New insights on how metal clusters in MIL-68-NH2 influence the photocatalytic nitrogen fixation

Abstract

Three isostructural MIL-68(M)-NH₂ frameworks (M = In, Fe, or Al) featuring different metal centers were synthesized to investigate their photocatalytic activities for nitrogen fixation. The influence of the metaloxo clusters on the crystalline structures, morphologies, and optical characteristics was examined. MIL-68(In)-NH₂ (MNI) with one-dimensional microrod structures has a larger surface area than bulk MIL-68(Fe)-NH₂ (MNF) and spherical MIL-68(Al)-NH₂ (MNA), and achieves an ammonia generation rate (140.34 μmol g_cat⁻¹ h⁻¹) ca. 3.4 and 1.6 times higher than those of MNF (37.25 μmol g_cat⁻¹ h⁻¹) and MNA (87.5 μmol g_cat⁻¹ h⁻¹), respectively. The superior performance can be attributed to its high crystallinity, strong coordination between the In−oxo clusters and ligands, and microrod morphology, which enhance electron transfer, improve mass transport, and extend visible light absorption. Moreover, MNI shows greater stability in ethanol aqueous solution than other sacrificial reagents, such as methanol, potassium sulfite, and triethanolamine. While slight surface corrosion of the microrods occurs in methanol and potassium sulfite solutions, complete collapse is observed in triethanolamine.