Engineering Single Cu Sites into Covalent Organic Framework for Selective Photocatalytic CO 2 Reduction.

Photocatalytic CO conversion into value-added chemicals is a promising route but remains challenging due to poor product selectivity. Covalent organic frameworks (COFs) as an emerging class of porous materials are considered as promising candidates for photocatalysis. Incorporating metallic sites into COF is a successful strategy to realize high photocatalytic activities. Herein, 2,2'-bipyridine-based COF bearing non-noble single Cu sites is fabricated by chelating coordination of dipyridyl units for photocatalytic CO reduction. The coordinated single Cu sites not only significantly enhance light harvesting and accelerate electron-hole separation but also provide adsorption and activation sites for CO molecules. As a proof of concept, the Cu-Bpy-COF as a representative catalyst exhibits superior photocatalytic activity for reducing CO to CO and CH without photosensitizer, and impressively, the product selectivity of CO and CH can be readily modulated only by changing reaction media. Experimental and theoretical results reveal the crucial role of single Cu sites in promoting photoinduced charge separation and solvent effect in regulating product selectivity, which provides an important sight onto the design of COF photocatalysts for selective CO photoreduction.