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Metal-organic framework as a host for synthesis of nanoscale Co3O4 as an active catalyst for CO oxidation
Catalysis Communications 12 (2011) 875–879
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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
Metal-organic framework as a host for synthesis of nanoscale Co3O4 as an active catalyst for CO oxidation Weixia Wang a, Yingwei Li a,⁎, Rongjun Zhang b, Dehua He b,⁎, Hongli Liu a, Shijun Liao a a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 26 January 2011 Accepted 1 February 2011 Available online 21 March 2011 Keywords: Metal-organic framework Cobalt oxide nanoparticles Carbon monoxide oxidation Catalyst stability
a b s t r a c t Co3O4 nanoparticles were prepared from cobalt nitrate that was accommodated in the pores of a metalorganic framework (MOF) ZIF-8 (Zn(MeIM)2, MeIM = 2-methylimidazole) by using a simple liquid-phase method. The ZIF-8 host was removed by pyrolysis under air and subsequently washing with an NH4Cl– NH3·H2O aqueous solution. Transmission electron microscopy (TEM) analysis shows that the obtained Co3O4 is composed of separate nanoparticles with a mean size of 18 nm. The Co3O4 nanoparticles exhibit excellent catalytic activity, cycling stability, and long-term stability in the low temperature CO oxidation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal-organic frameworks (MOFs) are an emerging, important class of porous materials, which have attracted considerable attention because of their wide range of applications, including catalysis, sensor, gas storage, and separation [1–3]. Owing to their open channels, permanent cavities, and ordered crystalline lattice, MOFs will provide great potential as hosts for various nanoparticles. Over the past decade, there have been a number of reports on the encapsulation of metal nanoparticles (e.g., Pd, Au, Ru,and Pt) in the pores of MOFs [4–9]. Very recently, Bhakta et al. demonstrated that MOFs could also be employed as effective hosts for nanoscale metal hydride such as NaAlH4 [10]. Fischer et al. were the first to show that MOFs could be used as hosts for transition-metal oxides nanoclusters (e.g., TiO2 and ZnO) by gas-phase infiltration with organometallic precursors [11,12]. In these reports, the MOF hosts were not removed after the loadings to enable a high dispersion of the nanoparticles. Thus, the stability of the MOF will play an important role in determining the constancy of the overall nanoparticles@MOF material. The decomposition of the MOF host will lead to a significant loss of the performance of the material [5,12]. On the other hand, there are two that reports have shown that MOFs could be used for the preparation of neat porous materials with permanent structures and excellent performance after the removal of the MOF
⁎ Corresponding authors. E-mail address: [email protected] (Y. Li). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.02.001
hosts [13,14]. Xu et al. introduced furfuryl alcohol into the pores of MOF-5 followed by polymerization and carbonization to prepare several nanoporous carbons [13,14]. To the best of our knowledge, there are currently no reports on the use of MOFs as hosts for the synthesis of neat metal-oxide nanoparticles. Herein, we report the first example of the use of MOF as host for preparing neat metal-oxide nanoparticles by a facile liquid-phase method. Using Co3O4 as an example, we show that the direct pyrolysis of cobalt nitrate accommodated in the pores of a MOF yields Co3O4 nanoparticles after removing the MOF. The prepared Co3O4 exhibits excellent catalytic activity and stability in the lowtemperature CO oxidation. It should be mentioned that Co3O4 is one of the most important functional materials among the transition metal oxides, which has attracted wide interest for its established activity in the oxidation reactions (e.g. CO oxidation) either as efficient catalyst supports or catalysts themselves [15]. The lowtemperature CO oxidation is an important reaction in heterogeneous catalysis that covers a variety of fields in practical applications including air purification, automotive emission control, fuel cells, and gas sensors. The ZIF-8 framework (Zn(MeIM)2, MeIM = 2-methylimidazole), one of the representative MOFs, was used as host in this work (Fig. S1, see Supplementary Data). We have chosen ZIF-8 because it holds an intersecting three-dimensional channel system, a large pore size (11.6 Å in diameter), and high thermal stability (N500 °C in N2) as well as good chemical resistance to water and organic solvents [16], which may be an appropriate host for preparing Co3O4 nanoparticles by using a solution-based method to introduce the cobalt precursor.
