Mesoporous silica supported cobalt catalysts for hydrogen generation in hydrolysis of ammonia borane

Mesoporous silica supported cobalt boride (Co-B) catalysts are rationally designed for hydrogen generation in ammonia-borane hydrolysis reactions under ambient conditions. Cobalt boride catalysts are supported on three different mesoporous silica, including beta-zeolite seeded MCM-41 (Co@M41S) and traditional MCM-41 (Co@M41T) via chemical adsorption onto functionalized surface with 3-trihydroxysilylpropylmethylphosphonate (THPMP), and one-step co-precipitation into mesoporous silica framework (Co@M41C). Our preparation strategies provide two insights to the reactions: first, cobalt oxide species are intrinsically deposited as ultra-small nanoparticles (<2 nm) on mesoporous silica supports; subsequently the nanoparticles are converted to active Co-B catalysts by reduction with sodium borohydride (SB). Three catalysts exhibit significant differences in catalytic reactivities with hydrogen production rates ranked in an order of Co@M41S > Co@M41T > Co@M41C. Detailed analysis of the coordination environments from in situ X-ray absorption spectroscopy (XAS) results confirm reducibility in SB. Amorphous nature of Co-B catalysts are responsible for efficient catalytic activity in Co@M41S and Co@M41T. Ammonia temperature programmed desorption (NH₃-TPD) demonstrates support acidity that correlates to the degree of high dispersity and effective reducibility to Co-B. Effects from catalyst sizes, reducibility in SB treatment and surface acidity are studied in detail to compare catalytic reactivities among three types of supports.