Molecular Uranium Dioxide-Mediated CO2 Photoreduction

The reduction of CO2 mediated by transition metals has garnered significant interest, yet little is known about the reduction of CO2 using f-element compounds. Herein, the reduction of CO2 to CO by tetravalent uranium (UIV) compound UO2 is investigated via matrix isolation infrared spectroscopy and quantum chemical study. Our results reveal that a stable carbonate intermediate OUIVCO3 (A) can be prepared at low temperatures (4-12 K). Through photolytic reactions of A under visible-light irradiations (495 nm < λ < 580 nm), the charge-separated pentavalent UV isomer [UVO2]+[(η2-O2C)]- (B) is produced through electron transfer from the quasi-atomic U-7s orbital to the CO2 moiety. Sequentially, one C═O bond in CO2 breaks by successive UV-visible irradiation (250 nm < λ < 580 nm), and the photolysis generates the products CO and hexavalent UVI compound UVIO3 following two intermediates UVIO3(CO) (C) and UVIO3(OC) (D) with a physiosorbed carbonyl group. Moreover, the evolution of oxidation states from electron-rich UIV to UVI on multiple potential energy surfaces of different electronic states involving configurations U(f1s1 → f2 → f1 → f0) is further demonstrated. Our findings unveil a mechanism for the photoreduction of CO2 by a UO2 molecule. This strategy can be used to design molecular and solid-state catalysts for depleted uranium for CO2 reduction reactions.