Control of pore size in shell of hollow silica–alumina composite spheres for hydrolytic dehydrogenation of ammonia borane

Abstract

In the present study, we attempted to control the shell pore size of hollow spheres and investigated the resulting effect on their acidic properties and activity for the acid-promoted hydrolytic dehydrogenation of ammonia borane. Silica shells were formed on polystyrene template particles via sol–gel method, followed by calcination. For controlling the pore structure, surfactants with various molecular lengths (decyltrimethylammonium bromide (DeTAB), cethyltrimethylammonium bromide (CTAB), and stearyltrimethylammonium bromide (STAB)) were used. The shells of all the hollow spheres included monodispersed and well-ordered pores, and the pore size increased with increase in the molecular length of the surfactant. In particular, the hollow spheres prepared with CTAB and STAB consisted of numerous well-ordered mesopores with sharp peaks centered at approximately 2.4 and 2.8 nm, respectively. The molar ratios of the hydrolytically evolved hydrogen to the initial NH3BH3 in the presence of the hollow spheres prepared with DeTAB, CTAB, and STAB were 0.9, 3.0, and 2.7, respectively, which indicated the stoichiometric amount of hydrogen was evolved in the presence of the hollow spheres prepared with CTAB and STAB. The amount and rate of hydrogen evolution depended on the pore structure, and the hollow spheres with well-ordered and appropriate-sized mesopores in the shells showed a large amount and high rate of hydrogen evolution.