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Magnetic field induced synthesis of amorphous CoB alloy nanowires as a highly active catalyst for hydrogen generation from ammonia borane
Catalysis Communications 84 (2016) 124–128
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Magnetic field induced synthesis of amorphous CoB alloy nanowires as a highly active catalyst for hydrogen generation from ammonia borane Jingjing Yan a, Jinyun Liao b, Hao Li b,⁎, Hui Wang a, Rongfang Wang a,⁎ a b
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China Department of Chemical Engineering, Huizhou University, Huizhou 516007, China
a r t i c l e
i n f o
Article history: Received 26 March 2016 Received in revised form 28 May 2016 Accepted 7 June 2016 Available online 25 June 2016 Keywords: Hydrogen generation Ammonia borane CoB alloy Nanowire Catalyst
a b s t r a c t CoB nanowires are prepared by a facile process free of any template or capping agent. The cleanly-surfaced CoB nanowires exhibit high catalytic activity in hydrolysis of ammonia borane, and the turnover frequency value is 4.88 LH2 min−1 g−1 catalyst, which is significantly higher than those of recently reported CoB nanocatalysts. The apparent activation energy of catalytic hydrolysis of ammonia borane is as low as 16.2 kJ mol−1, hinting the high catalytic activity of the CoB nanowires. The high catalytic performance and good recoverability make the CoB nanowires a strong catalyst candidate in the hydrolysis of ammonia borane for hydrogen generation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Amorphous alloys possessing pronounced short- and mediumrange order at the atomic scale have gained much attention from both academia and industry due to their multi-advantages including superior corrosion resistance in acid media, high mechanical toughness, magnetic, electronic, and catalytic properties compared to their crystalline counterparts, originated from its unique isotropic structural and highly flexible chemical composition [1,2]. In the field of heterogeneous catalysis, especially in various kinds of hydrogenation reactions, amorphous transition metal alloys (M_Ni, Co) have been extensively studied because of their excellent catalytic activity and selectivity [3]. Generally, some metalloids (e.g., B and P) were incorporated in amorphous alloys in order to form and stabilize the amorphous structure, which will thus significantly influence their physical and chemical properties [4]. For example, although Co in Co–B alloy acts as the active center in the catalytic hydrolysis of ammonia borane, the introduction of B into Co can significantly improve the catalytic activity of Co in the hydrolytic reaction [5, 6]. The kind of M–B amorphous alloy catalysts were often synthesize by chemical reduction of metallic ions with borohydride (BH− 4 ) in aqueous solution, but vigorous and exothermic reactions between metallic ions and BH− 4 usually result in the formation of M–B samples in the form of nonporous nanoparticles which generally exhibit irregular shapes and non-uniform sizes, thereby reducing their catalytic activity [7,8]. ⁎ Corresponding authors. E-mail addresses: [email protected] (H. Li), [email protected] (R. Wang).
http://dx.doi.org/10.1016/j.catcom.2016.06.019 1566-7367/© 2016 Elsevier B.V. All rights reserved.
Recently, uniform NiB [9], CoB [10] and NiP [11] amorphous alloy spheres have been achieved through microemulsions, using large amounts of organic phase and surfactants in the synthesis. However, the usage of organic phase or surfactant not only causes environmental problem in the synthetic process but also results in a “non-clean” surface of the M–B catalyst because effective removal of the adsorbed surfactants from the surface of metals or alloys is a difficult process [12]. In addition, the nanoparticles are also responsible for the ready crystallization of amorphous alloys [13], implying the necessity of fabricating the specific morphology except the particles. On the other hand, it is well-known that the catalytic activity of many nanomaterials is sensitively dependent on their morphology in view of the fact that those shape anisotropic nanocatalysts with more edges, corners and faces can provide more active site for catalytic reaction, which will exhibit much higher catalytic performance compared to the spherical counterparts in many cases [14]. However, little attention has been paid to the catalytic activity of the amorphous nanocatalyst with a specific morphology, perhaps, due to that the synthesis of the amorphous nanoalloys with a specific shape is still a challenge. Over the past decade, magnetic field, similar to the conventional reaction conditions such as temperature and pressure, has been introduced as a new parameter to synthesize and assemble special structure in chemical reactions because magnetic field force can control and influence the nucleation, growth, and assembly of magnetic materials [15]. A series of highly uniform Ni, Co, and its alloys were obtained by chemical method under magnetic fields [16–18]. However, the magnetic alloy materials in the above reports were crystalline phase, and
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alternately, and finally dried in vacuum oven at 60 °C for 6 h. Then, the final product denoted as MF-CoB was obtained. For comparison, a reference sample was also prepared by a similar process described above except that no magnetic field was introduced into the reaction system. The resultant sample was denoted as CoB. 2.2. Characterization XRD patterns were recorded on a Shimadzu XD–3A (Japan) using filtered Cu-Kα radiation generated at 40 kV and 30 mA. Scanning electron microscopy (SEM) images were obtained using a Carl Zeiss Ultra Plus. Transmission electron microscopy (TEM) images of the catalysts were obtained using a JEOL (JEM-2000 FX) microscope operating at 200 kV. Specific surface area was determined by Brunauer-Emmett-Teller (BET) method based on the sorption isotherms obtained on a Quantachrome Autosorb-1 volumetric analyzer. The molar ratios of Co to B in both CoB and MF-CoB samples were determined by a Varian 720 Inductively Coupled Plasma-Optical Emission Spectrometer (ICPOES). Fig. 1. XRD patterns of MF-CoB and CoB samples. Insets show the separation process of the MF-CoB sample with a magnet.
