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Carbon 45 (2007) 2159–2163

Catalytic synthesis of carbon nanostructures using layereddouble hydroxides as catalyst precursors

Yun Zhao a,*, Qingze Jiao a, Chunhua Li b, Ji Liang b

a School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, PR Chinab Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China

Received 24 May 2007; accepted 29 June 2007Available online 6 July 2007

Abstract

Layered double hydroxides with different components but similar iron content such as Fe0.1Mg2Al0.9, Fe0.1Zn2Al0.9 and Fe0.1Cu2Al0.9

were prepared using a coprecipitation reaction. Then mixed oxides were obtained by calcination of these layered double hydroxide pre-cursors, and their catalytic activities were examined during synthesis of various carbon nanostructures. It was found that single-walledcarbon nanotubes were synthesized using a Fe0.1Mg2Al0.9 mixed oxide catalyst, while multi-walled carbon nanotubes and carbon nano-fibers resulted from Fe0.1Zn2Al0.9 and Fe0.1Cu2Al0.9 mixed oxides as catalysts, respectively.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Layered double hydroxides (LDHs) are composed ofcharged brucite-like layers of divalent and trivalent metalhydroxides, whose excess positive charge is balanced byanions in the interlayer. The general formula for thesematerials is ½MII

1�xMIIIx ðOHÞ2�

xþðAn�Þx=n Æ yH2O, where MII

is a divalent cation, such as Mg2+, Zn2+, Ni2+, Co2+,Cu2+, MIII is a trivalent cation, such as Al3+, Fe3+, andAn� is the anion (often carbonate) [1–3]. They make up alarge class of isostructural materials. LDHs themselveshave attracted a great deal of interest as anion exchangers,adsorbents, ionic conductors and antacids [1]. Further-more, the mixed oxides obtained by calcination of LDHsat intermediate temperatures (723–873 K) are of consider-able interest in their own right as catalysts and catalyst sup-ports [1].

Various transition metal cations introduced into thebrucite-like layers of LDHs can be precursors of redox-type centers, showing attractive catalytic activity due tothe novel properties of the final catalysts, such as high

0008-6223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2007.06.071

* Corresponding author.E-mail address: zhaoyun@bit.edu.cn (Y. Zhao).

metal dispersion. The mixed oxide catalysts obtained bythermal decomposition of LDHs containing Co have beensuccessfully used in the removal of NOx and SOx from fluegasses [4] and hydrogenation of acetonitrile [5]. Ni/Mg/Almixed oxides originating from LDH decomposition havebeen investigated as catalysts in the oxidative dehydrogena-tion of n-butane and propene [6], partial oxidation of meth-ane to syngas [7] and CO2 reforming of CH4 [8].

Carbon nanotubes (CNTs) have been the focus of theconsiderable research due to their superior mechanical[9–11], thermal and electrical properties [12,13]. Potentialapplications include field-emission electron sources, fieldeffect transistors, etc. Transition metal particles obtainedby selective hydrogen reduction of oxide solid solutionssuch as Al1.8Fe0.2O3, Mg0.9Co0.1O and Mg1�xMxAl2O4

[14–16] are used as catalysts for preparing CNTs by thechemical vapor deposition method. We therefore have con-sidered Fe/Mg/Al, Fe/Zn/Al and Fe/Cu/Al mixed oxides(originating from corresponding LDH decomposition) aspromising catalysts for producing CNTs.

In this paper, ternary Fe/Mg/Al, Fe/Zn/Al and Fe/Cu/Al LDHs, which have almost the same iron content, werefirst prepared by coprecipitation. Then Fe/Mg/Al, Fe/Zn/Al and Fe/Cu/Al composite oxides were obtained by

b

a

PS)

CuO Cu(OH)2

ZnO

2160 Y. Zhao et al. / Carbon 45 (2007) 2159–2163

calcination of corresponding LDH precursors. Finally, car-bon nanostructures were synthesized in the catalyticdecomposition of methane using Fe/Mg/Al, Fe/Zn/Aland Fe/Cu/Al mixed oxides as catalysts. The influence ofcomponents of LDH precursors on the synthesis of carbonnanostructures was investigated.

