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An efficient strategy for highly loaded, well dispersed and thermally stable metal oxide catalysts
Catalysis Communications 12 (2011) 1075–1078
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
An efficient strategy for highly loaded, well dispersed and thermally stable metal oxide catalysts Changjin Tang a, Hongliang Zhang a, Chuanzhi Sun a, Jianchao Li a, Lei Qi a, Yangjian Quan c, Fei Gao b,⁎, Lin Dong a,b,⁎⁎ a b c
School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093, P.R. China Center of Modern Analysis, Nanjing University, Nanjing 210093, P.R. China Kuang Yaming Honors School, Nanjing University, Nanjing 210093, P.R. China
a r t i c l e
i n f o
Article history: Received 18 January 2011 Received in revised form 16 March 2011 Accepted 19 March 2011 Available online 26 March 2011 Keywords: Supported catalysts SBA-15 Metal oxides High loadings Homogeneous dispersion High thermal stability
a b s t r a c t A family of metal oxides (Co3O4, NiO and CeO2) confined in SBA-15 with high loadings (≥ 20 wt%) was prepared through a solvent-free method. Characterizations of X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that aggregation-free nanoparticles were obtained and N2 physisorption confirmed they were studded in mesopores. It was proposed that the intermediate molten salt phases ensured successful encapsulation and homogeneous dispersion of metal oxides. Lastly, the importance of the strategy was exemplified by NiO, and the high thermal stability together with superior performance in hydrodechlorination of chlorobenzene suggested great potential of these samples in heterogeneous catalysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The preparation of supported metal (oxide) catalysts with high loadings and homogenous dispersion is a field of great interest from both the industrial and fundamental points of view mainly because of their numerous active sites and hence remarkable performances in the production of chemicals, fuels and elimination of environment pollutants [1,2]. Conventionally, to suppress sintering and avoid leaching of the active species, the metals or their oxides are deposited onto an inert and thermostable support like Al2O3, SiO2 or activated carbon. Compared to solid supports, the supports with porous or hollow structures are preferable, as they usually possess larger surface areas and hence can provide more space for immobilization. More importantly, it is expected that these peculiar structures could offer confined environment to inhibit significant growth of guest species [3] and function as unique reactor for superior performance [4]. As a representative porous silica, SBA-15 with tunable pore diameters, thick pore walls and large surface areas is a desirable candidate for loading guest species [5]. Therefore, extensive researches have been focused on encapsulating nanoparticles into the pore
⁎ Correspondence to: L. Dong, School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093, P.R. China. Tel.: + 86 25 83592290; fax: + 86 25 83317761. ⁎⁎ Corresponding author. Tel.: + 86 25 83592290; fax: + 86 25 83317761. E-mail addresses: [email protected] (F. Gao), [email protected] (L. Dong). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.03.031
channels of SBA-15 to obtain thermostable nanocatalysts. Briefly, two routes are mainly developed to approach the goal. One is grafting method [6–8]. This route allows the effective fabrication of monodisperse noble metals embedded in SBA-15 tunnels. However, the drawback is also obvious. The low loadings (b5 wt%) together with complex processes will limit its further scale up. Another way is wet impregnation. Seemingly facile as the practical execution is, the fundamental phenomena underlying impregnation and drying are extremely complex. More often than not the interactions between precursor and inert support are limited, thereby causing partial migration of guest species with the evaporation of solvent and allowing redistribution of the active phase over the support [9]. As a result, the control over particle size is poor without special treatment [10,11]. From the discussion above, it is known that the efficient preparation of aggregation-free nanoparticles encapsulated in SBA15 is of great importance in heterogeneous catalysis but still remains a challenge, especially for high loadings. Hence, in the present study, a solvent-free method is extended to fabricate supported metal oxides with high loadings (≥20 wt%) and narrow particle size distribution (PSD). We first investigate the efficiency of solvent-free method for preparing a family of highly loaded metal oxides confined in SBA-15. Then, based on characterization results, the homogeneous dispersion of active species in the pores derived from the absence of solvent was emphasized. Lastly, the advantage of the present synthesis in catalysis is exemplified by that of NiO, and the high thermal stability together with superior performance in hydrodechlorination of chlorobenzene