little attention has been paid to the fabrication of the amorphous alloys under external magnetic fields. In this paper, we report a facile and cost-effective magnetic-fieldassisted process to synthesize wire-like amorphous CoB nanoalloys. To the best of our knowledge, room-temperature synthesis of CoB nanowires without the assistance of any templates or capping agents has not been reported previously. Furthermore, these CoB nanowires have a “clean” surface due to the non-use of templates or surfactants in the reaction system. The as-prepared CoB amorphous alloys exhibited much improved catalytic activity in the hydrolysis of ammonia borane (NH3BH3) than the amorphous CoB nanoparticles and recently reported CoB nanocatalysts in the literature. 2. Experimental 2.1. Synthesis All reagents were of analytic grade, and double-distilled water was used throughout the experiments. Ammonia borane complex (97%) was purchased from the company of Shanghai Weizhi. Firstly, 0.17 mmol CoCl2·6H2O was dissolved into 50 mL solution composed of deionized water and glycerin (Vwater:Vglycerin = 7:1) with intense mechanical stirring. Consequently, 50 mL NaBH4 (0.021 M) solution was dropped into the above solution. During the reaction process, a permanent magnet was placed under the bottom of the reaction vessel and the magnetic field intensity around the vessel is ca. 1.4 T. After reaction, the resultant was rinsed with double-distilled water and ethanol
2.3. Catalytic performance testing The catalytic hydrolytic dehydrogenation of ammonia borane was carried out in a three-necked glass container connected with a gas burette to measure the accumulative volume of H2 generated during the hydrolysis reaction. In a typical procedure, 2 mmol ammonia borane and 0.6 mmol NaOH were mixed into the reaction vessel containing 10 mL ultrapure water, followed by the addition of ca. 6 mg catalyst under mechanical stirring. The reaction temperature was fixed at 25 ° C in a thermostated reactor. 3. Results and discussion The XRD patterns of the as-prepared samples are shown in Fig. 1. As can be seen, only a broad peak appears at approximately 2θ = 45°. This wide, diffuse peak indicates that both the two samples have amorphous structure [19]. Inset in Fig. 1 presents a photograph of a certain amount of the MF-CoB sample dispersed into ultrapure water in a standard bottle. When a magnet is approaching to the bottle, the MF-CoB sample is separated from the solution within a few seconds, demonstrating that the as-prepared MF-CoB sample exhibits good magnetic recoverability. This is a technical advantage in switching off the catalysis reaction by the use of a magnet field. As can be seen in Fig. 2a, CoB is irregularly-shaped agglomeration composed of numerous small CoB nanoparticles with a typical size of ca. 30 nm. When the magnetic field is introduced, the obtained MFCoB product is nanowires with a length of 2–10 μm (Fig. S1, Please see Supporting Information). Almost all of nanowires are aligned into bundles, and the surface of the wires is not smooth. The enlarged SEM image in Fig. 2b further proves the presence of the fluffy surface outside of the
Fig. 2. SEM images of the CoB (a) and MF-CoB (b) samples. Inset in (b) is the TEM image of MF-CoB.
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Fig. 3. The formation scheme of CoB alloy nanowires under an external magnetic field.