2θ10 20 30 40 50 60 70

c

I(C

Fig. 1. XRD patterns for LDHs with different components: (a) Fe0.1-Mg2Al0.9; (b) Fe0.1Zn2Al0.9; (c) Fe0.1Cu2Al0.9.

2. Experimental

2.1. Preparation and calcination of LDHs

Fe0.1Mg2Al0.9-LDHs, Fe0.1Zn2Al0.9-LDHs and Fe0.1Cu2Al0.9-LDHswere prepared using a coprecipitation reaction. One hundred and fifty mil-liliters of a 1.6 M solution containing the nitrates of the elements (Fe/Mg/Al = 0.1/2/0.9, Fe/Zn/Al = 0.1/2/0.9, Fe/Cu/Al = 0.1/2/0.9 (molar ratio))was added, slowly and with vigorous stirring, to 150 mL of a solu-tion of NaOH and Na2CO3 {n(NaOH)/[n(M2+) + n(M3+)] = 1.6,nðCO2�

3 Þ=nðM3þÞ ¼ 2}. The resulting slurry was aged at 313 K for 0.5 h.The final precipitate was then filtered, washed thoroughly with waterand dried.

Fe0.1Mg2Al0.9, Fe0.1Zn2Al0.9 and Fe0.1Cu2Al0.9 mixed oxides wereobtained by calcination of corresponding LDH precursors at 873 K in N2.

Table 1Selected data for LDHs with different components

2.2. Synthesis of carbon nanostructures

Synthesis of carbon nanostructures was carried out in a fixed bed flowreactor. The Fe0.1Mg2Al0.9, Fe0.1Zn2Al0.9 and Fe0.1Cu2Al0.9 mixed oxidecatalysts (500 mg) were placed in a furnace and heated under a flowingAr (100 mL/min). On reaching 873 K, the Ar was switched to H2–CH4

gas mixture (18% mol of CH4, 250 mL/min). Then the mixed oxides werereduced in situ and treated with the H2–CH4 atmosphere from 873 K to1183 K at a rate of 5 K/min and maintained at 1183 K for 10 min, produc-ing carbon nanostructure-metal-oxide composite powders. The quality ofcarbon deposit was observed using transmission electron microscopy(TEM).

Sample Fe0.1Mg2Al0.9 Fe0.1Zn2Al0.9 Fe0.1Cu2Al0.9

d003 (nm) 0.7655 0.7538 0.7525d006 (nm) 0.3821 0.3767 0.3767d110 (nm) 0.1514 0.1534 0.1521Lattice parameter a (nm) 0.3028 0.3068 0.3042Lattice parameter c (nm) 2.2946 2.2608 2.2589

CuOZnMgO O

2.3. Characterization techniques

Powder X-ray diffraction (XRD) patterns were recorded on a RigakuD/max-RB diffractometer at 40 kV and 120 mA with CuKa radiation.TEM was performed with a Hitachi H-800 microscope. The Raman scat-tering spectrum was measured with Ar+ laser excitation at 632.7 nm usingRenishaw RM2000. High resolution transmission electron microscopy(HRTEM) was carried out on a JEOL JEM-2010 microscope.

2θ30 40 50 60 70

c

b

a

I(C

PS)

Fig. 2. XRD patterns for mixed oxides obtained by calcination of LDHprecursors with different compositions: (a) Fe0.1Mg2Al0.9; (b) Fe0.1Zn2-Al0.9; (c) Fe0.1Cu2Al0.9.

3. Results and discussion

3.1. Crystallite structure of LDHs and their calcination

products

Powder XRD patterns for as-synthesized Fe0.1Mg2Al0.9-LDHs, Fe0.1Zn2Al0.9-LDHs and Fe0.1Cu2Al0.9-LDHs areshown in Fig. 1. In each case, the XRD patterns exhibitthe characteristic reflections of an LDH material. A lowerdiffraction intensity is observed for the Fe0.1Mg2Al0.9-LDHsample than for others. However, there is no separatephase for the Fe0.1Mg2Al0.9-LDH sample. XRD patternsfor Fe0.1Zn2Al0.9-LDH and Fe0.1Cu2Al0.9-LDH samplesshow that they contain hydrotalcite as the main compo-nent, exhibiting sharp and symmetric reflections for thebasal (003) and (006). Little impurities (ZnO and CuO,

Cu(OH)2, respectively) also can be observed from XRDpatterns.

