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Carbon 45 (2007) 1645–1650

The effect of phase separation in Fe/Mg/Al/O catalystson the synthesis of DWCNTs from methane

Qiang Zhang, Weizhong Qian *, Qian Wen, Yi Liu, Dezheng Wang, Fei Wei

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University,

Beijing 100084, China

Received 26 January 2007; accepted 30 March 2007Available online 11 April 2007

Abstract

Double-walled carbon nanotubes (DWCNTs) were prepared by methane decomposition on Fe/Al/Mg/O catalysts with a fixed ironloading of 1.5%. Increasing Al/Mg ratio in the catalyst resulted in the formation of a new MgAl2O4 phase, which was characterized byXRD. The size of the MgO crystallites in the support was decreased, due to the phase separation, from 35 nm to 20 nm in the Al/Mgratio range of 0:1–4:1. At an Al/Mg ratio of 1:200, this effect prevented the sintering of iron on the MgO support and resulted in thesynthesis of high-purity DWCNTs in high-yield. Very high-Al/Mg ratio induced the formation of the MgAl2O4 phase, which becameanother catalyst support material. This had a negative effect on the synthesis of DWCNTs due to its acidity and hardness. Simulta-neously maintaining MgO as the dominant catalyst support and decreasing its particle size by the phase separation effect are importantfor good metal dispersion and, consequently, the yield and purity of DWCNTs.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Chemical vapor deposition (CVD) is one of the mostimportant methods to prepare double-walled carbon nano-tubes (DWCNTs), which have extraordinary electrical con-ductivity, thermal conductivity, mechanical properties, andmany potential applications. For CVD, many catalysts,e.g., Fe/MgO [1,2], Fe/Mo/MgO [3–7], Co/Mo/MgO por-ous catalysts [8–12], have been developed to decomposemethane to make DWCNTs. These give yields of 10%–200%. The important factors for a good catalyst are thecontrol of the loading of the active metal and maintaininga good metal dispersion on the catalyst support [1–3,5,9].Generally, methods to highly disperse the metal crystalliteson the support include tailoring the loading of the metal[3,6,12,19], protection using organic solvents during prepa-ration [13,14], using promoters (e.g., Mo) [15–17], and

0008-6223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2007.03.045

* Corresponding author. Fax: +86 10 62772051.E-mail addresses: qwz@flotu.org, qianwz@mail.tsinghua.edu.cn (W.

Qian).

using a stable and molecular-level uniform structure forthe support [2,18,19].

However, the temperature for DWCNT synthesis is nor-mally higher than 973 K where sintering of the metal crys-tallites and catalyst support are both serious due to theeffect of Ostwald ripening. Typically, the sintering and lossof the surface area of the catalyst support are the main rea-son for the sintering of the metals. It can be imagined thatwith a larger sized catalyst support, the sintering of themetal on it will be much worse. Thus, if the catalyst sup-port can be controlled to a relatively small size and the sup-port particles separated from one another, e.g. duringcatalyst preparation or the DWCNT synthesis process[7,20], sintering of the metal crystallites can be effectivelyinhibited, which is favorable for the growth of DWCNTswith high-selectivity. Furthermore, another advantage ofa small-sized catalyst support is that the growth of single-walled carbon nanotubes (SWCNTs) on it is relativelyfacile [7,21], which favors the synthesis of SWCNTs withhigh-purity and yield. Thus, a method that controls the sizeof catalyst support will be very useful.

MgAl2O4

MgO

1:200

1:50

1:5

1:2

4:1

Al /Mg

Inte

nsity

, a.u

.

