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Optical investigation of carbon nanotube agglomerate growth on single catalyst particles
Chemical Engineering Journal 234 (2013) 74–79
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Optical investigation of carbon nanotube agglomerate growth on single catalyst particles Kristian Voelskow a, Lena Nickelsen a, Michael J. Becker b, Wei Xia b, Martin Muhler b, Ulrich Kunz a, Alfred P. Weber c, Thomas Turek a,⇑ a b c
Institute of Chemical Process Engineering, Leibnizstr. 17, 38678 Clausthal-Zellerfeld, Germany Laboratory of Industrial Chemistry, Universitätsstr. 150, 44801 Bochum, Germany Institute of Mechanical Process Engineering, Leibnizstr. 19, 38678 Clausthal-Zellerfeld, Germany
h i g h l i g h t s A new setup for observation of MWCNT agglomerate growth on single catalyst particles was designed. Agglomerate growth is rapid and finished in less than 3 min. Agglomerate growth rate increases with ethene concentration and temperature.
a r t i c l e
i n f o
Article history: Received 3 April 2013 Received in revised form 11 August 2013 Accepted 15 August 2013 Available online 29 August 2013 Keywords: Multiwalled carbon nanotubes Agglomerate growth Single particle observation Cobalt catalyst
a b s t r a c t A setup for optically monitoring the agglomerate growth of multiwalled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition on single Co–Mn–Al–Mg oxide catalyst particles with ethene as carbon precursor has been developed. Ethene concentrations and temperatures were varied between 5 –75 Vol.% and 550–770 °C, respectively. It could be shown that the agglomerate growth is rapid and the final diameter is reached after a few ten seconds to about 3 min depending on the reaction conditions. The average enlargement factor of the agglomerates over all experiments was found to be 6.5 ± 1.2 compared to the original diameter of the catalyst particle. The growth rate is enhanced by both, reaction temperature and ethene concentration. Hence it is concluded that the agglomerate growth rate is associated with the reaction rate of MWCNT synthesis. Short time experiments and analysis of the resulting agglomerates have confirmed an earlier proposed growth mechanism. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction MWCNTs have many interesting properties such as high thermal and electrical conductivity as well as high mechanical strength combined with low density [1,2]. Therefore, the synthesis of MWCNTs has been among the most relevant topics in material science since their rediscovery in 1991 [3,4]. In the last two decades, the field of possible applications for MWCNTs has increased rapidly. Examples comprise composite materials, catalysis, electronics, and electrochemistry [5–7]. This growing interest towards industrial application results in a higher demand for inexpensive MWCNTs. The most promising route to low-cost MWCNTs appears to be the synthesis by catalytic chemical vapor deposition (CCVD) using transition metals, especially Co, Fe or Ni [8]. Typically, MWCNT production on industrial scale is carried out in fluidizedbed reactors with Showa Denko or Arkema having already
designed plants producing up to 400 tons/year.1,2 CCVD carried out in fluidized-bed reactors results in the production of agglomerated MWCNTs consisting of a complex three-dimensional network structure. Starting with small catalyst particles, MWCNT agglomerates are formed which have a much larger volume and a corresponding lower density than the starting material [9]. The original catalyst particles are consumed during the process, similar to catalysts used in polymerization processes which are left in the polymer product. The size and density of MWCNT agglomerates during the CCVD process is very important for their fluidization behavior [9]. Furthermore, the growth properties of the CNT agglomerates have strong impact on product quality such as the agglomerate density and the properties of the CNTs in the agglomerate (e.g. short and entangled or long and stiff). Especially the shape of the formed
1
Showa Denko K.K., http://www.sdk.co.jp/english/ml. April 01, 2013. Arkema. GraphistrengthÒ production capacity, http://prodawl.arkema.com/sites/ group/en/products/detailed_sheets/multi_wall_carbon_nanotubes_graphistrength/ production_capacity.page. April 01, 2013. 2
⇑ Corresponding author. Tel.: +49 5323 722184. E-mail address: [email protected] (T. Turek). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.08.068
K. Voelskow et al. / Chemical Engineering Journal 234 (2013) 74–79
