Dual heteroatom-doped carbons have attracted widespread research attention as catalysts in the field of energy storage and conversion due to their unique electronic structures and chemical tunability. In particular, boron and nitrogen co-doped carbon (B,N@C) has shown great potential for photo/electrocatalytic applications. However, more needs to be done for rational designing and regulating the structure of these materials to improve their catalytic performance. Herein, monodispersed hierarchical porous B,N@C nanocages were fabricated by pyrolyzing zeolite imidazole framework (ZIF) which was treated with ammonia borane or boric acid via an integrated double-solvent impregnation and nanocofined-etching method. The treated ZIF-8 provided an essential structural template to achieve B, N co-doped hierarchical structures with micro/meso/macro multimodal pore size distributions. The resultant B,N@C nanocages displayed high catalytic activities for electrochemical oxygen reduction reaction (ORR) in alkaline media, outperforming most carbon-based catalysts, particularly from the perspective of the half-wave potentials. Such high catalytic performance is due to the enhanced activity by the coexistence of B and N and the mass transfer promoted by the unique hierarchical porous structure.

In heterogeneous catalysis, the precise placement of active components to perform unique functions in cooperation with each other is a tremendous challenge. The migration of matter on micro/nano-scale caused by diffusion is a promising pathway for design of catalytic nanoreactors with precise active sites location and controllable microenvironment through compartmentalization and confinement effects. Herein, we report two categories of mesoporous ZnCoSiO_x hollow nanoreactors with different metal distributions and microenvironment engineered by the diffusion behavior of metal species in confined nanospace. Double-shelled hollow structures with well-distributed metal species were obtained by adopting core@shell structured ZnCo-zeolitic imidazolate framework (ZIF)@SiO₂ as a template and employing three stages of hydrothermal treatment including the decomposition of ZIF, diffusion of metal species into the silica shell, and Ostwald ripening. Additionally, the formation of yolk@shell structure with a collective (Zn-Co) metal oxide as the yolk was achieved by direct pyrolysis of ZnCo-ZIF@SiO₂. In CO₂ hydrogenation, ZnCoSiO_x with double-shelled hollow structures and yolk@shell structures respectively afford CO and CH₄ as main product, which is related with different dispersion and location of active sites in the two catalysts. This study provides an efficient method for the synthesis of catalytic nanoreactors on the basis of insights of the atomic diffusion in confined space at the mesoscale.