Abstract
This work studied about the coordination structure and activity hollow silica-alumina composite spheres for acid-promoted hydrogen evolution from aqueous ammonia borane solution. The hollow spheres were fabricated by adjusting temperature and time for coating their shells on spherical polystyrene templates, following removal of the templates through calcination process. We investigated the influence on the coordination state of active aluminum species through the 27Al MAS NMR spectra and on activity for the hydrogen evolution. The NMR spectra indicated that both the ratios of active tetra-coordinated aluminum species and inactive hexa-coordinated aluminum species also increased with increase of the coating temperature, however, improvement of the activity was not observed despite the increase of the active aluminum species. The ratios of active tetra-coordinated aluminum species and inactive hexa-coordinated aluminum species were also controlled by adjusting the coating time, and the hollow spheres with both high ratio of active tetra-coordinated aluminum species and low ratio of inactive hexacoordinated aluminum species showed high activity for the hydrogen evolution. The highly active hollow spheres also included relatively high ratio of penta-coordinated aluminum atoms, suggesting that the active sites for the hydrogen evolution were also included in the penta-coordinated aluminum species.
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