K10500 (c). TEM micrographs (b) are evidence of the carbon fiber nanostructures formed on K10500 and Na+-K10500.

Figure 2a shows a SEM micrograph of the carbonaceous products obtained in the isobutane CCVD over K10500. Only a few of randomly oriented carbon fibers, as evidenced by TEM analysis (Figure 2b), are visible. Different nanostructures, such as flakes, also form, and what is more, they represent the main product of reaction. Figure 2c, showing an SEM micrograph of the reaction products obtained over Na+-K10500, indicates that the formation of graphitic flakelike structures still prevails, but a larger relative amount of carbon fibers are observable. This suggests that the most active Bro¨nsted acid sites of K10 are chiefly responsible for the growth of graphitic flakelike structures, while carbon fibers form on clay’s iron sites, in agreement with previous findings reported for raw montmorillonite.16 3.2.2. Behavior of Iron-Loaded Catalysts. In order to investigate the effect of metal load, CCVD of isobutane was carried out over Fe/K10 and Fe/Na+-K10 reduced at 500 °C. As shown in Table 1, at lower metal load, Fe/K10500 catalyst leads to a limited increase of yield with respect to the parent support (∆YC ) +1.9 wt % going from sample K10500 to Fe5/ K10500). Instead, a further increase of wFe (sample Fe15/K10500) allows achieving a yield of 114 wt %. The addition of iron to Na+-K10 causes a strong enhancement of YC already at low load, while with further increasing wFe (sample Fe15/Na+K10500), the highest YC value (140 wt %) is obtained. The results of morphologic analyses on carbon deposits obtained over Fe/K10500 and Fe/Na+-K10500 catalysts are displayed in Figure 3. SEM and TEM analyses reveal that flakelike formations are prevailingly obtained over Fe5/K10500 together with a lower amount of MWCNT (Figure 3a and b). The selectivity toward the formation of MWCNT considerably

improves with increasing metal load (compare Figure 3c and d), demonstrating that the added iron sites are mainly responsible for their formation, in agreement with the literature data.15,16 The highest selectivity to MWCNT is obtained by CCVD over Fe/Na+-K10500 catalysts, regardless the metal content (Figure 3e-h). These findings suggest that Fe/K10500 acts as a bifunctional catalyst in the isobutane CCVD. The clay’s active sites are mainly responsible for the formation of flakelike nanostructures, whereas MWCNT form on added iron sites. The morphology of C deposits is, thus, the result of the competition between reactions taking place at the different growing-sites. When the reaction rate of clay-support is low, as in the case of Fe/Na+K10500, the i-C4H10 decomposition takes place preferentially at the loaded Fe sites, and higher selectivity toward MWCNT is always obtained, regardless the metal load. The same occurs at higher Fe load, since the activity of iron sites overcomes that of montmorillonite, as in the case of Fe15/ K10500 (Figure 3c). At lower metal loads, Fe-nanoparticles are not active enough with respect to the clay, and a loss of selectivity toward the MWCNT formation is observed, as in the case of Fe5/K10500 (Figure 3a). If the activity of iron is low, it may be further lowered owing to the partial covering (and consequent deactivation) of the metal sites by the graphite platelets faster growing on the clay’s acid sites. This phenomenon might account for the small increase of yield (∆YC ) +1.9 wt %) observed for sample Fe5/K10500, with respect to the active K10500 support. On the contrary, loading the same amount of iron on the less active Na+-K10500 produces a sizeably larger YC variation with respect to the unloaded support (∆YC ) +27.0 wt %).

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Figure 3. Morphology of carbonaceous products; as monitored by SEM (left side) and TEM (right side). The micrographs shown refer to reaction products obtained over catalysts Fe5/K10500 (a and b); Fe15/K10500 (c and d); Fe5/Na+-K10500 (e and f); and Fe15/Na+-K10500 (g and h), respectively.

As evidenced by TEM analysis, MWCNT formed over Fe5/ K10500 and Fe5/Na+-K10500 (Figures 3b and f) show comparable outer diameters (dMWCNT ) 10-25 nm) but different lengths. In particular, MWCNT grown over the less selective K10 based catalyst are much shorter than those prepared over Na+-K10 based catalyst, indicating that their formation is strongly hampered during the CCVD. Keeping in mind that MWCNT forms on added iron sites, the presence of carbon flakes in intimate contact with the very short MWCNT (Figure 3b) gives an evidence of the proposed iron site deactivation mechanism occurring on Fe5/K10500. Figure 3d and h demonstrates that, regardless the clay’s type (K10 or Na+-K10), very long (>1 µm) MWCNT form over the catalysts having the highest iron load. Moreover, they show similar outer diameters, ranging between 15 and 40 nm, owing to the comparable mean iron-particle size (Table 1). However, MWCNT formed over Fe15/Na+-K10500 are much straighter than those obtained over Fe15/K10500 hinting at the presence of a smaller amount of lattice defects.

