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Influence of preparation conditions of hollow titania–nickel composite spheres on their catalytic activity for hydrolytic dehydrogenation of ammonia borane
Materials Research Bulletin 52 (2014) 117–121
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Influence of preparation conditions of hollow titania–nickel composite spheres on their catalytic activity for hydrolytic dehydrogenation of ammonia borane Tetsuo Umegaki a,*, Takato Ohashi a, Qiang Xu b, Yoshiyuki Kojima a a
Department of Materials & Applied Chemistry, College of Science & Technology, 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
b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 September 2013 Received in revised form 6 January 2014 Accepted 12 January 2014 Available online 21 January 2014
The present work reports influence of preparation conditions of hollow titania–nickel composite spheres on their morphology and catalytic activity for hydrolytic dehydrogenation of ammonia borane (NH3BH3). The as-prepared hollow titania–nickel composite spheres were characterized by transmission electron microscopy (TEM). Catalytic activities of the hollow spheres for hydrolytic dehydrogenation of aqueous NaBH4/NH3BH3 solution improve with the decrease of Ti + Ni content. From the results of FTIR spectra and elemental analysis, the amount of residual polystyrene (PS) templates is able to be reduced by increasing aging time for the preparation, and the catalytic activity of the hollow spheres increases when the amount of residual PS templates decreases. The carbon content in the hollow spheres prepared with aging time = 24 h is 17.3 wt.%, and the evolution of 62 mL hydrogen is finished in about 22 min in the presence of the hollow spheres from aqueous NaBH4/NH3BH3 solution. The molar ratio of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of the hollow spheres is 2.7. ß 2014 Elsevier Ltd. All rights reserved.
Keywords: A. hollow spheres A. titania–nickel composite D. hydrolytic dehydrogenation of ammonia borane D. hydrogen evolution D. catalytic activity
1. Introduction There have been intensive efforts for developing safe and efficient methods for hydrogen storage, a key issue of the hydrogen economy [1–3]. Ammonia borane (NH3BH3) possesses a low molecular weight (30.9 g mol1) and high hydrogen content (19.6 wt.%) [4], making it an attractive candidate for chemical hydrogen storage application [5–25]. NH3BH3 is nontoxic, stable under ambient condition or in neutral or alkaline solution, and environmentally benign, can be handled at room temperature and can release hydrogen gas upon catalytic hydrolysis under mild conditions [4]. The hydrolysis occurs at appreciable rate in the presence of a suitable acid or a suitable catalyst at ambient temperature [12–25]. For the viewpoint of practical application, the development of efficient, low-cost, and stable catalysts to further improve the kinetic properties under moderate conditions is very important. It is reported that nickel based catalyst is one of highly active catalysts for this reaction [14,17,19–25]. However, there are several reports that have studied support or composite effect with metal oxides on nickelbased catalysts [14,17,21,22,24,25].
* Corresponding author. Tel.: +81 3 3259 0810; fax: +81 3 3293 7572. E-mail address: [email protected] (T. Umegaki). 0025-5408/$ – see front matter ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2014.01.017
Synthesis of micro- and nanosized hollow spheres is attracting continuous interest, since these hollow spheres have potential applications in catalysis, photonic crystal, chromatography, protection of biologically active agents, fillers (or pigments, coatings), waste removal, and dye-sensitized solar cells [26–31]. There have been developed several strategies, such as heterophase polymerization/combined with a sol–gel process [32,33], emulsion/interfacial polymerization approach [34], spray-drying method [35], self-assembly technique [36], and surface living polymerization process [37], to prepare hollow spheres comprising polymeric or ceramic materials. The most frequently used method is the template-based route, which is particularly interesting in fabricating hollow spheres with homogeneous, dense layers [38– 43]. In recent years, microsized, monodisperse, hollow silica [44] and titania [45] spheres were fabricated via a one-step process, which meant that the formation of the inorganic shells and the dissolution of core particles occur in the same medium. In this 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 [46] or by emulsifierfree emulsion polymerization using a,a0 -azodiisobutyramidine dihydrochloride (AIBA) as the initiator and poly(vinyl pyrrolidone) (PVP) as the stabilizer [47]. These small PS template particles
