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Support effects in catalytic wet oxidation of H2S to sulfur on supported iron oxide catalysts
Applied Catalysis A: General 284 (2005) 1–4 www.elsevier.com/locate/apcata
Support effects in catalytic wet oxidation of H2S to sulfur on supported iron oxide catalysts Eun-Ku Lee a, Kwang-Deog Jung b,*, Oh-Shim Joo b, Yong-Gun Shul a a
b
Department of Chemical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Eco-Nano Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Republic of Korea Received 4 August 2004; received in revised form 8 November 2004; accepted 24 December 2004 Available online 14 March 2005
Abstract Iron oxide catalysts supported on MgO, Al2O3, SiO2 and ZrO2 were prepared by an impregnation method. They were characterized by BET surface analysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and Mo¨ssbauer spectroscopy. Fe/MgO catalyst shows the highest H2S removal capacity in wet oxidation at room temperature. Both the XRD and the TPR analyses of the catalysts indicate that H2S removal capacity correlates with the dispersion of iron oxide on supports. Mo¨ssbauer spectroscopy shows that four kinds of Fe3+ species are present in the supported iron oxide catalysts: two highly dispersed species, Fe3+ oxide clusters, and a-Fe2O3 particles. Two highly dispersed species, Fe3+ cation and MgFe2O4, are present in Fe/MgO. Fe3+ oxide clusters and a-Fe2O3 crystals are evident in both Fe/SiO2 and Fe/ZrO2. a-Fe2O3 crystals are only observed in Fe/Al2O3. # 2004 Elsevier B.V. All rights reserved. Keywords: Iron oxide catalyst; Fe/MgO; Mo¨ssbauer spectroscopy; H2S removal capacity
1. Introduction The Claus process has been most commonly employed to remove H2S from natural gas in facilities or refinery plants. Claus plants generally convert 94–98% of sulfur compounds in the feed gas into elemental sulfur [1,2]. As the restrictions on sulfur emissions are annually strengthening worldwide, a number of tail gas clean-up processes have been developed to reduce sulfur emission to permissible levels [3]. The development of the new processes to deal with the Claus tail gas is based on the direct oxidation of remaining traces of H2S by oxygen or H2S absorption/recycling technologies [4]. Fe-based catalysts have been used for H2S oxidation. The heterogeneous catalytic systems for this reaction are commercialized. The Superclaus and BSR were developed for the direct oxidation of H2S above the sulfur dew point [5– 7]. The heterogeneous catalytic systems for the processes showed high activity, but the high reaction temperature resulted in the low stability of the catalysts due to the * Corresponding author. Tel.: +822 21233554; fax: +822 3126401. E-mail address: [email protected] (K.-D. Jung). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.034
sulfidation of metal and metal oxide components, as well as in the low selectivity due to the formation of SO2. H2S wet oxidation proceeds by the redox mechanism on Fe-chelating catalysts [8–15]. H2 S þ Fe3þ ! S þ Fe2þ Fe2þ þ O ! Fe3þ The Fe-chelating catalytic system requires pH control agents to stabilize the catalyst. The heterogeneous catalytic system, Fe/MgO, has been developed to oxidize H2S to sulfur at room temperature. The isolated Fe ion was proposed to be the active site for the reaction [16–18]. The catalytic system mimics the homogeneous catalyst and one need not control the pH of the reaction media. In the present study, the supported iron oxide catalysts were characterized using Mo¨ssbauer spectroscopy, XRD, and TPR techniques. These catalysts were investigated to obtain fundamental knowledge of catalytic behavior for the catalytic wet oxidation of H2S. Different kinds of Fe3+ species are found to be formed on the surface of the supports with the same Fe loading. The highly dispersed species of
2
E.-K. Lee et al. / Applied Catalysis A: General 284 (2005) 1–4
iron oxide are proposed to be responsible for the high removal capacity of H2S in catalytic wet oxidation.
