Simultaneous Production of Hydrogen and ... - ACS Publications

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Energy & Fuels 2000, 14, 1188-1194

Simultaneous Production of Hydrogen and Nanocarbon from Decomposition of Methane on a Nickel-Based Catalyst Yongdan Li,* Jiuling Chen, Yongning Qin, and Liu Chang Department of Catalysis Science and Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China Received April 17, 2000. Revised Manuscript Received September 5, 2000

From the decomposition of methane, hydrogen without carbon oxides can be produced with a high energy-efficiency, which is attractive for its suitability of utilization in the fuel cells. At a same time nanocarbon materials with attractive texture and structure can be produced in a large amount. Toward a simultaneous bulk production of hydrogen and nanocarbon, catalysts based on nanometer-scale nickel particles prepared from a hydrotalcite-like anionic clay precursor have been designed and tested to fit the process goals. For hydrogen production, as the equilibrium methane conversion of the reaction increases with the increase of the reaction temperature, the process is commercially more attractive if it can be operated at a temperature higher than 1073 K. However, a nickel catalyst has a maximum activity for nanocarbon production at 923 K. Modification of the catalyst with doping of copper increased the activation temperature and leads to a production of nanocarbon with an attractive structure. The feasibility and the challenges met for the coupling of the two process goals is discussed, and some promising results are presented in this work.

1. Introduction Hydrogen plays an important role and has been used with a very large volume in chemical, food, and refining industries.1 When it burns no pollutants are produced; with the increased concern of the environment, hydrogen has been expected to become a major source of energy. The recent advances in fuel cells have prompted the exploration for replacing traditional central large power plants, and for automobile use of the technique.2,3 Many recent publications focused on the hydrogen production for fuel cells.4-8 Steam reforming of methane (SRM) and other hydrocarbon feedstocks has been the largest and the most economical and popular technology for the production of hydrogen.1-3 However, this route makes hydrogen an indirect source of CO2. One of the products of steam reforming is CO; CO is removed by two subsequent steps, i.e., water-gas shift and methanation. A complete removal of CO is not economical, so that the hydrogen thus produced is not suitable for low-temperature fuel * Corresponding author. E-mail: [email protected]. (1) Armor, J. N. Appl. Catal. A 1999, 176, 159-176. (2) Andrews, L. D. J. Power Sources 1996, 61, 113-124. (3) Steinberg, M.; Cheng H. C. Int. J Hydrogen Energy 1989, 14, 797-820. (4) Williams, K. R.; Burstein G. T. Catal. Today 1997, 38, 401410. (5) Semin, G. L.; Belyaev, V. D.; Demin, A. K.; Sobyanin, V. A. Appl. Catal. A 1999, 181, 131-137. (6) Breen, J. P.; Ross, J. R. H. Catal. Today 1999, 51, 521-533. (7) Clarke, S. H.; Dicks, A. L.; Pointon, K.; Smith, T. A.; Swann, A.; Catal. Today 1997, 38, 411-423. (8) Acres, G. J. K.; Frost, J. C.; Hards, G. A.; Potter, R. J.; Ralph, T. R.; Thompsett, D.; Burstein, G. T.; Hutchings, G. J. Catal. Today 1997, 38, 393-400.