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without a positively charged comonomer could also be dissolved in the same medium subsequently, or even simultaneously, during the coating of silica shells to directly form the hollow spheres. We have revealed that hollow metal oxide–nickel composite spheres can be fabricated by the similar method [48]. The hollow titania–nickel composite spheres show higher catalytic activity for hydrolytic dehydrogenation of NH3BH3 than other hollow metal oxide–nickel composite spheres such as hollow zironica–nickel composite spheres and hollow silica–nickel composite spheres [48]. In the present study, we investigated the influence of preparation conditions of the hollow titania–nickel composite spheres using PS template method on their morphology, the amount of residual PS templates, and their catalytic activity for hydrolytic dehydrogenation of NH3BH3. 2. Experimental 2.1. Catalyst preparation Hollow titania–nickel composite spheres were prepared by polystyrene beads template method as follows [48]. 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 2,20 -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. The mixed aqueous solution of 0.03 g of Ni(NO3)26H2O (Kanto Chem. Co. >99.0%), 0– 36 mL of 28 wt.% aqueous ammonia solution (Kanto Chem. Co.), and 160 mL of ethyl alcohol were added into 20.0 g of the PS suspension, to which added was 0.245 mL of titanium-tetra-nbutoxide (Kanto Chem. Co. >97.0%). The sol–gel reaction was carried out at 323 K for 0.5–24 h, and the hollow titania–nickel composite spheres could be directly obtained. After drying in a desiccator overnight, the obtained light green fine powders were used as catalysts. 2.2. Characterization The morphologies of hollow titania–nickel composite spheres were observed using a Hitachi FE2000 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. Fourier transform infrared (FTIR) spectra of hollow titania–nickel composite spheres were recorded using a Fourier transform infrared spectrophotometer (FTIR-8400S, Shimadzu Co., Ltd.) with a resolution of 4 cm1. The carbon content in the as-prepared samples was measured using an elemental analyzer (Micro Corder JM10, J-Science Lab Co., Ltd.). 2.3. Experimental procedures for hydrolysis of ammonia borane A mixture of sodium borohydride (NaBH4, 5 mg, Kanto Chemical Reagent Co. >98.5%), ammonia borane (NH3BH3, 27.5 mg, Aldrich, 90%), and catalyst (NH3BH3/NaBH4/Ni = 1/0.17/ 0.05) was kept in a two-necked round-bottom flask. The ratio of NH3BH3/NaBH4/Ni was the same as some other nickel based catalysts previously reported [21,48]. 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, NH3BH3, and the catalyst, and the evolution of gas was monitored using the gas buret. The reactions were carried out at room temperature. All the samples after hydrolysis of NH3BH3 were centrifugally separated from the reaction solution, and then dried under vacuum condition for characterizations. 3. Results and discussion The morphologies of hollow titania–nickel composite spheres before hydrolysis of NH3BH3 were examined using TEM. The TEM images of hollow titania–nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol reveal that all the samples consist of spherical particles with ca. 200–300 nm diameter (Fig. 1). The hollow spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol mainly consist of spherical particles with about 190, 210, and 225 nm of diameter. Thus, the results indicate that the morphology of the hollow titania–nickel composite spheres is slightly different from each other. Moreover, Fig. 1c demonstrates their hollow feature indicated by the arrows, indicating that the spherical particles possess hollow structure. Fig. 2 shows the time course of the hydrogen evolution from aqueous NaBH4/NH3BH3 solution in the presence of hollow titania– nickel composite spheres. From the results, the reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The evolution of 