Sulfur-Bridged Iron and Molybdenum Catalysts for Electrocatalytic Ammonia Synthesis.

Carbon zero electrocatalytic nitrogen reduction reaction (NRR), converting N to NH under ambient temperature and pressure, offers a sustainable alternative to the energy-intensive Haber-Bosch process. Nevertheless, NRR still faces major challenges due to direct dissociation of the strong N≡N triple bond, poor selectivity, as well as other issues related to the inadequate adsorption, activation and protonation of N. In nature's nitrogen fixation, microorganisms are able to convert N to ammonia at ambient temperature and pressure, and in aqueous environment, thanks to the nitrogenase enzymes. The core NRR performance is achieved with sulfur-rich Fe transition metal clusters as active site cofactors to capture and reduce N, with optimum performance found for Fe-Mo clusters. Because of this reason, artificial analogs in Fe-Mo coordination chemistry have been explored. However, the studies of sulfur coordinated Fe, Mo catalysts for electrocatalytic ammonia synthesis are scarce. In this review, the recent progress of Fe-Mo sulfur-bridged catalysts (including sulfur-coordinated single-site catalysts in carbon frameworks and MoS-based catalysts) and their activities for the ammonia synthesis from nitrate reduction reaction (NO RR) and nitrogen reduction reaction (NRR) are summarized. Further existing challenges and future perspectives are also discussed.