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Chemically stable electrospun NiCu nanorods@carbon nanofibers for highly efficient dehydrogenation of ammonia borane
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Chemically stable electrospun NiCu nanorods@carbon nanofibers for highly efficient dehydrogenation of ammonia borane Ayman Yousef a,b, Nasser A.M. Barakat c,d,*, Mohamed El-Newehy e, Hak Yong Kim c,** a
BioNanosystem Engineering Department, Chonbuk National University, 664-14 1Ga Duckjin-dong, Jeonju 561-756, Republic of Korea Faculty of Engineering, Matteria, Helwan University, Cairo, Egypt c Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea d Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt e Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia b
article info
abstract
Article history:
We describe the preparation of bimetallic NiCu nanorods (NRs) incorporated on carbon
Received 27 July 2012
nanofibers (NFs). The synthesis nanofibers were prepared by low cost and facile technique;
Received in revised form
electrospinning. Typically, solegel consisting of nickel acetate, copper acetate, and poly
5 September 2012
(vinyl alcohol) was electrospun. Sintering of the electrospun nanofiber mats in argon
Accepted 7 September 2012
atmosphere led to partial elimination of the utilized polymer and abnormal decomposition
Available online 6 October 2012
of the metallic acetates to finally produce NiCu nanorods incorporated in carbon nanofibers. The as-obtained nanofibers were characterized by SEM, FE-SEM, XRD, TGA,
Keywords:
XPS, TEM, and TEM-EDX standard techniques. The introduced bimetallic nanofibers
Ammonia borane
revealed superior catalytic activity toward hydrogen release from ammonia borane.
Hydrogen release
Also, they showed a good chemical stability due to covering the bimetallic nanorods by
Electrospinning
carbon shells. Interestingly, nanofibers were reused for 6 successive cycles with good
Nickel copper nanorods
catalytic activity. Moreover, the prepared nanofibers showed low activation energy about
Carbon nanofibers
28.9 kJ/mol. Finally, development of new catalytic materials in the field of energy is considered as a key objective of the modern research. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Ammonia-borane complex (AB, H6BN) was identified as one of the leading candidates as a hydrogen reservoir owing to its low molecular weight (30.9 g mol1) and high hydrogen content (19.6%wt) which exceeds of gasoline [1,2], soluble/stable in aqueous solution and stable toward hydrolysis in aqueous solutions for at least 4 days. AB is a colorless molecular crystal
with a tetragonal structure under ambient conditions. Pure AB has a density of 0.74 g cm3 and a melting point of 110e125 C. It is first synthesized in the mid-1950s for developing boron hydride based high-energy fuels for jets and rockets. Most importantly, AB is nontoxic, stable, environmentally benign, and can be safely handled under ambient conditions, which greatly facilitate its application [2,3]. It can release hydrogen by two main routes; pyrolysis or a hydrolysis route. The pyrolysis
* Corresponding author. Tel.: þ82632702363; fax: þ82632702348. ** Corresponding author. E-mail addresses: [email protected] (N.A.M. Barakat), [email protected] (H.Y. Kim). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.038
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of solid AB, at potential application temperatures is unable to meet the recent targets and this approach is confounded by the practical disadvantages associated with delivery of a solid material [4]. Actually, for the pyrolysis methodology, complete decomposition of AB was performed at high temperatures and power consumption [5e8] which recommends utilizing the hydrolysis methodology. Recently, the hydrolysis of AB was accomplished by the use of noble and non-noble metallic, bimetallic, and trimetallic nanoparticles (NPs) due to their good catalytic activity and large surface area to volume ratios such as Pt, Rh, and Ru [9e13], Co(0) [14,15], Ni (0) [16], Fe (0) [17], Cu(0) [18], PdCo nanoparticles [19], and Cu@FeNi [20]. However, most of the reported NPs suffer from high aggregation during harsh reaction to release hydrogen from ammonia borane [21,22]. To circumvent this problem, loading of metal NPs on organic or inorganic matrix was reported [23e26]. Recently, one-dimensional (ID) nanostructures have received a good attention due to their optical, electrical, sensing, electrocatalytic, and catalytic property. ID nanostructures showed a superior catalytic activity in many reactions as well as a low aggregation as compared to NPs. Among of them, nanofibers and nanorods have a special interest in the field of catalyst. The high axial ratio aspect provides the nanofibrous morphology special interest especially in the catalytic applications [27e32]. In this study, NiCu nanorode-doped carbon nanofibers have been synthesized by using a low cost and high yield technique; electrospinning. Pure Ni and Cu showed good catalytic activities toward hydrolysis of AB. The chosen of bimetallic NiCu alloy depends on the catalytic activity of the counterparts. It is well known that bimetallic nanostructures showed an attractive property as a catalyst as compared to monometallic counterparts. Generally speaking, alloy materials have distinct binding properties with reactants in contrast to those for monometallic metal catalysts. The strong metalemetal interactions tune the bonding between the catalyst surfaces and the reactants, where the extra stabilization of the transition state on the alloy catalysts in comparison to the corresponding interaction on the monometallic catalyst surface is an additional benefit [33]. NiCu nanoalloys have been extensively studied as a catalyst and electrocatalyst in different reactions such as carbon nanotubes formation and fuel cells [34,35]. This alloy is capable to enhance not only the catalytic activity but also the electrochemical activity and long term stability [36,37]. Zhang et al, demonstrated that the high hydrogen generation from NaHB4 by using near-monodisperse Ni1xCux (x) 0.2e0.8) bimetallic nanocrystals [38]. It is noteworthy to mention that AB more stable than NaHB4 in aqueous solutions. To our best of our knowledge, NiCu NRs-doped carbon NFs have not yet been reported. Covering the effective bimetallic NRs using chemically resistant materials might be a good strategy to enhance the stability of the nanofibers during fast reaction of hydrogen release. Beside the good chemical resistance, carbon has the advantages of adsorption capacity, which helps to improve the contact between ammonia borane and the catalyst. NiCu NRs/Carbon nanofibers are prepared by electrospinning of an aqueous solution consists of nickel acetate tetrahydrate, copper acetate monohydrate, and poly
(vinyl alcohol). Then electrospun nanofiber mats were dried and calcined under vacuum atmosphere. The introduced nanofibers appeared as a superior catalytic activity of hydrogen release from ammonia borane at room temperature in a short time. Also, they showed lower activation energy. Moreover, the present nanofibers containing magnetic element Ni, so nanofibers can be separated easily from the solution and reused [39]. The introduced nanofibers revealed a good stability toward hydrogen release from AB.
