Modification of Ni state to promote the stability of Ni–Al2O3 catalyst in methane decomposition to produce hydrogen and carbon nanofibers

Journal of Solid State Chemistry 191 (2012) 107–113

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Modification of Ni state to promote the stability of Ni–Al2O3 catalyst in methane decomposition to produce hydrogen and carbon nanofibers Jiuling Chen n, Yuanhua Qiao, Yongdan Li Department of Catalysis Science & Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China

a r t i c l e i n f o

abstract

Article history: Received 3 February 2012 Received in revised form 11 March 2012 Accepted 12 March 2012 Available online 18 March 2012

The methodology was illustrated for modifying the state of Ni to promote the stability of the coprecipitated Ni–Al2O3 catalyst via incorporating ZnO and Cu in methane decomposition to produce hydrogen and carbon nanofibers. The influences of the incorporation on the state of Ni were examined with XRD, TPR, XPS and TEM. For the incorporation of ZnO, ZnAl2O4 spinel-like structure could be formed in the interface between ZnO and Al2O3. The interaction between Ni and the ZnAl2O4 structure can promote both the activity and the stability of Ni in methane decomposition. The formation of a Ni– Cu alloy from Ni and the incorporated Cu decreases the activity of Ni, however, promotes the stability pronouncedly. & 2012 Elsevier Inc. All rights reserved.

Keywords: Ni–Al2O3 catalyst Ni state ZnAl2O4 spinel Ni–Cu alloy Methane decomposition

1. Introduction Coprecipitated Ni–Al2O3 catalysts have been widely employed as catalysts in many processes in chemical industry, such as methane steam reforming, CO/CO2 methanation and hydrogenation/dehydrogenation of hydrocarbons [1]. Recently, the Ni–Al2O3 catalyst with a high Ni content was reported to possess a high activity in direct methane decomposition (CH4 ¼ Cþ2H2), which has been attracting intensive attentions as a potential route to producing hydrogen and nanocarbon materials because of being significantly simple, energy-saving and no CO2 emission by comparison with the conventional main production processes like methane steam reforming (CH4 þ2H2O¼CO2 þ4H2) and methane partial oxidation (CH4 þO2 ¼CO2 þ2H2) [2–22]. Since methane is the most stable hydrocarbon molecule, the noncatalytic thermal decomposition process has to be performed at high temperatures, e.g., thermal black process needs 1400 1C [12], however, the temperature can be lowered greatly on a Ni–Al2O3 catalyst. However, the Ni–Al2O3 catalyst is inevitably deactivated because the solid carbon produced from methane decomposition can accumulate up on the catalyst and encapsulate Ni particles from contacting the reactants [2–11,14–22]. The results in our lab and the published literature have shown that on the Ni catalyst with a high stability, solid carbon could be deposited as long nanofibers as possible with the corresponding Ni particles on

n Corresponding author. Present address: School of Chemical Engineering, the University of Queensland, St. Lucia, QLD 4072, Australia. Fax: þ61 7 33654199. E-mail address: [email protected] (J. Chen).

0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.03.019

their tops and so catalyst deactivation process can be delayed to times when the carbon amount on the catalyst is very high [2–11,14–22]. The produced carbon nanofibers have many unique properties, like high mechanical strength, high conductivity, high specific surface area and macroporous and mesoporous structure, which make them suitable for many potential applications such as polymer reinforcements for composites or breakthrough materials for energy storage, electronics and catalysis [23]. Therefore, the stability of the catalyst is a key factor to obtain a high production of hydrogen and solid carbon nanofibers. In principle, coprecipitated Ni–Al2O3 catalysts with a high Ni content possess a structure in which Ni dominates and the additive Al2O3 is uniformly incorporated into the Ni phase. The existence of Al2O3 in Ni phase can affect the crystallization of nickel particles and can cause the formation of abundant defects and dislocations in Ni crystallites. Therefore, Ni particles are active at low temperatures with a high growth rate of carbon nanofibers and a high methane conversion [4,8–11,15–22]. However, the investigation results show that the stability of Ni particles generally declines with the increasing methane decomposition rate because the diffusion rate of carbon in the bulk of Ni particles (the rate-limiting step for the growth of carbon nanofibers) becomes slower than the formation rate of carbon from methane decomposition on the surface of Ni particles and the produced solid carbon layers can not diffuse in time through the bulk of the nickel particles to precipitate as the highly-ordered carbon nanofibers and thus accumulate up and encapsulate the Ni surface [4,10,11,14–18]. This tendency becomes more severe with the increasing decomposition temperature to obtain a high conversion of methane decomposition [4,5,8–11,14–22]. Some

