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Production of Hydrogen and Carbon Nanostructures by Non-oxidative Catalytic Dehydrogenation of Ethane and Propane Naresh Shah,* Yuguo Wang, Devadas Panjala,† and Gerald P. Huffman University of Kentucky, 533 South Limestone Street, Lexington, Kentucky 40508-4005 Received December 10, 2003. Revised Manuscript Received February 19, 2004

Nanoscale, binary, M-Fe (M ) Mo, Ni, or Pd) catalysts supported on alumina have been shown to be very effective for the decomposition of undiluted methane to yield hydrogen and multiwalled nanotubes. The same suite of catalysts has been tested for non-oxidative catalytic dehydrogenation of undiluted ethane and propane. After prereduction at 700 °C, all three binary catalysts exhibited significantly lower decomposition temperatures and longer time-on-stream performances than either the nonmetallic alumina support or a mono-metallic Fe/alumina catalyst. Ethene, propene, water, and carbon oxides were absent, and only hydrogen, methane, and unreacted ethane or propane were observed in the product stream. The carbon deposit left on the catalyst bed showed two distinct forms, depending on the reaction temperature. Above 600 °C, the carbon deposits were in the form of multiwalled nanotubes with some irregularly spaced capping of the innermost walls. At or below 500 °C reactor temperature, carbon nanofibers with capped and truncated stacked cone structure were produced.

Introduction Historically, hydrogen was manufactured primarily for use in the production of ammonia and methanol. Over the years, however, the technological base for hydrogen utilization has expanded tremendously to incorporate applications in chemical and petroleum refining, metallurgy, hydrogenation of edible fats and oils, fuel cells, and manufacturing of high-quality electronic components. The demand for high purity hydrogen is expected to increase as more stringent environmental legislation requiring deep desulfurization of petroleum-based fuels in refineries is enforced and its use in fuel cells in automotive and power generation applications is increased. Traditionally, hydrogen has been produced by reforming or partial oxidation of hydrocarbons to produce synthesis gas, followed by the water-gas shift reaction to convert CO to CO2 and produce more hydrogen, followed by separation procedures. However, further purification steps are required to reduce CO to ppm levels tolerable by the catalysts used in fuel cells. Noncatalytic steam cracking of ethane has been commercially used for production of ethylene1 at reaction conditions of high (>800 °C) temperature and short contact times. Catalytic oxidative dehydrogenation processes2-7 are preferred due to lower temperatures, * Corresponding author. E-mail: [email protected]. † Currently at ConocoPhillips, Inc., Ponca City, OK 74601. (1) For example, U.S. Patents 5,763,725 and 5,990,370. (2) For example, U.S. Patents 5,759,946, 6,355,854, 6,471,422, and 6,436,871; World Patent 9633149A1. (3) Mulla, S. A. R.; Buyevskaya, O. V.; Baerns, M. Appl. Catal. A 2002, 226, 73-78. (4) Banares, M. A. Catal. Today 1999, 51, 319-348. (5) Venkataraman, K.; Redenius, J. M.; Schmidt, L. D. Chem. Eng. Science 2002, 51, 2335-2343.

