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Mesoporous activated alumina layers deposited on FeCrAl metallic substrates by an in situ hydrothermal method
Journal of Alloys and Compounds 396 (2005) 283–287
Mesoporous activated alumina layers deposited on FeCrAl metallic substrates by an in situ hydrothermal method Qi Wei, Zeng-Xiang Chen, Zuo-Ren Nie∗ , Ya-Li Hao, Jing-Xia Zou, Zhi-Hong Wang College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100022, PR China Received 30 November 2004; received in revised form 20 December 2004; accepted 21 December 2004 Available online 1 February 2005
Abstract Mesoporous activated alumina was prepared by a surfactant-templating route and the alumina layers were deposited on FeCrAl metallic substrates by an in situ hydrothermal method. The phase composition, pore structure and surface morphology of activated alumina layers were studied by means of small and wide angle X-ray diffraction, nitrogen adsorption and transmission electron microscopy, as well as fieldemission scanning electron microscopy. The resultant activated alumina layers possess a surface area and a pore volume much higher than that of sol–gel-derived alumina, and the activated alumina layers were deposited successfully on the substrates after hydrothermal reaction and heat treatment. © 2004 Elsevier B.V. All rights reserved. Keywords: Mesoporous activated alumina; FeCrAl substrate; In situ hydrothermal method; Surface area; Porosity
1. Introduction Activated alumina is an interesting material with a broad range of utilities. In catalysis, for example, it can be used as catalyst carriers in an automotive catalytic converter [1,2]. The application of alumina in catalyst carriers is to a great extent basically relying upon its surface area and porosity [3]. Since the discovery of ordered mesoporous silica molecular sieves (MCM-41) by Mobil group in 1992 [4], mesostructured alumina powders or gels with higher surface area and porosity than those of the traditional alumina have been successfully prepared by the supramolecule-templating route as used in the case of MCM-41. Cabrera et al. prepared mesostructured alumina using cationic cetyltrimethylammonium bromide as a surfactant-directing agent in a water/triethanolamine medium [5]. Mesoporous alumina with a surface area larger than 300 m2 /g has been prepared by gel synthesis containing non-ionic surfactants in the presence ∗
Corresponding author. Tel.: +86 10 67391536; fax: +86 10 67391536. E-mail address: [email protected] (Z.-R. Nie).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.12.030
of dipropylamine [3]. Zhao et al. synthesized mesoporous MSU-X alumina molecular sieves through the neutral N0 I0 assembly using aluminum sec-butoxide as the precursor and triblock copolymer as the structure-directing agent [6]. In contrast to the extensive investigation on the alumina powders or gels, less attention has been paid, to the best of our knowledge, to the supramolecule-templating alumina layers deposited on ceramic, glass or metal substrates. It is well known that the most important unit of an automotive catalytic converter is the honeycomb monolith made of ceramic or metal and the honeycomb should be coated with activated alumina layers impregnated with active catalysts before it is used to purify automobile exhaust [7]. Acting as a honeycomb, FeCrAl metals have a great advantage over cordierite ceramics due to their high thermal conductivity and heat capacity [8], so it is of considerable significance if supramolecule-templating alumina layers can be coated on FeCrAl metallic substrates. In this paper, we try to deposit supramolecule-templating alumina layers on FeCrAl metallic substrates by an in situ hydrothermal method and investigate the materials by means of X-ray diffraction, nitrogen
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adsorption and transmission electron microscopy. As far as we know, similar research has not been reported elsewhere so far.
