Mechanistic Insight into the Synergetic Interaction of Ammonia Borane and Water on ZIF-67-Derived Co@Porous Carbon for Controlled Generation of Dihydrogen

Regarding dihydrogen as a clean and renewable energy source, ammonia borane (NHBH, AB) was considered as a chemical H-storage and H-delivery material due to its high storage capacity of dihydrogen (19.6 wt %) and stability at room temperature. To advance the development of efficient and recyclable catalysts for hydrolytic dehydrogenation of AB with parallel insight into the reaction mechanism, herein, ZIF-67-derived fcc-Co@porous carbon nano/microparticles (cZIF-67_nm/cZIF-67_μm) were explored to promote catalytic dehydrogenation of AB and generation of H. According to kinetic and computational studies, zero-order dependence on the concentration of AB, first-order dependence on the concentration of cZIF-67_nm (or cZIF-67_μm), and a kinetic isotope effect value of 2.45 (or 2.64) for HO/DO identify the Co-catalyzed cleavage of the H-OH bond, instead of the H-BHNH bond, as the rate-determining step in the hydrolytic dehydrogenation of AB. Despite the absent evolution of H in the reaction of cZIF-67 and AB in the organic solvents (i.e., THF or CHOH) or in the reaction of cZIF-67 and water, Co-mediated activation of AB and formation of a Co-H intermediate were evidenced by theoretical calculation, infrared spectroscopy in combination with an isotope-labeling experiment, and reactivity study toward CO-to-formate/HO-to-H conversion. Moreover, the computational study discovers a synergistic interaction between AB and the water cluster (HO) on fcc-Co, which shifts the splitting of water into an exergonic process and lowers the thermodynamic barrier for the generation and desorption of H from the Co-H intermediates. With the kinetic and mechanistic study of ZIF-67-derived Co@porous carbon for catalytic hydrolysis of AB, the spatiotemporal control on the generation of H for the treatment of inflammatory diseases will be further investigated in the near future.