Energy Fuels 2009, 23, 4866–4870 Published on Web 08/25/2009
: DOI:10.1021/ef900339t
Catalytic Reactions of Ethylene and Hydrogen in a Fluidized-Bed Reactor with Ni Nanoparticles Woo Jin Lee† and Chun-Zhu Li* Department of Chemical Engineering, PO Box 36, Monash University, Victoria 3800, Australia. Present address: CSIRO Petroleum Resources, Private Bag 10, Clayton South, Vic 3169, Australia.
†
Received April 16, 2009. Revised Manuscript Received July 15, 2009
The catalytic reactions of ethylene and hydrogen have been investigated in a fluidized-bed reactor containing Ni nanoparticles and microsize SiO2 particles. Although significant amounts of carbon were formed on the Ni surface, the Ni catalyst was not completely deactivated. However, the formation of carbon could lead to defluidization. Increasing temperature from 665 to 800 °C drastically improved the fluidization. The catalytic activity of the Ni catalyst changed with reaction time, showing a maximum at 750 and 800 °C. The crystalline size of Ni as determined by X-ray diffraction also decreased with reaction time. Although the dehydrogenation of ethylene was the dominant reaction initially, the hydrogenation of ethylene to ethane became increasingly important with time. The purpose of this study is to investigate the reactions of ethylene and hydrogen using fluidized Ni nanoparticles as the catalyst. The choice of ethylene and hydrogen was based on the consideration that this reaction system would have flexibility to allow the easy formation of gaseous products from the hydrogenation of ethylene (e.g., to form ethane) and the dehydrogenation of ethylene (e.g., to form acetylene and solid carbon materials on the catalyst surface). This provides an opportunity to investigate the inter-relationship between catalyst property (e.g., changes in catalyst properties due to carbon formation) and reactivity. The fluidization of the Ni nanoparticles provides a means of eliminating the possible heat transfer limitation by having a uniform temperature within the fluidized bed.
1. Introduction The reactions of a hydrocarbon at an elevated temperature can be exceedingly complex, involving hundreds of elementary radical reactions. When a catalyst is used, the reaction system becomes even more complicated. In addition to gasphase reactions, reactions can also take place on the catalyst surface. Furthermore, radicals, as intermediates formed on the catalyst surface, can desorb into the gas phase to initiate/ participate in the gas-phase reactions.1-3 When a porous supported-catalyst (e.g., Ni/SiO2) is used, the porous structure becomes a major barrier for the transfer of radicals from the catalyst surface into the gas phase.1,4 Instead, repeated adsorption and desorption of radicals may take place within the pores. The use of a porous supported catalyst is thus not ideal for the fundamental understanding of the catalytic reactions involving hydrocarbons at high temperatures when both heterogeneous catalytic and homogeneous noncatalytic reactions are important. Nanoparticles (even with a diameter up to 100 nm) have relatively high ratios of surface area to mass, allowing for significant levels of conversion to be achieved.5 A nanoparticle also has a relatively simple, if at all, pore structure, reducing the resistance for the mass transfer of radicals from the catalyst surface into the gas phase.6 Therefore, the use of nanoparticles as a catalyst would provide additional information about the changes in catalyst activity and reaction pathways during the catalytic reaction of a hydrocarbon, without the complications caused by the porous structure of a support.
