Surface species and the stability of gamma-Mo2N

The surface species of passivated and nonpassivated Mo2N end their stability at elevated temperature in various environments have been investigated by thermal programmed desorption with mass spectrometry (TPD-MS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results demonstrate that two types of NHx and H species remain on fresh Mo2N, and a small amount of weakly adsorbed NHx species is present on passivated Mo2N. During heating at 1000 degrees C in He, fresh Mo2N transforms first from gamma-Mo2N to beta-Mo2N and then decomposes to metallic Mo. No obvious change of the crystalline phase occurs for passivated Mo2N before its decomposition or reduction. The XRD spectra showed a high stability of the Mo2N bulk structure against sulphidation. XPS data indicated that some changes of the surface composition occurred in sulphided Mo2N. Oxygen incorporated during passivation reacted with sulphur and formed a thin layer molybdenum of sulphide covering the nitride surface.