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Energy & Fuels 2008, 22, 1480–1484
Catalytic Decomposition of Methane over Ni/Al2O3 Catalysts: Effect of Plasma Treatment on Carbon Formation Xinli Zhu,*,† Dangguo Cheng, and Pingyu Kuai Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China ReceiVed December 9, 2007. ReVised Manuscript ReceiVed January 24, 2008
Two Ni/Al2O3 samples prepared using incipient wetness impregnations with and without argon glow discharge plasma treatment were tested for methane decomposition to H2 and carbon nanofibers. The plasma treatment induces a significant change in properties of catalytic methane decomposition. The plasma-treated sample shows a low CH4 decomposition rate and low final carbon yield, as a result of a higher concentration of a close-packed plane in the Ni particle, smaller Ni particle sizes, and stronger Ni-Al2O3 interactions. The carbon nanofiber growth pathway shifts from a mixture of tip and base growth for the nonplasma-treated sample to base growth for the plasma-treated sample. Very different from the carbon nanofibers obtained over the Ni catalyst prepared without plasma treatment, the tips of about 80% carbon nanofibers produced over the plasmatreated sample are open.
Catalytic decomposition of methane has attracted a lot of attention because of the great potential production of H2 and nanostructured carbon (carbon nanotubes and/or nanofibers).1–4 H2 produced from this reaction is free of CO,5–7 which is highly desirable for proton-exchange membrane (PEM) fuel cells. On the other hand, the carbon nanotubes/nanofibers have extensive potential applications in catalysis, energy-storage devices, selective adsorption, microreactors, and reinforcement materials.1,2 Furthermore, carbon atoms in methane are converted and stored as solid carbon, and zero CO2 emission is thus obtained. This property is very attractive because the greenhouse gas effect has become more serious in recent years because of the drastic increase in CO2 emissions all over the world. Although there are other ways for nanotube/nanofiber production, such as arc discharge and laser ablation,8,9 chemical vapor deposition (CVD) is recognized as a relatively cheap way to produce a large amount of nanotubes/nanofibers. Methane, as a major component of natural gas, has the highest H/C ratio among all hydrocarbons. Catalytic decomposition of methane is suitable for both hydrogen and nanostructured carbon production.
The supported Ni catalyst is the most active catalyst among Fe-, Co-, Ni-, and Cu-based catalysts for methane decomposition.10 Many studies have been focused on the effect of particle size on nanotube growth, which influences not only the diameter of carbon nanofibers but also the growth rate and final carbon yield.10–13 For example, Chen et al. studied methane decomposition over supported Ni catalysts with different Ni particle sizes.11 In their results, the optimized Ni size is around 36 nm and smaller sizes of Ni particles give lower carbon yield. Supports play an important role in the particle size, metal–support (or substrate) interactions, as well as Ni particle crystallographic orientations. Thus, the support has a significant effect on the structure of nanofibers and carbon yield. The influences of various supports have been well-documented.14–17 However, the influence of pretreatment of the catalyst or the catalyst precursor on Ni particle sizes, Ni-support interactions, and Ni morphology for methane decomposition has not been well-studied. Plasma treatment of the Ni catalyst has shown a promotion effect for several reactions. Ratanatawanate et al.18 showed that the RF plasma treatment of Ni/Al2O3 promotes the benzene hydrogenation reaction. In previous work, we studied the plasmatreated Ni/Al2O3 catalyst for CO2 reforming of methane.19,20
* To whom correspondence should be addressed. Telephone: 405-3250550. Fax: 1-405-325-5813. E-mail:
[email protected] or zzxxlly@ yahoo.com.cn. † Present address: School of Chemical, Biological, and Materials Engineering, The University of Oklahoma, Norman, OK 73019. (1) Choudhary, T. V.; Goodman, D. W. Catalysis 2006, 19, 164. and references therein. (2) Ledoux, M. J.; Vieira, R.; Pham-Huu, C.; Keller, N. J. Catal. 2003, 216, 333. (3) Zhang, T. J.; Amiridis, M. D. Appl. Catal., A 1998, 167, 161. (4) Rahman, M. S.; Croiset, E.; Hudgins, R. R. Top. Catal. 2006, 37, 137. (5) Zein, S. H. S.; Mohamed, A. R. Energy Fuels 2004, 18, 1336. (6) Muradov, M. Z. Energy Fuels 1998, 12, 41. (7) Shah, N.; Panjala, D.; Huffman, G. P. Energy Fuels 2001, 15, 1528. (8) Iijima, S. Nature 1991, 354, 56. (9) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Colbert, D. T.; Scuseria, G.; Tomanek, D.; Fisher, J. E.; Smalley, R. E. Science 1996, 273, 483.
