Well‐Defined Metal Nanoparticles@Covalent Organic Framework Yolk–Shell Nanocages by ZIF‐8 Template as Catalytic Nanoreactors

Abstract Yolk–shell nanoreactors have received considerable interest for use in catalysis. However, the controlled synthesis of continuous crystalline shells without imperfections or cracks remains challenging. Here, a strategy for the synthesis of yolk–shell metal nanoparticles@covalent organic framework (MNPs@COF) nanoreactors by using MNPs@ZIF‐8 core–shell nanostructures as a self‐template is designed and developed. The COF shell is formed through an amorphous‐to‐crystalline transformation process of a polyimine shell in a mildly acidic solution, while the ZIF‐8 is etched in situ, generating a void space between the MNPs core and the COF shell. With the protection of the COF shell, multiple ligand‐free MNPs are confined inside of the hollow nanocages. Importantly, the synthetic strategy can be generalized to engineer the functions and properties of the designed yolk–shell nanocages by varying the structure of the COF shell and/or the composition of the core MNPs. Representative Pd@H‐TpPa yolk–shell nanocages with active Pd NP cores and permeable TpPa shells exhibit high catalytic activity and stability in the reduction of 4‐nitrophenol by NaBH 4 at room temperature.