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2. Experimental 2.1. Catalyst Preparation 2.1.1. Synthesis of ZIFs ZIF-8 was prepared according to the reported procedures [16]. However, we employed a different protocol from that developed by Yaghi et al. to synthesize the ZIF-67 [1]. The detailed procedures were described in the Supplementary Data. 2.1.2. Synthesis of Co3O4 by Using ZIF-8 as Host Cobalt nitrate was introduced into the ZIF-8 by an impregnation method. Typically, 0.5 g of ZIF-8 was dispersed in 10 mL of 0.6 g Co (NO3)2·6H2O in ethanol and stirred at room temperature for 2 h. The obtained powder was washed thoroughly with ethanol and water. The resulting composite was heated at 200 °C for 5 h (the composite obtained at this step was denoted as Co3O4@ZIF-8), and then to 600 °C for 5 h with a heating rate of 2 °C/min in an air flow. The remaining powder was then dispersed in an NH4Cl (5 M)-NH3·H2O (2.5 M) aqueous solution to remove ZnO, which was formed from the decomposition of ZIF-8. The solid was washed several times with water and collected by centrifugation, finally dried at 100 °C overnight in an oven (the obtained Co3O4 was denoted as Co3O4-MOF). 2.1.3. Synthesis of Co3O4 by Thermolysis Co3O4 nanoparticles were synthesized by heating ZIF-67 at 600 °C for 5 h in air (the prepared Co3O4 was denoted as Co3O4-Ther). 2.2. Catalyst Characterization The as-synthesized materials were characterized by means of the XRD, SEM, HRTEM, EDX, BET, IR, elemental analysis, and TGA techniques. The detailed procedures were described in the Supplementary Data. 2.3. Catalytic Reactions The catalytic activities of CO oxidation were measured in a quartz tubular reactor (5 mm i.d.) under ambient pressure. Typically, 100 mg of catalyst was loaded and pretreated in 20 vol.% O2/He at 400 °C or 200 °C for 1 h. After cooling to room temperature, a gas mixture containing 1 vol.% CO, 20 vol.% O2 and 79 vol.% He was introduced into the reactor at a flow rate of 50 mL min−1 using mass flow controllers, −1 corresponding to a space velocity of 30,000 mL g−1 . The compocat h sition of effluent gas was monitored by an on-line gas chromatograph with a thermal conductivity detector (TCD). For the stability tests, the reactions were performed under the same reaction conditions as described above, except using the recovered catalyst. 3. Results and Discussion The XRD pattern of as-synthesized ZIF-8 matches well with the simulated as well as the already published XRD patterns (Fig. S2 of the Supplementary Data). The ZIF-8 was impregnated with Co(NO3)2, and then subjected to heating at 200 °C to decompose the nitrate [17]. It is clearly seen that the characteristic reflection pattern of the crystalline ZIF-8 is nicely maintained after the loading of Co(NO3)2 and subsequent heating treatment (Fig. S3 of the Supplementary Data). However, the small reductions in intensities and the observation of ZnO diffractions may also indicate a small deformation or a small quantity of defects of the framework. This could be due to the transformation and agglomeration of Co oxides during the heating treatments because ZIF-8 should be stable up to 325 °C under air (Fig. S4 of the Supplementary Data). The formation of Co3O4 phase is evident from the PXRD (Fig. S3). The relatively weak and broad diffraction peaks may be attributed to the low crystallinity of the Co3O4 phase annealed at a low temperature of 200 °C.
TEM images show that the Co3O4 nanoparticles were mostly in hexagonal shape (Fig. 1), which were highly dispersed on the ZIF8 with mean diameters of 16.4 ± 3.8 nm. The HRTEM image of an individual Co3O4 nanoparticle in Fig. 1c clearly shows that the spacing between lattice fringes is ca. 0.28 nm, corresponding to the (220) lattice spacing of Co3O4. The EDX shows that nitrogen element was not presented in the Co3O4@ZIF-8 sample, which was, however, detected in the Co(NO3)2 impregnated ZIF-8, indicating that the Co (NO3)2 has been completely transformed into cobalt oxides. The dark filed TEM image (Fig. 1d) shows that the Co3O4 nanoparticles were partially embedded in the networks of the ZIF-8, and the decomposition of Co(NO3)2 and growth of the Co3O4 particles led to the partial collapse of the framework structure. This is in good agreement with the PXRD results. The content of Co3O4 in the Co3O4@ZIF-8 was measured to be ca. 22.0 wt.%. The resulting composite was further heated to 600 °C in air. The ZnO phase with strong diffraction peaks shown in Fig. 2 was formed owing to the complete decomposition of the ZIF-8 (Figs. S4 and S5 of the Supplementary Data). The powder was then dispersed in an NH4Cl–NH3·H2O aqueous solution. The diffraction peaks of ZnO disappeared in the remaining solid (Fig. 2), indicating that ZnO has been removed. The absence of ZnO in the resulting Co3O4 can be further confirmed by FTIR and elemental analysis. The IR vibrational bands around 669 and 576 cm−1 can be attributed to the stretching vibrations of the Co–O bonds in Co3O4 (Fig. S6 of the Supplementary Data). Elemental analysis shows that the composition of the solid residue is (Co3O4)1.0(H2O)0.1. The presence of H2O in the residue might be due to the moisture adsorption when it was exposed to air. Therefore, all the diffraction peaks shown in the PXRD pattern (line b in Fig. 2) can be ascribed to the Co3O4 phase [18]. More structural details of the Co3O4 by using ZIF-8 as host (i.e. Co3O4-MOF) were investigated by means of SEM and TEM. SEM image