Highly Active Ni- and Co-Based Bimetallic Catalysts for Hydrogen Production From Ammonia-Borane

Ammonia-borane is one of the most promising candidates for hydrogen carriers. A series of Ni- and Co-based bimetallic catalysts supported on SiO₂ (Ni-M/SiO₂ and Co-M/SiO₂; M = Ga, Ge, Sn, Zn) was prepared and tested as catalysts for hydrogen production from ammonia-borane (AB) in water or methanol. Ni-Zn/SiO₂ and Co-Ge/SiO₂ exhibited catalytic activities much higher than those of monometallic Ni/SiO₂ and Co/SiO₂, respectively. Ni-Zn/SiO₂ showed a high catalytic activity when water was used as a solvent, where the reaction was completed within 6 min at room temperature with a specific reaction rate of 4.3 ml min^-1 mmol-cat^-1 mM-AB^-1. To the best of our knowledge, this is the highest value among those reported using 3d metal-based catalysts. Co-Ge/SiO₂ afforded a five-fold higher reaction rate than that of the corresponding monometallic Co/SiO₂. XRD, TEM, and HAADF-STEM-EDS analyses revealed that Ni_0.75Zn_0.25 and Co_0.8Ge_0.2 solid-solution alloys were formed with high phase purities. An XPS study showed that Co atoms in Co_0.8Ge_0.2 were electron-enriched due to electron transfer from Ge to Co, which may be the origin of the improved catalytic activity.