cells because CO poisons the electrocatalyst.2,3,9-11 Electrolysis or photocatalytic decomposition of water offers clean hydrogen; however, the former is only economical regionally and the latter is not an easy technical approach.1-3 Another alternative route is to crack hydrocarbons directly into hydrogen and carbon. In this case, formation of CO2 is avoided and the subsequent steps for removal of CO are not needed. Steinberg and Cheng3 and Muradov10 proposed that the route is superior to steam reforming from economical and energy efficiency points of views. Recently, nanocarbon materials, carbon nanotubes and nanofibers, have been paid considerable attention due to their excellent properties and potential utilizations.12-22 Two techniques have been used to produce nanocarbons: arc-discharge evaporation of graphite with metal presented in the vapor phase as catalyst,23-25 and catalytic growth of nanocarbon from decomposition (9) Zhang, T.; Amiridis, M. D. Appl. Catal. A 1998, 167, 161-172. (10) Muradov, N. Z. Int. J. Hydrogen Energy 1993, 18, 211-215. (11) Poirier, M. G.; Sapundzhiev, C. Int. J. Hydrogen Energy 1997, 22, 429-433. (12) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220-222. (13) Chen, P.; Wu X.; Lin, J.; Tan, K. L. Science 1999, 285, 91-93. (14) Ruoff, R. S.; Lorents, D. C. Carbon 1995, 33, 925-930. (15) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233-3250. (16) Kiang, C. H.; Goddard, W. A.; Beyers, R.; Bethune, D. S. Carbon 1995, 33, 903-914. (17) Iijima, S. Nature 1991, 354, 56-58. (18) Daumit, G. P. Carbon 1989, 27, 759-764. (19) Tibbetts, G. G.; Endo, M.; Beetz, C. P., Jr. Sampe J. 1986, September/October, 30-35. (20) Endo, M. Chemtech 1988, September, 568-576. (21) Lin, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127-1129. (22) Dagani, R. Chem. Eng. News 1999, June 7, 25-37. (23) Saito, Y. Carbon 1995, 33, 979-988.

10.1021/ef0000781 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000

Hydrogen and Nanocarbon from CH4 Decomposition

Energy & Fuels, Vol. 14, No. 6, 2000 1189

Table 1: Composition of the Catalysts Used in This Work sample 1 2 3 4 5 6

composition/atomic ratio Ni:Al Ni:Al Ni:Al Ni:Cu:Al Ni:Cu:Al Ni:Cu:Al

2:1 3:1 9:1 75:2:23 15:3:2 2:1:1

of hydrocarbons or disproportionation of carbon monoxide.15,26-29 The first technique produced attractive singlewalled and multiwalled nanotubes, however, the technique is difficult for scale-up and hence the demand of bulk use.23-25 The catalytic growth technique is easy for scale-up, and can be carried out at moderate temperatures, and is less energy consuming.15 The H/C ratio in methane is the highest among all hydrocarbons. A large amount of hydrogen is produced in the methane decomposition reaction (MDR). The catalytic technique for nanocarbon production would be more attractive if it can be coupled with bulk production of hydrogen. Nevertheless, to achieve the goal, the catalyst should be active and stable enough under conditions so that hydrogen with a high concentration and carbon with a large amount can be produced. The formation of nanocarbon from MDR has been investigated on a nickel-based nonsupported catalyst prepared from a precursor with a hydrotalcite-like (HC) anionic clay structure in our laboratory.30-33 It has been found that a large amount of nanocarbon can be produced and the morphology of the nanocarbon can be monitored. In this work, the possibility of simultaneous bulk production of hydrogen and nanocarbon from MDR is discussed. The activity and stability of catalyst and the morphology of produced nanocarbon are explored to adopt the process goals. 3. Experimental Section 3.1. Catalyst Samples. Catalyst precursors with a HC structure, or sometimes called Feitknecht compound structure, were prepared by coprecipitation from a mixed aqueous solution of nitrates with sodium carbonate. The precipitates were then washed, dried, calcined, and reduced. Details of the preparation and the structure of the precursor were given elsewhere.30,31 The HC structure of the precursors and the suitable preparation conditions ensure nanosized metal particles formed after the reduction.30-33 Table 1 lists the composition of the catalyst samples used. (24) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701-1703. (25) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; Lamy de la Chapelle, M.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756-758. (26) Chen, P.; Zhang, H. B.; Lin, G. D.; Hong, Q.; Tsai, K. R. Carbon 1997, 35, 1495-1501. (27) Avdeeva, L. B.; Goncharova, O. V.; Kochubey, D. I.; Zaikovskii, V. I.; Plyasova, L. M.; Novgorodov, B. N.; Shaikhutdinov, S. K. Appl. Catal. A 1996, 141, 117-129. (28) Fenelonov, V. B.; Derevyankin, A. Y.; Okkel, L. G.; Avdeeva, L. B.; Zaikovskii, V. I.; Moroz, E. M.; Salanov, A. N.; Rudina, N. A.; Likholobov, V. A.; Shaikhutdinov, Sh. K. Carbon 1997, 35, 1129-1140. (29) Nolan, P. E.; Lynch, D. C.; Culter, A. H. Carbon 1994, 32, 477483. (30) Li, Y. D.; Chen, J. L.; Chang, L. Appl. Catal. A 1997, 163, 4557. (31) Li, Y. D.; Chen, J. L.; Chang, L.; Qin, Y. N. J. Catal. 1998, 178, 76-83. (32) Li, Y. D.; Chen, J. L.; Zhao, J. S.; Chang, L. Stud. Surf. Sci. Catal. 1998, 118, 321-329. (33) Li, Y. D.; Chen, J. L.; Ma, Y.; Zhao, J. B.; Chang, L.; Qin, Y. N. Chem. Commun. 1999, 12, 1141-1142.

