Metal–Organic Frameworks
Tiny Pd@Co Core–Shell Nanoparticles Confined inside a Metal–Organic Framework for Highly Efficient Catalysis Yu-Zhen Chen, Qiang Xu, Shu-Hong Yu, and Hai-Long Jiang* Noble metal nanoparticles (NPs) have been the subject of intense research due to the unique physiochemical properties compared to their bulk counterparts and broad applications in energy conversion and storage, cancer therapy, sensing, etc., especially for catalysis.[1] However, the cost of noble metals is one of the key limitations for their practical application. One possible solution is minimum usage of noble metal via introducing a transition metal to afford bimetallic NPs, which, especially core–shell structured NPs, usually have improved catalytic activity compared to their monometallic counterparts and alloys.[2] The performance of the core–shell nanocatalyst was significantly affected by the core NPs although the catalytic reaction takes place on the shell. In addition, metal NPs with small sizes are desired with preferable catalytic activities, while one particular concern is that small NPs have high surface energies and readily aggregate to larger NPs with deteriorative performance. Therefore, much endeavors have been devoted to developing effective approaches to stabilize NPs.[3,4] Studies have demonstrated that porous materials (such as zeolite, mesoporous SiO2, carbon, etc.) are very efficient to restrict the growth of monometallic or alloy NPs.[4] However, as far as we know, very few attempts were made to define core–shell NPs in several nanometers with the help of porous materials.[5] As a relatively new type of porous materials, metalorganic frameworks (MOFs) have captured widespread research interest in recent two decades,[6] owing to their crystalline nature, intriguing structural topologies and potential applications for gas storage/separation, sensor, catalysis, drug delivery, and so on.[7–10] It has been demonstrated that MOFs can be elegantly employed as hosts to provide solid catalysts with fine monometallic or alloy NPs based on the confinement effect of their crystalline pore structure.[8e,11] However, Y.-Z. Chen, Prof. S.-H. Yu, Prof. H.-L. Jiang Hefei National Laboratory for Physical Sciences at the Microscale Collaborative Innovation Center of Suzhou Nano Science and Technology Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026, PR China E-mail: [email protected] Prof. Q. Xu National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563–8577, Japan DOI: 10.1002/smll.201401875 small 2014, DOI: 10.1002/smll.201401875
only two examples on stabilizing core–shell NPs on MOFs were reported thus far, in which we obtained Au@Ag and Pt@Pd NPs with sizes significantly larger than those of MOF pores via a stepwise reduction.[5] Considering that the NPs on the external surface of MOF are instable and prone to aggregate, the development of an effective strategy to obtain core–shell NPs inside the MOF pores under mild conditions is challenging but of great importance. On the other hand, the search for effective hydrogen storage materials is exigent for a hydrogen powered society as a long-term solution to guarantee a secure energy future. Ammonia borane (NH3BH3, AB) with a very high hydrogen content of 19.6 wt% makes it a promising candidate for chemical hydrogen storage. To meet its practical application, the most challenge is to develop efficient, economical, and recyclable catalysts to boost the kinetic and thermodynamic properties under moderate conditions.[12] In this work, for the first time, bimetallic core–shell NPs with sizes smaller than the pores were stabilized by a mesoporous MOF. The key to the success is that the MOF pores (>3 nm) were pre-incorporated with Pd2+ and Co2+ precursors, which were sequentially in situ reduced based on their different reduction potentials during the catalytic process to offer ∼2.5 nm Pd@Co core–shell NPs mostly embedded inside the MOF under mild conditions within a few minutes. Remarkably, compared to their monometallic counterparts and alloys as well as Pd@Co NPs on the external surface of the MOF, the resultant Pd@Co NPs inside MOF pores exert synergistic and superior catalytic activity as well as excellent recyclability in hydrolytic dehydrogenation of AB under mild conditions. The mesoporous MOF, Cr(III)-based MIL-101 with molecular formula Cr3F(H2O)2O[(O2C)C6H4(CO2)]3·nH2O (n = ∼25), was selected as a host matrix in this work to incorporate NPs because of its giant pores (2.9 to 3.4 nm) accessible through pore windows of ca. 1.2 and 1.6 nm, high specific surface area (BET, >3000 m2/g) and high stability,[13] which are desirable to encapsulate core–shell NPs for catalysis. The metal precursors are readily diffused into the cavities of MIL-101 via a double solvents approach (DSA).[11e] Given the hydrophilic nature of the inner pore surface of MIL-101, the Pd(NO3)2 and CoCl2 aqueous solution, with a volume slightly less than MIL-101 pore volume dispersed in a large amount of dry n-hexane, was readily incorporated into the pores of activated MIL-101 by capillary force during the impregnation process. The Pd2+ and Co2+ loaded MIL-101 was allowed to exposure to the aqueous solution involving a reducing agent (AB), where Pd NPs would be formed first
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
communications
Y.-Z. Chen et al.
Scheme 1. Synthesis of Pd@Co@MIL-101, Pd@Co/MIL-101, and PdCo@MIL-101 catalysts by different procedures and reducing agents.
and behaved as cores and in situ seeds for the subsequent reduction to give Co shell NPs. The trick in the formation of core–shell structure is to take advantage of the difference in reduction potentials of the two soluble metal salts (E°Pd2+/Pd = +0.915 eV vs SHE (Standard Hydrogen Electrode); E°Co2+/ Co = –0.28 eV vs. SHE) and a suitable reducing agent (AB), the latter of which can only reduce Pd2+ with high reduction potential and the Co2+ was subsequently reduced by M-H species produced during the AB hydrolysis.[2d] In this context, we can reasonably assume that the obtained Pd@Co core–shell NPs would stay inside the MOF pores during the reduction process to give Pd@Co@MIL-101. In contrast, the Pd@Co NPs were mostly deposited on the external surface of MIL-101 (denoted as Pd@Co/MIL-101) even under the same reduction conditions when the metal precursors have not been pre-incorporated into its pores (physical mixture form, see Experimental Section). The pre-incorporated metal precursors instead led to the formation of PdCo alloy NPs inside MOF pores (denoted as PdCo@MIL-101) when a stronger reduction agent, NaBH4, was applied, highlighting the critical roles of reducing agent and in situ reduction approach (Scheme 1). The monometallic NPs were also prepared by pre-incorporation of metal precursor followed by reduction to yield Pd@MIL-101 and Co@MIL-101. The synthesis progress for Pd@Co@MIL-101 can be visibly monitored by the evolution of the solution color (see Supporting Information, SI, Figure S1) and the structure has been evidenced by the transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS) and elemental mapping analyses (Figure 1). As shown in Figure 1a and b, the average sizes of Pd@Co NPs are ∼2.5 nm (SI, Figure S11), which are smaller than the pore sizes of MIL-101, revealing their confinement inside the giant cages, while very few NPs with a little bit larger sizes could escape onto the external surface during the reduction (Figure S2b). Although the core–shell structure is indistinguishable from TEM images due to the very small sizes (Figure 1a and b), a brighter core coated by a
2 www.small-journal.com
darker shell in each particle is quite clear in HAADF-STEM images by particularly observing relatively larger particles (Figure 1c; SI, Figure S2a). The core–shell structure has been further unambiguously approved by EDS mapping for Pd and Co NPs (Figure 1d–f). The in situ reduction for the physical mixture of MIL-101 and metal precursors by AB yielded Pd@Co NPs with a little bit larger sizes of ca. 3.0–3.5 nm (Figure 1g). It is understandable as the NPs could basically locate on the outer surface of MIL-101, where the porous surface structure may also offer steric restriction to prevent the NPs growth to some extent.[5] As displayed in Figure 1h,i; SI S11e and S11f, with precursors pre-incorporated into MIL101, the produced PdCo alloy or Pd NPs have ∼2.5 nm sizes, dominated by the pore confinement effect. The N2 adsorption/desorption has been carried out at 77 K (Figure 2). The BET (Brunauer–Emmett–Teller) surface areas are 3660, 2421, 2378, 2203, 3305, 2486 m2/g, respectively for as-synthesized MIL-101, Pd@MIL-101, Co@ MIL-101, Pd@Co@MIL-101, Pd@Co/MIL-101 and PdCo@ MIL-101. The slight decrease in the N2 sorption of Pd@Co/ MIL-101 and the appreciable decrease of Pd@Co@MIL-101 and PdCo@MIL-101 is in good agreement with the results described above that the former Pd@Co NPs possibly locate on the external surface whereas the latter Pd@Co and PdCo alloy NPs are mainly embedded inside the mesopores of MIL-101. The powder X-ray diffraction (PXRD) profiles show that there is no apparent loss of crystallinity and no identifiable peaks for metal NPs after the reduction, indicating the retained integrity of the MIL-101 framework and tiny metal NPs (SI, Figure S5). To compare the activity of Pd@Co NPs incorporated in and located on MIL-101 as well as those of Pd, Co, and PdCo alloy NPs, the hydrolytic dehydrogenation of AB was employed for evaluating their catalytic performances. The catalysts or the precursors with AB were placed in a flask and the reaction was initiated by introducing water under vigorous shaking or stirring. Figure 3 shows the H2 generation from aqueous AB in the presence of different catalysts.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201401875
Pd@Co Core–Shell Nanoparticles Confined inside a Metal–Organic Framework for Catalysis
Figure 1. TEM images for (a) Pd@Co@MIL-101, (g) Pd@Co/MIL-101, (h) PdCo@MIL-101 and (i) Pd@MIL-101. HAADF-STEM images for (b) Pd@ Co@MIL-101 showing the overall small sizes of Pd@Co NPs, and (c) Pd@Co@MIL-101 particularly presenting relatively larger particles, where the core–shell nanostructure with a brighter core coated by a darker shell for each particle is faintly observable. (d–f) The elemental mapping of Co (red with green circle) and Pd (green with red circle) for a Pd@Co NP highlighted with red square in Pd@Co@MIL-101.