2. Experimental 2.1. FeCrAl substrates pre-treatment The FeCrAl substrates should be pre-treated in order to remove impurities and obtain superficial oxides on their surface. The superficial oxides were proved to favor the adherence of deposited alumina layers to FeCrAl substrates because they have a thermal expansion coefficient close to that of activated alumina [9]. The FeCrAl alloy foils (Fe: 75 wt.%, Cr: 20 wt.%, Al: 5 wt.%) were first ultrasonically cleaned in 1 M NaOH, 0.5 M HCl and distilled water in sequence, each for 15 min, and then calcined at 900 ◦ C for 12 h. After calcination, the substrates were again ultrasonically cleaned in toluene and distilled water for 15 min, respectively. The substrates were immersed in a 0.1 M surfactant (cetyltrimethylammonium, CTAB) solution for 5 h at room temperature before they are coated with activated alumina layers. 2.2. Deposition of activated alumina layers The activated alumina layers were prepared by surfactanttemplating route as described by Cabrera et al. [5] and the corresponding layers were deposited on FeCrAl substrates by an in situ hydrothermal method. Sodium hydroxide (NaOH, 0.4 g) was dissolved in 2 ml distilled water and then the NaOH solution was added to 40 ml triethanolamine (TEA) and heated at 393 K for 5 min to evaporate the water. Aluminum sec-butoxide (10.372 g) was added dropwise into the TEA solution with stirring and heated at 423 K for 10 min to obtain solution I. Cetyltrimethylammonium (CTAB, 7.28 g) was dissolved in 120 ml H2 O at room temperature to form solution II. Solution I was slowly added to solution II with vigorous stirring at 333 K, and the mixture was stirred for another 5 h. Subsequently, the mixture was brought into a Teflon-lined autoclave and the surfactant-coated FeCrAl substrates were placed horizontally in the autoclave. After hydrothermal reaction at 343 K for 48 h, the reactant was filtered, washed with ethanol and dried in air. Finally the gels and the coated FeCrAl substrates were calcined at different temperatures for 6 h.
scanned from 0.6 to 6◦ , using Cu K␣ radiation. Nitrogen adsorption was measured with Micromeritics ASAP 2020 at −196 ◦ C. Before analysis, the samples were first degassed at 250 ◦ C for 5 h. The surface area was calculated according to the BET equation at a relative pressure ranging from 0.05 to 0.20 and the pore size distribution was obtained from the desorption branch of the isotherms using the BJH approach. The morphology of the alumina-coated FeCrAl materials and the alumina powders was examined by field-emission scanning electron microscopy (Qutan 200) and transmission electron microscopy (JEOL JEM-2010), respectively.
3. Results and discussion The wide angle X-ray diffraction patterns of the alumina layers are shown in Fig. 1. As can be seen from Fig. 1, the samples sintered at 400 ◦ C are composed of activated ␥-Al2 O3 phase, as demonstrated by two typical peaks appearing at the 2θ positions of 45◦ and 67◦ , respectively. Upon further heat treatment up to 800 ◦ C, the ␥-Al2 O3 phase is still retained and the intensity increases with increasing sintering temperatures. When the sintering temperature increases to 1000 ◦ C, the samples begin to transform into stable ␣-Al2 O3 , which suggests that the materials tend to loss their active properties. Therefore, the optimal sintering temperature should be limited to a range from 600 to 800 ◦ C in terms of phase composition and the consideration that most catalytic reactions occur in this temperature range. Fig. 2 shows the N2 adsorption–desorption isotherm and pore size distribution of the alumina sintered at 600 ◦ C. The isotherm can be categorized into type IV, characteristic of typical mesoporous materials. The steep increase in N2 volume adsorbed at a relative pressure of about 0.6 indicates a uniform mesopore size, which is further confirmed by the pore size distribution curve (insert, Fig. 2) with a narrow peak at a pore diameter of about 5 nm. The hysteresis loop suggests the existence of some necking in the pore structure.
2.3. Characterization It is difficult to directly examine the phase composition and pore structure of the alumina layers deposited on the FeCrAl substrates, so the activated alumina powder was used as a substitute to be studied. The phase composition was investigated by wide angle X-ray diffraction (Bruker D8/advance) with a scanning rate of 1.5◦ /min and a step of 0.02◦ . The order of pore arrangement was investigated by small angle X-ray diffraction with a resolution of 0.02◦ and a rate of 0.5◦ /min,
Fig. 1. XRD patterns of alumina layers sintered at different temperatures.
Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
285
Fig. 2. N2 adsorption–desorption isotherm and pore size distribution (insert) of the alumina layers sintered at 600 ◦ C.