49, 45, and 43 mL hydrogen from aqueous NaBH4 solution were finished in 25, 29, and 34 min in the presence of hollow titania–nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol, respectively. In the previous study [48], negligible amount of hydrogen was generated from aqueous NH3BH3 solution in the presence of hollow titania–nickel composite spheres. On the other hand, certain amount of hydrogen generated from aqueous NH3BH3 solution following hydrogen generation from aqueous NaBH4 solution. However, hydrogen evolution in the presence of the hollow spheres is not completed. Moreover, the amount of hydrogen evolution from aqueous NaBH4/NH3BH3 solution in the hollow spheres is much higher than those from aqueous NH3BH3 solution following hydrogen generation from aqueous NaBH4 solution. In the process, hydrogen is evolved via following the reactions (2) besides reaction (1); 4Ni2þ þ BH4 þ 2H2 O ! 4Ni þ BO2 þ 8Hþ
(1)
NaBH4 þ 2H2 O ! Naþ þ BO2 þ 4H2
(2)
Under the present reaction condition, about 12 mL of hydrogen (4.8 104 mol) is generated via reactions (2) from residual amount of NaBH4 consumed via the reaction (1) (1.3 104 (total amount of NaBH4) – 0.1 104 (amount of NaBH4 consumed via the reaction (1)) = 1.2 104 mol), experimentally. Hydrogen is also evolved from ammonia borane via reaction (3); NH3 BH3 þ 2H2 O ! NH4 þ þ BO2 þ 3H2
(3)
Under the present reaction condition, about 59 mL of hydrogen (24.0 104 mol) is generated via reaction (3), experimentally. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of hollow titania–nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol are 2.0, 1.8, and 1.7, respectively. These results indicate that the catalytic activity of the hollow titania–nickel composite spheres for hydrolysis of NH3BH3 decreases with the increase of Ti + Ni content. In this study, the same amount of PS templates were used for preparation of the hollow spheres with all the Ti + Ni contents, and thus the amount of Ti + Ni on one PS template particle increases with the increase of Ti + Ni content. Consequently, wall
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Fig. 1. TEM images of as-prepared hollow titania–nickel composite spheres prepared with Ti + Ni content = (a) 1.06, (b) 3.49, and (c) 5.82 103 mol.
thickness of the hollow spheres can be controlled, depending to the Ti + Ni content. The as-prepared hollow titania–nickel composite spheres were characterized by using FTIR spectroscopic methods to confirm that the difference in the catalytic activity depends on the amount of residual PS templates. Fig. 3 shows FTIR spectra for hollow titania– nickel composite spheres. These hollow spheres display the characteristic bands of PS at around 1600, 1492, 1449, 754, and 698 cm1 [45]. These bands indicate that the PS segments are included in the shell of hollow spheres. However, the intensities of all the bands depend on the samples. The figure shows that the intensity at 698 cm1 in the spectrum of hollow titania–nickel composite spheres increases with the decrease of Ti + Ni content.
Fig. 2. Hydrogen generation from the mixture of sodium borohydride and ammonia borane in the presence of hollow titania–nickel composite spheres prepared with Ti + Ni content = (a) 1.06, (b) 3.49, and (c) 5.82 103 mol. Ni/NH3BH3 = 0.05 (under ambient atomosphere).
By elemental analysis, the carbon content of the hollow titania– nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol are 58.0, 23.0, and 23.8 wt.%, respectively. These results indicate that the amount of PS residue of the hollow titania–nickel composite spheres with Ti + Ni content = 1.06 103 mol is the highest in all the hollow titania– nickel composite spheres. In order to reduce the amount of residual PS templates in the hollow titania–nickel composite spheres with Ti + Ni content = 1.06 103 mol, we investigate the effect of the various conditions for procedures during or after preparation. The carbon content in the hollow titania–nickel composite spheres prepared with the aging time = 0.5, 6, 17, and 24 mL are 55.0, 35.0, 23.8, and 17.3 wt.%, respectively. All the hollow titania–nickel composite spheres before hydrolysis of NH3BH3 consist of spherical particles of ca. 200–300 nm in diameter (Fig. 4), indicating that the
Fig. 3. FTIR spectra of hollow titania–nickel composite spheres prepared with Ti + Ni content = (a) 1.06, (b) 3.49, and (c) 5.82 103 mol.
