Ferrocene-boosting Zr-MOFs for efficient photocatalytic CO2 reduction: A trade-off between enhancing LMCT and frustrating Lewis acid

The introduction of rich-electron ligands is a promising strategy to improve the ligand-to-metal charge transfer (LMCT) ability of metal–organic frameworks (MOFs) for efficient photocatalytic CO2 reduction reactions (CO2RR), while the resulting impaired Lewis acid sites go against stable bonding with CO2 and hinder the electron transfering to CO2. It is thus the key to search a trade-off between increasing LMCT and frustrating Lewis acid. Herein, the ferrocene (Fc) modified UiO-66 series MOFs, NH2-UiO-66-Fc, have been prepared to optimize LMCT for boosting CO2RR through a simple solvent-assisted ligand incorporation (SALI) method. Theoretical calculation and systematic experiments, including photoelectrochemical measures, in situ electron paramagnetic resonance, fluorescence probe analysis and X-ray photoelectron spectroscopy indicate that with increasing Fc content, the LMCT energy of MOFs significantly decreases from 2.66 to 2.10 eV based on the established electron transfer double-channel mechanism, while the Lewis acidity of Zr sites reduces with the CO2 adsorption energy (Ead) ranging from −1.061 to −0.632 eV. Through introducing an optimal Fc content, NH2-UiO-66-Fc(2.0) integrating satisfactory LMCT ability and Lewis acidity shows superior photocatalytic CO2RR performance (CO yield: 90.65 μmol·g−1·h−1), about 13 times higher than that of the unmodified MOF and superior to most reported pristine MOF photocatalysts. DFT calculation demonstrates that the excellent performance of NH2-UiO-66-Fc(2.0) is attributed to the lowest energy barrier of 1.50 eV for the rate-limiting step of *COOH formation. In this work, such a simple post-modification strategy for facilitating the whole electron transfer path of photocatalytic CO2RR will provide a new way to construct high-performance MOFs-based CO2RR photocatalysts.