better process control, and higher ethylene yields. However, even in the presence of an oxidizing atmosphere, unintended carbon buildup and subsequent coking and fouling of the reactor and catalysts occurs.8 Consequently, special coatings to reduce this carbon buildup are required.9 Because of the oxidative atmosphere, a major portion of the extracted hydrogen is invariably lost in the process as water. Literature dealing exclusively with non-oxidative dehydrogenation of ethane is very sparse. Yang et al.10 have investigated ethylene yield and selectivity using a Cr2O3 catalyst supported on various supports with a 50% ethane-50% argon feed at various space velocities and temperatures in a fluidized bed apparatus. BASF is currently building the largest propane dehydrogenation plant capable of producing 350 000 Tons of propene/year in Tarragona, Spain. This plant will use UOP’s Oleflex process11 for propane dehydrogenation. Su¨d Chemie Inc.’s CATOFIN12 process uses fixed-bed chromia alumina catalyst to produce polymer-grade propylene from propane-rich stream. Grasselli et al.13 have reviewed several dehydrogenation processes to conclude that selective combustion of hydrogen produced is crucial for efficient production of olefins. Non-oxidative, catalytic decomposition of hydrocarbons is an alternative, one-step process to produce pure (6) Xu, Y.; Corberan, V. C. Progress in Natural Science 2000, 10 (1), 22-26. (7) Solymosi, F.; Nameth, R. Catal. Lett. 1999, 62, 197-200. (8) Rostrup-Nielsen, J. R. Catal. Today 1993, 18, 305-324. (9) For example, U.S. Patents 5,658,452 and 5,723,707. (10) Yang, H.; Xu, L.; Ji, D.; Wang, Q.; Lin, L. Reaction Kinetics and Catal. Lett. 2002 76 (1), 151-159. (11) http://www.uop.com/techsheets/oleflex.pdf. (12) See web site http://www.sud-chemieinc.com/houdry_propane.shtml.

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Table 1. Free Energies of Some of the Typical Ethane Reactions Used for Hydrogen Production free energy, ∆G kcal/mol 1 2 3 4 5 6 7

reaction

at 25 °C

at 500 °C

C2H6 f 2C + 3H2 C + O2 f CO2 C2H6 + 2H2O f 2CO + 5H2 CO + H2O f CO2 + H2 C2H6 + 4H2O f 2CO2 + 7H2 C2H6 + 2O2 f 2CO2 + 3H2 C2H6 + O2 f 2CO + 3H2

+7.85 -94.25 +51.56 -6.84 +37.89 -180.65 -57.71

-14.61 -94.54 -2.74 -2.52 -7.78 -203.70 -100.70

hydrogen. Nanoscale, binary, Fe-based catalysts supported on high surface area alumina [0.5%M-4.5%Fe)/ Al2O3, M ) Mo, Ni, or Pd] have been shown to have high activity for the catalytic decomposition of undiluted methane into pure hydrogen and multiwalled carbon nanotubes.14 One of the problems with non-oxidative dehydrogenation is coking of the catalyst and reactor due to carbon buildup. Under proper reaction conditions, however, these binary catalysts promote the growth of carbon nanotubes that transport carbon away from the catalyst surfaces, thereby preventing catalyst deactivation by coking as well as producing a potentially valuable byproduct. Ethane and propane are constituents of natural gas as well as byproducts of steam and catalytic cracking and fractional distillation of petroleum. They can also be present in significant amounts in Fischer-Tropsch reaction products. Currently, ethane is either oxidatively dehydrogenated to ethylene or burnt for its heating value. There is a well-established network for propane distribution for heating applications in this country. Propane is also used as an alternative fuel for automobile applications. The primary use of propane in commercial, nonrefinery settings is for heating applications. Refineries use propane dehydrogenation to produce ethylene or propylene monomers. Decomposition of alkanes to produce hydrogen requires external heat energy. Table 1 compares free energies of several reactions used for hydrogen production from ethane. Reaction 1 is the typical endothermic decomposition reaction used in this study. Carbon, one of the decomposition products carries its own potential energy (reaction 2) which can be recovered by either combustion or by using a direct carbon fuel cell. Compared to this scheme of hydrogen production, the commercially used steam reforming route, with or without the associated water-gas shift reaction (reactions 3, 4, and 5), is highly endothermic and requires additional heat input. Additionally, steam reforming consumes all the potential energy available in carbon to produce CO2. Preferential (and partial) oxidation of ethane (reaction 6 or 7) would be the ideal method for producing hydrogen. Unfortunately, not only has this reaction been impossible to accomplish without the associated oxidation of hydrogen, it is also highly uncontrollable and explosive in nature. In this work, the activities and rates of deactivation of the same suite of binary catalysts used in our earlier work on non-oxidative methane decomposition14 are (13) Grasselli, R. K.; Stern, D. L.; Tsikoyiannis, J. G. Appl. Catal. A: General 1999, 189 (1), 1-8 and 9-14. (14) Shah, N.; Panjala, D.; Huffman, G. P. Energy Fuels 2001, 15 (6), 1528-34.