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Fig. 4. TEM images of as-prepared hollow titania–nickel composite spheres prepared with the aging time = (a) 0.5, (b) 6, (c) 17, and (d) 24 h.
morphology of the hollow titania–nickel composite spheres is not significantly different from each other. Fig. 5 shows the catalytic activity over the hollow titania–nickel composite spheres prepared with the aging time = 0.5, 6, 17, and 24 h. The reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The reaction was completed with hydrogen evolution of 44, 50, 57, and 62 mL in 34, 30, 30, and 22 min in the presence of as-prepared hollow titania–nickel composite spheres prepared with the aging time = 0.5, 6, 17, and 24 h, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of hollow titania–nickel composite spheres with the aging time = 0.5, 6, 17, and 24 h are 1.8, 2.1, 2.4, and 2.7, respectively. The results indicate that the catalytic activity of hollow titania–nickel composite spheres increases with decrease of the amount of residual PS templates. The results also suggested that the amount of active sites of catalyst decreases with
increase of the amount of residual PS templates, or that some active nickel sites are covered with residual PS templates. The carbon content in the hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL are 89.5, 59.7, 45.2, and 35.0 wt.%, respectively. From the analysis of TEM images, all the samples prepared with the amount of aqueous ammonia consist of spherical particles with ca. 200–300 nm diameter. These results indicate that the amount of aqueous ammonia solution affected the amount of residual PS templates without the change of morphology. Fig. 6 shows the catalytic activity over the hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL. The reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The reaction was completed with hydrogen evolution of 18, 38, 42, and
Fig. 5. Hydrogen generation from the mixture of sodium borohydride and ammonia borane in the presence of hollow titania–nickel composite spheres prepared with the aging time = (a) 0.5, (b) 6, (c) 17, and (d) 24 h. Ni/NH3BH3 = 0.05 (under ambient atomosphere).
Fig. 6. Hydrogen generation from the mixture of sodium borohydride and ammonia borane in the presence of hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = (a) 0, (b) 9, (c) 18, and (d) 36 mL. Ni/ NH3BH3 = 0.05 (under ambient atomosphere).
T. Umegaki et al. / Materials Research Bulletin 52 (2014) 117–121
50 mL in 139, 131, 64, and 30 min in the presence of as-prepared hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of hollow titania– nickel composite spheres with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL are 0.5, 1.5, 1.7, and 2.1, respectively. The results indicate that the amount of aqueous ammonia solution also affects the amount of residual PS templates and the catalytic activity of hollow titania–nickel composite spheres. 4. Conclusion In this paper, we report the influence of preparation conditions of hollow titania–nickel composite spheres on their morphology and catalytic activity for hydrolytic dehydrogenation of NH3BH3. In the preparation method, titania–nickel composite shells are coated on PS particles by the sol–gel method and the PS templates are dissolved subsequently, or even synchronously, in the same medium to form hollow spheres. All the as-prepared titania– nickel composite spheres have the similar morphology identified by TEM measurement. The catalytic activities of hollow titania– nickel composite spheres prepared with various conditions for hydrolytic dehydrogenation of aqueous NaBH4/NH3BH3 solution were compared. The catalytic activity of the hollow titania–nickel composite spheres increases with decrease of Ti + Ni content. From the results of FTIR spectra, a certain amount of residual PS templates exists in hollow titania–nickel composite spheres which show highest activity for hydrolytic dehydrogenation of NH3BH3, and the amount of the residual PS templates was able to be reduced by increasing aging time during the preparation step. The catalytic activity of hollow titania–nickel composite spheres increases with decrease of the amount of residual PS templates. Acknowledgement The authors would like to thank Ms. M. Takagi for the measurements of elemental analyzer. References [1] Basic research needs for the hydrogen economy, report of the basic energy sciences workshop on hydrogen production, storage and use, May 13–15, 2003, Office of Science, U.S. Department of Energy, http://www.sc.doe.gov/bes/ hydrogen.pdf. [2] J. Turner, G. Sverdrup, M.K. Mann, P.G. Maness, B. Kroposki, M. Ghirardi, R.J. Evans, D. Blake, Int. J. Energy Res. 32 (5) (2008) 379–407. [3] A.W.C.V. Berg, C.O. Area´n, Chem. Commun. 27 (2008) 668–681. [4] Basic research needs catalysis for energy, report from the U.S. Department of Energy, basic energy sciences workshop report, August 6–8, 2007, http:// www.sc.doe.gov/bes/reports/list.html.