2.
Experimental
2.1.
Materials
Nickel (II) acetate tetrahydrate (NiAc, 98%), copper (II) acetate monohydrate (CuAc, 98%), and Ammonia borane complex (AB, 97.0%) were purchased from Aldrich Co., Milwaukee, WI. Poly (vinyl alcohol) (PVA) with a molecular weight (MW) ¼ 65000 g/ mol was obtained from DC chemical Co., South Korea without any further modifications. Distilled water was used as a solvent.
2.2.
Preparation of NiCu nanorods@carbon nanofibers
NiAc and CuAc aqueous solution were first prepared by dissolving NiAc:CuAc at ratio 80:20 wt% in 5 g distilled water and then mixed with a PVA/H2O solution (10 wt%). The polymer content in the prepared solegel was 7.14 wt%. Finally, the mixture was stirred at 50 C for 5 h. The prepared solegel was electrospun at a voltage of 20 kV using a high voltage DC power supply. The formed nanofiber mats were initially dried for 24 h at 60 C under vacuum and then calcined at 750 C for 3 h in argon atmosphere with a heating rate of 2.3 C/min.
2.3. Catalytic hydrolysis of AB by NiCu nanorods@carbon nanofibers The catalytic activity of the as-obtained nanofibers toward AB hydrolysis was evaluated by measuring the rate of hydrogen generation in a typical water-filled gas burette system. Before starting the catalytic activity test, a jacketed reaction flask (25 mL) containing a teflon-coated stir bar was placed on a magnetic stirrer and thermostated at a specific temperature. Then, a burette filled with water was connected to the reaction flask to measure the volume of the hydrogen gas to be evolved from the reaction. Next, the 75 mL aqueous dispersion of the catalyst was transferred into the reaction flask and 31.5 mg (1 mmol) AB (corresponding to the generation of a maximum 3 mmol ¼ 67 mL H2 gas at 25.0 1 C and 0.91 atm pressure) was added into the catalyst solution under 600 rpm stirring rate. The volume of hydrogen gas evolved was measured by recording the displacement of water level every minute. It is noteworthy mentioning that the water vapor has been estimated and deducted from the total gas obtained. The reaction was ceased when the no hydrogen gas generation was observed. For a control experiment, the same experiment was repeated without any catalytic material, it was observed that no appreciable hydrogen gas evolved. The reusability of the introduced nanofibers has been investigated by using the same catalyst without any regeneration process for several successive cycles.
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After every cycle, another equivalent (1 mmol) of AB was added to the reaction system and the hydrogen released was measured once again. This experiment was repeated up to six runs. Moreover, the hydrolysis of ammonia borane was performed at various temperatures (25, 30, 35 and 40 C) in order to estimate the activation energy (Ea) [19].
2.4.
Characterization
The surface morphology of the as-obtained nanofibers was studied by a JEOL JSM-5900 scanning electron microscope (SEM, JEOL Ltd., Japan) and field-emission scanning electron microscope (FE-SEM, Hitachi S-7400, Japan). The phase and crystallinity of the catalyst were characterized using a Rigaku X-ray diffractometer (XRD, Rigaku Co., Japan) with Cu Ka (l ¼ 1.54056 A) radiation over a range of 2q angles from 20 to 80 . The surface composition was detected with an X-ray photoelectron spectroscopy analysis (XPS, AXIS-NOVA, Kratos Analytical, UK) with the following conditions: base pressure of 6.5 109 Torr, resolution (pass energy) of 20 eV and scan step of 0.05 eV/step. High-resolution images and selected area electron diffraction patterns were observed by a JEOL JEM2200FS transmission electron microscope (TEM) operating at 200 kV equipped with EDX (TEM-EDX), JEOL Ltd., Japan).
3.
Results and discussion
Fig. 1A and B show the SEM and FE-SEM images, respectively of the electrospun nanofibers obtained from the prepared CuAC/NiAc/PVA solegel. As shown in these figures, smooth and continuous nanofibers were obtained. Fig. 2 depicts the SEM and FE-SEM images of the obtained powder after sintering of the electrospun nanofiber mats in argon atmosphere. It was showed that the proposed sintering condition did not affect in the nanofibrous morphology as the nanofibers structure was kept after the sintering process. Moreover, nanorods-like structure grown inside nanofibers after calcinations can be observed in Fig. 2C. The typical XRD pattern of the sintered powder was shown in Fig. 3. As shown in this figure, the peaks at 2qw43.9 , 51.0 , and 75.5 agree well with the crystal planes (111), (200), and (220), respectively. According to the JCDPS XRD data base, these peaks indicate
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formation of pristine Ni (JCDPS 04e0850) and Cu3.8Ni alloy (JCDPS 47e1406) and simultaneously match other reported literature [40,41]. Moreover, there is another peak can be observed at 2qw26 corresponding to graphite like-carbon (JCPDS 41e1487). According to the obtained XRD results, one can claim that the NRs and NFs represent NiCu and carbon, respectively. Nickel and copper have the same face centered cubic structure with space group glass (S.G) of Fm3m(225) and very close lattice parameters (a ¼ 3.523 for Ni and 3.616 for Cu), so these metals can form a solid solution alloy with a wide range of composition [40]. It is noteworthy that Ni and NiCu have the catalytic activity toward a graphitizing process of the utilized polymer [42], this might be a good explanation for carbon detection. Formation of pure nickel from the utilized precursor can be explained by the following equations [43]: Ni(CH3COO)2$4H2O / 0.86Ni(CH3COO)2$0.14Ni(OH)2 þ 0.28CH3COOH þ 3.72H2O
(1)
0.86 Ni(CH3COO)2$0.14Ni(OH)2 / NiCO3 þ NiO þ CH3COCH3 þ H2O
(2)
NiCO3 / NiO þ CO2
(3)
NiO þ CO / Ni þ CO2
(4)
Carbon monoxide in equation (4) comes from decomposition of the resultant acetic acid in the first equation. By the same way other authors explained the formation of pure copper from thermal decomposition of copper acetate under inert atmosphere [44] as follow: Cu(CH3COO)2.H2O / Cu(CH3COO)2 þ H2O
(5)
2Cu(CH3COO)2 / 2Cu þ 3CH3COOH þ CO2 þ H2 þ C
(6)
To estimate the carbon content in the obtained nanofibers, TGA analysis was carried out for the obtained nanofibers under an oxygen atmosphere. Fig. 4 displays the obtained results. As shown in the figure, the remaining residuals represent 63.71 wt% from the original sample. Considering
Fig. 1 e SEM; (A) and FE-SEM; (B) images of CuAC/NiAC/PVA mat after drying at 60 C for 24 h.
