Guidelines for the Molybdenum Oxidation State and Geometry from X‑ray Absorption Spectroscopy at the Mo L2,3-Edges

Molybdenum accepts oxidation states from −II to +VI. This versatility is highly beneficial for applications in catalysis, especially combined with sulfur to form the ubiquitous MoS2 material. X-ray absorption near-edge structure (XANES) is a particularly well-adapted technique to study simultaneously both elements since the K-edge of S (2472 eV) and the L2,3-edges of Mo (2520–2625 eV) have similar absorption energies. It provides information on both the electronic and local structures of metal-containing species and allows drawing structure–activity relationships. However, L2,3-edges are difficult to interpret, especially for 4d and 5d transition metals. In addition, only a few recent studies focus on the measurement of the signal of Mo-based reference compounds, meaning that the references from the literature do not benefit from recent technical progress. Notwithstanding theoretical tools that allow a deep understanding of such spectroscopic data, the lack of reference spectra prevents a quick yet reliable interpretation. In this work, we provide a method for the interpretation of X-ray absorption near-edge structure (XANES) data at the Mo L2,3-edges based on a library of spectra of simple Mo compounds. From our analysis, we suggest using the L3-edge to determine the oxidation state (in selected cases) and the L2-edge to gain insight on the geometry around Mo atoms. This method is then applied to a series of molybdenum sulfide compounds to rationalize their structures. Besides this example, these guidelines should help to qualitatively interpret XANES of Mo at L2,3-edges in future studies.