2. Experimental Section Nickel oxide (NiO, green, >99%) of the mean primary particle size of 100 nm (NanoAmor, Inc. USA) and SiO2 (fumed) of the mean particle size of 50-80 μm were used as a catalyst and a diluent, respectively. At a high reaction temperature for this reaction system (e.g., from 665 to 800 °C in this study), the fluidization of nickel oxide nanoparticles alone would be easily terminated within 5 min. When a few percent of NiO nanoparticles were added into silica, the mixture could be fluidized for long time (see below for details). The silica particles had little porous structure with a BET surface area of 0.1842 m2 g-1 and an average pore size of 13.6 A˚ (using a Micromeritics TriStar 3000). With these almost nonexistent pore structures, little chance exists for the Ni particle to enter the pores (if any at all) of silica particles. The concentration of NiO (1.5 or 3.0 wt %) in silica was chosen to simulate the composition range of a typical Ni/SiO2 supported catalyst. Prior to a catalytic experiment, the NiO and silica mixture was reduced in hydrogen (20% in Ar) atmosphere (200 mL min-1) at 500 °C for 30 min. Some loose agglomerates were formed during reduction, but they were then broken down by grinding the mixture gently in a mortar. The experimental data on the Ni particle sizes (see Figures 3, 4, and 6) confirm the absence of significant sintering of Ni during the reduction. All experiments were conducted in a quartz fluidized-bed reactor with an inner diameter of 35 mm. The mixture of the
*To whom correspondence should be addressed. Present address: Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845. Fax: þ61 8 9266 1138. E-mail:
[email protected]. (1) Quah, E. B. H.; Mathews, J. F.; Li, C.-Z. J. Catal. 2001, 197, 315– 323. (2) Quah, E. B. H.; Li, C.-Z. Appl. Catal. A: G. 2004, 258, 63–71. (3) Lee, W. J.; Li, C.-Z. Appl. Catal. A: G. 2007, 316, 90–99. (4) Couwenberg, P. M.; Chen, Q.; Marin, G. B. Ind. Eng. Chem. Res. 1996, 35, 3999–4011. (5) Pajonk, G. M. Appl. Catal. 1991, 72, 217–266. (6) Weber, A. P.; Seipenbusch, M.; Kasper, G. J. Nanopart. Res. 2003, 5, 293–298. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 4866–4870
: DOI:10.1021/ef900339t
Lee and Li
reduced Ni and SiO2 was loaded into the reactor on the quartz gas distributor through which the fluidization gas passed. The reactor was placed vertically in an electrically heated furnace and heated up slowly at 10 K min-1 from the room temperature to the required temperature. Prior measurement of temperature distribution ensured that the catalyst bed was placed in the isothermal region. The minimum fluidization gas velocity (Umf) was identified on the basis of the ratio of pressure drop (4P) to bed weight and the bed expansion ratio (H/H0). Pressure measurement was carried out using a digital manometer (Dwyer, Series 475 Mark III, USA). Gases used in this study were ethylene (>99.0%, from BOC), argon (>99.999%, from Linde), and hydrogen (20 vol % in argon, from Linde). All gas flow rates were controlled with mass flow controllers. All experiments were carried out at atmospheric pressure. During an experiment, argon was fed into the reactor while the reactor was heated up to the required temperature. Ethylene and hydrogen were then fed into the reactor. The volumetric ratio of C2H4/H2 in the reactant mixture was kept at 1, and the concentration of argon was 90 vol %. Gaseous products were analyzed using a HP 5890 GC equipped with a Heysep DB column (15 ft 1/8 in.) and a FID and a Perkin-Elmer GC equipped with a molecular sieve column (1 m 1/8 in.), a Porapack N column (3 m 1/8 in.) and a TCD. A filter was installed at the end of the gas line, and no discernible Ni could be detected. Therefore, no loss of Ni particles was observed during the experiments. The reactor including the catalyst was weighed before and after each experiment to determine the amount of carbon produced. No loss of Ni particles was observed during the experiments. X-ray diffraction (XRD) patterns were measured on a Philips PW1140/90 diffractometer with Cu KR radiation at a scan rate of 2°/min and a step size of 0.02°. The crystalline size of Ni based on the XRD results was calculated using the Scherrer equation. The values of full width at half height of the Ni (111) peak at around 44.5°, required in the Scherrer equation, were calculated using the GRAMS/32 AI commercial software. Spent catalysts were also examined with a Philips EM420 transmission electron microscope (TEM).
Figure 1. Conversion of ethylene as a function of reaction holding time in the presence of Ni nanocatalyst (i.e., 14.7 g of 1.5 wt % Ni in SiO2) or SiO2 only (14.7 g) at 750 °C in a fluidized-bed reactor. C2H4/H2/Ar = 0.05:0.05:0.9 at a total gas flow rate of 2 L min-1.