The pore structure data of the materials sintered from 400 to 800 ◦ C are listed in Table 1. Both the surface area and the pore volume decrease as the sintering temperature increases from 400 to 1000 ◦ C. The loss of surface area and pore volume may be attributed to the higher sintering temperature of the samples because sintering is a process of densification driven by the tendency of reducing the internal energy [10,11]. The phase transformation of activated ␥-Al2 O3 to stable ␣-Al2 O3 is also responsible for the decrease of both the surface area and the pore volume. The alumina prepared by a hydrothermal method have a much higher surface area (360.1 m2 /g, sintered at 600 ◦ C) than the samples synthesized by a sol–gel method (266.8 m2 /g, 600 ◦ C) [12], which indicates that the alumina by the former method is more activated than the traditionally sol–gel-derived alumina. The pore volume is also much larger than that of the sol–gel-derived alumina, which may be due to the cavities left behind after the organic additives (CTAB and TEA) in the mixture are removed by calcination. Therefore, the alumina layers prepared by the hydrothermal method are more desirable for the use as catalyst support in mobile exhaust purifiers than sol–gel-derived alumina. In order to examine the order of the pore arrangement of the alumina, small angle X-ray diffraction is employed to detect the structure of this material, and the result is depicted in Fig. 3. The small angle XRD pattern, with only one illdefined and relatively broad peak at a 2θ value of 1.5◦ , is
much similar to that of typical disordered HMS, MSU silica materials [13,14] and to disordered mesoporous alumina materials (denoted as MSU-X) with non-ionic surfactants as templates [15]. The disordered pore arrangement is also confirmed by the TEM image (Fig. 3), in which the pore packing motif seems to be wormhole-like. The disordered pore structure may result from the failure of the surfactant templates to self-assemble into an ordered micelles structure as in the case of M41S families. It is fortunate that the pore arrangement has no unfavorable influence on the use of the mesoporous alumina as catalyst carriers. The pore size measured by TEM is about 5 nm, in good agreement with that determined from N2 adsorption (Fig. 4). The surface morphology of the FeCrAl substrates and the alumina layers are shown in Fig. 5. For the as-prepared samples, almost all the substrate surfaces have been covered by the alumina monoliths with triangular or rectangular shape (Fig. 5b). A number of cracks are also observed, and the occurrence of these cracks may be attributed to the difference in thermal expansion behavior between the FeCrAl substrates
Table 1 Pore structure data of mesoporous alumina layers sintered at different temperatures Temperature (◦ C)
Surface area (m2 /g)
Pore volume (cm3 /g)
Mean pore diameter (nm)
400 600 800 1000
423.96 360.12 215.23 117.01
0.47 0.54 0.38 0.32
4.47 6.03 7.10 11.10
Fig. 3. Small angle XRD pattern of the mesoporous alumina layers sintered at 600 ◦ C.
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Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
into aluminum hydroxide grains and finally the grains grow closely to form a layer. The final mesoporous alumina layers will be obtained after the surfactants and the residual TEA as well as water are removed by calcination. Unfortunately, the adherence of the alumina layers to the substrates is not tight enough. As a consequence, some alumina monoliths begin to peel off from the substrates after ultrasonical cleaning for 10 min (Fig. 5c), and furthermore this tendency seems to continue as the cleaning time is extended to 20 min (Fig. 5d). Further study is necessarily required to explore a method to improve the coating adhesion between the alumina layers and the FeCrAl substrates.
4. Conclusion
Fig. 4. TEM image of the mesoporous alumina layers sintered at 600 ◦ C.
and the alumina layers. The in situ formation mechanism of alumina layers might be described by the following. First, the surfactant (CTAB) molecules self-organize into micelles and adhere to the surface of the FeCrAl substrates, and then aluminum moieties aggregate around the surfactant micelles due to the electrostatic interaction during the hydrolysis and condensation process of the aluminum sec-butoxide. Under hydrothermal condition, the aluminum species grow
Mesoporous activated alumina layers were deposited successfully on FeCrAl metallic substrates by an in situ hydrothermal method. Although the pore arrangement seems to be disordered, the alumina sintered at 600 ◦ C has a surface area of 360.1 m2 /g and a pore volume of 0.54 cm3 /g, much higher than sol–gel-derived alumina. The coverage of alumina layers on the substrates is considerably high, but further effort is necessarily required to improve the adherence between the top alumina layers and the FeCrAl substrates.
Acknowledgements The financial support of Doctoral Foundation of Beijing University of Technology (Grant No. KZ090200378) and
Fig. 5. SEM images of the FeCrAl substrate (a) and the alumina layers after ultrasonical cleaning for (b) 0 min, (c) 10 min, (d) 20 min.
Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
Beijing Foundation of New Star (Grant No. 953810200) is gratefully acknowledged.
References [1] R.D. Monte, P. Fornasiero, J. Kaspar, M. Graziani, J.M. Gatica, S. Bernal, A.G. Herrero, Chem. Commun. (2000) 2167–2168. [2] A. Piras, A. Trovarelli, G. Dolcetti, Appl. Catal. B 28 (2000) L77–L80. [3] V.G. Pena, I. Diaz, C.M. Alvarez, E. Sastre, J.P. Pariente, Micropor. Mesopor. Mater. 44/45 (2001) 203–210. [4] C.T. Kresge, M.E. Lenowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [5] S. Cabrera, J.E. Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. Dolores, P. Amoros, Adv. Mater. 11 (1999) 379–381.
287
[6] X.P. Zhao, Y.H. Yue, Y. Zhang, W.M. Hua, Z. Gao, Catal. Lett. 89 (2003) 41–47. [7] J. Kaspar, P. Fornasiero, N. Hickey, Catal. Today 77 (2003) 419– 449. [8] H. Bode (Ed.), Materials Aspects in Automotive Catalytic Converters, Wiley/VCH, Weinheim, Germany, 2002, pp. 1–281. [9] S. Zhao, J.Z. Zhang, D. Weng, X.D. Wu, Surf. Coat. Technol. 167 (2003) 97–105. [10] Q. Wei, D.W. Wang, S.G. Zhang, C.S. Chen, J. Alloys Comp. 325 (2001) 223–229. [11] J. Kim, Y.S. Lin, J. Membr. Sci. 139 (1998) 75–79. [12] Q. Wei, Z.X. Chen, Z.H. Wang, Y.L. Hao, J.X. Zou, Z.R. Nie, J. Alloys Comp. 387 (2005) 292–296. [13] R.J.P. Corrius, A. Mehdi, C. Reye, C. Thieuleux, Chem. Mater. 16 (2004) 159–166. [14] R. Richer, L. Mercier, Chem. Mater. 13 (2001) 2999–3008. [15] W.Z. Zhang, T.J. Pinnavaia, Chem. Commun. (1999) 1185–1186.
Mesoporous activated alumina layers deposited on FeCrAl metallic substrates by an in situ hydrothermal method Qi Wei, Zeng-Xiang Chen, Zuo-Ren Nie∗ , Ya-Li Hao, Jing-Xia Zou, Zhi-Hong Wang College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100022, PR China Received 30 November 2004; received in revised form 20 December 2004; accepted 21 December 2004 Available online 1 February 2005
Abstract Mesoporous activated alumina was prepared by a surfactant-templating route and the alumina layers were deposited on FeCrAl metallic substrates by an in situ hydrothermal method. The phase composition, pore structure and surface morphology of activated alumina layers were studied by means of small and wide angle X-ray diffraction, nitrogen adsorption and transmission electron microscopy, as well as fieldemission scanning electron microscopy. The resultant activated alumina layers possess a surface area and a pore volume much higher than that of sol–gel-derived alumina, and the activated alumina layers were deposited successfully on the substrates after hydrothermal reaction and heat treatment. © 2004 Elsevier B.V. All rights reserved. Keywords: Mesoporous activated alumina; FeCrAl substrate; In situ hydrothermal method; Surface area; Porosity
1. Introduction Activated alumina is an interesting material with a broad range of utilities. In catalysis, for example, it can be used as catalyst carriers in an automotive catalytic converter [1,2]. The application of alumina in catalyst carriers is to a great extent basically relying upon its surface area and porosity [3]. Since the discovery of ordered mesoporous silica molecular sieves (MCM-41) by Mobil group in 1992 [4], mesostructured alumina powders or gels with higher surface area and porosity than those of the traditional alumina have been successfully prepared by the supramolecule-templating route as used in the case of MCM-41. Cabrera et al. prepared mesostructured alumina using cationic cetyltrimethylammonium bromide as a surfactant-directing agent in a water/triethanolamine medium [5]. Mesoporous alumina with a surface area larger than 300 m2 /g has been prepared by gel synthesis containing non-ionic surfactants in the presence ∗
Corresponding author. Tel.: +86 10 67391536; fax: +86 10 67391536. E-mail address: [email protected] (Z.-R. Nie).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.12.030
of dipropylamine [3]. Zhao et al. synthesized mesoporous MSU-X alumina molecular sieves through the neutral N0 I0 assembly using aluminum sec-butoxide as the precursor and triblock copolymer as the structure-directing agent [6]. In contrast to the extensive investigation on the alumina powders or gels, less attention has been paid, to the best of our knowledge, to the supramolecule-templating alumina layers deposited on ceramic, glass or metal substrates. It is well known that the most important unit of an automotive catalytic converter is the honeycomb monolith made of ceramic or metal and the honeycomb should be coated with activated alumina layers impregnated with active catalysts before it is used to purify automobile exhaust [7]. Acting as a honeycomb, FeCrAl metals have a great advantage over cordierite ceramics due to their high thermal conductivity and heat capacity [8], so it is of considerable significance if supramolecule-templating alumina layers can be coated on FeCrAl metallic substrates. In this paper, we try to deposit supramolecule-templating alumina layers on FeCrAl metallic substrates by an in situ hydrothermal method and investigate the materials by means of X-ray diffraction, nitrogen
284
Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
adsorption and transmission electron microscopy. As far as we know, similar research has not been reported elsewhere so far.
