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 nm 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 Co ₃O₄ 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 1s 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.