MF-CoB nanowires. The above results indicate the one-dimensional wire-like morphology of the MF-CoB samples derived from the induction of the external magnetic field. The fine microstructure was also observed by TEM. As displayed in the inset in Fig. 2b, the surface of CoB nanowires is covered by numerous ultrathin flakes. These flakes would provide the large surface area, being favor of the catalytic process. Fig. S2a (Supporting Information) shows the SAED pattern of the sample. A continuous hollow ring which is a typical diffraction pattern of an amorphous material can be observed, confirming the amorphous structure of the MF-CoB sample [20]. The EDX spectrum in Fig. S2b (Supporting Information) indicates the MF-CoB sample consist of Co and B elements, and the molar ratio of Co:B is ca. 1:0.28. This value is close to the molar ratio determined by ICP-OES, which is ca. 1:0.3. Moreover, the distribution of Co and B elements in the sample observed in Fig. S3 (Supporting Information) is uniform. Overlapping elemental mapping of the Co and B elements further demonstrates the formation of CoB alloy structure. In the present study, it is found that the magnetic field in the synthesis plays an important role in forming the wire-like CoB alloy. In a contrast experiment, only some CoB nanoparticles are obtained when the magnet is removed. Reduction of Co2+ with NaBH4 results in the formation of primary CoB particles, which will be magnetized by the external magnetic field. Then, these particles tend to align along the magnetic force line. In such model, one-dimensional CoB nanowires are obtained. The formation scheme is displayed in Fig. 3. The porous structure of the CoB and MF-CoB samples was investigated by N2 isotherms (Fig. 4). For the two samples, the isotherms can be classified as type III isotherms in which the maximum adsorption is attained [21]. The shape of isotherms of the two samples indicates the presence of the disorder and multi-scale pores, which is proved by the
corresponding pore size distribution shown in the inset. The BET surface area of CoB and MF-CoB is 85.0 and 58.6 m2 g−1, respectively. The catalytic behaviors of two CoB samples in the hydrolysis reaction of ammonia borane are assessed and the results are shown in Fig. 5. For the both catalysts, the accumulative volume of H2 generated during NH3BH3 hydrolysis almost increases linearly as the reaction time, implying the hydrolysis reaction follows the pseudo-zero order kinetics. At hydrolytic temperature of 25 °C, for the CoB catalyst, the rate of hydrogen generation is about 16.0 mL min−1 with a turnover frequency (TOF) value of 2.67 LH2 min−1 g−1 catalyst. In contrast, the rate of hydrogen generation for the MF-CoB catalyst can reach 29.3 mL min−1 with a turnover frequency (TOF) value of 4.88 LH2 min−1 g−1 catalyst, which is 1.83 times as large as that of the CoB catalyst. Noting that the BET surface of the MFCoB catalyst is only ca. 69% of that of the CoB catalyst, the BET surface normalized rate constant of the MF-CoB catalyst is ca. 2.65 times as large as that of the CoB catalyst. This suggests that the MF-CoB catalyst has higher intrinsic catalytic activity to NH3BH3 hydrolysis. It has been well documented that those shape-anisotropic nanocatalysts with more edges, corners and faces can provide more active site for catalytic reaction, which will exhibit much higher catalytic performance [22]. In this study, it is clear that rough-surfaced CoB nanowires possess more edges, corners and faces in contrast to spherical CoB nanoparticles, resulting in an enhanced catalytic activity. As far as we know, the as-prepared MF-CoB catalyst is one of the highest active CoB catalysts to NH3BH3 hydrolysis in terms of TOF, which exhibits much better catalytic activity in NH3BH3 hydrolysis under similar conditions than many other CoB nanocatalysts reported in the literature, such −1as CoB powder (0.360 LH2 min−1 g−1 catalyst [23]), CoB/SiO2 (1.20 LH2 min −1 −1 −1 gcatalyst [24]), CoB hollow nanospindles (1.283 LH2 min gcatalyst [25]), 1 honeycomb-like CoB nanostructures (1.763 LH2 min−1 g− catalyst [26]), −1 −1 gcatalyst [27]), CoB supported on mesoporous SBA-15 (1.9 LH2 min CoB supported on mesoporous MCM-41 (1.15 LH2 min−1 g−1 catalyst [27]), CoB supported on mesoporous FSM-16 (1.20 LH2 min−1 g−1 catalyst [27]) and the CoB supported on mesoporous beta-zeolite seeded (4.0 LH2 min−1 g−1 catalyst [28]). The TOF value is also higher than those of other types of catalysts to NH3BH3 hydrolysis, such as CoP catalyst −1 −1 gcatalyst [30]), (2.0 LH2 min−1 g−1 catalyst [29]), Cu0.75B0.25(1.18 LH2 min [30]), Co-W-B-P/Ni(4.0 LH2 min−1Ni0.75B0.25(3.87 LH2 min−1 g−1 catalyst −1 gcatalyst [31]). However, the TOF value is still much lower than those of some noble-metal-based catalysts, such as Ru nanoparticles −1 −1 gcatalyst [33]). (8.8 LH2 min−1 g−1 catalyst [32]), Pt/CNTs-O-HT(53.7 LH2 min It should be mentioned that the hydrolytic reaction was carried out in a NaOH solution (0.6 M) in this study. In our preliminary experiments, it was found that a proper amount of NaOH can enhance the hydrolysis of ammonia borane. According to the hydrolysis reaction of ammonia borane, − H3NBH3(aq) + 2H2O(l) → NH+ 4 (aq) + BO2 (aq) + 3H2(g)
Fig. 4. N2 isotherms of the CoB and MF-CoB samples. Inset shows and the corresponding pore size distribution.
