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

• We investigated controlling method of wall thickness of hollow silica–alumina composite spheres.
• The hollow spheres prepared with l(+)-arginine show narrow pore size distribution.
• The hydrogen evolution rate depends on the pore size distribution or the wall thickness.

We investigated the influence that preparation conditions have on the morphology of hollow silica–alumina composite spheres and their ability for hydrolytic dehydrogenation of ammonia borane (NH3BH3). Silica–alumina composite shells were coated on polystyrene (PS) template particles by the sol–gel method using l(+)-arginine as a promoter for the reaction, followed by calcination. The hollow silica–alumina composite spheres were characterized by transmission electron microscopy, N2 adsorption/desorption isotherm analysis, and X-ray diffraction. Compared with the hollow spheres prepared with ammonia, hollow spheres prepared using l(+)-arginine showed higher surface area and narrower pore size distribution. The hollow spheres prepared using l(+)-arginine showed a higher rate of hydrogen evolution from aqueous NH3BH3 solution. To control the wall thickness of the hollow spheres prepared using l(+)-arginine, we adjusted the amount of PS suspension used during coating process. The time required to obtain homogeneously coated hollow spheres depends on the amount of PS suspension. Hollow spheres with the wall thickness of 20, 30, and 50 nm were obtained using 30.0, 15.0, and 7.5 g of PS suspension, respectively. The hydrogen evolution activity in the presence of the hollow spheres prepared using l(+)-arginine with the wall thickness of 20, 30, and 50 nm was higher than those of the hollow spheres prepared using ammonia.

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