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Low-temperature catalytic decomposition of ethylene into H2 and secondarycarbon nanotubes over Ni/CNTs

P.G. Savva a, K. Polychronopoulou a, V.A. Ryzkov b, A.M. Efstathiou a,*a Chemistry Department, Heterogeneous Catalysis Laboratory, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprusb Rosseter Holdings Ltd., P.O. Box 57220, Limassol, Cyprus

1. Introduction

The catalytic decomposition of light hydrocarbons intohydrogen and ‘‘carbon’’ products has been studied in recent yearsas a potential clean energy source for hydrogen production [1,2].In particular, the production of carbon nanofibers (CNFs) with alarge number of potential applications is of great interest [3]. Mostof the researches performed so far on the catalytic decompositionof hydrocarbons to produce CNFs were focused on the use ofmethane due to its large availability (natural gas) over supportedmetal catalysts. The support and metal employed, feedstockcomposition, and the operating conditions have been assessed foroptimum hydrogen production and carbon deposit properties [4].Supported metal catalysts limit the activity towards lighthydrocarbons due to the restricted space available around themetal active sites, which limits the growth of CNFs and prompts

the rapid deactivation of the catalyst. For instance, the highactivity of methane decomposition over silica support of lowsurface area was ascribed to the fact that the growth of CNFs wasnot impeded by the silica walls or the available space around them[5,6].

On the other hand, carbon nanotubes, which are also producedby catalytic decomposition of hydrocarbons, are very promisingmaterials for modern technological applications, such as catalysis,electronic devices (chemical sensors, MEMS), hydrogen storage,water purification and others [3,4]. The quality of ‘‘carbon’’produced from the catalytic decomposition of hydrocarbonslargely depends on the operation conditions and the type ofcatalyst used.

Catalytic decomposition of methane using supported Ni and Ni-Cu catalysts to produce hydrogen and novel carbonaceousmaterials has been reported by many authors [7–10]. However,methane catalytic decomposition presents thermodynamic limita-tions for hydrogen production at temperatures lower than 700 8C,which is a disadvantage for this process [4,11]. On the other hand,ethylene, one of the main products of petrochemical industry with

Applied Catalysis B: Environmental 93 (2010) 314–324

A R T I C L E I N F O

Article history:

Received 20 May 2009

Received in revised form 5 October 2009

Accepted 7 October 2009

Available online 13 October 2009

Keywords:

Carbon nanotubes

Ethylene decomposition

H2 production

XPS

HRTEM

A B S T R A C T

The present work reports on the production of H2 and secondary carbon nanotubes (CNTs) during

catalytic decomposition of ethylene over a novel catalytic system, namely, nickel supported on carbon

nanotubes (Ni/CNTs) at remarkably low-temperatures, e.g. 400 8C. A number of catalyst parameters were

investigated, namely the chemical nature of support, the Ni metal loading (0.1–10 wt%), the nature of

nickel metal precursor (organometallic vs. inorganic) used during catalyst synthesis, and the nature of

transition metal used (e.g. Co, Fe, Cu, Ni). Among the different Ni/CNT supported catalysts investigated,

0.5 wt% Ni/Ros1-B1 (Ros1-B1 a commercial CNT) presented the highest activity in terms of H2 production

(296 mol H2/gNi) and carbon capacity (3552 gC/gNi). In terms of transition metal used as active catalytic

phase, the activity (moles H2 per gram of metal) was found to decrease in the order Co� Fe > Cu. The

activity of supported Ni and Co catalysts was found to strongly depend on the metal loading. The

structural and morphological features of primary (catalytic support) and secondary carbon nanotubes

produced during ethylene decomposition at 400 8C were studied using X-ray Diffraction (XRD), scanning

electron microscopy (SEM), High-resolution Transmission Electron Microscopy (HRTEM), and X-ray

Photoelectron Spectroscopy (XPS). The production of secondary carbon nanotubes at 400 8C was

confirmed after using HRTEM and after a comparison with the primary carbon nanotubes of catalyst

support was made. Different regeneration conditions (use of oxygen or steam) were investigated in order

to remove by gasification the amorphous carbon deposited under reaction conditions. Oxygen appeared

to be a better regeneration reagent than steam, where after ten consecutive reaction/regeneration cycles

the 0.5 wt% Ni/Ros1-B1 catalyst showed high and stable activity with time on stream.

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* Corresponding author. Tel.: +357 22 892776; fax: +357 22 892801.

E-mail address: efstath@ucy.ac.cy (A.M. Efstathiou).

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annual production approaching 79 � 106 tons [12] has an envir-onmentally friendly potential as H2 production source by its directdecomposition into COx-free hydrogen and ‘‘carbon’’ productscompared to steam reforming of methane, and thus remains achallenging alternative. In addition, hydrogen could be separatedfrom ethylene in an easier and less costly way than methane (e.g.use of membrane technologies) [13].

Dıaz et al. [14] reported on the hydrogen production andcarbon nanofiber growth through catalytic decomposition ofethylene over Ni-Cu alloy catalysts. A reaction conversion of74% was achieved at 650 8C with hydrogen yield of about4000 molH2

=molNiþCu and coke yield of 1112 gC/gNi+Cu after 15 hof continuous reaction. Yu et al. [15] reported a large scalesynthesis (128 g/g of catalyst) of CNFs by ethylene decompositionon hydrotalcite derived catalysts at temperatures lower than650 8C. Takenaka et al. [16] reported the formation of CNFs fromethylene decomposition over highly dispersed micro-emulsionprepared silica-supported Co catalysts at relatively high tem-peratures (T > 700 8C). They reported that Multi-walled CarbonNanoTubes (MWCNTs) and bundles of single and double walledcarbon nanotubes were formed during ethylene and methanedecomposition, respectively.