pletely under similar experimental conditions.7 This is most likely due to the lack of a porous structure in our catalyst system, thus the growth of carbon (Figure 1) did not completely block the access of reactant molecules to the catalyst sites. Instead, the Ni particles would stay on the tips of carbon (see below) and would thus be available to the reactants to maintain their catalytic activities, as is shown in Figure 1. On the other hand, the growth of carbon inside the pores in a typical metal/SiO2 supported catalyst would have led to the complete blockage of the pores and therefore the complete loss of catalyst activity.7 Therefore, different from a traditional supported catalyst system, the fluidized Ni nano- and micrometer-sized SiO2 particles allow us to separate the effects of changes in Ni properties from the effects of changes in the (diffusion) resistance for the reactants to access the Ni sites inside a pore. However, closer examination of the data in Figure 1 shows that the decrease in ethylene conversion and that in the formation rate of carbon (derived from ethylene) were not parallel, indicating that the formation rates of products other than carbon increased with holding time. The quantification of reaction products shown in Figure 2 shows that the formation rate of ethane (C2H6), as the major gaseous product, more than tripled as the holding time was increased from zero to 140 min. The formation rates of other minor C-containing products such as CH4 and C3H6 also increased with increasing holding time (Figure 2). However, in agreement with the decreases in the formation rate of carbon (Figure 1), the formation rate of H2 (Figure 2) also decreased with holding time as both carbon and H2 originated from the dehydrogenation of C2H4. The above data in Figures 1 and 2 indicate that the properties of Ni catalyst nanoparticles must have changed with reaction holding time to result in the changes in the relative formation rates of hydrogenation products (e.g., C2H6) and dehydrogenation products (e.g., H2 and C) from C2H4, indicating changes in the dominant reaction pathways. This might be due to the changes in the crystalline size of Ni or the crystallographic reconstruction of the Ni surface. To gain insights into the changes in the catalyst properties, the spent bed samples including carbon materials were sieved
3. Results and Discussion 3.1. Catalytic Activity of Ni Nanoparticles in a FluidizedBed Reactor. Figure 1 shows the conversion of ethylene as a function of holding time at 750 °C. When only SiO2 was loaded into the fluidized-bed reactor as a “catalyst”, the conversion of ethylene was only around 2.5%, indicating that the conversion of ethylene due to homogeneous gasphase reactions and the catalytic effects of SiO2 were both negligibly small under the present experimental conditions. When Ni nanoparticles were present in the fluidized bed, the conversion level of ethylene increased drastically. These results indicate that the Ni nanoparticles mixed in micrometer-sized SiO2 are very effective in catalyzing the reactions between ethylene and hydrogen. The data in Figure 1 also show that the conversion of ethylene decreased gradually with increasing holding time, reaching an asymptotic conversion value of around 15% within the holding time range investigated. The examination of spent catalyst showed significant carbon formation; the average carbon formation rate as a function of holding time is also shown in Figure 1. It is apparent that the deactivation of this catalyst (i.e., Ni nanoparticles mixed with SiO2 particles) behaved quite differently from a typical metal/ SiO2 supported catalyst that would have deactivated com(7) Venegoni, D.; Serp, P.; Feurer, R.; Kihn, Y.; Vahlas, C.; Kalck, P. Carbon 2002, 40, 1799–1807.
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: DOI:10.1021/ef900339t
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using a metal sieve to separate carbon materials from SiO2, which were then analyzed using XRD (Figure 3). In Figure 3, “catalyst bed” samples represent those taken from the bed
located mainly in the bottom of the bed, and “carbon bed” samples refer to those taken from the top part of the bed. The peak at around 44.5° corresponds to metallic Ni (111). In Figure 3a, the Ni (111) peak heights of the catalyst bed samples decreased gradually with time. In contrast, the samples from the carbon bed (Figure 3b) showed increases in the Ni (111) peak height. Therefore, the data in Figures 3a and 3b indicate that the Ni particles have been somehow segregated along the fluidized bed. Figure 3c shows the crystalline size of Ni particles in the carbon bed as a function of holding time, which was calculated using the Scherrer’s equation. The lack of strong signals for the catalyst bed samples has precluded us from the accurate determination of Ni particle crystalline size. The data in Figure 3c clearly show that the average Ni crystalline size of Ni in the carbon bed decreased drastically, about halved, with reaction holding time. The spent bed samples in the carbon bed were also examined with TEM to further understand the possible reasons for the decreases of Ni particle size. Figure 4a shows the carbon nanotubes and carbon-encapsulated Ni particles, which are still attached on a SiO2 particle. It was observed during the TEM analysis that only small portion of carbon nanotubes and Ni particles have remained on the surface of SiO2 particles. The majority of these materials were seen in the carbon bed. The Ni particle could be located at the tip of a carbon tube (Figure 4b), indicating the tip growth mode for the
Figure 2. Product formation rates as a function of reaction holding time during the reaction of C2H4 and H2 in the presence of Ni nano catalyst (i.e., 14.7 g of 1.5 wt % Ni in SiO2) at 750 °C in a fluidizedbed reactor. C2H4/H2/Ar = 0.05:0.05:0.9 at a total gas flow rate of 2 L min-1.