CNTs is very important for safe production and handling, as Poland et al. showed that long and stiff MWCNTs possess asbestos-like pathogenic behavior [10]. Hence, the agglomerate structure and its evolution during MWCNT growth are of fundamental importance for understanding and improvement of MWCNT mass production. Only with the knowledge of MWCNT agglomerate growth it will be possible to scale-up reactors and to improve product quality. Until now, only a few studies have dealt with the structure and the morphology during MWCNT agglomerate growth. De Jong and Geus investigated the ‘‘tertiary structure’’ in Carbon Nanofibers and described the agglomerates as ‘‘bird nests’’, ‘‘open net’’ and ‘‘combed yarn’’ [11]. Bierdel et al. described the fast MWCNT agglomerate growth as ‘‘expanding universe mechanism’’ [12]. Qian et al. investigated the time-dependent evolution of defects in MWCNTs by applying SEM, TGA and Raman spectroscopy [13]. Hao et al. prepared MWCNTs in a fluidized-bed reactor and investigated the evolution of their macroscopic properties [14]. Based on these results, a schematic mechanism for MWCNT agglomerate growth has been proposed (Fig. 1). According to this mechanism, the original catalyst particle is crushed by MWCNT growth in the initial period forming small clusters with separated catalytic sites. With increasing carbon formation, these catalyst clusters disintegrate further giving rise to sub-agglomerates which are entangled among each other. During MWCNT growth, the sub-agglomerates are being pushed apart, leading to an increase in agglomerate size and a corresponding decrease in density. After the agglomerate has reached its final diameter, growth does only occur inside the agglomerate resulting in densification. Recently, Kasaliwal et al. observed such sub-agglomerates of 1–10 lm in size within MWCNTs from Bayer MaterialScience AG [15]. Philippe et al. performed growth experiments on a Fe/Al2O3 catalyst and observed an additional step of aligned MWCNT growth on the catalyst surface before the catalyst structure was destroyed [16]. In this study the growth of MWCNT agglomerates over single catalyst particles was optically observed in-situ by a camera using a specially designed flow cell. A Co–Mn–Al–Mg catalyst [17–19] with ethene as carbon precursor was employed for synthesis of the MWCNTs, Catalyst
C2 H4 ! 2CCNT þ 2H2
ð1Þ
Fig. 1. Schematic mechanism of MWCNT agglomeration according to Hao et al. [14]: (a) original catalyst particle; (b) catalyst particle structure crushed by MWCNT growth; (c) separation of catalytic sites and formation of sub-agglomerates; (d) fully developed agglomerates.
75
Ethene inlet concentration and reaction temperature were varied at ambient pressure in the range from 5 Vol.% to 75 Vol.% and 550 °C to 770 °C, respectively. Special emphasis was given to the early stage of the agglomerate growth. 2. Materials and methods The MWCNT particle growth experiments were performed in a flow cell with a channel length of 80 mm and a cross section of 10 mm 5 mm (Figs. 2 and 3). In the center of the cell a thermal element was introduced from the bottom. The top of the element was bent by 90° and served as support for single catalyst particles. The element was surrounded by a spiral coil made of a Fe/Cr/Al alloy which heated the particle by thermal radiation and convection to the desired temperature. The flow channel was covered by a window made of fused silica. Above the window a camera (l-Eye; IDS-Imaging) with a resolution of 1280 1024 px was mounted. An InfiniStix™ objective lens and spacer rings by Edmund Optics Inc. were used resulting in a magnification of ca. 100. At the outlet of the cell the gas concentration was monitored by a mass spectrometer (GAM 200; InProcess Instruments). With the aid of the mass spectrometer the residence time distribution of the cell was determined. Monitoring the outlet concentration after a step-change of the ethane inlet concentration revealed that the Bodenstein number describing the axial dispersion in the cell amounted to about 50. Hence, the flow pattern in the cell can be regarded as close to plug flow. At the beginning of each experiment a single catalyst particle with a diameter between 180 lm and 300 lm was placed with a fine brush on top of the thermocouple. The used Co–Mn–Al–Mg oxide catalyst was described in a former publication by Tessonnier et al. [18]. After mounting the fused silica window and complete filling of the cell with the reactant gas, the heating coil was powered and the particle growth was observed by the camera. The development of temperature as a function of time during a typical experiment is shown in Fig. 4. Only a small period of time, 10–15 s, is needed to achieve the desired temperature. In the following the temperature decreases slightly, which may be caused by the agglomerate growth and the resulting partial covering of the thermocouple. After shutdown of the current, the temperature drops down rapidly and the MWCNT synthesis is immediately stopped. The temperature was varied between 550 and 770 °C at an ethene concentration of 20 Vol.% in argon. In further experiments, the inlet concentration was varied between 5 and 75 Vol.% ethene in argon at a constant temperature of 650 °C. Typically, the growth experiments had a duration of 5 min.
