Ni nanoparticles supported on carbon as efficient catalyst for hydrolysis of ammonia borane

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1 Nano Research Nano Res 1 DOI /s Ni nanoparticles supported on carbon as efficient catalyst for hydrolysis of ammonia borane Limin Zhou, Tianran Zhang, Zhanliang Tao ( ), Jun Chen Nano Res., Just Accepted Manuscript DOI: /s on March Tsinghua University Press 2014 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Ni nanoparticles supported on carbon as efficient catalyst for hydrolysis of ammonia borane Limin Zhou, Tianran Zhang, Zhanliang Tao ( ) and Jun Chen Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Chemistry College, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin , China. Page Numbers. The font is ArialMT 16 (automatically inserted by the publisher) Ni nanoparticles (about 10-nm size) supported on carbon, which were prepared by the calcination of Ni-MOF at 700 C under Ar and the reduction of KBH 4, have been explored as efficient catalysts for hydrolysis of ammonia borane.

3 Nano Res DOI (automatically inserted by the publisher) Review Article/Research Article Research Article Ni nanoparticles supported on carbon as efficient catalyst for hydrolysis of ammonia borane Limin Zhou, Tianran Zhang, Zhanliang Tao( ), Jun Chen Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Chemistry College, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin , China. Fax: (+86) ; Tel: (+86) ; Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT We report on the preparation of three kinds of Ni nanoparticles supported on carbon (Ni/C) and their application in catalytic hydrolysis of ammonia borane (AB). Three Ni/C catalysts were prepared by the treatment of Ni metal-organic framework (Ni-MOF) with reduction by KBH4, calcination at 700 C under Ar and the combination of calcination and reduction, denoting as Ni/C-1, Ni/C-2 and Ni/C-3, respectively. The structure, morphology, specific surface area and element valence were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption-desorption measurement and X-ray photoelectron spectra (XPS). The results demonstrate that Ni/C-1 is amorphous Ni agglomeration on carbon, Ni/C-2 is characteristic of crystalline Ni nanoparticles (about 10-nm size) supported on carbon with Ni oxidized on the surface, while the surface of Ni in Ni/C-3 is less oxidized. The specific surface areas of Ni-MOF, Ni/C-1, Ni/C-2 and Ni/C-3 are 1239, 33.14, , m 2 g -1, respectively. The catalytic hydrolysis of AB with Ni/C-3 shows the hydrogen generation rate of 834 ml min -1 g -1 at room temperature and the activation energy of 31.6 kj/mol. This indicates that Ni/C-3 with more Ni crystal surface shows higher catalytic activity. This study is promising in the replacement of noble metal by Ni nanoparticles for AB hydrolysis under ambient condition. KEYWORDS Ni nanoparticles; ammonia borane; catalytic hydrolysis; hydrogen generation 1. Introduction With the development of science and technology, hydrogen as sustainable and clean energy has attracted much attention [1]. The reliable and efficient hydrogen generation and storage has become the necessary requisite toward the application of hydrogen energy [2]. Recently, ammonia borane (NH3BH3, AB) has become a promising hydrogen storage material because of its high hydrogen content (19.6 wt% H2) [3] and high stability in neutral aqueous solutions at room temperature [4]. The generation of hydrogen from Address correspondence to Z. L. Tao. Fax: (+86) ; Tel: (+86) ; taozhl@nankai.edu.cn 2