shows that the obtained Co3O4 is composed of separate nanoparticles with uniform shape (Fig. 3a). TEM (Fig. 3b) indicates that the average diameter of the Co3O4 is 18.1 ± 4.5 nm (Fig. S7 of the Supplementary Data), which shows only a small increase in size as compared with that of Co3O4@ZIF-8. The surface area of the Co3O4 was measured to be 50 m2 g−1 (Table S1 of the Supplementary Data). The theoretical calculation result of the surface area for the particle diameter of 18.1 nm is 54.2 m2 g−1 (S = 6/dρ, d is the particle diameter and ρ is the density of Co3O4 (6110 kg m−3)), which is very close to the experimentally measured value. For comparison, we also prepared Co3O4 by a thermal decomposition method (i.e. Co3O4-Ther) using a Co-MOF as precursor. Xu et al. reported the preparation of Co3O4 by converting cobalt oxide subunits in a Co-MOF (Co3(NDC)3, NDC = 2,6-naphthalene dicarboxylate) by pyrolysis in air [19]. The as-prepared Co3O4 was agglomerated with an average size around 250 nm [19]. In this work, ZIF-67 (Co(MeIM)2) [1] was used as the cobalt precursor for Co3O4, which is isostructural with ZIF-8 (except for different metal ions in the frameworks) (Fig. S1 and Table S2). The thermolysis mechanism of MOF-5 in vacuum has been systematically investigated by Hu et al. [20]. It was found that the decomposition of MOF-5 was due to breaking Zn–O bonds between carboxylic ligands and Zn4O clusters, and amorphous carbon as well as ZnO were the main solid products. In this study, Co3O4 would be the only product remained after pyrolysis because the 2methylimidazole ligands should be burned off at 600 °C in air. This can be confirmed by the TG analysis. The decomposition of ZIF-67 takes place between 250 and 325 °C with a cliffy weight loss in the TG curve (Fig. S4 of the Supplementary Data). The weight of the final product was 36.2%, which is very close to the calculated weight of Co3O4 (36.3%) produced from the oxidation of ZIF-67 (C8H10CoN4). From the PXRD analysis (Fig. 2), the as-prepared material is further confirmed to be Co3O4, and no other phases can be identified.
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Fig. 1. TEM images of Co3O4@ZIF-8. (a–c), bright field images; (d), black field image. Inset in a is the EDX pattern.
As reported by Xu et al. [19], the direct thermolysis of Co-MOFs resulted in agglomerated particles as identified by the SEM and TEM images (Fig. 3). Flocculent-like structures of the Co3O4 can be observed in the SEM image (Fig. 3c). From the TEM observation, it is apparent that most of the particles were agglomerated into large particles in the range of 50–200 nm (Fig. 3d). A comparison between particle sizes of the Co3O4 samples obtained by the two different methods used in this study suggests that the ZIF-8 framework may play an important role in preventing the migration or/and aggregation of cobalt precursor in the pores during the formation of Co3O4 upon heating. As reported, the direct thermolysis of cobalt nitrate in air usually yields bulk Co3O4 samples [21].
Fig. 2. PXRD patterns of Co3O4-MOF before dissolution of the zinc oxide (a), Co3O4-MOF (b), and Co3O4-Ther (c).
Catalytic activities of the Co3O4 samples in CO oxidation at different temperatures are shown in Fig. 4. It can be seen that the activity of Co3O4-MOF was greatly enhanced after dissolution of the ZnO. The light-off temperature (temperature of 50% conversion, T50) decreases by 72 °C (from 130 °C to 58 °C) after removing the ZnO (Table S1 of the Supplementary Data). Complete conversion of CO is achieved at 80 °C over the neat Co3O4 at a high space velocity of −1 30,000 ml g−1 . In contrast, the agglomerated Co3O4-Ther sample cat h shows a much lower activity. The T50 is 92 °C, which is 34 °C higher than that of Co3O4-MOF. The much higher activity of Co3O4-MOF may be attributed to the higher specific surface area and porosity of the sample (Table S1 of the Supplementary Data). It is interesting to note that the Co3O4@ZIF-8 exhibits very similar conversions of CO as neat Co3O4 after removing the ZIF-8 host under the same reaction conditions (Fig. 4). The catalytic activity can be related to the Co3O4 which were highly dispersed on the ZIF-8 because neither ZIF-8 nor ZnO was active in the CO oxidation up to 300 °C in our experiments. Considering the different contents of Co3O4 in the two samples (while the same weights were used for the reactions), the Co3O4 on ZIF-8 should have a higher specific activity towards CO oxidation. This can be easily understood as magnetic Co3O4 particles tended to gather after the removal of the ZIF-8 host, which resulted in a decrease of the quantity of exposed active Co3O4 sites for CO oxidation. It is well known that the experimental parameters, such as amount of catalyst, CO concentration, and flow rate are crucial for CO conversion. Therefore, it is not particularly well suited to use T50 as an index for the comparison of the catalytic activities of various catalysts under different reaction conditions. Instead, specific rate offers a more appropriate means of comparison [18]. Our Co3O4-MOF sample exhibits a high specific rate of 12.8 mmol g− 1 h− 1 at 70 °C (Table S1), which can be comparable to the very good Co3O4 catalysts reported in the literature by a nanocasting method using mesoporous
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Fig. 3. SEM (a, c) and TEM (b, d) images of Co3O4-MOF (a, b), and Co3O4-Ther (c, d). Inset in a is the EDX pattern.