2. Experimental 2.1. FeCrAl substrates pre-treatment The FeCrAl substrates should be pre-treated in order to remove impurities and obtain superficial oxides on their surface. The superficial oxides were proved to favor the adherence of deposited alumina layers to FeCrAl substrates because they have a thermal expansion coefficient close to that of activated alumina [9]. The FeCrAl alloy foils (Fe: 75 wt.%, Cr: 20 wt.%, Al: 5 wt.%) were first ultrasonically cleaned in 1 M NaOH, 0.5 M HCl and distilled water in sequence, each for 15 min, and then calcined at 900 ◦ C for 12 h. After calcination, the substrates were again ultrasonically cleaned in toluene and distilled water for 15 min, respectively. The substrates were immersed in a 0.1 M surfactant (cetyltrimethylammonium, CTAB) solution for 5 h at room temperature before they are coated with activated alumina layers. 2.2. Deposition of activated alumina layers The activated alumina layers were prepared by surfactanttemplating route as described by Cabrera et al. [5] and the corresponding layers were deposited on FeCrAl substrates by an in situ hydrothermal method. Sodium hydroxide (NaOH, 0.4 g) was dissolved in 2 ml distilled water and then the NaOH solution was added to 40 ml triethanolamine (TEA) and heated at 393 K for 5 min to evaporate the water. Aluminum sec-butoxide (10.372 g) was added dropwise into the TEA solution with stirring and heated at 423 K for 10 min to obtain solution I. Cetyltrimethylammonium (CTAB, 7.28 g) was dissolved in 120 ml H2 O at room temperature to form solution II. Solution I was slowly added to solution II with vigorous stirring at 333 K, and the mixture was stirred for another 5 h. Subsequently, the mixture was brought into a Teflon-lined autoclave and the surfactant-coated FeCrAl substrates were placed horizontally in the autoclave. After hydrothermal reaction at 343 K for 48 h, the reactant was filtered, washed with ethanol and dried in air. Finally the gels and the coated FeCrAl substrates were calcined at different temperatures for 6 h.
scanned from 0.6 to 6◦ , using Cu K␣ radiation. Nitrogen adsorption was measured with Micromeritics ASAP 2020 at −196 ◦ C. Before analysis, the samples were first degassed at 250 ◦ C for 5 h. The surface area was calculated according to the BET equation at a relative pressure ranging from 0.05 to 0.20 and the pore size distribution was obtained from the desorption branch of the isotherms using the BJH approach. The morphology of the alumina-coated FeCrAl materials and the alumina powders was examined by field-emission scanning electron microscopy (Qutan 200) and transmission electron microscopy (JEOL JEM-2010), respectively.