investigated for the catalytic cracking of undiluted ethane and propane. Additionally, the carbon nanostructures produced in the reactions at several temperatures are investigated by high-resolution transmission electron microscopy (HRTEM). Experimental Section Supported catalysts were prepared by two different methods: (1) coprecipitation and (2) incipient wetness. Coprecipitation catalysts were prepared by first adding an aqueous solution of appropriate catalyst metal salts (Fe(NO3)3‚9H2O, (NH4)6Mo7O24‚4H2O, Pd(NO3)2‚xH2O, and Ni(NO3)2‚6H2O) in the desired proportions to a slurry of γ-alumina (150 m2/g) and then precipitating the metal oxyhydroxide on alumina by raising the pH of the slurry with ammonia. The slurry was washed with distilled water, dewatered to form a paste, and extruded. Incipient wetness catalysts were prepared by adding an aqueous solution of appropriate metal salts to dry alumina powder. Enough additional water was added to form paste which could be easily extruded. Extruded pellets were vacuumdried and calcined (500 °C, 2-4 h) to decompose metal nitrates to metal oxides. Performance of catalysts produced by the incipient wetness technique was indistinguishable from the performance of catalysts produced by the precipitation method. The composition of the binary metal catalysts, M-Fe (M ) Mo, Pd, or Ni) was 0.5 wt % M and 4.5 wt % Fe with respect to the alumina support. As described in detail in our work on methane decomposition,14 the reactions were carried out in a fixed-bed, plug-flow quartz reactor. Approximately one gram of catalyst bed was centered in a vertical quartz reactor with quartz wool with an up-flow of the reactant/product stream. Prior to reaction, the catalysts were reduced in flowing hydrogen (50 mL/min) for 2 h at 700 °C. After reduction, the reactor was flushed with an inert gas until the GC showed no residual hydrogen peak (∼15 min). To quickly screen the effectiveness of various catalysts, an undiluted alkane stream was reacted with the prereduced catalysts and the yields of the different products were measured. The furnace temperature was ramped in 25 °C increments every 15 min, and no alkane conversion was observed below a reactor temperature of 300 °C. Such experiments were very useful in quick screening of catalysts and in identifying process variables for further detailed time-on-stream studies.

Results and Discussion Noncatalytic Dehydrogenation. The product distributions from thermal cracking of ethane and propane are shown in Figure 1. At temperatures above 500 °C, C-H bonds of alkanes start breaking to produce hydrogen and the respective alkenes. Above 600 °C, C-C bonds also start breaking to produce highly active methyl radicals. These radicals either react with hydrogen to produce methane or undergo further dissociation to produce C and hydrogen. Thus we see a rise in both methane and hydrogen concentrations. At temperatures over 650 °C, alkenes further dehydrogenate to produce C and hydrogen. It is worth noting that in our experiments we did not observe any dehydrogenation of alkenes to alkynes (or propadiene), probably because the temperature of the system was sufficiently high to instantaneously further dehydrogenate the products to carbon and hydrogen. At temperatures over 800 °C, methane starts thermally cracking to produce more carbon and hydrogen as we had observed previously.14 Catalytic Dehydrogenation. Figure 2 shows the product distribution of catalytic (0.5%Pd-4.5%Fe/Al2O3)

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Figure 1. Product distribution from thermal cracking of ethane (left) and propane (right).