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Fig. 2 e FE-SEM image of NiAC/CuAC/PVA mat nanofibers after sintering at 750 C for 3 h in argon atmosphere.
that Ni and Cu are active metals and can be easily converted to NiO and CuO, respectively upon heating in an oxygen atmosphere and the carbon content will evolve in the form CO and/ or CO2 gases. In other words, the remaining residuals consist of CuO and NiO. As Ni and Cu content in the original electrospun nanofibers are known, the carbon content in the sintered nanofibers can be roughly estimated using mathematical calculations to be 49.76 wt%.
To determine the valence state, chemical bonding, and the surface composition and oxidation state of the materials XPS analysis was used. It is sensitive to information on the surface layer up to 10 nm for the inorganic materials. Fig. 5 shows the high sensitivity survey spectrum of the obtained sintered nanofibers (the inset represents the low scan analyses within the Ni and Cu main ranges). The blue inset confirms the presence of Cu2p3/2 and Cu2p1/2 peaks lies at binding energies
Fig. 3 e XRD pattern of the calcined NiAC/CuAC/PVA mat nanofibers at 750 C for 3 h in argon atmosphere.
Fig. 4 e TGA analysis for the obtained NiCu NRs/carbon nanofibers under oxygen atmosphere.
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Fig. 5 e XPS spectrum of the synthesized nanofibers. The insets represent very low scan rate measurements at the Cu and Ni reported ranges.
932.7 and 952.55 eV, respectively, these values are closely match reported values in the literature for metallic copper [45e47]. Moreover, the red pattern inset demonstrates the emerged of Ni2p3/2 and Ni2p1/2 peaks lies at binding energies 852.7 and 870.25 eV, respectively, these values also matches previous results for metallic nickel [48e50].
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To investigate the hypothesis of the distributed of NiCu nanorods on the carbon nanofibers; TEM analysis technique was used. Fig. 6A, B and C show the normal TEM images, as shown in the figures nanorods are highly distributed along with the nanofibers. Fig. 6D indicates HR-TEM image. As seen in the image, the prepared NRs have different crystal lattice compared to the matrix nanofibers. Moreover, the NFs have an amorphous structure but the NRs have high crystalline one. According the obtained images, one can claim that the formed structure is bimetallic nanorods suspended in carbon nanofibers. In other words, the synthesized bimetallic nanorods are sheathed by carbon shell. The inset in Fig. 6B shows the characteristic rings in the electron diffraction pattern (SAED); a good crystalline of the synthesized nanofibers can be concluded which supports the HR-TEM result. To properly understand the composition and elementary analysis of the obtained NiCu NRs/carbon NFs, STEM and TEM-EDX have been carried out. The results are demonstrated in Fig. 7. Fig. 7A indicates an STEM image of the selected nanofiber. It is very clear from the image that NiCu nanorods envelope inside the carbon nanofiber. Fig. 7B demonstrates the line EDX result corresponding to the line shown in Fig. 7A. As shown in Fig. 7B, Cu and Ni have the same elemental distribution along with the chosen line which indicates good alloying characteristics of the obtained bimetallic NRs. Moreover, the carbon elemental distribution confirms complete covering of the bimetallic NRs as carbon presents at all points along within the chosen line. The presence of carbon might be effective to enhance the catalytic activity of the obtained
Fig. 6 e TEM analysis results for the sintered nanofiber. Panels A, B, and C represent the normal TEM images. The inset in panel B demonstrates the selected area electron diffraction pattern (SAED). Panel D represents the high resolution TEM image (HR-TEM).
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Fig. 7 e TEM analysis result of the calcined nanofibers; panel (A) Rrepresents the normal TEM image along with the line EDX analysis. Panel B demonstrate line analysis TEM EDX results for the line shown in A.
nanofibers which behaves as an adsorbent in any catalytic chemical reactions by increasing the attachment of the target with catalytic material.
3.1.
Hydrogen release from ammonia borane
Fig. 8 depicts the time course of hydrogen evolution by AB hydrolysis at the experiments were conducted at 25 C using different catalyst; NiCu NRs/carbon nanofibers and Ni and Cu NPs for comparison. The amount of each catalyst utilized was 30 mg, AB ¼ 31.5 mg and water ¼ 75 ml. As shown in the figure, the NPs of the utilized metals showed the low amount of hydrogen release as compared to introduced CuNi NRs/ carbon nanofibers. Within the relatively long experimental time, the pristine metallic nanoparticles could not completely
Fig. 8 e Hydrogen release from aqueous ammonia borane in the presence of from CuNi NRs-doped carbon nanofibers, Ni NPS, and Cu NPs catalysts at room temperature. The amount of the utilized catalyst was 30 mg for each formulation.
hydrolyze the ammonia borane. The volume of the hydrogen released from the synthesized nanofibers was higher than commercial Ni and Cu NPs. The good catalytic activity of the introduced nanofibers can be explicated by three expected reasons; first the adsorption capacity of carbon which can adsorb AB molecules on the surface of the nanofibers. Second, the bimetallics have catalytic activity more than monometallics. Finally, the as-obtained CuNi NRs loading on carbon nanofibers might decrease the agglomeration that regularly happens with the NPs. Fig. 9 displays the effect of catalyst loading on the catalytic reactivity of NiCu NRs/carbon nanofibers. Different catalyst: AB ratios (0.3, 0.6, and 1) have been used under the same aforementioned conditions. In each experiment, the ammonia borane amount was kept at 31.5 mg and the amount of catalyst was changed to obtain different ratios. As shown in
Fig. 9 e The effect catalyst to ammonia borane ratio on the hydrogen release from aqueous ammonia borane at ambient conditions.