2. Experimental Fe/MgO catalyst was prepared by an impregnation of MgO (Aldrich 22,036-1) with aqueous iron nitrate solutions, followed by drying at 100 8C and subsequent calcinations in air for 5 h at 460 8C. After the calcinations, 6 wt.% Fe/MgO was obtained. For comparison, 6 wt.% Fe/Al2O3, 6 wt.% Fe/ SiO2, and 6 wt.% Fe/ZrO2 were prepared by the same procedure as that used for 6 wt.% Fe/MgO. Activity measurements were carried out using a stirred batch tank reactor. The catalyst samples (3.0 g, if not specified) were dispersed in the reactor while distilled water (1.5 L, if not specified) and reactant gases were supplied through a perforated rubber plate at the bottom reactor. H2S concentrations from the reactor were measured with on-line GC with FPD detector which can detect up to 0.1 ppm H2S. A Porapak Q column (1/8 in. o.d. 2 m) was used for separating the product gases. The specific surface areas of the catalysts were obtained in an ASAP 2000 instrument by a BET method from the nitrogen adsorption isotherms at 77 K, taking a value of 0.164 nm2 for the cross-section of nitrogen. The XRD patterns were collected with Rint 2000 (Rigaku Co.) using Cu Ka radiation (l = 0.1542 nm). Temperature-programmed reduction experiments were carried out in a micro-reactor system with a TCD detector. The samples of 50 mg were first treated in argon at room temperature for 1 h. After that, the samples were reduced in a stream of 5% H2/Ar (30 ml/min) at a ramping rate of 20 8C/min from 100 to 1400 8C. Mo¨ ssbauer spectra were recorded using a conventional Mo¨ ssbauer spectrometer of the electromechanical type with a 30 mCi 57Co source in an Rh matrix.
Fig. 1. H2S removal capacities with Fe/MgO (^), Fe/Al2O3 (*), Fe/SiO2 (&), and Fe/ZrO2 (~) in H2S wet oxidation at room temperature. H2S flow rate: 5 ml/min; O2 flow rate: 100 ml/min; catalyst: 3 g.
highest activity and best selectivity above 450 K. The reaction at room temperature was shown to change the activity of the catalysts. The Fe/MgO catalyst with the lowest activity above 450 K shows the highest activity for the wet oxidation at room temperature, indicating that the mechanism can change with the reaction temperature. Table 1 shows the BET surface areas and pore volumes of the supported iron oxide catalysts. The BET surface area of Fe/SiO2 is the highest, at 137 m2/g, while that of Fe/SiO2 is the lowest, at 5.9 m2/g. The BET surface areas are in the order: Fe/SiO2 > Fe/MgO > Fe/ZrO2 > Fe/Al2O3. As shown in Fig. 1, the H2S removal capacity is not in
3. Results and discussion The support effects on H2S oxidation at room temperature are examined with Fe/MgO, Fe/Al2O3, Fe/SiO2 and Fe/ ZrO2 as shown in Fig. 1. The Fe/MgO catalyst shows a much higher removal capacity (2.6 g H2S/gcat) in wet oxidation of H2S. The high removal capacity of Fe/MgO cannot be ascribed to MgO, since MgO alone also shows low H2S removal capacity [16]. Interestingly, the activity values of H2S oxidation above 450 K were in the following order: Fe/SiO2 > Fe/ZrO2 > Fe/TiO2 > Fe/Al2O3 > Fe/MgO [19]. Especially, F2O3 supported on a SiO2 catalyst showed the Table 1 BET surface area and pore volume of the supported iron oxide catalysts Surface property 2
Surface area (m /g) Pore volume (cm3/g)
Fe/MgO
Fe/SiO2
Fe/ZrO2
Fe/Al2O3
74.16 0.34
231.3 0.993
8.6 0.04
5.9 0.007
Fig. 2. X-ray diffraction patterns of (a) Fe/MgO, (b) Fe/Al2O3, (c) Fe/SiO2, and (d) Fe/ZrO2; (*) a-Fe2O3.
