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Al molar ratio of hollow silica–alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 6 1 5 1 e6 1 5 7
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Influence of Si/Al molar ratio of hollow silicaealumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane Naoki Toyama, Tetsuo Umegaki*, Yoshiyuki Kojima Department of Materials & Applied Chemistry, College of Science & Technology, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308, Japan
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abstract
Article history:
In this work, we investigated influence of Si/Al molar ratio of hollow silicaealumina
Received 2 February 2015
composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane.
Received in revised form
The hollow spheres were prepared by polystyrene templates method. In this method, silica
6 March 2015
ealumina composite shell were coated on polystyrene particles via solegel reaction, and
Accepted 8 March 2015
the PS template particles were removed by calcination. From the result of transmission
Available online 3 April 2015
electron microscopy, the obtained hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were almost similar morphologies. The activities of these hollow spheres for the
Keywords:
hydrolytic dehydrogenation of ammonia borane were compared. The amount of hydrogen
Ammonia borane
evolution of hollow spheres with a Si/Al molar ratio of 49 was the highest of all the hollow
Hydrolytic dehydrogenation
spheres. From the result of solid-state
Si/Al molar ratio
amount of aluminum species play important roles in determining the amount of hydrogen
Hollow silicaealumina composite
evolution. Additionally, the amount of hydrogen evolution depends on the amount of
spheres
Brønsted acid sites of the hollow spheres.
Brønsted acid sites
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
27
Al MAS NMR and NH3-TPD, the dispersion and
reserved.
Introduction Hydrogen has attracted an increasing interest as a clean energy [1,2]. Developing efficient methods for hydrogen storage is a key requirement for the development of a hydrogen economy [3,4]. Ammonia borane (NH3BH3) is an attractive candidate for application to chemical hydrogen storage, because it has a high hydrogen density (19.6 wt%), a low molecular weight (30.9 g mol1), and nontoxic [5e10]. Additionally, the release of hydrogen from NH3BH3 can be obtained through thermal dehydrogenation in the solid state [11,12],
hydrolysis [13e16] or methanolysis in solution [17e19]. In each of these methods, NH3BH3 releases hydrogen via the hydrolysis reaction in the presence of suitable acids or catalysts under mild conditions (Eq. (1)) [20e29].
NH3BH3 þ 2H2O / NHþ 4 þ BO2 þ 3H2
(1)
Among the acids used for this reaction, it has been reported that solid acids such as H-type zeolites (Beta and Mordenite) had high activities [20]. However, only a few reports have investigated activities of these solid acids for hydrogen via the
* Corresponding author. Tel.: þ81 3 3259 0810; fax: þ81 3 3293 7572. E-mail address: [email protected] (T. Umegaki). http://dx.doi.org/10.1016/j.ijhydene.2015.03.021 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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hydrolysis of NH3BH3, and there is little information about such important factors as the structures and elemental compositions of solid acids. To clarify relationship between the activities and structure of the solid acids, our groups has focused on hollow spheres. Hollow spheres with a well-defined size have attracted interest because of their low density, large surface area, and good insulation [30e32]. They are used in a wide variety of applications such as catalysis, lithium-ion batteries, biomedical applications, and gas sensors [33e35]. One methods of preparing the hollow spheres is the template based route, which can be classified into two possible categories [36]: the soft template method [37e39] and the hard template method [40e42]. The hard template method has many advantageous properties; for example, it can be used to prepare monodisperse particles and easily control of the shell and void structures. Recently, microsized, monodisperse, hollow silica [43,44] and titania [45] spheres have been fabricated using polystyrene (PS) particles as templates. In this method, microsized, monodisperse, and positively charged PS particles were prepared through dispersion polymerization using the cationic monomer 2-(methacryloxy)-ethyltrimethylammonium chloride (MTC) as the co-monomer [46], or through an emulsifierfree emulsion polymerization using a,a00 -azodiisobutyramidine dihydrochloride (AIBA) as the initiator and poly vinyl pyrrolidone (PVP) as the stabilizer [47,48]. In previous works, we reported that homogeneous hollow silicaealumina composite spheres were successfully prepared by PS template method [26,28,29]. It has been confirmed that the acid strength and amount of acid sites of the silicaealumina catalyst can be changed by adjusting the Si/Al molar ratio [49]. Si/Al molar ratios expect to play important roles in determining the activity of the hollow spheres for the hydrolytic dehydrogenation of NH3BH3. In this work, we investigate the effect of the Si/Al molar ratio of hollow silicaealumina composite spheres on their activities for the hydrolytic dehydrogenation of NH3BH3.
