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Energy & Fuels 2004, 18, 1336-1345
Mn/Ni/TiO2 Catalyst for the Production of Hydrogen and Carbon Nanotubes from Methane Decomposition Sharif Hussein Sharif Zein and Abdul Rahman Mohamed* School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, S.P.S, Pulau Pinang, Malaysia Received November 12, 2003. Revised Manuscript Received May 21, 2004
The decomposition of methane into carbon and hydrogen over 15 mol % MnOx/20 mol %NiO/ TiO2 catalysts was investigated. The effects of catalyst synthesis and pretreatment of 15 mol % MnOx/20 mol % NiO/TiO2 catalysts for the activity and morphology of carbon from methane decomposition were identified, based on the parameters studied. The catalysts were characterized using X-ray diffractometry (XRD), temperature-programmed reduction (TPR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption. The activity tests were performed at atmospheric pressure in a stainless-steel fixed-bed reactor at a temperature of 725 °C and gas hourly space velocity (GHSV) of 2700 h-1. The 15 mol % MnOx/20 mol % NiO/TiO2 catalyst that was prepared using the impregnation method without any pretreatment showed high activity for the methane decomposition reaction at 725 °C and GHSV ) 2700 h-1. The regenerated samples showed no significant decrease in methane conversion after up to six cycles of decomposition and regeneration. The XRD patterns of the regenerated samples indicated that no apparent structural change occurred. Although different types of filamentous carbon formed on the various 15 mol % MnOx/20 mol % NiO/TiO2 catalysts prepared by different methods, an attractive carbon nanotube was observed in the case of the 15 mol % MnOx/20 mol % NiO/TiO2 catalyst prepared via the impregnation method and tested without any pretreatment.
Introduction Recently, nanocarbon materials have attracted considerable attention, because of their excellent properties and potential utilizations. Carbon nanotubes are one of the most innovative material technologies of the twentyfirst century, because of their many desirable material properties. They are ideal conductors of heat and electricity and are strong, chemically inert, and nontoxic. They can be used for energy storage, and they can be utilized as thermal materials, nanoelectronics, nanosensors, electrostatic discharge materials, and ultracapacitors. Other applications include hydrogen storage, sorbents, catalyst supports, industrial rubber, tires, plastics, inks, paints, structural composite materials, and in new areas.1-7 If encouraging results are obtained, in regard to hydrogen storage on carbon, it could have strong implications for future hydrogen storage sys* Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Shaikhutdinov, S. K.; Zaikovskii, V. I.; Avdeeva, L. B. Appl. Catal., A 1996, 148, 123-133. (2) Avdeeva, L. B.; Kochubey, D. I.; Shaikhutdinov, S. K. Appl. Catal., A 1999, 177, 43-51. (3) Ermakova, M. A.; Ermakov, D.Y.; Kuvshinov, G. G.; Plyasova, L. M. J. Catal. 1999, 187, 77-84. (4) Ermakova, M. A.; Ermakov, D. Y.; Kuvshinov G. G. Appl. Catal., A 2000, 201, 61-70. (5) Aiello, R.; Fiscus, J. E.; Zur Loye, H.-C.; Amiridis, M. D. Appl. Catal., A 2000, 192, 227-234. (6) Otsuka, K.; Kobayashi, S.; Takenaka, S. Appl. Catal., A 2001, 210, 371-379. (7) Otsuka, K.; Ogihara, H.; Takenaka, S. Carbon 2003, 41, 223233.
tems8,9 and might accelerate the introduction of hydrogen as a fuel.10 A recent review of the processing and properties of carbon nanotubes and their composites has been given by Thostenson et al.11 For the synthesis of carbon nanotubes, several methods have been developed (mainly arc discharge, laser ablation, and chemical vapor deposition). The development of a reliable source of large quantities of carbon nanotubes is dependent on better production methods. Natural gas, which contains primarily methane, is a natural resource that rivals liquid petroleum, in regard to abundance. Given the inevitable depletion of liquid petroleum and a concomitant increase in natural gas reserves, it is expected that methane will eventually become a major resource for chemicals and liquid fuels. The abundance of natural gas can be better utilized by increasing its use as a source of chemicals in place of its predominant use as a fuel. One of the major challenges is the development of a direct process for the conversion of natural gas to more-value-added chemicals. The decomposition of methane to hydrogen and carbon over supported nickel catalysts is of current interest as an alternative route to the production of carbon nanotubes from natural gas. Nickel is known as one of the active catalysts in the decomposition of methane to hydrogen and carbon.1-7 The direct cracking (8) Darkrim, F. L.; Malbrunot, P.; Tartaglia, G. P. Int. J. Hydrogen Energy 2002, 27, 193-202. (9) Ci, L.; Zhu, H.; Wei, B.; Xu, C.; Wu, D. Appl. Surf. Sci. 2003, 205, 39-43. (10) Ogden, J. M. Annu. Rev. Energy Environ. 1999, 24, 227-279. (11) Thostenson, E. T.; Ren, Z.; Chou, T.-W. Compos. Sci. Technol. 2001, 61, 1899-1912.