Figure 2. Product distribution from 0.5%Pd-4.5%Fe/Al2O3 catalytic cracking of ethane (left) and propane (right).

ethane and propane decompositions as a function of reactor temperature. This catalyst promotes C-C bond breaking in alkanes. Consequently, hydrogen as well as methane production initiates at much lower temperatures compared to thermal decomposition. At 500 °C, almost complete conversion of the alkanes to methane and hydrogen is observed. As we have shown previously,14 this catalyst is also highly active for methane dehydrogenation. Therefore, at temperatures above 500 °C, methane produced from ethane cracking starts cracking to produce carbon and hydrogen. Similar trends of high methane formation at relatively low temperatures (450-500 °C) followed by a high hydrogen production plateau at ∼650-825 °C were also observed for the other bimetallic catalysts (0.5%Ni-4.5%Fe or 0.5%Mo-4.5%Fe)/Al2O3. Very little alkene production was observed in catalytic reactions, whereas the thermal cracking produced significant amounts of alkenes at temperatures below the methane production peak. On the basis of the results of the ethane dehydrogenation experiments, the propane experiments were terminated when complete propane conversion was observed. Figure 3 compares the observed hydrogen concentrations as a function of temperature for the different catalysts used, each subjected to a reduction pretreatment at 700 °C. It is evident that the catalysts lower the temperature of alkane cracking significantly compared to thermal cracking over the alumina support.

The three bimetallic catalysts show similar trends and are all significantly more active for hydrogen production than the monometallic iron catalyst. Time-on-Stream Behavior at Lower Reaction Temperature. Time-on-stream (TOS) studies of all prereduced (H2, 700 °C, 2 h) catalysts were conducted at temperatures of 500 °C (for ethane) and 475 °C (for propane). Hydrogen and ethane concentrations (vol %) in the reactor product stream are shown as a function of time for a reactor temperature of 500 °C in Figure 4. At this reactor temperature, unreacted ethane, methane, and hydrogen comprised over 98 vol % of the product stream. As shown in Figures 1 and 3, there is no conversion of ethane at this temperature in the noncatalytic (alumina support only) reaction. However, even at this relatively low temperature, the metallic alloy phases are effective catalysts in decomposing ethane to produce substantial quantities of hydrogen and methane, even though the dehydrogenation reaction is not complete and the reactor exit stream still contains substantial amounts of unconverted ethane. As shown in Figure 4, the monometallic 5%Fe/Al2O3 shows rather weak activity in decomposing ethane at 500 °C. Furthermore, it rapidly loses its activity after a very brief exposure to the undiluted ethane stream. Previously, we have shown that the prereduction treatment at 700 °C is not very effective in reducing 5%Fe/ Al2O3 catalyst, and reduction at 1000 °C is required to

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Figure 3. Hydrogen production from catalytic and thermal (noncatalytic) cracking of ethane (left) and propane (right) as a function of temperature.

Figure 4. Hydrogen and unreacted ethane concentration (vol %) in the reactor exit stream for ethane cracking by various catalysts (prereduced at 700 °C) at 500 °C reactor temperature.

generate substantial quantities of Fe metal species needed for methane decomposition.14,15 On the other hand, all three binary catalysts are more easily reduced at 700 °C and yield catalytically active metallic alloy phases. XAFS and Mo¨ssbauer spectroscopy indicate that the active phase is an austenitic Fe-M (C) alloy.15 Both 0.5%Ni-4.5%Fe/Al2O3 and 0.5%Mo-4.5%Fe/ Al2O3 catalysts show a significant decrease in hydrogen production with increasing time-on-stream (TOS) due to deactivation. However, in the early stages of TOS, 0.5%Ni-4.5%Fe/Al2O3 is more effective in total ethane conversion to both methane and hydrogen. The 0.5%Mo4.5%Fe/Al2O3 catalyst, on the other hand, converts ethane directly to hydrogen with relatively low (less than 10 vol %) conversion to methane throughout the TOS study. The 0.5%Pd-4.5%Fe/Al2O3 catalyst showed the best stability in ethane conversion. Over most of the TOS experiment at 500 °C, it exhibited over 90% ethane conversion to over 60% methane and more than 30% hydrogen. It is worth noting, however, that if all of ethane is converted to methane and hydrogen, one would expect approximately equal amounts of hydrogen and methane in the product stream according to the reaction:

