Catalytic thermal decomposition of methane to COx-free hydrogen and carbon nanotubes over MgO supported bimetallic group VIII catalysts

Applied Surface Science 296 (2014) 100–107

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Catalytic thermal decomposition of methane to COx -free hydrogen and carbon nanotubes over MgO supported bimetallic group VIII catalysts A.E. Awadallah ∗ , A.A. Aboul-Enein, D.S. El-Desouki, A.K. Aboul-Gheit Process Development Division, Egyptian Petroleum Research Institute, Nasr City, Cairo 11787, Egypt

a r t i c l e

i n f o

Article history: Received 10 November 2013 Received in revised form 10 January 2014 Accepted 11 January 2014 Available online 20 January 2014 Keywords: Natural gas Hydrogen production Carbon nanotube MgO

a b s t r a c t Bimetallic Ni–Fe, Ni–Co and Fe–Co supported on MgO catalysts with a total metals content of 50 wt.% were evaluated for decomposition of methane to CO/CO2 free hydrogen and carbon nanomaterials. The catalytic runs were carried out at 700 ◦ C under atmospheric pressure using fixed bed horizontal flow reactor. The materials were characterized by XRD, TEM, Raman spectroscopy, surface analysis and TGA–DTG. The data showed that the bimetallic 25% Fe–25%Co/MgO catalyst exhibited remarkable higher activity and stability up to ∼10 h time-on-stream with respect to H2 production. However, the catalytic activity and durability was greatly declined after incorporating 25%Ni to either 25%Fe or 25%Co/MgO catalysts at all time on stream. The main reason for the catalytic inhibition of Ni containing catalysts is consuming NiO during the formation of rock-salt Mgx Ni(1−x) O solid solution. However, the almost complete segregation of Fe2 O3 and Co3 O4 oxides played an important role for the high activity of the Fe–Co based catalyst. TEM images illustrate that the accumulated carbon over all catalysts are multi-walled carbon nanotubes in nature. The TG data showed that a higher yield of MWCNTs was achieved over bimetallic Fe–Co catalyst compared to the Ni–Fe or Ni–Co containing catalysts. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Currently, hydrogen can be considered as an important raw material for manufacture of valuable products such as ammonia and methanol [1–3]. Additionally, it is considered as a clean energy source and its market demand is steadily increasing. Hydrogen can be extracted directly from hydrogen-rich sources such as natural gas, water, coal or ammonia [3–5]. At the present time, steam or dry reforming of natural gas is the main commercial process for H2 production [6–8]. The main problem for this process is the emission of CO or CO2 with the produced hydrogen. Therefore, hydrogen should be purified in order to be used in fuel cell. However, this problem can be solved by catalytic non-oxidative decomposition of natural gas (methane) [9–11]. An additional advantage of this process is the generation of highly valuable by-product namely, carbon nanotubes (CNTs) or carbon nanofibers (CNFs) [12–16]. The most prestigious catalysts used in the catalytic methane decomposition reaction are the transition metals of group VIII (Fe, Co and Ni) loaded on different supports, e.g., Al2 O3 , SiO2 and MgO [12,17–23]. The supported material and metal support interaction (MSI) play an essential role for the catalytic dissociation

∗ Corresponding author. Tel.: +202 01224540383; fax: +202 22747433. E-mail address: ahmedelsayed [email protected] (A.E. Awadallah). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.055

