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Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes
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Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on aminefunctionalized carbon nanotubes Kai Kang, Xiaojun Gu**, Lingling Guo, Penglong Liu, Xueli Sheng, Yanyan Wu, Jia Cheng, Haiquan Su* Inner Mongolia Key Laboratory of Coal Chemistry, School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China
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abstract
Article history:
A series of surfactant-free bimetallic AuM (M ¼ Co, Ni) nanoparticles (NPs) immobilized
Received 19 May 2015
on amine-functionalized carbon nanotubes (CNTs) have been successfully prepared using
Received in revised form
three synthesis methods and then used as effective catalysts for hydrolytic dehydroge-
4 July 2015
nation of ammonia borane (NH3BH3, AB). Surprisingly, compared to the ex situ and one-pot
Accepted 19 July 2015
seeding-growth synthesized AueCo catalysts, the in situ synthesized AueCo alloy catalyst
Available online xxx
exhibited remarkably enhanced activity, featuring turnover frequency (TOF) values of 36.05 1 at 298 and 328 K, respectively, comparable to the values and 148.51 molH2 mol1 cat min
Keywords:
acquired from the reported highly active Co-based catalysts for hydrolytic dehydrogena-
Hydrogen
tion of aqueous AB. The XRD investigations of catalysts after heating at different tem-
Ammonia borane
peratures in Ar atmosphere showed the different existence state of amorphous Co species
Bimetallic nanoparticles
of AueCo NPs. In addition, these catalysts could keep the high activity even after 14 h,
Nanocatalyst
indicating that they had high stability/durability. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As a globally accepted energy carrier with advantages of clean, lightweight and high chemical energy per mass, hydrogen (H2) has gained extensive attention for satisfying the increasing energy demand [1e7]. The search for effective hydrogen storage materials is one of the most difficult challenges toward a fuel-cell-based hydrogen economy [8e17]. Due to the high hydrogen content of 19.6 wt%, nontoxicity
and high stability at room temperature, ammonia borane (NH3BH3, AB) is currently a favourable candidate for chemical hydrogen storage applications [18e23]. There are two primary ways to release hydrogen from AB, namely, thermolysis and catalytic hydrolysis [24,25]. Compared to the thermolysis of AB, where the high temperature is usually required and the hydrogen is commonly released at a relatively low rate, the hydrolysis of AB can proceed rapidly with 100% of H2 selectivity when proper heterogeneous catalysts are employed [26e29].
* Corresponding author. Tel./fax: þ86 471 499 2981. ** Corresponding author. Tel./fax: þ86 471 499 2981. E-mail addresses: [email protected] (X. Gu), [email protected] (H. Su). http://dx.doi.org/10.1016/j.ijhydene.2015.07.081 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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So far, many catalyst systems have been developed for hydrogen production from hydrolysis of AB. In the early stage, heterogeneous noble-metal catalysts such as Pt, Ru and Pd with high performance were the mainstream, but they are too expensive to be widely applied in practical applications [30e34]. Afterwards, transition-metal catalysts such as Fe, Co and Ni have been explored; however, they have only moderate activity [35e39]. Recently, bimetallic nanoparticles (NPs) consisting of noble metal and inexpensive first-row transition metal have exhibited good catalytic performance in many fields mainly due to the synergistic interactions between two different metals [40e47]. Among the factors influencing the catalytic activities of bimetallic NPs, the structures featuring alloy or coreeshell morphologies are in the centre [48e54]. In view of this, various bimetallic NPs in the presence of surfactants, which prevent the NPs from aggregation, have been synthesized for hydrogen production from catalytic hydrolysis of AB [55e59]. However, the protective surfactant shells around the bimetallic NPs occupy the active sites for catalysis and then are unfavourable for catalytic applications. In order to solve this problem, suitable supports such as SiO2, Al2O3 and metal-organic frameworks [60e63], which can not only be beneficial for immobilizing active metal NPs but also be used to optimize the catalytic performance by modulating the electronic structures of active metals [64e66], have been chosen to support active metal NPs. However, the activities of this type of catalysts still need to be remarkably enhanced. Therefore, it is urgent for academic researches and practical use to develop highly active supported bimetallic catalysts for hydrogen production from the hydrolysis of AB through the rational choice of supports and in the meantime to elucidate the effect of microstructures of bimetallic NPs on the corresponding catalytic performance. Herein, we report successful the preparation and catalytic activities of a series of supported bimetallic AuM (M ¼ Co, Ni) NPs synthesized using three facial and general methods, namely in situ, ex situ and one-pot seeding-growth methods. The amine-functionalized CNT was selected as support since it had high surface area and excellent electrical conductivity and then could immobilize metal NPs through amino groups [67e69]. It is interesting that tuning the structures of bimetallic NPs through selecting different methods for catalyst synthesis leaded to the remarkable activity enhancement for hydrolytic dehydrogenation of aqueous AB.
