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Pd nanoparticles supported on amine-functionalized metal–organic framework for catalytic hydrolysis of ammonia borane
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Ag/Pd nanoparticles supported on aminefunctionalized metaleorganic framework for catalytic hydrolysis of ammonia borane Ning-Zhao Shang, Cheng Feng, Shu-Tao Gao, Chun Wang* College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China
article info
abstract
Article history:
In this study, a bi-metal based nanocatalyst (AgPd@UIO-66-NH2) was synthesized by
Received 29 July 2015
immobilizing AgPd nanoparticles onto the amino-functionalized metal-organic framework
Received in revised form
material (UIO-66-NH2). The catalytic activity of AgPd@UIO-66-NH2for the hydrolysis of
16 October 2015
ammonia borane was evaluated. Among all the prepared AgPd@UIO-66-NH2 catalysts, the
Accepted 16 October 2015
Ag1Pd4@UIO-66-NH2 exhibited the highest catalytic activity for the hydrolysis of ammonia
Available online xxx
borane with a turnover frequency value of 90 mol H2 mol catalyst1 min1, which is among the higher value of the previous reports.
Keywords:
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
UIO-66-NH2 AgPd bimetallic Ammonia borane Hydrolysis Hydrogen storage
Introduction With the aggravation of world energy crisis, developing new energy is getting more and more attention. Hydrogen (H2) as a clean and renewable energy vector is extremely promising amongst the various potential solutions. However, searching for the safe and efficient release of hydrogen under ambient condition still remains one of the most difficult challenges in the hydrogen generation field [1e3]. Recently, ammonia borane (NH3BH3, AB) is found to be a promising candidate for hydrogen generation with many advantages, such as low molecular weight (30.9 g mol1), high hydrogen content (19.6 wt%), and good stability at room temperature [4e7]. So far, there are numerous catalysts that could enhance the
hydrolysis of AB, such as non-noble metal nanoparticles (NPs) [8,9], noble metal NPs [10,11], metal oxides [12], and multicomponent metallic systems [13e16]. However, these catalysts usually suffer from serious leaching or aggregation of metal NPs during the reaction, and the catalytic performance is significantly reduced. Thus, there is still great interest in developing efficient and recyclable catalysts for hydrolysis of AB. A good approach is to anchor the NPs onto specific supports. As a relatively novel class of nanoporous materials, metalorganic frameworks (MOFs) have received significant attention in recent years. Due to the large internal surface areas, uniform but tunable pore size, MOFs have found potential applications in hydrogen storage, drug delivery, gas separation, sensing, and catalysis [17e19]. Currently, the
* Corresponding author. Tel.: þ86 312 7528291; fax: þ86 312 7528292. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.10.062 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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incorporation of metal nanoparticles in MOFs for catalysis of organic reactions have been extensively investigated since the porous structures of MOFs could restrain the aggregation of the metal NPs, and further increase their catalytic activity and recyclability [20]. Up to now, there are a few reports using monometallic or bimetallic NPs@MOF as catalysts for the hydrolysis of AB. For example, Ru@MIL-101, Pd@MIL-101, Ru/ MIL-96 [21e23], and Ni/ZIF-8 monometallic NPs [20] have been prepared to catalyze the hydrolysis reaction of AB. Xu and co-workers investigated the application of MIL-101 as matrix to anchor metal NPs including monometal (Pt) and bimetal (AuCo, AuNi, CuCo and PdCo) within the framework to prepare efficient catalysts for the hydrolysis of AB [24e28]. Bimetallic nanoparticles, a class of metal nanoparticles prepared via different synthesis approaches, have shown excellent catalytic properties for different chemical transformations. The addition of second metal is an important approach to improve the catalytic activity and selectivity [29,30]. Arlin and co-workers fabricated bimetallic PdPt NPs as catalyst for the hydrolysis of AB and the catalytic activities significantly depended on the shape of PdPt NPs [31]. Sun used CoPd as highly efficient catalyst for the hydrolysis of AB. The catalyst showed high catalytic activity when the molar ratio of Co and Pd was 35:65 [32]. Wang et al. reported a new kind of nanofibers, bimetallic AgPd supported on polyacrylonitrile, which exhibited high catalytic activity and durable stability for the reaction system [33]. Zr-MOF (UIO-66) can be obtained by self-assembly of [Zr6O4(OH)4(CO2)12] clusters and terephthalate ligands [34]. The framework possesses tetrahedral and octahedral cages, in a 2:1 ratio, with free dimensions close to 8 and 11 A, respectively. Due to their large surface area, high thermal and chemical stability, UIO-66 and amino-modified Zr-MOF (UIO66-NH2) have attracted increasing attention in many applications. Moreover, the amine groups in UiO-66-NH2 could provide coordination sites for metal ions [35,36]. In continuing our efforts to develop heterogeneous metal catalysts and new functional materials [37e40], herein, we reported the in situ co-reduction method to prepare UIO-66-NH2 supported AgPd nanoparticles, and it was used as catalyst for the hydrolysis of AB. The results demonstrated that Ag1Pd4@UIO-66-NH2 composite exhibited extraordinary catalytic activity toward the hydrolysis of AB under ambient condition.
