MO-99(BETA(-))TC-99 SPECTROSCOPY OF MOS2, LITHIATED MOS2, CO0.5MOS2, AND MOS2 SURFACE SPECIES

Nuclear quadrupole interactions (NQI) were measured for several molybdenum-sulfur compounds by rime differential perturbed angular correlation (TDPAC) spectroscopy using the Mo-99(beta(-))Tc-99 probe. Compounds having the composition MoS2 of different origins and having different degrees of crystallinity, Li-intercalated MoS2, Co0.5MoS2, and exfoliated single MoS2 slabs, were investigated, Crystalline and well-ordered 2H-MoS2 exhibits a characteristic NQI-frequency of 111 Mrad/s and axial symmetry. Dispersed MoS2 prepared by thermal decomposition of various precursor compounds shows lower frequencies (70-95 Mrad/s), and this effect is attributed to the presence of part of the Mo in octahedral coordination. Co0.5MoS2 and Li-intercalated MoS2 with Mo in octahedral coordination exhibit typical frequencies around 50 Mrad/s. Single MoS2 slabs prepared by exfoliation of LixMoS2 also give rise to a low frequency of 56 Mrad/s, this being indicative for octahedral Mo coordination. Highly disperse MoS2 samples typically contained additional minority species that have NQI-frequencies in the range 400 < omega < 650 Mrad/s. These fractions were identified as Mo centers that have sulfur vacancies which are located along the edges of MoS2 platelets. These data are relevant for the interpretation of TDPAC spectra of sulfided supported Mo/Al2O3 catalysts. We show that formation steps of the sulfided phase and aging processes can be followed by TDPAC. Moreover, the observation of high-frequency fractions at 400 < omega < 650 Mrad/s represents the first direct experimental detection of coordinatively unsaturated Mo-x+ sites at edges and corners of MoS2 slabs in these catalysts.