Manipulating Lewis acid-base pairs on carbon nitride with synergistic catalytic sites for photocatalytic aerobic uranyl reduction

Abstract: Photochemical reduction of hexavalent uranium (U(VI)) to insoluble tetravalent uranium (U(IV)) presents a sustainable catalytic conversion approach for the recovery of low-concentration uranium and the alleviation of growing nuclear environmental pollution. However, achieving superior U(VI) reduction activity is hindered by the challenges of low carrier separation efficiency, limited active sites, and unclear catalytic mechanisms. Here, we report the synthesis of P, K co-doped and C≡N modified carbon nitride (K/PD/g-CN) featuring multiple catalytic sites for photocatalytic U(VI) reduction. The Lewis base sites, provided by the terminal C≡N groups, facilitate the reduction of hexavalent U(VI). The electron-deficient K atom acts as the Lewis acid site to oxidize CHOH3 to CH2O. The coexistence of acid and base sites results in a strong synergistic effect during the photocatalytic aerobic reduction of U(VI). P doping sites not only causes charge redistribution but also boosting Lewis acid-base active sites by increasing K and C≡N group concentrations. Spectroscopy and density functional theory (DFT) calculations show that efficient charge separation is achieved through electron delocalization within the K/PD/g-CN structure. The combination of abundant active sites and effective carrier separation leads to an impressive 99.2% U(VI) reduction efficiency in 30 minutes under atmospheric air conditions, which is approximately 5.6 times faster than g-CN. This study presents a novel approach for advancing the high-efficiency reduction of uranyl with well-designed multisite Lewis acid-base catalysts.