Process Parameter Interaction Effects during Carbon Nanotube Synthesis inFluidized Beds

Chee Howe See,* Oscar M. Dunens, Kieran J. MacKenzie, and Andrew T. HarrisLaboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, UniVersity ofSydney, Australia, NSW 2006

The interaction effects between temperature, catalyst properties, fluidization conditions, and deposition timeduring carbon nanotube (CNT) synthesis by chemical vapor deposition in a fluidized bed were investigated.While numerous investigations have attempted to correlate process parameters with CNT characteristics,selectivity and yield, the interaction between process parameters is often ignored. Parametric interactions inthis process have been investigated using a factorial design methodology. Besides the main effects of synthesistemperature, deposition time, and catalyst type, the interaction parameters temperature-time andtemperature-catalyst were found to significantly influence the resultant carbon and CNT yields. These resultslay the foundation for a detailed parametric analysis toward the optimization of CNT synthesis in fluidizedbeds, which takes into account these interaction effects.

1. Introduction

Carbon nanotubes (CNTs) have been investigated for amultitude of applications including energy storage,1 fieldemission devices,2 and composite material strengthening,3because of their unique mechanical, optical, and electricalproperties.4 Both multiwalled (MWCNTs) and single-walled(SWCNTs) carbon nanotubes can be used, depending on thevalue-added functionality required in the product. Nevertheless,the use of CNTs in both research and end-use applications iscurrently inhibited by production throughput.5

Of the three dominant techniques used for CNT synthesis,that is, laser ablation, arc discharge, and chemical vapordeposition (CVD), the latter is recognized as having the mostpotential for large-scale, economically viable CNT production.6,7In particular, we concur with De Jong and Geus8 that the mostlikely low cost scenario for CNT production is to conduct theCVD reaction in a fluidized bed reactor. The most prominentadvantages of fluidized bed CVD (FBCVD) compared totraditional fixed-bed CVD are the enhanced heat and masstransfer, continuous operation, and scalability; characteristicswhich lead to lower costs of production.9 Despite its potential,we highlighted in a recent review, the dearth of researchinvestigating the (FBCVD) technique for large-scale CNTcapability.10

When the results of several studies are analyzed collec-tively,7,9-20 there appears to be no clear correlation betweenthe synthesis parameters and CNT characteristics, yield, andselectivity. Taken to an extreme, this suggests that the synthesisof CNTs in a fluidized bed is a noncontrollable process. This isalmost certainly incorrect, runs counterintuitive to the behaviorof similar fluidized bed processes, and at the very least, requiresverification. Hence, in this work, we have undertaken anexperimental study to elucidate the influential parameters;employing a statistical experimental design methodology toinvestigate the influence of (i) synthesis temperature, (ii)deposition time, (iii) catalyst type (Fe, Fe-Co), (iv) catalystloading (2.5, 5 wt %), and (v) total gas flow rate on carbonyield in a 0.5 kg/h FBCVD process.

Although fractional factorial design (FFD) has been employedin CNT research previously,21-23 we report for the first time,the use of this methodology for CNT synthesis via FBCVD.The advantage of using FFD is that statistically meaningfulinsights can be obtained using a reduced experimental set.Furthermore, besides the main effects, interaction effects(between different process parameters) can be analyzed simul-taneously. In addition, the FFD design tool provides a simplevalidity test of the optimal parametric envelope during reactorscale-up, again using minimal experiments. We note that, toour knowledge, this is the first time that process interactioneffects have been investigated for CNT synthesis via FBCVD,although this information may well exist in a commercialenvironment. The results from this study verify the degree ofinfluence of process parameters and their interactions on thedesired output criteria, laying the foundation for future detailedparametric analyses toward the optimization of CNT synthesisvia FBCVD.