4 AB can utilize either thermolysis or hydrolysis. However, the thermolysis of AB exists the following shortcomings: (1) the relatively high hydrogen releasing temperature at o C, (2) the emission of the volatile byproducts (such as aminoborane, borazine, ammonium, and diborane) [5]. Therefore, the hydrolysis of AB is extensively studied. At the same time, catalysts are required to accelerate the sluggish hydrolysis reaction for the hydrogen generation. Previous reports have demonstrated that noble metals such as Pt and Pt-based alloys diaplay excellent performance for AB hydrolysis [6-7]. However, this is difficult to be widely used due to the high cost and scarcity of noble metals. Thus, it is of great interest to develop low-cost catalysts with high catalytic performance. It has been evidenced that current Co-based materials perform excellent catalytic activity [8-10]. In comparison, recent studies have focused on Ni-based catalysts, which are cheaper and more environmentally benign. In general, those catalysts were prepared by infusion methods [11], electrodeposition process [12], microemulsion [13], and colloid methods [14-16]. In order to improve the catalyst dispersion, the prepared nanoparticles were supported on SiO2 [17], SBA-15 [18], Al2O3 [19], nickel foam [20], carbon [14] and TiO2 [21]. Recently, the dispersion of Ni nanoparticles on Ketjen carbon was identified as remarkable catalytic activity for hydrolysis of AB [14]. However, the issues on the particle agglomeration and the preparation simplification are still to be addressed on. In contrast, metal-organic frameworks (MOFs) can make up for the above deficiency because of their tunable porosities and regulated metal position. Based on the superiority of MOFs, we have developed a simple method to prepare nickel nanoparticles supported on carbon using the precursor Ni-MOF. At first, Ni-MOF was prepared by a hydrothermal method. Then, three ways, namely reduction, calcination, and the combination of calcination and reduction of Ni-MOF were employed to prepare Ni/C-1, Ni/C-2, and Ni/C-3. Ni/C-1 was prepared by the reduction of Ni-MOF with KBH4 aqueous solution. In comparison, Ni/C-2 was obtained by the calcination of Ni-MOF at 700 C in Ar atmosphere. Both Ni/C-1 and Ni/C-2 show poor catalytic activity for hydrolysis of AB due to the agglomerated Ni nanoparticles for Ni/C-1 and the surface metallic Ni oxidization for Ni/C-2. Therefore, Ni/C-3 was prepared by the reduction of Ni/C-2 in order to reduce the surface oxidation for enhancing the catalytic performance. The as-prepared Ni/C-3 as catalyst for hydrolysis of AB presents the hydrogen generation rate of 834 ml min -1 g-1 and a low activation energy of 31.6 kj/mol. The results indicate that the as-prepared Ni/C-3 nanocomposite is promising in the application as efficient, low-cost, and stable catalysts for hydrolysis of ammonia borane. 2. Experimental 2.1 Catalysts preparation The precursor in this work was Ni metal-organic framework Ni(4,4 -bipy)(hbtc) (4,4 -bipy = 4,4 -bipyridine; H3BTC = 1,3,5-benzenetricarboxylic acid), which was prepared by a solvothermal synthesis route similar to the work reported by Li s group [22]. All reagents were of analytical grade and used without further purification. A mixture of 4,4 -bipy (0.576 g, 3 mmol), H3BTC (0.633 g, 3 mmol) and NiNO3 6H2O (0.876 g, 3 mmol) dissolved in 60 ml DMF (DMF = N,N-dimethylformamide) was sealed in 100 ml Teflon-lined autoclave and kept at 353 K for 72 h. When cooled to room temperature, the solid was filtrated and washed thoroughly with DMF for three times. Finally, the solid was transferred to a beaker for vacuum drying at 80 C for 10 h. The synthesis of the as-prepared Ni/C is outlined in Scheme 1. The first approach was directly reducing Ni-MOF with KBH4 and the catalyst was marked with Ni/C-1. Ni-MOF (30 mg) aqueous solution (10 ml) was sonicated for 10 min. Then, 3