model JEM-2011(HR) at 200 kV. Scanning electron microscopy (SEM) was conducted with an S-4800 SEM instrument. The surface area, total pore volume and pore size distribution of the samples were measured at 77 K by nitrogen adsorption using a APP V-Sorb 2800P Surface Area and Porosity Analyzer. The XRD patterns of the samples were recorded with a Rigaku D/max 2500 X-ray diffractometer using Cu Ka radiation (40 kV, 150 mA) in the range 2q ¼ 5 e80 . X-ray photoelectron spectroscopy (XPS) was performed with a PHI 1600 spectroscope using Mg Ka X-ray source for excitation. The metal content of the materials was determined by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES) on Thermo Elemental IRIS Intrepid II.
Synthesis of Zr-MOF Zr-MOF was synthesized according to the reported procedure [41]. Generally, ZrCl4 (1250 mg, 5.4 mmol) was dissolved in 60 mL of DMF:HCl (5:1, v/v) under ultrasonication for 20 min. Then, NH2-BDC (1340 mg, 7.5 mmol) and 100 mL of DMF were added into the mixture and sonicated for 20 min. Subsequently, the solution was transferred to a 200 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated in an oven at 80 C overnight. After cooling to room temperature, the resulting solid was collected by centrifugation at 8000 rpm for 10 min and washed with DMF (3 30 mL) and EtOH (3 30 mL), respectively. The obtained UIO-66-NH2 was then immersed in an ethanol solution overnight to replace DMF molecules from the cavities of UIO-66-NH2. Finally, the UiO-66-NH2 was heated in an oven at 180 C in air.
Synthesis of AgPd@UIO-66 For preparation of Ag1Pd4@UIO-66-NH2, 95 mg of UIO-66-NH2 was added to 5 mL water, and the mixture was sonicated for 20 min until it became homogeneous. A aqueous solution of AgNO3 (1.7 mL, 1 mg mL1) was added and stirred at room temperature for 1 h, then a PdCl2 solution (7.1 mL, 1 mg mL1) was added and stirred for 4 h at 25 C. The resulting mixture
Experimental section Chemicals Palladium chloride (PdCl2), zirconium chloride (ZrCl4), 2aminoterephthalic acid (NH2-BDC), and ammonia borane were all obtained from Aladdin Reagent Limited Company. AgNO3, HCl, N, N-dimethyl formamide (DMF), ethanol and other reagents were of analytical grade. All the chemicals were used as received without any purification.
Physical characterizations The size and morphology of the nanoparticles were observed by transmission electron microscopy (TEM) using a JEOL
Fig. 1 e Powder X-ray diffraction patterns of UIO-66-NH2 and AgPd@UIO-66-NH2 with different metal molar.
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Fig. 2 e XPS patterns of the Ag1Pd4@UIO-66-NH2. was reduced by NaBH4 (1 mL, 0.5 mol L1) solution at room temperature for 1 h. After filtration, the resulting solid was washed with H2O (3 10 mL) and then washed with EtOH (3 10 mL). The obtained Ag1Pd4@UIO-66-NH2 was dried at 80 C under reduced pressure. The Pd@UIO-66-NH2, Ag1Pd2@UIO-66-NH2, Ag1Pd1@UIO-66-NH2, Ag2Pd1@UIO-66-NH2, Ag4Pd1@UIO-66-NH2 and Ag@UIO-66-NH2 catalysts were synthesized by using the similar procedure except that the molar ratios of Ag and Pd were 0:1, 1:2, 1:1, 2:1, 4:1 and 1:0, respectively. The contents of Ag and Pd in the composite were determined by ICP-AES.
Hydrogen generation The reaction between NH3BH3 and water was as follows: catalyst
NH3 BH3 þ 2H2 O !NH4 BO2 þ 3H2 The hydrolysis reactions of aqueous NH3BH3 catalyzed by as-prepared AgxPdy@UIO-66-NH2 catalysts were carried out at room temperature. The hydrogen production from AB solution was carried out in a 10 mL round-bottomed flask, which was placed in an oil bath with magnetic stirrer. A gas burette filled with water was connected to the reaction flask to measure the volume of released gas. Firstly, 20 mg catalyst was
Fig. 3 e N2 adsorptionedesorption isotherms of UIO-66NH2 and Ag1Pd4@UIO-66-NH2 at 77 K.
dispersed into 4 mL deionized water in a 10 mL roundbottomed flask. Then, the reaction started when 1.0 mL of the NH3BH3 (23.6 mg, 0.75 mmol) aqueous was injected into the mixture using a syringe. The molar ratio of metal and AB was theoretically fixed at n ¼ 0.0125 for all the catalytic reactions. The volume of the evolved gas was monitored by recording the displacement of water in the gas burette.
Recycle stability After the hydrolysis reaction was exhaustively completed, the catalyst was isolated by centrifugation, washed with water, and dried at 120 C under reduced pressure. Subsequently, the dry powder were weighed and redispersed in 4 mL deionized water. 1.0 mL of the NH3BH3 (23.6 mg, 0.75 mmol) aqueous was injected into the mixture for testing the catalytic activity of the collected catalyst.