Figure 1. Schematic diagram of the reactor system Table 2: Conditions for Constant-Temperature Reactions expt. no.

catalyst sample

reaction temperature/K

feed composition/vol ratio

1 2 3

3 5 6

773 873 1023

CH4:N2 ) 0.6:0.4 CH4 ) 1 CH4:N2 ) 0.37:0.63

3.2. Hydrogen and Nanocarbon Production. The methane and nitrogen used in the experiments were nominally 99.999% pure. The hydrogen was 99.99% in purity. All the reactions were carried out in a tubular reactor, which was made of a horizontal stainless steel tube with an internal diameter of 38 mm. The catalyst particles were put at the bottom of the middle part of the tube where the temperature gradient was small, as shown in Figure 1. A catalytic oxygenremoving purifier was incorporated before the reactor inlet. A 100 mg amount of catalyst particles of 200-260 mesh were used. The flow rate of each gas stream was measured by precalibrated rotameters. All the reactions were done under atmospheric pressure. The catalyst was reduced for 2 h for the Ni/Al2O3 catalyst at 1023 K and for the Ni-Cu/Al2O3 catalysts at 973 K, respectively, with a flow of H2/N2 mixed gas with a volume ratio of 1/3 and a flow rate 150 mL/min (STP). After switching to the reaction gas with a prescribed composition at a fixed temperature, the reaction was presumed started at a prescribed temperature. The total flow rate of reaction gas is 68 mL/min (STP). The conversion of methane was measured by a gas chromatograph (GC). 3.2.1. Reactions with Stepwise Heating. For catalyst samples 1-6, reactions were carried out with a stepwise heating mode. The reaction was started at 723 K, after a period the temperature was increased at 10 K/min for 50 K, and the reaction was carried out at each temperature for 1 h, respectively. The stepwise heating was continued until the conversion of methane became lower than the first step. The reaction gas was composed of 63 vol % nitrogen and 37 vol % methane. 3.2.2. Reactions at Constant Temperature. Reactions were carried out at constant temperatures for catalyst samples 3, 5, and 6 with different feed compositions. When the reaction rate was very low as indicated by the conversion, the reaction was stopped. The reactor was cooled naturally to ambient temperature, and afterward the carbon with the catalyst was removed from the reactor and characterized. The reaction conditions and the catalyst samples are listed in Table 2. 3.3. Characterization. The texture of nanocarbon formed from experiment 1 was observed with a Hitachi 650 scanning electron microscope (SEM), and the morphologies of nanocarbons formed in experiments 1-3 were recorded with a JEOL JEM-100CXII transmission electron microscope (TEM).

4. Results 4.1. MDR with Stepwise Heating. Figure 2 presents the initial conversions of methane on Ni-alumina catalysts of different Ni/Al ratio with a stepwise heating mode. The catalysts have the maximum initial conversions at around 923 K. While the three maximum values

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Figure 2. Conversion of methane on Ni-alumina catalysts with stepwise heating; CH4:N2 ) 0.37:0.63 (vol). ∆: Ni:Al ) 2:1; ]: Ni:Al ) 3:1; 0: Ni:Al ) 9:1.