The unit cell parameters (a and c) (see Table 1) forhydrotalcite structures with hexagonal crystal symmetryare calculated in all LDH samples from (003), (006) and(11 0) diffraction peaks. The value of lattice parametersvaries from the types of divalent cations.

The above LDH samples prepared with different compo-nents were separately calcined at 873 K for 3 h. The pow-der XRD patterns for the products of calcination of theseLDH precursors are shown in Fig. 2. Lower diffraction

Y. Zhao et al. / Carbon 45 (2007) 2159–2163 2161

intensity is observed for the Fe0.1Mg2Al0.9 sample. Onlythree broad peaks are observed. These can be attributedto a mixed oxide phase with an MgO-like structure. Theloading of iron oxide should be well-above the XRD detec-tion limit, e.g. 5%, while the absence of such a componentin the XRD spectrum indicates that iron oxide formed asolid solution with Mg and Al as reported by Shishidoet al. [8]. The peaks are broad because of the low crystallin-ity. Similarly, Fe0.1Zn2Al0.9 and Fe0.1Cu2Al0.9 samples canalso be indexed to a mixed oxide phase with ZnO-like andCuO-like structure, respectively. Standard references ofXRD spectra or indices of ZnO and CuO are also givento match the diffraction patterns of Fe0.1Zn2Al0.9 andFe0.1Cu2Al0.9 mixed oxides (to be incorporated in Fig. 2).There is no peak attributed to iron species. It indicates thatiron oxides formed solid solutions with Zn, Al and Cu, Al.

3.2. Carbon nanostructures

The above mixed oxides with different compositionswere reduced in situ and treated with a H2–CH4 gas mix-ture to synthesize carbon nanostructures. TEM observa-tions (Fig. 3) show some differences in the structures. Inthe case of the Fe0.1Mg2Al0.9 sample (Fig. 3a), the productsare single-walled carbon nanotubes (SWCNTs). In the caseof the Fe0.1Zn2Al0.9 sample (Fig. 3b), multi-walled carbonnanotubes (MWCNTs) are observed. In the Fe0.1Cu2Al0.9

Fig. 3. TEM micrographs for products using different mixed oxides ascatalysts: (a) Fe0.1Mg2Al0.9; (b) Fe0.1Zn2Al0.9; (c) Fe0.1Cu2Al0.9.

sample (Fig. 3c), a few carbon nanofibers and amorphouscarbons appear.

In order to confirm the SWCNTs in the sample ofFe0.1Mg2Al0.9, the Raman spectrum was measured. Theresult is shown in Fig. 4. The resonant Raman spectrumof SWCNTs shows two main features: radial breathingmode (RBM) band and G band. With the RBM frequency,information about the SWCNT diameters can be obtained.Based on the relationship between diameter and frequency[17], the diameters of the SWCNTs synthesized usingFe0.1Mg2Al0.9 mixed oxides as catalysts are 1.15, 1.09 and1.03 nm, respectively. From the HRTEM micrograph(Fig. 5), diameters of SWCNTs are about 1–3 nm. The dif-ference is probably due to the interaction between theSWCNTs in the bundles. The Raman spectrum indicatesthat quite a few disordered carbon phases exist. That isthe result of a substantial amount of amorphous carbon.It should not relate to the defective nature of SWCNTsbecause we do not observe it with HRTEM.

The effect of using different mixed oxides as catalystsleading to different carbon nanostructures was investi-gated. Mixed oxides with different compositions werereduced with H2 under the same conditions as carbonnanostructure synthesis. TEM micrographs for the reduc-tion products are shown in Fig. 6. It is found that the sizesof iron species in different mixed oxides are different. Theiron particles in Fe0.1Mg2Al0.9 mixed oxides are smallerin size (1–3 nm), and they are evenly dispersed in the

0 900 1000 1500 2000

0

5000

10000

15000

212.3

1322.5

1588

223.9200.8

Raman Shift (cm-1)

Fig. 4. Raman spectrum of SWCNTs synthesized using Fe0.1Mg2Al0.9

mixed oxides as catalysts.

