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Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor
Applied Catalysis A: General 260 (2004) 223–228
Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor Qian Weizhong a,∗ , Liu Tang a , Wang Zhanwen a , Wei Fei a , Li Zhifei a , Luo Guohua a , Li Yongdan b a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Department of Chemical Engineering, Tianjin University, Tianjin 300072, China
b
Received in revised form 29 September 2003; accepted 20 October 2003
Abstract Methane decomposition over a Ni/Cu/Al2 O3 catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Catalyst deactivation; Multi-stage fluidized bed reactor; Nickel catalyst; Methane decomposition
1. Introduction The catalytic decomposition of methane, under a condition where activated oxygen species (H2 O, CO, O2 , etc.) are absent, is a relatively short route to produce hydrogen without carbon oxides [1–7]. The hydrogen product has no pollutants released in burning, and can be used directly as fuel for proton exchange membrane fuel cell (PEMFC), with poor CO-tolerance (less than 10 ppm) [8]. To break the strong C–H bond of the methane molecular in this endothermic reaction, a high temperature is thermodynamically required and a catalyst with high activity is needed [1–3,9,10]. However, the solid carbon products simultaneously produced, in large amounts in high temperature [1–5,7,11], destroy the structure of the catalyst and make the catalyst deactive rapidly, through blocking the active pores of catalyst [2] or encapsulating the whole catalyst particles [7,11]. Chen et al. proposed that the catalyst deactivation should be attributed ∗ Correspondence author. Tel.: +86-10-62789041; fax: +86-10-62772051. E-mail address: [email protected] (Q. Weizhong).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.10.018
to the high carbon production rate and the relatively low carbon diffusion rate [7]. Therefore, for one to maintain the equilibrium between carbon production and diffusion for the continuous operation, the low temperature condition has to be adopted [2,3,7]. However, the carbon production rate and diffusion rate are both low and hence the conversion of methane is very low in such condition, and a large amount of methane has to be recycled. Since the methane decomposition was conducted in a uniform temperature condition in many previous studies [1–9], the carbon production and diffusion on the catalyst also occurs simultaneously in this condition. Thus it is impossible to combine the advantage of the high efficiency of methane decomposition at a high temperature condition and the high thermal stability of catalyst at a low temperature condition together. In this contribution, a novel two-stage fluidized bed reactor is adopted, which allows different temperature operations at different stages. Thus the catalytic cycle of carbon production and carbon diffusion over catalyst can be tailored when the catalyst moves alternatively in two different temperature conditions. And the conversion of methane and the stable life of catalyst in high temperature condition are increased significantly.
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Fig. 1. Two-stage nano-agglomerate fluidized bed reactor for methane decomposition.
2. Experimental The reactor is made of stainless steel, with an inner diameter of 40 mm and a height of 1000 mm (Fig. 1). A horizontal perforated plate with 10% holes is used to divide the reactor into two stages. This separation prevents the backmixing of gases between stages effectively, but allows the fluidized solid materials to move easily between stages [12,13]. The height of the lower and upper stage is 150 and 800 mm, respectively. A heat exchanger is adopted (about 50 mm high) between two stages. Thus the temperature of two stages can each be controlled accurately. The temperature of the low stage is fixed at 773 K, while the temperature of the high stage can be controlled from 773 to 1123 K. As an example, one catalyst used is a Ni/Cu/Al2 O3 catalyst, prepared from FC precursor [14]. Firstly, some catalyst (10 g) is reduced by hydrogen at 973 K for 2 h. Then methane (99.99%, 0.78 l/min, mixed with nitrogen (99.99%, 1 l/min)) is fed into the reactor and decomposed by the catalyst. An online gas chromatograph (HP4890D) is used to detect the gas products. For comparison, the methane decomposition is also studied in a single-stage fluidized bed reactor, as described in the previous study [15], following the above procedures. Similar to most previous studies [1–7], the gas products consist only of hydrogen and methane. And the yield of carbon nanotubes (CNTs) at different times is easily calculated from the conversion of methane. Through measuring the bulk density and the weight of CNTs at different times, the gross volume of CNTs can be determined following the manner described elsewhere [15]. During the reaction, the lower stage, having the fixed volume, is first occupied by
the CNTs and catalyst. And then the upper stage is gradually occupied by the CNTs and catalyst with increasing of the reaction times. Since the catalyst particle is at the tip of CNTs [1,3,4,15,16], it also disperses with the CNTs in the entire reactor. Considering the turbulent mixing of CNTs and catalyst in the fluidized state, it can be assumed rationally that the catalyst disperses uniformly in the reactor. Since the volume of the lower stage is fixed, the time-dependent volume ratio of the CNTs and catalyst in the lower stage to those in the entire reactor can be determined. The relatively residence time of the CNTs and catalyst in the lower stage, equal to the volume ratio of the CNTs and catalyst in the lower stage to those in the entire reactor, can also be determined. Thus one can use the time-dependent relative residence time of CNTs and catalyst in the lower stage and the time-dependent conversion of methane to evaluate the effect of the low temperature stage on the catalyst life. The morphologies of as-grown CNTs are characterized by using transmission electron microscopy (TEM, Philips, CM120) and high resolution TEM (HRTEM, JEOL2010). And their microstructures are characterized by Raman spectroscopy (RM2000, excited at 514.4 nm) and XRD (D/MAX-RB, target: Cu, 40 kV), respectively.
