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

Photocatalysis, which uses sunlight, N 2 and H 2 O to produce NH 3 , is a more sustainable approach to N 2 fixation than the Haber-Bosch process. However, its efficiency is severely limited by the difficulty of activating N N bonds. This work presents metal (M = Cu, Fe, V)-substituted MIL-125(Ti) (MIL-(MTi)) for photocatalytic N 2 fixation without using any sacrificial agents. Structural characterizations reveal that the active sites including oxygen vacancies (OV) and Ti 3+ species are formed by the resulting crystal distortion due to the partial substitution of Ti 4+ by other metal ions (Cu + , Fe 2+ , V 3+ ) in MIL-125(Ti). MIL-(CuTi) possesses a larger number of OV and Ti 3+ compared to MIL-(FeTi) and MIL-(VTi) due to the larger valence difference between Cu + and Ti 4+ . These active sites not only promote the adsorption and activation of N 2 and H 2 O, but also facilitate the photogenerated charge mobility. Photogenerated holes oxidize H 2 O to produce O 2 and H + . Photogenerated electrons reduce N 2 activated on Ti 3+ sites by combining with H + to form NH 4 + . Therefore, MIL-(CuTi) shows the highest NH 4 + 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 N 2 fixation systems by the construction of the active site.