2. Experimental Details

A typical apparatus setup is shown in Figure 1. The 52 mminternal diameter, 1000 mm long cylindrical fluidized bed,constructed entirely of Inconel 601 and enclosed within a hightemperature furnace, was operated as a batch reactor in thisstudy. It has an approximate capacity of 0.5 kg CNT/h. Whilethis diameter is less than the required minimum for manyclassical fluidized bed scale-up rules-of-thumb, rig commis-sioning fluidization tests show that well-developed fluidizationoccurs, and the diameter is sufficiently large to negate thesubstantial wall effects of very small reactors (e.g., 6.4 mm)16without requiring impracticably large catalyst volumes. Anexpansion unit, 100 mm in diameter and 500 mm long, wasaffixed to the top of the reactor to minimize particle entrainment.Particle scrubbers were incorporated to treat effluent gases priorto release. Gas flow to the reactor was controlled via a seriesof Alicat Scientific, Series 16 mass flow controllers.

Catalysts were prepared using impregnation.24 In brief, aweighed amount of sieved calcined alumina (-90 m, +106m) substrate was added to an ethanolic solution of Fe orFe-Co salts in the appropriate proportions to result in eitherFe or Fe-Co (1:1) catalyst, with a total metal loading of either

* To whom correspondence should be addressed. E-mail: c.see@usyd.edu.au. Tel.: +61-2-9036-6244. Fax: +61-2-9351-2854.

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2.5 or 5 wt %. These metal loadings are representative of ratiosreported in the literature.15,25 The mixture was air-driedovernight at 40 C prior to calcination in air for 12 h at 900 C.

Approximately 80 ( 5 g of calcined catalyst was used ineach experiment, corresponding to an initial bed height of 38( 2 mm. This was reduced in situ at 700 C in 1.5 SLPM,30% H2/N2 for 1 h before ethylene (the carbon source) wasintroduced. The synthesis of CNTs was carried out accordingto the conditions depicted in the full factorial design given inTable 1, investigating the effect of (i) synthesis temperature (600or 800 C), (ii) deposition time (20 or 60 min), (iii) catalysttype (Fe or Fe-Co), (iv) catalyst loading (2.5 or 5 wt %), and(v) total gas flow rate, reported as the ratio of the gas velocitythrough the bed to the minimum fluidization velocity at 800 Cunder N2 (U/Umf ) 3 or U/Umf ) 6). The output variables werethe total carbon yield, defined as TGA weight loss occurring inthe temperature range 250-800 C, and carbon nanotube yieldas defined in section 3.

A total of 32 runs were conducted in randomized order. Theas-synthesized products were analyzed using thermogravimetricanalysis (TGA; TA Instruments SDT Q600), transmissionelectron microscopy (TEM; Philips CM120, 120 kV) and Ramanspectroscopy. For TGA analysis, a sample weight of 60 mgwas used. The large sample size was chosen to negate the effectsof instrument error across the board, because in some instancesa weight loss of

excitation at 488 nm, 0.12 mW, for an exposure time of 20 sand 10 accumulations. An average of five different points persample was used.

3. Results and Discussion

In this work, we have investigated the effects of fiveinfluential parameters (synthesis temperature, catalyst type andloading, fluidization conditions, and deposition time), and theirinteractions, on the resulting carbon yield. To differentiatecarbon yield from CNT yield using TGA, we have used themethod first described by See and Harris.26 This involvesdeconvoluting the TGA weight loss profile into three generalgroupings: (i) amorphous carbon, (ii) CNT and carbon fibers,and (iii) graphitic particles. The temperature ranges of thesegroups have been subjectively determined on the basis ofextensive TEM imaging and Raman spectroscopy of severalcommercial samples and subsequently utilized as a quantitativemeasure of each constituent. Rather than addressing the quality ofproduct solely on the basis of the maximum burnoff temperature,which is highly dependent on the heating rate and impurities present(e.g., residual catalyst metals), this technique attempts to quantifythe amount of the constituent components contributing to the overallweight loss profile. This is best illustrated by the first orderderivative of the weight loss profile, shown in Figure 2. TEMimages of CNT samples corresponding to the first order derivativesin Figure 2 are illustrated in Figure 3.

The first order derivative shown in Figure 2b, obtained usinga commercial MWCNT sample (courtesy of Nanolabs; 95%purity) was used as a model MWCNT sample. The keyfeatures of the derivative function for a well-graphitizedMWCNT sample (Figure 3b) are (i) the relatively narrow peakwidth, (ii) high maximum burnoff temperature and (iii) onset

temperature. The shape of the curve (rate of change) is similarto a process function undergoing a forcing function (constantheating rate). A width at half-peak of 54 C was obtained forthis sample.