Experiment Preparation of hollow spheres The hollow silicaealumina composite spheres were fabricated by the PS template method as described previously [29]. The monodisperse PS particles were prepared by an emulsifierfree emulsion polymerization as follows: 9.0 mL of styrene (Kanto Chem. Co.), 1.5 g of PVP (K30 grade MW z 40000, Fluka), 0.26 g of cationic initiator AIBA (Kanto Chem. Co.), and 100 mL of deionized water were added into a 250 mL three-neck round-bottomed flask equipped with a mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, and a Graham condenser, and placed in an oil bath. The reaction solution was deoxygenated by bubbling nitrogen gas through the solution at room temperature for 1 h. The reaction was then, stirred at a rate of 250 rpm, and heated to 343 K for 24 h. The obtained PS suspension was centrifuged at 6000 rpm for 5 min and washed three times with ethanol; the content of the PS suspension was tailored through the addition of ethanol. A
0.0057 g of aluminum isopropoxide (Kanto Chem. Co., >99.0%), 3 mL of aqueous ammonia solution (28 wt%, Kanto Chem. Co.,), and 40 mL of ethanol were added to 15 g of the PS suspension, after which 155.1 mL of tetraethoxysilane (TEOS, Kanto Chem. Co., >99.9%) was added. The solegel reaction was carried out at 323 K for 1.5 h. After drying in a desiccator overnight, the hollow silicaealumina composite spheres were obtained by calcination in air at 873 K for 0 h at a heating rate of 0.5 K min1. The white powders obtained in this fashion were used without further treatment.
Characterization The chemical compositions of the hollow spheres (Si and Al) were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo ICAP 6500) using Ar to create the plasma. The specific surface areas, average pore sizes, and pore volumes of the hollow spheres were measured by N2 adsorption at 77 K with an ASAP 2010 automatic physical adsorption instrument. The crystalline phases and average crystallite sizes of the hollow spheres were determined by Xray powder diffraction using a Rigaku MultiFlex X-ray diffractometer. The morphologies of the hollow spheres were observed using a Hitachi FE-2000 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. Solid-state 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded on a JEOL ECA-500 spectrometer (11.75 T). The relaxation delay time was 2 ms, and the hollow spheres were spun at 15 kHz using a 4 mm ZrO2 rotor. Temperature-programmed desorption of ammonia (NH3-TPD) was carried out with a BELCAT-B instrument. The analysis was performed by loading 50 mg of the hollow spheres into a quartz reactor and drying the sample in a pure He flow at 783 K for 1 h followed by purging with pure He at the same temperature for 1 h. After the hollow spheres had cooled to 373 K under the He flow, the NH3 adsorption was carried out by exposing the hollow spheres to an NH3eHe gas mixture (95 vol.% He) at 373 K for 1 h. The hollow spheres were then purged with pure He, allowing the accurate detection of the desorbed NH3. The NH3-TPD measurements were conducted using a pure He flow by heating the sample from 373 to 873 K at a rate of 10 K min1, the desorbed NH3 molecules were detected by a thermal conductivity detector (TCD).
Experimental procedure for the hydrolysis of NH3BH3 A 0.8 g of hollow silicaealumina composite spheres was placed in a two-necked round-bottomed flask in air at room temperature, one neck was connected to a gas burette, and the other to an addition funnel. The reaction was started by stirring the mixture of the hollow spheres, and aqueous NH3BH3 (Aldrich, 90%) solution (0.14 wt%, 3.5 mL) was added from the addition funnel. Gas evolution was monitored using the gas burette.
