Fabrication of hollow nickel-silica composite spheres using l (+)-arginine and their catalytic performance for hydrolytic dehydrogenation of ammonia borane

Journal of Molecular Catalysis A: Chemical 371 (2013) 1–7

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Fabrication of hollow nickel-silica composite spheres using l(+)-arginine and their catalytic performance for hydrolytic dehydrogenation of ammonia borane Tetsuo Umegaki a,∗ , Chihiro Takei a , Yasuhiro Watanuki a , Qiang Xu b , Yoshiyuki Kojima b a b

Department of Materials & Applied Chemistry, College of Science & Engineering, Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

a r t i c l e

i n f o

Article history: Received 5 November 2012 Received in revised form 6 January 2013 Accepted 7 January 2013 Available online 29 January 2013 Keywords: Hollow nickel-silica composite spheres l(+)-Arginine Hydrolytic dehydrogenation Ammonia borane

a b s t r a c t In this paper, we report a facile and effective approach for fabrication of hollow nickel-silica composite spheres. In this approach, when nickel-silica composite shells were coated on polystyrene templates by the sol–gel method using l(+)-arginine as the promoter for the reaction to form nickel-silica composite shell, the polystyrene templates were dissolved subsequently, even synchronously, in the same medium to form hollow spheres. Neither additional dissolution nor a calcination process was needed to remove the polystyrene templates. The as-prepared nickel-silica composite spheres were characterized by transmission electron microscopy and N2 adsorption/desorption isotherm analysis. The effects of the amount of l(+)-arginine, the existence of hollow structure, and the kinds of promoter for sol–gel reaction on the morphology, porosity and mean pore size, and specific surface areas were systematically evaluated. The catalytic activity of the hollow nickel-silica composite spheres for hydrolytic dehydrogenation of ammonia borane was compared to that of the hollow nickel-silica composite spheres prepared with aqueous ammonia solution and nickel-silica composite spheres without hollow structure. The catalytic activity for the hydrogen evolution over the hollow spheres was higher than those of the hollow spheres prepared with aqueous ammonia solution and the nickel-silica composite spheres without hollow structure. Moreover, the results of diffuse reflectance UV–vis spectra indicate that the amount of hydrogen evolution was correlated with the reduction degree of nickel species after hydrolytic dehydrogenation of ammonia borane. © 2013 Elsevier B.V. All rights reserved.

1. Introduction There have been intensive efforts for developing safe and efficient methods for hydrogen storage, a key issue of the hydrogen economy [1–4]. Ammonia borane (NH3 BH3 ) possesses a low molecular weight (30.9 g mol−1 ) and high hydrogen content (19.6 wt%) [5,6], and therefore makes it an attractive candidate for chemical hydrogen storage application [5–30]. NH3 BH3 is nontoxic, stable, and environmentally benign, can be handled at room temperature and can release hydrogen gas upon catalytic hydrolysis under mild conditions [5]. The hydrolysis occurs at appreciable rate in the presence of a suitable acid or a suitable catalyst at ambient temperature [10–22]. From the viewpoint of practical application, the development of efficient, low-cost, and stable catalysts to further improve the kinetic properties under moderate conditions is therefore very important. Nowadays, there is a general interest in searching for more abundant first-row transition-metal-based catalysts, such as Fe, Co, Ni, and so on, to catalyze the hydrolysis of NH3 BH3 with high

∗ Corresponding author. Tel.: +81 3 3259 0810; fax: +81 3 3293 7572. E-mail address: [email protected] (T. Umegaki). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.01.014