A broadening of the width at half-peak to 82 C in Figure 2asuggests that a higher ratio of amorphous carbon and graphiticparticles are present. The lower maximum burnoff temperatureand pronounced skew toward lower temperatures was mostlikely due to a higher concentration of amorphous carbon and/or poorly graphitized CNTs; this cannot be differentiated usingthis technique. The even larger width at half-peak of 125 C inFigure 2c suggests that a broad range of products is present,and that the CNT/CNF component is rather low. The maximumburnoff temperature of 538 C compared to the pure CNTsample of 602 C is indicative of the higher amorphous contentin the sample. While we acknowledge that the oxidization range420-620 C is a quantitative measure of both CNTs and CNFs,we refer to the weight loss in subsequent discussion as CNTs.The reason for this simplistic categorization is analysis of TEMimages shows the majority of carbon product comprises CNTs.

While we concede that the error is potentially large usingthis approach, the results are inherently more accurate than thosestudies based solely on TEM or Raman spectroscopy toquantify the yield of these components. It is generally acceptedthat TGA is the only tool capable of quantitatively measuringthe yield, thermal stability, and homogeneity of a productsample.27-29 Because of the large TGA sample size requiredas previously noted, heat and mass transfer effects are intro-duced. Further optimization work, where CNT yields aresubstantially higher, allow the sample mass to be greatly reducedand hence this characterization protocol will provide far greaterresolution. Nonetheless, more work is necessary to tune the

Figure 2. First order derivatives of the weight loss profile measured using TGA, showing the definition of the amorphous carbon, CNTs/CNFs, and graphiticparticle regions. 2(a) and (c) are samples synthesized via FBCVD, whereas (b) is a commercial sample courtesy of Nanolabs, USA. A variety of curves areobtained depending on the ratio of individual components present within the sample.

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TGA results with qualitative tools, especially in the regiondefined between 420 and 620 C, in order to determine thequality of CNTs formed.

Raman spectroscopy was conducted on samples exhibiting aCNT-like TGA first order derivative to determine the degreeof graphitization. This is measured as the ratio between thegraphite (G band, 1590 cm-1) and amorphous (D band, 1370cm-1) Raman shifts. Typical Raman shifts of samples exhibitingTGA weight loss behavior to Figure 2a,b are correspondinglyshown in Figure 4 a,b.

The G/D ratio of 1.1 obtained is comparable to reportedvalues.30,31 However, Raman spectroscopy was carried out onlyon samples with CNTs detected using TEM and TGA becauseRaman spectroscopy does not differentiate between the graphiticstructures of CNTs, fibers, and graphitic particles. Consequently,

Figure 3. Panels a and c are representative TEM images of CNTs synthesized via FBCVD in this work, whereas panel b is a commercial sample courtesyof Nanolabs, USA. A higher degree of graphitization is observed in panel b than in panel a, which accounts for the higher maximum burnoff temperature,in the absence of graphitic particles in significant amounts. (c) Samples containing ill-defined nanotubes (amorphous-like) and large diameter fibers exhibitlower maximum burnoff temperatures with large-peak widths.

Figure 4. Raman shift of (left) CNT samples synthesized via FBCVD and(right) commercial sample synthesized via CVD. The lower G/D ratio of1.11 obtained in the left panel compared to 1.67 in the right panel suggestthat the samples synthesized via FBCVD are less graphitized than thecommercial sample.

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high G/D ratios could possibly reflect the quantity of graphiticparticles present and not the quality of CNTs. Likewise, a highD band may be due to the presence of a large quantity ofamorphous carbon.

3.1. Design Methodology. A full factorial design wasemployed in this work to verify the influence of (i) synthesistemperature, (ii) deposition time, (iii) catalyst type (Fe, Fe-Co),(iv) catalyst loading (2.5, 5wt %), and (v) total gas flow rate,on carbon and CNT yield (TGA weight loss from 420 to 620C was attributed as CNT yield). The factorial design was alsoused to examine the effect of higher order process interactions.We found that third- and fourth-order interaction effects werestatistically insignificant, agreeing with most reported chemicalprocesses.21 However, second-order interactions were notnegligible and required further investigation.