Fig. 2. Schematic of the experimental setup.
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K. Voelskow et al. / Chemical Engineering Journal 234 (2013) 74–79
catalyst particle was conserved during growth of the MWCNT agglomerates. Therefore, it was assumed that the agglomerates grow homogeneously in every direction of space. Thus the 2D picture can be used to derive the agglomerate diameter at each instant of time. The equivalent agglomerate diameter was determined with a coextensive ellipse (Fig. 5). It was checked through evaluating a certain number of results by different persons that the error of this manual procedure is less than 10%. 3.1. Agglomerate growth
Fig. 3. Left: top view of cell; right: thermocouple with catalyst particle surrounded by heating coil.
In a first series of experiments the reproducibility of particle growth was determined. All experiments were performed with a feed gas of 20 Vol.% ethene in argon at a temperature of 650 °C. The left diagram of Fig. 6 reveals that the enlargement factor of the agglomerates compared to the original catalyst particle size varies and has an average value of 6.7 ± 0.8. It was furthermore observed that the agglomerate growth started at different times (5–15 s) after the heating coil had reached the reaction temperature. A possible reason for this behavior is that the individual catalyst particles had different positions relative to the heating coil. To account for fluctuations of the initial and especially the final agglomerate diameter, the relative growth ratio v was defined:
vðdAgg Þ ¼
Fig. 4. Temperature as a function of time during start-up and shutdown of current.
The obtained MWCNT agglomerates were characterized by scanning electron microscopy (SEM) using a Helios NanoLAB™ 600 (FEI Company) and transmission electron microscopy (TEM) with a JEM-2100 (JEM). 3. Results and discussion The experiments were evaluated manually with the open source software ImageJ. The initial catalyst particles had different shapes. However, as shown in Fig. 5, the original shape of the
dAgg dCat dAgg;max dCat
ð2Þ
Here, dAgg is the agglomerate diameter varying with time, dCat the original catalyst particle diameter and dAgg,max the final agglomerate diameter. The right diagram of Fig. 6 shows the original data in normalized form with the onset time of each curve being averaged to 10 s. The gradients of the observed curves are similar during each run and vary within ±20%. It can be seen that the agglomerates reach their final diameter in a short time period and that the growth is almost completed within the first minute. This rapid growth can be watched in a video clip contained in the supplementary material. However, the ethene conversion during MWCNT synthesis still continues despite the stop of the agglomerate growth [20]. Accordingly, further MWCNT growth occurs within the developed agglomerates accompanied by an increase of density. An increase of concentration (Fig. 7) or temperature (Fig. 8) shows that the agglomerate growth is accelerated in either way. Consequently it can be assumed that the MWCNT agglomerate growth mechanism is linked to the reaction rate. Fig. 9 shows the growth rate v, which was defined as the average gradient of each curve between v = 0.1 and v = 0.7, as a function of the
Fig. 5. Top: differently shaped catalyst particles; bottom: resulting CNT agglomerates with conserved shape.
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K. Voelskow et al. / Chemical Engineering Journal 234 (2013) 74–79
Fig. 6. Reproducibility of experiments (20 Vol.% ethene in argon, T = 650 °C). Left: dimensionless diameter as a function of time; right: growth ratio v as a function of time with average onset time of 10 s; continuous line: average of the data points determined by a spline function.
Fig. 7. Growth ratio v as a function of time at different ethene inlet concentrations and constant temperature (T = 650 °C); hollow symbols: reproduced experiments at same conditions.
Fig. 9. Average growth rate v as a function of temperature at constant ethene inlet concentration (20 Vol.% in argon).
Fig. 8. Growth ratio v as a function of time at different temperatures and constant ethene inlet concentration (20 Vol.% in argon).