E.-K. Lee et al. / Applied Catalysis A: General 284 (2005) 1–4
3
Fig. 3. Mo¨ ssbauer spectra at 293 K of (a) Fe/MgO, (b) Fe/Al2O3, (c) Fe/ SiO2, and (d) Fe/ZrO2.
Fig. 4. Mo¨ ssbauer spectra at 13 K of (a) Fe/MgO, (b) Fe/Al2O3, (c) Fe/SiO2, and (d) Fe/ZrO2.
agreement with the order of BET surfaces of supported iron oxide catalysts. The XRD patterns of the prepared catalysts are presented in Fig. 2. The characteristic peaks in Fe/MgO are not found, indicating that a-Fe2O3 can be present in only small-sized form. Fe2O3 characteristic peaks in other catalysts are strong, indicating that the Fe2O3 particle sizes of the Fe/SiO2, Fe/ZrO2, Fe/TiO2 and Fe/Al2O3 are much larger than that of Fe/MgO [20]. It is well known that the particles below 5 nm in size cannot be observed by the XRD. Figs. 3 and 4 show the Mo¨ ssbauer spectra of the Fe/MgO, Fe/Al2O3, Fe/SiO2, and Fe/ZrO2 catalysts at 293 and 13 K, respectively. Table 2 shows the Mo¨ ssbauer parameters of these catalysts. The doublet (2 line) of the Fe/MgO catalyst at 293 K may be due to highly dispersed Fe3+ species, that is, paramagnetic Fe3+ cations and/or super-paramagnetic Fe3+ oxide clusters [17,21]. However, the Mo¨ ssbauer spectra
measured at 13 K indicate the formation of sextets. Such results indicate that the super-paramagnetic iron oxide particles and the paramagnetic Fe3+ cations coexist. The sextets can be mainly attributed to Fe3+ located at tetrahedral and octahedral sites in the spinel structure, although a small proportion of the a-Fe2O3 presence cannot be ruled out. The hyperfine field of Fe/MgO (H = 386.1 kOe for tetrahedral site and H = 454.0 kOe for octahedral site) is much smaller than that of pure a-Fe2O3 (ca. 525 kOe) and that of MgFe2O4 (H = 511 kOe for tetrahedral site and H = 538 kOe for octahedral site at 75 K). The much lower hyperfine field of the Fe/MgO can be attributed to the small size of the MgFe2O4 particles or the iron particles [22]. The spectra of the Fe/Al2O3 catalyst at 293 and 13 K are composed only of a sextet. Comparing the Mo¨ ssbauer parameters in Table 2 with those reported for Fe catalysts and reference compounds, we can conclude that the sextet should be attributed to large a-Fe2O3 particles [23,24].
Table 2 Mo¨ ssbauer parameters at 293 and 13 K of Fe/MgO, Fe/Al2O3, Fe/SiO2, and Fe/ZrO2 catalysts T (K)
298
13
Samples
Sextet
Doublet
Hhf (kOe)
EQ (mm/s)
d (mm/s)
Fe/MgO Fe/Al2O3 Fe/SiO2 Fe/ZrO2
517.77 492.90 510.99
0.11 0.13 0.11
0.26 0.26 0.25
Fe/MgO Fe/Al2O3 Fe/SiO2 Fe/ZrO2
386, 454 546.78 529.25 538.23
0.04, 0.10 0.17 0.09 0.11
0.31, 0.33 0.38 0.36 0.36
EQ (mm/s)
Area (%) d (mm/s)
6 line
2 line
0.65
0.27
0.52 0.77
0.23 0.19
– 100 48 70
100 – 52 30
0.77
0.39
24 100 100 100
76 – – –
4
E.-K. Lee et al. / Applied Catalysis A: General 284 (2005) 1–4
4. Conclusions Fe/MgO was a very effective catalyst for the wet oxidation of H2S to sulfur at room temperature. Characterization of supported iron oxide catalysts by Mo¨ ssbauer spectroscopy, XRD, and TPR leads us to conclude that four kinds of Fe3+ species are formed with the various supports: both Fe3+ cation and MgFe2O4 species produced in Fe/MgO, Fe3+ oxide clusters and a-Fe2O3 crystallites in Fe/SiO2 and Fe/ZrO2, and a-Fe2O3 crystallites in Fe/Al2O3. The high dispersion of Fe on MgO support should be ascribed to the high activity in the wet oxidation of H2S to sulfur.