We stress the importance of second-order interaction effectsbecause, in the vast majority of the literature, includingparametric studies of CNT synthesis, for example, Venegoni etal.7 and Corrias et al.,15 researchers employ the change onefactor at a time approach. However, not enough data isprovided to analyze the effect of interactions because of theunderlying assumption that the single variable at some initialfixed condition is optimized. While this could have beensolved iteratively as a multimodal optimization process, in thecase of the studies above, the authors do not explicitly statewhy their starting point was optimal. Hence, the reportedrelationships between process variables and their optimizedparameter are not generally valid unless parameter interactionsare not dominant.

Although the factorial design is a useful tool, it is importantto account for possible discontinuities in the system under study.In this work, it is clear that the different catalyst types may

cause discontinuities, for example, there may be an inactivecatalyst. Hence, it is more suitable to consider the catalysts ascategorical variables. Consequently, the system should beconsidered as either a (i) 2 2v5-1 design based on Fe orFe-Co catalysts or a (ii) 4 23 design by combining catalystloading and type, rather than as a full 25 factorial design. Wedecided to use the latter for analysis.

3.2. Influence of Process Variables on Carbon andCNT Yield. On the basis of the 4 23 design, we havecombined the catalyst type and loading as a new parameter foranalysis, which we simply call catalyst in the followingdiscussion. The results summarized in Figure 5 suggest that allfive processes parameters investigated in this work influenceboth the carbon and CNT yields significantly. In Figure 5, thecarbon and CNT yields have been normalized per unit weightof metal catalyst. This was done to prevent skewing the resultsin favor of higher metal loadings in the event that catalystloading was limiting carbon deposition under the same conditions.

Temperature has been reported to influence the formationof CNTs greatly, although the effect of temperature on CNTdiameter, quality, and yield remains unclear.10 On the basis ofreports utilizing fixed-bed reactors, it appears that the cause ofsuch discrepancies lies in the surface area available forheterogeneous reactions to take place. Zeng et al.32 showed thatunder the same conditions, the carbon yield was increasedsimply by spreading the same amount of catalyst over a largerarea, that is, using two quartz boats instead of one. This isstrongly indicative of heat and mass transfer limitations. In ourwork, we observed a positive gradient for both carbon and CNTyield with an increase in temperature, as a function of fluidiza-tion ratio and deposition time for all catalysts. However, theslope of carbon yield increases more rapidly than that for CNT

Figure 5. The gradient of ascent obtained for the four catalysts, Fe (2.5% and 5%) and Fe-Co (2.5% and 5%) weight loading on alumina, as a function offluidization ratio and deposition time, with increasing temperature: (---) CNT yield; (s) carbon yield.

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yield, across all experiments, with an increase in temperature.This suggests that the selectivity toward CNTs decreases withincreasing temperature. Nonetheless, we note that higher tem-peratures have been reported to produce better graphitizedCNTs.26 Hence, it is necessary to consider the tradeoff betweenyield, quality, and selectivity to CNTs.

Intuitively, one would expect that the quantity of carbondeposited would increase with deposition time. If it could beassumed that there is no lag time for CNT growth as reportedby Morancais et al.,31 we would expect that when the depositiontime is lengthened by three times, the carbon yield would alsoincrease ca. three times, in the absence of inhibiting factors,for example, catalyst deactivation. However, there does notappear to be a correlation across the board between the gain incarbon yield with deposition time; gains from 1 (Fe 2.5%, U )3Umf, 600 C) to 13 (Fe 2.5%, U ) 6Umf, 800 C) were observedwith an increase in deposition time, which suggests that a lagprobably does exist.