efficiency [10,12,15–21]. Studies of catalysts for hydrogen generation from aqueous ammonia borane solution show that dispersion of active metals and/or amorphousness of active phase play important roles in the catalytic performances [12,15–21]. In recent years, the preparation and study of core–shell solid and hollow microspheres with well-defined structures have attracted substantial interest because of their potential applications in controlled drug delivery system, lightweight fillers, catalysis, chromatography, vessels for confined reactions, and photonic band gap material [31–36]. A number of efforts to find new methods have been devoted to generating colloids with the core–shell structure, such as template-assisted sol–gel process [37–42], layer by layer (LBL) techniques [43–47], microemulsion/interfacial polymerization strategies [48–52]. Microsized, monodisperse, hollow silica [40] and titania [53] spheres were fabricated via a one-step process, which means that the formation of the inorganic shells and the dissolution of core particles occur in the same medium. In that method, microsized, monodisperse, positively charged polystyrene (PS) particles were prepared by dispersion polymerization using the cationic monomer 2(methacryloyl)-ethyltrimethylammonium chloride (MTC) as the comonomer [54] or by emulsifier-free emulsion polymerization

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using ␣,␣ -azodiisobutyramidine dihydrochloride (AIBA) as the initiator and poly(vinyl pyrrolidone) (PVP) as the stabilizer [40,55,56]. These small PS template particles without a positively charged comonomer could also be dissolved in the same medium subsequently, even simultaneously, during the coating of silica shells to directly form hollow spheres. Neither additional dissolution nor a calcination process was used to remove the PS cores to form hollow spheres. In the present study, we fabricated hollow nickel-silica composite spheres using PS template method and investigated their activity for hydrolytic dehydrogenation of ammonia borane. In addition, we firstly fabricated the hollow spheres using l(+)arginine, a kind of basic amino acid, as the promoter for hydrolysis of tetraethoxysilane (TEOS) [57] and for dissolution of PS templates. 2. Experimental 2.1. Catalyst preparation Hollow nickel-silica composite spheres were prepared by polystyrene beads template method as follows [53]. The monodisperse PS particles were prepared by emulsifier-free emulsion polymerization as follows: 10.0 g of styrene (Kanto Chem. Co.), 1.5 g of poly(vinyl pyrrolidone) (PVP) K30 (Mw ≈ 40,000, Fluka), 0.26 g of cationic initiator 2,2 -azobis-(isobutyramidine)dihydrochloride (AIBA, Kanto Chem. Co.), and 100.0 g of distilled water were charged into a 250-mL three-neck flask equipped with a mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, a Graham condenser, and a heating oil bath. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for 60 min. Then, under a stirring rate of 150 rpm, the reaction was carried out at 343 K for 24 h. The obtained PS suspension was centrifuged (6000 rpm, 5 min) and washed in ethyl alcohol for 3 times, and content of the PS suspension could be tailored through the addition of ethanol. Then, aqueous l(+)-arginine (1.78, 5.34, and 10.68 × 10−3 mol l(+)-arginine/9.0, 27.0, and 54.0 mL of deionized water, Acros Organics Co.) and nickel nitrate hexahydrate (0.03 g, 1.0 × 10−4 mol, Kanto Chem. Co.) mixed solution and tetraethoxysilane (TEOS, 215.2 ␮L, 9.6 × 10−4 mol, Kanto Chem. Co.) and 160.0 mL of ethyl alcohol were added into 20.0 g of the PS suspension, in which the sol–gel reaction was carried out at 323 K for 0.5–17.0 h, and the hollow nickel-silica composite spheres could be directly obtained. 2.2. Characterization The morphologies of the catalysts were observed using a Hitachi FE2000 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. Measurement of specific surface area and analysis of porosity for the nickel-silica composite products were performed through measuring N2 adsorption–desorption isotherms at 77 K with a Micromeritics Model ASAP 2010MC analyzer. Diffuse reflectance ultraviolet and visible (DRUV–vis) spectra were recorded on a V-670 (JASCO) UV–Vis–NIR spectrophotometer with barium sulfate as standard spectra over the range of 250–800 nm. The TPR–TGA (temperature-programmed reduction–thermogravimetric analysis) investigations were performed on a Rigaku TG8120 instrument. TPR profiles were recorded by passing a 10 vol% H2 in Ar (260 mL min−1 ) through the sample (≈2 mg) heated at a constant rate of 20 K min−1 up to 1173 K. 2.3. Experimental procedures for hydrolysis of ammonia borane A mixture of sodium borohydride (NaBH4 , 5 mg, Kanto Chemical Reagent Co., >98.5%) and ammonia borane (NH3 BH3 , 27.5 mg, Aldrich, 90%) with 28.8 mg of catalyst