Figure 2. N2 adsorption/desorption isotherms for different catalysts at 77 K. Filled and open symbols represent adsorption and desorption branches, respectively. The BET surface areas have been calculated by choosing N2 adsorption points in the P/P0 range of 0.05–0.3. small 2014, DOI: 10.1002/smll.201401875
The catalytic activities of bimetallic core–shell NPs outperform those of all Pd, Co and PdCo alloy NPs, revealing the synergistic effect between Pd and Co and also the particular advantage of core–shell NPs. It is worth emphasizing that, although the confined Pd@Co NPs have slightly higher activity than the core–shell NPs on the external surface of MIL-101, the former has much better stability (mentioned below). Evidently, Pd@Co@MIL-101 achieves the highest activity and the control experiments indicate that the catalyst with Pd/Co molar ratio of 0.3 is the most active (SI, Figure S6) and releases ∼100% H2 in ca. 6.5 min with (Pd + Co)/ AB = 0.011 in molar ratio, corresponding to a turnover frequency (TOF) value of 51 molH2·molcat−1·min−1 (SI, Figure S7), which is a very high value for such reaction reported by far and even higher than those of pure noble metal catalysts such as Ru (36.3 molH2·molcat−1·min−1) and much higher than those of non-noble metal-based catalysts such as Ni (8.8 molH2·molcat−1·min−1), CuCo (9.18 molH2·molcat−1·min−1).[2f,14] From the Arrhenius plots of the reaction rate constants over Pd@Co@MIL-101 obtained in the range of 286–303 K
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
communications
Y.-Z. Chen et al.
Figure 3. Plots of time vs volume of hydrogen generated from the catalytic hydrolysis of AB (0.875 mmol in 20 mL water) over Pd@Co@ MIL-101, Pd@Co/MIL-101, PdCo@MIL-101, Pd@MIL-101 and Co@MIL101 catalysts at 30 °C. The same Pd/Co = 0.3 (molar ratio) is applied and the total amount of (Pd+Co) is fixed to be (Pd+Co)/AB = 0.011 (molar ratio) for all catalysts involving bimetallic NPs. The amount of Pd in Pd@MIL-101 and Co in Co@MIL-101 is equal to their respective amount in Pd@Co@MIL-101.
(SI, Figure S9), the apparent activation energy Ea is calculated to be 22 kJ/mol, which is much lower than those of the reported catalysts such as Ni@Ru (44 kJ/mol) and Ru
(39 kJ/mol)[14b–d, 15] and further illuminates the synergistic catalytic effect of Pd and Co species and the superiority of the catalyst. It is safe to infer that the small sizes of surfactant-free NPs stabilized by MIL-101, the unique core– shell structure as well as synergistic effect between Pd and Co accounts for the observed high catalytic activity. The stability is of great importance for the practical application of catalysts. The initial catalytic activity of the Pd@Co@MIL-101 slightly surpasses that of Pd@Co/MIL-101, while there is a sharp contrast between their recycling performances (Figure 4a and c). Strikingly, the catalytic activity of Pd@Co@MIL-101 almost remains even after five runs without any treatment or activation. The catalytic activity of Pd@Co@MIL-101 displays slight drop in the 5th to 10th successive runs and the activity drop can be recovered to a large extent after a simple regeneration treatment for the catalyst (washing with H2O and ethanol, then drying at 50 °C for 2 h), suggesting its great recyclability, durability and longevity (SI, Figure S13). However, the H2 generation over Pd@Co/ MIL-101 presents considerable activity drop during the recycling experiments. The recycling results for both catalysts are well explained by TEM observations, which show that Pd@ Co NPs incorporated into MOF pores have well remained sizes after 5 runs, while the NPs on the pore surfaces present apparent aggregation to 5–10 nm, causing the deteriorated recycling performance of Pd@Co/MIL-101 (Figure 4b and 4d; SI, Figure S11). All these results perfectly demonstrate the
Figure 4. Durability test for the hydrogen generation from aqueous AB solution catalyzed by (a) Pd@Co@MIL-101 and (c) Pd@Co/MIL-101 ((Pd+Co)/ AB = 0.011) at 30 °C. Corresponding TEM images of (b) Pd@Co@MIL-101 and (d) Pd@Co/MIL-101 catalysts after five cycles of AB hydrolysis.
4 www.small-journal.com
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201401875
Pd@Co Core–Shell Nanoparticles Confined inside a Metal–Organic Framework for Catalysis
significance of pre-incorporation of metal precursors and the confinement effect of MIL-101. In summary, for the first time, we have demonstrated a novel methodology for a facile and rapid synthesis of tiny core–shell NPs stabilized by a MOF under mild conditions. The key factors to the successful synthesis are ascribed to the pre-incorporation of metal precursors into MOF followed by in situ reduction with a suitable reducing agent, AB. The resultant tiny Pd@Co NPs are low-cost, readily recyclable and exhibit synergistically enhanced catalytic performance over the monometallic counterparts and alloy NPs for hydrolytic dehydrogenation of AB. In addition, the Pd@Co@MIL-101 catalyst possesses superior catalytic activity and especially excellent cyclic stability compared to those of Pd@Co/MIL-101 because the NPs are basically confined and stabilized inside the MOF pores in the former catalyst. The electronic structure modification-induced synergistic effects between Pd and Co species in Pd@Co NPs and the confinement effect of MIL-101 are assumed to play crucial roles for the excellent catalytic activity and recyclability. Taking advantage of the unique reducing agent of AB, the relative reduction potentials of the metal ions as well as MOF confinement, the rational synthetic strategy employed here may be general and open up a new avenue for preparing surfactant-free bimetallic core–shell NPs with very small sizes, which will have potential applications in optics, magnetism, and electrics as well as heterogeneous catalysis appear here.