5 KBH4 (25 mg) aqueous solution (20 ml) was added dropwise to the above solution in ice-water bath. The solution was sonicated successively for 1 h keeping ice-water temperature. KBH4 as reducing agent converts divalent nickel ion to metallic nickel. At the same time, the evolution of hydrogen in KBH4 aqueous solution also facilitates the above reaction. The second approach was the calcination of Ni-MOF with a heating rate of 5 C/min from room temperature to 700 C under Ar environment (see Fig. S1(a) in the Electronic Supplementary Material (ESM)), which was marked with Ni/C-2. During the calcination, metal organic frameworks decompose into gas and carbon. The obtained carbon as reducing agent reduces the nickel ion. The third approach was reducing the surface nickel oxide of Ni/C-2 with KBH4 to produce Ni/C-3. In general, the calcined Ni-MOF (15 mg) was placed in a two-necked glass flask (25 ml). One of the necks was connected to a gas burette. The other neck of the flask was to introduce KBH4 aqueous solution. out using the typical water displacement method [23]. A certain amount of catalyst (10 mg) was placed in a three-neck round-bottom flask with adding 8 ml deionized water and stirring for a few minutes. An inverted, water-filled gas burette in a water-filled vessel was used to monitor the volume of the evolved H2. The hydrolysis started when 40 mg AB power was added to the flask. The reaction of AB hydrolysis can be described by the following equation [24]: NH3BH3+2H2O NH4 + +BO2 - +3H2 The hydrogen generation rate of Ni/C catalysts was based on the amount of Ni nanoparticles, without considering the weight of the carbon. A series of experiments for hydrolysis of AB (such as temperature, concentration and catalyst amount) were carried out to study the catalytic activity of the as-prepared Ni/C at room temperature. 3. Results and Discussion 3.1 Catalysts characterization Figure 1 shows the XRD patterns of the as-prepared Ni/C. Figure 1(a) shows the XRD pattern of Ni/C-1, from which we can see the weak and broad peaks in the ranges of o and o. This indicates that either carbon or Ni in Ni/C-1 is amorphous. Scheme 1 A schematic illustration of the morphological evolution process of the prepared Ni/C (The light yellow in Ni/C-2 represents the surface oxidization of Ni nanoparticles). 2.2 Catalysts Characterization The as-synthesized catalysts were characterized by powder X-ray diffraction (XRD, Rigaku MiniFlex600, Cu K radiation), scanning electron microscopy (SEM, JEOL JSM 7500F) and transmission electron microscopy (TEM, Philips Tecnai F20). The Brunauer-Emmett-Teller (BET) specific surface area was measured by using the nitrogen adsorption-desorption isotherms (BELSORP-Mini) at 77 K. The X-ray photoelectron spectra (XPS) was conducted on PHI 5000VersaProbe instrument. 2.3 Hydrogen generation test The hydrogen generation experiments were carried Figure 1 XRD patterns of as-prepared (a) Ni/C-1, (b) Ni/C-2, (c) Ni/C-3. Figures 1(b) and 1(c) show the XRD patterns of 4

6 Ni/C-2 and Ni/C-3. The peaks at 2θ=45 o, 52 o, 76 o can be readily indexed as (111), (200), and (220) crystal planes of face-centered cubic Ni (fcc Ni, JCPDS NO ). Thus, Ni/C-2 and Ni/C-3 are characteristic of high crystallinity. In addition, Ni/C-2 and Ni/C-3 also present the weak and broad peak at 2θ=22-28 o which can be attributed to the amorphous carbon similiar to Ni/C-1. It is understandable that the carbon peak appears in the calcination of Ni-MOF due to the breaking down of the organic ligand. Raman spectrum has been conducted to further verify the carbon in the prepared Ni/C. It is seen that Ni/C-1 displays two Raman peaks at 1563 cm -1 (G-band) and 1365 cm -1 (D-band) of carbon (Fig. S2(b) in the ESM). The results indicate that directly reduction of Ni-MOF evolved definitely carbon. In comparison, there is no apparent difference of carbon in Ni/C-2 and Ni/C-3 (Fig. S2(a) in the ESM), demonstrating that after calcination, the role of KBH4 is just to reduce the surface of nickel oxide. Figure 2 displays the morphologies of the various Ni/C. As shown in the SEM micrograph (Fig. 2(a)), Ni/C-1 is composed of agglomerated particles, which is a common phenomenon prepared by chemical treatment without adding dispersant. In comparison, typical SEM images of Ni/C-2 and Ni/C-3 (Figs. 2(d) and 2(g)) clearly reveal uniform nanoporous morphology. Figures 2(e) and 2(h) show the TEM image of Ni/C-2 and Ni/C-3, from which we can see that they consist of dispersive particles with mean size of ~10 nm. The dispersion of the nanoparticles is ascribed to the regular arrangement of nickel ion and ligand in Ni-MOF structure. The high-resolution (HRTEM) image of Ni/C-1 (Fig. 2(c)) shows no lattice fringe, which is in accordance with the amorphous state of XRD in Fig. 1(a). From the HRTEM images of Ni/C-2 and Ni/C-3 (Figs. 2(f) and 2(i)), it s obvious to observe the lattice fringes of Ni nanoparticles in Ni/C-2 and Ni/C-3. The measured distance between adjacent lattice fringes is approximately 0.21 nm, which is in consistent with the neighbouring spacing of (111) plane of nickel. Figure 2 (a,d,g) SEM, (b,e,h) TEM and (c,f,i) HRTEM images of Ni/C-1 (a-c), Ni/C-2 (d-f), and Ni/C-3 (g-i). The inset of e and h is the corresponding size distributions for Ni/C-2 and Ni/C-3 nanoparticles, respectively. 5

