Al2O3 catalyst

Journal of Natural Gas Chemistry 20(2011)84–89

A parametric study of methane decomposition into carbon nanotubes over 8Co-2Mo/Al2O3 catalyst Siang-Piao Chai1 ,

Choon-Ming Seah2 ,

Abdul Rahman Mohamed2∗

1. School of Engineering, Monash University, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor, Malaysia; 2. School of Chemical Engineering, Engineering Campus, University Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, S.P.S. Pulau Pinang, Malaysia [ Manuscript received August 9, 2010; revised November 9, 2010 ]

Abstract The effects of reaction temperature, partial pressure of methane, catalyst weight and gas hourly space velocity (GHSV) on methane decomposition were reported. The decomposition reaction was performed in a vertical fixed-bed reactor over 8Co-2Mo/Al2 O3 catalyst. The experimental results show that these four process parameters studied had vital effects on carbon yield. As revealed by the electron microscopy and Raman spectroscopy analyses, the reaction temperature and GHSV governed the average diameter, the diameter distribution and the degree of graphitization of the synthesized carbon nanotubes (CNTs). Also, an evidence is presented to show that higher temperatures and higher GHSV favored the formation of better-graphitized CNTs with larger diameters. Key words methane; chemical vapor deposition; Co-Mo/Al2 O3 ; carbon nanotubes; parametric study

1. Introduction Carbon nanotubes (CNTs) have been found to possess many distinguished properties and potential applications [1−3]. During the last decades, various CNT synthesis techniques have been proposed and among these techniques, chemical vapor deposition (CVD) appears to be the most popular one in synthesizing CNTs. The synthesis of CNTs by CVD involves the decomposition of carbon precursor (e.g., CH4 , C2 H2 or C6 H6 ) catalyzed by transition metals from group VIII (Fe, Co or Ni) in the periodic table. The CVD process is sensitive to the used catalysts as well as the employed process parameters. The studies on the effects of catalysts on the morphology of CNTs synthesized from CVD have been widely reported in the literatures [4−7]. However, only limited articles studied the influence of the process parameters on CNT formation. According to Kuo et al. [8], reaction temperature and reactant flow rate are the two important factors governing the diameter of the formed CNTs. Kukovecz et al. [9] also reported that volumetric flow rates for both carrier gas and reactant are the factors controlling the quality of CNTs, whereas reaction temperature affects both the quality and quantity of the formed CNTs. Methane de-

composition over catalytic materials depends on a multitude of process variables. The effects of reaction temperature, partial pressure of methane and catalyst weight on methane decomposition were examined in this study as they have been found to have significant influences on the yield and the morphology of the synthesized CNTs [9−11]. Previously, we have reported that 8Co-2Mo/Al2O3 catalyst is efficient in growing CNTs with narrow diameter distribution [12,13]. We have also shown that moderate catalyst reduction is essential to give rise to the formation of higher graphitized CNTs with higher yield [14]. Following up the previous studies, the present study aims at investigating the effects of various process parameters, i.e., reaction temperature, partial pressure of methane, catalyst weight and gas hourly space velocity (GHSV), on the yield and the morphology of the synthesized CNTs on the reduced Co-Mo/Al2 O3 catalyst via methane decomposition process. 2. Experimental The catalytic material used for the decomposition of methane into CNTs was Co-Mo/Al2 O3 catalyst (molar ratio of Co : Mo = 8 : 2, and the weight percentage of Co and Mo is

∗

Corresponding author. Tel: +604-5996410; Fax: +604-5941013; E-mail: [email protected] (A. R. Mohamed) This work was supported by the Malaysian Technology Development Corporation (MTDC) under the Commercialization of Research & Development Fund (CRDF) (Project A/C No. MBF065-USM/05) and the Monash Internal Seed Grant (A/C no: E-9-09). Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60151-X

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10 wt%), reduced in situ by hydrogen at 550 ◦ C for 1 h and denoted as 8Co-2Mo/Al2O3 . The catalyst composition, the catalyst preparation procedures and the experimental setup have been described previously [12−15]. The product gases from methane decomposition were analyzed using an on-line gas chromatograph (GC) (Hewlett-Packard Series 6890, USA). The carbons deposited on the catalyst were analyzed using transmission electron microscope (TEM) (Philips CM12) and field emission scanning electron microscope (FESEM) (LEO Supra 50 VP). Raman spectra of the deposited carbons were measured with 514.55 nm excitation line of an argon ion laser at room temperature using Raman spectrometer (Jobin Yvon Horiba HR800UV) with power laser output of 20 mW. In the present study, the reaction conditions were set at: pressure, 1 atm; reaction time, 2 h; reaction temperatures, 650 to 800 ◦ C; partial pressure of methane (PCH4 ), 0.25 to 0.75 atm; catalyst weight, 0.2 to 0.4 g; GHSV, 3000 to 18000 ml/(h·gcat ). GHSV is defined as the volumetric flow rate of methane to the catalyst weight as given below:

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temperatures below 700 ◦ C thermodynamically do not favor the methane decomposition reaction [16].