Results and discussion Characterization of the catalysts Because of its high specific surface area (BET, >1000 m2 g1) and good stability [41], the microporous Zr-MOF, UIO-66-NH2, was selected as a host matrix to incorporate Pd NPs. UiO-66NH2 is obtained by connecting Zr6O4(OH)4 inorganic cornerstones with NH2-BDC. The framework itself comprises octahedral and tetrahedral cages in a ratio of 1:2. The free diameters of cages are 8 A and 11 A, respectively, and the cages are connected by triangular windows with a diameter of 6 A. The large pore volume of UiO-66-NH2 is appropriate for the loading of metal precursors. UiO-66-NH2 was dispersed in AgNO3 and PdCl2 solution, and then stirred at room temperature. Subsequently, NaBH4 was added into the above mixture for the reduction of Agþ and Pd2þ. This method could improve the dispersion and stabilization of metal nanoparticles and prevent metal nanoparticles from escaping or agglomerating from cavities. To characterize the prepared catalysts, the powder X-ray diffraction (PXRD) pattern was conducted. The PXRD pattern (Fig. 1) of the as-synthesized UIO-66-NH2 is similar to that
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Fig. 4 e SEM (a), TEM (b and c) images and EDS spectrum (d) of Ag1Pd4@UIO-66-NH2. reported in the literature [35], which suggest that UIO-66-NH2 was successfully obtained. Fig. 1 shows that Ag1Pd4@UIO-66NH2 exhibit good crystallinity, indicating that integrity of the UIO-66-NH2 framework was maintained well during the catalyst preparation process. For Ag4Pd1@UIO-66-NH2 and Ag1Pd1@UIO-66-NH2 catalysts, the characteristic peaks intensity of UIO-66-NH2 lowered significantly, which might be ascribed to that AgPd bimetallic nanoparticles weathering or overlap the UIO-66-NH2 tiny crystal face during the catalysts preparation process. For the Ag1Pd1@UIO-66-NH2 and
Fig. 5 e Hydrogen generation from AB in the presence of different catalysts at 25 C, catalyst/AB ¼ 0.0125.
Ag4Pd1@UIO-66-NH2, the obvious XRD peak between the characteristic peaks for Ag (111) (2q ¼ 38.03 ) and Pd (111) (2q ¼ 40.10 ) indicate the high crystallinity of AgePd alloy. Furthermore, no significant diffraction peak of metal species is detected from the PXRD pattern of Ag1Pd4@UIO-66-NH2 (Fig. 1), which can be ascribed to the well dispersed metal nanoparticles and low metal loading. The XPS images of Ag1Pd4@UIO-66-NH2 were shown in Fig. 2. The 3d5/2 and 3d3/2 peak of Pd0 appear at 335.9 eV and 341.2 eV; the 3d5/2 and 3d3/2 peak of Ag0 appear at 373.8 eV and 368.0 eV. No obvious peaks of Agþ and Pd2þ were observed. The N2 adsorptionedesorption isotherms were carried out at 77 K (Fig. 3). The BrunauereEmmetteTeller (BET) surface areas and the total pore volume of as-synthesized UIO-66-NH2 and Ag1Pd4@UIO-66-NH2 were 1170 m2 g1, 0.87 cm3 g1 and 780 m2 g1, 0.51 cm3 g1, respectively. The decrease in the N2 adsorption and the pore volume of Ag1Pd4@UIO-66-NH2 suggested that the metal NPs possibly located on the external surface and inside the mesopores of UIO-66-NH2. The Ag1Pd4@UIO-66-NH2 was also characterized by SEM (Fig. 4a), TEM (Fig. 4b and c) and energy-dispersive spectroscopy (EDS) (Fig. 4d). As shown in Fig. 4a, the AgPd NPs was well dispersed on the surface of MOF, and the mean diameter of AgPd nanoparticles were in the range of 5e9 nm. A representative high-resolution TEM image (Fig. 4c) showed a dspacing of 0.229 nm, which is between the (111) lattice spacing of face-centered cubic Ag (0.24 nm) and Pd (0.22 nm),
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Table 1 e Catalytic activity of palladium nanoparticles with different support. Catalyst
Metal/AB Ratio (mol/mol)
Co35Pd65/C annealed 2.1 wt% RGO@Pd Pd/zeolite [email protected] 4 wt%Pd@MIL-101 Pd69Sn31 NPs/C RGO-Cu75Pd25 Pd-Pt @PVP NPs NiAgPd/C Pd(0)NPs/n-HAp Pd-Rh@PVP NPs Pd(0)/SiO2eCoFe2O4 Pd(0)/SiO2 [email protected] RGO/Pd Pd-PVB-TiO2 Pd2þ-HAP Pd-PVB-TiO2 PSSA-co-MA stabilized Pd nanoclusters Ag1Pd4@UIO-66-NH2
TOF Ref. (mol H2 mol catalyst1 min1)
0.024 0.006 0.02 0.02 0.0189 0.0275 0.003 0.03 0.012 0.01 0.003 0.0186 0.0186 0.037 0.04 0.03 0.02 0.06 e
35.7 26.3 6.25 40.89 45 13.64 29.9 125.0 93.8 11 1333.0 254 10 14.52 6.25 17.85 13.5 6 20
0.0125
90
[32] [42] [43] [44] [22] [45] [46] [47] [48] [49] [50] [51] [51] [52] [53] [54] [55] [56] [57]
This study
suggesting that Ag and Pd was formed as an alloy structure. The EDS data (Fig. 4d) further confirmed the presence of the Ag and Pd NPs in the Ag1Pd4@UIO-66-NH2.
