Co-porphyrin/Ru-pincer complex coupled polymer with Z-scheme molecular junctions and dual single-atom sites for visible light-responsive CO2 reduction

Covalent organic polymer has been considered as the promising organic semiconductor for the photocatalytic CO2 reduction reaction (CO2RR) due to its unique features such as adjustable framework, durable stability and feasible bandgap energy, et al. Herein, we report a novel CoII-porphyrin/RuII-pincer complex coupled polymer (CoPor-RuN3), which exhibits efficient visible light-responsive CO2RR activity with the average CO/CH4 yields of 37.1/1.57 μmol g-1h−1 and a total photoactivity of 86.8 μmol g-1h−1, 25.5 and 4.5 times that of the CoPor and RuN3 monomers, respectively. It is found that the two metal centers (Co/Ru) and the conjugated framework of the polymer are essential for efficient CO2RR. A series of in-situ characterizations prove that periodic Z-scheme molecular junctions are formed by the linkage of amide group in the CoPor-RuN3 polymer to facilitatie the photogenerated electron transfer from CoPor units to RuN3 ones, in which Ru and Co serve as the reduction and oxidative single-atom sites, respectively. Moreover, the wavelet transform plots of X-ray absorption spectra of Ru K-edge shows that two CH3CN ligands are easily dissociated from the Ru center during the photocatalytic process, leaving empty sites for the adsorption of CO2. Based on the experimental observations and theoretical calculations, a plausible photocatalytic CO2 reduction mechanism involving a pincer-typed N′NN′-Ru-CO intermediate (N′NN′ = 2,6-bis(benzimidazol-2-yl)pyridine) as the real reduction center is clarified. This work gives a new thread for exploring broadband-responsive covalent organic polymers with uniformly dispersed single-atom catalytic sites for CO2 photoreduction.