γ-Al2O3 catalyst in methane

Journal of Natural Gas Chemistry 19(2010)617–621

In situ carbon nanotube synthesis by the reduction of NiO/γγ -Al2O3 catalyst in methane Dongyan Xu∗ ,

Haizhen Wang,

Qingjie Guo

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, Shandong, China [ Manuscript received July 12, 2010; revised July 30, 2010 ]

Abstract The synthesis of carbon nanotubes (CNTs) via chemical vapour deposition of methane on NiO/γ-Al2 O3 catalyst has been investigated. The reduction behavior of NiO/γ-Al2 O3 by methane was studied using thermogravimetric (TG) and X-ray diffraction (XRD) techniques. It was found that the NiO supported on γ-Al2 O3 , was reduced to Ni0 in methane atmosphere in the temperature range of 710–770 ◦ C. The catalytic activity of NiO/γ-Al2 O3 for CNTs synthesis by in situ chemical vapour deposition of methane during the reduction was also investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the CNTs produced at various reduction temperatures. The results indicated that the reduction temperature exhibits obvious influence on the morphology and the yield of CNTs. CNTs with the diameter of about 20 nm were obtained at reduction temperature of 750 ◦ C, and higher reduction temperature (such as 800 and 850 ◦ C) led to an increase in CNTs diameter and a decrease in CNTs yield. Key words chemical vapour deposition; carbon nanotubes; NiO/γ-Al2 O3 catalyst; reduction temperature

1. Introduction Since carbon nanotubes (CNTs) were discovered by Iijima in 1991 [1], they have attracted extensive attentions because of their unique structural, electronic, thermal, and mechanical properties [2,3]. Up to now, various synthetic methods have been developed for the production of CNTs, including arc discharge, laser vaporization and catalytic chemical vapour deposition (CCVD) [4]. Compared to other synthetic methods, CCVD is a simpler and more economic technique in CNT synthesis. The advantages of CCVD for growing CNTs can be attributed to low reaction temperature, pure and a lot of aligned carbon nanotube products, and possibly high yield for large-scale applications [5–9]. The CCVD utilizes hydrocarbon gases as carbon sources and metal nanoparticles as the catalytic seeds for CNTs growth. Synthesis of CNTs via CCVD consists essentially of two steps, an initial catalyst nucleation, followed by CNTs growth, where the catalyst plays an important role in the process. Nanoparticles of transition metals are considered as the most effective catalysts. Usually, reduction of metal oxides by hydrogen is conducted prior to synthesis of CNTs [10–12]. In

this paper, methane, instead of hydrogen, was used to reduce NiO/γ-Al2 O3 catalyst, and in situ synthetic behavior of CNTs at different temperatures was investigated. 2. Experimental 2.1. Catalyst preparation The nickel catalysts were prepared by impregnating commercial γ-Al2 O3 templates with an aqueous solution of Ni(NO3 )2 ·6H2 O (Shenyang Chemical Agents Plant, China). The impregnated samples were dried overnight at 120 ◦ C and calcined at 700 ◦ C for 4 h in static air of a furnace. The theoretical content of nickel is 12 wt%. 2.2. Synthesis of carbon nanotubes Synthesis of CNTs was carried out in a horizontal reactor in order to avoid sub-pressure during the formation of CNTs. In a typical experiment, ca. 200 mg of NiO/γ-Al2 O3 catalyst was uniformly distributed in a quartz boat inside the

∗

Corresponding author. Tel: 0532-84022506; Fax: 0532-84022757; E-mail: [email protected] This work was supported by the Key Project of Chinese Ministry of Education (No. 208076), the Foundation for Outstanding Young Scientist in Shandong Province (BS2009CL004) and the Open Foundation of Chemical Engineering Subject of Qingdao University of Science & Technology (No. 20100102). Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60121-3