(NH3 BH3 /NaBH4 /Ni = 1/0.17/0.06) was kept in a two-necked round-bottom flask. One neck was connected to a gas buret, and the other was fitted with a septum inlet to introduce distilled water (5 mL). The reaction started when distilled water was introduced to the mixture of NaBH4 , NH3 BH3 , and the catalyst, and the evolution of gas was monitored using the gas buret. The reactions were carried out at room temperature in air. All the samples after hydrolysis of NH3 BH3 were centrifugally separated from the reaction solution, and then dried in desiccator under vacuum condition for characterizations. 3. Results and discussion The monodisperse PS template beads were prepared by emulsifier free emulsion polymerization using the AIBA as the cationic initiator and PVP as the stabilizer. Fig. 1 illustrates the TEM images of various obtained spheres. The diameter of original PS particles was ca. 200–300 nm (Fig. 1a). Fig. 1b–d shows the TEM images of the solid products formed with l(+)-arginine = 1.78 × 10−3 mol after different times of the coating (1.5, 7.5, and 17.0 h) before hydrolysis of NH3 BH3 . After the aging at 323 K for 1.5 h as shown in Fig. 1b, no particles were observed. After 7.5 h, particles of irregular shape were found, as shown in Fig. 1c. After 17 h, the spherical particles of ca. 200–300 nm in diameter (Fig. 1d) were obtained, and the inset of Fig. 1d demonstrates their hollow feature indicated by the arrows. The morphology and diameters of the hollow spheres were almost the same as the solid products prepared without PS (Fig. 1e) and the solid products prepared with aqueous ammonia solution (Fig. 1f). Fig. 2a–c shows the TEM images of the solid products formed with l(+)-arginine = 5.34 × 10−3 mol after different times (0.5, 1.5, and 3 h) before the hydrolysis of ammonia borane. After 0.5 h, particles of irregular shape were found, as shown in Fig. 2a. After more than 1.5 h, the spherical particles with the diameter of 200–300 nm (Fig. 2b and c) were obtained. The morphology of the particles is not significantly different from the solid products prepared with l(+)arginine = 10.68 × 10−3 mol before hydrolytic dehydrogenation of NH3 BH3 (Fig. 2d). These results indicate that l(+)-arginine has promoting effect on the formation of nickel-silica shell, and the formation rate of the shell increases with the increase in the amount of l(+)-arginine. The specific surface area, average pore size, and pore volume of the as-samples were determined by BET and BJH method (listed in Table 1). The pore size distribution, the average pore diameter and the pore volume were calculated by using the desorption branch of the isotherm. It could be seen that the BET surface area and pore volume of the solid products prepared with l(+)arginine do not depend on the amount of l(+)-arginine, while the solid products prepared with higher amount of l(+)-arginine (5.34 and 10.68 × 10−3 mol) has larger average pore size than the solid products prepared with lower amount of l(+)-arginine (1.78 × 10−3 mol). The surface area of the solid sample prepared without PS is about four times lower than that of the solid products prepared with PS, while the average pore size and the pore volume of the solid products prepared without PS are almost the same as those of the solid products prepared with PS, suggesting the existence of hollow structure in the solid product prepared with PS. The specific surface area, average pore size, and pore volume of the solid product prepared with aqueous ammonia solution are almost the same as those of the solid products prepared with l(+)-arginine = 1.78 × 10−3 mol. Fig. 3 shows the time course of hydrogen evolution from aqueous NaBH4 /NH3 BH3 solution in the presence of hollow nickel-silica composite spheres prepared with PS (l(+)-arginine = 1.78, 5.34, and 10.68 × 10−3 mol), nickel-silica composite spheres prepared without PS (l(+)-arginine = 1.78 × 10−3 mol), and hollow nickel-silica