C2H6 f CH4 + H2 + C

However, for the 0.5%Pd-4.5%Fe/Al2O3 catalyst, the methane/hydrogen volume ratio is over 3 at 500 °C (Figures 2 and 4). Since no ethylene or higher hydrocarbons are detected in the product stream, it appears that the carbon deposited on the catalyst contains some hydrogen, either in the form of very large hydrocarbons (existing in solid phase at 500 °C) or as dangling aliphatic groups on the edges of graphene sheets. The latter concept was originally suggested by Franklin et al. in the early 1950s.16 This phenomenon can also explain the gradual increase in hydrogen production during the TOS experiment (Figure 4). Similar to Figure 4, Figure 5 shows hydrogen and propane concentrations in the reactor exit stream as a function of time-on-stream exposure to propane feed at a 475 °C reactor temperature. There was no propane conversion by 5%Fe/Al2O3 or thermal, noncatalytic alumina support, and so the corresponding time-onstream trends are not plotted in Figure 5. The 0.5%Mo4.5%Fe/Al2O3 catalyst is very weakly active for hydrogen production and was the only catalyst to show almost no methane production at this temperature. Activity trends of propane dehydrogenation by 0.5%Ni-4.5%Fe/ (15) Shah, N.; Pattanaik, S.; Huggins, F. E.; Panjala, D.; Huffman, G. P. Fuel Process. Technol. 2003, 83 (1-3),163. (16) Franklin, R. E. Acta Crystallogr. 1950, 3, 107 and 1951, 4, 253.



Figure 5. Hydrogen and unreacted propane concentration (vol %) in the reactor exit stream for propane cracking by various catalysts (prereduced at 700 °C) at 475 °C reactor temperature.

Figure 6. Hydrogen and unreacted ethane concentration (vol %) in the reactor exit stream for ethane cracking by various catalysts (prereduced at 700 °C) at 650 °C reactor temperature.

Al2O3 and 0.5%Pd-4.5%Fe/Al2O3 catalysts were similar to those observed for ethane dehydrogenation. The 0.5%Ni-4.5%Fe/Al2O3 catalyst initially showed very high propane conversion to methane and to a lesser extent to hydrogen. After an incubation period of about an hour, the hydrogen production increased due to conversion of methane but the total propane conversion started declining. The 0.5%Pd-4.5%Fe/Al2O3 catalyst showed high hydrogen production immediately, which further increased with time-on-stream with conversion of methane to hydrogen, even when the overall propane conversion was decreasing. Time-on-Stream Behavior at Higher Reaction Temperature. Catalytic decomposition of ethane is much higher at 650 °C than at 500 °C. This is not surprising since it exhibits significant decomposition even in the absence of a catalyst at 650 °C (Figures 1 and 6). The activities of the bimetallic catalysts at 650 °C showed only modest decay in TOS experiments. In contrast, the monometallic 5%Fe/Al2O3 catalyst has high activity for hydrogen production initially, but deactivates within a few hours to the level of thermal decomposition alone. Also, a gradual increase of the amount of ethylene in the exit stream to the level seen in thermal decomposition (∼15 vol %) was observed.

Both the nonmetallic Al2O3 support (Thermal) and mono-metallic 5%Fe/Al2O3 catalysts showed poor overall conversion to hydrogen as well as methane. The three binary metal catalysts showed almost identical performance with nearly complete ethane conversion at 650 °C reactor temperature. However, there were noteworthy changes in methane production in the TOS experiments. Since there was nearly complete ethane conversion and insignificant ethylene production, these changes in methane production are inversely reflected in hydrogen production. Among the three, the 0.5%Ni-4.5%Fe/ Al2O3 catalyst showed the highest methane production averaging about 35 vol % throughout the TOS experiment. The reaction at 650 °C using the 0.5%Pd-4.5%Fe/ Al2O3catalyst showed steady hydrogen and methane production (∼25 vol % methane and ∼75 vol % hydrogen) during the 5-h TOS experiment. The 0.5%Mo4.5%Fe/Al2O3 catalyst exhibited an increase in methane production from 15 to 30 vol % over 3 h, followed by decrease to ∼8 vol % in less than 1 h. Subsequently, methane production was steady but there was gradual decrease in ethane conversion and hydrogen production. It is worth noting that in our previous experiments on undiluted methane all three catalysts showed reasonable activity for undiluted methane decomposition at