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demonstrate great potential of these catalysts in heterogeneous catalysis. 2. Experimental 2.1. Preparation of SBA-15 All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., China (except P123 from Aldrich) and used as received. Synthesis of SBA-15 follows Zhao's process [5]. Basically, 4 g of P123 was added to 120 ml of 2 M HCl aqueous solution with stirring, followed by 30 ml H2O. After the surfactant was dissolved, 9 g of TEOS was added and the solution was stirring for 24 h at 40 °C. Then the solution was transferred into an autoclave, sealed and aged at 100 °C for 24 h. The resulting white precipitate was filtrated and washed with de-ionized water and ethanol for several times. The rinsed powder was dried and air-calcined at 550 °C for 5 h with a ramp rate of 1 °C min− 1. 2.2. Synthesis of mesopore confined metal oxide catalysts The solvent-free method to pore confined metal oxide catalysts was a simple and time-conserving process, as it only comprised manual grinding of the mixture of metal precursor and SBA-15 and the subsequent air-calcination. The precursors for Co, Ni, Ce are Co (NO3)2·6H2O, Ni(NO3)2·6H2O, Ce(NO3)3·6H2O, respectively. In a typical synthesis, calculated metal nitrates (Co: 0.091 g, Ni: 0.091 g, Ce: 0.1357 g) were manually ground with 0.1 g SBA-15 in an agate mortar for about 10 min. Then the homogeneously mixed powder was transferred into a crucible, which was placed in the muffle furnace. The thermal treatment in air started at room temperature with a ramp of 1 °C min− 1 to 450 °C and maintained at that temperature for 4 h, then cooled naturally. The obtained samples were denoted as MxOy@SBA-15, where M represented the metals. As expected, there are almost no loss of metal oxides and for Co3O4@SBA-15, NiO@SBA-15 and CeO2@SBA-15, the nominal loadings were 20 wt%, 20 wt% and 35 wt%, respectively. To determine the thermal stability of NiO, the NiO@SBA-15 was further calcined at higher temperatures (550 °C and 650 °C) for 4 h. As a comparison, the conventional wet impregnation route to supported NiO sample was employed. The precursor, calcination temperature and loading were identical to that of NiO@SBA-15 and the final product was named as NiO/SBA-15. 2.3. Characterization The powder X-ray diffraction patterns of samples were collected on a Philips X'pert X-ray diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15418 nm).The X-ray tube was operated at 40 kV and 40 mA. Nitrogen physisorption was measured at − 196 °C using a Micromeritics ASAP 2020 system. The samples were degassed for 160 min at 300 °C in the degas port of the adsorption analyzer. The mesoporosity was determined from BJH method. Transmission electron microscopy (TEM) images were taken on a JEM-2100 instrument at an acceleration voltage of 200 kV. The samples were crushed and dispersed in A.R. grade ethanol and the resulting suspensions were allowed to dry on carbon film supported on copper grids. 2.4. Evaluation of catalytic performance The gas phase hydrodechlorination (HDC) of chlorobenzene to benzene was carried out in a continuous fixed-bed reactor at atmospheric pressure. The prepared catalyst (27.5 mg, 60–80 mesh) was placed in a tubular quartz reactor between two quartz plugs and a layer of glass beads above the catalyst bed to ensure that the reactants
Fig. 1. The small-angle XRD patterns of pristine SBA-15, Co3O4@SBA-15, NiO@SBA-15 and CeO2@SBA-15.
reached the reaction temperature before contact with the catalyst. Before starting the reaction, the catalyst was activated at 400 °C for 2 h in hydrogen at a flow rate of 40 ml min− 1 and then cooled to the reaction temperature (300 °C). Chlorobenzene (0.12 ml h− 1) was fed into the reactor using a microfeeder and the hydrogen flow was maintained by a mass flow controller. Product analysis was performed on an on-line gas chromatograph with flame ionization detector. 3. Results and discussion Fig. 1 shows the small-angle XRD patterns of MxOy@SBA-15 samples. As comparison, the pattern of pristine SBA-15 is displayed. For SBA-15, four peaks corresponding to the (100), (110), (200) and (210) interplanar diffractions of the hexagonally arranged structures are displayed, indicating high periodicity of the mesopores. After introduction of metal oxides, the (210) peak is vanished but the (100), (110) and (200) peaks are still displayed, despite their weakened intensity as compared to the pristine SBA-15. It tells us that the ordered mesostructures do not suffer from severe damage during grinding and the subsequent calcination. Taking into account the involvement of guest species, the weakening of diffraction peaks is probably due to worse scattering contrasts between silica walls and the encapsulated pores, which is a common practice for occluded ordered mesoporous materials [12,13]. In contrast to reduced peak intensities, the unit cells of samples (Table 1) are almost not changed, further confirming the ordered mesopores are not much disturbed. Characterization of mesoporosity by N2 physisorption is informative for the determination of guest species in SBA-15 [14] and the results are summarized in Table 1. It can be found that both surface area and pore volume are greatly decreased with the introduction of metal oxides. As small-angle XRD result shows that the ordered mesostructures are preserved, the obvious reduction in surface area and pore volume verifies the insertion of metal oxides into the pores, which qualitatively supports the small-angle XRD deduction and proves that the dry metal nitrate precursors can be driven into the
Table 1 The texture properties of various samples. Sample
Pore volume (cm3 g− 1)
Surface area
Unit cell
Pristine SBA-15 Co3O4@SBA-15 NiO@SBA-15 CeO2@SBA-15 NiO/SBA-15
1.01 0.45 0.30 0.27 0.76
801 316 285 315 534
11.1 11.3 11.3 11.0 11.0
C. Tang et al. / Catalysis Communications 12 (2011) 1075–1078
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Fig. 2. The wide-angle XRD patterns of Co3O4@SBA-15, CeO2@SBA-15, NiO@SBA-15 and NiO/SBA-15.