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Fig. 1. TEM images of the various obtained microspheres. (a) Pure PS spheres; hollow nickel-silica composite spheres prepared with l(+)-arginine = 1.78 × 10−3 mol stirring for (b) 1.5, (c) 7.5, and (d) 17.0 h; (e) nickel-silica composite spheres prepared without PS (l(+)-arginine = 1.78 × 10−3 mol, stirring time: 17.0 h), and (f) hollow nickel-silica composite spheres prepared with aqueous ammonia solution (NH3 = 237 × 10−3 mol, stirring time: 1.5 h).

composite spheres prepared with PS using aqueous ammonia solution. The reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The evolution of 60, 57, 50, 50, and 19 mL hydrogen were finished in 30, 105, 100, 125, and 180 min in the presence of the hollow nickel-silica composite spheres prepared with l(+)-arginine = 1.78, 5.34, and 10.68 × 10−3 mol, the nickel-silica composite spheres prepared without PS, and the hollow nickel-silica composite spheres prepared with aqueous ammonia solution, respectively. The effect of NaBH4 has been reported about Fe catalyst for hydrolysis of NH3 BH3 [15]. In the

present reaction system, NaBH4 was mixed with H2 O, NH3 BH3 , and catalyst. Hydrogen is evolved via following two reactions (reactions (2) and (3)) besides reaction (1); 4Ni2+ + BH4 − + 2H2 O → 4Ni + BO2 − + 8H+

(1)

−

+

NaBH4 + 2H2 O → Na + BO2 + 4H2

(2)

−

+

NH3 BH3 + 2H2 O → NH4 + BO2 + 3H2

(3)

Under the present reaction condition, about 12 mL of hydrogen (4.8 × 10−4 mol) is generated via reaction (2) from

Table 1 Characteristics of hollow nickel-silica composite spheres.

l(+)-Arginine

NH3 a b

Amount [×10−3 mol]

Stirring time [h]

BET surface area [m2 g−1 ]a

Average pore size [nm]a , b

Pore volume [cm3 g−1 ]a , b

1.78 5.34 10.68 1.78 (without PS) 237

17.0 1.5 1.5 17.0 1.5

19.4 14.6 17.1 4.5 23.9

12.0 21.3 23.8 11.2 9.6

0.076 0.083 0.107 0.039 0.075

Measured by N2 adsorption at 77 K with the BJH method. Calculated by the BJH method from the adsorption curves.

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Fig. 2. TEM images of hollow nickel-silica composite spheres prepared with l(+)-arginine (5.34 × 10−3 mol) stirring for (a) 0.5, (b) 1.5, (c) 3.0 h, and (d) with l(+)arginine = 10.68 × 10−3 mol stirring for 1.5 h.

residual amount of NaBH4 consumed via the reaction (1) (1.3 × 10−4 (total amount of NaBH4 ) – 0.1 × 10−4 (amount of NaBH4 consumed via the reaction (1)) = 1.2 × 10−4 mol), and about 59 mL of hydrogen (24.0 × 10−4 mol) is generated via reaction (3), experimentally. The molar ratio of hydrolytically generated hydrogen to the initial NH3 BH3 in the presence of the hollow nickel-silica composite spheres prepared with l(+)-arginine = 1.78, 5.34, and 10.68 × 10−3 mol, the nickel-silica composite spheres prepared without PS, and the

Fig. 3. Hydrogen evolution from aqueous NaBH4 /NH3 BH3 solution in the presence of hollow nickel-silica composite spheres prepared with l-arginine = (a) 1.78 × 10−3 mol (stirring time: 17.0 h), (b) 5.34 × 10−3 mol (stirring time: 1.5 h), (c) 10.68 × 10−3 mol (stirring time: 17.0 h), (d) nickel-silica composite spheres prepared without PS (l(+)-arginine = 1.78 × 10−3 mol, stirring time: 17.0 h), and (e) hollow nickel-silica composite spheres prepared with aqueous ammonia solution (NH3 = 237 × 10−3 mol, stirring time: 1.5 h).

