99Mo(.beta.-)99Tc Spectroscopy of MoS2, Lithiated MoS2, Co0.5MoS2, and MoS2 Surface Species

Abstract

Nuclear quadrupole interactions (NQI) were measured for several molybdenum-sulfur compounds by time differential perturbed angular correlation (TDPAC) spectroscopy using the 99Mo@-)99Tc probe. Compounds having the composition MoS2 of different origins and having different degrees of crystallinity, Li-intercalated MoS2, C00.5MoS2, and exfoliated single MoS2 slabs, were investigated. Crystalline and well-ordered 2H-MoS2 exhibits a characteristic NQI-frequency of 11 1 Mrads and axial symmetry. Dispersed MoSz prepared by thermal decomposition of various precursor compounds shows lower frequencies , and this effect is attributed to the presence of part of the Mo in octahedral coordination. Co0,sMoSz and Liintercalated MoS2 with Mo in octahedral coordination exhibit typical frequencies around 50 Mrad/s. Single MoS2 slabs prepared by exfoliation of Li,MoSz also give rise to a low frequency of 56 Mrads, this being indicative for octahedral Mo coordination. Highly disperse MoS2 samples typically contained additional minority species that have NQI-frequencies in the range 400 < w < 650 Mrads. 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 < w < 650 Mrads represents the first direct experimental detection of coordinatively unsaturated MoX+ sites at edges and comers of MoS2 slabs in these catalysts.

Intercalation Systems as Nanostructured Functional Materials

“…95,96 X-ray investigations on colloid MoS 2 layers prepared by exfoliation of lithium-intercalated molybdenum disulfide in polar solvents 97 showed that the single MoS 2 layers exhibit distorted octahedral coordination with a (2 × 1) superlattice; 98 nuclear quadrupole interaction measurements revealed coordinatively unsaturated Mo sites (sulfur vacancies) at the prismatic sites of the MoS 2 slabs. 99 The direct intercalation of polyaniline via colloid suspensions into MoS 2 has been reported (Figure 6). 83 Recently it was shown that in situ formation of polypyrrole/MoS 2 can be achieved via an oxidative encapsulation reaction and polymer growth under kinetically restricted conditions.…”

Heterogeneous Catalysis and Solid Catalysts

“…The time differential observation of the perturbed angular correlation of g rays emitted from radioactive nuclei (TDPAC) [368,370,371] is a g-spectroscopic technique which also allows the determination of hyperfine interactions such as nuclear electric quadrupole interactions (NQI). The NQI parameters enable local structural information around the g emitter to be extracted.…”

Gamma Spectroscopy

“…Therefore, the NQI exhibits axial symmetry. Such high-frequency components have also frequently been observed in highly disperse MoS 2 powders and in surface species obtained by sulfidation of oxidic precursors [49]. As the particle size decreases, this precession frequency decreases by almost a factor of two for particles of about 50Å diameter.…”