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reactor. The catalyst inside the vessel was gradually heated under nitrogen flow (100 ml/min) from room temperature to the desired reduction temperature. Then, methane (99.99%, 100 ml/min) was introduced to the reactor to reduce the catalyst for 1 h. During the reduction process, CNTs were simultaneously formed on the generated Ni0 sites. After reduction and deposition, the methane flow was cut off, and the reactor was cooled to the room temperature under a flow of nitrogen. 2.3. Purif ication of synthesized carbon nanotubes The as-produced carbon nanotubes were purified in two steps. About 200 mg of as-synthesized carbon sample was firstly calcined under low temperature to remove the amorphous carbon. Then, the sample was treated with concentrated hydrochloric acid (37 vol%) to dissolve the metal particles. The washed and filtered material was dried at 100 ◦ C for 5 h in air. 2.4. Characterization TG/DTG experiments were performed using a thermogravimetric analyzer (NETZSCH STA 409). About 25 mg of the as-prepared NiO/γ-Al2 O3 sample was loaded into a 5 ml alumina crucible. Initially, the system was heated from room temperature to 420 ◦ C at a rate of 30 ◦ C/min under a stream of nitrogen in a flow rate of 10 ml/min. Then, a mixed gas containing of 10%CH4 in Ar was switched to the system at a flow rate of 10 ml/min. The heating rate was controlled at 10 ◦ C/min, and a temperature range from 420 to 900 ◦ C was chosen to probe the temperature-programmed reaction. X-ray powder diffraction (XRD) patterns of the assynthesized samples were recorded by a D/MAX-2500/PC X-Ray diffractometer with Cu Kα radiation. The CNTs were examined by transmission electron microscopy (TEM, JEOL JEM-2000EX) and field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). 3. Results and discussion

reduction temperature of NiO supported on NiAl2 O4 under 10% CH4 was also reported [14]. From Figure 1, there is a marked weight gain at temperature higher than 780 ◦ C, indicating that carbon deposition occurred on the metal particles in situ as soon as the active Ni0 clusters were available. Jin et al. [15] have investigated the interaction of CH4 with NiO catalysts by CH4 -TPSR and suggested that the interaction followed a two-step mechanism. First, CH4 was oxidized to CO, CO2 and H2 O, in accompany with the reduction of NiO to Ni0 ; then, CH4 dissociation took place over Ni0 sites, generating H2 . In the metal oxide reduction reactions with methane, the reduction process begins with the adsorption of methane on active sites of the oxide surface and its decomposition to adsorbed carbon and hydrogen [16]. Thus, the reduction of NiO by methane can be expected to proceed through the following steps: CH4 (g) −→ CH4 (ad)

(1)

CH4 (ad) −→ CH3 (ad) + H(ad)

(2)

CH3 (ad) −→ CH2 (ad) + H(ad)

(3)

CH2 (ad) −→ CH(ad) + H(ad)

(4)

CH(ad) −→ C(ad) + H(ad)

(5)

C(ad) + NiO −→ CO(ad) + Ni0

(6)

CO(ad) + NiO −→ CO2 (ad) + Ni0

(7)

2H(ad) + NiO −→ H2 O(ad) + Ni0

(8)

2H(ad) −→ H2 (g)

(9)

where C (ad) represents active carbon species adsorbed on the solid surface and is substantially different from deposited solid carbon. And C (ad) can lead to blockage of pores in the oxide pellets, and thus a significant decrease in the rate of reduction happens. Obviously, the reduction temperature is also the synthesis temperature of CNTs.

3.1. Reduction of NiO/γ-Al2 O3 with methane The TG and DTG profiles of NiO/γ-Al2 O3 under the methane flow are shown in Figure 1. The thermogram shows two distinct weight losses. The initial weight loss at temperature up to 120 ◦ C represents desorption of the adsorbed water on the sample. The second weight loss from 710 to 770 ◦ C can be ascribed to the reduction of NiO. The reduction of pure NiO by methane was investigated by Alizadeh et al. using thermogravimetric technique in the temperature range of 600–725 ◦ C at atmospheric pressure. Their result showed that the complete reduction of NiO was achieved in 11 min at 725 ◦ C [13]. In this study, the reduction temperature of NiO supported on Al2 O3 is higher than that of pure NiO due to the interaction between NiO and Al2 O3 . Likewise, a little higher