inorganic materials as templates [18]. It should be mentioned that the specific rate of the Co3O4 on the Co3O4@ZIF-8 was calculated to be as high as 58.2 mmol g− 1 h− 1 at 70 °C, based on the Co3O4 active phase. The results indicate that the Co3O4-MOF sample is highly reactive towards CO oxidation. Co3O4 is known to suffer from deactivation, and thus the catalytic stability has also been evaluated over the Co3O4 nanoparticles. The catalytic activity remains unchanged after three runs of reactions and no loss of CO conversion is observed after 20 h on stream at 80 °C (Fig. S8 of the Supplementary Data). Therefore, the Co3O4 exhibits both good cycling and long-term stability.
4. Conclusions In summary, we have, for the first time, employed a MOF as host to prepare neat Co3O4 nanoparticles. The thermolysis of cobalt nitrate accommodated in the MOF host at a low temperature gave hexagonal Co3O4 nanoparticles that were highly dispersed on the well-retained MOF networks. The MOF host could be fully removed to yield neat Co3O4 nanoparticles. It is suggested that the MOF host could play an important role in preventing the migration or/and aggregation of cobalt precursor in the pores during the formation of cobalt oxides. The mean diameter of the Co3O4 is around 18 nm, which is much smaller than that synthesized by direct pyrolysis of a Co-MOF. The Co3O4 exhibits excellent catalytic activity, cycling stability, and long-term stability in CO oxidation. We are now developing a more effective synthesis way and extending the MOF methods to the preparation of other metal-oxide nanoparticles with a variety of metal centers, pore dimensions, and physical–chemical properties for various applications. Acknowledgment This work was supported by NSF of China (20803024, 20936001, and 21073065), Doctoral Fund of Ministry of Education of China (200805611045), Guangdong Natural Science Foundation (8151064 101000094 and 10351064101000000), the Fundamental Research Funds for the Central Universities (2009ZZ0023), and the program for New Century Excellent Talents in Universities (NCET-08-0203). Appendix A. Supplementary Data
Fig. 4. Conversion as a function of reaction temperature for CO oxidation over Co3O4@ZIF-8 (⋄), Co3O4-MOF before dissolution of the zinc oxide (▲), Co3O4-MOF (●), and Co3O4-Ther (■).
Supplementary data to this article can be found online at doi:10.1016/j.catcom.2011.02.001.
W. Wang et al. / Catalysis Communications 12 (2011) 875–879
References [1] R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O'Keeffe, O.M. Yaghi, Science 319 (2008) 939. [2] S. Shimomura, M. Higuchi, R. Matsuda, K. Yoneda, Y. Hijikata, Y. Kubota, Y. Mita, J. Kim, M. Takata, S. Kitagawa, Nat. Chem. 2 (2010) 633. [3] C. Wu, A. Hu, L. Zhang, W.B. Lin, J. Am. Chem. Soc. 127 (2005) 8940. [4] M. Sabo, A. Henschel, H. Fröde, E. Klemm, S. Kaskel, J. Mater. Chem. 17 (2007) 3827. [5] F. Schröder, D. Esken, M. Cokoja, M.W.E. van den Berg, O.I. Lebedev, G.V. Tendeloo, B. Walaszek, G. Buntkowsky, H.H. Limbach, B. Chaudret, R.A. Fischer, J. Am. Chem. Soc. 130 (2008) 6119. [6] D. Farrusseng, S. Aguado, C. Pinel, Angew. Chem. Int. Ed. 48 (2009) 7502. [7] H. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J. Am. Chem. Soc. 131 (2009) 11302. [8] B. Yuan, Y.Y. Pan, Y.W. Li, B.L. Yin, H.F. Jiang, Angew. Chem. Int. Ed. 49 (2010) 4054. [9] H.L. Liu, Y.L. Liu, Y.W. Li, Z.Y. Tang, H.F. Jiang, J. Phys. Chem. C 114 (2010) 13362.