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Figure 7. Hydrogen and unreacted propane concentration (vol %) in the reactor exit stream for propane cracking by various catalysts (prereduced at 700 °C) at 625 °C reactor temperature.

700 °C.14 Consequently, at a little higher temperature, enhanced conversion of methane to hydrogen should increase the total hydrogen yield from an ethane feed stream. The amount of carbon nanotubes produced by propane decomposition at higher (625 °C) temperature was so high that the reactor started developing back-pressure in about half an hour using our normal plug flow reactor configuration. The data shown in Figure 7 were obtained by using about 0.3 g of catalysts distributed in five catalyst beds (about 60 mg/bed) spaced about one inch apart from each other. Even with these low catalyst loadings, the spent catalyst beds filled up all available reactor space with spongy mass that contained about 90% carbon by weight. All three binary catalysts showed very good hydrogen production initially. However, the hydrogen production decreased fairly rapidly to the level produced by thermal cracking of propane (∼10 volume percent) after approximately 1 h for the 0.5%Pd4.5%Fe/Al2O3 catalyst, 3 h for the 0.5%Ni-4.5%Fe/Al2O3 catalyst and 4 h for the 0.5%Mo-4.5%Fe/Al2O3 catalyst. It appears that the carbon supply from the thermal decomposition of the propane reactant has exceeded the capability of the catalysts to carry the carbon away from the active sites in the form of nanotubes. The oversupply of the carbon is then deposited as amorphous carbon. The fouling by the amorphous carbon deposition disrupts the supply of fresh propane to the active sites and the catalyst is rapidly deactivated. Transmission Electron Microscopy (TEM) Observations. As in our previous study of catalytic methane decomposition,14 most of the carbon produced by ethane and propane decomposition is in the form of carbon nanotubes and nanofibers. These nanostructures grow away from the surfaces of the binary catalyst particles, which remain anchored to the alumina support by the formation of hercynite (FeAl2O4)15 during the reduction pretreatment. To examine the nature of these carbon nanostructures, small amounts of the used catalyst bed material from the time-on-stream experiments were ultrasonicated in acetone. Single drops of the suspension were then placed on holey carbon substrates of TEM grids and examined by high-resolution TEM. Two distinct groups of carbon nanostructures were observed. Figure 8 is a schematic showing these two structures. Nanotubes formed at higher decomposi-

Figure 8. Schematic representation of concentric cylindrical multiwalled nanotubes (left) and stacked truncated and/or capped cones nanofiber (right) structures observed as reaction products during ethane decomposition.

tion temperature (Figures 9 and 10) were similar to those observed in our earlier study of methane decomposition and observed by other investigators investigating nanotube formation by typical chemical vapor deposition (CVD) methods.17,18 The walls of these nanotubes consist of concentric graphene cylinders of differing diameters. The difference in diameter of two adjacent cylinder walls is 6.79 Å, the same as the c-axis distance in graphite 2H (JCPDS PDF #42-1487, ICSD #31170) unit cell. In most of the multiwalled nanotubes, we also observed capping of one or more innermost cylindrical walls at irregular intervals. Consequently, the number of walls and the inner diameter of the MWNT are different on the either side of such caps along the nanotube axis. This capping of the innermost walls is different from the reported “bamboo” structure19,20 where the capping is at regular intervals, the (17) Ijima, S. Nature 1991, 354 (Nov 7), 56-58. (18) Ajayan, P. M. Chem. Rev. 1999, 99, 1787-1799. (19) Lee, C. J.; Park, J. Carbon 2001, 39, 1891-1896.