1646 Q. Zhang et al. / Carbon 45 (2007) 1645–1650

In the present work, we report a method to decrease thesize of the catalyst support during the catalyst preparation,and its use in the synthesis of DWCNTs. The conceptualidea originates from the phase separation phenomenon inan alloy with the increase of the proportion of one compo-nent. In particular, an Fe/MgO catalyst was used as themodel catalyst and MgO was the dominant support. Onintroducing Al species during catalyst preparation and tai-loring of the Al/Mg ratio, the formation of a new MgAl2O4

phase in the MgO phase was observed. Due to the latticemismatch between the MgO and MgAl2O4 phases, the sizeof the MgO support was gradually decreased by a phaseseparation effect. Using the catalyst with the optimumAl/Mg ratio, the yield and purity of DWCNTs can be dra-matically increased. In addition, the complex relationshipbetween the catalyst (with different Al/Mg ratios and acid-ity) and the different purities and yields of the as-growncarbon nanotubes is also reported. These findings providenew concepts for the controlled synthesis of DWCNTs ata potentially low-cost.

20 40 60 80

0:1

2-theta

Fig. 1. XRD patterns of Fe/Mg/Al/O catalysts.

30

35

40

talli

ne /n

m

2. Experimental

The Fe/Mg/Al/O catalyst was prepared by co-precipitation. A solutionof Fe(NO3)3 Æ 9H2O, Mg(NO3)2 Æ 6H2O and Al(NO3)3 Æ 6H2O was precip-itated from the base solution containing (NH4)2CO3 at 293–353 K. Afterfiltration, drying at 383 K for 20 h, and calcination at 673 K for 5 h, thecatalysts were obtained as faintly yellow powders. Further details of thepreparation of the catalyst were similar to that in Qian et al. [22].

To make CNTs, 100 mg catalyst was put into a quartz boat inserted inthe center of a quartz tube reactor (id: 25 mm, length: 1200 mm). The reac-tor and the catalyst were heated to 873 K in Ar (flow rate of 600 ml/min),and H2 (flow rate of 50 ml/min) was then introduced into the reactor toreduce the catalyst for 30 min. After the catalyst reduction, the reactorwas heated to 1273 K and a mixture of CH4 and H2 (100/50 ml/min,v/v) was introduced into the reactor. CH4 was decomposed on the catalystto synthesize CNTs. This was continued for 60 min. Finally the reactorwas cooled to ambient conditions in Ar.

The catalyst was characterized by powder X-ray diffraction (XRD,O8DISCOVER diffractometer, nickel-filtered Cu Ka radiation, datarecorded between 10� and 90�). The crystallite size of the catalyst was cal-culated using the Scherrer equation.

The morphology of the as-grown CNTs was characterized by high-res-olution scanning electron microscopy (HRSEM, JSM 7401F, at 5.0 kV)and high-resolution transmission electron microscopy (HRTEM, JEM2010, at 200.0 kV). Scattering Raman characterization of the CNTs wasobtained using a Raman Microscope (Renishaw, RM2000, He–Ne laserexcitation line at 633.0 nm) at ambient conditions. TGA (TGA-2050)was used to characterize the purity and quality of the DWCNTs.

10-5 10-4 10-3 10-2 10-1 100

20

25

MgO

cry

s

Al/Mg ratio

Fig. 2. Change of MgO crystalline size with the Al/Mg ratio in thecatalyst.

3. Results and discussion

Fig. 1 presented the XRD data for catalysts with differ-ent Al/Mg ratios before reduction. The catalyst withoutany Al species only had the peaks of the MgO crystal.The iron species at the low-loading (1.5%) was in an oxi-dized state and mainly formed an alloy with MgO, and dis-persed uniformly in the support phase [23,24]. The size ofthe iron oxide crystallites was smaller than 3 nm, whichwas beyond the detection limit of XRD. However, on add-ing Al species into the Fe/MgO catalyst by the co-precipita-

tion method, the existence of the MgAl2O4 phase wasclearly indicated in the XRD pattern when the Al ratiowas higher than 20%. There was no obvious MgAl2O4

phase in the XRD pattern when the Al/Mg ratio was inthe range of 1:50–1:200. Calculation of the size of theMgO crystallites indicated that the size was monotonouslydecreased with increasing Al/Mg ratio (Fig. 2). Specifically,the size of the MgO crystallites could be decreased from35 nm to 20 nm by changing the Al/Mg ratio to 4:1. Since

Fig. 3. SEM images of carbon nanotubes on the Fe/Mg/Al/O catalysts.(Fe mole fraction was fixed at 0.015, (a) Al/Mg = 0:1; (b) Al/Mg = 1:200;(c) Al/Mg = 1:50; (d) Al/Mg = 1:5; (e) Al/Mg = 1:2 and (f) Al/Mg = 4:1).

