Microsoft Word - Vol 5-1-pages 13-24.DOC IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 13 THE EFFECT OF CATALYST SUPPORT ON THE DECOMPOSITION OF METHANE TO HYDROGEN AND CARBON 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 *e-mail: chrahman@eng.usm.my Abstract: Decomposition of methane into carbon and hydrogen over Cu/Ni supported catalysts was investigated. The catalytic activities and the lifetimes of the catalysts were studied. Cu/Ni supported on TiO2 showed high activity and long lifetime for the reaction. Transmission electron microscopy (TEM) studies revealed the relationship between the catalyst activity and the formation of the filamentous carbon over the catalyst after methane decomposition. While different types of filamentous carbon formed on the various Cu/Ni supported catalysts, an attractive carbon nanotubes was observed in the Cu/Ni supported on TiO2. Key Words: Methane decomposition, carbon nanotube, Cu/Ni supported catalysts. 1. INTRODUCTION All conventional options of hydrogen production from natural gas, mainly methane e.g., steam reforming, partial oxidation, and autothermal reforming, involve CO2 production at some point in the technological chain of the process. Another approach is to decompose methane into hydrogen and carbon. It is a technologically simple one-step process without energy and material intensive gas separation stages and shows the potential to be a CO2-free hydrogen production process [1]. Although the decomposition of methane over metal-oxide transition metal catalysts produces high initial hydrogen concentration, their activity rapidly drops because of the surface deposition of carbon. However, the generally accepted mechanism of the growth of carbon nanotube proved that this kind of carbon deposition occurs without encapsulating of the metal surfaces, hence, maintaining the catalyst activity for a period of a time. Thus, efficient catalyst for methane decomposition could be developed when the condition of filamentous carbon growth are provided. Cu/Ni is known as one of the active catalysts in methane decomposition [2-5]. When active metal species are deposited on different supports, it is generally accepted that the catalytic performance of metal species depends on the types of supports. This can be attributed to the change of the structure or electronic state of the metal species due to the IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 14 interaction with the supports [6]. It is also reported that methane decomposition depend on type of support and this might be the case with which carbon migration occurred from metal support [7]. Therefore, it is worth examining the best support and its combination of the active component in order to design a catalyst having a high catalytic performance for the decomposition of methane. The objective of this study is to identify the effect of Cu/Ni supported on MgO, Al2O3, TiO2, and SiO2 towards methane decomposition to hydrogen and carbon. 2. EXPERIMENTAL The catalysts used in this study were 15 mol%CuO/20mol%NiO catalyst supported on MgO, Al2O3, TiO2, and SiO2. The catalysts were prepared using impregnation method. The experiments were carried out at atmospheric pressure in a stainless steel fixed bed reactor system. A schematic diagram of the reactor system is shown in Fig. 1. The reactor was fabricated from a stainless steel tube (O.D. 12.7 mm, I.D. 10.92 mm and 600 mm length). A thermocouple of type K in an inconel tube, with 3 mm diameter and 600 mm long was used to measure the temperature of the catalyst bed in the reactor. The catalyst layer was situated in the centre of the reactor. The free space before and after the catalyst layer was filled with quartz particles (RDH) in order to minimize the reactor dead volume. Furnace used was a single zone (model Carbolite VST 11) with temperature controller and was supplied by Carbolite, U.K. A pressure gauge (Ashcroft, USA) located just above the reactor was used to read the inlet pressure. Methane (supplied by Malaysian Oxygen Sdn. Bhd.) with 99.999% purity and argon (supplied by Sitt Tatt Industrial Gasses Sdn Bhd.) with 99.999% purity were mixed before entering the reactor. Argon was used as a diluent gas as nitrogen might react with the hydrogen at high temperatures. Flow of methane was regulated using a mass flow