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investigations have shown that the state of Ni can be modified to improve the stability of Ni–Al2O3 via incorporating the third components such as Fe, Co, Cu, CeO2, ZnO and MgO [4,5,9–11,14–20]. However, the methodology for selecting the third components is still not systematically illustrated up to now. In the present work, ZnO and Cu are employed as the typical third components to show how to modify the state of Ni. In an ideal solid solution of NiO and Al2O3 obtained from calcination of a coprecipitated basic carbonate of catalyst precursor containing Ni2 þ and Al3 þ ions, the strong interaction between NiO and Al2O3 originates from the formation of an inverse NiAl2O4 spinellike structure from the neighbor Ni2 þ and Al3 þ along with the elimination of hydroxyl and carbonate ions in the calcination process [1,9–11,14–17,20–22,24,25]. In the lattice of this NiAl2O4 structure, Ni2 þ ions are in the octahedral sites and Al3 þ ions are in the adjacent octahedral sites and tetrahedral sites in a close-packed O2 ionic lattice. After reduction, Al2O3 units remain trapped in the Ni crystal lattice as the energy barrier and the strong interaction between NiO and Al2O3 units was inherited [1,9–11,14–17,20– 22,24]. Therefore, in principle, to modify the existing state of Ni, the third components should at least either have an interaction with Ni or with Al2O3 to influence the previous interaction between Ni and Al2O3 inherited from the coprecipitated precursor. It has been found that ZnAl2O4 spinel structure is much more easily formed than NiAl2O4 when Zn2 þ , Ni2 þ and Al3 þ ions coexist in an ideal solid solution of ZnO–NiO–Al2O3 because Zn2 þ ions are much more preferable to enter into the tetrahedral sites of the interstices between O2  ions than Ni2 þ in the closepacked O2  ionic lattice [26,27]. After the formation of ZnAl2O4, the interaction between NiO and Al2O3 can be modified. Cu atoms, which can form an alloy with Ni atoms in the whole range of the atomic ratio, are rich in electrons of the d orbits and are good electron donators [28]. In a Ni–Cu alloy, electrons can transfer from Cu atoms to fill the d holes of the adjacent Ni atoms. Thus the electronic energy state of Ni atoms can be modified directly through this alloying effect [14,15,17,20,28]. Some research results have shown the incorporation of Cu can efficiently promote the stability of Ni at high temperatures [4,9–11,14,15,17–20]. The modification effects were also compared between the incorporation of ZnO and that of Cu in this work. To make the interaction between the different components to reach a maximum, Feitknecht compounds (FCs) were prepared as the precursors of Ni–Al2O3 catalysts in our lab and other research groups, which have brucite-like layers containing octahedrally coordinated bivalent and trivalent cations, as well as interlayer anions, carbonates and hydroxyls, and water [1,4,9–11,14–17,20–22,24]. Due to the difficulty of solid phase diffusion of metal ions, this precursor results in a well mixed phase of active components and promoters and subsequently, a maximum interaction between components was reached after careful calcination and reduction. The influences of the incorporation of ZnO or Cu on the state of Ni were observed using X-ray diffraction (XRD), H2 temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The activity and the stability of methane decomposition were investigated in an insitu thermal balance reactor (TBR) by recording the weight growth of the solid carbon with the time extension. The carbon nanofibres produced on the catalysts were observed with a transmission electron microscope (TEM).

2. Experimental 2.1. Catalyst preparation and characterization Catalyst precursors, FCs, were prepared by coprecipitation from a mixed aqueous solution of nitrates with sodium carbonate.