Results and discussions Hollow silicaealumina composite spheres prepared with Si/Al precursor molar ratios of 12.5, 25, 50, 100, and 200 were
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 6 1 5 1 e6 1 5 7
measured by ICP-AES analysis, and the Si/Al molar ratios of the obtained hollow spheres were found to be about 12, 23, 49, 92, and 186, respectively. Table 1 shows physiochemical properties of the obtained hollow spheres. Specific surface area, average pore size, and pore volume were measured by N2 adsorptionedesorption using the Brunauer-Emmett-Teller and Barrett-Joyner-Halenda methods. From the results, specific surface areas, average pore sizes, and pore volumes of the hollow spheres with Si/Al molar ratio of 12, 23, 49, 92, and 186 were as follows: specific surface areas: 388, 393, 365, 404, and 379 m2 g1, average pore size: 4.3, 5.0, 4.8, 4.7, and 4.8 nm, and pore volumes: 0.42, 0.50, 0.46, 0.48, and 0.45 cm3 g1, respectively. The results indicate that the specific surface area, average pore size, and pore volume of all the hollow spheres were approximately the same levels. The morphology of the composites was examined using a TEM measurement. Fig. 1 shows the TEM images of the composites with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. From the result, homogeneous hollow spheres were obtained at all the Si/Al molar ratios. The shell thickness of all the obtained hollow spheres were about 5e7 nm, and the diameters of the hollow spheres were about 210e230 nm. The result indicates that all the hollow spheres were similar morphologies. The crystalline structure of the composites was investigated by powder X-ray diffraction measurements. Fig. 2 shows the XRD patterns of the obtained hollow spheres with various Si/Al molar ratios. The XRD pattern of the hollow spheres shows the characteristic peaks of silica in an amorphous phases between 2q ¼ 20 and 40 [50], indicating that the hollow spheres consist of amorphous silica phases. The activity for the hydrolytic dehydrogenation of NH3BH3 was determined in the presence of the obtained hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. Fig. 3 shows time course of hydrogen generation from aqueous NH3BH3 solution in the presence of hollow silicaealumina composite spheres with various Si/Al molar ratios. From the result, the evolution of 7.0, 10.0, 10.2, 5.0, and 4.0 mL hydrogen was occurred in 10, 12, 12, 6, and 4 min in the presence of hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 was 1.7, 2.6, 2.7, 1.3, and 1.0 in the presence of the hollow spheres with Si/ Al molar ratio of 12, 23, 49, 92, and 186, respectively. From the results, the amount of hydrogen evolution of the hollow spheres with Si/Al molar ratio of 49 shows the highest of all the hollow spheres. Additionally, hydrogen evolution rate
Table 1 e Physicochemical properties of hollow silicaealumina composite spheres with various Si/Al molar ratio. Si/Al molar Specific surface Average pore Pore volume ratio [-] area [m2 g1] size [nm] [cm3 g1] 12 23 49 92 189
388 393 365 404 379
4.3 5 4.8 4.7 4.8
0.42 0.50 0.46 0.48 0.45
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calculated with the data up to 50% of the amount of hydrogen at reaction completion. The hydrogen evolution rate in the presence of the hollow spheres with Si/Al molar ratio of 12, 23, 49, 92, and 186 was 2.3, 1.7, 2.2, 2.0, and 2.0 mL min1, respectively. From the results, the hydrogen evolution rate of all the hollow spheres was almost the same level. These results indicate that Si/Al molar ratio of the hollow spheres influences on the amount of hydrogen evolution. However, all the hollow spheres was incomplete hydrogen evolution because acidic Hþ probably exchanged into NHþ 4 ion on the acid sites of the hollow spheres during hydrolytic dehydrogenation of NH3BH3. The activation process of NH3BH3 occurs when the acidic protons from the solid acids in the aqueous medium, which have a higher activity than boric acid, promote the dissociation of the BeN bond and the hydrolysis of BH3 species. This produces borate ions, along with the release of hydrogen, after which the borate ions react with Hþ to produce boric acid [20]. These results show that the effective acid sites for the hydrolytic dehydrogenation of NH3BH3 are likely Brønsted acid sites. To confirm the coordination number of the obtained hollow spheres, hollow silicaealumina composite spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were examined using solid-state 27Al MAS NMR. Fig. 4 shows the solid-state 27 Al MAS NMR profiles of hollow silicaealumina composite spheres with various Si/Al molar ratios. It has been reported that 