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Table 1 e Activation energy (Ea) values for hydrolysis of AB catalyzed by different catalysts. Catalyst
Fig. 10 e Effect of recyclying on the catalytic activity of NiCu NRs-doped carbon nanofibers on the catalytic activity. The experiments were conducted at room temperature and the catalyst to ammonia borane ratio was kept at 1.
the figure, increase the catalyst: AB ratio from 0.3 to 1 has a distinct impact as a considerable difference in the obtained hydrogen can be observed. Increase the ratio to 1 had a distinct influence as most of the theoretical hydrogen embedded in AB was released within almost 15 min as shown in the figure. It is noteworthy to mention that reusing the catalyst did not affect the catalytic activity as can be concluded from Fig. 10 which demonstrates the obtained hydrogen after using the introduced NiCu NRs/carbon nanofibers for six successive cycles. As shown in Fig. 10, the result appears same catalytic activity toward hydrogen release upon 6 cycles. This finding explains that CuNi NRs-doped carbon nanofibers are more stable than Ni and Cu NPs as well as avoid the agglomeration problem of the NPs.
Ni powder Co/Ɣ-Al2O3 Ni0.97Pt0.03 hollow spheres Hydrogel supported Ni (0) nanoparticles Hydrogel supported Cu (0) nanoparticles Hydrogel supported Co (0) nanoparticles Zeolite-confined Cu NiAg Laurate-stabilized Ru Ni0.03Pt0.97 alloys [email protected]/C Ni0.65Pt0.35 alloys Ni0.35Pt0.65 alloys [email protected]/C [email protected] Ni0.88Pt0.12 hollow spheres Co-B/C NiCu nanorods@ C nanofibers PdCo nanoparticles Ru/Ɣ-Al2O3 Rh/Ɣ-Al2O3 Pt/Ɣ-Al2O3
Ea (kJ/mol)
Ref.
70 62 57 52.8
[51] [52] [51] [25]
48.8
[25]
47.7
[25]
51.8 1.8 51.5 47.0 2.2 43.7 41.5 39.3 39 33 32.9 30 29.2 28.9 27 23 21 21
[18] [53] [13] [54] [55] [54] [54] [56] [20] [51] [57] This study [19] [52] [52] [52]
Fig. 11A shows the effect of temperature in the generated hydrogen. As shown in the figure, the amount of hydrogen released increased with increasing the temperature from 298 to 313 K. Because of the chemical reaction rate increased with increasing the temperature. The activation energy (Ea) of the AB hydrolysis by the synthesized nanofibers was calculated from the Arrhenius plot ln K vs.1/T (Fig. 11B), from which Ea 28.9 K J/mole, this value of activation energy is lower than most of the reported Ea values of the noble and non-noble metals (Table 1), which indicates a superior catalytic performance of the synthesized NiCu NRs/carbon nanofibers.
Fig. 11 e Hydrogen release from aqueous ammonia borane in the presence NiCu NRs/carbon nanofibers catalysts at different temperatures; (A), Arrhenius plot of ln K vs (1/T ); (B).
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4.
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Conclusion
Electrospinning process can be used to prepare bimetallic NRs-carbon nanofibers in a single production step. The prepared NiCu NRs/carbon nanofibers used as effective and chemically stable catalyst. The introduced nanofibers showed a superior catalytic activity toward hydrogen release from ammonia borane as well as low activation energy was obtained. Moreover, the produced nanofibers are easy to be reused because of the presence of magnetic element (Ni). Reusing the introduced nanofibers does not influence the corresponding catalytic activity as all the embedded hydrogen was obtained after six successive cycles.
Acknowledgment This research was supported by NPST program by King Saud University project number 11-ENE1721-02. Following are results of a study on the “Leaders in Industry-University Cooperation” Projected, Supported by the Ministry of Education, Science &Technology (MEST) and National Research Foundation of Korea (NRF). We thank Mr. T. S. Bae, Mr. J. C. Lim, Dr. Lee Young-Boo, KBSI, Jeonju branch for taking highquality FESEM, TEM-EDX and TEM images, respectively.
