Effects of surface imperfections on the binding of CH3OH and H2O on FeS2(100): using adsorbed Xe as a probe of mineral surface structure

Studies are presented that investigate the adsorption and binding of CH3OH and H2O on the atomically clean (100) crystallographic plane of pyrite, FeS2. Temperature programmed desorption suggests that both reactants adsorb molecularly at 90 K and desorb thermally between 170 and 400 K depending on the surface coverage. Photoemission of adsorbed xenon (PAX) suggests that the surface of pyrite is heterogeneous and contains a significant fraction of defect sites that are believed to be, at least in part, anion vacancy or sulfur-deficient sites. An upper limit of 0.2 is proposed for the fraction of surface sites that are defects on FeS2(100). PAX indicates that these defect sites at low adsorbate coverage serve as the exclusive binding sites for H2O and CH3OH adsorbate, We speculate, on the basis of our ability to interpret PAX data for pyrite, that PAX may be of use for understanding the effect of short range order on adsorbate binding on other complex mineral surfaces. On the basis of high resolution electron energy loss spectroscopy, it is found that some dissociation of the adsorbate occurs on the pyrite. Vibrational data obtained with this technique suggests that Fe-O species result from the adsorbate decomposition, After saturation of the defect sites, further molecular adsorption is accommodated on the less reactive surface that we postulate is largely disulfide, the characteristic structural group of pyrite. (C) 1997 Elsevier Science B.V.