Photoemission of Adsorbed Xenon, X-ray Photoelectron Spectroscopy, and Temperature-Programmed Desorption Studies of H₂O on FeS₂(100)

Abstract

The reaction of H2O with the (100) crystallographic plane of pyrite, FeS2, has been investigated in the vacuum environment with photoemission of adsorbed xenon (PAX), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). TPD data indicate that H2O desorbs from FeS2(100) in a broad range of temperatures (150−300 K). XPS data suggests that the vast majority of H2O that initially adsorbs on pyrite at 79 K desorbs from pyrite during thermal annealing to 300 K. PAX, a technique that is sensitive to the short range order of a surface, has been used to elucidate the types of sites that are available on FeS2(100) for the binding of adsorbate. Within the resolution of our PAX data, the surface of FeS2(100) consists of at least two types of sites. It is proposed that these two types of sites are associated with the stoichiometric surface and defect (i.e., sulfur-deficient or anion vacancy) sites. PAX further suggests that at low adsorbate coverage, H2O predominately resides on defect sites. As the coverage of H2O is increased, defect sites become saturated and additional adsorption occurs on the less reactive stoichiometric surface.

Ab initio simulation of H2S adsorption on the (100) surface of pyrite

“…Instead, they should take place on defect sites, where dissociation of H 2 S is more likely to happen. 10 …”

“…Experimentally, dissociative adsorption has been observed at temperatures above 500 K and only for molecules previously physisorbed at low temperature on defect sites ͑presumably steps or kinks͒. 10,11 On the other hand, molecules adsorbed on normal sites on surface terraces do not dissociate even at high temperatures. Our calculations agree with this observation by predicting the dissociative chemisorption on the ͑100͒ surface ͑at coverage of 1/4 monolayer͒ to be indeed strongly endothermic (ϩ7.21 kcal/mol), and thus energetically very unfavorable.…”

“…5 However, the adsorption of other molecules also has relevance in the chemistry of pyrite. Experimentally a number of molecules sorbing on pyrite have been investigated including water, [6][7][8][9][10][11] oxygen, 9 H 2 S, 10,11 and heavy metals. 1,12 It has been shown, e.g., that at low temperature H 2 O can easily adsorb first on the defect sites, then on the stoichiometric surface, but desorbs quickly at room temperature.…”

“…[17][18][19] It has been shown, for instance, that adsorption of H 2 S or its dissociated anions can change the surface charge state 19 and the hydrophobicity of pyrite. 20 Guevremont et al 10,11 analyzed the thermal behavior of adsorbed H 2 S molecules in comparison with adsorbed H 2 O. It was found that adsorption on a stoichiometric surface occurs in a similar fashion for both H 2 O and H 2 S. However, while all H 2 O desorbs below 300 K, it has been proposed 11 that some H 2 S molecules still adsorbed on defect sites dissociate into H and SH fragments, which remain bound to the surface.…”

DFT-D2 simulations of water adsorption and dissociation on the low-index surfaces of mackinawite (FeS)

“…An understanding of the interactions of water with the surfaces of FeS may also provide detailed insights into the underlying reaction mechanisms of its facile oxidation, [36][37][38] which remains a major problem that severely limits the potential applications of these materials. Earlier experimental [38][39][40][41][42] and theoretical [43][44][45] reaction of iron sulfides with water have focused extensively on the pyrite surfaces. The catalytic dissociation of water on the low-index surfaces of violarite, FeNi 2 S 4 , has also been reported recently using density functional theory (DFT) methods.…”