10.1021/ef0340864 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/22/2004
H2 and Carbon Nanotubes from CH4 Decomposition
of dilute methane with helium over a 16.4 wt % Ni/SiO2 catalyst has been studied.12 The initial high activity of catalyst at 550 °C yielded a methane conversion of 35%. Filamentous carbon was also produced and eventually deactivated the catalyst. Nickel-loaded catalysts (up to 90 wt %), which are used to produce hydrogen and filamentous carbon, have been reported. The methane conversion was 8% at 500 °C and 15% at 550 °C. However, complete deactivation was observed at 600 °C.3 Nickel gauze was used as a catalyst to decompose methane at 500 °C.13 However, the catalyst deactivated, because of coke deposition. The optimum reaction performance was for reaction periods of 4 min, followed by a 4 min period of regeneration. The reduction of an oxide phase by methane to catalytically active metal and carbide phases, followed by depletion of these active phases before reaching steady-state methane decomposition to yield 20% hydrogen, was reported when Fe2O3 and NiO supported on alumina at 850 °C were used.14 Shah et al.15 determined that the Fe/Al2O3 catalyst pretreated with hydrogen gave the highest activity, compared to the catalysts that were pretreated by oxygen or carburized. In a previous study, we investigated the effect of a promoter on 20 mol % NiO/TiO2.16 To select a suitable promoter, experiments were conducted for the decomposition of methane to hydrogen and carbon with a 20 mol % NiO/TiO2 catalyst with different promoters, viz., MnOx, FeO, CoO, and CuO at 725 °C and a gas hourly space velocity (GHSV) of 2700 h-1. MnOx promoted onto a 20 mol % NiO/TiO2 catalyst was observed to maintain its activity for a time-on-stream of 180 min. The activity changes were most probably due to the mechanism of formation of carbon on the catalysts. Hence, carbon samples obtained on 20 mol % NiO/TiO2 catalysts were studied using transmission electron microscopy (TEM). The results elucidated that doping on CuO, MnOx, FeO, and CoO influenced the carbon morphology remarkably. Different transition metals produced different types of filamentous carbon. Although different types of filamentous carbon formed on the various nickel-containing catalysts, an attractive filamentous carbon was observed on the Mn/Ni/TiO2-based catalyst. Therefore, a nickelbased catalyst promoted by manganese was chosen, based on its performance in regard to the yield of hydrogen and the type of carbon that forms from the methane decomposition reaction. The focus of this study is to identify the effect of the Mn/Ni/TiO2-based catalyst preparation method and pretreatment conditions toward methane decomposition to hydrogen and carbon. Experimental Section Impregnation (IM), poly(vinyl) (PV), and sol-gel (SG) methods were used to create the 15 mol % MnOx/20 mol % NiO/TiO2 catalysts. The IM- and PV-type catalysts were both prepared via conventional impregnation method, with the exception that, in the latter, poly(vinyl alcohol) was used as a solvent. A desired amount of transition-metal oxides were (12) Zhang, T.; Amiridis, M. D. Appl. Catal., A 1998, 167, 161-172. (13) Monnerat, B.; Kiwi-Minsker, L.; Renken, A. Chem. Eng. Sci. 2001, 56, 633-639. (14) Muradov, N. Z. Energy Fuels 1998, 12, 41-48. (15) Shah, N.; Panjala, D.; Huffman, G. P. Energy Fuels 2001, 15, 1528-1534. (16) Zein, S. H. S. Ph.D. Thesis, Universiti Sains Malaysia, 2003.
