Regulating Electron Transfer in a POM-Based MOF Photocatalyst with Dual Active Sites for Enhanced N₂ Oxidation

Nitric acid (HNO₃) serves as a vital industrial raw material and ranks among the most widely manufactured chemicals. Recently, photocatalytic systems have been explored as an environmentally friendly pathway for direct N₂ utilization in oxidation reactions. However, progress has been hindered by the high activation barrier of the N≡N bond and severe electron-hole recombination in catalysts. Here, we highlight that polyoxometalates (POMs)-based metal-organic frameworks (MOFs) (Mo₇₂Cr₃₀/UiO-66) enable highly efficient HNO₃ synthesis, owing to a dual-site mechanism. The results reveal that oxygen vacancies on UiO-66 capture electrons from Mo₇₂Cr₃₀, inducing adjacent metals to form Zr³⁺ with unpaired electrons, which transfer to N₂ antibonding orbitals, thereby lowering the activation barrier. Furthermore, the holes enrich on the Mo₇₂Cr₃₀ surface to oxidize water for generating strong oxidative reactive oxygen species hydroxyl radical (•OH), thus facilitating HNO₃ production. The distinct site synergy of Mo₇₂Cr₃₀ and UiO-66 for N₂ activation results in an exceptional activity of 646.3 μg g^-1 h^-1, which far surpasses the performance of Mo₇₂Cr₃₀ and UiO-66 alone by approximately 18-fold and 6-fold, respectively. Our study offers a novel design vision for photocatalytic materials and presents a promising approach for advancing sustainable artificial N₂ fixation.