Anion (O, N, C, and S) vacancies promoted photocatalytic nitrogen fixation

Mild-condition nitrogen fixation using green solar energy, merely requiring a solar-to-NH3 (STA) efficiency of 0.1% for potential use, is a promising alternative to the Haber-Bosch process but remains a great challenge. The bottleneck lies in the ultra-low efficiency originating from the sluggish surface reaction involving 6e(-)/6H(+), specifically, the rate-limiting reductive activation of nitrogen. To meet this challenge, a common approach adopted by natural nitrogenase or artificial catalysts is to craft an active center (transition metal) of electron-rich nature, thus enriching the LUMO (lowest unoccupied molecular orbital) electron density of N-2 via the e(-) -> pi*-orbital (N) transition to facilitate the molecular nitrogen activation. Recent progress suggests that anion vacancies such as O, C, N, and S vacancies, which inherently bear excess electrons and coordinatively unsaturated metal ions, could be explored in promoting the molecular nitrogen activation in photocatalysis. In this review, we summarize the in depth anion vacancy-dominated nitrogen photo-fixation systems, and then focus on the mechanical understanding of how an anion vacancy kinetically affects the nitrogen reductive activation including N-N triple bond weakening, N-2 adsorption and activation energies, and kinetic pathways, and finally propose the future challenges and prospects of this emerging area.