activity of methane to hydrogen and carbon materials. Strong metal support interaction inhibits the reduction rate of active metal oxide via increasing the reduction temperature or by formation of metal-support species that is difficult to reduce or even irreducible [24,25]. Moreover, a strong MSI inhibits the aggregation of metal particles on the support surface and consequently the metal dispersion is enhanced. Takenaka et al. [26] studied the performance of Ni catalyst supported on different supports, i.e. SiO2 , TiO2 , ZrO2 , MgO/SiO2 , MgO, SiO2 /Al2 O3 , Al2 O3 and graphite and observed that the SiO2 , TiO2 and graphite supports exhibited higher catalytic activity and stability, while Al2 O3 , MgO and SiO2 –MgO catalysts are inactive. In the active catalysts, Ni is present in the metallic state, whereas in the inactive catalysts Ni forms oxide compounds ( O Ni O M ; M = Mg, Al and Si) with the support [26]. The same authors [27] examined the methane decomposition reaction over Co-based catalysts supported on various supports, i.e. MgO, Al2 O3 , SiO2 and TiO2 , where the results showed that Co/Al2 O3 and Co/MgO catalysts have higher activities than Co/TiO2 and Co/SiO2 . They observed that Co particles in Co/Al2 O3 and Co/MgO possess smaller particle sizes of 10–30 nm which enhance the growth of CNFs [27]. Monzón et al. [28] used bimetallic Ni–Co/Mg–Al catalyst for methane decomposition reaction to H2 production. They observed that the bimetallic Ni–Co/Mg–Al catalyst was more stable at reaction temperatures below 580 ◦ C. It was possible to produce hydrogen and CNTs under state conditions, which is a critical

A.E. Awadallah et al. / Applied Surface Science 296 (2014) 100–107

factor for the industrial development of the process. Huffman et al. [29] have studied H2 production by catalytic decomposition of methane over a series of Fe (4.5%)–M (0.5%)/Al2 O3 catalysts, where M = Pd, Mo or Ni. All the bimetallic Fe–M catalysts reduced the reaction temperature by 400–500 ◦ C relative to non-catalytic thermal decomposition and exhibited higher activity than monometallic Fe/Al2 O3 catalyst. With respect to the catalytic performance, the Pd–Fe was the most effective catalyst, achieving H2 yield of 80% at 700 ◦ C compared to 75% and 65% on Mo–Fe and Ni–Fe, respectively [29]. The same authors have studied also the relationship between the reaction temperature and the type of deposited carbon material. In the temperature range 700–800 ◦ C, multi-walled CNTs (MWCNTs) were the dominant form of carbon, whereas at higher temperatures, the activity of the bimetallic catalysts decreased and amorphous carbon, carbon flakes, and carbon fibers were obtained [29]. Shen et al. [30] have examined the bimetallic Fe–Ni supported on Mg(Al)O for methane catalytic decomposition reaction at 600–700 ◦ C. They concluded that the nanoparticle structure of the active metals were more easily reduced at a lower temperature (600 ◦ C in H2 ) and exhibited enhanced methane decomposition activity and stability. The nanoparticles of active metals are found to favor the production of relatively more uniform MWCNTs. Ermakova et al. [31] have shown that the incorporation of Cu to silica supported nickel catalysts containing 85–95% Ni have enhanced the catalytic efficiency compared to NiO–MgO catalysts. They attributed the lower activity of MgO containing catalysts to the presence of strong metal-support interactions and the formation of NiO–MgO solid solution. Similar conclusions have been presented by Tomishige et al. [32] who observed good activity of Ni/Al2 O3 catalysts but much lower performance of the Ni–MgO catalysts. Recently, we have examined the catalytic performance of Co and Ni supported on a novel aluminosilicate hollow sphere for production of hydrogen via methane decomposition [33]. We observed the superior activity of Co based catalyst compared to the Ni based catalyst, due to the formation of nickel silicates in the catalyst structure. The difficult reducibility of Ni2 SiO4 species was the main reason for inhibiting the catalytic activity of Ni containing catalyst [33]. We may confirm that no previous studies have been carried out for comparing the bimetallic combinations of group VIII metals toward H2 production via methane decomposition, since these metals were devoted only for the synthesis of carbon nanomaterials. Therefore, the present work focuses on the evaluation of bimetallic combinations of Ni–Fe, Ni–Co and Fe–Co/MgO not only for production of pure hydrogen but also for the formation of carbon nanotubes. Also, the typical compositions of the current catalysts have never been discussed before in this reaction. Finally, the role of metal distribution on the catalyst surface as well as their interactions with the support on the catalyst performance has been widely discussed.