The precipitates were then washed by water, dried at 393 K for 5 h, calcinated in air at 723 K for 10 h and sieved into particles in different size distribution. The catalyst samples were expressed as aNi–bCu–cZn–dAl where a, b, c and d mean the atomic percentage of different metal elements in the catalyst samples and the sum of them is 100. Seven samples of different composition were prepared in this work. A D/MAX 2038 XRD with Fe Ka was employed to get the XRD profiles of the precursors and the corresponding metal oxide samples after calcination. TPR of the metal oxide samples was carried out in a ‘‘U’’ tube whose inner diameter was 4 mm and a sample of 30 mg between 60–80 mesh was used for each run in a heating rate of 10 K min  1 and with a H2/N2 ¼1/9 (vol.) gas stream of 25 ml min  1 (STP). XPS analysis was performed using a PHI 1600 XPS system operated at 1.2  10  8 Torr under Mg-Ka radiation with a power of 300 W. The binding energy was corrected by taking C1s, 284.6 eV, as a criteria. Before XPS measurement, the catalyst samples were first reduced at 973 K in a H2/N2 ¼1/2 (vol.) gas mixture and then cooled to the ambient temperature in N2 and passivated in a N2 flow saturated with ethanol steam for 2 h before unloaded. 2.2. Methane decomposition An in-situ TBR was used to investigate the carbon growth on the catalysts. The set-up details can be found in Refs. [16,17,21,22]. Three milligrams of catalyst particles of 260– 270 mesh in the oxidized state were used for each experiment. The reactor system was first purged with N2 and then heated in a rate of 5 K min  1 to 973 K in a H2/N2 ¼1/3 (vol.) gas stream of 45 ml min  1 (STP). The temperature was held for 30 min to allow the catalyst to be reduced fully and then the temperature was decreased to the preset values in N2. In a temperature-programmed heating experiment of carbon growth, the temperature was first decreased to 473 K and then the gas flow was switched to a CH4/N2 ¼1/2 (vol.) reaction gas of 45 ml min  1 and the temperature of the catalyst sample was increased in a heating rate of 5 K min  1 for methane decomposition. In an isothermal experiment, the temperature was decreased to 873 K and then the same reaction gas was switched on for carbon growth. The weight of solid carbon produced over the catalysts was measured and recorded continuously by TBR. The buoyancy effect can be neglected during the measurement. All the reactions ended when the weight of carbon was not increased. The product gas from TBR in the isothermal experiments was collected using gas sample bags possessing a volume of 1000 ml and analyzed with a gas chromatography (GC) equipped with a thermal conductivity detector. A JEOL JEM-100 CXII TEM was used to observe the morphology of the carbon nanofibers formed on the catalyst samples. The methane and nitrogen used in all the experiments were nominally 99.999% pure. The hydrogen was 99.99% in purity. All the reactions were performed under atmospheric pressure.

3. Results and discussion 3.1. Influences on structure of Ni/Al2O3 The XRD patterns of the seven different coprecipitated catalyst precursors showed that they were all crystallized FC structure, which indicated atomic uniform dispersion of all the metal ions, Ni2 þ , Zn2 þ , Cu2 þ and Al3 þ in these samples (Fig. 1). The 2y degrees of the diffraction peaks of the samples containing Zn2 þ or Cu2 þ , shift to the corresponding lower positions with the replacement of Ni2 þ by Zn2 þ or Cu2 þ in FC phase of 75Ni–25Al. The

J. Chen et al. / Journal of Solid State Chemistry 191 (2012) 107–113

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G G

I/I0

I/I0

F E

F E D

D C B

C B

A A

20

30

40

50 2θ (°)

60

70

80

90

Fig. 1. XRD patterns of the seven coprecipitated precursors (Fe Ka); (A) 75Ni– 25Al; (B) 73Ni–2Zn–25Al; (C) 56Ni–19Zn–25Al; (D) 50Ni–25Zn–25Al; (E) 50Ni– 12.5Zn–12.5Cu–25Al; (F) 50Ni–8Zn–17Cu–25Al; (G) 50Ni–25Cu–25Al; X: Ni6Al2(OH)16CO3.4H2O (JCPDS#150087).