Q. Zhang et al. / Carbon 45 (2007) 1645–1650 1647

the composite Al/Mg/O alloy did not change its size, weproposed here that MgAl2O4 phase, though in smallamount, could be formed when the Al/Mg ratio was assmall as 1:200. The results confirmed that a phase separa-tion of the MgAl2O4 phase and MgO phase occurred inthe entire range of Al/Mg ratio in the present work and con-firmed our hypothesis that this effect is effective in decreas-ing the size of the catalyst support. It was also clear thatwhen the Al/Mg ratio of the catalyst support was small,the MgO support was the dominant support, and whenthe Al/Mg ratio was higher than 1:4, the MgAl2O4 phaseand MgO phase were both the dominant catalyst supports.

The SEM images in Fig. 3 showed the morphology of thecatalyst and as-grown CNTs on them. It was clear that thecatalysts with low-Al/Mg ratios were sheet-like products(Fig. 3a and b). The catalysts with larger Al/Mg ratios inFig. 3e and f were randomly distributed particles. Theseresults also suggested that the size of the crystallites in thecatalyst support was effectively decreased and these parti-cles were kept separated from each other. Meanwhile, itwas observed that CNTs in the form of relatively straightbundles were grown in large-amounts on the catalyst parti-cles. In these images, no obvious multi-walled carbon nano-tubes (MWCNTs) with large-diameters were observed,indicating the effective dispersion of the iron nanoparticleson the catalyst supports.

HRTEM images (Fig. 4) showed that the products con-tained DWCNTs for all the catalysts used. The diameter ofan individual tube was mainly 1.7–3.0 nm. The diameter ofthe DWCNT bundle was about 10–30 nm. For the imagesin Fig. 4, we were unable to see any amorphous carbon thatmay adhere to the outer wall of the DWCNTs, which indi-cated the high-purity of the DWCNT products. It can alsobe deduced that if there existed amorphous carbon in theproducts, its formation will be at different active sites fromthose for the DWCNT growth. When the Al/Mg ratio waslower than 1:50, the outer diameters of most DWCNTswere near 2.0 nm. Increasing the Al ratio to higher than1:5, the outer diameter of DWCNT was increased a littleand the defects on these were also increased. As indicatedby the XRD, the MgAl2O4 phase was formed and couldbe detected, which meant it had crystallites that reacheda certain size and also was a catalyst carrier besides theMgO. Some active metal catalyst particles were presenton the surface and had different sizes, which were sitesfor large-diameter CNTs or amorphous carbon due tothe surface acidity [8]. Both the separation effect and thecomposition of the carrier affected the DWCNTs growthbehavior.

The Raman spectra in Fig. 5 provided a macroscopicanalysis of the as-grown CNTs synthesized on theFe/Mg/Al/O catalysts. There were RBM (radial breathingmode) peaks for all carbon products indicated that they allcontained SWCNTs or DWCNTs. The very strong inten-sity of RBM peaks, for DWCNTs grown on the catalystwithout Al and with the Al/Mg ratio of 1:200, indicatedthe strong excitation of DWCNTs of certain diameters by

the laser, as compared with those of DWCNTs grown onother catalysts. Furthermore, we compared the intensityratio of the D band to G band (ID/IG value) of the differentsamples (Fig. 6). It was clear that the ID/IG value wassmallest for the DWCNTs grown on the catalyst with anAl/Mg ratio of 1:200, indicating the low-content of amor-phous carbon and defects in this sample. However, whenthe Al/Mg ratio was increased, the ID/IG value alsoincreased. Especially, when the Al/Mg ratio of the catalystwas 4:1, the ID/IG value of the as-grown carbon productwas 0.8, a value that is close to that of MWCNTs [25].