4-coordinated aluminum species were ascribed to Brønsted acid sites, while 5- and 6-coordinated aluminum species were ascribed to Lewis acid sites [51e53]. From the results, all the hollow spheres and particles have Brønsted and Lewis acid sites. We calculated the peak areas of the 4-, 5-, and 6-coordinated aluminum species of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 with a Gaussian function. The ratios of the peak areas of 4-coordinated aluminum species to the total peak areas of 4-, 5-, and 6coordinated aluminum species (I4/Iall) of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were 0.21, 0.23, 0.24, 0.26, and 0.26, respectively. From these results, the ratio of Brønsted acid sites increases with increase of the Si/Al molar ratio of the hollow spheres. It has been reported that the agglomerated aluminum species increase with increase of amount of aluminum species [54]. It is concluded that the dispersion of aluminum species increases with increase of Si/ Al molar ratio of the hollow spheres. The amount of Brønsted acid sites of hollow silicaealumina composite spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 was measured using NH3-TPD. Fig. 5 shows NH3-TPD profile of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. It has been reported that the NH3-TPD profile shows a peak around 400e600 K, indicating the presence of Brønsted acid sites, and a peak over 650 K, indicating the Lewis acid sites [51e53]. On the other hands, NH3-TPD profile of all the hollow spheres was observed first peak at around 400e523 K and the broad peak at around 523e600 K. This temperature range was ascribed to the presence of Brønsted acid sites. The amount of the Brønsted acid sites calculated from the peak area in the temperature range of 400e600 K of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were 0.15, 0.18, 0.19, 0.08 and
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Fig. 1 e TEM images of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
Fig. 2 e Powder XRD pattern of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
Fig. 3 e Hydrogen evolution from aqueous NH3BH3 solution (0.14 wt%, 3.5 mL) in the presence of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
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Fig. 4 e Solid-state 27Al MAS NMR spectra of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
0.06 mmol g1, respectively. From the result, the amount of Brønsted acid sites of the hollow spheres with Si/Al molar ratio of 49 was the highest of all the hollow spheres. The results of Figs. 4 and 5 indicate that both the amount of aluminum species and their dispersion play the important roles in determining the amount of hydrogen evolution of the hydrolytic dehydrogenation of NH3BH3.
Fig. 5 e NH3-TPD profile of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
Fig. 6 shows the H2/NH3BH3 molar ratios of hydrogen generated from the aqueous NH3BH3 solution versus the amount of Brønsted acid sites of the hollow silicaealumina composite spheres with various Si/Al molar ratios. From the result, the amount of hydrogen evolution increases with increase of the amount of Brønsted acid sites of the hollow spheres. On the other hands, the hydrogen evolution rate was almost same level. These results indicate that the hydrogen evolution does not depend on the amount of Brønsted acid sites.
Fig. 6 e The H2/NH3BH3 molar ratios of hydrogen generated from aqueous NH3BH3 solution (0.14 wt%, 3.5 mL) versus amount of Brønsted acid sites of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
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Conclusion In this work, we successfully prepared hollow silicaealumina composite spheres with various Si/Al molar ratios. In this approach, the silicaealumina composites shell was coated on PS template particles, and the hollow spheres were prepared by calcination. The result of ICP-AES analysis shows that the elemental component ratios (Si/Al) of the obtained hollow spheres were about 12, 23, 49, 92, and 186, respectively. The result of TEM shows that all the obtained hollow spheres were almost similar morphologies. Activities for the hydrolytic dehydrogenation of NH3BH3 in the presence of the hollow spheres with various Si/Al molar ratios were compared. The amount of hydrogen evolution of hollow spheres with Si/Al molar ratio of 49 was the highest of all the hollow spheres. The result of solid-state 27Al MAS NMR and NH3-TPD experiments show that the dispersion and amount of aluminum species play important roles in determining the amount of hydrogen evolution, which also depends on the amount of Brønsted acid sites of the hollow spheres.