Figure 3. Conversion of methane on Ni-Cu alumina catalyst with stepwise heating; CH4:N2 ) 0.37:0.63 (vol). ∆: Ni:Cu:Al ) 75:2:23; ]: Ni:Cu:Al ) 15:3:2; 0: Ni:Cu:Al ) 2:1:1.

have nearly the same temperature, the conversions at the points are quite different, indicating that higher nickel content favored higher methane conversion. Below and above the temperature, the initial conversions of methane on the three catalysts are close. It should be noticed that the time periods for stable conversion of methane are different for different catalysts and for different reaction temperatures, and here, the reaction time for each point is limited to 1 h. Figure 3 depicts the initial conversions of methane on the three copper-doped catalysts with a stepwise heating mode. It shows that the change tendency of the conversion of methane on the three catalysts with the increase of reaction temperature is similar to that on

Li et al.

Figure 4. Decomposition of methane on a Ni-alumina catalyst at 773 K. Ni:Al ) 9:1 (mol); CH4:N2 ) 0.6:0.4 (vol). ∆: conversion of methane; ]: concentration of hydrogen; 0: amount of nanocarbon formed.

the undoped catalysts. The conversion of methane is lower than that on the undoped catalysts at low temperatures and the temperatures with maximum conversion go higher with the increase of the copper content. The temperatures of maximum conversion for the three catalysts are 923, 973, and 1023 K, respectively. During these reactions for doped and undoped catalysts, hydrogen was found to be the only gas product. 4.2. MDR at Constant Temperature. Figure 4 illustrates the conversion of methane, the hydrogen concentration in the product, and the amount of carbon formed at a constant temperature 773 K on the catalyst sample 3 with a feed composition of 60 vol % methane and 40 vol % nitrogen. It can be seen that the conversion of methane keeps close to 20% in a rather long time period and the conversion is lowered slowly. The conversion is still above 6% after 90 h. The final amount of carbon formed is 244 g-C/g-Ni, when the reaction was stopped, weighed by a balance after removal of the solid from the reactor. As carbon and hydrogen are the only two products, according to the carbon and hydrogen balance, the curve of the amount of carbon formed and hydrogen concentration in the product are obtained, as shown in Figure 4. The calculated final amount of carbon formed is 241 g-C/g-Ni, very close to the measured one. Figure 5 gives the conversion of methane at 873 K with a feed of pure methane on the catalyst sample 5, which has a ratio of Ni/Cu/Al as 75:15:10. The conversion increased slowly during the beginning 40 h and then lowered gradually, it kept above 20% for around 50 h. However, the conversion lowers close to zero when the reaction has been on for 99 h. The amount of carbon formed during this reaction is 585 g-C/g-Ni by weighing, and is 615 g-C/g-Ni by mass balance calculation. The conversion of methane on catalyst sample 6 at 1023 K with a feed of CH4/N2 as 0.37:0.63 is plotted in Figure 6. The conversion of methane keeps a stable level of around 70% during about 13 h. After 17 h, it lowered to 2%. The amount of nanocarbon formed is 191 g-C/gNi by weighing, and is 194 g-C/g-Ni by calculation.



was noticed, and in some cases bulk mass of carbon stuck to the reactor tube was formed. Nanocarbon formed on catalyst sample 3 at a constant temperature 773 K was observed with a SEM, shown in Figure 7A. The nanocarbon is fibrous and grows in a random direction and tends to take the form of loops. Many nanofibers are woven into each other randomly to become lumps. Figure 7B is the micrograph obtained with a TEM. It shows the morphology of nanofibers. A catalyst particle is located at the tip of a fiber, indicating the growth of the fiber from a metal particle. The inner diameter of the fiber is very thin. The morphology of the nanocarbons formed on catalyst sample 5 at a constant temperature 873 K was complex, as illustrated in Figure 8. Octopus-shaped, branch-shaped, and stickshaped nanocarbons are found coexist in the solid mass obtained. In the reaction with catalyst sample 6 at a constant temperature of 1023 K, only tubular nanofibers are found formed, as seen in Figure 9. Figure 5. Decomposition of methane on a Ni-Cu-alumina catalyst at 873 K with a feed of pure methane; Ni:Cu:Al ) 15:3:2 (mol); ∆: conversion of methane; ]: concentration of hydrogen; 0: amount of nanocarbon formed.