Figure 9. HRTEM image of carbon nanotubes deposited on a spent 0.5%Mo-4.5%Fe/Al2O3 catalyst (prereduced at 700 °C) exposed to undiluted ethane at 650 °C reactor temperature (left) and exposed to undiluted propane at 625 °C reactor temperature (right).

Figure 10. HRTEM image of carbon nanotubes deposited on a spent 0.5%Ni-4.5%Fe/Al2O3 catalyst (left) and 0.5%Pd-4.5%Fe/ Al2O3 catalyst (right), both prereduced at 700 °C and exposed to undiluted ethane at 650 °C reactor temperature.

walls are not parallel to the nanotube axis, and the inner diameter remains the same on either side of the caps. As discussed earlier, at higher decomposition temperatures both ethane and propane exhibit some carbon formation by thermal, noncatalytic decomposition. Carbon formed by this mechanism is amorphous in nature and can be seen in these micrographs (Figures 9 and 10) as deposits on the outer walls of the already-formed nanotubes. Nanotubes formed at lower decomposition temperature showed the stacked-cone structure shown schematically in Figure 8. Because such structures do not have hollow continuous cores, they are usually referred to as nanofibers and not nanotubes. Typical TEM micrographs of such stacked-cone nanofibers produced from ethane and propane are shown in Figures 11-13. Several investigators have observed such structures using different reaction conditions and catalysts.21-25 In general, the conical nanostructures exhibit lower parallelism between the graphene sheets because the (20) Zhang, X. X.; Li, Z. Q.; Wen, G. H.; Fung, K. K.; Chen, Y.; Li, J. Chem. Phys. Lett. 2001, 333, 509-514.

sheets themselves are not very flat. The strain induced due to the conical structure of graphene sheets is relieved by local nonuniformity in the hexagonally packed planar structure. This local disorder can cause ripples in the planar surface of the cone that cause lower parallelism between the graphene layers. One explanation for this higher degree of disorder in conical nanostructures is the lower reaction temperature. It is wellknown that higher-temperature, long duration treatment is required to produce highly graphitic structures from glassy and amorphous carbon preforms. Thermal vibrations during annealing at higher temperatures can move disordered carbon atoms (as well as vacancy defects) to (21) Rodriguez, N. M. J. Mater. Res. 1993, 8 (12), 3233-3250. (22) Geus, J. W.; van Dillen, A. J.; Hoogenraad, M. S. Mater. Res. Soc. Symp. Proc. 1995, 368, 87-98. (23) Terrones, H.; Hayashi, T.; Munoz-Navia, M.; Terrones, M.; Kim, Y. A.; Grobert, N.; Kamalakaran, R.; Dorantes-Davila, J.; Escudero, R.; Dresselhaus, M. S.; Endo, M. Chem. Phys. Lett. 2001, 343, 241250. (24) Merkulov, V. I.; Hensley, D. K.; Melecheko, A. V.; Guillorn, M. A.; Lowndes, D. H.; Simpson, M. L. J. Phys. Chem. B 2002, 106, 1057010577. (25) Endo, M.; Kim, Y. A.; Hayashi, T.; Fukai, Y.; Oshida, K.; Terrones, M.; Yanagisawa, T.; Higaki, S.; Dresselhaus, M. S. Appl. Phys. Lett. 2002, 80 (7) (Feb 18), 1267-1269.

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Figure 11. HRTEM image of carbon nanotubes deposited on spent 0.5%Mo-4.5%Fe/Al2O3 catalyst (prereduced at 700 °C) exposed to undiluted ethane at 500 °C reactor temperature (left) and exposed to undiluted proane at 475 °C reactor temperature (right).