7 Figure 3 shows the N2 adsorption-desorption isotherms of the as-synthesized Ni-MOF and Ni/C. It can be seen that Ni-MOF has a specific surface area of 1239 m 2 g -1. While the specific surface area of Ni/C-1, Ni/C-2 and Ni/C-3 are 33.14, , m 2 g -1, respectively. This analysis illustrates that Ni/C-1 has relatively small specific surface area compared with the precursor Ni-MOF, further confirming that Ni/C-1 seriously exists the accumulation of amorphous nickel particles because of the damage of Ni-MOF. However, the calcination removes almost all of the organic ligands and maintains the Ni particles so that Ni/C-2 and Ni/C-3 have increased specific surface area than that of Ni/C-1. This result is highly coincided with the morphology images analysis of Ni/C-2 and Ni/C-3 (Figs 2(d) and 2(g)). Figure 3 N 2 adsorption-desorption isotherms of (a) as-prepared Ni-MOF, (b) Ni/C-1, (c) Ni/C-2, (d) Ni/C Catalytic activity We have investigated the catalytic activities of Ni/C for hydrogen generation from the hydrolysis of AB solution. As shown in Fig. 4, Ni/C-3 possesses the highest hydrogen generation rate at room temperature. It should be noted that the excellent catalytic performance of Ni/C-3 depends on its relatively disperse particles and uniform pores. In comparison, Ni/C-1 exhibits a lower catalytic activity. The reason why Ni/C-1 performs sluggish catalytic rate is mainly due to that the accumulation of nickel particles decrease the specific surface area and the low activity of the amouphous Ni of Ni/C-1. Figure 4 Hydrogen generation from AB solution (0.5 wt%, 8 ml) catalyzed by three kinds of Ni/C at room temperature. It is noted that the hydrolysis reaction system using Ni/C-2 catalyst shows a distinct induction period at the beginning. In catalytic hydrolysis reaction of 6

8 sodium borohydride, this phenomenon was also observed using cobalt tungsten boron/nickel foam catalyst [25]. To investigate the reason of the slow hydrogen generation rate, X-ray photoelectron spectroscopy (XPS) analysis was used for searching discrepancy of the surface composition between Ni/C-2 and Ni/C-3. The Ni 2p3/2 XPS peaks of the prepared Ni/C-2 and Ni/C-3, as shown in Fig. S3 in the ESM, can be divided into two peaks at ev and ev, indicating the existence of both metallic Ni (852.3 ev) and trivalent Ni (856.1 ev). The intensity of trivalent Ni peak (856.1 ev) is much higher than that of zero Ni (852.3 ev) for Ni/C-2 (Fig. S3(a) in the ESM). In comparison, the intensity of zero valence Ni peak is stronger in Ni/C-3 (Fig. S3(b) in the ESM). This demonstrates that the surface nickel (Ш) oxide is readily formed when the calcined sample is exposed in air for Ni/C-2; While Ni/C-3 addresses the issue to reduce the surface nickel (Ш) oxide. This is the reason why Ni/C-2 shows the poor activity with an induction period phenomenon. Therefore, further reduction was applied for the enhancement of the catalytic capabilities. Figure 5 (a) Temperature effect on hydrogen generation rate from AB solution (0.5 wt%, 8 ml) using 10 mg Ni/C-3 nanoparticles and (b) the corresponding Arrhenius plots of lnk versus absolute temperature 1/T in the temperature range of o C, (c) Hydrogen generation from the hydrolysis of different AB solution (0.250, 0.375, 0.500, wt%) in the presence of 10 mg Ni/C-3 at room temperature and (d) the corresponding plot of hydrogen generation rate versus the concentration of AB (both on logarithmic scales), (e) Hydrogen generation from the hydrolysis of AB solution (0.5 wt%) in the presence of 5, 10, 15, 20 mg Ni/C-3 catalysts at room temperature and (f) the corresponding plot of hydrogen generation rate versus the amount of Ni/C-3 (both on logarithmic scales). A series of experiments considering the factors of temperature, concentration and catalyst amount were carried out for the hydrolysis of AB using Ni/C-3 as the catalyst. Figure 5(a) shows the effect of temperature on the hydrogen generation rate of Ni/C-3. It can be seen that the rates of hydrogen generation rise dramatically with the increase of temperature. The hydrolysis of AB went on at various temperatures (25-55 o C) in the presence of 0.5 wt% AB and 10 mg catalysts. The hydrogen generation rate constant can be determined by the slope of the fitting liner region in each plot. The amount of nickel in Ni/C-3 was determined according to the TG curve of Ni/C-3 measured under air environment (Fig. S1(c) in the ESM). The quality content ratio of Ni to C can be seen from the curve, 3:2, which is employed to calculate the activation energy concerning the catalyst amount. In contrast to the TG curve of Ni/C-2 (Fig. S1(b) in the ESM), there is no significant difference between Ni/C-2 and 7

