Nickel Incorporation into Fresh Iron Sulfide Precipitates Modulates Selectivity in Nitrite, Carbon Dioxide, and Proton Reduction

Nickel and iron sulfide minerals are ubiquitous in natural environments and are central to Earth's biogeochemical cycles. Both metals are found in metalloenzyme active sites that catalyze key redox reactions and have been proposed as protocatalysts in prebiotic chemistry. Here, we investigate how coprecipitated Ni-Fe sulfides differ in their ability to catalyze and directly reduce CO₂, NO₂^-, and protons and how these differences relate to their structural and compositional properties. Sulfides were synthesized by direct precipitation and characterized by X-ray absorption spectroscopy (XAS) at the Ni and Fe K-edges. At low Ni²⁺:Fe²⁺ ratios, nickel substitutionally incorporates into mackinawite (FeS_<i>m</i>) to form [Ni_<i>n</i>Fe_<i>m</i>]S_<i>x</i> phases, while high Ni²⁺ concentrations suppress FeS formation and favor NiS₂-like structures. Increasing Ni content enhances the CO₂ and proton reduction as well as direct NO₂^- reduction, whereas Fe-rich samples favor NO₂^- reduction by Fe²⁺ while suppressing H₂ evolution. These contrasting behaviors indicate distinct mechanisms for proton, CO₂, and nitrite reduction. The observed reactivity trends mirror biological metal utilization, where nickel catalyzes CO₂ fixation and hydrogen evolution, underscoring the potential role of mixed Ni-Fe sulfides as protoenzymatic catalysts in early metabolic networks.