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[10] R.K. Bhakta, J.L. Herberg, B. Jacobs, A. Highley, R. Behrens Jr., N.W. Ockwig, J.A. Greathouse, M.D. Allendorf, J. Am. Chem. Soc. 131 (2009) 13198. [11] M. Müller, X. Zhang, Y. Wang, R.A. Fischer, Chem. Commun. (2009) 119. [12] M. Müller, S. Hermes, K. Kähler, M.W.E. van den Berg, M. Muhler, R.A. Fischer, Chem. Mater. 20 (2008) 4576. [13] B. Liu, H. Shioyama, T. Akita, Q. Xu, J. Am. Chem. Soc. 130 (2008) 5390. [14] B. Liu, H. Shioyama, H. Jiang, X. Zhang, Q. Xu, Carbon 48 (2010) 456. [15] X.W. Xie, Y. Li, Z.Q. Liu, M. Haruta, W.J. Shen, Nature 458 (2009) 746. [16] K.S. Park, Z. Ni, A.P. Côté, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O'Keeffe, O.M. Yaghi, PNAS 103 (2006) 10186. [17] M. Zheng, J. Cao, S. Liao, J. Liu, H. Chen, Y. Zhao, W. Dai, G. Ji, J. Cao, J. Tao, J. Phys. Chem. C 113 (2009) 3887. [18] S. Sun, Q. Gao, H. Wang, J. Zhu, H. Guo, Appl. Catal. B Environ. 97 (2010) 284. [19] B. Liu, X. Zhang, H. Shioyama, T. Mukai, T. Sakai, Q. Xu, J. Power Sources 195 (2010) 857. [20] L. Zhang, Y.H. Hu, J. Phys. Chem. C 114 (2010) 2566. [21] F. Teng, W. Yao, Y. Zhu, G. Gao, D.D. Meng, J. Non-Cryst. Solids 355 (2009) 2375.
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
Metal-organic framework as a host for synthesis of nanoscale Co3O4 as an active catalyst for CO oxidation Weixia Wang a, Yingwei Li a,⁎, Rongjun Zhang b, Dehua He b,⁎, Hongli Liu a, Shijun Liao a a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 26 January 2011 Accepted 1 February 2011 Available online 21 March 2011 Keywords: Metal-organic framework Cobalt oxide nanoparticles Carbon monoxide oxidation Catalyst stability
a b s t r a c t Co3O4 nanoparticles were prepared from cobalt nitrate that was accommodated in the pores of a metalorganic framework (MOF) ZIF-8 (Zn(MeIM)2, MeIM = 2-methylimidazole) by using a simple liquid-phase method. The ZIF-8 host was removed by pyrolysis under air and subsequently washing with an NH4Cl– NH3·H2O aqueous solution. Transmission electron microscopy (TEM) analysis shows that the obtained Co3O4 is composed of separate nanoparticles with a mean size of 18 nm. The Co3O4 nanoparticles exhibit excellent catalytic activity, cycling stability, and long-term stability in the low temperature CO oxidation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal-organic frameworks (MOFs) are an emerging, important class of porous materials, which have attracted considerable attention because of their wide range of applications, including catalysis, sensor, gas storage, and separation [1–3]. Owing to their open channels, permanent cavities, and ordered crystalline lattice, MOFs will provide great potential as hosts for various nanoparticles. Over the past decade, there have been a number of reports on the encapsulation of metal nanoparticles (e.g., Pd, Au, Ru,and Pt) in the pores of MOFs [4–9]. Very recently, Bhakta et al. demonstrated that MOFs could also be employed as effective hosts for nanoscale metal hydride such as NaAlH4 [10]. Fischer et al. were the first to show that MOFs could be used as hosts for transition-metal oxides nanoclusters (e.g., TiO2 and ZnO) by gas-phase infiltration with organometallic precursors [11,12]. In these reports, the MOF hosts were not removed after the loadings to enable a high dispersion of the nanoparticles. Thus, the stability of the MOF will play an important role in determining the constancy of the overall nanoparticles@MOF material. The decomposition of the MOF host will lead to a significant loss of the performance of the material [5,12]. On the other hand, there are two that reports have shown that MOFs could be used for the preparation of neat porous materials with permanent structures and excellent performance after the removal of the MOF
⁎ Corresponding authors. E-mail address: [email protected] (Y. Li). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.02.001
hosts [13,14]. Xu et al. introduced furfuryl alcohol into the pores of MOF-5 followed by polymerization and carbonization to prepare several nanoporous carbons [13,14]. To the best of our knowledge, there are currently no reports on the use of MOFs as hosts for the synthesis of neat metal-oxide nanoparticles. Herein, we report the first example of the use of MOF as host for preparing neat metal-oxide nanoparticles by a facile liquid-phase method. Using Co3O4 as an example, we show that the direct pyrolysis of cobalt nitrate accommodated in the pores of a MOF yields Co3O4 nanoparticles after removing the MOF. The prepared Co3O4 exhibits excellent catalytic activity and stability in the lowtemperature CO oxidation. It should be mentioned that Co3O4 is one of the most important functional materials among the transition metal oxides, which has attracted wide interest for its established activity in the oxidation reactions (e.g. CO oxidation) either as efficient catalyst supports or catalysts themselves [15]. The lowtemperature CO oxidation is an important reaction in heterogeneous catalysis that covers a variety of fields in practical applications including air purification, automotive emission control, fuel cells, and gas sensors. The ZIF-8 framework (Zn(MeIM)2, MeIM = 2-methylimidazole), one of the representative MOFs, was used as host in this work (Fig. S1, see Supplementary Data). We have chosen ZIF-8 because it holds an intersecting three-dimensional channel system, a large pore size (11.6 Å in diameter), and high thermal stability (N500 °C in N2) as well as good chemical resistance to water and organic solvents [16], which may be an appropriate host for preparing Co3O4 nanoparticles by using a solution-based method to introduce the cobalt precursor.