3. Results and discussion Since the temperature in the lower stage is fixed at 773 K, when the temperature at high stage is 773 K, the temperature in the two stages is the same. As shown in Fig. 2, the conversion of methane at 773 K is very low, which indicates that the carbon produced in the lower stage at any time is very
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small. With increasing the temperature at the high stage, the gross conversion of methane is increased (Fig. 2). This result indicates that, in this reactor, the conversion of methane and yield of CNTs is mostly dedicated to methane decomposition in higher temperature stage. At 773–1023 K using the same catalyst, the conversion of methane in the single-stage fluidized bed reactor is higher than that in two-stage one, when the temperature in the upper stage of the latter reactor is the same as that in the former. Such a result indicates that the existence of the low temperature condition influences the conversion of methane to some degree. And the conversion of methane (or the yields of hydrogen) in the fluidized bed reactor is lower than that in fixed bed reactor [3], probably due to the difference in the gas flow characteristics between the fixed bed (plug flow) and the fluidized bed reactor (some gas backmixing). However, when the temperature is higher than 1023 K, the conversion of methane decreases sharply, indicating the rapid catalyst deactivation in the fixed bed [3] and in the single-stage fluidized bed reactor in the present work (curve (a) in Fig. 2). The conversion of methane increases steadily with temperature in the entire temperature range of 773–1123 K in the two-stage fluidized bed reactor (curve (b) in Fig. 2). Furthermore, a detailed result of operation at 1123 K in the two-stage fluidized bed reactor is shown in Fig. 3. After the continuous operation for nearly 17 h, the conversion of methane only decreases from 43.6 to 36.5%. The tendency of catalyst deactivation is rather slow, significantly different from the other studies [1,2,4,7], but similar to that at 873 K using the same catalyst [3]. However, we note that the average conversion of methane is about 40%, far higher than that (20%) at 873 K for stable methane decomposition [3]. Such a result indicates that the life of catalyst and the conversion of methane are both increased significantly in the two-stage fluidized bed reactor with different temperature operations. As compared with the previous studies [1–7,14,15], the catalytic decomposition of methane in the present work is conducted in a special temperature condition. As the fluidized catalyst and CNTs move freely in two stages, and are exposed to the high and low temperature condition alterna-
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Time, min Fig. 3. Time dependence of the conversion of methane and the percentage time of CNTs and catalyst in low temperature (LT) stage (operated at 1123 K) in two-stage fluidized bed reactor.
