Ti³⁺-mediated MIL-125(Ti) by metal substitution for boosting photocatalytic N₂ fixation

Photocatalysis, which uses sunlight, N₂ and H₂O to produce NH₃, is a more sustainable approach to N₂ fixation than the Haber-Bosch process. However, its efficiency is severely limited by the difficulty of activating NN bonds. This work presents metal (M = Cu, Fe, V)-substituted MIL-125(Ti) (MIL-(MTi)) for photocatalytic N₂ fixation without using any sacrificial agents. Structural characterizations reveal that the active sites including oxygen vacancies (OV) and Ti³⁺ species are formed by the resulting crystal distortion due to the partial substitution of Ti⁴⁺ by other metal ions (Cu⁺, Fe²⁺, V³⁺) in MIL-125(Ti). MIL-(CuTi) possesses a larger number of OV and Ti³⁺ compared to MIL-(FeTi) and MIL-(VTi) due to the larger valence difference between Cu⁺ and Ti⁴⁺. These active sites not only promote the adsorption and activation of N₂ and H₂O, but also facilitate the photogenerated charge mobility. Photogenerated holes oxidize H₂O to produce O₂ and H⁺. Photogenerated electrons reduce N₂ activated on Ti³⁺ sites by combining with H⁺ to form NH₄⁺. Therefore, MIL-(CuTi) shows the highest NH₄⁺ production rate 46.5 µmol·h^-1·g^-1, which is much higher than that (1.2 µmol·h^-1·g^-1) of the pristine MIL-125(Ti). This work provides a new insight into rational design for artificial N₂ fixation systems by the construction of the active site.