Fig. 10. Average growth rate temperature (T = 650 °C).
temperature. Below 700 °C the growth rate increases exponentially while above 700 °C no further effect on the particle growth can be noticed and the growth rate reaches a maximum. The increase of the ethene concentration also results in a raising growth rate (Fig. 10), however, with a smaller influence than the temperature. No clear tendency could be observed regarding the enlargement factor of the agglomerates during these experiments. On average, the final agglomerates were found to be 6.5 ± 1.2 larger than the
v
as a function of ethene concentration at constant
original catalyst particles. As the standard deviation of 1.2 is slightly higher than observed during the reproducibility experiments (0.8, Fig. 6), temperature and/or ethene concentration may have an impact on the enlargement factor. 3.2. Agglomerate morphology Additional experiments were performed where the temperature was dropped rapidly after short periods of time to investigate the
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K. Voelskow et al. / Chemical Engineering Journal 234 (2013) 74–79
Fig. 11. SEM pictures after different times (T = 650 °C, 20 Vol.% ethene in argon).
early phase of the MWCNT agglomerate growth. A temperature of 650 °C and an ethene concentration of 20 Vol.% were chosen. The resulting MWCNT agglomerates were analyzed by SEM (Fig. 11). In the very early phase (t = 14 s) the catalyst structure is still intact, but first short MWCNTs are generated. Only a few seconds later (t = 21 s), the original catalyst structure can no longer be identified. As Hao et al. [14] have already shown, sub-agglomerates were formed which stick together due to entangled MWCNTs (cf. Fig. 1). These sub-agglomerates form together the fully developed MWCNT agglomerate. With progress in time the sub-agglomerates disappear and a homogeneous structure of entangled MWCNTs emerges. The core–shell structure reported by Philippe et al. [16] was not observed in this work, which is caused by the different catalyst system. Additional TEM investigations (not shown) revealed that the formed MWCNTs have the typical thickness of 10–20 nm with about 10 walls as expected for the used type of catalyst [12,18]. 4. Conclusions An experimental setup has been developed to monitor optically the MWCNT agglomerate growth emerging from single cobaltbased catalyst particles. It could be shown that agglomerate growth is rapid and that the final diameter is reached after a period between a few ten seconds to about 3 min depending on the reaction conditions. As both increasing reaction temperatures and ethene concentrations enhance the growth rate, it can be concluded that agglomerate growth is determined by the reaction rate of MWCNT synthesis. Additional short time experiments have confirmed the MWCNT growth mechanism proposed by Hao et al. [14]. Acknowledgement We thank the German Federal Ministry of Education and Research (BMBF) and Bayer MaterialScience AG for financial support through the CarboScale Project (Grants 03X0040G and 03X0040I).
Appendix A. Supplementary meterial Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.08.068.
References [1] R. Andrews, D. Jacques, D.L. Qian, T. Rantell, Multiwall carbon nanotubes: synthesis and application, Acc. Chem. Res. 35 (2002) 1008–1017. [2] A. Jorio, G. Dresselhaus, M.S. Dresselhaus, Carbon Nanotubes: Advanced Topics in the Synthesis, Structure, Properties, and Applications, Springer, New York, 2008. [3] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [4] M. Monthioux, V.L. Kuznetsov, Who should be given the credit for the discovery of carbon nanotubes?, Carbon 44 (2006) 1621–1623 [5] A.T. Harris, C.H. See, J. Liu, O. Dunens, K. MacKenzie, Towards the large-scale synthesis of carbon nanotubes in fluidised beds, J. Nanosci. Nanotechnol. 8 (2008) 2450–2457. [6] P. Serp, Carbon nanotubes and nanofibers in catalysis, Appl. Catal. A 253 (2003) 337–358. [7] M. Endo, T. Hayashi, Y.A. Kim, M. Terrones, M.S. Dresselhaus, Applications of carbon nanotubes in the twenty-first century, Philos. Trans. R. Soc. Lond. Ser. A – Math. Phys. Eng. Sci. 362 (2004) 2223–2238. [8] C.H. See, A.T. Harris, A review of carbon nanotube synthesis via fluidized-bed chemical vapor deposition, Ind. Eng. Chem. Res. 46 (2007) 997–1012. [9] Q. Zhang, J.-Q. Huang, M.-Q. Zhao, W.