Fig. 4. The wide-angle XRD patterns of NiO@SBA-15 calcined at different temperatures.
mesopores. Moreover, it is noticeable that the surface area and pore volume of NiO/SBA-15 are lower than NiO@SBA-15, suggesting more NiO are introduced into mesopores by the solvent-free method. The efficiency of solvent-free method to fabricate highly loaded metal oxides in SBA-15 is comprehensively investigated by XRD and TEM characterizations. The wide-angle XRD patterns of samples are presented in Fig. 2. For comparison, the pattern of NiO/SBA-15 is also listed. In general, all samples display peaks which can be indexed to the diffraction peaks of the corresponding metal oxides, and no reflections from other species demonstrate the complete conversion of metal nitrates into metal oxides. Noticeably, for the samples prepared from solvent-free method, the line broadening of diffraction peaks is obvious, suggesting the formation of particles with nanometric dimension. By roughly calculating with Scherrer equation (d = Kλ/βcosθ), the mean grain sizes of these particles are in the range of 4–7 nm. In contrast, for NiO/SBA-15, its diffraction peaks are more
intensive than that of NiO@SBA-15, demonstrating the inferiority of the conventional impregnation method to disperse NiO. The dispersion of metal oxides is further determined by TEM characterization and the result is shown in Fig. 3. Generally, for the samples prepared from solvent-free method, the guest species are well scattered on the support and no bulk or rod-like particles are observed, powerfully demonstrating the advantage of solvent-free method for preparing aggregation-free nanoparticles. However, for NiO/SBA-15, in line with the wide-angle XRD result, the NiO particles are poorly dispersed. There are bulk particles with size of tens of nanometers and they are all deposited on the exterior of SBA-15. Meanwhile, rod-like particles confined in the pores are also observed, indicating the random distribution of guest species through the conventional impregnation method. On the basis of the above characterizations, the dispersion and location of metal oxides from the solvent-free method are
Fig. 3. Typical TEM images of (a) Co3O4@SBA-15, (b) NiO@SBA-15, (c) CeO2@SBA-15 and (d) NiO/SBA-15. The metal oxide nanoparticles are indicated by arrows in the images.
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activity. Catalytic hydrodechlorination (HDC) is one of the most effective routes to remove these pollutants [20]. Hence, to preliminarily evaluate the catalytic application of the prepared samples, the gas phase hydrodechlorination of chlorobenzene to benzene is employed and result is shown in Fig. 5. For both catalysts (NiO@SBA-15 and NiO/SBA-15), benzene is the only organic product and the activities are not changed within 2 h, indicating that steady state is quickly established and nickel-based catalysts are stable. It is clear that the conversion of chlorobenzene to benzene for NiO@SBA15 is much higher than that of NiO/SBA-15. The calculated conversions are ca. 80% and 60% for NiO@SBA-15 and NiO/SBA-15, respectively, showing a great enhancement in performance by the present facile solvent-free method. 4. Conclusions
Fig. 5. The conversions of chlorobenzene to benzene as a function of reaction time for NiO@SBA-15 and NiO/SBA-15.