GHSV (ml/(h · gcat)) = Volumetric flow rate of methane (ml/h) Catalyst weight (gcat ) The amount of produced carbon was estimated based on the degree of methane conversion determined by the GC, assuming that methane conversion to hydrogen and carbon proceeded stoichiometrically (CH4 → C + 2H2 ). The percentage of carbon yield is defined as follows: Carbon yield (%) = Weight of deposited carbon on the catalyst × 100 Weight of the metal portion of the catalyst 3. Results and discussion

Figure 1. (a) Kinetic curves of methane conversion as a function of time on stream and (b) carbon yield as a function of reaction temperatures. PCH4 = 0.5 atm and W cat = 0.2 g

3.2. Ef fect of partial pressure of methane Figure 2(a) shows the effect of PCH4 on methane conversion over 8Co-2Mo/Al2O3 catalyst. It can be observed that an increase in PCH4 lowered the degree of methane conversion. It is known that an increase in PCH4 reduces the residence time of methane in the reactor and the contact time for methane and the catalyst, thus decreasing the conversion of methane.

3.1. Ef fect of reaction temperature Methane conversion as a function of time on stream over 8Co-2Mo/Al2O3 catalyst was measured at reaction temperatures ranging from 650 to 800 ◦ C and the results are presented in Figure 1(a). It was noted that an increase in reaction temperature led to an increase in the initial methane conversion. The conversion of methane at 800 ◦ C declined rapidly after 30 min on stream, indicating that the catalyst lost its activity and stability at high temperatures. The result also shows that an increase in reaction temperature increased both the activation and deactivation of 8Co-2Mo/Al2O3 catalyst in methane decomposition. Carbon yields were calculated based on the degree of methane conversion and the results are presented in Figure 1(b). The highest carbon yield, being 585%, was obtained at 750 ◦ C. Obviously, carbon yield at 800 ◦ C was lower than that at 750 ◦ C. This means that carrying out the reaction at higher temperatures was not apposite due to the severe catalyst deactivation. It was shown that the catalyst tested at 650 ◦ C gave the lowest carbon yield. This is because

Figure 2. (a) Kinetic curves of methane conversion as a function of time on stream and (b) carbon yield as a function of partial pressure of methane. T = 750 ◦ C and W cat = 0.2 g

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Otherwise, methane would have a longer contact time with catalyst at lower PCH4 condition. This is the reason why the highest methane conversion was obtained at a PCH4 of 0.25 atm. In all cases, it was found that the conversion of methane increased in the PCH4 order of 0.75<0.65<0.50<0.25 (atm). Figure 2(b) shows the carbon yields as a function of PCH4 . Although PCH4 at 0.25 atm gave the highest conversion of methane, the yield of carbon recorded was the lowest, i.e., 488%. Undoubtedly, only a small amount of methane was fed into the reactor at low PCH4 and this caused less amount of produced carbon. It can be noted that an increase in PCH4 to 0.5 atm increased carbon yield to a maximum amount of 585%. It was also found that subsequent increase in the PCH4 slightly decreased the yields of carbon. In this regard, the yields of carbon at PCH4 of 0.65 and 0.75 atm were reduced to 564% and 530%, respectively. According to Villacampa et al. [11], increasing PCH4 leads to the formation of polymeric carbon species which can encapsulate and deactivate the catalyst. This explains well that a high PCH4 lowered the carbon yield as observed in this study. 3.3. Ef fect of catalyst weight The effect of catalyst weight on methane conversion over 8Co-2Mo/Al2O3 catalyst was studied by varying the catalyst weight from 0.2 to 0.4 g at the reaction temperature of 750 ◦ C and PCH4 of 0.5 atm. As shown in Figure 3(a), an increase in catalyst weight led to an increase in methane conversion. The highest methane conversion was achieved for a catalyst weight of 0.4 g. The finding shows that catalyst weight has a profound effect on the catalyst lifetime, i.e., the catalyst deactivated fast when a small amount of catalyst was used, whereas a large amount of catalyst gave rise to higher catalytic activity and longer catalyst lifetime in methane decomposition.