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Influence of preparation conditions of hollow titania–nickel composite spheres on their catalytic activity for hydrolytic dehydrogenation of ammonia borane Tetsuo Umegaki a,*, Takato Ohashi a, Qiang Xu b, Yoshiyuki Kojima a a
Department of Materials & Applied Chemistry, College of Science & Technology, 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
b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 September 2013 Received in revised form 6 January 2014 Accepted 12 January 2014 Available online 21 January 2014
The present work reports influence of preparation conditions of hollow titania–nickel composite spheres on their morphology and catalytic activity for hydrolytic dehydrogenation of ammonia borane (NH3BH3). The as-prepared hollow titania–nickel composite spheres were characterized by transmission electron microscopy (TEM). Catalytic activities of the hollow spheres for hydrolytic dehydrogenation of aqueous NaBH4/NH3BH3 solution improve with the decrease of Ti + Ni content. From the results of FTIR spectra and elemental analysis, the amount of residual polystyrene (PS) templates is able to be reduced by increasing aging time for the preparation, and the catalytic activity of the hollow spheres increases when the amount of residual PS templates decreases. The carbon content in the hollow spheres prepared with aging time = 24 h is 17.3 wt.%, and the evolution of 62 mL hydrogen is finished in about 22 min in the presence of the hollow spheres from aqueous NaBH4/NH3BH3 solution. The molar ratio of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of the hollow spheres is 2.7. ß 2014 Elsevier Ltd. All rights reserved.
Keywords: A. hollow spheres A. titania–nickel composite D. hydrolytic dehydrogenation of ammonia borane D. hydrogen evolution D. catalytic activity
1. Introduction There have been intensive efforts for developing safe and efficient methods for hydrogen storage, a key issue of the hydrogen economy [1–3]. Ammonia borane (NH3BH3) possesses a low molecular weight (30.9 g mol1) and high hydrogen content (19.6 wt.%) [4], making it an attractive candidate for chemical hydrogen storage application [5–25]. NH3BH3 is nontoxic, stable under ambient condition or in neutral or alkaline solution, and environmentally benign, can be handled at room temperature and can release hydrogen gas upon catalytic hydrolysis under mild conditions [4]. The hydrolysis occurs at appreciable rate in the presence of a suitable acid or a suitable catalyst at ambient temperature [12–25]. For the viewpoint of practical application, the development of efficient, low-cost, and stable catalysts to further improve the kinetic properties under moderate conditions is very important. It is reported that nickel based catalyst is one of highly active catalysts for this reaction [14,17,19–25]. However, there are several reports that have studied support or composite effect with metal oxides on nickelbased catalysts [14,17,21,22,24,25].
* Corresponding author. Tel.: +81 3 3259 0810; fax: +81 3 3293 7572. E-mail address: [email protected] (T. Umegaki). 0025-5408/$ – see front matter ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2014.01.017
Synthesis of micro- and nanosized hollow spheres is attracting continuous interest, since these hollow spheres have potential applications in catalysis, photonic crystal, chromatography, protection of biologically active agents, fillers (or pigments, coatings), waste removal, and dye-sensitized solar cells [26–31]. There have been developed several strategies, such as heterophase polymerization/combined with a sol–gel process [32,33], emulsion/interfacial polymerization approach [34], spray-drying method [35], self-assembly technique [36], and surface living polymerization process [37], to prepare hollow spheres comprising polymeric or ceramic materials. The most frequently used method is the template-based route, which is particularly interesting in fabricating hollow spheres with homogeneous, dense layers [38– 43]. In recent years, microsized, monodisperse, hollow silica [44] and titania [45] spheres were fabricated via a one-step process, which meant that the formation of the inorganic shells and the dissolution of core particles occur in the same medium. In this 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 [46] or by emulsifierfree emulsion polymerization using a,a0 -azodiisobutyramidine dihydrochloride (AIBA) as the initiator and poly(vinyl pyrrolidone) (PVP) as the stabilizer [47]. These small PS template particles