Hydrolysis of AB catalyzed by the prepared catalysts Generally, the properties of alloy NPs can be well tuned by varying the composition and atomic ordering. According to the first principles studies, the synergistic effect on the alloy NPs is subject to surface electronic states, which are greatly altered by change of the catalyst's geometric parameters. The AgPd supported on UIO-66-NH2 with different molar ratio were used as catalysts for the hydrolysis reaction of AB. As shown in Fig. 5, the catalytic activities of the catalysts were strongly depended on the composition of AgePd NPs. The Ag1Pd4@UIO-66-NH2 exhibited the highest catalytic activity, with the turnover frequency (TOF) value of 90 mol H2 mol catalyst1 min1, which is among the highest value of the previously reported Pd based catalysts (Table 1).
Fig. 7 e Stability test for the hydrogen generation from aqueous AB in the presence of Ag1Pd4@UIO-66-NH2 catalyst at 25 C, catalyst/AB ¼ 0.0125.
To get the activation energy (Ea) of the AB hydrolysis catalyzed by Ag1Pd4@UIO-66-NH2, the hydrolysis reactions at different temperature ranging from 293 to 308 K were carried out. The values of rate constant k at different temperatures were calculated from the slope of the linear part of each plot from Fig. 6a. The Arrhenius plot of ln k vs. 1/T for the catalyst was plotted in Fig. 6b, from which the apparent activation energy was determined to be 51.77 kJ mol1. As we all know, the reusability of catalyst is crucial in the practical application. We tested the reusability of the Ag1Pd4@UIO-66-NH2 catalyst in the decomposition of AB. As shown in Fig. 7, the catalytic activity of Ag1Pd4@UIO-66-NH2 catalyst was mostly retained after three runs. But experimental results also showed that the as-prepared catalyst displayed a significant loss of activity after being used repetitively for 5 times. The main reasons for activity decrease may be the agglomeration of AgPd nanoparticles grown on the external surfaces of MOFs, and the weight loss of catalyst during the separation process.
Conclusions In conclusion, we have demonstrated the applicability of amino functionalized UIO-66-NH2 supported AgPd bimetallic nanoparticles for the catalytic hydrolysis of AB. By tuning the
Fig. 6 e (a) Volume of the generated gas (H2) versus time, (b) Arrhenius plot (ln (k) vs. 1/T). Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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molar ratio of AgPd precursors, a series of AgxPdy@UIO-66-NH2 composite catalysts were synthesized, among which Ag1Pd4@UIO-66-NH2 exhibited the highest catalytic activity for the catalytic hydrolysis of AB, with the TOF value of 90 mol H2 mol catalyst1 min1 at 298 K.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 31171698, 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), Natural Science Foundation of Hebei Province (B2015204003) and the Natural Science Foundation of Agricultural University of Hebei (LG201404, ZD201506).
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€ Composition-controlled catalysis of € rmez K, Metin O. [46] Gu¨ngo reduced graphene oxide supported CuPd alloy nanoparticles in the hydrolytic dehydrogenation of ammonia borane. App Catal A Gen 2015;494:22e8. [47] Rakap M. Poly(N-vinyl-2-pyrrolidone)-stabilized palladiumeplatinum nanoparticles-catalyzed hydrolysis of ammonia borane for hydrogen generation. J Power Sources 2015;276:320e7. [48] Hu L, Zheng B, Lai Z, Huang KW. Room temperature hydrogen generation from hydrolysis of ammoniaeborane over an efficient NiAgPd/C catalyst. Int J Hydrogen Energy 2014;39:20031e7. [49] Karatas Y, Yurderi M, Gulcan M, Zahmakiran M, Kaya M. Palladium(0) nanoparticles supported on hydroxyapatite nanospheres: active, long-lived, and reusable nanocatalyst for hydrogen generation from the dehydrogenation of aqueous ammonia-borane solution. J Nanopart Res 2014:16. [50] Rakap M. The highest catalytic activity in the hydrolysis of ammonia borane by poly(N-vinyl-2-pyrrolidone)-protected palladiumerhodium nanoparticles for hydrogen generation. Appl Catal B Environ 2015;163:129e34. € [51] Akbayrak S, Kaya M, Volkan M, Ozkar S. Palladium(0) nanoparticles supported on silica-coated cobalt ferrite: a highly active, magnetically isolable and reusable catalyst for hydrolytic dehydrogenation of ammonia borane. Appl Catal B Environ 2014;147:387e93. [52] Shen X, Dai M, Gao M, Wang Z, Zhao B, Ding W. Core-Shell strutured Cu@CoCr catalyst: synthesis and catalytic performance for hydrolysis of ammonia borane aqueous solution. Chin J Inorg Chem 2013;29:999e1006. [53] Xi P, Chen F, Xie G, Ma C, Liu H, Shao C, et al. Surfactant free RGO/Pd nanocomposites as highly active heterogeneous catalysts for the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. Nanoscale 