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Methane Decomposition on La2O3-Promoted Raney-Type Fe Catalysts ´ rfa˜o, and J. L. Figueiredo* A. F. Cunha, N. Mahata, J. J. M. O Laborato´rio de Cata´lise e Materiais, Laborato´rio Associado LSRE/LCM, Departamento de Engenharia Quı´mica, Faculdade de Engenharia, UniVersidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ReceiVed April 30, 2009. ReVised Manuscript ReceiVed June 9, 2009
The catalytic decomposition of methane into hydrogen and carbon was studied on La2O3-promoted Raneytype Fe catalysts. Promoted catalysts were synthesized by in situ thermal treatment of physical mixtures of Raney Fe and La2O3. The catalysts were characterized by particle size measurement, Brunauer-Emmett-Teller method (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). La2O3 acts as a textural promoter. XPS reveals charge transfer from La2O3 to Fe. Promoted Raney Fe catalyst with an optimum loading of La2O3 exhibited good performance in methane decomposition.
1. Introduction The catalytic decomposition of methane is an interesting route to the production of hydrogen (CH4 f C + 2H2).1-3 A total of 1 mol of methane produces 2 mol of hydrogen together with 1 mol of carbon. Methane is the preferred reactant among the hydrocarbons because the hydrogen/solid carbon ratio is in large favor to hydrogen. The production of hydrogen by catalytic decomposition of methane does not release any harmful products to the atmosphere. Hence, it is environmentally preferable over conventional processes, such as steam reforming of natural gas or coal gasification. Furthermore, this process produces COxfree hydrogen, which can be used in fuel cell systems to produce energy, avoiding cumbersome and expensive purification process. Besides, the formation of carbon in the form of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) makes this process more interesting, because these materials hold scientific and technological potential.4-6 Fe, Co, and Ni, being elements of the groups 8-10 in the periodic table, show nearly the same physical and chemical properties, which reflects also on the catalytic decomposition of methane.3,7-9 Nevertheless, some differences are there, especially with Fe. Fe shows quite different properties in methane decomposition.10-13 It has been observed in our previous work that Co or Fe perform worse than Ni.3 Relatively * To whom correspondence should be addressed. Telephone: +351-22508-1663. Fax: +351-22-508-1449. E-mail:
[email protected]. (1) Muradov, N. Z.; Veziroglu, T. N. Int. J. Hydrogen Energy 2005, 30, 225–237. (2) Ha¨ussinger, P.; Lohmu¨ller, R.; Watson, A. M. Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed.; Wiley-VCH: Weinheim, Germany, 2000. ´ rfa˜o, J. J. M.; Figueiredo, J. L. Appl. Catal., A 2008, (3) Cunha, A. F.; O 348, 103–112. ´ rfa˜o, J. J. M.; Figueiredo, J. L. Appl. (4) Mahata, N.; Cunha, A. F.; O Catal., A 2008, 351, 204–209. ´ rfa˜o, J. J. M.; Figueiredo, J. L. Catal. (5) Mahata, N.; Cunha, A. F.; O Commun. 2009, 10, 1203–1206. (6) Carbon Materials for Catalysis; Serp, P., Figueiredo, J. L., Eds.; Wiley: Hoboken, NJ, 2009. (7) Yang, R. T.; Chen, J. P. J. Catal. 1989, 115, 52–64. (8) Koertz, T.; van Santen, R. A. J. Mol. Catal. 1992, 74, 185–191. (9) Avdeeva, L. B.; Kochubey, D. I.; Shaikhutdinov, Sh. K. Appl. Catal., A 1999, 177, 43–51. (10) Muradov, N. Z. Energy Fuels 1998, 12, 41–48.
