Highly Selective and Sharp Volcano-type Synergistic Ni_2Pt@ZIF-8-Catalyzed Hydrogen Evolution from Ammonia Borane Hydrolysis

摘要

Ammonia borane hydrolysis is considered as a potential means of safe and fast method of H_(2) 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 H_(2) release than either Ni or Pt analogues alone on this support, and for instance the best catalyst Ni_(2)Pt@ZiF-8 achieves a TOF of 600 mol_(H_(2))·mol_(catal)~(–1)·min~(–1) and 2222 mol_(H_(2))·mol_(Pt)~(–1)·min~(–1) under ambient conditions, which overtakes performances of previous Pt-base catalysts. The presence of NaOH boosts H_(2) evolution that becomes 87 times faster than in its absence with Ni_(2)Pt@ZiF-8, whereas NaOH decreases H_(2) 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 D_(2)O 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 Ni_(2)Pt@ZiF-8 has been exploited for successful tandem catalytic hydrogenation reactions using ammonia borane as H_(2) source. In conclusion the selective and remarkable synergy disclosed here together with the mechanistic results should allow significant progress in catalyst design toward convenient H_(2) generation from hydrogen-rich substrates in the close future.