Fig. 5. TPR profiles of (a) Fe/MgO, (b) Fe/SiO2, (c) Fe/ZrO2, and (d) Fe/ Al2O3.
This is consistent with the XRD results in Fig. 3 showing the strong peak intensity due to a-Fe2O3 crystallites. In the spectra of both Fe/SiO2 and Fe/ZrO2 catalysts at 293 K, the signals are composed of two components: a doublet and a magnetic-split sextet. The doublet is attributed to highly dispersed Fe3+ species (Fe3+ oxide cluster), while the sextet is assigned to the a-Fe2O3 crystallites [25]. The spectra of the same catalysts are only composed of a sextet when measured at 13 K. These spectra at 13 K confirm the existence of super-paramagnetic Fe oxide particles (Fe3+ oxide cluster). The parameters are close to the values reported for Fe3+ oxide cluster and for a-Fe2O3 crystallites, respectively. The most reliable percentages of each species (doublets and sextets) can be estimated at low temperature, since the probable differences in the recoil-free factors are minimal. Fig. 5 shows the TPR profiles of the supported iron oxide catalysts. For the Fe/MgO sample, the TPR profile shows two peaks at 650 and 850 8C assignable to the Fe3+ cation and MgFe2O4 species, respectively. For the Fe/SiO2 and Fe/ ZrO2 samples, there are two broad peaks at relatively higher temperature compared to those of Fe/MgO. The peak at low temperature can be attributed to Fe3+ oxide clusters and the one at high temperature can be ascribed to a-Fe2O3 crystallites. In the case of Fe/Al2O3 sample, only one peak is observed at 800 8C. This can be assigned to the reduction of a-Fe2O3 crystallites. The XRD and Mo¨ ssbauer spectra support the conclusion that these species are formed for the supported iron oxide catalysts.
References [1] J.W. Estep, G.T. McBride, J.R. West, Advances in Petroleum Chemistry and Refining, vol. 6, Interscience, New York, 1962, p. 315. [2] B.G. Goar, Oil Gas J. 25 (1975) 96. [3] Anon., Sulfur 20 (1995) 236. [4] N. Keller, C.P. Huu, M.J. Ledoux, Appl. Catal. 217 (2001) 205. [5] J.A. Lagas, J. Borboom, P.H. Berben, J.W. Geus, European Patent Application No. 0,242,006 (1988). [6] P.H. Berben, J.W. Geus, US Patent 4,818,740 (1989). [7] P.H. Berben, J.W. Geus, European Patent Application No. 0,242,920 (1987). [8] G.E. Vrieland, C.B. Murchison, Appl. Catal. 134 (1996) 101. [9] J. Ogonowski, Chem. Stosow. 33 (1989) 281. [10] J. Ogonowski, Nafta-Gas 49 (1993) 269. [11] V.P. Luk’yanenko, D.N. Tmenov, V.S. Solodkaya, Neftepererab. Neftekhim. (Kiev) 46 (1994) 14. [12] L.E. Cadus, M.C. Abello, M.F. Gomez, J.B. Rivarla, Ind. Eng. Chem. Res. 35 (1996) 14. [13] F.C. Meunier, A. Yasmeen, J.R.H. Ross, Catal. Today 37 (1997) 33. [14] K.H. Lee, Y.S. Yoon, W. Ueda, Y. Moro-Oka, Catal. Lett. 46 (1997) 267. [15] J. Shimada, T. Sato, Y. Yoshimura, J. Hiraishi, A. Nishijima, J. Catal. 110 (1988) 275. [16] K.D. Jung, O.S. Joo, S.H. Cho, S.H. Han, Appl. Catal. 240 (2001) 213. [17] K.D. Jung, O.S. Joo, C.S. Kim, Catal. Lett. 84 (2002) 53. [18] K.D. Jung, Characterization of Fe/MgO for Wet Catalytic Oxidation, ACS Meeting, Chicago, 2001. [19] R.J.A.M. Tero¨ rde, P.J. van den Brink, L.M. Visser, A.J. van Dillen, J.W. Geus, Catal. Today 17 (1993) 217. [20] Y. Okamoto, T. Kubota, Y. Ohto, S. Nasu, J. Catal. 