Acknowledgment This work was supported by NIMS microstructural characterization platform (NMCP) as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We are grateful to Mr. Ohki, Mr. Deguchi, and Ms. Wada for using solid state NMR measurement. Additionally, we would like to thank Dr. Yoneda for using NH3-TPD measurement.
references
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Influence of Si/Al molar ratio of hollow silicaealumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane Naoki Toyama, Tetsuo Umegaki*, Yoshiyuki Kojima Department of Materials & Applied Chemistry, College of Science & Technology, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308, Japan
article info
abstract
Article history:
In this work, we investigated influence of Si/Al molar ratio of hollow silicaealumina
Received 2 February 2015
composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane.
Received in revised form
The hollow spheres were prepared by polystyrene templates method. In this method, silica
6 March 2015
ealumina composite shell were coated on polystyrene particles via solegel reaction, and
Accepted 8 March 2015
the PS template particles were removed by calcination. From the result of transmission
Available online 3 April 2015
electron microscopy, the obtained hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were almost similar morphologies. The activities of these hollow spheres for the
Keywords:
hydrolytic dehydrogenation of ammonia borane were compared. The amount of hydrogen
Ammonia borane
evolution of hollow spheres with a Si/Al molar ratio of 49 was the highest of all the hollow
Hydrolytic dehydrogenation
spheres. From the result of solid-state
Si/Al molar ratio
amount of aluminum species play important roles in determining the amount of hydrogen
Hollow silicaealumina composite
evolution. Additionally, the amount of hydrogen evolution depends on the amount of
spheres
Brønsted acid sites of the hollow spheres.
Brønsted acid sites
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
27
Al MAS NMR and NH3-TPD, the dispersion and
reserved.
Introduction Hydrogen has attracted an increasing interest as a clean energy [1,2]. Developing efficient methods for hydrogen storage is a key requirement for the development of a hydrogen economy [3,4]. Ammonia borane (NH3BH3) is an attractive candidate for application to chemical hydrogen storage, because it has a high hydrogen density (19.6 wt%), a low molecular weight (30.9 g mol1), and nontoxic [5e10]. Additionally, the release of hydrogen from NH3BH3 can be obtained through thermal dehydrogenation in the solid state [11,12],
hydrolysis [13e16] or methanolysis in solution [17e19]. In each of these methods, NH3BH3 releases hydrogen via the hydrolysis reaction in the presence of suitable acids or catalysts under mild conditions (Eq. (1)) [20e29].
NH3BH3 þ 2H2O / NHþ 4 þ BO2 þ 3H2
(1)
Among the acids used for this reaction, it has been reported that solid acids such as H-type zeolites (Beta and Mordenite) had high activities [20]. However, only a few reports have investigated activities of these solid acids for hydrogen via the
* Corresponding author. Tel.: þ81 3 3259 0810; fax: þ81 3 3293 7572. E-mail address: [email protected] (T. Umegaki). http://dx.doi.org/10.1016/j.ijhydene.2015.03.021 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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hydrolysis of NH3BH3, and there is little information about such important factors as the structures and elemental compositions of solid acids. To clarify relationship between the activities and structure of the solid acids, our groups has focused on hollow spheres. Hollow spheres with a well-defined size have attracted interest because of their low density, large surface area, and good insulation [30e32]. They are used in a wide variety of applications such as catalysis, lithium-ion batteries, biomedical applications, and gas sensors [33e35]. One methods of preparing the hollow spheres is the template based route, which can be classified into two possible categories [36]: the soft template method [37e39] and the hard template method [40e42]. The hard template method has many advantageous properties; for example, it can be used to prepare monodisperse particles and easily control of the shell and void structures. Recently, microsized, monodisperse, hollow silica [43,44] and titania [45] spheres have been fabricated using polystyrene (PS) particles as templates. In this method, microsized, monodisperse, and positively charged PS particles were prepared through dispersion polymerization using the cationic monomer 2-(methacryloxy)-ethyltrimethylammonium chloride (MTC) as the co-monomer [46], or through an emulsifierfree emulsion polymerization using a,a00 -azodiisobutyramidine dihydrochloride (AIBA) as the initiator and poly vinyl pyrrolidone (PVP) as the stabilizer [47,48]. In previous works, we reported that homogeneous hollow silicaealumina composite spheres were successfully prepared by PS template method [26,28,29]. It has been confirmed that the acid strength and amount of acid sites of the silicaealumina catalyst can be changed by adjusting the Si/Al molar ratio [49]. Si/Al molar ratios expect to play important roles in determining the activity of the hollow spheres for the hydrolytic dehydrogenation of NH3BH3. In this work, we investigate the effect of the Si/Al molar ratio of hollow silicaealumina composite spheres on their activities for the hydrolytic dehydrogenation of NH3BH3.