NH+ 4 ions will be produced as the hydrolysis reaction proceeds, which will be consumed by OH− by the following reaction. − NH+ 4 + OH → NH3 + H2O. So, the rate of hydrolysis reaction was enhanced in the presence of OH−. However, NaOH at a too large
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Fig. 5. Hydrogen evolution in the presence of CoB (a) or MF-CoB (b) as a catalyst at different temperatures; Insets show the determination of apparent activation energy in the temperature range from 25 to 40 °C.
concentration (N 0.6 M) have negative effect on ammonia borane hydrolysis. As the increase of NaOH concentration, the viscosity of the reaction solution will increase, which may result in the slow transportation of ammonia borane from bulk solution to the catalyst surface. In this case, the hydrolysis reaction was retarded and corresponding hydrolysis rate decreased. To investigate the effect of temperature on the hydrogen generation rate, NH3BH3 hydrolysis was also carried out at temperature of 30, 35, and 40 °C, and the results are also shown in Fig. 5. As can be observed, there is a remarkable increase in the hydrogen generation rate with the increase in reaction temperature for both samples. For example, when CoB acts as catalyst, it will take ca. 10 min to generate 160 mL H2 at 25 °C, while it will take only ca. 5 min to produce the same amount of H2 at 40 °C. According to the classical Arrhenius equation, the apparent activity energy (Ea) can be figured out based on the data of reaction rates at different temperature (the inset in Fig. 5a and b), which is 38.7 kJ mol−1 for the CoB catalyst, and 16.2 kJ mol−1 for the MF-CoB catalyst. The relatively low Ea of the MF-CoB catalyst further demonstrates that it exhibit higher catalytic activity than the CoB catalyst. The cycling performance is very important to a catalyst. In this study, after the catalytic reaction, the CoB catalyst was separated from the reaction solution. After washing, the recovered catalyst was then re-dispersed in another newly-prepared ammonia borane solution. In such model, the reusability and stability of the CoB catalyst is checked. Fig. S4 (Supporting Information) displays the ratio of the rate constant in the different run (kn) to that in the first run (k1). It is found that there is a ca. 36% loss in the hydrolysis rate in the sixth run. That is, the rate of hydrogen production decreases from 29.3 mL min−1 in the first run to the 18.8 mL min−1 in the sixth run. However, if the catalytic activity is expressed in terms of TOF, there is only ca. 20% loss in the sixth run. The main reason for the loss of the rate is the mass loss of our catalyst during the separation process. In addition, the produced metaborate in the hydrolysis process tends to adsorb on the surface of catalyst, which will occupy the active sites of catalyst and thus result in the decrease of catalytic activity [34]. 4. Conclusion Amorphous CoB nanowires with a diameter of ca. 80 nm were prepared at room temperature by a cost-effective magnetic-field-assisted process independent of any template and capping agent, such as surfactant, complexant or polymer stabilizer. When the as-prepared amorphous CoB nanowires was used as the catalyst in the hydrolysis of ammonia borane for hydrogen production, TOF is 4.88 LH2 min−1 g−1 catalyst and Ea is 16.2 kJ mol−1. Compared to the values of recently reported CoB nanocatalysts towards the hydrolysis of ammonia borane, high TOF and
low Ea is indicative of the high catalytic activity of the amorphous CoB nanowires, which is related to its unique architecture. The high catalytic performance makes them a promising catalyst in the hydrolysis of ammonia borane for hydrogen generation. Acknowledgements This work was financially supported by the Natural Science Foundation of Guangdong Province (No. 2016A030313120), the High-level Talent Project of the University in Guangdong Province (No. 184), the Excellent Youth Foundation of the University in Guangdong Province (No. YQ2015154), and the National Natural Science Foundation of China (21363022 and 51362027). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.06.019. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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[24] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 195 (2010) 8209–8214. [25] D.G. Tong, X.L. Zeng, W. Chu, D. Wang, P. Wu, J. Mater. Sci. 45 (2010) 2862–2867. [26] D.G. Tong, W. Chu, P. Wu, L. Zhang, RSC Adv. 2 (2012) 2369–2376. [27] N. Patel, R. Fernandes, S. Gupta, R. Edla, D.C. Kothari, A. Miotello, Appl. Catal. B 140141 (2013) 125–132. [28] Y.C. Luo, Y.H. Liu, Y. Hung, X.Y. Liu, C.Y. Mou, Int. J. Hydrog. Energy 38 (2013) 7280–7290.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Magnetic field induced synthesis of amorphous CoB alloy nanowires as a highly active catalyst for hydrogen generation from ammonia borane Jingjing Yan a, Jinyun Liao b, Hao Li b,⁎, Hui Wang a, Rongfang Wang a,⁎ a b
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China Department of Chemical Engineering, Huizhou University, Huizhou 516007, China
a r t i c l e
i n f o
Article history: Received 26 March 2016 Received in revised form 28 May 2016 Accepted 7 June 2016 Available online 25 June 2016 Keywords: Hydrogen generation Ammonia borane CoB alloy Nanowire Catalyst