above tendency shows that the layer distance of FC precursors becomes wider after the incorporation of Zn2 þ or Cu2 þ due to the ˚ and Cu2 þ (0.72 A) ˚ size of the ionic diameter of Zn2 þ (0.74 A) ˚ [29]. being bigger than that of Ni2 þ (0.69 A) Among the XRD patterns of the seven mixed metal oxides obtained after the calcination of the corresponding FC precursors, only broadened diffraction peaks of NiO phase were seen and ZnO, CuO or Al2O3 phases were not identified even when the atomic percentage of Zn2 þ , Cu2 þ or Al3 þ are as high as 25% (Fig. 2). Additionally, the 2y degrees of the corresponding NiO diffraction peaks of six samples containing Ni2 þ , Zn2 þ and/or Cu2 þ and Al3 þ shift to lower positions than those of 75Ni–25Al. Since the crystal structure of NiO (cubic, face-centered, JCPDS#040835) is different from that of ZnO (hexagonal, primitive, JCPDS#800074) and that of CuO (monoclinic, end-centered, JCPDS#801916), and the 2y degrees of the diffraction peaks of NiO are different from those of g-Al2O3 (cubic, face-centered, JCPDS#011307), the above results indicate that in the seven samples, poorly-crystallized NiO dominates the system with the uniform dispersion of ZnO, CuO and Al2O3 in the lattice of NiO, which causes the distance of the crystal lattice of NiO become wider. Therefore, a maximum interaction can be formed among NiO, ZnO and/or CuO, and Al2O3. However, no phases of the wellcrystallized spinels NiAl2O4, ZnAl2O4 or CuAl2O4 were identified by XRD. There is only one big peak of hydrogen consumption in TPR curves of the four samples, 75Ni–25Al, 73Ni–2Zn–25Al, 56Ni– 19Zn–25Al and 50Ni–25Zn–25Al (Fig. 3(A)–(D)), which can be assigned to the reduction of NiO. Two peaks can be seen for the other three samples, 50Ni–12.5Zn–12.5Cu–25Al, 50Ni–8Zn– 17Cu–25Al and 50Ni–25Cu–25Al, in which one in low temperature range should be attributed to the reduction of CuO, and the other in high temperature range should correspond to that of NiO because ZnO and Al2O3 are unreducible in hydrogen under these conditions. By comparison with those of 75Ni–25Al, with the increase of ZnO content, the initial temperature and the maximum temperature of the NiO reduction peaks are increased from about 580 to 620 K and from about 770 to 880 K, respectively (Fig. 3(A)–(D)), which suggests that the incorporation of ZnO cause the reduction of NiO become more difficult. On the one hand, the unreducible ZnO in NiO particles may obstruct the diffusion and the

30

40

50

60 2θ (°)

70

80

90

Fig. 2. XRD patterns of the seven metal oxides obtained after calcination of the coprecipitated precursors at 723 K (Fe Ka); (A) 75Ni–25Al; (B) 73Ni–2Zn–25Al; (C) 56Ni–19Zn–25Al; (D) 50Ni–25Zn–25Al; (E) 50Ni–12.5Zn–12.5Cu–25Al; (F) 50Ni–8Zn–17Cu–25Al; (G) 50Ni–25Cu–25Al; X: NiO (JCPDS#040835); .: ZnO (JCPDS#800074); }: g-Al2O3 (JCPDS#011307); ~: CuO (JCPDS#801916).

G

H2 consumption

10

F E

D C B

400

600

800 1000 Reduction temperature (K)

A

1200

Fig. 3. TPR profiles of the seven metal oxides obtained after calcination of the coprecipitated precursors at 723 K in a heating rate of 10 K min  1 in a H2/N2 ¼1/9 (vol.) gas flow; (A) 75Ni–25Al; (B) 73Ni–2Zn–25Al; (C) 56Ni–19Zn–25Al; (D) 50Ni–25Zn–25Al; (E) 50Ni–12.5Zn–12.5Cu–25Al; (F) 50Ni–8Zn–17Cu–25Al; (G) 50Ni–25Cu–25Al.