Fig. 5. HRTEM micrograph for SWCNTs using Fe0.1Mg2Al0.9 mixedoxides as catalysts.

Fig. 6. TEM micrographs for the reduction products of different mixed oxides: (a) Fe0.1Mg2Al0.9; (b) Fe0.1Zn2Al0.9; (c) Fe0.1Cu2Al0.9.

2162 Y. Zhao et al. / Carbon 45 (2007) 2159–2163

matrix. The iron particles in Fe0.1Zn2Al0.9 mixed oxides arelarger in size (15–50 nm). The iron particles in Fe0.1Cu2-Al0.9 mixed oxides are not easy to determine due to thereduction of Cu2+, so Cu and Fe particles are mixedtogether. They are also larger in size (50–200 nm). The car-bon nanostructures are related to the size of the active spe-cies in the catalysts. The different size of iron species indifferent mixed oxides led to the different carbon nano-structures. The smaller iron species produced CNTs withsmaller diameter, i.e. SWCNTs. The larger iron particlesize resulted in MWCNTs with larger diameter. The influ-ence of Cu on Fe made the formation of carbon nanofibers[18,19].

Reasons leading to different size of iron species usingdifferent mixed oxides as catalysts might include the com-positions in the LDH sheet and the purity of the LDH sam-ples. From the XRD patterns of LDH precursors (Fig. 1),Fe0.1Mg2Al0.9-LDHs showed high purity. No other impu-rity existed in the sample. So Mg2+ and Al3+ acted as goodsupports. As a result, Fe3+ could be distributed uniformlyin the LDH sheet. It was not easy for Fe to conglomerateand grow up in the course of CNT synthesis. Therefore,SWCNTs were obtained using Fe0.1Mg2Al0.9 mixed oxidecatalysts.

There was a ZnO impurity in the Fe0.1Zn2Al0.9-LDHsample and CuO and Cu(OH)2 impurities in the system ofFe0.1Cu2Al0.9-LDHs. The amount of Zn2+ and Cu2+ form-ing LDH sheets decreased due to the existence of theseimpurities, which made a dense distribution of iron speciesin the matrix. When iron oxides were reduced, it was easyfor them to agglomerate and grow up. Additionally, animprovement in reducing CuO would help eliminate someof the confusion. CuO is easier to reduce than ZnO, soreduction of Cu2+ in Fe0.1Cu2Al0.9mixed oxides also ledto the agglomeration of iron particles easily. Therefore thesize of iron and copper species in Fe0.1Cu2Al0.9 mixed oxi-des was larger than iron particles in Fe0.1Zn2Al0.9 mixedoxides. So Fe0.1Zn2Al0.9 mixed oxides produced MWCNTs.

For Fe0.1Cu2Al0.9 mixed oxides, reduction of a largeamount of Cu2+ occurred when Fe3+ was reduced in thesynthesis of carbon nanostructures. Thus small quantitiesof carbon nanofibers were obtained. It was shown thatcompositions in the LDH sheet and the purity of theLDH samples influenced the distribution and reduction ofthe active element, and then further affected the propertiesof carbon nanostructures.

4. Conclusions

Fe0.1Mg2Al0.9-LDHs, Fe0.1Zn2Al0.9-LDHs and Fe0.1Cu2-Al0.9-LDHs were prepared by introducing Fe3+ into LDHlayers. Carbon nanostructures were synthesized usingFe0.1Mg2Al0.9, Fe0.1Zn2Al0.9 and Fe0.1Cu2Al0.9 mixed oxi-des as catalysts obtained by calcination of these LDH pre-cursors. The different size of iron species in different mixedoxides led to different carbon nanostructures. The smalleriron species in Fe0.1Mg2Al0.9 mixed oxides produced theSWCNTs, the larger iron particle size in Fe0.1Zn2Al0.9

mixed oxides resulted in the MWCNTs with larger diame-ter. The influence of Cu on Fe improved the formation ofcarbon nanofibers.

Acknowledgement

Financial support for this work was provided by Excel-lent Young Scholars Research Fund of Beijing Institute ofTechnology (000Y05-22), China.

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