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2. Experimental 2.1. Catalyst Preparation 2.1.1. Synthesis of ZIFs ZIF-8 was prepared according to the reported procedures [16]. However, we employed a different protocol from that developed by Yaghi et al. to synthesize the ZIF-67 [1]. The detailed procedures were described in the Supplementary Data. 2.1.2. Synthesis of Co3O4 by Using ZIF-8 as Host Cobalt nitrate was introduced into the ZIF-8 by an impregnation method. Typically, 0.5 g of ZIF-8 was dispersed in 10 mL of 0.6 g Co (NO3)2·6H2O in ethanol and stirred at room temperature for 2 h. The obtained powder was washed thoroughly with ethanol and water. The resulting composite was heated at 200 °C for 5 h (the composite obtained at this step was denoted as Co3O4@ZIF-8), and then to 600 °C for 5 h with a heating rate of 2 °C/min in an air flow. The remaining powder was then dispersed in an NH4Cl (5 M)-NH3·H2O (2.5 M) aqueous solution to remove ZnO, which was formed from the decomposition of ZIF-8. The solid was washed several times with water and collected by centrifugation, finally dried at 100 °C overnight in an oven (the obtained Co3O4 was denoted as Co3O4-MOF). 2.1.3. Synthesis of Co3O4 by Thermolysis Co3O4 nanoparticles were synthesized by heating ZIF-67 at 600 °C for 5 h in air (the prepared Co3O4 was denoted as Co3O4-Ther). 2.2. Catalyst Characterization The as-synthesized materials were characterized by means of the XRD, SEM, HRTEM, EDX, BET, IR, elemental analysis, and TGA techniques. The detailed procedures were described in the Supplementary Data. 2.3. Catalytic Reactions The catalytic activities of CO oxidation were measured in a quartz tubular reactor (5 mm i.d.) under ambient pressure. Typically, 100 mg of catalyst was loaded and pretreated in 20 vol.% O2/He at 400 °C or 200 °C for 1 h. After cooling to room temperature, a gas mixture containing 1 vol.% CO, 20 vol.% O2 and 79 vol.% He was introduced into the reactor at a flow rate of 50 mL min−1 using mass flow controllers, −1 corresponding to a space velocity of 30,000 mL g−1 . The compocat h sition of effluent gas was monitored by an on-line gas chromatograph with a thermal conductivity detector (TCD). For the stability tests, the reactions were performed under the same reaction conditions as described above, except using the recovered catalyst. 3. Results and Discussion The XRD pattern of as-synthesized ZIF-8 matches well with the simulated as well as the already published XRD patterns (Fig. S2 of the Supplementary Data). The ZIF-8 was impregnated with Co(NO3)2, and then subjected to heating at 200 °C to decompose the nitrate [17]. It is clearly seen that the characteristic reflection pattern of the crystalline ZIF-8 is nicely maintained after the loading of Co(NO3)2 and subsequent heating treatment (Fig. S3 of the Supplementary Data). However, the small reductions in intensities and the observation of ZnO diffractions may also indicate a small deformation or a small quantity of defects of the framework. This could be due to the transformation and agglomeration of Co oxides during the heating treatments because ZIF-8 should be stable up to 325 °C under air (Fig. S4 of the Supplementary Data). The formation of Co3O4 phase is evident from the PXRD (Fig. S3). The relatively weak and broad diffraction peaks may be attributed to the low crystallinity of the Co3O4 phase annealed at a low temperature of 200 °C.