Another technologically important aspect of the catalytichydrocarbon decomposition is the restoration of catalyst’s activityusing appropriate agents to gasify the deposited coke on thesurface. Pinilla et al. [17] studied the regeneration of carbon-basedcatalysts used in methane decomposition using CO2 as catalystregeneration agent. Muradov et al. [18] used steam as regenerationagent, and reported the increase of catalyst surface area and of therate of methane decomposition. It has been reported that thesurface area increase is accompanied by the formation of oxygen-containing surface groups. It is believed that these groups areprecursors of active radicals participating in the methanedecomposition reaction via attacking methane molecules at thehigh reaction temperatures used, leading to the formation ofmethyl radicals [17].

In our previous work [19] a novel catalyst, nickel supported on acommercially produced CNT [20,21] led to the direct decomposi-tion of ethylene at the low-temperature of 400 8C to produce COx-free hydrogen. This catalyst consisted of 0.5 wt% Ni supported on aCNT (code: Ros1-B1) that presented the highest hydrogen yield(mol H2/gNi) and carbon capacity (gC/gNi) ever reported in theliterature for ethylene decomposition at 400 8C. In particular, the0.5 wt% Ni/Ros1-B1 catalyst exhibited 50 times higher hydrogenyield compared to a 0.3 wt% Ni/SiO2 catalyst which has beenexamined under the same experimental conditions. According tothe literature [22] a very active 40 wt% Ni/SiO2 catalyst could give acarbon nanofiber yield of 491 gC/gNi during methane decomposi-tion at 500 8C. Using niobium oxide to support 50.8 wt% Ni and21.2 wt% Cu, the yield of deposited carbon following methanedecomposition at 600 8C was found to be 743 gC/gNi [23].

The present work attempts to gain fundamental knowledge onthe ethylene interaction with metal supported catalysts on CNTsupports, and aspires to provide valuable information on ethylenecatalytic decomposition into H2 and carbon nanofibers and CNTsproduction far beyond the already published work on methanedecomposition reaction. In particular, for the first time theproduction of secondary CNTs at the low-temperature of 400 8C isproved over a novel 0.5 wt% Ni supported on CNTs. The structuraland morphological characteristics of the catalyst were studiedusing XRD, SEM, XPS and HRTEM. Characterization of the secondary

carbon nanotubes produced following ethylene decompositionwas performed using XRD, SEM, and HRTEM. Furthermore,different regeneration conditions (oxygen or steam) were inves-tigated for the removal by gasification of the amorphous carbondeposited under reaction conditions.

2. Experimental

2.1. Catalyst preparation

The carbon nanotubes coded as Ros1-B1, Ros2-B2, Ros5, andRos1-Brox85 and which were used as supports of Ni metal weresupplied by Rosseter Holdings Ltd. [20,21]. The main character-istics of the different supports used are given in Table 1. Inparticular, Ros1-B1 support consists of 50 wt% multi-walledcarbon nanotubes as shown in the HRTEM image of Fig. 1, 20–50 wt% carbon polyhedral nanoparticles, and disordered graphite.Ros1-Brox85 is mainly an oxidation product of Ros1-B1 with opencaps. Traces of Fe (�0.05 wt%) and Br (<0.05 wt%) were identifiedin Ros1-Brox85. Ros2-B2 consists of 50 wt% MWCNTs with largerconcentrations of Fe, Ni and Co (�0.3 wt%) compared to Ros1-B1.Ros5 mainly consists of disordered graphite. In all cases, the‘‘carbon’’ support was calcined in air at 250 8C for 4 h before metaldeposition. The x wt% Ni/support catalysts were prepared by thewet impregnation method using nickel (II) acetylacetonate(Ni(acac)2�4H2O (99%, Aldrich)) as metal precursor and water assolvent. After impregnation and drying (overnight at 120 8C), thecatalyst was calcined in air at 300 8C for 2 h and then reduced inpure H2 (1 bar) at 300 8C for 2 h. In the case of use of othertransition metals (e.g. Fe, Co, and Cu), inorganic and metal-organicprecursors were used, namely, iron nitrate (Fe(NO3)3�9H2O) andiron acetylacetonate Fe(III)(acac)3, copper nitrate (Cu(NO3)2�5H2O)and copper acetate (Cu(ac)�H2O), and cobalt nitrate (Co(N-O3)2�6H2O) and cobalt acetylacetonate Co(II)(acac)2. In all casesthe same synthesis and calcination steps were followed as for Ni.

2.2. Catalyst characterization

2.2.1. XRD studies

The crystal structure of the 0.5 wt% Ni/Ros1-B1 catalyst beforeethylene decomposition reaction and after reaction and regenera-tion (20% O2/He) was checked by XRD (Shimadzu 6000 series) afteremploying CuKa radiation (l = 1.5418 A).

2.2.2. SEM studies

Characterization of the particle dimensions and surfacemorphology of the Ros1-B1 support and that of supported nickelcatalyst were examined using a JEOL JSM 5200 scanning electronmicroscope (25 kV). Powdered specimens were spread on the SEMslabs and were sputtered with gold. EDX microanalysis wasperformed for qualitative elemental analysis.

2.2.3. TEM and HRTEM studies

TEM studies were conducted over the 0.5 wt% Ni/Ros1-B1catalyst as prepared, after 3 h and 14 h of continuous ethylenedecomposition reaction (in the latter case complete deactivationwas achieved), and after regeneration. The as prepared catalystswere calcined in air at 400 8C for 2 h and then reduced in H2 (1 bar) at300 8C for 2 h before TEM measurements. A 5-mg powder samplewas dispersed in 1 ml of methanol and kept in an ultrasonic bath for1 h. The sample was then deposited on a copper grid and dried at25 8C. A JEOL 1010 electron microscope operated at an accelerationvoltage of 80 kV was used. The specimen for High-resolutionTransmission Electron Microscopy (HRTEM) analyses was preparedfrom a powder sample following conventional crushing in an agatemortar and dispersing the resulting powder in acetone. It was thenplaced on holey carbon microgrids. Micrographs were takenusing a JEOL-JEM-300F field emission electron microscopeoperated at 200 kV.