3.2.3. Effect of Reduction Temperature. For Fe/K10 catalysts, the increase of the reduction temperature from 500 to 700 °C causes an improvement in selectivity toward MWCNT. As shown in Figure 4, a larger abundance of MWCNT is observed analyzing the reaction products obtained over both Fe5/K10700 (Figure 4a) and Fe15/K10700 (Figure 4b) with respect to the low temperature reduced samples (Figure 3a and c). In agreement with previous findings, the enhancement of selectivity, more marked at lower metal loads (compare Figures 3a and 4a), results from the suppression of the clay support activity, a condition upon which the decomposition of isobutane principally occurs at the most selective Fe sites. TEM analysis reveals that, in addition to MWCNT (Figure 4c), with outer diameters ranging between 10 and 20 nm carbon nanofibers (CNF) of bigger size (dCNF ) 50-100 nm) also form over Fe5/K10700 (Figure 4d). As the diameter of filamentous carbon mirrors the metal particle size, the growth of CNF is likely to occur on the larger iron nanoparticles, formed upon reduction at higher temperature. Also, the increase of iron load favors the agglomeration of iron nanoparticles. In fact, a further increase of the mean particle size is evidenced over Fe15/K10700 (Table 1), which provokes an enlargement of the outer tube diameters up to 40 nm (Figure 4e) and the formation of graphitic shells encapsulating metal particles with size exceeding 100 nm (Figure 4f). As shown in Table 1, the increase of the reduction temperature causes a strong decrease of YC moving from sample Fe15/ K10500 to Fe15/K10700. The loss of C yield upon increase of reduction temperature is mainly due to the average size enlargement undergone by iron nanoparticles upon catalyst treatment at higher temperature (Table 1), in agreement with previous findings on isobutane CCVD over Fe/Al2O3 catalysts.26 Contrarily, no substantial change in YC is found between catalysts Fe5/K10500 and Fe5/K10700, whose activity does not seem, at a first glance, to be influenced by the increase of the reduction temperature. The complex reaction pathway occurring on Fe5/K10500 catalyst might be responsible for the behavior observed for this sample. In all the probabilities, the deactivation of metal sites occurring in Fe5/K10500, owing to the partial covering by the graphite platelets faster growing on the clay’s acid sites, casually, leads to a YC value close to that found on the catalyst reduced at higher temperature. The comparison between YC values obtained by using catalysts Fe5/Na+-K10500 and Fe5/K10700 strongly supports this picture (Table 1). Once the contribution arising from the Bro¨nsted sites’ activity of the bifunctional catalyst is strongly reduced by thermal treatment at higher temperature (Fe5/K10700) or by Na+-substitution (Fe5/Na+-K10500), an evident diminishing of YC with the increase of the reduction temperature is found. 4. Crystalline Quality and Thermal Stability of MWCNT The average crystalline quality of MWCNT produced with the most selective catalysts (Fe15/K10500, Fe5 and Fe15/K10700, and Fe5 and Fe15/Na+-K10500) is evaluated by Raman scattering measurements monitoring the shape evolution of the main spectral features: the graphite-like in-plane optical mode at 1580 cm-1 (G-band); the band, at 1350 cm-1, originating from lattice defects (vacancies, pentagons, heptagons, ...) that break the basic graphene-layer symmetry (D-band)28,29 and its overtone at 2700 cm-1 (G′-band) that, conversely, is detected only in nanotubes


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Figure 4. Morphology of carbonaceous products; as monitored by SEM (upper) and TEM (lower). The micrographs shown refer to reaction products obtained over catalysts Fe5/K10700 (a, c, and d) and Fe15/K10700 (b, e, and f), respectively. Table 2. Crystalline Quality of MWCNT as Respectively Monitored by the D/G (ID/IG), G′/G (IG′/IG), and G′/D (IG′/ID) Integrated Intensity Ratio Derived from the Quantitative Analysis of Raman Spectra catalyst

ID/IG

IG′/IG

IG′/ID

Fe5/K10700 Fe15/K10500 Fe15/K10700 Fe5/Na+-K10500 Fe15/Na+-K10500

1.88 1.53 1.11 0.86 0.52

0.44 0.46 0.99 1.15 1.27

0.23 0.30 0.90 1.34 2.43

constituted by a sequence of smooth graphene sheets,30 i.e. in the presence of graphitic long-range order. After background subtraction, the spectral features are fitted to Lorentzian bands and the integrated-intensity ratios are calculated. The extent of structural defects is monitored by the D/G intensity ratio (ID/IG)14,31 that, for fixed excitation energy, increases with decreasing in-plane correlation length32,33 (i.e., mean interdefect distance32,34). Instead, the G′/G intensity ratio (IG′/IG) is generally regarded as an indicator of long-range order.30,31 Thus, the overall crystalline quality, which improves with increasing mean interdefect distance and/or unundulated tube-wall number (i.e., with increasing G/D and/or G′/G ratios, respectively), is pictorially described by the G′/D intensity ratio (IG′/ID).26,31 The results obtained are shown in Table 2. The spectral features of the investigated samples (spectra a-c in Figure 5) clearly show that in the MWCNT produced on Fe/K10 based catalysts the quality improves following the catalyst order Fe5/K10700 < Fe15/K10500 < Fe15/K10700, as demonstrated by the appreciably D-band weakening and the outstandingly G-band and G′-band intensifying. As can be seen in Table 2, the ID/IG ratio correspondingly decreases from 1.88 down to 1.11, while, on the contrary, IG′/ IG increases from 0.23 to 0.90. These changes signal that a significant reduction in the extent of lattice defects accompanies the establishment of long-range graphitic order. The crystalline quality of MWCNT grown over iron based Na+-K10500 catalysts results to be even better. As shown in

Figure 5. Shape evolution of Raman spectra of MWCNT obtained over catalysts Fe5/K10700 (a); Fe15/K10500 (b); Fe15/K10700 (c); Fe5/Na+-K10500 (d); and Fe15/Na+-K10500.