Figure 3(b) shows the carbon yield as a function of catalyst weight. A decrease in the yield of carbon with increasing catalyst weight was observed. 3.4. Ef fect of GHSV at dif ferent temperatures Carbon yield data were plotted as a function of GHSV at temperatures of 700, 750 and 800 ◦ C and presented in Figure 4. The presented graph shows a similar trend in the yield of carbon at all temperatures. An increase in GHSV from 3000 to 9000 ml/(h·gcat ) increased the carbon yield exponentially, indicating that the growth process is apparently limited by the carbon supply. Subsequent increases in the GHSV reduced the carbon yield gradually. This indicates that 1 g of the catalyst has the ability to uptake the maximum methane flow rate of 9000 ml/h. Consequently, an increase in GHSV, within the range of 3000−9000 ml/h, increased greatly the yield of carbon. However, when the GHSV was further increased above 9000 ml/(h·gcat ), the methane flow rate exceeded the maximum amount that the catalyst could accommodate. The excessive methane molecules deactivated the catalyst by forming either encapsulated carbon or amorphous carbon [10], resulting in lower carbon yield as observed in the experiment. This also shows that the optimum GHSV for the catalyst within the tested temperature range was 9000 ml/(h·gcat ). This value is identical to the findings when partial pressure of 0.75 atm and catalyst weight of 0.4 g applied. It was also noted that an increase in the reaction temperature from 700 to 750 ◦ C led to the highest carbon yield. This can be explained that decomposition of methane is an endothermic reaction in which higher reaction temperatures are favored. However, the yield was reduced with an increase in temperature from 750 to 800 ◦ C. This is mainly caused by the fast deactivation of 8Co-2Mo/Al2O3 catalyst at high temperatures as mentioned previously.

Figure 4. The effects of reaction temperature and GHSV on carbon yield up to 2 h reaction over 8Co-2Mo/Al2 O3 catalyst

Figure 3. (a) Kinetic curves of methane conversion as a function of time on stream and (b) carbon yield as a function of catalyst weight. T = 750 ◦ C and PCH4 = 0.5 atm

3.5. Electron microscopy analysis The CNTs deposited on all the catalysts tested at different

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GHSV and reaction temperatures were firstly characterized using TEM. The diameters for more than a hundred randomly chosen CNTs were counted from the TEM images. Figure 5 shows the average diameters and the standard deviations of the synthesized CNTs. It was clearly observed that the average diameters and standard deviations of CNTs synthesized at 800 ◦ C were slightly larger than those obtained at 700 ◦ C. This was expected that higher temperatures would result in the severe agglomeration of the metal component, which appeared in quasi-liquid state, into larger sized metal clusters [17], leading to the formation of CNTs of larger diameters and wider diameter distribution. The examination of the effect of GHSV on the diameter distribution shows that CNTs with smaller diameter were synthesized at lower GHSV, and an increase in GHSV led to the increase of CNT diameter. It has been reported that an increase in GHSV increases the solubility of carbon in the metal catalyst [18]. Probably, this makes the metal catalyst, which was supersaturated with carbon, sinter easily at high temperatures. When GHSV exceeded 9000 ml/(h·gcat ), only negligible effect on the diameter distribution was noticed. It is believed that the excess amount of methane did not take part in the reaction for growing CNTs, and thus the effect of GHSV exceeding 9000 ml/(h·gcat ) on the diameter of CNTs can be disregarded. Figure 6 shows the TEM and SEM images of CNTs synthesized at 9000 ml/(h·gcat ) at both 700 and 800 ◦ C. These images revealed that CNTs grew in high density, forming interwoven covering. It is important to note that there is insignificant morphological discrepancy of the CNTs synthesized under all reaction conditions and the observed CNTs shows structural similarities such as being long, entangled and of uniform diameter.