5. Discussion 5.1. MDR as a Hydrogen Production Process. To facilitate the discussion, a short review of the hydrogen production processes, i.e., MDR and SRM processes, may be necessary, though many excellent reviews have been published recently.1-3 MDR can be written as eq 1:

CH4(g) ) C + 2H2(g) +75 kJ/mol

(1)

which is moderately endothermic. SRM shown as eq 2:

CH4(g) + H2O(g) ) CO(g) + 3H2(g) + 206 kJ/mol

(2)

is a highly endothermic reaction. As CO needs to be eliminated, a water-gas shift converter needs to be incorporated following the reformer. The reaction is expressed as eq 3:

CO(g) + H2O(g) ) H2(g) + CO2(g) - 41 kJ/mol

(3)

The global reaction is as eq 4:

Figure 6. Decomposition of methane on a Ni-Cu-alumina catalyst at 1023 K. Ni:Cu:Al ) 2:1:1 (mol); CH4:N2 ) 0.37: 0.63 (vol); ∆: conversion of methane; ]: concentration of hydrogen; 0: amount of nanocarbon formed.

It should be specified that in the results shown in Figures 4 and 6, the nitrogen used as a diluent is ignored in the gas product expression. The weighing of the solid product and the calculation gives a good closure of the carbon balance (100 ( 5%) in these reactions. The rates of methane consumption and hydrogen formation were found to be in a ratio of 1:2. 4.3. Morphology and Texture of Nanocarbon Formed. The carbon formed in the experiments looks like granulates. The size of the granulate depends on the size of the particle of catalyst put into the reactor and the amount of carbon formed on unit mass of catalyst. It seems likely that each catalyst particle grows into a carbon granulate with a dimension in millimeters. When the amount of carbon formed is large, the merging or weaving into each other of the fibers of granulates

CH4(g) + 2H2O(g) ) CO2(g) + 4H2(g) +165 kJ/mol (4) In commercial practice, to reach a high conversion of methane and to depress the formation of filamentous carbon, which deactivates and crushes the catalyst, a much higher ratio of steam than the stoichiometry of eq 2 is used.34 The ratio of H2O/CH4 in SRM is often around 2.5-3.1. The temperature of the outlet of the reformer is normally around 1123 K to ensure total conversion of methane. Two water-gas shift processes are often employed in industry. A high-temperature one with ferromagnetic iron and chromium mixed oxides used as catalyst is operated at 623-723 K, and a lowtemperature one with Cu-Zn-Al mixed oxides used as catalyst carried out at 453-553 K. The high-temperature one is suitable for removing high concentration CO, and can reduce the CO to 2-4 vol %, while the lowtemperature one is used for a further reduction of the CO concentration to 0.1 to 1 vol %. The rest of the CO (34) Trimm, D. L. Catal. Rev.sSci. Eng. 1977, 16, 155-189.

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Figure 7. Nanofibers formed on catalyst sample 3 at 773 K in experiment 1. A: SEM micrograph; B: TEM micrograph.

Figure 8. Nanocarbon formed on catalyst sample 5 at 873 K in experiment 2. A: octopus-shaped; B: stick-shaped; C: branchshaped.

is removed in a methanation process, i.e., the reverse of reaction 2.1-3 The CO2 formed in the water-gas shift is removed with a pressurized washing process. However, there is limit for an economical removal of CO2. The ultimate purity of the hydrogen produced by MDR and SRM processes at a temperature around 1123 K is comparable. However, the MDR process saves over 40% of the energy input than that the SRM process for unit hydrogen volume production.35 It is likely that a major part of the energy input to the SRM route is used to heat the steam and to activate the water. Another important advantage for the MDR process over the SRM process is that no carbon oxides are produced. MDR as a process both for hydrogen and carbon materials production needs just one step. The advan(35) Chen, J. L. Study on the Production and Utilization of Carbon Nanofibers from Catalytic Decomposition of Methane (Thesis), Tianjin University, China, 1999.

tages over the other competitive routes are obvious. Steinburg and Cheng3 made a comprehensive technoeconomical analysis of existing hydrogen production technologies, and proposed with convincing data that thermal decomposition of methane, without assuming to employ a catalyst, and with assuming to burn the carbon as the energy source of the reaction heat of eq 1, is the most economical route for hydrogen production. Here, Figure 10 depicts the equilibrium conversion of methane and the equilibrium concentration of hydrogen in the product gas. Though for many hydrogenation and fuel cell use purposes, hydrogen with a substantial amount of methane is acceptable, e.g., Poirier and Sapundzhiev11 mentioned that hydrogen containing 4050% of methane is usable in some kinds of fuel cells in a cyclic mode; however, it is favorable for the reaction being carried out at a temperature between 1073 and 1123 K and for the production of hydrogen with near


Figure 9. Tubular nanocarbon formed on catalyst sample 6 at 1023 K in experiment 3.