2.2.4. X-ray photoelectron spectroscopy studies

X-ray Photoelectron Spectroscopy (XPS) studies were con-ducted using a VG Escalab 200 R spectrometer equipped with a

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hemispherical electron analyzer (SPECS LH-10) and an MgKa(1253.6 eV) X-ray source. The XPS spectrometer (base pressure8 � 10�10 mbar) was equipped with an environmental chamber forsample treatment under controlled gas atmosphere and attemperatures lower than 700 8C. A certain region of the XPspectrum was scanned a number of times in order to obtain a goodsignal-to-noise ratio. The binding energies (BE) were referenced tothe C 1s peak (285 eV) as internal standard to take into accountcharging effects.

2.3. Catalytic studies

The gas flow system used for performing catalytic measure-ments at 1 atm total pressure consisted of a mass flow measuringand control system (MKS Instruments, Model 247C), mixingchambers, a quartz fixed-bed micro-reactor (2 ml nominalvolume), and a mass spectrometer (Omnistar-Balzers) analysissystem. The flow system, the micro-reactor and the analysissystem used have been described in detail elsewhere [24]. A feedstream consisting of 1.12 vol% C2H4 in He was used in all catalyticexperiments. The amount of supported Ni catalyst used was 30 mgdiluted in 120 mg of SiO2 (150 mg of total mass) and the total flowrate was 50 NmL/min, resulting in a GHSV of about 20,000 h�1.

Regeneration of the completely deactivated supported Nicatalyst was carried out as follows. Initially, the deactivated

catalyst sample was treated in a 20% O2/He gas mixture at 400 8Cand the CO (m/z = 28) and CO2 (m/z = 44) signals were recordedcontinuously by on line mass spectrometer. Oxidation wasconsidered complete if no further CO2 and/or CO production wereobserved. The catalyst sample was then reduced by H2 pulses at400 8C until no further consumption of hydrogen was noticed. Asubsequent reaction/regeneration cycle was then initiated. Theamount of hydrogen used during catalyst reduction step was0.18 mol H2/gNi in the case of 0.5 wt% Ni/Ros1-B1. This amount isabout 1% of the hydrogen produced in every reaction/regenerationcycle.

The hydrogen product yield (moles H2/g metal) wasestimated by integrating the H2 gas concentration transientresponse curve recorded at the exit of the micro-reactor with on

line mass spectrometer, and using the metal loading of thecatalyst prepared. The ‘‘carbon’’ capacity (g ‘‘carbon’’/g metal)was estimated by performing a carbon material balance basedon the continuous recording of C2H4 signal from the massspectrometer and any COx or other hydrocarbons formed. Itshould be noted that no hydrocarbon (CxHy) or CO/CO2 gaseousspecies were found under the present catalytic decomposition ofethylene reaction conditions. Therefore, the carbon analysiswith time on stream was based solely on the ethylene signal. Anatomic H material balance considering the C2H4 feed and outletfrom the reactor composition, and that of H2 produced wasfound to close satisfactorily within 4%. This strongly suggeststhat the ‘‘carbon’’ accumulated on the catalyst contained verylittle H, if any.

3. Results and discussion

3.1. Catalyst characterization

3.1.1. X-ray Diffraction (XRD) studies

Fig. 2 presents comparative X-ray diffractograms of the 0.5 wt%Ni/Ros1-B1 catalyst before ethylene decomposition reaction at400 8C and after reaction followed by regeneration with 20% O2/Hegas mixture at 400 8C. The sharp XRD peak at 268 2u observed forthe fresh catalyst and the smaller peaks at 42 and 538 2ucorrespond to well graphitized CNTs, graphitic onions, and turbo-strated graphite (DOG) which cannot be distinguished from eachother in the XRD patterns [25,26]. As mentioned before (seeSection 2.1), Ros1-B1 support consists of �50 wt% multi-walledcarbon nanotubes which give rise to XRD peaks in the region ofcrystalline carbon and carbon polyhedral nanoparticles. Inaddition, amorphous carbon has been partially removed fromthe solid catalyst during oxidation pre-treatment at 300 8C. Themain peak at 268 2u is the fingerprint of carbon nanotubes andgraphite according to the literature [27,28]. In the XRD pattern ofthe catalyst after ethylene decomposition followed by regenera-tion in 20% O2/He, some new peaks emerged at 35.2, 44.2, and 51.582u (Fig. 2) which correspond to Ni nanoparticles and graphite-likenanofibers (secondary nanotubes formed). The widening of the(0 0 2) reflection (268 2u) is due to amorphous-like carbon formed

Table 1Composition of carbon nanotubes supports used in the present work.

Commercial names

of CNT-based supports

CNT content Other carbon structures content Metal content

Ros1-B1 �50 MWNT �50% carbon polyhedral nanoparticles (onions) –

Ros2-B2 �50% MWNT (a) �20% carbon polyhedral nanoparticles Fe, Ni, Co (�0.3 wt%)

(b) Different graphitic forms (highly disordered

graphite, single graphite sheets, graphite platelets)

Ros5 – Disordered graphite –

Ros1-Brox85 �60% MWNT Fe (�0.05 wt%), Br <0.05 wt%)

Fig. 1. HRTEM image obtained on the as synthesized Ros1-B1 support.

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along with the secondary CNTs. These carbon structures wereresistant to oxidation at 400 8C.

3.1.2. X ray Photoelectron Spectroscopy (XPS) studies

Fig. 3a presents C 1s core level spectra (after deconvolution)obtained on the fresh 0.5 wt% Ni/Ros-1-B1 catalyst. The maincontribution arises from the component at 284.6 eV whichcorresponds to C having sp2 hybridization, namely, a chemicalenvironment of C55C bond. The second peak at 286.0 eVcorresponds to C having sp3 hybridization (C–C bond environment)and which originates from the presence of structural defects on thesurface of carbon nanotubes [29–31]. The other XP peaks observedat 287.3 and 289 eV correspond to surface carbon atoms bonded tooxygen atoms [29–31]. As seen in Fig. 3b and Table 2, O 1s peakconsists of two components, one appeared in the 531.7–531.9 eVrange corresponding to C55O bond [29], and the other one appearedin the 533.6–533.7 eV range corresponding to hydroxyl groups[32]. It is worthwhile to mention that no Ni–O bond was detectedfrom the O 1s core level spectrum, and this suggests that Nicrystallites are covered with carbon deposits having a thicknessgreater than the escape depth (�2.0–2.5 nm) of photoelectronscoming from the metal, blocking, therefore, their escape anddetection.

