A metal-covalent organic framework nanosheet constructed from an asymmetric tridentate ligand structure was designed for the photoreduction of CO₂ to syngas under different contents of CO₂ and natural sunlight. The carboxyl group in the asymmetric tridentate ligand structure facilitated dissociation of water to generate *H by hydrogen bond. Moreover, *H assists CO₂ (g) adsorption and lowers the energy barrier for *COOH formation.

Abstract

Photocatalytic conversion of CO₂ into syngas is a promising approach to solving energy and environmental challenges. However, the current studies are mainly conducted by using neat CO₂ where CO₂ enrichment and purification is an energy-intensive process. Herein, we report a Co-COOH-COF with an asymmetric tridentate ligand for syngas synthesis from different contents of CO₂. The nanosheets of the resulting champion catalyst, Co_17.7-COOH-COF enable an outstanding syngas production rate of 151.1 mmol g⁻¹ h⁻¹ from neat CO₂ and 180.5 mmol g⁻¹ h⁻¹ from 40 vol% CO₂ under visible light, while it is up to 112.3 mmol g⁻¹ h⁻¹ from neat CO₂ and 116.0 mmol g⁻¹ h⁻¹ from 40 vol% CO₂ under natural sunlight, which surpasses most of the previously reported state-of-the-art photocatalysts. Impressively, the catalyst also exhibits a widely and continuously adjustable CO/H₂ molar ratio from 4:1 to 1:21. Experimental and theoretical studies indicate that the asymmetric tridentate ligand with carboxyl groups increases Co loading in the covalent organic frame (COF) by the formation of atomically dispersed N–Co–O₄ sites and enhances dissociation of water and CO₂ adsorption by hydrogen bonding, thus facilitating CO₂ transformation and hydrogen evolution. This work presents a new pathway for the design of high-performance catalysts for the photoreduction of CO₂ to syngas.