Influence of preparation conditions on morphology of in-situ synthesized hollow ruthenium-silica composite spheres for hydrolytic dehydrogenation of ammonia borane

Abstract

Hollow ruthenium-silica composite spheres were synthesized from spherical ruthenium-silica composite particles prepared by sol-gel method, followed by in-situ activation in an aqueous sodium borohydride (NaBH4)/ammonia borane (NH3BH3) solutions. Through the preparation of the spherical particles, we investigated influence of promotors (L(+)-arginine and ammonia) on the sol-gel reaction in terms of the morphology of the spherical particle precursors and the hollow spheres. Average particle size of the precursors drastically increased by increasing the amount of L(+)-arginine used, though this also increased the solution pH. Average particle size of the precursors did not significantly increase when concentration of ammonia increased. These results indicate that L(+)-arginine promotes particle growth more effectively than ammonia. The spherical particles prepared with L(+)-arginine shows a higher hydrogen evolution rate and a higher quantity of evolved hydrogen from the aqueous NaBH4/NH3BH3 solution than the spherical particles prepared with ammonia. The spherical particles resulting from in-situ synthesis with sizes ranging from 100 to 950 nm possess hollow voids. UV-Vis spectra of the in-situ synthesized samples indicated that the activity depends on the reducibility of the active ruthenium species. The ruthenium species included in the sample prepared using L(+)-arginine was more metallic than that included in the sample prepared using ammonia.