Experimental Section Preparation of MIL-101: MIL-101 was synthesized according to the previous report with modifications. Typically, a mixture of terephthalic acid (332 mg, 2.0 mmol) with Cr(NO3)3·9H2O (800 mg, 2.0 mmol) in the presence of aqueous HF (0.1 mL, 0.5 mmol) and de-ionized water (9.5 mL) was reacted at 200 °C for 8 h. The reaction produced microcrystalline green powder of MIL-101 with formula Cr3X(H2O)2O[(O2C)C6H4(CO2)]3·nH2O (X = F or OH, n ≤ 25). MIL-101 was purified in water at reflux temperature for 12 h followed by in ethanol at 100 °C for 24 h for twice and washed with hot ethanol, and was further purified by NH4F solution. The resultant green solid was finally dried overnight at 150 °C under vacuum prior to the further use. Preparation of Pd2+@MIL-101, Co2+@MIL-101 and Pd2+Co2+@ MIL-101 via a double solvents approach: Typically, 200 mg of activated MIL-101 was suspended in 40 mL hydrophobic solvent of dry n-hexane and the mixture was sonicated for around 20 min until it became homogeneous. After being stirred for a certain time, 0.32 mL of hydrophilic aqueous Pd(NO3)2·2H2O and/or CoCl2·6H2O solution with desired concentrations was added dropwise with a syringe pump over 20 min during constant vigorous stirring. Subsequently, the resultant solution was continuously stirred for 3 h. When the stirring stopped, the solid settled down to the bottom of the flask was harvested from the supernatant by decanting and simply drying in air at room temperature. The synthesized sample was further dried at 150 °C under vacuum for 12 h. The molar ratios of Pd2+/(Pd2+ + Co2+) were changed with various values (0.09, small 2014, DOI: 10.1002/smll.201401875
0.23, 0.36) with a fixed molar amount of (Pd2+ + Co2+) to 0.044 for the activity optimization of the bimetallic Pd@Co catalysts. Preparation of Co@MIL-101 and PdCo@MIL-101: To the dried Co2+@MIL-101 and Pd2+Co2+@MIL-101 samples, 5 mL freshly prepared aqueous NaBH4 solution (0.6 mol/L) was introduced during the vigorous stirring under the condition of ice bath to afford Co@ MIL-101 and PdCo@MIL-101, respectively. Preparation of Pd@MIL-101: A certain amount of Pd2+@MIL101 was stirring in 20 mL aqueous solution of ammonia borane (30 mg). The Pd2+ was rapidly reduced during the hydrolytic degeneration of ammonia borane to yield Pd@MIL-101. Preparation of Pd@Co@MIL-101: A certain amount of Pd2+Co2+@MIL-101 was allowed to exposure to the aqueous solution (20 mL) involving a reducing agent (ammonia borane, 30 mg), where the Pd core NPs would be formed first and behaved as cores and in situ seeds for the subsequent deposition of Co shell NPs to give Pd@Co@MIL-101 during the hydrolytic degeneration of ammonia borane. Preparation of Pd@Co/MIL-101: The aqueous solution of Pd(NO3)2·2H2O (41 µL, 0.056 mol/L) and CoCl2·6H2O (8 µL, 0.966 mol/L) with 20 mL of pure water was charged into a mixture of MIL-101 and ammonia borane (30 mg) that placed in the bottom of flask. The reaction proceeds during similar to that for Pd@Co@ MIL-101, the Pd2+ was first reduced by ammonia borane to behave as cores and in situ seeds for the subsequent reduction of Co2+ as shell to produce Pd@Co core–shell NPs basically located on the external surface of MIL-101, Pd@Co/MIL-101. It should be noted that magnetic stirring was not preferred and shaking was employed for the reaction in order to avoid the attachment of Co species onto the magnetic stirring bar. Catalytic Activity Characterization: In general, a mixture of catalyst (45.4 mg) and NH3BH3 (0.875 mmol, 30 mg) in a twonecked round-bottomed flask (50 mL) was placed in a water bath at required temperature under ambient atmosphere. A gas burette filled with water was connected to the reaction flask to measure the volume of hydrogen evolved. The reaction started when 20 mL water was injected into the mixture using a separating funnel. The volume of the evolved hydrogen gas was monitored by recording the displacement of water in the gas burette. The reaction was completed when there was no more gas generated. For the catalytic recycling/durability experiments, the same amount of NH3BH3 (30 mg) was added into the flask to initiate the reaction.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors are grateful to Prof. Ming Gong and Dr. Yue Lin for TEM measurements and financial support by the National Natural Science Foundation of China (Grants 21371162 and 51301159), the National Key Basic Research Program of China (Grant
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
5
communications
Y.-Z. Chen et al.
2014CB931803), the NSF of Anhui Province (Grant 1408085MB23), the Recruitment Program of Global Experts and the Fundamental Research Funds for the Central Universities (WK2060190026).
[1] a) N. L. Rosi, C. A. Mirkin, Chem. Rev. 2005, 105, 1547; b) P. K. Jain, X. Huang, I. H. El-Sayed, M. A. El-Sayed, Acc. Chem. Res. 2008, 41, 1578; c) D. Astruc, in Nanoparticles and Catalysis, Vol. 2, Wiley-VCH, Weinheim, 2008, pp.1–640; d) Y.-G. Guo, J.-S. Hu, L.-J. Wan, Adv. Mater. 2008, 20, 2878; e) H. Zhang, M. Jin, Y. Xia, Chem. Soc. Rev. 2012, 41, 8035; f) C. Gao, Z. Lu, Y. Liu, Q. Zhang, M. Chi, Q. Cheng, Y. Yin, Angew. Chem. Int. Ed. 2012, 51, 5629; g) A. M. Alkilany, S. E. Lohse, C. J. Murphy, Acc. Chem. Res. 2013, 46, 650. [2] a) O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez, R. M. Crooks, J. Am. Chem. Soc. 2005, 127, 1015; b) F. Tao, M. E. Grass, Y. Zhang, D. R. Butcher, J. R. Renzas, Z. Liu, J. Y. Chung, B. S. Mun, M. Salmeron, G. A. Somorjai, Science 2008, 322, 932; c) S. Alayoglu, A. U. Nilekar, M. Mavrikakis, B. Eichhorn, Nat. Mater. 2008, 7, 333; d) J. Yan, X. Zhang, T. Akita, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2010, 132, 5326; e) D. Wang, Y. Li, Adv. Mater. 2011, 23, 1044; f) Q.-L. Zhu, J. Li, Q. Xu, J. Am. Chem. Soc. 2013, 135, 10210. [3] a) S. H. Joo, J. Y. Park, C.-K. Tsung, Y. Yamada, P. Yang, G. A. Somorjai, Nat. Mater. 2009, 8, 126; b) E. Boisselier, A. K. Diallo, L. Salmon, C. Ornelas, J. Ruiz, D. Astruc, J. Am. Chem. Soc. 2010, 132, 2729; c) Z. Chen, Z. Guan, M. Li, Q. Yang, C. Li, d) X. Chen, H. Häkkinen, J. Am. Chem. Soc. 2013, 135, 12944; e) G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. Yang, J. T. Hupp, F. Huo, Nat. Chem. 2012, 4, 310; f) M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao, Z. Tang, J. Am. Chem. Soc. 2014, 136, 1738. [4] a) C.-m. Yang, M. Kalwei, F. Schüth, K.-J. Chao, Appl. Catal., A 2003, 254, 289; b) M. Zahmakıran, Y. Tonbul, S. Özkar, J. Am. Chem. Soc. 2010, 132, 6541; c) Z. Wu, Y. Lv, Y. Xia, P. A. Webley, D. Zhao, J. Am. Chem. Soc. 2012, 134, 2236; d) C. Jiang, K. Hara, A. Fukuoka, Angew. Chem. Int. Ed. 2013, 52, 6265; e) J. Wang, A.-H. Lu, M. Li, W. Zhang, Y.-S. Chen, D.-X. Tian, W.-C. Li, ACS Nano 2013, 7, 4902. [5] a) H.-L. Jiang, T. Akita, T. Ishida, T. Haruta, Q. Xu, J. Am. Chem. Soc. 2011, 133, 1304; b) A. Aijaz, T. Akita, N. Tsumori, Q. Xu, J. Am. Chem. Soc. 2013, 135, 16356. [6] a) G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, Acc. Chem. Res. 2005, 38, 217; b) S. Horike, S. Shimomura, S. Kitagawa, Nat Chem. 2009, 1, 695; c) J. R. Long, O. M. Yaghi, Chem. Soc. Rev. 2009, 38, 1213; d) H.-C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673; e) H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, Science 2013, 341, 974. [7] a) H. Wu, W. Zhou, T. Yildirim, J. Am. Chem. Soc. 2009, 131, 4995; b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae, J. R. Long, Chem. Rev. 2012, 112, 724; c) M. P. Suh, H. J. Park, T. K. Prasad, D.-W. Lim, Chem. Rev. 2012, 112, 782; d) J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 2012, 112, 869; e) Y.-X. Tan, Y.-P. He, J. Zhang, ChemSusChem 2012, 5, 1597; f) Y. Hu, W. M. Verdegaal, S.-H. Yu, H.-L. Jiang, Chem Sus Chem 2014, 7, 734. [8] a) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim, Nature 2000, 404, 982; b) L. Ma, C. Abney, W. Lin, Chem. Soc. Rev. 2009, 38, 1248; c) D. Farrusseng, S. Aguado, C. Pinel, Angew. Chem. Int. Ed. 2009, 48, 7502; d) A. Corma, H. García, F. X. Llabrés i Xamena, Chem. Rev. 2010, 110, 4606; e) H.-L. Jiang, Q. Xu, Chem. Commun. 2011, 47, 3351; f) Y. Fu, D. Sun, Y. Chen,