Energy & Fuels, Vol. 18, No. 5, 2004 1337 dissolved in deionized water or poly(vinyl alcohol). The selected dopant concentrations were actually relative to the molar quantity of the support. The resulting paste was then dried in an oven and then calcined in a ceramic crucible at 900 °C. The catalysts were then sieved to a size of 400-500 µm. The SG-type catalyst was prepared using a conventional sol-gel preparation method. Manganese nitrate hexahydrate solution and nickel nitrate hexahydarate solution were added to a mixture of titanium tetraisopropoxide solution. The solution was then stirred until it became progressively more viscous. At a defined point, the particles coalesced to form an elastic gel. The resulting gel was air-dried in a preheated, wellventilated oven and spread thinly (∼10 mm deep) onto a silica tray and air-calcined in a preheated furnace. The calcined catalyst was then crushed and sieved to a size of 400-500 µm. The activity tests for the catalysts in the decomposition of methane were conducted at atmospheric pressure in a stainless-steel fixed-bed reactor. Pure argon was allowed into the reactor, to create an inert atmosphere in the reactor. The catalysts were subjected to the following pretreatments before the reaction: (a) Oxidation. Catalysts were oxidized under flowing oxygen (20 mL/min) at 725 °C for 2 h. After oxidation, the experimental setup was flushed with argon until gas chromatography (GC) showed a complete disappearance of oxygen. (b) Reduction. Same as pretreatment (a), except hydrogen was used instead of oxygen. (c) Fresh. Catalysts were tested without any of the aforementioned pretreatments. The methane decomposition rig system consisted of a gas mixing system, a reactor, and on-line analysis system. A schematic diagram of the methane decomposition rig system is shown in Figure 1. In the gas mixing system, methane (99.999% purity, supplied by Malaysian Oxygen Sdn. Bhd.) was mixed with argon (99.999% purity, supplied by Sitt Tatt Industrial Gases Sdn. Bhd.) before entering the reactor. Nitrogen can react with the hydrogen at high temperature; therefore, for safety reasons, argon was used as a diluent gas. Oxygen (98.5% purity, supplied by Malaysian Oxygen Sdn. Bhd.) was used for catalyst pretreatment and regeneration study and hydrogen (99.999% purity, supplied by Malaysian Oxygen Sdn. Bhd.) was used for catalyst pretreatment study. The flow of methane was regulated using a mass flow controller (MKS) and the argon, oxygen, and hydrogen flows were regulated by different mass flow controllers (Brooks, model 5850E). The outlet gas flow was monitored by a gas flow meter (Alexander Wright, model DM3 B). A thermocouple was used to measure the temperature of the catalyst bed in the reactor. The furnace was equipped with a temperature controller and was supplied by Carbolite, UK. A pressure gauge (Ashcroft, USA), located just above the reactor, was used to read the inlet pressure. The product gases were analyzed using an on-line gas chromatograph (Hewlett-Packard Series 6890, USA). The gas chromatograph was controlled on-line, using HP ChemStation Revision A. 06.01.[403] software. The standard gas mixture was supplied by BOC Gases, UK. X-ray diffractometry (XRD) refined by the Rietveld method was used to characterize the catalyst structure. Room-temperature XRD was conducted on a Siemen D-5000 diffractometer, using Cu KR radiation and a graphite secondary beam monochromator. The specimen was prepared by packing sample powder in a glass holder. Intensity was measured by step scanning in the 2θ range of 10°-90°, with a step of 0.02° and a measuring time of 2 s per point. The diffraction lines of the XRD pattern were used to identify the formation of a solid solution by comparing the 2θ values of the materials with those of phases from the powder diffraction files. Temperatureprogrammed reduction (TPR) was performed using CHEMBET-3000. Samples with a mass of 10 mg each were reduced in the TPR mode, using a quartz U-shaped reactor, by heating
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Figure 1. Schematic diagram of the methane decomposition rig system. from room temperature to 950 °C. The temperature of the catalysts was linearly increased at a heating rate of 5 °C/min, in 5% v/v hydrogen in helium. The gas mixture flow rate was 20 mL/min. The distributions of pore diameters of the different samples were determined via nitrogen adsorption/desorption isotherms at liquid nitrogen temperature (77 K) using an automated gas sorption system (Autosorb I, QuantoChrome Corporation, USA). All samples were degassed at a temperature of 573 K for 3 h prior to the measurements. Computer programs (Micropore, Version 2.46) allowed for rapid numerical results for the surface area and pore texture from adsorptiondesorption isotherm. For scanning electron microscopy (SEM) measurements, a finely ground sample was spread evenly on top of an aluminum sample stub stacked with a double-sided carbon tab. The sample was then placed into the specimen chamber under vacuum (5 atm). A Philips model XL 40 SEM system was used to determine the sample morphology. The sample was bombarded using an electron beam at 25 kV. The image was recorded using a graphic video recorder (Philips, model GP850). Spent catalysts were analyzed via a TEM system (Philips, model CM12) that used an accelerating voltage of 80 kV to extract electrons and Soft Imaging System model SIS 3.0. In preparation for the TEM experiments, a few particles of each sample were dispersed in distilled water, and then a drop of the suspension was deposited on a coated copper grid.
Results and Discussion Figure 2 shows that the thermal decomposition of methane in the temperature range of 400-900 °C was negligible, because the conversion was