(10) Avdeeva, L. B.; Reshetenko, T. V.; Ismagilov, Z. R.; Likholov, V. A. Appl. Catal., A 2002, 228, 53. (11) Chen, D.; Christensen, K. O.; Ochoa-Fernández, E.; Yu, Z. X.; Tøtdal, B.; Latorre, N.; Monzón, A.; Holmen, A. J. Catal. 2005, 229, 82. (12) Dai, H.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471. (13) Cheung, C. L.; Kurtz, A.; Park, H.; Lieber, C. M. J. Phys. Chem. B 2002, 106, 2429. (14) Park, C.; Keane, M. A. J. Catal. 2004, 221, 386. (15) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otuska, K. J. Phys. Chem. B 2004, 108, 7656. (16) Van Wal, R. L.; Ticich, T. M.; Curtis, V. E. Carbon 2001, 39, 2277. (17) Li, X. N.; Zhang, Y.; Smith, K. J. Appl. Catal., A 2004, 264, 81. (18) Ratanatawanate, C.; Macias, M.; Jang, B. W. L. Ind. Eng. Chem. Res. 2005, 44, 9868. (19) Zhu, X. L.; Zhang, Y. P.; Liu, C. J. Catal. Lett. 2007, 118, 306. (20) Zhu, X. L.; Huo, P. P.; Zhang, Y. P.; Cheng, D. G.; Liu, C. J. Appl. Catal., B 2008, in press, doi: 10.1016/j.apcatb.2007.11.042.
1. Introduction
10.1021/ef700746g CCC: $40.75 2008 American Chemical Society Published on Web 02/29/2008
Methane Decomposition oVer Plasma-Treated Ni/Al2O3
Figure 1. Effect of the reaction temperature on methane conversion over Ni/Al2O3 catalysts.
Figure 2. Effect of the reaction temperature on carbon yield over Ni/ Al2O3 catalysts.
The plasma treatment induced smaller Ni particle sizes, a stronger Ni-support interaction, and an enhanced concentration of close-packed planes. These changes lead to a catalyst with great coke resistance property for CO2 reforming. In this work, we attempt to study, for the first time, the influence of glow discharge plasma treatment of the catalyst precursor of Ni/Al2O3 on the methane decomposition to H2 and carbon nanofibers. 2. Experimental Section 2.1. Catalyst Preparation. Two Ni/Al2O3 (5:100 Ni/Al2O3) catalysts were prepared by incipient wetness impregnations with and without plasma treatment.19,20 Briefly, γ-Al2O3 was impregnated with an aqueous solution of Ni(NO3)2, dried at 110 °C, treated with or without Ar glow discharge plasma treatment for 1 h, and finally calcined at 600 °C for 4 h. The obtained Ni/Al2O3 catalysts with and without plasma treatment are denoted as NiAl-PC and NiAl-C, respectively. The Ar glow discharge plasma system used for catalyst treatment has been described elsewhere.19–21 (21) Zhu, X. L.; Huo, P. P.; Zhang, Y. P.; Liu, C. J. Ind. Eng. Chem. Res. 2006, 45, 8604.
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Figure 3. Temperature-programmed catalytic methane decomposition over Ni/Al2O3 catalysts.
Figure 4. XRD patterns of Ni/Al2O3 catalysts after methane decomposition at 500 °C.
2.2. Activity Test. The catalytic performance was carried out in a micro quartz tubular reactor at atmospheric pressure. After the catalyst sample (50 mg) was reduced by flowing H2 at 700 °C for 2 h, 30 cm3/min gases of CH4 and Ar with a molar ratio of 1:2 were introduced into the micro reactor by mass flow controllers at certain temperatures. The CH4 and Ar gases are ultrahigh pure grade (>99.999%) and are used without further purification. The gas residue was analyzed online by a gas chromatograph (GC, Agilent 6890), equipped with a thermal conductivity detector (TCD) and a TDX-01 packed column. The reaction was stopped when the catalyst was thoroughly deactivated. For the temperature-programmed CH4 decomposition, after the catalyst (50 mg) was reduced by flowing H2 at 700 °C for 2 h, the temperature was decreased to 100 °C in Ar flow. Then, reactant (1:2 CH4/Ar, 30 cm3/min) was introduced, and the reaction temperature was increased from 100 to 1000 °C at a rate of 10 °C/min. Products were monitored by a mass spectrometer (GSD301, Omnistar). It should be noted that H2 is the only gaseous product for methane decomposition in the present study. 2.3. Catalyst Characterizations. X-ray powder diffraction (XRD) patterns of the catalyst samples were recorded by a Rigaku D/max-2500 diffractometer at a scanning speed of 4°/min, with a Cu KR radiation source (λ ) 1.540 56 Å).
