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Catalysis Today 216 (2013) 254– 260

Contents lists available at SciVerse ScienceDirect

Catalysis Today

j our nal homep ag e: www.elsev ier .com/ locate /ca t tod

e particles on the tops of carbon nanofibers immobilized ontructured carbon microfibers for ammonia decomposition

ian Ji a, Xuezhi Duana,∗, Gang Qiana, Ping Lia, Xinggui Zhoua, De Chenb, Weikang Yuana

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, ChinaDepartment of Chemical Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway

a r t i c l e i n f o

rticle history:eceived 28 February 2013eceived in revised form 3 June 2013ccepted 7 June 2013vailable online 6 July 2013

a b s t r a c t

Fe particles on the tops of carbon nanofibers immobilized on structured carbon microfibers (Fe-CNFs/CMFs) were prepared in situ by the catalytic decomposition of CO over Fe/CMFs catalyst, and theirparticle sizes, shapes and crystal phases were adjusted by changing the growth time of CNFs and thepartial pressure of hydrogen. The as-obtained Fe-CNFs/CMFs were used as a catalyst for ammonia decom-position, which showed higher activity when they were prepared with the longer growth time of CNFs

eywords:tructured Fe catalyste particles on the tops of CNFs-Fe(C)e3Cmmonia decomposition

and the higher partial pressure of hydrogen. Moreover, the behavior of the typical Fe-CNFs/CMFs catalystwas also investigated when changing the experimental conditions.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Ammonia decomposition has been extensively studied in bothhe fundamental understanding in catalysis and the (potential)pplications in energy and environmental industries [1–3]. How-ver, NH3 is a very stable molecule, and its thermal decompositions difficult to occur because of the extremely high activation energy3]. Various catalysts were thus developed to catalyze ammoniaecomposition. Although Fe catalysts had lower activity than com-only used Ru and Ni catalysts, their low cost and abundant

esources have encouraged us to develop highly efficient Fe cat-lysts [4–12]. Typically, dispersed Fe particles incorporated intoNFs (Fe-CNFs) were prepared by the catalytic decomposition ofarbon-containing gases over Fe catalysts using catalytic chemicalapor deposition (CCVD) method [8,12]. These new types of Fe-NFs catalysts were active for ammonia decomposition especially

or Fe particles on the tops of platelet carbon nanofibers (Fe-PCNFs)12].

It is well-known that compared with the powdered catalysts,he structured catalysts are promising for the practical applica-

ions because of the intensified heat and mass transfer, desirable

echanical properties and ease of use in portable applications.revious studies showed that the structured Ru and Ni catalysts

∗ Corresponding author. Tel.: +86 21 64250937; fax: +86 21 64253528.E-mail address: xzduan@ecust.edu.cn (X. Duan).

920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2013.06.008

exhibited higher activities [13–16]. For the above mentioned Fe-PCNFs catalysts, there are two methods to form the structuredFe-PCNFs catalysts: shaping the Fe-PCNFs catalysts with bindersand in situ growing Fe-PCNFs on the structured substrate. The lat-ter method seems to be simpler and more attractive for practicalapplication. In the previous works, the carbon nanofibers immo-bilized on structured carbon microfibers were prepared using thelatter method, and used as both supports to support Ir catalystsfor hydrazine decomposition and catalysts for the oxidative dehy-drogenation of ethylbenzene with higher catalytic performances[17–21]. However, whether or not, the residual metal particlesover the carbon nanofibers immobilized on structured carbonmicrofibers could be directly used as catalysts, was less concerned.

In this work, Fe-CNFs/CMFs catalysts were prepared in situ bythe catalytic decomposition of CO over Fe/CMFs catalyst using CCVDmethod where the structured CMFs substrate was easily tailored topre-defined shapes and sizes [21,22], and Fe particle sizes, shapesand crystal phases were adjusted by changing the growth timeof CNFs and the partial pressure of hydrogen. The as-obtainedFe-CNFs/CMFs catalysts were directly used to catalyze ammoniadecomposition. The dependence of the activity on the growth timeof CNFs and the partial pressure of H2 was determined, and thebehavior of the typical Fe-CNFs/CMFs catalyst varying experimen-

tal conditions (i.e., different NH3 fractions and flow rates) wasinvestigated. In addition, a comparison of the activities betweenthe structured Fe-CNFs/CMFs catalyst and the commercial catalystswas given.