adsorption of hydrogen on the adjacent NiO so as to increase the reduction temperature. On the other hand, small units of ZnAl2O4 spinel-like structure may be formed from ZnO and Al2O3 to trap NiO particles and to make them more stable although XRD could not identify the presence of ZnAl2O4 phase (Fig. 2) [30]. For the other three samples containing CuO, with the increasing content of CuO instead of ZnO, the initial reduction temperature of NiO is decreased from about 580 to 540 K. This may be because H2 molecules are first adsorbed and activated on the reduced Cu atoms and then diffuse to the surface of the adjacent NiO promoting the reduction of NiO [17]. Although ZnO is contained in two samples 50Ni–12.5Zn–12.5Cu–25Al and 50Ni–8Zn–17Cu– 25Al, the presence of Cu still decreases the reduction temperature of NiO. The influence of the incorporation of ZnO or Cu on the state of Ni was further investigated using XPS to measure the binding

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energy of Ni2p3/2 and Zn2p3/2 in 75Ni–25Al, 50Ni–25Zn–25Al and 50Ni–25Cu–25Al (Fig. 4) after the oxidized surface layers were peeled off via being bombarded by high-energy Ar þ ions under high vacuum. The Ni2p3/2 binding energy was 853.4, 852.4 and 852.1 eV for Ni0, and 856.2, 855.4 and 855.1 eV for Ni2 þ , in 75Ni– 25Al, 50Ni–25Zn–25Al and 50Ni–25Cu–25Al, respectively. The detected Ni2 þ atoms may be attributed to those unreduced during the reduction process or re-oxidized in the passivation process. It can be seen that the incorporation of ZnO or Cu into Ni–Al2O3 decreases the binding energy of Ni2p3/2 of both Ni0 and Ni2 þ . The binding energy of Zn2p3/2 in 50Ni–25Zn–25Al is 1022.0 eV, similar to that in ZnAl2O4 spinel (1021.9 eV) [31], indicating the formation of ZnAl2O4 spinel-like structure in the interface between ZnO and Al2O3 although XRD cannot identify the segregated phase of well-crystallized ZnAl2O4 spinel (Fig. 2). The formation of a Ni–Cu alloy in 50Ni–25Cu–25Al has been found using XRD and selected area electron diffraction techniques in Ref. [20]. In Ni–Al2O3, the strong interaction between Ni and Al2O3 can cause the core-hole relaxation effects of Ni atoms and the positive shift of binding energy of Ni [32]. However, after the formation of ZnAl2O4 spinel-like structure, the previous interaction between Ni and Al2O3 is replaced by that between Ni and ZnAl2O4, causing a negative shift of the binding energy of Ni. For the incorporation of Cu, the electrons can be transferred from Cu to Ni in the Ni–Cu alloy to decrease the binding energy of Ni. By comparison, the alloy effect is slightly more effective to modify the energy state of Ni under the employed conditions.

3.2. Promoting effects in methane decomposition The methane decomposition reaction is an endothermic process and the conversion of methane and the concentration of hydrogen products are both increased with the decomposition temperature. Therefore, the carbon growth from methane decomposition at different temperatures was investigated on the seven samples shown in Fig. 5. It should be specified that the reaction quickly enters into the internal diffusion control and the impact of internal diffusion on the apparent activation energy of carbon growth is increased with the continuous accumulation of solid carbon on Ni particles [33]. Therefore, it is difficult to extract the information of the reaction kinetics under the present conditions. According to Refs. [3,10,15–17], the activity of the catalysts in this work can be expressed as the number of milligrams of the formed solid carbon per milligram of nickel per minute, mgC mgNi  1 min  1. In Fig. 5, the growth rates of solid carbon on the seven samples show the similar trend, first increasing with the growth temperature, then reaching a peak value and finally decreasing. On 75Ni– 25Al, 73Ni–2Zn–25Al, 56Ni–19Zn–25Al and 50Ni–25Zn–25Al, the initial temperatures for carbon growth are all around 660 K and the growth rates reach the peak values at 803, 841, 875 and 890 K, respectively. It can be seen that except that of 73Ni–2Zn– 25Al, the curves of the growth rate shift to the high temperature range with the increasing ZnO content. This indicates that the incorporation of the suitable amount of ZnO could decrease the carbon growth rate in the low temperature range; however, improves the stability in the high temperature range. On

Ni2p3/2

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855.1

860

Ni2p3/2

855.4

850

848

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1024

1022

1020

1018

Binding energy (eV)

Fig. 4. Binding energy of Ni2p3/2 and Zn2p3/2 in the reduced samples, 75Ni–25Al (A), 50Ni–25Zn–25Al (B) and (D) and 50Ni–25Cu–25Al (C).

