Fabrication of hollow metal oxide–nickel composite spheres and their catalytic activity for hydrolytic dehydrogenation of ammonia borane

In this paper, we report an effective approach to the fabrication of hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres. In this approach, metal oxide–nickel composite shells were coated on polystyrene particles by the sol–gel method and the polystyrene templates were dissolved subsequently, or 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 hollow metal oxide–nickel composite spheres were characterized by transmission electron microscopy. The catalytic activities of hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres for hydrolytic dehydrogenation of aqueous NaBH4/NH3BH3 solution were compared. The evolutions of 64, 58, and 18 mL hydrogen were finished in about 49, 69, and 162 min in the presence of the hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres from aqueous NaBH4/NH3BH3 solution, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH3BH3 both in the presence of hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres are 2.8, 2.4, and 0.1 (the theoretical value of 3.0), respectively, indicating that the hollow titania–nickel composite spheres and hollow zirconia–nickel composite spheres show much higher hydrogen evolution rates and the amount of hydrogen evolution via hydrolytic dehydrogenation of ammonia borane than the hollow silica–nickel composite spheres. From the results of ATR-IR spectra, a certain amount of residual PS templates exists in hollow silica–nickel composite spheres, and the amount of the residual PS templates were able to be reduced by increasing the amount of aqueous ammonia solution used for the preparation. The catalytic activity of hollow silica–nickel composite spheres increases when the amount of residual PS templates decreases.