J. Ji et al. / Catalysis Today 216 (2013) 254– 260 255

ure CM

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Fig. 1. (a) XRD patterns of the reduced and calcined Fe/CMFs catalysts and p

. Experimental

.1. Catalyst preparation

The structured CMFs substrate was commercially available fromhe Shanghai Q-Carbon Material Co. Ltd. and made of skeins ofraphite fibers, and its specific surface area was less than 1 m2 g−1.he precursor of 15 wt% Fe/CMFs catalyst was prepared by usingncipient wetness impregnation method, dried in stagnant air at20 ◦C for 12 h, calcined at 500 ◦C for 4 h in argon atmosphere,nd then activated at 600 ◦C for 3 h in Ar/H2 (v/v = 4/1) mixture.fter the catalyst activation, CO mixed with H2 was introduced

nto the reactor to prepare Fe-CNFs/CMFs catalysts, and the detailedynthetic conditions and physical properties were summarized inable 1.

.2. Catalyst characterization

The crystal phases of CMFs, calcined and reduced Fe-CMFs, ande-CNFs/CMFs were determined by X-ray diffraction (XRD) on aigaku D/Max2550VB/PC X-ray diffractometer (Rigaku, Japan) withu K� radiation (� = 1.54056 A), in which the reduced Fe-CMFs cat-lyst was first passivated in a steam of 0.98 vol% O2/Ar at roomemperature for 4 h. Hydrogen temperature-programmed reduc-ion (H2-TPR) experiments of the calcined Fe-CMFs catalyst andure CMFs were carried out on Autochem II 2920 (Micromeritics,SA). The microstructures of Fe catalysts were characterized byeld emission scanning electron microscopy (FE-SEM, FEI NOVAano SEM 450, Netherlands) and high-resolution transmission

lectron microscopy (HRTEM, JEOL JEM-2100F, Japan) with anccelerating voltage of 200 kV and a point resolution of 0.19 nm.he nitrogen adsorption isotherms were measured at −196 ◦C onriStar II 3020 V1.04 (Micromeritics, USA) after the evacuation ofamples at 160 ◦C for 6 h.

Fs, and (b) H2-TPR profiles of the calcined Fe/CMFs catalyst and pure CMFs.

2.3. Catalytic experiments

The catalytic testing of Fe-CNFs/CMFs catalyst (ca. 0.5 mL) wascarried out in a continuous-flow fixed-bed quartz reactor forammonia decomposition at 400–650 ◦C. The concentrations of N2and NH3 in the effluent were analyzed at room temperature by anonline gas chromatography equipped with a thermal conductivitydetector and a 3 m Poropak Q packed column using H2 as a carriergas. All the data were taken from the reaction time of 4 h with theammonia conversion approaching a steady state. NH3 conversionwas calculated by a normalization method based on the nitrogenatom counting [23]. No signal of CH4 from the methanation of car-bon supports was detected. Moreover, blank tests showed that NH3conversion in a blank reactor or over the supports was less than 1.0%at 600 ◦C.

3. Results and discussion

3.1. Structured Fe3C catalysts for ammonia decomposition

3.1.1. CharacterizationThe calcined and reduced Fe/CMFs catalysts, and pure CMFs

were characterized by XRD to identify the crystal phases of C, Feand iron oxide, and the results were shown in Fig. 1a. All the XRDpatterns indicated the hexagonal phase of graphite. For the calcinedand reduced Fe/CMFs catalysts, the hexagonal phase of �-Fe2O3(JCPDS 33-0664) and the cubic phase of �-Fe (JCPDS 06-0696) wererespectively observed, indicating that the �-Fe2O3 species for thecalcined Fe/CMFs catalyst were fully reduced at the designatedreduction condition. This was further confirmed by H2-TPR resultof the calcined Fe/CMFs catalyst (Fig. 1b) where the temperatures

corresponding to the two reduced peaks of �-Fe2O3 were lowerthan 600 ◦C. Moreover, the average Fe crystal size of the reducedFe/CMFs catalyst was ca. 37 nm calculated by using the Scherrerequation based on the Fe(1 1 0) peak.

256 J. Ji et al. / Catalysis Today 216 (2013) 254– 260

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ig. 2. Back-scattering electron images of (a) Fe-CNFs/CMFs-1, (b) Fe-CNFs/CMFs-2,f Fe particles.

