Tailoring Defect Density via Ultraviolet Illumination in UiO-66 for Efficient Photocatalytic Nitrogen Fixation

Modulation of the electronic properties and active sites of catalysts by defect engineering provides an effective way to improve photocatalytic activity in nitrogen fixation. Photoillumination treatment has been reported as an effective approach to fabricating defects on a substrate. Herein, UiO-66 as one of the most typical MOFs has been selected as a substrate to modulate its organic linker defects via controlling the UV irradiation conditions and solvent composition. With the removal of linkers in the MOFs, the pore size along with the specific surface area has been greatly increased. Moreover, the photocatalytic activity of linker-defective UiO-66 in the N₂ fixation to produce ammonia has also been greatly improved, with an ammonia productivity of 47.65 μmol h^-1 g^-1, which is nearly 2.5-fold enhanced compared to that of pristine UiO-66. The enhancement may result from the enriched reactive sites with the formation of linker defects. Specifically, the defects optimize the electronic structure around the coordinatively unsaturated Zr sites, which facilitates the injection of its d-orbital electrons into the π* antibonding orbitals of N₂. The initial hydrogenation of the N₂ step is the rate-determining step, and the conversion to ammonia is preferentially carried out along the associative alternating pathway at the exposed Zr site. This work provides a new approach for the controllable construction of defects, as well as is expected to offer a reference for the precise design and molecular-level modulation of ammonia synthesis photocatalysts.