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Energy & Fuels 2001, 15, 1528-1534
Hydrogen Production by Catalytic Decomposition of Methane Naresh Shah,*,† Devadas Panjala,‡ and Gerald P. Huffman§ University of Kentucky, 533 South Limestone Street, Room 111, Lexington, Kentucky 40508-4005 Received July 31, 2001. Revised Manuscript Received October 1, 2001
Traditionally, hydrogen is produced by reforming or partial oxidation of methane to produce synthesis gas, followed by the water-gas shift reaction to convert CO to CO2 and produce more hydrogen, followed in turn by a purification or separation procedure. This paper presents results for the catalytic decomposition of undiluted methane into hydrogen and carbon using nanoscale, binary, Fe-M (M ) Pd, Mo, or Ni) catalysts supported on alumina. All of the supported Fe-M binary catalysts reduced methane decomposition temperature by 400-500 °C relative to noncatalytic thermal decomposition and exhibited significantly higher activity than Fe or any of the secondary metals (Pd, Mo, and Ni) supported on alumina alone. At reaction temperatures of approximately 700-800 °C and space velocities of 600 mL g-1 h-1, the product stream was comprised of over 80 volume % of hydrogen, with the balance being unconverted methane. No CO, CO2, or C2 and higher hydrocarbons were observed in the product gas. High-resolution SEM and TEM characterization indicated that almost all carbon produced in the temperature range of 700-800 °C is in the form of potentially useful multiwalled nanotubes. At higher temperatures (>900 °C), hydrogen production decreases and carbon is deposited on the catalyst in the form of amorphous carbon, carbon flakes, and carbon fibers. In the noncatalytic thermal decomposition mode, at temperatures above 900 °C, graphitic carbon film is deposited everywhere in the reactor. Thus, the morphology of the carbon produced may be the controlling parameter in catalytic decomposition of methane. The efficient removal of the carbon from the catalyst surface in the form of nanotubes may be the key factor influencing catalyst performance.
Introduction Currently, refineries are processing heavier and more sour crude oils to increase the degree of conversion. At the same time, environmental regulations are forcing refineries to reduce sulfur to ever lower levels and to produce more reformulated gasoline and deeper hydrotreated diesel. Therefore, refinery demand for hydrogen is increasing significantly. Hence, production of pure hydrogen from hydrocarbons, particularly methane, the major component of natural gas, has great practical importance. Traditionally, dry (with CO2) reforming, wet (with H2O) reforming, and partial oxidation of methane are employed to produce synthesis gas. Converting CO in synthesis gas using the water-gas shift reaction then produces a relatively pure hydrogen stream. However, this hydrogen still contains enough CO to poison the catalysts used in PEM electrochemical fuel cells. A reverse methanation reaction has to be carried out to reduce the CO concentration to sub-ppm levels. Non-oxidative catalytic decomposition of methane is an alternate source for producing a pure hydrogen stream from natural gas. However, solid carbon deposits from gas-phase methane can cause severe fouling of the reactor, catalyst, and gas handling systems. Conse* Corresponding author. † Fax: (859) 257-7215. E-mail:
[email protected]. ‡ E-mail:
[email protected]. § E-mail:
[email protected].
quently, non-oxidative catalytic decomposition of methane has neither been utilized commercially nor studied extensively. Steinberg1,2 has proposed a noncatalytic fossil fuel decarbonization process at temperatures above 800 °C to produce particulate carbon as a means of producing H2 for use as an energy source and thereby reducing the greenhouse gas, CO2. Muradov3,4 has also attempted catalytic pyrolysis of methane to produce CO2-free hydrogen using alumina-supported Fe2O3 and NiO (10 wt %) at 850 °C. He proposes reduction of 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 volume % hydrogen. Bromberg et al.5-7 have proposed thermal reforming of natural gas and diesel with and without catalysts using plasma as a heat source to produce hydrogen in a fuel converter for internal combustion engines. Fouling due to carbon (1) Steinberg, M. Int. J. Hydrogen Energy 1999, 24, 771-777. (2) Steinberg, M.; Grohse, E. W. U.S. Patent 5,427,762, June 27, 1995. (3) Muradov, N. Z. Energy Fuels 1998, 12, 41-48. (4) Muradov, N. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2001, 46 (1), 29-30. (5) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; Alexeev, N. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2001, 46 (1), 1-4. (6) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; Alexeev, N. Int. J. Hydrogen Energy 1999, 24, 1131-1137. (7) Rabinovich, A.; Cohn, D. R.; Bromberg, L. U.S. Patent 5,437,250, August 1, 1995.
10.1021/ef0101964 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/03/2001
H2 Production by Catalytic Decomposition of CH4
deposition was avoided by passing air with the fuel, which produced CO. Dahl et al.8 have carried out thermal dissociation of methane to hydrogen and carbon black using solar energy at temperatures of 400 °C to 1000 °C. Kurosaka et al.9 have reported very small (50 wt %) Ni catalysts prepared from hydrotalcite-like (HC) structure by coprecipitating mixed aqueous solution of nickel nitrate with Na2CO3 for production of hydrogen and nano-carbon from decomposition of methane. They found that the maximum methane conversion occurs at 650 °C irrespective of Ni loading, though the maximum value of conversion increases with the increase in Ni content. Addition of Cu (up to 25 wt %) to the catalyst increased both the temperature of maximum conversion and the maximum conversion percentage. They also reported that the morphology of nano-carbon is influenced by hydrogen in the feed itself. Gao et al.14 have used the hydrogen storage alloy LaNi5 as a catalyst to grow carbon nanotubes by catalytic decomposition of methane at 670 °C. Choudhary et al.15 have also used 10 wt % Ni on SiO2, H-ZSM-5, and HY supports for hydrogen production by catalytic decomposition of diluted (20 volume %) methane in a stainless steel reactor. They observed maximum CH4 conversions of 45% at 550 °C and observed prolonged release of substantial concentrations of CO (2500 ppm going down to 750 ppm) and CO2 (700 °C was required to oxidize this tenacious film. The form of carbon produced by catalytic decomposition of methane at lower (∼700-800 °C) temperatures is quite different. As illustrated by the electron micrographs (Figures 9-13), most of this carbon is in the form of potentially valuable multiwalled nanotubes. Figure 9 is an SEM image showing a profusion of carbon nanotubes produced during catalytic decomposition of methane using the binary Mo-Fe catalyst. A substantial amount (>90%) of the catalytically produced carbon deposits (at ∼700-800 °C) were in the form of multiwalled carbon nanotubes. Carbon fibers were observed in small amounts (