a b s t r a c t CoB nanowires are prepared by a facile process free of any template or capping agent. The cleanly-surfaced CoB nanowires exhibit high catalytic activity in hydrolysis of ammonia borane, and the turnover frequency value is 4.88 LH2 min−1 g−1 catalyst, which is significantly higher than those of recently reported CoB nanocatalysts. The apparent activation energy of catalytic hydrolysis of ammonia borane is as low as 16.2 kJ mol−1, hinting the high catalytic activity of the CoB nanowires. The high catalytic performance and good recoverability make the CoB nanowires a strong catalyst candidate in the hydrolysis of ammonia borane for hydrogen generation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Amorphous alloys possessing pronounced short- and mediumrange order at the atomic scale have gained much attention from both academia and industry due to their multi-advantages including superior corrosion resistance in acid media, high mechanical toughness, magnetic, electronic, and catalytic properties compared to their crystalline counterparts, originated from its unique isotropic structural and highly flexible chemical composition [1,2]. In the field of heterogeneous catalysis, especially in various kinds of hydrogenation reactions, amorphous transition metal alloys (M_Ni, Co) have been extensively studied because of their excellent catalytic activity and selectivity [3]. Generally, some metalloids (e.g., B and P) were incorporated in amorphous alloys in order to form and stabilize the amorphous structure, which will thus significantly influence their physical and chemical properties [4]. For example, although Co in Co–B alloy acts as the active center in the catalytic hydrolysis of ammonia borane, the introduction of B into Co can significantly improve the catalytic activity of Co in the hydrolytic reaction [5, 6]. The kind of M–B amorphous alloy catalysts were often synthesize by chemical reduction of metallic ions with borohydride (BH− 4 ) in aqueous solution, but vigorous and exothermic reactions between metallic ions and BH− 4 usually result in the formation of M–B samples in the form of nonporous nanoparticles which generally exhibit irregular shapes and non-uniform sizes, thereby reducing their catalytic activity [7,8]. ⁎ Corresponding authors. E-mail addresses: [email protected] (H. Li), [email protected] (R. Wang).
http://dx.doi.org/10.1016/j.catcom.2016.06.019 1566-7367/© 2016 Elsevier B.V. All rights reserved.
Recently, uniform NiB [9], CoB [10] and NiP [11] amorphous alloy spheres have been achieved through microemulsions, using large amounts of organic phase and surfactants in the synthesis. However, the usage of organic phase or surfactant not only causes environmental problem in the synthetic process but also results in a “non-clean” surface of the M–B catalyst because effective removal of the adsorbed surfactants from the surface of metals or alloys is a difficult process [12]. In addition, the nanoparticles are also responsible for the ready crystallization of amorphous alloys [13], implying the necessity of fabricating the specific morphology except the particles. On the other hand, it is well-known that the catalytic activity of many nanomaterials is sensitively dependent on their morphology in view of the fact that those shape anisotropic nanocatalysts with more edges, corners and faces can provide more active site for catalytic reaction, which will exhibit much higher catalytic performance compared to the spherical counterparts in many cases [14]. However, little attention has been paid to the catalytic activity of the amorphous nanocatalyst with a specific morphology, perhaps, due to that the synthesis of the amorphous nanoalloys with a specific shape is still a challenge. Over the past decade, magnetic field, similar to the conventional reaction conditions such as temperature and pressure, has been introduced as a new parameter to synthesize and assemble special structure in chemical reactions because magnetic field force can control and influence the nucleation, growth, and assembly of magnetic materials [15]. A series of highly uniform Ni, Co, and its alloys were obtained by chemical method under magnetic fields [16–18]. However, the magnetic alloy materials in the above reports were crystalline phase, and
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alternately, and finally dried in vacuum oven at 60 °C for 6 h. Then, the final product denoted as MF-CoB was obtained. For comparison, a reference sample was also prepared by a similar process described above except that no magnetic field was introduced into the reaction system. The resultant sample was denoted as CoB. 2.2. Characterization XRD patterns were recorded on a Shimadzu XD–3A (Japan) using filtered Cu-Kα radiation generated at 40 kV and 30 mA. Scanning electron microscopy (SEM) images were obtained using a Carl Zeiss Ultra Plus. Transmission electron microscopy (TEM) images of the catalysts were obtained using a JEOL (JEM-2000 FX) microscope operating at 200 kV. Specific surface area was determined by Brunauer-Emmett-Teller (BET) method based on the sorption isotherms obtained on a Quantachrome Autosorb-1 volumetric analyzer. The molar ratios of Co to B in both CoB and MF-CoB samples were determined by a Varian 720 Inductively Coupled Plasma-Optical Emission Spectrometer (ICPOES). Fig. 1. XRD patterns of MF-CoB and CoB samples. Insets show the separation process of the MF-CoB sample with a magnet.