J. Chen et al. / Journal of Solid State Chemistry 191 (2012) 107–113

Carbon growth rate (mgC.mgNi-1.min-1)

Carbon amount (mgC.mgNi-1)

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Growth temperature (K)

Fig. 5. Carbon growth amount (A) and growth rate (B) from decomposition of CH4/N2 ¼ 1/2 (vol.) on the seven samples in a temperature-programmed heating mode in a rate of 5 K min  1 in TBR; (a) 75Ni–25Al; (b) 73Ni–2Zn–25Al; (c) 56Ni–19Zn–25Al; (d) 50Ni–25Zn–25Al; (e) 50Ni–12.5Zn–12.5Cu–25Al; (f) 50Ni–8Zn–17Cu–25Al; (g) 50Ni–25Cu–25Al.

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Fig. 6. Carbon growth amount (A and B) and growth rate (C and D) from decomposition of CH4/N2 ¼ 1/2 (vol.) on the seven samples at 873 K in TBR; (a) 75Ni–25Al; (b) 73Ni–2Zn–25Al; (c) 56Ni–19Zn–25Al; (d) 50Ni–25Zn–25Al; (e) 50Ni–12.5Zn–12.5Cu–25Al; (f) 50Ni–8Zn–17Cu–25Al; (g) 50Ni–25Cu–25Al.

50Ni–12.5Zn–12.5Cu–25Al, 50Ni–8Zn–17Cu–25Al and 50Ni– 25Cu–25Al, the initial temperatures for carbon deposit are all around 750 K and the growth rates reach the peak values at 963, 965 and 1019 K, respectively. Compared with that of ZnO, the incorporation of Cu clearly decreases the carbon growth rate on Ni and shifts the peak temperature to much higher positions. Additionally, it can be seen when ZnO and Cu are both

incorporated, the latter shows more impacts on the performance of Ni, which is consistent with the results of TPR. When the carbon growth was carried out at a constant temperature, 873 K, their growth rates show the similar trend as that in the above temperature-programmed mode (Fig. 6). The catalytic properties of the seven samples for the growth of solid carbon in methane decomposition were listed in Table 1. From

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Table 1 Catalytic properties of the different samples in methane decomposition at 873 K in TBR. Catalyst samples

75Ni–25Al 73Ni–2Zn–25Al 56Ni–19Zn–25Al 50Ni–25Zn–25Al 50Ni–12.5Zn– 12.5Cu–25Al 50Ni–8Zn–17Cu– 25Al 50Ni–25Cu–25Al a b

Extension time rmaxd (min) (mgC mgNi  1 min  1)

ta t1/2b

tc

10 28 8 27 14 32 20 48 3 105

50 54 56 98 250

We (mgC mgNi  1)

1.24 1.25 1.55 1.62 0.92

43 36 56 98 97

97

260 0.91

105

10 480

780 0.28

133

3

t, time to reach the peak value of the growth rate; t1/2, time for the decrease to half of the corresponding peak value of the

growth rate; c t, the total growth time; d rmax, the peak value of the growth rate; e W, the number of milligrams of the carbon deposition per milligram Ni.

