Cu/Co3O4 Nanoparticles as Catalysts for Hydrogen Evolution from Ammonia Borane by Hydrolysis

A series of nanosized Co3O4 particles in which Cu was loaded on the surface were examined as robust catalysts for hydrogen evolution by ammonia borane hydrolysis. Their catalytic activity was dependent on the shape and size of nanosized Co3O4. The shape of nanosized Co3O4 was cube, hexagonal sheet, or uncontrolled. Among these, the Co3O4 in the shape of hexagonal sheet showed the highest catalytic activity. To investigate the size dependence of the catalytic reactivity, Co3O4 particles with the controlled size of about 4, 20, or 500 rim were examined, and it was found that the one in the size about 4 nm showed the highest activity although the size dependence was not remarkable compared with the shape dependence. The robustness of the catalyst was assured by no significant activity loss after 10 times repetitive reactions. The structural characterizations of Cu/Co3O4 composite in the fresh and used conditions were performed by X-ray photoelectron spectroscopy, Auger spectroscopy, and powder X-ray diffraction spectroscopy. The X-ray diffraction patterns assigned to Co3O4 were observed for both fresh and used catalysts, indicating that the Co3O4 form was maintained at the core part of each particle after the reaction. On the other hand, the XPS peaks or Auger peak for Cu 2p, Cu L3M45M45, Co 2p, and O Is of the used catalyst suggested that its surface was reduced or hydrolyzed to Cu2O, Co metals, and Co(OH)(2) during the reaction. The observed Cu2O and Co metals are regarded as active species for ammonia borane hydrolysis.