Oxidation of Nanodispersed MoS2 in Ambient Air: The Products and the Mechanistic Steps

Oxidation of finely divided molybdenum sulfide in ambient air has been studied for air exposure times from 10 min to 1 year to clarify the nature of the reaction products and the mechanistic steps. At the initial steps, for air exposure times from several minutes to several hours, rapid oxidation of MoS2 edges occurs with simultaneous formation of hydroxyl species and surface disulfide S-2(2-) moieties as attested by H-1 NMR, X-ray photoelectron spectra, and temperature-programmed reduction. Prolonged air exposure of MoS2 nanodispersions leads to deep oxidation. According to the results of X-ray absorption spectroscopy, UV-visible, and electron paramagnetic resonance spectroscopies, the main oxidation products are soluble paramagnetic molybdenum blue species and sulfuric acid. As shown by EXAS fitting, the major product is oxo-bridged dimolybdenyl Mo(V, VI) species. Ambient moisture plays an important role in the oxidation process as it contributes to the formation of sulfuric acid which leads to liquescence of the material and to deep oxidation without formation of a protective passivation layer.