If the rate of carbon deposition was a function of the activatedstate of the catalysts, then the rate of carbon deposition shoulddecrease over time because of catalyst deactivation. Thus, thegain of carbon deposited should decrease over time. However,in a few instances, a gain much greater than the increase indeposition time was obtained. Therefore, it is probable thatcarbon deposition slows during the first 20 min; perhaps a lagtime exists, or the rate of carbon gasification is quicker.Nonetheless, a more pronounced increase in carbon yield isobserved for experiments conducted at the higher fluidizationratio than at the lower ratio with increasing deposition time.This seems to suggest that the higher gas flow rate induced bettermixing, leading to greater heat and mass transfer between solidand gas particles. This is true of most fluidized bed processesoperating in the bubbling fluidization regime.33

For a deposition time of 60 min, we observed an increase incarbon yield as the fluidization ratio was doubled. Although adoubling in the carbon yield was expected since the total carbonfeed was also doubled, we observed an increase of less than 2times. In contrast, a general decrease in carbon yield for acatalyst loading of 2.5%, for a deposition time of 20 min, wasobserved. This counterintuitive observation can also be attributedto the uncertainty as to whether or not there is a lag time beforeCNT synthesis begins.

An analysis of variance (ANOVA) was conducted at a 95%confidence interval to verify the statistical significance of processparameters using both carbon and CNT yields as the parametersfor optimization. It was found that there was sufficient data forthe analysis to be statistically valid in both response variables (i.e.,model P < 0.05) and the main effects; that is, (i) synthesistemperature, (ii) run duration, and (iii) catalyst, and second-ordereffects, (iv) temperature-time and (v) temperature-catalyst, weresignificant. The results of the ANOVA are given in Table 2.

The insignificance of fluidization ratio in the experimentalruns suggests that the experiments were likely conducted in thesame fluidization regime. Therefore, the heat and mass transfercoefficients are somewhat comparable for both flow ratesinvestigated. Nonetheless, the actual fluidization regime at whichthe experiments were undertaken is uncertain. This can beattributed to changes in (i) gas density and viscosity of thereactive gas mixture, (ii) rate of reactions, and (iii) bed stability.We have observed in further experiments (See, C. H.; Harris,A. T. Large-scale production of multiwalled carbon nanotubesby fluidized bed chemical vapor deposition: Parametric inves-tigation of interaction pairs, In preparation) that while the flowrates employed were at the respective fluidization ratios under

nitrogen, this is not the case during the CNT deposition reaction.A step change in carbon yield was observed at about U/Umf) 15, which we believe is the actual start of the bubblingregime under typical reaction conditions in our setup. Hence,the flow rate that was employed is likely to be bordering onthe intermediate fluidization regime, that is, close to Umf.

The catalyst type and their interaction with temperature wasfound to have a significant effect on CNT yield only, signifyingthat the selectivity of CNTs over other carbon products is highlydependent on the catalyst used. This is why numerous catalystshave been investigated to date.34 In our work, it appeared thata higher carbon yield per unit weight of metal was obtained for2.5 wt % loadings than for 5 wt % loadings, regardless ofcatalyst type. However, the binary Fe-Co catalysts seem to bemore selective toward CNTs than the singular Fe catalyst.

4. Conclusions

We have investigated for the first time, the effect ofparametric interactions on carbon and CNT yield for CNTsynthesis via FBCVD. This was achieved using a full factorialdesign investigating the effects of (i) synthesis temperature, (ii)deposition time, (iii) fluidization ratio, and (iv) catalyst. Besidesthe main effects of synthesis temperature, deposition time, andcatalyst, the interactions between temperature-time andtemperature-catalyst were found to influence both CNT andcarbon yield. The effects of interaction parameters have not beenadequately studied in the literature to date, even duringparametric studies of CNT synthesis. The work reported hereforms the basis for further in-depth parametric analytical studiesinvestigating both main effects and two-factor interactions.

Acknowledgment

C.H.S. gratefully acknowledges the financial support of theCommonwealth of Australia and the University of Sydney forproviding the Endeavour International Postgraduate ResearchScholarship and International Postgraduate Award respectively.The authors are grateful to Nanolabs (USA) for providing CNTsamples for comparative analysis.

Literature Cited(1) Sankaran, M.; Viswanathan, B. The role of heteroatoms in carbon

nanotubes for hydrogen storage. Carbon 2006, 44, 28162821.(2) Maeda, Y.; Sato, Y.; Kako, M.; Wakahara, T.; Akasaka, T.; Lu, J.;

Nagase, S.; Kobori, Y.; Hasegawa, T.; Motomiya, K.; Tohji, K.; Kasuya,A.; Wang, D.; Yu, D.; Gao, Z.; Han, R.; Ye, H. Preparation of single-

Table 2. ANOVA for Selected Factorial Model Analysis of VarianceTable [Classical Sum of Squares, Type II]a

response 1: carbon yield response 2: CNT yieldF p-value F p-value

source value prob > F value prob > F

model 6.3

walled carbon nanotube-organosilicon hybrids and their enhanced fieldemission properties. Chem. Mater. 2006, 18, 42054208.

