The interaction of pyrite {100} surfaces with O-2 and H2O: Fundamental oxidation mechanisms

The interaction of gaseous O-2, H2O, and their mixtures with clean {100} su rfaces of pyrite (FeS2) were investigated in ultra-high vacuum using scanni ng tunneling microscopy and spectroscopy (STM-STS), ultraviolet photoelectr on spectroscopy (UPS) and ab initio calculations. He I UPS spectra of O-2 e xposed surfaces show that the density of states decreases at the top of the valence band but increases deeper in the valence band. These changes indic ate oxidative consumption of low binding energy electrons occupying danglin g bond surface states localized on surface Fe atoms, and the formation of F e-O bonds. No such changes in the valence band spectra are observed for pyr ite surfaces exposed to H2O. However, UPS spectra of surfaces exposed to mi xtures of O-2 and H2O demonstrate that the combined gases more aggressively oxidize the surface compared to equivalent exposures of pure O-2. Atomical ly resolved STM images of O-2 and O-2-H2O exposed surfaces show discrete ox idation "patches" where reacted surface Fe sites have lost surface state de nsity to the sorbed species. STS spectra show the removal of highest occupi ed and lowest unoccupied surface state density associated with dangling bon d states consistent with the interaction of sorbates with surface Fe sites. Ab initio cluster calculations of adsorption energies and the interaction of O-2 and water species with the surface show that O-2 dissociatively sorb s and H2O molecularly sorbs to surface Fe. For the mixtures, the calculatio ns indicate that H2O dissociatively sorbs when O-2 is present on the surfac e. Charge population analyses also show that the surface S sites become mor e electropositive in this environment which should allow for easier formati on of S-O surface bonds, thus promoting the production of sulfate during ox idation.