determined. To obtain supported catalysts with homogeneously distributed active species, it is reported that the suppression of redistribution of precursors during the drying step is an effective way [15–17]. Previously, such strategies like creation of strong interactions between support and precursor and employment of chelating salts or viscosity-increasing agent to increase the viscosity were carried out. In essence, the redistribution is a result of the migration of precursor with the evaporation of solvent under a certain temperature range (80–150 °C). Thus, if the solvent is absent and the precursor can still be inserted into the mesopores without the aid of solvent, the redistribution of precursor should be greatly inhibited. As proposed elsewhere [18] and confirmed by the N2 physisorption result, certain kinds of materials without the presence of solvent can be introduced into the porous matrix through melt infiltration. In the present case, the precursors are metal nitrates and they will turn into molten salt phase when the temperature reaches their melting points. Besides their own unique property of mobility, the molten salts are also viscous, both of which are supposed to be key parameters in determining the uniform dispersion of metal oxide particles confined in the mesoporous support. When increasing the temperature to the melting point of those metal nitrates, the dry metal nitrates are transformed into its molten salt phase and the filling of mesopores starts and proceeds. Probably, with the aid of capillary imbibition [19], all of the molten salts are sucked into the pore channels. In contrast to the widely employed wet impregnation, the absence of solvent evaporation during drying process and the high viscosity of molten salts greatly reduce the redistribution of guest species under thermal treatment. Consequently, uniform and aggregation-free metal oxide nanoparticles confined in the pores are formed when the temperatures arrive to and surpass the decomposition temperatures. To evaluate the effect of pore confinement, the thermal stability of the confined metal oxides is exemplified by that of NiO and the result of wide-angle XRD is shown in Fig. 4. The line broadening of NiO diffraction peaks is still obvious and almost no coarsening of NiO occurred when the thermal treatments are elevated to higher temperatures (550 °C and 650 °C). The result clearly depicts the high thermal stability of the pore-confined nanocatalysts. Chloroaromatics are among the most undesirable industrial effluents because of their persistence and carcinogenic and mutagenic
In this work, a facile solvent-free method is reported to be efficient in preparing highly loaded, well dispersed and thermally stable nanocatalysts. By manually grinding the precursors (metal nitrates) with SBA-15, a family of aggregation-free metal oxides (Co3O4, NiO and CeO2) with high loadings (≥20 wt%) are encapsulated into the mesopores, which endow them with high thermal stability. It is proposed that owing to the absence of solvent, the redistribution of precursors are greatly reduced, which allows the preparation of nanocatalysts with homogenous dispersion. Acknowledgement The financial supports of the National 973 program of China (No.2009CB623500), the National Natural Science Foundation of China (No. 20873060, 20973091) and Nanjing University Talent Development Foundation are gratefully acknowledged. References [1] Z.H. Zhou, S.L. Wang, W.J. Zhou, G.X. Wang, L.H. Jiang, W.Z. Li, S.Q. Song, J.G. Liu, G.Q. Sun, Q. Xin, Chem. Commun. (2003) 394–395. [2] Z.W. Huang, F. Cui, H.X. Kang, J. Chen, X.Z. Zhang, C.G. Xia, Chem. Mater. 20 (2008) 5090–5099. [3] J.M. Sun, D. Ma, H. Zhang, X.M. Liu, X.W. Han, X.H. Bao, G. Weinberg, N. Pfander, D.S. Su, J. Am. Chem. Soc. 128 (2006) 15756–15764. [4] R.Y. Zhang, W. Ding, B. Tu, D.Y. Zhao, Chem. Mater. 19 (2007) 4379–4381. [5] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548–552. [6] Z. Li, C. Kübel, V.I. Pârvulescu, R. Richards, ACS Nano 2 (2008) 1205–1212. [7] M. Boutros, Z. Maoui, H. Sfihi, V. Viossat, A. Gédéon, F. Launay, Micropor. Mesopor. Mater. 108 (2008) 247–2578 90. [8] X.Y. Liu, A.Q. Wang, X.F. Yang, T. Zhang, C.Y. Mou, D.S. Su, J. Li, Chem. Mater. 21 (2009) 410–418. [9] T. Toupance, M. Kermarec, C. Louis, J. Phys. Chem. B 104 (2000) 965–972. [10] M.Y. Cheng, C.J. Pan, B.J. Hwang, J. Mater. Chem. 19 (2009) 5193–5200. [11] J.R.A. Sietsma, J.D. Meeldijk, J.P. den Breejen, M. Versluijs-Helder, A.J. van Dillen, P.E. de Jongh, K.P. de Jong, Angew. Chem. Int. Ed. 46 (2007) 4547–4549. [12] B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Micropor. Mater. 6 (1996) 375–383. [13] J. Sauer, F. Marlow, B. Spliethoff, F. Schuth, Chem. Mater. 14 (2002) 217–224. [14] L. Vradman, M.V. Landau, D. Kantorovich, Y. Koltpin, A. Gedanken, Micropor. Mesopor. Mater. 79 (2005) 307–318. [15] L. Jiao, J.R. Regalbuto, J. Catal. 260 (2008) 329–341. [16] A.J. van Dillen, R.J.A.M. Terorde, D.J. Lensveld, J.W. Geus, K.P. de Jong, J. Catal. 216 (2003) 257–264. [17] M. Kotter, L. Riekert, Stud. Surf. Sci. Catal. 3 (1979) 51–63. [18] P. Dobriyal, H.Q. Xiang, M. Kazuyuki, J.T. Chen, H. Jinnai, T.P. Russell, Macromolecules 42 (2009) 9082. [19] B.M. Kim, S.Z. Qian, H.H. Bau, Nano Lett. 5 (2005) 873–878. [20] B.F. Hagh, D.T. Allen, in: H.M. Freeman (Ed.), Innovative Hazardous Waste Treatment Technology, Technomic, Vol. 1, Lancaster, PA, 1990, p. 45.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