tively and frequently, the temperature condition so controlled will exert different effects on the carbon production and diffusion from the effects of previous reactors [2,3,7,9,10]. As mentioned above, the carbon is mainly produced in the high temperature stage. Though there is some carbon produced in the low temperature stage, the amount of the carbon produced is too small to be neglected. However, as pointed by Baker et al. [17] and Yang and Yang [18,19], the temperature gradient inside the metal particles is one of the driving forces for the nucleation of CNTs. Therefore, the carbon diffusion is not only stopped as the CNTs and catalyst stay in the low temperature region at 773 K, but also is probably enhanced in such temperature condition. Thus the effective time for carbon diffusion in two stages is longer than that for carbon formation mainly in the high temperature stage. In this case, even if the carbon production rate is higher than the carbon diffusion rate in high temperature, the excess carbon can effectively migrate from the gas–metal interface to the carbon–metal precipitation interface during their stay in the low temperature stage. Thus the properly catalytic cycle of methane decomposition and hydrogen production and the stable CNTs growth can be achieved by adopting the novel reactor in this paper, even when the upper limit of the temperature for this catalyst is increased from 1023 K [3] to 1123 K in the present work. Following the method above-mentioned, the timedependent relative residence time of CNTs and catalyst in the lower stage is calculated as using the volume ratio of the CNTs and catalyst in the lower stage to those in the entire reactor at different growth periods [15]. As shown in Fig. 3, when the reaction time is 100, 200, 300 or 900 min, the relative residence time of the CNTs and catalyst in low temperature stage is 55, 40, 30 or 10% of the total time of CNTs in this reactor, respectively. Therefore the slight catalyst deactivation is observed with the increasing time of CNTs and catalyst in high temperature stage. However, the catalyst deactivation is not serious, even if the time of the CNTs and catalyst in low temperature stage is very short by the end of the reaction. Such a result indicates that the
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Fig. 4. TEM image of CNTs prepared at different reactors: (a) in single-stage reactor and (b) in two-stage reactor.
low temperature condition is effective to alter the catalytic cycle for methane decomposition [17–19]. As compared with the previous studies [1–7,15], the novel reactor such as reported here provides great flexibility for the stable methane decomposition over a wide temperature range. As the yield of hydrogen is increased, the morphology of as-grown CNTs is also improved in such condition. As shown in Fig. 4, the products from the single stage fluidized
bed reactor at 1123 K are relatively rough (Fig. 4(a)). There is amorphous carbon outside the CNTs bundles. And it is not in the clear tube form and has a short length. The tube characteristics become clear, as from the two-stage fluidized bed reactor with low stage at 773 K and high stage at 1123 K (Fig. 4(b)). Detailed comparison of the HRTEM images of two products is shown in Fig. 5. The cross-section of the product (a) has the obvious fish bone characteristic, which
Fig. 5. High resolution TEM image of CNTs prepared at different reactors: (a) in single-stage reactor and (b) in two-stage reactor.
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the XRD measurement shows that the CNTs (b) have a very intense response at 26.44◦ (Fig. 7). The interlayer distance of carbon atom is 0.3378 nm, which is very close to that of the highly oriented graphite carbon (about 0.335 nm). This is far better than other CNTs prepared in the similar temperature range [1,3,15,20–23]. These results all indicate the temperature condition so controlled does favor the effective carbon diffusion to form an ordered structure with high graphitization degree. The improved CNTs may have better properties and find much wider applications.
(a)
4. Conclusion 800
1200
1600 Raman Shift, cm-1
2000
Fig. 6. Raman spectra of CNTs prepared at 1123 K: (a) in single-stage reactor, ID /IG : 1.18 and (b) in two-stage reactor, ID /IG : 0.77.
is similar to those in work of Li [3,14]. The angle between the carbon layer and the axis is about 45◦ . And the outer wall of the single tube is also rough. Also the tube exhibits the obvious dissymmetry, if one measures the number of the carbon layers between the left and right sides (Fig. 5(a)). Comparatively, the product (b) is relatively perfect, considering its smooth outer wall, relatively long carbon layer and the symmetry of its carbon layers (Fig. 5(b)). Further characterization by Raman spectroscopy shows that there are obviously different scattering characteristics of the microstructures of two carbon products. As shown in Fig. 6, the peak at 1343 cm−1 (D band) represents the disorder structures inside the carbon layers, including the lattice defects or poly-microcrystalline of carbon. And the peak at 1576 cm−1 (G band) means the graphitization of CNTs [20–23]. It is clear that, as compared with product (a) at the single-stage fluidized bed at 1123 K, the CNTs (b) have weaker D bands and stronger G bands, indicating its relatively fewer defects. According to the relationship between the in-plane carbon length and the intensity ratio of D band to G band in Raman spectra [20,21], the in-plane carbon length of CNTs (a) and (b) is 3 and 6 nm, respectively. Also
Intensity (counts)
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10 20 30 40 50 60 70 80 90 2-Theta(degree) Fig. 7. XRD pattern of the CNTs prepared at 1123 K in two-stage fluidized bed reactor.
