Unraveling the photocatalytic mechanisms for U(VI) reduction by TiO2

Photocatalytic reduction of U(VI) to U(IV) by TiO2 offers a promising avenue to meet the requirements of nuclear fuels and environmental control. In this study, DFT-D3 calculations are used to unravel photocatalytic mechanisms for U(VI) reduction by rutile and anatase, two major TiO2 polymorphs. Photocatalytic mechanisms depend on TiO2 polymorphs: Stepwise for rutile and concerted for anatase. For both TiO2 polymorphs, U(IV) represents the dominant product and is produced facilely under ambient conditions, thus favoring immobilization. Anatase has higher photocatalytic activity than rutile, due to the position of photoexcited electrons at top-surface rather than sub-surface. U(III) emerges as by-product only for anatase whereas the protonated U(IV) species exists for rutile. All steps of photocatalytic reduction are characterized by strong asynchronicity of electron and proton transfers. Surface electronic modulation of rutile alters electron distribution by shifting photoexcited electrons to the top-surface, and greatly enhances photocatalytic activity. Compared to the position of photoexcited electrons, surface structure difference between rutile and anatase is less critical to photocatalytic activity. Results rationalize experimental observations available, and also provide significantly new insights for photocatalytic reduction of U(VI) by TiO2 that facilitate the development of efficient photocatalysts for uranium management and utilization.