Catalytic application of shape-controlled Cu2O particles protected by Co3O4 nanoparticles for hydrogen evolution from ammonia borane

Cu2O particles are active catalysts for hydrogen evolution from ammonia borane (AB) by hydrolysis, however, Cu2O particles easily form agglomerates as a result of highly reduced conditions during the reaction. In order to suppress agglomerate formation, capping of Cu2O with organic reagents or inorganic materials was performed and the catalytic reactivity in AB hydrolysis was examined. Among the examined methods, capping of Cu2O particles with Co3O4 nanoparticles was the most effective to avoid agglomerate formation of Cu2O particles. The finding enabled us to examine the shape effect of Cu2O particles on the catalytic reactivity in AB hydrolysis in the presence of Co3O4 nanoparticles. Comparisons of turnover frequencies for hydrogen evolution of Cu2O-Co3O4 composites, in which Cu2O particles were in the shape of 50-facets, cube, octahedron or rhombicuboctahedron, indicated that the composite with Cu2O with the shape of 50-facets showed more than 7-fold higher hydrogen evolution rate normalized by surface area than the composite with Cu2O with the octahedral shape. The size and shape effects of Co3O4 nanoparticles were also investigated on their ability to protect Cu2O from agglomeration. Comparisons of the catalytic reactivity of Cu2O particles decorated with Co3O4 nanoparticles of different sizes and shapes in terms of amounts and rates of hydrogen evolved by AB hydrolysis indicated that the size of Co3O4 nanoparticles is more important than the shape to exhibit high catalytic reactivity.