Four Fe-CNFs/CMFs catalysts prepared at different growth timesf CNFs (i.e., Fe-CNFs/CMFs-1, Fe-CNFs/CMFs-2, Fe-CNFs/CMFs-3nd Fe-CNFs/CMFs-4) were characterized by FE-SEM, and theorresponding back-scattering electron images were shown inig. 2a–d where the bright spots indicated the Fe particles. Twoypes of Fe particles were clearly observed. One was the smaller Fearticles remained on the surface of CMFs without catalyzing the

ormation of CNFs, which may be due to the unfavorable formationf CNFs over the smaller Fe particles [24], while another was thearger Fe particles located on the tops of CNFs, indicating a tip

able 1ynthetic conditions and physical properties of Fe-CNFs/CMFs catalysts.

Samples CO/H2 (mL min−1) Growth time (h)

Fe-CNFs/CMFs-1 160/40 1

Fe-CNFs/CMFs-2 160/40 2

Fe-CNFs/CMFs-3 160/40 6

Fe-CNFs/CMFs-4 160/40 12

Fe-CNFs/CMFs-5 160/80 6

-CNFs/CMFs-3 and (d) Fe-CNFs/CMFs-4, and (e) the corresponding size distribution

growth mechanism [25,26]. With the increase of the growth timeof CNFs, the smaller Fe particles without growth of CNFs wereeasily coalesced into the larger particles favorable growth of CNFs,leading to the decrease of the number of the smaller Fe particlesand the increase of the amount of CNFs. Fig. 2e shows the particlesize distributions of the four Fe-CNFs/CMFs catalysts. Their averageparticle sizes (Table 1) were 106, 116, 122 and 146 nm, respectively.

Fig. 3 further shows the typical TEM images of the four Fe-CNFs/CMFs catalysts. All the Fe particles were found to locate onthe tops of CNFs, in agreement with the results of FE-SEM (Fig. 2).

Fe loading (wt%) dFe (nm) SBET(m2 g−1)

9.8 106 60.46.9 116 110.73.2 122 135.81.9 146 115.13.4 119 59.6

J. Ji et al. / Catalysis Today 216 (2013) 254– 260 257

F -3 andd

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ig. 3. TEM images of (a) Fe-CNFs/CMFs-1, (b) Fe-CNFs/CMFs-2, (c) Fe-CNFs/CMFsotted circles.

uring the growth of CNFs, significant reconstruction of Fe parti-les obviously took place. For the CNFs growth time of 1 h, both themaller Fe particles with the semi-spherical shape and the largere particles with the irregular morphologies (marked by dotted cir-les in Fig. 3a) were observed over Fe-CNFs/CMFs-1 catalyst. Whenhe growth time of CNFs was further increased, the larger Fe parti-les with the irregular morphologies were not observed (Fig. 3b–d),

hich possibly broke up into small ones with the semi-spherical

hape because of the stresses induced by the different rates of CNFsrowth on different Fe crystal faces [27,28].

ig. 4. XRD patterns of (a) Fe-CNFs/CMFs-1, (b) Fe-CNFs/CMFs-2, (c) Fe-CNFs/CMFs- and (d) Fe-CNFs/CMFs-4.

(d) Fe-CNFs/CMFs-4. Fe particles with irregular morphologies were marked with

Moreover, all the four Fe-CNFs/CMFs catalysts (Fig. 4) showedthe orthorhombic phase of Fe3C (JCPDS 35-0772). The BET surfacearea of Fe-CNFs/CMFs catalyst increased from 60.4 to 135.8 m2 g−1

with the growth time of CNFs from 1 to 6 h, possibly due to theincrease in the fraction of Fe-CNFs over Fe-CNFs/CMFs composites.However, with the further increase in the growth time of CNFs, theBET surface area of Fe-CNFs/CMFs catalyst decreased, which could

be related to the decrease in the amount of amorphous carbon onthe edges of carbon nanofibers.

Fig. 5. H2 formation rates of four Fe-CNFs/CMFs catalysts with an NH3 flow rate of60 mL min−1: (�) Fe-CNFs/CMFs-1, (©) Fe-CNFs/CMFs-2, (�) Fe-CNFs/CMFs-3 and(�) Fe-CNFs/CMFs-4.

258 J. Ji et al. / Catalysis Today 2

Fig. 6. NH3 conversion of Fe-CNFs/CMFs-3 catalyst (ca. 0.2 g) as a function of reactiontemperature under total gas flow rate of 60 mL min−1 and different fractions of NH3

i1g

3

f

n NH3-Ar mixture: (�) 5% NH3, (©) 25% NH3, (�) 50% NH3, (�) 75% NH3, and (�)00% NH3; NH3 conversion of Fe-CNFs/CMFs-3 catalyst (ca. 0.2 g) as a function ofas hourly space velocity of pure NH3 (GHSVNH3 , �) at 650 ◦C.

