Redox Properties of Iron Sulfides: Direct versus Catalytic Reduction and Implications for Catalyst Design

Abstract In electrocatalysis we seldom think about the competing direct reduction reactions that may happen alongside catalytically mediated reduction‐ with direct redox chemistry often happening slower but in competition with, catalysis. One class of compounds of interest from this perspective are iron sulfides. In addition to being structurally similar to many metalloproteins, iron sulfides are also among nature‘s strongest chemical reductants and reported to act as catalysts for key chemical reactions including proton, nitrite, and nitrate reduction. It is important that iron sulfides can act as catalysts because they are also strong enough reductants to mediate some of the same reactions directly. This is paradoxical because in order to be a catalyst for reduction, an iron sulfide cannot also be oxidised. To investigate this phenomenon further, we assembled a test set of iron sulfides spanning both amorphous iron sulfide (FeSam) as well as the crystalline iron sulfides greigite, pyrite, and troilite. These were used to explore the relationship between direct reduction and catalysis of a reduction reaction with a secondary electron source, NO2− was chosen as a test substrate. The trends in direct reduction followed the least stable material (FeSam) to the most stable material (FeS2). Of the phases studied, troilite (FeS) showed the largest difference between direct and catalytic reduction, however amorphous iron sulfide showed the greatest selectivity for NH3/NH4+ production as both a direct reductant and a catalyst.