2012;4:5597e601. € [54] Rakap M, Kalu EE, Ozkar S. Hydrogen generation from hydrolysis of ammonia-borane using Pd-PVB-TiO2 and CoNi-P/Pd-TiO2 under stirred conditions. J Power Sources 2012;210:184e90. € [55] Rakap M, Ozkar S. Hydroxyapatite-supported palladium(0) nanoclusters as effective and reusable catalyst for hydrogen generation from the hydrolysis of ammonia-borane. Int J Hydrogen Energy 2011;36:7019e27. € [56] Rakap M, Kalu EE, Ozkar S. Polymer-immobilized palladium supported on TiO2 (Pd-PVB-TiO2) as highly active and reusable catalyst for hydrogen generation from the hydrolysis of unstirred ammoniaeborane solution. Int J Hydrogen Energy 2011;36:1448e55. € Sahin S, Ozkar € [57] Metin O, S. Water-soluble poly(4styrenesulfonic acid-co-maleic acid) stabilized ruthenium(0) and palladium(0) nanoclusters as highly active catalysts in hydrogen generation from the hydrolysis of ammoniaborane. Int J Hydrogen Energy 2009;34:6304e13.
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Ag/Pd nanoparticles supported on aminefunctionalized metaleorganic framework for catalytic hydrolysis of ammonia borane Ning-Zhao Shang, Cheng Feng, Shu-Tao Gao, Chun Wang* College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China
article info
abstract
Article history:
In this study, a bi-metal based nanocatalyst (AgPd@UIO-66-NH2) was synthesized by
Received 29 July 2015
immobilizing AgPd nanoparticles onto the amino-functionalized metal-organic framework
Received in revised form
material (UIO-66-NH2). The catalytic activity of AgPd@UIO-66-NH2for the hydrolysis of
16 October 2015
ammonia borane was evaluated. Among all the prepared AgPd@UIO-66-NH2 catalysts, the
Accepted 16 October 2015
Ag1Pd4@UIO-66-NH2 exhibited the highest catalytic activity for the hydrolysis of ammonia
Available online xxx
borane with a turnover frequency value of 90 mol H2 mol catalyst1 min1, which is among the higher value of the previous reports.
Keywords:
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
UIO-66-NH2 AgPd bimetallic Ammonia borane Hydrolysis Hydrogen storage
Introduction With the aggravation of world energy crisis, developing new energy is getting more and more attention. Hydrogen (H2) as a clean and renewable energy vector is extremely promising amongst the various potential solutions. However, searching for the safe and efficient release of hydrogen under ambient condition still remains one of the most difficult challenges in the hydrogen generation field [1e3]. Recently, ammonia borane (NH3BH3, AB) is found to be a promising candidate for hydrogen generation with many advantages, such as low molecular weight (30.9 g mol1), high hydrogen content (19.6 wt%), and good stability at room temperature [4e7]. So far, there are numerous catalysts that could enhance the
hydrolysis of AB, such as non-noble metal nanoparticles (NPs) [8,9], noble metal NPs [10,11], metal oxides [12], and multicomponent metallic systems [13e16]. However, these catalysts usually suffer from serious leaching or aggregation of metal NPs during the reaction, and the catalytic performance is significantly reduced. Thus, there is still great interest in developing efficient and recyclable catalysts for hydrolysis of AB. A good approach is to anchor the NPs onto specific supports. As a relatively novel class of nanoporous materials, metalorganic frameworks (MOFs) have received significant attention in recent years. Due to the large internal surface areas, uniform but tunable pore size, MOFs have found potential applications in hydrogen storage, drug delivery, gas separation, sensing, and catalysis [17e19]. Currently, the
* Corresponding author. Tel.: þ86 312 7528291; fax: þ86 312 7528292. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.10.062 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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incorporation of metal nanoparticles in MOFs for catalysis of organic reactions have been extensively investigated since the porous structures of MOFs could restrain the aggregation of the metal NPs, and further increase their catalytic activity and recyclability [20]. Up to now, there are a few reports using monometallic or bimetallic NPs@MOF as catalysts for the hydrolysis of AB. For example, Ru@MIL-101, Pd@MIL-101, Ru/ MIL-96 [21e23], and Ni/ZIF-8 monometallic NPs [20] have been prepared to catalyze the hydrolysis reaction of AB. Xu and co-workers investigated the application of MIL-101 as matrix to anchor metal NPs including monometal (Pt) and bimetal (AuCo, AuNi, CuCo and PdCo) within the framework to prepare efficient catalysts for the hydrolysis of AB [24e28]. Bimetallic nanoparticles, a class of metal nanoparticles prepared via different synthesis approaches, have shown excellent catalytic properties for different