Binding of oxygen on vacuum fractured pyrite surfaces: Reactivity of iron and sulfur surface sites

Synchrotron radiation excited photoelectron spectroscopy (SXPS) has been used to study the interaction of oxygen with vacuum fractured pyrite surfaces. Especially valence band spectra obtained with 30 eV photon energy were analyzed to provide a mechanism of the incipient steps of pyrite oxidation. These spectra are far more sensitive to the oxidation than sulfur or iron core level spectra. It is shown that oxygen is adsorbed on Fe(II) surface sites restoring the octahedral coordination of the Fe(II) sites. This process leads to the removal of two surface states in the valence band which are located at the low and high binding energy sides of the outer valence band, respectively. The existence of these surface states which have been proposed by calculations is experimentally proven. Furthermore, it is shown, that the sulfur sites are more reactive than expected. Sulfite like species are already formed after the lowest oxygen exposure of 10 L. This oxidation occurs at sulfur sites neighboring the Fe(II) surface sites. Oxidation of the S2 − surface sites which were considered as the most reactive species in former studies is second. No iron(III) oxides are formed during oxygen exposure, supporting the assumption that water plays an important role in the oxidation mechanism of pyrite surfaces.

► Study of valence band and S 2p spectra of fractured pyrite surfaces exposed to O2 ► 30 eV valence band spectra by far most surface sensitive ► Detection of a surface state at valence band edge and its removal by O2 adsorption ► Oxidation of sulfur detectable despite absence of water