Fig. 4. TEM images of carbon nanotubes synthesized on the Fe/Mg/Al/O catalysts (iron loading is 1.5 mol.%, (a) Al/Mg = 0:1; (b) Al/Mg = 1:200;(c) Al/Mg = 1:50; (d) Al/Mg = 1:5; (e) Al/Mg = 1:2 and (f) Al/Mg = 4:1).

0 500 1000 1500 2000

4:1

1:2

1:5

1:50

1:200

0:1Al/Mg

Inte

nsity

(a.u

.)

Raman shift (cm-1)

Fig. 5. Raman spectra of CNTs synthesized on the Fe/Mg/Al/O catalysts(iron loading is 1.5 mol %).

10-6 10-4 10-2 100

0.1

0.2

0.3

0.2

0.4

0.6

0.8

Yiel

d of

DW

CN

T

Al/MgI D

/I G

Fig. 6. Relationship between ID/IG value and yield of carbon and thecatalyst with different Al/Mg ratios.

1648 Q. Zhang et al. / Carbon 45 (2007) 1645–1650

In order to determine whether the carbon product wasamorphous carbon or MWCNTs that was grown on thecatalyst with high-Al/Mg ratios of 1:1 to 4:1, TGA analysisfor these carbon products were performed and shown inFig. 7. It was clear that the carbon products on the catalystwith large-Al/Mg ratios (1:1–4:1, especially for the later)were easily burnt in the low-temperature range (<400 �C).This confirmed the high-content of amorphous carbon inthe products on the catalyst with large-Al/Mg ratios (1:1–4:1). The results indicated that the MgAl2O4 phase wasmainly responsible for the formation of amorphous car-bon. In comparison, the carbon products synthesized bythe catalysts with low-Al/Mg ratios had higher thermaloxidative stability and were burnt only at the high-temper-

ature range (>600 �C). This indicated its high-content ofDWCNTs with fewer defects in the product. These resultswere in agreement with the Raman spectra and SEMresults.

The formation of amorphous carbon on the catalystswith high-Al/Mg ratios can be mainly attributed to theacidity of the MgAl2O4 phase, as compared with that ofthe MgO phase. It is well known that alumina-containingsupports (e.g., Al2O3, MgAl2O4 and many kinds of Al/Sizeolite) are acidic and are able to decompose a carbonsource to produce amorphous carbon, as distinct fromMgO and SiO2 supports [21]. Notably, this behavior isindependent of the presence of metal crystallites on the

200 400 600 800 1000

0.75

0.80

0.85

0.90

0.95

1.00

4:1

1:200

4:11:5

1:11:500:1

Al/Mg

Wei

ght p

erce

ntag

e

Temperature /ºC

Fig. 7. TGA of CNTs over the Fe/Mg/Al/O catalysts (iron loading is1.5 mol.%).

Q. Zhang et al. / Carbon 45 (2007) 1645–1650 1649

support. It is in agreement with our above observation thatamorphous carbon does not adhere to the wall of theDWCNTs, but are from other active sites on the catalyst.Also, the diameter of the DWCNT and defect density ofDWCNT also increased, as indicated by the HRTEMimages. This is due to the differences in catalyst behaviorof the metal catalyst on the MgAl2O4 support. Fromthis viewpoint, the MgAl2O4 support is not suitable forthe preparation of DWCNTs, in contrast to the MgOsupport.

