Unraveling Polymorphic Pyrrhotite Electrochemical Oxidation by Underlying Electronic Structures

Metal sulfide oxidation is a common, yet poorly understood, phenomenon that significantly affects surface properties. In this paper, we studied the electrochemical oxidation of polymorphic pyrrhotites (Fe_1–xS) to gain insights into the relationship between their electrochemical oxidation rate and electronic structures. Surface composition of oxidized pyrrhotites, as shown by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), suggested that one key step for pyrrhotite oxidation is the outward diffusion of metal cations to form polysulfide and oxides. This diffusion process involves the rupture of Fe–S bonds and, hence, depends on the Fe–S bond strength. According to the ToF-SIMS, the Fe–S bond strength in the defective layer (>100 nm), the layer right underneath the polysulfide layer (∼20 nm), was modified by the incorporation of oxygen atoms, which mainly existed in the form of OH^– and H₂O. It was found that oxygen anions are much more abundant in the defective layer of monoclinic pyrrhotite (Fe₇S₈) than that of hexagonal pyrrhotite (Fe₉S₁₀), resulting in a much weaker Fe–S bond with the former than the latter. The oxygen abundance difference can be explained by their electronic structures. Density functional theory (DFT) calculation showed that monoclinic pyrrhotite (Fe₇S₈) get a higher Fe 3d and S 3p band center than hexagonal pyrrhotite (Fe₉S₁₀). Therefore, monoclinic pyrrhotite could incorporate oxygen atoms easier than hexagonal pyrrhotite. This presented a clear relation between polymorphic pyrrhotite electronic structures and their electrochemical oxidation rate and also fundamentally explained why the sulfides with a slight bulk metal–sulfur bond strength difference could demonstrate a significant oxidation rate difference.