Nitrate and Proton Reduction Catalyzed by Iron-and Nickel-Sulfides under Conditions Simulating Hydrothermal Vents

The reduction of small molecules, such as protons (H +), nitrate (NO3-), and nitrite (NO2-) to dihydrogen (H2) and ammonium (NH4 +) are essential reactions for life. Extant enzymes such as hydrogenase, nitrogenase, nitrate reductase, and nitrite reductase catalyze these critical reactions with iron-sulfur (FeS), iron-nickel-sulfur (FeNiS), or nickel-sulfur (NiS) clusters as the active sites or electron transfer cofactors. FeS, FeNiS, and NiS minerals are abundant in hydrothermal vents and have been suggested to function as early catalysts for these reduction reactions, serving as precursors to the enzymes that catalyze them today. Here, we show that a suite of FeS, NiS, and FeNiS compounds that simulate minerals can serve as catalysts for the reduction of H + and NO3in an electrochemical system that mimics the potentials expected in hydrothermal vent environments. Synthetic FeS and FeNiS compounds were found to reduce NO3⁻ to NO2and NH4 + and protons to H2, while NiS compounds exclusively catalyzed H + reduction to H2. Specific activities for NO3⁻ reduction to NH4 + by FeS and FeNiS compounds ranged from 70 to 193 nmol NH4 + /mgcat/min at-0.4 V vs the reversible hydrogen electrode (RHE) with Faradaic efficiencies for NH4 + formation of 50-75% at 25℃. NiS compounds exhibited H2 formation specific activities ranging from 559 to 1509 nmol H2/mgcat/min with 100% H2 Faradaic efficiency at 25℃. Together, these findings show that FeS, FeNiS, and NiS compounds can act as catalysts for the reduction of NO3⁻ to NO2and NH4 + , and H + to H2 under conditions consistent with hydrothermal vent environments.