Chemistry of SO2, H2S, and CH3SH on Carbide-Modified Mo(110) and Mo2C Powders: Photoemission and XANES Studies

Abstract

The chemistry of SO2, H2S, and CH3SH on carbide-modified Mo(110) and powders of bulk Mo2C was studied using synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES). These studies reveal that molybdenum carbide is very reactive toward sulfur-containing molecules, being able to break S−O, S−H, and S−C bonds at temperatures below 300 K. Upon adsorption of sulfur dioxide on molybdenum carbide at 150 or 300 K, there is dissociation of the adsorbate into S and O, and also formation of SO3 or SO4 by reaction with O adatoms or disproportionation of SO2. The adsorbed SO3 and SO4 species decompose upon heating to 500 K leaving a heavily sulfided and oxidized carbide. In the case of H2S adsorption, the cleavage of the first H−S bond occurs at 80−100 K and the resulting HS intermediate transforms into S at temperatures below 250 K. Finally, for CH3SH on MoCx, the breaking of the C−S bond takes place at temperatures between 250 and 400 K. From 100 to 350 K, CH3S and a second S-containing species (CHyS) coexist on the surface of the carbide. A comparison to results reported in the literature indicates that the chemical reactivity of molybdenum carbide is similar to that of metallic Mo and much larger than that of molybdenum sulfide. The differences in the chemical reactivities of molybdenum carbide and molybdenum sulfide correlate well with changes in the electronic properties of Mo in these systems.