New Strategy for the Persistent Photocatalytic Reduction of U(VI): Utilization and Storage of Solar Energy in K⁺ and Cyano Co‐Decorated Poly(Heptazine Imide)

Abstract

The photocatalytic conversion of soluble U(VI) into insoluble U(IV) is a robust strategy to harvest aqueous uranium, but remains challenging owing to the intermittent availability of solar influx and reoxidation of U(IV) without illumination. Herein, a dual platform based on K + and cyano group co-decorated poly(heptazine imide) (K-CN-PHI) is reported that can drive persistent U(VI) extraction upon/beyond light. K-CN-PHI achieves the photocatalytic reduction of U(VI) with a reaction rate of 0.89 min −1 , being 47 times greater than that over pristine carbon nitride (PCN). This system can further be triggered by light to form long-living radicals, driving the reduction of U(VI) in the dark for over 3 d. The flexible structural K + as counterions stabilize the electrons trapped by cyanamide groups, enabling the long lifetime of the generated radicals. The results collectively prove K-CN-PHI to be a novel and efficient photocatalyst enabling persistent U(VI) extraction around the clock, and broadening the practical applications of the photocatalytic extraction of U(VI).

Strong Electron Transfer in Covalently Integrating Cu(I)-Organic Frameworks Enabling Effective Radionuclide Capture

“…Generally, uranium­(VI) capturing the photogenerated e – would lead to the reduction to uranium­(IV), which can be commonly detected by XPS spectra. As shown in Figure b, the uranium captured by Cu-CTF-1 gave two peaks at 380.9 and 391.8 eV for U 4f 7/2 and U 4f 5/2 , respectively, which is clearly below those of uranium­(VI) commonly with corresponding values of 382.4 and 393.4 eV, − suggesting complete capture of uranium­(IV) on Cu-CTF-1 through photocatalytic reduction of uranium­(VI). At the same time, copper in uranium-loaded samples was found to give four strong peaks including Cu 3d 5/2 (955.2 eV), Cu 3d 3/2 (935.1 eV), plus two satellite peaks (Figure c), and higher binding energy, in contrast to the pristine sample of Cu-CTF-1, showing a typical Cu­(II) nature.…”

“…However, the adsorption ability can be largely enhanced with a removal efficiency up to 99.9% under visible light irradiation (Figure a,b), which is equal to 28.5-fold of that under adsorption, implying its ultrahigh photocatalytic ability. To the best of our knowledge, such high and complete photocatalytic efficiency observed in Cu-CTF-1 is still scarce in the literature, − since established photocatalysts often covered both adsorption and photocatalytic reduction. By contrast, Cu-CTF-2 and Cu-COF-1 can only result in 69.2% and 33% removal efficiency under light irradiation (Figure a,b), respectively, suggesting relatively low ability in the photocatalytic reduction of uranium­(VI).…”

“…For the former, the photocatalytic reduction of uranium is now viewed to be one of the best alternatives for the uranium recovery or capture from spent fuel or seawater, due to its low-cost nature from using visible light as driving power, high uptake efficiency, little pH dependence, big selectivity, and good recycle use. − In contrast to the adsorbent-based adsorption technique that commonly captures highly soluble and migratory UO 2 2+ ions, − the photocatalytic reduction route could largely reduce its soluble and migratory nature through U­(VI)-to-U­(IV) reduction with the formation of UO 2 precipitation. However, as a commonly encountered problem in all photocatalysts, the established photocatalysts for U­(VI) reduction are also seriously restricted by the weak spatial separation of electron–hole pairs that leads to the fast recombination of photogenerated electron–hole pairs, due to weak electron transfer in these materials, − and weak spatial function separation that will suffer the addition of sacrificial agent. In this regard, it is highly desirable to design a photocatalyst with strong electron transfer and spatial function separation.…”

Low-Temperature Synthesis of Cyano-Rich Modified Surface-Alkalinized Heterojunctions with Directional Charge Transfer for Photocatalytic In Situ Generation and Consumption of Peroxides

“…A smaller arc radius indicates lower impedance in charge transfer and higher efficiency in photogenerated charge transfer. 59 Notably, NCN/ ZnO demonstrated the lowest charge transfer impedance owing to the structural optimization of CN through Na + incorporation and the creation of a heterojunction with Table 3. Scope of the Benzylamine Oxidative Coupling Substrate in the NCN/ZnO Catalytic System a a Reaction conditions: CM (10 mmol), initiator (2 mmol), substrate (1 mmol), and photocatalyst (0.1 g) under white light (10 W) in the presence of an oxygen balloon at room temperature (18 h).…”

Photochargeable Semiconductors: in “Dark Photocatalysis” and Beyond

“…Polymeric carbon nitrides (PCN) represent the most promising alternative to Ti and Zn-based semiconductors for lightdriven electron storage. Several examples of PCN photocharging have been reported, [17,22,28,[37][38][39] with the most valuable results found for ionic carbon nitrides, that is PCN containing Na + , K + , or Li + . The presence of cations in the polymeric structure is proposed to be at the foundation of photocharging.…”