Tuning Catalyst Selectivity for Ammonia vs Hydrogen: An Investigation into the Coprecipitation of Mo and Fe Sulfides

Abstract

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 (NO 2 − ) 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 (NH 4 + /NH 3 ) production from NO 2 − and minimizing competitive hydrogen (H 2 ) formation from protons (H + ) with a secondary reductant.

Abiotic transformations of nitrogen mediated by iron sulfides and related species from early Earth to catalyst design

“…96 Additionally, the incorporation of ∼10% of molybdenum into the amorphous mackinawite structure increased the selectivity of nitrite reduction to ammonia. 97…”

“…The Mo/FeS x samples with 10%Mo and 90% iron show the highest ammonia production and this is an interesting result because a similar ratio is found in nitrogenase. 96,97…”

“…Thus, key thermodynamic factors play a role in the selectivity of products in nitrite reduction mediated by Fe x S y materials, including the concentration of the substrate relative to the catalyst, the amount of a secondary metal incorporated into the Fe x S y structure as well as the electron source and its chemistry. 96,97…”