X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively) and scanning tunneling microscopy (STM) were used to observe the initial oxidation of pyrite surfaces in air. The results show the growth of oxide-like oxidation products, with minor contributions from sulfate. UPS shows a decrease in the density of electronic states in the uppermost valence band of pyrite, corresponding to oxidation of surface Fe2+. This allows reliable interpretation of STM images, which show that initial surface oxidation of Fe2+ proceeds by growth of oxidized patches. The borders of oxidized patches contain small segments oriented in the (110) and (100) directions. STM of as-received pyrite cube surfaces, oxidized in air for years, also show the importance of the (110) crystallographic directions, on the surface, in controlling reaction progress.
A model in which oxidation probabilities for Fe2+ surface sites are proportional to the number of nearest-neighbor oxidized (Fe3+ ) sites was tested using a Monte Carlo approach and reproduces the surface patterns observed in STM.
An oxidation mechanism consistent with the XPS, UPS, STM, and Monte Carlo results is proposed. The rate constant for electron transfer from surface-exposed pyrite Fe2+ to O2 is small. Electron transfer is more rapid from pyrite Fe2+ to Fe3+ present on the surface as an oxidation product, such as in the patches we observed. Fe2+ in oxide is a better reductant than Fe2+ in pyrite, so electron transfer to O2 from the oxide is also fast. However, this two-step mechanism is faster overall only if electron transfer to the surface oxide patches is irreversible (e.g., because of S2 oxidation or electron hopping within the surface oxide patches). Cycling of Fe between the Fe2+ and Fe3+ forms, particularly along borders between oxidized and unoxidized areas, is thus a key feature of the pyrite oxidation mechanism. An understanding of the surface electronic and band structure aids definition of the redox potentials of electrons in various surface states. Rates of electron transfer from these states to O2 are estimated using a kinetic theory of elementary heterogeneous electron transfer.