little attention has been paid to the fabrication of the amorphous alloys under external magnetic fields. In this paper, we report a facile and cost-effective magnetic-fieldassisted process to synthesize wire-like amorphous CoB nanoalloys. To the best of our knowledge, room-temperature synthesis of CoB nanowires without the assistance of any templates or capping agents has not been reported previously. Furthermore, these CoB nanowires have a “clean” surface due to the non-use of templates or surfactants in the reaction system. The as-prepared CoB amorphous alloys exhibited much improved catalytic activity in the hydrolysis of ammonia borane (NH3BH3) than the amorphous CoB nanoparticles and recently reported CoB nanocatalysts in the literature. 2. Experimental 2.1. Synthesis All reagents were of analytic grade, and double-distilled water was used throughout the experiments. Ammonia borane complex (97%) was purchased from the company of Shanghai Weizhi. Firstly, 0.17 mmol CoCl2·6H2O was dissolved into 50 mL solution composed of deionized water and glycerin (Vwater:Vglycerin = 7:1) with intense mechanical stirring. Consequently, 50 mL NaBH4 (0.021 M) solution was dropped into the above solution. During the reaction process, a permanent magnet was placed under the bottom of the reaction vessel and the magnetic field intensity around the vessel is ca. 1.4 T. After reaction, the resultant was rinsed with double-distilled water and ethanol
2.3. Catalytic performance testing The catalytic hydrolytic dehydrogenation of ammonia borane was carried out in a three-necked glass container connected with a gas burette to measure the accumulative volume of H2 generated during the hydrolysis reaction. In a typical procedure, 2 mmol ammonia borane and 0.6 mmol NaOH were mixed into the reaction vessel containing 10 mL ultrapure water, followed by the addition of ca. 6 mg catalyst under mechanical stirring. The reaction temperature was fixed at 25 ° C in a thermostated reactor. 3. Results and discussion The XRD patterns of the as-prepared samples are shown in Fig. 1. As can be seen, only a broad peak appears at approximately 2θ = 45°. This wide, diffuse peak indicates that both the two samples have amorphous structure [19]. Inset in Fig. 1 presents a photograph of a certain amount of the MF-CoB sample dispersed into ultrapure water in a standard bottle. When a magnet is approaching to the bottle, the MF-CoB sample is separated from the solution within a few seconds, demonstrating that the as-prepared MF-CoB sample exhibits good magnetic recoverability. This is a technical advantage in switching off the catalysis reaction by the use of a magnet field. As can be seen in Fig. 2a, CoB is irregularly-shaped agglomeration composed of numerous small CoB nanoparticles with a typical size of ca. 30 nm. When the magnetic field is introduced, the obtained MFCoB product is nanowires with a length of 2–10 μm (Fig. S1, Please see Supporting Information). Almost all of nanowires are aligned into bundles, and the surface of the wires is not smooth. The enlarged SEM image in Fig. 2b further proves the presence of the fluffy surface outside of the
Fig. 2. SEM images of the CoB (a) and MF-CoB (b) samples. Inset in (b) is the TEM image of MF-CoB.
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Fig. 3. The formation scheme of CoB alloy nanowires under an external magnetic field.