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without a positively charged comonomer could also be dissolved in the same medium subsequently, or even simultaneously, during the coating of silica shells to directly form the hollow spheres. We have revealed that hollow metal oxide–nickel composite spheres can be fabricated by the similar method [48]. The hollow titania–nickel composite spheres show higher catalytic activity for hydrolytic dehydrogenation of NH3BH3 than other hollow metal oxide–nickel composite spheres such as hollow zironica–nickel composite spheres and hollow silica–nickel composite spheres [48]. In the present study, we investigated the influence of preparation conditions of the hollow titania–nickel composite spheres using PS template method on their morphology, the amount of residual PS templates, and their catalytic activity for hydrolytic dehydrogenation of NH3BH3. 2. Experimental 2.1. Catalyst preparation Hollow titania–nickel composite spheres were prepared by polystyrene beads template method as follows [48]. 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 2,20 -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. The mixed aqueous solution of 0.03 g of Ni(NO3)26H2O (Kanto Chem. Co. >99.0%), 0– 36 mL of 28 wt.% aqueous ammonia solution (Kanto Chem. Co.), and 160 mL of ethyl alcohol were added into 20.0 g of the PS suspension, to which added was 0.245 mL of titanium-tetra-nbutoxide (Kanto Chem. Co. >97.0%). The sol–gel reaction was carried out at 323 K for 0.5–24 h, and the hollow titania–nickel composite spheres could be directly obtained. After drying in a desiccator overnight, the obtained light green fine powders were used as catalysts. 2.2. Characterization The morphologies of hollow titania–nickel composite spheres were observed using a Hitachi FE2000 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. Fourier transform infrared (FTIR) spectra of hollow titania–nickel composite spheres were recorded using a Fourier transform infrared spectrophotometer (FTIR-8400S, Shimadzu Co., Ltd.) with a resolution of 4 cm1. The carbon content in the as-prepared samples was measured using an elemental analyzer (Micro Corder JM10, J-Science Lab Co., Ltd.). 2.3. Experimental procedures for hydrolysis of ammonia borane A mixture of sodium borohydride (NaBH4, 5 mg, Kanto Chemical Reagent Co. >98.5%), ammonia borane (NH3BH3, 27.5 mg, Aldrich, 90%), and catalyst (NH3BH3/NaBH4/Ni = 1/0.17/ 0.05) was kept in a two-necked round-bottom flask. The ratio of NH3BH3/NaBH4/Ni was the same as some other nickel based catalysts previously reported [21,48]. 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, NH3BH3, and the catalyst, and the evolution of gas was monitored using the gas buret. The reactions were carried out at room temperature. All the samples after hydrolysis of NH3BH3 were centrifugally separated from the reaction solution, and then dried under vacuum condition for characterizations. 3. Results and discussion The morphologies of hollow titania–nickel composite spheres before hydrolysis of NH3BH3 were examined using TEM. The TEM images of hollow titania–nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol reveal that all the samples consist of spherical particles with ca. 200–300 nm diameter (Fig. 1). The hollow spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol mainly consist of spherical particles with about 190, 210, and 225 nm of diameter. Thus, the results indicate that the morphology of the hollow titania–nickel composite spheres is slightly different from each other. Moreover, Fig. 1c demonstrates their hollow feature indicated by the arrows, indicating that the spherical particles possess hollow structure. Fig. 2 shows the time course of the hydrogen evolution from aqueous NaBH4/NH3BH3 solution in the presence of hollow titania– nickel composite spheres. From the results, the reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The evolution of 49, 45, and 43 mL hydrogen from aqueous NaBH4 solution were finished in 25, 29, and 34 min in the