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 CO 2 reduction reactions (CO 2 RR), while the resulting impaired Lewis acid sites go against stable bonding with CO 2 and hinder the electron transfering to CO 2 . 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, NH 2 -UiO-66-Fc, have been prepared to optimize LMCT for boosting CO 2 RR 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 CO 2 adsorption energy ( E ad ) ranging from −1.061 to −0.632 eV. Through introducing an optimal Fc content, NH 2 -UiO-66-Fc (2.0) integrating satisfactory LMCT ability and Lewis acidity shows superior photocatalytic CO 2 RR 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 NH 2 -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 CO 2 RR will provide a new way to construct high-performance MOFs-based CO 2 RR photocatalysts.