Experimental section
Synthesis and catalytic study One type of CNT-supported AuM alloy NPs with different Au/ M and different transition metals, which were labelled as AuM/CNT-1, were synthesized using a simple in situ synthesis method [70e72]. The molar ratio for AB:NaBH4:metal remained constant at 1:0.04:0.02. The typical synthesis process of AuCo alloy catalyst with 1/7 of Au/Co and its catalytic study was as follows: a mixture of a 4.7 mL of aqueous solution of HAuCl4,4H2O (0.0043 mmol) and CoCl2,6H2O (0.030 mmol) was stored in a two-necked round-bottom flask. CNTs (10 mg) activated by heating at 150 C for 12 h under dynamic vacuum were added into the solution, which was constantly and vigorously stirred for 4 h. After stirring, 1.5 mL of aqueous solution of AB (1.71 mmol) and NaBH4 (0.068 mmol) was injected into the flask through the rubber plug using a syringe, and then the reaction immediately began. One neck of the round-bottom flask was connected to a gas burette, while the other neck was sealed with a rubber plug. Gas evolution was monitored using the gas burette. The reaction finished upon the termination of gas generation. The reactions were initiated at different temperatures (298 K, 308 K, 318 K, and 328 K) with AuCo/CNT-1 under ambient atmosphere. The atmospheric pressure in Hohhot, Inner Mongolia is 88.8 kPa. The other type of CNT-supported AuM alloy catalysts, which were labelled as AuM/CNT-2, were prepared using an ex situ synthesis method under the same conditions as in the in situ synthesized catalysts AuM/CNT-1 except that only NaBH4 was used as the reductant. Once the target alloy catalysts generated, the AB solution was immediately added into the solution containing as-synthesized catalysts to study their catalytic hydrolysis of AB. The catalysts with coreeshell structures of AuM NPs, which were labelled as Au@AuM/CNT, were prepared through a one-pot seeding-growth method under the same conditions as in the in situ synthesized catalysts AuM/CNT-1 except that only AB was used as the reductant. Once AB was added into the suspension containing metal precursors and CNTs, the catalytic reaction immediately began.
Recycle stability Once the first hydrogen generation reaction completed, the aqueous solution containing equivalent AB (1.14 M, 1.5 mL) was added into the reaction flask. The gas evolution was monitored using the gas burette. The cycle experiments on the AuCo/CNT-1, AuCo/CNT-2 and Au@AuCo/CNT catalysts were repeated at room temperature.
Chemicals Catalyst characterization All chemicals were commercial and were used without further purification. AB (Aldrich, 97%), tetrachloroauric (III) acid (HAuCl4$4H2O, Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd, >99%), cobalt (II) chloride hexahydrate (CoCl2$6H2O, Sinopharm Chemical Reagent Co., Ltd, >99%), nickel (II) chloride hexahydrate (NiCl2$6H2O, Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd, >99%), aminofunctionalized multiwall CNTs (Chengdu Organic Chemicals Co., Ltd), and sodium borohydride (NaBH4, J&K Chemical, 98%) were obtained. Deionized water was utilized in all experiments.
Powder X-ray diffraction (XRD) studies were conducted using a Panalytical X-Pert X-ray diffractometer with a Cu-Ka source (40 kV, 20 mA). The surface area measurements were performed by N2 adsorption/desorption at liquid N2 temperature (77 K) after dehydration under vacuum at 150 C for 12 h using automatic volumetric adsorption equipment (Autosorb-iQ2MP). The X-ray photoelectron spectra (XPS) were acquired with an ESCALAB250 (Thermo VG Corp.) equipped with an AlKa X-ray excitation source (1486.6 eV) that operated at 15 kV
Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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and 20 mA. The morphologies of all the samples were observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM, JEM-2010) equipped with an energy dispersive X-ray spectrometer (EDS) for elemental analysis. TEM samples were prepared by depositing one or two droplets of the catalyst suspensions onto amorphous carbon-coated copper grids.
Results and discussion Synthesis and characterization In order to increase the active sites for hydrolytic dehydrogenation of AB, the amine-functionalized CNT was selected as support. In addition, in order to investigate the
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relationship between the structures and properties of supported bimetallic catalysts for hydrolysis of AB, three types of catalysts were synthesized using three methods. In the experiments, a one-pot seeding-growth method was used to synthesize a series of Au@AuM/CNT catalysts with coreeshell structures of bimetallic NPs. The in situ and ex situ methods, where mixed NaBH4 and AB as well as only NaBH4 were selected as reduction reagents, respectively, were used to synthesize two series of CNT-supported AuM alloy catalysts AuM/CNT-1 and AuM/CNT-2, respectively. It should be noted that the two different types of alloy catalysts exhibited remarkably different microstructures, as is testified by XRD results as follows. The morphologies of CNT-supported AuM NPs were characterized by TEM and EDS (Figs. 1 and S1). The results showed that the bimetallic NPs were dispersed on CNTs
Fig. 1 e Representative TEM images and EDS of as-synthesized catalysts (a, b) AuCo/CNT-1, (c, d) AuCo/CNT-2 and (e, f) Au@AuCo/CNT (Au/Co ¼ 1:7). Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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despite very few aggregated NPs being observed, and the sizes of bimetallic NPs in in situ synthesized AuCo/CNT-1, ex situ synthesized AuCo/CNT-2 and one-pot seeding-growth synthesized Au@AuCo/CNT catalysts were in the ranges of 4.0e8.5, 2.0e7.5, and 5.0e11.5 nm, respectively. In the catalyst Au@AuCo/CNT, a distinct contrast of core and shell metals was observed. The EDS spectra of the catalysts showed the presence of metallic species Au, Co or Ni. The SEM investigation showed that all the catalysts featured the typical tubelike morphologies (Fig. S4). The XPS investigation of the bimetallic catalysts exhibited that all the Au species was in the metal state and the Co and Ni species was in oxidation state, indicating that metallic Co and Ni were oxidized (Fig. 2). Moreover, the peaks of Au and Co species could be clearly observed in the XPS patterns of AuCo/ CNT-1, AuCo/CNT-2 and Au@AuCo/CNT, indicating the existence of bimetallic alloy structures in the catalysts. However, on the basis of the above TEM image (Fig. 1e), XPS patterns (Fig. 2e and f) and the reported bimetallic coreeshell structured catalysts prepared using the same reduction method towards
mixed AuCl4 and Co2þ ions [73,80,81], it could be concluded that the components of shell in the present coreeshell structured bimetallic catalyst were Au and Co and the core was Au. In order to further confirm the microstructure of bimetallic NPs in the coreeshell structured catalyst, we synthesized another Au@AuCo/CNT with the mole ratio of Au/Co (6/94) and carried out the XPS measurements. The results exhibited that the XPS patterns of Au@AuCo/CNT with the mole ratio of Au/Co (6/94) were similar to those of Au@AuCo/CNT with mole ratio of Au/ Co (1/7) (Fig. S6). This comparison further confirmed that the components of the shell in the bimetallic coreeshell structured NPs were Au and Co. The XRD patterns showed that Au was crystalline while Co was amorphous in all the bimetallic catalysts (Fig. 3), which was similar to the previous reports [73,74]. In our experiments, the reduction of Co2þ ions by NaBH4 generated metallic Co, which was easily oxidized in the air. It should be noted that no Co2B and Co3(BO3)2 generated in this reduction reaction on the basis of the XRD patterns and EDS, which leaded to remarkably different catalytic properties as bellow [75e78]. The N2
Fig. 2 e XPS spectra for three AueCo catalysts: (a, b) AuCo/CNT-1; (c, d) AuCo/CNT-2; (e, f) Au@AuCo/CNT (Au/Co ¼ 1/7). Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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Fig. 3 e XRD patterns of (a) AuCo/CNT-1, (b) AuCo/CNT-2 and (c) Au@AuCo/CNT; (d) AuCo/CNT-1, (e) AuCo/CNT-2 and (f) Au@AuCo/CNT after heat treatment at 873 K for 4 h in Ar atmosphere; (g) AuCo/CNT-1, h) AuCo/CNT-2 and (i) Au@AuCo/CNT after heat treatment at 1173 K for 4 h in Ar atmosphere.