hollow nickel-silica composite spheres prepared with aqueous ammonia solution are 2.5, 2.3, 1.9, 1.9, and 0.4, respectively. The results indicate that the hollow nickel-silica composite spheres with PS show higher activity and hydrogen evolution rates than nickel-silica composite spheres prepared without PS and the hollow nickel-silica composite spheres with PS using aqueous ammonia solution. Fig. 4 shows the diffuse reflectance of UV–vis (DRUV–vis) absorption spectra of the nickel-silica composite spheres. Transitions assigned as 3 T1g (P) ← 3 A2g (F) and 3 T1g (F) ← 3 A2g (F) are located at 350–400 nm and at 550–650 nm for Ni2+ , respectively [58], indicating that all the samples before the reaction include octahedral coordination of Ni(II). The band assigned to 3 T (P) ← 3 A (F) transition is observed at around 360 nm in the 1g 2g spectra of hollow nickel-silica composite spheres prepared with l(+)-arginine and nickel-silica composite spheres prepared without PS templates, while the band is observed at around 400 nm in the spectrum of hollow nickel-silica composite spheres prepared with aqueous ammonia solution. The result indicates that the band of the hollow spheres prepared with l(+)-arginine and the composite spheres prepared without PS templates is blue shifted compared with the band of the hollow spheres prepared with aqueous ammonia solution probably because of the different coordination environments surrounding the nickel species between in the samples prepared with l(+)-arginine and in the samples prepared with aqueous ammonia solution. Compared with the spectra of each sample before the reaction, the intensity of those peaks after the reaction decreased, and the degree of the intensity decrease depends on the sample. Fig. 5 shows the relationship of the amount of hydrogen evolution and the ratio of peak intensity of the samples after to before reaction at 350–400 nm. With increasing the amount of hydrogen evolution, the ratio of peak intensity decreases, indicating that the catalytic activity for hydrolysis of ammonia borane is correlated to the reduction degree of nickel species. Compared with reduction degree of nickel species in the hollow spheres prepared with

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Fig. 4. DRUV–vis spectra of nickel-silica composite spheres before and after the reaction. Hollow nickel-silica composite spheres prepared with l(+)-arginine = (a) 1.78 × 10−3 mol (stirring time: 17.0 h), (b) 5.34 × 10−3 mol (stirring time: 1.5 h), (c) 10.68 × 10−3 mol (stirring time: 17.0 h), (d) nickel-silica composite spheres prepared without PS (l(+)-arginine = 1.78 × 10−3 mol, stirring time: 17.0 h), and (e) hollow nickel-silica composite spheres prepared with aqueous ammonia solution (NH3 = 237 × 10−3 mol, stirring time: 1.5 h).

Fig. 5. The ratio of peak intensity of the samples after to before reaction at 350–400 nm vs. the amount hydrogen evolution. Hollow nickel-silica composite spheres prepared with l(+)-arginine = (a) 1.78 × 10−3 mol (stirring time: 17.0 h), (b) 5.34 × 10−3 mol (stirring time: 1.5 h), (c) 10.68 × 10−3 mol (stirring time: 17.0 h), (d) nickel-silica composite spheres prepared without PS (l(+)arginine = 1.78 × 10−3 mol, stirring time: 17.0 h), and (e) hollow nickel-silica composite spheres prepared with aqueous ammonia solution (NH3 = 237 × 10−3 mol, stirring time: 1.5 h).

