A large π-conjugated ligand in metal-organic framework as opticalswitch to regulate the electron transfer pathway for highly selective reduction of CO2 to CH4

Here, we report a Cu-based metal-organic framework (Cu-DBC), constructed by the large π-conjugated ligand dibenzo-[g,p]chrysene-2,3,6,7,10,11,14,15-octaol and the square-pyramidal CuO5 nodes, as the photo-coupled electrocatalysts for CO2 reduction to CH4. Under visible light, it exhibits high performance for photocatalytic reduction of CO2 to CH4 with selectivity of 100% and rate of 1.04 μmol g-1 h-1, without additional photosensitizer. Importantly, at -1.4 V vs. RHE, it exhibits high performance for photo-coupled electroreduction of CO2 to CH4 with a Faradaic efficiency (CH4) of 93% and current density of 10.4 A g-1. Theoretical calculations, in-situ infrared spectroscopy investigation and Mott–Schottky measurements showed that the large conjugated ligand in Cu-DBC has the suitable lowest unoccupied molecular orbital (LUMO) to match well with the reduction potential of CO2/CH4 and serves as a photoswitch to regulate electron transfer pathway to the metal center, resulting highly selective photocatalytic reduction or photo-coupled electroreduction of CO2 to CH4.