Naoki Toyama

Control of Shell Thickness of Hollow Silica-Alumina Composite Spheres and their Activity for Hydrolytic Dehydrogenation of Ammonia Borane

Herein, we investigated the influence of the shell thickness of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane (NH3BH3). Silica-alumina composite shells were fabricated by coating on polystyrene (PS) template particles and then completely removing the PS by calcination. Based on the transmission electron microscopy images, the shell thicknesses of the hollow spheres prepared using 5, 10, and 15 g of PS suspensions and coating times of 24.0, 7.0, and 1.5 h were 20, 13, and 5 nm, respectively. The results indicated that the shell thickness of homogeneous hollow spheres was controlled by adjusting the amount of PS suspension and the coating time. In the presence of the hollow spheres with shell thicknesses of 5, 13, and 20 nm, 10.0, 9.5, and 9.0 mL of hydrogen was evolved from aqueous NH3BH3 solutions for 13, 12, and 13 min, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 in the presence of the hollow spheres with shell thicknesses of 5, 13, and 20 nm were 2.6, 2.5, and 2.3, respectively. From these results, it can be inferred that the activity for the hydrolytic dehydrogenation of NH3BH3 increase with decrease of the shell thickness of the hollow spheres. The result of the temperature-programmed desorption profile of ammonia showed that in the number of amount of acid sites of the hollow spheres increase with decrease of the shell thickness of the hollow spheres. The results indicate the activity depends on the number of acid sites of the hollow spheres.

The influence of the pore structure of hollow silica–alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

The effect of the pore structures of hollow silica–alumina composite spheres on their activity for the hydrolytic dehydrogenation of ammonia borane has been studied. Following a calcination process that resulted in hollow spheres, the spheres’ shells were coated on spherical polystyrene particle templates. The pore structures of the hollow spheres were controlled by adding a surfactant, cetyltrimethylammonium bromide (CTAB); the volume and homogeneity of the mesopores in the shell of the hollow spheres increased with increasing amount of aqueous CTAB solution. The hollow spheres with a large volume and homogeneous mesopores showed high rates of hydrogen evolution from aqueous ammonia borane solution. The highly active hollow spheres prepared with 5 mL of aqueous CTAB solution showed a considerably high turnover frequency (TOF), although the hollow spheres did not have a significantly high BET surface area or a large quantity of acid sites. This suggests that the activity for hydrogen evolution from aqueous ammonia borane solution in the presence of the hollow spheres with homogeneous mesopore structures was unexpectedly high.

Influence of morphology of hollow silica–alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

Hollow silica–alumina composite spheres were prepared by a polystyrene (PS) template method using various amounts of PS suspension. Homogeneous hollow spheres prepared using 40 g were found to be with a diameter of about 300 nm in scanning electron microscopy, and transmission electron microscopy demonstrated their hollow sphere morphology. From the nitrogen adsorption isotherm results, the homogeneous hollow spheres prepared using 40 g of the PS suspension were found to be an ordered pore structure. The activities of the hollow spheres prepared using various amounts of the PS suspension for hydrolytic dehydrogenation of ammonia borane were compared. The results showed that 10, 7, and 6 mL of hydrogen were evolved from the aqueous ammonia borane solution in about 40 min in the presence of the hollow spheres prepared using 40, 80, and 120 g of PS suspension, respectively. The homogeneous hollow spheres with an ordered pore structure showed the highest activity among all the hollow spheres. The amount of acid sites and the coordination number of aluminum active species were characterized using neutralization titration and solid-state 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The homogeneous hollow spheres with an ordered pore structure had high amount of acid sites and 4-coordinated aluminum species. The relative proportion of 4-coordinated aluminum species was related to the dispersion of aluminum species. These results indicate that the homogeneous hollow spheres with an ordered pore structure showed the high activity because of high amount of acid sites induced by the highly dispersed aluminum species.

Fabrication of Hollow Silica-Alumina Composite Spheres Using L(+)-Arginine and their Catalytic Performance for Hydrolytic Dehydrogenation of Ammonia Borane

The present study reports a facile and effective approach for fabrication of hollow silica-alumina composite spheres. In this approach, silica-alumina composite walls were coated on polystyrene template by the sol-gel method using L(+)-arginine as a promoter for the reaction followed by calcination procedure. Using L(+)-arginine as a promoter of coating process, homogeneous hollow silica-alumina composite spheres are obtained and the wall thickness is larger than that of the hollow spheres prepared with ammonia. The hollow spheres shows high activity for hydrolytic dehydrogenation of ammonia borane compared with spherical silica-alumina composite particles without hollow structure, the hollow spheres prepared with ammonia, and conventional H-BEA zeolite. The results indicate that hollow structure plays important role to show high activity.