Highly conjugated and chemically stable three-dimensional covalent organic frameworks for efficient photocatalytic CO2 reduction

Abstract

Covalent organic frameworks (COFs), as efficient photocatalysts, can convert CO₂ and H₂O into value-added fuel, thus improving the deteriorating ecological environment. However, achieving high photocatalytic efficiency and selectivity without relying on noble metals or sacrificial agents remains a significant challenge. Here, a highly conjugated three-dimensional (3D) COF 3D-COF-1 with imine linkage is constructed by the combination of two rigid and conjugated orthogonal building blocks (spirobifluorene and bicarbazole) with good photoactivites. Through a simple post-synthetic reduction, a chemically stable and amine-linked 3D-COF-2 which maintains excellent crystallinity and porosity can be obtained. Notably, the 3D-COF-2 exhibits excellent performance in CO₂ reduction and exceptional selectivity of CO due to the highly conjugated structure and abundant amine groups as chemisorption sites for selectively capturing CO₂. Under the irradiation of visible light and without noble metals and sacrificial agents, 3D-COF-2 produces 1070 μmol g^-1 of carbon monoxide in 4.5 h, and the selectivity is close to 100%.