XPS and XRD studies of fresh and sulfided Mo2N

Mo surface species of molybdenum nitride and their changes under sulfiding conditions were investigated by XRD and XPS. Mo2N was synthesized by temperature-programmed reaction of MoO3 with NH3. The decomposition of the Mo3d spectra gave a Mo3d doublet which corresponded to Moδ+ (2≤δ<4), Mo4+ and Mo5+ or Mo6+ species. The BE of the Mo species of passivated Mo2N shifted to higher energy level compared with the freshly prepared Mo2N due to the oxidation of Mo nitride during passivation. When Mo2N was contacted for 4 h with a 15% H2S–H2 mixture at 400°C, the XRD spectra did not reveal any new phase, which indicates a high stability of Mo2N against sulfidation, but XPS data showed the presence of sulfur, including S0 and S2− species, and a decrease of the N/Mo atomic ratio revealed some changes in surface composition. More than one monolayer of Mo2N was transformed to sulfide. It is probable that the oxygen incorporated during passivation reacted with sulfur and formed a thin layer of molybdenum sulfide on the Mo2N surface.