In summary, as the carbon production and carbon diffusion on the catalyst in micro scale are controlled in the different temperature conditions in one reactor, the mismatch of carbon production rate with the carbon diffusion rate at high temperature is effectively eliminated. Thus the catalyst life is significantly prolonged at high temperature operation. Also, the conversion of methane for long time operation is increased from 20% at 873 K to 40% at 1123 K, which leads to the high concentration of hydrogen in gas products. Furthermore, characterized by Raman spectroscopy and XRD, the effective migration of carbon over catalyst also leads to the better graphitization degree of CNTs. These results indicates that the two-stage fluidized bed reactor with different temperature operations not only favors the gas phase processing for the continuous hydrogen production, but also favors the solid phase processing to make CNTs with relatively perfect microstructures.
Acknowledgements This work is partly supported by Natural and Science Foundation of China (No. 20236020).
References [1] T.J. Zhang, M.D. Amiridis, Appl. Catal. A: Gen. 167 (1998) 161. [2] G.G. Kuvshinov, Y.I. Mogilnykh, D.G. Kuvshinov, Catal. Today 42 (1998) 357. [3] Y.D. Li, J.L. Chen, Y.N. Qin, L. Chang, Energy Fuels 14 (2000) 1188. [4] M.A. Ermakova, D.Y. Ermakov, G.G. Kuvshinov, Appl. Catal. A: Gen. 201 (2000) 61. [5] B. Monnerat, L. Kiwi-Minsker, A. Renken, Chem. Eng. Sci. 56 (2001) 633. [6] N. Muradov, Int. J. Hydrogen Energy 26 (2001) 1165. [7] P. Chen, H.B. Zhang, G.D. Lin, Q. Hong, K.R. Tsai, Carbon 35 (1997) 1495. [8] T.V. Choudhary, D.W. Goodman, Catal. Today 77 (2002) 65. [9] J.W. Snocek, G.F. Froment, M. Fowles, J. Catal. 169 (1997) 240. [10] J.W. Snocek, G.F. Froment, M. Fowles, J. Catal. 169 (1997) 250. [11] C.H. Baratholomew, Appl. Catal. A: Gen. 212 (2000) 17. [12] C.S. Kannan, S.S. Rao, Y.B.G. Varma, Powder Technol. 78 (1994) 201.
228
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[13] K. Krishnaiah, Y. Pydisetty, Y.B.G. Varma, Chem. Eng. Sci. 37 (1982) 1371. [14] Y.D. Li, J.L. Chen, C. Liu, Appl. Catal. A: Gen. 163 (1997) 45. [15] W.Z. Qian, F. Wei, Z.W. Wang, T. Liu, H. Yu, G.H. Luo, L. Xiang, X.Y. Deng, AIChE J. 49 (2003) 619. [16] R.T.K. Baker, Carbon 27 (1989) 315. [17] R.T.K. Baker, M.A. Barber, P.S. Harris, J. Catal. 12 (1972) 185. [18] R.T. Yang, K.L. Yang, J. Catal. 93 (1985) 182.
[19] R.T. Yang, K.L. Yang, J. Catal. 115 (1989) 52. [20] T.C. Chieu, M.S. Dresselhaus, M. Endo, Phys. Rev. B 26 (1982) 5867. [21] W.Z. Qian, F. Wei, T. Liu, Z.W. Wang, Y.D. Li, J. Chem. Phys. 118 (2003) 878. [22] L.J. Ci, B.Q. Wei, C.L. Xu, J. Liang, D.H. Wu, S.S. Xie, J. Cryst. Growth 233 (2001) 823. [23] W.Z. Qian, T. Liu, F. Wei, H.Y. Yuan, Carbon 41 (2003) 1644.
Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor Qian Weizhong a,∗ , Liu Tang a , Wang Zhanwen a , Wei Fei a , Li Zhifei a , Luo Guohua a , Li Yongdan b a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Department of Chemical Engineering, Tianjin University, Tianjin 300072, China
b
Received in revised form 29 September 2003; accepted 20 October 2003
Abstract Methane decomposition over a Ni/Cu/Al2 O3 catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Catalyst deactivation; Multi-stage fluidized bed reactor; Nickel catalyst; Methane decomposition
1. Introduction The catalytic decomposition of methane, under a condition where activated oxygen species (H2 O, CO, O2 , etc.) are absent, is a relatively short route to produce hydrogen without carbon oxides [1–7]. The hydrogen product has no pollutants released in burning, and can be used directly as fuel for proton exchange membrane fuel cell (PEMFC), with poor CO-tolerance (less than 10 ppm) [8]. To break the strong C–H bond of the methane molecular in this endothermic reaction, a high temperature is thermodynamically required and a catalyst with high activity is needed [1–3,9,10]. However, the solid carbon products simultaneously produced, in large amounts in high temperature [1–5,7,11], destroy the structure of the catalyst and make the catalyst deactive rapidly, through blocking the active pores of catalyst [2] or encapsulating the whole catalyst particles [7,11]. Chen et al. proposed that the catalyst deactivation should be attributed ∗ Correspondence author. Tel.: +86-10-62789041; fax: +86-10-62772051. E-mail address: [email protected] (Q. Weizhong).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.10.018
to the high carbon production rate and the relatively low carbon diffusion rate [7]. Therefore, for one to maintain the equilibrium between carbon production and diffusion for the continuous operation, the low temperature condition has to be adopted [2,3,7]. However, the carbon production rate and diffusion rate are both low and hence the conversion of methane is very low in such condition, and a large amount of methane has to be recycled. Since the methane decomposition was conducted in a uniform temperature condition in many previous studies [1–9], the carbon production and diffusion on the catalyst also occurs simultaneously in this condition. Thus it is impossible to combine the advantage of the high efficiency of methane decomposition at a high temperature condition and the high thermal stability of catalyst at a low temperature condition together. In this contribution, a novel two-stage fluidized bed reactor is adopted, which allows different temperature operations at different stages. Thus the catalytic cycle of carbon production and carbon diffusion over catalyst can be tailored when the catalyst moves alternatively in two different temperature conditions. And the conversion of methane and the stable life of catalyst in high temperature condition are increased significantly.
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Fig. 1. Two-stage nano-agglomerate fluidized bed reactor for methane decomposition.
2. Experimental The reactor is made of stainless steel, with an inner diameter of 40 mm and a height of 1000 mm (Fig. 1). A horizontal perforated plate with 10% holes is used to divide the reactor into two stages. This separation prevents the backmixing of gases between stages effectively, but allows the fluidized solid materials to move easily between stages [12,13]. The height of the lower and upper stage is 150 and 800 mm, respectively. A heat exchanger is adopted (about 50 mm high) between two stages. Thus the temperature of two stages can each be controlled accurately. The temperature of the low stage is fixed at 773 K, while the temperature of the high stage can be controlled from 773 to 1123 K. As an example, one catalyst used is a Ni/Cu/Al2 O3 catalyst, prepared from FC precursor [14]. Firstly, some catalyst (10 g) is reduced by hydrogen at 973 K for 2 h. Then methane (99.99%, 0.78 l/min, mixed with nitrogen (99.99%, 1 l/min)) is fed into the reactor and decomposed by the catalyst. An online gas chromatograph (HP4890D) is used to detect the gas products. For comparison, the methane decomposition is also studied in a single-stage fluidized bed reactor, as described in the previous study [15], following the above procedures. Similar to most previous studies [1–7], the gas products consist only of hydrogen and methane. And the yield of carbon nanotubes (CNTs) at different times is easily calculated from the conversion of methane. Through measuring the bulk density and the weight of CNTs at different times, the gross volume of CNTs can be determined following the manner described elsewhere [15]. During the reaction, the lower stage, having the fixed volume, is first occupied by
the CNTs and catalyst. And then the upper stage is gradually occupied by the CNTs and catalyst with increasing of the reaction times. Since the catalyst particle is at the tip of CNTs [1,3,4,15,16], it also disperses with the CNTs in the entire reactor. Considering the turbulent mixing of CNTs and catalyst in the fluidized state, it can be assumed rationally that the catalyst disperses uniformly in the reactor. Since the volume of the lower stage is fixed, the time-dependent volume ratio of the CNTs and catalyst in the lower stage to those in the entire reactor can be determined. The relatively residence time of the CNTs and catalyst in the lower stage, equal to the volume ratio of the CNTs and catalyst in the lower stage to those in the entire reactor, can also be determined. Thus one can use the time-dependent relative residence time of CNTs and catalyst in the lower stage and the time-dependent conversion of methane to evaluate the effect of the low temperature stage on the catalyst life. The morphologies of as-grown CNTs are characterized by using transmission electron microscopy (TEM, Philips, CM120) and high resolution TEM (HRTEM, JEOL2010). And their microstructures are characterized by Raman spectroscopy (RM2000, excited at 514.4 nm) and XRD (D/MAX-RB, target: Cu, 40 kV), respectively.