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Figure 5. The effects of reaction temperature and GHSV on the average diameters of the synthesized CNTs at 700 and 800 ◦ C (a) and standard deviations of CNTs diameter distributions (b)

Figure 6. TEM (a, b) and SEM (c, d) images of the synthesized CNTs at 700 ◦ C (a, c) and 800 ◦ C (b, d). GHSV = 9000 ml/(h·gcat )

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3.6. Raman spectroscopy analysis Figure 7 shows the Raman spectra of CNTs synthesized at different reaction temperatures and GHSV. For all Raman spectra, two major bands, representing D- and G-bands, were clearly seen. The D-band, observed at 1350 cm−1 , is known as either the disorder induced by the wall disorder or the presence of amorphous carbon deposited on the outer surface of nanotubes. The G-band (observed at 1590 cm−1 ) can represent the degree of graphitization of CNTs. The ratios of the intensity of D- to G-bands (ID /IG ), which is regarded as an index of the crystalline order of CNTs [19], were calculated from the Raman spectra and presented in Figure 8. D’ band was assigned to the imperfect graphite or disordered carbons [20].

Raman analysis shows that CNTs synthesized at a lower temperature of 700 ◦ C on the whole presented a higher ID /IG ratio than the CNTs synthesized at 800 ◦ C. Higher ID /IG ratio indicates less graphitization and/or higher structural defects in CNTs. It was also noted that the ID /IG ratio decreased with an increase in GHSV for CNTs synthesized at 700 ◦ C. Apparently, high temperatures and high GHSV are the indispensable factors for forming CNTs with better-graphitized wall structures. It has been reported that increases in reaction temperature and GHSV can increase the solubility of carbon in the metal catalyst [11,18]. It seems that the increased solubility of carbon in the metal catalyst contributed to the formation of CNTs with highly graphitized wall structures. For the CNTs synthesized at 800 ◦ C, the ID /IG ratio decreased from 0.787 to 0.765 with an increase in GHSV from 3000 to 9000 ml/(h·gcat ). However, the ID /IG ratio for CNTs synthesized at 18000 ml/(h·gcat ) was estimated to be 0.780, and this was slightly higher than that of CNTs synthesized at 9000 ml/(h·gcat ). The temperature as high as 800 ◦ C can lead to greater extent of thermal decomposition of methane especially when the reaction was conducted in an environment with high methane concentration, such as 18000 ml/(h·gcat ). It is known that higher thermal decomposition of hydrocarbon gases can lead to the formation of amorphous carbons [21,22]. The deposition of amorphous carbon resulted from the thermal decomposition of methane on the CNT materials is probably the cause contributing to the increase of ID /IG ratio in this experiment. 4. Conclusions

Figure 7. Raman spectra for the synthesized CNTs at temperatures (◦ C) and GHSV (ml/(h·gcat )) of (1) 700 and 18000, (2) 700 and 9000, (3) 700 and 6000, (4) 700 and 3000, (5) 800 and 18000, (6) 800 and 9000, (7) 800 and 6000 and (8) 800 and 3000

Figure 8. Changes in the ID /IG ratio as a function of GHSV at both 700 and 800 ◦ C

The present study has demonstrated that reaction temperature, partial pressure of methane, catalyst weight and GHSV affected the carbon yield over 8Co-2Mo/Al2O3 catalyst in methane decomposition. It can be noted that temperatures of either 800 or 650 ◦ C are not apposite because the former caused an increased catalyst deactivation rate and the latter did not contribute much to the catalyst activation. The suitable temperature for methane decomposition over 8Co2Mo/Al2O3 catalyst was 750 ◦ C at which the highest carbon yield was obtained. Electron microscopy and Raman spectroscopy analyses show that the average diameter, the diameter distribution and the graphitization of CNTs synthesized from methane decomposition depend greatly on the reaction temperature and GHSV. It was found that higher temperatures and higher GHSV favored the formation of better-graphitized CNTs. Also, increases in both temperature and GHSV increased the average diameter and the standard deviation of the synthesized CNTs. This is because of the agglomeration of metal catalyst into larger particles and the formation of CNTs with larger diameter. The optimum GHSV for CNT formation was found to be 9000 ml/(h·gcat ) for all temperatures tested in this study. Further increases in GHSV did not contribute to higher CNT yield because the excessive amount of methane did not take part in growing CNTs. Conversely, excessive methane contributed to the formation of the encap-

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sulated carbon and/or amorphous carbon that deactivated the 8Co-2Mo/Al2O3 catalyst and lowered the yield of CNTs. Acknowledgements The authors gratefully acknowledge the financial support provided by the Malaysian Technology Development Corporation (MTDC) under the Commercialization of Research & Development Fund (CRDF) (Project A/C No. MBF065-USM/05) and the Monash Internal Seed Grant (A/C no: E-9-09).

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