The relationship between the DWCNT yield and the Al/Mg ratio of the catalyst was calculated from the TGAresults in Fig. 7. As shown in Fig. 6, the yield of DWCNTswas about 15% and 28% when grown from the catalystwithout Al species and with an Al/Mg ratio of 1:200,respectively. The 70% increase in the DWCNT yield indi-cated the effectiveness of the phase separation effectdecreasing the size of the MgO support from 35 nm to30 nm. In this case, the MgO support was still the domi-nant support, and the newly formed MgAl2O4 phase pre-sented in small amounts acts only to separate theircrystallites from each other. In this case, the acidity ofthe MgAl2O4 phase did not seriously influence the purityof the DWCNTs. However, with the further increase inthe Al/Mg ratio of the catalyst, the situation was signifi-cantly different. Although there was a monotonic decreaseof the size of the MgO support with increasing Al/Mgratio, the proportion of the MgAl2O4 phase also increasedand it became another support for the iron particles. In thiscase, its acidity resulted in the transformation of the carbonsource not into DWCNTs but into amorphous carbon, asdiscussed above. The diameter was increased, and thedefects of DWCNTs were also increased with higher pro-portion of the MgAl2O4 phase. The purity of the DWCNTswas significantly decreased. Even worse, MgAl2O4 is a sta-ble spinel structure material with a Mohs hardness of 7–8,which is higher than that of MgO (about 4–6). When

MgAl2O4 was used as the support and iron crystallites dis-persed on it or in its inner pores, DWCNTs did not growfacilely in its inner pores, which had been discussed indetail in Ref. [7], which caused more defects on DWCNTsas-grown products. Thus, the yield of DWCNTs wasdecreased (Fig. 6), a similar phenomenon to that can beseen in Refs. [7,24]. However, our analysis on the forma-tion of defective DWCNTs from MgAl2O4 phase was qual-itative. To this date, we are unable to distinguish thecontribution of defective DWCNTs in Raman and TGAanalyses quantitatively from those of amorphous carbon,though the content of amorphous carbon had the domi-nant effect on the low-purity of entire carbon products.Further investigation is needed. In this case, we proposedthat the simultaneous maintenance of MgO as the domi-nant support material and decreasing its size were impor-tant for an increased yield and purity of DWCNTs.From this viewpoint, the addition of a small amount ofAl species was sufficient to break the texture of the MgOsupport and consequently to increase the purity and yieldof the DWCNTs.

Finally, it is noted that our catalyst is only a model cat-alyst used to show the phase separation effect. It showedthat the effect can significantly increase the yield ofDWCNTs by 70%, though the actual carbon yield is rela-tively low. We believe that better results are possible if acatalyst with an intrinsically high-activity is used. Further-more, our work also implies that the addition of other com-ponents in a small amount in the Fe/MgO catalyst can alsobe effective in decreasing the size of the MgO support. Thisprovides a new alternative to increase the yield and purityof DWCNTs. The method is simple and reproducible andthe catalyst can be mass produced for the controlledgrowth of DWCNTs at low-cost [26,27].

4. Conclusions

The introduction of Al species into an Fe/MgO catalystby the co-precipitation method gave a decrease in the sizeof the MgO crystallites of the support from 35 nm to20 nm with an increasing Al/Mg ratio, due to a phase sep-aration effect between the newly formed MgAl2O4 phaseand the MgO phase. The effect gave better dispersion ofthe metal crystallites on the MgO support. Consequently,the yield and purity of DWCNTs grown on the catalystwere increased by using a catalyst with an Al/Mg ratio of1:200. However, further increase in the Al/MgO ratio ofthe catalyst led to the newly formed MgAl2O4 phasebecoming another support material. Its acidity and hard-ness were both unfavorable for the formation of DWCNTsin high-yield and high-purity, although in its presence, thesize of the MgO crystallites of the support was significantlydecreased. The results indicated that the keeping of MgOas the dominant support and decreasing the size of its crys-tallites simultaneously were important for the dispersion ofthe metal crystallites to favor the growth in DWCNTs withhigh-yield and high-purity.

1650 Q. Zhang et al. / Carbon 45 (2007) 1645–1650

Acknowledgements

The work was supported by A Foundation for theAuthor of National Excellent Doctoral Dissertation ofPR China (No. 200548), Natural Scientific Foundation ofChina (No. 20606020), China National program (No.2006CB932702), Key Project of Chinese Ministry of Edu-cation (No. 106011) and National Center for Nanoscienceand Technology of China.

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