Table 2 presents O/C surface atom ratios for the 0.5 wt% Ni/CNTcatalysts, whereas CNT refers to Ros1-B1, Ros1-E13, Ros1-Brox85and Ros2-B2, where Ni(acac)2�4H2O was used as precursor of Nimetal. The O/C ratio provides valuable information for thepopulation of O-containing active groups, such as COOH, C–OH,C–O–C and C55O on the surface of the catalyst. It is seen that0.5 wt% Ni/Ros1-Brox85 presents the highest O/C ratio, and this isbelieved to be linked to the synthesis procedure of this particularCNT support according to which the latter is the oxidation productof Ros1-B1 [21]. The numbers in parentheses shown in Table 2correspond to the relative abundance (%) of atomic carbon andoxygen species.

3.1.3. Scanning Electron Microscopy (SEM) studies

Fig. 4 presents SEM micrographs of the Ros1-B1 support(Fig. 4a) and the supported Ni catalyst (Fig. 4b). It is clearly seenthat Ros1-B1 consists of carbon nanotubes, carbon nanoparticlesstructured in polyhedral configuration (onions-like shapedparticles) and plates of disordered graphitic carbon labelled as1, 2 and 3, respectively, in the SEM images of Fig. 4. The presenceof amorphous carbon cannot be excluded but due to its very low

concentration (<1 wt%) it escapes SEM detection and imaging.In the case of 0.5 wt% Ni/Ros1-B1 catalyst, the amount ofamorphous carbon is significantly reduced since the catalysthas been pre-treated in air at 300 8C (see Section 2.1) prior to theSEM measurements.

3.2. Catalytic studies

3.2.1. Effect of support on catalyst activity

Fig. 5a presents the H2 product yield (mol H2/gNi) obtained onthe 0.5 wt% Ni supported catalysts after reaction with a 1.12 mol%C2H4/He gas mixture at 400 8C until complete deactivation of thecatalyst, as a function of CNT support chemical composition. Asclearly shown Ni/Ros1-B1 presents the highest catalytic activitytowards hydrogen production (296 mol H2/gNi). Possible reasonsfor this exceptional performance could be an electronic effect ofsupport on Ni particles via metal–support interactions and/or thefact that the support could provide sites for the transfer of ‘‘carbon’’formed on the Ni surface to the support under reaction conditions

Fig. 2. XRD patterns of the fresh and regenerated 0.5 wt% Ni/Ros1-B1 supported

catalyst. Sharp peak at 268 2u corresponds to well graphitized CNTs, graphitic

onions, and turbo-strated graphite (DOG). Main diffraction peaks at 35.28, 44.28, and

51.58 2u (&) obtained after ethylene decomposition at 400 8C and oxygen

regeneration over Ni/Ros1-B1 correspond to new CNTs formed.

Fig. 3. X-ray photoelectron spectroscopy core level spectra of C 1s (a) and O 1s (b)

obtained on the fresh 0.5 wt% Ni/Ros1-B1 catalyst.

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(geometrical factor) [33,34]. Also, it could be stated that intrinsicsupport structural and morphological properties favouredcatalytic activity since Ros1-B1 support is composed of CNTs,graphitic nano-onions, and disordered graphite (DOG) each likelyinfluencing the reaction rate and contributing also to theincreasing longevity of active nickel sites.

It is also of great importance to point out the remarkable carboncapacity (3552 gC/gNi) obtained with this catalyst composition

compared to the other catalysts reported as depicted in Fig. 5b. Forexample, Takenaka et al. [22] reported a carbon yield of 491 gC/gNi

on a 40 wt% Ni/SiO2 catalyst during methane decomposition at500 8C. Li et al. [23] prepared a 50.8 wt% Ni/Nb2O5 with 21.2 wt%Cu as promoter, where the carbon yield was found to be 743 gC/gM

at 600 8C. It was also reported by Reshetenko et al. [35] that 75 wt%Ni–15 wt% Cu/Al2O3 catalyst gave a carbon yield of 700 gC/gM. It isalso interesting to note that a carbon yield larger than 1000 gC/gM

was reported [14] in the case of ethylene decomposition in the600–650 8C range over unsupported Ni/Cu (4:1, w/w) alloy.

The technological importance of the present result shown inFig. 5b (Ni/Ros1-B1) is even greater considering the low-temperature of 400 8C used in ethylene decomposition comparedto the higher temperatures used in many other studies[14,22,23,35]. The main factors which contribute to the formationof carbon-capacious Ni/CNT systems could be [35]: (a) theselection of the appropriate precursor compound used for catalystsynthesis, (b) the preferential textural properties of the support(porosity, size of pores), and (c) the increase liability of the systemdue to weak metal–support interactions. It could be speculatedthat the remarkable carbon-capacious behaviour of the 0.5 wt% Ni/Ros1-B1 could also be due to the activation by H2 of CNT supporttowards its increase of meso- and macro-pore volume, and, as aresult of this, the enhanced storage of ‘‘carbon’’ formed underreaction conditions.

Table 2C 1s and O 1s (eV) binding energies and O/C atomic surface ratios for the various

0.5 wt% Ni/CNT catalysts investigated. The numbers in parentheses correspond to

the relative abundance (%) of atomic C and O species.