absorption/desorption measurements showed that the Brunauer-Emmertt-Teller (BET) surface areas of aminefunctionalized CNT, AuCo/CNT-1, AuCo/CNT-2, and Au@AuCo/CNT were 199.019, 176.071, 126.342, and 173.796 cm3,g1, respectively. The appreciable decrease in the amount of N2 adsorption of catalysts indicated that the channels of CNTs were occupied by AuCo NPs and/or blocked by the AuCo NPs located at their surface (Fig. 4).
Catalytic activity To study the different catalytic behaviours, the assynthesized bimetallic catalysts using different methods have been applied as catalysts for hydrolysis of AB. All the catalytic reactions were conducted under the same conditions. As displayed in Fig. 5a, the ex situ synthesized AuCo/ CNT-2 had a dehydrogenation capability of 17.7 min, and a
Fig. 4 e N2 sorption isotherms of support and assynthesized AueCo catalysts at 77 K.
Fig. 5 e Plots of time versus volume of generated H2 from AB aqueous solution (0.276 M, 6.2 mL) over (a) AueCo catalysts and (b) AueNi catalysts at room temperature. The mole ratio of metal/AB ¼ 0.02.
discernible increase in the H2 evolution rate was observed in Au@AuCo/CNT synthesized by one-pot seeding-growth method. Surprisingly, compared to the above two catalysts, the catalytic activity of in situ synthesized AuCo/CNT-1 was remarkably enhanced with a total TOF value of 36.05 1 molH2,mol1 cat,min , comparable to the values acquired from the reported highly active Co-based catalysts for hydrolytic dehydrogenation of aqueous AB (Table 1). This phenomenon indicated that different synthesis methods for synthesizing supported bimetallic catalysts have significant effect on their catalytic activities and the present in situ synthesis method is very effective for highly efficient hydrolysis of AB. In addition, no gas was detected from the hydrolysis of AB using bare CNT as catalyst (Fig. S7). In order to verify the contribution of functional groups in CNT support to catalytic dehydrogenation of aqueous AB, the activities of bare CNT and aminefunctionalized CNT-immobilized AuCo NPs were compared. The results showed that the bare CNT-based catalysts featured lower activities than the amine-functionalized CNTbased catalysts prepared using the same synthesis method (Figs. S8eS10). In general, besides the structures, the components of catalysts remarkably influenced their activities. In view of this, CNT-supported AueNi nanocatalysts were also synthesized using the three methods and they displayed remarkably different activities. As shown in Fig. 5b, the AueNi catalysts were ordered in terms of catalytic activity: Au@AuNi/
Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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CNT > AuNi/CNT-1 > AuNi/CNT-2. It should be noted that as a whole, the AueCo catalysts had higher activities than AueNi catalysts, which could be attributed to the more excellent activity of Co NPs than those of Ni NPs toward hydrogen generation from AB [79,80]. In addition, in the catalytic system of supported AueCo catalysts, the in situ synthesized AuCo/ CNT exhibited the highest activity; however, in the system of supported AueNi catalysts, the one-pot seeding-growth synthesized Au@AuNi/CNT exhibited the highest activity. This indicated that both the components and structures of catalysts affected their catalytic performance such as activity. It is reported that the synergistic effect of two different metal components can improve the catalytic activity of corresponding bimetallic catalysts. The ex situ and one-pot seeding-growth synthesized AueCo catalysts exhibited much higher catalytic activity for hydrolysis of AB than their monometallic counterparts (Fig. S11), indicating a synergetic effect between the binary components. However, in situ synthesized AuCo/CNT and Co/CNT exhibited similar activities. It should be noted that compared to monometallic Co/CNT-2 and Co/CNT, Co/CNT-1 had the highest catalytic activity 1 (Table featuring the TOF value of 32.68 molH2,mol1 cat,min S1), indicating that the in situ synthesis method can be applied to non-noble-metal catalysts for remarkably improving the catalytic hydrolytic dehydrogenation of AB. In addition, the activities of as-synthesized bimetallic AueCo catalysts almost did not change when the ratio of Au/Co was greatly tuned (Fig. S12), which was different from the reported bimetallic catalysts for hydrolysis of AB [81,82]. In order to determine the different state of as-synthesized three types of catalysts, we analysed the XRD patterns of AuCo/CNT-1, AuCo/CNT-2 and Au@AuCo/CNT samples after heating at different temperatures in Ar atmosphere. After heat treatment at 873 K for 4 h, the Co species in Au@AuCo/