Experiment Preparation of hollow spheres The hollow silicaealumina composite spheres were fabricated by the PS template method as described previously [29]. The monodisperse PS particles were prepared by an emulsifierfree emulsion polymerization as follows: 9.0 mL of styrene (Kanto Chem. Co.), 1.5 g of PVP (K30 grade MW z 40000, Fluka), 0.26 g of cationic initiator AIBA (Kanto Chem. Co.), and 100 mL of deionized water were added into a 250 mL three-neck round-bottomed flask equipped with a mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, and a Graham condenser, and placed in an oil bath. The reaction solution was deoxygenated by bubbling nitrogen gas through the solution at room temperature for 1 h. The reaction was then, stirred at a rate of 250 rpm, and heated to 343 K for 24 h. The obtained PS suspension was centrifuged at 6000 rpm for 5 min and washed three times with ethanol; the content of the PS suspension was tailored through the addition of ethanol. A
0.0057 g of aluminum isopropoxide (Kanto Chem. Co., >99.0%), 3 mL of aqueous ammonia solution (28 wt%, Kanto Chem. Co.,), and 40 mL of ethanol were added to 15 g of the PS suspension, after which 155.1 mL of tetraethoxysilane (TEOS, Kanto Chem. Co., >99.9%) was added. The solegel reaction was carried out at 323 K for 1.5 h. After drying in a desiccator overnight, the hollow silicaealumina composite spheres were obtained by calcination in air at 873 K for 0 h at a heating rate of 0.5 K min1. The white powders obtained in this fashion were used without further treatment.
Characterization The chemical compositions of the hollow spheres (Si and Al) were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo ICAP 6500) using Ar to create the plasma. The specific surface areas, average pore sizes, and pore volumes of the hollow spheres were measured by N2 adsorption at 77 K with an ASAP 2010 automatic physical adsorption instrument. The crystalline phases and average crystallite sizes of the hollow spheres were determined by Xray powder diffraction using a Rigaku MultiFlex X-ray diffractometer. The morphologies of the hollow spheres were observed using a Hitachi FE-2000 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. Solid-state 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded on a JEOL ECA-500 spectrometer (11.75 T). The relaxation delay time was 2 ms, and the hollow spheres were spun at 15 kHz using a 4 mm ZrO2 rotor. Temperature-programmed desorption of ammonia (NH3-TPD) was carried out with a BELCAT-B instrument. The analysis was performed by loading 50 mg of the hollow spheres into a quartz reactor and drying the sample in a pure He flow at 783 K for 1 h followed by purging with pure He at the same temperature for 1 h. After the hollow spheres had cooled to 373 K under the He flow, the NH3 adsorption was carried out by exposing the hollow spheres to an NH3eHe gas mixture (95 vol.% He) at 373 K for 1 h. The hollow spheres were then purged with pure He, allowing the accurate detection of the desorbed NH3. The NH3-TPD measurements were conducted using a pure He flow by heating the sample from 373 to 873 K at a rate of 10 K min1, the desorbed NH3 molecules were detected by a thermal conductivity detector (TCD).
Experimental procedure for the hydrolysis of NH3BH3 A 0.8 g of hollow silicaealumina composite spheres was placed in a two-necked round-bottomed flask in air at room temperature, one neck was connected to a gas burette, and the other to an addition funnel. The reaction was started by stirring the mixture of the hollow spheres, and aqueous NH3BH3 (Aldrich, 90%) solution (0.14 wt%, 3.5 mL) was added from the addition funnel. Gas evolution was monitored using the gas burette.