aqueous ammonia solution, the reduction degree of nickel species in the hollow spheres prepared with l(+)-arginine is much higher, indicating that l(+)-arginine has promoting effect on the reduction of nickel species. However, the reduction degree of the nickel species in the hollow spheres prepared with l(+)-arginine decreases with the increase in the amount of l(+)-arginine, indicating that excess amount of l(+)-arginine has negative effect on reduction degree of nickel species. In order to obtain further information about reducibility of active nickel species, the temperature programmed reduction (TPR) profiles observed upon treatment of the as-prepared samples in H2 are obtained by TGA. Fig. 6 shows the DTG curves of the asprepared samples. The curves involved three main domains that point to the presence of different kinds of nickel species with diverse reducibility: one reduced at the lowest temperature (Tmax at about 525 K), one reduced at moderate temperatures (Tmax at about 710–750 K), and one most difficult to reduce (Tmax at about 840 K). Fig. 6(a) represents the reduction profile of hollow nickel-silica composite spheres prepared with aqueous ammonia solution. The clearly defined peaks at 722 K and 843 K appear in the DTG curve. These are assigned to nickel species contacting with silica matrix and to nickel species strongly interacting with silica matrix or nickel phyllosilicate [59–61]. Contrary to the sample, a low temperature peak at about 528 K which is ascribed to

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Acknowledgements The research was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society. Special thanks are given to Dr. H. Ikake for the use of DRUV–vis spectrometer. References

Fig. 6. DTG curve registered upon H2 -TPR of (a) hollow nickel-silica composite spheres prepared with aqueous ammonia solution (NH3 = 237 × 10−3 mol, stirring time: 1.5 h), (b) nickel-silica composite spheres prepared without PS (l(+)arginine = 1.78 × 10−3 mol, stirring time: 17.0 h), and hollow nickel-silica composite spheres prepared with l(+)-arginine = (c) 1.78 × 10−3 mol (stirring time: 17.0 h), (d) 5.34 × 10−3 mol (stirring time: 1.5 h), and (e) 10.68 × 10−3 mol (stirring time: 17.0 h).

the presence of clustered nickel species [60] is well resolved in the reduction profiles of nickel-silica composite spheres prepared without PS templates and hollow nickel-silica composite spheres prepared with l(+)-arginine = 1.78 × 10−3 mol. These results indicate that nickel species in hollow nickel-silica composite spheres prepared with l(+)-arginine are well dispersed compared with these in hollow nickel-silica composite spheres prepared with aqueous ammonia solution, and more reducible nickel species exist in the hollow spheres prepared with l(+)-arginine than the nickel species in the hollow spheres prepared with aqueous ammonia solution. On the other hand, the reduction profiles of hollow nickelsilica composite spheres prepared with l(+)-arginine = 5.34 and 10.68 × 10−3 mol exhibit a large peak at around 710–750 K and a small peak at around 515–540 K. In addition, the peak temperatures (750 and 545 K) in the profile of hollow nickel-silica composite spheres prepared with l(+)-arginine = 10.68 × 10−3 mol are higher than that (725 and 524 K) in the profile of hollow nickel-silica composite spheres prepared with l(+)-arginine = 5.34 × 10−3 mol. These results indicate that the less reducible nickel species contacting with silica matrix increase with increase of l(+)arginine for preparation of hollow nickel-silica composite spheres. 4. Conclusions Highly active hollow nickel-silica composite spheres have been successfully synthesized via a sol–gel method using l(+)-arginine as the promoter for sol–gel reaction. The kind of promoter for sol–gel reaction, the amount of l(+)-arginine, and the existence of hollow structure influenced the morphology of the hollow spheres. As the amount of l(+)-arginine increased, the average pore size increased. The amount of hydrogen evolution and the hydrogen evolution rate in the presence of hollow nickel-silica composite spheres prepared with l(+)-arginine are much higher than those in the presence of nickel-silica composite spheres without hollow structure and hollow nickel-silica composite spheres prepared with aqueous ammonia solution. From DRUV–vis spectra before and after reaction, the amount of hydrogen evolution is correlated with the reduction degree of active nickel species, and the higher reduction degree of nickel species during hydrolytic dehydrogenation in aqueous NaBH4 /NH3 BH3 solution corresponds to higher amount of hydrogen evolution.

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