MF-CoB nanowires. The above results indicate the one-dimensional wire-like morphology of the MF-CoB samples derived from the induction of the external magnetic field. The fine microstructure was also observed by TEM. As displayed in the inset in Fig. 2b, the surface of CoB nanowires is covered by numerous ultrathin flakes. These flakes would provide the large surface area, being favor of the catalytic process. Fig. S2a (Supporting Information) shows the SAED pattern of the sample. A continuous hollow ring which is a typical diffraction pattern of an amorphous material can be observed, confirming the amorphous structure of the MF-CoB sample [20]. The EDX spectrum in Fig. S2b (Supporting Information) indicates the MF-CoB sample consist of Co and B elements, and the molar ratio of Co:B is ca. 1:0.28. This value is close to the molar ratio determined by ICP-OES, which is ca. 1:0.3. Moreover, the distribution of Co and B elements in the sample observed in Fig. S3 (Supporting Information) is uniform. Overlapping elemental mapping of the Co and B elements further demonstrates the formation of CoB alloy structure. In the present study, it is found that the magnetic field in the synthesis plays an important role in forming the wire-like CoB alloy. In a contrast experiment, only some CoB nanoparticles are obtained when the magnet is removed. Reduction of Co2+ with NaBH4 results in the formation of primary CoB particles, which will be magnetized by the external magnetic field. Then, these particles tend to align along the magnetic force line. In such model, one-dimensional CoB nanowires are obtained. The formation scheme is displayed in Fig. 3. The porous structure of the CoB and MF-CoB samples was investigated by N2 isotherms (Fig. 4). For the two samples, the isotherms can be classified as type III isotherms in which the maximum adsorption is attained [21]. The shape of isotherms of the two samples indicates the presence of the disorder and multi-scale pores, which is proved by the
corresponding pore size distribution shown in the inset. The BET surface area of CoB and MF-CoB is 85.0 and 58.6 m2 g−1, respectively. The catalytic behaviors of two CoB samples in the hydrolysis reaction of ammonia borane are assessed and the results are shown in Fig. 5. For the both catalysts, the accumulative volume of H2 generated during NH3BH3 hydrolysis almost increases linearly as the reaction time, implying the hydrolysis reaction follows the pseudo-zero order kinetics. At hydrolytic temperature of 25 °C, for the CoB catalyst, the rate of hydrogen generation is about 16.0 mL min−1 with a turnover frequency (TOF) value of 2.67 LH2 min−1 g−1 catalyst. In contrast, the rate of hydrogen generation for the MF-CoB catalyst can reach 29.3 mL min−1 with a turnover frequency (TOF) value of 4.88 LH2 min−1 g−1 catalyst, which is 1.83 times as large as that of the CoB catalyst. Noting that the BET surface of the MFCoB catalyst is only ca. 69% of that of the CoB catalyst, the BET surface normalized rate constant of the MF-CoB catalyst is ca. 2.65 times as large as that of the CoB catalyst. This suggests that the MF-CoB catalyst has higher intrinsic catalytic activity to NH3BH3 hydrolysis. It has been well documented that those shape-anisotropic nanocatalysts with more edges, corners and faces can provide more active site for catalytic reaction, which will exhibit much higher catalytic performance [22]. In this study, it is clear that rough-surfaced CoB nanowires possess more edges, corners and faces in contrast to spherical CoB nanoparticles, resulting in an enhanced catalytic activity. As far as we know, the as-prepared MF-CoB catalyst is one of the highest active CoB catalysts to NH3BH3 hydrolysis in terms of TOF, which exhibits much better catalytic activity in NH3BH3 hydrolysis under similar conditions than many other CoB nanocatalysts reported in the literature, such −1as CoB powder (0.360 LH2 min−1 g−1 catalyst [23]), CoB/SiO2 (1.20 LH2 min −1 −1 −1 gcatalyst [24]), CoB hollow nanospindles (1.283 LH2 min gcatalyst [25]), 1 honeycomb-like CoB nanostructures (1.763 LH2 min−1 g− catalyst [26]), −1 −1 gcatalyst [27]), CoB supported on mesoporous SBA-15 (1.9 LH2 min CoB supported on mesoporous MCM-41 (1.15 LH2 min−1 g−1 catalyst [27]), CoB supported on mesoporous FSM-16 (1.20 LH2 min−1 g−1 catalyst [27]) and the CoB supported on mesoporous beta-zeolite seeded (4.0 LH2 min−1 g−1 catalyst [28]). The TOF value is also higher than those of other types of catalysts to NH3BH3 hydrolysis, such as CoP catalyst −1 −1 gcatalyst [30]), (2.0 LH2 min−1 g−1 catalyst [29]), Cu0.75B0.25(1.18 LH2 min [30]), Co-W-B-P/Ni(4.0 LH2 min−1Ni0.75B0.25(3.87 LH2 min−1 g−1 catalyst −1 gcatalyst [31]). However, the TOF value is still much lower than those of some noble-metal-based catalysts, such as Ru nanoparticles −1 −1 gcatalyst [33]). (8.8 LH2 min−1 g−1 catalyst [32]), Pt/CNTs-O-HT(53.7 LH2 min It should be mentioned that the hydrolytic reaction was carried out in a NaOH solution (0.6 M) in this study. In our preliminary experiments, it was found that a proper amount of NaOH can enhance the hydrolysis of ammonia borane. According to the hydrolysis reaction of ammonia borane, − H3NBH3(aq) + 2H2O(l) → NH+ 4 (aq) + BO2 (aq) + 3H2(g)
Fig. 4. N2 isotherms of the CoB and MF-CoB samples. Inset shows and the corresponding pore size distribution.