Results and discussions Hollow silicaealumina composite spheres prepared with Si/Al precursor molar ratios of 12.5, 25, 50, 100, and 200 were
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measured by ICP-AES analysis, and the Si/Al molar ratios of the obtained hollow spheres were found to be about 12, 23, 49, 92, and 186, respectively. Table 1 shows physiochemical properties of the obtained hollow spheres. Specific surface area, average pore size, and pore volume were measured by N2 adsorptionedesorption using the Brunauer-Emmett-Teller and Barrett-Joyner-Halenda methods. From the results, specific surface areas, average pore sizes, and pore volumes of the hollow spheres with Si/Al molar ratio of 12, 23, 49, 92, and 186 were as follows: specific surface areas: 388, 393, 365, 404, and 379 m2 g1, average pore size: 4.3, 5.0, 4.8, 4.7, and 4.8 nm, and pore volumes: 0.42, 0.50, 0.46, 0.48, and 0.45 cm3 g1, respectively. The results indicate that the specific surface area, average pore size, and pore volume of all the hollow spheres were approximately the same levels. The morphology of the composites was examined using a TEM measurement. Fig. 1 shows the TEM images of the composites with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. From the result, homogeneous hollow spheres were obtained at all the Si/Al molar ratios. The shell thickness of all the obtained hollow spheres were about 5e7 nm, and the diameters of the hollow spheres were about 210e230 nm. The result indicates that all the hollow spheres were similar morphologies. The crystalline structure of the composites was investigated by powder X-ray diffraction measurements. Fig. 2 shows the XRD patterns of the obtained hollow spheres with various Si/Al molar ratios. The XRD pattern of the hollow spheres shows the characteristic peaks of silica in an amorphous phases between 2q ¼ 20 and 40 [50], indicating that the hollow spheres consist of amorphous silica phases. The activity for the hydrolytic dehydrogenation of NH3BH3 was determined in the presence of the obtained hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. Fig. 3 shows time course of hydrogen generation from aqueous NH3BH3 solution in the presence of hollow silicaealumina composite spheres with various Si/Al molar ratios. From the result, the evolution of 7.0, 10.0, 10.2, 5.0, and 4.0 mL hydrogen was occurred in 10, 12, 12, 6, and 4 min in the presence of hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 was 1.7, 2.6, 2.7, 1.3, and 1.0 in the presence of the hollow spheres with Si/ Al molar ratio of 12, 23, 49, 92, and 186, respectively. From the results, the amount of hydrogen evolution of the hollow spheres with Si/Al molar ratio of 49 shows the highest of all the hollow spheres. Additionally, hydrogen evolution rate
Table 1 e Physicochemical properties of hollow silicaealumina composite spheres with various Si/Al molar ratio. Si/Al molar Specific surface Average pore Pore volume ratio [-] area [m2 g1] size [nm] [cm3 g1] 12 23 49 92 189
388 393 365 404 379
4.3 5 4.8 4.7 4.8
0.42 0.50 0.46 0.48 0.45
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calculated with the data up to 50% of the amount of hydrogen at reaction completion. The hydrogen evolution rate in the presence of the hollow spheres with Si/Al molar ratio of 12, 23, 49, 92, and 186 was 2.3, 1.7, 2.2, 2.0, and 2.0 mL min1, respectively. From the results, the hydrogen evolution rate of all the hollow spheres was almost the same level. These results indicate that Si/Al molar ratio of the hollow spheres influences on the amount of hydrogen evolution. However, all the hollow spheres was incomplete hydrogen evolution because acidic Hþ probably exchanged into NHþ 4 ion on the acid sites of the hollow spheres during hydrolytic dehydrogenation of NH3BH3. The activation process of NH3BH3 occurs when the acidic protons from the solid acids in the aqueous medium, which have a higher activity than boric acid, promote the dissociation of the BeN bond and the hydrolysis of BH3 species. This produces borate ions, along with the release of hydrogen, after which the borate ions react with Hþ to produce boric acid [20]. These results show that the effective acid sites for the hydrolytic dehydrogenation of NH3BH3 are likely Brønsted acid sites. To confirm the coordination number of the obtained hollow spheres, hollow silicaealumina composite spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were examined using solid-state 27Al MAS NMR. Fig. 4 shows the solid-state 27 Al MAS NMR profiles of hollow silicaealumina composite spheres with various Si/Al molar ratios. It has been reported that 4-coordinated aluminum species were ascribed to Brønsted acid sites, while 5- and 6-coordinated aluminum species were ascribed to Lewis acid sites [51e53]. From the results, all the hollow spheres and particles have Brønsted and Lewis acid sites. We