Catalyst support (CNT) C 1s O 1s O/C atom ratio

Ros1-B1 284.6 (73)

286.0 (18) 531.9 (64)

287.3 (6) 533.7 (36) 0.015

289.1 (3)

Ros1-E13 284.6 (76)

286.0 (16) 531.7 (64)

287.3 (6) 533.6 (36) 0.011

289.0 (2)

Ros1-Brox85 284.6 (74)

286.0 (18) 531.8 (63)

287.3 (6) 533.7 (37) 0.024

289.0 (2)

Ros2-B2 284.6 (74)

286.0 (18) 531.9 (63)

287.4 (6) 533.6 (37) 0.011

289.0 (2)

Fig. 4. SEM photographs of (a) Ros1-B1 support, and (b) 0.5 wt% Ni/Ros1-B1 catalyst

pre-treated in air at 300 8C. The numbers 1, 2 and 3 correspond to MWNTs, carbon

polyhedral nanoparticles (onions), and plates of disordered graphite (DOG).

Fig. 5. (a) Effect of chemical composition of CNT support on the H2 production

(mol H2/gNi) following reaction of 1.12 mol% C2H4/He at 400 8C (GHSV = 20,000 h�1)

over 0.5 wt% Ni/CNT. (b) Comparison of carbon capacity (g C/g Ni) obtained for the

present Ni/Ros1-B1 and other supported catalysts reported in the literature

following direct catalytic decomposition of ethylene or other hydrocarbons.

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3.2.2. Effect of Ni metal loading on catalyst activity

Fig. 6 presents the effect of Ni loading on the catalytic activity interms of total H2 production per gram of Ni in the case of Ros1-B1(Fig. 6a) and Ros2-B2 (Fig. 6b) supports. It is clearly shown that forNi/Ros1-B1 and Ni/Ros2-B2 catalytic systems there is a maximumin hydrogen production at low nickel loadings, namely 0.5 and0.7 wt%, respectively. It is worthwhile to mention here that Nidispersions for the 0.5 wt% Ni/Ros1-B1 and 10 wt% Ni/Ros1-B1catalysts were 55 and 10%, respectively. The significant increase ofcatalytic activity over the low-loading catalysts could be attributedto the small Ni particles formed. According to Park and Keane [36]stronger metal–support interactions could potentially result insuppression of Ni crystallites growth under activation of thecatalyst and reaction conditions. It can be stated that the increasedH2 production could be due to the fact that at low metal loadings(small Ni clusters) metal–support interactions are favourable [37],which may control the electronic states of Ni surface against cokingformation. It is known that one of the major problems inhydrocarbons’ decomposition reactions is carbon deposition,which leads to catalyst deactivation. This reaction is controlledby metal particle size, and it is usually favoured over large metalparticle sizes [38].

Takenaka et al. [39] have studied the methane decompositionover Ni-Pd/CNF catalysts using different Pd and Ni loadings in the10–90 wt% range. The authors reported that the reaction wasfavoured at relatively low metal loadings (30–40 wt%). Li et al. [40]

reported that methane decomposition over Ni catalysts wasstrongly affected by the crystalline size of Ni. The authors reportedalso that supported Ni catalysts with particle size of about 11 nmpresented the highest carbon and hydrogen yields. A gradualincrease of Ni particle size led to deterioration of catalytic activityin terms of carbon and hydrogen yields, where the main reason forthis was the change in Ni particle size that was accompanied by achange in particle’s morphology. According to the literature [40] Niparticles with sizes larger than 100 nm are rather inactive formethane decomposition, and this seems to be also the case for thepresent C2H4 decomposition reaction with Ni loadings larger than1 wt%. Similar results to those reported in Fig. 6 were obtained alsowith the other CNT supports listed in Table 1.

The fact that there is a critical Ni particle size (ensemble sizecontrol) favouring ethylene decomposition can be understood byconsidering the steps of ethylene decomposition reaction on Nicrystal surfaces. Ethylene is firstly adsorbed and decomposed onfavourable crystallographic planes of Ni, followed by diffusion anddeposition of carbon atoms on the surface of Ni crystals. Threeimportant parameters are involved in these processes which affecthydrogen and carbon nanotubes formation: (a) the rate of ethylenedecomposition (RDC2H4), (b) the rate of carbon diffusion onto thesupport (RdC), and (c) metal–support interaction phenomena. Theoptimum case is that when the metal phase is catalytically activewith long time on stream, and this is achieved when the rate ofethylene decomposition is kept lower than the rate of carbondiffusion onto the support. An optimum Ni particle size conditionseems to be the crossover between large Ni particle sizes over whichthe unwanted coke formation is favoured, and intermediate size Niparticles. Very small Ni particles appeared to be inactive for thereaction under study since the necessary condition for secondaryCNFs formation is that large enough Ni particles should be present.Furthermore, small Ni particles are prone to strong metal–supportinteractions (SMSI) leading to particles distortions [9]. Anothercondition which regulates the activity of small Ni particles seems tobe the reaction temperature. It has been shown that for methanedecomposition towards CNFs the smaller Ni particles become moreactive as the reaction temperature increased [22].

Takenaka et al. [41] studied the structural changes of a Ni/SiO2

catalyst used for methane decomposition using XANES. Theyreported that Ni species (fresh catalyst) are transformed into Nicarbides after reaction. Given the fact that Ni carbides formationoccurs either through diffusion on the surface or diffusion throughthe surface of Ni particles, it is speculated that enhanced diffusionlimitations due to restricted size (small Ni particles) could slowdown the process of Ni carbides formation and consequently thedecomposition reaction rate [41].

3.2.3. Effect of the nature of transition metal on catalytic activity

The effect of transition metal on ethylene catalytic decom-position (1.12 mol% C2H4/He) at 400 8C was investigated for thecatalysts M/Ros1-B1 (M = Ni, Cu, Co, and Fe), and results arepresented in Table 3. The selection of the specific catalyticsupport and temperature was based on previous studies [19]which showed that Ros1-B1 support and the reaction tempera-ture of 400 8C led to an optimum H2 production. It is clear thatcatalytic activity decreases in the order: Co� Fe > Cu. Nickelmetal appears to be more active compared to Co in the loadingrange 0.3–0.7 wt% but less active in the 1–10 wt% range(compare Fig. 6a and Table 3) in agreement with other results[42–44]. For the decomposition of unsaturated hydrocarbons theproduction of filamentous carbon has been reported over Cocatalysts to be similar with Ni catalysts [45–47]. Also, it hasbeen reported that Co is active mainly at the initial stage ofreaction [48,49], whereas it deactivates with time on stream dueto carbon deposition.