references
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[29] Barakat NAM, Farage TE, Kanjwal MA, Park SJ, Arjmand FA, Kim HY. Silver Nanofibres by a novel electrospinning process: nanofibres with plasmon resonance in the IR region and thermal hysteresis electrical conductivity features. Eur J Inorg Chem 2010;10:1481e8. [30] Yousef A, Barakat NAM, Amna T, Al-Deyab SS, Hassan MS, Abdel-hay A, et al. Inactivation of pathogenic Klebsiella pneumoniae by CuO/TiO2 nanofibers: a multifunctional nanomaterial via one-step electrospinning. Ceram Int 2012; 38:4525e32. [31] Yousef A, Barakat NAM, Khalil KA, Unnithana AR, Panthia G, Panta B, et al. Photocatalytic release of hydrogen from ammonia borane-complex using Ni (0)-doped TiO2/C electrospun nanofibers. Colloids Surf A 2012;410:59e65. [32] Barakat NAM, Kanjwa MA, Al-Deyab SS, Chronakis IS, Kim HY. Influence of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Mater Sci App 2011;2:1180e7. [33] Jiang HL, Xu Q. Recent progress in synergistic catalysis over heterometallic nanoparticles. J Mater Chem 2011;21:13705e25. [34] Ding R, Liu J, Jiang J, Wu F, Zhu J, Huang X. Tailored NieCu alloy hierarchical porous nanowire as a potentialefficient catalyst for DMFCs. Catal Sci Technol 2011;1:1406e11. [35] Reshetenko TV, Avdeeva LB, Ismagilov ZR, Chuvilin AL, Ushakov VA. Carbon capacious NieCueAl2O3 catalysts for high-temperature methane decomposition. Appl Catal A 2003;247:51e63. [36] Nishimura K, Machida K, Enyo M. Electrooxidation of formate and formaldehyde on electrooxide of alloys between Pd and groups 1B metals in alkaline media: part11: the possibility of complete oxidation of formaldehyde in weak alkali. J Electroanal Chem 1988;251:117e25. [37] Morallon E, Cases FJ, Vazquez JL. Irreversible adsorption of methanol on Pt (110) in carbonate solution. Electrochim Acta 1992;37:1883e6. [38] Zhang Y, Huang W, Habas SE, Kuhn JN, Grass ME, Yamada Y, et al. Near-monodisperse Ni-Cu bimetallic nanocrystals of variable composition: controlled synthesis and catalytic activity for H2 generation. J Phys Chem C 2008;112:12092e5. [39] Saeed GH, Radiman MS, Gasaymeh SS, Lim HN, Huang NM. Mild hydrothermal synthesis of NieCu nanoparticles. J Nanomaterials 2010:2010. http://dx.doi.org/10.1155/2010/ 184137. [40] Danaee I, Jafarian M, Forouzandeh F, Gobal F, Mahjani MG. Electrochemical impedance studies of methanol oxidation on GC/Ni and GC/NiCu electrode. Int J Hydrogen Energy 2009; 34:859e69. [41] Alper M, Kockar H, Safak M, Celalettin Baykul M. Comparison of NieCu alloy films electrodeposited at low and high pH levels. J Alloys Compd 2008;453(1e2):15e9. [42] Otani S, Okuda K, Matsuda HS. Carbon fiber, Vol. 1. Tokyo: Kindai Henshu Ltd; 1986. 8e9.
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Chemically stable electrospun NiCu nanorods@carbon nanofibers for highly efficient dehydrogenation of ammonia borane Ayman Yousef a,b, Nasser A.M. Barakat c,d,*, Mohamed El-Newehy e, Hak Yong Kim c,** a
BioNanosystem Engineering Department, Chonbuk National University, 664-14 1Ga Duckjin-dong, Jeonju 561-756, Republic of Korea Faculty of Engineering, Matteria, Helwan University, Cairo, Egypt c Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea d Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt e Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia b
article info
abstract
Article history:
We describe the preparation of bimetallic NiCu nanorods (NRs) incorporated on carbon
Received 27 July 2012
nanofibers (NFs). The synthesis nanofibers were prepared by low cost and facile technique;
Received in revised form
electrospinning. Typically, solegel consisting of nickel acetate, copper acetate, and poly
5 September 2012
(vinyl alcohol) was electrospun. Sintering of the electrospun nanofiber mats in argon
Accepted 7 September 2012
atmosphere led to partial elimination of the utilized polymer and abnormal decomposition
Available online 6 October 2012
of the metallic acetates to finally produce NiCu nanorods incorporated in carbon nanofibers. The as-obtained nanofibers were characterized by SEM, FE-SEM, XRD, TGA,
Keywords:
XPS, TEM, and TEM-EDX standard techniques. The introduced bimetallic nanofibers
Ammonia borane
revealed superior catalytic activity toward hydrogen release from ammonia borane.
Hydrogen release
Also, they showed a good chemical stability due to covering the bimetallic nanorods by
Electrospinning
carbon shells. Interestingly, nanofibers were reused for 6 successive cycles with good
Nickel copper nanorods
catalytic activity. Moreover, the prepared nanofibers showed low activation energy about
Carbon nanofibers
28.9 kJ/mol. Finally, development of new catalytic materials in the field of energy is considered as a key objective of the modern research. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Ammonia-borane complex (AB, H6BN) was identified as one of the leading candidates as a hydrogen reservoir owing to its low molecular weight (30.9 g mol1) and high hydrogen content (19.6%wt) which exceeds of gasoline [1,2], soluble/stable in aqueous solution and stable toward hydrolysis in aqueous solutions for at least 4 days. AB is a colorless molecular crystal
with a tetragonal structure under ambient conditions. Pure AB has a density of 0.74 g cm3 and a melting point of 110e125 C. It is first synthesized in the mid-1950s for developing boron hydride based high-energy fuels for jets and rockets. Most importantly, AB is nontoxic, stable, environmentally benign, and can be safely handled under ambient conditions, which greatly facilitate its application [2,3]. It can release hydrogen by two main routes; pyrolysis or a hydrolysis route. The pyrolysis
* Corresponding author. Tel.: þ82632702363; fax: þ82632702348. ** Corresponding author. E-mail addresses: [email protected] (N.A.M. Barakat), [email protected] (H.Y. Kim). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.038