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Zhu et al. diameter size distributions were obtained by measuring approximately 150 nanofibers from different TEM photos.
3. Results and Discussion
Figure 5. Raman spectra of Ni/Al2O3 catalysts after methane decomposition at 500 °C.
Figure 6. TEM images of carbon nanofibers produced at 500 °C over the NiAl-C sample. (A) Typical morphology. (B) Ni particle at the tip of a carbon nanofiber.
Raman spectra of the catalyst samples were recorded on a micro Raman spectrometer (In via, Renishaw) with the 514.5 nm line of an argon laser. The incident laser power at the sample was 5 mW, and the spectrum resolution is 4 cm-1. Transmission electron microscopy (TEM) observations of the carbon nanofibers formed were performed on a Philips TECNAI G2F20 system operated at 200 kV. The catalyst powder after the reaction was directly dispersed ultrasonically in ethanol without grinding to avoid any damage in the structure of carbon nanofibers during grinding. A drop of the suspension was deposited on a carbon-coated copper grid for TEM analysis. Carbon nanofiber outer
3.1. Effect of the Reaction Temperature on Methane Decomposition. Figure 1 shows the effect of the reaction temperature on the CH4 conversion. For both samples, the initial CH4 conversion is increased with the reaction temperature increasing. However, the deactivation rate is increased at the same time. At each reaction temperature, the CH4 conversion rate for NiAl-PC is always lower than that for NiAl-C, probably as a result of the Ni particles in the NiAl-PC having a higher concentration of close-packed plane,19 because CH4 methane is more easily decomposed over defect sites than over close-packed planes. The effect of the reaction temperature on the final carbon yield is shown in Figure 2. The carbon yield is decreased with the reaction temperature increasing, as a result of fast deactivation at higher reaction temperatures.4 The carbon yield is also lower for NiAl-PC than that for NiAl-C at each reaction temperature. The lower carbon yield for NiAl-PC can be related to the lower CH4 decomposition rate, smaller Ni particle size (7.1 nm for NiAl-PC and 11.1 nm for NiAl-C),19 and the stronger Ni-Al2O3 interactions.20 As suggested by Chen et al.,11 a smaller Ni particle is unfavorable for a large amount carbon nanofiber formation. In addition, the stronger metal– support interactions would limit the carbon removal from the Ni surface by carbon filament formation.17 3.2. Temperature-Programmed Methane Decomposition. To obtain the property of CH4 decomposition over a wide temperature range, temperature-programmed CH4 decomposition was conducted. Because H2 is the only gaseous product of CH4 decomposition, Figure 3 is presented in the H2 production form. H2 production is initiated at 286 °C for both samples. An intense H2 evolution peak centered at 563 °C is visible for both samples, with a shoulder at the lower temperature side. Because the catalyst amount used is essentially the same (50 mg) for both samples, it is safe to compare the peak intensity. The main peak is more intense for NiAl-C, indicating that NiAl-C is more favorable for CH4 decomposition and nanofiber growth. When the reaction temperature is increased higher than 707 °C, H2 evolution is increased again. This feature was also observed in the Ni/CeO2 catalyst prepared by coprecipitation during CH4 decomposition, but no reason was given.22 It can not be ascribed to CH4 pyrolysis at high temperatures, because pyrolysis is unselective and the intensity of H2 evolution should be the same for both samples. However, the H2 evolution signal is more intense for NiAl-C, indicating again that NiAl-C is more favorable for CH4 decomposition. Therefore, it is most likely due to the fact that the H2 produced from CH4 decomposition reduces the unreduced Ni species during prereduction at 700 °C for 2 h. This new reduced Ni gives active sites for CH4 decomposition at higher temperatures. The temperatureprogrammed CH4 decomposition results indicate that the most favorable temperature for CH4 decomposition is lower than 563 °C, over which fast deactivation will occur. The fast deactivation at higher temperatures is because the CH4 decomposition rate is much faster than the rate of carbon diffuse and nucleation to form carbon nanofibers over Ni particles.11 Therefore, encapsulation of the Ni particle by carbon will take place, resulting in the deactivation of the catalyst. 3.3. Characterization of Carbon Nanofibers Produced at 500 °C. The carbon nanofibers formed at 500 °C were (22) Li, Y.; Zhang, B. C.; Tang, X. L.; Xu, Y. D.; Shen, W. J. Catal. Commun. 2006, 7, 380.
