Construction of isolated Ni sites on nitrogen-doped hollow carbon spheres with Ni–N₃ configuration for enhanced reduction of nitroarenes

Designing and synthesizing high-efficiency non-precious metal-based catalysts having uniform active sites increases the reactivity and selectivity of materials and provides a platform for an in-depth understanding of their catalytic reaction mechanism. In this study, we provided an approach for fabricating isolated nickel single-atom sites (Ni SAs) with high loading (4.9 wt.%) stabilized on nitrogen-doped hollow carbon spheres (NHCS) using a core–shell structured Zn/Ni bimetallic zeolitic imidazolate framework (ZIF) composite as the sacrificial template. The as-fabricated Ni SAs/NHCS catalyst shows superior activity, selectivity, and recycling durability for the catalytic transfer hydrogenation of nitrobenzene to aniline, thus achieving 100% yield of aniline with a turn-over frequency (TOF) value as high as 29.9 h⁻¹ under mild conditions. This TOF value is considerably superior to the supported Ni nanoparticle catalysts. The experiments designed show that the hollow structure feature of NHCS facilitates accessible active sites and mass transfer, which thus contributes to the enhancement of the catalytic performance of Ni SAs/NHCS. Density functional theory calculations show the high chemo-selectivity and activity of the Ni SAs catalyst, arising from the unique role of the single Ni-N₃ site on simultaneously activating the H donor (N₂H₄) and substrate, as well as the hydrogenation of the –NOH group as the rate-determining step.