Figure 3. XRD profiles of (a) samples from the catalyst bed and (b) samples from the carbon bed as well as (c) the changes in the Ni crystalline size of samples from the carbon bed as a function of holding time during the reaction of C2H4 and H2 in the presence of Ni nanocatalyst (i.e., 14.7 g of 1.5 wt % Ni in SiO2) at 750 °C in a fluidizedbed reactor. C2H4/H2/Ar = 0.05:0.05:0.9 at a total gas flow rate of 2 L min-1. þ: graphitic; *: Ni0.
Figure 4. TEM microphotographs for (a) the carbon formed on the surface of SiO2, (b) the carbon nanotube, and (c) the diameter distribution of Ni particles after the reaction of C2H4 and H2 in the presence of Ni nanocatalyst (i.e., 14.7 g of 3 wt % Ni in SiO2) at 750 °C in a fluidized-bed reactor. C2H4/H2/Ar = 0.05:0.05:0.9 at a total gas flow rate of 2 L min-1.
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: DOI:10.1021/ef900339t
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Figure 5. Effects of reaction temperature on (a) the C2H4 conversion, (b) the fluidization time and C formation rate, and (c) the formation rates of gaseous products during the conversion of ethylene (C2H4/H2/Ar = 0.05:0.05:0.9 with the gas flow rate of 2 L min-1) in a fluidized-bed reactor containing 14.7 g of 3 wt % Ni in SiO2. Fluidization time refers to the length of time during which the bed was fluidized (i.e., before defluidization occurred).
pathway of ethylene. Andersson et al.12 have reported that the reconstruction of Ni surface at step edges could take place due to carbon coverage. Vang et al.13 have found that blocking Ni step sites makes it easier for ethylene to form vinyl (CHCH2) rather than the direct CdC bond breakage to form methylene (CH2) on Ni surface. Recently, similar results were also reported from the kinetic simulation work.14 Although the breakage of CdC bonds on catalyst surface would tend to favor the formation of either carbon (through the dehydrogenation of CH2) or CH4 (through the hydrogenation of CH2), the formation of vinyl radicals on catalyst surface would provide better chance for the formation of C2H6 from the hydrogenation of CHCH2. 3.2. Effects of Reaction Temperature. Figure 5 shows the effects of reaction temperature on the catalytic reactions of ethylene and hydrogen. As expected, the increases in temperature have led to corresponding increases in the catalyst activity and thus increases in the observed conversion of ethylene, as is shown in Figure 5a. However, at each
carbon nanotubes. In addition, it revealed that a large amount of Ni particles were encapsulated by carbon. The size distribution of Ni particles was determined based on the TEM images after the reaction (Figure 4c), confirming that the Ni particles remained at the nano size range, as expected. The formation of carbon on the Ni surface and the subsequent dissolution of carbon into the Ni phase would have decreased the Ni crystalline size.8 Therefore, the TEM microphotographs like the one shown in Figure 4b provide convincing evidence/explanation that the Ni particles were broken down with the growth of carbon, as is shown in Figure 3c. Indeed, past studies 9,10 also clearly showed that Ni particles undergo fragmentation upon carbon formation thereon. The formation of carbon as well as dissolution of carbon into Ni11 would necessarily mean changes in Ni surface structure. This would in turn affect the preferential reaction (8) Ermakova, M. A.; Yu. Ermakov, D.; Plyasova, L. M.; Kuvshinov, G. G. Catal. Lett. 1999, 62, 93–97. (9) Toebes, M. L.; Bitter, J. H.; van Dillen, A. J.; de Jong, K. P. Catal. Today 2002, 76, 33–42. (10) Park, G. S.; Kim, M. Y.; Baik, H. S.; Song, S. A.; Han, I. T.; Lee, N. S.; Han, J. H.; Yoo, J. B. J. Appl. Phys. 2002, 92, 7459–7461. (11) Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1972, 26, 51–62.
(12) Andersson, M. P.; Abild-Pedersen, F. Surf. Sci. 2007, 601, 649– 655. (13) Vang, R. T.; Honkala, K.; Dahl, S.; Vestergaard, E. K.; Schnadt, J.; Lægsgaard, E.; Clausen, B. S.; Nørskov, J. K.; Besenbacher, F. Surf. Sci. 2006, 600, 66–77. (14) Tan, X.; Yang, G. W. J. Phys. Chem. C 2008, 112, 4219–4225.