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∼90 ◦ C, then dried overnight at 120 ◦ C and ground into fine powders before calcination at 600 ◦ C for 4 h. 2.2. Materials characterization Powder X-ray diffraction experiments were performed for the current catalysts before and after the catalytic runs using X’Pert PRO PANalytical. The patterns were recorded using CuK␣ radiation ( = 0.1541 nm) and 2 range from 10◦ to 70◦ at a rate of 0.05 s−1 . The BET surface area was measured by physical adsorption of N2 at 77 K using Autosorb 1, Quantachrome instrument. Before each measurement, the catalyst was degassed at 200 ◦ C for 4 h to remove adsorbed impurities. Temperature programmed reduction analyses (TPR) were carried out in a quartz reactor under 5% H2 /N2 flow (30 ml/min) from ambient temperature to 1000 ◦ C at a heating rate of 10 ◦ C/min. A thermal conductivity detector was used to follow the H2 consumption. The morphology of accumulated carbon materials have been investigated by transmission electron microscopy (TEM, Model JEM-200CX, JEOL, Japan). A small quantity of the sample was dispersed in 10 ml ethanol and sonicated for 10 min. A Few drops of the resulting suspension were placed on a covered copper grid and photographed. Raman spectra of as-grown carbon nanotube samples were conducted at room temperature using SENTERRA Dispersive Raman Microscope (Bruker) equipped with a diode Nd:YAG laser and a wavelength of 532 nm from 10 to 2000 cm−1 . TGA and DTA were operated using SDT; Q600 apparatus using ∼20 mg of used catalysts at a heating rate of 10 ◦ C min−1 in an air flow of 50 cm3 /min. Carbon yield for each catalyst was calculated according to the following equation: Carbon yield % =

 % weight loss by carbon oxidation  % of residue after oxidation

× 100

2.3. Methane decomposition reaction

2. Experimental

The catalytic decomposition runs using the current catalysts were carried out in a horizontal fixed-bed quartz reactor (100 cm length and 1.5 cm ID) which was located in the centre of a tubular electric furnace. The catalyst (0.5 g) was placed in the middle of the reactor and then reduced in situ by hydrogen at 700 ◦ C for 2 h. The catalytic runs were carried out under atmospheric pressure at 700 ◦ C using undiluted high purity CH4 (99.995%) at a flow rate of 50 sccm. The effluent gases were analyzed by an online gas chromatograph (Perkin Elmer Calrus 500, USA) equipped by Alumina BOND/Na2 SO4 capillary column (30 m length and 0.53 mm ID) and thermal conductivity detector to analyze hydrogen and unreacted methane. The concentration of hydrogen and methane were determined using calibrated data. No gaseous products other than hydrogen and unreacted methane were detected using this system. After the reaction, the reactor was cooled down to room temperature in the adjusted flow of nitrogen.

2.1. Preparation of catalysts

3. Results and discussion

The monometallic 50%Ni/MgO catalyst (reference catalyst) and the current bimetallic catalysts containing 25%Ni–25%Fe, 25%Ni–25%Co and 25%Fe–25%Co supported on MgO were prepared by traditional impregnation and co-impregnation methods, respectively. Typically, the calculated quantities of metal nitrates (Analytical grade, Sigma–Aldrich) were dissolved in sufficient quantities of deionized water then added to the required amount of MgO support and mixed well to get homogeneous mixture. The mixtures were then evaporated gently under continuous stirring at

3.1. Characterization of fresh catalysts XRD patterns of the current fresh binary catalysts are presented in Fig. 1. It is observed that the diffraction peaks at 2 = 36, 43, 62, 74 and 78◦ are principally related to the MgO support or the rocksalt Mgx Ni(1−x) O solid solution. The NiO and MgO components in the catalyst can easily form a Mgx Ni(1−x) O solid solution due to the excellent mutual solubility between them [34–36]. Accordingly, the NiO is nearly consumed during the formation of solid solution. On

dV(r), cc/Å/g

A.E. Awadallah et al. / Applied Surface Science 296 (2014) 100–107

Volume, cc/g

102

0

5

10 15 Pore volume, nm

20

Fe-Co/MgO Ni-Fe/MgO Ni-Co/MgO 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Relative pressure, P/Po Fig. 2. Nitrogen adsorption/desorption isotherms and pore volume distribution (inset) for the fresh bimetallic group VIII/MgO catalysts. Table 1 Surface characteristics of fresh bimetallic Ni–Fe, Ni–Co and Fe–Co/MgO catalysts measured by N2 physisorption.

