Highly Selective and Sharp Volcano-type Synergistic Ni₂Pt@ZIF-8-Catalyzed Hydrogen Evolution from Ammonia Borane Hydrolysis

Abstract

Ammonia borane hydrolysis is considered as a potential means of safe and fast method of H₂ production if it is efficiently catalyzed. Here a series of nearly monodispersed alloyed bimetallic nanoparticle catalysts are introduced, optimized among transition metals, and found to be extremely efficient and highly selective with sharp positive synergy between 2/3 Ni and 1/3 Pt embedded inside a zeolitic imidazolate framework (ZIF-8) support. These catalysts are much more efficient for H₂ release than either Ni or Pt analogues alone on this support, and for instance the best catalyst Ni₂Pt@ZiF-8 achieves a TOF of 600 mol_H₂·mol_catal^-1·min^-1 and 2222 mol_H₂·mol_Pt^-1·min^-1 under ambient conditions, which overtakes performances of previous Pt-base catalysts. The presence of NaOH boosts H₂ evolution that becomes 87 times faster than in its absence with Ni₂Pt@ZiF-8, whereas NaOH decreases H₂ evolution on the related Pt@ZiF-8 catalyst. The ZIF-8 support appears outstanding and much more efficient than other supports including graphene oxide, active carbon and SBA-15 with these nanoparticles. Mechanistic studies especially involving kinetic isotope effects using D₂O show that cleavage by oxidative addition of an O-H bond of water onto the catalyst surface is the rate-determining step of this reaction. The remarkable catalyst activity of Ni₂Pt@ZiF-8 has been exploited for successful tandem catalytic hydrogenation reactions using ammonia borane as H₂ source. In conclusion the selective and remarkable synergy disclosed here together with the mechanistic results should allow significant progress in catalyst design toward convenient H₂ generation from hydrogen-rich substrates in the close future.