.1.2. Ammonia decomposition activityThe H2 formation rates of the four Fe-CNFs/CMFs catalysts as a

unction of temperature were presented in Fig. 5. Clearly, the H2

Fig. 7. TEM images of (a and b) Fe-CNFs/CMFs-5

16 (2013) 254– 260

formation rates increased with the temperature because ammoniadecomposition is an endothermic reaction. Notably, the H2 forma-tion rates also increased with the growth time of CNFs, indicatingthat Fe3C particles on the tops of CNFs were more active than Feparticles supported on CMFs. The semi-spherical shape and inter-stitial carbon atoms [12] may endow the structured Fe3C catalystwith a high ammonia decomposition activity.

To further understand ammonia decomposition over the struc-tured Fe3C catalyst, Fig. 6 illustrates the NH3 decomposition activityof the typical Fe-CNFs/CMFs-3 catalyst as a function of the volumefraction of NH3 and GHSVNH3 . The gap between the curves becamenarrower as the NH3 concentration increased, which may be dueto the inhibition of H2 to the reaction [16]. Moreover, the depend-ence of NH3 conversion on GHSVNH3 at 650 ◦C showed that the NH3was fully decomposed when the flow rate was 10 mL min−1 at aGHSVNH3 of ca. 2700 mL h−1 gcat

−1.

3.2. Structured ˛-Fe(C) catalyst for ammonia decomposition

During the growth of carbon nanofibers, the partial pressureof hydrogen in the feed gas usually influenced the catalyst crystalmorphology, size, composition and structure [29], and thus the cat-alytic performance. By increasing the partial pressure of hydrogenfrom 0.20 to 0.33 atm, the Fe-CNFs/CMFs-5 catalyst was prepared,

and its typical TEM images were shown in Fig. 7a and b. Clearly, theFe particles over Fe-CNFs/CMFs-5 catalyst had polyhedral shapes,while those over Fe-CNFs/CMFs-3 catalyst (Fig. 7c and d) had semi-spherical shapes. The XRD results from the comparative study on

and (c and d) Fe-CNFs/CMFs-3 catalysts.

J. Ji et al. / Catalysis Today 216 (2013) 254– 260 259

NFs/C

Fsptitpbtp

C1lrsF

Fa

Fig. 8. (a) XRD pattern and (b) back-scattering electron image of Fe-C

e-CNFs/CMFs-5 and Fe-CNFs/CMFs-3 catalysts with different Fehapes showed the two different Fe crystal phases, i.e., the cubichase of �-Fe (JCPDS 87-0721) for Fe-CNFs/CMFs-5 (Fig. 8a) andhe orthorhombic phase of Fe3C for Fe-CNFs/CMFs-3. These resultsndicated the strongly reducing environment unfavorable forma-ion of Fe3C. Moreover, because the Fe-CNFs/CMFs-5 catalyst wasrepared by CCVD method, the carbon atoms (i.e., interstitial car-on atoms) were inevitably existed in the Fe particles according tohe growth mechanism of carbon nanofibers, in which the Fe crystalhase of Fe-CNFs/CMFs-5 catalyst was denoted as �-Fe(C).

Fig. 8b shows the back-scattering electron image of Fe-NFs/CMFs-5 catalyst. The average size of Fe particles was ca.19 nm (Table 1), and almost all the Fe particles were found to

ocate on the tops of CNFs, in good agreement with the TEMesults from Fig. 7a and b. In addition, increasing the partial pres-ure of hydrogen obviously influenced the BET surface areas ofe-CNFs/CMFs catalysts (Table 1), and the BET surface area of

ig. 9. (a) Activity of Fe-CNFs/CMFs-5 (filled) and Fe-CMFs/CMFs-3 catalysts (unfilled) as

nd Fe-CMFs/CMFs-3 (�) catalysts as a function of reaction temperature time on stream a

MFs-5 catalyst. The insert shows the size distribution of Fe particles.

Fe-CNFs/CMFs-5 was much smaller than that of Fe-CNFs/CMFs-3,possibly due to the presence of amorphous carbon on the edges ofcarbon nanofibers.