192 (2000) 412. [21] R. Spretz, S.G. Marchetti, M.A. Ulla, E.A. Lombardo, J. Catal. 194 (2000) 167. [22] G. Bond, K.C. Molloy, F.S. Stone, Solid State Ion. 101 (1997) 697. [23] Y.L. Huang, D.S. Xue, P.H. Zhou, Y. Ma, F.S. Li, Mater. Sci. Eng. 359 (2003) 332. [24] M.L. Cubeiro, H. Morales, M.R. Goldwasser, M.J. Pe´ rez-Zurita, F. Gonza´ lez-Jime´ nez, C. Urbina de N, Appl. Catal. 189 (1999) 87. [25] C.D. Julia´ n, G.A.P. Alca´ zar, F. Cebollada, M.I. Montero, J.M. Gonza´ lez, J.F. Marco, J. Magn. Magn. Mater. 203 (1999) 175.
Support effects in catalytic wet oxidation of H2S to sulfur on supported iron oxide catalysts Eun-Ku Lee a, Kwang-Deog Jung b,*, Oh-Shim Joo b, Yong-Gun Shul a a
b
Department of Chemical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Eco-Nano Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Republic of Korea Received 4 August 2004; received in revised form 8 November 2004; accepted 24 December 2004 Available online 14 March 2005
Abstract Iron oxide catalysts supported on MgO, Al2O3, SiO2 and ZrO2 were prepared by an impregnation method. They were characterized by BET surface analysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and Mo¨ssbauer spectroscopy. Fe/MgO catalyst shows the highest H2S removal capacity in wet oxidation at room temperature. Both the XRD and the TPR analyses of the catalysts indicate that H2S removal capacity correlates with the dispersion of iron oxide on supports. Mo¨ssbauer spectroscopy shows that four kinds of Fe3+ species are present in the supported iron oxide catalysts: two highly dispersed species, Fe3+ oxide clusters, and a-Fe2O3 particles. Two highly dispersed species, Fe3+ cation and MgFe2O4, are present in Fe/MgO. Fe3+ oxide clusters and a-Fe2O3 crystals are evident in both Fe/SiO2 and Fe/ZrO2. a-Fe2O3 crystals are only observed in Fe/Al2O3. # 2004 Elsevier B.V. All rights reserved. Keywords: Iron oxide catalyst; Fe/MgO; Mo¨ssbauer spectroscopy; H2S removal capacity
1. Introduction The Claus process has been most commonly employed to remove H2S from natural gas in facilities or refinery plants. Claus plants generally convert 94–98% of sulfur compounds in the feed gas into elemental sulfur [1,2]. As the restrictions on sulfur emissions are annually strengthening worldwide, a number of tail gas clean-up processes have been developed to reduce sulfur emission to permissible levels [3]. The development of the new processes to deal with the Claus tail gas is based on the direct oxidation of remaining traces of H2S by oxygen or H2S absorption/recycling technologies [4]. Fe-based catalysts have been used for H2S oxidation. The heterogeneous catalytic systems for this reaction are commercialized. The Superclaus and BSR were developed for the direct oxidation of H2S above the sulfur dew point [5– 7]. The heterogeneous catalytic systems for the processes showed high activity, but the high reaction temperature resulted in the low stability of the catalysts due to the * Corresponding author. Tel.: +822 21233554; fax: +822 3126401. E-mail address: [email protected] (K.-D. Jung). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.034