TEM images show that the Co3O4 nanoparticles were mostly in hexagonal shape (Fig. 1), which were highly dispersed on the ZIF8 with mean diameters of 16.4 ± 3.8 nm. The HRTEM image of an individual Co3O4 nanoparticle in Fig. 1c clearly shows that the spacing between lattice fringes is ca. 0.28 nm, corresponding to the (220) lattice spacing of Co3O4. The EDX shows that nitrogen element was not presented in the Co3O4@ZIF-8 sample, which was, however, detected in the Co(NO3)2 impregnated ZIF-8, indicating that the Co (NO3)2 has been completely transformed into cobalt oxides. The dark filed TEM image (Fig. 1d) shows that the Co3O4 nanoparticles were partially embedded in the networks of the ZIF-8, and the decomposition of Co(NO3)2 and growth of the Co3O4 particles led to the partial collapse of the framework structure. This is in good agreement with the PXRD results. The content of Co3O4 in the Co3O4@ZIF-8 was measured to be ca. 22.0 wt.%. The resulting composite was further heated to 600 °C in air. The ZnO phase with strong diffraction peaks shown in Fig. 2 was formed owing to the complete decomposition of the ZIF-8 (Figs. S4 and S5 of the Supplementary Data). The powder was then dispersed in an NH4Cl–NH3·H2O aqueous solution. The diffraction peaks of ZnO disappeared in the remaining solid (Fig. 2), indicating that ZnO has been removed. The absence of ZnO in the resulting Co3O4 can be further confirmed by FTIR and elemental analysis. The IR vibrational bands around 669 and 576 cm−1 can be attributed to the stretching vibrations of the Co–O bonds in Co3O4 (Fig. S6 of the Supplementary Data). Elemental analysis shows that the composition of the solid residue is (Co3O4)1.0(H2O)0.1. The presence of H2O in the residue might be due to the moisture adsorption when it was exposed to air. Therefore, all the diffraction peaks shown in the PXRD pattern (line b in Fig. 2) can be ascribed to the Co3O4 phase [18]. More structural details of the Co3O4 by using ZIF-8 as host (i.e. Co3O4-MOF) were investigated by means of SEM and TEM. SEM image shows that the obtained Co3O4 is composed of separate nanoparticles with uniform shape (Fig. 3a). TEM (Fig. 3b) indicates that the average diameter of the Co3O4 is 18.1 ± 4.5 nm (Fig. S7 of the Supplementary Data), which shows only a small increase in size as compared with that of Co3O4@ZIF-8. The surface area of the Co3O4 was measured to be 50 m2 g−1 (Table S1 of the Supplementary Data). The theoretical calculation result of the surface area for the particle diameter of 18.1 nm is 54.2 m2 g−1 (S = 6/dρ, d is the particle diameter and ρ is the density of Co3O4 (6110 kg m−3)), which is very close to the experimentally measured value. For comparison, we also prepared Co3O4 by a thermal decomposition method (i.e. Co3O4-Ther) using a Co-MOF as precursor. Xu et al. reported the preparation of Co3O4 by converting cobalt oxide subunits in a Co-MOF (Co3(NDC)3, NDC = 2,6-naphthalene dicarboxylate) by pyrolysis in air [19]. The as-prepared Co3O4 was agglomerated with an average size around 250 nm [19]. In this work, ZIF-67 (Co(MeIM)2) [1] was used as the cobalt precursor for Co3O4, which is isostructural with ZIF-8 (except for different metal ions in the frameworks) (Fig. S1 and Table S2). The thermolysis mechanism of MOF-5 in vacuum has been systematically investigated by Hu et al. [20]. It was found that the decomposition of MOF-5 was due to breaking Zn–O bonds between carboxylic ligands and Zn4O clusters, and amorphous carbon as well as ZnO were the main solid products. In this study, Co3O4 would be the only product remained after pyrolysis because the 2methylimidazole ligands should be burned off at 600 °C in air. This can be confirmed by the TG analysis. The decomposition of ZIF-67 takes place between 250 and 325 °C with a cliffy weight loss in the TG curve (Fig. S4 of the Supplementary Data). The weight of the final product was 36.2%, which is very close to the calculated weight of Co3O4 (36.3%) produced from the oxidation of ZIF-67 (C8H10CoN4). From the PXRD analysis (Fig. 2), the as-prepared material is further confirmed to be Co3O4, and no other phases can be identified.
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Fig. 1. TEM images of Co3O4@ZIF-8. (a–c), bright field images; (d), black field image. Inset in a is the EDX pattern.
As reported by Xu et al. [19], the direct thermolysis of Co-MOFs resulted in agglomerated particles as identified by the SEM and TEM images (Fig. 3). Flocculent-like structures of the Co3O4 can be observed in the SEM image (Fig. 3c). From the TEM observation, it is apparent that most of the particles were agglomerated into large particles in the range of 50–200 nm (Fig. 3d). A comparison between particle sizes of the Co3O4 samples obtained by the two different methods used in this study suggests that the ZIF-8 framework may play an important role in preventing the migration or/and aggregation of cobalt precursor in the pores during the formation of Co3O4 upon heating. As reported, the direct thermolysis of cobalt nitrate in air usually yields bulk Co3O4 samples [21].
