Unraveling Polymorphic Pyrrhotite Electrochemical Oxidation by Underlying Electronic Structures

Abstract

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 (Fe1–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 def...