Figure 5 (spectra d and e), on MWCNT produced with Fe5/ Na+-K10500 the G-band further sharpens, the D-band is weaker, and the G′-band is more intense. IG′/ID increases by a factor of 3 /2, with respect to the best MWCNT sample prepared with Fe15/ K10700 (Table 2) while IG′/ID diminishes, reaching a value of 0.86. It is worth noting that on this sample, the extent of structural defects, as monitored by the ID/IG ratio is lower than 0.95 reported for MWCNT synthesized by Fe-catalyzed decomposition of acetylene on raw montmorillonite14 and, rather, closer to that obtained, under the same conditions, over laponite (0.82).14 As expected on the basis of evidence provided by electron microscopy analyses (Figure 3g and h), the results obtained over catalyst Fe15/Na+-K10500 are really outstanding. The crystalline quality of MWCNT further improves (IG′/ID nearly doubles,

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exchangeable Bro¨nsted acid sites (H+) introduced in the clay upon the precommercialization acid treatment. Iron-loaded K10 behaves as a bifunctional catalyst in the CCVD of i-C4H10. Hydrocarbon decomposition occurring on metal sites preferentially leads to the MWCNT formation while Bro¨nsted acid sites of the support are responsible for the formation of graphite flakes. Therefore, MWCNT formation is favored by the iron that has been deposited on the clay surface. It is now controlling the reaction outcome. For Fe/K10 catalysts reduced at 500 °C, this condition is achieved by increasing the iron load up to 14 wt %. The yield of carbon deposited is strongly enhanced with respect to the support and MWCNT preferentially form. On the catalysts with lower metal load (6 wt %), instead, the amount of carbon produced does not substantially differ from that obtained on the support (K10500) and also selectivity to MWCNT remains very poor. The low activity of low loaded catalyst is due to the deactivation of iron sites owing to the partial covering by the graphite platelets growing on the clay’s acid sites. At a given metal load, the increase of the reduction temperature up 700 °C or the use of Na+-exchanged K10 as support generally leads to an enhancement of the selectivity to MWCNT, because of the diminishing of the support active sites. However, with increasing reduction temperature, as the effect of the iron particle size increase, other carbon nanostructures, such as CNF and iron encapsulating graphite shells form and the yield to carbon deposited decreases. Instead, the use of Na+ exchanged K10 allows obtaining the most active and selective iron catalysts, under the present reaction conditions. MWCNT copiously form, both at low and high metal load, and exhibit the highest structural order. Figure 6. Thermal stability of MWCNT; as monitored by DTG curves. The curves shown refer to MWCNT prepared over (a) Fe5/Na+-K10500 (dash); Fe15/Na+-K10500 (solid); and (b) Fe5/K10500 (solid); Fe15/K10500 (dashed-dotted); Fe15/K10700 (dashed).

increasing from 1.34 up to 2.43), thanks to the development of quite smooth graphene sheets. The long-range graphitic order establishes on a larger scale (as reported in Table 2 IG′/IG undergoes a further increase). The crystalline perfection of these MWCNT exceeds that of MWCNT reported as the best crystallized ever obtained by CCVD over clay-based catalysts (ID/IG is only 0.52 against 0.74-0.9914). The direct comparison with the D/G intensity ratio reported for MWCNT synthesized by the use of iron as catalyst and raw montmorillonite as clay support14 (0.52 against 0.95) is particularly satisfactory. The results attained by TG analysis, displayed in Figure 6, confirm the outstanding crystalline quality of MWCNT prepared with Fe/Na+-K10 based catalysts. Indeed, a narrow single peak, in the temperature range 400-600 °C is obtained in the combustion of MWCNT produced on the above catalysts (Figure 6a). On the contrary, MWCNT produced on Fe/K10 catalysts, show a complex profile due to the overlap of more oxidation processes (Figure 6b). The combustion starts at lower temperature due to the presence of carbon nanostructures with lowered thermal stability, i.e. worse crystalline quality.35 5. Conclusions The results of the present study show that, upon reduction at 500 °C, K10 catalyzes isobutane decomposition. Few carbon fibers accompany the prevailing carbon flake formation. Upon Na+ exchange or by increasing the reduction temperature, the activity of the clay is strongly lowered. It has been argued the catalytic activity of K10500 substantially originates from the

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ReceiVed for reView November 18, 2009 ReVised manuscript receiVed February 24, 2010 Accepted February 24, 2010 IE9018275

K10 Montmorillonite Based Catalysts for the Growth of Multiwalled

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