CNT and AuCo/CNT-2 could be well crystallized into CoO, while the Co species in AuCo/CNT-1 still kept the amorphous state (Fig. 3). When the temperature was raised to 1173 K, all the Co species in three catalysts could be well crystallized into metallic Co. These phenomena suggested that existence states of Co species in the three catalysts were different. For the practical application of catalysts, the stability/ durability is the key point. In this sense, the durability of the catalysts AuCo/CNT-1, AuCo/CNT-2 and Au@AuCo/CNT was testified by additional aliquot of AB (1.71 mmol) at room temperature. As shown in Fig. 6, after 5 runs, the productivity of H2 remained almost unchanged, indicating that the three supported bimetallic catalysts had long durability, which can be attributed to the formation of bimetallic NPs immobilized by CNTs. Compared to the stable activities of AuCo/CNT-2 and Au@AuCo/CNT in the five runs, the activity of AuCo/CNT-1
Table 1 e TOF and Ea values for hydrolysis of AB catalysed by different catalysts at 298 K. Catalyst
AuCo/CNT-1 AuCo/CNT-2 Au@AuCo/CNT In situ Co Co(0) nanoclusters Au@Co Electroplated CoeP AuCo alloy Co/g-Al2O3 Co/hydroxyapatite CuCo@MIL-101 Cu@Co AueCo@CN AueCo@CN with light AuCo@MIL-101 Co/PEI-GO Ru@Co/graphene Pd@Co/graphene Cu@FeCo Ag@Co/graphene Cu@FeNi
TOF Ea 1 (molH2,mol1 cat, (kJ,mol ) min1) 36.05 8.47 13.24 39.8 25.7 13.7 10 6.0 2.08 4.54 19.6 15 28.4 48.28 23.5 39.9 40.46 40.9 10.5 10.23 4.69
38.82 34.83 41.91 e 34 e 22 e 62 50 ± 2 e e e e e 28.2 e e 38.75 20.03 32.9
Ref.
This work This work This work [72] [39] [73] [23] [73] [83] [7] [38] [80] [74] [74] [82] [37] [52] [56] [53] [54] [59]
Fig. 6 e Durability test for the generation of H2 from aqueous AB solution over as-synthesized catalysts after addition of the same amounts of AB (1.71 mmol) at room temperature: (a) AuCo/CNT-1; (b) AuCo/CNT-2; (c) Au@AuCo/CNT.
Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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after the first run was lowered; however, its activity kept constantly in the rest four runs. This phenomenon was likely to result from the metastable state of AuCo alloy NPs in the in situ synthesis process [73,74]. In order to further explore the stability/durability of the catalysts, the time of duration of the activity test was remarkably extended. The results showed that the three catalysts had no deactivation and there was only some decrease in activities even after 14 h (Figs. S13eS15). Temperature plays an important role in the hydrolysis of AB. The catalytic dehydrogenation rates of three AueCo catalysts towards the AB solution at various temperatures were investigated (Fig. 7). It can be clearly seen that the H2 evolution rate increased gradually with increasing solution temperatures from 298 to 328 K, indicating that a high reaction temperature was beneficial for enhancing the dehydrogenation rate of AB. According to the Arrhenius plot of ln r versus 1/T, the obtained apparent activation energy (Ea) of the catalytic processes involving AuCo/CNT-1, AuCo/CNT-2, and Au@AuCo/CNT were 38.82, 34.83, and 41.91 kJ,mol1, respectively. In comparison with the reported Ea values for the same
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reaction using various catalysts, the present Ea values were in the medium range (Table 1). This result indicated that there probably existed a preferred pathway for AB molecules to form activated species on NPs surface in the three catalysts, which drove us to further explore the possible catalytic mechanism.
Conclusions In summary, three facial methods have been used to prepare a series of amine-functionalized CNT-supported bimetallic AueCo and AueNi catalysts for hydrolytic dehydrogenation of AB. Among all the as-synthesized bimetallic catalysts, the in situ synthesized AueCo alloy catalyst exhibited superior catalytic activity, featuring the high TOF value in the Co-based catalysts for hydrolytic dehydrogenation of aqueous AB. The remarkably different catalytic activities were resulted from the different microstructures of active supported AueCo NPs. In addition, all the as-synthesized catalysts exhibited good
Fig. 7 e Plots of time versus volume of generated H2 from AB aqueous solution (0.276 M, 6.2 mL) and Arrhenius plots and TOF values of AB dehydrogenation over (a, b) AuCo/CNT-1, (c, d) AuCo/CNT-2 and (e, f) Au@AuCo/CNT (Au/Co ¼ 1/7) at different temperatures. Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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recycle stability and were expected to be used in fuel cells, metal/air batteries, and electrochemical sensors. The present facial synthesis methods might be applied to prepare highperformance supported metal catalysts including noblemetal free catalysts for hydrogen generation from AB and other heterogeneous catalysis reactions.
Acknowledgements The authors gratefully acknowledge financial support from the Program for New Century Excellent Talents in University of the Ministry of Education of China (grant no. NCET-130846), the National Natural Science Foundation of China (grant no. 21101089), the Inner Mongolia Natural Science Foundation (2011JQ01), and the Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (grant no. NJYT-13-A01).
Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.07.081.