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of solid AB, at potential application temperatures is unable to meet the recent targets and this approach is confounded by the practical disadvantages associated with delivery of a solid material [4]. Actually, for the pyrolysis methodology, complete decomposition of AB was performed at high temperatures and power consumption [5e8] which recommends utilizing the hydrolysis methodology. Recently, the hydrolysis of AB was accomplished by the use of noble and non-noble metallic, bimetallic, and trimetallic nanoparticles (NPs) due to their good catalytic activity and large surface area to volume ratios such as Pt, Rh, and Ru [9e13], Co(0) [14,15], Ni (0) [16], Fe (0) [17], Cu(0) [18], PdCo nanoparticles [19], and Cu@FeNi [20]. However, most of the reported NPs suffer from high aggregation during harsh reaction to release hydrogen from ammonia borane [21,22]. To circumvent this problem, loading of metal NPs on organic or inorganic matrix was reported [23e26]. Recently, one-dimensional (ID) nanostructures have received a good attention due to their optical, electrical, sensing, electrocatalytic, and catalytic property. ID nanostructures showed a superior catalytic activity in many reactions as well as a low aggregation as compared to NPs. Among of them, nanofibers and nanorods have a special interest in the field of catalyst. The high axial ratio aspect provides the nanofibrous morphology special interest especially in the catalytic applications [27e32]. In this study, NiCu nanorode-doped carbon nanofibers have been synthesized by using a low cost and high yield technique; electrospinning. Pure Ni and Cu showed good catalytic activities toward hydrolysis of AB. The chosen of bimetallic NiCu alloy depends on the catalytic activity of the counterparts. It is well known that bimetallic nanostructures showed an attractive property as a catalyst as compared to monometallic counterparts. Generally speaking, alloy materials have distinct binding properties with reactants in contrast to those for monometallic metal catalysts. The strong metalemetal interactions tune the bonding between the catalyst surfaces and the reactants, where the extra stabilization of the transition state on the alloy catalysts in comparison to the corresponding interaction on the monometallic catalyst surface is an additional benefit [33]. NiCu nanoalloys have been extensively studied as a catalyst and electrocatalyst in different reactions such as carbon nanotubes formation and fuel cells [34,35]. This alloy is capable to enhance not only the catalytic activity but also the electrochemical activity and long term stability [36,37]. Zhang et al, demonstrated that the high hydrogen generation from NaHB4 by using near-monodisperse Ni1xCux (x) 0.2e0.8) bimetallic nanocrystals [38]. It is noteworthy to mention that AB more stable than NaHB4 in aqueous solutions. To our best of our knowledge, NiCu NRs-doped carbon NFs have not yet been reported. Covering the effective bimetallic NRs using chemically resistant materials might be a good strategy to enhance the stability of the nanofibers during fast reaction of hydrogen release. Beside the good chemical resistance, carbon has the advantages of adsorption capacity, which helps to improve the contact between ammonia borane and the catalyst. NiCu NRs/Carbon nanofibers are prepared by electrospinning of an aqueous solution consists of nickel acetate tetrahydrate, copper acetate monohydrate, and poly
(vinyl alcohol). Then electrospun nanofiber mats were dried and calcined under vacuum atmosphere. The introduced nanofibers appeared as a superior catalytic activity of hydrogen release from ammonia borane at room temperature in a short time. Also, they showed lower activation energy. Moreover, the present nanofibers containing magnetic element Ni, so nanofibers can be separated easily from the solution and reused [39]. The introduced nanofibers revealed a good stability toward hydrogen release from AB.
2.
Experimental
2.1.
Materials
Nickel (II) acetate tetrahydrate (NiAc, 98%), copper (II) acetate monohydrate (CuAc, 98%), and Ammonia borane complex (AB, 97.0%) were purchased from Aldrich Co., Milwaukee, WI. Poly (vinyl alcohol) (PVA) with a molecular weight (MW) ¼ 65000 g/ mol was obtained from DC chemical Co., South Korea without any further modifications. Distilled water was used as a solvent.
2.2.
Preparation of NiCu nanorods@carbon nanofibers
NiAc and CuAc aqueous solution were first prepared by dissolving NiAc:CuAc at ratio 80:20 wt% in 5 g distilled water and then mixed with a PVA/H2O solution (10 wt%). The polymer content in the prepared solegel was 7.14 wt%. Finally, the mixture was stirred at 50 C for 5 h. The prepared solegel was electrospun at a voltage of 20 kV using a high voltage DC power supply. The formed nanofiber mats were initially dried for 24 h at 60 C under vacuum and then calcined at 750 C for 3 h in argon atmosphere with a heating rate of 2.3 C/min.
2.3. Catalytic hydrolysis of AB by NiCu nanorods@carbon nanofibers The catalytic activity of the as-obtained nanofibers toward AB hydrolysis was evaluated by measuring the rate of hydrogen generation in a typical water-filled gas burette system. Before starting the catalytic activity test, a jacketed reaction flask (25 mL) containing a teflon-coated stir bar was placed on a magnetic stirrer and thermostated at a specific temperature. Then, a burette filled with water was connected to the reaction flask to measure the volume of the hydrogen gas to be evolved from the reaction. Next, the 75 mL aqueous dispersion of the catalyst was transferred into the reaction flask and 31.5 mg (1 mmol) AB (corresponding to the generation of a maximum 3 mmol ¼ 67 mL H2 gas at 25.0 1 C and 0.91 atm pressure) was added into the catalyst solution under 600 rpm stirring rate. The volume of hydrogen gas evolved was measured by recording the displacement of water level every minute. It is noteworthy mentioning that the water vapor has been estimated and deducted from the total gas obtained. The reaction was ceased when the no hydrogen gas generation was observed. For a control experiment, the same experiment was repeated without any catalytic material, it was observed that no appreciable hydrogen gas evolved. The reusability of the introduced nanofibers has been investigated by using the same catalyst without any regeneration process for several successive cycles.
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After every cycle, another equivalent (1 mmol) of AB was added to the reaction system and the hydrogen released was measured once again. This experiment was repeated up to six runs. Moreover, the hydrolysis of ammonia borane was performed at various temperatures (25, 30, 35 and 40 C) in order to estimate the activation energy (Ea) [19].
2.4.
Characterization
The surface morphology of the as-obtained nanofibers was studied by a JEOL JSM-5900 scanning electron microscope (SEM, JEOL Ltd., Japan) and field-emission scanning electron microscope (FE-SEM, Hitachi S-7400, Japan). The phase and crystallinity of the catalyst were characterized using a Rigaku X-ray diffractometer (XRD, Rigaku Co., Japan) with Cu Ka (l ¼ 1.54056 A) radiation over a range of 2q angles from 20 to 80 . The surface composition was detected with an X-ray photoelectron spectroscopy analysis (XPS, AXIS-NOVA, Kratos Analytical, UK) with the following conditions: base pressure of 6.5 109 Torr, resolution (pass energy) of 20 eV and scan step of 0.05 eV/step. High-resolution images and selected area electron diffraction patterns were observed by a JEOL JEM2200FS transmission electron microscope (TEM) operating at 200 kV equipped with EDX (TEM-EDX), JEOL Ltd., Japan).
3.