6 www.small-journal.com
[9]
[10]
[11]
[12]
[13] [14]
[15]
R. Huang, Z. Ding, X. Fu, Z. Li, Angew. Chem. Int. Ed. 2012, 51, 3364; g) J. Gascon, A. Corma, F. Kapteijn, F. X. Llabrés i Xamena, ACS Catal. 2014, 4, 361; h) C.-H. Kuo, Y. Tang, L.-Y. Chou, B. T. Sneed, C. N. Brodsky, Z. Zhao, C.-K. Tsung, J. Am. Chem. Soc. 2012, 134, 14345; i) Y. Liu, W. Zhang, S. Li, C. Cui, J. Wu, H. Chen, F. Huo, Chem. Mater. 2014, 26, 1119. a) B. Chen, S. Xiang, G. Qian, Acc. Chem. Res. 2010, 43, 1115; b) Y. Takashima, V. Martínez, S. Furukawa, M. Kondo, S. Shimomura, H. Uehara, M. Nakahama, K. Sugimoto, S. Kitagawa, Nat Commun. 2011, 2, 1; c) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T. Hupp, Chem. Rev. 2012, 112, 1105; d) R.-B. Lin, F. Li, S.-Y. Liu, X.-L. Qi, J.-P. Zhang, X.-M. Chen, Angew. Chem. Int. Ed. 2013, 52, 13429; e) Z. Hu, B. J. Deibert, J. Li, Chem. Soc. Rev. 2014, DOI: 10.1039/ C4CS00010B. a) Z. Wang, S. M. Cohen, Chem. Soc. Rev. 2009, 38, 1315; b) S. C. Sahoo, T. Kundu, R. Banerjee, J. Am. Chem. Soc. 2011, 133, 17950; c) J. D. Rocca, D. Liu, W. Lin, Acc. Chem. Res. 2011, 44, 957; d) P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Férey, R. E. Morris, C. Serre, Chem. Rev. 2012, 112, 1232. a) S. Hermes, M.-K. Schröter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer, R. A. Fischer, Angew. Chem. Int. Ed. 2005, 44, 6237; b) Y. K. Hwang, D.-Y. Hong, J.-S. Chang, S. H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre, G. Férey, Angew. Chem. Int. Ed. 2008, 47, 4144; c) H.-L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J. Am. Chem. Soc. 2009, 131, 11302; d) B. Yuan, Y. Pan, Y. Li, B. Yin, H. Jiang, Angew. Chem. Int. Ed. 2010, 49, 4054; e) A. Aijaz, A. Karkamkar, Y. J. Choi, N. Tsumori, E. Rönnebro, T. Autrey, H. Shioyama, Q. Xu, J. Am. Chem. Soc. 2012, 134, 13926; f) J. Hermannsdörfer, M. Friedrich, N. Miyajima, R. Q. Albuquerque, S. Kümmel, R. Kempe, Angew. Chem. Int. Ed. 2012, 51, 11473; g) A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 2012, 41, 5262; h) H. R. Moon, D.-W. Lim, M. P. Suh, Chem. Soc. Rev. 2013, 42, 1807. a) P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, Nature 2002, 420, 302; b) W. Grochala, P. P. Edwards, Chem. Rev. 2004, 104, 1283; c) A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C. Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, M. Gutowski, T. Autrey, Angew. Chem. Int. Ed. 2005, 44, 3578; d) M. E. Bluhm, M. G. Bradley, R. Butterick lll, U. Kusari, L. G. Sneddon, J. Am. Chem. Soc. 2006, 128, 7748; e) C. W. Hamilton, R. T. Baker, A. Staubitz, I. Manners, Chem. Soc. Rev. 2009, 38, 279; f) R. J. Keaton, J. M. Blacquiere, R. T. Baker, J. Am. Chem. Soc. 2007, 129, 1844; g) H.-L. Jiang, S. K. Singh, J.-M. Yan, X.-B. Zhang, Q. Xu, ChemSusChem 2010, 3, 541; h) P.-Z. Li, A. Aijaz, Q. Xu, Angew. Chem. Int. Ed. 2012, 51, 6753. G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé, I. Margiolaki, Science 2005, 309, 2040. a) Ö. Metin, V. Mazumer, S. Özkar, S. Sun, J. Am. Chem. Soc. 2010, 132, 1468; b) M. Chandra, Q. Xu, J. Power Sources 2007, 168, 135; c) G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Commun. 2012, 48, 8009; d) F. Qiu, G. Liu, L. Li, Y. Wang, C. C. Xu, C. An, C. C. Chen, Y. Xu, Y. Huang, Y. Wang, L. Jiao, H. Yuan, Chem. Eur. J. 2014, 9, 487; e) Y. Yamada, K. Yano, Q. Xu, S. Fukuzumi, J. Phys. Chem. C. 2010, 114, 16456. a) Ö. Metin, S. Özkar, S. Sun, Nano Res. 2010, 3, 676; b) C.-Y. Cao, C.-Q. Chen, W. Li, W.-G. Song, W. Cai, ChemSusChem 2010, 3, 1241; c) S. Karahan, M. Zahmakiran, S. Özkar, Chem. Commun. 2012, 48, 1180.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: June 26, 2014 Revised: August 12, 2014 Published online:
small 2014, DOI: 10.1002/smll.201401875
Tiny Pd@Co Core–Shell Nanoparticles Confined inside a Metal–Organic Framework for Highly Efficient Catalysis Yu-Zhen Chen, Qiang Xu, Shu-Hong Yu, and Hai-Long Jiang* Noble metal nanoparticles (NPs) have been the subject of intense research due to the unique physiochemical properties compared to their bulk counterparts and broad applications in energy conversion and storage, cancer therapy, sensing, etc., especially for catalysis.[1] However, the cost of noble metals is one of the key limitations for their practical application. One possible solution is minimum usage of noble metal via introducing a transition metal to afford bimetallic NPs, which, especially core–shell structured NPs, usually have improved catalytic activity compared to their monometallic counterparts and alloys.[2] The performance of the core–shell nanocatalyst was significantly affected by the core NPs although the catalytic reaction takes place on the shell. In addition, metal NPs with small sizes are desired with preferable catalytic activities, while one particular concern is that small NPs have high surface energies and readily aggregate to larger NPs with deteriorative performance. Therefore, much endeavors have been devoted to developing effective approaches to stabilize NPs.[3,4] Studies have demonstrated that porous materials (such as zeolite, mesoporous SiO2, carbon, etc.) are very efficient to restrict the growth of monometallic or alloy NPs.[4] However, as far as we know, very few attempts were made to define core–shell NPs in several nanometers with the help of porous materials.[5] As a relatively new type of porous materials, metalorganic frameworks (MOFs) have captured widespread research interest in recent two decades,[6] owing to their crystalline nature, intriguing structural topologies and potential applications for gas storage/separation, sensor, catalysis, drug delivery, and so on.[7–10] It has been demonstrated that MOFs can be elegantly employed as hosts to provide solid catalysts with fine monometallic or alloy NPs based on the confinement effect of their crystalline pore structure.[8e,11] However, Y.-Z. Chen, Prof. S.-H. Yu, Prof. H.-L. Jiang Hefei National Laboratory for Physical Sciences at the Microscale Collaborative Innovation Center of Suzhou Nano Science and Technology Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026, PR China E-mail: [email protected] Prof. Q. Xu National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563–8577, Japan DOI: 10.1002/smll.201401875 small 2014, DOI: 10.1002/smll.201401875
