21 Often, the first formed precipitates are very reactive, and not as well characterised. 21c However, variations in conditions can promote the formation of other phases. This becomes more ordered with time under anoxic and reducing conditions, 21 slowly forming crystalline mackinawite, greigite and then pyrite (Figure 2). In most geological conditions, the first precipitated phase is a nanoparticulate and amorphous hydrated mackinawite-like material, 20 often referred to as amorphous iron sulfide FeS am. The structures of four well known and crystallographically determined iron sulfides, mackinawite, 16b greigite, 17 pyrite, 18 and troilite 19 are given in Figure 1e-h. 10 In such cases, transient, or reversible direct reduction is likely associated with a dissolution and reformation process may be a key part of the catalyst mechanism. Further, it is increasingly clear that not all catalysts function in the same way some catalysts function through a self-healing reaction where the catalyst reforms as part of the catalytic cycle. 9 However, the reduction chemistry and the ability to transfer electrons and the relative kinetics/timescales of these reactions are undoubtedly important in catalyst design because key target substrates (for example N 2 and CO 2) are often very difficult to chemically reduce. This is, in part, because catalysis is often thought about as a surface sorption process. We do not usually think about reduction chemistry mediated by iron sulfide and catalytic chemistry as having a direct relationship. A good example of this is chemolithotrophic life which draws its energy from the oxidation of iron sulfides coupled to the chemical reduction of CO 2. In some cases, biological processes also uses sacrificial or direct redox chemistry to mediate small molecule transformations. It is often overlooked that biology does not just use the iron sulfides as catalysts in reactions that involve molecules such as CO 2 and N 2. Getting electrons to and from a catalyst and then to the substrate is a key part of many catalytic reduction reactions, and in nature these electron transfer processes are also achieved with iron sulfur clusters of ferredoxins which have been noted to have exceptionally fast electron transfer rates as there are little geometric changes associated with oxidation and reduction. Significantly, this enzyme also catalytically reduces other key substrates, including nitrite (NO 2 −), 4 nitrous oxide (N 2O), 5 and carbon dioxide (CO 2), 6 implying there is not a significant substrate selectivity in this enzyme. 15 Lower row: Comparison of the structure of the representative (e) mackinawite (FeS m), 16 (f) greigite (Fe 3S 4), 16a, 17(g) pyrite (FeS 2), 16a, 18 (h) and troilite (FeS). Upper row: Comparison of the key active sites of (a) nitrogenase, 12 (b) carbon monoxide dehydrogenase, 13 (c) hydrogenase, 14 (d) and ferredoxin. Of the phases studied, troilite (FeS) showed the largest difference between direct and catalytic reduction, however amorphous iron sulfide showed the greatest selectivity for NH 3/NH 4 + production as both a direct reductant and a catalyst. The trends in direct reduction followed the least stable material (FeS am) to the most stable material (FeS 2).
These were used to explore the relationship between direct reduction and catalysis of a reduction reaction with a secondary electron source, NO 2 − was chosen as a test substrate. To investigate this phenomenon further, we assembled a test set of iron sulfides spanning both amorphous iron sulfide (FeS am) as well as the crystalline iron sulfides greigite, pyrite, and troilite. This is paradoxical because in order to be a catalyst for reduction, an iron sulfide cannot also be oxidised. 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. 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.
One class of compounds of interest from this perspective are iron sulfides. 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.
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