5:15 PM - 7:15 PM
[U12-P10] Site-specific chemical reactions on transition metal sulfide surfaces: MoS2 and FeS2 studied by XPS
Keywords:transition metal sulfide, surface, chemical reaction, X-ray photoelectron spectroscopy
Transition metal sulfides (TMS) have been widely used as catalysts in various chemical reactions. In particular, TMSs play an important role in the petroleum refining processes including the hydrodesulfurization and hydrodenitrogenation reactions. Recently, TMSs are attracting much attention in electrochemical reactions such as hydrogen evolution reactions and oxygen evolution reactions. In addition, TMSs have been proposed as important catalysts in prebiotic chemistry. Some sulfides have the ability to catalyze the conversion of inorganic molecules to organic molecules, and play an important role in hypotheses regarding the origin of life. In particular, these minerals supply electrons and promote the reduction of CO etc. in the environment of deep-sea hydrothermal vents.
In this study, we prepared well-defined surfaces of typical transition metal sulfides, i.e., molybdenum disulfide (MoS2) and pyrite (FeS2), and investigated their electronic states and chemical properties. We prepared the edge surface by cutting MoS2 single crystals with ultrashort pulse laser, and the electronic states of the edge surface are selectively observed by XPS. The valence band spectrum of the edge showed the metallic states. In addition, the Mo 3d XPS spectra show the components corresponding to the coordination unsaturated Mo atoms. We have elucidated that water molecules are dissociated only on the edge surface at room temperature and the under-coordinated Mo atoms of the edge are the active sites for water dissociation. In addition, CO2 is dissociated on the MoS2 edge surface, but not on the basal surface. In the case of FeS2, the S-vacancy on FeS2 surface plays an important role in NO dissociation. The XPS analysis of NO adsorbed at 300 K on low/high-defected FeS2(100) surface was conducted. The results have shown that after exposure to 2 torr of NO at 300 K, the low-defected surface retained partially unoxidized iron, while only fully oxidized iron was observed on the high-defected surface; S-vacancy facilitated the dissociation of NO on the FeS2(100) surface. In addition, two oxygen species remained on the surface, and a small amount of nitrogen remained on the surface. Based on these results, the oxygen species remain on the surface, but nitrogen atoms are recombined into N2, following desorption from the surface.
In this study, we prepared well-defined surfaces of typical transition metal sulfides, i.e., molybdenum disulfide (MoS2) and pyrite (FeS2), and investigated their electronic states and chemical properties. We prepared the edge surface by cutting MoS2 single crystals with ultrashort pulse laser, and the electronic states of the edge surface are selectively observed by XPS. The valence band spectrum of the edge showed the metallic states. In addition, the Mo 3d XPS spectra show the components corresponding to the coordination unsaturated Mo atoms. We have elucidated that water molecules are dissociated only on the edge surface at room temperature and the under-coordinated Mo atoms of the edge are the active sites for water dissociation. In addition, CO2 is dissociated on the MoS2 edge surface, but not on the basal surface. In the case of FeS2, the S-vacancy on FeS2 surface plays an important role in NO dissociation. The XPS analysis of NO adsorbed at 300 K on low/high-defected FeS2(100) surface was conducted. The results have shown that after exposure to 2 torr of NO at 300 K, the low-defected surface retained partially unoxidized iron, while only fully oxidized iron was observed on the high-defected surface; S-vacancy facilitated the dissociation of NO on the FeS2(100) surface. In addition, two oxygen species remained on the surface, and a small amount of nitrogen remained on the surface. Based on these results, the oxygen species remain on the surface, but nitrogen atoms are recombined into N2, following desorption from the surface.