controller (MKS) and argon flow was regulated by Brooks mass flow controller (model 5850E). Outlet gas flow was monitored by a gas flow meter (Alexander Wright DM3 B). The product gases were analyzed using an on-line Gas Chromatograph (GC) (Hewlett- Packard Series 6890, USA). The GC was controlled on-line using HP ChemStation Rev. A. 06.01. [403] software. Porapaq N and Molecular Sieve 5Å (1/8” diameter, 6 feet long length) stainless steel columns, situated in a series with the Porapaq N column located in front were used. The Porapaq N column was used to separate carbon dioxide, ethane, ethylene and propylene and the Molecular Sieve 5 Å column for hydrogen, oxygen, carbon monoxide, nitrogen and methane. Since higher hydrocarbons and carbon dioxide can ruin the Molecular Sieve 5 Å column, two valves operated at 333 K were used to control the outlet gas from the Porapaq N column to the detectors while avoiding passing through the Molecular Sieve 5 Å. Valve 1 functioned as sampling mechanism and valve 2 to control the flow of the product through the Molecular Sieve 5 Å column. When one of the valve is turned off, it indicates that the gas is allowed to flow through the Molecular Sieve 5 Å column. The gas chromatograph injector temperature was set at 313 K. The initial and the final temperature of the oven were set at 313 and 473 K, respectively. A heating rate of 5 K/min was used. The detector temperature was kept at 473 K. Pure argon gas (99.999%) was used as a carrier gas. The total analysis time was 25 minutes for each injection. Standard gas was injected into the gas chromatograph and the area of each of the component in the standard gas was determined. The standard gas mixture was supplied by IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 15 BOC Gases, UK. Chromatogram of hydrocarbons such as methane, ethylene, ethane, and propylene were obtained using a flame ionization detector (FID) whereas hydrogen, oxygen, carbon monoxide and carbon dioxide were detected using thermal conductivity detector (TCD). Pore size and surface area measurements 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 hours prior to the measurements. Computer programs (Micropore version 2.46) allowed for rapid numerical results for the surface area and pore texture from adsorption-desorption isotherm. X-ray diffraction refined by Reitfield method was used to characterize the catalyst structure. Room- temperature XRD was conducted on a Siemen D-5000 diffractometer, using CuK radiation, and a graphite secondary beam monochromator. Specimen was prepared by packing sample powder in a glass holder. Intensity was measured by step scanning in the 2 range between 10 – 90, with a step of 0.02 and a measuring time of 2 second per point. The diffraction lines of the XRD pattern were used to identify the formation of solid solution by comparing the 2 values of the materials with those of phase from the powder diffraction files. Spent catalysts, covered with carbon were analyzed using a transmission electron microscope (Philips TEM CM12). In preparation for TEM experiments, a few samples of the spent catalyst were dispersed in distilled water, and then a drop was deposited on a coated copper grid. The conversion of methane and the yield of hydrogen are defined as follows: 100 inputmethaneofMole reactedmethaneofMole (%)Conversion  (1) 010 2inputmethaneofMole producedhydrogenofMole (%)Yield *    (2) 24 2HCCH   3. RESULTS AND DISCUSSION Thermal decomposition of methane converts methane to hydrogen and solid carbon at high temperatures. It is believed that methyl radicals polymerize to form cyclic and aromatic precursors to graphitic soot particles. However, the form of carbon produced by catalytic decomposition of methane depends on the catalyst used and the reaction parameters. Table 1 shows the physical properties of the fresh Cu/Ni supported on TiO2, SiO2, MgO, and Al2O3, respectively. The surface area of the Cu/Ni on Al2O3, Cu/Ni on MgO, Cu/Ni on SiO2, and Cu/Ni on TiO2 was 14.27, 11.27, 5.28, and 4.79 m2/g, respectively. The total pore volume shows the same trend as in the surface area and with the value of 0.009, 0.008, 0.004, and 0.003 cc/g for the Cu/Ni on Al2O3, Cu/Ni on MgO, Cu/Ni on SiO2, and Cu/Ni on TiO2, respectively. The Cu/Ni on MgO had the largest pore diameter IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 16 (~27 Å), while Cu/Ni on Al2O3 and Cu/Ni on TiO2 had average pore diameter (~ 26 Å). The Cu/Ni on SiO2 had the smallest average pore diameter (~ 24 Å). This might be the change of the structure of the catalyst due to the support. Reactor Catalyst Bed Furnace Gas Trap Gas Flow Vent Gas Chromatograp Recorder Methane Argon GAS MIXING REACTOR SYSTEM ON - LINE SYSTEM ANALYSIS Mass Flow Meters Mixer Thermocouple Meter Gas Chromatograph Data GAS MIXING SYSTEM REACTOR SYSTEM ON - LINE SYSTEM ANALYSIS Pre-Heater Fig. 1: Schematic diagram of the reactor system. Table 1: The physical properties of the fresh Cu/Ni supported on different supports. Catalyst Surface Area (m2 /g) Total Pore Volume (Vp) (cc/g) Average Pore Diameter (Å) Cu/Ni on MgO 11.27 0.008 26.99 Cu/Ni on Al2O3 14.27 0.009 25.73 Cu/Ni on TiO2 4.79 0.003 26.07 Cu/Ni on SiO2 5.28 0.004 23.81 Table 2 shows the performance of Cu/Ni catalyst supported onto TiO2, Al2O3, MgO, and SiO2 supports for methane dissociation to hydrogen and carbon at 998 K and gas hourly space velocity (GHSV) of 2700 h-1. Generally, methane decomposition proceeded over all the catalysts upon contact with the catalysts. The ratio of methane conversion and hydrogen formation was found to be in a ratio of 1:2. In fact, hydrogen was the only gas detected after five minutes on stream. The initial methane decomposition obtained within the first 5 minutes of reaction decreased in the order SiO2  Al2O3  TiO2  MgO support. IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 17 These data should be relatively free of deactivation effects due to blocking of active sites by the decomposition products. After 60 minutes on stream, the order of activity changed to TiO2  SiO2  MgO support and pressure build up was observed on Al2O3 support. At 120 minutes on stream, the TiO2 system maintained its activity and the MgO decreased from 16% to 8% while pressure build up in the SiO2 supported catalyst system. Table 2: The effect of the catalyst support on hydrogen production via catalytic decomposition of methane at 998 K (GHSV = 2700 h-1). Catalyst Conversion (%) & H2 Yield (%) Conversion (%) & H2 Yield (%) Conversion (%) & H2 Yield (%) 5 min 60 min 120 min Cu/Ni on MgO 52 16 8 Cu/Ni on Al2O3 73 nd nd Cu/Ni on TiO2 65 61 62 Cu/Ni on SiO2 74 49 nd nd = Not determined. When the inlet pressure exceeds 1 atm. The methane decomposition over Cu/Ni supported onto TiO2, Al2O3, MgO, and SiO2 supports was significant, as can be seen from Table 2. This can be explained in terms of the relative magnitudes of the rate of methane decomposition and the rate of migration of the deposit from the metal to the support. It is assumed that the rate of methane decomposition decreases with increasing coverage while the rate of migration of the deposit from the metal to the support increases with coverage. Initially, the observed rate is determined by the rate of methane decomposition since all metal sites are vacant. However, as coverage of the active metal by carbonaceous deposit increases, the rate of migration of the deposit from the metal to the support is increased whereas the methane decomposition rate is decreased. When monolayer coverage is achieved, the observed rate is controlled by the rate of migration of the carbonaceous deposit from the metal to the support [8]. When the rate of migration at monolayer coverage is lower than the initial rate of decomposition and as coverage of metal increases, the rate controlling step changes from decomposition of methane to migration of the deposit from the metal to the support. Accordingly, as the carbonaceous species migrate from the metal to the support, metal sites are regenerated and further reaction can occur. Hence, the mobility of the carbonaceous deposit from the metal to the support must be an important factor. Thus, in the parameters studied, it was concluded that the carbonaceous deposit migrates onto support more readily for TiO2  SiO2  MgO  Al2O3 support and the cumulative methane decomposition is therefore greater for Cu/Ni supported on TiO2  SiO2  MgO  Al2O3 support. IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 18 The X-ray diffraction (XRD) profiles of the Cu/Ni loaded on different supports are shown in Fig. 2 (a) – (d). As shown in Fig. 2 (a) and (b), the Cu/Ni on TiO2 