only two examples on stabilizing core–shell NPs on MOFs were reported thus far, in which we obtained Au@Ag and Pt@Pd NPs with sizes significantly larger than those of MOF pores via a stepwise reduction.[5] Considering that the NPs on the external surface of MOF are instable and prone to aggregate, the development of an effective strategy to obtain core–shell NPs inside the MOF pores under mild conditions is challenging but of great importance. On the other hand, the search for effective hydrogen storage materials is exigent for a hydrogen powered society as a long-term solution to guarantee a secure energy future. Ammonia borane (NH3BH3, AB) with a very high hydrogen content of 19.6 wt% makes it a promising candidate for chemical hydrogen storage. To meet its practical application, the most challenge is to develop efficient, economical, and recyclable catalysts to boost the kinetic and thermodynamic properties under moderate conditions.[12] In this work, for the first time, bimetallic core–shell NPs with sizes smaller than the pores were stabilized by a mesoporous MOF. The key to the success is that the MOF pores (>3 nm) were pre-incorporated with Pd2+ and Co2+ precursors, which were sequentially in situ reduced based on their different reduction potentials during the catalytic process to offer ∼2.5 nm Pd@Co core–shell NPs mostly embedded inside the MOF under mild conditions within a few minutes. Remarkably, compared to their monometallic counterparts and alloys as well as Pd@Co NPs on the external surface of the MOF, the resultant Pd@Co NPs inside MOF pores exert synergistic and superior catalytic activity as well as excellent recyclability in hydrolytic dehydrogenation of AB under mild conditions. The mesoporous MOF, Cr(III)-based MIL-101 with molecular formula Cr3F(H2O)2O[(O2C)C6H4(CO2)]3·nH2O (n = ∼25), was selected as a host matrix in this work to incorporate NPs because of its giant pores (2.9 to 3.4 nm) accessible through pore windows of ca. 1.2 and 1.6 nm, high specific surface area (BET, >3000 m2/g) and high stability,[13] which are desirable to encapsulate core–shell NPs for catalysis. The metal precursors are readily diffused into the cavities of MIL-101 via a double solvents approach (DSA).[11e] Given the hydrophilic nature of the inner pore surface of MIL-101, the Pd(NO3)2 and CoCl2 aqueous solution, with a volume slightly less than MIL-101 pore volume dispersed in a large amount of dry n-hexane, was readily incorporated into the pores of activated MIL-101 by capillary force during the impregnation process. The Pd2+ and Co2+ loaded MIL-101 was allowed to exposure to the aqueous solution involving a reducing agent (AB), where Pd NPs would be formed first
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
communications
Y.-Z. Chen et al.
Scheme 1. Synthesis of Pd@Co@MIL-101, Pd@Co/MIL-101, and PdCo@MIL-101 catalysts by different procedures and reducing agents.
and behaved as cores and in situ seeds for the subsequent reduction to give Co shell NPs. The trick in the formation of core–shell structure is to take advantage of the difference in reduction potentials of the two soluble metal salts (E°Pd2+/Pd = +0.915 eV vs SHE (Standard Hydrogen Electrode); E°Co2+/ Co = –0.28 eV vs. SHE) and a suitable reducing agent (AB), the latter of which can only reduce Pd2+ with high reduction potential and the Co2+ was subsequently reduced by M-H species produced during the AB hydrolysis.[2d] In this context, we can reasonably assume that the obtained Pd@Co core–shell NPs would stay inside the MOF pores during the reduction process to give Pd@Co@MIL-101. In contrast, the Pd@Co NPs were mostly deposited on the external surface of MIL-101 (denoted as Pd@Co/MIL-101) even under the same reduction conditions when the metal precursors have not been pre-incorporated into its pores (physical mixture form, see Experimental Section). The pre-incorporated metal precursors instead led to the formation of PdCo alloy NPs inside MOF pores (denoted as PdCo@MIL-101) when a stronger reduction agent, NaBH4, was applied, highlighting the critical roles of reducing agent and in situ reduction approach (Scheme 1). The monometallic NPs were also prepared by pre-incorporation of metal precursor followed by reduction to yield Pd@MIL-101 and Co@MIL-101. The synthesis progress for Pd@Co@MIL-101 can be visibly monitored by the evolution of the solution color (see Supporting Information, SI, Figure S1) and the structure has been evidenced by the transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS) and elemental mapping analyses (Figure 1). As shown in Figure 1a and b, the average sizes of Pd@Co NPs are ∼2.5 nm (SI, Figure S11), which are smaller than the pore sizes of MIL-101, revealing their confinement inside the giant cages, while very few NPs with a little bit larger sizes could escape onto the external surface during the reduction (Figure S2b). Although the core–shell structure is indistinguishable from TEM images due to the very small sizes (Figure 1a and b), a brighter core coated by a
2 www.small-journal.com
darker shell in each particle is quite clear in HAADF-STEM images by particularly observing relatively larger particles (Figure 1c; SI, Figure S2a). The core–shell structure has been further unambiguously approved by EDS mapping for Pd and Co NPs (Figure 1d–f). The in situ reduction for the physical mixture of MIL-101 and metal precursors by AB yielded Pd@Co NPs with a little bit larger sizes of ca. 3.0–3.5 nm (Figure 1g). It is understandable as the NPs could basically locate on the outer surface of MIL-101, where the porous surface structure may also offer steric restriction to prevent the NPs growth to some extent.[5] As displayed in Figure 1h,i; SI S11e and S11f, with precursors pre-incorporated into MIL101, the produced PdCo alloy or Pd NPs have ∼2.5 nm sizes, dominated by the pore confinement effect. The N2 adsorption/desorption has been carried out at 77 K (Figure 2). The BET (Brunauer–Emmett–Teller) surface areas are 3660, 2421, 2378, 2203, 3305, 2486 m2/g, respectively for as-synthesized MIL-101, Pd@MIL-101, Co@ MIL-101, Pd@Co@MIL-101, Pd@Co/MIL-101 and PdCo@ MIL-101. The slight decrease in the N2 sorption of Pd@Co/ MIL-101 and the appreciable decrease of Pd@Co@MIL-101 and PdCo@MIL-101 is in good agreement with the results described above that the former Pd@Co NPs possibly locate on the external surface whereas the latter Pd@Co and PdCo alloy NPs are mainly embedded inside the mesopores of MIL-101. The powder X-ray diffraction (PXRD) profiles show that there is no apparent loss of crystallinity and no identifiable peaks for metal NPs after the reduction, indicating the retained integrity of the MIL-101 framework and tiny metal NPs (SI, Figure S5). To compare the activity of Pd@Co NPs incorporated in and located on MIL-101 as well as those of Pd, Co, and PdCo alloy NPs, the hydrolytic dehydrogenation of AB was employed for evaluating their catalytic performances. The catalysts or the precursors with AB were placed in a flask and the reaction was initiated by introducing water under vigorous shaking or stirring. Figure 3 shows the H2 generation from aqueous AB in the presence of different catalysts.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201401875
Pd@Co Core–Shell Nanoparticles Confined inside a Metal–Organic Framework for Catalysis
Figure 1. TEM images for (a) Pd@Co@MIL-101, (g) Pd@Co/MIL-101, (h) PdCo@MIL-101 and (i) Pd@MIL-101. HAADF-STEM images for (b) Pd@ Co@MIL-101 showing the overall small sizes of Pd@Co NPs, and (c) Pd@Co@MIL-101 particularly presenting relatively larger particles, where the core–shell nanostructure with a brighter core coated by a darker shell for each particle is faintly observable. (d–f) The elemental mapping of Co (red with green circle) and Pd (green with red circle) for a Pd@Co NP highlighted with red square in Pd@Co@MIL-101.