Fig. 1. XRD patterns of fresh bimetallic catalysts (a) Ni–Fe, (b) Ni–Co and (c) Fe–Co/MgO catalysts.

the contrary, there are several peaks related to both Co3 O4 and Fe2 O3 appeared separately in the XRD patterns (Fig. 1). This manifests that the solid solutions between Co3 O4 or Fe2 O3 and MgO are greatly unfavoured. The applied calcination temperature of 600 ◦ C for the current catalysts inhibits the solubility of Co3 O4 or Fe2 O3 in the MgO lattice. Wang and Ruckenstein [37] found that the formation of a solid solution between CoO and MgO is observed at a calcination temperature of 900 ◦ C. Conversely, any calcination temperature above 500 ◦ C induces a complete dissolution of NiO into the MgO lattice, reflecting a strong metal-support interaction. Moreover, it is interesting to see that the intensities of all diffraction peaks of Co–Fe/MgO catalyst are significantly decreased compared to both Ni containing catalysts (Ni–Fe and Ni–Co/MgO). This behavior indicates that the metal particles of Fe or Co are well dispersed in the MgO support. Nitrogen adsorption–desorption isotherms for the current bimetallic catalysts are depicted in Fig. 2. It is shown that the adsorption–desorption isotherms for both Ni–Fe and Ni–Co/MgO catalysts display type IV isotherms with an H1 hysteresis loops at high relative pressure, which is a characteristic of capillary condensation within mesoporous structure. Moreover, a sharp curvature in P/P0 is observed from 0.45 to 0.85 in both isotherms of binary Ni–Fe and Ni–Co containing catalysts, providing further evidence on the enhancing of mesoporous structure after incorporation of metallic precursors. Our result certifies that the inclusion of nickel nitrate in either Ni–Fe or Ni–Co/MgO catalysts creates a mesopours structure via destructing the MgO framework to form the corresponding Mgx Ni(1−x) O solid solution. On the other hand, the adsorption-desorption isotherms of Fe–Co/MgO catalyst represent type III isotherm, implying a nonporous material (Fig. 2). Also, the disappearance of hysteresis loops at the whole pressure range; indicates that almost all MgO pores were blocked by Fe2 O3 and Co3 O4 oxides. This occupation is mainly due to the physical adsorption rather than chemical adsorption as

Catalyst

SBET , m2 /g

Vp , cc/g

Dv (r), Å

Ni–Fe/MgO Ni–Co/MgO Fe–Co/MgO

52.7 68.2 24.1

0.082 0.078 0.035

29.75 21.90 16.64

in case of Ni containing catalysts. This emphasis that both iron and cobalt oxides are located separately (non-interacting) on the support surface. This finding is greatly consistent with XRD data in Fig. 1. Furthermore, the textural parameters of current bimetallic catalysts were distinctly changed by adding the NiO in the catalysts, where a considerable increase in BET surface area and pore volume are observed (Table 1). For instance, the BET surface area are 68.2 and 52.7 m2 /g for Ni–Co and Ni–Fe/MgO catalysts, respectively, but decreases to 24.1 m2 /g for the Fe–Co/MgO catalyst. Accordingly, the pore volume of Fe–Co/MgO catalyst shrinks to 0.035 cm3 /g after amounting to 0.082 and 0.078 cm3 /g for the Ni–Fe and Ni–Co/MgO catalysts, respectively. Pore size distributions presented as BJH plots are inserted in Fig. 2, which shows most of the pores in both Ni based catalysts are wider than Fe–Co/MgO catalyst. According to the above results, it can be predicted that the distribution of metal particles on the MgO surface is represented in Fig. 3. It is assumed that the NiO oxide is highly immersed in the framework of MgO support, whereas both Fe2 O3 and Co3 O4 appeared to

Fig. 3. Suggested distribution of metal particles on the MgO surface (a) Ni–Fe, (b) Ni–Co and (c) Fe–Co/MgO catalysts.

