Co-Encapsulation of Rhenium and Ruthenium Complexes into the Scaffolds of Metal-Organic Framework to Promote CO₂ Reduction

The molecular complexes of Re(4,4 '-dcbpy)(CO)(3)Cl (dcbpy = dicarboxylicacid-2,2'-bipyridyl) and [Ru(dcbpy)(3)](2+) are co-assembled into UiO-66 scaffolds as structural imperfects for CO2 photocatalytic reduction (named as Re-Ru@U). The prepared catalysts are characterized by XRD, Fourier-Transform infrared (FTIR) spectra, X-ray photoelectron spectra (XPS) and N-2 adsorption-desorption isotherms. The intact structure of molecular complexes within the matrix are monitored by H-1 nuclear magnetic resonance (NMR) spectra through a totally digesting catalyst. The optical properties are studied via absorption and photoluminescence spectra, and the single-electron reduction in Re and Ru complexes is detected by electron paramagnetic resonance (EPR) spectra. An excellent photocatalytic performance is obtained with steady and sustained CO evolution and a turnover number (TON) value of 15 (11 h). The CO activity irradiating by single wavelength presents the absorption-intensity-dependent changing tendency, where the absorption intensity is superposed by Re and Ru complexes. The two radicals related to Re and Ru, respectively, are simultaneously detected in the Re-Ru@U catalyst. It is suggested that the ReC2 component serves as both a photosensitizer and a catalyst, and the RuC2 component works as an additional photosensitizer to supply the second electron for CO2 reduction. The co-assembling of dual metals Re and Ru in the matrix promotes the electron transfer from the reductive Ru centres to one-electron-reduced Re centres and accounts for the superior activity of CO evolution. Our results demonstrate a strategy to develop the multimetallic catalysts via facile assembling into MOF scaffolds to promote photocatalytic performance.