presence of hollow titania–nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol, respectively. In the previous study [48], negligible amount of hydrogen was generated from aqueous NH3BH3 solution in the presence of hollow titania–nickel composite spheres. On the other hand, certain amount of hydrogen generated from aqueous NH3BH3 solution following hydrogen generation from aqueous NaBH4 solution. However, hydrogen evolution in the presence of the hollow spheres is not completed. Moreover, the amount of hydrogen evolution from aqueous NaBH4/NH3BH3 solution in the hollow spheres is much higher than those from aqueous NH3BH3 solution following hydrogen generation from aqueous NaBH4 solution. In the process, hydrogen is evolved via following the reactions (2) besides reaction (1); 4Ni2þ þ BH4 þ 2H2 O ! 4Ni þ BO2 þ 8Hþ
(1)
NaBH4 þ 2H2 O ! Naþ þ BO2 þ 4H2
(2)
Under the present reaction condition, about 12 mL of hydrogen (4.8 104 mol) is generated via reactions (2) from residual amount of NaBH4 consumed via the reaction (1) (1.3 104 (total amount of NaBH4) – 0.1 104 (amount of NaBH4 consumed via the reaction (1)) = 1.2 104 mol), experimentally. Hydrogen is also evolved from ammonia borane via reaction (3); NH3 BH3 þ 2H2 O ! NH4 þ þ BO2 þ 3H2
(3)
Under the present reaction condition, about 59 mL of hydrogen (24.0 104 mol) is generated via reaction (3), experimentally. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of hollow titania–nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol are 2.0, 1.8, and 1.7, respectively. These results indicate that the catalytic activity of the hollow titania–nickel composite spheres for hydrolysis of NH3BH3 decreases with the increase of Ti + Ni content. In this study, the same amount of PS templates were used for preparation of the hollow spheres with all the Ti + Ni contents, and thus the amount of Ti + Ni on one PS template particle increases with the increase of Ti + Ni content. Consequently, wall
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Fig. 1. TEM images of as-prepared hollow titania–nickel composite spheres prepared with Ti + Ni content = (a) 1.06, (b) 3.49, and (c) 5.82 103 mol.
thickness of the hollow spheres can be controlled, depending to the Ti + Ni content. The as-prepared hollow titania–nickel composite spheres were characterized by using FTIR spectroscopic methods to confirm that the difference in the catalytic activity depends on the amount of residual PS templates. Fig. 3 shows FTIR spectra for hollow titania– nickel composite spheres. These hollow spheres display the characteristic bands of PS at around 1600, 1492, 1449, 754, and 698 cm1 [45]. These bands indicate that the PS segments are included in the shell of hollow spheres. However, the intensities of all the bands depend on the samples. The figure shows that the intensity at 698 cm1 in the spectrum of hollow titania–nickel composite spheres increases with the decrease of Ti + Ni content.
Fig. 2. Hydrogen generation from the mixture of sodium borohydride and ammonia borane in the presence of hollow titania–nickel composite spheres prepared with Ti + Ni content = (a) 1.06, (b) 3.49, and (c) 5.82 103 mol. Ni/NH3BH3 = 0.05 (under ambient atomosphere).
By elemental analysis, the carbon content of the hollow titania– nickel composite spheres with Ti + Ni content = 1.06, 3.49, and 5.82 103 mol are 58.0, 23.0, and 23.8 wt.%, respectively. These results indicate that the amount of PS residue of the hollow titania–nickel composite spheres with Ti + Ni content = 1.06 103 mol is the highest in all the hollow titania– nickel composite spheres. In order to reduce the amount of residual PS templates in the hollow titania–nickel composite spheres with Ti + Ni content = 1.06 103 mol, we investigate the effect of the various conditions for procedures during or after preparation. The carbon content in the hollow titania–nickel composite spheres prepared with the aging time = 0.5, 6, 17, and 24 mL are 55.0, 35.0, 23.8, and 17.3 wt.%, respectively. All the hollow titania–nickel composite spheres before hydrolysis of NH3BH3 consist of spherical particles of ca. 200–300 nm in diameter (Fig. 4), indicating that the
Fig. 3. FTIR spectra of hollow titania–nickel composite spheres prepared with Ti + Ni content = (a) 1.06, (b) 3.49, and (c) 5.82 103 mol.
