Synthesis of highly dispersed ultra-small cobalt nanoparticles within the cage-type mesopores of 3D cubic mesoporous silica via double agent reduction method for catalytic hydrogen generation

Ultra-small cobalt nanoparticles (NPs) are successfully confined within the mesopores of cubic mesoporous silica KIT-6 via a facile chemical reduction process using double reducing agents sodium borohydride (NaBH ₄ ) and ammonia borane (NH ₃ BH ₃ , AB). When the Co NPs embedded KIT-6 catalysts are used as catalysts for generation of hydrogen from hydrolysis of NH ₃ BH ₃ a superior turnover frequency (TOF) of 20.05 mol _{H 2} mol _Co ^-1 min ^-1 and lower activation energy of 32.6 kJ mol ⁻¹ , better than most of the Co-based catalysts, are achieved. The higher catalytic activity is mainly attributed to the ultra-small size (2.5 nm) of Co NPs and uniform dispersion of Co NPs in the mesopores of KIT-6. Co NPs are also prepared using KIT-6 as the support with another two synthesis routes, namely, single reducing agent NaBH ₄ and thermal reduction method for comparative study. However, the resulting Co NPs are much larger in size and hence have less catalytic activities. The Co NPs are synthesized within the hexagonal mesopores of SBA-15 as well using double reducing agents in order to study the effect of the pore structure of the support on hydrogen generation. KIT-6 supported Co NPs can produce hydrogen much faster than the SBA-15 supported Co NPs as the 3D interconnected porous structure of KIT-6 allows homogenous transportation and easy diffusion of NH ₃ BH ₃ molecules to the active sites. The Co NPs in KIT-6 exhibits good recycle ability up to 5 cycles, and the catalyst can be recovered economically from the aqueous solution due to its magnetic nature.