Defective UiO-66-NH2 Functionalized with Stable Superoxide Radicals toward Electrocatalytic Nitrogen Reduction with High Faradaic Efficiency

The electrocatalytic nitrogen reduction reaction (NRR) to NH3 is limited by low Faradaic efficiency (FE). Herein, defective UiO-66-NH2 functionalized with quite stable superoxide radicals (O-2(center dot)) is developed as a highly active NRR catalyst. The experimental and computational results show that one linker per Zr-6 node is missed and two Zr atoms are exposed in the defective UiO-66-NH2. One of the two exposed Zr atoms can stably adsorb O-2(center dot), and thus, a Zr-OO center dot site forms during the preparations without light excitation or postoxidation, while the other Zr atom is activated as an active site. The synergistic effects of the two Zr sites in the defective UiO-66-NH2 suppress hydrogen and hydrazine evolutions considerably. They are as follows: (i) due to repulsion of the proton on the active Zr site and stabilization of the proton on the Zr-OO center dot site, the active Zr site is unfavorable for the adsorption of the proton with a high energy barrier, which is the HER rate-determining step (RDS); (ii) under the assistance of the OO center dot of the Zr-OO center dot site, the first hydrogenation step of *N-2 (i.e., NRR RDS) on the active Zr site is promoted; and (iii) relying on the assistance of the OO center dot of the Zr-OO center dot site, the continuous hydrogenation of *NH2NH2 to produce NH3 on the active Zr site is spontaneously exothermic, whereas its desorption to hydrazine is blocked. Accordingly, an extremely high FE of similar to 85.21% has been realized along with a high yield rate of NH3 (similar to 52.81 mu g h(-1) mg(cat)(-1)). To the best of our knowledge, it is the highest FE that has been achieved in recent years. Radical scavenging treatment of the defective UiO-66-NH2 and detailed investigations of two categories of control samples further verify the favorable effects of the O-2(center dot) that closely correlates with the missed linkers on the performance of the NRR to NH3. This work opens a new way toward highly efficient NRR catalysts, i.e., stable radical-activating defective metal-organic frameworks.

Automated summary

This study achieved extremely high Faradaic efficiency and a high NH3 production rate by using defective UiO-66-NH2 catalyzed with stable superoxide radicals.