sulfidation of metal and metal oxide components, as well as in the low selectivity due to the formation of SO2. H2S wet oxidation proceeds by the redox mechanism on Fe-chelating catalysts [8–15]. H2 S þ Fe3þ ! S þ Fe2þ Fe2þ þ O ! Fe3þ The Fe-chelating catalytic system requires pH control agents to stabilize the catalyst. The heterogeneous catalytic system, Fe/MgO, has been developed to oxidize H2S to sulfur at room temperature. The isolated Fe ion was proposed to be the active site for the reaction [16–18]. The catalytic system mimics the homogeneous catalyst and one need not control the pH of the reaction media. In the present study, the supported iron oxide catalysts were characterized using Mo¨ssbauer spectroscopy, XRD, and TPR techniques. These catalysts were investigated to obtain fundamental knowledge of catalytic behavior for the catalytic wet oxidation of H2S. Different kinds of Fe3+ species are found to be formed on the surface of the supports with the same Fe loading. The highly dispersed species of
2
E.-K. Lee et al. / Applied Catalysis A: General 284 (2005) 1–4
iron oxide are proposed to be responsible for the high removal capacity of H2S in catalytic wet oxidation.
2. Experimental Fe/MgO catalyst was prepared by an impregnation of MgO (Aldrich 22,036-1) with aqueous iron nitrate solutions, followed by drying at 100 8C and subsequent calcinations in air for 5 h at 460 8C. After the calcinations, 6 wt.% Fe/MgO was obtained. For comparison, 6 wt.% Fe/Al2O3, 6 wt.% Fe/ SiO2, and 6 wt.% Fe/ZrO2 were prepared by the same procedure as that used for 6 wt.% Fe/MgO. Activity measurements were carried out using a stirred batch tank reactor. The catalyst samples (3.0 g, if not specified) were dispersed in the reactor while distilled water (1.5 L, if not specified) and reactant gases were supplied through a perforated rubber plate at the bottom reactor. H2S concentrations from the reactor were measured with on-line GC with FPD detector which can detect up to 0.1 ppm H2S. A Porapak Q column (1/8 in. o.d. 2 m) was used for separating the product gases. The specific surface areas of the catalysts were obtained in an ASAP 2000 instrument by a BET method from the nitrogen adsorption isotherms at 77 K, taking a value of 0.164 nm2 for the cross-section of nitrogen. The XRD patterns were collected with Rint 2000 (Rigaku Co.) using Cu Ka radiation (l = 0.1542 nm). Temperature-programmed reduction experiments were carried out in a micro-reactor system with a TCD detector. The samples of 50 mg were first treated in argon at room temperature for 1 h. After that, the samples were reduced in a stream of 5% H2/Ar (30 ml/min) at a ramping rate of 20 8C/min from 100 to 1400 8C. Mo¨ ssbauer spectra were recorded using a conventional Mo¨ ssbauer spectrometer of the electromechanical type with a 30 mCi 57Co source in an Rh matrix.
Fig. 1. H2S removal capacities with Fe/MgO (^), Fe/Al2O3 (*), Fe/SiO2 (&), and Fe/ZrO2 (~) in H2S wet oxidation at room temperature. H2S flow rate: 5 ml/min; O2 flow rate: 100 ml/min; catalyst: 3 g.