calculated the peak areas of the 4-, 5-, and 6-coordinated aluminum species of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 with a Gaussian function. The ratios of the peak areas of 4-coordinated aluminum species to the total peak areas of 4-, 5-, and 6coordinated aluminum species (I4/Iall) of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were 0.21, 0.23, 0.24, 0.26, and 0.26, respectively. From these results, the ratio of Brønsted acid sites increases with increase of the Si/Al molar ratio of the hollow spheres. It has been reported that the agglomerated aluminum species increase with increase of amount of aluminum species [54]. It is concluded that the dispersion of aluminum species increases with increase of Si/ Al molar ratio of the hollow spheres. The amount of Brønsted acid sites of hollow silicaealumina composite spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 was measured using NH3-TPD. Fig. 5 shows NH3-TPD profile of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186, respectively. It has been reported that the NH3-TPD profile shows a peak around 400e600 K, indicating the presence of Brønsted acid sites, and a peak over 650 K, indicating the Lewis acid sites [51e53]. On the other hands, NH3-TPD profile of all the hollow spheres was observed first peak at around 400e523 K and the broad peak at around 523e600 K. This temperature range was ascribed to the presence of Brønsted acid sites. The amount of the Brønsted acid sites calculated from the peak area in the temperature range of 400e600 K of the hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were 0.15, 0.18, 0.19, 0.08 and
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Fig. 1 e TEM images of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
Fig. 2 e Powder XRD pattern of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
Fig. 3 e Hydrogen evolution from aqueous NH3BH3 solution (0.14 wt%, 3.5 mL) in the presence of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
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Fig. 4 e Solid-state 27Al MAS NMR spectra of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
0.06 mmol g1, respectively. From the result, the amount of Brønsted acid sites of the hollow spheres with Si/Al molar ratio of 49 was the highest of all the hollow spheres. The results of Figs. 4 and 5 indicate that both the amount of aluminum species and their dispersion play the important roles in determining the amount of hydrogen evolution of the hydrolytic dehydrogenation of NH3BH3.
Fig. 5 e NH3-TPD profile of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
Fig. 6 shows the H2/NH3BH3 molar ratios of hydrogen generated from the aqueous NH3BH3 solution versus the amount of Brønsted acid sites of the hollow silicaealumina composite spheres with various Si/Al molar ratios. From the result, the amount of hydrogen evolution increases with increase of the amount of Brønsted acid sites of the hollow spheres. On the other hands, the hydrogen evolution rate was almost same level. These results indicate that the hydrogen evolution does not depend on the amount of Brønsted acid sites.
Fig. 6 e The H2/NH3BH3 molar ratios of hydrogen generated from aqueous NH3BH3 solution (0.14 wt%, 3.5 mL) versus amount of Brønsted acid sites of hollow silicaealumina composite spheres with Si/Al molar ratios of (a):12, (b):23, (c):49, (d):92, and (e):186.
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Conclusion In this work, we successfully prepared hollow silicaealumina composite spheres with various Si/Al molar ratios. In this approach, the silicaealumina composites shell was coated on PS template particles, and the hollow spheres were prepared by calcination. The result of ICP-AES analysis shows that the elemental component ratios (Si/Al) of the obtained hollow spheres were about 12, 23, 49, 92, and 186, respectively. The result of TEM shows that all the obtained hollow spheres were almost similar morphologies. Activities for the hydrolytic dehydrogenation of NH3BH3 in the presence of the hollow spheres with various Si/Al molar ratios were compared. The amount of hydrogen evolution of hollow spheres with Si/Al molar ratio of 49 was the highest of all the hollow spheres. The result of solid-state 27Al MAS NMR and NH3-TPD experiments show that the dispersion and amount of aluminum species play important roles in determining the amount of hydrogen evolution, which also depends on the amount of Brønsted acid sites of the hollow spheres.
Acknowledgment This work was supported by NIMS microstructural characterization platform (NMCP) as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We are grateful to Mr. Ohki, Mr. Deguchi, and Ms. Wada for using solid state NMR measurement. Additionally, we would like to thank Dr. Yoneda for using NH3-TPD measurement.
references
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