Dramatic acceleration by visible light and mechanism of AuPd@ZIF-8-catalyzed ammonia borane methanolysis for efficient hydrogen production

Abstract

The generation of H₂ from materials with a high content of H atoms is attractive for both sustainable energy and convenient hydrogenation. We report that the novel synthesized AuPd@ZIF-8-alloyed nanoparticles (ZIF = zeolitic imidazolate framework) in which the AuPd nanoparticles (NPs) have a size of 2.43 nm and are shown by Brunauer–Emmett–Teller (BET) surface to be encapsulated into ZIF-8. The AuPd and ZIF-8 nanoparticle is an excellent nanocatalyst for the evolution of H₂ in the methanolysis reaction of aminoborane (AB) under visible light irradiation. Visible-light-induced acceleration is due to the Au plasmonic excitation provoking hot electron transfer from Au to Pd-substrate ensemble, whereas the reactions catalyzed by monometallic Au@ZIF-8 or Pd@ZIF-8 undergo only few minutes or no acceleration (respectively) under visible light irradiation. Three mol H₂ per mol AB are produced in 6 min at 25 °C (TOF: 86.8 mol H₂ mol_atom⁻¹ min⁻¹) with AuPd@ZIF-8 under visible light compared with incomplete H₂ formation in 30 min in the dark. A comparison of various heterogeneous supports shows that the ZIF-8 encapsulation of the nanoalloy is by far the best support for this reaction. The large primary kinetic isotope effect (KIE) k_H/k_D = 3.4 with visible-light irradiation, the high turnover frequency (TOF) under light illumination (3.7 times higher than in the dark), and density functional theory (DFT) calculations confirm the mechanism and illustrate the more difficult O–H oxidative addition on Pd in methanol than in water in this process. This is due to the weaker acidity of methanol compared to that of water. Coupling labeling and tandem reactions with styrene hydrogenation showed that one H atom of H₂ formed is provided by AB, while the other one is from methanol. The cleanness of the H₂ generated and the recyclability of the NH₄B(OMe)₄ product render AB methanolysis attractive for low-temperature H₂ production devices and tandem reactions.