A.E. Awadallah et al. / Applied Surface Science 296 (2014) 100–107

be free on the MgO surface in case of Ni–Fe and Ni–Co based catalysts. Conversely, almost all metal particles in the Fe–Co/MgO are presented separately in the MgO surface. The metal distribution as well as the degree of metal interaction with the MgO support can be considered as one of the most important factors influencing the catalytic activity of the current catalysts. 3.2. Catalytic activity Fig. 4 presents the catalytic thermal decomposition of methane into hydrogen as a function of time-on-stream at 700 ◦ C over binary Fe–Co, Ni–Fe and Ni–Co/MgO catalysts as well as the monometallic 50%Ni/MgO as a reference catalyst. It is evident that there is a significant difference in the catalytic decomposition activities of methane over these catalysts. The Fe–Co/MgO catalyst displays a tremendous efficiency and stability toward H2 production compared to the other binary catalysts especially at longer time-onstream. During the initial period of reaction, this catalyst exhibits a moderate activity, where the overall H2 yield amounts to 45% at 5 min, then increases gradually with increasing the reaction time to reach a maximum of 86% at 270 min and maintains its activity and stability up to 570 min. On the other hand, all Ni based catalysts, i.e., 50%Ni, 25%Ni–25%Fe or 25%Ni–25%Co showed similar trends in methane catalytic decomposition activity, where these catalysts exhibited higher activity (∼70–76%) at the initial stage of reaction (5 min), beyond which these catalysts encountered drastic drop to reach as low as ∼10% after running 270 min (Fig. 4). Afterwards, the activity slightly increases with a further increase of time-on-stream to reach ca. 17% at 570 min using Ni–Fe/MgO catalyst, whereas the H2 yield remained almost unchanged on using the Ni–Co/MgO catalyst. Moreover, at longer reaction time the yield of hydrogen is close to the reaction equilibrium using the Fe–Co/MgO catalyst. In general, the Fe–Co/MgO catalyst is the most active with an overall yield of ca. 75% over that obtained on the other two bimetallic catalysts (Fig. 4). The higher catalytic methane dissociation activity and durability of Fe–Co/MgO catalyst is mainly due to the existence of large numbers of non-interacting Fe2 O3 and Co3 O4 oxide phases on the MgO surface as evident from XRD data (Fig. 1). Subsequently, the higher adsorption and faster solubility of methane molecules associated with decomposition reaction is achieved. Therefore, the active Fe or Co metals remain exposed to the reactant gas, leading to

103

enhancement in H2 production for relatively longer reaction time. The moderate activity of Fe–Co/MgO catalyst during ∼200 min is attributed to the reduction completion of Fe2 O3 to the corresponding Fe0 , where the pre treatment is sufficient for reducing all Co3 O4 oxide to its active metallic form. This means that after running 270 mim, the metallic phases dominate in the catalyst composition. The dramatic inhibition in the catalytic decomposition activities over 50%Ni, 25%Ni–25%Fe and 25%Ni–25%Co/MgO catalysts is principally related to the consumption of NiO through the formation of Mgx Ni(1−x) O solid solution. Accordingly, the outer d orbital of Ni is presumed to be occupied which inhibits the solubilization of hydrocarbon molecules in Ni particles. Furthermore, it is difficult to exclude Ni0 metal from Mgx Ni(1−x) O phase due to the excellent mutual solubility between NiO and MgO. Hence, the amount of metal particles required for methane feed was extremely low, i.e., nearly 25 wt.% of the remaining iron or cobalt oxides is responsible for the decomposition reaction. It has been proved that a nickel catalyst obtained through the reduction of the solid solution of NiO and MgO has very small nickel particle size [38]. This has been further [39] reconfirmed proving that small nickel crystallites have strong interaction with magnesia and have very high initial methane catalytic decomposition reaction rate. Ermakova et al. [31] got superior activity with a silica supported Ni catalyst containing 85–95% Ni with Cu as a dopent to the NiO–MgO catalyst. They attributed the lower activity of the NiO–MgO catalyst to the presence of a strong MSI and the formation of a NiO–MgO solid solution.