Figure 2. N2 adsorption/desorption isotherms for different catalysts at 77 K. Filled and open symbols represent adsorption and desorption branches, respectively. The BET surface areas have been calculated by choosing N2 adsorption points in the P/P0 range of 0.05–0.3. small 2014, DOI: 10.1002/smll.201401875
The catalytic activities of bimetallic core–shell NPs outperform those of all Pd, Co and PdCo alloy NPs, revealing the synergistic effect between Pd and Co and also the particular advantage of core–shell NPs. It is worth emphasizing that, although the confined Pd@Co NPs have slightly higher activity than the core–shell NPs on the external surface of MIL-101, the former has much better stability (mentioned below). Evidently, Pd@Co@MIL-101 achieves the highest activity and the control experiments indicate that the catalyst with Pd/Co molar ratio of 0.3 is the most active (SI, Figure S6) and releases ∼100% H2 in ca. 6.5 min with (Pd + Co)/ AB = 0.011 in molar ratio, corresponding to a turnover frequency (TOF) value of 51 molH2·molcat−1·min−1 (SI, Figure S7), which is a very high value for such reaction reported by far and even higher than those of pure noble metal catalysts such as Ru (36.3 molH2·molcat−1·min−1) and much higher than those of non-noble metal-based catalysts such as Ni (8.8 molH2·molcat−1·min−1), CuCo (9.18 molH2·molcat−1·min−1).[2f,14] From the Arrhenius plots of the reaction rate constants over Pd@Co@MIL-101 obtained in the range of 286–303 K
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
communications
Y.-Z. Chen et al.
Figure 3. Plots of time vs volume of hydrogen generated from the catalytic hydrolysis of AB (0.875 mmol in 20 mL water) over Pd@Co@ MIL-101, Pd@Co/MIL-101, PdCo@MIL-101, Pd@MIL-101 and Co@MIL101 catalysts at 30 °C. The same Pd/Co = 0.3 (molar ratio) is applied and the total amount of (Pd+Co) is fixed to be (Pd+Co)/AB = 0.011 (molar ratio) for all catalysts involving bimetallic NPs. The amount of Pd in Pd@MIL-101 and Co in Co@MIL-101 is equal to their respective amount in Pd@Co@MIL-101.
(SI, Figure S9), the apparent activation energy Ea is calculated to be 22 kJ/mol, which is much lower than those of the reported catalysts such as Ni@Ru (44 kJ/mol) and Ru
(39 kJ/mol)[14b–d, 15] and further illuminates the synergistic catalytic effect of Pd and Co species and the superiority of the catalyst. It is safe to infer that the small sizes of surfactant-free NPs stabilized by MIL-101, the unique core– shell structure as well as synergistic effect between Pd and Co accounts for the observed high catalytic activity. The stability is of great importance for the practical application of catalysts. The initial catalytic activity of the Pd@Co@MIL-101 slightly surpasses that of Pd@Co/MIL-101, while there is a sharp contrast between their recycling performances (Figure 4a and c). Strikingly, the catalytic activity of Pd@Co@MIL-101 almost remains even after five runs without any treatment or activation. The catalytic activity of Pd@Co@MIL-101 displays slight drop in the 5th to 10th successive runs and the activity drop can be recovered to a large extent after a simple regeneration treatment for the catalyst (washing with H2O and ethanol, then drying at 50 °C for 2 h), suggesting its great recyclability, durability and longevity (SI, Figure S13). However, the H2 generation over Pd@Co/ MIL-101 presents considerable activity drop during the recycling experiments. The recycling results for both catalysts are well explained by TEM observations, which show that Pd@ Co NPs incorporated into MOF pores have well remained sizes after 5 runs, while the NPs on the pore surfaces present apparent aggregation to 5–10 nm, causing the deteriorated recycling performance of Pd@Co/MIL-101 (Figure 4b and 4d; SI, Figure S11). All these results perfectly demonstrate the
Figure 4. Durability test for the hydrogen generation from aqueous AB solution catalyzed by (a) Pd@Co@MIL-101 and (c) Pd@Co/MIL-101 ((Pd+Co)/ AB = 0.011) at 30 °C. Corresponding TEM images of (b) Pd@Co@MIL-101 and (d) Pd@Co/MIL-101 catalysts after five cycles of AB hydrolysis.
4 www.small-journal.com
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201401875
Pd@Co Core–Shell Nanoparticles Confined inside a Metal–Organic Framework for Catalysis
significance of pre-incorporation of metal precursors and the confinement effect of MIL-101. In summary, for the first time, we have demonstrated a novel methodology for a facile and rapid synthesis of tiny core–shell NPs stabilized by a MOF under mild conditions. The key factors to the successful synthesis are ascribed to the pre-incorporation of metal precursors into MOF followed by in situ reduction with a suitable reducing agent, AB. The resultant tiny Pd@Co NPs are low-cost, readily recyclable and exhibit synergistically enhanced catalytic performance over the monometallic counterparts and alloy NPs for hydrolytic dehydrogenation of AB. In addition, the Pd@Co@MIL-101 catalyst possesses superior catalytic activity and especially excellent cyclic stability compared to those of Pd@Co/MIL-101 because the NPs are basically confined and stabilized inside the MOF pores in the former catalyst. The electronic structure modification-induced synergistic effects between Pd and Co species in Pd@Co NPs and the confinement effect of MIL-101 are assumed to play crucial roles for the excellent catalytic activity and recyclability. Taking advantage of the unique reducing agent of AB, the relative reduction potentials of the metal ions as well as MOF confinement, the rational synthetic strategy employed here may be general and open up a new avenue for preparing surfactant-free bimetallic core–shell NPs with very small sizes, which will have potential applications in optics, magnetism, and electrics as well as heterogeneous catalysis appear here.