Figure 1. TG-DTG profiles of Ni/Al2 O3 catalyst at methane ambience

Journal of Natural Gas Chemistry Vol. 19 No. 6 2010

3.2. XRD characterization of catalyst and CNTs Figure 2 shows the XRD patterns of the as-prepared NiO/Al2 O3 catalyst and the catalysts reduced by methane at 750, 800 and 850 ◦ C, respectively. Distinct diffraction peaks as exhibited in the XRD patterns are attributed to NiO and γ-Al2 O3 . The peaks located at 2θ = 37.1o, 43.3o, and 63.1o are ascribed to NiO while the peaks as marked by “” are attributed to Al2 O3 . For the reduced catalysts using methane as the reductant, the peaks of NiO disappeared and the characteristic peaks of Ni0 appeared at 2θ = 44.4o , 51.7o, and 76.3o (Figure 2(2)–(4)). The average size of crystalline NiO was estimated to be 6.7 nm. However, the mean crystallite sizes of the Ni particles reduced at temperatures of 750, 800 and 850 ◦ C were calculated by Scherrer’s equation, as 11, 15 and 18 nm, respectively. The results demonstrated that high reduction temperature can increase the size of Ni particles. In addition, the XRD patterns of the reduced samples display a strong peak at 2θ = 25.7o, which are assigned to (0 0 2) reflections of typical graphite. This result indicates that CNTs are well graphitized [17].

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tubes synthesized at 850 ◦ C is about 50 nm, which is much larger than that of the CNTs formed at 750 ◦ C in the range of from 20 to 30 nm.

Figure 2. XRD patterns of (1) the fresh catalyst and the catalysts reduced with methane at (2) 750 ◦ C, (3) 800 ◦ C and (4) 850 ◦ C

3.3. SEM and TEM characterization of CNTs The SEM images of as-synthesized CNTs at reduction temperature of 750, 800 and 850 ◦ C are shown in Figure 3. For the sample reduced at 750 ◦ C (Figure 3a), a large amount of CNTs along with catalyst particles at the tips of these CNTs are observed. However, when the reduction temperature increases, the amount of CNTs decreases evidently (Figure 3b and Figure 3c). From the mass variation of catalyst sample before and after the synthesis of CNTs, the yields of assynthesized CNTs at reduction temperature of 750, 800 and 850 ◦ C were calculated to be 55.3%, 50.3%, and 48.6%, respectively. As shown in Figure (3c), the diameter of these

Figure 4 shows the TEM images of the as-synthesized carbon materials without purification. It is observed that the reduction temperature exerts a noticeable effect on the formation of CNTs. The sample reduced and synthesized at 750 ◦ C (Figure 4a) presents much more CNTs than that reduced at 850 ◦ C (Figure 4c), which is in agreement with the observation from SEM images and the experimental results. It was reported that the amount of CNTs decreased significantly with reaction temperature (625–750 ◦ C) by methane CVD on Ni/SiO2 probably due to catalyst deactivation [18]. But in low temperature range, it was found that the CNTs yield increased with the rise of reaction temperature for Ni/Al catalysts [11].

Figure 3. SEM images of CNTs synthesized at reduction temperature of (a) 750 ◦ C, (b) 800 ◦ C and (c) 850 ◦ C

TEM images of the purified CNTs are shown in Figure 5. It is obvious that amorphous carbon has been removed effectively. After purification, only small Ni particles are present on the CNT, which are mostly encapsulated by the CNTs. For the purified CNTs sample synthesized at 750 ◦ C, these CNTs have a small diameter range from 20 to 30 nm

(Figure 5a). A few similar globular catalyst nanoparticles can be observed on the CNTs. The diameter of these catalyst nanoparticles is approximately equal to that of the corresponding CNTs. The CNT samples synthesized at 800 and 850 ◦ C exhibit much larger diameter than that synthesized at 750 ◦ C due to the formation of large Ni particles at relatively high re-

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duction temperature, which is also the synthesis temperature of CNTs in this study (Figure 5b and Figure 5c). Therefore,

the diameter of CNTs can be controlled by varying the size of nickel nanoparticles through the reduction process.

Figure 4. TEM images of as-synthesized CNTs at reduction temperature of (a) 750 ◦ C, (b) 800 ◦ C and (c) 850 ◦ C

Figure 5. TEM images of purified CNTs at reduction temperature of (a) 750 ◦ C, (b) 800 ◦ C and (c) 850 ◦ C

4. Conclusions

References

In the present work, nickel oxide supported on γ-Al2 O3 was successfully reduced to Ni0 by methane. The reduction of NiO to Ni0 occurred at temperature of 710–770 ◦ C. CNT was synthesized in situ by methane through the reduction of NiO/γ-Al2 O3 catalyst. The diameter of the CNT can be controlled by varying nickel nanoparticle size through the reduction temperature. The CNTs with diameters about 20 nm were obtained at the reduction temperature of 750 ◦ C. Reduction at higher temperature led to a decrease of CNTs in yield and an increase in CNTs diameter.

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