Fig. 6. Effect of Ni metal loading (wt%) on the H2 yield (mol H2/gNi) following

reaction of 1.12 mol% C2H4/He at 400 8C (GHSV = 20,000 h�1) over (a) x wt% Ni/

Ros1-B1 and (b) x wt% Ni/Ros2-B2 catalysts.

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From the results presented in Table 3 the following remarks canbe made: (a) for all the catalytic systems investigated maximum H2

product yield was found at a metal loading of 0.5 wt% due to thehigh dispersion of metal particles achieved, and (b) the use oforganometallic instead of inorganic (nitrate salts) metal precursorcompounds for the impregnation of active metal phase withinthe carbon nanotube support porous structure led to catalyticsystems with increased catalytic performance. The latter could beattributed to the higher dispersion of the catalysts achieved usingorganometallic than nitrate precursors, and/or to the stabilizationof Ni particles to a favourable critical size, likely due toorganometallic entities–CNT interactions.

It was reported [42] that the chemical nature of the metalmainly affects the quality of carbon nanotubes. A correlationbetween the size of metal particle and the inner diameter of thetubes was reported [50,51]. Also, it has been reported that sincecarbon precipitates at the (1 1 1) face of fcc metals (e.g. Ni), thisleads to a correlation between the metal shape and carbonstructure. The shape of metal particles is also very important forthe given reaction. The shape of nm-sized particles in equilibriumconditions was determined and it was found that the distributionof surface planes in such conditions in fcc metal particles is 25%(1 0 0), 5% (1 1 0) and 70% (1 1 1) [35].

The kinetics of hydrocarbon decomposition and diffusion overdifferent exposed crystallographic planes appears to be different[52], where square symmetry formation of intermediate speciessuch as vinyl (CHCH2), vinylidene (CCH2), acetylene (CHCH), andethynyl (CCH) is more likely to occur. Ethylidyne (CCH3), which isthe most abundant intermediate on the (1 1 1) surfaces of manymetals [53,54] it cannot be traced on (1 0 0) metal surfaces withthe exception of rhodium. The distribution of surface crystal-lographic planes is highly influenced by metal particle size, whichcan be different in the transition metals studied (Ni, Co, Cu, Fe), andwhich in turn has an impact on the strength of metal–supportinteractions [35].

Baker and co-workers [55–58] reported that the nature ofbimetallic catalysts and the carbon-containing feed gas havesignificant effect on the crystalline structure of formed carbon.According to the results presented in Fig. 6a and Table 3, Nicatalysts appear to be superior to Fe, Co, and Cu towards H2

production for metal loadings lower than 1 wt%. This could beexplained by considering a different ‘‘carbon quality’’ producedin the present cases. It can be stated that in Ni-based catalysts thecarbon formed could be less amorphous than in the other cases.This amorphous carbon is responsible for the encapsulation ofactive metal phase and final deterioration of catalyst activity.Park et al. [59] studied the filamentous carbon formation over Ni-Fe bimetallic catalysts during decomposition of CO in thepresence of H2 (CO/H2 feed mixtures) at 600 8C using HRTEM.They found that catalysts compositions with Ni being the majorcomponent are prone towards formation of the maximum

amount of solid carbon at lower temperatures compared toFe-rich catalysts. Above this optimum temperature a decrease incatalytic activity was reported [59]. This supports the view thatthe optimum temperature for increased catalytic activity differsaccording to the transition metal used which cannot be excludedin the present work.

3.2.4. Effect of feed gas composition on catalyst activity

The best performed lab-synthesized catalytic system, 0.5 wt% Ni/Ros1-B1 was used for the evaluation of the effect of feed gascomposition in ethylene decomposition towards H2 production.Keeping the rest of parameters constant, namely: catalyst mass,reaction temperature, and total flow rate, the feed molar composi-tion of C2H4 was varied in the 0.58 to 2.32 vol% range. It is clearlyseen in Fig. 7 that H2 production passes through a maximum inethylene feed gas composition (�1.1 vol%). It is of interest tomention that in the 0.6–1.1 vol% ethylene feed composition rangethe reaction selectivity towards H2 formation was 100%; no other gasproducts were noticed. Further increase of ethylene feed concen-tration led to significant reduction in hydrogen yield (Fig. 7). In the1.4–2.3 vol% C2H4 feed composition range, formation of methaneand acetylene by-products was detected. This result is in agreementwith the work of Rodriguez et al. [57] who reported on thepromotional effect of CO on hydrogen production through ethylenedecomposition over Fe-based catalysts. This catalytic behaviour wasattributed to the lower rate of carbon deposition occurring at lowerethylene feed concentrations. By increasing ethylene feed concen-

Table 3Effect of transition metal and precursor compound used in the M/Ros1-B1 supported metal catalyst synthesis on hydrogen product yield (mol H2/g metal) for the ethylene

decomposition at 400 8C.

Metal Precursor salt Metal loading (wt%)

0.15 0.3 0.5 0.7 1 7 10

mol H2/g metal

Fe Fe(NO3)3�9H2O 0.02 0.04 0.05 0.03 0.02 0.01 0

Fe(C5H8O2)3 0.03 0.06 0.08 0.05 0.02 0.01 0

Cu Cu(NO3)2�5H2O 0.01 0.02 0.03 0.02 0.01 0 0

Cu(C2H3O2)�1H2O 0.01 0.02 0.03 0.01 0.01 0 0

Co Co(NO3)2�6H2O 3.5 4.9 5.6 4.2 3.1 1.2 0.5

Co(C5H8O2)2 4.4 5.2 6.5 4.8 4.1 2.6 1.3

Fig. 7. Effect of ethylene feed gas composition (mol%) on the hydrogen yield

(mol H2/gNi) obtained over the 0.5 wt% Ni/Ros1-B1 catalyst following reaction of

x mol% C2H4/He at 400 8C.