3. Results and discussion Since the temperature in the lower stage is fixed at 773 K, when the temperature at high stage is 773 K, the temperature in the two stages is the same. As shown in Fig. 2, the conversion of methane at 773 K is very low, which indicates that the carbon produced in the lower stage at any time is very
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small. With increasing the temperature at the high stage, the gross conversion of methane is increased (Fig. 2). This result indicates that, in this reactor, the conversion of methane and yield of CNTs is mostly dedicated to methane decomposition in higher temperature stage. At 773–1023 K using the same catalyst, the conversion of methane in the single-stage fluidized bed reactor is higher than that in two-stage one, when the temperature in the upper stage of the latter reactor is the same as that in the former. Such a result indicates that the existence of the low temperature condition influences the conversion of methane to some degree. And the conversion of methane (or the yields of hydrogen) in the fluidized bed reactor is lower than that in fixed bed reactor [3], probably due to the difference in the gas flow characteristics between the fixed bed (plug flow) and the fluidized bed reactor (some gas backmixing). However, when the temperature is higher than 1023 K, the conversion of methane decreases sharply, indicating the rapid catalyst deactivation in the fixed bed [3] and in the single-stage fluidized bed reactor in the present work (curve (a) in Fig. 2). The conversion of methane increases steadily with temperature in the entire temperature range of 773–1123 K in the two-stage fluidized bed reactor (curve (b) in Fig. 2). Furthermore, a detailed result of operation at 1123 K in the two-stage fluidized bed reactor is shown in Fig. 3. After the continuous operation for nearly 17 h, the conversion of methane only decreases from 43.6 to 36.5%. The tendency of catalyst deactivation is rather slow, significantly different from the other studies [1,2,4,7], but similar to that at 873 K using the same catalyst [3]. However, we note that the average conversion of methane is about 40%, far higher than that (20%) at 873 K for stable methane decomposition [3]. Such a result indicates that the life of catalyst and the conversion of methane are both increased significantly in the two-stage fluidized bed reactor with different temperature operations. As compared with the previous studies [1–7,14,15], the catalytic decomposition of methane in the present work is conducted in a special temperature condition. As the fluidized catalyst and CNTs move freely in two stages, and are exposed to the high and low temperature condition alterna-
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Time, min Fig. 3. Time dependence of the conversion of methane and the percentage time of CNTs and catalyst in low temperature (LT) stage (operated at 1123 K) in two-stage fluidized bed reactor.