3. Results and discussion The wide angle X-ray diffraction patterns of the alumina layers are shown in Fig. 1. As can be seen from Fig. 1, the samples sintered at 400 ◦ C are composed of activated ␥-Al2 O3 phase, as demonstrated by two typical peaks appearing at the 2θ positions of 45◦ and 67◦ , respectively. Upon further heat treatment up to 800 ◦ C, the ␥-Al2 O3 phase is still retained and the intensity increases with increasing sintering temperatures. When the sintering temperature increases to 1000 ◦ C, the samples begin to transform into stable ␣-Al2 O3 , which suggests that the materials tend to loss their active properties. Therefore, the optimal sintering temperature should be limited to a range from 600 to 800 ◦ C in terms of phase composition and the consideration that most catalytic reactions occur in this temperature range. Fig. 2 shows the N2 adsorption–desorption isotherm and pore size distribution of the alumina sintered at 600 ◦ C. The isotherm can be categorized into type IV, characteristic of typical mesoporous materials. The steep increase in N2 volume adsorbed at a relative pressure of about 0.6 indicates a uniform mesopore size, which is further confirmed by the pore size distribution curve (insert, Fig. 2) with a narrow peak at a pore diameter of about 5 nm. The hysteresis loop suggests the existence of some necking in the pore structure.
2.3. Characterization It is difficult to directly examine the phase composition and pore structure of the alumina layers deposited on the FeCrAl substrates, so the activated alumina powder was used as a substitute to be studied. The phase composition was investigated by wide angle X-ray diffraction (Bruker D8/advance) with a scanning rate of 1.5◦ /min and a step of 0.02◦ . The order of pore arrangement was investigated by small angle X-ray diffraction with a resolution of 0.02◦ and a rate of 0.5◦ /min,
Fig. 1. XRD patterns of alumina layers sintered at different temperatures.
Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
285
Fig. 2. N2 adsorption–desorption isotherm and pore size distribution (insert) of the alumina layers sintered at 600 ◦ C.
The pore structure data of the materials sintered from 400 to 800 ◦ C are listed in Table 1. Both the surface area and the pore volume decrease as the sintering temperature increases from 400 to 1000 ◦ C. The loss of surface area and pore volume may be attributed to the higher sintering temperature of the samples because sintering is a process of densification driven by the tendency of reducing the internal energy [10,11]. The phase transformation of activated ␥-Al2 O3 to stable ␣-Al2 O3 is also responsible for the decrease of both the surface area and the pore volume. The alumina prepared by a hydrothermal method have a much higher surface area (360.1 m2 /g, sintered at 600 ◦ C) than the samples synthesized by a sol–gel method (266.8 m2 /g, 600 ◦ C) [12], which indicates that the alumina by the former method is more activated than the traditionally sol–gel-derived alumina. The pore volume is also much larger than that of the sol–gel-derived alumina, which may be due to the cavities left behind after the organic additives (CTAB and TEA) in the mixture are removed by calcination. Therefore, the alumina layers prepared by the hydrothermal method are more desirable for the use as catalyst support in mobile exhaust purifiers than sol–gel-derived alumina. In order to examine the order of the pore arrangement of the alumina, small angle X-ray diffraction is employed to detect the structure of this material, and the result is depicted in Fig. 3. The small angle XRD pattern, with only one illdefined and relatively broad peak at a 2θ value of 1.5◦ , is
much similar to that of typical disordered HMS, MSU silica materials [13,14] and to disordered mesoporous alumina materials (denoted as MSU-X) with non-ionic surfactants as templates [15]. The disordered pore arrangement is also confirmed by the TEM image (Fig. 3), in which the pore packing motif seems to be wormhole-like. The disordered pore structure may result from the failure of the surfactant templates to self-assemble into an ordered micelles structure as in the case of M41S families. It is fortunate that the pore arrangement has no unfavorable influence on the use of the mesoporous alumina as catalyst carriers. The pore size measured by TEM is about 5 nm, in good agreement with that determined from N2 adsorption (Fig. 4). The surface morphology of the FeCrAl substrates and the alumina layers are shown in Fig. 5. For the as-prepared samples, almost all the substrate surfaces have been covered by the alumina monoliths with triangular or rectangular shape (Fig. 5b). A number of cracks are also observed, and the occurrence of these cracks may be attributed to the difference in thermal expansion behavior between the FeCrAl substrates
Table 1 Pore structure data of mesoporous alumina layers sintered at different temperatures Temperature (◦ C)
Surface area (m2 /g)
Pore volume (cm3 /g)
Mean pore diameter (nm)
400 600 800 1000
423.96 360.12 215.23 117.01
0.47 0.54 0.38 0.32
4.47 6.03 7.10 11.10
Fig. 3. Small angle XRD pattern of the mesoporous alumina layers sintered at 600 ◦ C.