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tration leads to deactivation of Ni active sites. Possible reasons forthis could be the higher rates of amorphous carbon deposition whichcauses encapsulation of the metal, and/or possible H2-inducedchanges of Ni surface towards generation of crystallographic faceswith lower reactivity.

3.3. Catalyst regeneration studies

3.3.1. The use of O2/He

Fig. 8a presents the effect of oxygen feed concentration on therate of removal of amorphous carbon as a function of time in theO2/He stream during the first cycle of regeneration at 400 8Cfollowing C2H4/He reaction at 400 8C. It is seen that by increasingthe concentration of oxygen the time needed for the removal ofcarbon decreases. The quantity of CO2 produced after integration ofthe CO2 transient response curves shown in Fig. 8a was found to bevery similar (ca. 1200 g ‘‘C’’/gNi), result that illustrates that the useof 5% O2/He results in the least consumption of oxygen. It is alsovery important to mention that carbon removed during oxidativeregeneration is the ‘‘carbon’’ deposited onto the catalyst surfaceunder reaction conditions and not the amorphous carboncontained in the virgin carbon support, since prior to the catalystsynthesis the support had undergone oxidative treatment at 300 8Cwhich removed completely its amorphous carbon constituent.

In order to gain a more quantitative view of the carbondeposited on the catalytic surface, it is worthwhile to mention that

after reaction for 14 h (until complete deactivation of the catalyst)the carbon yield was found to be 3552 g ‘‘C’’/gNi. The latter quantityis larger than that obtained from the O2/He regenerationexperiments, suggesting that only part of the ‘‘carbon’’ (�34%)was removed by oxygen regeneration. This is in harmony with theHRTEM studies (see Section 3.4) which convincingly proved thepresence of secondary CNTs formed and which apparently do notreact with oxygen at 400 8C. The ‘‘carbon’’ removed is suggested torepresent ‘‘amorphous’’ carbon.

Fig. 8b presents the catalytic activity in terms of H2 and CO2

production (mol/gNi) of the fresh and regenerated 0.5 wt% Ni/Ros1-B1 catalysts after ethylene decomposition at 400 8C as afunction of the number of regeneration cycles performed. TheH2-yield related to the 1st regeneration cycle is that obtainedusing the fresh catalyst sample. CO2 production relates to thatobtained during regeneration of the catalyst with 20% O2/He gasmixture at 400 8C (see Section 2.3). It is seen that the catalystappears to be stable after the sixth reaction/regeneration cycle.After the tenth consecutive reaction/regeneration cycle the Ni/Ros1-B1 catalyst presents a maximum H2 production of56 mol H2/gNi, which is still the highest value ever reported inthe literature for both methane and ethylene decompositionreactions over supported Ni catalysts at the low temperature of400 8C. In Fig. 8b it is also shown that there is a good correlationbetween the carbonaceous deposits removed under O2 regen-eration conditions and the H2 product yield obtained at the endof reaction (before regeneration) as a function of the number ofregeneration cycles.

3.3.2. The use of H2O/He

Fig. 9 presents the effect of steam composition (x vol% H2O/He)on the removal of carbonaceous deposits from the 0.5 wt% Ni/Ros1-B1 catalyst surface at 400 8C following ethylene decomposi-tion performed at 400 8C as a function of the number ofregeneration cycles. During regeneration with steam formationof CO and CO2 was noticed. It is seen that by increasing the steamconcentration in the feed a more effective removal of depositedamorphous carbon is achieved. Comparing the oxygen and watercatalyst regeneration treatments (Figs. 8 and 9), it is seen thatoxygen is more effective. Steam appeared also to be less effective in‘‘carbon’’ oxidation towards CO2 formation, where CO is also

Fig. 8. (a) Rate of CO2 formation (mol/gNi min) during regeneration of the 0.5 wt%

Ni/Ros1-B1 catalyst at 400 8C with x mol% O2/He gas mixture (x = 5–20). (b) H2 and

CO2 yields (mol/gNi) over the fresh and regenerated 0.5 wt% Ni/Ros1-B1 catalyst. H2

yield corresponds to 1.12 mol% C2H4/He reaction at 400 8C, whereas CO2 yield

corresponds to regeneration (20% O2/He, 400 8C) conditions.

Fig. 9. CO2 production (mol/gNi) during regeneration of 0.5 wt% Ni/Ros1-B1 catalyst

at 400 8C with x vol% H2O/He gas mixture (x = 10–30).

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formed, according to the following reaction scheme:

C þ H2O @ CO þ H2 (1)

C þ 2H2O @ CO2þ2H2 (2)

It is important to notice that the use of steam as regenerative agentof Ni/Ros1-B1 catalyst caused complete deactivation of it after thesixth cycle of reaction/regeneration, whereas in the case of oxygenthe catalytic activity remains stable and at comparable level to theinitial value (Fig. 8a).

3.4. Characterization of secondary CNTs using transmission electron

microscopy

Fig. 10a presents TEM images of 0.5 wt% Ni/Ros1-B1 catalyst after14 h of continuous ethylene decomposition reaction. From this

image the presence of amorphous carbon and carbon nanotubes(indicated by arrows) which are partially covered by carbon layersare shown. According to our previous findings [19] the carbonnanotubes mainly present in the Ros1-B1 support do not exceed225 nm and 8 nm in length and diameter, respectively, althoughcarbon nanotubes which emerged after ethylene decomposition(Fig. 10a) appeared to be larger in length and diameter.

Based on these results a more detailed TEM study wasperformed. TEM images were obtained over the 0.5 wt% Ni/Ros1-B1 catalyst after ethylene decomposition at 400 8C for 3 hfollowed by oxidative regeneration (20% O2/He, 400 8C) until noCO/CO2 production was seen. Results are presented in Fig. 10b andc. The new secondary carbon nanotubes produced from ethylenedecomposition at 400 8C can be clearly observed. This is inagreement with other studies reported [10,60]. The secondary

carbon nanotubes formed appear longer (few mm) in comparison

Fig. 10. TEM images of 0.5 wt% Ni/Ros1-B1 catalyst (a) after 14 h of continuous ethylene decomposition (1.12 mol% C2H4/He, 400 8C). (b and c) TEM images after 3 h of

continuous ethylene decomposition at 400 8C followed by regeneration with 20% O2/He gas mixture.

