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

Abstract

while protecting them from aggregation is highly desirable.Yolk-shell nanostructures, which are composed of MNPs cores within a hollow cavity surrounded by an outer shell, [3] offer great potential to fulfill the optimal balance between the catalytic activity and stability of MNPs, [4] in which the MNPs cores with ligand-free surfaces provide the active sites for catalytic reaction, while the shell serves as a barrier to prevent aggregation of the MNPs. [5] To obtain a highly efficient and stable yolk-shell catalyst, a shell with structural stability and permeability is of critical importance. [6] The shells in most earlier reports were amorphous. [7] In fact, the crystallinity of the shell has a substantial effect on the catalytic performance of yolk-shell nanocomposites. [8] Previously reported yolk-shell MNPs@MOF composites have illustrated the successful utility of crystalline shells with ordered porosity for heterogeneous catalysis. [9] However, the conglomerate of the nanocrystals of MOFs is the primary limiting factor for the formation of a well-packed and continuous shell in yolk-shell MNPs@MOF nanostructures, [10] as imperfections or cracks will result in undesirable diffusion pathways and/or MNP leaching. Moreover, the MOF shells in most previous yolk-shell MNPs@MOF nanostructures are microporous. [9a,d] The small pore size would limit the diffusion and mass transport efficiency of large molecules and thus hinder their access to the internal active sites. Therefore, the rational design of hierarchical porous crystalline shells hosting uncapped MNPs is a crucial issue for the development of yolkshell nanocatalysts.Covalent organic frameworks (COFs), [11] known as crystalline porous networks, [12] show great potential as carriers for metal ions or MNPs due to their well-defined tunable structures. [13] Considering the unique characteristics of COFs, such as their high porosity, easy modification, and well-defined and designable topologies, [14] the encapsulation of MNPs inside the cavities of hollow COFs could be a promising strategy for developing stable and active yolk-shell catalysts. The sizes and shapes associated with the chemical and physical properties of the presynthesized MNPs can be feasibly tuned by their well-established syntheses, while the adequate coordination 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...

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