highest activity and best selectivity above 450 K. The reaction at room temperature was shown to change the activity of the catalysts. The Fe/MgO catalyst with the lowest activity above 450 K shows the highest activity for the wet oxidation at room temperature, indicating that the mechanism can change with the reaction temperature. Table 1 shows the BET surface areas and pore volumes of the supported iron oxide catalysts. The BET surface area of Fe/SiO2 is the highest, at 137 m2/g, while that of Fe/SiO2 is the lowest, at 5.9 m2/g. The BET surface areas are in the order: Fe/SiO2 > Fe/MgO > Fe/ZrO2 > Fe/Al2O3. As shown in Fig. 1, the H2S removal capacity is not in
3. Results and discussion The support effects on H2S oxidation at room temperature are examined with Fe/MgO, Fe/Al2O3, Fe/SiO2 and Fe/ ZrO2 as shown in Fig. 1. The Fe/MgO catalyst shows a much higher removal capacity (2.6 g H2S/gcat) in wet oxidation of H2S. The high removal capacity of Fe/MgO cannot be ascribed to MgO, since MgO alone also shows low H2S removal capacity [16]. Interestingly, the activity values of H2S oxidation above 450 K were in the following order: Fe/SiO2 > Fe/ZrO2 > Fe/TiO2 > Fe/Al2O3 > Fe/MgO [19]. Especially, F2O3 supported on a SiO2 catalyst showed the Table 1 BET surface area and pore volume of the supported iron oxide catalysts Surface property 2
Surface area (m /g) Pore volume (cm3/g)
Fe/MgO
Fe/SiO2
Fe/ZrO2
Fe/Al2O3
74.16 0.34
231.3 0.993
8.6 0.04
5.9 0.007
Fig. 2. X-ray diffraction patterns of (a) Fe/MgO, (b) Fe/Al2O3, (c) Fe/SiO2, and (d) Fe/ZrO2; (*) a-Fe2O3.
E.-K. Lee et al. / Applied Catalysis A: General 284 (2005) 1–4
3
Fig. 3. Mo¨ ssbauer spectra at 293 K of (a) Fe/MgO, (b) Fe/Al2O3, (c) Fe/ SiO2, and (d) Fe/ZrO2.
Fig. 4. Mo¨ ssbauer spectra at 13 K of (a) Fe/MgO, (b) Fe/Al2O3, (c) Fe/SiO2, and (d) Fe/ZrO2.
agreement with the order of BET surfaces of supported iron oxide catalysts. The XRD patterns of the prepared catalysts are presented in Fig. 2. The characteristic peaks in Fe/MgO are not found, indicating that a-Fe2O3 can be present in only small-sized form. Fe2O3 characteristic peaks in other catalysts are strong, indicating that the Fe2O3 particle sizes of the Fe/SiO2, Fe/ZrO2, Fe/TiO2 and Fe/Al2O3 are much larger than that of Fe/MgO [20]. It is well known that the particles below 5 nm in size cannot be observed by the XRD. Figs. 3 and 4 show the Mo¨ ssbauer spectra of the Fe/MgO, Fe/Al2O3, Fe/SiO2, and Fe/ZrO2 catalysts at 293 and 13 K, respectively. Table 2 shows the Mo¨ ssbauer parameters of these catalysts. The doublet (2 line) of the Fe/MgO catalyst at 293 K may be due to highly dispersed Fe3+ species, that is, paramagnetic Fe3+ cations and/or super-paramagnetic Fe3+ oxide clusters [17,21]. However, the Mo¨ ssbauer spectra
measured at 13 K indicate the formation of sextets. Such results indicate that the super-paramagnetic iron oxide particles and the paramagnetic Fe3+ cations coexist. The sextets can be mainly attributed to Fe3+ located at tetrahedral and octahedral sites in the spinel structure, although a small proportion of the a-Fe2O3 presence cannot be ruled out. The hyperfine field of Fe/MgO (H = 386.1 kOe for tetrahedral site and H = 454.0 kOe for octahedral site) is much smaller than that of pure a-Fe2O3 (ca. 525 kOe) and that of MgFe2O4 (H = 511 kOe for tetrahedral site and H = 538 kOe for octahedral site at 75 K). The much lower hyperfine field of the Fe/MgO can be attributed to the small size of the MgFe2O4 particles or the iron particles [22]. The spectra of the Fe/Al2O3 catalyst at 293 and 13 K are composed only of a sextet. Comparing the Mo¨ ssbauer parameters in Table 2 with those reported for Fe catalysts and reference compounds, we can conclude that the sextet should be attributed to large a-Fe2O3 particles [23,24].