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Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on aminefunctionalized carbon nanotubes Kai Kang, Xiaojun Gu**, Lingling Guo, Penglong Liu, Xueli Sheng, Yanyan Wu, Jia Cheng, Haiquan Su* Inner Mongolia Key Laboratory of Coal Chemistry, School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China
article info
abstract
Article history:
A series of surfactant-free bimetallic AuM (M ¼ Co, Ni) nanoparticles (NPs) immobilized
Received 19 May 2015
on amine-functionalized carbon nanotubes (CNTs) have been successfully prepared using
Received in revised form
three synthesis methods and then used as effective catalysts for hydrolytic dehydroge-
4 July 2015
nation of ammonia borane (NH3BH3, AB). Surprisingly, compared to the ex situ and one-pot
Accepted 19 July 2015
seeding-growth synthesized AueCo catalysts, the in situ synthesized AueCo alloy catalyst
Available online xxx
exhibited remarkably enhanced activity, featuring turnover frequency (TOF) values of 36.05 1 at 298 and 328 K, respectively, comparable to the values and 148.51 molH2 mol1 cat min
Keywords:
acquired from the reported highly active Co-based catalysts for hydrolytic dehydrogena-
Hydrogen
tion of aqueous AB. The XRD investigations of catalysts after heating at different tem-
Ammonia borane
peratures in Ar atmosphere showed the different existence state of amorphous Co species
Bimetallic nanoparticles
of AueCo NPs. In addition, these catalysts could keep the high activity even after 14 h,
Nanocatalyst
indicating that they had high stability/durability. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As a globally accepted energy carrier with advantages of clean, lightweight and high chemical energy per mass, hydrogen (H2) has gained extensive attention for satisfying the increasing energy demand [1e7]. The search for effective hydrogen storage materials is one of the most difficult challenges toward a fuel-cell-based hydrogen economy [8e17]. Due to the high hydrogen content of 19.6 wt%, nontoxicity
and high stability at room temperature, ammonia borane (NH3BH3, AB) is currently a favourable candidate for chemical hydrogen storage applications [18e23]. There are two primary ways to release hydrogen from AB, namely, thermolysis and catalytic hydrolysis [24,25]. Compared to the thermolysis of AB, where the high temperature is usually required and the hydrogen is commonly released at a relatively low rate, the hydrolysis of AB can proceed rapidly with 100% of H2 selectivity when proper heterogeneous catalysts are employed [26e29].
* Corresponding author. Tel./fax: þ86 471 499 2981. ** Corresponding author. Tel./fax: þ86 471 499 2981. E-mail addresses: [email protected] (X. Gu), [email protected] (H. Su). http://dx.doi.org/10.1016/j.ijhydene.2015.07.081 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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So far, many catalyst systems have been developed for hydrogen production from hydrolysis of AB. In the early stage, heterogeneous noble-metal catalysts such as Pt, Ru and Pd with high performance were the mainstream, but they are too expensive to be widely applied in practical applications [30e34]. Afterwards, transition-metal catalysts such as Fe, Co and Ni have been explored; however, they have only moderate activity [35e39]. Recently, bimetallic nanoparticles (NPs) consisting of noble metal and inexpensive first-row transition metal have exhibited good catalytic performance in many fields mainly due to the synergistic interactions between two different metals [40e47]. Among the factors influencing the catalytic activities of bimetallic NPs, the structures featuring alloy or coreeshell morphologies are in the centre [48e54]. In view of this, various bimetallic NPs in the presence of surfactants, which prevent the NPs from aggregation, have been synthesized for hydrogen production from catalytic hydrolysis of AB [55e59]. However, the protective surfactant shells around the bimetallic NPs occupy the active sites for catalysis and then are unfavourable for catalytic applications. In order to solve this problem, suitable supports such as SiO2, Al2O3 and metal-organic frameworks [60e63], which can not only be beneficial for immobilizing active metal NPs but also be used to optimize the catalytic performance by modulating the electronic structures of active metals [64e66], have been chosen to support active metal NPs. However, the activities of this type of catalysts still need to be remarkably enhanced. Therefore, it is urgent for academic researches and practical use to develop highly active supported bimetallic catalysts for hydrogen production from the hydrolysis of AB through the rational choice of supports and in the meantime to elucidate the effect of microstructures of bimetallic NPs on the corresponding catalytic performance. Herein, we report successful the preparation and catalytic activities of a series of supported bimetallic AuM (M ¼ Co, Ni) NPs synthesized using three facial and general methods, namely in situ, ex situ and one-pot seeding-growth methods. The amine-functionalized CNT was selected as support since it had high surface area and excellent electrical conductivity and then could immobilize metal NPs through amino groups [67e69]. It is interesting that tuning the structures of bimetallic NPs through selecting different methods for catalyst synthesis leaded to the remarkable activity enhancement for hydrolytic dehydrogenation of aqueous AB.