Experimental Section Preparation of MIL-101: MIL-101 was synthesized according to the previous report with modifications. Typically, a mixture of terephthalic acid (332 mg, 2.0 mmol) with Cr(NO3)3·9H2O (800 mg, 2.0 mmol) in the presence of aqueous HF (0.1 mL, 0.5 mmol) and de-ionized water (9.5 mL) was reacted at 200 °C for 8 h. The reaction produced microcrystalline green powder of MIL-101 with formula Cr3X(H2O)2O[(O2C)C6H4(CO2)]3·nH2O (X = F or OH, n ≤ 25). MIL-101 was purified in water at reflux temperature for 12 h followed by in ethanol at 100 °C for 24 h for twice and washed with hot ethanol, and was further purified by NH4F solution. The resultant green solid was finally dried overnight at 150 °C under vacuum prior to the further use. Preparation of Pd2+@MIL-101, Co2+@MIL-101 and Pd2+Co2+@ MIL-101 via a double solvents approach: Typically, 200 mg of activated MIL-101 was suspended in 40 mL hydrophobic solvent of dry n-hexane and the mixture was sonicated for around 20 min until it became homogeneous. After being stirred for a certain time, 0.32 mL of hydrophilic aqueous Pd(NO3)2·2H2O and/or CoCl2·6H2O solution with desired concentrations was added dropwise with a syringe pump over 20 min during constant vigorous stirring. Subsequently, the resultant solution was continuously stirred for 3 h. When the stirring stopped, the solid settled down to the bottom of the flask was harvested from the supernatant by decanting and simply drying in air at room temperature. The synthesized sample was further dried at 150 °C under vacuum for 12 h. The molar ratios of Pd2+/(Pd2+ + Co2+) were changed with various values (0.09, small 2014, DOI: 10.1002/smll.201401875
0.23, 0.36) with a fixed molar amount of (Pd2+ + Co2+) to 0.044 for the activity optimization of the bimetallic Pd@Co catalysts. Preparation of Co@MIL-101 and PdCo@MIL-101: To the dried Co2+@MIL-101 and Pd2+Co2+@MIL-101 samples, 5 mL freshly prepared aqueous NaBH4 solution (0.6 mol/L) was introduced during the vigorous stirring under the condition of ice bath to afford Co@ MIL-101 and PdCo@MIL-101, respectively. Preparation of Pd@MIL-101: A certain amount of Pd2+@MIL101 was stirring in 20 mL aqueous solution of ammonia borane (30 mg). The Pd2+ was rapidly reduced during the hydrolytic degeneration of ammonia borane to yield Pd@MIL-101. Preparation of Pd@Co@MIL-101: A certain amount of Pd2+Co2+@MIL-101 was allowed to exposure to the aqueous solution (20 mL) involving a reducing agent (ammonia borane, 30 mg), where the Pd core NPs would be formed first and behaved as cores and in situ seeds for the subsequent deposition of Co shell NPs to give Pd@Co@MIL-101 during the hydrolytic degeneration of ammonia borane. Preparation of Pd@Co/MIL-101: The aqueous solution of Pd(NO3)2·2H2O (41 µL, 0.056 mol/L) and CoCl2·6H2O (8 µL, 0.966 mol/L) with 20 mL of pure water was charged into a mixture of MIL-101 and ammonia borane (30 mg) that placed in the bottom of flask. The reaction proceeds during similar to that for Pd@Co@ MIL-101, the Pd2+ was first reduced by ammonia borane to behave as cores and in situ seeds for the subsequent reduction of Co2+ as shell to produce Pd@Co core–shell NPs basically located on the external surface of MIL-101, Pd@Co/MIL-101. It should be noted that magnetic stirring was not preferred and shaking was employed for the reaction in order to avoid the attachment of Co species onto the magnetic stirring bar. Catalytic Activity Characterization: In general, a mixture of catalyst (45.4 mg) and NH3BH3 (0.875 mmol, 30 mg) in a twonecked round-bottomed flask (50 mL) was placed in a water bath at required temperature under ambient atmosphere. A gas burette filled with water was connected to the reaction flask to measure the volume of hydrogen evolved. The reaction started when 20 mL water was injected into the mixture using a separating funnel. The volume of the evolved hydrogen gas was monitored by recording the displacement of water in the gas burette. The reaction was completed when there was no more gas generated. For the catalytic recycling/durability experiments, the same amount of NH3BH3 (30 mg) was added into the flask to initiate the reaction.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors are grateful to Prof. Ming Gong and Dr. Yue Lin for TEM measurements and financial support by the National Natural Science Foundation of China (Grants 21371162 and 51301159), the National Key Basic Research Program of China (Grant
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
5
communications
Y.-Z. Chen et al.
2014CB931803), the NSF of Anhui Province (Grant 1408085MB23), the Recruitment Program of Global Experts and the Fundamental Research Funds for the Central Universities (WK2060190026).
[1] a) N. L. Rosi, C. A. Mirkin, Chem. Rev. 2005, 105, 1547; b) P. K. Jain, X. Huang, I. H. El-Sayed, M. A. El-Sayed, Acc. Chem. Res. 2008, 41, 1578; c) D. Astruc, in Nanoparticles and Catalysis, Vol. 2, Wiley-VCH, Weinheim, 2008, pp.1–640; d) Y.-G. Guo, J.-S. Hu, L.-J. Wan, Adv. Mater. 2008, 20, 2878; e) H. Zhang, M. Jin, Y. Xia, Chem. Soc. Rev. 2012, 41, 8035; f) C. Gao, Z. Lu, Y. Liu, Q. Zhang, M. Chi, Q. Cheng, Y. Yin, Angew. Chem. Int. Ed. 2012, 51, 5629; g) A. M. Alkilany, S. E. Lohse, C. J. Murphy, Acc. Chem. Res. 2013, 46, 650. [2] a) O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez, R. M. Crooks, J. Am. Chem. Soc. 2005, 127, 1015; b) F. Tao, M. E. Grass, Y. Zhang, D. R. Butcher, J. R. Renzas, Z. Liu, J. Y. Chung, B. S. Mun, M. Salmeron, G. A. Somorjai, Science 2008, 322, 932; c) S. Alayoglu, A. U. Nilekar, M. Mavrikakis, B. Eichhorn, Nat. Mater. 2008, 7, 333; d) J. Yan, X. Zhang, T. Akita, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2010, 132, 5326; e) D. Wang, Y. Li, Adv. Mater. 2011, 23, 1044; f) Q.-L. Zhu, J. Li, Q. Xu, J. Am. Chem. Soc. 2013, 135, 10210. [3] a) S. H. Joo, J. Y. Park, C.-K. Tsung, Y. Yamada, P. Yang, G. A. Somorjai, Nat. Mater. 