Thermal chemistry of H2S and H2O on the (100) plane of pyrite:: Unique reactivity of defect sites

The interaction of a natural face of FeS2 (100), cleaned in ultra-high vacuum (UHV), with H2O and H2S has been investigated with X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and the photoemission of adsorbed xenon (PAX). PAX is sensitive to the short-range order of the surface and allows the effects of defects on the surface reactivity of FeS2(100) to be studied. PAX results suggest that both H2S and H2O bind most strongly to defect sites that we propose are, at least in part, sulfur anion vacancy sites. Whereas the majority of H2O adsorbate desorbs from these sites in the temperature interval of 200-300 K, H2S dissociates upon heating to 500 K into adsorbed surface hydrogen, S, and SH. This dissociation occurs on defect sites that then release part of the dissociation fragment, which is thought to be surface hydrogen, onto other regions of the pyrite surface that are proposed to be stoichiometric FeS2. Heating to 600 K causes further reaction of S containing dissociation fragments with sulfur-deficient sites to form new surface sites that resemble FeS2. The results also suggest that surface hydrogen dissolves into the pyrite bulk upon heating to 600 K.