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

Nuclear quadrupole interactions (NQI) were measured for several molybd enum-sulfur compounds by rime differential perturbed angular correlati on (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 exfoli ated single MoS2 slabs, were investigated, Crystalline and well-ordere d 2H-MoS2 exhibits a characteristic NQI-frequency of 111 Mrad/s and ax ial symmetry. Dispersed MoS2 prepared by thermal decomposition of vari ous precursor compounds shows lower frequencies (70-95 Mrad/s), and th is effect is attributed to the presence of part of the Mo in octahedra l coordination. Co0.5MoS2 and Li-intercalated MoS2 with Mo in octahedr al coordination exhibit typical frequencies around 50 Mrad/s. Single M oS2 slabs prepared by exfoliation of LixMoS2 also give rise to a low f requency of 56 Mrad/s, this being indicative for octahedral Mo coordin ation. Highly disperse MoS2 samples typically contained additional min ority species that have NQI-frequencies in the range 400 < omega < 650 Mrad/s. These fractions were identified as Mo centers that have sulfu r 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 sul fided phase and aging processes can be followed by TDPAC. Moreover, th e observation of high-frequency fractions at 400 < omega < 650 Mrad/s represents the first direct experimental detection of coordinatively u nsaturated Mo-x+ sites at edges and corners of MoS2 slabs in these cat alysts.