Results and discussion
Fig. 1A and B show the SEM and FE-SEM images, respectively of the electrospun nanofibers obtained from the prepared CuAC/NiAc/PVA solegel. As shown in these figures, smooth and continuous nanofibers were obtained. Fig. 2 depicts the SEM and FE-SEM images of the obtained powder after sintering of the electrospun nanofiber mats in argon atmosphere. It was showed that the proposed sintering condition did not affect in the nanofibrous morphology as the nanofibers structure was kept after the sintering process. Moreover, nanorods-like structure grown inside nanofibers after calcinations can be observed in Fig. 2C. The typical XRD pattern of the sintered powder was shown in Fig. 3. As shown in this figure, the peaks at 2qw43.9 , 51.0 , and 75.5 agree well with the crystal planes (111), (200), and (220), respectively. According to the JCDPS XRD data base, these peaks indicate
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formation of pristine Ni (JCDPS 04e0850) and Cu3.8Ni alloy (JCDPS 47e1406) and simultaneously match other reported literature [40,41]. Moreover, there is another peak can be observed at 2qw26 corresponding to graphite like-carbon (JCPDS 41e1487). According to the obtained XRD results, one can claim that the NRs and NFs represent NiCu and carbon, respectively. Nickel and copper have the same face centered cubic structure with space group glass (S.G) of Fm3m(225) and very close lattice parameters (a ¼ 3.523 for Ni and 3.616 for Cu), so these metals can form a solid solution alloy with a wide range of composition [40]. It is noteworthy that Ni and NiCu have the catalytic activity toward a graphitizing process of the utilized polymer [42], this might be a good explanation for carbon detection. Formation of pure nickel from the utilized precursor can be explained by the following equations [43]: Ni(CH3COO)2$4H2O / 0.86Ni(CH3COO)2$0.14Ni(OH)2 þ 0.28CH3COOH þ 3.72H2O
(1)
0.86 Ni(CH3COO)2$0.14Ni(OH)2 / NiCO3 þ NiO þ CH3COCH3 þ H2O
(2)
NiCO3 / NiO þ CO2
(3)
NiO þ CO / Ni þ CO2
(4)
Carbon monoxide in equation (4) comes from decomposition of the resultant acetic acid in the first equation. By the same way other authors explained the formation of pure copper from thermal decomposition of copper acetate under inert atmosphere [44] as follow: Cu(CH3COO)2.H2O / Cu(CH3COO)2 þ H2O
(5)
2Cu(CH3COO)2 / 2Cu þ 3CH3COOH þ CO2 þ H2 þ C
(6)
To estimate the carbon content in the obtained nanofibers, TGA analysis was carried out for the obtained nanofibers under an oxygen atmosphere. Fig. 4 displays the obtained results. As shown in the figure, the remaining residuals represent 63.71 wt% from the original sample. Considering
Fig. 1 e SEM; (A) and FE-SEM; (B) images of CuAC/NiAC/PVA mat after drying at 60 C for 24 h.
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Fig. 2 e FE-SEM image of NiAC/CuAC/PVA mat nanofibers after sintering at 750 C for 3 h in argon atmosphere.
that Ni and Cu are active metals and can be easily converted to NiO and CuO, respectively upon heating in an oxygen atmosphere and the carbon content will evolve in the form CO and/ or CO2 gases. In other words, the remaining residuals consist of CuO and NiO. As Ni and Cu content in the original electrospun nanofibers are known, the carbon content in the sintered nanofibers can be roughly estimated using mathematical calculations to be 49.76 wt%.
To determine the valence state, chemical bonding, and the surface composition and oxidation state of the materials XPS analysis was used. It is sensitive to information on the surface layer up to 10 nm for the inorganic materials. Fig. 5 shows the high sensitivity survey spectrum of the obtained sintered nanofibers (the inset represents the low scan analyses within the Ni and Cu main ranges). The blue inset confirms the presence of Cu2p3/2 and Cu2p1/2 peaks lies at binding energies
Fig. 3 e XRD pattern of the calcined NiAC/CuAC/PVA mat nanofibers at 750 C for 3 h in argon atmosphere.
Fig. 4 e TGA analysis for the obtained NiCu NRs/carbon nanofibers under oxygen atmosphere.
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Fig. 5 e XPS spectrum of the synthesized nanofibers. The insets represent very low scan rate measurements at the Cu and Ni reported ranges.
932.7 and 952.55 eV, respectively, these values are closely match reported values in the literature for metallic copper [45e47]. Moreover, the red pattern inset demonstrates the emerged of Ni2p3/2 and Ni2p1/2 peaks lies at binding energies 852.7 and 870.25 eV, respectively, these values also matches previous results for metallic nickel [48e50].
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To investigate the hypothesis of the distributed of NiCu nanorods on the carbon nanofibers; TEM analysis technique was used. Fig. 6A, B and C show the normal TEM images, as shown in the figures nanorods are highly distributed along with the nanofibers. Fig. 6D indicates HR-TEM image. As seen in the image, the prepared NRs have different crystal lattice compared to the matrix nanofibers. Moreover, the NFs have an amorphous structure but the NRs have high crystalline one. According the obtained images, one can claim that the formed structure is bimetallic nanorods suspended in carbon nanofibers. In other words, the synthesized bimetallic nanorods are sheathed by carbon shell. The inset in Fig. 6B shows the characteristic rings in the electron diffraction pattern (SAED); a good crystalline of the synthesized nanofibers can be concluded which supports the HR-TEM result. To properly understand the composition and elementary analysis of the obtained NiCu NRs/carbon NFs, STEM and TEM-EDX have been carried out. The results are demonstrated in Fig. 7. Fig. 7A indicates an STEM image of the selected nanofiber. It is very clear from the image that NiCu nanorods envelope inside the carbon nanofiber. Fig. 7B demonstrates the line EDX result corresponding to the line shown in Fig. 7A. As shown in Fig. 7B, Cu and Ni have the same elemental distribution along with the chosen line which indicates good alloying characteristics of the obtained bimetallic NRs. Moreover, the carbon elemental distribution confirms complete covering of the bimetallic NRs as carbon presents at all points along within the chosen line. The presence of carbon might be effective to enhance the catalytic activity of the obtained
Fig. 6 e TEM analysis results for the sintered nanofiber. Panels A, B, and C represent the normal TEM images. The inset in panel B demonstrates the selected area electron diffraction pattern (SAED). Panel D represents the high resolution TEM image (HR-TEM).