high masses of Co or Fe are required to maintain catalyst stability. In the case of Fe, a hydrogen pre-reduction process is also required to avoid large induction periods, corresponding to the reduction of iron oxides. However, because of low cost and wide availability, Fe can be an attractive alternative to Ni. Also, the application of suitable promoters can enhance the activity of Fe. Several reports are available where La2O3 has been applied as a promoter for Ni catalysts in methane decomposition.14,15 Rivas et al. also observed that in situ-generated La2O3 acts as excellent stabilizer for nickel particles in the same reaction.16 The objective of this work is to study the promotional effect of La2O3 on Raney-type Fe skeletal catalysts during catalytic decomposition of methane at well-defined operating conditions. Physical mixtures of Raney Fe and La2O3 were subjected to in situ thermal treatment under diluted hydrogen to facilitate close interaction between the active phase and the promoter. 2. Experimental Section 2.1. Catalysts Preparation. Raney-type catalysts were prepared from Fe-Al alloys of two types, supplied by H.C. Starck GmbH (Amperkat SK alloys). The nominal Fe content in the alloys was 35 or 50 wt %. The alloy containing 50% Fe was conventional.17 Whereas, the alloy powder containing 35% Fe was obtained by a fast quenching method.18 The Raney-type catalysts were prepared by leaching out Al with a concentrated NaOH solution at room temperature.19 The obtained samples were subsequently washed several times with distilled (11) Ermakova, M. A.; Ermakov, D. Y.; Chuvilin, A. L.; Kuvshinov, G. G. J. Catal. 2001, 201, 183–197. (12) Ogihara, H.; Takenaka, S.; Yamanaka, I.; Tanabe, E.; Genseki, A.; Otsuka, K. J. Catal. 2006, 238, 353–360. (13) Sharma, R.; Moore, E.; Rez, P.; Treacy, M. M. J. Nano Lett. 2009, 9, 689–694. (14) Gao, J.; Hou, Z.; Guo, J.; Zhu, Y.; Zheng, X. Catal. Today 2008, 131, 278–284. (15) Jiang, P.; Shang, Y.; Cheng, T.; Bi, Y.; Shi, K.; Wei, S.; Xu, G.; Zhen, K. J. Nat. Gas Chem. 2003, 12, 183–188. (16) Rivas, M. E.; Fierro, J. L. G.; Guil-Lu¨pez, R.; Pen˜a, M. A.; La Parola, V.; Goldwasser, M. R. Catal. Today 2008, 133-135, 367–373. (17) Raney, M. U.S. Patent 1,628,190, 1927. (18) Birkenstock, U.; Scharschmidt, J.; Kunert, P.; Meinhardt, H.; Hausel, P.; Maier, P. U.S. Patent 5,090,997, 1992.
10.1021/ef900385e CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
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Table 1. Average Particle Sizes (d) and BET Surface Areas (SBET) of Selected Catalysts catalyst
d (µm)
SBET (m2 g-1)
50Fe 50FeLa(20) 35Fe 35FeLa(20)
13 13 29 12
5 11 6 11
water. The catalysts were doped with La2O3 (Merck). In a typical procedure, a calculated amount of La2O3 was suspended in 50 mL of distilled water, added to the Raney catalyst, and left on an ultrasonicator for 1 h. Subsequently, the samples were transferred carefully to a tubular reactor, avoiding exposure to air, and dried under N2 flow (100 Ncm3 min-1) at 100 °C for 3 h. The samples were then pretreated at 600 °C for 2 h under 10% H2 in N2 prior to each run, to facilitate close interaction between active metal and La2O3. For a direct comparison, unpromoted Raney Fe catalysts were also pretreated under the same conditions. Two series of catalysts having 5, 10, 20, and 50 wt % La2O3 loadings, along with unpromoted ones, were prepared starting from both types of Fe-Al alloys. The unpromoted catalysts were designated as xFe, and the promoted catalysts were designated as xFeLa(y). x stands for Fe wt % value in the original alloys, and y stands for wt % of La2O3 in the catalysts. 2.2. Catalysts Characterization. The particle size distributions of the catalysts were measured with a laser particle size analyzer. Surface areas were calculated by the Brunauer-Emmett-Teller (BET) method from the N2 adsorption isotherms at 77 K. X-ray diffraction (XRD) measurements were conducted using Cu KR radiation (λ ) 0.154 nm). Intensity was measured by step scanning in the 2θ range of 10-90°. Crystallite size was calculated applying the Scherrer equation. X-ray photoelectron spectroscopy (XPS) analysis was performed using Mg KR radiation (1253.6 eV). The charging effect was corrected using the C 1s level (285.0 eV) as a reference. Quantitative results were obtained by fitting the experimental peaks to Gaussian-Lorentzian curves with Shirley background, measuring the spectral areas corresponding to each chemical element of interest and applying their empirical sensitivity factors. Scanning electron microscopy (SEM) examination was carried out using an electron beam of 20 kV. 2.3. Catalysts Testing. As described in the Catalysts Preparation section, the catalysts were pretreated in situ before the actual reaction run. The catalysts were evaluated in the decomposition of methane in a continuous-flow reactor consisting of a 75 cm long silica tube with 2.7 cm inner diameter, inserted into a furnace with a photoionization detector (PID) temperature controller. The flow rates of methane (99.999%), nitrogen (99.995%), and hydrogen (99.999%) were controlled by mass flow controllers. The feed consisted of a mixture of methane and nitrogen (molar ratio of 1:10), with a total gas flow rate of 110 Ncm3 min-1. During the reaction, the composition of the outgoing gas stream was analyzed with a gas chromatograph equipped with a capillary column (Carboxen 1010 Plot, Supelco) and a thermal conductivity detector.