An efficient strategy for highly loaded, well dispersed and thermally stable metal oxide catalysts Changjin Tang a, Hongliang Zhang a, Chuanzhi Sun a, Jianchao Li a, Lei Qi a, Yangjian Quan c, Fei Gao b,⁎, Lin Dong a,b,⁎⁎ a b c
School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093, P.R. China Center of Modern Analysis, Nanjing University, Nanjing 210093, P.R. China Kuang Yaming Honors School, Nanjing University, Nanjing 210093, P.R. China
a r t i c l e
i n f o
Article history: Received 18 January 2011 Received in revised form 16 March 2011 Accepted 19 March 2011 Available online 26 March 2011 Keywords: Supported catalysts SBA-15 Metal oxides High loadings Homogeneous dispersion High thermal stability
a b s t r a c t A family of metal oxides (Co3O4, NiO and CeO2) confined in SBA-15 with high loadings (≥ 20 wt%) was prepared through a solvent-free method. Characterizations of X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that aggregation-free nanoparticles were obtained and N2 physisorption confirmed they were studded in mesopores. It was proposed that the intermediate molten salt phases ensured successful encapsulation and homogeneous dispersion of metal oxides. Lastly, the importance of the strategy was exemplified by NiO, and the high thermal stability together with superior performance in hydrodechlorination of chlorobenzene suggested great potential of these samples in heterogeneous catalysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The preparation of supported metal (oxide) catalysts with high loadings and homogenous dispersion is a field of great interest from both the industrial and fundamental points of view mainly because of their numerous active sites and hence remarkable performances in the production of chemicals, fuels and elimination of environment pollutants [1,2]. Conventionally, to suppress sintering and avoid leaching of the active species, the metals or their oxides are deposited onto an inert and thermostable support like Al2O3, SiO2 or activated carbon. Compared to solid supports, the supports with porous or hollow structures are preferable, as they usually possess larger surface areas and hence can provide more space for immobilization. More importantly, it is expected that these peculiar structures could offer confined environment to inhibit significant growth of guest species [3] and function as unique reactor for superior performance [4]. As a representative porous silica, SBA-15 with tunable pore diameters, thick pore walls and large surface areas is a desirable candidate for loading guest species [5]. Therefore, extensive researches have been focused on encapsulating nanoparticles into the pore
⁎ Correspondence to: L. Dong, School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093, P.R. China. Tel.: + 86 25 83592290; fax: + 86 25 83317761. ⁎⁎ Corresponding author. Tel.: + 86 25 83592290; fax: + 86 25 83317761. E-mail addresses: [email protected] (F. Gao), [email protected] (L. Dong). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.03.031
channels of SBA-15 to obtain thermostable nanocatalysts. Briefly, two routes are mainly developed to approach the goal. One is grafting method [6–8]. This route allows the effective fabrication of monodisperse noble metals embedded in SBA-15 tunnels. However, the drawback is also obvious. The low loadings (b5 wt%) together with complex processes will limit its further scale up. Another way is wet impregnation. Seemingly facile as the practical execution is, the fundamental phenomena underlying impregnation and drying are extremely complex. More often than not the interactions between precursor and inert support are limited, thereby causing partial migration of guest species with the evaporation of solvent and allowing redistribution of the active phase over the support [9]. As a result, the control over particle size is poor without special treatment [10,11]. From the discussion above, it is known that the efficient preparation of aggregation-free nanoparticles encapsulated in SBA15 is of great importance in heterogeneous catalysis but still remains a challenge, especially for high loadings. Hence, in the present study, a solvent-free method is extended to fabricate supported metal oxides with high loadings (≥20 wt%) and narrow particle size distribution (PSD). We first investigate the efficiency of solvent-free method for preparing a family of highly loaded metal oxides confined in SBA-15. Then, based on characterization results, the homogeneous dispersion of active species in the pores derived from the absence of solvent was emphasized. Lastly, the advantage of the present synthesis in catalysis is exemplified by that of NiO, and the high thermal stability together with superior performance in hydrodechlorination of chlorobenzene
1076
C. Tang et al. / Catalysis Communications 12 (2011) 1075–1078