chemical transformations. The addition of second metal is an important approach to improve the catalytic activity and selectivity [29,30]. Arlin and co-workers fabricated bimetallic PdPt NPs as catalyst for the hydrolysis of AB and the catalytic activities significantly depended on the shape of PdPt NPs [31]. Sun used CoPd as highly efficient catalyst for the hydrolysis of AB. The catalyst showed high catalytic activity when the molar ratio of Co and Pd was 35:65 [32]. Wang et al. reported a new kind of nanofibers, bimetallic AgPd supported on polyacrylonitrile, which exhibited high catalytic activity and durable stability for the reaction system [33]. Zr-MOF (UIO-66) can be obtained by self-assembly of [Zr6O4(OH)4(CO2)12] clusters and terephthalate ligands [34]. The framework possesses tetrahedral and octahedral cages, in a 2:1 ratio, with free dimensions close to 8 and 11 A, respectively. Due to their large surface area, high thermal and chemical stability, UIO-66 and amino-modified Zr-MOF (UIO66-NH2) have attracted increasing attention in many applications. Moreover, the amine groups in UiO-66-NH2 could provide coordination sites for metal ions [35,36]. In continuing our efforts to develop heterogeneous metal catalysts and new functional materials [37e40], herein, we reported the in situ co-reduction method to prepare UIO-66-NH2 supported AgPd nanoparticles, and it was used as catalyst for the hydrolysis of AB. The results demonstrated that Ag1Pd4@UIO-66-NH2 composite exhibited extraordinary catalytic activity toward the hydrolysis of AB under ambient condition.
model JEM-2011(HR) at 200 kV. Scanning electron microscopy (SEM) was conducted with an S-4800 SEM instrument. The surface area, total pore volume and pore size distribution of the samples were measured at 77 K by nitrogen adsorption using a APP V-Sorb 2800P Surface Area and Porosity Analyzer. The XRD patterns of the samples were recorded with a Rigaku D/max 2500 X-ray diffractometer using Cu Ka radiation (40 kV, 150 mA) in the range 2q ¼ 5 e80 . X-ray photoelectron spectroscopy (XPS) was performed with a PHI 1600 spectroscope using Mg Ka X-ray source for excitation. The metal content of the materials was determined by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES) on Thermo Elemental IRIS Intrepid II.
Synthesis of Zr-MOF Zr-MOF was synthesized according to the reported procedure [41]. Generally, ZrCl4 (1250 mg, 5.4 mmol) was dissolved in 60 mL of DMF:HCl (5:1, v/v) under ultrasonication for 20 min. Then, NH2-BDC (1340 mg, 7.5 mmol) and 100 mL of DMF were added into the mixture and sonicated for 20 min. Subsequently, the solution was transferred to a 200 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated in an oven at 80 C overnight. After cooling to room temperature, the resulting solid was collected by centrifugation at 8000 rpm for 10 min and washed with DMF (3 30 mL) and EtOH (3 30 mL), respectively. The obtained UIO-66-NH2 was then immersed in an ethanol solution overnight to replace DMF molecules from the cavities of UIO-66-NH2. Finally, the UiO-66-NH2 was heated in an oven at 180 C in air.
Synthesis of AgPd@UIO-66 For preparation of Ag1Pd4@UIO-66-NH2, 95 mg of UIO-66-NH2 was added to 5 mL water, and the mixture was sonicated for 20 min until it became homogeneous. A aqueous solution of AgNO3 (1.7 mL, 1 mg mL1) was added and stirred at room temperature for 1 h, then a PdCl2 solution (7.1 mL, 1 mg mL1) was added and stirred for 4 h at 25 C. The resulting mixture
Experimental section Chemicals Palladium chloride (PdCl2), zirconium chloride (ZrCl4), 2aminoterephthalic acid (NH2-BDC), and ammonia borane were all obtained from Aladdin Reagent Limited Company. AgNO3, HCl, N, N-dimethyl formamide (DMF), ethanol and other reagents were of analytical grade. All the chemicals were used as received without any purification.
Physical characterizations The size and morphology of the nanoparticles were observed by transmission electron microscopy (TEM) using a JEOL
Fig. 1 e Powder X-ray diffraction patterns of UIO-66-NH2 and AgPd@UIO-66-NH2 with different metal molar.
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Fig. 2 e XPS patterns of the Ag1Pd4@UIO-66-NH2. was reduced by NaBH4 (1 mL, 0.5 mol L1) solution at room temperature for 1 h. After filtration, the resulting solid was washed with H2O (3 10 mL) and then washed with EtOH (3 10 mL). The obtained Ag1Pd4@UIO-66-NH2 was dried at 80 C under reduced pressure. The Pd@UIO-66-NH2, Ag1Pd2@UIO-66-NH2, Ag1Pd1@UIO-66-NH2, Ag2Pd1@UIO-66-NH2, Ag4Pd1@UIO-66-NH2 and Ag@UIO-66-NH2 catalysts were synthesized by using the similar procedure except that the molar ratios of Ag and Pd were 0:1, 1:2, 1:1, 2:1, 4:1 and 1:0, respectively. The contents of Ag and Pd in the composite were determined by ICP-AES.