To understand the difference in the catalytic activities of theabove two structured Fe catalysts (Fig. 7) with different Fe shapesand crystal phases, the H2 formation rate as a function of both thereaction temperature and the time on stream were tested, and theresults were shown in Fig. 9. Compared to the Fe-CNFs/CMFs-3 cat-alyst, the Fe-CNFs/CMFs-5 catalyst exhibited not only higher H2formation rate, but also shorter induction period. Herein, the usedFe-CNFs/CMFs-3 and Fe-CNFs/CMFs-5 catalysts were characterizedby XRD and TEM to study whether or not there were any changes(e.g., phase transformation or structural changes). Interestingly, for

the two types of Fe catalysts, their particle shapes before and afterthe reaction were similar, while their crystal phases (Fig. 10) werecompletely changed to the orthorhombic phase of Fe2N (JCPDS 06-0656). Generally, the nitridation of Fe particles was complete only

a function of reaction temperature, and (b) NH3 conversion of Fe-CNFs/CMFs-5 (�)t 650 ◦C.

260 J. Ji et al. / Catalysis Today 2

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ig. 10. XRD patterns of the used Fe-CNFs/CMFs-5 (up) and Fe-CMFs/CMFs-3 cata-ysts (down).

n few minutes [30], while that of Fe3C underwent a long activationeriod [8]. Therefore, for the Fe-CNFs/CMFs-3 and Fe-CNFs/CMFs-5atalyst, the difference in the nitridation processes of Fe3C and �-e(C) may be responsible for the difference in the induction periods,nd the difference in the Fe2N particle shapes may be one maineason for the difference in the ammonia decomposition activities.

In addition, the H2 formation rates normalized to metalontents of both Fe-CNFs/CMFs-5 and commercial catalysts (i.e.,800, G43-A and 146) [31] at 600 ◦C were compared. It cane seen that the H2 formation rate of Fe-CNFs/CMFs-5 cat-lyst (4.3 mmol s−1 gmetal

−1) was much higher than those of800 (Raney Ni) (0.02 mmol s−1 gmetal

−1) and G43-A (Ni-Pt/Al2O3)0.5–1.4 mmol s−1 gmetal

−1), but slightly lower than that of 146Ru/Al2O3) (5.8 mmol s−1 gmetal

−1). This was possibly due to thenique advantages of structured catalyst mentioned above, and theolyhedral shape and interstitial carbon atoms of the Fe2N particlesn the tops of CNFs.

. Conclusions

In summary, a series of structured Fe-CNFs/CMFs catalysts haveeen prepared by changing the growth time of CNFs and the partialressure of hydrogen from the decomposition of CO over Fe/CMFsatalyst using CCVD method. Under the low partial pressure ofydrogen (i.e., 0.20 atm), both the coalescence of small Fe particlesithout growth of CNFs and the fragmentation of large Fe particlesith irregular morphologies on the tops of CNFs were observedith the growth time of CNFs, which were inclined to form the

emi-spherical shape Fe particles with Fe3C crystal phase on theops of CNFs following a tip growth mechanism. When the partialressure of hydrogen increased from 0.20 to 0.33 atm under theNFs growth time of 6 h (corresponding Fe catalysts changed frome-CNFs/CMFs-3 to Fe-CNFs/CMFs-5), the Fe particles still locatedn the tops of CNFs, while the shape and crystal phase were changednto the polyhedral shape and �-Fe(C), respectively.

The as-obtained structured Fe-CNFs/CMFs catalysts were usedo catalyze ammonia decomposition, which showed higher activ-ty when they were prepared with the longer growth time of CNFsnd the higher partial pressure of hydrogen. Interestingly, for the

e-CNFs/CMFs-3 and Fe-CNFs/CMFs-5 catalysts with the similarverage Fe particle size and Fe loading, the latter catalyst showedigher activity and shorter induction periods. TEM and XRD resultsf the two structured Fe catalysts before and after the reaction

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16 (2013) 254– 260

showed that the two different Fe shapes were almost unchanged,while the two different Fe crystal phases were changed into Fe2N. Inaddition, the Fe-CNFs/CMFs-5 compared to the commercial ammo-nia decomposition catalysts had a comparable activity. Therefore,the higher activity of Fe-CNFs/CMFs-5 catalyst could be related tothe unique advantages of structured catalyst, and the polyhedralshape and interstitial carbon atoms of the Fe2N particles on thetops of CNFs.

Acknowledgements

This work is financially supported by the China PostdoctoralScience Foundation (2012M520041 and 2013T60428), the NaturalScience Foundation of China (21276077 and 21106047), the Funda-mental Research Funds for the Central Universities (WA1214020,WG1213011 and WA1114006), the Shanghai Natural Science Foun-dation (12ZR1407300) and the 111 Project of Ministry of Educationof China (B08021).

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