100

H2 Yield, %

80

50%Ni/MgO 25%Fe-25%Co/MgO 25%Ni-25%Fe/MgO 25%Ni-25%Co/MgO

60

40

20

0 0

100

200

300

400

500

600

TOS, min Fig. 4. Hydrogen production via methane decomposition as a function of time-onstream over bimetallic group VIII/MgO catalysts.

Fig. 5. XRD patterns of deposited carbon over used catalysts (a) Ni–Fe, (b) Ni–Co and (c) Fe–Co/MgO.

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Fig. 6. TEM images of MWCNTs grown on the catalysts: (a and b) Fe–Co/MgO, (c and d) Ni–Co/MgO, and (e and f) Ni–Fe/MgO (arrows refer to chain-like carbon nanomaterials).

3.3. Characterization of deposited carbon materials The XRD patterns of the bimetallic catalysts Ni–Fe, Ni–Co and Fe–Co/MgO are shown in Fig. 5. It is clear that the (0 0 2) diffraction peak appearing at 2 = 26◦ is attributed to growth of graphitic-like structures on the surface of all the bimetallic catalysts. Although the XRD patterns are not suitable to differentiate microstructural details between CNTs and other similar graphitic-like structures, since the CNT diffraction peaks are close to those of graphite [40], they provide a primary evidence of graphitic-like deposits. The intensity of the (0 0 2) diffraction peak can be related to the graphitization degree of as produced carbon products (Fig. 5). Generally, less ideal graphitized materials exhibit a decreased (0 0 2) diffraction peak [41]. In the present study, the Fe–Co/MgO catalyst shows the formation of (0 0 2) peak with high intensity, implying higher activity of this catalyst for the formation of CNTs compared to the other Ni containing catalysts. Moreover, the CNTs peaks are sharp over the current catalysts, indicating there are few defects in the structure of carbon nanotubes [42]. Furthermore, many diffraction peaks attributed to the metallic forms for Fe, Ni and Co appeared, which indicate reduction of metals oxides species to form the metallic species. For the bimetallic catalysts containing Fe, the peaks due to FeO disappeared completely and many peaks due to ␣-Fe and Fe3 C appear. These results indicated that Fe2 O3 in the fresh catalyst would be reduced stepwise with methane during the reaction, i.e., Fe2 O3 → Fe3 O4 → FeO → ␣-Fe metal and Fe3 C [43]. It is likely that the decomposition of Fe3 C into ␣-Fe and carbons produced filamentous carbons, as proposed by several research groups [44,45]. The resulting data are consistent with the catalytic decomposition activity toward H2 production (Fig. 4). Table 2 displays the XRD data of graphitic carbon peak at 2 ≈ 26◦ over the bimetallic used catalysts. It is shown that the relative intensity of this peak is 100% for all catalysts, reflecting the higher growth activity. Furthermore, the d-spacing of CNTs are nearly close to the ideal distance of two graphite layers (0.3354 nm),

implying the higher crystallinity of as grown CNTs over all bimetallic catalysts. Fig. 6 shows the TEM images of the bimetallic catalysts, namely: Fe–Co, Ni–Co and Ni–Fe supported on MgO. Both carbon nanotubes and carbon nanofiber are grown on the catalysts containing Fe component (Figs. 6a and 6e). As can be seen in Fig. 6a, b, e and f, filamentous carbons with different structures are formed by methane decomposition over iron-containing catalysts (Fe–Co/MgO and Ni–Fe/MgO). These carbons can be classified into two types on the basis of the structures. One is multiwalled carbon nanotubes as shown in Fig. 6a. Outer and hollow diameters for multiwalled carbon nanotubes observed in TEM images ranged from 12 to 75 nm. Another type was filamentous carbons with a chain-like structure, as can be observed in Figs. 6a and 6e [44,46]. The wall of a chainlike carbon fiber is of uneven structures and the hollows of this carbon fiber were divided into many cells. In addition, chain-like carbons fibers containing some cells filled with metallic particle is also observed (Fig. 6e). Moreover, using the catalyst containing Ni–Fe, chain-like CNFs with large diameter (∼150 nm) in addition to CNTs are formed. On the other hand, MWCNTs with lower density are formed on the Ni–Co/MgO catalyst (Figs. 6c and 6d). Significant hollow cores could be observed for the produced MWCNTs. Comparing the TEM pictures of all bimetallic catalysts, we can observe that the CNTs grown on the Ni–Co/MgO catalyst acquires low diameter among the other bimetallic catalysts (Fe-containing). Fig. 7 shows