2009, 8, 126; b) E. Boisselier, A. K. Diallo, L. Salmon, C. Ornelas, J. Ruiz, D. Astruc, J. Am. Chem. Soc. 2010, 132, 2729; c) Z. Chen, Z. Guan, M. Li, Q. Yang, C. Li, d) X. Chen, H. Häkkinen, J. Am. Chem. Soc. 2013, 135, 12944; e) G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. Yang, J. T. Hupp, F. Huo, Nat. Chem. 2012, 4, 310; f) M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao, Z. Tang, J. Am. Chem. Soc. 2014, 136, 1738. [4] a) C.-m. Yang, M. Kalwei, F. Schüth, K.-J. Chao, Appl. Catal., A 2003, 254, 289; b) M. Zahmakıran, Y. Tonbul, S. Özkar, J. Am. Chem. Soc. 2010, 132, 6541; c) Z. Wu, Y. Lv, Y. Xia, P. A. Webley, D. Zhao, J. Am. Chem. Soc. 2012, 134, 2236; d) C. Jiang, K. Hara, A. Fukuoka, Angew. Chem. Int. Ed. 2013, 52, 6265; e) J. Wang, A.-H. Lu, M. Li, W. Zhang, Y.-S. Chen, D.-X. Tian, W.-C. Li, ACS Nano 2013, 7, 4902. [5] a) H.-L. Jiang, T. Akita, T. Ishida, T. Haruta, Q. Xu, J. Am. Chem. Soc. 2011, 133, 1304; b) A. Aijaz, T. Akita, N. Tsumori, Q. Xu, J. Am. Chem. Soc. 2013, 135, 16356. [6] a) G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, Acc. Chem. Res. 2005, 38, 217; b) S. Horike, S. Shimomura, S. Kitagawa, Nat Chem. 2009, 1, 695; c) J. R. Long, O. M. Yaghi, Chem. Soc. Rev. 2009, 38, 1213; d) H.-C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673; e) H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, Science 2013, 341, 974. [7] a) H. Wu, W. Zhou, T. Yildirim, J. Am. Chem. Soc. 2009, 131, 4995; b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae, J. R. Long, Chem. Rev. 2012, 112, 724; c) M. P. Suh, H. J. Park, T. K. Prasad, D.-W. Lim, Chem. Rev. 2012, 112, 782; d) J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 2012, 112, 869; e) Y.-X. Tan, Y.-P. He, J. Zhang, ChemSusChem 2012, 5, 1597; f) Y. Hu, W. M. Verdegaal, S.-H. Yu, H.-L. Jiang, Chem Sus Chem 2014, 7, 734. [8] a) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim, Nature 2000, 404, 982; b) L. Ma, C. Abney, W. Lin, Chem. Soc. Rev. 2009, 38, 1248; c) D. Farrusseng, S. Aguado, C. Pinel, Angew. Chem. Int. Ed. 2009, 48, 7502; d) A. Corma, H. García, F. X. Llabrés i Xamena, Chem. Rev. 2010, 110, 4606; e) H.-L. Jiang, Q. Xu, Chem. Commun. 2011, 47, 3351; f) Y. Fu, D. Sun, Y. Chen,
6 www.small-journal.com
[9]
[10]
[11]
[12]
[13] [14]
[15]
R. Huang, Z. Ding, X. Fu, Z. Li, Angew. Chem. Int. Ed. 2012, 51, 3364; g) J. Gascon, A. Corma, F. Kapteijn, F. X. Llabrés i Xamena, ACS Catal. 2014, 4, 361; h) C.-H. Kuo, Y. Tang, L.-Y. Chou, B. T. Sneed, C. N. Brodsky, Z. Zhao, C.-K. Tsung, J. Am. Chem. Soc. 2012, 134, 14345; i) Y. Liu, W. Zhang, S. Li, C. Cui, J. Wu, H. Chen, F. Huo, Chem. Mater. 2014, 26, 1119. a) B. Chen, S. Xiang, G. Qian, Acc. Chem. Res. 2010, 43, 1115; b) Y. Takashima, V. Martínez, S. Furukawa, M. Kondo, S. Shimomura, H. Uehara, M. Nakahama, K. Sugimoto, S. Kitagawa, Nat Commun. 2011, 2, 1; c) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T. Hupp, Chem. Rev. 2012, 112, 1105; d) R.-B. Lin, F. Li, S.-Y. Liu, X.-L. Qi, J.-P. Zhang, X.-M. Chen, Angew. Chem. Int. Ed. 2013, 52, 13429; e) Z. Hu, B. J. Deibert, J. Li, Chem. Soc. Rev. 2014, DOI: 10.1039/ C4CS00010B. a) Z. Wang, S. M. Cohen, Chem. Soc. Rev. 2009, 38, 1315; b) S. C. Sahoo, T. Kundu, R. Banerjee, J. Am. Chem. Soc. 2011, 133, 17950; c) J. D. Rocca, D. Liu, W. Lin, Acc. Chem. Res. 2011, 44, 957; d) P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Férey, R. E. Morris, C. Serre, Chem. Rev. 2012, 112, 1232. a) S. Hermes, M.-K. Schröter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer, R. A. Fischer, Angew. Chem. Int. Ed. 2005, 44, 6237; b) Y. K. Hwang, D.-Y. Hong, J.-S. Chang, S. H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre, G. Férey, Angew. Chem. Int. Ed. 2008, 47, 4144; c) H.-L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J. Am. Chem. Soc. 2009, 131, 11302; d) B. Yuan, Y. Pan, Y. Li, B. Yin, H. Jiang, Angew. Chem. Int. Ed. 2010, 49, 4054; e) A. Aijaz, A. Karkamkar, Y. J. Choi, N. Tsumori, E. Rönnebro, T. Autrey, H. Shioyama, Q. Xu, J. Am. Chem. Soc. 2012, 134, 13926; f) J. Hermannsdörfer, M. Friedrich, N. Miyajima, R. Q. Albuquerque, S. Kümmel, R. Kempe, Angew. Chem. Int. Ed. 2012, 51, 11473; g) A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 2012, 41, 5262; h) H. R. Moon, D.-W. Lim, M. P. Suh, Chem. Soc. Rev. 2013, 42, 1807. a) P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, Nature 2002, 420, 302; b) W. Grochala, P. P. Edwards, Chem. Rev. 2004, 104, 1283; c) A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C. Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, M. Gutowski, T. Autrey, Angew. Chem. Int. Ed. 2005, 44, 3578; d) M. E. Bluhm, M. G. Bradley, R. Butterick lll, U. Kusari, L. G. Sneddon, J. Am. Chem. Soc. 2006, 128, 7748; e) C. W. Hamilton, R. T. Baker, A. Staubitz, I. Manners, Chem. Soc. Rev. 2009, 38, 279; f) R. J. Keaton, J. M. Blacquiere, R. T. Baker, J. Am. Chem. Soc. 2007, 129, 1844; g) H.-L. Jiang, S. K. Singh, J.-M. Yan, X.-B. Zhang, Q. Xu, ChemSusChem 2010, 3, 541; h) P.-Z. Li, A. Aijaz, Q. Xu, Angew. Chem. Int. Ed. 2012, 51, 6753. G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé, I. Margiolaki, Science 2005, 309, 2040. a) Ö. Metin, V. Mazumer, S. Özkar, S. Sun, J. Am. Chem. Soc. 2010, 132, 1468; b) M. Chandra, Q. Xu, J. Power Sources 2007, 168, 135; c) G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Commun. 2012, 48, 8009; d) F. Qiu, G. Liu, L. Li, Y. Wang, C. C. Xu, C. An, C. C. Chen, Y. Xu, Y. Huang, Y. Wang, L. Jiao, H. Yuan, Chem. Eur. J. 2014, 9, 487; e) Y. Yamada, K. Yano, Q. Xu, S. Fukuzumi, J. Phys. Chem. C. 2010, 114, 16456. a) Ö. Metin, S. Özkar, S. Sun, Nano Res. 2010, 3, 676; b) C.-Y. Cao, C.-Q. Chen, W. Li, W.-G. Song, W. Cai, ChemSusChem 2010, 3, 1241; c) S. Karahan, M. Zahmakiran, S. Özkar, Chem. Commun. 2012, 48, 1180.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: June 26, 2014 Revised: August 12, 2014 Published online:
small 2014, DOI: 10.1002/smll.201401875