NH+ 4 ions will be produced as the hydrolysis reaction proceeds, which will be consumed by OH− by the following reaction. − NH+ 4 + OH → NH3 + H2O. So, the rate of hydrolysis reaction was enhanced in the presence of OH−. However, NaOH at a too large
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Fig. 5. Hydrogen evolution in the presence of CoB (a) or MF-CoB (b) as a catalyst at different temperatures; Insets show the determination of apparent activation energy in the temperature range from 25 to 40 °C.
concentration (N 0.6 M) have negative effect on ammonia borane hydrolysis. As the increase of NaOH concentration, the viscosity of the reaction solution will increase, which may result in the slow transportation of ammonia borane from bulk solution to the catalyst surface. In this case, the hydrolysis reaction was retarded and corresponding hydrolysis rate decreased. To investigate the effect of temperature on the hydrogen generation rate, NH3BH3 hydrolysis was also carried out at temperature of 30, 35, and 40 °C, and the results are also shown in Fig. 5. As can be observed, there is a remarkable increase in the hydrogen generation rate with the increase in reaction temperature for both samples. For example, when CoB acts as catalyst, it will take ca. 10 min to generate 160 mL H2 at 25 °C, while it will take only ca. 5 min to produce the same amount of H2 at 40 °C. According to the classical Arrhenius equation, the apparent activity energy (Ea) can be figured out based on the data of reaction rates at different temperature (the inset in Fig. 5a and b), which is 38.7 kJ mol−1 for the CoB catalyst, and 16.2 kJ mol−1 for the MF-CoB catalyst. The relatively low Ea of the MF-CoB catalyst further demonstrates that it exhibit higher catalytic activity than the CoB catalyst. The cycling performance is very important to a catalyst. In this study, after the catalytic reaction, the CoB catalyst was separated from the reaction solution. After washing, the recovered catalyst was then re-dispersed in another newly-prepared ammonia borane solution. In such model, the reusability and stability of the CoB catalyst is checked. Fig. S4 (Supporting Information) displays the ratio of the rate constant in the different run (kn) to that in the first run (k1). It is found that there is a ca. 36% loss in the hydrolysis rate in the sixth run. That is, the rate of hydrogen production decreases from 29.3 mL min−1 in the first run to the 18.8 mL min−1 in the sixth run. However, if the catalytic activity is expressed in terms of TOF, there is only ca. 20% loss in the sixth run. The main reason for the loss of the rate is the mass loss of our catalyst during the separation process. In addition, the produced metaborate in the hydrolysis process tends to adsorb on the surface of catalyst, which will occupy the active sites of catalyst and thus result in the decrease of catalytic activity [34]. 4. Conclusion Amorphous CoB nanowires with a diameter of ca. 80 nm were prepared at room temperature by a cost-effective magnetic-field-assisted process independent of any template and capping agent, such as surfactant, complexant or polymer stabilizer. When the as-prepared amorphous CoB nanowires was used as the catalyst in the hydrolysis of ammonia borane for hydrogen production, TOF is 4.88 LH2 min−1 g−1 catalyst and Ea is 16.2 kJ mol−1. Compared to the values of recently reported CoB nanocatalysts towards the hydrolysis of ammonia borane, high TOF and
low Ea is indicative of the high catalytic activity of the amorphous CoB nanowires, which is related to its unique architecture. The high catalytic performance makes them a promising catalyst in the hydrolysis of ammonia borane for hydrogen generation. Acknowledgements This work was financially supported by the Natural Science Foundation of Guangdong Province (No. 2016A030313120), the High-level Talent Project of the University in Guangdong Province (No. 184), the Excellent Youth Foundation of the University in Guangdong Province (No. YQ2015154), and the National Natural Science Foundation of China (21363022 and 51362027). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.06.019. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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