-Z. Qian, F. Wei, Carbon nanotube mass production: principles and processes, ChemSusChem 4 (2011) 864– 889. [10] C.A. Poland, R. Duffin, I. Kinloch, A. Maynard, W.A.H. Wallace, A. Seaton, V. Stone, S. Brown, W. MacNee, K. Donaldson, Carbon nanotubes introduced into abdominal cavity of mice show asbestos-like pathogenicity in a pilot study, Nat. Nanotechnol. 3 (2008) 423–428. [11] K.P. de Jong, J.W. Geus, Carbon nanofibers: catalytic synthesis and applications, Catal. Rev. 42 (2000) 481–510. [12] M. Bierdel, S. Buchholz, V. Michele, L. Mleczko, R. Rudolf, M. Voetz, A. Wolf, Industrial production of multiwalled carbon nanotubes, Phys. Status Solidi B 244 (2007) 3939–3943. [13] W. Qian, The evaluation of the gross defects of carbon nanotubes in a continuous CVD process, Carbon 41 (2003) 2613–2617. [14] Y. Hao, Z. Qunfeng, W. Fei, Q. Weizhong, L. Guohua, Agglomerated CNTs synthesized in a fluidized bed reactor: agglomerate structure and formation mechanism, Carbon 41 (2003) 2855–2863. [15] G.R. Kasaliwal, S. Pegel, A. Göldel, P. Pötschke, G. Heinrich, Analysis of agglomerate dispersion mechanisms of multiwalled carbon nanotubes during melt mixing in polycarbonate, Polymer 51 (2010) 2708–2720. [16] R. Philippe, B. Caussat, A. Falqui, Y. Kihn, P. Kalck, S. Bordère, D. Plee, P. Gaillard, D. Bernard, P. Serp, An original growth mode of MWCNTs on alumina supported iron catalysts, J. Catal. 263 (2009) 345–358.
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[19] M.J. Becker, W. Xia, K. Xie, A. Dittmer, K. Voelskow, T. Turek, M. Muhler, Separating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor, Carbon. 58 (2013) 107–115. [20] A. Jess, C. Kern, K. Schrögel, A. Jung, W. Schütz, Herstellung von Kohlenstoff- Nanotubes und -fasern durch Gasphasenabscheidung, Chem. Ing. Tech. 78 (2006) 94–100.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Optical investigation of carbon nanotube agglomerate growth on single catalyst particles Kristian Voelskow a, Lena Nickelsen a, Michael J. Becker b, Wei Xia b, Martin Muhler b, Ulrich Kunz a, Alfred P. Weber c, Thomas Turek a,⇑ a b c
Institute of Chemical Process Engineering, Leibnizstr. 17, 38678 Clausthal-Zellerfeld, Germany Laboratory of Industrial Chemistry, Universitätsstr. 150, 44801 Bochum, Germany Institute of Mechanical Process Engineering, Leibnizstr. 19, 38678 Clausthal-Zellerfeld, Germany
h i g h l i g h t s A new setup for observation of MWCNT agglomerate growth on single catalyst particles was designed. Agglomerate growth is rapid and finished in less than 3 min. Agglomerate growth rate increases with ethene concentration and temperature.
a r t i c l e
i n f o
Article history: Received 3 April 2013 Received in revised form 11 August 2013 Accepted 15 August 2013 Available online 29 August 2013 Keywords: Multiwalled carbon nanotubes Agglomerate growth Single particle observation Cobalt catalyst
a b s t r a c t A setup for optically monitoring the agglomerate growth of multiwalled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition on single Co–Mn–Al–Mg oxide catalyst particles with ethene as carbon precursor has been developed. Ethene concentrations and temperatures were varied between 5 –75 Vol.% and 550–770 °C, respectively. It could be shown that the agglomerate growth is rapid and the final diameter is reached after a few ten seconds to about 3 min depending on the reaction conditions. The average enlargement factor of the agglomerates over all experiments was found to be 6.5 ± 1.2 compared to the original diameter of the catalyst particle. The growth rate is enhanced by both, reaction temperature and ethene concentration. Hence it is concluded that the agglomerate growth rate is associated with the reaction rate of MWCNT synthesis. Short time experiments and analysis of the resulting agglomerates have confirmed an earlier proposed growth mechanism. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction MWCNTs have many interesting properties such as high thermal and electrical conductivity as well as high mechanical strength combined with low density [1,2]. Therefore, the synthesis of MWCNTs has been among the most relevant topics in material science since their rediscovery in 1991 [3,4]. In the last two decades, the field of possible applications for MWCNTs has increased rapidly. Examples comprise composite materials, catalysis, electronics, and electrochemistry [5–7]. This growing interest towards industrial application results in a higher demand for inexpensive MWCNTs. The most promising route to low-cost MWCNTs appears to be the synthesis by catalytic chemical vapor deposition (CCVD) using transition metals, especially Co, Fe or Ni [8]. Typically, MWCNT production on industrial scale is carried out in fluidizedbed reactors with Showa Denko or Arkema having already