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Fig. 4. TEM images of as-prepared hollow titania–nickel composite spheres prepared with the aging time = (a) 0.5, (b) 6, (c) 17, and (d) 24 h.
morphology of the hollow titania–nickel composite spheres is not significantly different from each other. Fig. 5 shows the catalytic activity over the hollow titania–nickel composite spheres prepared with the aging time = 0.5, 6, 17, and 24 h. The reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The reaction was completed with hydrogen evolution of 44, 50, 57, and 62 mL in 34, 30, 30, and 22 min in the presence of as-prepared hollow titania–nickel composite spheres prepared with the aging time = 0.5, 6, 17, and 24 h, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of hollow titania–nickel composite spheres with the aging time = 0.5, 6, 17, and 24 h are 1.8, 2.1, 2.4, and 2.7, respectively. The results indicate that the catalytic activity of hollow titania–nickel composite spheres increases with decrease of the amount of residual PS templates. The results also suggested that the amount of active sites of catalyst decreases with
increase of the amount of residual PS templates, or that some active nickel sites are covered with residual PS templates. The carbon content in the hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL are 89.5, 59.7, 45.2, and 35.0 wt.%, respectively. From the analysis of TEM images, all the samples prepared with the amount of aqueous ammonia consist of spherical particles with ca. 200–300 nm diameter. These results indicate that the amount of aqueous ammonia solution affected the amount of residual PS templates without the change of morphology. Fig. 6 shows the catalytic activity over the hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL. The reaction rate and the amount of hydrogen evolution significantly depend on the catalysts. The reaction was completed with hydrogen evolution of 18, 38, 42, and
Fig. 5. Hydrogen generation from the mixture of sodium borohydride and ammonia borane in the presence of hollow titania–nickel composite spheres prepared with the aging time = (a) 0.5, (b) 6, (c) 17, and (d) 24 h. Ni/NH3BH3 = 0.05 (under ambient atomosphere).
Fig. 6. Hydrogen generation from the mixture of sodium borohydride and ammonia borane in the presence of hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = (a) 0, (b) 9, (c) 18, and (d) 36 mL. Ni/ NH3BH3 = 0.05 (under ambient atomosphere).
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50 mL in 139, 131, 64, and 30 min in the presence of as-prepared hollow titania–nickel composite spheres prepared with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of hollow titania– nickel composite spheres with the amount of aqueous ammonia solution = 0, 9, 18, and 36 mL are 0.5, 1.5, 1.7, and 2.1, respectively. The results indicate that the amount of aqueous ammonia solution also affects the amount of residual PS templates and the catalytic activity of hollow titania–nickel composite spheres. 4. Conclusion In this paper, we report the influence of preparation conditions of hollow titania–nickel composite spheres on their morphology and catalytic activity for hydrolytic dehydrogenation of NH3BH3. In the preparation method, titania–nickel composite shells are coated on PS particles by the sol–gel method and the PS templates are dissolved subsequently, or even synchronously, in the same medium to form hollow spheres. All the as-prepared titania– nickel composite spheres have the similar morphology identified by TEM measurement. The catalytic activities of hollow titania– nickel composite spheres prepared with various conditions for hydrolytic dehydrogenation of aqueous NaBH4/NH3BH3 solution were compared. The catalytic activity of the hollow titania–nickel composite spheres increases with decrease of Ti + Ni content. From the results of FTIR spectra, a certain amount of residual PS templates exists in hollow titania–nickel composite spheres which show highest activity for hydrolytic dehydrogenation of NH3BH3, and the amount of the residual PS templates was able to be reduced by increasing aging time during the preparation step. The catalytic activity of hollow titania–nickel composite spheres increases with decrease of the amount of residual PS templates. Acknowledgement The authors would like to thank Ms. M. Takagi for the measurements of elemental analyzer. References [1] Basic research needs for the hydrogen economy, report of the basic energy sciences workshop on hydrogen production, storage and use, May 13–15, 2003, Office of Science, U.S. Department of Energy, http://www.sc.doe.gov/bes/ hydrogen.pdf. [2] J. Turner, G. Sverdrup, M.K. Mann, P.G. Maness, B. Kroposki, M. Ghirardi, R.J. Evans, D. Blake, Int. J. Energy Res. 32 (5) (2008) 379–407. [3] A.W.C.V. Berg, C.O. Area´n, Chem. Commun. 27 (2008) 668–681. [4] Basic research needs catalysis for energy, report from the U.S. Department of Energy, basic energy sciences workshop report, August 6–8, 2007, http:// www.sc.doe.gov/bes/reports/list.html.
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