(3) Coleman, J. N.; Khan, U.; Blau, W. J.; Gunko, Y. K. Small butstrong: A review of the mechanical properties of carbon nanotube-polymercomposites. Carbon 2006, 44, 16241652.

(4) Dresselhaus, M. S.; Dresselhaus, G., Carbon nanotubes: Synthesis,Structure, Properties and Applications; Springer-Verlag: Berlin, 2001.

(5) Schrand, A. M.; Tolle, T. B., Carbon nanotube and epoxy compositesfor military applications. In Carbon Nanotechnology: Recent DeVelopmentsin Chemistry, Physics, Materials Science and DeVice Applications; Dai,L., Ed.; Elsevier B. V.: Amsterdam, The Netherlands, 2006; Vol. 1, pp633-675.

(6) Journet, C.; Bernier, P. Production of carbon nanotubes. Appl. Phys.,A 1998, 67, 19.

(7) Venegoni, D.; Serp, P.; Feurer, R.; Kihn, Y.; Vahlas, C.; Kalck, P.Parametric study for the growth of carbon nanotubes by catalytic chemicalvapor deposition in a fluidized bed reactor. Carbon 2002, 40, 17991807.

(8) de Jong, K. P.; Geus, J. W. Carbon nanofibers: Catalytic synthesisand applications. Catal. ReV. 2000, 42, 481510.

(9) Wang, Y.; Wei, F.; Luo, G.; Yu, H.; Gu, G. The large-scaleproduction of carbon nanotubes in a nano-agglomerate fluidized-bed reactor.Chem. Phys. Lett. 2002, 364, 568572.

(10) See, C. H.; Harris, A. T. A review of carbon nanotube synthesisvia fluidized-bed chemical vapor deposition. Ind. Eng. Chem. Res. 2007,46, 9971012.

(11) Weidenkaff, A.; Ebbinghaus, S. G.; Mauron, P.; Reller, A.; Zhang,Y.; Zuttel, A. Metal nanoparticles for the production of carbon nanotubecomposite materials by decomposition of different carbon sources. Mater.Sci. Eng., C 2002, 19, 119123.

(12) Qian, W.; Wei, F.; Wang, Z.; Liu, T.; Yu, H.; Luo, G.; Xiang, L.Production of carbon nanotubes in a packed bed and a fluidized bed. AIChEJ. 2003, 49, 619625.

(13) Li, Y. L.; Kinloch, I. A.; Shaffer, M. S. P.; Geng, J.; Johnson, B.;Windle, A. H. Synthesis of single-walled carbon nanotubes by a fluidized-bed method. Chem. Phys. Lett. 2004, 384, 98102.

(14) Yu, H.; Zhang, Q.; Wei, F.; Qian, W.; Luo, G. Agglomerated CNTssynthesized in a fluidized bed reactor: Agglomerate structure and formationmechanism. Carbon 2003, 41, 28552863.

(15) Corrias, M.; Caussat, B.; Ayral, A.; Durand, J.; Kihn, Y.; Kalck,P.; Serp, P. Carbon nanotubes produced by fluidized bed catalytic CVD:First approach of the process. Chem. Eng. Sci. 2003, 58, 44754482.

(16) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Lucas, A. A.Fe-catalyzed carbon nanotube formation. Carbon 1996, 34, 12491257.

(17) Qian, W.; Liu, T.; Wei, F.; Wang, Z.; Li, Y. Enhanced productionof carbon nanotubes: Combination of catalyst reduction and methanedecomposition. Appl. Catal., A 2004, 258, 121124.

(18) Perez-Cabero, M.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Char-acterization of carbon nanotubes and carbon nanofibers prepared by catalyticdecomposition of acetylene in a fluidized bed reactor. J. Catal. 2003, 215,305316.