tively and frequently, the temperature condition so controlled will exert different effects on the carbon production and diffusion from the effects of previous reactors [2,3,7,9,10]. As mentioned above, the carbon is mainly produced in the high temperature stage. Though there is some carbon produced in the low temperature stage, the amount of the carbon produced is too small to be neglected. However, as pointed by Baker et al. [17] and Yang and Yang [18,19], the temperature gradient inside the metal particles is one of the driving forces for the nucleation of CNTs. Therefore, the carbon diffusion is not only stopped as the CNTs and catalyst stay in the low temperature region at 773 K, but also is probably enhanced in such temperature condition. Thus the effective time for carbon diffusion in two stages is longer than that for carbon formation mainly in the high temperature stage. In this case, even if the carbon production rate is higher than the carbon diffusion rate in high temperature, the excess carbon can effectively migrate from the gas–metal interface to the carbon–metal precipitation interface during their stay in the low temperature stage. Thus the properly catalytic cycle of methane decomposition and hydrogen production and the stable CNTs growth can be achieved by adopting the novel reactor in this paper, even when the upper limit of the temperature for this catalyst is increased from 1023 K [3] to 1123 K in the present work. Following the method above-mentioned, the timedependent relative residence time of CNTs and catalyst in the lower stage is calculated as using the volume ratio of the CNTs and catalyst in the lower stage to those in the entire reactor at different growth periods [15]. As shown in Fig. 3, when the reaction time is 100, 200, 300 or 900 min, the relative residence time of the CNTs and catalyst in low temperature stage is 55, 40, 30 or 10% of the total time of CNTs in this reactor, respectively. Therefore the slight catalyst deactivation is observed with the increasing time of CNTs and catalyst in high temperature stage. However, the catalyst deactivation is not serious, even if the time of the CNTs and catalyst in low temperature stage is very short by the end of the reaction. Such a result indicates that the
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Fig. 4. TEM image of CNTs prepared at different reactors: (a) in single-stage reactor and (b) in two-stage reactor.
low temperature condition is effective to alter the catalytic cycle for methane decomposition [17–19]. As compared with the previous studies [1–7,15], the novel reactor such as reported here provides great flexibility for the stable methane decomposition over a wide temperature range. As the yield of hydrogen is increased, the morphology of as-grown CNTs is also improved in such condition. As shown in Fig. 4, the products from the single stage fluidized
bed reactor at 1123 K are relatively rough (Fig. 4(a)). There is amorphous carbon outside the CNTs bundles. And it is not in the clear tube form and has a short length. The tube characteristics become clear, as from the two-stage fluidized bed reactor with low stage at 773 K and high stage at 1123 K (Fig. 4(b)). Detailed comparison of the HRTEM images of two products is shown in Fig. 5. The cross-section of the product (a) has the obvious fish bone characteristic, which
Fig. 5. High resolution TEM image of CNTs prepared at different reactors: (a) in single-stage reactor and (b) in two-stage reactor.
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(b)
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the XRD measurement shows that the CNTs (b) have a very intense response at 26.44◦ (Fig. 7). The interlayer distance of carbon atom is 0.3378 nm, which is very close to that of the highly oriented graphite carbon (about 0.335 nm). This is far better than other CNTs prepared in the similar temperature range [1,3,15,20–23]. These results all indicate the temperature condition so controlled does favor the effective carbon diffusion to form an ordered structure with high graphitization degree. The improved CNTs may have better properties and find much wider applications.
(a)
4. Conclusion 800
1200
1600 Raman Shift, cm-1
2000
Fig. 6. Raman spectra of CNTs prepared at 1123 K: (a) in single-stage reactor, ID /IG : 1.18 and (b) in two-stage reactor, ID /IG : 0.77.
is similar to those in work of Li [3,14]. The angle between the carbon layer and the axis is about 45◦ . And the outer wall of the single tube is also rough. Also the tube exhibits the obvious dissymmetry, if one measures the number of the carbon layers between the left and right sides (Fig. 5(a)). Comparatively, the product (b) is relatively perfect, considering its smooth outer wall, relatively long carbon layer and the symmetry of its carbon layers (Fig. 5(b)). Further characterization by Raman spectroscopy shows that there are obviously different scattering characteristics of the microstructures of two carbon products. As shown in Fig. 6, the peak at 1343 cm−1 (D band) represents the disorder structures inside the carbon layers, including the lattice defects or poly-microcrystalline of carbon. And the peak at 1576 cm−1 (G band) means the graphitization of CNTs [20–23]. It is clear that, as compared with product (a) at the single-stage fluidized bed at 1123 K, the CNTs (b) have weaker D bands and stronger G bands, indicating its relatively fewer defects. According to the relationship between the in-plane carbon length and the intensity ratio of D band to G band in Raman spectra [20,21], the in-plane carbon length of CNTs (a) and (b) is 3 and 6 nm, respectively. Also
Intensity (counts)
26.44
10 20 30 40 50 60 70 80 90 2-Theta(degree) Fig. 7. XRD pattern of the CNTs prepared at 1123 K in two-stage fluidized bed reactor.