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Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
into aluminum hydroxide grains and finally the grains grow closely to form a layer. The final mesoporous alumina layers will be obtained after the surfactants and the residual TEA as well as water are removed by calcination. Unfortunately, the adherence of the alumina layers to the substrates is not tight enough. As a consequence, some alumina monoliths begin to peel off from the substrates after ultrasonical cleaning for 10 min (Fig. 5c), and furthermore this tendency seems to continue as the cleaning time is extended to 20 min (Fig. 5d). Further study is necessarily required to explore a method to improve the coating adhesion between the alumina layers and the FeCrAl substrates.
4. Conclusion
Fig. 4. TEM image of the mesoporous alumina layers sintered at 600 ◦ C.
and the alumina layers. The in situ formation mechanism of alumina layers might be described by the following. First, the surfactant (CTAB) molecules self-organize into micelles and adhere to the surface of the FeCrAl substrates, and then aluminum moieties aggregate around the surfactant micelles due to the electrostatic interaction during the hydrolysis and condensation process of the aluminum sec-butoxide. Under hydrothermal condition, the aluminum species grow
Mesoporous activated alumina layers were deposited successfully on FeCrAl metallic substrates by an in situ hydrothermal method. Although the pore arrangement seems to be disordered, the alumina sintered at 600 ◦ C has a surface area of 360.1 m2 /g and a pore volume of 0.54 cm3 /g, much higher than sol–gel-derived alumina. The coverage of alumina layers on the substrates is considerably high, but further effort is necessarily required to improve the adherence between the top alumina layers and the FeCrAl substrates.
Acknowledgements The financial support of Doctoral Foundation of Beijing University of Technology (Grant No. KZ090200378) and
Fig. 5. SEM images of the FeCrAl substrate (a) and the alumina layers after ultrasonical cleaning for (b) 0 min, (c) 10 min, (d) 20 min.
Q. Wei et al. / Journal of Alloys and Compounds 396 (2005) 283–287
Beijing Foundation of New Star (Grant No. 953810200) is gratefully acknowledged.
References [1] R.D. Monte, P. Fornasiero, J. Kaspar, M. Graziani, J.M. Gatica, S. Bernal, A.G. Herrero, Chem. Commun. (2000) 2167–2168. [2] A. Piras, A. Trovarelli, G. Dolcetti, Appl. Catal. B 28 (2000) L77–L80. [3] V.G. Pena, I. Diaz, C.M. Alvarez, E. Sastre, J.P. Pariente, Micropor. Mesopor. Mater. 44/45 (2001) 203–210. [4] C.T. Kresge, M.E. Lenowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [5] S. Cabrera, J.E. Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. Dolores, P. Amoros, Adv. Mater. 11 (1999) 379–381.
287
[6] X.P. Zhao, Y.H. Yue, Y. Zhang, W.M. Hua, Z. Gao, Catal. Lett. 89 (2003) 41–47. [7] J. Kaspar, P. Fornasiero, N. Hickey, Catal. Today 77 (2003) 419– 449. [8] H. Bode (Ed.), Materials Aspects in Automotive Catalytic Converters, Wiley/VCH, Weinheim, Germany, 2002, pp. 1–281. [9] S. Zhao, J.Z. Zhang, D. Weng, X.D. Wu, Surf. Coat. Technol. 167 (2003) 97–105. [10] Q. Wei, D.W. Wang, S.G. Zhang, C.S. Chen, J. Alloys Comp. 325 (2001) 223–229. [11] J. Kim, Y.S. Lin, J. Membr. Sci. 139 (1998) 75–79. [12] Q. Wei, Z.X. Chen, Z.H. Wang, Y.L. Hao, J.X. Zou, Z.R. Nie, J. Alloys Comp. 387 (2005) 292–296. [13] R.J.P. Corrius, A. Mehdi, C. Reye, C. Thieuleux, Chem. Mater. 16 (2004) 159–166. [14] R. Richer, L. Mercier, Chem. Mater. 13 (2001) 2999–3008. [15] W.Z. Zhang, T.J. Pinnavaia, Chem. Commun. (1999) 1185–1186.