the results in Fig. 6(A) and (C) and Table 1, it can be seen that except that on 73Ni–2Zn–25Al, the carbon growth rate is always higher on 56Ni–19Zn–25Al and 50Ni–25Zn–25Al than that on 75Ni–25Al. However, even on 73Ni–2Zn–25Al, before reaching the peak value at 8 min, the growth rate is also higher than that on 75Ni–25Al. Additionally, with the incorporating amount of ZnO, the peak values of the growth rate are increased and except that on 73Ni–2Zn–25Al, the time for the decrease of the peak values to their halves is extended as well. These results show that the incorporation of the suitable amount of ZnO can improve both the activity and the stability of Ni particles in Ni–Al2O3. As illustrated in Fig. 6(B) and (D), the carbon growth rates and their corresponding peak values on the three catalysts containing Cu are much lower than those on 75Ni–25Al. However, the growth time and the time for the decrease of the peak values of the growth rate to their halves are extended pronouncedly, especially on 50Ni–25Cu–25Al (Table 1). Obviously the activity of Ni particles is decreased after the incorporation of Cu; however, the stability is enhanced. This balances in the highest amount of the solid carbon formed on 50Ni–25Cu–25Al among the seven samples. When ZnO and Cu are both incorporated, the latter shows more impacts on the performance of Ni. The product gas from TBR was collected in gas sample bags and analyzed with GC. The GC analysis showed that hydrogen is the only gas product and no CO/CO2 was found to be produced. 3.3. Morphology of formed carbon nanofibers Fig. 7(A) shows a typical carbon nanofiber formed at 873 K on 75Ni–25Al, whose inner diameter is thin and a droplet-like catalyst particle is located in its tip, similar to that reported in Refs. [2–4,14–17]. The morphology of carbon formed on 73Ni– 2Zn–25Al is similar to that on 75Ni–25Al. The carbon formed on 56Ni–19Zn–25Al also possesses the similar morphology, however, with a clearly hollow inner diameter (Fig. 7(B)). On 50Ni– 25Zn–25Al, carbon nanofibers grew in two different directions originating from one polyhedral catalyst particle (Fig. 7(C)). The appearance of the droplet-like catalyst particles indicates that they may be in the quasi-liquid state and have a good interfacial wetting effect with the produced carbon layers under the reaction conditions, while the polyhedral catalyst particles may imply that they may not enter into the quasi-liquid state. We have suggested that the nickel particles in the quasi-liquid state may show a weak stability and be easily encapsulated by the growing carbon

Fig. 7. Morphology of carbon nanofibers formed from methane decomposition at 873 K on four samples, 75Ni–25Al (A), 56Ni–19Zn–25Al (B), 50Ni–25Zn–25Al (C) and 50Ni–25Cu–25Al (D).

layers [20]. The incorporation of ZnO in a suitable content to Ni– Al2O3 could delay the appearance of the quasi-liquid state and weaken the wetting effect with the carbon layers, which may be attributed to the interaction between the formed ZnAl2O4 spinellike structure and Ni particles. On 50Ni–12.5Zn–12.5Cu–25Al, 50Ni–8Zn–17Cu–25Al and 50Ni–25Cu–25Al, the carbon is octopus-shaped where several solid fibrous carbon grown from a polyhedral catalyst particle (Fig. 7(D)) [4,5,15,17,18,20]. This result suggests that the mechanism of carbon growth on the samples containing Cu should be different from that on 75Ni– 25Al or the samples only containing Ni, Zn and Al. Additionally, when ZnO and Cu are both incorporated, the latter shows more impacts on the mechanism of carbon growth, which is consistent with the results of the activity test (Figs. 5 and 6). It has been suggested that the catalyst particles of 50Ni–25Cu–25Al may have been reconstructed and re-crystallized before growing carbon nanofibers in the induction period, which can efficiently prohibit Ni particles entering into quasi-liquid state in methane decomposition [20]. On the one hand, Cu, which is not active to adsorb methane, is generally enriched in the surface of Ni–Cu alloy and decreases the available number of Ni atoms to decompose methane [28]. On the other hand, the adsorption strength of methane on Ni atoms can be weakened because the electronlacking state of Ni is relieved owing to attracting electrons from the adjacent Cu atoms. Additionally, Cu could enhance the hydrogen mobility on Ni particles, which was favorable to the gasification of the accumulating carbon layers produced from methane decomposition on the surfaces of Ni particles [34,35]. The above roles of Cu are advantageous to decreasing the rate of methane decomposition into carbon layers on Ni surface. Further, Cu has a high affinity with the graphite structure, from which the dissolution process of carbon into Ni particles may be enhanced [33]. Therefore, the formation and deposition of carbon layers from methane decomposition on Ni can be retarded and the diffusion of carbon in the bulk of Ni particles can be enhanced, resulting in the timely growth of carbon nanofibers and the delay of the deactivation process.

4. Conclusions In the mixed metal oxides of NiO, ZnO, CuO and Al2O3 obtained from calcination of the coprecipitated precursors, NiO phase dominates and the other components are uniformly dispersed in

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