PtNi@ZIF-8 nanocatalyzed high efficiency and complete hydrogen generation from hydrazine borane: origin and mechanistic insight

Abstract

The rational design of highly efficient nanocatalysts and understanding of the hydrogen generation process for selective and complete decomposition of hydrazine borane (HB) remain challenging. Here, a series of monodispersed, ultrasmall, highly efficient Pt_xNi_y alloyed nanoparticles were prepared using ZIF-8 as the MOF template. Upon scrutinizing the optimal Pt, Ni and support proportions, remarkable synergy of volcano-type is disclosed between these two metals in their alloy and with the ZIF-8 support. The high catalyst efficiency with complete gas evolution is among the best ever recorded, also showing promising utilization of this catalyst in other tandem reactions involving hydrogen. The experimental and DFT simulation results indicate that alloying Pt with Ni in PtNi@ZIF-8 with an optimized surface d-band center optimizes absorption and activation of the HB molecule, thereby reducing the energy barrier of the hydrolysis of the BH₃ group in HB. By systemically comparing the hydrolytic decomposition processes of HB over the nanocatalysts PtNi@ZIF-8 and RhNi@ZIF-8, the desorption of the borate is identified as the rate-determining step. In the following decomposition of the N₂H₄ moiety, the nanocatalyst PtNi@ZIF-8 always shows a smaller Gibbs free energy gap than RhNi@ZIF-8, rationalizing the faster kinetics for complete hydrogen evolution. The principle and results generated in the present study should contribute to optimizing the design of highly efficient nanocatalysts for hydrogen generation from hydrogen storage materials.