demonstrate great potential of these catalysts in heterogeneous catalysis. 2. Experimental 2.1. Preparation of SBA-15 All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., China (except P123 from Aldrich) and used as received. Synthesis of SBA-15 follows Zhao's process [5]. Basically, 4 g of P123 was added to 120 ml of 2 M HCl aqueous solution with stirring, followed by 30 ml H2O. After the surfactant was dissolved, 9 g of TEOS was added and the solution was stirring for 24 h at 40 °C. Then the solution was transferred into an autoclave, sealed and aged at 100 °C for 24 h. The resulting white precipitate was filtrated and washed with de-ionized water and ethanol for several times. The rinsed powder was dried and air-calcined at 550 °C for 5 h with a ramp rate of 1 °C min− 1. 2.2. Synthesis of mesopore confined metal oxide catalysts The solvent-free method to pore confined metal oxide catalysts was a simple and time-conserving process, as it only comprised manual grinding of the mixture of metal precursor and SBA-15 and the subsequent air-calcination. The precursors for Co, Ni, Ce are Co (NO3)2·6H2O, Ni(NO3)2·6H2O, Ce(NO3)3·6H2O, respectively. In a typical synthesis, calculated metal nitrates (Co: 0.091 g, Ni: 0.091 g, Ce: 0.1357 g) were manually ground with 0.1 g SBA-15 in an agate mortar for about 10 min. Then the homogeneously mixed powder was transferred into a crucible, which was placed in the muffle furnace. The thermal treatment in air started at room temperature with a ramp of 1 °C min− 1 to 450 °C and maintained at that temperature for 4 h, then cooled naturally. The obtained samples were denoted as MxOy@SBA-15, where M represented the metals. As expected, there are almost no loss of metal oxides and for Co3O4@SBA-15, NiO@SBA-15 and CeO2@SBA-15, the nominal loadings were 20 wt%, 20 wt% and 35 wt%, respectively. To determine the thermal stability of NiO, the NiO@SBA-15 was further calcined at higher temperatures (550 °C and 650 °C) for 4 h. As a comparison, the conventional wet impregnation route to supported NiO sample was employed. The precursor, calcination temperature and loading were identical to that of NiO@SBA-15 and the final product was named as NiO/SBA-15. 2.3. Characterization The powder X-ray diffraction patterns of samples were collected on a Philips X'pert X-ray diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15418 nm).The X-ray tube was operated at 40 kV and 40 mA. Nitrogen physisorption was measured at − 196 °C using a Micromeritics ASAP 2020 system. The samples were degassed for 160 min at 300 °C in the degas port of the adsorption analyzer. The mesoporosity was determined from BJH method. Transmission electron microscopy (TEM) images were taken on a JEM-2100 instrument at an acceleration voltage of 200 kV. The samples were crushed and dispersed in A.R. grade ethanol and the resulting suspensions were allowed to dry on carbon film supported on copper grids. 2.4. Evaluation of catalytic performance The gas phase hydrodechlorination (HDC) of chlorobenzene to benzene was carried out in a continuous fixed-bed reactor at atmospheric pressure. The prepared catalyst (27.5 mg, 60–80 mesh) was placed in a tubular quartz reactor between two quartz plugs and a layer of glass beads above the catalyst bed to ensure that the reactants
Fig. 1. The small-angle XRD patterns of pristine SBA-15, Co3O4@SBA-15, NiO@SBA-15 and CeO2@SBA-15.
reached the reaction temperature before contact with the catalyst. Before starting the reaction, the catalyst was activated at 400 °C for 2 h in hydrogen at a flow rate of 40 ml min− 1 and then cooled to the reaction temperature (300 °C). Chlorobenzene (0.12 ml h− 1) was fed into the reactor using a microfeeder and the hydrogen flow was maintained by a mass flow controller. Product analysis was performed on an on-line gas chromatograph with flame ionization detector. 3. Results and discussion Fig. 1 shows the small-angle XRD patterns of MxOy@SBA-15 samples. As comparison, the pattern of pristine SBA-15 is displayed. For SBA-15, four peaks corresponding to the (100), (110), (200) and (210) interplanar diffractions of the hexagonally arranged structures are displayed, indicating high periodicity of the mesopores. After introduction of metal oxides, the (210) peak is vanished but the (100), (110) and (200) peaks are still displayed, despite their weakened intensity as compared to the pristine SBA-15. It tells us that the ordered mesostructures do not suffer from severe damage during grinding and the subsequent calcination. Taking into account the involvement of guest species, the weakening of diffraction peaks is probably due to worse scattering contrasts between silica walls and the encapsulated pores, which is a common practice for occluded ordered mesoporous materials [12,13]. In contrast to reduced peak intensities, the unit cells of samples (Table 1) are almost not changed, further confirming the ordered mesopores are not much disturbed. Characterization of mesoporosity by N2 physisorption is informative for the determination of guest species in SBA-15 [14] and the results are summarized in Table 1. It can be found that both surface area and pore volume are greatly decreased with the introduction of metal oxides. As small-angle XRD result shows that the ordered mesostructures are preserved, the obvious reduction in surface area and pore volume verifies the insertion of metal oxides into the pores, which qualitatively supports the small-angle XRD deduction and proves that the dry metal nitrate precursors can be driven into the
Table 1 The texture properties of various samples. Sample
Pore volume (cm3 g− 1)
Surface area
Unit cell
Pristine SBA-15 Co3O4@SBA-15 NiO@SBA-15 CeO2@SBA-15 NiO/SBA-15
1.01 0.45 0.30 0.27 0.76
801 316 285 315 534
11.1 11.3 11.3 11.0 11.0
C. Tang et al. / Catalysis Communications 12 (2011) 1075–1078
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Fig. 2. The wide-angle XRD patterns of Co3O4@SBA-15, CeO2@SBA-15, NiO@SBA-15 and NiO/SBA-15.