Table 2 XRD data of graphite carbon produced over used catalysts. Catalyst Ni–Fe/MgO Ni–Co/MgO Fe–Co/MgO

2

FWHM

d-spacing (nm)

Relative intensity (%)

26.09 25.21 26.28

0.8659 0.5510 0.3936

0.340202 0.339985 0.339049

100 29 100

A.E. Awadallah et al. / Applied Surface Science 296 (2014) 100–107

105

G band

D band

400

600

ID/IG= 0.41

(c)

ID/IG= 0.57

(b)

ID/IG= 0.92

(a)

800

1000

1200

1400

1600

1800

2000

Raman shift (cm-1)

Fig. 7. HRTEM image of MWCNT deposited over Fe–Co/MgO catalyst.

the high resolution TEM image of MWCNT grown over Fe–Co/MgO catalyst. It is clearly seen that the number of graphene layers constituting a MWCNTs amounts to approximately 50 layers. This behavior is closely compatible with the corresponding hydrogen yield, indicating that the length and diameter of MWCNTs is directly proportional to the hydrogen yield. It is generally reported that the longer reaction time produces longer carbon nanotubes as well as larger number of graphene layers [47]. The TGA analysis (Fig. 8 and Table 3) determines the thermal stability, yield and purity of as-grown CNTs using the

Fig. 8. TGA and DTA analysis of CNTs produced over bimetallic group VIII/MgO catalysts.

Fig. 9. Raman spectra of as-grown CNTs over (a) Ni–Fe, (b) Ni–Co and (c) Fe–Co/MgO catalysts.

bimetallic catalysts as evident from the weight loss curves. Generally, all catalysts show similar oxidation behavior with single step degradation, indicating the absence of amorphous carbon and hence high purity MWCNTs were obtained. The oxidative stability of carbon nanotubes is influenced by defect sites in graphite walls [48] and nanotube diameters [49]. The onset, inflection and offset (end) temperatures in Table 3 represent the temperature at the initial weight loss, the maximum weight loss, and the final weight loss, respectively [41]. It is clear that the Fe–Co/MgO catalyst exhibits the highest yield of CNTs compared to the Ni based catalysts, which is in good agreement with catalytic decomposition activity (Fig. 4). Indeed, the growth of CNTs is directly proportional to the catalytic decomposition activity. CNTs with the highest inflection temperature were grown on the bimetallic Ni–Fe/MgO catalyst, indicating the highest thermal stability of carbon nanomaterial. Raman spectra are also useful for evaluating the quality and crystallinity of carbon nanomaterials. The Raman spectra of CNTs produced over the current bimetallic catalysts are depicted in Fig. 9. Two distinct bands are observed for all catalysts, the Dband appeared at 1336 cm−1 , due to the disorder carbon such as the wall defects or the presence of amorphous carbon. Based on the TGA data, the D band in our samples are due to the disordered structure of the nanotubes rather than amorphous carbon. Anyhow, the appearance of D-band refers to the formation of multiwalled CNTs. The G-band around 1568 cm−1 ; assigned to the presence of crystalline graphitic carbon. Accordingly, the intensity ratio of ID /IG ratio gives us valuable information about the graphitization degree as well as the crystallinity of CNTs [50]. It is clear that the Fe–Co/MgO catalyst produces high quality CNTs, where the ID /IG ratio of 0.41 is achieved (Fig. 9). On the other hand the ID /IG ratios for Ni based catalysts, i.e. Ni–Fe and Ni–Co/MgO catalysts are 0.92 and 0.57, respectively, indicating a somewhat lower crystallinity and graphtization degree of as-grown CNTs.