designed plants producing up to 400 tons/year.1,2 CCVD carried out in fluidized-bed reactors results in the production of agglomerated MWCNTs consisting of a complex three-dimensional network structure. Starting with small catalyst particles, MWCNT agglomerates are formed which have a much larger volume and a corresponding lower density than the starting material [9]. The original catalyst particles are consumed during the process, similar to catalysts used in polymerization processes which are left in the polymer product. The size and density of MWCNT agglomerates during the CCVD process is very important for their fluidization behavior [9]. Furthermore, the growth properties of the CNT agglomerates have strong impact on product quality such as the agglomerate density and the properties of the CNTs in the agglomerate (e.g. short and entangled or long and stiff). Especially the shape of the formed
1
Showa Denko K.K., http://www.sdk.co.jp/english/ml. April 01, 2013. Arkema. GraphistrengthÒ production capacity, http://prodawl.arkema.com/sites/ group/en/products/detailed_sheets/multi_wall_carbon_nanotubes_graphistrength/ production_capacity.page. April 01, 2013. 2
⇑ Corresponding author. Tel.: +49 5323 722184. E-mail address: [email protected] (T. Turek). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.08.068
K. Voelskow et al. / Chemical Engineering Journal 234 (2013) 74–79
CNTs is very important for safe production and handling, as Poland et al. showed that long and stiff MWCNTs possess asbestos-like pathogenic behavior [10]. Hence, the agglomerate structure and its evolution during MWCNT growth are of fundamental importance for understanding and improvement of MWCNT mass production. Only with the knowledge of MWCNT agglomerate growth it will be possible to scale-up reactors and to improve product quality. Until now, only a few studies have dealt with the structure and the morphology during MWCNT agglomerate growth. De Jong and Geus investigated the ‘‘tertiary structure’’ in Carbon Nanofibers and described the agglomerates as ‘‘bird nests’’, ‘‘open net’’ and ‘‘combed yarn’’ [11]. Bierdel et al. described the fast MWCNT agglomerate growth as ‘‘expanding universe mechanism’’ [12]. Qian et al. investigated the time-dependent evolution of defects in MWCNTs by applying SEM, TGA and Raman spectroscopy [13]. Hao et al. prepared MWCNTs in a fluidized-bed reactor and investigated the evolution of their macroscopic properties [14]. Based on these results, a schematic mechanism for MWCNT agglomerate growth has been proposed (Fig. 1). According to this mechanism, the original catalyst particle is crushed by MWCNT growth in the initial period forming small clusters with separated catalytic sites. With increasing carbon formation, these catalyst clusters disintegrate further giving rise to sub-agglomerates which are entangled among each other. During MWCNT growth, the sub-agglomerates are being pushed apart, leading to an increase in agglomerate size and a corresponding decrease in density. After the agglomerate has reached its final diameter, growth does only occur inside the agglomerate resulting in densification. Recently, Kasaliwal et al. observed such sub-agglomerates of 1–10 lm in size within MWCNTs from Bayer MaterialScience AG [15]. Philippe et al. performed growth experiments on a Fe/Al2O3 catalyst and observed an additional step of aligned MWCNT growth on the catalyst surface before the catalyst structure was destroyed [16]. In this study the growth of MWCNT agglomerates over single catalyst particles was optically observed in-situ by a camera using a specially designed flow cell. A Co–Mn–Al–Mg catalyst [17–19] with ethene as carbon precursor was employed for synthesis of the MWCNTs, Catalyst
C2 H4 ! 2CCNT þ 2H2
ð1Þ
Fig. 1. Schematic mechanism of MWCNT agglomeration according to Hao et al. [14]: (a) original catalyst particle; (b) catalyst particle structure crushed by MWCNT growth; (c) separation of catalytic sites and formation of sub-agglomerates; (d) fully developed agglomerates.
