Two-Dimensional Kagome Covalent Organic Frameworks with Single Atomic Co Sites for Superior Photocatalytic CO₂ Reduction

Covalent organic frameworks (COFs) have attracted great attention as pivotal photocatalysts for efficient CO 2 photoreduction into value-added fuels, which hold great promise for simultaneously mitigating global warming and the energy crisis. However, the synthesis of COFs with a high crystalline state and hierarchically porous structure to boost CO 2 photoreduction is still an enormous challenge and rarely reported, probably because of the great dependence upon monomers and rigorous preparation conditions. Herein, a series of functional kagome ( kg m ) topologic 2D COFs with high crystallinity and porosity were synthesized based on the condensation of 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)tetraaniline (ETTA) and 2,2′-bipyridyl-5,5′-dialdehyde (Bpy-CHO) building units combined with a postmodification strategy, named ETTA-Bpy-COF-M (M = H, Fe, Co, Ni, or Cu). Stimulated by the unique kg m topologized framework with well-ordered hierarchical micropores and mesopores, abundant exposed atomic Co sites, and remarkable photoelectrical performance, ETTA-Bpy-COF-Co is used as a photocatalyst for catalyzing the CO 2 -to-CO photoconversion and exhibits a high CO yield rate (9398.14 μmol g –1 h –1 ), a large CO selectivity (92.73%), and good durability. Experimental and theoretical analyses demonstrated that the superior performance for CO 2 photoreduction catalyzed by ETTA-Bpy-COF-Co was attributed to the desirable cooperative contribution of kg m topological structure with hexagonal and triangular pores as well as atomic Co active sites, which can promote the photoexcited charge carrier kinetics, enhance the CO 2 adsorption and activation, as well as reduce the energy barriers of *COOH generation and CO desorption. This work opens a new way to enhance COF photosynthesis for CO 2 reduction and offers precious insights into related studies in the future.