3. Results and Discussion 3.1. Catalysts Characterization. Table 1 lists average particle sizes and BET surface areas of some selected catalysts. The average particle size of catalysts originating from the alloy containing 35% Fe decreases upon La2O3 promotion, whereas it remains constant in the catalysts originating from the alloy containing 50% Fe. The BET surface areas of the unpromoted catalysts, obtained from both alloys, are pretty low (5-6 m2 g-1), which indicates that the catalysts went through a sinterization process during hydrogen pretreatment. Untreated Raney Fe catalysts exhibited BET areas in the range of 25-60 m2 g-1.3 The addition of La2O3 avoids sinterization to a certain extent, and the promoted catalysts exhibit comparatively higher BET areas (11 m2 g-1). It can be inferred that La2O3 acts as a textural promoter.
Figure 1. Fe 2p XPS spectra of selected catalysts.
XRD spectra were obtained for selected catalysts. All of the catalysts show a sharp diffraction line at a 2θ value of ∼44.5° corresponding to Fe(100).20,21 Minor peaks are also observed at 2θ values of ∼65° and ∼82° corresponding to Fe(200) and Fe(211) diffraction planes, respectively. Only very weak diffraction lines corresponding to La2O3 or La(OH)3 are observed with the La2O3-promoted catalysts. Both 50FeLa(20) and 35FeLa(20) show the presence of La2O3(101) at a 2θ value of ∼29°. In addition, 35FeLa(20) shows diffraction lines at 2θ values of ∼16°, ∼27°, and ∼28° corresponding to (100), (110), and (101) planes of La(OH)3. The crystallite size of Fe was calculated applying the Scherrer equation. The most intensive peak was considered for calculation, and the obtained crystallite size was considered as an average crystallite size. The catalysts obtained from the alloy containing 50% Fe exhibited a Fe crystallite size of ∼15 nm, whereas the catalysts obtained from the alloy containing 35% Fe exhibited a Fe crystallite size of ∼25 nm. The crystallite size of Fe in the catalysts is well in the optimum range for filamentous carbon formation during hydrocarbon decomposition.22,23 The catalysts were subjected to Fe 2p, La 3d, and Al 2p XPS analysis. Figure 1 shows XPS spectra of the Fe 2p region of some selected samples. The iron spectrum clearly reveals that surface Fe is mainly present in the oxidized state. The 2p3/2 photoelectron peak observed at 710.2 ( 0.5 eV can be assigned to a combination of Fe2+ and Fe3+. Reported binding energy (BE) values of Fe 2p3/2 in FeO and Fe2O3 are 709.5 ( 0.3 and 710.5 ( 0.1 eV, respectively.24-26 Hence, surface iron mainly consists of FeO and Fe2O3 in the present catalysts. A comparison of the spectra of La2O3-promoted catalysts to those of unpromoted catalysts shows a negative shift of BE in the promoted catalysts, indicating charge transfer from La2O3 to iron. Table 2 summarizes the Fe 2p3/2, La 3d5/2, and Al 2p BE values along with the surface concentration of iron. The catalysts were also characterized by SEM, with typical micrographs being shown in Figure 2. Both promoted and unpromoted catalysts exhibit rough surfaces. The bright points, observed in 50Fe, indicate the presence of FexOy on the surface. (19) Birkenstock, U.; Holm, R.; Reinfandt, B.; Storp, S. J. Catal. 1985, 93, 55–67. (20) Yin, S.; Li, C.; Blan, Q.; Lu, M. Mater. Sci. Eng., A 2008, 496, 362–365. (21) Mondal, B. N.; Basumallick, A.; Chattopadhyay, P. P. J. Magn. Magn. Mater. 2007, 309, 290–294. (22) Chen, D.; Christensen, K. O.; Ochoa-Ferna´ndez, E.; Yu, Z.; Tøtdal, B.; Latorre, N.; Monzo´n, A.; Holmen, A. J. Catal. 2005, 229, 82–96. (23) Li, Y.; Zhang, B.; Xie, X.; Liu, J.; Xu, Y.; Shen, W. J. Catal. 2006, 238, 412–424. (24) Aronniemi, M.; Sainio, J.; Lahtinen, J. Surf. Sci. 2005, 578, 108– 123. (25) Lee, D. H.; Yoon, S. Y.; Kim, J. H.; Suh, S. J. Thin Solid Films 2005, 475, 251–255. (26) Keller, P.; Strehblow, H.-H. Corros. Sci. 2004, 46, 1939–1952.