Figure 10. Equilibrium conversion of methane and equilibrium concentration of hydrogen under atmospheric pressure at different temperatures. ]: Methane conversion; 4: hydrogen concentration.

100% concentration. The potential uses of the nanocarbons depend on their textures and structures; therefore, another goal of the process is to monitor the features of the carbon.15,18,19 5.2. The MDR Reaction in This Work. The three Ni/Al2O3 catalysts without copper doping show the maximum methane conversion at nearly a same temperature irrespective of the nickel-to-aluminum ratio, though the maximum values of the conversion increase with the increase of the nickel content in the catalyst. This result indicates that the nickel derived from a HC structured precursor has nearly the same energetic state and the same ability for activation of methane and for nanocarbon formation. In a piece of previous work,30 carbon formation reactions on these catalysts were carried out at the same and constant temperature in a thermobalance reactor, and it was found that the amount of carbon formed before the deactivation of the catalyst was almost proportional to the nickel content in the catalyst. The results in Figure 2 indicate also that after the temperature of the maximum conversion the activity of the catalyst goes down sharply as the increase of the temperature. Though the conversions of methane


in the temperature range of lower than the maximum conversion point in Figure 2 can be expected be increased further than that shown in the figure, with the effort in optimizing the configuration of the reactor, however, the room is not large because of the thermodynamic equilibrium limitation. It is likely that with these catalysts undoped with copper, only a low concentration of hydrogen can be obtained. Modification of the catalyst with doping copper increases the temperature of the maximum methane conversion and increases the maximum value of the conversion, as shown in Figure 3. This brings some light for getting a high concentration of hydrogen with a single pass. Several remarks can be made on the basis of the results of the reactions with constant temperature and different catalyst presented in Figures 4-6. The period of stable activity is different for different catalysts. The conversion of methane and the concentration of hydrogen are parallel and are stable during several tens of hours; however, these are strongly dependent on the catalyst and reaction temperature. Hydrogen with a commercially interesting concentration and a very large amount nanocarbon with respect to the amount of catalyst can be formed. After a time of stable activity for a specific catalyst, the conversion of methane goes down slowly, indicating that deactivation of the catalyst happened. However, it needs to be remarked that the configuration of the reactor in this work is far from optimized. From the appearance of the carbon and catalyst removed from the reactor, it can be seen that the nanocarbon wove into rather dense granulates with a bulk crushing strength higher than activated carbon. It is still doubtful whether the catalyst was deactivated or not, and the diminishing of the activity is very likely to be caused by the diffusion barrier for the reactant and the spatial limitation of the growth. Anyway, the result obtained in this work is already promising for practical use. The hydrogen obtained in experiment 3 is usable in many commercial hydrogenation processes and fuel cells. The morphology of the nanocarbon formed determines its usefulness for different purposes. For utilization as strengthening and modifying materials for polymer composites, tubular carbon with a concentric layered graphite structure is the most attractive one, however, the other morphologies may find utilization in other fields. The results in this work and in previous publications indicate that the morphology of nanocarbon is determined by many factors, such as the reaction conditions and the structure and composition of the metal catalyst.15,30-33 It has been reported that hydrogen in the feed has an influence on the morphology of nanocarbon formed on nickel.30,31 Therefore, to obtain pure morphology of nanocarbon, the reaction conditions should be kept uniform in the whole catalyst bed including temperature, gas composition, structure and composition of catalyst, etc. Moreover, due to the hydrogen formation, the catalyst in the rear of the reactor is located in the mixed atmosphere of methane and hydrogen. All these factors can influence the morphology of nanocarbon formed. The nanocarbons presented in Figures 7-9 have very different structure and may find different use.