9 Ni/C-3, again illustrating the role of KBH4 is just to reduce the surface nickel oxide. Figure 5(b) shows the Arrhenius plots of lnk versus the reciprocal absolute temperature (1/T). The slope of the straight line gives apparent activation energy (Ea= 31.6 kj/mol). This value is relatively low in Table 1. Figure 5(c) shows the effect of AB concentration on the hydrolysis. The slope is determined to be 0.32 with respect to the AB concentration (Fig. 5(d)). The value is close to zero, proving the hydrolysis of AB to be zero-order reaction with respect to the concentration of AB [14]. The effect of catalyst amount on the hydrolysis of AB has also been studied. Figure 5(e) displays the hydrogen generation rate with the increasing catalysts amount from 5 mg to 20 mg. In addition, ln (Rate) versus ln (Catalysts amount) is plotted in Fig. 5(f), from which we can learn that the hydrolysis rate of AB presents linear change considering the catalysts amount. The slope of the line is 0.94, closing to 1.0, indicating that the hydrolysis of AB is quasi first-order reaction with respect to the catalyst amount. Table 1 Activation energy values for hydrolysis of AB catalyzed by different catalyst Catalysts Ea (kj/mol) Ref. 3.2 nm Ni/C 28 [14] Our Ni/C 31.6 [this study] Ni 0.67 /C 33 [26] Ni/SiO 2 34 [27] Ni 0.03 Pt 0.97 alloys 43.7 [28] Ni power 70 [6] 4. Conclusions In summary, Ni/C composite can be prepared by either reduction, calcination, or the combination of calcinations and reduction of Ni-MOF. The reduction of Ni-MOF by KBH4 produces amorphous Ni/C composite. The calcination of Ni-MOF at 700 C under Ar results in homogeneous dispersion of crystalline Ni nanoparticles (about 10-nm size) with oxidized surface on carbon. The combination of the first calcination and then the reduction offers an approach to obtain Ni nanoparticles (about 10-nm size) with less oxidized surface that are uniformly dispersed on carbon. Moreover, Ni/C composite with less oxidized surface affords a high hydrogen release rate of 834 ml min -1 g -1 at room temperature and a low activation energy of 31.6 kj/mol for AB hydrolysis. Such Ni/C catalyst is promising to replace noble metals such as Pt for the catalytic hydrolysis of AB. Acknowledgements This work was supported by the Programs of MOST (2010CB631301, 2012AA and 2012AA051901), NSFC ( and ), and Tianjin High-Tech (12JCQNJC03900 and 13SJCZDJC26500). Electronic Supplementary Material: Supplementary material (TG curve for Ni-MOF, Ni/C-2 and Ni/C-3 (Fig. S1), carbon in Ni/C-1, Ni/C-2 and Ni/C-3 (Fig. S2), and Ni 2p XPS spectrum for Ni/C-2 and Ni/C-3 (Fig. S3)) is available in the online version of this article at (automatically inserted by the publisher). References [1] Grochala, W.; Edwards, P. P. Thermal decomposition of the non intersticial hydrides for the storage and production of hydrogen. Chem. Rev. 2004, 104(3), [2] Peng, B.; Chen, J. Ammonia borane as an efficient and lightweight hydrogen storage medium. Energy Environ. Sci. 2008, 1, [3] Graham, T. W.; Tsang, C. W.; Chen, X. H.; Guo, R. W.; Jia, W. L.; Lu, S. M.; Sui-Seng, C.; Ewart, C. B.; Lough, A.; 8

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