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Figure 7. TEM images of carbon nanofibers produced at 500 °C over the NiAl-PC sample. (A) Typical morphology. (B) Open tip. (C) Closed tip. (D) Onion-like carbon.
Figure 8. Carbon nanofiber outer diameter distributions of NiAl-C and NiAl-PC samples produced at 500 °C.
characterized because the highest carbon yield was obtained at 500 °C. Figure 4 shows XRD patterns of NiAl-C and NiAl-PC after methane decomposition at 500 °C. For the purpose of comparison, the XRD patterns were normalized by one of Al2O3 peaks (2θ ) 66.7°). Strong peaks at 2θ ) 26° are observed for both samples, which is related to the (002) plane of material of the graphitic structure. The peak intensity of NiAl-C is 2 times stronger than that of NiAl-PC, which is in good agreement of
the higher carbon yield for NiAl-C. The graphitization degrees, calculated from the Maire and Mering formula23 using d002, are 36 and 24% for NiAl-C and NiAl-PC, respectively. This result suggests that, although both samples contain a high concentration of disorder, the graphitization degree is higher for the NiAl-C sample. The Raman spectra show that typical multiwalled carbon nanofiber structures are formed (Figure 5). D and G bands are observed at ca. 1345 and 1580 cm-1, respectively, for both samples. No feature was observed in the range of 50–400 cm-1, which is the character region of radial breathing mode (RBM) of the single- or few-walled carbon nanotube. The D band has arisen from structure defects or imperfection of graphite, whereas the G band is associated with a splitting of the E2g stretching mode of graphite.24 In addition, a very weak D′ band is present at ca. 1600 cm-1 as a shoulder of the G band, which has stemmed from the dangling band of disorder graphite.24 The ratio of the integrated intensity of the G to D band (IG/ID) is indicative of the graphitization degree.24 The IG/ID values, obtained after Gauss curving fitting, are 0.76 and 0.67 for NiAl-C and NiAl-PC, respectively, indicating a higher graphitization degree for the NiAl-C sample, which is in line with XRD results. Figure 6 illustrates the TEM photos of carbon nanofibers produced over NiAl-C after CH4 decomposition at 500 °C. Ni particles (darker spots) can be clearly seen at the tip of nanofibers (Figure 6A); also some tips of nanofibers are absent of the Ni particle, indicating a mixture of the tip and base growth (23) Ermakova, M. A.; Ermakov, D. Y.; Chuvilin, A. L.; Kuvshinov, G. G. J. Catal. 2001, 201, 183. (24) Biris, A. R.; Biris, A. S.; Lupu, D.; Trigwell, S.; Dervishi, E.; Rahman, Z.; Marginean, P. Chem. Phys. Lett. 2006, 429, 204.