support and Cu/Ni on MgO support contained 20 mol%NiO, but large NiO particle was not observed on their XRD profiles, which indicated NiO on these two catalysts was highly dispersed. It is well known that MgO and NiO could form a solid solution due to very good mutual solubility between MgO and NiO. This can be brought out by the results of XRD determination of the catalyst precursor NiO-MgO, in which does not have difference in comparison with that of the pure MgO and NiO phase but the crystal cell of this newly formed phase is between those of NiO and MgO, perhaps implying the formation of NixMg1-xO formation solid solution [9]. Since the Cu/Ni on TiO2 support was more active than Cu/Ni on MgO support, in this case the Ni+2 ions in the NiO-MgO system may be highly dispersed and evenly distributed in the lattice of MgO due to the mutual solubility between NiO and MgO, so that the Ni-component in the NixMg1-xO would be inactive in methane decomposition in the studied process. The NiO characteristic peaks (2 =37.4, 43.4 and 63.1) were not observed. The characteristic peaks of NiO are obvious in the XRD spectrum of Cu/Ni on SiO2 support catalyst which indicates that the Ni is comparatively inconsistently loaded on this support and its surface becomes rough (Fig. 2 (c)). This phenomenon is probably caused by sintering of the catalyst and is also responsible for the loss in catalytic activity. The XRD pattern of Cu/Ni on SiO2 support revealed peaks at 2 =37.3, 43.3 and 62.9 indicated large crystal of NiO particles were formed [10]. The Cu/Ni on Al2O3 support catalyst was found to be the least crystalline as shown in Fig. 2 (d). This might caused interaction between Cu/Ni and the Al2O3 support. As a result, the catalyst was not stable during reaction. Fig. 2: The XRD pattern of the fresh catalyst containing Cu/Ni supported on (a) TiO2, (b) MgO, (c) SiO2, and (d) Al2O3. Table 3 shows the physical properties of used Cu/Ni catalysts supported on TiO2, SiO2, MgO, and Al2O3. Carbon formation affected the physical properties of the catalysts. The (a) (b) (c) (d) IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 19 surface area of the Cu/Ni supported on Al2O3, MgO, SiO2, and TiO2 was 24.38, 25.86, 22.10, and 38.23 m2/g, respectively. The total pore volume was 0.015, 0.017, 0.014, and 0.025 cc/g for and the Cu/Ni supported on Al2O3, MgO, SiO2, and TiO2, respectively. The average pore diameter for the Cu/Ni on MgO, Cu/Ni on Al2O3, and Cu/Ni on TiO2 gave ~ 26 Å. The Cu/Ni on SiO2 had the smallest average pore diameter (~ 25 Å). Table 3: The physical properties of the used Cu/Ni catalysts supported on different supports. These catalysts were used in the methane decomposition reaction at 998 K and GHSV of 2700h-1. Carbon samples obtained on Cu/Ni catalysts loaded into various supports were further studied using TEM. The result obtained elucidated that introduction of the support influence the carbon morphology remarkably. The TEM micrograph of carbon synthesized on Cu/Ni on MgO support is shown in Fig. 3. It looks like small tubes connected to each other. Figure 4 shows the TEM images of the carbon synthesized on Cu/Ni on Al2O3 support. A major part of the carbon looks like short and broken small tubes and are free of metal particles, and only empty ends of tubes are seen in the micrographs. On the other hand, the TEM image of Cu/Ni on SiO2 support shows that the carbon nanotube looks like not developed fully yet and also the presence of black spots which might be the sintered NiO particle (Fig. 5) although it has a catalyst particle at the tip of the carbon. The carbon formed using Cu/Ni on TiO2 support catalyst (Fig. 6) was the best among the supported catalysts. It shows well developed long carbon nanotubes with a catalyst particle located at the tip of the carbon and clear image of the pore where reactants and the products can flow. Data presented above demonstrate that long-lived catalysts could be developed when the conditions of filamentous carbon growth are provided. Thus, the catalytic activity depended strongly the kind of support. A mechanistic interpretation for the growth of carbon on the catalyst was proposed [11, 12]. The result in Fig. 5 support the proposed mechanism for the growth of carbon nanotube on the catalyst. Methane decomposes on the front surface of certain