Construction of Single Ni Atom-Immobilized ZIF-8 with Ordered Hierarchical Pore Structures for Selective CO₂ Photoreduction

The rational construction of single-atom-modified porous metal-organic frameworks (MOFs) for efficient CO2 photoconversion is a promising avenue, yet the one that poses a significant challenge. In this study, we demonstrate the successful construction of a Ni single-atom-immobilized ZIF-8 with an ordered hierarchical (macro-micro) pore structure for highly efficient photocatalytic CO2 reduction to CO. The interpenetrated macroporous structure in microporous ZIF-8 is built up by employing polystyrene (PS) opal as a hard template, while the high stability of the framework is achieved through the double-solvent-induced crystallization. The hierarchical porous structure enables rapid mass transport and strong adsorption of CO2, while Ni single atoms as active sites and electron-trapping sites optimize the reaction energy and hinder the competitive H2 evolution. As a result, the Ni single-atom-immobilized hierarchical porous ZIF-8 achieves high stability and an activity of 4.2 mmol g-1 h-1 for CO yield with an electronic selectivity of 94%, which shows a 47-fold CO yield of that for ZIF-8. This work highlights the significance of constructing a hierarchical porous structure of MOFs and single-atom-active sites to improve the activity and selectivity of CO2 photoreduction for realizing carbon neutralization.