Experimental section
Synthesis and catalytic study One type of CNT-supported AuM alloy NPs with different Au/ M and different transition metals, which were labelled as AuM/CNT-1, were synthesized using a simple in situ synthesis method [70e72]. The molar ratio for AB:NaBH4:metal remained constant at 1:0.04:0.02. The typical synthesis process of AuCo alloy catalyst with 1/7 of Au/Co and its catalytic study was as follows: a mixture of a 4.7 mL of aqueous solution of HAuCl4,4H2O (0.0043 mmol) and CoCl2,6H2O (0.030 mmol) was stored in a two-necked round-bottom flask. CNTs (10 mg) activated by heating at 150 C for 12 h under dynamic vacuum were added into the solution, which was constantly and vigorously stirred for 4 h. After stirring, 1.5 mL of aqueous solution of AB (1.71 mmol) and NaBH4 (0.068 mmol) was injected into the flask through the rubber plug using a syringe, and then the reaction immediately began. One neck of the round-bottom flask was connected to a gas burette, while the other neck was sealed with a rubber plug. Gas evolution was monitored using the gas burette. The reaction finished upon the termination of gas generation. The reactions were initiated at different temperatures (298 K, 308 K, 318 K, and 328 K) with AuCo/CNT-1 under ambient atmosphere. The atmospheric pressure in Hohhot, Inner Mongolia is 88.8 kPa. The other type of CNT-supported AuM alloy catalysts, which were labelled as AuM/CNT-2, were prepared using an ex situ synthesis method under the same conditions as in the in situ synthesized catalysts AuM/CNT-1 except that only NaBH4 was used as the reductant. Once the target alloy catalysts generated, the AB solution was immediately added into the solution containing as-synthesized catalysts to study their catalytic hydrolysis of AB. The catalysts with coreeshell structures of AuM NPs, which were labelled as Au@AuM/CNT, were prepared through a one-pot seeding-growth method under the same conditions as in the in situ synthesized catalysts AuM/CNT-1 except that only AB was used as the reductant. Once AB was added into the suspension containing metal precursors and CNTs, the catalytic reaction immediately began.
Recycle stability Once the first hydrogen generation reaction completed, the aqueous solution containing equivalent AB (1.14 M, 1.5 mL) was added into the reaction flask. The gas evolution was monitored using the gas burette. The cycle experiments on the AuCo/CNT-1, AuCo/CNT-2 and Au@AuCo/CNT catalysts were repeated at room temperature.
Chemicals Catalyst characterization All chemicals were commercial and were used without further purification. AB (Aldrich, 97%), tetrachloroauric (III) acid (HAuCl4$4H2O, Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd, >99%), cobalt (II) chloride hexahydrate (CoCl2$6H2O, Sinopharm Chemical Reagent Co., Ltd, >99%), nickel (II) chloride hexahydrate (NiCl2$6H2O, Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd, >99%), aminofunctionalized multiwall CNTs (Chengdu Organic Chemicals Co., Ltd), and sodium borohydride (NaBH4, J&K Chemical, 98%) were obtained. Deionized water was utilized in all experiments.
Powder X-ray diffraction (XRD) studies were conducted using a Panalytical X-Pert X-ray diffractometer with a Cu-Ka source (40 kV, 20 mA). The surface area measurements were performed by N2 adsorption/desorption at liquid N2 temperature (77 K) after dehydration under vacuum at 150 C for 12 h using automatic volumetric adsorption equipment (Autosorb-iQ2MP). The X-ray photoelectron spectra (XPS) were acquired with an ESCALAB250 (Thermo VG Corp.) equipped with an AlKa X-ray excitation source (1486.6 eV) that operated at 15 kV
Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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and 20 mA. The morphologies of all the samples were observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM, JEM-2010) equipped with an energy dispersive X-ray spectrometer (EDS) for elemental analysis. TEM samples were prepared by depositing one or two droplets of the catalyst suspensions onto amorphous carbon-coated copper grids.
Results and discussion Synthesis and characterization In order to increase the active sites for hydrolytic dehydrogenation of AB, the amine-functionalized CNT was selected as support. In addition, in order to investigate the
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relationship between the structures and properties of supported bimetallic catalysts for hydrolysis of AB, three types of catalysts were synthesized using three methods. In the experiments, a one-pot seeding-growth method was used to synthesize a series of Au@AuM/CNT catalysts with coreeshell structures of bimetallic NPs. The in situ and ex situ methods, where mixed NaBH4 and AB as well as only NaBH4 were selected as reduction reagents, respectively, were used to synthesize two series of CNT-supported AuM alloy catalysts AuM/CNT-1 and AuM/CNT-2, respectively. It should be noted that the two different types of alloy catalysts exhibited remarkably different microstructures, as is testified by XRD results as follows. The morphologies of CNT-supported AuM NPs were characterized by TEM and EDS (Figs. 1 and S1). The results showed that the bimetallic NPs were dispersed on CNTs
Fig. 1 e Representative TEM images and EDS of as-synthesized catalysts (a, b) AuCo/CNT-1, (c, d) AuCo/CNT-2 and (e, f) Au@AuCo/CNT (Au/Co ¼ 1:7). Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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despite very few aggregated NPs being observed, and the sizes of bimetallic NPs in in situ synthesized AuCo/CNT-1, ex situ synthesized AuCo/CNT-2 and one-pot seeding-growth synthesized Au@AuCo/CNT catalysts were in the ranges of 4.0e8.5, 2.0e7.5, and 5.0e11.5 nm, respectively. In the catalyst Au@AuCo/CNT, a distinct contrast of core and shell metals was observed. The EDS spectra of the catalysts showed the presence of metallic species Au, Co or Ni. The SEM investigation showed that all the catalysts featured the typical tubelike morphologies (Fig. S4). The XPS investigation of the bimetallic catalysts exhibited that all the Au species was in the metal state and the Co and Ni species was in oxidation state, indicating that metallic Co and Ni were oxidized (Fig. 2). Moreover, the peaks of Au and Co species could be clearly observed in the XPS patterns of AuCo/ CNT-1, AuCo/CNT-2 and Au@AuCo/CNT, indicating the existence of bimetallic alloy structures in the catalysts. However, on the basis of the above TEM image (Fig. 1e), XPS patterns (Fig. 2e and f) and the reported bimetallic coreeshell structured catalysts prepared using the same reduction method towards
mixed AuCl4 and Co2þ ions [73,80,81], it could be concluded that the components of shell in the present coreeshell structured bimetallic catalyst were Au and Co and the core was Au. In order to further confirm the microstructure of bimetallic NPs in the coreeshell structured catalyst, we synthesized another Au@AuCo/CNT with the mole ratio of Au/Co (6/94) and carried out the XPS measurements. The results exhibited that the XPS patterns of Au@AuCo/CNT with the mole ratio of Au/Co (6/94) were similar to those of Au@AuCo/CNT with mole ratio of Au/ Co (1/7) (Fig. S6). This comparison further confirmed that the components of the shell in the bimetallic coreeshell structured NPs were Au and Co. The XRD patterns showed that Au was crystalline while Co was amorphous in all the bimetallic catalysts (Fig. 3), which was similar to the previous reports [73,74]. In our experiments, the reduction of Co2þ ions by NaBH4 generated metallic Co, which was easily oxidized in the air. It should be noted that no Co2B and Co3(BO3)2 generated in this reduction reaction on the basis of the XRD patterns and EDS, which leaded to remarkably different catalytic properties as bellow [75e78]. The N2
Fig. 2 e XPS spectra for three AueCo catalysts: (a, b) AuCo/CNT-1; (c, d) AuCo/CNT-2; (e, f) Au@AuCo/CNT (Au/Co ¼ 1/7). Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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Fig. 3 e XRD patterns of (a) AuCo/CNT-1, (b) AuCo/CNT-2 and (c) Au@AuCo/CNT; (d) AuCo/CNT-1, (e) AuCo/CNT-2 and (f) Au@AuCo/CNT after heat treatment at 873 K for 4 h in Ar atmosphere; (g) AuCo/CNT-1, h) AuCo/CNT-2 and (i) Au@AuCo/CNT after heat treatment at 1173 K for 4 h in Ar atmosphere.
