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

Abstract

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.