Enhanced Photocatalytic Nitrogen Fixation of an In Situ-Constructed Fe Mesh-Based La/TiO₂/MIL-88A Heterojunction

Abstract

Photocatalytic nitrogen (N2) fixation provides a clean and sustainable route to achieve an efficient conversion of N2 to ammonia (NH3) under mild conditions. However, the photocatalytic N2 fixation performance remains very low due to the rate-limiting reductive activation of a robust N≡N bond, high carrier recombination rate, and low solubility of N2 in water. Herein, a novel heterostructure has been successfully constructed based on the coupling of a metal–organic framework (MIL-88A) with a doped semiconductor (La/TiO2). Meanwhile, a gas-phase photoreactor for N2 reduction using water vapor to provide a proton source is developed. MIL-88A(Fe) possesses efficient visible light absorption ability, with abundant Fe(III) unsaturated sites on its surface, which is conducive to achieving advanced oxidation and reduction reactions as a photocatalyst. The synergistic effects of MIL-88A(Fe) and La/TiO2 further accelerate the transfer and separation of photogenerated electrons and holes and improve the light absorption performance of the catalyst. Meanwhile, the composite catalyst surface has abundant oxygen vacancies, which can significantly reduce the activation energy of N2 and regulate the transfer path of photogenerated charges, effectively extending the life of the photogenerated electron–hole pairs. In addition, La can form an impurity energy level, further optimizing the carrier's N2 activation ability. The results show that the construction of the heterojunction and La doping made the composite photocatalyst La/TiO2/MIL-88A/Fe mesh with good photoresponsiveness, and the synergistic effects among the components suppressed the complexation of photogenerated carriers, increasing the N2 fixation efficiency by about 11.6 times compared with the MIL-88A/Fe mesh. Furthermore, an anticipated mechanism for N2 photofixation is proposed.