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Iron sulfides are key materials in metalloprotein catalysis. One interesting aspect of iron sulfides in biology is the incorporation of secondary metals, for example, Mo, in nitrogenase. These secondary metals may provide vital clues as to how these enzymes first emerged in nature. In this work, we examined the materials resulting from the coprecipitation of molybdenum with iron sulfides using X-ray absorption spectroscopy (XAS). The materials were tested as catalysts, and direct reductants using nitrite (NO2–) and protons (H+) as test substrates. It was found that Mo will coprecipitate with iron as sulfides, however, in distinct ways depending on the stoichiometric ratios of Mo, Fe, and HS–. It was observed that the selectivity of reduction products depends on the amount of molybdenum, with the presence of approximately at 10% Mo optimizing ammonium/ammonia (NH4+/NH3) production from NO2–and minimizing competitive hydrogen (H2) formation from protons (H+) with a secondary reductant.