75
Ethene inlet concentration and reaction temperature were varied at ambient pressure in the range from 5 Vol.% to 75 Vol.% and 550 °C to 770 °C, respectively. Special emphasis was given to the early stage of the agglomerate growth. 2. Materials and methods The MWCNT particle growth experiments were performed in a flow cell with a channel length of 80 mm and a cross section of 10 mm 5 mm (Figs. 2 and 3). In the center of the cell a thermal element was introduced from the bottom. The top of the element was bent by 90° and served as support for single catalyst particles. The element was surrounded by a spiral coil made of a Fe/Cr/Al alloy which heated the particle by thermal radiation and convection to the desired temperature. The flow channel was covered by a window made of fused silica. Above the window a camera (l-Eye; IDS-Imaging) with a resolution of 1280 1024 px was mounted. An InfiniStix™ objective lens and spacer rings by Edmund Optics Inc. were used resulting in a magnification of ca. 100. At the outlet of the cell the gas concentration was monitored by a mass spectrometer (GAM 200; InProcess Instruments). With the aid of the mass spectrometer the residence time distribution of the cell was determined. Monitoring the outlet concentration after a step-change of the ethane inlet concentration revealed that the Bodenstein number describing the axial dispersion in the cell amounted to about 50. Hence, the flow pattern in the cell can be regarded as close to plug flow. At the beginning of each experiment a single catalyst particle with a diameter between 180 lm and 300 lm was placed with a fine brush on top of the thermocouple. The used Co–Mn–Al–Mg oxide catalyst was described in a former publication by Tessonnier et al. [18]. After mounting the fused silica window and complete filling of the cell with the reactant gas, the heating coil was powered and the particle growth was observed by the camera. The development of temperature as a function of time during a typical experiment is shown in Fig. 4. Only a small period of time, 10–15 s, is needed to achieve the desired temperature. In the following the temperature decreases slightly, which may be caused by the agglomerate growth and the resulting partial covering of the thermocouple. After shutdown of the current, the temperature drops down rapidly and the MWCNT synthesis is immediately stopped. The temperature was varied between 550 and 770 °C at an ethene concentration of 20 Vol.% in argon. In further experiments, the inlet concentration was varied between 5 and 75 Vol.% ethene in argon at a constant temperature of 650 °C. Typically, the growth experiments had a duration of 5 min.
Fig. 2. Schematic of the experimental setup.
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K. Voelskow et al. / Chemical Engineering Journal 234 (2013) 74–79
catalyst particle was conserved during growth of the MWCNT agglomerates. Therefore, it was assumed that the agglomerates grow homogeneously in every direction of space. Thus the 2D picture can be used to derive the agglomerate diameter at each instant of time. The equivalent agglomerate diameter was determined with a coextensive ellipse (Fig. 5). It was checked through evaluating a certain number of results by different persons that the error of this manual procedure is less than 10%. 3.1. Agglomerate growth
Fig. 3. Left: top view of cell; right: thermocouple with catalyst particle surrounded by heating coil.
In a first series of experiments the reproducibility of particle growth was determined. All experiments were performed with a feed gas of 20 Vol.% ethene in argon at a temperature of 650 °C. The left diagram of Fig. 6 reveals that the enlargement factor of the agglomerates compared to the original catalyst particle size varies and has an average value of 6.7 ± 0.8. It was furthermore observed that the agglomerate growth started at different times (5–15 s) after the heating coil had reached the reaction temperature. A possible reason for this behavior is that the individual catalyst particles had different positions relative to the heating coil. To account for fluctuations of the initial and especially the final agglomerate diameter, the relative growth ratio v was defined:
vðdAgg Þ ¼
Fig. 4. Temperature as a function of time during start-up and shutdown of current.