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Table 3 TGA data of deposited carbon over bimetallic Ni–Fe, Ni–Co and Fe–Co/MgO catalysts. Catalyst

Onset temp., ◦ C

Inflection temp., ◦ C

End temp., ◦ C

Carbon yield, wt.%

Ni–Fe/MgO Ni–Co/MgO Fe–sCo/MgO

474 567 505

510 675 595

547 719 659

340 411 766

4. Conclusion Several important conclusions could be obtained from the present work as follows: 1. The bimetallic combinations of group VIII metals (Fe, Co and Ni) supported on MgO have been prepared and investigated as catalysts for production of pure hydrogen and carbon nanotubes via catalytic decomposition of methane. 2. Incorporation of Ni metal in either Fe or Co/MgO catalysts enhances surface area via generating a mesoporous structure due to the formation of rock-salt Mgx Ni(1−x) O solid solution. On the contrary, Fe and Co particles could block the MgO pores leading to a nonporous structure. This behavior was attributed to the low metal support interaction in the Fe–Co/MgO catalyst. 3. The results demonstrated that the catalyst stability, activity as well as the hydrogen yield strongly depends on the amount and distribution of non-interacting metal particles on the surface of MgO support. Accordingly, the Fe–Co/MgO catalyst the most effective and most stable for hydrogen production by methane decomposition at 700 ◦ C. Hydrogen yield of as high as 86% was achieved over this catalyst due to the presence of a large number of non-interacting Fe and Co particles. On the other hand, both Ni containing catalysts showed a significant inhibition of the catalytic activities. The formation of Mgx Ni(1−x) O solid solution was the main reason for such catalytic inhibition. 4. TEM images and XRD patterns of used catalysts showed that all current bimetallic catalysts have a greater ability to accumulate MWCNTs on their surface. TEM images indicated also that the Fe based catalysts promote the formation carbon nanofibers beside carbon nanotubes. Additionally, Raman spectra revealed that the as-grown CNTs over Fe–Co/MgO catalyst acquired higher crystallinity and graphitization degree compared to the Ni containing catalysts. References [1] M. Conte, A. Iacobazzi, M. Ronchetti, R. Vellone, Hydrogen economy for a sustainable development: state-of-the-art and technological perspectives, J. Power Sources 100 (2001) 171–187. [2] M. Momirlan, T.N. Veziroglu, Current status of hydrogen energy, Renew. Sust. Energy Rev. 6 (2002) 141–179. [3] I. Dincer, Technical, environmental and exergetic aspects of hydrogen energy systems, Int. J. Hydrogen Energy 27 (2002) 265–285. [4] T.V. Choudhary, C. Sivadinarayana, D.W. Goodman, Catalytic ammonia decomposition: COx -free hydrogen production for fuel cell applications, Catal. Lett. 72 (2001) 197–201. [5] F.R. Garcia-Garcia, Y.H. Ma, I. Rodriguez-Ramos, A. Guerrero-Ruiz, High purity hydrogen production by low temperature catalytic ammonia decomposition in a multifunctional membrane reactor, Catal. Commun. 9 (2008) 482–486. [6] N. Sun, X. Wen, F. Wang, W. Peng, N. Zhao, F. Xiao, W. Wei, Y. Sun, J. Kang, Catalytic performance and characterization of Ni–CaO–ZrO2 catalysts for dry reforming of methane, Appl. Surf. Sci. 257 (2011) 9169–9176. [7] U. Izquierdo, V.L. Barrio, J.F. Cambra, J. Requies, M.B. Güemez, P.L. Arias, G. Kolb, R. Zapf, A.M. Gutiérrez, J.R. Arraibi, Hydrogen production from methane and natural gas steam reforming in conventional and microreactor reaction systems, Int. J. Hydrogen Energy 28 (2012) 7026–7033. [8] A.R. González, Y.J.O. Asencios, E.M. Assaf, J.M. Assaf, Dry reforming of methane on Ni–Mg–Al nano-spheroid oxide catalysts prepared by the sol–gel method from hydrotalcite-like precursors, Appl. Surf. Sci. 280 (2013) 876–887. [9] K.G. Ione, Problems of industrial hydrogen production in comparison with the problems of natural hydrogen extraction, Chem. Sustain. Dev. 11 (2003) 907–917. [10] T. Sandset, J. Sogge, T. Starm, Evaluation of natural gas based synthesis gas production technologies, Catal. Today 21 (1994) 269–278.

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