absorption/desorption measurements showed that the Brunauer-Emmertt-Teller (BET) surface areas of aminefunctionalized CNT, AuCo/CNT-1, AuCo/CNT-2, and Au@AuCo/CNT were 199.019, 176.071, 126.342, and 173.796 cm3,g1, respectively. The appreciable decrease in the amount of N2 adsorption of catalysts indicated that the channels of CNTs were occupied by AuCo NPs and/or blocked by the AuCo NPs located at their surface (Fig. 4).
Catalytic activity To study the different catalytic behaviours, the assynthesized bimetallic catalysts using different methods have been applied as catalysts for hydrolysis of AB. All the catalytic reactions were conducted under the same conditions. As displayed in Fig. 5a, the ex situ synthesized AuCo/ CNT-2 had a dehydrogenation capability of 17.7 min, and a
Fig. 4 e N2 sorption isotherms of support and assynthesized AueCo catalysts at 77 K.
Fig. 5 e Plots of time versus volume of generated H2 from AB aqueous solution (0.276 M, 6.2 mL) over (a) AueCo catalysts and (b) AueNi catalysts at room temperature. The mole ratio of metal/AB ¼ 0.02.
discernible increase in the H2 evolution rate was observed in Au@AuCo/CNT synthesized by one-pot seeding-growth method. Surprisingly, compared to the above two catalysts, the catalytic activity of in situ synthesized AuCo/CNT-1 was remarkably enhanced with a total TOF value of 36.05 1 molH2,mol1 cat,min , comparable to the values acquired from the reported highly active Co-based catalysts for hydrolytic dehydrogenation of aqueous AB (Table 1). This phenomenon indicated that different synthesis methods for synthesizing supported bimetallic catalysts have significant effect on their catalytic activities and the present in situ synthesis method is very effective for highly efficient hydrolysis of AB. In addition, no gas was detected from the hydrolysis of AB using bare CNT as catalyst (Fig. S7). In order to verify the contribution of functional groups in CNT support to catalytic dehydrogenation of aqueous AB, the activities of bare CNT and aminefunctionalized CNT-immobilized AuCo NPs were compared. The results showed that the bare CNT-based catalysts featured lower activities than the amine-functionalized CNTbased catalysts prepared using the same synthesis method (Figs. S8eS10). In general, besides the structures, the components of catalysts remarkably influenced their activities. In view of this, CNT-supported AueNi nanocatalysts were also synthesized using the three methods and they displayed remarkably different activities. As shown in Fig. 5b, the AueNi catalysts were ordered in terms of catalytic activity: Au@AuNi/
Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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CNT > AuNi/CNT-1 > AuNi/CNT-2. It should be noted that as a whole, the AueCo catalysts had higher activities than AueNi catalysts, which could be attributed to the more excellent activity of Co NPs than those of Ni NPs toward hydrogen generation from AB [79,80]. In addition, in the catalytic system of supported AueCo catalysts, the in situ synthesized AuCo/ CNT exhibited the highest activity; however, in the system of supported AueNi catalysts, the one-pot seeding-growth synthesized Au@AuNi/CNT exhibited the highest activity. This indicated that both the components and structures of catalysts affected their catalytic performance such as activity. It is reported that the synergistic effect of two different metal components can improve the catalytic activity of corresponding bimetallic catalysts. The ex situ and one-pot seeding-growth synthesized AueCo catalysts exhibited much higher catalytic activity for hydrolysis of AB than their monometallic counterparts (Fig. S11), indicating a synergetic effect between the binary components. However, in situ synthesized AuCo/CNT and Co/CNT exhibited similar activities. It should be noted that compared to monometallic Co/CNT-2 and Co/CNT, Co/CNT-1 had the highest catalytic activity 1 (Table featuring the TOF value of 32.68 molH2,mol1 cat,min S1), indicating that the in situ synthesis method can be applied to non-noble-metal catalysts for remarkably improving the catalytic hydrolytic dehydrogenation of AB. In addition, the activities of as-synthesized bimetallic AueCo catalysts almost did not change when the ratio of Au/Co was greatly tuned (Fig. S12), which was different from the reported bimetallic catalysts for hydrolysis of AB [81,82]. In order to determine the different state of as-synthesized three types of catalysts, we analysed the XRD patterns of AuCo/CNT-1, AuCo/CNT-2 and Au@AuCo/CNT samples after heating at different temperatures in Ar atmosphere. After heat treatment at 873 K for 4 h, the Co species in Au@AuCo/