Fig. 2. PXRD patterns of Co3O4-MOF before dissolution of the zinc oxide (a), Co3O4-MOF (b), and Co3O4-Ther (c).
Catalytic activities of the Co3O4 samples in CO oxidation at different temperatures are shown in Fig. 4. It can be seen that the activity of Co3O4-MOF was greatly enhanced after dissolution of the ZnO. The light-off temperature (temperature of 50% conversion, T50) decreases by 72 °C (from 130 °C to 58 °C) after removing the ZnO (Table S1 of the Supplementary Data). Complete conversion of CO is achieved at 80 °C over the neat Co3O4 at a high space velocity of −1 30,000 ml g−1 . In contrast, the agglomerated Co3O4-Ther sample cat h shows a much lower activity. The T50 is 92 °C, which is 34 °C higher than that of Co3O4-MOF. The much higher activity of Co3O4-MOF may be attributed to the higher specific surface area and porosity of the sample (Table S1 of the Supplementary Data). It is interesting to note that the Co3O4@ZIF-8 exhibits very similar conversions of CO as neat Co3O4 after removing the ZIF-8 host under the same reaction conditions (Fig. 4). The catalytic activity can be related to the Co3O4 which were highly dispersed on the ZIF-8 because neither ZIF-8 nor ZnO was active in the CO oxidation up to 300 °C in our experiments. Considering the different contents of Co3O4 in the two samples (while the same weights were used for the reactions), the Co3O4 on ZIF-8 should have a higher specific activity towards CO oxidation. This can be easily understood as magnetic Co3O4 particles tended to gather after the removal of the ZIF-8 host, which resulted in a decrease of the quantity of exposed active Co3O4 sites for CO oxidation. It is well known that the experimental parameters, such as amount of catalyst, CO concentration, and flow rate are crucial for CO conversion. Therefore, it is not particularly well suited to use T50 as an index for the comparison of the catalytic activities of various catalysts under different reaction conditions. Instead, specific rate offers a more appropriate means of comparison [18]. Our Co3O4-MOF sample exhibits a high specific rate of 12.8 mmol g− 1 h− 1 at 70 °C (Table S1), which can be comparable to the very good Co3O4 catalysts reported in the literature by a nanocasting method using mesoporous
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Fig. 3. SEM (a, c) and TEM (b, d) images of Co3O4-MOF (a, b), and Co3O4-Ther (c, d). Inset in a is the EDX pattern.
inorganic materials as templates [18]. It should be mentioned that the specific rate of the Co3O4 on the Co3O4@ZIF-8 was calculated to be as high as 58.2 mmol g− 1 h− 1 at 70 °C, based on the Co3O4 active phase. The results indicate that the Co3O4-MOF sample is highly reactive towards CO oxidation. Co3O4 is known to suffer from deactivation, and thus the catalytic stability has also been evaluated over the Co3O4 nanoparticles. The catalytic activity remains unchanged after three runs of reactions and no loss of CO conversion is observed after 20 h on stream at 80 °C (Fig. S8 of the Supplementary Data). Therefore, the Co3O4 exhibits both good cycling and long-term stability.
4. Conclusions In summary, we have, for the first time, employed a MOF as host to prepare neat Co3O4 nanoparticles. The thermolysis of cobalt nitrate accommodated in the MOF host at a low temperature gave hexagonal Co3O4 nanoparticles that were highly dispersed on the well-retained MOF networks. The MOF host could be fully removed to yield neat Co3O4 nanoparticles. It is suggested that the MOF host could play an important role in preventing the migration or/and aggregation of cobalt precursor in the pores during the formation of cobalt oxides. The mean diameter of the Co3O4 is around 18 nm, which is much smaller than that synthesized by direct pyrolysis of a Co-MOF. The Co3O4 exhibits excellent catalytic activity, cycling stability, and long-term stability in CO oxidation. We are now developing a more effective synthesis way and extending the MOF methods to the preparation of other metal-oxide nanoparticles with a variety of metal centers, pore dimensions, and physical–chemical properties for various applications. Acknowledgment This work was supported by NSF of China (20803024, 20936001, and 21073065), Doctoral Fund of Ministry of Education of China (200805611045), Guangdong Natural Science Foundation (8151064 101000094 and 10351064101000000), the Fundamental Research Funds for the Central Universities (2009ZZ0023), and the program for New Century Excellent Talents in Universities (NCET-08-0203). Appendix A. Supplementary Data
Fig. 4. Conversion as a function of reaction temperature for CO oxidation over Co3O4@ZIF-8 (⋄), Co3O4-MOF before dissolution of the zinc oxide (▲), Co3O4-MOF (●), and Co3O4-Ther (■).
Supplementary data to this article can be found online at doi:10.1016/j.catcom.2011.02.001.
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