In summary, as the carbon production and carbon diffusion on the catalyst in micro scale are controlled in the different temperature conditions in one reactor, the mismatch of carbon production rate with the carbon diffusion rate at high temperature is effectively eliminated. Thus the catalyst life is significantly prolonged at high temperature operation. Also, the conversion of methane for long time operation is increased from 20% at 873 K to 40% at 1123 K, which leads to the high concentration of hydrogen in gas products. Furthermore, characterized by Raman spectroscopy and XRD, the effective migration of carbon over catalyst also leads to the better graphitization degree of CNTs. These results indicates that the two-stage fluidized bed reactor with different temperature operations not only favors the gas phase processing for the continuous hydrogen production, but also favors the solid phase processing to make CNTs with relatively perfect microstructures.
Acknowledgements This work is partly supported by Natural and Science Foundation of China (No. 20236020).
References [1] T.J. Zhang, M.D. Amiridis, Appl. Catal. A: Gen. 167 (1998) 161. [2] G.G. Kuvshinov, Y.I. Mogilnykh, D.G. Kuvshinov, Catal. Today 42 (1998) 357. [3] Y.D. Li, J.L. Chen, Y.N. Qin, L. Chang, Energy Fuels 14 (2000) 1188. [4] M.A. Ermakova, D.Y. Ermakov, G.G. Kuvshinov, Appl. Catal. A: Gen. 201 (2000) 61. [5] B. Monnerat, L. Kiwi-Minsker, A. Renken, Chem. Eng. Sci. 56 (2001) 633. [6] N. Muradov, Int. J. Hydrogen Energy 26 (2001) 1165. [7] P. Chen, H.B. Zhang, G.D. Lin, Q. Hong, K.R. Tsai, Carbon 35 (1997) 1495. [8] T.V. Choudhary, D.W. Goodman, Catal. Today 77 (2002) 65. [9] J.W. Snocek, G.F. Froment, M. Fowles, J. Catal. 169 (1997) 240. [10] J.W. Snocek, G.F. Froment, M. Fowles, J. Catal. 169 (1997) 250. [11] C.H. Baratholomew, Appl. Catal. A: Gen. 212 (2000) 17. [12] C.S. Kannan, S.S. Rao, Y.B.G. Varma, Powder Technol. 78 (1994) 201.
228
Q. Weizhong et al. / Applied Catalysis A: General 260 (2004) 223–228
[13] K. Krishnaiah, Y. Pydisetty, Y.B.G. Varma, Chem. Eng. Sci. 37 (1982) 1371. [14] Y.D. Li, J.L. Chen, C. Liu, Appl. Catal. A: Gen. 163 (1997) 45. [15] W.Z. Qian, F. Wei, Z.W. Wang, T. Liu, H. Yu, G.H. Luo, L. Xiang, X.Y. Deng, AIChE J. 49 (2003) 619. [16] R.T.K. Baker, Carbon 27 (1989) 315. [17] R.T.K. Baker, M.A. Barber, P.S. Harris, J. Catal. 12 (1972) 185. [18] R.T. Yang, K.L. Yang, J. Catal. 93 (1985) 182.
[19] R.T. Yang, K.L. Yang, J. Catal. 115 (1989) 52. [20] T.C. Chieu, M.S. Dresselhaus, M. Endo, Phys. Rev. B 26 (1982) 5867. [21] W.Z. Qian, F. Wei, T. Liu, Z.W. Wang, Y.D. Li, J. Chem. Phys. 118 (2003) 878. [22] L.J. Ci, B.Q. Wei, C.L. Xu, J. Liang, D.H. Wu, S.S. Xie, J. Cryst. Growth 233 (2001) 823. [23] W.Z. Qian, T. Liu, F. Wei, H.Y. Yuan, Carbon 41 (2003) 1644.