Acceleration of Photoinduced Electron Transfer by Modulating Electronegativity of Substituents in Stable Zr-Metal-Organic Frameworks to Boost Photocatalytic CO Reduction.

Photoreduction of CO with water into chemical feedstocks of fuels provides a green way to help solve both the energy crisis and carbon emission issues. Metal-organic frameworks (MOFs) show great potential for CO photoreduction. However, poor water stability and sluggish charge transfer could limit their application. Herein, three water-stable MOFs functionalized with electron-donating methyl groups and/or electron-withdrawing trifluoromethyl groups are obtained for the CO photoreduction. Compared with UiO-67--CF-CH and UiO-67--(CF), UiO-67--(CH) achieves excellent performance with an average CO generation rate of 178.0 μmol g h without using any organic solvent or sacrificial reagent. The superior photocatalytic activity of UiO-67--(CH) is attributed to the fact that compared with trifluoromethyl groups, methyl groups could not only elevate CO adsorption capacity and reduction potential but also promote photoinduced charge separation and migration. These are evidenced by gas physisorption, photoluminescence, time-resolved photoluminescence, electrochemical impedance spectroscopy, transient photocurrent characteristics, and density functional theory calculations. The possible working mechanisms of electron-donating methyl groups are also proposed. Moreover, UiO-67--(CH) demonstrates excellent reusability for the CO reduction. Based on these results, it could be affirmed that the strategy of modulating substituent electronegativity could provide guidance for designing highly efficient photocatalysts.