Influence of Morphology of Silica-Alumina Composites on Their Activity for Hydrolytic Dehydrogenation of Ammonia Borane

Abstract

In this work, we investigate the influence of the morphology of silica-alumina composites on their activity for hydrolytic dehydrogenation of ammonia borane. Three type of composites, hollow spheres, fine particles, and spherical particles are prepared by sol-gel method. The hollow spheres were prepared by using polystyrene particles as templates. The morphology of composites were observed by transmission electron microscopy. The activity of each type of composite for hydrolytic dehydrogenation of ammonia borane was compared. In the presence of the hollow spheres, the fine particles, and the spherical particles, 10, 2.5, and 1.5 mL of hydrogen was released with the completion times of the reaction being 12, 2, and 1 min, respectively. The amount of hydrogen evolution from the hollow spheres was much higher as compared to those from the fine particles and the spherical particles. Temperature-programmed desorption of ammonia suggested that the hollow spheres possess both weak and strong Brønsted acid sites, while the fine particles and the spherical particles possess only the weak Brønsted acid sites. These results indicate that the morphology of the silica-alumina composites influences their acidic properties, and the strong Brønsted acid sites are more effective for hydrolytic dehydrogenation of ammonia borane, as compared to the weak Brønsted acid sites.