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 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.