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 (U^IV) compound UO₂ is investigated via matrix isolation infrared spectroscopy and quantum chemical study. Our results reveal that a stable carbonate intermediate OU^IVCO₃ (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 U^V isomer [U^VO₂]⁺[(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 U^VI compound U^VIO₃ following two intermediates U^VIO₃(CO) (C) and U^VIO₃(OC) (D) with a physiosorbed carbonyl group. Moreover, the evolution of oxidation states from electron-rich U^IV to U^VI 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.