Hydrogen generation The reaction between NH3BH3 and water was as follows: catalyst
NH3 BH3 þ 2H2 O !NH4 BO2 þ 3H2 The hydrolysis reactions of aqueous NH3BH3 catalyzed by as-prepared AgxPdy@UIO-66-NH2 catalysts were carried out at room temperature. The hydrogen production from AB solution was carried out in a 10 mL round-bottomed flask, which was placed in an oil bath with magnetic stirrer. A gas burette filled with water was connected to the reaction flask to measure the volume of released gas. Firstly, 20 mg catalyst was
Fig. 3 e N2 adsorptionedesorption isotherms of UIO-66NH2 and Ag1Pd4@UIO-66-NH2 at 77 K.
dispersed into 4 mL deionized water in a 10 mL roundbottomed flask. Then, the reaction started when 1.0 mL of the NH3BH3 (23.6 mg, 0.75 mmol) aqueous was injected into the mixture using a syringe. The molar ratio of metal and AB was theoretically fixed at n ¼ 0.0125 for all the catalytic reactions. The volume of the evolved gas was monitored by recording the displacement of water in the gas burette.
Recycle stability After the hydrolysis reaction was exhaustively completed, the catalyst was isolated by centrifugation, washed with water, and dried at 120 C under reduced pressure. Subsequently, the dry powder were weighed and redispersed in 4 mL deionized water. 1.0 mL of the NH3BH3 (23.6 mg, 0.75 mmol) aqueous was injected into the mixture for testing the catalytic activity of the collected catalyst.
Results and discussion Characterization of the catalysts Because of its high specific surface area (BET, >1000 m2 g1) and good stability [41], the microporous Zr-MOF, UIO-66-NH2, was selected as a host matrix to incorporate Pd NPs. UiO-66NH2 is obtained by connecting Zr6O4(OH)4 inorganic cornerstones with NH2-BDC. The framework itself comprises octahedral and tetrahedral cages in a ratio of 1:2. The free diameters of cages are 8 A and 11 A, respectively, and the cages are connected by triangular windows with a diameter of 6 A. The large pore volume of UiO-66-NH2 is appropriate for the loading of metal precursors. UiO-66-NH2 was dispersed in AgNO3 and PdCl2 solution, and then stirred at room temperature. Subsequently, NaBH4 was added into the above mixture for the reduction of Agþ and Pd2þ. This method could improve the dispersion and stabilization of metal nanoparticles and prevent metal nanoparticles from escaping or agglomerating from cavities. To characterize the prepared catalysts, the powder X-ray diffraction (PXRD) pattern was conducted. The PXRD pattern (Fig. 1) of the as-synthesized UIO-66-NH2 is similar to that
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Fig. 4 e SEM (a), TEM (b and c) images and EDS spectrum (d) of Ag1Pd4@UIO-66-NH2. reported in the literature [35], which suggest that UIO-66-NH2 was successfully obtained. Fig. 1 shows that Ag1Pd4@UIO-66NH2 exhibit good crystallinity, indicating that integrity of the UIO-66-NH2 framework was maintained well during the catalyst preparation process. For Ag4Pd1@UIO-66-NH2 and Ag1Pd1@UIO-66-NH2 catalysts, the characteristic peaks intensity of UIO-66-NH2 lowered significantly, which might be ascribed to that AgPd bimetallic nanoparticles weathering or overlap the UIO-66-NH2 tiny crystal face during the catalysts preparation process. For the Ag1Pd1@UIO-66-NH2 and
Fig. 5 e Hydrogen generation from AB in the presence of different catalysts at 25 C, catalyst/AB ¼ 0.0125.