active sites of the Cu/Ni/TiO2 based catalyst and the carbon formed diffuses through the metal and precipitates at the rear surface. The driving force which pushed the carbon diffusion was suggested to originate from the concentration gradient of dissolved carbon between the two interfaces i.e. the metal-gas interface to the metal-nanocarbon interface. Catalyst Surface Area (m2 /g) Total Pore Volume (Vp) (cc/g) Average Pore Diameter (Å) Cu/Ni on MgO 25.86 0.017 26.12 Cu/Ni on Al2O3 24.38 0.015 25.89 Cu/Ni on TiO2 38.23 0.025 25.76 Cu/Ni on SiO2 22.01 0.014 25.12 IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 20 Fig. 3: The transmission electron microscope image of the nanotube produced on Cu/Ni supported on MgO at 998 K and GHSV of 2700 h-1. Fig. 4: The transmission electron microscope image of the nanotube produced on Cu/Ni supported on Al2O3 at 998 K and GHSV of 2700 h-1. IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 21 Fig. 5: The transmission electron microscope image of the nanotube produced on Cu/Ni supported on SiO2 at 998 K and GHSV of 2700 h-1. Fig. 6: The transmission electron microscope image of the nanotube produced on Cu/Ni supported on TiO2 at 998 K and GHSV of 2700 h-1. Catalyst Particle Carbon Nanotube IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 22 4. CONCLUSION Hydrogen production is of great significance on seeking a better way to use natural gas resources. Catalytic decomposition of methane to hydrogen and carbon is a technologically simple single step process without energy and material intensive gas separation stages. It produces only hydrogen and solid carbon. It was found that the catalyst activity in the methane decomposition depended on the filamentous carbon formed. The best catalyst obtained was Cu/Ni supported on TiO2. TiO2 was found to be an effective support for the catalytic decomposition of methane into hydrogen and carbon, giving high activity, attractive carbon nanotube as well as the longest catalyst lifetime. ACKNOWLEDGMENT The authors acknowledge the financial support provided by Ministry of Education, Malaysia and Universiti Sains Malaysia under Fundamental Research Grant Scheme (Project: A/C No: 6070014). LIST OF ABBREVIATIONS FID Flame ionisation detector GC Gas chromatograph GHSV Gas hourly space velocity PB Pressure build up. When the inlet pressure exceeds 1 atm TCD Thermal conductivity detector TEM Transmission electron microscope Vp Total pore volume XRD X-ray diffraction REFERENCES [1] N.Z. Muradov, “CO2-free production of hydrogen by catalytic pyrolysis of hydrocarbon fuel”, Energy Fuels, Vol. 12, No. 1, Jan. 1998. [2] V.V. Chesnokov, V.I. Zaikovskii, R.A. Buyanov, V.V. Molchanov, L.M. Plyasova, “Morphology of carbon from methane on nickel-containing catalysts”, Catalysis Today, Vol. 24, No. 3, pp. 265-267, June 1995. [3] L.B. Avdeeva, O.V. Goncharova, D.I. Kochubey, V.I. Zaikovskii, L.M. Plyasova, B.N. Novgrodov, and Sh.K. Shaikhutdinov, “Coprecipitated Ni-alumina and Ni-Cu-alumina IIUM Engineering Journal, Vol. 5, No. 1, 2004 S. H. Sharif Zein and A. R. Mohamed 23 catalysts of methane decomposition and carbon deposition. II. Evolution of the catalysts in reaction”. Applied Catalysis A: General, Vol. 141, No. 1-2, pp. 117-129, 4 July 1996. [4] V.B. Fenelonov, A.Yu. Derevyankin, L.G. Okkel, L.B. 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Kuvshinov, “Decomposition of methane over iron catalysts at the range of moderate temperatures: the Influence of structure of the catalytic systems and the reaction conditions on the yield of carbon and morphology of carbon filaments”. Journal of Catalysis, Vol. 201, No. 2, pp 183-197, July 2001. BIOGRAPHIES Abdul Rahman Mohamed is an Associate Professor and the dean of the school of chemical engineering, Universiti Sains Malaysia. He got his Ph.D. from University of New Hampshire, USA in the area of chemical reaction engineering. His research interest is in the area of chemical reaction engineering and air pollution control. Sharif Hussein Sharif Zein is a lecturer in the school of chemical engineering, Universiti Sains Malaysia. He got his Ph.D. from Universiti Sains Malaysia in the area of reaction engineering and catalysis. His research interest is in the field of reaction engineering and catalysis, natural gas processing and catalyst development.