La2O3-Promoted Raney-Type Fe Catalysts
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Table 2. XPS Results of Selected Catalysts BE (eV) catalyst
Fe 2p3/2
50Fe 50FeLa(20) 35Fe 35FeLa(20)
710.6 709.7 710.6 710.3
La 3d5/2 835.1 835.6
Al 2p
fractional amount of Fe
73.8 72.6 73.8 73.5
0.25 0.15 0.24 0.18
La2O3 agglomerates on the iron surface are clearly visible in the promoted catalyst. 3.2. Catalysts Performance. Catalytic decomposition of methane was carried out isothermally at 600 °C. The results are shown in Figure 3. The catalysts were evaluated for 5 h on stream. The carbon yields obtained in all of the experimental runs are in agreement with the methane conversion values. Figure 3a shows the effect of La2O3 loading on Raney Fe obtained from Fe-Al alloy containing 50% Fe, while Figure 3b shows the effect of La2O3 loading on Raney Fe obtained from Fe-Al alloy containing 35% Fe. The methane conversion increases initially, attains a maximum value, and then drops down over all of the catalysts. The initial induction period is related to the reduction of surface FexOy. XPS analysis confirms that surface layers mainly consist of Fe in oxidized states, even though XRD reveals that bulk Fe is mainly in the metallic state. Conversion maxima over all of the catalysts are in the range of 50-55%. Rapid deactivation beyond conversion maxima is not unexpected, because blocking of active sites by initial carbon deposition can be significant. However, the drop in methane conversion is much slower at later stages of the reaction. This performance of
Figure 3. Performance of the catalysts in methane decomposition: (a) catalysts originated from 50:50 Fe/Al alloy and (b) catalysts originated from 35:65 Fe/Al alloy. Reaction conditions: amount of catalysts, 0.5 g of Raney Fe (+promoter); total flow rate of the CH4 + N2 gas mixture, 110 Ncm3 min-1 (CH4/N2 ) 0.1); temperature, 600 °C.
Figure 2. SEM images of catalysts: (a) 50Fe and (b) 50FeLa(20).
the catalysts, with very slow deactivation, implies that deposited carbon is mainly filamentous (nanofiber or nanotube) rather than encapsulating carbon. In fact, SEM analysis of catalysts after the reaction confirms that a large fraction of deposited carbon is filamentous. A comparison between panels a and b of Figure 3 reveals that catalysts from the 50:50 Fe/Al alloy exhibited better performance than those obtained from the 35:65 Fe/Al alloy. Also, it is evident that La2O3 promotes the activity of Raney iron up to a certain loading. However, excess La2O3 decreases the activity, by blocking active Fe sites. Among the selected samples, Raney Fe promoted with 20 wt % La2O3 is the best catalyst for decomposition of methane to hydrogen and carbon. After 5 h on stream, 50FeLa(20), obtained from the 50:50 Fe/Al alloy and promoted by 20 wt % La2O3, exhibited a nearly stable performance with 42% methane conversion. However, 35FeLa(20), the analogue system obtained from the 35:65 Fe/Al alloy, showed inferior performance with 28% methane conversion and not reaching a steady state after 5 h on stream. La2O3 enhances the activity of Raney iron in two ways. First, it acts as a good textural promoter, La2O3 promoted catalysts showing higher BET surface areas than the unpromoted catalysts (Table 1). Second, it acts as an electronic promoter, as shown by a negative shift of BE of iron in the promoted catalysts, indicating charge transfer from La2O3 to iron (Figure 1 and Table 2). An increased electron density over iron seems to enhance electron transfer to the σ* antibonding orbital of adsorbed methane, thereby facilitating C-H bond
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splitting. This argument is further supported by the fact that the enhancement of the activity after 5 h, compared to the respective unpromoted counterparts, is more pronounced in 50FeLa(20) than in 35FeLa(20). After 5 h on stream, 50FeLa(20) exhibited 20% higher methane conversion than 50Fe, whereas 35FeLa(20) exhibited only 9% higher conversion than 35Fe. Table 2 shows that the Fe 2p3/2 BE shift in 50FeLa(20) is -0.9 eV compared to -0.3 eV in 35FeLa(20), meaning higher electron density over Fe in 50FeLa(20) than 35FeLa(20). 4. Conclusions La2O3-promoted Raney Fe catalysts for methane decomposition can be synthesized by in situ thermal treatment of physical mixtures of Raney Fe and La2O3.
Cunha et al.
La2O3 acts as a textural promoter by increasing the BET surface area. It also acts as an electronic promoter by transferring charge to iron. The activity and stability of Raney Fe can be improved by adequate loading of La2O3. However, excess La2O3 blocks the active sites. The best systems, among those tested, correspond to 20 wt % La2O3-promoted Raney Fe. Acknowledgment. The authors are grateful to Dr. Rainer-Leo Meisel, H.C. Starck GmbH, who kindly supplied the Amperkat SK alloys used in this work. Financial support was provided by Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT) with contribution from FEDER (POCTI/1181). A.F.C. and N.M. thank FCT for doctoral (SFRH/BD/16035/2004) and postdoctoral (SFRH/BPD/ 14804/2003) grants, respectively. EF900385E