Fig. 4. The wide-angle XRD patterns of NiO@SBA-15 calcined at different temperatures.
mesopores. Moreover, it is noticeable that the surface area and pore volume of NiO/SBA-15 are lower than NiO@SBA-15, suggesting more NiO are introduced into mesopores by the solvent-free method. The efficiency of solvent-free method to fabricate highly loaded metal oxides in SBA-15 is comprehensively investigated by XRD and TEM characterizations. The wide-angle XRD patterns of samples are presented in Fig. 2. For comparison, the pattern of NiO/SBA-15 is also listed. In general, all samples display peaks which can be indexed to the diffraction peaks of the corresponding metal oxides, and no reflections from other species demonstrate the complete conversion of metal nitrates into metal oxides. Noticeably, for the samples prepared from solvent-free method, the line broadening of diffraction peaks is obvious, suggesting the formation of particles with nanometric dimension. By roughly calculating with Scherrer equation (d = Kλ/βcosθ), the mean grain sizes of these particles are in the range of 4–7 nm. In contrast, for NiO/SBA-15, its diffraction peaks are more
intensive than that of NiO@SBA-15, demonstrating the inferiority of the conventional impregnation method to disperse NiO. The dispersion of metal oxides is further determined by TEM characterization and the result is shown in Fig. 3. Generally, for the samples prepared from solvent-free method, the guest species are well scattered on the support and no bulk or rod-like particles are observed, powerfully demonstrating the advantage of solvent-free method for preparing aggregation-free nanoparticles. However, for NiO/SBA-15, in line with the wide-angle XRD result, the NiO particles are poorly dispersed. There are bulk particles with size of tens of nanometers and they are all deposited on the exterior of SBA-15. Meanwhile, rod-like particles confined in the pores are also observed, indicating the random distribution of guest species through the conventional impregnation method. On the basis of the above characterizations, the dispersion and location of metal oxides from the solvent-free method are
Fig. 3. Typical TEM images of (a) Co3O4@SBA-15, (b) NiO@SBA-15, (c) CeO2@SBA-15 and (d) NiO/SBA-15. The metal oxide nanoparticles are indicated by arrows in the images.
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activity. Catalytic hydrodechlorination (HDC) is one of the most effective routes to remove these pollutants [20]. Hence, to preliminarily evaluate the catalytic application of the prepared samples, the gas phase hydrodechlorination of chlorobenzene to benzene is employed and result is shown in Fig. 5. For both catalysts (NiO@SBA-15 and NiO/SBA-15), benzene is the only organic product and the activities are not changed within 2 h, indicating that steady state is quickly established and nickel-based catalysts are stable. It is clear that the conversion of chlorobenzene to benzene for NiO@SBA15 is much higher than that of NiO/SBA-15. The calculated conversions are ca. 80% and 60% for NiO@SBA-15 and NiO/SBA-15, respectively, showing a great enhancement in performance by the present facile solvent-free method. 4. Conclusions
Fig. 5. The conversions of chlorobenzene to benzene as a function of reaction time for NiO@SBA-15 and NiO/SBA-15.