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Figure 6. XRD profiles of (a) the freshly reduced (No. 1) and used samples (No. 2-4) from the carbon bed after being used for different temperatures (2: 720 °C; 3: 750 °C; 4: 800 °C) and (b) the effect of temperature on crystalline size of Ni after the reaction of ethylene and hydrogen (C2H4/H2/Ar = 0.05:0.05:0.9 with the gas flow rate of 2 L min-1) in a fluidized-bed reactor containing 14.7 g of 3 wt % Ni in SiO2.
temperature, the catalyst activity as shown by ethylene conversion was not constant but changed with reaction time, particularly obvious at 750 and 800 °C. Our visual observation of the reactor after the reaction revealed the presence of dry black traces on the inner reactor wall. The lower temperature was used, the more dry traces were observed. It is believed that viscous and/or liquid products (e.g., polyaromatic hydrocarbons) must have formed during the experiment: they were most likely heavy (aromatic) hydrocarbons and then polymerized into carbon. From the gas analysis, C2H2 was detected only during the reactions conducted at 665 and 690 °C. At temperatures above 690 °C, no C2H2 was detected. The observations of C2H2 and heavy hydrocarbons as a function of temperature agree well with the hypothesis that the higher molecular weight polycyclic aromatic compounds would have been formed mainly from C2H2.15 The formation of heavy viscous/liquid-like materials at low reaction temperatures would have enhanced the rate of carbon formation. The actual measurement of carbon formation rates, as is shown in Figure 5b, does show that the carbon formation rate decreased with increasing temperature. These viscous/liquid-like materials apparently have also caused the fluidized bed to defluidize: the time that the bed of SiO2/Ni could be fluidized increased drastically when the carbon formation rates decreased with increasing temperature. The catalytic reactions of ethylene and hydrogen at high temperatures (e.g., 750 and 800 °C) showed a clear maximum conversion of ethylene as a function of reaction time. As was shown in Figures 3 and 4, the formation of carbon on Ni nanoparticle surface resulted in the fragmentation of the Ni nanoparticles. The XRD results in Figure 6 have shown that the final Ni particles size after reaction tended to decrease with increasing temperature, confirming that the Ni particles were fragmented at 720 and 800 °C as they did at 750 °C. It is therefore possible that the Ni surface area available for the reaction would increase initially with time, due to carbon formation, which in turn suggests that the observed ethylene conversion would increase with time. However, the further growth of carbon would mean that some Ni sites would be
encapsulated/buried by the carbon, leading to decreases in the catalytic reaction rates and ethylene conversion (Figure 5a). Figure 5c shows the effects of temperature on the average formation rates of gaseous products. Corresponding to the decreases in the average formation rate of carbon, the average formation rate of H2 from the dehydrogenation of C2H4 decreased with increasing temperature. However, the average formation rate of ethylene hydrogenation product (C2H6) increased with increasing temperature. The increased CH4 formation rate may be a result of the thermal decomposition of C2H6 at high temperature.3 It is clear that the hydrogenation of ethylene was intensified relative to the direct decomposition on Ni surface with increasing temperature. 4. Conclusion Our experimental results show that the fluidized bed of Ni nanoparticles and micrometer-sized SiO2 particles can act as an effective catalyst for the catalytic reactions of ethylene and hydrogen. The nonporous catalyst system allowed us to investigate the changes in catalyst activity due to catalyst properties without the complications due to the changes in the diffusion resistance within pores. Although significant amounts of carbon were formed on the Ni surface, the Ni catalyst was not completely deactivated. However, the formation of carbon could lead to defluidization. Fluidization of Ni and SiO2 mixture could be drastically improved by increasing temperature. The catalytic activity of the Ni catalyst changed with reaction time, showing a maximum at 750 and 800 °C. The crystalline size of Ni as determined by X-ray diffraction (XRD) also decreased with reaction time. Although the dehydrogenation of ethylene was the dominant reaction initially, the hydrogenation of ethylene to ethane became increasingly important with time. The changes in the dominant reaction pathways are believed to be due to the changes in the Ni crystalline size and Ni surface composition associated with the formation of carbon on the nickel surface. Acknowledgment. The authors gratefully acknowledge the financial support of this study by the Australian Research Council (DP0556095).
(15) Glacier, G. F.; Filfil, R.; Pacey, P. D. Carbon 2001, 39, 497–506.
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