Table 2 Mo¨ ssbauer parameters at 293 and 13 K of Fe/MgO, Fe/Al2O3, Fe/SiO2, and Fe/ZrO2 catalysts T (K)
298
13
Samples
Sextet
Doublet
Hhf (kOe)
EQ (mm/s)
d (mm/s)
Fe/MgO Fe/Al2O3 Fe/SiO2 Fe/ZrO2
517.77 492.90 510.99
0.11 0.13 0.11
0.26 0.26 0.25
Fe/MgO Fe/Al2O3 Fe/SiO2 Fe/ZrO2
386, 454 546.78 529.25 538.23
0.04, 0.10 0.17 0.09 0.11
0.31, 0.33 0.38 0.36 0.36
EQ (mm/s)
Area (%) d (mm/s)
6 line
2 line
0.65
0.27
0.52 0.77
0.23 0.19
– 100 48 70
100 – 52 30
0.77
0.39
24 100 100 100
76 – – –
4
E.-K. Lee et al. / Applied Catalysis A: General 284 (2005) 1–4
4. Conclusions Fe/MgO was a very effective catalyst for the wet oxidation of H2S to sulfur at room temperature. Characterization of supported iron oxide catalysts by Mo¨ ssbauer spectroscopy, XRD, and TPR leads us to conclude that four kinds of Fe3+ species are formed with the various supports: both Fe3+ cation and MgFe2O4 species produced in Fe/MgO, Fe3+ oxide clusters and a-Fe2O3 crystallites in Fe/SiO2 and Fe/ZrO2, and a-Fe2O3 crystallites in Fe/Al2O3. The high dispersion of Fe on MgO support should be ascribed to the high activity in the wet oxidation of H2S to sulfur.
Fig. 5. TPR profiles of (a) Fe/MgO, (b) Fe/SiO2, (c) Fe/ZrO2, and (d) Fe/ Al2O3.
This is consistent with the XRD results in Fig. 3 showing the strong peak intensity due to a-Fe2O3 crystallites. In the spectra of both Fe/SiO2 and Fe/ZrO2 catalysts at 293 K, the signals are composed of two components: a doublet and a magnetic-split sextet. The doublet is attributed to highly dispersed Fe3+ species (Fe3+ oxide cluster), while the sextet is assigned to the a-Fe2O3 crystallites [25]. The spectra of the same catalysts are only composed of a sextet when measured at 13 K. These spectra at 13 K confirm the existence of super-paramagnetic Fe oxide particles (Fe3+ oxide cluster). The parameters are close to the values reported for Fe3+ oxide cluster and for a-Fe2O3 crystallites, respectively. The most reliable percentages of each species (doublets and sextets) can be estimated at low temperature, since the probable differences in the recoil-free factors are minimal. Fig. 5 shows the TPR profiles of the supported iron oxide catalysts. For the Fe/MgO sample, the TPR profile shows two peaks at 650 and 850 8C assignable to the Fe3+ cation and MgFe2O4 species, respectively. For the Fe/SiO2 and Fe/ ZrO2 samples, there are two broad peaks at relatively higher temperature compared to those of Fe/MgO. The peak at low temperature can be attributed to Fe3+ oxide clusters and the one at high temperature can be ascribed to a-Fe2O3 crystallites. In the case of Fe/Al2O3 sample, only one peak is observed at 800 8C. This can be assigned to the reduction of a-Fe2O3 crystallites. The XRD and Mo¨ ssbauer spectra support the conclusion that these species are formed for the supported iron oxide catalysts.
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