determined. To obtain supported catalysts with homogeneously distributed active species, it is reported that the suppression of redistribution of precursors during the drying step is an effective way [15–17]. Previously, such strategies like creation of strong interactions between support and precursor and employment of chelating salts or viscosity-increasing agent to increase the viscosity were carried out. In essence, the redistribution is a result of the migration of precursor with the evaporation of solvent under a certain temperature range (80–150 °C). Thus, if the solvent is absent and the precursor can still be inserted into the mesopores without the aid of solvent, the redistribution of precursor should be greatly inhibited. As proposed elsewhere [18] and confirmed by the N2 physisorption result, certain kinds of materials without the presence of solvent can be introduced into the porous matrix through melt infiltration. In the present case, the precursors are metal nitrates and they will turn into molten salt phase when the temperature reaches their melting points. Besides their own unique property of mobility, the molten salts are also viscous, both of which are supposed to be key parameters in determining the uniform dispersion of metal oxide particles confined in the mesoporous support. When increasing the temperature to the melting point of those metal nitrates, the dry metal nitrates are transformed into its molten salt phase and the filling of mesopores starts and proceeds. Probably, with the aid of capillary imbibition [19], all of the molten salts are sucked into the pore channels. In contrast to the widely employed wet impregnation, the absence of solvent evaporation during drying process and the high viscosity of molten salts greatly reduce the redistribution of guest species under thermal treatment. Consequently, uniform and aggregation-free metal oxide nanoparticles confined in the pores are formed when the temperatures arrive to and surpass the decomposition temperatures. To evaluate the effect of pore confinement, the thermal stability of the confined metal oxides is exemplified by that of NiO and the result of wide-angle XRD is shown in Fig. 4. The line broadening of NiO diffraction peaks is still obvious and almost no coarsening of NiO occurred when the thermal treatments are elevated to higher temperatures (550 °C and 650 °C). The result clearly depicts the high thermal stability of the pore-confined nanocatalysts. Chloroaromatics are among the most undesirable industrial effluents because of their persistence and carcinogenic and mutagenic
In this work, a facile solvent-free method is reported to be efficient in preparing highly loaded, well dispersed and thermally stable nanocatalysts. By manually grinding the precursors (metal nitrates) with SBA-15, a family of aggregation-free metal oxides (Co3O4, NiO and CeO2) with high loadings (≥20 wt%) are encapsulated into the mesopores, which endow them with high thermal stability. It is proposed that owing to the absence of solvent, the redistribution of precursors are greatly reduced, which allows the preparation of nanocatalysts with homogenous dispersion. Acknowledgement The financial supports of the National 973 program of China (No.2009CB623500), the National Natural Science Foundation of China (No. 20873060, 20973091) and Nanjing University Talent Development Foundation are gratefully acknowledged. References [1] Z.H. Zhou, S.L. Wang, W.J. Zhou, G.X. Wang, L.H. Jiang, W.Z. Li, S.Q. Song, J.G. Liu, G.Q. Sun, Q. Xin, Chem. Commun. (2003) 394–395. [2] Z.W. Huang, F. Cui, H.X. Kang, J. Chen, X.Z. Zhang, C.G. Xia, Chem. Mater. 20 (2008) 5090–5099. [3] J.M. Sun, D. Ma, H. Zhang, X.M. Liu, X.W. Han, X.H. Bao, G. Weinberg, N. Pfander, D.S. Su, J. Am. Chem. Soc. 128 (2006) 15756–15764. [4] R.Y. Zhang, W. Ding, B. Tu, D.Y. Zhao, Chem. Mater. 19 (2007) 4379–4381. [5] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548–552. [6] Z. Li, C. Kübel, V.I. Pârvulescu, R. Richards, ACS Nano 2 (2008) 1205–1212. [7] M. Boutros, Z. Maoui, H. Sfihi, V. Viossat, A. Gédéon, F. Launay, Micropor. Mesopor. Mater. 108 (2008) 247–2578 90. [8] X.Y. Liu, A.Q. Wang, X.F. Yang, T. Zhang, C.Y. Mou, D.S. Su, J. Li, Chem. Mater. 21 (2009) 410–418. [9] T. Toupance, M. Kermarec, C. Louis, J. Phys. Chem. B 104 (2000) 965–972. [10] M.Y. Cheng, C.J. Pan, B.J. Hwang, J. Mater. Chem. 19 (2009) 5193–5200. [11] J.R.A. Sietsma, J.D. Meeldijk, J.P. den Breejen, M. Versluijs-Helder, A.J. van Dillen, P.E. de Jongh, K.P. de Jong, Angew. Chem. Int. Ed. 46 (2007) 4547–4549. [12] B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Micropor. Mater. 6 (1996) 375–383. [13] J. Sauer, F. Marlow, B. Spliethoff, F. Schuth, Chem. Mater. 14 (2002) 217–224. [14] L. Vradman, M.V. Landau, D. Kantorovich, Y. Koltpin, A. Gedanken, Micropor. Mesopor. Mater. 79 (2005) 307–318. [15] L. Jiao, J.R. Regalbuto, J. Catal. 260 (2008) 329–341. [16] A.J. van Dillen, R.J.A.M. Terorde, D.J. Lensveld, J.W. Geus, K.P. de Jong, J. Catal. 216 (2003) 257–264. [17] M. Kotter, L. Riekert, Stud. Surf. Sci. Catal. 3 (1979) 51–63. [18] P. Dobriyal, H.Q. Xiang, M. Kazuyuki, J.T. Chen, H. Jinnai, T.P. Russell, Macromolecules 42 (2009) 9082. [19] B.M. Kim, S.Z. Qian, H.H. Bau, Nano Lett. 5 (2005) 873–878. [20] B.F. Hagh, D.T. Allen, in: H.M. Freeman (Ed.), Innovative Hazardous Waste Treatment Technology, Technomic, Vol. 1, Lancaster, PA, 1990, p. 45.