Ag4Pd1@UIO-66-NH2, the obvious XRD peak between the characteristic peaks for Ag (111) (2q ¼ 38.03 ) and Pd (111) (2q ¼ 40.10 ) indicate the high crystallinity of AgePd alloy. Furthermore, no significant diffraction peak of metal species is detected from the PXRD pattern of Ag1Pd4@UIO-66-NH2 (Fig. 1), which can be ascribed to the well dispersed metal nanoparticles and low metal loading. The XPS images of Ag1Pd4@UIO-66-NH2 were shown in Fig. 2. The 3d5/2 and 3d3/2 peak of Pd0 appear at 335.9 eV and 341.2 eV; the 3d5/2 and 3d3/2 peak of Ag0 appear at 373.8 eV and 368.0 eV. No obvious peaks of Agþ and Pd2þ were observed. The N2 adsorptionedesorption isotherms were carried out at 77 K (Fig. 3). The BrunauereEmmetteTeller (BET) surface areas and the total pore volume of as-synthesized UIO-66-NH2 and Ag1Pd4@UIO-66-NH2 were 1170 m2 g1, 0.87 cm3 g1 and 780 m2 g1, 0.51 cm3 g1, respectively. The decrease in the N2 adsorption and the pore volume of Ag1Pd4@UIO-66-NH2 suggested that the metal NPs possibly located on the external surface and inside the mesopores of UIO-66-NH2. The Ag1Pd4@UIO-66-NH2 was also characterized by SEM (Fig. 4a), TEM (Fig. 4b and c) and energy-dispersive spectroscopy (EDS) (Fig. 4d). As shown in Fig. 4a, the AgPd NPs was well dispersed on the surface of MOF, and the mean diameter of AgPd nanoparticles were in the range of 5e9 nm. A representative high-resolution TEM image (Fig. 4c) showed a dspacing of 0.229 nm, which is between the (111) lattice spacing of face-centered cubic Ag (0.24 nm) and Pd (0.22 nm),
Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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Table 1 e Catalytic activity of palladium nanoparticles with different support. Catalyst
Metal/AB Ratio (mol/mol)
Co35Pd65/C annealed 2.1 wt% RGO@Pd Pd/zeolite [email protected] 4 wt%Pd@MIL-101 Pd69Sn31 NPs/C RGO-Cu75Pd25 Pd-Pt @PVP NPs NiAgPd/C Pd(0)NPs/n-HAp Pd-Rh@PVP NPs Pd(0)/SiO2eCoFe2O4 Pd(0)/SiO2 [email protected] RGO/Pd Pd-PVB-TiO2 Pd2þ-HAP Pd-PVB-TiO2 PSSA-co-MA stabilized Pd nanoclusters Ag1Pd4@UIO-66-NH2
TOF Ref. (mol H2 mol catalyst1 min1)
0.024 0.006 0.02 0.02 0.0189 0.0275 0.003 0.03 0.012 0.01 0.003 0.0186 0.0186 0.037 0.04 0.03 0.02 0.06 e
35.7 26.3 6.25 40.89 45 13.64 29.9 125.0 93.8 11 1333.0 254 10 14.52 6.25 17.85 13.5 6 20
0.0125
90
[32] [42] [43] [44] [22] [45] [46] [47] [48] [49] [50] [51] [51] [52] [53] [54] [55] [56] [57]
This study
suggesting that Ag and Pd was formed as an alloy structure. The EDS data (Fig. 4d) further confirmed the presence of the Ag and Pd NPs in the Ag1Pd4@UIO-66-NH2.
Hydrolysis of AB catalyzed by the prepared catalysts Generally, the properties of alloy NPs can be well tuned by varying the composition and atomic ordering. According to the first principles studies, the synergistic effect on the alloy NPs is subject to surface electronic states, which are greatly altered by change of the catalyst's geometric parameters. The AgPd supported on UIO-66-NH2 with different molar ratio were used as catalysts for the hydrolysis reaction of AB. As shown in Fig. 5, the catalytic activities of the catalysts were strongly depended on the composition of AgePd NPs. The Ag1Pd4@UIO-66-NH2 exhibited the highest catalytic activity, with the turnover frequency (TOF) value of 90 mol H2 mol catalyst1 min1, which is among the highest value of the previously reported Pd based catalysts (Table 1).
Fig. 7 e Stability test for the hydrogen generation from aqueous AB in the presence of Ag1Pd4@UIO-66-NH2 catalyst at 25 C, catalyst/AB ¼ 0.0125.
To get the activation energy (Ea) of the AB hydrolysis catalyzed by Ag1Pd4@UIO-66-NH2, the hydrolysis reactions at different temperature ranging from 293 to 308 K were carried out. The values of rate constant k at different temperatures were calculated from the slope of the linear part of each plot from Fig. 6a. The Arrhenius plot of ln k vs. 1/T for the catalyst was plotted in Fig. 6b, from which the apparent activation energy was determined to be 51.77 kJ mol1. As we all know, the reusability of catalyst is crucial in the practical application. We tested the reusability of the Ag1Pd4@UIO-66-NH2 catalyst in the decomposition of AB. As shown in Fig. 7, the catalytic activity of Ag1Pd4@UIO-66-NH2 catalyst was mostly retained after three runs. But experimental results also showed that the as-prepared catalyst displayed a significant loss of activity after being used repetitively for 5 times. The main reasons for activity decrease may be the agglomeration of AgPd nanoparticles grown on the external surfaces of MOFs, and the weight loss of catalyst during the separation process.
Conclusions In conclusion, we have demonstrated the applicability of amino functionalized UIO-66-NH2 supported AgPd bimetallic nanoparticles for the catalytic hydrolysis of AB. By tuning the
Fig. 6 e (a) Volume of the generated gas (H2) versus time, (b) Arrhenius plot (ln (k) vs. 1/T). Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062
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molar ratio of AgPd precursors, a series of AgxPdy@UIO-66-NH2 composite catalysts were synthesized, among which Ag1Pd4@UIO-66-NH2 exhibited the highest catalytic activity for the catalytic hydrolysis of AB, with the TOF value of 90 mol H2 mol catalyst1 min1 at 298 K.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 31171698, 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), Natural Science Foundation of Hebei Province (B2015204003) and the Natural Science Foundation of Agricultural University of Hebei (LG201404, ZD201506).
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Please cite this article in press as: Shang N-Z, et al., Ag/Pd nanoparticles supported on amine-functionalized metaleorganic framework for catalytic hydrolysis of ammonia borane, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.10.062