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Fig. 7 e TEM analysis result of the calcined nanofibers; panel (A) Rrepresents the normal TEM image along with the line EDX analysis. Panel B demonstrate line analysis TEM EDX results for the line shown in A.
nanofibers which behaves as an adsorbent in any catalytic chemical reactions by increasing the attachment of the target with catalytic material.
3.1.
Hydrogen release from ammonia borane
Fig. 8 depicts the time course of hydrogen evolution by AB hydrolysis at the experiments were conducted at 25 C using different catalyst; NiCu NRs/carbon nanofibers and Ni and Cu NPs for comparison. The amount of each catalyst utilized was 30 mg, AB ¼ 31.5 mg and water ¼ 75 ml. As shown in the figure, the NPs of the utilized metals showed the low amount of hydrogen release as compared to introduced CuNi NRs/ carbon nanofibers. Within the relatively long experimental time, the pristine metallic nanoparticles could not completely
Fig. 8 e Hydrogen release from aqueous ammonia borane in the presence of from CuNi NRs-doped carbon nanofibers, Ni NPS, and Cu NPs catalysts at room temperature. The amount of the utilized catalyst was 30 mg for each formulation.
hydrolyze the ammonia borane. The volume of the hydrogen released from the synthesized nanofibers was higher than commercial Ni and Cu NPs. The good catalytic activity of the introduced nanofibers can be explicated by three expected reasons; first the adsorption capacity of carbon which can adsorb AB molecules on the surface of the nanofibers. Second, the bimetallics have catalytic activity more than monometallics. Finally, the as-obtained CuNi NRs loading on carbon nanofibers might decrease the agglomeration that regularly happens with the NPs. Fig. 9 displays the effect of catalyst loading on the catalytic reactivity of NiCu NRs/carbon nanofibers. Different catalyst: AB ratios (0.3, 0.6, and 1) have been used under the same aforementioned conditions. In each experiment, the ammonia borane amount was kept at 31.5 mg and the amount of catalyst was changed to obtain different ratios. As shown in
Fig. 9 e The effect catalyst to ammonia borane ratio on the hydrogen release from aqueous ammonia borane at ambient conditions.
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Table 1 e Activation energy (Ea) values for hydrolysis of AB catalyzed by different catalysts. Catalyst
Fig. 10 e Effect of recyclying on the catalytic activity of NiCu NRs-doped carbon nanofibers on the catalytic activity. The experiments were conducted at room temperature and the catalyst to ammonia borane ratio was kept at 1.
the figure, increase the catalyst: AB ratio from 0.3 to 1 has a distinct impact as a considerable difference in the obtained hydrogen can be observed. Increase the ratio to 1 had a distinct influence as most of the theoretical hydrogen embedded in AB was released within almost 15 min as shown in the figure. It is noteworthy to mention that reusing the catalyst did not affect the catalytic activity as can be concluded from Fig. 10 which demonstrates the obtained hydrogen after using the introduced NiCu NRs/carbon nanofibers for six successive cycles. As shown in Fig. 10, the result appears same catalytic activity toward hydrogen release upon 6 cycles. This finding explains that CuNi NRs-doped carbon nanofibers are more stable than Ni and Cu NPs as well as avoid the agglomeration problem of the NPs.
Ni powder Co/Ɣ-Al2O3 Ni0.97Pt0.03 hollow spheres Hydrogel supported Ni (0) nanoparticles Hydrogel supported Cu (0) nanoparticles Hydrogel supported Co (0) nanoparticles Zeolite-confined Cu NiAg Laurate-stabilized Ru Ni0.03Pt0.97 alloys [email protected]/C Ni0.65Pt0.35 alloys Ni0.35Pt0.65 alloys [email protected]/C [email protected] Ni0.88Pt0.12 hollow spheres Co-B/C NiCu nanorods@ C nanofibers PdCo nanoparticles Ru/Ɣ-Al2O3 Rh/Ɣ-Al2O3 Pt/Ɣ-Al2O3
Ea (kJ/mol)
Ref.
70 62 57 52.8
[51] [52] [51] [25]
48.8
[25]
47.7
[25]
51.8 1.8 51.5 47.0 2.2 43.7 41.5 39.3 39 33 32.9 30 29.2 28.9 27 23 21 21
[18] [53] [13] [54] [55] [54] [54] [56] [20] [51] [57] This study [19] [52] [52] [52]
Fig. 11A shows the effect of temperature in the generated hydrogen. As shown in the figure, the amount of hydrogen released increased with increasing the temperature from 298 to 313 K. Because of the chemical reaction rate increased with increasing the temperature. The activation energy (Ea) of the AB hydrolysis by the synthesized nanofibers was calculated from the Arrhenius plot ln K vs.1/T (Fig. 11B), from which Ea 28.9 K J/mole, this value of activation energy is lower than most of the reported Ea values of the noble and non-noble metals (Table 1), which indicates a superior catalytic performance of the synthesized NiCu NRs/carbon nanofibers.
Fig. 11 e Hydrogen release from aqueous ammonia borane in the presence NiCu NRs/carbon nanofibers catalysts at different temperatures; (A), Arrhenius plot of ln K vs (1/T ); (B).
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4.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 7 1 5 e1 7 7 2 3
Conclusion
Electrospinning process can be used to prepare bimetallic NRs-carbon nanofibers in a single production step. The prepared NiCu NRs/carbon nanofibers used as effective and chemically stable catalyst. The introduced nanofibers showed a superior catalytic activity toward hydrogen release from ammonia borane as well as low activation energy was obtained. Moreover, the produced nanofibers are easy to be reused because of the presence of magnetic element (Ni). Reusing the introduced nanofibers does not influence the corresponding catalytic activity as all the embedded hydrogen was obtained after six successive cycles.
Acknowledgment This research was supported by NPST program by King Saud University project number 11-ENE1721-02. Following are results of a study on the “Leaders in Industry-University Cooperation” Projected, Supported by the Ministry of Education, Science &Technology (MEST) and National Research Foundation of Korea (NRF). We thank Mr. T. S. Bae, Mr. J. C. Lim, Dr. Lee Young-Boo, KBSI, Jeonju branch for taking highquality FESEM, TEM-EDX and TEM images, respectively.
references
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