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with those originally present in Ros1-B1 support, the formerhaving larger diameter, up to 55 nm in comparison with 8 nm inthe case of primary CNTs. According to the literature [13,36,61], thediameter of these carbon nanotubes largely depends on the metalparticle size, and as a consequence, the size distribution of carbonnanotubes implies a Ni particle size distribution. Also, the diameterof CNTs depends on the reaction temperature.

Fig. 11 presents EDX microanalysis spectrum (Fig. 11a) andHRTEM image (Fig. 11b) of a Ni crystallite which appeared in thesecondary carbon nanotubes. Based on the EDX microanalysis, it isobserved that apart from Ni and C, small amounts of Fe are alsopresent which are inherent impurities in the Ros1-B1 supportmaterial. The presence of Cu is attributed to the grid used assample holder in the particular kind of experiment. According toFig. 11b, small Ni crystallites appear at the edges of carbonnanotubes, in agreement to what reported [60], according to whicha tip-growth method for nanotubes is facilitated over Ni-basedcatalysts. It should also be mentioned that after ethylenedecomposition carbon could built up around Ni particles, leadingtherefore to misinterpretation of the actual Ni particle size(diameter of 45 nm) due to the lack of contrast between depositedcarbon and Ni particles.

Fig. 11c presents HRTEM image of secondary carbon nanotubes

produced during ethylene decomposition reaction over Ni/Ros1-B1. Ni particles are located on the top of the growing filament (tip-growth mode of CNT formation). The primary carbon nanotubes

have diameters of about 8–10 nm, whereas Ni crystallites havediameters of about 12 nm and appear at the edge of the secondaryproduced carbon nanotubes (d � 7 nm). The differences appearingin the primary and secondary CNTs dimensions could bepotentially attributed to the intrinsic three modal distributionsin lengths and diameters in the primary CNTs [20], and in the Nimetal particle size distribution which cannot be excluded underthe wet impregnation synthesis conditions applied. This is due tothe fact that Ni metal particles are potential nucleation sites forCNTs growth.

As the secondary filamentous carbon begins to grow underreaction conditions the catalytic particle escapes from the originalsupport surface. According to the EDX microanalyses performed(not shown), carbon is the most abundant element with Ni clearlyvisible, while traces of Fe were found in agreement with previousreports that these elements are inherent impurities in the Ros1-B1product in the 0.01–0.1 wt% range [20,21].

4. Conclusions

Nickel supported on different in composition CNTs has led tosignificant variations in the intrinsic catalytic activity andselectivity towards hydrogen formation in ethylene decomposi-tion. Among the different carriers used Ros1-B1 was found tolargely influence the activity of supported Ni mainly through anensemble size effect, and likely via electronic modifications of the

Fig. 11. (a) EDX microanalysis spectrum, and (b) HRTEM image of a Ni crystallite of the 0.5 wt% Ni/Ros1-B1 catalyst following 1.12 mol% C2H4/He reaction at 400 8C. (c) HRTEM

image of secondary carbon nanotubes produced by ethylene decomposition at 400 8C over the 0.5 wt% Ni/Ros1-B1 catalyst.

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nickel surface. In addition, the following conclusions are derivedfrom the present work:

(a) The performance of the catalysts in terms of H2 yield (mol/gmetal) and sustainable regeneration ability was found tostrongly depend on the nature of transition metal used. Thefollowing order was found: Co� Fe > Cu. In the case of Ni- andCo-supported catalysts, their activity in terms of H2 yield wasfound to be dependent on the metal loading regime.

(b) There is an optimum loading of Ni in the 0.15–10 wt% rangeinvestigated, namely 0.5–0.7 wt% for maximum H2-yield in theethylene decomposition reaction investigated.

(c) A 0.5 wt% Ni/Ros1-B1 catalyst was found to exhibit a significant‘‘carbon’’ capacity compared not only to the other catalystsinvestigated but also to those reported in the literature for thesame reaction at 400 8C.

(d) The activity of the present 0.5 wt% Ni/CNTs catalysts was foundto depend on the metal precursor used (nickel nitrate vs. nickelacetate) for depositing nickel clusters within the pore system ofCNTs. Ni(acac)2�4H2O seems to lead to more favourable metalprecursor–carbon nanotube interactions towards stabilizationof Ni particles in their critical size for the reaction of ethylenedecomposition.

(e) Regeneration of the 0.5 wt% Ni/Ros1-B1 catalyst at 400 8C in20% O2/He gas mixture was found to be much more effectivethan x vol% H2O/He (x = 10–30) used at the same temperature.The activity of the 0.5 wt% Ni/Ros1-B1 after ten consecutivereaction/regeneration cycles was found to drop by a factor ofsix compared to the fresh catalyst (296 vs. 56 mol/gNi). OnlyCO2 but not any CO formation was found under oxygenregeneration conditions.

(f) XRD and HRTEM studies revealed the growth of secondary

carbon nanotubes over the 0.5 wt% Ni/Ros1-B1 catalyst afterethylene decomposition at 400 8C. HRTEM confirmed a tip-growth formation of these new carbon nanotubes, where theirdiameter and length were found to be larger than thedimensions of the carbon nanotubes originally present in thecatalyst support.

Acknowledgements

Financial support by the Research Committee of the Universityof Cyprus is gratefully acknowledged. The research group of Dr. K.Kyriakou at the Cyprus Institute of Neurology is also acknowledgedfor performing TEM measurements. Professor Jose-Luis G. Fierro ofthe Institute of Catalysis and Petrochemistry (ICP/CSIC, Madrid—Spain) is acknowledged for XPS and HRTEM measurements andfruitful discussions.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.apcatb.2009.10.005.

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