Figure 12. HRTEM image of carbon nanotubes deposited on spent 0.5%Ni-4.5%Fe/Al2O3 catalyst (prereduced at 700 °C) exposed to undiluted ethane at 500 °C reactor temperature.

crystalline close-packed hexagonal arrangement with lower free energy. Conical, less-ordered nanostructures

are almost always observed at lower reaction temperatures, and cylindrical structures with higher parallelism between the walls are observed at higher reaction temperatures. Further rearrangement and alignment of the graphene layers is expected if these disordered conical nanostructures are post-treated at higher temperatures. Indeed, the carbon nanostructure produced by the (0.5%Ni-4.5%Fe)/Al2O3 catalyst (prereduced at 700 °C) exposed to undiluted ethane at a 650 °C reactor temperature appears to represent a transition between these two structures (Figure 10). One other noteworthy nanostructure was observed in the case of ethane decomposition at 500 °C reactor temperature by the (0.5%Pd-4.5%Fe)/Al2O3 catalyst (Figure 13). The “cones” in this particular nanofiber are completely flattened to form graphene sheets. Because there is little internal strain in such a structure, there is a high degree of parallelism between the graphene layers. The effectiveness of these binary catalysts for hydrogen production is not just because of their activity for dehydrogenation by breaking C-H bonds. It is also because these catalysts are very effective in stabilizing

Figure 13. HRTEM image of carbon nanotubes deposited on spent 0.5%Pd-4.5%Fe/Al2O3 catalyst (prereduced at 700 °C) exposed to undiluted ethane at 500 °C reactor temperature (left) and exposed to undiluted propane at 475 °C reactor temperature (right).


and removing one of the reaction productsscarbonsin the form of nanotubes and nanofibers from the active catalytic sites. Previously,14 an apparently thermodynamically contradictory behavior of increase in hydrogen production with lowering of the reaction temperature for an endothermic methane decomposition reaction was demonstrated. This apparently contradictory behavior was explained by noting that the form of carbon produced at lower temperature (nanotubes) was effectively transported away from the active catalytic site, while the form produced at higher temperature (graphitic coke), covered the catalytic site and markedly lowered hydrogen production. Conclusions After prereduction at 700 °C, all three aluminasupported binary catalysts [(0.5%M-4.5%Fe)/Al2O3, M ) Mo, Ni, or Pd] significantly lowered ethane and propane decomposition temperatures and exhibited longer time-on-stream performances than either the nonmetallic alumina support or a mono-metallic Fe/ alumina catalyst. This hydrogen stream does not contain any oxides of carbon and hence should not require expensive purification and separation steps for use in PEM fuel cell applications. The effectiveness of these catalysts is believed to be due to their activity for dehydrogenation by breaking C-H bonds and in forming carbon nanotubes that carry carbon away from the active metallic sites. Only thermal decomposition experiments or those using a mono-metallic Fe/Al2O3 catalyst showed any significant ethylene production.


Depending on the reactor temperature utilized, two distinct forms of nanostructures were observed. At higher reaction temperatures, multiwall nanotubes with a high degree of parallelism between the graphene wall layers were produced. This appears to be an efficient mechanism for transporting carbon away from the active catalytic sites, leading to high hydrogen production and retaining good activity for over 5 h time-onstream. At lower reaction temperatures, the catalysts displayed lower conversion and hydrogen production. The carbon produced was in the form of nanofibers consisting of stacked truncated cones, which exhibited higher disorder and lower parallelism between graphene sheets. This form appears to be less efficient in carrying carbon away from the active catalytic site than the multiwalled nanotubes produced at higher reaction temperature (>650 °C). At higher reactor temperature there is some deposition of amorphous carbon due to noncatalytic, thermal decomposition of alkanes. In the case of propane decomposition at 625 °C, this carbon supply apparently exceeds the catalytic capacity to produce nanotubes and the resulting fouling of the active sites deactivates the catalysts within a few hours. Acknowledgment. This research was supported by the U.S. Department of Energy through the Office of Fossil Energy (FE), National Energy Technology Laboratory (NETL), under Contract No. DE-FC26-02FT41594. EF034100C

Production of Hydrogen and Carbon Nanostructures by Non-oxidative

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