(19) Mauron, P.; Emmenegger, C.; Sudan, P.; Wenger, P.; Rentsch, S.;Zuttel, A. Fluidised-bed CVD synthesis of carbon nanotubes on Fe2O3/MgO. Diamond Relat. Mater. 2003, 12, 780785.

(20) Liu, B. C.; Gao, L. Z.; Liang, Q.; Tang, S. H.; Qu, M. Z.; Yu,Z. L. A study on carbon nanotubes prepared from catalytic decompositionof C2H2 or CH4 over the pre-reduced LaCoO3 perovskite precursor. Catal.Lett. 2001, 71, 225228.

(21) Montgomery, D. C., Design and Analysis of Experiments, 5th ed.;John Wiley and Sons Inc.: New York, 2001.

(22) Kukovecz, A.; Mehn, D.; Nemes-Nagy, E.; Szabo, R.; Kiricsi, I.Optimization of CCVD synthesis conditions for single-wall carbon nano-tubes by statistical design of experiments (doe). Carbon 2005, 43, 28422849.

(23) Yang, Y.; York, J. D.; Xu, J.; Lim, S.; Chen, Y.; Haller, G. L.Statistical design of c10-Co-mcm-41 catalytic template for synthesizingsmaller-diameter single-wall carbon nanotubes. Microporous MesoporousMater. 2005, 86, 303313.

(24) Schwarz, J. A.; Contescu, C.; Contescu, A. Methods for preparationof catalytic materials. Chem. ReV. 1995, 95, 477510.

(25) Kathyayini, H.; Nagaraju, N.; Fonseca, A.; Nagy, J. B. Catalyticactivity of Fe, Co, and Fe/Co supported on Ca and Mg oxides, hydroxidesand carbonates in the synthesis of carbon nanotubes. J. Mol. Catal., A 2004,223, 129136.

(26) See, C. H.; Harris, A. T. On the development of fluidised bedchemical vapour deposition for large-scale carbon nanotube synthesis:Influence of synthesis temperature. Aust. J. Chem. 2007, 60 (7), 541546.

(27) Bom, D.; Andrews, R.; Jacques, D.; Anthony, J.; Chen, B.; Meier,M. S.; Selegue, J. P. Thermogravimetric analysis of the oxidation ofmultiwalled carbon nanotubes: Evidence for the role of defect sites in carbonnanotube chemistry. Nano Lett. 2002, 2, 615619.

(28) McKee, G. S. B.; Vecchio, K. S. Thermogravimetric analysis ofsynthesis variation effects on CVD generated multiwalled carbon nanotubes.J. Phys. Chem. B 2006, 110, 11791186.

(29) Arepalli, S.; Nikolaev, P.; Gorelik, O.; Hadjiev, V. G.; Holmes,W.; Files, B.; Yowell, L. Protocol for the characterization of single-wallcarbon nanotube material quality. Carbon 2004, 42, 17831791.

(30) Lamouroux, E.; Serp, P.; Kihn, Y.; Kalck, P. Identification of keyparameters for the selective growth of single or double wall carbonnanotubes on femo/Al2O3 CVD catalysts. Appl. Catal., A 2007, 323, 162173.

(31) Morancais, A.; Caussat, B.; Kihn, Y.; Kalck, P.; Plee, D.; Gaillard,P.; Bernard, D.; Serp, P. A parametric study of the large scale productionof multi-walled carbon nanotubes by fluidized bed catalytic chemical vapordeposition. Carbon 2007, 45, 624635.

(32) Zeng, X.; Sun, X.; Cheng, G.; Yan, X.; Xu, X. Production of multi-wall carbon nanotubes on a large scale. Phys. B 2002, 323, 330332.

(33) Rhodes, M., Introduction to Particle Technology; John Wiley andSons: Chichester, U.K., 1998.

(34) Jodin, L.; Dupuis, A. C.; Rouviere, E.; Reiss, P. Influence of thecatalyst type on the growth of carbon nanotubes via methane chemical vapordeposition. J. Phys. Chem. B 2006, 110, 73287333.

ReceiVed for reView December 31, 2007ReVised manuscript receiVed March 26, 2008

Accepted August 12, 2008

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