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

Abstract

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 (O2•) is developed as a highly active NRR catalyst. The experimental and computational results show that one linker per Zr6 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 O2•, and thus, a Zr-OO• 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• 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• of the Zr-OO• site, the first hydrogenation step of *N2 (i.e., NRR RDS) on the active Zr site is promoted; and (iii) relying on the assistance of the OO• of the Zr-OO• 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 ∼85.21% has been realized along with a high yield rate of NH3 (∼52.81 μg h-1 mgcat-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 O2• 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.