CNT and AuCo/CNT-2 could be well crystallized into CoO, while the Co species in AuCo/CNT-1 still kept the amorphous state (Fig. 3). When the temperature was raised to 1173 K, all the Co species in three catalysts could be well crystallized into metallic Co. These phenomena suggested that existence states of Co species in the three catalysts were different. For the practical application of catalysts, the stability/ durability is the key point. In this sense, the durability of the catalysts AuCo/CNT-1, AuCo/CNT-2 and Au@AuCo/CNT was testified by additional aliquot of AB (1.71 mmol) at room temperature. As shown in Fig. 6, after 5 runs, the productivity of H2 remained almost unchanged, indicating that the three supported bimetallic catalysts had long durability, which can be attributed to the formation of bimetallic NPs immobilized by CNTs. Compared to the stable activities of AuCo/CNT-2 and Au@AuCo/CNT in the five runs, the activity of AuCo/CNT-1
Table 1 e TOF and Ea values for hydrolysis of AB catalysed by different catalysts at 298 K. Catalyst
AuCo/CNT-1 AuCo/CNT-2 Au@AuCo/CNT In situ Co Co(0) nanoclusters Au@Co Electroplated CoeP AuCo alloy Co/g-Al2O3 Co/hydroxyapatite CuCo@MIL-101 Cu@Co AueCo@CN AueCo@CN with light AuCo@MIL-101 Co/PEI-GO Ru@Co/graphene Pd@Co/graphene Cu@FeCo Ag@Co/graphene Cu@FeNi
TOF Ea 1 (molH2,mol1 cat, (kJ,mol ) min1) 36.05 8.47 13.24 39.8 25.7 13.7 10 6.0 2.08 4.54 19.6 15 28.4 48.28 23.5 39.9 40.46 40.9 10.5 10.23 4.69
38.82 34.83 41.91 e 34 e 22 e 62 50 ± 2 e e e e e 28.2 e e 38.75 20.03 32.9
Ref.
This work This work This work [72] [39] [73] [23] [73] [83] [7] [38] [80] [74] [74] [82] [37] [52] [56] [53] [54] [59]
Fig. 6 e Durability test for the generation of H2 from aqueous AB solution over as-synthesized catalysts after addition of the same amounts of AB (1.71 mmol) at room temperature: (a) AuCo/CNT-1; (b) AuCo/CNT-2; (c) Au@AuCo/CNT.
Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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after the first run was lowered; however, its activity kept constantly in the rest four runs. This phenomenon was likely to result from the metastable state of AuCo alloy NPs in the in situ synthesis process [73,74]. In order to further explore the stability/durability of the catalysts, the time of duration of the activity test was remarkably extended. The results showed that the three catalysts had no deactivation and there was only some decrease in activities even after 14 h (Figs. S13eS15). Temperature plays an important role in the hydrolysis of AB. The catalytic dehydrogenation rates of three AueCo catalysts towards the AB solution at various temperatures were investigated (Fig. 7). It can be clearly seen that the H2 evolution rate increased gradually with increasing solution temperatures from 298 to 328 K, indicating that a high reaction temperature was beneficial for enhancing the dehydrogenation rate of AB. According to the Arrhenius plot of ln r versus 1/T, the obtained apparent activation energy (Ea) of the catalytic processes involving AuCo/CNT-1, AuCo/CNT-2, and Au@AuCo/CNT were 38.82, 34.83, and 41.91 kJ,mol1, respectively. In comparison with the reported Ea values for the same
7
reaction using various catalysts, the present Ea values were in the medium range (Table 1). This result indicated that there probably existed a preferred pathway for AB molecules to form activated species on NPs surface in the three catalysts, which drove us to further explore the possible catalytic mechanism.
Conclusions In summary, three facial methods have been used to prepare a series of amine-functionalized CNT-supported bimetallic AueCo and AueNi catalysts for hydrolytic dehydrogenation of AB. Among all the as-synthesized bimetallic catalysts, the in situ synthesized AueCo alloy catalyst exhibited superior catalytic activity, featuring the high TOF value in the Co-based catalysts for hydrolytic dehydrogenation of aqueous AB. The remarkably different catalytic activities were resulted from the different microstructures of active supported AueCo NPs. In addition, all the as-synthesized catalysts exhibited good
Fig. 7 e Plots of time versus volume of generated H2 from AB aqueous solution (0.276 M, 6.2 mL) and Arrhenius plots and TOF values of AB dehydrogenation over (a, b) AuCo/CNT-1, (c, d) AuCo/CNT-2 and (e, f) Au@AuCo/CNT (Au/Co ¼ 1/7) at different temperatures. Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081
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recycle stability and were expected to be used in fuel cells, metal/air batteries, and electrochemical sensors. The present facial synthesis methods might be applied to prepare highperformance supported metal catalysts including noblemetal free catalysts for hydrogen generation from AB and other heterogeneous catalysis reactions.
Acknowledgements The authors gratefully acknowledge financial support from the Program for New Century Excellent Talents in University of the Ministry of Education of China (grant no. NCET-130846), the National Natural Science Foundation of China (grant no. 21101089), the Inner Mongolia Natural Science Foundation (2011JQ01), and the Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (grant no. NJYT-13-A01).
Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.07.081.
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Please cite this article in press as: Kang K, et al., Efficient catalytic hydrolytic dehydrogenation of ammonia borane over surfactant-free bimetallic nanoparticles immobilized on amine-functionalized carbon nanotubes, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.081