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The idea of catalyst design in this work is to get stabilized nanoscale metal particles, acting as the growth tip of the nanocarbon. The precursor has a general formula M2+xM3+yCO3(OH)z‚4H2O and has a layered anionic clay structure. After calcination and reduction, rather uniformly nanometer-sized metal particles are obtained.33 The trivalent Al3+ does not act as a support, but as an irreducible component holding some oxygen and distorting the crystal lattice of the nickel and facilitating the formation of paracrystalline nano-metal particles.30,31,36 This catalyst shows superior activity and produces more controllable carbon structure than the silica-supported nickel catalyst employed by literature.9,37 More details of the properties of these catalysts have been and will be published elsewhere.30-33 5.3. Perspectives. MDR for the production of hydrogen has proven to be promising because of its low cost and low energy input compared to the conventional steam reforming technology. This process is extremely attractive to be used for the production of hydrogen for fuel cells. The nickel-based catalyst has been proven to be effective for the MDR, and to be sensitive to the doping modification by other metals. The structure and the morphology of the produced nanocarbon are also sensitive to the properties of the catalyst and the reaction conditions.10,25-28 There is much possibility to optimize the structure of the carbon through designing the nanometal particle.30-33,38 The catalytically grown nanocarbon has high mechanical strength, low electronic resistance, etc.15,18-20 The potential use of nanocarbon as a component of composites and adsorbents may make nanocarbon a product of bulk utilization.22,39-41 However, to realize the coupling of the two process goals, there is much work to be done. The authors think the reactor design and testing, the modification and understanding of nanometal catalysts, and the understanding (36) Twigg, M.; Richardson, J. T. Appl. Catal. A 2000, 190, 61-72. (37) Aiello, R.; Fiscus, J. E.; zur Loye, H. C.; Amiridis, M. D. Appl. Catal. A 2000, 192, 227-234. (38) Chen, J. L.; Li, Y. D.; Ma, Y. M.; Qin, Y. N.; Chang, L. Carbon, submitted. (39) Makoa, M. P.; Coville, N. J.; Sokolovskii, V. D. Catal. Today 1999, 49, 11-16. (40) Fenelonov, V. B.; Avdeeva, L. B.; Goncharova, O. V.; Okkel, L. G.; Simonov, P. A.; Derevyankin, A. Y.; Likholobov, V. A. Stud. Surf. Sci. Catal. 1995, 91, 825-831. (41) Hoogenraad, G. S. The Growth and Utilization of Carbon Fibrils(thesis), Universiteit Utrecht, The Netherlands, 1995.

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on the properties of nanocarbons are the most important shortOLINIT-term tasks. Incorporation of a membrane separator after the MDR reactor may provide more room for the optimization of the carbon produced. Conclusions A concept of simultaneous production of hydrogen and nanocarbon from the MDR reaction has been elaborated on the basis of process analysis and preliminary experimental results. It is possible to get a high concentration of hydrogen and a large amount of nanocarbon with a single pass reaction. For the hydrogen production, due to the thermodynamic equilibrium limitation, higher reaction temperature favors formation of high-concentration hydrogen, which is often expected by commercial processes. However, for a catalyst there exists a definite suitable reaction temperature range based on its chemical nature, which does not always fit the purpose of hydrogen production. The structure and morphology of the nanocarbon depend on the catalyst and reaction conditions, and have many possibilities. If the goal is to couple the two production purposes by a single pass, work needs to be done to modify the catalyst and to optimize the reactor design. Fortunately the nanosized nickel catalyst is very sensitive to doping and to structural modifications, this provides a room for the coupling of the process purposes. In this work, with doping of the catalyst with copper, a stable production of hydrogen with a over 80 vol % purity for over 10 h, and at the same time 180 g of nanocarbon production on 1 g of catalyst with a tubular morphology, are achieved. The hydrogen is usable in many hydrogenation processes and some fuel cells. The structure of the carbon is also attractive. To implement the process, catalysts with high activity at around 1073 K and high structural selectivity for the nanocarbon are called for, and the exploration of the commercial use of nanocarbon is needed. Acknowledgment. The financial support for this work from a national 973 project under Contract No. G199902240 is gratefully acknowledged. EF0000781

Simultaneous Production of Hydrogen and ... - ACS Publications

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