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pathway25,26 for carbon nanofiber formation. Apparently, the larger particle with a weak interaction with support is easy to form tip-growth-type nanofibers. An enlarged TEM image of the pear-like Ni particle at the tip of the nanofiber is shown in Figure 6B. The rear side of the Ni particle is parallel to the graphene layers of the nanofiber. In addition, the front side of the Ni particle is covered by several layers of graphene, indicating the deactivation is due to encapsulation. The Ni particle confined in the nanofiber is occasionally observed, as marked by an arrow in Figure 6A. For NiAl-PC, no Ni particles can be found at the tip of nanofibers (Figure 7A), suggesting a base growth pathway for carbon nanofiber growth.25,26 Actually, about 80% of tips are characterized by the ring-like structure. An enlarged TEM image of these structures (Figure 7B) reveals that the tip of the nanofiber is open. The other tips of nanofibers are closed (Figure 7C). Whether carbon nanofiber formation follows tip or base growth is largely dependent upon the metal–support interactions.25 A strong metal–support interaction favors the base growth pathway, whereas a weak metal–support interaction favors the tip growth pathway.25 The carbon nanofiber formation pathway shifts from a mixture of tip and base growth for NiAl-C to base growth for NiAl-PC and is in good agreement with the stronger Ni-Al2O3 interactions for the NiAl-PC sample.20 The open-tip fibers/tubes have been occasionally found over Co, Mo, and Ni catalysts,2,27–29 but no growth model has been given to distinguish the open and closed fibers/tubes thus far. For example, in Baker’s25 growth model for carbon filament formation, it only gives tip or base growth without prediction of whether the tip is open or closed. Cheung et al.13 studied single-walled nanotube growth over Fe nanoparticles. In their base growth model, the formation of the closed carbon cap is necessary for carbon nanotube formation. Therefore, the tip of the carbon nanotube is closed. The growth condition is different for single-walled carbon nanotubes and carbon nanofibers, because the metal particles for nanofiber growth are usually much larger than that for nanotube growth and the reaction temperature is usually higher for the single-walled nanotube. It is more difficult for the closed carbon cap growth over a particle with a large diameter at relative lower temperatures. Therefore, the formation of a closed or open tip is most likely related to the morphology of the Ni particle, because both the carbon nucleation rate and graphene layer stacking structure of the nanofiber growth are determined by the exposed crystallographic orientations.30 The morphology changes (high concentration of (25) Baker, R. T. K. Carbon 1989, 27, 315. (26) Melechko, A. V.; Merkulov, V. I.; Lowndes, D. H.; Guillorn, M. A.; Simpson, M. L. Chem. Phys. Lett. 2002, 356, 527. (27) Chai, S. P.; Zein, S. H. S.; Mohamed, A. E. Chem. Phys. Lett. 2006, 426, 345. (28) Burns, S.; Gallagher, J. G.; Hargreaves, J. S. J.; Harris, P. J. F. Catal. Lett. 2007, 116, 122. (29) Li, Z. Q.; Chen, J. L.; Zhang, X. X.; Li, Y. D.; Fung, K. K. Carbon 2002, 40, 409. (30) Hofmann, S.; Sharma, R.; Rucati, C.; Du, G. H.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. Nano Lett. 2007, 7, 602.
Zhu et al.
the close-packed plane) of plasma-treated supported Ni catalysts have been revealed by infrared spectroscopy of adsorbed CO.19 There are some particles that cannot grow naonfibers but give onion-like graphite. As shown in Figure 7D, the Ni particle is surrounded by layers of graphite carbon. These particles are generally smaller than 6 nm, indicating that smaller particles are difficult to form carbon nanofibers in the present condition (low reaction temperature). The structure of carbon nanofibers formed over the two samples is the typical fishbone structure of Ni catalysts, with the angle around 45° of the graphene plane to the fiber axis (Figure 6B and parts B and C of Figure 7). Figure 8 compares the diameters of carbon nanofibers formed over NiAl-C and NiAl-PC at 500 °C. The NiAl-PC gives a narrower nanofiber diameter size distribution and a smaller average diameter with respect to that of NiAl-C, as a result of size-controlled growth.13 The manufacture of high-purity open-tip nanofibers with controlled diameter has potential usage in some special occasions, such as acting as a host for small nanoparticles. Also, the study of the formation of this kind of structure will shed more light on the more detailed mechanism of carbon nanofiber formation. 4. Conclusions The Ni/Al2O3 catalyst sample prepared with plasma treatment shows a low CH4 conversion rate and low final carbon yield in methane decomposition with respect to that of the nonplasmatreated sample, as a consequence of a higher concentration of the close-packed plane in Ni particles, smaller Ni particle sizes, and stronger Ni-Al2O3 interactions induced by plasma treatment. The changes in the Ni particle size, Ni particle morphology, and Ni-support interactions also shift the carbon nanofiber growth pathway from a mixture of tip and base growth for the nonplasma-treated sample to a base growth for the plasmatreated sample. Interestingly, about 80% carbon nanofibers produced over the plasma-treated sample are characterized by the open-tip structure. The reason that the open-tip nanofiber formation is most likely related to the morphology changes induced by the plasma treatment. Further studies on the formation mechanism of the open-tip carbon nanofiber are needed. These open-tip nanofibers probably have potential usage in some special occasions. Our results also indicate that the metal-support interactions play an important role on various types of carbon-material formation. Acknowledgment. Prof. C. J. Liu in Tianjin University is greatly appreciated. Support from the National Natural Science Foundation of China (20490203) and the Key Fundamental Research Project of the Ministry of Science and Technology of China (2005CB221406) is greatly appreciated. The instrument for high-voltage generation and measurement was donated by ABB Switzerland Ltd., which is greatly appreciated. EF700746G
