Breaking the Methanol Synthesis Barrier in CO2 Photoreduction: The Synergistic Effect of Single Atom Copper Within Covalent Organic Frameworks.

The main challenges in the selective reduction of CO2 to methanol (MeOH) via photocatalysis with covalent organic frameworks (COFs) as catalysts are slow charge transfer kinetics and limited CO2 adsorption and activation. Herein, COF-TPDA is designed to securely anchor isolated Cu single atoms at its imine N sites, resulting in the formation of the Cu1/COF-TPDA single atom catalyst. Cu1/COF-TPDA demonstrates robust photocatalytic activity, achieving a MeOH yield of 872.5 μmol·gcat-1·h-1 for the selective conversion of CO2 in neutral water under simulated sunlight, without the use of sacrificial reagents. The Cu single atom acts as an electron acceptor, while the COF-TPDA serves as an electron donor, thereby enhancing charge separation efficiency. Meanwhile, the multielectron reduction of CO2 to MeOH is facilitated by the well-matched conduction band potential (-0.53 eV vs NHE), effectively minimizing competitive side reactions. In-situ characterization and theoretical calculations have shown that Cu single-atom sites within the Cu-N coordination structure alter the reaction pathway. Compared to the CO* hydrogenation pathway catalyzed by N sites in COF-TPDA (free energy barrier of 1.37 eV), a more direct reduction pathway, hydrogenation of HCOOH*, is provided over the Cu1/COF-TPDA, where the energy barrier for generating the HCOO* is reduced to 0.76 eV. The presence of unsaturated Cu sites enhances catalytic activity, making MeOH synthesis thermodynamically more favorable. This research shows that the photocatalytic performance of COFs can be efficiently tuned by anchoring single metal atoms, offering universal guidelines for creating COF-based photocatalysts that are both highly active and scalable.