The obtained MWCNT agglomerates were characterized by scanning electron microscopy (SEM) using a Helios NanoLAB™ 600 (FEI Company) and transmission electron microscopy (TEM) with a JEM-2100 (JEM). 3. Results and discussion The experiments were evaluated manually with the open source software ImageJ. The initial catalyst particles had different shapes. However, as shown in Fig. 5, the original shape of the
dAgg dCat dAgg;max dCat
ð2Þ
Here, dAgg is the agglomerate diameter varying with time, dCat the original catalyst particle diameter and dAgg,max the final agglomerate diameter. The right diagram of Fig. 6 shows the original data in normalized form with the onset time of each curve being averaged to 10 s. The gradients of the observed curves are similar during each run and vary within ±20%. It can be seen that the agglomerates reach their final diameter in a short time period and that the growth is almost completed within the first minute. This rapid growth can be watched in a video clip contained in the supplementary material. However, the ethene conversion during MWCNT synthesis still continues despite the stop of the agglomerate growth [20]. Accordingly, further MWCNT growth occurs within the developed agglomerates accompanied by an increase of density. An increase of concentration (Fig. 7) or temperature (Fig. 8) shows that the agglomerate growth is accelerated in either way. Consequently it can be assumed that the MWCNT agglomerate growth mechanism is linked to the reaction rate. Fig. 9 shows the growth rate v, which was defined as the average gradient of each curve between v = 0.1 and v = 0.7, as a function of the
Fig. 5. Top: differently shaped catalyst particles; bottom: resulting CNT agglomerates with conserved shape.
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Fig. 6. Reproducibility of experiments (20 Vol.% ethene in argon, T = 650 °C). Left: dimensionless diameter as a function of time; right: growth ratio v as a function of time with average onset time of 10 s; continuous line: average of the data points determined by a spline function.
Fig. 7. Growth ratio v as a function of time at different ethene inlet concentrations and constant temperature (T = 650 °C); hollow symbols: reproduced experiments at same conditions.
Fig. 9. Average growth rate v as a function of temperature at constant ethene inlet concentration (20 Vol.% in argon).
Fig. 8. Growth ratio v as a function of time at different temperatures and constant ethene inlet concentration (20 Vol.% in argon).
Fig. 10. Average growth rate temperature (T = 650 °C).
temperature. Below 700 °C the growth rate increases exponentially while above 700 °C no further effect on the particle growth can be noticed and the growth rate reaches a maximum. The increase of the ethene concentration also results in a raising growth rate (Fig. 10), however, with a smaller influence than the temperature. No clear tendency could be observed regarding the enlargement factor of the agglomerates during these experiments. On average, the final agglomerates were found to be 6.5 ± 1.2 larger than the
v
as a function of ethene concentration at constant
original catalyst particles. As the standard deviation of 1.2 is slightly higher than observed during the reproducibility experiments (0.8, Fig. 6), temperature and/or ethene concentration may have an impact on the enlargement factor. 3.2. Agglomerate morphology Additional experiments were performed where the temperature was dropped rapidly after short periods of time to investigate the
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Fig. 11. SEM pictures after different times (T = 650 °C, 20 Vol.% ethene in argon).
early phase of the MWCNT agglomerate growth. A temperature of 650 °C and an ethene concentration of 20 Vol.% were chosen. The resulting MWCNT agglomerates were analyzed by SEM (Fig. 11). In the very early phase (t = 14 s) the catalyst structure is still intact, but first short MWCNTs are generated. Only a few seconds later (t = 21 s), the original catalyst structure can no longer be identified. As Hao et al. [14] have already shown, sub-agglomerates were formed which stick together due to entangled MWCNTs (cf. Fig. 1). These sub-agglomerates form together the fully developed MWCNT agglomerate. With progress in time the sub-agglomerates disappear and a homogeneous structure of entangled MWCNTs emerges. The core–shell structure reported by Philippe et al. [16] was not observed in this work, which is caused by the different catalyst system. Additional TEM investigations (not shown) revealed that the formed MWCNTs have the typical thickness of 10–20 nm with about 10 walls as expected for the used type of catalyst [12,18]. 4. Conclusions An experimental setup has been developed to monitor optically the MWCNT agglomerate growth emerging from single cobaltbased catalyst particles. It could be shown that agglomerate growth is rapid and that the final diameter is reached after a period between a few ten seconds to about 3 min depending on the reaction conditions. As both increasing reaction temperatures and ethene concentrations enhance the growth rate, it can be concluded that agglomerate growth is determined by the reaction rate of MWCNT synthesis. Additional short time experiments have confirmed the MWCNT growth mechanism proposed by Hao et al. [14]. Acknowledgement We thank the German Federal Ministry of Education and Research (BMBF) and Bayer MaterialScience AG for financial support through the CarboScale Project (Grants 03X0040G and 03X0040I).
Appendix A. Supplementary meterial Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.08.068.
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